Impact of the Valence Charge of Transition Metals on the Cobalt- and Rhodium-Catalyzed Synthesis of Indenamines, Indenols, and Isoquinolinium Salts: A Catalytic Cycle Involving M/M [M = Co, Rh] for [4 + 2] Annulation.

Reaction mechanisms for the synthesis of indenamines, indenols, and isoquinolinium salts through cobalt- and rhodium-catalysis were investigated using density functional theory calculations. We found that the valence charge of transition metals dramatically influences the reaction pathways. Catalytic reactions involving lower-oxidation-state transition metals (M/M, M = Co and Rh) generally favor a [3 + 2] cyclization pathway, whereas those involving higher oxidation states (M/M) proceed through a [4 + 2] cyclization pathway. A catalytic cycle with novel M/M as a crucial species was successfully revealed for isoquinolinium salts synthesis, in which highly valent M was encountered not only in the [RhCp*]-catalysis but also in the [CoCp*]-catalysis.