Computational Study of the Substitution Effect on the Mechanism for the Gold(I)‐Catalyzed Ring Expansions of Unactivated Alkynylcyclopropanes

The ring expansion reactions of unactivated alkynylcyclopropanes X-C≡C-C3H5 X-C=C4H5 (X = H, F, Cl, Me, OMe, NMe2, CMe3) were examined using the density functional theory calculations. All of the structures were completely optimized at the B3LYP/6-311++G** level of theory. For clarify the effect of the cationic gold(I), we also added AuPH3+ as the catalyst into the system and the structures for Au were calculated at the B3LYP/LANL2DZ level of theory. The main finding of this work is that the singlet-triplet splitting of X-C≡C-C3H5 play an important role in determining the kinetic and thermodynamic stability of the unactivated ring expansion reactions. When X-C≡C-C3H5 with a smaller singlet-triplet splitting is utilized, the reaction has a smaller activation energy and a larger exothermicity.