Quantum-Chemical Study of the Assembly Mechanism of 1-Pyrrolines from N-Benzylaldimines and Arylacetylenes in KO^tBu/DMSO Superbasic Medium

By using a quantum-chemical approach, B2PLYP-D2/6-311+G**//B3LYP/6-31+G*, we have carried out a detailed study of the assembly of 1-pyrrolines from N-benzyl-1-phenylmethanimine and phenylacetylene in the superbasic medium KOtBu/dimethyl sulfoxide (DMSO). In this way, we have considered, both theoretically and experimentally, the mechanisms of the assembly through a concerted and stepwise nucleophilic cycloaddition and have addressed the side processes accompanying the assembly. It is found that the assembly via the concerted cycloaddition is kinetically more favorable than that via the stepwise cycloaddition. At the same time, the reaction of C-vinylation of aldimine with phenylacetylene occurs with a similar activation energy as the concerted cycloaddition and leads to the formation of 2-aza-1,4-pentadiene. The anion of 2-aza-1,4-pentadiene is an intermediate for the side processes leading to the formation of triarylpyridines and 1,3-diarylpropan-1-ones. Triarylpyridines are formed through the concerted cycloaddition of the next phenylacetylene molecule to 2-aza-1,4-pentadiene, while 1,3-diarylpropan-1-ones are formed as a result of the hydrolysis of 2-aza-1,4-pentadienes. It is found out that the mild conditions for the assembly of 1-pyrrolines (60 °C, 15 min) relate to the formation of complexes in the KOtBu/DMSO superbasic medium, where the anion is readily accessible for the nucleophilic attack by the phenylacetylene molecule.