Investigation of the Mechanism of Alkane Reductive Elimination and Skeletal Isomerization in Tp‘Rh(CNneopentyl)(R)H Complexes: The Role of Alkane Complexes

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Abstract
Experiments are described that provide indirect evidence for the involvement of alkane σ-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-d6 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 α- and ω-positions of the alkyl group monitored by 2H 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 σ-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.

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