From Dirhodium to Monorhodium: Identification of the True Active Catalyst in Dirhodium Catalyzed Carbonyl Arylation

Dirhodium(II) carboxylates play a central role in catalysis; nevertheless, achieving high enantioselectivity through the use of external chiral ligands in dirhodium systems has remained elusive until recent years. While these reports using chiral bisphosphines represent landmark breakthroughs, the nature of the active species has not been established, and the proposed coordination modes of phosphines have lacked direct experimental validation. Here, we unambiguously identify the true active catalyst in the dirhodium(II) tetracarboxylate/chiral Josiphos-catalyzed carbonyl arylation reaction. Through a rigorous combination of variable-temperature 31 P{ 1 H} NMR (VT-31 P{ 1 H} NMR) spectroscopy, electron spray ionization high-resolution mass spectrometry (ESI-HRMS), single-crystal X-ray diffraction (scXRD), and density functional theory (DFT) calculations, we demonstrate that catalysis proceeds via an unprecedented additive-assisted cleavage of the Rh-Rh bond. Under the reaction conditions, the dirhodium(II) core dissociates to form a chiral, orthometalated Rh(III) complex that constitutes the catalytically competent species. This new complex exhibits higher reactivity and selectivity compared to dirhodium(II), Rh(I), and other Rh(III) complexes. DFT calculations elucidate the origin of high stereocontrol in the CC bond-forming event. This study reveals, for the first time, that dirhodium(II) catalysts can dissociate under catalytically relevant conditions to generate a highly active chiral monorhodium(III) species, thereby overturning the prevailing mechanistic model of dirhodium(II) catalysis. Such monomeric species may constitute the true active catalysts in other dirhodium(II)-catalyzed reactions, thereby opening new avenues for mechanistic exploration in dirhodium(II) catalysis. Moreover, the chiral orthometalated Rh(III) complex identified herein presents significant opportunities for the development of new asymmetric transformations.