Revisiting the mechanism of acetylenic amine N-Oxide rearrangement catalysed by Gold(I) complexes from a DFT perspective

In this study, we used density functional theory (DFT) to reinvestigate the mechanism proposed by Houk and Zhang et al. (J. Am. Chem. Soc. 2012, 134, 1078) for piperidinone formation through rearrangement of an acetylenic amine N-oxide catalysed by phosphine gold(I) complexes. For this rearrangement, the C-C coupling was proposed to be the rate-determining step with activation energy as high as 35.8 kcal/mol. Such a barrier seems inconsistent with the fact that the actual reaction proceeds under very mild conditions (0 °C, 1 h, in CH²Cl²). In the original report, it was proposed that the C-C coupling takes place via a mechanism which we called “front-side addition”. Interestingly, we found that the C-C coupling step becomes energetically more favourable if it occurs via another mechanism called “back-side addition”. We explored the effect of different phosphine ligands on all conceivable steps of the catalytic reaction and found that while the other steps are not highly sensitive to the phosphine identity, the C-C coupling one shows a considerable degree of dependency; the more electron-donating the phosphine ligand, the lower the rate-limiting step barrier.