Activation of arene carbon-hydrogen bonds. Highly regioselective, electrophilic aromatic metalation with rhodium(III) porphyrin and subsequent cleavage of carbon-rhodium bond

Article abstract of DOI:10.1021/om00132a030

(Octaethylporphyrinato)rhodium(III) chloride, (OEP)Rh^IIICl, reacts with benzene in the presence of AgClO₄ or AgBF₄ to give the phenyl-rhodium(III) complex. Anisole, toluene, and chlorobenzene are similarly metalated exclusively at the para positions. The metalation of methyl benzoate, on the other hand, gives a 92:8 mixture of meta- and para-metalated isomers. The reactivities of arenes follow the Hammett equation with the ρ value of -5.43. The observed substituent effects both on reactivity and orientation unambiguously characterize the present reaction as an electrophilic aromatic metalation with a [(OEP)Rh^III]⁺ intermediate generated from the anion exchange of (OEP)Rh^IIICl with silver salts. The slow step in the reaction is coordination of arene to form [(OEP)Rh(arene)]⁺, which subsequently loses a proton. Arenes thus activated with rhodium undergo photochemical homolysis of the C-Rh bond as well as its halogen-induced heterolysis. The photolysis in benzene leads to 4-substituted biphenyls and the halogenolysis gives p-haloarenes with extremely high regioselectivities (>99%).