Silacyclopropane rearrangement

It is tempting to assume, that the facile formation of silylene 2 from cyclotrisilane 1 is due to the effective stabilization of 2 by intramolecular coordination of the dimethylamino group to the silicon centre [10], which should lower the activation energy of a dissociation process from 1 to 2. Reaction of 1 with benzylvinylether resulted in a complex reaction mixture, from which 12 % of vinylsilane 4 was isolated 4 is presumably formed by rearrangement of the unstable oxy-substituted silacyclopropane 3c. 

Stirring 1 for 12 h at 40 °C with excess styrene led quantitatively to silaindane 13 [12], The silacyclopropane 3d was identified as an intermediate in this reaction by its Si NMR shift (5 = -82.5 ppm) [6]. Thus, 13 appears to be formed by initial formation of 3d, which rearranges to intermediate 12. Rearomatization eventually yields 13 (Scheme 2). This pathway resembles the well known mechanism of the reaction of silylenes with conjugated olefins via initial formation of vinylsilacyclopropanes [3]. 

In the case of silacyclopropanes derived from vinyl ethers, addition of alcohol resulted in a stereoselective rearrangement. For example, cw-methoxyhexene (99 1) was converted to vinyl silane (34) in 22% yield with greater than 85% cis selectivity (Scheme 1) <81JA4170>. 

Silyl transfer. l,l-Di(r-butyl)silacyclopropanes readily submit the di-r-butylsilyl residue to a-keto esters to form 4-aIkoxy-l,3,2-dioxasiloles. In the case of an aUyloxy ester the situation is set up for the Ireland-Claisen rearrangement. ... 

If an aryl group is attached to the sp2-hybridized carbon remote from silicon, different behavior is observed. Reversible photoisomerization of cis to trans alkenes occurs, and on further photolysis insertion-rearrangement reactions lead to silacyclopropanes and silaindanes45 (equation 27). The generality of these rearrangements has not yet been explored in depth. 

Di-7r-silane photochemistry is apparently different from di-7r-methane photochemistry.21 No isomerization to silacyclopropanes was observed on irradiation of the di-TT-silanes (290), the sole products being the cis, tra/w-isomers (291) and (292). Rearrangement together with solvent incorporation was observed,... 

Pyrolysis of 1,2-dimethyltetramethoxydisilane at 350 ""C gave methoxymethyl-silane which reacts with cis,cis-cyclo-octa-l,3-diene to give the silacyclopropane (173) which rearranges under the reaction conditions to give 1-methoxy-1-methyl-1-silacyclonona-2,8-diene by a free-radical cleavage of the C-1—C-8 bond and trans-annular hydrogen transfer. ... 

The a-ketoester 134 participates in a novel metal-catalyzed silylene transfer in the presence of silacyclopropane to generate silacarbonyl ylide 137 which undergoes 6 r-electrocyclization to provide the silylketene acetal 138. Subsequent [3,3]-sigmatropic rearrangement proceeds via the chair-like... 

C-Labelling showed that the thermal conversion of the phenyl trimethylsilyl diazo-compound (168) into (169) involved isomerization of a phenyl carbene into a cycloheptatrienylidene, silacyclopropanes were not involved. It was shown that the P-naphthyl carbene obtained by rearrangement of 4,5-benzocycloheptatrienylidene enters the singlet-triplet manifold as a singlet. Evidence has been reported that aryl carbenes can show both electrophilic and nucleophilic properties in their intramolecular ring expansion. Low-temperature photolysis of the sodium salt of the toluene-p-sulphonyl hydrazone (170) gave, by intramolecular carbene addition, the dibenzobicyclo[4,l,0]heptatriene (171) which was trapped by buta-l,3-diene. ... 

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