K_2CO_3?Promoted Consecutive Carbon-Hydrogen and Carbon- Carbon Bond Activation of Cycloheptane with Rhodium(III) Porphyrin Complexes: Formation of Rhodium Porphyrin Cycloheptyl and Benzyl

摘要

K_2CO_3-promoted carbon-hydrogen and carbon- carbon bond activations of cycloheptane are achieved with rhodium(III) tetrakis(4-tolyl)porphyrin chloride (Rh-(ttp)Cl) at 120 ℃ to give Rh(ttp) cycloheptyl and benzyl complexes. On the basis of mechanistic studies, Rh(ttp)Cl first reacts by ligand substitution to give Rh(ttp)OH, which then undergoes reductive elimination to give Rh~(Ⅱ) _2(ttp)_2. The metalloradical Rh~(Ⅱ)(ttp), formed via dissociation of Rh~(Ⅱ) _2(ttp)_2, activates the CH bond of cycloheptane to form Rh(ttp)(cycloheptyl) and Rh(ttp)H. Rh(ttp)(cycloheptyl) slowly yields Rh(ttp)(cycloheptatrieneyl) by successive β-hydride elimination to olefins and Rh(ttp)H. K_2CO_3 promoted the dehydrogenation of Rh(ttp)H to give Rh~(Ⅱ) _2(ttp)_2 and H_2. Both Rh(ttp)H and Rh~(Ⅱ) _2(ttp)_2 activate the cycloheptatriene to give Rh(ttp)(cycloheptatrienyl), which further undergoes a Rh~(Ⅱ)(ttp)-catalyzed skeletal rearrangement to form Rh(ttp)Bn with rate enhancement much faster than that of the analogous organic isomerization of cycloheptatriene to toluene.