Werner Complexes with [omega]-Dimethylaminoalkyl Substituted Ethylenediamine Ligands: Bifunctional Hydrogen-Bond-Donor Catalysts for Highly Enantioselective Michael Additions

The racemic carbonate complex [Co(en)2O2CO]+ Cl- (en=1,2-ethylenediamine) and (S)-[H3NCH((CH2)nNHMe2)CH2NH3]3+ 3Cl- (n=1-4) react (water, charcoal, 100°C) to give [Co(en)2((S)-H2NCH((CH2)nNHMe2)CH2NH2)]4+ 4Cl- (3a-dH4+ 4Cl-) as a mixture of [Lambda]/[Delta] diastereomers that separate on chiral-phase Sephadex columns. These are treated with NaOH/Na+ BArf- (BArf=B(3,5-C6H3(CF3)2)4) to give lipophilic [Lambda]- and [Delta]-3a-d3+ 3BArf-, which are screened as catalysts (10mol%) for additions of dialkyl malonates to nitroalkenes. Optimal results are obtained with [Lambda]-3c3+ 3BArf- (CH2Cl2, -35°C; 98-82% yields and 99-93% ee for six [beta]-arylnitroethenes). The monofunctional catalysts [Lambda]- and [Delta]-[Co(en)3]3+ 3BArf- give enantioselectivities of