Radical Philicity Inversion in Co- and Fe-Catalyzed Hydrogen-Atom-Transfer-Initiated Cyclizations of Unsaturated Acylsilanes

[1,2]-Radical Brook rearrangement (RBR) has been identified
as
a viable pathway in M–H (M = Co, Fe) catalyzed hydrogen-atom-transfer
reactions involving unsaturated acylsilanes. Guided by the same concept,
we have explored two transformations, namely, a Co-catalyzed cycloisomerization
reaction and a Fe-catalyzed cyclization/Giese addition reaction. Both
reactions involve the generation of a versatile α-siloxy radical
intermediate via concomitant philicity inversion and radical translocation,
which is mechanistically distinct from coupling reactions involving
fragmentation/reduction pathways. Synthesis of cyclic silyl enol ethers
and sterically congested cyclopentanol derivatives have been thus
achieved with high regio- and diastereo-selectivity.