Carbon with silacyclobutanes

Carbon-Carbon Bond Formation with Silacyclobutanes... 

In contrast, in the excited state the primary cleavage mechanism in silacyclobutanes like 5 involves the breaking of a silicon-carbon bond23. The initially formed silyl radicals 15 and 16 are stabilized by an intramolecular disproportionation reaction giving the silenes 17 and 18 and the homoallylsilane 19.17 and 18 were identified by their trapping products (20, 21) with methanol (equation 5)23. From pyrolysis of Z-5 a different set of products from 1,4-diradical disproportionation is obtained, which can be attributed to predominant cleavage of the carbon-carbon bond23. 

Very low pressure thermolysis of (2-silapropen-2-yl)benzene (CH2 Si(Me)C6H5) causes a 1,3-hydrogen shift from the aryl carbon to the sp2 silicon with the formation of (53) (940M4661). In a related reaction, thermolysis of the silacyclobutane (54) yields (55) by a 1,4-hydrogen shift after the four-membered ring has decomposed by loss of ethene (95JOMC4). 

Ring expansion in conjunction with Tamao-type oxidation of carbon-silicon bonds provides access to 1,4-diols. The l-(l-iodoalkyl)-l-silacyclobutanes are available from 1-chlorosilacyclobutanes (addition of vinyl, Scheme 34) <1991TL6383>. The utility of silacyclopentanes formed by the ring expansion of SCB for the synthesis of diols has been reported <1992TL7031, 1995BCJ625>. 

Silacyclobutanes as well as silacyclopropanes undergo aldehyde insertion under catalysis by /-BuOK (Equation (77)).292 The reaction of silacyclobutanes with lithium carbenoids such as dihalomethyllithium and oxiranyllithium gives 2-substituted silacyclopentanes (Equation (78)). Treatment of l-(l-iodoalkyl)- and 1-oxiranyl-silacyclobutanes with a stoichiometric amount of an alkali alkoxide leads to silacyclopentanes by anionic 1,2-shift of the ring carbon adjacent to silicon. These ring-expansion reactions proceed probably via a pentacoordinate silane intermediate. 

Dipolar reagents diazomethane and methylenetrimethylphosphorane act as carbon nucleophiles in their reactions with silacyclopropanes. Reaction of diazomethane with hexamethylsilacyclopropane gave silacyclobutane (40) in 70% yield (Scheme 5), whereas reaction with methylenetrimethylphosphorane afforded phosphorus ylide (41) in 78% yield (Scheme 6). These... 

The silicon-carbon bonds of silacyclobutanes are readily activated by Pd and Pt complexes [691]. This notable characteristic has been used for carbon-carbon bond formation using silacyclobutanes [692-695]. The Pd-catalyzed reaction of 191a with alkynes affords silacyclohexenes and allylvinylsilanes (Scheme 10.262) [693]. A plausible mechanism for formation of the cyclic product involves three steps - oxidative addition of 191 a to a Pd(0) species, alkyne insertion into the Si-Pd bond, and reduc-... 

Recently, 1-silacyclobutane with the planar configuration of the cycle, namely, racemic l,l-(rac-l,l -bi-2-naphthoxy)-l-silacyclobutanehas been synthesized [48]. It was shown that the silicon-carbon cycle is planar, and the distance between the... 

---