Cycloaddition Reaction of Vinylphenylfurans and Dimethyl Acetylenedicarboxylate to [8 + 2] Isomers via Tandem [4 + 2]/Diradical Alkene–Alkene Coupling/[1,3]‑H Shift Reactions: Experimental Exploration and DFT Understanding of Reaction Mechanisms

An experimental test of designed
[8 + 2] reaction of vinylphenylfuran and dimethyl acetylenedicarboxylate
(DMAD) has been carried out, showing that the reaction gave unexpected
addition products under different conditions. When the reaction was
conducted under thermal conditions in toluene, expoxyphenanthrene,
which was named as a [8 + 2] isomer, was generated. The scope of this
reaction has been investigated in the present study. In addition,
experiments and DFT calculations have been conducted to investigate
how the reaction between vinylphenylfuran and DMAD took place. Surprisingly,
the reaction did not involve the expected [8 + 2] intermediate, o-quinodimethane. Instead, the reaction starts from intermolecular
Diels–Alder reactions between DMAD and the furan moiety of
vinylphenylfuran, followed by unexpected intramolecular alkene–alkene
coupling. This step generates a diradical species, which then undergoes
[1,3]-H shift to give the experimentally observed expoxyphenanthrene.
DFT calculations revealed that, the [8 + 2] cycloadduct cannot be
obtained because the [1,5]-H shift process from the [1,5]-vinyl shift
intermediate is disfavored kinetically compared to the [1,3]-H shift
to the [8 + 2] isomer.