Stereochemistry of intermediates in homogeneous hydrogenation catalysed by tristriphenylphosphinerhodium chloride, employing nuclear magnetic resonance magnetisation transfer

Abstract

The DANTE technique has been employed to observe intra- and inter-molecular exchange equilibria in Wilkinson's catalyst, ClRh(PPh3)3 and its dihydride. The parent compound undergoes automerisation much faster than dissociative exchange with free PPh3. The rate of the latter process, 0.31 s–1, is in accord with that value deduced from the kinetics of H2 addition and by other techniques. For ClH2Rh(PPh3)3 the rapid exchange of PPh3trans to hydride occurs through an intermediate with effective C2v symmetry in which the two hydrides are equivalent. A slower, previously unobserved exchange of the mutually trans-phosphines with free PPh3 occurs, which is sufficiently rapid that the intermediate so generated can participate in the catalytic cycle of homogeneous hydrogenation. These results provide a basis for suggesting that the key intermediates in homogeneous hydrogenation have two mutually cis-biphosphine ligands bound to rhodium. Further justification arises from molecular modelling of transient olefin-bound intermediates based on authentic crystal structures. For several common olefins, the bound state involving trans-disposed triphenyl-phosphines is severely destabilished by unavoidable van der Waals repulsions, whereas the corresponding intermediate with cis phosphines is stable in catalytically realistic geometry.