Esters borate, formation

In the case of L-rhamnulose-1-phosphate aldolase (RhaD), we found that the problem of phosphorylated substrate requirement (dihydroxyactone phosphate (DHAP)) could be overcome by a simple change in buffer. Thus, when using borate buffer, reversible borate ester formation created a viable substrate out of dihydroxyacetone, which is not otherwise accepted by the wild-type enzyme (Figure 6.6) [23]. The process was used in a one-step synthesis of... 

Figure 6.6 In situ borate ester formation obviates the need for phosphorylated substrates with RhaD...

Let hydroxyl groups (alcohols) from the two polymer chains play the role of the two alcohols in the equation provided for formation of borate esters. Borate ester formation cross-links the two polymer chains as follows ... 

These are microscopic p.K1di values for a particular mode of borate ester formation where a sugar forms more than one type of 1 1 complex, the macroscopic association constant (1/A1diO is the sum of all the microscopic association constants [E(l/.Ki3i)]. 

It is believed that the apiofiiranose (Api residue of side chain A, but not that of side-chain B, is involved in RG-II dimerization. Recently mutant Arabidopsis murl strains, which are defective in L-fucose biosynthesis, have been shown to result in small changes in chain A of RG-II the L-fucose residue is replaced by L-galactose. This seemingly minor modification in a complex oligosaccharide structure results in a significant reduction in RG-II dimer formation and has a dramatic impact of plant morphology(/P). It is not yet clear which structural features of RG-II are key to borate ester formation and stability. 

Borate ester formation (polymer cross-linking)... 

Various-borate ester formation, polymer cross-linker, alkaline pH buffer, corrosion inhibition, sparingly soluble salt formation, biostatic effect, oxidizing property... 

NMR spectroscopy has been used to determine the thermodynamics of borate ester formation by three readily grafted carbohydrates. Boron specia-tion in borate-poly(amido amine) dendrimer has been performed using B NMR spectroscopy. The role of inositol in the synthesis of FAU- and LTA-type zeolites has been investigated using H, C, Na, and Al NMR spectroscopy. Ulvan-boron complex formation has been studied using B and NMR spectroscopy. The complexation of borate ions and humic acid fractions has been analysed with B and H NMR spectroscopy. ... 

FIGURE 15 Borate ester formation. c/s-Hydroxyls are preferred. The extent of reaction was originaiiy monitored by fbiiowing the change in conductivity of borate soiutions on the addition of saccharide. 

Boron compounds used as fire retardants may also owe much of their effectiveness to such heat sink effects due to dehydration or, for ammonium salts, loss of ammonia. A second mode of action may be connected with the formation of a glassy layer at the surface. Boron compounds, however, cannot be regarded as inert, since there is evidence of borate ester formation with cellulose, for example. The ultimate effect is increased char formation, which impedes heat transfer. 

A practical, inexpensive one-step procedure was developed for the RhaD-catalyzed gram-scale synthesis of L-fructose. The requirement for DHAP as the donor substrate was circumvented by use of borate buffer, presumably by in situ formation of borate esters as a phosphate ester mimic. Racemic glyceraldehyde was also used, as the enzyme preferentially accepted the L-enantiomer as a substrate. The method can also be apphed to other products, including L-rhamnulose, and towards a two-step synthesis of L-iminocychtols. ... 

It has long been recognized that boron is required by higher plants [61, 62], and recent research indicates the involvement of boron in three main aspects of plant physiology cell wall structure, membrane function, and reproduction. In vascular plants, boron in solution moves in the transpiration stream from the roots and accumulates in the stems and leaves. Once in the leaves, the translocation of boron is limited and requires a phloem transport mechanism. The nature of this mechanism was only recently elucidated with the isolation of a number of borate polyol compounds from various plants [63-65]. These include sorbitol-borate ester complexes isolated from the floral nectar of peaches and mannitol-borate ester complexes from the phloem sap of celery. The implication is that the movement of boron in plants depends on borate-polyol ester formation with the particular sugar polyol compounds used as transport molecules in specific plants. 

Oxidation catalyzed with boric acid (160-175°C, 8-10 atm) results in the formation of cyclohexyl esters.878,879 After hydrolysis a product mixture containing mainly cyclohexanol is formed.878 Since borate esters are less sensitive to further oxidation, better yields are usually achieved (85% at 12% conversion).872... 

The conversion of methanol to ethanol with carbon monoxide and hydrogen has attracted considerable attention. Further carbonylation to higher alcohols occurs much more slowly, but acetic acid formation is a competing reaction and this leads to ester formation. Using CoI2 in presence of PBu 3 as catalyst, the selectivity to ethanol was improved by addition of the borate ion B4072. 399 This was attributed to an enhanced carbene-like nature of an intermediate cobalt-acyl complex by formation of a borate ester (equation 76). This would favour hydrogenolysis to... 

Hydrolysis of a methyl ether.1 Treatment of either 1 or 2 with BF3 etherate yields exclusively 3 (aklavinone). One explanation for the result is formation of a cyclic borate ester intermediate involving the C7- and C9-hydroxyl groups. Under similar conditions, trifluoroacetic acid converts either 1 or 2 into an 8 1 mixture of 3 and 7-epi-3. BF3 etherate was also used to convert the C,0-epimer of 2 into 3. 

M. A. O Neill, D. Warrenfeltz, K. Kates, P. Pellerin, T. Doco, A. G. Darvill, and P. Alber-sheim, Rhamnogalacturonan-II, a pectic polysaccharide in the walls of growing plant cell, forms a dimer that is covalently cross-linked by a borate ester. In vitro conditions for the formation and hydrolysis of the dimer, J. Biol. Chem., 271 (1996) 22923-22930. 

In hydrocarbon oxidation processes to produce alcohol, there is always a degree of overoxidation. The alcohol is often further oxidized to waste carboxylic acids and carbon oxides. If boric acid is introduced to the reactor, the alcohol reacts to form a borate ester, which protects the alcohol from further oxidation. The introduction of boric acid terminates the by-product formation pathway and greatly increases the product yield. The borate ester of alcohol is then hydrolyzed, releasing boric acid for recycle back to the process. This kind of reaction pathway control has been applied to a commercial process, resulting in about a 50% reduction in waste generation once the process was optimized. 

Myers et al. reported that partially dehydrated APB is an effective intumescent flame retardant in thermoplastic polyurethane.77 APB at 5-10 phr loading in TPU can provide 7- to 10-fold improvement in burn-through test. It is believed that in the temperature range of 230°C-450°C, the dehydrated APB and its released boric oxide/boric acid may react with the diol and/or isocyanate, the decomposed fragments from TPU, to produce a highly cross-linked borate ester and possibly boron-nitrogen polymer that can reduce the rate of formation of flammable volatiles and result in intumescent char. 

Some other important aspects of boric acid chemistry are summarized in Fig. 5-26. Among these is the formation of borate esters [B(OR)3, R = alkyl or aryl], usually obtained as colorless liquids, on treatment with alcohols and H2S04. The vast literature on these compounds falls generally within the purview of organic chemistry, and will not be developed here however, it will be noted that a well-known qualitative test for boron involves treatment of the sample with methanol to form B(OMe)3, which produces a bright green color in a Bunsen burner flame. A major early discovery in this area was the synthesis of boronic acids by E. Frankland in 1862, via partial oxidation of trialkylboranes, with subsequent hydrolysis of the ester ... 

Recently, it was been found that RhuA accepts DHA in the presence of boric-borate buffer, presumably by reversible in situ formation of DHA borate ester [39]. Most recently, the group of Wong selected an RhuA mutant enzyme that accepts DHA as a donor [40],... 

---