Diastereomeric and Isotopic Scrambling in (Hydrido)alkyliridium Complexes. Evidence for the Presence of a Common "Alkane Complex" Intermediate

Abstract

An alternative explanation for the scrambling of isotopic labels in the alkylmetal hydride complexes is the direct dyotropic rearrangement over the transition state. In an attempt to clarify the mechanism involved in these deuterium-scrambling reactions we have carried out kinetic investigations of the interconversion of diastereomeric (hydrido)(dimethylcyclopropyl)iridium complexes 1a and 1b and the deuterium scrambling of their {alpha}-deuterio analogs. A dramatic difference in behavior of the Rh and Ir systems yields significant conclusions regarding the reaction coordinate for alkane oxidative addition reactions to metal centers. Comparison of the results from the iridium system discussed here and the analogous rhodium system studied earlier yields the following interesting conclusion: with iridium, the barrier for interconversion of 4a and 4b is comparable to the barrier for C-H insertion to re-form 1a,b whereas the corresponding barrier for interconversion of the rhodium analogs of 4a and 4b is high compared to the barrier for C-H insertion. This difference may be caused by the different temperatures at which the two reactions are run; another possibility, however, is that deuterium scrambling in the rhodium system occurs by a dyotropic rearrangement. 27 refs., 3 figs.