Dimethylsilylene

POLYETHERS - TETRAHYDROFURAN AND OXETANE POLYTffiRS] (Vol 19) Poly[oxy(dimethylsilylene)] [9016-00-6]... 

Relatively few examples are known which utilize an oxidative addition reaction of metal hydrides to necessarily low valent silicon compounds. Seyfert s hexame-thylsilirane (31) could be used as a source of dimethylsilylene to perform an... 

The structurally simplest silicon reagent that has been used to reduce sulphoxides is the carbene analog, dimethylsilylene (Me2Si )29. This molecule was used as a mechanistic probe and did not appear to be useful synthetically. Other silanes that have been used to reduce sulphoxides include iodotrimethylsilane, which is selective but unstable, and chlorotrimethylsilane in the presence of sodium iodide, which is easy to use, but is unselective since it cleaves esters, lactones and ethers it also converts alcohols into iodides. To circumvent these complications, Olah30 has developed the use of methyltrichlorosilane, again in the presence of sodium iodide, in dry acetonitrile (equation 8). A standard range of sulphoxides was reduced under mild conditions, with yields between 80 and 95% and with a simple workup process. The mechanism for the reaction is probably very similar to that given in equation (6), if the tricoordinate boron atoms in this reaction scheme are replaced... 

Surprisingly, the most highly strained and, at first glance, least stable hexa-methyl[3]pericyclinosilane 74 could be obtained by pyrolytic threefold extrusion of dimethylsilylene from the ethynyl-expanded permethylcyclohexasilane 73 under drastic conditions in strikingly good yield (68%) (Scheme 14) [24]. 

In a detailed study of the IR transitions of matrix-isolated 1-methylsilene and its isomer dimethylsilylene the structures of these species154 have been clearly defined. 

Research in polysilane polymers grew slowly at first after this reawakening. But within the past few years, both the unusual scientific interest and the technological possibilities of the polysilanes have been recognized, and activity in this field has increased sharply. Commercial manufacture of both poly dimethylsilylene) and "polysilastyrene" is now being carried out in Japan, so that these two polymers are readily available in quantity. 

The mechanism to be dicussed resembles that given by Weber [9] for the reaction of dimethylsilylene with protic substrates. Zwitterionic compounds, formed by interaction of the lone pair of the substrate with the vacant p orbital at silicon, are regarded as intermediates. 

Other organosilicon polymer precursors for ceramics have either been prepared or improved by means of transition metal complex-catalyzed chemistry. For instance, the Nicalon silicon carbide-based ceramic fibers are fabricated from a polycarbosilane that is produced by thermal rearrangement of poly(dimethylsilylene) [18]. The CH3(H)SiCH2 group is the major constituent of this polycarbosilane. 

Photolysis of acyldisilanes at A > 360 nm (103,104) was shown, based on trapping experiments, to yield both silenes 22 and the isomeric siloxy-carbenes 23, but with polysilylacylsilanes only silenes 24 are formed, as shown by trapping experiments and NMR spectroscopy (104,122-124) (see Scheme 4). These silenes react conventionally with alcohols, 2,3-dimethylbutadiene (with one or two giving some evidence of minor amounts of ene-like products), and in a [2 + 2] manner with phenyl-propyne. Ketones, however, do not react cleanly. Perhaps the most unusual behavior of this family of silenes is their exclusive head-to-head dimerization as described in Section V. More recently it has been found that these silenes undergo thermal [2 + 2] reactions with butadiene itself (with minor amounts of the [2 + 4] adduct) and with styrene and vinyl-naphthalene. Also, it has been found that a dimethylsilylene precursor will... 

A rearrangement of some interest involving a very simple silene is that of the interconversion of 1-methylsilene and dimethylsilylene [Eq. (17)]. 

Evidently Seyferth was the first to recognize the new opportunities of such variations in organosilicon chemistry.16 He generated dimethylsilylene in the presence of ketones and phosphines. According to authors hypothesis multi-step transformation takes place in the reaction (Scheme 2). 

In this chapter, the general term polysilanes will be used, and homopolymers and co-polymers will be named according to the examples of 1 and 2 above as poly(methyl- -propylsilylene) and poly(dimethylsilylene-c -di- -propylsilylene), respectively. 

Photochemical behaviour of compounds 83-86 [33] in the gas phase has been reported, in order to distinguish between silyl radical and silylene formation. Photolysis of the noncyclic precursors 83 and 84 gave products derived from silyl radicals, which come from a direct Si—Si bond homolysis, with a little evidence of silylene formation. In contrast, dimethylsilylene (Mc2Si ) was observed as a direct photoproduct from the cyclic precursors 85 and 86. The reaction sequence including a Sni step shown in Scheme 6.18 for the formation of dimethylsilylene was proposed to explain the different observations for cyclic and noncyclic systems. 

Pyrolysis of polysilanes played an important role in the discovery of silylene reactions. Through the pyrolysis of alkoxydisilanes in the presence of diphenylacety-lene, Atwell and Weynberg obtained a product regarded as a dimer of dimethylsilylene adduct. 1,2-Shift of a methoxy group in disilanes takes place under relatively mild conditions (Scheme 14.1). ... 

In contrast to the somewhat complicated thermal behavior of siUranes, photolysis of silirene (4) readily leads to the loss of dimethylsilylene (Scheme 14.5... 

In spite of its long-assumed intermediacy in several reactions, no carbon-substituted silylene was directly observed for many years. In 1979, however, Drahnak et al. detected a broad ultraviolet (UV) absorption band (3-niax = 450 nm) after the photolysis of dodecamethylcyclohexasilane (6) in 3-methylpentane. This band was assigned to dimethylsilylene (5). Many different approaches to this intermediate, either photochemicaUy or thermally, were examined (Scheme 14.6). ... 

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