A Quantum‐Chemical Study of the Mechanism of 1‐Azadienes Formation from Aldimines and Arylacetylenes in the KOtBu/DMSO Superbasic Medium

The mechanism of 1‐azadienes formation from aldimines and arylacetylenes in the KOtBu/DMSO superbasic medium has been explored theoretically using a DFT approach [B2PLYP(D2)/6‐311+G(d,p)//B3LYP/6‐31+G(d)] and a sophisticated model taking the potassium tert‐butoxide and the nearest solvation shell of the potassium cation explicitly into account. The main focus is placed on (i) a detailed analysis of the kinetic and thermodynamic characteristics of the isomerization of intermediate propargylamine into 1‐azadienes, and (ii) the factors determining the isomeric composition of 1‐azadienes with different substituents in the starting arylaldimines and arylacetylenes. Our findings clearly indicate that the mild experimental conditions of 1‐azadienes formation are accounted for by the relatively small activation barrier (ΔG≠≤16.2 kcal/mol) of the limiting step (ethynylation of aldimine with arylacetylenes). The rapid isomerization of propargylamine to 1‐azadiene is found to be due to low activation barriers (ΔG≠≤9.1 kcal/mol) related to the protons transfer. The isomeric composition in the case of various substituted 1‐azadienes is determined solely by the difference in the thermodynamic stability of the 1E,2E‐ and 1Z,2E‐forms. The formation mechanism of 1‐azadienes from aldimines and arylacetylenes in the KOtBu/DMSO superbase has been explored in some detail. The mild experimental conditions of the formation are explained by the low activation barrier of the limiting step (ethynylation reaction), which relates to a special influence of the superbase. The ratio of 1E,2E‐/1Z,2E isomers is determined by their relative thermodynamic stability.