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

The racemic carbonate complex [Co(en)(2)O2CO](+) Cl- (en=1,2-ethylenediamine) and (S)-[H3NCH((CH2)(n)NHMe2)CH2NH3](3+) 3Cl(-) (n=1-4) react (water, charcoal, 100 degrees C) to give [Co(en)(2)((S)-H2NCH((CH2)(n)NHMe2)CH2NH2)](4+) 4Cl(-) (3a-dH(4+) 4Cl(-)) as a mixture of / 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 - and -3a-d(3+) 3BAr(f)(-), which are screened as catalysts (10mol%) for additions of dialkyl malonates to nitroalkenes. Optimal results are obtained with -3c(3+) 3BAr(f)(-) (CH2Cl2, -35 degrees C; 98-82% yields and 99-93% ee for six -arylnitroethenes). The monofunctional catalysts - and -[Co(en)(3)](3+) 3BAr(f)(-) give enantioselectivities of <10% ee with equal loadings of Et3N. The crystal structure of -3aH(4+) 4Cl(-) provides a starting point for speculation regarding transition-state assemblies.