Cyclobutanes 1.3- disila

Conlin and co-workers have also studied the fragmentation of a siletane (silacyclobutane). In this case, both the E- and Z-isomers of 1,1,2,3-tetra-methylsilane 45 were prepared and thermolyzed (Scheme 8).144 Both E-and Z-isomers of 45 led to the same products in slightly different ratios the major products were propene with silene 46, and E- and Z-2-butenes with silene 47. Silene formation was inferred from detection of the disila-cyclobutane products. During these processes, the stereochemical integrity of the compounds was largely preserved. 

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. 

Silylene 85 reacts also with imines to give 1 1 adducts or 2 1 adducts, depending on the imine used and on the reaction conditions <1998QM1378>. Reaction of silylene 85 with pyridine gave the l-aza-2,3-disila-cyclobutane 141,... 

Table 11. Synthetic route for l,l,3,3-Tetramethyl-l,3-disila-2(methylene)-cyclobutane...

The syntheses of l,l,3,3-tetramethyl-l,3-trisilacyclopentene 87 and of 1,1,3,3-tetramethyl-l,3-disila-4-methylene-cyclobutane 88 are described in Section C. By adding Br2 or HBr, the ring system is cleaved as a first reaction step (35). For syntheses of carbosilanes, information on the corresponding six- or seven-membered ring systems was required. Therefore, the synthesis of the compounds 179 and 180 was investigated (54). 

The photolysis of l-trimethylsilylethynyl-l,l-diphenyltrimethyldisilane (63) is of considerable interest, because of a novel 1,2-disilacyclobutene is produced (107). When a benzene solution of 63 is photolyzed by irradiation with a low-pressure mercury lamp bearing a Vycor filter, yellow crystals of 1,1,2, 2-tetraphenyl-3,4-bis[bis(trimethylsilyl)methylene]-1,2-disila-cyclobutane (64) are formed. This can be readily isolated by evaporation of the solvent from the resulting mixture. The formation of 64 can be explained in terms of head-to-head dimerization of transiently formed sila-propadiene. 

SCHEME 18.9 Synthesis of polysilaethylene by Pt-catalyzed ring opening polymerization (ROP) of l,3-disila-l,l,3,3-tetrachloro cyclobutane. ... 

This does not, however, explain the preferential formation of cyclic carbo-silanes, such as Si-hexamethylcyclocarbosilane. The isolation of 1,3-disila-cyclobutane (compound 2, Table III) appears to be especially significant in this connection. Its formation may be thought of as associated with Eq. (2) ... 

Sila- and 1,3-disila cyclobutanes possess an increased reactivity because of their ring strain [106] and can be transformed into polycarbosilanes by ring opening polymerization using either thermal activation [107], anionic polymerization with organolithium compounds [108], or mostly platinum [109] or other late transition metal catalysts [110]. 

Setzt man Trimethylphosphinmethylen 264 mit Dimethyldichlorsilan 269 im Molverhaltnis 6 2 ura, so erhalt man nach folgender Summen-gleichung Tetramethylphosphoniumchlorid 263 und das cyclische Bis-ylid 1.1.3.3.-Tetramethyl-2.4-bis-trimethylphosphoranyliden-1.3-disila-cyclobutan 270... 

---