Selective Activation of Benzylic Carbon−Hydrogen Bonds of Toluenes with Rhodium(III) Porphyrin Methyl: Scope and Mechanism

Toluenes underwent selective benzylic carbon−hydrogen bond
activation (BnCHA) with rhodium(III) porphyrin methyl. The ortho-, meta-, and para-substituted toluenes yielded the corresponding rhodium porphyrin
benzyls in high yields in solvent-free conditions as well as in benzene
solvent. Mechanistically, Rh(ttp)Me likely undergoes a σ-bond metathesis pathway. The small value of the kinetic
isotope effect (2.7) indicates a bent transition state. The negative
slope (−1.1) of the linear free energy relationship Hammett
plot supports that the benzylic carbon builds up a positive charge
in the transition state.