Abstract
The ‘β-silyl effect’ of ground-state chemistry has been found to be operative in excited-state
systems as well, where it exerts just as remarkable of an effect on the yields and
selectivity of the intermolecular arene-olefin photocyclization reaction. Benzyltrimethylsilane
has been found to undergo intermolecular arene-olefin photocyclization with alicyclic
hydrocarbon alkenes to provide 2,6-meta -adducts with yields of 65-90% in a regio- and stereoselective manner. Particularly
encouraging are the reactions with cyclohexene and with cis -cyclooctene, as both of these substrates give anomalous results with many arenes,
but fall perfectly in line when coupling with benzyltrimethylsilane. These data augur
well for the use of β-silyl directing groups in more complex photocyclization reactions.
The ability of a C-Si sp3 bond to stabilize an incipient positive charge in the β-position by hyperconjugation
apparently also allows it to stabilize a transiently electron-deficient excited state,
in the same way that an electron-donating group (e.g., a methyl or a methoxy group)
has been observed to direct the regiochemistry of the arene-olefin photocyclization
reaction. To fully characterize the photoadducts, they have been elaborated via radical
addition of substituted thiophenols to crystalline compounds and X-ray crystal structures
obtained.
Key words
photochemistry - cycloadditions - neighboring group effects - hyperconjugations -
radical reactions
