Boric acid-diol complexes

Boric acid forms complexes with a number of inorganic ions and organic molecules. Ammonia, hydrazine, hydroxides and oxyhalides from complexes with boric acid. The organics include diols, thiols, dioxane, pyridine and many other solvents in which boric acid dissolves. 

Most of the work on the boric acid-diol reaction during the last twenty years has been done to determine the coordination number of the diol (number of diol molecules) in the complex and to evaluate the equilibrium constant (often called a stability constant) for a number of diol-boric acid reactions. Several techniques have been used to study these questions, including polarimetry (7), optical rotatory dispersion (8), polarography (9), conductivity (3), vapor pressure osmometry (10), and electrochemistry (II, 12, 13). The most frequently studied system has been the electrochemical (pH) titration of boric acid or borax solutions with various diols. 

Novel tetrahedral boronate complexes have been prepared as potential ligands for affinity chromatography either from the amination (LiN(TMS)2) of a-haloalkylboronates followed by A-acetyl-ation (i.e., (132)) or, alternatively, from the a-iodo derivatives and acetamidine (i.e., (133)) (Scheme 17). The 1,3,2-dioxaborinanes proved to be less stable than the pinanediol derivatives to silica gel. "B NMR as well as other data confirmed their chelated nature (e.g., (132) d 6.8) <92JOM(43i)255>. Spiroborates (e.g. (134)) have also been prepared from boric acid, diols, and enolizable 1,3-dicarbonyl compounds <86JPR755>. Vinylzirconocenes (135) are converted to vinylboronates (136) with chloroboranes <910M3777>. The hydrozirconation of 5-vinylboronates (137) produces the mixed... 

The system has been used as an analytical tool (4), because the complex formed shows greater conductivity than the sum of the conductivities of the boric acid and the diol compound. Favorably situated diols have a pronounced effect in increasing the conductivity. cis-l,2-Diols fixed in position by molecular structure—e.g., o-dihydroxyphenols and cis-dicyclic polyols—are favorable structures. The reaction is depicted in Figure 2. 

Reduction of the carboxylic acid group passes through the intermediate aldehyde. For a number of examples in the heterocyclic series, the aldehyde becomes a major product because it is trapped as the hydrated vfc.-diol form. Examples include imidazole-2-caiboxylic acid [139], thiazole-2-carboxylic acid [140] and pyridine-4-carboxylic acid [141] reduced in dilute aqueous acid solution. Reduction of imidazole-4-carboxylic acid proceeds to the primary alcohol stage, the aldehyde intermediate is not isolated. Addition of boric acid and sodium sulphite to the electrolyte may allow the aldehyde intermediate to be trapped as a non-reducible complex, Salicylaldehyde had been obtained on a pilot plant scale in this way by... 

Diols and polyols can participate in equilibria with boric acid in aqueous solution. The stability of polyolborates is determined by the number of OH groups in cis positions. Complexes with polyols are more stable than with diols, and 1,2-diol complexes are more stable than their 1,3-diol counterparts (Table 10) since the resulting five-membered chelate ring is unstrained.75120 In the case of 1,3,5-triols stable cage-like structures (5) and (6) are favored. Open-chain or five-membered cyclic polyols form more stable chelate complexes than their six-membered counterparts.120 Thus, chelates from alditols and ketohexoses are more stable than the corresponding aldose chelates (Table 10). Many polyols allow quantitative titrimetric determination of boric acid. Of these, mannitol remains the most widely used reagent on the basis of availability, cost and ease of handling.75... 

Bachelier, N. and J.-F. Verchere, Formation of neutral complexes of boric acid with 1,3-diols in organic solvents and in aqueous solution. Polyhedron, 1995. 14(13) p. 2009-2017. 

Boric acid has the particular ability to form stable complexes with compounds that present cis-hydroxyl groups (cis-diol groups) [117]. Several compounds, such as sugars and their derivatives and some phenolics (o-diphenols) have these cis-diol groups and therefore can form stable complexes with B [117]. 

The reactions of boric acid solutions with diols have been used for almost a century to examine structural differences among carbohydrates. The complexity of these reactions seems to arise not only from simple structural differences but also from differences in carbohydrate configuration and conformation. The precise nature of these reactions is not clear. Recent studies of the chemistry of polyol-boric acid solutions have clarified some aspects of these reactions that have important bearing on the structure of carbohydrates in solution. Nevertheless, some of the most fundamental questions about the nature of the reaction are still unanswered. 

At equilibrium the diol-boric acid ester or complex can have a diol to boron ratio of 1 to 1, 2 to 1, or both. 

An example of some recent data which was interpreted to support this assumption is the work of Knoeck and Taylor (12). Using PMR spectra of mannitol-boric acid solutions, they observed that a decrease in pH resulted in a decrease in the mannitol-boric acid complex concentration. These results are supported by nB nuclear magnetic resonance (NMR) spectroscopy (16) which showed that the complex between mannitol and boric acid increased with increasing pH. However, Knoeck and Taylor (12) reasoned that since an increase in pH resulted in an increase in the borate anion concentration, as well as an increase in the complex concentration, the diol reacts only with the borate ion to produce the complex. 

Expressions VII and VIII are identical in form they differ only in the meaning of the constants they contain. In both cases an increase in the borate ion concentration would result in an increase in the diol-boric acid complex. Therefore, an examination of the effect of pH on the equilibrium concentrations of various components of the system cannot be used to determine which of the two boroxy species actually reacts with the diol. 

Isotope labeling experiments indicate that the B—O bond is broken and not the C—O bond in the formation of the diol-boric acid complexes (18). This indicates that the initial step in the mechanism may be an attack on the boron atom by an oxygen of the diol, followed by the release of water. This could occur without developing any charge separation. If such a mechanism were correct, it would seem that an attack on the boron atom would be easier for trigonal boric acid than for the tetrahedral borate anion (Figure 1). 

Determination of the Coordination Number from Diol-Titration Experiments. Early in the research on the diol-boric acid reaction it was recognized that two possible complexes could form (3), one with a diol coordination number of one, the other with a coordination number of two. A generalized expression for the overall complex reaction can be written as follows,... 

Through the years there has been some conflict in the literature over the determination of the coordination numbers of various diol-boric acid complexes. Most of these conflicts can be resolved by making the proper approximations in these three conservation equations as determined by the experimental conditions. 

Under alkaline conditions, boric acid (or at this pH, boron tetrahydrate) forms complexes with diols 11) The formation of such a complex between two polysaccharide molecules can lead to crosslinking. The use of borax can, therefore, be expected to increase the stability of the bonds between the hemicellulose and between the hemicellulose and cellulose fibers in the paper. 

Ebelman and Bouquet prepared the first examples of boric acid esters in 1846 from boron trichloride and alcohols. Literature reviews of this subject are available. B The general class of boric acid esters includes the more common orthoboric acid based trialkoxy- and triaryloxyboranes, B(0R)3 (1), and also the cyclic boroxins, (ROBO)3, which are based on metaboric acid (2). The boranes can be simple trialkoxyboranes, cyclic diol derivatives, or more complex trigonal and tetrahedral derivatives of polyhydric alcohols. Nomenclature is confusing in boric acid ester chemistry. Many trialkoxy- and triaryloxyboranes such as methyl, ethyl, and phenyl are commonly referred to simply as methyl, ethyl, and phenyl borates. The lUPAC boron nomenclature committee has recommended the use of trialkoxy- and triaryloxyboranes for these compounds, but they are referred to in the literature as boric acid esters, trialkoxy and triaryloxy borates, trialkyl and triaryl borates or orthoborates, and boron alkoxides and aryloxides. The lUPAC nomenclature will be used in this review except for relatively common compounds such as methyl borate. Boroxins are also referred to as metaborates and more commonly as boroxines. Boroxin is preferred by the lUPAC nomenclature committee and will be used in this review. 

Borate buffers The useful pH range for borate buffers is 8.5-10.0. Boric acid is poisonous in high concentration, and it complexes with vicinal diols like ribose. 

Earlier fluorometric methods for analysis of urinary free catecholamines have been replaced by HPLC methods that allow selective quantitation of epinephrine, norepinephrine, and dopamine. Preliminary extraction of urine is stid required and numerous preanalytical cleanup techniques are available. An alumina extraction procedure is typically coupled with ion-exchange or adsorption chromatography. Alumina pretreatment usually involves a batch extraction technique in which catechols are first adsorbed at pH 8.6 and then eluted with boric acid, which forms a complex with cis-diol groups. Purification on boric acid affinity gels provides an alternative procedure for selective adsorption of catecholamines. 

Few medicines based on boron are known, in general boric acid or a boronic acid serve to esterify an a-diol or an ortho-diphenol. This is the case for the emetic antimony borotartrates of the ancient pharmacopoieas, for the injectable catecholamine solutions, for tolboxane " , that is close to meprobamate and that was commercially available as a tranquilhzer some decades ago, or also for the phenylbo-ronic esters of chloramphenicol. " Boromycine was the first natural product containing boron isolated. " It is a complex between boric acid and a polyhydroxylated tet-radentate macrocycle. " Another natural product is aplas-momycin with antibiotic properties. " " ... 

Boric acid and borates form very stable complexes exceedingly rapidly with polyols10 and a-hydroxy carboxylic acids.11 These are mainly 1 1 and of type (8-V). The acidity of boric acid is thereby increased glycerol is commonly used analytically as the acid can then be titrated by aqueous NaOH. Steric considerations are very critical in the formation of these complexes. Thus 1,2- and 1,3-diols in the cw-form only, such as c/j-l,2-cyclo-pentanediol, are active, and only o-quinols react. Indeed, the ability of a diol to affect the acidity of boric acid is a useful criterion of the configuration where cis-trans-isomers are possible. 

TLC is commonly used for the separation of different classes of wax components or for analysis of monomers from cutin and suberin depolymerization. A typical separation is shown in Fig. 6.12. By such methods, it is possible to separate hydrocarbons, wax esters, primary alcohols, secondary alcohols and /8-diketones from plant waxes (von Wettstein-Knowles, 1979). Products of hydrogenolysis from cutin can be separated by TLC into alkan-l-ols, alkane-a,ft>-diols, Cis triols, Ci6 triols and Cis tetrols (Kolattukudy, 1980). Unsaturated components can be resolved by argentation-TLC (Tulloch, 1976) and threo or erythro diastereoisomers separated by boric acid/silica gel TLC (Eglinton and Hunneman, 1968). Straight-chain compounds can be preferentially removed from branched compounds as their urea complexes (Kolattukudy, 1980). 

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