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The 'b-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 areneolefin 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 b-silyl directing groups in more complex photocyclization reactions. The ability of a C-Si sp 3 bond to stabilize an incipient positive charge in the b-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 Xray crystal structures obtained.

Die Salze (III), (IV), (VI) und (VII) werden gemäß Formelschema dargestellt (keine Ausb.‐Angaben) und IR‐ sowie Raman‐Spektren aufgenommen.

Organo-silicon compounds S 0060β-Silyl Arenes in the Arene-Olefin Photocyclization Reaction. -Benzyltrimethylsilane undergoes intermolecular arene-olefin photocyclization with cycloalkenes to give predominantly single products, even with problematical cyclohexene and cis-cyclooctene. The presence of a β-silyl group has a powerful stabilization effect on the electron-deficient early transition state. It thus allows the reactions to be run to complete consumption of the starting arene without significant polymer formation. -(DELONG*, M. A.; MATHEWS, J. A.; COHEN, S. L.; GUDMUNDSDOETTIR, A.; Synthesis 2007, 15, 2343-2350; Dep. Chem., Univ., Cincinnati, OH 45221, USA; Eng.) -Bartels