Silylenes formation

This silylene formation from 27 under mild conditions permits the synthesis of a variety of interesting carbo- and heterocycles, most of which are new types of compounds. The results are summarized in Schemes 5 and 6. The reactions with benzene and naphthalene represent the first examples of [2+1] cycloadditions of a silylene with aromatic C=C double bonds.59 623 The reactions with carbon disulfide and isocyanide (Scheme 6) are also of great interest because of their unusual reaction patterns.62b... 

The product 33 (Amax = 345 nm) was formed as a film (estimated by the optical density of the UV spectrum to be several tens of nanometres thick) on a target quartz plate, the surface of which had been treated with 10 wt.% NaOH solution, followed by dichlorodimethylsilane and finally LiAlH4 to afford a surface terminated with -OSiMe2H groups. On the basis of trapping experiments (to confirm the formation of silylenes) and photo-CVD in the presence of styrene (to confirm the absence of radical species), a mechanism based on silylene formation and insertion into the surface Si-H bond was shown to be likely, as outlined in Scheme 19. 

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. 

Insertion, addition and abstraction reactions of free silicon atoms can lead to the formation of silylenes (equation 9)1-5. Silylene formation for reaction, spectroscopic and... 

As mentioned in Section HD, an important publication bearing on the question of dichlorosilane-metal reactions appeared in 198856. Boudjouk and coworkers treated t-Bu2SiX2, X = Cl, Br and I, with lithium and ultrasound activation in the presence of Et3SiH. Good yields (ca 60%) of EtjSi—SiH(Bu-t)2 were obtained in all three cases. This is rather persuasive evidence in favor of silylene formation, since Si—H bond insertion is characteristic for silylenes and it is difficult to see how the disilane could arise from the reaction of Et3SiH with a silylenoid95. 

The formation of silacyclopentenes from dichlorosilanes, alkali metals and 1,3-dienes is sometimes taken as evidence for silylene formation. Lithium and ultrasound were used in one such study, in which R2Si moieties were intercepted by 2,3-dimethylbutadiene (equation 48) albeit in low yield the result was interpreted in terms of a silylene intermediate96. 

The reaction of a diol with di-terf-butyldichlorosilane [Scheme 3.118]223 in the presence of 1-hydroxy-benzotriazole (HOST) was the first method used to prepare di-fert-butylsilylene derivatives. Di-ferf-butylsilyl bisftrifluoromethanesulfo-nate),224 225 in the presence of a base such as 2,6-lutidine, effects silylene formation more rapidly and under milder conditions than the less reactive dichloride [Scheme 3.1I9].110... 

The Diels-Alder adduct formed by treating furan with hexafluorobut-2-yne has been subjected to a retro-Diels-Alder reaction acetylene is eliminated and the reaction provides a route to 3,4-bis(trifluoromethyl)furan. The corresponding cycloadduct (154) formed from l,l-dimethyl-2,5-diphenyl-sllacyclopentadiene and the butyne was decomposed in an attempt to generate dimethylsilylene by a symmetry-allowed cheletropic fragmentation (Scheme 50) 836 evidence for silylene formation could be obtained unless the decomposition was effected photochemically or in refluxing cumene, under which conditions the addition of tolan enabled dimethylsilylene to be trapped... 

Calculations suggest that the Si-C bond of MeSiH should be 3 pm longer than observed from its microwave spectrum, while that of allylsilane supports the skew isomer. 1 silanes and disilanes decompose with silylene formation, while IR photochemistry of alkyl and aryl silanes supports SiH formation first.The IR band... 

Three unusual and interesting examples of silylene formation from photolyses are shown in Scheme 54. In the first, Fink and coworkers photolyzed the trisilane 294 at 254 nm and produced the relatively stable silylene 295. This on further photolysis gave rise to the sterically crowded silacyclobutadiene 296 which was trapped with several reagents. In the second example Michl and coworkers photolyzed the matrix-isolated bis-azide 297 to form the cyclic silylene 298, and this on further photolysis at selected wavelengths, using matrices and low temperatures, isomerized to the silacyclopentadienes 299 and 300 and finally to the l-sUa-2,4-cyclopentadiene 301. Finally, Sakurai and coworkers were able to convert the trisUane 302 to the cyclic divinylsilylene 303. 

Basically the same methods known from the synthesis of classical metal-silyl complexes can also be applied to the preparation of low valent Si compounds. The procedures given here are summarized with the focus on silylene complexes These are a) reactions of appropriate metal anions with halosilanes, which are the most important methods for the formation of M-Si bonds. Alternatively, silyl... 

An interesting variant of metal-silicon bond formation is the combination of metal halides with silyl anions. Since silyl dianions are not available, only one metal-silicon bond can be formed directly. The silylene complexes are then accessible by subsequent reaction steps [113], An example of this approach is given by the reaction of cis-bistriethylphosphaneplatinumdichloride 25 with diphenylsilylli-thium, which yields, however, only dimeric platinadisilacyclosilanes 26a-c [114]. 

In 1971, a short communication was published [54] by Kumada and co-workers reporting the formation of di- and polysilanes from dihydrosilanes by the action of a platinum complex. Also the Wilkinson catalyst (Ph3P)3RhCl promotes hydrosilation. If no alkenes are present, formation of chain silanes occurs. A thorough analysis of the product distribution shows a high preference for polymers (without a catalyst, disproportionation reactions of the silanes prevail). Cross experiments indicate the formation of a silylene complex as intermediate and in solution, free silylenes could also be trapped by Et3SiH [55, 56],... 

Also the titanium hydride can react with PhSiH3 to form 63, followed by the extrusion of silylene again with formation of the hydride. 

Developments in the synthesis and characterization of stable silylenes (RiSi ) open a new route for the generation of silyl radicals. For example, dialkylsilylene 2 is monomeric and stable at 0 °C, whereas N-heterocyclic silylene 3 is stable at room temperature under anaerobic conditions. The reactions of silylene 3 with a variety of free radicals have been studied by product characterization, EPR spectroscopy, and DFT calculations (Reaction 3). EPR studies have shown the formation of several radical adducts 4, which represent a new type of neutral silyl radicals stabilized by delocalization. The products obtained by addition of 2,2,6,6-tetramethyl-l-piperidinyloxy (TEMPO) to silylenes 2 and 3 has been studied in some detail. ... 

Triphenylphosphonium ylide reacts with the silylene complex 93 which has a highly electrophilic silicon center, to give the corresponding cationic adduct 94 [115]. The lengthening of the PC bond indicates a loss of the double bond character of the ylide and corresponds to the formation of a tetrahedral silicon center with four covalent bonds (Scheme 28). 

The trapping of silacyclopentadienes has also been reported recently.115 Using the pyrolysis of 27, or photolysis or pyrolysis of 28, the formation of the silylene 29 was inferred. Further photolysis or thermolysis converted the silylene into silene 30, which could be photochemically isomer-... 

Grobe15 has described the pyrolysis of 1 -methyl-1 -vinyl- and 1,1 -diviny 1-1-silacyclobutanes 166 which led to the formation of methylvinylsilene and divinylsilene, respectively. Under the experimental conditions used, it was suggested that the silenes rearrange to exo-methylene- 1-silacyclo-propanes 167 which extrude methylsilylene or vinylsilylene, respectively. In support of this proposal, when the reactions were carried out in the presence of 2,3-dimethylbutadiene, the anticipated silylenes were trapped as their respective l-silacyclopent-3-enes 168. 

The disilene synthesis by the photolysis of linear trisilanes proceeds via initial formation of a silylene followed by its dimerization (Eq. 1). Disilene 1 has now become a common organometallic reagent. A detailed synthetic procedure employing photolysis of the corresponding linear trisilane is described in Inorganic Syntheses (Eq. 2).6... 

More recently, a new mode of cis-trans isomerization of a disilene has been suggested for the extremely hindered disilene 27. As will be detailed in Section VIII. B, 27 undergoes thermal dissociation into the corresponding silylenes. Monitoring the thermolysis of (Z)-27 at 50°C by H and 29Si NMR reveals a competitive formation of the isomerized ( >27 and benzosilacyclobutene 37, which is most likely formed by intramolecular insertion of silylene 36 into the C—H bond of the o-bis(trimethylsilyl)-methyl group (Scheme 3).22,59 This suggests the possible occurrence of cis-trans isomerization via a dissociation-association mechanism. 

As shown in Scheme 3, disilene 27 undergoes thermal dissociation into silylene 36 under very mild conditions (about 50°C).22,59 This represents the first example of such dissociation. The formation of 36 was confirmed... 

Secondary Photochemical Processes. While the nature of the primary photochemical step may be described as still uncertain, the nature of the subsequent secondary steps is best characterized as obscure. A previous trapping study during exhaustive irradiation (30) demonstrated that silylenes are formed somewhere along the line and implicated silyl radicals as well since the formation of Si-H bonds was observed, presumably by hydrogen atom abstraction. 

The first step in the polymerization is the electron transfer from sodium to dichlorosilane and the formation of the corresponding radical anion. The latter upon elimination of the chloride anion is transformed to the silyl radical. To fit the chain growth mechanism, the reactivities of the macromolecular radicals must be higher than the reactivities of the monomeric radicals. The latter after electron transfer and elimination of chloride anion could be transformed to the reactive silylenes. Thus, in principle, two or more mechanisms of chain growth are possible ... 

Polysilanes with alkoxy groups are more light sensitive than conventional polysilanes. They degrade rapidly in the presence of light in agreement with the facile formation of silylene from dialkoxydisilanes. Properties of these polymers are currently being investigated. 

The formation of silaneselone 57 was evidenced by the trapping reaction with mesitonitrile oxide leading to the corresponding cycloadduct 58 and was also supported by the observation of a remarkably downfield 29Si chemical shift (8Si = 174) indicative of the Si=Se double bond of 57. Although this direct selenation of silylene 55 with an equimolar amount of selenium was not reproducible, the use of excess amount of elemental selenium resulted in the formation of a new cyclic diselenide, diselenasilirane 59, as a stable compound (8Si = -44 and... 

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