Boron-stabilized carboxylation

The carboxylation of boron-stabilized carbanions followed by acidification has been reported to give malonic acids in yields of 65-70% (equation 39). The carboxylation of (39), however, did not yield any of the correspon ng malonic acids. ... 

An efficient aqueous phase Suzuki-Miyaura reaction of activated aryl chlorides with aryl boronic acids has been reported. The method uses a new D-glucosamine-based dicyclohexylarylphosphine ligand for the palladium catalyst and works well with nitro-and cyano-activated chlorides.32 The aryl fluoride bond has been considered inert to palladium-catalysed substitution reactions. However, a computational study, backed up by experiment, shows that the presence of a carboxylate group ortho to fluorine will allow reaction both with phenylboronic acids in a Suzuki-type reaction and with organotin reagents in a Stille-type reaction the presence of the adjacent oxyanion stabilizes the transition state.33... 

An improvement over this earlier system was attained with the addition of boron compounds such as boric acid or borate salts [106-108], It has been hypothesized that boric acid and calcium form intramolecular bonds which effectively crosslink or staple an enzyme molecule together [107,108], The use of polyols such as propylene glycol, glycerol, and sorbitol in conjunction with the boric acid salts further enhances the stability of these enzymes [109-111], The patent literature contains numerous examples of enzyme stabilization systems that utilize borates, polyols, carboxylate salts, calcium, and ethanolamines, or combinations thereof [91,112-115],... 

The mechanism was postulated to involve a Cu(l)-carboxylate as the active species, which promotes oxidative addition of the thioimide. Subsequent transme-talation and C-S reductive elimination generates the thioether product. An excess of boronic acid is often required, as copper catalysts may competitively oxidize aryl substituted boronic acids to the corresponding phenol in the presence of adventitious water [21]. The rate of acceleration observed with amino acids and carboxylate-based ligands, such as 3-methylsalicylate, is attributed to stabilization of a 7i-Cu intermediate generated through a nucleophilic aromatic substitution type mechanism (Scheme 1) [72]. The amino acid or carboxylate ligand may also simply stabilize putative Cu(lll) intermediates. 

In 1995, Barthel and Gores reported a new class of inexpensive and chemically, elec-trochanically, and thermally stable salts based upon boron chelate complex anions with aromatic or aliphatic diols or carboxylic acids. The first such salt reported was lithium bis(l,2-benzenediolato(2-)-0,0 )borate (LiBBE) (Fig. 1.26a) [372]. The acid with this anion was originally reported in 1949 by Schafer [388]. The LiBBB salt has a high solubility in aprotic solvents (>1 M), but the oxidative stability is relatively low. A number of other nonfluorinated lithium salts with benzenediol nonfluorinated... 

The nanocomposites obtained firom phenolic resin and carboxylated MWNTs showed an improvement of the thermal stability than the neat phenolic resin. The highest thermal stability was obtained in the case of the nanocomposites obtained by in situ polymerization, due to the quality of dispersion of the functionalized MWCNTs [94]. An enhancement of the thermal stability was also obtained in the case of nanocomposites containing boron phenolic resin and MWCNTs modified with nitric acid, 4,4 -diaminodiphenyl methane and boric acid. This effect was ascribed to better interfacial interactions between modified MWCNTs and the resin matrix [95]. 

The value of rate and stability constants are not only dependent on pH there is also a clear dependence on the acidity of the reacting ligand. While the binding of diols by trigonal boronic acids can be considered to be almost negligible, this is not the case when boronic acids complex with more acidic poly-hydroxyl species such as 1,2-diphenols, a-hydroxy carboxylic acids and dicarboxylic acids. ... 

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