No abstract

Dinuclear transition-metal catalysis is a distinctive approach for cross-coupling reactions, but Rh­(I)–Rh­(III) redox catalysis has rarely been considered. Here, a dinuclear Rh­(III)–Rh­(III) redox catalytic cycle for [Rh­(coe)2Cl]2-catalyzed remote terminal arylation of activated olefins is proposed and validated by density functional theory. Calculations reveal that the catalytic cycle of terminal arylation involves phosphine-directed oxidative addition of the indole C7­(aryl)–H bond, alkene insertion, Rh-walking, and reductive elimination. Throughout the catalytic cycle, the dinuclear rhodium complex remains intact, and each of the rhodium atoms have important roles. One acts as the active catalytic center and undergoes Rh­(I)–Rh­(III) redox cycles; the other remains in the +3 oxidation state after the pre-catalytic cycle and is not directly involved in the reactions but affects the chemoselectivity by adjusting the coordination environment of the active rhodium atom. The hydroarylation regioselectivity is controlled by steric effects, which distinguish different reactive sites in the dinuclear rhodium pathway. The mononuclear rhodium catalytic cycle is unfavorable because of the endothermic monomerization process. A tether effect in the reductive elimination step also restricts generation of the δ-arylation product in the mononuclear pathway.

Density functional theory (DFT) calculations have been performed to provide mechanistic insight into the Rh/Cu co-catalyzed multicomponent annulation of indoles, diazo compounds, and α,β-unsaturated esters.