Isolable endocyclic silenes by thermal Brook rearrangement

Abstract

In the following study, we investigated thermally and photolytically induced transformations of acylcyclotetrasilanes (Me3Si)2Si4Me8(Me3Si)COR (compounds 9a-c; R = alkyl, aryl). Mild thermolysis of 9a-c below 130 °C cleanly and selectively afforded the corresponding endocyclic silenes 10b-d irrespective of the nature of the substituents at the carbonyl moiety. Thus, the previously unknown heterocyclopolysilane 10b could be isolated by crystallization and fully characterized including single crystal X-ray crystallography. Upon photolysis with light of λ ≥ 300 nm 9a-c showed Brook type 1,3-Si → O migration reactions under formation of the silenes 10a-c with exo- and endocyclic SiC double bonds. In this case the substituent at the carbonyl moiety largely determines the type of the resulting silene. Contrary to alkyl substituted educts, which form both types of silenes, aryl substituted acylcyclotetrasilanes exclusively afford the endocyclic derivative. 10b,c were photochemically unstable and underwent different degradation processes while 10d was surprisingly stable under similar conditions. Photolysis of 9a-c, in the presence of methanol, cleanly afforded the expected 1,2-addition products of the SiC double bond.