The reaction of triallroxyboranes 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 Grignard reagent to form diethylisopropoxyborane at-40" C. The selectivity of this reaction is kgely controlled by the stability of the bromomagnesium diethyldiisopropoxyborate, MgBr [Et, B (O'Pr),]. Triisopropoxyborane ...