Disilene

Polymerization ofiVIasked Disilenes. A novel approach, namely, the anionic polymerization of masked disilenes, has been used to synthesize a number of poly(dialkylsilanes) as well as the first dialkylamino substituted polysilanes (eq. 13) (111,112). The route is capable of providing monodisperse polymers with relatively high molecular weight M = lO" — 10 ), and holds promise of being a good method for the synthesis of alternating and block copolymers. 

Disilenes, containing the grouping >Si=Si<, can be isolated as thermally stable yellow or orange crystalline compounds provided that the substituents are sufficiently large to prevent... 

Rarely has any research area so continuously been in the focus of scientific interest as organosilicon chemistry. A rapidly increasing number of publications, review articles, and scientific conferences reflects this development. The initiating factor was the spectacular discovery of stable molecules with SiSi double bonds, disilaethenes (disilenes) by R. West and S. Masamune in 1981 and 1982 [1-7]. 

Recently, a variety of silylenes were generated and characterized by matrix isolation techniques. The observed loose donor adducts between silylenes and the matrix molecules (THF, CO) are only stable at very low temperatures. Melting of the matrix induces polymerization of the silylenes which proceeds through disilenes. However, 0->Si transfer reactions do not occur only in the case of 1-methyl-THF has an insertion of the silylene into the C —O bond been observed [155-158],... 

The chemistry of the disilenes (disilaethenes) has developed very rapidly since the discovery of stable compounds. It was an obvious challenge to explore also the possibility of a n-coordination of disilenes to transition metals. According to the Dewar-Chatt-Duncason bonding model, a high stability for a disilene complex should result. 

However, it was about 8 years after the first synthesis of tetramesityldisilene before stable coordination compounds became known. The main reason for this is the kinetic stabilization of the known disilenes by bulky substituents, which effectively prevents the coordination of the double bond to a metal fragment. Thus, a direct coordination of stable disilenes appeared to be reasonable only if metals with very low coordination numbers were used. 

Evidence for a 7t-coordination was obtained through the reaction of various disilenes with Hg(OCOCF3)2, a reaction which leads regioselectively to bis(tri-fluoracetyl)disilanes. A disilene n-complex (79), which is stable up to — 50 °C, could be identified as an intermediate by spectroscopic methods. 

A particularly elegant pathway to stable coordination compounds of disilenes has been found with the reaction of the Pt-phosphine complex 80 with 81 ab, which... 

Very recently, synthesis and structure of molybdenum and tungsten complexes of the relatively unhindered disilene Si2Me4 were reported. The x-ray structure of 84 shows a metallacyclosilane structure with W — Si = 2.606(2) A and Si —Si = 2.260(3) A. The W — Si bond length is within the range of various estimates of the Si and W covalent radii and the Si —Si distance falls midway between the expected values for a single (2.35 A) and a double bond (2.14 A) (Fig. 13). 

Kira, Mitsuo, Progress in the Chemistry of Stable Disilenes. 

R R M were prepared by reduction of the dibromides R R MBr2 with lithium naphthalenide (method A), by thermolysis of a disilene (method B), and the ligand exchange of divalent group 14 element species (method C). In all cases except for the synthesis of Tip2PbS4, exclusively tetrasulfides R R MS4... 

Using CO-saturated hydrocarbon matrices, Pearsall and West" photolyzed sily-lene precursors at 77 K and monitored CO coordination to the silylenes by UV-vis spectroscopy (Scheme 13). Bis(trimethylsilyl)silanes 44a-c or SifiMcji were irradiated at 254 nm to create silylenes 45a-d, which reacted with CO, causing new peaks to ca. 290 and 350 nm, which were attributed to complex 46a-d, a resonance structure of silaketene 47a-d. Silylene adducts form fairly weak bonds, as seen by warming of the matrices. In the case of silylene adducts where one R = Mes, the CO dissociates and the corresponding disilene 48a-c peaks in the UV-vis spectra observed upon warming (R2 = Me most likely produced silane rings Si, Me6. etc.). 

The reactivity of a silylene 103 with isocyanides was probed by Okazaki et a . in 1997. When disilene 102 is heated to 60 C in THF or CfiDf, it dissociates... 

After decades of unsuccessful attempted syntheses, Gusel nikov and Flowers in 1967 reported the first compelling evidence for the existence of silenes, compounds containing a double bond between silicon and carbon. This initiated a renewed interest in the synthesis and behavior of stable silenes,8 disilenes,b iminosilanes,c phosphasilenesd and their heavier homologs. 

The first silaethenes and disilenes were synthesized at the beginning of the 1980s, and the first iminosilanes were reported in the mid-1980s. The difference in the electronegativity between Si (1.8) and N (3.0) gives the iminosilanes an ylidic nature, which makes them susceptible to oligomerization and dimerization. 

Besides disilenes, the metastable phosphasilenes 15 (and the analogous arsasilenes) are the only known compounds with (p-p) 7r-bonds between main-group elements that react with P4 under relatively mild conditions (40oC).10b 33 Thus 15a reacts with P4 in a 2 1 molar ratio to build up the SiP3 butterfly-like compound 22 (Eq. 6). 

Analogous adducts, generated by the reaction of cyanides with disilenes, are also known.44... 

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