Allene Trifluoroacetoxylation with a 2-Phosphinoimidazole-Derived Bimetallic Rh(II) Catalyst

We report that a 2-phosphinoimidazole-scaffolded bimetallic Rh(II) complex enables the addition of trifluoroacetic acid across an allene under conditions where monometallic Rh(I) or bimetallic Rh(II) tetracarboxylate catalysts fail. The resulting allyl trifluoroacetate products are isolated in good yield across a range of allene substrates. Mechanistic studies suggest that reversible hydrometallation of the allene is followed by reductive elimination of the trifluoroacetate, which only occurs with our bimetallic catalyst. Monometallic catalysts undergo beta-hydride elimination to give an isomerized diene as the only product rather than undergoing the kinetically slow C-O reductive elimination. DFT studies suggest that the barrier for C-O reductive elimination is much lower in the case of our bimetallic catalyst due to electron-sharing and substrate coordination across both metal centers in the transition state. These studies help explain why the otherwise kinetically disfavored reductive elimination is only observed with our bimetallic catalyst (and not with monometallic Rh catalysts) to give the more thermodynamically stable allylic trifluoroacetate over the isomerized diene product.

Automated summary

In this study, we report a new bimetallic catalyst that enables the addition of trifluoroacetic acid to various allene substrates. Mechanistic studies reveal that the unique structure of the bimetallic catalyst lowers the barrier for C-O reductive elimination, leading to selective formation of allyl trifluoroacetate products.