Silylenes, intramolecular insertion

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. 

The rearrangements of silylenes, like those of carbenes, can involve H shifts and the shifts of C—C bonds (intramolecular insertion and ring expansion) or cyclization by intramolecular addition to C=C jr-bonds5. The mechanism discovered by Barton for... 

Stepwise addition of two pyridine molecules to Ph-Si , whose reversibility was established in collision-induced dissociation (CID) experiments, seems to be due to the formation of one bond at a time, the monovalent silicon cation reacting as a Lewis acid. That two, but no more than two, pyridine molecules are accepted by Ph-Si points to the silicon atom as the site of addition. In this scenario, addition of the first pyridine forms a distonic silylene. That this is a plausible process is indicated by the reaction of the parent silanetriyl cation H-Si with diethylamine HNEt2 CID of the product ion established its structure as a four-membered ring whose most likely source is a two-step process formation of a silylene intermediate by a Lewis acid-Lewis base reaction followed by intramolecular insertion of the silylene into a methyl C-H bond. Three bonds are formed in a single reactive encounter, but the stepwise process is much more likely than the more interesting concerted reaction. 

Incidentally, the silylene SiR 2, generated by dehalogenation of R 2SiX2 (X = halogen), does not dimerize due to steric reasons (R = R in 1 is not possible). Instead, it decomposes by an intramolecular insertion of the silylene Si atom into a CH bond of supersilyl. 

The pathways to compounds 2-12 probably require that in the first step the silylene 1 behaves towards MX as a nucleophile yielding an intermolecular donor-acceptor adduct B, followed by subsequent intramolecular insertion of either 1 into the M-X bond or X into the Si-M bond (Scheme 7), resulting in C. 

Insertion reactions of silylene into a number of single bonds have been observed. The bonds include Si—O, Si N, Si H, Si halogen, strained C O, Si Si, O—H, N—H, and C—H (intramolecular only). Insertion into an X H bond can be initiated by the formation of silaylide (50) with the donation of a pair of electrons from a heteroatom X to form a bond to a divalent sihcon atom (Scheme 14.26). 

When di(t-butyl)silylene 321, generated in a 3-methylpentane glass at 77 K or in an argon matrix at 10 K by photolysis of a precursor bis-azide, was irradiated with 500-nm light, intramolecular C—H insertion occurred yielding the silacyclopropane 322 (equation 26)161. 

Insight into the mechanism for the formation of allylic disilane 201 and silane 202 was obtained from a series of control experiments (Scheme 7.57). Submission of allylic silane 202 to reaction conditions did not produce disilane 201. Woerpel and coworkers interpreted this result to indicate that disilane formation does not occur through subsequent silylene insertions. Crossover experiments established that no dissociation of the alkoxy group occurs during the reaction to suggest that silylene insertion is intramolecular. 

Quantum-chemical calculations have been used to probe all the characteristic chemical reactions of CAs (at least in the case of silylenes and germylenes). The theoretical studies cover intramolecular rearrangements, insertions into 0-bonds, additions to double and triple bonds and dimerizations. Note that experimental data on the mechanisms of these reactions are still scarce and the results of theoretical studies are needed to understand the main trends in the reactivity of germylenes, stannylenes and plumbylenes. 

Silylenes only undergo rearrangements at elevated temperatures, and the high barriers for their intramolecular reactions have made possible the convenient study of their intermolecular chemistry. Alkylcarbenes undergo rapid H-shifts, converting them to olefins even at room temperature, and thus the intermolecular insertions and additions of alkyl carbenes have remained rather obscure. 

Whether carbene, germylene, and silylene are justifiable terms for stabilized versions of the reactive species is a very debatable question [28], However, for synthetic chemists, the most important issue is to know whether these stable species feature the reactivity of transient carbenes. Formation of azaphospholidines upon thermolysis (intramolecular carbene insertion into a carbon-hydrogen bond), cyclopropanation reactions, and [l + l]-addition to isocyanides giving keteneimines... 

We would like to suggest, that an attack of a chlorine takes place first at one of the silicon atoms of the tri- or tetrasilane under discussion to form a pentacoordinated silicon. In a second step, a silylene is generated which inserts into a silicon-chlorine bond of the disilane while the silicon skeleton of the oligosilane is shortened by one silicon atom. The source of the attacking chlorine is not yet known An intramolecular reaction with a chlorine of a chloromethylsilyl-sidechain and the central silicon might occur, or remaining traces of the aluminium trichloride, used for the chlorination, might induce the reaction. This is possible since only catalytic amounts would be necessary to cause the decomposition. 

This mechanism is quite general for this substitution reaction in transition metal hydride-carbonyl complexes [52]. It is also known for intramolecular oxidative addition of a C-H bond [53], heterobimetallic elimination of methane [54], insertion of olefins [55], silylenes [56], and CO [57] into M-H bonds, extmsion of CO from metal-formyl complexes [11] and coenzyme B12- dependent rearrangements [58]. Likewise, the reduction of alkyl halides by metal hydrides often proceeds according to the ATC mechanism with both H-atom and halogen-atom transfer in the propagation steps [4, 53]. 

The r-butyl analog showed additional reactions, since the silylene which was formed in part during the photolysis was inserted intramolecularly into a C-H bond to give a silirane, which in turn was unambiguously trapped with labelled alcohol (equation 103). 

The mechanism of catalysis involves the insertion of the vinyl-silicon dienes into M-H and M-Si bond, followed by p-Si and P-H elimination, to yield ethene and two isomeric trans and gem- bis(vinylsilyl)ethenes respectively. In the presence of [RuCl2(CO)3] as a catalyst, rraw5-E-CH2=CH[Si]CH=CH[Si]CH=CH2 is exclusively formed [23]. Under optimum conditions the well-defined trans-iaciic (A) silylene (siloxylene)-vinylene (M = 6350-8650 M /Mn = 1.12-1.16) [23] and silazanylene-vinylene (Mw = 813) [18] polymers can be prepared. On the other hand, gem- dimeric products can be effectively formed with (B) when the reaction is catalyzed by [(cod)RuX]2 (where X = Cl, OSiMes) but finally intramolecular ring closure takes place to yield respective cyclotetrasiloxane, cyclotetrasilazane and cyclohexacarbosilanes [22]. 

---