Base-Promoted Selective Activation of Benzylic Carbon-Hydrogen Bonds of Toluenes with Rhodium(III) Porphyrin Chloride: Synthetic Scopes and Mechanism

Toluenes underwent base-promoted selective benzylic carbon-hydrogen bond activation (CHA) with rhodium porphyrin chlorides (Rh(por)Cl). In the absence of nucleophilic base, both aryl and benzylic rhodium porphyrins were formed. In the presence of nucleophilic base, the selectivity, rates and functional group compatibilities of benzylic activation reactions were enhanced. K2CO3 was found to be the optimal base to achieve the best yields. Ortho-, meta- and para-substituted toluenes in the presence of K2CO3 yielded the corresponding rhodium porphyrin benzyls in high yields both in solvent-free conditions and in benzene solvent. Mechanistically, in the absence of nucleophilic base, a cationic rhodium(III) porphyrin species together with some rhodium(II) porphyrin are the most likely intermediates to account both the aryl and benzylic CHA. In the presence of a base, Rh(por)OH is generated by ligand substitution with Rh(por)Cl and rapidly undergoes reduction to give rhodium(II) porphyin dimer and H2O2. The key rhodium porphyrin intermediates for benzylic CHA were found to be rhodium(II) porphyrin dimer and hydrides as observed by H-1 NMR spectroscopy, which underwent parallel benzylic CHA reactions with the rhodium(II) porphyrin dimer being the more reactive intermediate.