Investigation of the mechanism of alkane reductive elimination and skeletal isomerization in Tp'Rh(CNneopentyl)(R)H complexes: The role of alkane complexes

Experiments are described that provide indirect evidence for the involvement of alkane sigma -complexes in oxidative addition/reductive elimination reactions of Tp'Rh(L)(R)H complexes (Tp' = tris-3,5-dimethylpyrazolylborate, L = CNCH2CMe3). Reductive elimination rates in benzene-d(6) were determined for loss of alkane from Tp'Rh(L)(R)H, where R = methyl, ethyl, propyl, butyl, pentyl, and hexyl, to generate RH and Tp'Rh(L)(C6D5)D. The isopropyl hydride complex Tp'Rh(L)(CHMe2)H was found to rearrange to the n-propyl hydride complex Tp'Rh(L)(CH2CH2CH3)H in an intramolecular reaction. The sec-butyl complex behaves similarly. These same reactions were studied by preparing the corresponding metal deuteride complexes, Tp'Rh(L)(R)D, and the scrambling of the deuterium label into the alpha- and omega -positions of the alkyl group monitored by H-2 NMR spectroscopy. Inverse isotope effects observed in reductive elimination are shown to be the result of an inverse equilibrium isotope effect between the alkyl hydride(deuteride) complex and the sigma -alkane complex. A kinetic model has been proposed using alkane complexes as intermediates and the selectivities available to these alkane complexes have been determined by kinetic modeling of the deuterium scrambling reactions.