Bis(pyridyl)-silane and -methanol ligands IV. Catalytic application of nickel(II) complexes in the Kumada cross-coupling reaction

Treatment of the bis(2-pyridyl)silane ligands MeRSi(2-C5H4N)2 with (MeCN)2-NiBr2 afforded {MeRSi(2-C5H4N)2}NiBr2 (3a, R = Me; 3b, R = Ph) in excellent yield. The latter complexes were bright purple crystalline solids with paramagnetic line broadening in the NMR spectra; although, no ESR signal could be detected in solution or in the solid state. Complex 3a was found to be an effective catalyst for the cross-coupling of Grignard reagents with aryl and vinyl halides. The catalytic system also coupled alkyl Grignard reagents with chlorobenzene provided a dipolar aprotic solvent such as HMPA was employed. Optically pure ligands, {6-alkoxy-2-pyridyl}(Ar)CH(OMe) (5), were employed in the asymmetric cross-coupling reaction of (1-phenylethyl)magnesium chloride and bromoethene. Conversion of the Grignard reagent ranged from 73 to 88% with ∼50% chemical yields of 3-phenyl-1-butene with optical yields ranging from ∼0 to 11%.