Dynamically Generated Carbenium Species via Photoisomerization of Cyclic Alkenes: Mild Friedel–Crafts Alkylation

The torsional strain of trans-configured
medium-sized
(6–8) cycloalkenes imparts substantial potential energy efficiently
toward ionic additions through generated reactive carbenium species.
These reactions have been underexplored due to a historical necessity
for harsh ultraviolet irradiation. We report here the Friedel–Crafts
(FC) type reactivity of arylcycloalkenes (ACs) and π-nucleophiles
for the first time with weak Brønsted acid and visible light
energy transfer catalysis. Following optimizations using p-fluorophenyl cyclohexene as the AC and 2-methylfuran as the nucleophile,
model conditions were obtained to probe the respective influence of
the acid catalyst, aryl component of AC, nucleophile, and alicyclic
component of AC on the desired FC reactivity. Each parameter was found
to critically influence the course of the reaction. Ultimately, a
mild, visible light-driven method for the preparation of a variety
of 1,1-diarylcyclohexane and 4,4-diarylpiperidine derivatives that
is mechanistically distinct from and complementary to other methods
of preparation is outlined.