DFT Study on the Gold(I)-Catalyzed Dehydrogenative Heterocyclization of 2-(1-Alkynyl)-2-alken-1-ones to form 2,3-Furan-Fused Carbocycles: Effects of Additives C5H5NO vs. PhNO

A computational study with the M06/B3LYP density functional is carried out to explore the effects of additives C₅H₅NO vs. PhNO on the gold-catalyzed dehydrogenative heterocyclization of 2-(1-alkynyl)-2-alken-1-ones to form 2,3-furan-fused carbocycles. The following three conclusions are obtained based on our theoretical calculations. (a) The Au(I) catalyst plays a crucial role on the intramolecular cyclization reaction. (b) Both additives C₅H₅NO and PhNO as the proton shuttle can assist proton-transfer through a two-step proton-transfer mechanism including the protonation of additive and the deprotonation of additive-H⁺, whereas the catalytic capability of PhNO is weaker than that of C₅H₅NO (energy barrier: 90.6 vs. 33.2 kJ/mol). (c) C₅H₅NO-H⁺ has stronger stability comparing with PhNO-H⁺ because the basicity of C₅H₅NO is stronger than that of PhNO, which cause that the energy barrier of ts3 + PhNO-H⁺ (131.5 kJ/mol) is higher than that of ts3 + C₅H₅NO-H⁺ (60.5 kJ/mol) in the intermolecular addition. Therefore, the base strength is the primary factor that controls the catalytic capability of additives C₅H₅NO vs. PhNO. These studies are expected to improve our understanding of Au(I)-catalyzed reactions involving additive as the cocatalyst and to provide guidance for the future design of new catalysts and new reactions.