Interconversion of diastereomeric complexes involved in Rh(I)‐catalyzed asymmetric hydrogenation: A (31P, 31P) EXSY NMR study

Abstract

AbstractRhodium(I) complexes containing chiral chelating diphosphines, e.g. R,R‐dipamp [(R,R)‐1,2‐bis(o‐methoxy‐phenylphenylphosphino)ethane] or S,S‐chiraphos [(2S,3S)‐bis(diphenylphosphino)butane] and a prochiral olefin, e.g. methyl (Z)‐α‐acetamidocinnamate (mac), form diastereomeric complexes which differ only in which enantiotopic face of the olefin is bound to rhodium and can be observed by 31P NMR. The interconversion of the two diastereomers has been observed with (31P,31P)‐{1H} 2D EXSY techniques for various mixing times, Tm, and in three different solvents (CD2Cl2, CD3OD and CD3COCD3). The results clearly demonstrate that both inter‐ and intramolecular ligand exchange processes are occurring, and that diastereomeric interconversion cannot proceed exclusively by complete mac dissociation followed by readdition to the opposite enantiotopic olefin face. The new results are consistent with previous DANTE NMR experiments. The differing results in CD2Cl2, CD3OD and CD3COCD3 solutions suggest the possibility of solvent involvement in the rate‐limiting step of ligand exchange. The 2D EXSY method appears to be the method of choice for elucidating the qualitative exchange pathway of this complex system.