The Role of Xantphos in forming an Elusive dirhodium-η1-allyl Intermediate in a Rh(II)-Catalyzed Allylic Alkylation: A Combined Computational and Experimental Study

The use of dirhodium tetracarboxylate catalysts in multicomponent reactions involving allylic alkylation has been a formidable challenge to synthetic chemists. A unique strategy by means of catalyst structure modification in the presence of an external ligand, Xantphos, has recently enabled their efficient use in one-pot reactions involving carbene insertion into X–H bonds followed by allylic alkylation. However, the origin of the novel reactivity and the mechanism of such reactions remain unclear. Herein, we report a combined computational and experimental mechanistic study to shed light on the ligand-enabled catalyst structure modification and its implication in catalysis. This unique reactivity is enabled by the dissociation of an octanoate bridge driven by κ2-Xantphos ligation to the dirhodium core of the catalyst. This in turn allows for a hitherto unknown oxidative addition with the Rh(II) catalyst resulting in a dirhodium-η1-allyl species. For the first time, we confirm the presence of such a species in solution through in situ NMR and cyclic voltammetry experiments in line with DFT calculations. Alongside, we study the role of the base and solvent in generating the nucleophilic partner that can trap the electrophilic allylic species. This study is expected to guide future catalyst design, including chiral variants, for exploring newer modes of reactivity and selectivity using dirhodium catalysis.