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6.1.38.21.4 Variation 4: Alkylation of Other α-Heterosubstituted Carbanions

DOI: 10.1055/sos-SD-006-00903

Zaidlewicz, M.; Krzeminski, M.Science of Synthesis, (200561171.

α-Heterosubstituted carbanions with leaving groups containing nitrogen, oxygen, and sulfur, react with trialkylboranes providing alcohols, aldehydes, ketones, carboxylic acids, and functionalized alkenes and dienes (Table 10).[‌405‌,‌407‌,‌410‌,‌461‌‌470‌] Thus, direct synthesis of carboxylic acids through a two-carbon-atom homologation can be readily achieved by reaction with the dianion of phenoxyacetic acid.[‌461‌] Alkanoic, alkenoic, and alkynoic acids can be prepared from the corresponding alkenes, dienes, and enynes, respectively, via hydroboration. The reaction is tolerant to substituents and functional groups such as chloro, sulfide, ether, acetal, and thioacetal. Tri-sec-alkylboranes react but give low yields; these yields can be increased to 3540% by the use of 9-sec-alkyl-9-borabicyclo[3.3.1]nonane derivatives. However, in certain cases a competitive migration of the cyclooctyl unit takes place. Although the reaction of trialkylboranes with 2-lithio-1,3-dithiane is sluggish, 1-lithio-1,1-bis(phenylsulfanyl)alkanes react readily providing the corresponding aldehydes or ketones in good yields.[‌405‌,‌407‌] Consequently, such lithiated thioacetals, which are convenient acyl carbanion equivalents, can also be used for reactions with trialkylboranes. The use of less hindered 2-lithio-2-alkyl-1,3-benzodithioles instead of acyclic thioacetals is preferred when the steric requirements of the alkyl groups are increased, for example, when dialkyl(1,1,2-trimethylpropyl)boranes are employed. Transfer of the second alkyl group from boron to carbon requires the addition of mercury(II) chloride. 2-Lithiofurans and 2-lithiopyridines react stereoselectively, with the ring opening providing functionalized alkenes and dienes, respectively.[‌462‌,‌471‌] A simple synthesis of difluoromethyl ketones via 2,2-difluoroalkenylboranes by oxidation with hydrogen peroxide in the presence of sodium methoxide has been developed. The intermediates can also be transformed into 1,1-difluoroalk-1-enes, 1,1-difluoro-2-iodoalk-1-enes, β,β-difluoro-α,β-unsaturated ketones, 2,2-difluoro- and 2-(fluorovinyl)phosphorus compounds.[‌463‌,‌472‌]

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