SOME DIAMINES AND POLYAMIDES
CONTAINING CYCLOHEXANE RINGS
H. VAN BREDERODE
SOME D I A M I N E S AND P O L Y A M I D E S C O N T A I N I N G C Y C L O H E X A N E R I N G S
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"0 Ul CD BIBLIOTHEEK TU Delft P 1831 5356SOME DIAMINES AND POLYAMIDES
CONTAINING CYCLOHEXANE RINGS
PROEFSCHRIFT
TER VERKRIJGING VAN DE GRAAD VAN DOCTOR IN DE TECHNISCHE WETENSCHAPPEN
AAN DE TECHNISCHE HOGESCHOOL DELFT, OP GEZAG VAN DE RECTOR MAGNIFICUS
Ir. H. B. BOEREMA,
HOOGLERAAR IN DE AFDELING DER ELEKTROTECHNIEK, VOOR EEN COMMISSIE AANGEWEZEN DOOR HET
COLLEGE VAN DEKANEN TE VERDEDIGEN OP WOENSDAG 4 JUNI 1975 TE 16 00 UUR
DOOR
HENDRIK VAN BREDERODE
SCHEIKUNDIG INGENIEURGEBOREN TE RIJSWIJK (ZH) / A> 1/
1975
Dit proefschrift is goedgekeurd door de promoter: PROF. DR. D.W. VAN KREVELEN
Aan mijn ouders Aan Renate
D A N K B E T U I G I N G
Aan de Raden van Bestuur van Akzo nv en Akzo R e s e a r c h & Engi-neering bv betuig ik mijn dank voor de toestemming tot publikatie van dit onderzoek.
Mijn erkentelijkheid gaat voorts uit naar alle medewerkers van Akzo Research Laboratories Arnhem die er toe hebben bijgedragen dat dit proef-schrift tot stand is gekomen. In het bijzonder dank ik Dr. W. J . Mijs voor zijn waardevolle adviezen en daadwerkelijke steun, alsmede de heer D.A. Scheer voor zijn enthousiaste medewerking aan het praktische deel van het onderzoek.
C O N T E N T S
Page CHAPTER 1 INTRODUCTION
1.1 Aim and scope 11 1.2 Thermal properties of polyamides 13
1.3 Polyamides containing cyclohexane rings in the
main chain 16 1.4 References 19
CHAPTER 2 SYNTHESIS OF 1, 2-BIS(4-AMINOCYCLOHEXYL)-ETHANE 2 . 1 Introduction 21 2.2 Hydrogenations of 4,4'-dinitrobibenzyl 22 2 . 2 . 1 Onestep hydrogenation of 4 , 4 ' d i n i t r o -bibenzyl to BACE 23 2 . 2 . 1 . 1 Results ^3 2 . 2 . 1 . 2 Structure assignments of products 26
2 . 2 . 1 . 3 Discussion 28 2 . 2 . 2 Hydrogenation of 4,4'-dinitrobibenzyl to
4 , 4 ' -diaminobibenzyl 31 2 . 3 Hydrogenations of 4,4'-diaminobibenzyl 31
2 . 3 . 1 Results 31 2 . 3 . 2 Structure assignments of products 33
2 . 3 . 3 Discussion 37 2.4 Stereoisomeric composition of BACE 37
2 . 4 . 1 Identification and determination of isomer
Page 2 . 4 . 2 Effect of the hydrogenation temperature 41 2 . 4 . 3 Other methods of varying the isomer
distribution 43 2 . 4 . 4 Discussion 45 2. 5 Experimental part 46 2.6 References 50
CHAPTER 3 SYNTHESIS OF 1,2-BIS(4-AMINOMETHYLCYCLO-HEXYL)ETHANE
3.1 Introduction 53 3.2 Hydrogenations of 4,4'-dicyanobibenzyl 54
3 . 2 . 1 Results 56 3 . 2 . 2 Structure assignments of products 58
3. 2. 3 Discussion 61 3.3 Hydrogenations of 4,4'-bis(aminomethyl)bibenzyl 62
3 . 3 . 1 Results 63 3 . 3 . 2 Structure assignments of products 65
3 . 3 . 3 Discussion 69 3.4 Stereoisomeric composition of BAMCE 70
3 . 4 . 1 Identification and determination of isomer
distributions 71 3 . 4 . 2 Effect of the hydrogenation temperature 75
3 . 4 . 3 Other methods of varying the isomer
distribution 76 3 . 4 . 4 Discussion 78 3.5 Experimental part 78 3.6 References 84
CHAPTER 4 PREPARATION OF POLYAMIDES
4 . 1 Introduction 87 4 . 2 Preparation of BACE-12 polyamides 88
4 . 2 . 1 Results and discussion 88 4 . 2 . 2 Verification of isomer distribution by
PMR spectroscopy 90 4 . 3 Preparation of BAMCE-6 polyamides 92
4 . 3 . 2 Verification of isomer distribution by PMR spectroscopy 4 . 4 Experimental part 4. 5 References Page 93 94 96
CHAPTER 5 PROPERTIES OF POLYAMIDES
5 . 1 Introduction 97 5.2 BACE-12 polyamides 97 5 . 2 . 1 Thermal properties 97 5.2.2 X - r a y diffraction 100 5 . 3 BAMCE-6 polyamides 102 5 . 3 . 1 Thermal properties 102 5 . 3 . 2 X - r a y diffraction 106 5.4 Discussion on the effects of s t e r e o i s o m e r i s m 107
5 . 4 . 1 Glass transition temperature 107 5.4.2 Melting point and x - r a y diffraction 108
5. 5 Experimental part 113 5.6 References 114
CHAPTER 6 COMPARISON OF THERMAL PROPERTIES OF POLYAMIDES CONTAINING CYCLOHEXANE RINGS IN THE MAIN CHAIN
6 . 1 Introduction 116 6 . 2 The ratio between glass transition temperature
and melting point 118 6 . 3 Glass transition t e m p e r a t u r e s 120 6 . 4 Melting points 123 6. 5 Discussion 126 6.6 References 127 APPENDK 128 SUMMARY 129 SAMENVATTING 132
C H A P T E R 1
INTRODUCTION
1 . 1 . AIM AND SCOPE
This thesis d e s c r i b e s a study into structure-property relations of polyamides containing cyclohexane rings in the main chain. The aim was to mvestigate the influence on the polyamide p r o p e r t i e s , particularly on t h e r -mal properties and crystallization behaviour, of the s t e r e o i s o m e r i s m of 1,4-disubstituted cyclohexane r i n g s .
1,4Disubstituted cyclohexane compounds can occur in two s t e r e o i s o -m e r i c for-ms, viz. cis and t r a n s . The confor-mational equilibria of both for-ms a r e depicted in Fig. 1.1, where only chair conformations for the cyclohexane ring a r e assumed. CIS B TRANS A-Figure 1.1
As the monomers some diamines containing two cyclohexane rings were chosen, viz. 1, 2-bis(4-aminocyclohexyl)ethane (I), abbreviated as BACE,
and 1, 2-bis(4-aminomethylcyclohexyl)ethane (II), abbreviated as BAMCE (see
Fig. 1.2). Because of the presence of two cyclohexane rings in the diamines, there a r e three s t e r e o i s o m e r s , viz. c i s c i s (cc), c i s t r a n s (ct) and t r a n s -t r a n s (-t-t).
BACE H2N-^])-CH2-CH2-<3>-NH2 (I )
BAMCE H2N-CH2-<3-CH2-CH2^^^CH2-NH2 (K)
Figure 1.2
The syntheses of BACE and BAMCE, as well as the methods of v a r y -ing and analys-ing the stereoisomeric compositions a r e described in the t e r s 2 and 3. The polyamide syntheses a r e described in Chapter 4. Chap-ter 5 deals with the p r o p e r t i e s of the polyamides including a discussion on the effect of the s t e r e o i s o m e r i c composition on thermal properties and c r y s
-PACM* H 2 N - < 3 - C H 2 - ( 3 - N H 2 ( m )
CDA H2N-(2)-NH2 (ET)
BAMC H2N-CH2-O-CH2-NH2 ( ^ )
BAEC H2N-CH2-CH2-Q-CH2-CH2-NH2 (3Zr)
Figure 1 3
This abbreviation is commonly used in the literature It will also be used here, although BACM seems a more logical abbreviation
tallization behaviour. In Chapter 6 the thermal properties a r e compared with those of knov^m polyamides containing 1,4-disubstituted cyclohexane r i n g s . These polyamides a r e derived from bis(4-aminocyclohexyl)methane (III), 1,4cyclohexanediamine (IV), l,4bis(aminomethyl)cyclohexane (V) and 1 , 4 b i s -(2-aminoethyl)cyclohexane (VI) (see Fig. 1.3).
1. 2. THERMAL PROPERTIES OF POLYAMIDES
The main transition t e m p e r a t u r e s of polymers a r e the glass(-rubber) transition t e m p e r a t u r e (T ) and the melting temperature (T ). The T r e p -r e s e n t s a change in the amo-rphous -regions of the polyme-r, whe-reas at T
m the crystalline phase t u r n s into liquid. The crystalline melting t e m p e r a t u r e i s a first o r d e r transition point, at which both phases a r e in equilibrium, thus givii^ the relation T AS = AH for the changes in entropy (S) and enthalpy (H). The g l a s s r u b b e r transition has the similarity of a second o r -d e r transition, showing a -discontinuity in the -derivatives of S, H an-d V (volume) . However, there a r e kinetic factors which prevent the glassy state from reaching thermodynamic equilibrium, so that the g l a s s - r u b b e r transition
2
i s not a real thermodynamic second order transition . Consequently, a r e l a -tionship between T and T is not obvious from a thermodynamic point of view. However, an empirical relationship has been found , viz.
T / T = 0 . 50 for symmetrical polymers, and T / T = 0.67 for unsymmetrical polymers,
where t e m p e r a t u r e s a r e expressed in K. Unsymmetrical polymers a r e d e -fined as those containing a main chain atom which does not have two identi-cal substituents, whereas the other polymers a r e regarded as symmetriidenti-cal. Recently, Lee and Knight have reported a study of the T / T r a t i o s of
132 poljrmers . Their conclusion is that there is no sharp division between the T /T values observed for symmetrical and unsymmetrical polymers: the arithmetic means a r e 0.63 and 0.69, respectively. Therefore, the s y m -m e t r y of poly-mers as defined above does not see-m to be a good guide in studying differences in T / T values. To be able to discuss this in more J -o g m detail, one should realize what p r o c e s s e s on molecular scale occur at T and T , and what p a r a m e t e r s of the chemical structure influence these p r o c e s s e s .
Above T , polymer chain segments in the amorphous r e g i o n s exhibit mobility due to rotational i s o m e r i s m As the t e m p e r a t u r e is lowered to and below T , these motions a r e frozen in, or, in other words, an increased
S 5 cooperativity between the motions of neighbouring chain segments a r i s e s
Andrews suggested intermolecular cohesive (secondarv) bonding below T to be predominant. The loosening of this bonding at T allows, a s an inci-dental associated effect, the molecular motions to take place Although this h5rpothesis is perhaps a r a t h e r fundamental approach, it deprives u s of the
1 3
phenomenal correlation between T and chain stiffness ' Therefore, we will adhere to the intermolecular forces as well as the chain stiffness as molecular p a r a m e t e r s which affect T Additionally, geometrical factors
3
such a s the presence of side groups may play a r o l e In fact the same 3
p a r a m e t e r s affect T , at which temperature the chain segments in the c r y s -talline regions move away from the totally regular lattice The breakdown of this lattice r e q u i r e s m o r e energy because of the long-range o r d e r
In polyamides the intermolecular forces a r i s e particularly from the 7
hydrogen bonds of the amide groups. Miyake studied 24 polyamides bv x-rav diffraction as well a s by infrared spectroscopy, and came to the conclusion that "the amide groups form hydrogen bonds almost completelv, irrespective of the number of methylene groups" Even in the amorphous r e g i o n s (below T ) hydrogen bonds a r e apparently present, forming a three-dimensional
net-^ 8 work, which is disrupted at T , as has been suggested by Gordon on the
b a s i s of Differential Thermal Analysis experiments From his experiments we might conclude that for the aliphatic polyamides the suggestion of Andrews - predominant intermolecular bonding - is valid t h e r e is a small endothermal effect near T , and the transition temperature i n c r e a s e s after
g
annealing p r o c e d u r e s . This increase might be explained by a s t r o n g e r net-work of hydrogen bonds Additional evidence that the T of aliphatic poly-amides is connected with a disruption of a hydrogen-bonded network has
9 10
been found by Northolt ' through an investigation of the deformation be-haviour of amorphous polyamide films. It should be emphasized that this network of hydrogen bonds in the non-crystalline regions does not have the long-range order which c h a r a c t e r i z e s the crystalline regions T h i s would mean that a change of the molecular geometry which particularly affects this long-range order may influence the T / T r a t i o
L i t e r a t u r e data on T and T values of aliphatic polyamides (see Table 1.1) indicate that an increasing chain length between the amide groups
Table 1 1
11 12 Polyamides T data T data T /T
g m g m (°C) °C) (K/K)
poly(6-aminocaproic acid) (nylon 6) 50-75 214-233 0 . 6 4 - 0 . 7 1 poly(7-am:noenanthic acid) (nylon 7) 62 217-233 0 . 6 6 - 0 . 6 8 poly(8-aminocaprylic acid) (nylon 8) 50 185-209 0 . 6 7 - 0 . 7 1 poly(9-aminopeIargonic acid) (nylon 9) 50 194-209 0 6 7 - 0 . 6 9 poly(10-aminocapric acid) (nylon 10) 42 177-192 0 . 6 8 - 0 . 7 0 poIy{ll-aiiiinoundecanoic acid) (nylon 11) 46 182-220 0 . 6 5 - 0 . 7 0 poIy(12-aniinolaunc acid) (nylon 12) 37 179 0 . 6 9 poly(hexamethylene adipamide) (nylon 66) —57 265-270 " - 0 61
poly(liexamethylene pimelamide) (nylon 67) 58 202-228 0 . 6 6 - 0 . 7 0 poly(hexamethylene azelamide) (nylon 69) 58 185-226 0 . 6 6 - 0 . 7 2 poly(hexamethylene sebacamide) (nylon 610) SO 215-233 0 . 6 4 - 0 . 6 6
gives r i s e to a decreasing tendency of T and T , in spite of the fact that g ™ 4 11 12 the data show more scatter as more publications a r e available ' ' . In addition, an alternating effect can be observed for the T values of even
13 ™ and odd nylons . However, a comparison of the T / T values of the v a r
-ious polyamides r e v e a l s no striking differences (see Table 1.1). All these aliphatic polyamides have r a t h e r flexible chain segments, which r e s u l t s in comparable crystallization capabilities. The question a r i s e s what happens if the chain flexibility is reduced, for instance by the incorporation of ring s t r u c t u r e s in the main chain. F i r s t of all this will have an increasing effect on T . The occurrence of a similar effect on T , however, will d e
-g m ' pend on the geometry in that this allows conformations of the polymer chain,
which are favourable for packing in a crystal lattice. It will for instance be shown that c i s - l , 4 - d i s u b s t i t u t e d cyclohexane rings in the main chain of a polyamide give r i s e to a melting point lower than in the case of the t r a n s i s o m e r . The cisconfiguration has a l e s s favourable influence on c r y s t a l l i z a -tion. Since T„ is not very much affected by the i s o m e r i s m , different T / T
g J J ' g m
1. 3. POLYAMIDES CONTAINING CYCLOHEXANE RINGS IN THE MAIN CHAIN
Levme and Temin studied the influence of the incorporation of 3 -and 4-aminomethylcyclohexanecarboxylic acid in nylon 6 The copolyamides of e-caprolactam and 4-aminomethylcvclohexanecarboxylic acid (0-50 mole-%) all show higher melting points than nylon 6 On the contrary, 3-amino-methylcyclohexanecarboxylic acid causes depression of the nylon 6 melting point. In the latter case the crystalline lattice of nylon 6 is disturbed, whereas the 1,4-isomer is tolerated to a large extent in the lattice Prince
18
et al. also studied the copolyamides of e-caprolactam and 4aminomethylcyclohexanecarboxylic acid and concluded from x r a y diffraction m e a s u r e -ments, that the nylon 6 lattice can accommodate up to 30 mole-% of the cycloaliphatic compound, whereas at greater than 50 mole% the crystalline s t r u c -ture IS very similar to that of the homopolymer of 4-aminomethylcyclo-hexanecarboxyhc acid. The c i s / t r a n s i s o m e r i s m of this monomer had been taken into consideration. However, it appeared that isomerization took place during the polymerization at 250 C, leading to an equilibrium ratio of 20/80 c i s / t r a n s .
P a r t i a l replacement of 1,6-hexanediamine or adipic acid in nylon 66 19
by ring containing monomers was investigated by Ridgway . 1,4-Cyclo-hexanedicarboxylic acid (10-30 mole-%) causes both T and T to increase, even more than terephthalic acid does. The cycloaliphatic diacid, which is assumed to be present in the polymer as the t r a n s - i s o m e r consequent on
20
isomerization , does not seem to disrupt the chain packing very much and introduces somewhat more rigidity than the aromatic diacid does, which is probably due to the puckered chair conformation. Ridgway found the behav-iour of trans-l,4-bis(aminomethyl)cyclohexane (V), replacing 1,6-hexane-diamine, to be similar. Polyamides of this diamine (BAMC) were already studied in comparison with those of l,4-bis(ammomethyl)benzene by Bell et
21
al. . The polyamides of trans-BAMC show higher melting points than those of the aromatic analogue. This was also believed to be due to the chair conformation, the puckered structure of which r e s u l t s in a slightly shorter r e -peating unit. The effect of the c i s / t r a n s i s o m e r i s m of BAMC is described
21
in the same paper . The polyamides of cis-BAMC melt lower than those of the t r a n s - i s o m e r (see Table 1.2) Copolyamides of c i s - and trans-BAMC with adipic acid (BAMC-6) show a eutectic melting point at a ratio of 80/20
T.!ble 1.2 polyami BAMC-6 90% BAMC-10 95% BAMC-12 90% CDA-10 CDA-12 87K 73% BAEC-6 PA CM-12 d e ^ ' trans cis cis trans cis cis trans cis cis trans cis trans CIS trans CIS c c / c t / t t 0 / 2/98 5/25/70 9/40/51 g (°C) (ref ) 113 (22) 101 (22) 91 (22) 87 (22) 85 (22) 72 (22) 107 (25) 103 (25) 135 (29) 135 (29) 125 (29) T m (°C) 345 347 242-246 237 295-300 285-296 205-208 207 275-278 280 205-215 181-202 420-430 250-255 397 232 348 172 300 290 264 (ref. ) (21) (22) (21) (22) (21) (22) (21) (22) (21) (22) (21) (22) (24) (24) (25) (25) (26) (26) (29) (29) (29) T /T g ni (K/K) 0 . 6 2 0. 72-0 73 0 . 6 4 - 0 65 0 75 0 65 0 71-0.76 0 58 0 . 7 4 0.71 0 . 7 3 0 . 7 4
a ) The structures and abbreviations of the diamine parts are given in Fig. 1.3. The designations of the diacid parts are as follows 6 - adipic acid, 10 = decanedioic acid; 12 = dodecanedioic acid.
c i s / t r a n s . The authors conclude that the two i s o m e r s a r e m e m b e r s of a 22
nonisomorphous system Prince et al prepared polyamides from BAMC of varying c i s / t r a n s ratios and adipic acid, decanedioic acid and dodecane-dioic acid. Their diagrams do not show any eutectic composition The same
23
polyamides were also investigated by x - r a y diffraction Within each series of BAMC polyamides, i . e . BAMC-6, BAMC-10 and BAMC-12, the inter-planar spacings and relative intensities of the crystalline reflections are practically independent of the isomer ratio between 30% and 100% t r a n s
-23
BAMC So, the conclusion of Prince et al, is that the polyamides of c i s -and trans-BAMC crystallize isomorphically within the given r a n g e .
The T / T ratios of BAMC polyamides in correlation with the iso-g m
m e r i s m a r e given in Table 1 2. The glass transition temperatures are " e s -22
sentially independent" of the diamine i s o m e r i s m , whereas the cis-BAMC polyamides melt much lower than the corresponding polyamides of the t r a n s i s o m e r . The same phenomenon a r i s e s with polyamides of cis and t r a n s -1,4-cyclohexanediamine (IV, CDA), viz. a large difference between the melting points, and practically no effect on the g l a s s transition (see Table
9fi 97
1.2). Recently, Ridgway ' reported on polyamides from l , 4 b i s ( 2 a m i n o -ethyl)cyclohexane (VI, BAEC) The melting point of the polyadipamide de-c r e a s e s linearly with the t r a n s - i s o m e r de-content, the de-crystallinity bede-coming very low for cis-BAEC.
Polyamides of bis(4-aminocyclohexyl)methane (III, PACM) have been described extensively in patent l i t e r a t u r e . In it, however, the effect on T and T of the isomer distribution ( c i s - c i s / c i s - t r a n s / t r a n s - t r a n s ) has never
^ 28
been elucidated. Barkdoll et al mentioned that the polyadipamide of tt-PACM melts higher than those of the c c - and c t - i s o m e r In 1971 Prince
29
and P e a r c e presented a paper in which they d e s c r i b e polyamides of PACM in t h r e e isomer ratios with adipic acid 6), decanedioic acid (PACM-10) and dodecanedioic acid (PACM-12). The PACM-12 polyamides (see Table 1.2) show a difference in T of 10°C For the PACM-10 and PACM-6
poly-o poly-o
amides the differences a r e 14 and 20 C, respectively The a u t h o r s refer to this as a "marked effect" on the T of the PACM isomer distribution (cf the differences in T for the BAMC polyamides, in which case the same
S 22 authors speak of "essentially independent" of diamine isomerism) Prince
29
and P e a r c e observed r a t h e r small melting point depressions at lowering the tt-PACM content (cf 98% and 70% of t t - i s o m e r in PACM-12, see Table
For the investigation of the polyamide properties as a function of the s t e r e o i s o m e r i s m of 1,4-disubstituted cyclohexane rings our choice fell upon BACE (I) and BAMCE (II). The advantage of cycloaliphatic diamines over diacids containing carboxyl groups directly attached to the cyclohexane ring is that the diamines do not isomerize during polymerization even at high
. 22,30 temperatures
Our experimental work on BACE and its polyamides was in its final 31
stage when Komoto et al. launched an article on the same subject. This 32
was the reason why we presented a preliminary paper .
1.4. REFERENCES
1 M C SHEN a n d A EISENBERG, R u b b e r C h e m T e c h n o l 4 3 , 9 5 ( 1 9 7 0 ) 2 A J S T A V E R M A N , R h e o l o g i c a A c t a 5, 283 ( 1 9 6 6 ) 3 R F BOYER, R u b b e r C h e m T e c h n o l 3 5 , 1 3 0 3 ( 1 9 6 3 ) 4 W A LEE a n d G ] K N I G H T , Brit P o l y m J 2 , 73 ( 1 9 7 0 ) 5 Yu A S H A R O N O V a n d M V V O L ' K E N S H T E I N , V y s o k o m o l S o e d m 2 , 9 1 7 ( 1 9 6 2 ) , Sov Phys S o l i d S t a t e 5, 4 2 9 ( 1 9 6 3 ) 6 R D A N D R E W S , J P o l y m Sci C , 1 4 , 261 ( 1 9 6 6 ) 7 A M I Y A K E , ] P o l y m S c i 4 4 , 2 2 3 ( 1 9 6 0 ) 8 G A G O R D O N , J P o l y m S c i A - 2 , 9, 1693 ( 1 9 7 1 ) 9 M G N O R T H O L T , J P o l y m S c i C , 3 8 , 2 0 5 ( 1 9 7 2 ) 10 M G N O R T H O L T , B J T A B O R a n d ] J V A N A A R T S E N , C o l l o i d a n d P o l y m S c i , m press 11 W A LEE a n d G ] K N I G H T , P o l y m e r H a n d b o o k (Eds ] B R A N D R U P a n d E H I M M E R G U T ) , I n t e r s c i e n c e N e w Y o r k ( 1 9 6 6 ) , 111-61 12 R L MILLER, i b i d l l l - l13 D R H O L M E S , C W BUNN a n d D J S M I T H , J P o l y m Sci IJ, 159 ( 1 9 5 5 ) 14 H G WEYLAND, P ] H O F T Y Z E R a n d D W V A N KREVELEN, P o l y m e r H , 79 ( 1 9 7 0 ) 1 5 D W VAN KREVELEN, P r o p e r t i e s of P o l y m e r s , C o r r e l a t i o n s w i t h C h e m i c a l S t r u c t u r e , E l s e -v i e r , A m s t e r d a m ( 1 9 7 2 ) , p 1 0 9 16 V P P R I V A L K O a n d Yu S L I P A T O V , P o l y m Sci USSR ^ 3 , 3 0 7 5 ( 1 9 7 1 ) 1 7 M 1£VINE a n d S C T E M I N , J P o l y m Sci 4 9 , 241 ( 1 9 6 1 ) 18 F R P R I N C E , E M PEARCE a n d R J F R E D E R I C K S , J P o l y m Sci A - 1 , 8^, 3 5 3 3 ( 1 9 7 0 ) 19 J S R I D G W A Y , J P o l y m S c i A - 1 , 8 , 3 0 8 9 ( 1 9 7 0 ) 20 T M FRUNZE, V V K O R S H A K a n d V A M A K A R K I N , V y s o k o m o l S o e d i n J_, 3 4 9 ( 1 9 5 9 ) , C h e m Abstr 5 4 , 7 2 1 7 d ( 1 9 6 0 ) 21 A BELL, J G S M I T H a n d C J KIBLER, J P o l y m S c i A , 3 , 19 ( 1 9 6 5 )
22 F R PRINCE F A TURI and E M PEARCE J Polym Sci A-1 10 465 (1972) 23 F R PRINCE and R J FREDERICKS, Macromolecules 5^ 168 (1972)
24 V D KALMYKOVA, M N BOGDANOV N P OKROMCHFDLIDZE, I \ ZHMAYEVA and V \ 1 YEFREMOV Polym Sci USSR 9 2872 (1967)
25 H M SCHMIDT Internal Report Akzo Reseirch 26 J S RIDGWAY J Appl Polym Sci 1_S 1517 (1974) 27 J S RIDGWAY J Polvm Sci Polvm Chem 1_2 2003 (1974)
28 A F BARKDOIL H \\ GRAY, W KIRK, Ir D C PEASE and R S SCHREIBER J Amer Chem Soc 75 1238 (1953)
29 F R PRINCE and E M PEARCE Macromolecules 4 347 (1971) 30 F T WALLENBERGER U S 3,472,818 (Oct 14, 1969)
31 H KOMOTO F HAYANO T TAKAMI and S YAMATO J Polym Sci A 1 9 2983(1971) 32 H VAN BREDERODE, D A SCHEER and W J MIJS J Polym Sci A I U) 2197 (1972)
C H A P T E R 2
SYNTHESIS OF 1, 2-BIS(4-AMINOCYCLOHEXYL)ETHANE
2 . 1 . INTRODUCTION
The most promising synthetic route to l,2-bis(4-aminocyclohexyl)-ethane (BACE) s t a r t s from toluene (see Scheme 2.1 and the literature survey given below). Within the scope of our investigations 4, 4'-dinitrobibenzyl was the starting m a t e r i a l . Our aim was to effect its hydrogenation to BACE in high yield, and to study the possibilities of changing the distribution of the BACE stereoisomers: c i s - c i s (cc), c i s - t r a n s (ct) and t r a n s - t r a n s (tt). In particular we aimed at a gradually decreasing con-tent of the t t - i s o m e r , because we ex-pected this i s o m e r to show the greatest ability to form crystalline polymers (stretched conformation; cf. Fig. 2.5).
Literature survey (Scheme 2.1)
The nitration of toluene had never led to high yields of 4-nitrotoluene, until an investigation of the mercury-catalysed nitration showed better yields
0 2 N - ( Q ^ ^ ^ ^ 2 - C H 2 ^ Q > - N 0 2
" 2 ^ - 0 ^ CH2-CH2-(Q>-NHj
^ I ^
BACE
cis-cis (cc), cis-trans (ct), trans-trans (tt)
due to an improv ed ortho/para ratio (about 0 5) .
The oxidative coupling of 4-nitrotoluene to 4,4'-dinitrobibenzvl under 2
basic conditions has been known since 1893 It has been established that the reaction of 4-nitiotoluene with a base in the presence of oxygen leads to 4,4'dinitrobibenzyl, along with 4nitrobenzoic acid and 4 , 4 ' d i n i t r o
-the 5,6
stilbene . In the absence of oxygen the maximum yield of 4 , 4 ' d i n i t r o
-7 h been described bv House It is based on the method of Green et al t r e a t
-ment of 4-nitrotoluene with methanolic potassium hydroxide and air How-ever, Ijubbling air through a solution of 4-nitrotoluene in methanolic sodium hydroxide has recently been found to gi\ e even better yields of 4 , 4 ' d m i t r o -bibenzyl (86'0"^.
For the reduction of 4,4'-dinitrobibenzvl to 4,4'-diaminobibenzyl sev-eral methods have been described- by metal/acid' , bv hydrazine "
9
and by catalytic hydrogenation with Raney nickel and with palladium-on-14
carbon , All methods afforded yields of over 90%.
Before the recent publication of Komoto et al. the hydrogenation of 4 , 4 ' -15-17 diaminobibenzyl to BACE had only been described in patent literature It was mamlv mentioned m connection with the hydrogenation of bis(4-amino-phenyl)methane to bis(4-aminocyclohexyl)methane (PACM). The catalyst which
1 8
was first described for this hydrogenation consisted of cobaltic oxide, calcium oxide and sodium carbonate, and required t e m p e r a t u r e s of over 200 C Ruthenium dioxide was found to allow hydrogenation at temperatures as low as 100 C ' . The cobalt catalyst mentioned a s v\ell as a ruthenium c a t a lyst were used by Komoto et al. for the hydrogenation of 4 , 4 ' d i a m i n o -bibenzyl to BACE. 4,4'-Dinitrobibenzvl, however, could not be hydrogenated by the cobalt catalyst, whereas several ruthenium catalysts proved to be suitable foi the direct formation of BACE from 4,4'-dinitrobibenzyl . This one-step hydrogenation by a ruthenium catalvst was already known from patent literature
2 . 2 . HYDROGENATIONS OF 4,4'-DINnROBlBENZYL
The synthesis of BACE was attempted by the one-step hydrogenation of 4,4'-dinitrobibenzyl as well as by the two-step procedure isolating the intermediate 4,4'-diaminobibenzyl. The r e s u l t s of the first method and those of the first step of the second method a r e described in this section
2 . 2 , 1 . O n e - s t e p h y d r o g e n a t i o n of 4 , 4 ' - di n 11 r o b i b e n z yl t o B A C E
2 . 2 . 1 . 1 . R e s u l t s
For the hydrogenation of 4,4'-dinitrobibenzyl to BACE in one step a ruthenium catalyst was selected, particularly for the saturation of the a r o matic ring. The use of ruthenium catalysts for hydrogenations of 4 , 4 ' -diaminobibenzyl - the intermediate product in this case - as well as of the comparable compound bis(4-aminophenyl)methane is known to be favour-able ' . Ruthenium is also an excellent catalyst for the hydrogenation of aniline . Under the conditions necessary for the saturation of the a r o -matic ring - high t e m p e r a t u r e and high p r e s s u r e - the reduction of the nitro
22 23
group was expected to proceed readily ' . The water formed by the nitro group reduction was not expected to be unfavourable, since it has been r e -ported that water may have promoting effects m ruthenium-catalysed
hv-27 drogenations
4,4'-Dinitrobibenzyl was submitted to high p r e s s u r e hydrogenation with ruthenium-on-alumina as the catalyst, at t e m p e r a t u r e s varying from 180 to 250 C. 1,4Dioxane was used as the solvent. P r i o r to use the r u t h e -nium catalyst was activated by treatment with hydrogen at room t e m p e r a t u r e .
In some first experiments the effect of ammonia addition to the r e a c -tion mixture was studied. The r e s u l t s a r e shown in Table 2 . 1 and Fig. 2 . 1 . No hydrogen uptake could be observed at room temperature, as appears from Fig. 2 . 1 . When the temperature was raised, the p r e s s u r e increased until above 100 C hydrogen uptake started. After a short period, however, hydrogen absorption temporarily ceased. Any interruption of the reaction at
Table 2 I
Effect of added ammonia m liydro^enations of 4, 4 ' d m i t r o -bibenzyl over rulhenium-on-alumma
hydrogenation conditions yield of BACE
(after distillation)
see Fig 2 1, solvent 1,4-dioxane 58%, 63^^
see Fig 2 1, solvent 1,4-dioxane
containing ammonia 77"g, 81% (0 8 mole/I)
ki
i
I
s 200 150 100 0 ' ^ ^ ^ 5 ^.,.n?e(h)8 r n ^ O Figure 2. 1Effect of added ammonia in hydrogenations of 4,4'-dinitrobibenzyl over m t h e n i u m - o n - a l u m m a ; hydrogen pressure (1 MPa ;^ 9 87 atm) vs time for the reactions in 1,4-dioxane (a) and m 1, 4-dioxane/ammonia (b), both at the temperature indicated.
this stage afforded a product mainly consisting of 4,4'-diaminobibenzyl. Hy-drogen uptake increased again after some time (about 1 h at 180 C), but then it was slower in the experiments in which ammonia had been added. However, upon distillation of the reaction products BACE was obtained in higher yields when the solvent had been 1,4-dioxane/ammonia (see Table 2.1). The lower yields in the experiments without ammonia were caused by the formation of l a r g e r amounts of high-boiling compounds.
Therefore, the hydrogenation experiments were continued with 1,4-dioxane/ammonia as the solvent. Reactions were carried out at different t e m p e r a t u r e s and the reaction products were analysed by gas chromatography (GC). The r e s u l t s a r e given in Table 2 . 2 . GC analyses showed, besides the peak of BACE (III), three peaks of by-products (I, II, IV). No more than t r a c e amounts of 4,4'-diaminobibenzyl were detected. By-product IV was ex-pected to be l-(4-aminocyclohexyl)-2-(4-aminophenyl)ethane. This compound, however, was detected after some hydrogenations of 4,4'-diaminobibenzyl
Table 2 2 Hydrogenations of 4, 4'-dinitrobibenzyl. Hydrogenation conditions a) initial hydrogen pressure MPa (1 MPa « 9 87 a t m ) 14 7 14 7 14.7 14 2 13 2 temper-ature °C 180 180 190 200 250 c) time h 8 22 8 8 6
Composition of reaction product by GC analysis ,d) „d) III d) ,d) 0 0 0 0 5 1 4 4 . 3 2 0 3 6 6 0 9 2 . 3 93 6 92.2 92 7 88 0 6.3 2.1 5 8 3 . 7 4 5
a) Catalyst: S% r u t h e n m m - o n - a l u m m a ; solvent- 1 ^ 4-dioxane/ammonia b) See experimental part,
c) Reaction time at temperature indicated. d) I - IV are given in the retention sequence;
1
III
IV
(see Section 2 . 3 ) , and appeared to have a different retention time (GC). We found compound IV to be l-(4-aminophenyl)-2-(4-hydroxycyclohexyl)ethane (see Section 2 . 2 . 1 . 2 ) , which was also formed after a hydrogenation of 4 , 4 ' -diaminobibenzyl in the presence of water (see Section 2.3). The GC peaks I and II, the f o r m e r of which was only detected after the hydrogenation at 250 C, appeared to relate to products of hydrogenolysis: 1, 2dicyclohexylethane and l(4aminocyclohexyl)2cyclohexyl2dicyclohexylethane, respectively. The s t r u c -t u r e assignmen-ts a r e discussed in Sec-tion 2 . 2 . 1 . 2 .
Complete purification of BACE by distillation in vacuo could not be accomplished. Although after a second distillation no more by-products were detected by GC analysis, the PMR spectrum indicated that BACE was not pure Further analysis (see Section 2 2.1.2) showed the impurity to be l-(4-aminocvclohexyl)-2-(4-hydroxycyclohexyl)ethane, a p r e c u r s o r of which, namely by-product IV, had been found before
2 . 2 . 1 . 2 S t r u c t u r e a s s i g n m e n t s of p r o d u c t s
The structure of BACE, the main product of the ruthenium-catalysed hydrogenations described above, was ascertained by PMR and IR spectroscopy. The spectra of pure BACE a r e shown in Section 2 . 3 . The elementary analysis of its bis(trifluoroacetamide) is given in the experimental part
The PMR spectrum of BACE as obtained in one step from 4 , 4 ' -dinitrobibenzyl and distilled twice is shown in Fig 2 2 Although GC analy s i s showed onlanaly one peak, the PMR signal at 6 3 5 ppm indicated the p r e s ence of an impurity. All protons of BACE but two show chemical shifts b e
-WVr^
,—S(p<^f
Figure 2 2
100 MHz PMR spectrum (CCI4) of BACE obtained from a one step hy drogenation of 4, 4'-dinitrobibenzyl
tween 6 0.8 and 1. 9 ppm. The chemical shift of the protons in the 4-posi-tions IS different for the two possible configura4-posi-tions of the 1,4-disubstituted cyclohexane ring- o 2 . 5 ppm for a trans-substituted ring, and 6 2.9 ppm for a cis-substituted ring (see Section 2 . 4 . 1 ) . The observed signal at 6 3. 5 ppm does not belong to BACE. It was assigned to the methine proton of a cvclohexanol moiety. The PMR spectrum of cvclocvclohexanol shows the methine p r o
-QO
ton signal at 6 3.6 ppm . Further evidence for the presence of a cvclo-hexanol group containing molecule was obtained from IR spectroscopy, which revealed an absorption at 3300 cm much stronger than in the spectrum of pure BACE (Fig. 2.4). More information about the structure of the impurity was obtained with the aid of m a s s spectroscopy. The product of the hy-drogenation c a r r i e d out at 250 C (see Table 2. 2) was analysed by a combi-nation of gas chromatography and mass spectroscopy. Each time a GC peak appeared a m a s s spectrum was taken. The r e s u l t s a r e compiled in Table 2 . 3 . During the appearance of GC peak III two m a s s spectra were recorded, both being at variance with the m a s s spectrum of pure BACE (Table 2.5). In ad-dition to the parent ion of BACE (m/e 224) and its fragmentation pattern, the spectra have intensities at m / e 225 and several lower m a s s values which show a relative increase in the second scan. This indicates the presence of a compound that has about the same GC retention time as BACE and is characterized by a molecular weight of 225. This result and the indications obtained from PMR and IR led to the conclusion that not only BACE but also l-(4-aminocyclohexyl)-2-(4-hydroxycyclohexyl)ethane must have been formed in the hydrogenations of 4,4'-dinitrobibenzyl.
A p r e c u r s o r of it, namely l-(4-aminophenyl)-2-(4-hydroxycyclohexyl)-ethane, was also found in the hydrogenation products, as was indicated by the m a s s spectroscopic analysis of GC peak IV. Here, the parent ion ( m / e 219) shows l o s s of water (to m / e 201), a c h a r a c t e r i s t i c for a cyclohexanol
29
moiety , but it mainly fragments into m / e 106, assigned to the 4-amino-30 benzyl cation or its r e a r r a n g e m e n t product, the aminotropylium ion
The m a s s spectrum taken during the appearance of GC peak I shows m / e 194 as the parent ion and corresponds to the known m a s s spectrum of
31
1,2-dicyclohexylethane . GC peak II was assigned to l-(4-aminocyclohexvl)-2-cyclohexylethane, because the corresponding m a s s spectrum shows m / e 209 as the parent ion, and m / e 192 (loss of ammonia) and m / e 56 as frag-ments. Loss of ammonia and the appearance of m / e 56 a r e c h a r a c t e r i s t i c s
29
Table 2 3
Analysis of products by direct coupling of gas chromatography and mass spectroscopy
GC peak .a) mass spectroscopy m /e (relative intensities) assigned structure (molecular weight) 194(12), 166(12), 110(21), 109(18), 96(75), 83(54),82(100), 81(64), 67(28), 55(16), 43(20)
<3<H-CH^-Q
(194) 209(12), 192(40), 180(10), 163(15), 138(48), 110(42), 96(100), 82(32), 81(54), 67(17), 56(52), 43(13) (209) HI A 225(14), 224(39), 207(100), 195(18), 190(17), 178(24), 169(42), 166(23), 161(14), 154(15), 152(31 ), 136(15), 127(11), 124(26), 112(21), 110(26), 98(51), 96(59), 94(30), 84(15), 82(31), 81(26), 57(21), 56(81), 43(13) 225(24), 224(21), 207(77), 195(9), 190(29), 178(13), 169(20), 166(12), 161(18), 154(38), 152(16), 136(12), 127(19), 124(15), 112(19), 110(25), 98(34), 96(45), 94(55), 84(27), 82(21), 81(26), 57(31), 56(100), 43(18) ,=) (224) (225) IV 219(24),201(10), 106(100), 96(34), 94(32), 84(17), 82(19),81(21) (219) a) Compare Table 2 2 b) See experimental part c) A first scan, B second scan2 . 2 . 1 . 3 . D i s c u s s i o n
The addition of ammonia to the solvent 1,4-dioxane in the ruthenium-catalysed hydrogenation of 4,4'-dinitrobibenzyl improved the yield of BACE. On the other hand, it led to some retardation of the reaction, particularly in the second hydrogenation step, the reduction of the aromatic ring These effects a r e in accordance with l i t e r a t u r e data on the hydrogenation of aniline
25 26
by dicyclohexylamine, the formation of which can be reduced by the addition of ammonia. It i s accepted that the secondary amine is formed via the enamine intermediate, the tautomeric imine structure of which is attacked by aniline or cyclohexylamine, whereupon ammonia is eliminated (see Scheme
0^ '?(\ 1'? '\'\
2.2) ' ' ' . The suppressing effect of ammonia on the formation of s e c -ondary amine h a s been ascribed to the occurrence of the ketimine s t r u c t u r e , depicted in the centre of Scheme 2 . 2 , which is in equilibrium with
cyclo-25 33
hexylamine and the intermediate imine ' . The formation of secondary 34
amine has also been reported by Durland and Adkins , who c a r r i e d out the complete hydrogenation of 2,2'-dinitrobibenzyl in two steps over Raney nickel. Intramolecular secondary amine formation afforded perhydrodibenzoLb,f J-azepine in r a t h e r high yield.
< ^ N H 2
2H21 I
H2_II
NH2 H2 - N H 3 NH2 NH30=^0
H2 -NH3O N H ^
3H-,- O - H ^
NH2 Scheme 2. 2It is highly probable that during our hydrogenations intermolecular secondary amine formation occurred, leading to products of relatively high molecular weight. It may be assumed that the higher yields of BACE, when ammonia had been present during the hydrogenation, were effected by a d e c r e a s e of secondary amine formation.
The formation of l-(4-aminophenyl)-2-(4-hydroxycyclohexyl)ethane and l-(4-aminocyclohexyl)-2-(4-hydroxycyclohexyl)ethane i s to be attributed to the presence of water, which is formed by the nitro group reduction. Indeed, the same compounds were obtained when 4,4'-diaminobibenzyl had been hy-drogenated in the presence of water (see Section 2 . 3 ) . The fragmentation pattern of the m a s s spectrum belonging to GC peak FV clearly indicates that the hydroxyl group is attached to the cyclohexane ring: the parent ion (m/e 219) loses water and shows fragmentation to the 4-aminobenzyl cation (amino-tropylium ion, m / e 106). This is consistent with the postulation that water attacks the six-membered ring in its partly or completely hydrogenated state. Probably the imine intermediates a r e hydrolysed to the cyclohexanone
32 group, which is then further reduced (see Scheme 2 . 3 ) . Debus and Jungers proposed such a mechanism for the formation of cyclohexanone and cyclo-hexanol from dehydrogenation of cyclohexylamine in the presence of water. Also some hydrogenations of aniline and toluidines led to cyclohexanols '
•CH2-0=NH _ ^ - c H 2 O 0 ^ - C H 2 O 0 H
H2O
- C H 2 - ( 3 - N = < | 2 ) ^ C H 2 - ^ - - C H 2 H ( 3 ~ ' ^ ^ 2 + - C H 2 - ( 3 = 0
Scheme 2. 3Another side reaction observed in our hydrogenation experiments is the formation of l-(4-aminocyclohexyl)-2-cyclohexylethane, and, at higher t e m p e r a t u r e s , 1,2-dicyclohexylethane. This hydrogenolysis of the carbon-nitrogen bond is known to occur in hydrogenations of aniline over ruthenium at about 200 C and seems not to be diminished by the addition of ammonia .
It may be concluded that the main disadvantage of the one-step hy-drogenation of 4,4'-dinitrobibenzyl to BACE is the partial introduction of hydroxyl groups, caused by water which is formed by the nitro group reduc-tion. This prevents the simple purification of BACE by distillareduc-tion. There-fore, a two-step hydrogenation procedure, with intermediate isolation of 4,4'-diaminobibenzyl, has been p r e f e r r e d .
2 . 2 . 2 . H y d r o g e n a t i o n of 4 , 4 ' d i n i t r o b i b e n z y 1 t o 4 , 4 ' -d i a m i n o b i b e n z y l
For the preparation of 4,4'-diaminobibenzyl from 4, 4'-dinitrobibenzyl the catalytic hydrogenation over palladium (57o on carbon), according to the
14
method of Lyman et a l . , was selected. We, too, used N, N-dimethyl-formamide a s the solvent, because at room temperature the solubilities of the starting material and the product a r e moderate and good, respectively. The hydrogenation was preferably carried out in an autoclave at 2 MPa (20 atm) hydrogen p r e s s u r e . The exothermic reaction raised the t e m p e r a -ture up to about 40 C, and hydrogen uptake ceased within 1 h. The yield of 4,4'-diaminobibenzyl after recrystallization was 90 - 94%.
2 . 3 . HYDROGENATIONS OF 4,4'-DIAMINOBIBENZYL
4,4'-Diaminobibenzyl was converted into BACE by hydrogenation over ruthenium-on-alumina. The effect of ammonia addition to the solvent 1,4-dioxane a s well a s the influence of the hydrogenation temperature on the product composition and the yield of BACE a r e described in this section. The hydrogenation temperature was varied because we aimed at obtaining differences in the isomer distribution of BACE. The possibilities concerning this isomer variation a r e described separately (Section 2.4).
2 . 3 . 1 . R e s u l t s
The hydrogenations of 4,4'-diaminobibenzyl were c a r r i e d out at high hydrogen p r e s s u r e (about 14 MPa ?« 140 atm) and at t e m p e r a t u r e s between 140 and 200 C. Here, too, the ruthenium catalyst (5% on alumina) was pretreated with hydrogen immediately before u s e . The effect of adding a m -monia or ammonium hydroxide to the solvent 1,4-dioxane was investigated in hydrogenation experiments at 180 C for 5 h. The reaction products were analysed by GC. The r e s u l t s a r e shown in Table 2 . 4 . Comparison between the hydrogenations c a r r i e d out with and without added ammonia (no. 2 and 1) showed a less complete reduction for the experiment in which ammonia had been present. The compound showing GC peak V appeared to be the partly hydrogenated product l-(4-aminocyclohexyl)-2-(4-aminophenyl)ethane (see Section 2 . 3 . 2 . ) , whereas GC peak VI was found to belong to the starting
CO to Table 2 4 Hydrogenations of 4, 4'-diaminobibenzyl. n o 1 2 3 4 5 6 7 8 9 10 h y d r o g e n a t i o n t e m p e r a t u r e °C 180 180 180 140 150 ISO 160 180 180 2 0 0 t i m e h 5 5 5 20 6 20 20 14 20 20 c o n d i t i o n s b) a d d i t i o n -N H j ^ ' NH '^*,H 0 N H j " ' d o d o d o d o d o d o d) Il''> % 1 1-1 1 2 - 1 2 0 0 5 0 1 5 1 0 2 8 3 1 3 8 4 4 9 6 8 7 III 5 9 -88 9 3 4 3 9 7 9 7 9 6 9 6 9 6 1 ) 9 6 8 8 8 2 7 0 9 7 9 2 GC 7 6 , e a n a l y s i s iv''' % 0 0 4 2 0 0 0 0 0 0 0 f) V ^ ' 2 1 2 6 8 - 8 2 9 3 3 4 7 3 1 5 0 8 0 5 0 0 2 0 h ) VI ' 0 2 7 - 3 4 2 1 3 0 9 0 0 0 3 0 0 0 u p o n y i e l d of BACE, % 6 0 - 6 4 7 0 - 7 1 -78 -81 82 79 82 74 d i s t i l l a t i o n p u r i t y of BACE, / . ^ ' > 9 9 9 5 - 9 6 98 •>99 > 9 9 . > 9 9 > 9 9 9 9
a) Catal^-st S% ruthenium-on-alumina solvent 1,4-dioxane, initial hydrogen pressure 13-15 MPa ^ 130-150 atm
b) Reaction time at temperature indicated c) 0 8 mole '1
d) 2 8 m o l e / I
e) See experimental part
f) Retention sequence II, III, V, IV, VI. g) Determined by GC analysis
h) II
III
^ C H - C H ^ - ^ N H ^
m a t e r i a l . However, upon hydrogenation without added ammonia BACE was obtained in relatively low yield, leaving a r a t h e r large amount of high-boiling compounds. When ammonia and water had been added to the hy-drogenation m i x t u r e (no. 3), GC analysis of the reaction product showed the p r e s e n c e of l-(4-aminophenyl)-2-(4-hydroxycyclohexyl)ethane (IV), which com-pound had already been detected after hydrogenations of 4,4'-dinitrobibenzyl
(see Section 2 . 2 . 1 ) .
Although longer reaction times were needed, the hydrogenation e x p e r i -m e n t s were continued with added a-m-monia, because it resulted in higher yields of BACE (see Table 2.4). Of the by-products formed the hydrogenolysis product l-(4-aniinocyclohexyl)-2-cyclohexylethane (II) had also been detected before (see Section 2 . 2 . 1 ) . Only small amounts were formed, but an in-c r e a s e with inin-creasing reain-ction temperature in-could be observed. The in-content of BACE in the reaction product was scarcely affected by the differences in reaction t e m p e r a t u r e , unless the reaction time had been too short (no. 5).
As is shown in Table 2.4, BACE of good purity could be obtained in yields of about 80%. F r e s h l y distilled samples were semi-solids, showing a melting range from room temperature or below up to about 105 C. After standing in contact with a i r , they completely solidified by reaction with c a r -bon dioxide. Storage under nitrogen was therefore n e c e s s a r y .
2 . 3 . 2 . S t r u c t u r e a s s i g n m e n t s of p r o d u c t s
PMR and ER spectroscopy confirmed the s t r u c t u r e of BACE. The d e -picted spectra ( F i g s . 2.3 and 2.4) pertain to a mijcture of s t e r e o i s o m e r s obtained by the hydrogenation at 150 C for 20 h. The PMR spectrum of BACE is discussed in detail in Section 2 . 4 . 1 . The m a s s spectrum of BACE i s given in Table 2. 5. As BACE appeared to r e a c t with carbon dioxide r a t h e r quickly, i t s bis(trifluoroacetamide) was used for elementary analysis (see experimental p a r t ) .
The GC analysis of some hydrogenation products showed the presence of the starting m a t e r i a l , 4,4'-diaminobibenzyl (VI). This was proved by GC analyses of a sample to which 4,4'-diaminobibenzyl had been added. In the s a m e way the components II and IV were found to be identical with by-products obtained after hydrogenations of 4,4'-dinitrobibenzyl (see Section 2 . 2 . 1 ) , namely l(4aminocyclohexyl)2cyclohexylethane and l ( 4 a m i n o -phenyl)-2-(4-hydroxycyclohexyl)ethane, respectively.
To elucidate the structure of component V the reaction product con-taining 47.3'~o of it (no. 5 in Table 2.4) was submitted to the d i r e c t coupling of gas chromatography and mass spectroscopy. The mass s p e c t r u m (see Table 2. 6) showed m / e 218 a s the parent ion, and the fragment m / e 106 ascribed to the 4-aminobenzyl cation (aminotropylium ion ). It showed the compound to be l-(4-aminocyclohexyl)-2-(4-aminophenyl)ethane. This assign-ment was confirmed by the PMR spectrum of the reaction product assign-mentioned, which showed the presence of a corresponding number of a r o m a t i c protons.
The r e s u l t s a r e compiled in Table 2 . 6 .
\J
L
2 1 -i(ppm)
Figure 2 . 3
2000 1S00
WAVENUMBERIcm'l
Figure 2.4
m / e 224 207 195 178 169 166 152 136 124 Mass Table 2 spectrum rel. intensity 10 22 4 4 8 4 7 4 7 5 of BACE m / e 112 110 98 96 82 81 67 56 43 rel. intensity 6 7 15 24 20 22 16 100 38 Table 2 . 6
Structure assignment of products formed by hydrogenation of 4, 4'-diaminobibenzyl.
GC peak .a) assigned structure data
^]^H-CH^ NH see Section 2 , 2 . 1 2 (BACE) PMR - F i g 2 3 IR Fig. 2 . 4 MS . Table 2 5 elementary analysis b) IV H O ^ ^ H ^ - C H ^ - ^ ^ H ^ see Section 2,2 1 2 V H^N^^^^_CH^-^Q>^H^ b) mass spectrum (directly after GC) 218(31), 201(17), 159(18), 158(17), 147(18), 106(90), 98(22), 96(30), 81(25), 73(56), 64(48), 56(100), 43(19)
VI
»2''^^»2-^»2-Q^<'»,
identical with starting materiala) Compare Table 2 4, b) See experimental part.
2. 3. 3 D i s c u s s i o n
The hydrogenation of 4,4'-diaminobibenzyl resulted in higher vields of BACE when ammonia had been added to the hydrogenation mixture, a s has previously been observed in hydrogenations of 4,4'-dinitrobibenzyl This effect IS a s c r i b e d to suppressed formation of secondarv amines in the p r e s ence of ammonia, a s discussed in Section 2 2 1.3 On the other hand, a m monia r e t a r d e d the hydrogenation to some extent, as was shown by the s o m e -what higher amounts of aromatic compounds obtained after 5 h reaction at 180°C.
The additional presence of water during the hydrogenation of 4 , 4 ' -diaminobibenzyl led to the formation of the same by-products as were found after the hydrogenations of 4,4'-dinitrobibenzyl l-(4-aminophenyl)-2-(4-hydroxycyclohexyl)ethane and l-(4-aminocyclohexyl)-2-(4-hydroxvcyclohexyl)-ethane. The hydroxyl groups a r e presumably introduced by hydrolysis of imine intermediates and subsequent reduction of the cyclohexanone (see Scheme 2.3) .
As had already been noticed in the experiments with 4 , 4 ' d i n i t r o b i -benzyl, the ruthenium-catalysed hydrogenation of the aromatic diamine to BACE IS accompanied by some hydrogenolysis Particularly at the higher reaction t e m p e r a t u r e s amino groups a r e split off, though only m an amount of a few p e r cent According to the literature the addition of ammonia does not s e e m to lead to a decrease of hydrogenolysis.
F o r the preparation of pure BACE in good vields by hydrogenation of 4,4'-dinitrobibenzyl the two-step procedure via 4,4'-diaminobibenzyl is indeed m o r e favourable than the one-step procedure described in Section 2 . 2 . 1 . The two-step procedure described here afforded BACE of s 999{ pu-rity in an overall yield of about 75%.
2 . 4 . STEREOISOMERIC COMPOSITION OF BACE
The CIS-trans i s o m e r i s m resulting from the 1,4-disubstitution of a cyclohexane ring gives r i s e to the occurrence of three BACE stereoisomers* CIS-CIS (cc), CIS-trans (ct) and t r a n s - t r a n s (tt). Assuming only chair confor-mations the substituents of both cyclohexane rings in tt-BACE mainly occupy the equatorial positions (see Fig. 2.5), because the axial positions a r e high-ly unfavourable a s a r e s u l t of the 1, 3-interaction between an axial substituent
37 and the syn-axial hydrogen atoms .
TRANS-SUBSTITUTION CIS SUBSTITUTION
H
Figure 2 5
The cis-substituted ring in ct-BACE will have one of the substituents in equatorial position (e) and the other in axial position (a), the two possible conformations (e,a and a,e) being in equilibrium (see F i g 2.5). In cc-BACE this IS the case in both cyclohexane r i n g s .
Section 2 . 4 . 1 describes in what way the t h r e e stereoisomers could be identified and differentiated from each other, so that a quantitative de-termination of the isomer distribution could be accomplished.
The stretched conformation of tt-BACE was expected to be the most favourable for the formation of crystalline p o l y m e r s . Therefore, in view of our study of the polyamide properties in relation to the stereoisomerism, our aim was to synthesize BACE with gradually varied contents of its t t -i s o m e r . The most attract-ive method would be the adjustment by the hy-drogenation conditions, particularly by the hyhy-drogenation temperature.
Indeed, the analogous ruthenium-catalysed hydrogenation of bis(4-aminophenyl)methane to bis(4-aminocyclohexyl)methane (PACM) shows a de-pendence of the PACM isomer ratio on the hydrogenation temperature- at 100 -120 C mainly c c - and ct-PACM a r e formed, while the tt-content
in-20
However, the maximum content of the 38,39 c r e a s e s at higher t e m p e r a t u r e s
t t - i s o m e r in PACM directly obtained from hydrogenation is about 54% The equilibrium ratio of approximately 6% c c - , 40% c t - and 54%, tt-PACM IS reached both after hydrogenations at 190 -290 C and after isomerizations of PACM with higher tt-contents. F o r the preparation of PACM containing
m o r e than 54% of t t - i s o m e r several methods have been developed. Fraction-19 al crystallizations of the diformyl-derivative lead to the pure t t - i s o m e r . A similar procedure with the diamine in the presence of water or alcohols
40-43
gives enrichment in tt-PACM . The preferential crystallization of the 44 45 t t i s o m e r also holds for salts of PACM and dicarboxylic acids ' . C r y s -tallizations of the carbamate, which is formed from PACM and carbon
di-46 47 oxide, also offers a way of obtaining high contents of tt-PACM '
Section 2 . 4 . 2 d e s c r i b e s to what extent the isomer distribution of BACE could be influenced by the temperature at which 4,4'-diaminobibenzyl was hydrogenated. The other methods used to alter the isomer distribution a r e described in Section 2 . 4 . 3 .
2 . 4 . 1 . I d e n t i f i c a t i o n a n d d e t e r m i n a t i o n of i s o m e r d i s t r i -b u t i o n s
In the PMR spectrum of BACE as a mixture of i s o m e r s (Fig. 2.3) all protons show chemical shifts between 5 0.8 and 1.9 ppm, except for the two methine protons in the 4-positions. These protons show signals at 5 2. 5 and 2.9 ppm (see Fig. 2.6c, in which this part of the spectrum is depicted again). The signal at 2. 5 ppm is assigned to the proton H(4) in a t r a n s substituted ring, because this occupies mainly the axial position. The c o r -responding proton in trans-4-t-butylcyclohexylamine is known to absorb at 6 2.47 - 2.52 ppm, whereas in cis-4-t-butylcyclohexylamine, where it is predominantly in equatorial position, it exhibits a signal at 6 3. 05 - 3.15
48
ppm . The proton H(4) in a cis-substituted ring of BACE absorbs at 2.9 ppm, because its occupation of the equatorial position is important but not predominant; the rapid inversion between both chair conformations at room temperature brings the proton in axial as well as in equatorial position. The assignment of the signals at 6 2.5 and 2. 9 ppm to H(4) of t r a n s - and cis-substituted r i n g s , respectively, is supported by the fact that the first signal is broader than the second. An axial proton H(4) is expected to show a triplet of t r i p l e t s which is broad a s a r e s u l t of the axial-axial coupling constant of m o r e than 11 Hz. If the proton is in the equatorial position a
48 49 quintet with spacings of about 3 Hz can be expected '
The relative intensities of the signals at 6 2.5 and 2.9 ppm present the ratio between t r a n s - and cis-substituted r i n g s . This ratio is equal to (2 tt + ct)/(2 cc + ct). However, additional information required for the d e
-a b c PMR spectra of
(HpN^Q^ CH2-)
\ i 3 2^.j
1
I
1 0k
4 3 2 1 0 A / \ / part of Fig 2 3 /. •} 7 1 n - — 5(pp )m) GC analysis of0
retention sequence peak 1 peak 2 peak 30% 1% 99%
5% 94% 1 %
14% 47% 39%
• see experimental part Figure 2 6
termination of the ratio between c c - , c t - and tt-BACE could not be obtained f r o m the PMR spectra. Even after addition of the paramagnetic shift reagent tris(dipivalomethanato)europium we obtained insufficient information, although we used this method successfully in the determination of the isomer d i s t r i -bution of PACM .
Analysis of a mixture of BACE s t e r e o i s o m e r s by gas chromatography (GC) appeared to be possible if its bis(trifluoroacetamide) (BACE-TFA) was u s e d . BACE was converted quantitatively into this derivative by the action of trifluoroacetic acid anhydride. GC analysis then showed three peaks. F o r the determination of the retention sequence of the t h r e e i s o m e r s at least two samples each enriched in a different isomer were needed. R e c r y s t a l l i -zation of BACE-TFA from N, N-dimethylformamide afforded an almost pure i s o m e r , corresponding with the latest GC peak (see Fig. 2.6a, peak 3). Subsequent hydrolysis yielded the diamine whose PMR spectrum showed it to be tt-BACE: only resonance at 6 2.5 ppm for the protons H(4).
Further recrystallizations of the residual BACE-TFA from ethanol/ hexane resulted in a sample which showed mainly GC peak 2. The PMR s p e c t r u m of the diamine demonstrated that both methine proton signals were in a 1:1 ratio, which proved this isomer to be ct-BACE (see Fig. 2.6b).
Thus, GC analysis of BACE-TFA offered the three i s o m e r s in the following retention sequence: c c - i s o m e r , c t - i s o m e r and t t - i s o m e r . Their distribution could be determined from the relative peak a r e a s . Correlation with the PMR spectra could be made by calculating the ratio (2 tt + c t ) / (2 cc + ct) and comparing it with the relative intensities of the PMR signals at 6 2. 5 ppm and 5 2.9 ppm. For instance, the PMR spectrum depicted in F i g . 2.3 and partly in Fig. 2.6c shows an "overall" t r a n s / c i s ratio of 1.6 - 1.7, while the isomer distribution of c c / c t / t t = 14/47/39 indicates t h i s ratio to be 1.67.
2 . 4 . 2 . E f f e c t of t h e h y d r o g e n a t i o n t e m p e r a t u r e
In view of the desired variation of the isomer distribution of BACE it was investigated to what extent this distribution could be influenced by the hydrogenation conditions. P a r t i c u l a r l y the t e m p e r a t u r e at which 4 , 4 ' -diaminobibenzyl was hydrogenated was expected to be an important factor. In addition, the reaction time might have some influence.
4,4'-diaminobibenzyl, most of which experiments have been described in Section 2 . 3 , were determined by GC analysis of the bis(trifluoroacetamides). Hydrogenations for 20 h at t e m p e r a t u r e s varying from 140 to 200 C as well a s some hydrogenations at 180°C for different reaction times w e r e taken into account. The r e s u l t s a r e shown in Table 2 . 7 . Also some isomer d i s tributions of BACE obtained by onestep hydrogenations of 4 , 4 ' d i n i t r o b i -benzyl a r e mentioned. Although l-(4-aminocyclohexyl)-2-(4-hydroxycyclo-hexyl)ethane had to be present there (see Section 2 . 2 . 1 ) , GC analysis after trifluoroacetylation showed only three peaks, the intensity r a t i o of which is mentioned in the table.
F r o m Table 2.7 it appears that the isomer distribution of BACE can indeed be influenced by the hydrogenation conditions. The formation of the trans-configuration i n c r e a s e s with increasing hydrogenation temperature a s well a s with increasing reaction t i m e . However, the percentage tt-BACE never exceeds 56%.
Table 2 . 7
The isomer distribution of BACE influenced by the hydrogenation conditions.
b) c) starting hydrogenation conditions isomer distribution of BACE
temperature time cc ct tt °C h % % % DABB DABB DABB DABB DABB DABB DABB DNBB DNBB 140 150 160 180 180 180 200 180 180 20 ao 20 s 14 20 20 8 22 26 14 11 12 8 8 7 11 7 47 47 42 46 40 36 38 41 38 27 39 47 42 52 56 55 48 55
a) DABB = 4, 4'-diaminobibenzyl; DNBB = 4, 4'-din]trQbibenzyl
b) All experiments were carried out m 1, 4-dioxane/ammonia with ruthenium (5% on alumina) as the catalyst, starting with 13-15 MPa (130-150 atm) hydrogen pressure
c) Determined by GC analysis of BACE-TFA, d) For each percentage + 1%.
The supposition that this is due to the attainment of an equilibrium was verified by isomerization experiments. BACE samples were submitted to the same conditions as used in the hydrogenations of 4,4'-diaminobibenzyl. The r e s u l t s a r e given in Table 2 . 8 . Particularly the decrease in tt-content from 97% to 61% by treatment at 200 C for 24 h shows that it is impossible to obtain high contents of ttBACE directly from the hydrogenation of 4 , 4 ' -diaminobibenzyl. a) conditions temperature °c 200 200 180 180 220 270 Table 2 8 Isomerization experiments time h 24 24 24 120 120 24 before cc % 14 1 8 8 8 8 with somer treatment ct % 47 2 40 40 40 40 tt % 39 97 52 52 52 52 BACE distribution after cc % 8 5 6 5 5 7 treatment ct % 36 34 40 36 38 38 tt % 56 61 54 59 57*" 5 5 " '
a) Treatment over mthenium (5% on alumina) in 1,4-dioxane/ammonia at 15 MPa (150 atm) hydrogen pressure
b) By-products were formed
2 . 4 . 3 . O t h e r m e t h o d s of v a r y i n g t h e i s o m e r d i s t r i b u t i o n
Recrystallizations, preferably of well-crystallizing derivatives of BACE, were found to be effective for changing the isomer distribution of BACE. As already described in Section 2 . 4 . 1 , crystallization from a solution of BACETFA in N, Ndimethylformamide yielded the t t i s o m e r in p r a c -tically pure form: c c / c t / t t = 1/2/97. If this was recrystallized again the t t - i s o m e r content r o s e to 99%. In Section 2 . 4 . 1 it was also mentioned that the c t - i s o m e r was isolated in 94% purity after several crystallizations of the residual bis(trifluoroacetamide). The diamines could be recovered by alkaline hydrolysis.
Enrichment in tt-BACE could also be effected by recrystallization of the salt of BACE and adipic acid, on the analogy of the methods described
44 45
for PACM ' . After preparation of the adipic acid salt in ethanol and two successive recrystallizations from ethanol/water mixtures the resulting salt contained 97% tt-BACE. Continued recrystallizations of the residual amounts of the adipic acid salt gave BACE with t t - i s o m e r contents of 64%, 50%, 35% and 19%.
The complete list of isomer distributions found after the r e c r y s t a l l i -zations - the one with 81% tt-BACE was obtained by mixing - is shown in Table 2 . 9 .
The freshly distilled samples of BACE were s e m i - s o l i d s , except for those of the higher t t i s o m e r contents. The melting point of ttBACE (108 -109 C) was determined immediately after distillation.
Table 2 . 9
Isomer distributions of BACE.
o o o o a) m p 302 - 3 0 2 . 5 C m p 108 -109 C m . p . 300°-302°C b p. 137°-140°C/66, 7 Pa''' cc % 0 1 1 2 8 5 9 11 5 ct % 1 2 18 34 40 45 56 70 94 tt 99 97 81 64 52 50 35 19 1
data of BACE-TFA data of BACE
b p 130 -133 C / 5 3 . 3 Pa b . p . 1 2 5 ° - 1 3 0 ° C / 4 0 . 0 Pa o o o o a) m . p . 155 -270 C m . p 25 -105 C b . p . 1 1 0 ° - 1 1 5 ° C / 1 3 . 3 Pa b . p 1 1 8 ° - 1 2 3 ° C / 2 6 . 7 Pa b . p . ! 2 4 ° - 1 2 8 ° C / 4 0 . 0 Pa o o m . p . 165 -170 C
a) Determined immediately after distillation. b) 10 Pa S3 0 . 0 7 5 mm Hg.
2 . 4 . 4 . D i s c u s s i o n
Our method of determining the isomer distribution of BACE by GC analysis of the bis(trifluoroacetamide) seems more accurate and more con-venient than the method described by Komoto et al. . They estimated the i s o m e r distribution by combining the ratio (2 tt + ct)/(2 cc + ct) presented by the PMR spectrum with the content of tt-BACE determined by formylation and subsequent fractional crystallization.
The increase in ttBACE content with increasing hydrogenation t e m -p e r a t u r e is also observable from the r e s u l t s of Komoto et al. . However, the highest percentage tt-BACE reported by them is 45%, found after hy-drogenation of 4,4'-diaminobibenzyl at 200°C over a cobalt catalyst. As far as the isomer distributions obtained from ruthenium-catalysed hydrogenations are mentioned in their publication, hydrogenation at 170 C resulted in the highest tt-BACE content: 30%. Generally, the tt-BACE content as ,a function of the hydrogenation t e m p e r a t u r e is lower than reported h e r e . This might be due to the relatively short reaction times in the hydrogenation e x p e r i -ments of Komoto et al. . Indeed we found that longer reaction times lead to higher tt-BACE contents. On the other hand, their method of analysis, which includes fractional crystallization, might have resulted in too low values of the tt-BACE content.
The influence of the hydrogenation t e m p e r a t u r e on the isomer d i s t r i bution of BACE shows similarity to the case of PACM: increase in t t i s o -mer content with increasing hydrogenation t e m p e r a t u r e up to a certain
max-20 38 39
imum. For PACM this maximum is 54% of the t t - i s o m e r ' ' , whereas for BACE we found 56% as the highest value. The isomerization e x p e r i -ments indicate that here the configurational equilibrium is reached. As ex-pected from conformational considerations, the t t - i s o m e r (all substituents in equatorial position; cf. Fig. 2.5) i s the most stable (55-59%), followed by the c t - i s o m e r (36-387o) and the c c - i s o m e r (5-8%).
The fact that the configurational isomerization could be achieved un-der the conditions used ("hydrogenation conditions") implicates the occurr e n c e of isomeoccurrization duoccurring the fooccurrmation of BACE foccurrom 4 , 4 ' d i a m i n o bibenzyl. If the intermediate enamineketimine (see Scheme 2.2), the o c c u r -rence of which is indicated by the observed side reaction with water (Scheme 2.3), i s desorbed from the catalyst surface, readsorption can lead to the formation of the trans-configuration. The same intermediate might be in-volved in the isomerizations of BACE.
2 . 5 . EXPERIMENTAL PART
Melting points were determined under a Reichert melting point m i -croscope and a r e uncorrected.
Gas chromatographic (GC) analyses were carried out on a Varian Aerograph 1520B gas chromatograph with flame ionization detection. Peak a r e a s were determined with the aid of an integrator, type Infotronics CRS-101. The stationary phases and conditions used a r e mentioned below.
Mass spectra were measured on a Varian-MAT CH-5 mass spectrometer at 70 eV and an inlet temperature of 220 C. In the investiga-tions in combination with GC helium was used as the c a r r i e r g a s . The cou-pling between the gas chromatograph and the m a s s s p e c t r o m e t e r consisted of a Watson-Biemann molecular s e p a r a t o r .
Proton magnetic resonance (PMR) spectra were obtained at 25 C with a Jeol 4H-100 (100 MHz) spectrometer, using carbon tetrachloride as the solvent and tetramethylsilane as the internal standard.
Infrared (IR) spectra were recorded using a P e r k i n - E l m e r 457 Grating Infrared Spectrophotometer.
The elementary analysis was performed by Mr. W . J . Buis of the Institute for Organic Chemistry TNO (Utrecht).
4 , 4 ' -dinitr obibenzyl
The preparation of this compound was c a r r i e d out by the oxidative 7
coupling of 4-nitrotoluene, according to the literature procedure .
4,4'-diaminobibenzyl
The catalytic hydrogenation of 4,4'-dinitrobibenzyl was carried out 14
according to the method of Lyman et al. :
In a 2-litre stainless steel autoclave (Andreas Hofer, Miilheim/Ruhr) equipped with a reciprocating vertical s t i r r e r 100 g of 4,4'-dinitrobibenzyl were hy-drogenated at 2 MPa (20 atm) using 5 g of 5% palladium-on-carbon (Koch-Light Laboratories Ltd.) as the catalyst and 1.5 1 of N, N-dimethylformamide as the solvent. The reaction was completed within 1 h, the temperature rising to about 40 C due to the exothermic reaction. The catalyst was r e -moved by filtration and the diamine was isolated by precipitation with water.
After recrystallization from ethanol/water 4,4'-diaminobibenzyl was obtained in yields of 90 - 94%, m . p . 137. 5°-138. 5°C (literature: 137° - 138°C"'^^; 141° - 142°C (after sublimation)^^).
1, 2-bis(4-aminocyclohexyl)ethane (BACE)
The hydrogenations of 4,4'-dinitrobibenzyl and 4,4'-diaminobibenzyl were carried out in an 0 . 2 5 - l i t r e stainless steel autoclave (Andreas Hofer, Miilheim/Ruhr) equipped with a reciprocating vertical s t i r r e r .
1,4-Dioxane was used as the solvent. It was purified from acetic acid by saturation with gaseous ammonia, followed by filtration and distilla-tion. The distillation was omitted when 1, 4-dioxane/ammonia was used as the solvent. In those c a s e s the concentration of ammonia was 0. 8 mole/1, as determined by titration with hydrochloric acid.
5% Ruthenium-on-alumina (Koch-Light Laboratories Ltd.) was used as the catalyst.
Generally, the procedure was as follows:
The autoclave was charged with 1.5 g of the ruthenium catalyst and 150 ml of the solvent. The catalyst was activated at room temperature and 10 MPa (100 atm) hydrogen p r e s s u r e for at least 2 h. Hereafter 15 g of the substrate were added and the hydrogen p r e s s u r e was adjusted to 13 15 MPa (130 -150 atm). Then the temperature was raised to the required level and main-tained (+ 3 C) during the required reaction time (for details, see Tables 2.2 and 2.4). After the reaction mixture had been cooled, it was removed from the autoclave, and the catalyst was filtered off. The solvent was evap-orated and the residue was distilled in vacuo and/or analysed by GC.
For the GC analysis TENAX GC was used as the column packing. The temperature was programmed from 200 to 350 C (20 C/min).
Hydrogenations of 4,4'-dinitrobibenzyl
The conditions under which these hydrogenations were c a r r i e d out in accordance with the general procedure described above a r e shown in Fig. 2 . 1 and Table 2 . 2 . After the hydrogenations in the absence of ammonia, c a r r i e d out at 180 C for 8 h, distillation in vacuo resulted in obtaining BACE as a white semi-solid in the following yields:
1. 7.8 g (63% of theory) at 124°-133°C/40. 0 Pa (0.3 mm Hg); distiUation residue: 3.9 g.
