Cobalt-alkyne complexes with diphosphine ligands as mechanistic probes for the Pauson-Khand reaction

Enantiomerically pure (3,3-dimethylbutyne)[(R)-BINAP]Co-2(CO)(4) (6a) was obtained via thermal ligand exchange in 27% yield as a single diastereomer. X-ray crystal structure analysis of 6a revealed a basal bridged coordination of the BINAP ligand and a distorted geometry of the central Co2C2 core. Comparison of 6a with the structurally related (phenylacetylene)dppm)Co-2(CO)(4) (7) revealed that the geometric distortion is mainly caused by the BINAP ligand. Due to the replacement of basal CO ligands anti to the tert-butyl group in 6a by the diphosphine, the Pauson-Khand reaction of 6a with norbornene is completely suppressed. Comparison with (3,3-dimethylbutyne)(dppm)Co-2(CO)(4) (6b), (3,3-dimethylbutyne)(dppe)Co-2(CO)(4) (6c), 7, (3,3-dimethylbutyne)Co-2(CO)(6) (8a), and (3,3-dimethylbutyne)(PPh3)Co-2(CO)(5) (8b) showed a retardation of the reaction rate caused by phosphines. The mechanistic consequences for the Pauson-Khand reaction employing chiral phosphine ligands are discussed.