BORATE ESTERS: IDENTIFICATION,
STRUCTURE, STABILITY, AND CATION
COORDINATING ABILITY.
£>-Martin van Duin
TR diss
J . Böeseken, professor of organic chemistry at
Delft, 1907-1938: suggested structure of the
potassium(l) borate diester of salicylic acid.
ïi} /1 s y
BORATE ESTERS: IDENTIFICATION,
STRUCTURE, STABILITY, AND CATION
COORDINATING ABILITY.
STRUCTURE, STABILITY, AND CATION
COORDINATING ABILITY.
Proefschrift
ter verkrijging van de graad van doctor aan de Technische Universiteit
Delft, op gezag van de rector magnificus, prof.dr. J . M . Dirken,
in het openbaar te verdedigen ten overstaan van het College van Dekanen
op dinsdag 2 december 1986 te 16.00 uur
door
Martin van Duin,
geboren te 's-Gravenhage,
scheikundig ingenieur.
2 Promc- !-.-=:.ir-i'3in 1 ™
Delft University Press, 1986.
TR diss
1514
prof.dr.ir. H. van Bekkum, eerste promotor, prof.dr.ir. A.P.G. Kieboom, tweede promotor, dr.ir. J.A. Peters.
by the Netherlands Foundation for Chemical Research (SON) with financial aid from the Netherlands Organization for the Advancement of Pure Research (ZWO).
Typing: Mrs. M.A.A. van der Kooij-van Leeuwen Drawings: Mr. W.J. Jongeleen
1. INTRODUCTION 1 Esters of boric acid and borate: a general introduction 1
Applications of esters of boric acid and borate 3
Analytical chemistry 3 Organic synthesis 7 Chemical technology 10 Cheanical products 11 Biological occurrence and applications 14
Scope of the thesis 15 References and notes 17
2. THE pH DEPENDENCE OF THE STABILITY OF ESTERS OF BORIC ACID AND
BORATE IN AQUEOUS ALKALINE MEDIUM AS STUDIED WITH UB NMR 25
Introduction 25 Results and discussion 30
General 30 Glycol 30 Glycolic acid 31 Oxalic acid 33 Glyceric acid 34 Chemical shifts 35 Association constants 36 "Charge rule" for predicting the optimum pH stability 37
Experimental 38 References and notes 39
LATES AND RELATED POLYOLS IN AQUEOUS ALKALINE MEDIUM AS STUDIED
WITH UB NMR 43
Introduction 43 Results and discussion 44
General 44 Chemical shifts 49 Line widths 52 Association constants 53 Conclusions 58 Experimental 59 References and notes 59
4. STRUCTURAL ANALYSIS OF BORATE ESTERS OF POLYHYDROXYCARBOXYLATES
IN WATER USING 1 3C AND*H NMR 63
Introduction 63 Results and discussion 67
Spectral assignment 67 13
Structure determination of borate esters using C NMR 72 Conformational changes upon borate ester formation as
studied with lK NMR 75
Conclusions 76 Experimental 77 References and notes 77
5. THE AQUEOUS D-GLUCARATE-BORATE-CALCIUM(II) SYSTEM AS STUDIED
WITH 1H , 1 1B , AND 1 3C NMR 81
Introduction 81 Results and discussion 82
The system D-glucarate-borate 82 The system D-glucarate-borate-calcium(II) 86
Effect of concentration, pH, and temperature on the
D-glucarate-borate-calcium(II) system 91
Experimental 94 References and notes 94
niet-gebonden interact ies in de twee gauche vormen van
meso-2,3-butaan-diol gelijk.
M.F. Grenier-Loustalot, J. Bonastre en P. Grenier, J. Molec. Struct. 65
(1980) 249.
5. De redenering van Mbabazi met betrekking tot de structuurafhankelijkheid van de stabiliteit van oxyzuuranionesters van polyolen bevat elementaire fouten.
J. Mbabazi, Polyhedron 4 (1985) 75.
6. Het is bepaald onverwacht dat de associatieconstanten van de telluraat-esters van D- en I.-arabinose verschillen; Antikainen en Huttunen beste den hier ten onrechte geen aandacht aan.
P.J. Antikainen en E. Huttunen, Suom. Kemi B46 (1973) 184.
7. Door de parameters van de Haasnoot-De Leeuw-Altona vergelijking te optimaliseren, begeven Masamune et al. zich in een cirkelredenering.
S. Masamune, P. Ma, R.E. Moore, T. Fujiyoshi, C. Jaime en E. Osawa, J. Chem. S o c , Chen. Commun. (1986) 261.
8. Een denticiteit voor tartraat in kation-tartraat complexen van drie is realistischer dan één van vier, zoals door Brittain en Ransom voorgesteld, en verklaart tevens het verschil in stabiliteit van mescr
en rac.-tartraat complexen.
1. In tegenstelling tot wat diverse auteurs beweren, zal D-glucaraat een kation niet gelijktijdig met beide carboxylaatgroepen coördineren.
C L . Mehltretter, B.H. Alexander en C E . Rist, Ind. Eng. Chem. 45 (1953)
2782.
C G . Macarovici en L. Czegledi, Rev. Roum. Chim. 9 (1964) 411.
P. Spacu, E. Antonescu en S. Plostinaru, Rev. Roum. Chim. 11 (1966) 327.
J.G. Velasco, J. Ortega en J. Sancho, J. Inorg. Nuel. Chem. 36 (1976)
889.
R.J. Motekaitis en A.K. Mortell, Inorg. Chem. 23 (1984) 18.
2. Diverse auteurs realiseren zich onvoldoende dat met behulp van potentio-metrie in principe alleen het aantal ionisatiestappen en de bijbehorende pK 's bepaald kunnen worden, maar niet de structuren van de aanwezige species.
R.L. Pecsok en R.S. Juvet, J. Chem. Soc. (1955) 202. R.L. Pecsok en J. Sandera, J. Chem. Soc. (1955) 1489. A. Sonesson, Acta Chem. Scand. 13 (1959) 998.
C G . Macarovici en M. Volusniuc-Birou, Rev. Roum. Chim. 16 (1971) 823.
R.J. Motekaitis en A.E. Martell, Inorg. Chem. 23 (1984) 18.
P.B. Abdullah en C.B. Monk, J. Chem. Soc, Faraday Trans. I 81 (1985)
983.
3. Aruga neemt ten onrechte aan dat de verandering van de optische rotatie-eigenschappen van alditolen onder invloed van boraat zeer langzaam is, ten opzichte van de tijd welke nodig is voor de calorimetrische metin gen.
nieerd. Verder verdient het aanbeveling om meer specifieke termen als associatie-, dissociatie- en ionisatieconstante te gebruiken.
10. Anders dan Akitt en McDonald stellen, zijn de criteria die het elek trisch veldgradiëntnulpunt in een tetraëder vastleggen OP = / V a r en d = </2 r.
J.W. Akitt en W.S. McDonald, J. Magn. Res. 58 (1984) 401.
11. Behalve van homo sapiens of homo ludens kan men ook van homo structurens spreken.
12. Wetenschapsgeschiedenis en -filosofie zouden niet mogen ontbreken bij wetenschappelijke opleidingen.
13. Het verdient aanbeveling sollicitatieformulieren samen te stellen uit een gestandaardiseerd en een specifiek gedeelte.
14. Met behulp van de verhoudingsfactoren 1.10 voor vrouwen/mannen en 5.00 en 0.53 voor zwemmen/hardlopen en schaatsen/hardlopen kan men met hoge correlatie uit de wereldrecords bij mannen voor hardlopen de overeenkom stige records bij vrouwen en voor zwemmen en schaatsen voorspellen.
M. van Duin 2 december 1986
CARBOXYLATE SYSTEMS 97 Introduction 97 Results and discussion 99
Calcium(II) sequestering capacities 99 Origin of the synergic calcium(II) coordination in the
borate-polyhydroxycarboxylate system 100 Quantitative analysis of the
calcium(II)-borate-poly-hydroxycarboxylate MIS 106 Synergic calcium(II) coordination as a function of the
polyhydroxycarboxylate 110
Experimental 112 References and notes 113
7. INTERACTIONS OF CATIONS WITH OXYACID ANION BRIDGED ESTERS OF
D-GLUCARATE IN AQUEOUS ALKALINE MEDIUM 117
Introduction 117 Results and discussion 119
Cation addition to the boric acid- D-glucaric acid
system at pH = 10.5 119 Calcium(II) coordination in oxyacid anion- D-glucarate
systems 125 Experimental 129 References and notes 129
8. A GENERAL COORDINATION-IONIZATION SCHEME FOR
POLYHYDROXYCAR-BOXYLIC ACIDS IN WATER 133 Introduction 133 Data and discussion 134 References and notes 142
9. A 1H , 1 1B , AND 1 3C NMR STUDY OF BORATE ESTERS OF 1,3-DIONES,
INCLUDING CURCUMIN 147 Introduction 147 Results and discussion 150
Experimental 152 References and notes 155
Introduction 157 Results and discussion 158
Experimental 160 References 160
11. THE CONFORMATIONS OF THE SIMPLE VICINAL DIOLS AS STUDIED WITH
MOLECULAR MECHANICS CALCULATIONS 161
Introduction 161 Calculations 162 Discussion 164 General 164 1,2-Ethanediol 165 (S)-l,2-Propanediol 167 1,3-Propanediol 168 (.ff,S)-2,3-Butanediol 168 (,S,S)-2,3-butanediol 169 Steric interactions in the series of vicinal diols 170
Conclusions 171 References and notes 172
12. CONFORMATIONS AND PSEUDOROTATION OF 1,3-DIOXOLANE AND SOME METHYL SUBSTITUTED DERIVATIVES AS STUDIED WITH MOLECULAR
MECHANICS CALCULATIONS 175 Introduction 175 Experimental 177 Calculations 177 *H NMR 180 Discussion 181 General 181 1,3-Dioxolane 181 4,5-Dimethyl substituted 1,3-dioxolanes 182
2,2-Dimethyl substituted 1,3-dioxolanes 182
H NMR experiments 183 References and notes 184
SIMPLE VICINAL DIOLS AS STUDIED WITH MOLECULAR MECHANICS .
CALCULATIONS 189 Introduction 189
Calculations 190 Discussion 191
Configuration of the B(OH)„ moiety 191 Pseudorotation of the borate monoester ring 193
Stability of borate monoesters 194
References and notes 195
SUMMARY 197
SAMENVATTING 201
DANKWOORD 205
CHAPTER 1
INTRODUCTION
Esters of boric acid and borate: a general introduction.
The distinction between organic and inorganic chemistry is a historical one and is at least partly artificial. For instance, compounds characterized by an X-O-C moiety range from "pure" organic compounds, such as ethers, esters, and anhydrides (X = C) to coordination compounds like carboxylates and alcoholates (X = metal ion). Esters of bor(on)ic acid and bor(on)ate (Figure 1 ) , often denoted as complexes, are in an intermediate position (X = B ) .
R — B
R = R' = O H boric acid ester borate monoester borate diester R = alkyl/aryl: R' = O H boronlc acid ester boronate ester
R = R'= alkyl/aryl - borinate ester
Figure 1. Cyclic esters of bor(on)ic acid and bor(on)ate.
In analogy with the organic esters, esters of boric acid and borate are obtained by condensation of a hydroxyl donating species (boric acid or börate) and a hydroxy compound with loss of water. Here, cleavage and
1 2
formation of B-0 bonds occur. ' Formally the central boron is a trivalent 3
cation. But its ionic radius is so small and its charge/radius density is so large, that boron penetrates the electron cloud of oxygen upon bond
4
formation, resulting in orbital overlap and in a B-0 bond having a predominantly covalent character. However, the bondmoment is 0,8 D (C-O:
7
B-- Ov /
ooc
ooc
coo
coo
OHFigure 2. Variation of the denticity (number of coordinating donor atoms) of the organic hydroxy compound in the solid state structures of the
10 borate tetraester of methanol (a), -tartrate
the borate diester of
(5,5)-11 12 (b), the diborate ester of scyiVo-inositol (c), and
13
boromycine (d), respectively
Just as coordination compounds, the esters of boric acid and borate are 2 in fast equilibrium with the parent compounds in aqueous solution (k = 10 --10 M s at 25 °C). The stability of these esters in water is low except when chelation occurs, resulting in one or more rings. Borate esters of mono-, bi-, tri-, and tetradentate alcohols have been shown to exist (Figure 2 ) .
From 1920 to 1940, Böeseken and coworkers in this laboratory have been the first to study the chemistry of esters of boric acid and borate ("Böeseken complexes") systematically, using conductometry and
polarimetry. In the following decades refractometry, solubility and cryoscopic measurements, calorimetry, chromatography (GC and LC), electro-phoresis, spectroscopie techniques (IR, Raman, UV/Visible, fluorescence, luminescence, phosphorescence, and NMR), X-ray diffraction, polarography,
15-24
and thermal analysis have been applied as analytical tools. Recently, the development of new techniques, such as high field multinuclear NMR, have provided new stimuli to study these esters. This thesis should be regarded in this perspective.
Applications of esters of boric acid and borate.
The countries with the largest production of boron containing mineral are the United States (1972: 1.1 million ton with a value of 96 million US $ ) , Turkey, and Argentina. Boric acid and simple inorganic salts comprise over
24 99% of the boron containing products.
The number of applications of esters of boric acid and borate is overwhelming. It is well known that product properties and process characteristics, involving hydroxyl containing compounds, can be affected by addition of boric acid or borate. The effects have usually been studied in a rather empirical way.
In this paragraph a concise review is presented of the applications of esters of bor(on)ic acid and bor(on)ate in analytical chemistry and chemical synthesis. A survey of technical applications in chemical processes and
24
products is restricted to general topics and literature from the last decade. A brief summary of biological aspects is given, as these have been
25 reviewed by Kliegel.
Analytical chemistry
Applications of esters of bor(on)ic acid and bor(on)ate in analytical chemistry are found in quantitative determination, structure elucidation, and separation techniques.
The formation of esters of boric acid and borate enables the quantitative determination of boric acid/borate as well as of chelating dihydroxy compounds. The acidity of boric acid can be enhanced by addition of
alditols, such as D-glucitol and D-mannitol, * or even better by
o p OQ
calcium(II) D-gluconate. ' In this way direct titrimetric determination of boric acid with a strong base is possible. Boronic acid and borinic acid
24
are determined in a similar way. The decrease of pH upon iron(III) complexation by borogluconate (a reaction mixture of boric acid and D-gluconate at adjusted pH) is a quantitative measure for the amount of
30
iron(III) present. Precipitation of barium(II) borotartrate has been 24
exploited in the gravimetric determination of boron. Precipitation of boric acid as a borotartrate, followed by filtration and oxidation with 31 cerium(IV) is a rather cumbersome procedure for boric acid determination. More straightforward methods are the spectroscopie boric acid determinations
or ,34 32 24 33 in chloranilic- D-glucaric acid, anthraquinone, azomethin-H (with or without extraction with 2-ethyl-l,3-hexanediol), and curcumin-acid
(mineral or oxalic acid) systems (Figure 3 ) . The latter systems are also 24
used in the determination of boronic and borinic acid.
OH
OCH,
O O
Figure 3. Mixed borate diester of curcumin and oxalic acid.
The curcumin-boric acid system is used in dicarboxylic and a-hydroxycar-35
boxylic acid determinations. In liquid chromatography carbohydrates can be determined fluorometrically with borate-taurine and borate-
2-aminopropio-O C 0*7
nitril fumarate reagents. ' Invert sugar determination is possible with 38
potentiometry after addition of borax. A non-destructive catechol assay in 39
polymers is based on borate ester formation. Fluorescence in systems of borate and (hydr)oxy derivatives of anthraquinone, tetracycline, and
25 flavonoids is used in quantitative analysis.
Reaction of polyols with BEt„ results in diethylborinic and ethylboronic acid esters with evolution of ethane. The amount of released ethane is a
40 41 measure for the number of hydroxyls present in the polyol. '
HO HO-3, OH
I
HO y^\ o • B(OH)3 . H20 - V J • H ö A . 149 HO BtOH),-K-Figure 4. Enhancement of the conductivity of a boric acid solution (expres-14 sed as A) upon addition of cis- and tra«.s-l,2-cyclopentanediol.
The strict spatial requirements for multidentate ligands upon ester formation with bor(on)ic acid and bor(on)ate, i.e. the proximity of two or more hydroxyl functions, have made these boron containing compounds useful in studying structural, configuratlonal, and conformational problems. BUeseken and coworkers have applied this principle in the carbohydrate field
14
using conductometry and polarimetry. Cis- and tra/7s-l,2-cyclopentanediol
have been used as model compounds for the furanose sugars (Figure 4 ) . These studies might be considered as the origin of conformational analysis, although later chemists have for the most part been unaware and not appreciative of this research.
23 43
Mass spectra of boronic acid esters, ' chromatographic and 17 44 45
electrophoretic data for borate containing mobile phases, ' ' and changes in selectivity of reactions, such as periodate oxidation in the
O O AC
presence of borate ' also provide direct answers to structural questions. 13
Boric acid and borate are used as NMR shift reagents (mainly C NMR) for 47 48
the assignment of the signals of both diols ' and carbohydrates (and 49 50
derivatives) ' and for the elucidation of the configuration of ,. n 51-53
diols.
Esters of bor(on)ic acid and bor(on)ate are of importance in the removal of boric acid/borate and in the isolation of hydroxy compounds. A classical method for the removal of bor(on)ic acid is the formation of B(0Me)„ and boronic acid esters of glycol or 1,3-propanediol, repectively, followed by
14 54
2-ethyl-l,3-55 56 24 2-ethyl-l,3-55
-hexanediol, ' 2,2,4-trimethyl-l,3-pentanediol, ' or diol functiona-54
lized resins (cf. technical processes). Preeoncentration before boron 33
determination is achieved in this way. Boronic acids can. be purified and 23 54
characterized as the catechol esters. '
The separation and purification of carbohydrates via distillation or fractional crystallization of esters of boronic acid is also known for a
23
long time. With respect to modern analytical separation techniques of hydroxy compounds, bor(on)ic acid and bor(on)ate have found widespread use
22 23 25 57 in gas and liquid chromatography and in electrophoresis. ' ' ' GC of polar compounds, such as carbohydrates (and derivatives) is difficult, unless they are derivatized, for instance by methylation, acetylation, or trimethylsilylation. These derivatizations result in different anomeric forms with overlapping peaks and thus long retention times. A good alternative is the formation of volatile butyl- and phenylboronic acid esters. Standard procedures have been developed for both GC and GC-MS of carbohydrates. Analytical enantiomeric separation of aliphatic diols is possible through their boronic acid esters, using complexation GC with
59 chiral nickel(II) complexes as stationary phase.
These derivatizations are rather time consuming and separation techniques for the liquid phase, despite the lower separation capacities, have superseded GC in a number of cases. In this respect, applications of
1R
bor(on)ate are found in zone electrophoresis and in thin layer, paper, and
oo no cfj
anion exchange chromatography. ' ' After t h e i n i t i a l use of simple
1 rj MC crj cf\ fjl
borate buffers, > > > > refinement of these applications has resulted in subtle pH gradients in polyol-borate buffers for zone electrophoresis and
62—64 anion exchange chromatography.
In LC immobilization of boronate by coupling to an insoluble polymer has been introduced to separate mixtures of more complex molecules, such as
22 25 57 nucleosides, nucleotides, glycoproteins and -lipids, and enzymes ' ' and
65
in the analysis of the effluent of paper factories. The properties of phenylboronate columns are affected by the nature of the support matrix (cellulose, dextran, acrylamide, and polystyrene) ' and by substitution of the benzene ring. One of the most recent developments is the ligand
57
mediated chromatography (Figure 5 ) . The affinity ligand, that offers possible binding sites for the retardation of macromolecules (shaded form), is bound via its diol function to the boronate resin. Finally, uphill
J&
H O H O -O B ( O H ) , HO Oo-i
H O ^ O _ | |^
Figure 5 . Schematic diagram of ligand-mediated chromatography (cf. t e x t ) . 57
transport of monosaccharides between two liquid phases of different pH 67
across an organic, liquid membrane is mediated by phenylboronate.
Organic synthesis
Apart from a role as intermediates in the preparation of boron containing 24
compounds, esters of bor(on)ic acid and bor(on)ate are applied to affect the reactivity and selectivity of reactions involving hydroxy compounds, especially in the carbohydrate field. It has been reported that the presence of bor(on)ic acid and bor(on)ate in solutions of reducing sugars can shift the isomerization equilibria (increased ketose/aldose ratio: D-fructose/
R8—74
/D-glucose, D-maltulose/D-maltose, and D-lactulose/D-lactose ) and may 75
increase the amount of acyclic forms. Furthermore, diol functions can be protected selectively, with the advantage of easy deprotection. '
Thus upon addition of bor(on)ic acid or borate an enhancement of selectivity has been observed in a variety of reactions with carbohydrates (and derivatives), such as esterification [(chloro)acetylation,
benzoyla-23 77
tion, /7-tosylation, and phosphorylation], ' etherification (methyla-2*3 78—80 81 82
tion), isopropylidation, bromine oxidation, ' hydrogenation 83 84
(Figure 6 ) , and dehydration. Polystyrene boronic acid resins are also applied for this purpose, ' with the advantage of simple purification. Noteworthy is the application of these resins in the preparation of
7G 87
otherwise difficult accessible carbohydrates. ' Similarly, the reactivity of carbohydrates can be decreased in the presence of borate, for instance
25
copper(II), iron(III), and iodine oxidation and the alkaline degradation 88
of carbohydrates can be inhibited by borate addition.
Cu Cu
Figure 6. Enhanced selectivity of the D-glucose hydrogenation to D-mannitol: chemisorption of the borate diester of p-D-fructofuranose and
Q O
anti-ring-0 side H-attack.
reaction with BEt„ (cf. analytical chemistry), have been applied in the 89—92
selective synthesis and in the purification of polyols by
41 41 distillation. BEt„ can be applied in the dehydration of carbohydrates.
Bor(on)ic acid and bor(on)ate have been shown to catalyze condensations, additions, and hydrolyses by formation of a bor(on)ate ester in the vicinity
25 93
of the reactive site (Figure 7 ) . ' Such a scheme is also suggested for the NaBH. reduction of (hydr)oxy esters. It may be noted that these types of
25 93 reactions are studied as model systems for enzyme catalysis. '
The (stereo)selectivity of aldol condenstations is increased when the ketone is converted into the borinyl acid ester of its enol form, prior to addition of the aldehyde. The transition state is supposed to resemble the
94
dialkylborinate e3ter of the p-hydroxyketone product (Figure 7a). More recently, mixed boric acid esters of the enol form of the ketone and glycol
L L L 0 . . L SB H , > B 7 > B ' / O ( O O xO O O ^PVCHR. / i \ ^ \ x v ^ ^ „ ^Y~> 2 Rf®^f ^ R , ' ^~^ O' Me Me
F i g u r e 7. Suggested r o l e of boron in t h e s t e r e o s e l e c t i v e a l d o l condensation ( a ) , t h e o-hydroxymethylation of phenol w i t h formaldehyde ( b ) , and t h e NaBH. reduction of (hydr)oxy e s t e r s ( c ) .
95-97
have been applied. The product composition of the formose reaction, i.e. the oligomerization of formaldehyde in aqueous alkaline solutions to the so-called formose sugars, is also affected by borate. Substituted
102 103 1,7-dipheny1-5-hydroxyhepta-l,4,6-triene-3 -ones, such as curcumin (cf.
Figure 3 ) , are obtained by condensation of acetylacetone with two equiva lents of an aldehyde in the presence of boric acid.
An example of a boric acid catalyzed addition is the o-hydroxymethylation of phenol with formaldehyde. The transition state once again is
25
characterized by a cyclic borate ester structure (Figure 7b). The (intramolecular) borate catalyzed hydrolysis of hydroxyesters is assumed to follow a similar pathway. '
Usually, carboxylic acid esters and lactones are essentially inert towards KBH. and NaBH- reduction. However, several exceptions are known. Esters with complexing, neighbouring functions, such as (hydr)oxyesters, can be reduced with these borohydrides. A probable explanation is the formation of -C-0-BH„ structures (Figure 7c), followed by
93
intramolecular reduction. The NaBH. reduction of carbohydrates and the 112
trialkylborane reduction of 0-hydroxyketones are stereoselective for the same reason. An alternative is the addition of reagents, for instance primary alcohols, that increase the reductive power of NaBH., due to NaBH. (OR) formation. ' ' The use of special solvents, such as lflfi polyethylene glycol 400, is also effective, probably for the same reason.
Thermal decomposition of boric acid esters of tertiary alcohols gives the corresponding alkenes. This principle is used to separate mixtures of secondary and tertiary alcohols, via conversion of the alcohols into the boric acid esters, followed by pyrolysis of the tertiary alkyl boric acid
24 esters.
Some miscellaneous reactions involving bor(on)ic acid or bor(on)ate esters have been reported recently. Chain extension of boronic acid esters
115
is effected with LiCHCl„. In the presence of boronic acid esters of pinanediol, enantioselective synthesis of insect feromones and aminoalcohols
11 fi
is achieved. Borate has also been used as a template in the synthesis of 117
macrocyclic compounds (Figure 8 ) . Boric acid esters are used as 24
alkylating agents in Friedel-Crafts reactions. Boric acid is a catalyst 118
for the Friedel-Crafts acylation of catechol. Benzyl bromide is carboxy-119
CHO CHO CHO
Dl
h
- O H - O H H O H O -= N - w v N -=-IQ
U
Figure 8. Synthesis of macrocyclic compounds with borate as template 117acid esters with the flavoenzyme cyclohexanone oxygenase, boric acid esters 120
are obtained with conservation of chirality.
Role in technical processes
In the last decade the main technical applications of esters of bor(on)ic acid and bor(on)ate have been in the large scale oxidation of paraffins and in extraction processes. paraHin
4
alkine wash<P~
NaOH + H 20T
hydrogenation boric acid oxidation Q pa ratlin ," cycle flu gasI
■=>-alkaline wash NaOH T— distillation hydrolysis I distillation 1 hydrogen-vViboric acid cycle
sec. alcohols
Figure 9. Flow sheet for the Bahkirov oxidation process of paraffins (reproduced with permission from reference 121).
Boric acid esters of secondary fatty alcohols are obtained in the liquid phase oxidation of paraffins with air at 150-170 °C in the presence of stoichiometric amounts of boric acid or boron oxides (Figure 9 ) , thus
121 122
preventing through oxidation. ' The alcohols, obtained by hydrolysis, 121-123
are precursors for non-ionic surfactants. The esters B(OR)„ with R = Pr, Bu, Ph, and cyclohexyl are applied for the same selectivity purpose. The oxidation of 1,3-butadiene to butanediols and of cycloalkanes to cycloalcohols ' ' are also carried out in the presence . of these boron compounds. Olefins are oxidized to epoxides in the presence
128 129 of boric acid or with Hiutyl hydroperoxide-borate. '
Extraction of boric acid or borate is achieved by solvent extraction or with functionalized resins and is of importance in the boron production and recovery, in purification of MgCl„ brines, and in waste water
24 130-132
plants. ' The solvent extraction processes are characterized by boric acid/borate transport from an aqueous solution to an organic solvent via ester formation. In the case of boric acid in (acidic) leaching
55 133 solutions, 1,3-diols such as trimethyl-l,3-butanediol,
2-ethyl-l,3-130 134 -hexanediol, or alkoxy-l,3-butanediols are dissolved in the organic phase. In the case of borate, aromatic diols like alkylcatechols, alkylsalicylic acids, and 2-chloro-4-(l,l,3,3-tetramethylbutyl)-6-hydroxy
24 133
methylphenol are used. ' Polymers of phenol, aldehyde, aminopolyols, and aliphatic polyamines, poly(vinyl alcohol) resins, and resins
137
functionalized with hexose moieties are applied as boric acid exchange material. Aqueous borax solutions, on the other hand, are used to extract
24 polyhydroxycarboxylates from petroleum oxidation products.
Finally, esters of boric acid and borate have potentials as a catalyst for the polymerization of alkenes, dienes, and isocyanates and for
24 124
crosslinking epoxy resins. ' Boric acid (esters) are used in the 138
dehydration of 1,3-diols to conjugated dienes.
Chemical products
Esters of boric acid and borate as such are available as commercial products, but are also used as additive to improve product properties.
The cation sequestering capacity of the biodegradable and non-toxic polyhydroxycarboxylates for calcium(II), iron(III), and aluminium(III) in
aqueous alkaline solutions is increased upon addition of boric acid/borate. Borogluconate (Glucona BV., Ter Apelkanaal, The Netherlands) and boroglucoheptonate (Croda Chemicals Ltd., Cowick Hall, United Kingdom) are available as commercial sequestering agents and have a wide range of
139 applications (Table 1 ) .
Table 1. Function and sequestering agents
field of
139
application of borogluco(hepto)nate
function field of application
degreasing, desoxidation, paintstrip-ping, alkaline etching, electropla te ., 140 , . ... ting, and corrosion inhibition alkaline cleaning and descaling
set retardation and plasticization additive in bleaching and dyeing process
141 142 triphosphate substitute '
increasing the solubility of calcium(II)
galvanic industry
glass industry, breweries, and dairy industry
cement and concrete industry textile industry
detergent industry
treatment of hypocalcaemic condi tions (milkfever and parturient and delivery paresis)
Increase of the viscosity or gelation of aqueous solutions of polyols, such as polyvinyl alcohol, dextran, glycan, starch, and gelatin upon addition of boric acid or borate - resulting from crosslinking due to borate ester formation and hydrogen bonding - is applied in thickening and gelation
24 143-146
agents. ' These gels are used as drill fluids in oil winning processes, to encapsulate pesticides and other agrochemicals for slow
146 147
release purposes, ' and as adhesives, amongst others 124 143 148 149
for linerboards. ' ' ' Paper envelopes, containing detergent additives, can be impregnated with polyols. The presence of borate in the washing water results in dissolution of the polyols, followed by release of the additives 150 Furthermore, these high viscosity liquids are sold as childrens plaything ("Slime").
24 Borate esters of tanning agents are used in the leather treatment. Borate esters of polyols are used as resist or discharge agents in the
152
printing process of synthetic fabrics. In some fabric softeners, negatively charged borate diesters act as counterion for benzylammonium
153
compounds to increase the solubility in water. Borate esters of a variety of hydroxy compounds are used as surface active agents, amongst others in
24 detergents.
The properties of lubricating oils, greases, and hydraulic fluids, for instance as found in brake systems, can be improved upon addition of 24 124 154-156 bor(on)ic acid esters of (polyethylene) glycol or B(Oalkyl)„ ' '
24
and are claimed to be relatively water insensitive. The presence of esters of boric acid and borate in (propylene)glycol brake and antifreeze liquids
157
counteracts the influence of water and the borate diesters of glycerol 124
and D-glucitol are applied as anticorrosion additives. Solutions containing triborate polyol esters (for instance pentaerythritol monooleate)
158 159 or borax-disaccharides mixtures are applied as lubricating oil. ' Borate diesters of 3-mercapto-l,2-propanediol are antiwear additives for
i in
24
The applications in polymers are also numerous (Table 2 ) . Liquid crystals 33 B
24 124
lubricating oils. Boric acid esters of glycol are applied in automobile fuels to increase the octane number and as antiicing additive.'
5 (T8 1 RT
based upon phenylboronic acid esters are known. Borate esters of glycol and catechol can be used as capacitor electrolytes.
25
Biological occurrence and applications
Boron is present as boric acid, borate, esters, and salts in biological systems. It is essential for higher plants. In lower plants, boron sometimes is not found at all. Some boron containing antibiotics are known [boromycine (cf. Figure 2) and aplasmomycine], that are used against gram-positive bacteria. Boron is always present in animals, but it is unclear whether it is essential. The lethal dosis of boric acid for higher animals is about 1-2 g/kg.
The larger part of the biological boric acid and borate is present as esters of polyols, polyhydroxycarboxylates, carbohydrates (and derivatives), and aromatic alcohols. It may be noted that the use of boric acid or borate
164
Table 2. Applications of esters of boric acid and borate in polymers.
function additive applied in 124
vulcanizing agent B(Oalkyl)„ and borate 24 c u r i n g agent a n t i o x i d a n t p l a s t i c i z e r a n t i s t a t i c s u r f a c e a c t i v e compound heat s t a b i l i z e r flame r e t a r d a n t d i e s t e r s of p y r o c a t e c h o l b o r a t e e s t e r s of methanol, c a t e c h o l , c r e s o l , and i • i ^ 24 s a l i c y l a t e b o r a t e d i e s t e r of p y r o c a t e c h o l ,1 2 4 B(Oalkyl) 2 4 and 24 a r y l b o r o n i c a c i d e s t e r s B(Oalkyl) 24
borate esters of glycolstea-. glycolstea-. glycolstea-.glycolstea-. glycolstea-. 160-162 rate and -paImitate
B(Oalkyl)„, borate diester of glycerol, and borate esters of polyol-fatty acid esters borate diester of brominated
,. , 124 diols neoprene, chloroprene, and butadiene-styrene rubber epoxy, pheno1/urea formaldehyde resins (brominated) butyl rubber and polyvinyl chloride
natural and synthetic resins
polyethene
polyvinyl chloride and polystyrene
124
effects.
133 Boric acid is toxic for vegetation when it exceeds the 5 ppm level. Thus, the boric acid ester of 1,2-propanediol is applied in wood conservation and the borate esters of polyols in the conservation of samples of biological origin, such as urine during transport or storage and also
167 124 as fungicide. B(0alkyl)_ is applied as insect sterilant. Alditols, on
the other hand, are used as antidote for boric acid poisoning.
In solutions of boric acid/borate and a dihydroxy compound, the 165 solubility of both species is enhanced by borate ester formation.
1 RR
Applications of borotartrate in liquid fertilizers and of borate esters 169
of carbohydrates as grain seed developer are based on this principle. The solubility of alkaloids and antibiotics (caffeine, procaine, epinephrine, and Amphotericine B) is increased when borogluconate is the
170 171
counterion. ' The application of calcium(II) borogluconate in the treatment of low calcium(II) plasm levels in cattle has already been mentioned.
B labelling of dopamine is used in the cancer research to mark 172
melanomes and in the neutron capture therapy of tumors (Figure 10).
Figure 10. Structure of borate monoester of L-dopa.
Scope of the thesis
The first part of this thesis (chapters 2-9) deals with the identifi cation, structure, and stability of borate esters in aqueous medium. It provides a chemical basis for understanding the effects of boric acid/borate upon product properties or process characteristics, as summarized in the previous paragraph. The central theme is the (synergic) cation coordinating ability of borate esters of polyhydroxycarboxylates in aqueous solution (cf. Table 1 ) , as studied by multinuclear NMR and calcium(II) ion selective electrode measurements. Borate esters of 1,3-diones are also studied because
103
of their importance in the synthesis of curcumin and in the spectroscopie 24 33-35
determination in boric acid-curcumin systems. ' In the second part of this thesis (chapter 10-13) a more detailed study on the structure and conformation of both borate esters and the parent compounds is presented.
The effect of pH on the stability of esters of boric acid and borate with dihydroxy compounds in water is studied with B NMR (chapter 2 ) . The results are explained with a "charge rule" that allows prediction of the pH region of optimum stability for a particular ester. The various borate esters that exist for a compound containing several diol functions are identified with B NMR (chapter 3 ) . The factors that determine the B chemical shift and line width and the stability of the borate esters are
discussed. The borate binding sites in polyhydroxycarboxylates and the 13 occurrence of diastereomerism in the borate diesters are determined with C NMR (chapter 4 ) . The confonnational changes of the polyhydroxycarboxylates upon borate ester formation are determined with H NMR.
11 13 The D-glucarate-borate-calcium(II) system is studied using B, C, and H NMR, including the effects of pH, concentration, and temperature (chapter 5 ) . The species responsible for the synergic calcium(II) coordination are determined and the composition of the calcium(II) coordinating sites is discussed. The conclusions for this system are generalized for calcium(II)--borate-polyhydroxycarboxylate systems (chapter 6 ) . Fitting the data on the
(synergic) calcium(II) coordination, obtained with calcium(II) ion selective electrode measurements, with a model, containing all important species of these systems, allows quantification of the phenomena.
The scope of the synergic cation coordination is studied for various oxyacid anion- D-glucarate systems (chapter 7 ) . The effects of the cations on the relative borate diester stability are correlated with their charge/radius densities and polarizing abilities, whereas the effects of the oxyacid anions are explained with the pH rule of thumb derived for borate in chapter 2. Finally, a general coordination-ionization scheme for polyhy-droxycarboxylate complexes, based on literature data for 40 metal ions and oxyacid anions, is given (chapter 8 ) . A fundamental explanation for the phenomena is provided.
Borate esters of 1,3-diones are studied with B, C, and H NMR (chapter 9 ) . They are expected to have different stability effects in comparison with those of diols, because of the opposite charges of the respective esters.
A H NMR study of free polyhydroxycarboxylates is presented (chapter 10). The chemical shifts and vicinal coupling constants are explained in terms of substitution and preferred conformations.
Molecular mechanics calculations using the MM2 force field give detailed insight in the conformations and confonnational energies of some simple diols (chapter 11). Steric hindrance, the gauche effect, and hydrogen bonding are included in the discussion. An empirical force field study is performed on the pseudorotation and conformations of substituted 1,3-dioxo-lanes, which are used as model compounds of borate monoesters (chapter 12). Vicinal H coupling constants are calculated from the molecular mechanics
results and compared with experimental data. Finally, some calculations on borate monoester geometries are described and compared with X-ray structures (chapter 13). Calculated stabilities of borate monoesters are compared with experimental data.
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165. C A . Zittle, Advanc. Enzym. 12 (1951) 493.
166. G.0. Hentschel and T. Carolussen, Austr. Pat. 534,128 (1984); Chem. Abstr. 101 (1984) 86888.
167. J. Ploquin and C. Ploquin, Ann. Pharm. France 20 (1962) 201.
168. M. Hatori, Jap. Pat. 7,375,351 (1973); Chem. Abstr. 80 (1974) 14073. 169. J. Ploquin and C. Ploquin, Ann. Pharm. France 20 (162) 305.
170. D. Curtis, US Pat. 2,582,191 (1952); Chem. Abstr. 46 (1952) 7586. 171. G. Strauss and F. Krai, Biopolymers 21 (1982) 459.
CHAPTER 2
THE pH DEPENDENCE OF THE STABILITY OF ESTERS OF BORIC ACID AND BORATE IN AQUEOUS MEDIUM AS STUDIED WITH UB NMR*
Introduction
Esters of boric acid and borate have found.widespread use as a tool in configurational analysis, for instance of carbohydrates, and in a variety
2
of separation and chromatography techniques. This class of compounds has
3 4 been studied for more than a century, using several analytical techniques.
The possible equilibria between boric acid, borate, and the corresponding esters are summarized in Figure 1. Boric acid (B ) is a Lewis acid and can bind a hydroxyl ion, forming the borate anion (B ). Both boric acid and borate can react with a suitable dihydroxy compound (L), resulting in the boric acid ester (B L) and the borate monoester (B L ) , respectively.
Subse-B'
B'L
B~L
2Figure 1. Equilibria between boric acid, borate, and a dihydroxy compound in aqueous medium.
Cf. M. van Duin, J.A. Peters, A.P.G. Kieboom, and H. van Bekkum, Tetrahedron 40 (1984) 2901.
B"
^OH O He HO^o^OH HO-BC; ■- / B ^ O H HO OHB L
•OH -OH OH OH O^e^OHo'N)H
OH _ rr°~
L-o'
-°1
L glycol glycolic acid oxalic acid B ° L I J Ï B - O H ^ B - O H ^ B - O H B~L O ' ^ - O '
Co>-°
H
T°>-°
H o
X
L-o^
""-OHL
0/ \
O HnJ-o'
OHB-L,
I
_;B; II >- I
T ° ^ °
/ ^ N r
Figure 2. Esters of boric acid and borate for glycol, glycolic acid, and oxalic acid.
quently, these two esters can react with another dihydroxy compound to give the borate diester (B L„).
Throughout this thesis a distinction between esters of boric acid and of borate is made by the use of B L and B L (n = 1,2), respectively. The indices "o" and "-" do not always stand for the actual charges of the esters, but denote the charges of the B0„ or BO. moieties. As dihydroxy compounds diols, hydroxycarboxylic acids, and dicarboxylic acids are possible. The corresponding esters possess structures as shown in Figure 2.
5 Carbonyl functions can be estérified after hydration or enolization.
The association constants for the various equilibria involved are defined as follows: B° + OH" Ï=Z B" K | - = [ B " ] / ( [ B ° ] [ O H_] ) = 8.5*104 M_ 1 ( l )6 O B° + L ^ B°L + 2 H20 KgOL = [B°L]/( [B°] [L]) (2) B~ + L ^ B~L + 2 H20 KJj-L = [B~L]/( [B~] [L]) (3) B~L + L ^ B~L2 + 2 H20 K J - J = [ B " L2] / ( [ B " I ] [L]) (4)
Boric acid and boric acid esters are neutral compounds. In the crystal line phase the central boron atom is surrounded by three oxygen atoms in an almost planar fashion with O-B-0 valency angles of about 118°. Although three-coordinated boron compounds form adducts with suitable nucleophiles,
o boric acid does not form B(OH)„(OH„) adducts in aqueous solution. The central boron atom in borate and borate esters is coordinated by four oxygen atoms. The geometry is tetrahedral, because the O-B-0 valency angles are
7
circa 109°. Borate and borate esters are negatively charged.
A mechanism for the formation of borate esters from boric acid has been 9
proposed by Pizer and Selzer. A more extended mechanism is shown in Figure 3. Boric acid and boric acid-like structures are considered to be the reactive species in the esterification, due to the ease of the attack by a dihydroxy compound in comparison with the substitution of a hydroxyl ion in borate or borate-like structures. The rate of exchange between boric acid and borate is very fast and diffusion controlled. ' All the other steps involving proton or hydroxyl transfer are assumed to be fast too. As a result of the chelate effect the ring closure is relatively fast, in comparison with the attack of a free dihydroxy compound at a reactive boric acid-like species. Thus the latter steps are considered to be the rate determining steps in the formation of esters of boric acid and borate. The equilibria between B and B L and between B L and B L , as given in Figure 1, are therefore not realistic from a kinetical point of view, but can be defined thermodynamically.
Some 50 papers have dealt with the stability of esters of boric acid and borate. The influence of the pH on the stability, however, has not been studied systematically. In addition, most of the techniques used (such as potentiometry and polarimetry) give no direct information concerning the identity of the esters involved.
In our laboratory several multinuclear NMR techniques are applied to 12 study cation complexation phenomena of poly(hydr)oxycarboxylic acids and of combinations of these acids with borate. B NMR is an obvious choice in the study of esters of boric acid and borate, especially because this technique provides direct information concerning ester formation. Table 1 gives some important physical data of the two boron isotopes, B and B, which are both NMR sensitive.
B NMR is more suitable than B NMR for two reasons. Firstly, the NMR sensitivity in a 1.0 M solution for B is approximately 30 times larger than that for B. This is due to the differences in natural abundance and in the NMR sensitivity for an equal number of nuclei. Secondly, the resolution in B NMR spectra is better. This is a result of the higher resonance frequency of B NMR and of the smaller electric quadrupole moment
11 14 of B, which rules the line width.
B ( O H )3 OH"
Si
- 0 - B ( O H )2 Ï E = H O -1 H HO-J H © B(OH)< ® 6 0 - B ( O H ) j O—B(OH)3 B ( O H ) , Q0> B ( O H ) ^ = O H ' L0>B(OH)2 HO - I H O - B ^ | OH HO -1 H © © . O - , O - B C <©H) ^ O —I HOJ'
> - < : =
OH" HOJ
© ^ O -O - B ' ~| (-OH) ^O—' p O ^ f / O - ,L®
0/B^
0J
I H r - O ^ g / O - ,L
0/
B^ o J
Table 1. Physical data for B and B nuclei. 10B 3
21.49
18.83
UB 3/264.19
81.17
nucleus spin resonance frequency at 4.70 T (MHz) natural abundance (.%)relative sensitivity at constant field for an equal number of
nuclei (XH = 1.00) 0.0188 0.165
relative sensitivity in 1.0 M.
aqueous solution ( H = 1.00) 0.0036 0.12 —28 2
electric quadrupole moment (10 m ) 0.111 0.0355
Until now B NMR has mostly been used in the studies on boranes, carboranes, heterocyclic (N, 0, and P) boron compounds, and metallo derivatives in organic solvents. ' B NMR studies on compounds with boron coordinated only by oxygen atoms are more scarce and usually have been performed in organic solvents. B NMR has been used to study the equilibria between boric acid, borate, and several polyborate species in water, especially with respect to the dependence of the total boron concentration, the pH, and the presence of metal ions. ' In dilute solutions only one average signal is observed and, therefore, the exchange between boric acid (8 = 0.0 ppm) and borate (8 = -17.6 ppm) is fast on the B NMR time scale. Furthermore, it has been demonstrated that polyborate formation 21 occurs only at total boron concentrations above 0.2 M. Henderson et al. have been the first to use B NMR for identifying borate esters of diols in
22
water. The results have been applied in the carbohydrate field. Borate ester formation and hydrolysis is slow on the B NMR time scale and thus chemical shifts for borate mono- and diesters of 1,2- and 1,3-diols can easily be determined. The calculated association constants show good
21 correspondence with values obtained by other techniques.
In this chapter a B NMR study is presented dealing with the influence of the pH on the stability of esters of boric acid and borate in aqueous solution. As dihydroxy compounds a 1,2-diol (glycol), an a-hydroxycarboxylic acid (glycolic acid), a dicarboxylic acid (oxalic acid), and a compound combining a 1,2-diol and an a-hydroxycarboxylic acid function (glyceric acid = 2,3-dihydroxypropanoic acid) are used. A general rule for determining pH dependent concentration optima for esters of boric acid and borate is put forward.
Results and discussion
General
In Figures 4-7 the influence of the pH on the concentration of the 23
different boron compounds is given for L = glycol, glycolic acid, oxalic acid, and glyceric acid, respectively. The points were determined experi mentally with B NMR, while the curves were calculated from the association constants obtained (except for L = glyceric acid). Chemical shifts
[6(B°) = 0.0 ppm] are given in Table 2. The association constants (Table 3) were calculated using Equations (l)-(4), together with the pK's of the dihydroxy compounds and the following material balance equations:
CB = [B°] + [B~] + [B°L] + [B~L] + [B"L2] (5)
CL = [L] + [B°L] + [B~L] + 2[B_L2] (6)
Glycol
For L = glycol (Figure 4) only one signal was observed at pH < 8. This signal showed an upfield shift upon an increase of the pH and is assigned to the equilibrium between B and B , which is fast on the B NMR time scale. At pH > 8 two new signals were observed, which are assigned to B L and B L„-Thus the exchange between B and B L and between B L and B L9 is slow on the
0-10 c[M] 0 0 5 O 0 4 8 12 ► p H
Figure 4. Glycol (L: 1.0 M) and boric acid/borate (B°/B~: 0.1 M ) : distribu-23 tion of boron containing species as a function of the pH.
B NMR time scale. A further increase of the pH above 11 had no effect on the concentrations of B L and B L„.
These phenomena can be explained by rewriting the equilibria from Figure 1 as:
B° + L ?=£ B~L + H+ + H„0
B° + 2 L ^ B_L2 + H+ + 3 H20
B L and B L„ are formed at high pH, which means in practice that B is present in solution. Consequently, borate esters of glycol can only be expected in the region where pH > pK(boric acid) = 9.1; experimentally pH > 8 is found. At pH = 11, [B ] a 0 and a further increase of the pH has
o - - 24 no effect on [B ]. Therefore, [B L] and [B L„] reach maxima.
The presence of B L could not be demonstrated. When this ester does exist
B° o
KDoT should be small or S(B L) a 0 ppm. In the latter case the experimental signal at 6 = 0.0 ppm has to be assigned to the equilibrium between B and B L (fast exchange on the B NMR time scale). Paal, however, has shown
B°
that the former explanation is correct: K_oT » 0.D L
Glycolic acid
When L = glycolic acid the pH dependence was quite different (Figure 5 ) . At low pH a single signal was observed with a chemical shift between +0.4 and 0.0 ppm. It is assigned to the equilibrium between B and B L, which is fast on the B NMR time scale. Rise of the pH resulted in a second signal, attributed to B L„ with a maximum intensity at pH - 3. A third signal
appeared for B L with a maximum intensity at pH = 7. A further increase of the pH resulted in the disappearance of the B L and B L„ signals and only the signal of B remained. The difference in the pH dependence for L = gly colic acid in comparison with L = glycol is due to the easy deprotonation of the former compound:
0- 1 01-.
c[M]
0 05
-Figure 5. Glycolie aci d (L: 1.0 M) and boric acid/borate (B /B : 0.1 M ) : distribution of boron containing species as a function of the pH.
The amount of B L will be large when [B ] and [L] are large, therefore, at pH < pK(glycolic acid). When both [L] and [L ] are large, formation of B L„ will be optimum according to:
B + L + L ï ; B 12 + 3 HjO
At pH = pK(glycolic acid), where [L] = [L ], a maximum in [B L„] is reached. In principle, B L can be formed either from B and L or from B and L. The latter possibility, however, can be excluded because pK(glycolic acid) < < pK(boric acid). This means that B L exists when L is present, i.e. upon dissociation of glycolic acid:
B°+ L~
B L + H20When the pH reaches pK(boric acid), [B ] increases. The equilibrium for B L may be rewritten as:
B + L 2 ^ B L + OH + H„0
It is obvious that an increase of the pH causes dissociation of B L. In other words, the pH optimum for B 1 is attained at pK(glycolic acid) < pH < < pK(boric acid). Our experimental results fully agree with this picture.2 7 , 2 9
stabili-ties of the borate esters of glycol and glycolic acid can be understood by the difference in pK (»14 and 3.8, respectively). This is in agreement with
31
the results of Sienkiewicz and Roberts, who have studied the influence of the pH on the stability of the phenylboronate esters of 4,5-dihydroxynaphta-lene-2,7-disulfonic acid and 2,3-dihydroxynaphtalene-6-sulfonic acid. Boronate monoester formation with these aromatic diols showed also pH optima, viz. at pH = 6-7 and 8-9, which are close to the corresponding pK values (5.5 and 8.1, respectively).
Oxalic acid
When t = oxalic acid (Figure 6) only one signal could be observed,
besides the signal for the equilibrium between B and B . This signal is attributed to B L. A maximum for [B L] has to be found at the pH where [L ] and [B ] are maximum:
Bu + L ^=a B L + H20
Since oxalic acid is a dicarboxylic acid (pK, = 1.2; pK„ = 4.2 ) , the pH optimum for B L has to be pH = (pK. + pK„)/2 = 2.7. This value is close to
- 11
the experimental maximum for [B L] at pH = 3. B NMR gave no indication for
the occurrence of B L„, which would be expected at pH = pK,. Therefore, only
the upper limit for K_- could be calculated (Table 3 ) . Again B°L could
not be detected. 0-10 i-c[M] 0 0 5 B~L 12 — pH
Figure 6. Oxalic acid (L: 0.25 M) and boric acid/borate (B°/B : 0.1 M ) : distribution of boron containing species as a function of the pH.
Glyceric acid
Although glyceric acid is the simplest molecule in which 1,2-diol and oc-hydroxycarboxylic acid functions are combined, its picture for ester formation as a function of the pH is rather complicated as shown in Figure 7. Here two types of esters are possible as depicted in Figure 8, which holds generally for polyhydroxycarboxylic acids. The notations L,. , and L-)H . , indicate the way glyceric acid is esterified, viz. as a 1,2-diol or
0 10
c[M]
0 05
B"L(aOHacid)+ B"+ B"
Figure 7. Glyceric acid (L: 1.0 M) and boric acid/borate (Bu/B : 0.1 M ) : distribution of boron containing species as a function of the pH.
B L ( d i o l )0 ^ BL(oOHzr)0 pH«4 pH«9
L/\
BL(diol) pH-7 ^ BL(oOHzr)" B L ( d i o l ) L ( a O H z r ) " pH»4 L(oOHzr) •> L(aOHzr)"Figure 8. Equilibria between boric acid, borate, glyceric acid, and glycerate in aqueous solution.
