ORGANOBORANES .55. IMPROVED PROCEDURE FOR THE CONVERSION OF REPRESENTATIVE ACHIRAL AND CHIRAL ALKYLBORONATES, (E)-1-ALKENYLBORONATES, (Z)-1-ALKENYLBORONATES, AND ARYLBORONATES INTO THE CORRESPONDING ORGANYLDICHLOROBORANES

Diethyl alkylboronates, R*B(OEt)2, of essentially 100% enantiomeric purity, prepared by asymmetric hydroboration of readily available prochiral alkenes, were effectively converted into the corresponding chiral alkyldichloroboranes, R*BCl2, by treatment with boron trichloride (1 M solution in dichloromethane) in the presence of a catalytic amount of anhydrous ferric chloride (3 mol %). This reaction is quite general, proceeds well without detectable racemization, and is applicable to essentially optically pure boronic esters of widely varied structural requirements. The reaction is also applicable to achiral boronates, such as 1-hexyl, and hindered alkyl, such as tert-butyl. It is also applicable to the conversion of (E)- and (Z)-1-hexenylboronates, representative of the 1-alkenyl derivatives, and to phenylboronates, representative of aryl derivatives. Consequently, this procedure appears to be broadly applicable to the conversion of organylboronates, RB(OR')2, into the corresponding organyldichloroboranes, RBCl2.