CATALYTIC ENANTIOSELECTIVE MICHAEL ADDITION-REACTIONS OF ALPHA-NITROESTERS TO ALPHA,BETA-UNSATURATED KETONES

Enantioselective Michael additions of alpha-nitroesters 2a-d with alph a,beta-unsaturated ketones were carried out in the presence of a catal ytic amount of chiral Al-Li-(R,R')-2,2'-dihydroxy-1,1'-binaphthyl ('Al LiBINOL') 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 en antiomer of 40 was found when the 1,4-addition was performed at -23 de grees C. Solvent variation showed that tetrahydrofuran gave the highes t 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 combinati on with Al-27 NMR studies showed that 'AlLiBINOL' is a mixture of alum inium complexes in solution. (C) 1997 Elsevier Science Ltd.