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 MIII/MV [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 (MI/MIII, M = Co and Rh) generally
favor a [3 + 2] cyclization pathway, whereas those involving higher
oxidation states (MIII/MV) proceed through a
[4 + 2] cyclization pathway. A catalytic cycle with novel MIII/MV as a crucial species was successfully revealed for
isoquinolinium salts synthesis, in which highly valent MV was encountered not only in the [RhCp*]-catalysis but also in the
[CoCp*]-catalysis.