ASYMMETRIC MICHAEL REACTIONS OF AMINOCARBENE COMPLEX ANIONS

The first Michael addition reactions of the anions of aminocarbene complexes to alpha,beta-unsaturated carbonyl compounds are reported. Unlike their corresponding amide enolates, these anions give exclusive 1,4-addition to a number of enones. For chiral complex 27, it was shown that the formation of the 1,4-addition product is not reversible and that the initial 1,4-addition is thus kinetically controlled. The anions of aminocarbene complexes are much more effective in this regard than the anions of alkoxycarbene complexes. It was demonstrated that aminocarbene complexes can serve as synthons for amides in Michael additions since the metal can be liberated from the Michael adducts to give amide products by oxidation with either DMSO or dimethyldioxirane. The steric bulk of the metal unit in the aminocarbene complex anions plays a role in the facial selectivity in the addition to 1,4-diphenyl-2-penten-1-one which produces a 21:1 mixture of diastereomers in the case of methyl pyrrolidino complex 12. This is the highest facial selectivity that has ever been observed in the Michael additions of enolates to this enone. The Michael additions of the chiral amino complex 27 derived from prolinol methyl ether with several cyclic enones were investigated and represents the first examples of asymmetric reactions of any type of the 'enolates'' of either alkoxy- or amino-stabilized group 6 Fischer carbene complexes. Both enantiopodes of 27 were examined with cyclohexenone and found to give asymmetric induction in the range of 65-75% ee which is comparable with the best induction that has yet been reported for the addition of a chiral acetaldehyde equivalent to cyclohexenone.