On Ni Catalysts for Catalytic, Asymmetric Ni/Cr-Mediated Coupling Reactions

The importance of the Ni catalyst in achieving catalytic asymmetric Ni/Cr-mediated coupling reactions effectively is demonstrated. Six phenanthroline–NiCl2 complexes 1a–c and 2a–c and five types of alkenyl iodides A–E were chosen for the study, thereby demonstrating that these Ni catalysts display a wide range of overall reactivity profiles in terms of the degree of asymmetric induction, geometrical isomerization, and coupling rate. For three types of alkenyl iodides A–C, a satisfactory Ni catalyst(s) was found within 1a–c and 2a–c. For disubstituted (Z)-alkenyl iodide D, 2c was identified as an acceptable Ni catalyst in terms of the absence of Z → E isomerization and the degree of asymmetric induction but not in terms of the coupling rate. Two phosphine-based Ni catalysts, [(Me)3P]2·NiCl2 and [(cy)3P]2·NiCl2, were found to meet all three criteria for D. The bond-forming reaction at the C16–C17 position of palytoxin was used to demonstrate the usefulness of the Ni catalysts thus identified.