Photochemical reactions. 148^th communication. Photochemistry of acylsilanes: Preparation, photolyses, and thermolyses of α,β‐unsaturated silyl ketones

Abstract

Abstract The syntheses, photolyses, and thermolyses of the α,β‐unsaturated silyl ketones ( E / Z )‐7, ( E )‐ 8 , and ( E )‐ 9 are described. On n,π*‐excitation (λ > 347 mm), the aforementioned compounds undergo ( E / Z )‐isomerization followed by γ‐H abstraction. The intermediate enols are trapped intermolecularly by siloxycarbenes leading to the dimeric acetals 27A + B, 30A + B , and 31A + B . In addition, the acylsilanes ( E / Z )‐ 7 undergo photoisomerization by δ‐H abstraction furnishing the acylsilanes 29A + B . Flash vacuum thermolyses (FVT) of ( E / Z )‐ 7 , ( E / Z )‐ 8 , and ( E )‐ 9 give rise to intramolecular reactions of the siloxycarbene intermediates. Thus, FVT (520°) of ( E )‐ and ( Z )‐ 7 selectively leads to the enol silyl ethers 32 and ( E )‐ 33 , respectively, arising from carbene insertion into an allylic C–‐H bond. FVT of ( E / Z )‐ 8 (560°) and ( E )‐ 9 (600°) affords the trienol silyl ethers 34A + B and the cyclic silyl ethers 37A + B , respectively, which are formed by CH insertion of the siloxycarbenes. As further products of ( E )‐ 8 and ( E )‐ 9 , the bicyclic enol ethers 35 and 36 are formed, presumably via siloxycarbene addition to the cyclohexene CC bond.