Silacyclobutanes, allylic

When simple alkenes were employed as reaction partners for silenes of the type (Me3Si)2Si=C(OSiMe3)R1, silacyclobutanes were obtained, provided that no allylic hydrogen is present in the alkene. In the reaction with alkenes with allylic hydrogens the ene reaction becomes predominant (see Table 11). Thus, while the reaction with styrene exclusively gives the four-membered ring compound 454, with 1-methylstyrene the ene products 455 were obtained (equation 144). Similarly, from the reaction of 150 with 1-octene only the ene product 456 was isolated (equation 145). 

In another study several simple silenes RR Si=CH2 (R, R = Me, Vinyl etc.) were formed by laser-powered pyrolysis and were found to form linear polymers, in contrast to the usual behavior of silenes which yield cyclodimers when formed by conventional thermolysis techniques16. Reactions of the silenes in the presence of several monomers such as vinyl acetate, allyl methyl ether and methyl acrylate were also studied. Laser-induced decomposition of silacyclobutane and 1,3-disilacyclobutane gave rise to silenes and other oxygen-sensitive deposits17,18. 

Remarkable dependence of the regioselectivity on the catalyst species employed is observed in the intramolecular hydrosilylation of allylic /V-dimethylsilylamines 63, i.e. the Rh-catalyzed reactions proceed via 5-endo cyclization to give l-aza-2-silacyclopentanes 64 whereas the Pt-catalyzed reactions yield l-aza-2-silacyclobutanes 65 via A-exo... 

When deuterium-labeled 2-methylene-l-silacyclobutane was investigated, scrambling of deuterium between the allylic methylene and the terminal vinyl was observed. This fact has been explained by ring opening to the 1,4-diradical at temperatures below those required for the rearrangement to the carbene <1995JA11695>. 

The resulting 5-methylene-2-oxa-l-silacyclohexanes are insufficiently Lewis acidic to react with a second equivalent of the carbonyl compound. However, the incipient allylsilane does react with dimethyl acetals in decent yields in the presence of external Lewis acids including BF3-Et20 or AICI3. Based on these results, double allylation of dicarbonyl compounds with 3-methylene-l,l-diphenyl-l-silacyclobutane was examined, leading to the formation of 3-methylene-oxabicyclo[3.2.1]octanes. This transformation proceeded in one pot and in the presence of BF3-Et20 (Scheme 42). 

The reaction of l-allyl-l-(cyclohexyloxy)silacyclobutane with ct-hydroxy carbonyl compounds proceeded at a lower reaction temperature. The alkoxy group on silicon enhances the Lewis acidity of the allylsilacyclobutane and presents the possibility for ligand exchange to preceed the allylation event, which then occurs intramolecularly (Scheme 67). 

The stereochemical outcome of allylic silacyclobutanes is explained in terms of a six-membered chairlike transition state18 (Scheme 3.2p). The aldehyde would coordinate to the Lewis acidic silicon19 to form a pentacoordinated complex, from which the allyl group transfer occurs subsequently to give homoallylic alcohols. In transition states A and B, the aryl or alkyl group (R) of the aldehydes prefers to occupy an equatorial position to produce anti- and. syn-alcohols from the (E)-and (Z)-allylic silacyclobutanes, respectively. 

The anionic ring-opening polymerization of I is unexpected because under similar conditions, allyltrimethylsilane undergoes metallation to yield an a-trimethylsilyl-substituted allyl anion (14-16). Relief of ring strain may be an important factor in facilitating this ring-opening polymerization. For example, the polymerization of silacyclobutanes is catalyzed by various nu-... 

Silacyclobutane monomers with aryl and alkylaryl substituents were prepared in multi-gram quantities from reaction of commercially available 1,1 -dichloro-1 -silacyclobutane and 1 -chloro-1 -methyl-1 -silacyclobutane precursors with suitable Grignard reagents. Allyl-substituted AB and AB2 type monomers were prepared from treatment of intermediate diorganochlorosilanes and organodichlorosilanes, respectively, with allylmagnesium bromide. Selected structures of the monomers used in this work are represented in Figure 2. 

The platinum-catalyzed or thermal ring opening polymerization of the substituted silacyclobutane monomers yields linear polycarbosilanes. The platinum-catalyzed hydrosilations of the AB (allyl) and AB2 (diallyl) monomers yield linear and hyperbranched polycarbosilanes, respectively. A wide range of random copolymers are readily available from polymerization of mixtures of the silacyclobutane or AB and AB2 monomers. The preparation of random copolymers is often of interest for achieving desired physical properties. 

On the other hand, Finkelstein et al. reported that ADMET reaction of di(allyl, butenyl)silanes, silacyclobutanes and siloxanes as well as diallyltetramethyldisiloxanes... 

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