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

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.