Studies on the Himbert Intramolecular Arene/Allene Diels–Alder Cycloaddition. Mechanistic Studies and Expansion of Scope to All-Carbon Tethers

The
unusual intramolecular arene/allene cycloaddition described
30 years ago by Himbert permits rapid access to strained polycyclic
compounds that offer great potential for the synthesis of complex
scaffolds. To more fully understand the mechanism of this cycloaddition
reaction, and to guide efforts to extend its scope to new substrates,
quantum mechanical computational methods were employed in concert
with laboratory experiments. These studies indicated that the cycloadditions
likely proceed via concerted processes; a stepwise biradical mechanism
was shown to be higher in energy in the cases studied. The original
Himbert cycloaddition chemistry is also extended from heterocyclic
to carbocyclic systems, with computational guidance used to predict
thermodynamically favorable cases. Complex polycyclic scaffolds result
from the combination of the cycloaddition and subsequent ring-rearrangement
metathesis reactions.