Theoretical insight into the different reactivities of aliphatic and aromatic terminal alkynes towards homopropargyl alcohols via Au(I) catalysis

Abstract

Au(I)-catalyzed intermolecular condensations between terminal alkynes and homopropargyl alcohols were reported as an effective strategy for the synthesis of hydrooxepine skeletons. It was found that only aliphatic terminal alkynes furnish the desired product in high yields (84%), in contrast to very low yields (<5%) with aromatic terminal alkynes. This work aims at understanding the different reactivities of aliphatic and aromatic terminal alkynes by performing DFT calculations. The results show that the reaction of the aliphatic terminal alkyne occurs via the intermolecular-addition-first mechanism, while that of the aromatic terminal alkyne proceeds via the intramolecular-cyclization-first mechanism. The overall barrier is 21.7 kcal/mol for the reaction of the former and 27.3 kcal/mol for that of the latter, rationalizing the experimental observation. The poorer reactivity of the aromatic terminal alkyne is attributed to the weaker electrophilicity of the proximal carbon in phenylacelene resulted from the conjugative effect between phenyl and alkynyl group.