Dramatic improvement in catalyst loadings and molar ratios of coupling partners for Ni/Cr-mediated coupling reactions: heterobimetallic catalysts.

Two new ligands 1a,b are reported. Upon treatment with 1 equiv of NiCl(2) x (MeOCH(2))(2), 1a,b give the corresponding Ni complexes. X-ray analysis of 1a x NiCl(2) established that the NiCl(2) is selectively coordinated to the phenanthroline nitrogens. Ni/Cr heterobimetallic catalysts 1a,b x CrCl(2)/NiCl(2), prepared from 1a,b x NiCl(2), have been shown to behave exceptionally well in catalytic asymmetric Ni/Cr-mediated couplings, with highlights including the following: (1) 1-2 mol % catalyst is sufficient to complete the coupling; (2) only negligible amounts of the dimers, byproducts formed through the alkenyl Ni species, are observed; (3) the coupling goes to completion even with a 1:1 molar ratio of the coupling partners; and (4) the asymmetric induction is practically identical with that obtained from the coupling with the Cr catalysts prepared from (S)-sulfonamides 2a,b. The scope of the new Ni/Cr heterobimetallic catalysts was briefly studied using four additional aldehydes. The applicability of the new catalysts to polyfunctional substrates was demonstrated by two C-C bond formations chosen from the halichondrin/E7389 synthesis as examples.