2,2′-bis[bis(3,5-di-tert-butyl-4-methoxyphenyl)phosphino]-6, 6′-dimethoxy- 1,1′-biphenyl in intramolecular rhodium(I)-catalyzed asymmetric Pauson-Khand-type reactions

A cationic rhodium(I)/2,2′-bis[bis(3,5-ditert- butyl-4-methoxyphenyl) phosphino]-6,6′-dimeth-oxy-1,1′-biphenyl (DTBM-MeO-BIPHEP) catalyst was highly efficient for the asymmetric catalytic Pauson-Khand reaction, especially for those substrates containing aryl group-substituted alkynes. The formation of the products that were derived from a β-hydride eliminated intermediate 5 was completely suppressed over a wide range of substrates. This reaction was a serious process competing reaction with the migratory CO insertion that led to the Pauson-Khand reaction product and often substantially ruined the chemical yield of the Pauson-Khand reaction. The advantages of this system were clearly demonstrated for previously troublesome substrates, N-tosyl- (1b) and malonate-tethered 1,6-enynes (1c), that exhibited a higher enantioselectivity without a loss in the chemical yields. The obvious beneficial effects were attributed to the synergic effect of various factors, such as the electron density of the phosphorus of the ligand, the dihedral angles of the atropisomeric ligand, and the substitution on the phosphine aryl rings which play a crucial role in the stereochemical outcome of Rh-catalyzed Pauson-Khand reaction.