Reactions of unsaturated carbonyl compounds with organoboranes

The kinetic data indicate that both the hydroboration of a terminal alkene and the reduction of a carbonyl compound with 9-BBN exhibit first-order kinetics. However, the reactivities of alkenes and ketones toward borane reagents are not equal. Thus, a study of competition reactions between carbon-carbon unsaturated bonds and carbonyl groups toward organoboranes has been carried out. Unhindered borane reagents such as 9-BBN are less chemoselective toward carbon-carbon unsaturated bonds and carbonyl groups than hindered borane reagents, such as dicyclohexylborane. Dicyclohexylborane selectively hydroborates a terminal alkene in the presence of either a ketone or an aldehyde. The hydroborations of olefinic ketones and aldehydes with dicyclohexylborane give the corresponding hydroxy ketones and aldehydes in good yields after oxidation with sodium perborate. Both terminal and internal alkynyl ketones and aldehydes can be selectively hydroborated by dicyclohexylborane in the presence of the carbonyl group. The reaction affords the corresponding olefins in high yields after protonation with acetic acid or dicarbonyl compounds in good yields after oxidation with sodium perborate.

The reaction of α,β-acetylenic ketones with dicyclohexylborane occurs via a 1,4-addition. Allenoxyborinates are generated as intermediates which then react with excess a,p-acetylenic ketones or other electrophiles. The reactions afford highly functionalized trisubstituted olefins in high yields.