A study of selectivity in the reactions of unsymmetrical trialkylboron compounds

The reaction of disiamylborane with enol acetates derived from an aldehyde bearing two α-hydrogen atoms involves a slow anti-Markownikoff hydroboration followed by rapid elimination and re-hydroboration reactions. Selectivity between reaction of the cis and trans isomers is far smaller than for the corresponding olefinic hydrocarbons. Enol acetates derived from ketones are generally unreactive to disiamylborane. The extent of reaction of propionic acid with a representative series of trialkylboranes depends on the steric resistance of the borane to the initial co-ordination of the acid. The extent of protonolysis decreases with an increase in the total number of alkyl substituents at the α- and β-carbon atoms to boron. With an unsymmetrical trialkylborane the selectivity between the breaking of secondary and primary carbon-to-boron bonds is small. A very large selectivity is found in favour of the removal of a primary—rather than a tertiary—alkyl group. This is thought to be due to steric and not electronic factors. [Continues.]