Insertion of silylene

Insertion of Silylenes into Metal-Hydrogen Bonds... 

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. 

Insertion into Si H and Si Si Bonds. Silylenes, generated by thermolysis of cyclotrisilanes, inserted into the Si—Cl or Si—H bonds of monosilane to yield a variety of disilanes, which could be further functionalized. In contrast to carbenes, the insertion of silylenes into C—H bonds has not been observed. However, the insertion into Si—H bonds has been studied extensively. The occurrence of direct insertion has been indicated by formation of nongeminate homocoupling products. ... 

Theoretical calculations suggested three steps for the insertion of silylene into a Si—H bond, that is (a) the formation of the complex with the interaction of the hydrogen atom of a silane and the empty orbital of a silylene (b) the formation... 

Not surprising, the insertion of silylene into the S-H bond of hydrogen sulfide shows qualitatively the same features as the reaction with water. However, the complexation energy (36.0 kJ mol-1) and the activation energy (56.1 kJ moL1) of the rearrangement are lower at the MP4SDTQ/6-31G // 3-21G level.14 A compi increases to 55.3 kj mol-1 when calculated at the MP2/CEP-31g(2d,p)//MP2/CEP-31g(2d,p) level.83... 

Kinetic isotope effects (KIE) of silylene insertion reactions were determined in solution as well as in the gas phase. Steele and Weber39 found KIEs between 1.8 and 2.3 for the insertion of silylene 13 into various alcohols no significant variation of these values was observed with a change of solvent from cyclohexane to THF. These results are consistent with two mechanisms (Scheme 8), both of them proceeding via a nonlinear or triangular transition state. Mechanism B, which is favored over mechanism A by the authors, is in agreement with theoretical results, which, in general,... 

Tamao et al,83 found that a higher coordinated silylene 119 can be formed from penta-coordinated silane 118 (Scheme 31). Warming a solution of 118 in toluene or dimethylformamide in the presence of diphenylacetylene or 2,3-dimethyl-l,3-butadiene resulted in the formation of silylene-trapping products 120 and 121. Interestingly, no 1 1 reaction product between the silylene and the acetylene was isolated. Thus, it must be concluded that the insertion of silylene 119 into a Si-C bond of initially formed silacyclopro-pene is faster than the addition to the triple bond of the acetylene so that the silacyclopropene cannot be isolated under the reaction conditions. 

The silylene Tbt(Ph)Si reacts with selenium to give the 6-membered heterocycle 30 through insertion of silylene, once the 5-membered derivative 31 has been formed... 

A more complex reaction sequence may be responsible for the formation of the products of formal insertion of silylenes into the H—C bonds of carbaldimines110 116. As shown in equation 56, an initial Lewis base or TT-bond adduct can undergo rearrangement to both of the products observed from reactions of diarylsilylenes and pyridine-2-carbaldimines. A common intermediate is suggested by the thermal isomerization of the insertion product to the cycloadduct which can be the exclusive product from addition of a silylene to a 2,2/-bipyridyl (see Section ni.B)117. 

While investigating trisubstituted allylic ethers, Woerpel and coworkers observed insertion of silylene into the allylic C-O bond (Scheme 7.56).136 The product formation was dependent on the identity of the catalyst. Silver trifluor-oacetate promoted the formation of allylic disilane 201, whereas copper(I) triflate... 

Woerpel and coworkers interpreted the results of these mechanistic experiments as evidence that the insertion of silylene into the C-O bond occurs through a [1,2]-Stevens rearrangement of oxonium ylide 198 and that a competitive [2,3]-sigmatropic rearrangement of 198 could account for allylic transposition. 

These results clearly indicate that the insertion of silylene species into conjugated dienes proceeds via a 1,2-addition followed by rearrangement to the silacyclopentene. 

In recoil-atom experiments, disilane has been found to be five times more reactive than silane (42). Statistically, a reactivity ratio of 3 2 would be expected. Also, trisilane has been found to produce n-tetra-silane and i-tetrasilane in the ratio 4.4-5.2 l, much higher than the value expected on the basis of statistical considerations (14). These results seem to suggest the competing insertion of silylene into the Si—Si bonds in these reactions. However, the fact that Si—H bond strengths in different silanes vary would also result in a statistically unexpected ratio (51). 

Silver phosphonate was also used as catalyst for silylene transfer to 1-heteroatom-substituted alkynes, resulting in alkoxy-, amino-, and alkylthiolate-substituted silacyclopropenes 61. Silver-catalyzed silylene transfer to 1-halo- and 1-sulfonyl-substituted alkynes led to insertion of silylene into the carbon-heteroatom bond. Attempts of Cu-catalyzed... 

Complicated reactions occur in the absence of trapping agents. Many signals can be observed besides those of aminosilanes in the Si spectrum, possibly oligosilanes by insertion of silylenes into Si-N bonds [8]. More investigations are necessary. 

Insertion of silylenes into C-Cl bond forms methylchlorosilanes Scheme 1. The silylene-mechanism for the formation of methylchlorosilanes. 

A number of reviews have focused on the insertion of silylenes into a wide range of a bonds ... 

LFP studies of the N-H insertion of silylenes have shown that the reaction proceeds by formation of an acid - base complex at almost a diffusion rate with a noticeable dependence of the steric hindrance of the amine. Rhodium-catalysed hydrosilyation of ketones by diphenylsilane has been found to occur via the intermediacy of the rhodium silylene (158). ... 

As represented in equation (78), the insertion of a silylene, SiRR, into an X-Y bond gives SiRR XY as a product. The insertion of silylenes into single bonds is the most well-established type of reactions for these divalent species. It is the only silylene process which leads directly to stable products in a single step. On the other hand, the immediate reaction products of either polymerization or addition reactions of silylenes are supposed to be intermediates, which may either undergo further reactive encounters or require subsequent rearrangements to give the final stable products. 

The possible insertion of silylenes into Si-Si bonds is still a question of debate with evidence for and against its existence. In 1970, Bowrey and Purnell observed that for SiHj insertion into Si,H the yield ratio, n-tetra-silane/isotetrasilane, lies between the limits 4.4 and 5.2, significantly above the statistical value of 3. This result suggested that some insertion into the Si-Si bond, which can only give n-tetrasilane as shown in equation (88), was... 

Even in the presence of such negative evidence, the insertion of silylenes into Si-Si bonds is not necessarily an absolutely impossible reaction. The breeding modes of producing silylenes from trisilanes and polysilanes as observed by Sakurai and coworkers and by Ishikawa, Kumada, and coworkers are actually the reverse of such Si-Si bond insertion reactions, and they point to the possibility of a transition state as shown in equation (50). 

To date no experimental evidence is available regarding the insertion of silylene into the H2 molecule, however, according to the theoretical estimates in [75], the lower limit of the activation barrier amounts to 5 kcal/mol. Other theoretical estimates [70, 76] are consistent with this value (see Table 8.4). An analysis of structural alterations in the course of the reaction SiH2 H- H2 ->SiH4 has revealed that they are practically identical to those found in the reaction... 

Insertion of silylene in the C-O bond of propargylic oxiranes followed by reaction of the intermediate silaoxetane with aldehydes has been shown to yield eight-membered ring bent allene in a diastereoselective manner (Scheme 7). ... 

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