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 involvemen t of alkane sigma -complexes in oxidative addition/reductive elimination re actions 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, but yl, pentyl, and hexyl, to generate RH and Tp'Rh(L)(C6D5)D. The isopropyl hy dride complex Tp'Rh(L)(CHMe2)H was found to rearrange to the n-propyl hydri de complex Tp'Rh(L)(CH2CH2CH3)H in an intramolecular reaction. The sec-buty l complex behaves similarly. These same reactions were studied by preparing the corresponding metal deuteride complexes, Tp'Rh(L)(R)D, and the scrambl ing of the deuterium label into the alpha- and omega -positions of the alky l group monitored by H-2 NMR spectroscopy. Inverse isotope effects observed in reductive elimination are shown to be the result of an inverse equilibr ium isotope effect between the alkyl hydride(deuteride) complex and the sig ma -alkane complex. A kinetic model has been proposed using alkane complexe s as intermediates and the selectivities available to these alkane complexe s have been determined by kinetic modeling of the deuterium scrambling reac tions.