The importance of the Ni catalyst in achieving catalytic asymmetric Ni/Cr-mediated coupling reactions effectively is demonstrated. Six phenanthroline-NiCl _2 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· NiCl _2 and (cy) _3P _2·NiCl _2, 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.
展开▼
