Assessing Ligand and Counterion Effects in the Noble Metal Catalyzed Cycloisomerization Reactions of 1,6-Allenynes: a Combined Experimental and Theoretical Approach

Abstract

1,6-Allenynes are useful mechanistic probes in noble-metal catalysis, since they can give rise to very distinct products in a highly selective fashion. Various cycloisomerization reactions have been described, and discrete mechanisms have been postulated. Of particular interest, whereas Alder-ene types of products can be obtained in a variety of ways using noble-metal catalysts (Au, Pt, Rh, ...), hydrindienes have been reported solely with gold and platinum under specific conditions. It was shown in a previous study that this intriguing transformation required the presence of chloride ligands at the active catalytic species. Herein, the factors governing the fate of 1,6-allenynes under cycloisomerization conditions have been studied more thoroughly, revealing a much more complex scenario. The nature of ligands, counterions, and metals was examined, showing that hydrindienes can be isolated in the absence of halides, using electron-rich, bulky triorganophosphines or carbene ligands. This crucial finding could also be used to access hydrindienes in high yields, not only with gold or platinum but also with silver. On the basis of mass spectrometry, NMR spectroscopy, and computations, refined mechanistic scenarios have been put forward, also rationalizing counterion effects. Notably, a metal vinylidene intermediate has been proposed for the formation of the hydrindiene derivatives. Finally, in the presence of tris((triphenylphosphine)gold)oxonium tetrafluoroborate as catalyst, a new pathway has been unveiled, involving gold alkyne σ,π complexes and leading to previously unobserved [2 + 2] cycloaddition compounds.