A Distinct Mode of Strain-Driven Cyclic Allene Reactivity: Group Migration to the Central Allene Carbon Atom

Strained cyclic allenes are reactive species that can
be trapped
in a variety of complementary fashions that capitalize on their inherent
high potential energy. 1,2,4-Cyclohexatrienes represent a subclass
of allenes that, notably, can be conveniently generated by a net [4
+ 2] cycloaddition within a 1,3-enyne bearing a tethered alkyne via
a tetradehydro-Diels-Alder reaction. A limitation to the use of this
type of thermally generated cyclic allene as a construct for the introduction
of molecular complexity is their propensity to isomerize to benzenoids
via a simple net 1,5-hydrogen atom migration. We have discovered that
when the enyne component of the substrate is modified as an enol silyl
ether (or an enol ester), migration of the silyl (or acyl) group can
become the predominant event. Specifically, an appropriately electrophilic
group can migrate from the O atom to the central allene carbon adjacent
to the 1-siloxy(acyloxy) substituent. This process leads to highly
substituted phenolic products (e.g., o-silyl phenols)
following tautomerization of the intermediate cyclohexa-2,4-dienone.
Experimental studies show that this novel mode of reactivity is general;
DFT studies reveal the unimolecular nature of the group migration.