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 C_2v 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.