Decarboxylative Cyclization of Proline with o -Alkynylbenzaldehyde through an Unexploited 8π-Electrocyclization: A DFT Study.

We employed density functional theory (DFT) methods to investigate the most plausible mechanism of cyclization/ring expansion of proline with o -alkynylbenzaldehyde. This one-pot reaction starts with the in situ formation of azomethine ylide, which can undergo three different reaction pathways to form the final product. Two mechanisms are based on nucleophilic addition and 4π-electrocyclization of the azomethine ylide, and our results indicate that the rate-determining step (RDS) of these two cyclizations are 40.1 and 40.2 kcal/mol, respectively. The third novel pathway relies upon 8π-electrocyclization as the key step of this reaction; interestingly, the RDS of this cyclization is ∼20.6 kcal/mol, which shows this route to be most feasible. Thus, we introduce a novel mechanism for the electrocyclization reaction of conjugated azomethine ylides that can help chemists to design and access a new series of compounds.