Catalytic role of dinuclear σ,π-acetylide gold(I) complexes in the hydroamination of terminal alkynes: theoretical insights.

A detailed density functional theory (DFT) investigation of the hydroamination of 1-octyne with aniline mediated by a σ,π-digold(I) bulky phosphine-based complex was undertaken in order to shed light on the mechanistic aspects of such processes. With the purpose to probe whether the performance that the cationic digold complexes exhibit is superior to those of mononuclear complexes, the same hydroamination reaction was explored by considering separately the reaction of aniline with both the monogold(I) complexes formed by π- and σ-coordination of 1-octyne to the dialkylbiarylphosphine Au(I) precatalyst. The outcomes of the computational analysis presented here show that, when the σ,π-digold alkynide complex can be formed, the reaction is not necessarily assisted by such a complex, as the computed energy barrier is almost equal to that found when the π-coordinated alkyne mononuclear gold complex is involved in the hydroamination process. The catalytic assistance of the Au(I)-σ-alkynyl complex, instead, can be surely excluded as the hydroamination product is formed by overcoming an energy barrier significantly higher than that computed when both σ,π-digold and π-coordinated alkyne monogold complexes assist the reaction. Moreover, regardless of the implicated gold(I) species, the investigated mechanism accounts for the Markovnikov selectivity of the reaction, confirming the experimental evidence. The proposed mechanisms for the conversion of Au(I) π-coordinated alkyne complexes into the corresponding σ,π-digold alkynide complexes were also explored.