Theoretical Study on the Rh(III)-Catalyzed [5 + 1] Annulation of 2-Alkenylanilide and Allenyl Acetate

A DFT study investigates the mechanism of the Rh(III)-catalyzed [5 + 1] annulation of 2-alkenylanilide and allenyl acetate. In the presence of AgSbF6, the complex RhCp*(SbF6)2 (CAT-A) is determined to be the active catalyst. The catalytic cycle involves [CH3COO]- coordination, N-H activation, C-C coupling, ene reaction and hydrogen shift, C-N coupling, and formation of 3. Energy Span Model (ESM) analysis identifies the transition state of the C-C coupling step as the turnover-determining transition state (TDTS), with an apparent activation free energy barrier of 26.3 kcal mol-1. In the presence of NaOAc, the acetate additive plays a dual role by participating in ligand exchange and forming the ionic species [RhCp*(OAc)][Na2(OAc)3] (CAT-B). CAT-B operates via a mechanism similar to CAT-A, except that the initial [CH3COO]- coordination is obviated by the intrinsic proton-accepting acetate ligand. This leads to an apparent activation free energy barrier of 25.8 kcal mol-1. Calculations on [RhCp*(OAc)]+ (CAT-B') reveal that removal of the counteranion raises the barrier to 31.6 kcal mol-1, demonstrating the essential role of the counteranion in catalytic activity. The neutral complex RhCp*(OAc)2 (CAT-C) is found to be unviable, with a high C-C coupling barrier of 54.3 kcal mol-1.