Oxidation carbon-boron bonds

The syn steroselectivity was based on the assumption that the oxidation step—the transformation of the carbon-boron bond to carbon-oxygen bond—took place with retention of configuration. More recent NMR studies of alkylboranes formed in hydroborating labeled alkenes indeed confirmed the validity of the earlier conclusion.351... 

Treatment of [4.S ]-4-/-butyl-l-mcthylcyclohcxcnc with borane-THF followed by oxidation with F Ch-NaOH gives a mixture of [ 1.S , 2.S ,5.S ]-5-/-butyl-2-methylcyclohcxan-1 -ol and [ 1 A,2A,5.S -5-/-butyl-2-methylcyclohexan-l-ol and no other diastereomers. There are two steps in this process—addition to give an organoborane and oxidation which cleaves the carbon-boron bond to an alcohol. 

Oxidation of alkylboranes by alkaline hydrogen peroxide produces alcohols. The reaction is essentially quantitative and has been successfully applied to a wide variety of alkylboranes (Equations B2.3-5). It is important to note that the stereochemistry of the carbon atom attached to the boron atom is retained in this conversion of a carbon-boron bond to a carbon-oxygen bond (Equation B2.5). 

Carbon-boron bonds are generally rather easily oxidized and indeed volatile trialkylboranes such as trimethylborane and triethylborane are spontaneously inflammable when exposed to air. Less volatile or-ganylboranes do not spontaneously inflame but are nevertheless readily oxidized by oxygen and a variety of other reagents. Consequently, it is normally necessary to carry out organoborane reactions in an inert atmosphere. 

Schuster and co-workers discovered that 1,4-dicyanonaphthalene solutions containing an alkyltriphenylborate salt, when irradiated, yield one-eleetron oxidation of the alkyltriphenylborate leading to carbon-boron bond cleavage and formation of free alkyl radicals [23]. In Gottschalk s hands [24, 25], it was shown that ionic salt pairs formed from cyanine dyes and alkyltriphenylborates (Figure 1) could be used as photoinitiators [26] that were active in the visible region of the spectrum. 

The oxidation of carbon-boron bond converts boranes into alkyl or aryl borates, which may be hydrolyzed subsequently to alcohols and boric acid [991], The oxidation is carried out with hydrogen peroxide [183,1201, 1202] or trimethyiamine oxide [991, 992]. Phenylboronic acids are oxidized to phenols biochemically [1034]. 

Monohydroboration of 1-alkynes followed by oxidation gives the corresponding aldehydes in high yields.-" Oxidation of the vinyl carbon-boron bond produces the enol, which then tautomerizes to the carbonyl group. To minimize aldol condensation of the aldehyde formed during oxidation, the reaction should be carried out at pH 8 or in buffered medium. " ... 

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