Organometallic reactions. Part XI. The stereochemistry of the oxidative and hydrolytic cleavage of the 1-phenylethyl–boron bond

Abstract

Optically active 1-phenylethylboronic compounds undergo hydroxydeboration with trimethylamine oxide with retention of configuration. Deuteriodeboration with sodium deuteroxide gives 54% net inversion, and with deuterio-octanoic acid it proceeds with retention. It is proposed that these reactions involve nucleophilic attack of the reagent at boron, followed by, respectively, a nucleophilic 1,2-rearrangement of the alkyl group from boron to oxygen, an S_E1 reaction at the carbon centre, and a six-centre cyclic substitution. The diethanolamine ester of 1-phenylethylboronic acid is oxidised by alkaline silver nitrate or thallic chloride to give approximately equimolar amounts of erythro- and threo-2,3-diphenylbutane. This is compatible with the formation of an intermediate 1-phenylethylmetallic compound, which then breaks down homolytically.