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.