1,2-Borate-diol complex

Plants contain signiFcant concentrations of polysaccharides of which the potentially negatively charged oxygen functions can bind cations electrostatically or chelate them via polyhydroxy groups [89]. Particular attention was attracted by a structurally complex pectic polysaccharide rhamnogalacturonan-II (RG-II) [90]. This ubiquitous component of primary plant cell walls forms dimers cross-linked by 1 2 borate diol esters (dRG-II) that were found to complex in vitro sped be divalent cations and the majority of Ba, Pb, Sr, and rare earth elements (REEs) in fruit and vegetables [45, 91]. 

Detection of side products generated by Rubisco is accomplished by various means. XuBP (30) and pyruvate (36) are both conveniently detected spectrophotometrically by coupling to NADH oxidation with appropriate enzymes. Alternatively, our chromatographic procedure (27) gives a complete profile of all RuBP-derived products. Resolution of these compounds is enhanced by inclusion of 10 mM sodium borate, which complexes vic-diols, in elution buffers. Since our initial report of the separation of borohydride-reduced misprotonation products (27), we have observed that borate also effects complete separation of unreduced RuBP and XuBP (37). Thus, the analysis is simplified by circumventing the necessity to deduce the amounts of misprotonation-derived bisphosphate based on ratios of ribitol-, arabinitol-, and xylitol-1,5-bisphosphates. 

The borate liberated on dissolution of the perborate contributes to the alkalinity, cm act as a buffer, and may help solubilize some molecules via reactions such as borate/cis diol complexation. For example, during the alkaline hydrolysis of fats and natural oils, complexation of borate with, for example, glycerol may help shift the equilibrium favorably. The cis diol functionality is very common on naturally occurring molecules. Borate is a weak builder, but this effect may be boosted by adding other species (e.g., saccharate) [12]. 

SCHEME 50.1. Borate-imine complex formed by the reaction of a diol, 2-formylphenylboronic acid, and 1-phenylethylamine. 

The effect of polyhydroxy compounds has been explained on the basis of the formation of 1 1 and 1 2-mole ratio complexes between the hydrated borate ion and 1,2- or 1,3-diols ... 

Figure 2. Mechanism of complex formation of diols by borates...

This is analogous to the preferred formation of borate complexes from syn-1,2- and, yyn-l,3-diols rather from their ctntt-isomers (and terminal glycols). The latter observation has... 

Addition of anhydrous zinc chloride and warming to 20 C causes rearrangement of the borate complex 3 to the x-chloro boronic ester 4. If an (S,5)-diol boronic ester 1 or 2 as illustrated is used, the product is generally — 99 % (a/ )-a-chloro boronic ester 4 based on NMR analyses. The enantiomeric (/ ,/ )-diol boronic ester yields the (aS)-a-ehloro boronic ester4. 

Sinton, S. W., Complexation chemistry of sodium borate with poly(vinyl alcohol) and small diols. A UB NMR study. Macromolecules, 1987. 20(10) p. 2430-2441. 

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). 

A second important breakthrough was in the acetylation step. With 1 -methylimidazole as the catalyst, complete acetylation can be achieved without removing the borate formed from sodium borohydride in the prior reduction step (Blakeney et al., 1983 Harris et al., 1988). Borate complexes with ds-diol groups in the monosaccharides. In earlier methods (Albersheim et al., 1967 Selvendran etal., 1979), the borate had to be removed or acetylation would not go to completion (Wolfram and Thompson, 1963). The reduction and acetylation reactions are summarized in Figure E3.2.2. 

A chiral Lewis acid-catalysed method for the 1,2-migration of (dichloromethyl)borate complexes to provide synthetically useful (a-chloroalkyl)boronates has been developed,557 and the diastereoselective rearrangement of the a,a-dichloromethylboronate derivatives of 1,2-diols, (427) —> (429), has been explained558 on the basis of a bidentate interaction between the catalytic Lewis acid and the substrate, leading to a favoured transition state (428). 

Recent studies on complexing of w c-diols with borate and with cuprammonium hydroxide have also suggested that there is little correlation between 0—0 distance and complex formation H. Kwart and G. C. Gatos, J. Am. Chem. Soc., 80, 881 (1958). 

---