Hydroboration alkynylsilanes

Borane-dimethyl sulfide is a particularly useful reagent. The complex is more stable than borane-THF and can be prepared in neat form. It is soluble in a range of organic solvents and retains sufficient looseness to allow it to react readily with alkenes. It carries out most of the reactions of borane-THF, perhaps somewhat more slowly, and gives the same selectivity almost irrespective of solvent. It has been used to hydroborate alkynylsilanes at the a-position. Borane-thioxane is an alternative to borane-dimethyl sulfide. °... 

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). 

The size of the silyl groups in alkynylsilanes 48 effects their regio- and chemose-lectivity in hydroboration with 9-borabicyclo[3.3.1]nonane125 (9-BBN). Alkynylsilanes... 

If alkyldichloroboranes are specifically required, dichloroborane-dimethyl sulfide is the reagent of choice. It is more stable and more convenient than the dichloroborane-diethyl ether complex, but its hydroborating properties are very similar." Dichloroborane complexes ethers even more strongly than monochloroborane, and its reactions with alkenes in this solvent are slow and lead to mixtures. Therefore, it is generally used in pentane and trichloroborane is added to liberate uncomplexed dichloroborane. Under these conditions it readily gives alkyldichloroboranes on reaction with alkenes or alkenyldichloro-boranes on reaction with alkynes. " " The latter reaction has been applied to alkynylsilanes (equation 44)." ... 

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. 

Silicon.—Alkenyl, Vinyl-, and Allyl-silanes. Hydroboration of alkynylsilanes proceeds stereospecifically, subsequent carbodeboronation leading to stereochemically defined vinylsilanes (Scheme 26). A similar route using hydroalumination can also be employed, but the stereochemistry is dependent upon the conditions of the hydroalumination step (product of cw-hydroalumination in ether-hexane, transin hexane alone). ... 

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