Silenes with carbonyl groups

The UV absorptions of the tri- or tetrasubstituted silenes are red-shifted compared to the parent silene 2 (X = 258 nm) but occur at shorter wavelengths than in the highly substituted silenes of Brook (X = ca 340 nm). The introduction of trimethylsilyl groups on the carbon atoms results in slight red-shifts just as the conjugation of the silene system with carbonyl groups. 

Brook et al. 5X1 observed such reactions during the formation of siienes by photolysis. Using radiation with A > 360 nm, they photolyzed acylsi-lanes such as 127, which bears a mesityl group attached to the carbonyl carbon. On prolonged photolysis of the initially formed silene 128, the C—H bond of the ortho methyl group of the mesityl group added to the silicon-carbon double bond to form the benzocyclobutane 129. Alternatively a 1,5-H shift would lead to the species 130, which would also yield the benzocyclobutane on electrocyclic rearrangement. 

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