Hydroboration-oxidation of alkynylsilanes

A convenient route to both saturated and unsaturated acylsilanes lies in the hydroboration-oxidation of alkynylsilanes (Chapter 7). Recent improvements (6) to this method involve the use of the borane-dimethyl sulphide complex for hydroboration, and of anhydrous trimethylamine A-oxide for the oxidation of the intermediate vinyl boranes. 

Oxidation of organoboranes (6, 624 8, 507). A new synthesis of acylsilanes (2) involves oxidation of the hydroboration products of alkynylsilanes (I).1 Oxidation with the commercially available dihydrate of trimethylamine N-oxidc is sluggish however, anhydrous reagent2 is satisfactory for this purpose (equation I). 

Hydroboration of alkynylsilanes with dicyclohexylborane proceeds in a stereo-and regioselective manner, placing the boron at the silicon-bearing carbon. Oxidation of the resultant alkenylborane with excess alkaline hydrogen peroxide produces, via an acyl silane intermediate (see Section 7.9), the carboxylic acids in greater than 80% yields. The alkynylsilanes may be generated in situ as illustrated by the one-pot transformation below. 

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