Catalytic Ring Expansions of Cyclic Alcohols Enabled by Proton-Coupled Electron Transfer

We
report here a catalytic method for the modular ring expansion
of cyclic aliphatic alcohols. In this work, proton-coupled electron
transfer activation of an allylic alcohol substrate affords an alkoxy
radical intermediate that undergoes subsequent C–C bond cleavage
to furnish an enone and a tethered alkyl radical. Recombination of
this alkyl radical with the revealed olefin acceptor in turn produces
a ring-expanded ketone product. The regioselectivity of this C–C
bond-forming event can be reliably controlled via substituents on
the olefin substrate, providing a means to convert a simple N-membered
ring substrate to either n+1 or n+2 ring adducts in a selective fashion.