Silacyclobutanes 1,2-Disilacyclobutanes

Keywords ROMP, addition polymerization, silicon containing norbomenes, silacyclobutanes, disilacyclobutanes. 

Generally, only simple silenes having small groups (H, Me, CH2=CH) are obtained as transient species from the thermolysis of silacyclobutanes. In part this is due to the high temperatures (usually above 450°C) required for the ring cleavage. Substitution on the carbon atom adjacent to silicon in the ring can lead to carbon-substituted silenes. 1,3-Disilacyclobutanes do not readily revert to silenes under thermal conditions, but examples... 

A more useful thermolytic polymerization which produces linear polysilmethylenes is that of 1,3-disilacyclobutanes carried out in the liquid phase. Such polymerization of l,l,3,3,-tetramethyl-l,3-disilacyclobutane was reported first by Knoth [17] (eq. 7). This process was studied in some detail by Russian workers [18]. l,l,3,3-Tetramethyl-l,3-disila-cyclobutane is more thermally stable than 1,1-dimethyl-l-silacyclobutane. 

Anionic polymerization of 1,3-disilacyclobutanes also is possible. Solid KOH and alkali metal silanolates were mentioned as being effective by Russian authors [18, 19. 20]. However, alkyllithiums, which can initiate polymerization of silacyclobutanes (eq. 8) [21], do not initiate polymerization of 1,3-disilacyclobutanes [18, 22]. The problem is one of steric hindrance. 

The first conclusive evidence for the generation of silaethylenes (silenes) from silacyclobutanes came in 1967. If heated to 400 °C in the absence of trapping agents, ethylene and 1,3-disilacyclobutane resulted. Copyrolysis of a mixture of two silacyclobutanes with different substituents on silicon gave three 1,3-disilacyclobutanes (Schemes 81 and 82) (67CC864, 68JCS(B)419, 68JCS(B)1396). 

The laser flash photolysis of gaseous silacyclobutanes 2224 and 2325 and 1,3-disilacyclobutane 24 produced the transient silenes 25 (from 22 and 24) and 26 (from 23) as the major primary product. The silenes 25 and 26 were identified by their UV spectra with Xmax s=ss 260 nm. Rate constants for the decay processes of the transient silenes were also measured. 

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. 

Nucleophilic attack of carbanions on silicon in silacyclobutane rings results in breaking of the Si-C bond in the ring leading to the recovery of the carbanionic center. Disilacyclobutane 38 was polymerized by addition of alkyllithium as initiator in tetrahydrofuran (THF) at — 78 °G in the presence of hexamethylphosphoramide (HMPA) acting as an activator (Equation 14) <2000MI805>. 

Both of these reactions were expected to profit from relief of strain in the starting material and from a strain-free heterocycle in the product. Owing to the polar nature of the ylide it was desirable, however, to introduce at least 1 heteroatom into the cyclobutane ring in order to facilitate heterolytic cleavage of the system. The silacyclobutanes seemed to be an excellent choice, and, consequently, a project on ylide cleavage reaction of mono-and disilacyclobutanes was initiated. Although no stable cyclic pentaalkylphosphorane was obtained, it was possible to confirm the appearance of... 

The 1,3-dilithio-compounds exhibit "normal" reactivity with monofunctional reagents (Scheme 5). 1,1- and 1,2-dilithio-compounds do not always react in the expected way of a alkyllithium-compoimd [6]. 1,3-Disilacyclobutanes can be synthesized by the reaction of 18 with dichlorosilanes (e.g., 21, Scheme 5), illustrating the synthetic potential of these new building blocks for the preparation of silacyclobutanes. 

Both silacyclobutane and 1,1-dichlorosilacyclobutane have been shown by electron diffraction to be puckered to about the same extent as cyclobutane. The dihedral angle a is about 30 5°, and is much more than the 14° in l,l,3,3-tetrachloro-l,3-disilacyclobutane (Figure 1) (67DOK(177)1084). 

Synthesis of 1,3-Disilacyclobutanes, 1,3-Digermacyclo-butanes, and l-Germa-3-silacyclobutanes with New 1,3-Dimetallated Organoelement Building Blocks... 

Keywords 1,3-Disilacyclobutanes / l-Gemia-3-silacyclobutanes / 1,3-Digermacyclo-butanes / Bis(lithiomethyl)silanes... 

There is no synthetic route available for selective synthesis of unsymmetrically substituted 1,3-disilacyclobutanes, l-germa-3-silacyclobutanes, or 1,3-digermacyclobutanes. 

We present two crystal structures of 1,3-disilacyclobutanes and the structure of a 1-germa-3-silacyclobutane. The first examples of a homologous pair of 1,3-disila- and 1,3-digermacyclobutanes are shown in Figures 1 and 2... 

C. Strohmann, E. Wack, "Synthesis of 1,3-Disilacyclobutanes, 1,3-Digermacyclobutanes and I-Germa-3-silacyclobutanes with New 1,3-Dimetallated Organoelement Building Blocks" in Organosilicon Chemistry III From Molecules to Materials (Eds. N. Auner, J. Weis), Wiley-VCH, Weinheim, 1997, this volume p. 217. 

The pyrolysis of 1,1-disubstituted silacyclobutanes has been used as a method for the preparation of 1,1,3,3-tetrasubstituted 1,3-disilacyclobutanes183. Copyrolysis of two different 1,1-disubstituted silacyclobutanes yielded mixtures containing 1,3-disilacyclobutanes arising from all possible combinations of the two silenes present (equation 51). The preparative method failed in the case of cyclopentadienyl-substituted silacyclobutanes183, 184, presumably due to competing intramolecular reactions of the silene. The thermolysis of trimethylsilylcyclopentadiene may also proceed via a silene184. 

---