Silylenes, reaction with furans

Cyclotrisilanes are easily converted to a mixture of silylene and disilene. Reactions of the silylene (l-Bu)2Si and the disilene l-Bu2Si=Si(Bu-l)2 produced by photolysis of the cyclotrisilane 293 with cyclopentadiene, furan, and thiophene have been reported recently152, as were their reactions with 2,2 -bipyridyl, pyridine-aldimine and ketoimines153. 

The photolysis of cyclotrisilane 137 in the presence of furan gives an unusual tricyclic product 138 (among other products), presumably by reaction of furan with both the disilene Bu 2Si=SiBu 2 and the silylene Bu 2Si generated by the photolysis of 137 (Equation 10) <19950M5695>. 

Further cycloaddition reactions of silylenes generated by the photolysis of cyclotrisilanes have been published since Weidenbruch and coworkers summarized these reactions in an excellent review. Different siliranes were prepared by [2+1]-cycloaddition of di-t-butylsilylene to various alkenes and dienes (Scheme 6)46. Quite interesting results are obtained from the photolysis of hexa-i-butylcyclotrisilane in the presence of unsaturated five-membered ring compounds47 (Scheme 7). With cyclopentadiene and furane, [4 + 2]-cycloaddition of the photolytically generated disilene occurs only as a side reaction. Furthermore, [2 + 1]-cycloaddition of the intermediately formed silylene is highly favored and siliranes are primarily obtained. A totally different course is observed for the reaction in the presence of thiophene. The disilene abstracts the sulfur atom with the formation of the 1,2-disilathiirane as the major product with an extremely short Si—Si distance of 230.49 pm. 

The Diels-Alder adduct formed by treating furan with hexafluorobut-2-yne has been subjected to a retro-Diels-Alder reaction acetylene is eliminated and the reaction provides a route to 3,4-bis(trifluoromethyl)furan. The corresponding cycloadduct (154) formed from l,l-dimethyl-2,5-diphenyl-sllacyclopentadiene and the butyne was decomposed in an attempt to generate dimethylsilylene by a symmetry-allowed cheletropic fragmentation (Scheme 50) 836 evidence for silylene formation could be obtained unless the decomposition was effected photochemically or in refluxing cumene, under which conditions the addition of tolan enabled dimethylsilylene to be trapped... 

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