Density Functional Theory Study of Mechanisms of [8 + 2] Cycloadditions of Dienylfurans/Dienylisobenzofurans with DMAD.

The mechanisms of [8 + 2] cycloaddition reactions between dienylfurans/dienylisobenzofurans and the activated alkyne, DMAD (dimethyl acetylenedicarboxylate), have been investigated by DFT calculations. The former [8 + 2] reaction is stepwise, starting from attack of the diene substituent on furan, not the furyl moiety in dienylfurans, to DMAD to give a diradical intermediate, which then undergoes ring closure to form the second bond between DMAD and the furan moiety, generating the final [8 + 2] cycloadducts. In contrast, the latter [8 + 2] reaction starts from [4 + 2] cycloaddition of the diene in the furan ring of dienylisobenzofurans toward DMAD, followed by the rate-determining stepwise [1,5]-vinyl shift, forming the [8 + 2] products. The different mechanisms of [8 + 2] reactions are attributed to the facts that for dienylfurans, the reactive diene part is the diene substituent on furan, but in the case of dienylisobenzofurans, it is the diene in the furan ring (its reaction with DMAD to generate an aromatic benzene ring is the driving force for this regiochemistry). Consequently, the [8 + 2] reactions begin with the reaction of the most reactive part of tetraene (either the diene substituent on furan for dienylfurans or the diene in the furan ring for dienylisobenzofurans) with DMAD. FMO analysis and kinetic study have been carried out to gain more information of the reaction mechanisms. Two [8 + 2] reactions of dienylisobenzofurans with different substituents toward DMAD have also been further analyzed by DFT calculations in this paper.