SELECTIVE PREPARATION OF BORINIC ESTERS FROM GRIGNARD-REAGENTS AND SELECTED TRIALKOXYBORANES

The reaction of trialkoxyboranes with ethylmagnesium bromide was investigated for the selective alkylation to the symmetrical borinic esters, R2BOR'. Triisopropoxyborane was found to react cleanly with 2 equiv of the Gringnard reagent to form diethylisopropoxyborane at -40-degrees-C. The selectivity of this reaction is largely controlled by the stability of the bromomagnesium diethyldiisopropoxyborate, MgBr[Et2B(O(i)Pr)2]. Triisopropoxyborane was found to be the most selective borane examined, yielding symmetrical borinic esters for primary and aryl derivatives with high selectivities. Secondary alkyl groups showed lower selectivities. This reaction has been developed into a general procedure for preparation of diorganyl-alkoxyboranes from readily available organomagnesium reagents, especially for those containing organic groups which are not accessible via hydroboration.