Catalytic enantioselective Michael addition reactions of alpha-nitroesters to alpha,beta-unsaturated ketones

Enantioselective Michael additions of alpha-nitroesters 2a-d with alpha,beta-unsaturated ketones were carried out in the presence of a catalytic amount of chiral Al-Li-(R,R')-2,2'-dihydroxy-1,1'-binaphthyl ('AlLiBINOL') complex prepared in situ from LiAlH4 and 2.45 equiv. of (R,R')-BINOL. The enantioselectivity of the Michael addition proved to be extremely temperature dependent: Michael adduct 4a showed 7% e.e. when the reaction was performed at RT, whereas 72% e.e. of the opposite enantiomer of 40 was found when the 1,4-addition was performed at -23 degrees C. Solvent variation showed that tetrahydrofuran gave the highest selectivity (up to 80% e.e.), whereas the highest enantioselectivity for the opposite enantiomer was found in methylene chloride (up to 25%). X-Ray structure analysis of the AlLi3BINOL3 complex 6 in combination with Al-27 NMR studies showed that 'AlLiBINOL' is a mixture of aluminium complexes in solution. (C) 1997 Elsevier Science Ltd.