Enantioselective alkylation of carbonyl compounds. From stoichiometric to catalytic asymmetric induction

Multinuclear organometallic species play a significant role in alkylation of carbonyl compounds. Enantioselective alkylation of aldehydes using a 1:1 reagent/substrate stoichiometry is achievable by chirally modified lithium/magnesium binary organometallic reagents. For example, diethylmagnesium treated with di-O-lithio-(S)-2,2' ‘-dihydroxy-1,1’-binaphthyl reacts with benzaldehyde in a THF—dimethoxyethane mixture at-100 °C to give (S)-1-phenyl-1-propanol in 92% ee. In the presence of a catalytic quantity of (-)-3-exo-(dimethylamino)isoborneol (DAIB), reaction of dialkylzincs and aldehydes in nonpolar media is accelerated greatly to lead to the corresponding S alcohols in very high enantiomeric excesses (up to 99% ee). The enantioselective catalysis involves fluxional organozinc species and the product-forming dinuclear intermediate possesses DAIB auxiliary, an aldehyde ligand, and three alkyl groups, where it is the bridging alkyl group, rather than the terminal alkyls, that migrates from zinc to the carbonyl carbon.