Selectivity Switch in the Synthesis of Vinylgold(I) Intermediates

An unexpected regioselectivity reversal was observed in the synthesis of vinylgold(I) complexes from propargyl carboxamides. The use of [Au(IPr)(OH)] affords preferentially vinylgold(I) complexes resulting from a 5-exo-dig cyclization, whereas the use [Au(IPr)](+) species, generated in situ starting from [Au(IPr)(Cl)] and AgOTf, leads to vinylgold(I) complexes which form via a 6-endo-dig cyclization, This unexpected selectivity switch for this cyclization led us to propose two different reaction pathways enabling the formation of the two different products. One mechanism involves the [Au(IPr)](+) species with classical pi activation of the alkyne framework of the carboxamides, and the second represents an unusual use of the basic Bronsted character of [Au(IPr)(OH)). Mechanistic and DFT studies support the mechanistic hypotheses.