β-Silyl Arenes in the Arene-Olefin Photocyclization Reaction

Abstract

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.

Organische Chemie 2007

“…Aufmerksamkeit unter den Synthetikern erregte die Aren‐Photochemie21 unter anderem durch die Verwendung neuer Substratklassen wie Benzyltrimethylsilan 22. Weiterhin wichtig war die Oxadi‐ð‐Methanumlagerung in Kombination mit einer vorgelagerten Biotransformation (Aromatenoxidation) durch Anwendungen in der Synthese der Triquinane ( )‐Hirsutsäure und (‐)‐Complicatsäure 23…”

Synthetic methods : Part (iii) Heteroatom chemistry

“…DeLong and co-workers carried out an interesting study of an arene-olefin photocyclisation reaction (Scheme 35). 84 Silyl groups placed at the b-position were thought to stabilise the positive charge formed during the transition state leading to polycyclic adducts 54. In contrast, silyl groups placed at the a-position led to complex mixtures.…”