Mechanistic twist of the [8+2] cycloadditions of dienylisobenzofurans and dimethyl acetylenedicarboxylate: stepwise [8+2] versus [4+2]/[1,5]-vinyl shift mechanisms revealed through a theoretical and experimental study

Recently, it was reported that both dienylfurans and dienylisobenzofurans could react with dimethyl acetylenedicarboxylate (DMAD) to give [8+2] cycloadducts. Understanding these [8+2] reactions will aid the design of additional [8+2] reactions, which have the potential for the synthesis of 10-membered and larger carbocycles. The present Article is aimed to understand the detailed mechanisms of the originally reported [8+2] cycloaddition reaction between dienylisobenzofurans and alkynes at the molecular level through the joint forces of computation and experiment. Density functional theory calculations at the (U)B3LYP/6-31+G(d) level suggest that the concerted [8+2] pathway between dienylisobenzofurans and alkynes is not favored. A stepwise reaction pathway involving formation of a zwitterionic intermediate for the [8+2] reactions between dienylisobenzofurans that contain electron-donating methoxy groups present in their diene moieties and DMAD has been predicted computationally. This pathway is in competition with a Diels-Alder [4+2] reaction between the furan moieties of dienylisobenzofurans and DMAD. When there is no electron-donating group present in the diene moieties of dienylisobenzofurans, the [8+2] reaction occurs through an alternative mechanism involving a [4+2] reaction between the furan moiety of the tetraene and DMAD, followed by a [1,5]-vinyl shift. This computationally predicted novel mechanism was supported experimentally.