A Practical Catalytic Asymmetric Addition of Alkyl Groups to Ketones

Many catalysts will promote the asymmetric addition of alkylzinc reagents to aldehydes. In contrast, there are no reports of additions to ketones that are both general and highly enantioselective. We describe herein a practical catalytic asymmetric addition of ethyl groups to ketones. The catalyst is derived from reaction of camphor sulfonyl chloride and trans-1,2-diaminocyclohexane. The resulting diketone is reduced with NaBH₄ to give the C₂-symmetric exo diastereomer. Use of this ligand with titanium tetraisopropoxide and dialkylzinc at room temperature results in enantioselective addition of the alkyl group to the ketone. The resulting tertiary alcohols are isolated with high enantiomeric excess (all cases give greater than 87% ee, except one). The reaction has been run with 37 mmol (5 g) 3-methylacetophenone and 2 mol % catalyst to afford 73% yield of the resulting tertiary alcohol with 99% ee.