The catalytic resting state of asymmetric homogeneous hydrogenation. Exchange processes delineated by nuclear magnetic resonance saturation-transfer (DANTE) techniques

The DANTE magnetisation transfer technique has been employed to demonstrate site exchange in the 31P n.m.r. spectra of dehydroamino acid and ester rhodium diphosphine complexes. When the ligand is bis(1,2-diphenylphosphino)ethane, this occurs by dissociation of the enamide followed by recombination from the solute pool. The reaction is quite general but its rate is dependent on the nature of the enamide, with a pivalamide forming the least readily dissociated complex, and a t-butyl ester the most readily dissociated. Rate constants span the range 0.15–10 s–1 at 300 K. Similar observations were made for rhodium enamide complexes derived from RR-1,2-bis-(o-methoxyphenylphenylphosphino)ethane (dipamp) which dissociate more slowly so that experiments were carried out at 325 rather than 300 K. A further intramolecular reaction whereby two diastereoisomeric enamide complexes were interconnected by olefin dissociation and recombination was observed; the amide group remains co-ordinated throughout this process.