Carbanions boron stabilization

Reaction between aldehydes and boron-stabilized carbanions... 

See also Borates Boric acid Sodium borates boron oxides, 4 246-249 boron oxides table,4 242t environmental concerns, 4 284—285 health and safety factors, 4 285-288 occurrence, 4 245—246 Boron perchlorates, 18 278 Boron phosphate, 4 242t, 283 Boron removal, from water, 14 418 Boron-stabilized carbanions, 13 660-661 Boron subhalides, 4 141 Boron suboxide, 4 242t Boron tribromide, 4 138 manufacture, 4 145—146 physical properties of, 4 139-140t, 325 reactions, 4 141 specifications, 4 147t uses of, 4 149 Boron trichloride, 4 138 manufacture, 4 145—146 physical properties of, 4 139-140t reactions, 4 141... 

Diboryl compounds do not yield aldehydes with alkaline hydrogen peroxide. Instead there is r d hydrolysis, presumably via a boron-stabilized carbanion (see Volume 1, Ch ter 2.6) which is protonated and then oxidized to the alcohol (equation 16). ... 

This molecule presents a lithium-boratirane (C-B-Li) ring formed by 1,2-migration of a trimethylsilyl group to a neighboring boron atom, generating a boron-stabilized carbanion. The X-ray stmcture of this derivative revealed two independent molecules per asymmetric unit. The two boratirane derivatives have been thoroughly characterized by multinuclear NMR and X-ray diffraction analysis. 

Since the degree of stabilization of boron-stabilized carbanions is similar to that for anions stabilized by a formyl or a cyano group, the comparative lack of knowledge of boron-stabilized carbanions must be due either to kinetic factors associated with their production or to the availability and nature of their precursors. 

There are currently three general methods for the production of boron-stabilized carbanions, each of which has analogies in caibonyl chemistry. The cleavage of a 1,1-diboiylalkane by base readily yields the desired anion in a fashion similar to the base cleavage of a -dicaibonyl compound. Deprotonation a to a boron atom can be accomplished directly in special circumstances, as can the addition of an organo-metallic compound to a vinylborane, which is similar to conjugate addition to an a,p-unsaturated carbonyl compound. Each of these methods is treated in detail in Ae next three sections. 

Although cleavage of gem-diboryl compounds is a mild and general process for die production of boron-stabilized carbanions, it nevertheless suffers from two disadvantages. Firstly, although... 

Boron-stabilized carbanions may also be produced by selective cleavage of a heteroatom group from an a-substituted organoborane such as a borylstannylmethane (equations... 

In general, all types of boron-stabilized carbanions are readily alkylated by primary alkyl halides. The situation for the i ylation of dimesitylboryl compounds is sununarized in Scheme 2. These reactions represent highly efficient homologation processes. Oxidation of the tertiary all l organoboranes is slow, but use of 4-methoxy-2,6-dimethylphenyl groups instead of mesityl groups renders the products sensitive to solvolysis. ... 

Alkylations of boron-stabilized carbanions have been carried out with primary alkyl halides containing acetal, alkene, alkyne, chloride, cyano, ester and tosylate groups, though a ketone group was not tolerated. ... 

Acylation of (2) with benzonitrile gives an intermediate which yields acetophenone (52%) on hydrolysis with 3 M HCl. This seems to be the only recorded reaction of a boron-stabilized carbanion with a nitrile. ... 

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. ... 

It was early reported that boron-stabilized carbanions reacted with benzaldehyde and ketones to yield alkenes (equation 40). The reaction gave a 45-50% yield in the one case quantified, with ( ) (Z) ratios varying over a wide range with temperature. Benzophenone gave a similar yield but the yields with aliphatic ketones were considerably less. °... 

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