Hydroboration. 79. Preparation and Properties of Methylborane and Dimethylborane and Their Characteristics as Hydroborating Agents. Synthesis of Tertiary Alcohols Containing Methyl Groups via Hydroboration.

Article abstract of DOI:10.1021/jo00375a031

Lithium methylborohydride and lithium dimethylborohydride are readily prepared from the corresponding boronic and borinic esters.Methylborane and dimethylborane are cleanly liberated from these borohydrides by protonation with hydrogen chloride in ether, thus permitting the ready liberation of these methylboranes from the stable methylborohydrides.In contrast to the earlier gas-phase studies, methylborane in solution is remarkably stable to disproportionation, forming a strong dimer complex.However, dimethylborane in solution is less stable, undergoing some disproportionation at room temperature.By liberating these boranes in appropriate solvents in the presence of the alkenes, the redistribution can be controlled and a satisfactory preparation of dialkylmethylboranes achieved.These organoboranes can be carbonylated-oxidized to form the corresponding tertiary alcohols containing one or two methyl groups, the first time these valuable derivatives have been available via hydroboration.Gas chromatographic examination of these tertiary alcohols established the absence of significant redistribution.The regioselectivity of these two new hydroborating agents was investigated with representative alkenes, and excellent selectivities with terminal and trisubstituted alkenes were achieved.