SUBSTITUENT EFFECTS I N THE HYDROGENATION ON PALLADIUM
Proefschrift
ter verkrijging van de graad van doctor in de techni-sche wetenschappen aan de Technitechni-sche Hogeschool Delfts op gezag van de rector magnificus ir, H.H. van Nauta Lemkey hoogteraar in de A f dueling der elektrotechniek, voor een commissie uit de senaat te verdedigen op
woensdag 24 november 1971 te 16.00 uur door
Antonius Petrus Gerardus Kieboom
scheikundig ingenieur geboren te Rotterdam
/ ^
iS- 3/52
1971
Dit proefschrift is goedgekeurd door de promotor PROF, DR. IR, H. V M BEKKOM
c c
N T E N T S INTRODUCTION I. II. III. IV. V. VI.ELECTRONIC AND STERIC El-'J<EU'i'b IN THE HYDROGENATION OF ALKÏ1 ARYL KETONKS ON PALLADIUM
Introduction Experimental Results Discussion
Additional comments
SUBSTITUENT EJ-'Jf'KCTS IN THE HYDROGENOLYSIS OF BENZYL ALCOHOL DERIVAnVRR OVFH? PATT.ADIUM
Introduction Experimental Kinetics Results Discussion Conclusions Additional comments
ELECTRONIC SUBSTITUENT EFFECTS ON THE ADSORPTION AUD HYDROGENATION OF THE OLEFINIC BOND ON PALLADIUM Introduction
Experimental Kinetics
Results and Discussion Conclusions
Additional comments
A SIMPLE AUTOMATIC HYDROGENATION APPARATUS Experimental
Results
SUBSTITUENT EFFECTS IN THE DEHYDRATION OF 2-ARYL-3-METHYL-2-BUTANOLS AND IN THE SUBSEQUENT ISOMERIZATION OF 2-ARYL-3-METHYL-1-BUTENES IN SULiUHIC ACID-ACETIC ACID AND IN DIMETHYL SULFOXIDE
Introduction Results Discussion Experimental
Additional comments
SYNTHESIS OF 2-METHYL-1-TETRALONES FROM 1-TETRALONES Additional comments GENERAL CONCLUSIONS SAMENVATTING 5 5 5 6 7 11 15 16 16 17 17 19 20 24 25 26 27 28 29 33 38 39 40 40 42 43 43 45 48 54 56 57 61 62 63
I N T R O D U C T I O N
This thesis describes a kinetic study of a number of palladiiim catalyzed hydrogenation reactions the results of which have been published (i,£) or are in the press (2,). An automatic hydrogenation apparatus has been developed (^) to perform the fcLnetic experiments. In two additional papers {^,6) the synthesis of a number of compounds used in these studies has been described.
Up to the present, many phenomena in the field of heterogeneous catailysis can be neither understood nor predicted. Most of the information obtained concerning surface reactions has just become available by empiric methods. Therefore, one of the major puirposes in this field is a better understanding of the reaction mechanism in order to develop somewhat less empiric approximations. In principle, this aim may be accomplished in two different ways (2»fi)i
(i) by testing a series of catalysts by means of a standard reaction of j\xst one substrate, and
(ii) by studying the reaction of a series of compoimds on a single catalyst.
Studies of the latter type are much less frequent and systematic than those of the first type. Therefore, it was decided to investigate some examples of the latter type more extensively in order to see to what extent approach (ii) may be a further suitable tool for mechanistic studies in heterogeneously catalyzed reactions.
In this thesis studies of type (ii) have been devoted to the liquid-phase hydrogenation of the C=0 and C=C bond and the liqiiid-phaae hydrogenolysis of the C-0 bond using palladium on carbon as the
catalyst. Systematic variation of the substrates has been accomplished by the introduction of siiitable substituents to benzenoid compounds bearing the above-mentioned functional groups. In particular, quantitative correlations between the electronic character of
substituents at the meta- and para-positions of the benzene ring and some kinetic parameter were expected to provide valuable information regarding the reaction mechanism. The influence of steric effects was studied by introduction of alkyl substituents into both the reaction center and the ortho-position of the benzene ring. In this way a kinetic study has been made of steric and electronic substituent
4
effects in the palladium catalyzed hydrogenation of alkyl aiyl ketones (Chapter l ) , benzyl alcohol derivatives (Chapter II), and styrenes (Chapter III). Furthermore, the adsorbed state in these reactions on palladium was studied by means of both deuteration and competition hydrogenation experiments. Most of the kinetic experiments have been carried out with the automatic hydrogenation apparatus described in Chapter IV.
In connection with the above-mentioned studies the conversion of 2-aiyl-3-^ethyl-2-butanols into the corresponding 2-aryl-3-methyl-2-butenes has been investigated in detail including isomerization phenomena (Chapter V ) . Also, a simple three-step synthesis of 2-methyl-1-tetralones from the corresponding 1-tetralones has been developed (Chapter Vl).
REFERENCES
1 . H. van Bekkum. A.P.G. Kieboom. and K«J.G. van de Pgtte. Rec. Trav. Chim. Pays-Bas ^ , 52 (l969).
2. A.P.G. Kieboom. J.F. de Kreuk, and H. van Bekkum. J. Catal. |0, 58 (1971).
3. A.P.G. Kieboom and H. van Bekkum. J. Catal., in press. 4. G.W.H.A. Mansveld. A.P.G. Kieboom. W.Th.M. de Groot, and H^
van Bekkum. Anal. Chem. ^ , 813 (l970).
5. A.P.G. Kieboom and H. van Bekkum. Rec. Trav. Chim. Pays-Bas
m, 1424 (1969).
6. A.P.G. Kieboom and H. van Bekkum. Synthesis 1970, 476. 7. M. Boudart. Chem. Eng. Progr. ^(s), 33 (l96l). 8. M. Kraus. Advan. Catal. Relat. Subj. H f 75 (l967).
5
I
ELECTRONIC AND STERIC EFFECTS IN THE HYDROGENATION OF ALKYL ARYL KETONES ON PALLADIUM *
The hydrogenation of some forty aromatic ketones to the corresponding alcohols has been studied in ethanol using carbon^supported palladium as the catalyst. Zero order kinetics with respect to fcetooe is observed. For aceto-phenones eVectroaic substituent ellects may be expressed in terms of a Hammett'
Yukawa relationship with Q = 0.7. 2-Alkylacetophenones react very slowly. With alkyl phenyl ketones increase of size of the alkyl group causes increasing rate of hydrogenation. The adsorbed state is discussed on the basis of com-petition experiments.
Introduction
Several investigators have been engaged on the problem whether ifl the hfitwogeneous catalytic hydrogenation of unsalurated organic groups the rate of hydrogenation is correlated ia some way with the eleptron density at the OT-bond(s) in question. Such electronic effects can be studied upon introducing suitable electron-withdrawing or -donating substituents into the system under investigation,
Earlier studies including the hydrogenatioo of substituted alkeoes^, nitrobenzenee ^~^ and benzalanilines ^ on platinum, rhodium and palladium failed to correlate hydrogenation rate with electron shifting ability of the substituents. Recently, however, Ptnkelstein reported a linear correlation of log rate of hydrogenation and solvatochromic effect * jn the hydrogena-tion of substituted benzaldehydes, nitrobenzenes, azobenzenes and N-nitrosoanilines on palladium, rhodium and Raney nickel.
* H. van Bekkum, A . P . G . Kieboom, and K.J.G. von de Putte, Rec. Trav. Chim. Pays-Bas 88. 52 (1969).
» ƒ. Jaritme and F. J. McQuiUin, } . Chem. Soe, C HHiS, 45«,
« HtieM-Cheng Yao and P. H. Enmiett. J. Am, Chem. Sgc. 81,4US (1959). » L. Hernandez and F. F. Nord, Experientia 3, 4.89 (1947).
< Idem, J. Colloid Sci. 3, 370 (1948).
5 A. Roe and / . A. Montgomery, J. Am. Chem. Soc. 75, 910 (1953).
« A. V. Finkelstein, Rjeactionoaya Spowboort Organ, Sroedin. 4, 3Jfl (19(67); A, V. Finkeistein and K V. iranw, ibid. 4,605 (1967) and earlier papers referred to ip these references. Use is made of A» (heptane, ethanol) for the K-band of tfie electronic spectra.
/
In this paper we describe the palladium-catalyzed hydrogenation of a series of acetophenones
/ i - i
A solution of potassium hydroxide (0.5 M) in ethanol has been used as the reaction medium in order to avoid subsequent hydrogenolysis^. Steric effects in hydrogenation of e.g. alkenes are well recognized*. In order to evaluate the role of such factors in the hydrogenation of alkyl aryl ketones on palladium we have examined the effect of varying size of alkyl groups. 2-Alkylacetophenones also have been included.
In addition, some competition experiments have been carried out in order to obtain information regarding the relative strength of adsorption of aromatic ketones on palladium.
Experimental
Materials
Palladiiun, 10% on carbon, was purchased from Drijfhout N.V., Amsterdam. Total surface 690 m ^ g , palladium area 11.1 m^/g. 99.5% ethanol (Baker Analyzed Reagent) was used. Most of the starting carbonyl compounds were commercial samples or prepara-tions made by standard procedures. Solid ketones were purified by crystallization. Liquid ketones were purified through their semicarbazones or oximes. Physical constants closely agreed with literature values. Some new preparations are given below.
'i-i-Butylacetophenone (b.p. 135°/20 mm; nS" 1.5173; semicarbazone, m.p. 195.5-196°)
was prepared from 3-f-butylbenzoic acid and methyllithiimi. 2-t-Butylacetophenone (b.p. 109710 mm; nï," 1.5145) was synthesized from methyl 2-/-butylbenzoate and methyl-magnesium iodide. A-t-Butylpivalophenone (m.p. 45.3-45.6°) was prepared as described by Pearson » and purified through the thiourea adduct. Methyl i-acetylbenzoate (m.p. 39-39.5°) was obtained by treating 3-acetylbenzoic acid with diazomethane.
Apparatus
Hydrogenations were carried out in a 50 ml reaction vessel equipped with thermo-meter, magnetic stirrer, injection rubber, gas-inlet and -outlet tubes, and a revolving tubular device for adding solids. The gas-inlet was connected to a hydrogen burette containing dibutyl phthalate as the displacing liquid. Experiments were performed at atmospheric pressure. Reaction vessel and gas burette were equipped with a thermostat mantle and were kept at 25.0 ± 0 . 1 ° .
' K. Kindler, H. Helling, and E. Sussner, Ann. 605, 205 (1957).
* B. B. Corson, in Catalysis (P. H. Emmett Ed.), Vol. Ill, Reinhold, New York, 1955, Chapter 3.
7
Procedure
The catalyst (50-500 mg) and 15 ml of solvent were placed in the reaction vessel. After evacuation, hydrogen was admitted and this procedure repeated twice. The mixture was stirred for 30 min and then reaction was started by introducing the substrate (2-4 mmoles). Reactions were followed by measuring the uptake of hydrogen. Formulations were choosen so as to prevent diffusion limitation.
Hydrogenations were found to be zero order in ketone and first order with respect to catalyst. From the slope of the plot of the hydrogen uptake — corrected to 760 mm — versus time, pseudo zero order rate constants were calculated. Assuming reactions to be first order with respect to hydrogen pressure ^^ these figures were corrected for deviations of pressure from 760 mm and for the vapour pressure of the solvent. Rate constants proved reproducible to within 5%.
Competition experiments
Competition hydrogenations were carried out in the same way using an equimolar mixture of a pair of ketones A and B. Reactions were followed by measuring the uptake of hydrogen as well as by GLC analysis of samples. If the reduction of an equimolar mixture of two ketones is carried out, it follows that,
A (in solution) + B (adsorbed) ^ A (adsorbed) + B (in solution) and one may write A
an equilibrium constant as follows: ATA.B = —- in which dx and OB are the fractions
9B
of the active surface covered by ketones A and B. Also it may be written itc = itAÖA + itBÖB and ÖA 4- ÖB = 1
in which A: A and kv represent the pseudo zero order rate constants of A and B, respectively, and kc is the experimental rate constant as determined at the beginning of the competitive hydrogenation 11. Combining these equations gives,
6A = and
*A — ATB
and it follows that,
Results
The results of hydrogenations of substituted acetophenones on palladium are given in Table I. Up to conversions of 60-70% hydrogenations were found to be zero order in ketone indicating relatively strong adsorption of the ketones compared with the hydroxylic products and the solvent. At low concentrations which occurred with some slightly soluble aceto-phenones, deviations from zero order kinetics appeared.
Bn kc kx * A -- * B -Arc -ATB
" D. V. SokoPskii, Hydrogenation in Solutions, Oldboume Press, London, 1964, p. 388. 1' H. A. Smith and C. P. Rader, Proc. Intern. Congr. on Catalysis, 2nd, Paris, 1960 1,
8
Table I
Hydrogenation of substituted acetophenones on palladium*
Substituent H 2-CH3 3-CH3 4-CH3 4-C2H5 4-isoC3H7 2-t-CiH, 3-r-C4H9 4-(-C4H9 3,5-di-<-C4H9'' 4-CaH5'' 3-CH(OH)CH3 4-CH(OH)CH3 3-OCH3
Pseudo zero order rate constant (ml min-i g catalyst-^) 10.7 1.24 10.9 10.2 11.1 12.3 0.2 13.9 15.7 5.4 5.8 9.4 8.7 18.6 Substituent 4-OCH3 304 0 -3-NH2 4-NH2 4-N(CH3)2 3-COO-4-COO-'' 3-COOCH3 4-COOCH3 3-COCH3 4-COCH3 3-CF3 4-CF3
Pseudo zero order rate constant (ml min~i g catalyst'^) 4.5 9.7 0.44 16.1 1.16 0.61 11.7 15 11.3 22.6 12.8 24.4 18.8 29.8
» Temperature 25 °C; atmospheric H2 pressure; solvent ethanol (0.5 M KOH); Pd 10% on carbon.
" Deviation from zero order kinetics.
The data show that the hydrogenation is accelerated by electron-with-drawal and retarded by electron-donation by substituents.
Nitro-, cyano- and halogen-substituted acetophenones proved to be inapplicable because of hydrogenation or hydrogenolysis of the sub-stituents.
Rates of hydrogenation of 4- and 3-alkylacetophenones show that steric effects are also involved: increasing the size of the alkyl group gives rise to some acceleration. 2-Alkyl substituents proved highly inhibitory.
9
Table II
Hydrogenation of other oirbonyl compounds on palladium •
Substrate benzaldehyde acetophenone propiophenone (1, R = Et) isobutyrophenone (1, R = iPr) pivalophenone (1, R = tBu) 4-f-butylpivalophenone * 1-tetralone (2) 2-inethyl-l-tetralone 2,2-diniethyl-l -tetralone 7-r-butyl-l-tetralone'> benzophenone benzil (1, R = CeHsCO) benzoin (1, R = C.HsCHOH) methyl 1-naphthyI ketone methyl 2-naphthyl ketone phenylacetone
acetone »
/-butyl methyl ketone " cyclohexyl methyl ketone " /-butyl cyclohexyl ketone •>
Pseudo zero order rate constant (ml min-i g catalyst"') 40 10.7 14.3 16.6 30.4 19 3.2 3.2 3.1 Z2 7.9 148 29 0.83 6.0 6.3 23 4.3 5.4 0.02 a, b: See legend Table I.
10
Data on other ketones are collected in Table II. In the series of alkyl phenyl ketones increase of size of the alkyl group causes increasing rate of hydrogenation. In particular the high reaction rate of pivalophenone is noteworthy.
In the case of saturated systems the reverse is true as shown by the data on methyl and /-butyl cyclohexyl ketones and of acetone compared with /-butyl methyl ketone. The observed deviation from zero order kinetics is due to the weak adsorption of these ketones.
With diketones, i.e., benzil and the diacetylbenzenes, selective hydro-genation to the monoketone stage was observed. This enabled us to measure the rate of the first and second hydrogenation step in one experiment.
Table III
Competitive adsorption equilibrium constants
No. 1 2 3 4 5 6 7 8 9 10 11 12 13 14 Ketone acetophenone 2-methylacetophenone 4-methylacetophenone 4-ethylacetophenone 4-/-butylacetophenone 3-methoxyacetophenone 4-methoxyacetophenone 4-aminoacetophenone 4-hydroxyacetophenone •> p-diacetylbenzene pivalophenone phenylacetone acetone
cyclohexyl methyl ketone
-A^ketooe. acetophenone ^ 1.00 0.24 0.32 0.29 0.11 0.45 0.32 0.30 0.29 > 7 0.10 0.07 < 0.01 < 0.01 1
» From competitive hydrogenation on palladium (10% on C) in ethanol (0.5 M KOH) at 25°C of the following pairs of ketones: 1-4. 1-10, 2-4, 3-7, 3-8, 3-9, 4-5, 4-7, 4-11, 6-7, 11-12, 11-14 and 12-13.
Results of competition experiments (see Table III) show acetophenone preferentially adsorbed with respect to a number of substituted aceto-phenones. Somewhat more pronounced effects are observed when comparing acetophenone with pivalophenone, phenylacetone or cyclohexyl methyl ketone.
Discussion Adsorbed state
Data from the competition experiments would allow of some conclusions being drawn regarding the adsorbed state of the ketones investigated.
4-Substituted acetophenones with substituents of corresponding size, such as methyl, methoxy, amino andf hydroxy, are found to differ only slightly in strength of adsorption. This implies that contrary to current ideas i^ the three last-mentioned substituents, in spite of their lone electron pairs, do not contribute significantly to the adsorption on palladium, i.e. they behave as alkyl groups of corresponding size. Furthermore, it may be concluded that the adsorption of the acetophenone system is hardly influenced by electronic substituent effects. Consequently, the adsorbed state is still conjugated considerably. In fact, the occurrence of carbonyl group and benzene ring in conjugation proves favourable for adsorption as shown by the relatively weak adsorption of phenylacetone. The picture thus obtained for adsorbed acetophenones consists of a flat eight-atom conjugated system orientated parallel to the palladium surface. With the exception of p-diacetylbenzene, substituted acetophenones are adsorbed more weakly than acetophenone. First, this is due to extension of the system with surface-screening but weakly adsorbing substituents causing less efficient covering of the palladium. Secondly, depending on the size of the substituents, van der Waals interaction between substituent and palladium may occur in combination with some disturbance of optimal adsorption of the remainder of the molecule. It may be noted that similar differences in strength of adsorption have been observed ^^ between toluene and the xylenes on platinum and rhodium. The strong adsorption of p-diacetylbenzene will be due to extension of the conjugated 7i-system with a second carbonyl group.
Electronic ejfects on rate of hydrogenation
Table I shows a considerable spread in rate of hydrogenation of 3- and 4-substituted acetophenones. That electronic effects are involved is shown,
" G. C. Bond, Chem. Ind. London 1967, 2018.
'3 H. A. Smith and W. E. Campbell, Proc. Intern. Congr. on Catalysis, 3rd, Amsterdam, 1964 2, 1383 (1965).
12
for instance, by the divergent effects exhibited by methoxy or hydroxy substituents in the 3- and 4-position.
An attempt at correlating substituent effects in terms of a Hammett relationship ^* using cr" values ^* depicted a rather poor correlation (cor-relation coefficient 0.75). In particular, since electron-donating 4-sub-stituents were found to deviate, the Yukawa extension of the Hammett equation i* was applied. Using a resonance parameter of 0.8 i ' a fairly satisfactory correlation is obtained for this heterogeneous reaction i* as shown in Fig. 1. A rho value 0.7 is calculated (correlation coefficient 0.94).
I I I I I i_
2 0 -1.5 - 1 0 -0.5 0 0.5
Fig. 1. Log plotted against a" + 0.8 A(Ta+ for the hydrogenation of substituted ko
acetophenones on palladium. Oosed circles are for para substituents, open circles for meta. (T" and A<rR+ values derived from references''•!••".
" L. P. Hammett, Physical Organic Chemistry, McGraw-Hill, New York, 1940. 15 H. van Bekkum, P. E. Verkade, and B. M. Wepster, Rec. Trav. Chim. 78, 815 (1959). 1» Y. Yukawa, Y. Tsuno, and M. Sawada, Bull. Chem. Soc. Japan 39, 2274 (1966). 1' Computer calculations showed optimal correlation in case of a resonance
para-meter 1.6 (correlation coefficient 0.97). However, this value is considered to be much too high for the present reaction. Failure will be due to the fact that beside electronic effects other effects are operative (see text).
" For a review dealing with substituent effects in heterogeneous reactions see: M. Kraus, Advan. Catalysis 17, 75 (1967).
i« / . Hine, J. Am. Chem. Soc. 82, 4879 (1960).
13
In view of kinetics ana substituent effects we accept as the rate-deter-mining step in the hydrogenation process the attachment of a hydrogen to the carbonyl carbon 2". On inspection of adsorbed state (I) and transition state of hydrogenation (2) it will be clear that in this step the system looses the greater part of the conjugation between aromatic nucleus and carbonyl
b o n d ^ i .
CHj
1 2
An electron-donating substituent will favour this conjugation. Hence, the loss of conjugation energy in the transition state will be more sub-stantial and the rate of reaction will be low compared with that of aceto-phenone. The reverse holds for an electron-withdrawing substituent.
Fig. 1 reveals several deviations from the Hammett line. As substituents with differing steric requirements are involved in the present surface reaction, a treatment on the basis of electronic effects alone cannot be other than an approximation.
The effect of polar substituents attached directly to the carbonyl function (cf. benzil) can be interpreted in an analogous way as above.
Steric ejfects
The retarding influence of 2-alkyl substituents is ascribed mainly to steric factors. 2-Methylacetophenone is supposed to adsorb with its carbonyl bond directed towards the 2-methyl group. Since the directions of approach of hydrogen will be restricted by the presence of the substituent, the entropy of activation will be unfavourable as compared with acetophenone. The low rate of hydrogenation of methyl 1-naphthyl ketone is interpreted in the same way. In the case of 2-/-butylacetophenone flat adsorption of ring and carbonyl bond would seem difficult.
In spite of rate-reducing factors as electron donation and increased surface demand of the molecule, branched 4- or 3-alkyl substituents accelerate the hydrogenation of acetophenone. As mentioned before, we consider optimum flat adsorption of the acetophenone system to be disturbed
'° It may be noted that the kinetics observed excludes the possibility that enolization is rate determining. Accordingly, pivalophenone and acetophenone showed practically the same ratio of hydrogenation rates when using ethanol with or without alkali. ^^ The same argument applies in the case of a tri-adsorbed acetyl group (cf. D. Cornet
14
by the large alkyl groups. The accelerating effect observed impUes relatively less disturbance in the transition state of hydrogenation. We conclude that the optimum ketone-palladium distance is greater in the transition state than in the adsorbed state. Apparently this accelerating effect overrides the above-mentioned rate-reducing factors.
The acceleration in the series of alkyl phenyl ketones by to-substitution cannot be ascribed to such an effect alone. This appears from the absence of differences in reaction rate between 1-tetralone, 2-methyl- and 2,2-dimethyl-1-tetralone. In the case of these compounds the system has a rather fixed conformation of the carbonyl fragment with respect to the benzene ring. By contrast the preferred conformation in alkyl phenyl ketones is known ^^ to have the carbonyl bond rotated out of the plane of the ring according as the size of the alkyl group increases. Let us take pivalophenone as example for the discussion of the hydrogenation of alkyl phenyl ketones. If pivalophenone is flatly adsorbed, van der Waals strain will occur, due to interference of the alkyl group with a 2-hydrogen atom of the benzene ring. In the transition state the system is on the way to the tetrahedral configuration and this steric strain will be — at least partly — released. This explains ^^ the high rate of hydrogenation observed.
Alternatively, if the pivalophenone system is not flatly adsorbed, it lacks, relatively with respect to acetophenone, an amount of conjugation energy. This can account for the high rate of hydrogenation, on the analogy of some homogeneous reactions, e.g. sodium borohydride reduction of pivalophenone 2*. These explanations are further supported by data of methyl and cyclohexyl alkyl ketones. In the case of these two compounds where the above-mentioned factors are absent the hydrogenation is retarded by increased branching of alkyl substituents of the carbonyl bond, in accordance with the literature**.
Acknowledgements
The investigation was supported by the Delfts Hogeschool Fonds. Experimental assistance received from Mr. A. M. van Wijk is acknowledged. Thanks are due to Ir. Th. J. Osinga of the Unilever Research Laboratory Vlaardingen for measuring the palladium surface.
" G. D. Hedden and W. G. Brown, J. Am. Chem. Soc. 75, 3744 (1953).
2' For some other examples of acceleration of hydrogenation caused by steric strain
see: H. van Bekkum, T. J. Nieuwstad, J. van Bameveld, and B. M. Wepster,
Tetra-hedron Letters 24, 2269 (1967); B. van de Graaf, H. van Bekkum, and B. M. Wepster,
Rec. Trav. Chim. 87, 777 (1968).
" H. C. Brown and K. Ichikawa, J. Am. Chem. Soc. 84, 373 (1962).
Additional comments
In this study no contribution of substituents with lone electron pairs — like methoxy, amino, and hydroxy — to the adsorption on palladium was observed. Nevertheless, in view of the results
obtained in Chapter III, we must conclude now that these substituents will be adsorbed to some extent on the palladitm catalyst. Probably, this contribution to the adsorption of the acetophenones bearing these substituents will be obscured by the relatively strong adsorption of the eight-atom conjugated aryl ketone system, the
electronic interaction of the substituents with the carbonyl function, and the use of 0.5 M potassium hydroxide in ethanol as the reaction meditim.
16
I I
Substituent Effects in the Hydrogenolysis of Benzyl Alcohol Derivatives Over Palladium *
A kinetic study of substituent effects in the liquid-phase hydrogenolysis of benzyl oxygen compounds has been made using carbon-supported palladium as the catalyst. The reaction is zero order in substrate and is progressively inhibited by its product. Proton catalysis is essential for the hydrogenolysis of benzyl alcohols and alkyl benzyl ethers. For the hydrogenolysis of benzyl alcohols and 2-aryl-3-methyl-2-butanols the effects of substituents in the aromatic ring may be expressed in terms of a Hammett-Yukawa relationship with p = —0.37 and —1.43, respectively. «-Sub-stituents retard the rate of hydrogenolysis of benzyl alcohol. The order of reactivity for different leaving groups is: OH, OAlkyl < OAryl < *OHAlkyl < *0H., OAc < OCOCFs. I t is concluded that the hydrogenolysis reaction involves hydride attack at the benzyhc carbon displacing the leaving group as its anion. For primary alco-hols an SN2 mechanism is operative, whereas tertiary alcoalco-hols show SNI type charac-ter. The adsorbed state ia discussed on the basis of deuterolysis experiments,
INTRODUCTION
In recent years a number of mechanistic studies concerning the stereochemistry of the catalytic hydrogenolysis of benzyl al-cohol derivatives have been reported (^-4). However, only a few kinetic data of the hydrogenolysis reaction are available.
The hydrogenolysis over palladium was generally found to proceed with inversion of configuration. This has been explained
However, we felt that the results also might be explained by an SNI type reaction mechanism, i.e., a preliminary cleavage of the catbon-oxygen bond to give a car-bonium ion, which then reacts with the hydrogen on the surface of the catalyst. For the latter mechanism high stereoselec-tivity is to be expected because (i) rota-tion around the Caryi-C bond will be diffi-cult due to steric interactions of Ri and Rj
SN2 < @ ^ C ^ F ? 2
(S) by an SN2 type reaction mechanism, i.e., a one-step attack in which the reducing agent displaces the oxygen group from its attachment to the carbon atom.
with the catalyst surface and (ii) exclusive hydrogen attack from the catalyst surface will result in inversion.
An important difference between these
- ^ R j
ORi
- <s>cA
17 reaction mechanisms is the much greater electron demand of the reaction centre in the transition state of the SNI reaction. We, therefore, have examined the effect of sub-stituents in the aromatic nucleus and of Ri, R2, and R3 on the rate of hydrogenolysis in order to obtain more information re-garding the mechanism involved. In par-ticular, the application of linear free energy relationships to ring-substituted benzyl alcohol derivatives was expected to be very valuable for the elucidation of the reaction mechanism. Furthermore, some deuterolysis experiments have been included.
EXPERIMENTAL
Materials
Palladium, 10% on carbon, was pur-chased from Johnson & Matthey Chemicals Ltd., London. Palladium, 1% on carbon, was prepared by hydrogenation of pal-ladium chloride in water in the presence of carbon (5). Deuterized palladium, 10% on carbon, was prepared in a similar way using deuterium and deuterium oxide. Acetic acid (99.8%), trifluoroacetic acid (99.5%), and ethanol (99.5%) were Baker Analyzed Reagents. Acetic acid-d was pre-pared from acetic anhydride (Baker Analyzed Reagent) and deuterium oxide
(Merck A.G.).
Most of the substrates were commercial samples or preparations made by standard procedures. The compounds were purified by repeated distillation or crystallization. Physical constants agreed with literature values.
The preparation of a number of 2-aryl-3-methyl-2-butanols has been recently de-scribed (6). In the same way 2-(4-iso-propylphenyl) -3-methyl-2-butanol (bp 129-13rC/8 mm, mp 46-47"'C), 2-(4-t-butylphenyl)-3-methyl-2-butanol (mp 82-83°C), and 2-(3-methoxyphenyl)-3-methyl-2-butanol (bp 72°C/0.1 mm, HD" 1.5200) were obtained. l,l,l,3,3,3-Hexadeutero-2-phenyl-2-propanol (5.89 d) and 2-(penta-deuterophenyl)-2-propanol (4.91 d) were prepared as described for the undeuterated
compound (7) using hexadeuteroacetone (Merck A.G.) and pentadeuterobromoben-zene (Fluka A.G.), respectively.
Apparatus
The hydrogenolyses were carried out with magnetic stirring in a 25-ml reaction ves-sel provided with a thermostat jacket, in-jection rubber, gas-inlet and gas-outlet tubes, and a revolving tubular device for adding solids. The gas-inlet was connected to an automatic hydrogenation apparatus as described elsewhere (8). Experiments were performed at atmospheric pressure. Temperature was held constant within
o.rc.
ProcedureThe catalyst and 5 ml of solvent were placed in the reaction vessel. After evacu-ation, hydrogen was admitted and this procedure repeated twice. The mixture was stirred for 30 min and then reaction was started by introducing the substrate (1 mmole). Reactions were followed by mea-suring the uptake of hydrogen. No reduc-tion of the aromatic ring was observed. Formulations were chosen to prevent dif-fusion limitation.
Deuterolyses were carried out in a similar way using deuterium and deuterized pal-ladium, 10% on carbon. The products were analyzed by NMR and mass spectrometry.
KINETICS
In all cases the hydrogenolysis of the benzyl-oxygen bond over palladium was found to proceed at a progressively de-creasing rate. However, the initial rate proved to be independent of the concentra-tion of the substrate, as is shown for benzyl alcohol and 3-methyl-2-phenyl-2-butanol in Table 1.
Clearly, the hydrogenolysis is zero order in substrate and is inhibited by the hydro-carbon as it is formed in increasing amounts (9). We may describe this phe-nomenon by the following adsorption equilibrium of the alcohol (A) and its product (B):
18 A {in solution) -|- B (adsorbed)
? i ^ (adsorbed) -|- B(in solution) and one may write an equilibrium constant K = ÖACB/OBCA, in which OA and 9B are the fractions of the active catalyst surface, covered by A and B, and CA and CB are the concentrations of A and B in solution. Both t h e zero-order kinetics with respect
CB/CA = Kr,/r, - K, or
w...-r,=.(f)y(a-.,
in which V and Vmai are the volume and the maximum volume of hydrogen con-sumed. All hydrogenolysis reactions proved to obey this relation with K values vary-ing from 0.7 to 2.4. Both K and {dV/dt)oTABLE 1
E F F E C T OF THE CONCENTRATION OF THE SUBSTRATE ON THE INITIAL RATE OF HTDROGENOLTSIB Benzyl alcohol"'
Concentration Hydrogen uptake (mole liter"') (ml min~')
3-Methyl-2-phenyl-2-butanol'' Concentration (mole liter"') Hydrogen uptake (ml min"') 0.15 0.26 0.77 3.0 3.2 3.2 0.13 0.22 0.69 2.1 2.2 2 . 0
" Solvent 90% AcOH (5 ml); temperature 30°C; atmospheric Hj pressure. ' P d , 1% on carbon (300 mg).
' Pd, 10% on carbon (100 mg).
to A and the retarding influence of B in-dicate relative strong adsorption of A and B on the catalyst, so t h a t we may write OA + OB = 1 and rj = TOÖA, in which r» and rt are the rates of reaction a t time zero and time t, respectively. Combining these equations gives
can be derived from the plot of V/ (V„,a. — y ) vs l/(dV/dt)t as is shown in Fig. 1.
Furthermore, hydrogenolyses were found to be first order with respect t o catalyst. Assuming reactions to be also first order with respect to hydrogen pressure, the
1
19
than its product, while the strength of adsorption of the tertiary alcohol and of its product is roughly the same.
liie introduction of a-substituents re-tards the rate of hydrogenolysis of benzyl alcohol (Table 3). In the alkyl series the rate is strongly dependent on the size of the alkyl group. In some cases the retard-ing effect is caused by an unfavourable entropy of activation as compared with that of benzyl alcohol. The activation parameters were calculated from the Arrhenius plots, which are presented in Fig. 2.
Table 4 shows the results of the hydro-genolysis of various benzyl oxygen com-pounds in acid and in basic medium. The hydrogenolysis of benzyl alcohol and alkyl benzyl ethers is strongly inhibited by adding base. In contrast, the rate of reaction of aryl benzyl ethers and benzyl esters is hardly influenced. The reaction rate is strongly dependent on the nature of the leaving group OR3, especially in basic medium.
The results of the deuterolysis of benzyl alcohol and 2-phenyl-2-propanol over
TABLE 2
HYDROGENOLYSIS or SUBSTITUTED BENZYL ALCOHOLS" AND OF
2-ABYL-3-METHYL-2-BÜTAN0L8' OVER PALLADIUM
Benzyl alcohols 2-Aryl-3-methyl-2-butanols
Substituent H 2 - C H . 3-CH, 4-CH, 4-C.H. 4-i-C,H, 4-t-C4H, 2 - N H , + 3 - O C H , 4 - O C H , 4-OC2H5 3-CF, 4-CF, lO^k (mole s~' g catalyst"') 6.7 4.4 7.1 8.0 7.1 0.5 5.8 12.4 10.0 4.4 4.2 K' 1.9 1.9 2.1 2.0 1.5 2.2 1.7 1.9 2.0 1.8 1.8 Wk (mole s"' g catalyst"') 13.9 12.6 23.1 22.4 20.5 10.4 6.3 100 2.45 K' 0.8 0.7 1.0 0.7 0.8 0.7 1.0 0.8 1.2
action rate constant fc(mole s"' g cata-lyst"^) was calculated from
^ = 2 0 3 X 1 0 - ( ^ ^ 3 ^ ( f ) ; in which p is the pressure in mm, p, is the vapour pressure of the solvent, w is the amount of catalyst in g, and (dV/dt),, is the uptake of hydrogen (ml min"^) at p mm and T °K. It may be noted that
a deviation from first-order kinetics with respect to the hydrogen pressure will affect the results only to a slight extent, since p — Ps is fairly constant in all cases (cf. Table 4).
RESULTS
The results are given in Table 2 for a series of substituted benzyl alcohols and of 2-aryl-3-methyl-2-butanols. The data show that the hydrogenolysis is retarded by electron-withdrawal and accelerated by electron-donation by substituents. The effects are more pronounced for the ter-tiary alcohol. The K values show that the primary alcohol is more strongly adsorbed
" Temperature 30°C; atmospheric H2 pressure; solvent 90% AcOH; Pd, 1% on carbon. ' Idem, Pd, 10% on carbon.
20 TABLE 3
EFFECT OF «-SUBSTITUTION ON THE HYDROGENOLYSIS OF BENZYL ALCOHOLS OVER PALLADIUM"
Ri H CHa C^Hi, i-C,H, t-C4H. CH, CH, CH, C H , CJIs P h -4-CH,OCai4 Cai5 ••« See R i / -C—OH \ R2 R2 H H H H H CH3 i-C,H, COOH COGCjHs H H CJH5 legend Table 2. ' Relative to benzyl alcohol.
10«fc (mole 8"' g catalyst"') 6.7 2.61 1.22 0.080 <0.003 1.14 0.063 <0.002 <0.002 0.78 4.6 1.92 K' 1.9 1.0 1.0 0.8 1.2 0.8 1.7 1.0 0.8 -* AH* (kcal mole~') 9.8 9.2 7.9 8.0 12.3 M AAS» (eu) 0.0 - 3 . 9 - 7 . 3 - 1 0 . 2 -1-3.5
palladium are collected in Table 5. Acetic acid-d was used as the solvent, since slow exchange occurs between the acidic proton of the solvent and deuterium {10). Isotopic dilution due to the rapid exchange of the alcoholic proton of the substrate and the acidic proton of the solvent will be less
FIG. 2. Arrhenius plots for the hydrogenolysis of benzyl alcohol (D), 1-phenylethanol (O), 2-phenyl-2-propanol ( # ) , diphenylcarbinol (A), and triphenylcarbinol (A) over palladium.
than 7%. In another experiment 1,1,1,3,3,3-hexadeutero-2-phenyl-2-propanol was sub-jected to deuterolysis in acetic acid-d resulting in 0.94 d at the «rposition. Clearly, the rather low deuterium content at the of-position of cumene from the deu-terolysis of 2-phenyl-2-propanol is due to the attack of exchanged /8-hydrogen atoms. Under the same reaction conditions and times toluene and cumene showed only a small H / D exchange at the «-position (0.2 and 0.1 d, respectively). In all cases deuterium incorporation in the aromatic ring was not observed. This was further confirmed by hydrogenolysis of 2-(penta-deuterophenyl)-2-propanol (4.91 d) in ace-tic acid yielding 2,3,4,5,6-pentadeuterocu-mene with almost the same isotopic labeling
(4.87 d).
DISCUSSION
Efject of the Medium and of the Leaving Group
Proton catalysis proved to be essential for the hydrogenolysis of benzyl alcohol and the alkyl benzyl ethers. Apparently,
21
TABLE 4
HYDROGENOLYSIS OF CtHsCHjOR," IN ACID AND IN BASIC MEDIUM
R . H CH, CHjCOOCHs GIiaOjHö C ^ 5 4-CH.OCJH4 4-CH,OCOC,H4 CHjCO CF3CO Solvent: p - p. (mm) 10«fc (mole s" CF3COOH 737 27 15 4.6 24 ' g catalyst"') 90% AcOH 735 6.7 2.4 0.38 0.64 0 41 0.80 0 60 7.9 70 0.56 M N H , in 96% EtOH 673 0.00070* 0.00098' 0.00154' 0.00037' 0.76 0.87 2.00 10 90 K' 1.9 1.8 1.8 2.4 2 . 0 2.4 2.2 2 . 0
" Temperature SO'C; atmospheric H2 pressure; Pd, 1% on carbon.
' From experiments over Pd, 10% on carbon, by comparison with benzyl phenyl ether.
' Adsorption equilibrium constant in 90% AcOH. in this way a better leaving group is formed in acid medium, i.e., •'OHR3 in-stead of OR3. On the other hand, proton catalysis is of much less importance in the hydrogenolysis of aryl benzyl ethers and of benzyl esters.
Summarizing the results of Table 4, we may formulate the following order of re-activity: OR3 = OH, OAlkyl <^ OAryl < •^OHAlkyl < -^OHa, OAc < OCOCF3. The
ease of displacement clearly parallels the ability of the leaving group to bear a negative charge. This suggests a hydride displacement of the leaving group as its anion. McQuillin et al. (4) discarded such a mechanism because of the rapid hydro-genolysis of benzyl esters in apolar media. However, in apolar solvents the reaction may proceed in a more concerted fashion without the formation of the free anion:
TABLE 5
DEUTEROLYSIS OF BENZYL ALCOHOL AND 2-PHENYL-2-PROPANOL'" ISOTOPIC CONTENT (%) AT 6 0 % CONVERSION
Compound: dt di dt d, dt d, d, d, D «-D 18-D Benzyl alcohol 84 12 4 0.20 0.20 Toluene 25 54 17 4 1.00 1.00 2-Phenyl-2-propanol 100 0.00 0.00 0.00 Cumene 16 47 13 9 7 4 3 1 1.76 0.69 1.07
° Pd-D, 10% on carbon; temperature 30°C; atmospheric D2 pressure; concentration of substrate 1.5 mole/ liter; solvent AcOD (90% in D2O).
R 22
r
Effects of Substituentsin the Aromatic Ring
The effects of substituents in the 3- and 4-position of the aromatic ring on the rate of hydrogenolysis of benzyl alcohols and of 2-aryl-3-methyl-2-butanols (Table 2) may be correlated in terms of the Ham-mett equation (11). However, since elec-tron-donating 4-substituents were found to deviate, the Yukawa extension of the Hammett equation (12) was applied.
Computer calculations showed optimal correlation (correlation coefficient 0.98) with resonance parameters of 0.71 and 0.64 for the hydrogenolysis of benzyl alcohols and of 2-aryl-3-methyl-2-butanols, respectively. The corresponding p values proved to be —0.37 and —1.43, respectively, and were in excellent agree-ment with the p values derived from the Hammett relation without the 4-alkoxy and 4-alkyl substituents (viz., —0.38 and — 1.40). The Hammett-Yukawa relations are shown in Fig. 3.
The negative p values and the high
H 0 : = f - ^ H H
resonance parameters are consistent with an electron deficient transition state, which will be stabilized by electron do-nation by the aromatic system.
The small p value of —0.37 for substi-tuted benzyl alcohols indicates that no highly charged structure will be involved during the reaction. This implies a direct displacement of the hydroxyl group by the hydride, i.e., and SN2 type reaction mechanism. At first approximation, the transition state may be described by II, on the analogy of homogeneous displace-ment reactions
{14)-® cfH
From the deuterolysis experiments we may conclude that the aromatic ring will be TT-bonded on the catalyst, while at
-4.0 log<r 4.5 --5.0 -5.5 4-OCH3
FIG. 3. Hammett^Yukawa relations for the hydrogenolysis of substituted benzyl alcohols (O, r = 0.71) and 2-aryl-3-methyl-2-butanols ( 9 , r = 0.64) over palladium, a' and Airs''" values derived from references (/«, 13).
least in part dissociative adsorption occurs at the benzylic carbon. The some-what stronger adsorption of benzyl alcohol with respect to toluene points to a weak contribution of the oxygen to the adsorp-tion of benzyl alcohol in the initial state
( I ) . Such a geometry is rejected for the transition state in view of both the high resonance parameter and the inversion of configuration generally found for the hydrogenolysis over palladium
{1-4)-The p value of —1.43 for the hydro-genolysis of the tertiary alcohol series implies a more substantial charge at the benzylic carbon in the transition state. However, a benzylic cation will not be involved, because then a p value of —3 to —4 had to be expected {6). Although the reaction will not proceed via a real SNI mechanism, we may ascertain t h a t the C - 0 bond will be at least partly broken before the hydride attacks the benzylic carbon.
Finally, it may be noted t h a t the ob-served substituent effects for the hydro-genolysis reaction are opposite to those found for the hydrogenation of aceto-phenones to the corresponding 1-aryl-ethanols over palladium {16). I n the latter case reaction occurs via an sn'' —* s p ' change in hybridization at the benzylic carbon. The present picture for the hydro-genolysis reaction involves an s p ' - » s p ' change, in accordance with the conclusion of McQuillin et al. (4).
Effects of Substituents at the Benzylic Carbon
Further support for a transition state with an electron deficient benzylic carbon in the hydrogenolysis reaction is found by the low reaction rate of atrolactic acid
(Table 3 : R, = CH3 and R^ = COOH) and of its ethyl ester, due to the electron-withdrawing carboxyl group. This is also shown by the rate-enhancing effect of t h e 4-methoxy group on the hydrogenolysis of diphenyl carbinol.
The series with R2 = H and R i varying from methyl to t-butyl in Table 3 shows a remarkable decrease in rate of reaction with increasing number of /S-methyl groups. Comparison of this phenomenon with the effects of ^-methyl groups on the rates of homogeneous SN2 displacement reactions reflects a striking resemblance (Table 6 ) .
At first sight, one would conclude t h a t the hydrogenolysis will be an SN2 type reaction. Nevertheless, the rate of reaction of 2-phenyl-2-propanol is much higher than has to be expected for an SN2 re-action. Apparently, an SNl-like mechanism becomes more important going from t h e primary to the tertiary alcohol as has been concluded before.
a-Alkyl substitution results mainly in a decrease in the entropy of activation. Probably, the freedom of motion in the transition state is decreased by the
inter-TABLE 6
EFFECT OF /3-METHYL SUBSTITUTION ON THE RELATIVE RATES OF HYDROOENOLYSIS AND SN2 DISPLACEMENT REACTIONS
R Hydrogenolysis of SN2 displacement of Ph 1 R—C—OH 1 H H R—C—X" 1 H CH. 1.0 1.0 C,H, 0.47 0.4 i-C,H, 0.031 0 03 t-C.H, <0.0011 0 00001
24 action of the alkyl substituents with the catalyst surface. This is supported by the high degree of exchange of the /8-hydro-gens upon deuterolysis of 2-phenyl-2-propanol. The very close approach of the a-methyl groups to the catalyst surface in the transition state will result in a con-tribution of dissociative.adsorption of the alkyl group.
\ i
The leaving group is displaced as its anion by hydride attack from palladium at the benzylic carbon. Simultaneously, a
9_ (» ^\ H » * * H H V - H eïO- 6.0-etc. OR3 \ H®
This formation of an a,/8-diadsorbed species leads to a lower entropy of the transition state as compared to the initial state. The more substantial decrease of the entropy of activation for diphenyl carbinol will be due to the stronger adsorption of the mole-cule by its extension of the ^-system with a second phenyl group. The reverse in acti-vation parameters for triphenyl carbinol suggests t h a t the reaction occurs via another pattern avoiding the unfavourable entropy of .activation. Perhaps an SNi-type
mechanism (5) is involved, in which just one phenyl group is adsorbed on the catalyst.
CONCLUSIONS
The transition state of hydrogenolysis may be generally described by
in which important overlap occurs between the electron deficient p-orbital of the benzylic carbon and the ir-orbitals of the benzene ring.
For primary alcohols displacement occurs in a concerted fashion, i.e., an SN2 type mechanism. Tertiary alcohols show a greater degree of bond-breaking than of bond-making at the transition state, i.e., somewhat Snl-type character.
neighbouring hydrogen atom adsorbed on the catalyst goes into solution as a proton.
REFERENCES
1. MITSUI, S., AND IMAIZUMI, S., Bull. Chem.
Soc. Jap. 34, 774 (1961).
2. MITSUI, S., FUJIMOTO, M . , NAGAHISA, Y., AND SuKEGAWA, T., Chem. Ind. (London) 241, 1969.
3. MITSUI, S., KUDO, Y., AND KOBAYASHI, M . ,
Telrahedron 25, 1921 (1969).
i. K H A N , A. M., MCQUILLIN, F . J., AND JARDINE,
I., / . Chem. Soc. C. 136, 1967.
B. AUGUSTINE, R . L., "Catalytic Hydrogenation."
p. 152. Marcel Dekker, New York, 1965.
6. KIEBOOM, A. P. G., AND VAN B E K K U M , H . ,
Rec. Trav. Chim. Pays-Bas. 88, 1424 (1969).
7. TissiER, M., AND GBIONABD, V., C. R. H. Acad.
Sa. 132, 1182 (1901).
8. MANSVELD, G . W . H . A., KIEBOOM, A. P. G.,
D E GROOT, W . T H . M . , AND VAN B E K K U M , H . , Anal. Chem. 42, 813 (1970).
0. MESCHKE, R . W . , AND HARTUNO, W . H . ,
/ . Org. Chem. 25, 137 (1960).
10. MACDONALD, C . G., AND SHANNON, J. S., Ami.
J. Chem. 18, 1009 (1965).
11. HAMMETT, L . P., "Physical Organic
Chem-istry," McGraw-Hill, New York, 1940.
lê. YUKAWA, Y . , TSUNO, Y . , AND SAWADA, M . ,
Bull. Chem. Soc. Jap. 39, 2274 (1966).
13. VAN B E K K U M , H . , VERKADE, P . E., AND
WEPSTER, B . M . , Rec. Trav. Chim. Pays-Bas. 78, 815 (1959).
14. STREITWIESBR, JR., A., "Solvolytic
Displace-ment Reactions," p. 11-31, McGraw-Hill, New York, 1962.
IB. VAN B E K K U M , H . , KIEBOOM, A. P. G., AND
VAN DE PUTTE, K . J. G., Rec. Trav. Chim.
Additional comments
Mitsui. Imaiztimi. and Esashi (l6) recently reported a further study concerning the catalytic hydrogenolysis of benzyl-type alcohols and their derivatives. Most of their results and conclusions with respect to the reaction on palladium on carbon as the catalyst are in agreement with ours:
(i) the reaction proceeds through ir-adsorption of the benzene ring, followed by the formation of a ir-benzylic complex with palladium, and ^
(ii) a change towards sp -hybridization at the benzylic carbon resulting in cleavage of the carbon-oxygen bond.
However, despite the suggestion of these authors that the hydrogen will attack an adsorbed substrate from the side of the catalyst surface, the possibility of an Si-type reaction mechanism for the hydrogenolysis reaction was not considered. Furthermore, the authors assume an electron rather than a hydride as the reducing agent.
16. S. MitsTii. S. Imaizumi. and Y. Esashi. Bull. Chem, Soc, Jap. 42» 2143 (1970).
26
III
Electronic Substituent Effects on the Adsorption and Hydrogenation of the Olefinic Bond on Palladium
A kinetic study of substituent effects in the liquid-phase hydrogenation of the olefinic bond has been made using palladium on carbon as the catalyst. Individual and competition hydrogenations have been carried out with 2-aryl--3-methyl-2-butenes (j) and substituted 3,U-dihydro-1,2-diBethylnaphthalene8 (g) in basic, neutral,and acid medium.
Electronic substituent effects on the rate of hydrogenation are found to be rather small. The adsorbed state in basic medium is not seriously influenced by the substituents. More substantial substituent effects on the strength of
adsorption are observed in neutral and acid medim. Co-adsorption of the dimethylamino and etho:7carbonyl group on palladium takes place.
It is concluded that the electronic nature of the transition state of hydrogenation on palladium is quite similar to that of the initial state. Higher n-electron density at the olefinic bond favours the strength of adsorption;
this is consistent with a n-bonded adsorbed olefin.
INTRODUCTION
'Phe mechanism of the heterogeneously catalyzed hydrogenation of the olefinic bond has been studied very intensively during the last forty years (!.)• However, the influence of electronic substituent effects on the rate of hydrogenation has been studied (2;-it,) just recently with a series of l-substituted propenes and 2-methylpropenes. No clear picture was obtained from this work because the rate data could not be
correlated by means of Taft's polar free energy equation.
In order to diminish the serious interference of the substituents with the reaction center, which is a troiiblesone factor in the above mentioned series, we have studied the palladium-catalyzed hydrogenation of substituted 2-aryl-3-methyl-2-butenes (^) and 3,'*-dihydro-1,2-dimethyl-naphthalenes (£).
CH, ^ CH3
The aromatic ring, which is not reduced, serves (i) to separate the substituent from the reaction center, i_.e_. to avoid the influence of direct steric effects, and (ii) to pass on, via its n-electrons, the polar effects of the substituents to the reaction center.
The influence of substituents on both the rate of reaction and the strenght of adsorption of J. and 2 has been studied over palladium on carbon in basic (0.5 M potassium hydroxide in ethanol), neutral (n-heptane), and acid medium (acetic and trifluoroacetic acid) at 30^Jand atmospheric hydrogen pressure.
28
EXPERIMEHTAL
The mass s p e c t r a were measured w i t h a Varian MAT SM-1 and t h e PMR s p e c t r a w i t h a Varian A-60 and T-60 s p e c t r o m e t e r . Gas-chromatographic a n a l y s e s were performed w i t h SE-30 o r PEG-1000 as t h e s t a t i o n a r y p h a s e . S t r u c t u r e s o f new compounds were confirmed by e l e m e n t a l a n s d y s i s and PMR s p e c t r a .
M a t e r i a l s
Ethanol ( 9 9 . 5 Ï ) , a c e t i c a c i d ( 9 9 . 8 ; t ) , and t r i f l u o r o a c e t i c a c i d ( 9 9 . 5 / t ) were Baker Analyzed R e a g e n t s . n-Heptane, t r e a t e d w i t h c o n c e n t r a t e d s u l f u r i c a c i d and w a t e r , was d i s t i l l e d and d r i e d w i t h m o l e c u l a r s i e v e s . Psdladium {tO% on carbon) was purchased from D r i j f h o u t N.V. , Amsterdam.
2 - ( 3 - t - B u t y l p h e n y l ) - 3 - i n e t h y l - 2 - b u t a n o l (bp 125-130° C/8 mm, n?^ 1 . 5 0 2 8 ) , 2 - ( U - b r o m o p h e n y l ) - 3 - m e t h y l - 2 - b u t a n o l (bp lUo° C/10 mm,
25
n^ 1 . 5 ' t 7 3 ) , and 2 ( 3 N , N d i m e t h y l a m i n o p h e n y l ) 3 m e t h y l 2 b u t a n o l (bp 9 6 -97 C / 0 . 2 mm; m e t h i o d i d e , rap 2 0 1 . 5 - 2 0 2 C) were prepared as d e s c r i b e d e a r l i e r
f o r a number o f o t h e r 2 - a r y l - 3 - i i i e t h y l - 2 - b u t a n o l s (^, 6 ) .
2 - A r y l - 3 - p e t h y l - 2 - b u t e n e s {±)
The 2 - a r y l - 3 - m e t h y l - 2 - b u t a n o l s were dehydrated w i t h s u l f u r i c a c i d i n a c e t i c a c i d ( ^ ) . The crude ailkenes were p u r i f i e d by chromatography on s i l i c a g e l impregnated w i t h s i l v e r n i t r a t e (30?) u s i n g h e x a n e , t e t r a , or b e n z e n e / e t h e r ( 9 : 1 ) as t h e e l u e n t .
Compounds 1, X=3- and U-hydroxy were o b t a i n e d i n a one s t e p procedure from t h e c o r r e s p o n d i n g acetophenones amd a l a r g e e x c e s s o f isopropylmagnesium bromide; 1 , X=3- and U-ceurboxy were prepared from 1,, X»3- and U-bromo through t h e Grigaard confound and carbon d i o x i d e . The e t h y l e s t e r s were prepared i n t h e u s u a l way.
3 . * * - D i h y d r o - 1 . 2 - d i m e t h y l n a p h t h a l e n e s ( 2 )
Compounds 2 , X=5- and 6-methoxy, 7 - t - b u t y l , and 6-acetamido were
o b t a i n e d from t h e corresponding 2 - m e t h y l - 1 - t e t r a l o n e s (j[) and methylmagnesium c h l o r i d e , f o l l o w e d by dehydration of t h e crude a l c o h o l s with formic a c i d
29
as described for 3,'t-dihydro-l ,2-dimethylnaphthalene ( 8 ) .
The physical constcuits of J and 2 are summsirized in Table 1.
Apparatus and Procedure
The kinetic experiments were carried out as described earlier {6, 10).
The substrate (l mmole) in the solvent (5 ml) was hydrogenated over lOÏ palladium on carbon (10-500 mg) at 30.0 +0.1 C and atmospheric pressure
(760 + 20 mm). Formulations were chosen to prevent diffusion limitation. Ccanpetition hydrogenations were carried out with mixtures of two
alkenes (0.5 mmole of each) in the solvent (5 ml) under the same conditions. Samples were withdrawn from the reaction mixture and euialyzed by GLC. The percentage of conversion of 2, X=6-acetamido, which could not be determined by GLC, was calculated from the conversion of the other substrate and the total hydrogen uptake.
Reduction of the aromatic ring was not observed. No hydrolysis of the ethoxycarbonyl group occurred in 0.5 M potassium hydroxide in ethanol. Tri-fluoroacetylation of the alkenes was observed in trifluoroacetic acid, e_.£. the formation of 2-methyl-2-trifluoroacetyl-3-(3-trifluoromethylphenyl)-butane (bp 80-82° C/9 mm) from 3-methyl-2-(3-trifluoromethylphenyl)-2-butene. This reaction was found to be very fast for alkenes with electron-donating substituents at the aromatic ring. Therefore only a few 2-aiyl-3-methyl-2--butenes with electron-withdrawing substituents could be measiured in this medium.
KINETICS
Individual hydrogenations
Hydrogenations were found to be first order with respect to catalyst and were assumed to be first order with respect to the hydrogen pressure in order to correct the reaction rate for deviations from atmospheric pressure. So we may write for a compound A, using Langmuir kinetics,
30
TABLE 1
2-ARYL-3-METHrL-2-BUTENES ( 1) AND 3,'*-DIHYDR0-1,2-DIMETHYIJlAPHTHALENES (2)
S u b s t r a t e No 1 S -X H^ 3-CH2 U-CHj U-C2H5 U-isoC^H.^ 3-t-C^H^ ••-t-Ci^Hg 3-OH U-OH^ 3-OCH U-OCH J^ 3-N(CH2)2 U-N(CH2)2 3-CF2 3-COOH U-COOH*^ 3-COOC2H^ li-COOC^H H* 5-OCH2 6-OCH 6-NHCOCH f-i-c^Hg Bp ( ° C/imn) 8U-85/17 9 7 - 9 8 / 1 5 99-100/15 112-113/15 121/15 7 0 / 1 . 2 131/15 8 6 - 9 2 / 0 . 8 8 6 - 8 7 / 0 . 6 121-122/16 116-117/1'» 7'*/0.3 7 6 - 7 7 / 0 . u 86-87/16 mp 100-102 mp 101-103 8 5 / 0 . 6 8 9 - 9 0 / 0 . 6 123/15 9 6 / 1 102 /O.7 mp 108-110 101-103/0.9
4'
1.5203 1.5189 1.5198 1.5173 1.513U 1.5092 1.51'*0 1.51*71* 1.51*86 1.5280 1.5298 1.5507 1.5590 1.1*61*5 1.5203 1.5261 1.571*7 1.5786 1.5780 1.51*21 * L i t . ( 2 ) , bp 191-192° C, n^^ 1.5202. ^ L i t . ( 9 ) . bp 81-8U° C/0.5 nun. d ^ i t . ( 9 ) , mp 1 0 1 . 5 - 1 0 2 . 5 ° C. " L i t . ( 9 ) , bp 231*° C, n^^ I.530U. ^ L i t . (8_), bp 101° C/1.5 nun.-IT
-W'^^^s^ =
1 + b,c,
*
^bc ^'^
A A
in which C, is the concentration of A in solution, k. is the reaction rate
A ' A constant, fl is the fraction of the active catalyst surface covered by A,
w is the amount of cateOyst, p is the pressure, p is the vapoxur pressure of the solvent, and J b C is the sum of the contributions of the solvent, the hydrogenated product, and the hydrogen to the denominator of the Langmuir expression.
In n-heptane and in acetic acid the reaction was found to be independ-ent on the concindepend-entration of the substrate, i.e. b.C. >> 1 + 5 b C .
-1 - 1 * *
The reaction rate constant (mole s g catalyst ) was calculated from:
k = 203 X 10 P ^ (2) (P-Pg) T w t
in which V is the amount of hydrogen (ml at p, T) consumed during time t (min). P and p are expressed in mm, T in K, and w in g.
s
In 0.5 M potassium hydroxide in ethanol the order with respect to the substrate varied from 0.2-0.7, i...e. b.C, > 1 + 2 b C . The decreasing rate of hydrogenation is not caused by co-euisorption on the catalyst of the product formed. Namely, toluene did not affect the reaction rate. Therefore, we con-clude that the basic medium is responsible for the relatively weak adsorption of the substrate. Since the order in substrate is rather low ('v 0.2-0.3) for most of the substrates in the (initial) concentration region used and the initial concentration was taken constant (0.2 mole/l) for all the experiments, the k values in basic medium were derived from equation (2) in which V/t has been replaced by (dV/dt )^ , i.e. the rate of the hydrogen uptake at time
t=o
zero. It may be noted, that these k values will be infected in a measure by the Langmuir adsorption term and are not quite the true reaction rate constants. Competition hydrogenations
For a competition reaction of A and B we may write equation (1) for both A and B, in which the denominator has been enlarged by the term b„C„. Dividing
a a the two equations gives (11)
32
or, in the integrated form,
- A _ „ ^A , ^B log ^ - K T - log —
u^ A,a kg Lg
(M
in which C and C_ are the concentrations of A and B in solution at time zero and K is the adsorption equilibrium constant of A with respect to B, i-e^. b./b . Thus, K can be calculated from the slope of the plot of log (C /C ) vs. log (C /Cg),and t.^ie reaction rate constants k and k as derived from the individual hydrogenations of A and B. The results for a number of competition experiments between some 2-aryl-3-methyl-2-butenes and 2-(l»-ethylphenyl)-3-methyl-2-butene are shown in Fig. 1.
0 O.A o.e 1.2 1.6
-'-''' -^ -ZZ^^^i
^ %: t - * r ^ ^ ^
^^.--^^OOCjHs r/Y /J 1 ^ / ' 1 \ / 4-OCH3 / / ^ - ^ " ' - ^ 3-N(CH3). 1 logrc,-c,Vc;-c,«J^ . . . r r H , ^ , ^ \ 1 -1.6 -1.2 -0.8 -0.4FIG. 1. Plots for the competition hydrogenations of some 2-aryl-3-methyl-2--butenes (^^X) and 2-(U-ethylphenyl)-3-methyl-2-butene (1,1*-C H^) over palladium in n.-heptane at 30.0° C and 1 atm.
RESULTS AND DISCUSSION
The r e s u l t s of t h e h y d r o g e n a t i o n of 2 - a r y l - 3 - m e t h y l - 2 - b u t e n e s {^} and s u b s t i t u t e d 3,'+-dihydro-l , 2 - d i m e t h y l n a p h t h a l e n e s ( 2 ) over p a l l a d i u m a r e p r e s e n t e d i n Table 2. The fourth column shows t h e s e l e c t i v i t y f a c t o r s
(k„b / k b ) o b t a i n e d from c o m p e t i t i o n h y d r o g e n a t i o n s . These d a t a t o g e t h e r w i t h t h e r e a c t i o n r a t e c o n s t a n t s g e n e r a t e d t h e a d s o r p t i o n c o n s t a n t s (K., „) A , n w i t h r e s p e c t t o 3 - m e t h y l - 2 - p h e n y l - 2 - b u t e n e . The p o l a r c h a r a c t e r of t h e s u b s t i t u e n t s i s e x p r e s s e d by t h e i r Hammett o v a l u e s (12-1U). D e u t e r a t i o n of 3 - m e t h y l - 2 - p h e n y l - 2 - b u t e n e over p a l l a d i u m i n e t h a n o l - d a t 30 C and 1 atm gave 3-methyl-2-phenylbutane with t h e following i s o t o p i c c o n t e n t (MS and PMR): 0.e9 0.93 H H
<0^i-i-CH.
I CHj CHj S u b s t i t u e n t E f f e c t s on t h e Adsorbed S t a t e S t e r i c e f f e c t s . In o r d e r t o g e t a c l e a r p i c t u r e of t h e i n f l u e n c e of e l e c t r o n i c e f f e c t s on t h e s t r e n g h t of a d s o r p t i o n , one has t o r u l e out any s t e r i c f a c t o r s . The s e r i e s with H, CH ( o r C„H ) , euid t^C.Hq as t h e s u b s t i -t u e n -t s would seem u s e f u l -t o e s -t i m a -t e -t h e i n f l u e n c e of -t h e s -t e r i c f a c -t o r s i n t h e case of t h e o t h e r s u b s t i t u e n t s .In b a s i c medium t h e i n f l u e n c e of s t e r i c f a c t o r s i n t h e s e r i e s ^, X=alkyl on t h e a d s o r p t i o n prove t o be m o d e r a t e , so t h a t h a r d l y a c o r r e c t i o n has t o be made. In n - h e p t a n e and a c e t i c a c i d , however, s t e r i c f a c t o r s a r e much more pronounced. S u b s t i t u t i o n a t t h e 3 - p o s i t i o n has l e s s e f f e c t than a t t h e U-- p o s i t i o n . I t may be n o t e d t h a t s i m i l a r d i f f e r e n c e s i n s t r e n g h t of a d s o r p t i o n have been observed for t o l u e n e and t h e xylenes on p l a t i n u m and rhodium (15)
34
TABLE 2
HYDROGERATIOa OF 1 AID 2 OVEB PALLADHM*
S u b s t r a t e
1
He i.u
ik
X H 3-CHj U-CH3 •-C^H^ U-isoCjH.^ 3-t-C^H^ l-t-C^H^ 3-OCH3 U-OCB 3 0 -U-0» 3-H(CH3)2 ^-«(CHj)^ S-COOCgH It-COOCgH^ 3-COO* U-coo* 3-CF3 H 5-OCHj 6-OCH T-t-C^Hg e-HHCOCHj Acetophenone 0 . 5 M KOB i n ethanol lO^k 11.0 9 . 6 9 . 7 9 . 6 9 . 3 2 . 7 2.1. 1I. 18 7.1 11.5 6 13.5 3 . 3 5 . 5 3 . 6 U.l U.7 21 25 29 II 17 7 . 9 c 1.0 0 . 6 8 0.61 0.86 0.56 3.5 2 . 2 0 . 5 2 2 . 0 1.9 3 . 3 2 . 6 68 i - K e p t a n e,oC
635 560 570 510 1(50 1)60 350 635 830 620 800 570 700 760 2100 2200 300 c 1.0 0 . 5 2 0.3lt 0.37 0 . 3 0 0 . 1 8 0 . 1 2 0 . 5 8 O.Uo 8.7 10.8 0 . 3 0 0 . 1 6 0.016 2 . 7 3 . 9 100«^ *
^h/
0 . 5 M KOH i n e t h a n o l 1.0 0 . 5 9 0 . 1 5 I.U 0 . 5 9 11.3 1.12 0 . 2 2 1..3 8 . 5 k 1.91
B-hept-ane 1.0 0.U6 0.30 0.30 0.21 0 . 1 3 0 . 0 6 7 0 . 5 8 0 . 5 2 8.5 13.6 0 . 2 7 0 . 1 8 0.019 8 . 8 13.6 U6 f AcOH 1.0 0.25 0.09 0 . 2 3 1.3'' 0 . 0 5 0 . 0 0 6 2 . 7 U.2 f CF COOH ( 1 . 0 ) 8 0 . 0 0 3 ' ' -0 . -0 2 0.01 e \ 0 " 0 . 0 : O.OT 0 . 1 2 0 . 1 3 0.16 0 . 0 7 0 . 1 7 0.08 0 . 1 0 •0.1*7 0.81 0 . 0 5 0 . 3 ^ 0 . 3 7 0.U6 0 . 0 2 0 . 1 1 O.ltT 0.00 0.08 •0.10 •0.07 0 . 1* Temperature 30.0° C; atmospheric H pressure; Pd, 10Ï on carbon. Reaction rate constant (male s~1 g catalyst'1).
Equilibrium conatant of adsorption of X with respect to 2, X»H. Slope of the plot of log(Cx/CxO) versus log(C.^H/CiO „ ) . ' From references (J2-_1U). *' * *
Reaction rate independent on the substituent. * Estimated value with r<.spect to J, X"U-C00C2H5.
. X-lt-H (CH3)2H with o"--0.55 ii.)-^ Mean valiSe from references (J2, Vj),
and for substituted acetophenones on palladium (_16). Disturbemce of optimal flat adsorption of the Il-system by steric repulsion of the substituent with the catalyst surface is clearly a quite general phenomenon.
Electronic effects. The electronic character of a substituent will influence the Il-electron density of both the aromatic nucleus and the olefinic bond. Since the contribution of the Il-electrons of the aromatic nucleus as such to the adsorption is small - as appears from the weak adsoz^tion of toluene with respect to 2,3-dimethyl-2-butene - we may interprete differences in strength of adsorption by electronic factors in terms of adsorption of the olefinic bond.
We define the trifluoromethyl group as a stemdard for the inteiT)ret-ation of electronic effects, i,.£. being a strongly electron-withdrawing substituent without any contribution of its own to the adsorption. Further-more, the size of the trifluoromethyl group is well defined ( vd Waeils radius 2.8 A) because of its symmetry. Correction for steric factors may be easily achieved by comparison with the methyl (2.3 A) and ^-butyl
(3.8 A) groups , providing a minimum and maximum correction factor, respectively.
In 0.5 M potassium hydroxide in ethanol the electronic nature of the substituent scarcely influences the adsorption constant of 1 as follows from the small effect of the trifluoromethyl group. The higher values in this series for the U-dimethylamino and It-ethoxycarbonyl groups - which possess opposite electronic effects - will be due to co-adsorption of the substituents through their free electron pair and n-electrons, respectively. No significant contribution of this type is observed for both the methoxy group tuid the phenolate anion.
The strength of adsorption of the trifluoromethyl compound in n-heptane is much smaller than that of the corresponding methyl and t-butyl compounds. After correction for the small electron-donating character of the 3-alkyl group a Hammett rho VELIUC of about -2 is obtained. According to this picture we would expect about the same strength of adsorption in the case of the equally strong electron-withdrawing ethoxycarbonyl group as the substituent. The much stronger adsorption found (10-20 times stronger than trifluoromethyl)
