Photochemical reactions. 139 th Communication . Photochemistry of acylsilanes: 1. Siloxcarbene formation versus γ‐H‐abstraction

The Syntheses and the photolyses of the acylsilane 1 and the corresponding methyl ketone 2 are described. On n,π*‐excitation, the silyl ketone 1 as well as the methyl ketone 2 undergo a Norrish type II reaction involving γ‐H‐abstraction and fragmentation to the diene 12 , and acetone ( 20 ) or the acylsilane 26 , respectively. The methyl ketone 2 , but not the acylsilane 1 , isomerizes to cyclobutanols ( 21A ‐ D ). Additionally, compound 1 shows photochemical behavior typical of acylsilanes undergoing rearrangement to the siloxycarbene intermediate c . Insertion of c into the OH‐ond of the enol 28 leads to compound 13 . Initial trapping of the siloxycarbene c by H 2 O, however, gives rise to the formation of compounds 16 – 18 . As minor photolysis products of 1 , compounds 14 and ( Z )‐ 15 were formed; however, on vapor phase thermolysis (520°) of 1 , compounds 14 and ( E / Z )‐ 15 were obtained in 92% combined yield. To a small extent the acylsilane 1 also undergoes Norrish type I cleavage leading to the acid 19 .