Tautomerization, hydrogen peroxide-water oxide

Hydroboration-oxidation of alkynes preparation of aldehydes and ketones Hydroboration-oxidation of terminal alkynes gives syn addition of water across the triple bond. The reaction is regioselective and follows anti-Markovnikov addition. Terminal alkynes are converted to aldehydes, and all other alkynes are converted to ketones. A sterically hindered dialkylborane must be used to prevent the addition of two borane molecules. A vinyl borane is produced with anU-Markovnikov orientation, which is oxidized by basic hydrogen peroxide to an enol. This enol tautomerizes readily to the more stable keto form. 

Oxidation of the vinylborane (using basic hydrogen peroxide) gives a vinyl alcohol (end), resulting from anti-Markovnikov addition of water across the triple bond. This end quickly tautomerizes to its more stable carbonyl (keto) form. In the case of a terminal alkyne, the keto product is an aldehyde. This sequence is an excellent method for converting terminal alkynes to aldehydes. 

The addition of borane to alkenes is stereospecifically cis and leads to the formation of tri-alkylboranes. These may be oxidized to alcohols using the anion of hydrogen peroxide. Overall addition of water is achieved, in a c/s-stereospecific, anti-Markovnikov manner. Hydroboration/oxidation of alkynes gives ketones, after tautomerization of the enol formed. c/s-Dihydroxylation of alkenes is accomplished with catalytic OSO4 plus an oxidant such as NMO or K3[Fe(CN)g]. This contrasts with the formation of frans-diols by epoxidation of alkenes followed by the opening of the epoxide with hydroxyl ion. 

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