Hydrocarbon C-H bond activation by rhodium porphyrins

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Rhodium porphyrins provide a variety of C-H bond reactions with both aromatic and aliphatic hydrocarbons that acquire unusual selectivity in part through the steric requirements of the porphyrin ligand. Rhodium(III) porphyrins selectively react with aromatic C-H bonds by electrophilic substitution with the virtual exclusion of aliphatic C-H bond activation. Rhodium(II) porphyrins react by a metal-centered radical pathway with alkyl aromatics and alkanes selectively at the alkyl C-H bond with total exclusion of aromatic C-H bond activation. Reactions of rhodium(II) metalloradicals with alkyl C-H bonds have large deuterium isotope effects, small activation enthalpies and large negative activation entropies consistent with a near linear symmetrical four-centered transition state ( Rh ˙⋯ H ⋯ C ⋯˙Rh). The nature of this transition state and the dimensions of rhodium porphyrins provide steric constraints that preclude aromatic C-H bond reactions and give high kinetic preference for methane activation as the smallest alkane substrate. Rhodium(II) tethered diporphyrin bimetalloradical complexes convert the C-H bond reactions to bimolecular processes with dramatically increased reaction rates and high selectivity for methane activation.