Photolysis of acylsilanes

In view of the evident reactivity of the Brook-type silenes toward carbonyl compounds and the fact that these silenes were prepared by the photolysis of acylsilanes, it is natural to ask why the silenes apparently did not react with their acylsilane precursors. This question has been answered recently. On the one hand, as shown in Scheme 19, the silene Ph2Si=C(OSiMe3)Ad apparently did add in a [2 + 2] manner to its acylsi-... 

Silenes of the family Me3SiR1Si=C(OSiMe3)Ad-l 137 undergo a complex silene-to-silene photoisomerization reaction90,94,96. When silenes 137 are generated by photolysis of acylsilanes 138, the isomeric silenes 139 and 140 are formed in a subsequent reaction. The reaction was followed by UV and NMR spectroscopy. The disappearance of 138 cleanly follows first-order kinetics and the overall kinetics were consistent with the transformation 138 -> 137 -> 139. 137 as well as 139 were characterized by NMR spectroscopy and, in addition, the structure of 137 was established by trapping with methanol. The identity of 139 and 140 was confirmed by the isolation of their head-to-tail dimers from which crystals, suitable for X-ray analyses, were isolated (equation 34)90. 

Photolysis of acylsilanes produces siloxysubstituted carbenes, which could be trapped by an excess of electrondeficient olefins. This route to the resulting alkyl 2-siloxycyclopropanecarboxylates seems not to be of preparative value, however 67). 

Silaselenirane (13), another example of group 14 metal-containing seleniranes, was prepared by the selenation of the corresponding silene (12), which was generated by the photolysis of acylsilane (22) (Scheme 10) <92OM3088>. 

Photolysis of a,/i-unsaturated acylpolysilanes of the general structure (Me3Si)3-SiCOCR=CR2 failed to yield significant amounts of the isomeric siladienes, and gave instead products derived from the Norrish type 1 cleavage of the acylsilane into the radicals (Me3Si)3Si and COCR=CR, 124. 

Acylsilanes, compounds in which a carbonyl group is directly attached to a silicon atom, have been much investigated using photochemical methods. The chemistry of acylsilanes has been reviewed94 and an excellent review and discussion of all the details of acylsilane photolysis has recently been published1. 

The cyclic siloxycarbene was also trapped by carbonyl compounds giving epoxides105, and photolysis of the acylsilane in the presence of electron-deficient alkenes such as diethyl fumarate or maleate gave cyclopropanes which contained the siloxycarbene skeleton106 (equation 69). However, detailed mechanistic photochemical studies showed... 

A different type of photochemical rearrangement was observed with the mesitylsilene shown in equation 83, derived from photolysis of the acylsilane. In this case the initially... 

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