Domino Fragmentations in Traceless Directing Groups of Radical Cascades: Evidence for the Formation of Alkoxy Radicals via C–O Scission

Direct
evidence for the formation of alkoxy radicals is reported
in radical cascades using traceless directing groups. Despite the
possibility of hydrogen abstraction in the fragmenting step, followed
by loss of R-OH, β-scission is preferred for
the formation of alkoxy radicals. For the first time, the C-O radical
was intermolecularly trapped using a silyl enol ether. Various C–X
fragmenting groups were explored as possible traceless directing groups
for the preparation of extended polyaromatics. Computational evidence
shows that a combination of aromatization, steric and stereoelectronic
effects assists the fragmentation to alkoxy radicals. Additionally,
a new through-space interaction was discovered between O and Sn in
the fragmentation as a specific transition state stabilizing effect.