Rerouting Radical Cascades: Intercepting the Homoallyl Ring Expansion in Enyne Cyclizations via C–S Scission

The
switch from 5-exo- to 6-endo-trig selectivity in the radical cyclization
of aromatic enynes was probed via the combination of experimental
and computational methods. This transformation occurs by kinetic self-sorting
of the mixture of four equilibrating radicals via 5-exo-trig cyclization,
followed by homoallyl (3-exo-trig/fragmentation) ring expansion to
afford the benzylic radical necessary for the final aromatizing C–C
bond fragmentation. The interception of the intermediate 5-exo-trig
product via β-scission of a properly positioned weak C–S
bond provides direct mechanistic evidence for the 5-exo cyclization/ring
expansion sequence. The overall cascade uses alkenes as synthetic
equivalents of alkynes for the convenient and mild synthesis of Bu3Sn-functionalized naphthalenes.