The cyclic hydroboration of dienes. A simple convenient route to heterocyclic organoboranes

Abstract

ABSTRACT A systematic study of the cyclic hydroboration of a representative series of dienes with borane in tetrahydrofuran as well as with thexylborane is undertaken. The elucidation of the structures of the major products derived from 1.3butadiene and borane and of the mechanistic paths leading to their formulation has been accomplished. Several new analytical techniques, such as gas—liquid partition chromatography examination of B-methoxyboracyclanes and the carbonylation—oxidation of organoboranes, are introduced. By employing these new techniques, the extent of cyclic hydroboration of a representative series of dienes with borane is determined. Based on the findings in this study. simple syntheses of representative bisboracyclanes. such as bisborinane and 9-borabicyclo[3.3.l]nonane, are accomplished. The hydroboration of olefins with these bisboracyclanes represents a convenient and general procedure for the preparation of the corresponding B-alkylboracyclanes. The cyclic hydroboration of dienes with thexylborane in many cases provides the corresponding B-thexylboracyclanes in high yields. Finally, the usefulness of B-alkylboracyclanes as valuable intermediates in the organic synthesis via hydroboration is briefly discussed.