An Experimental Stereoselective Photochemical [1s,3s]-Sigmatropic Silyl Shift and the Existence of Silyl/Allyl Conical Intersections.
2020
We report an experimental discovery
and computational investigation
of the first photochemical stereoselective [1,3]-sigmatropic silyl
shift of an allylsilane. An organocatalytic enantioselective cascade
annulation generates a trimethylsilyl-o-isotoluene
reactant in >99:1 e.r., and this trimethylsilyl-o-isotoluene contains an allylic silane moiety that undergoes a stereoselective
photochemical [1,3]-silyl shift to form a benzylsilane with 96:4 e.r.
The mechanism of this unprecedented [1,3]-silyl shift has been elucidated
by a series of experimental studies and CASSCF, DFT, and TD-DFT calculations
on model systems and the experimental system. The highly stereoselective
photoreaction is proposed to occur via a singlet silyl/allyl conical
intersection. This is a new demonstration of the role of conical intersections
in selective photochemistry.
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