Reactive intermediates silylenes

In the course of this development, knowledge about low valent (in the sense of formal low oxidation states) reactive intermediates has significantly increased [26-30]. On the basis of numerous direct observations of silylenes (silanediyles), e.g., by matrix isolation techniques, the physical data and reactivities of these intermediates are now precisely known [31], The number of kinetic studies and theoretical articles on reactive intermediates of silicon is still continuously growing... 

Very recently, the coordination chemistry of low valent silicon ligands has been established as an independent, rapidly expanding research area. With the discovery of stable coordination compounds of silylenes [35-38], a major breakthrough was achieved. Within a short time a variety of stable complexes with a surprising diversity of structural elements was realized. Besides neutral coordination compounds (A, B) [35, 36, 38], and cationic compounds (C) [37], also cyclic bissilylene complexes (D) [39,40] exist. A common feature of the above-mentioned compounds is the coordination of an additional stabilizing base (solvent) to the silicon. However, base-free silylene complexes (A) are also accessible as reactive intermediates at low temperatures. 

Donor free silylene complexes are reactive intermediates in a variety of chemical reactions. In many cases, evidence for the coordinated silylenes involved has been obtained indirectly by means of trapping experiments [49-60]. 

With the stable donor adducts of silylene complexes, valuable model compounds are now available for reactive intermediates which otherwise cannot be observed directly. For example, a side reaction occurring in the hydrosilation process [61 -63], is the dehydrogenative coupling of silanes to disilanes. This reaction could be explained in terms of a silylene transfer reaction with a coordinated silylene as the key intermediate. 

A variety of further reactions are known in which silylenes are transferred to a substrate in the presence of a transition-metal catalyst. In most cases, silylene complexes can now be identified as reactive intermediates. For a detailed discussion refer to Sects. 2.5.3 and 2.5.4. 

The dehydrogenative coupling of silanes does not stop at the stage of disilanes in the coordination sphere of early transition metals like Zr and Hf, but chain polymers of low molecular weight are formed. As reactive intermediates in this reaction, silylene complexes are also assumed. However, alternative mechanisms have been discussed (sect. 2.5.4). 

At this stage of the discussion it is obvious that stable donor adducts of silylene complexes show a modified silylene reactivity and can thus be considered as model compounds for otherwise inaccessible reactive intermediates. 

Base-free uncomplexed silylene complexes are so far only known as reactive intermediates which are generated at low temperatures and trapped by suitable reagents. Several publications related to this subject are known, but most of the work is now summarized in review articles [95]. 

From this result it has been concluded that the reactive intermediate is an insertion product with a structure similar to that of the nickel compound 34 and not a silylene complex as postulated in an earlier publication.36 The molecular structures of 34 and 35 are presented in Fig. 6. 

The photochemical cleavage of Si-Si bonds of cyclotetrasilanes has been reported to generate several reactive intermediates. For example, Nagai and co-workers reported that silylene and cyclotrisilane are generated during the photolysis of a cyclotetrasilane with a folded structure.73 Shizuka, Nagai, West, and co-workers reported that the photolysis of planar cyclotetrasilanes gives two molecules of disilene.74... 

Photochemical transformations of cyclic and short chain polysilane oligomers have been intensively investigated (39). Irradiation of these materials in the presence of trapping reagents, such as silanes or alcohols, has suggested that substituted silylenes and silyl radicals are primary reactive intermediates. The former have been... 

Divalent silylenes, silicon congeners of carbenes, are well-established reactive intermediates, whose chemistry has been... 

The photolysis of cyclic polysilanes results in ring contraction with concomitant extrusion of a silylene fragment. Although the formation of two reactive intermediates potentially complicates mechanisms for product formation, it has provided a useful method for the synthesis of both unstable and stable disilenes... 

Carbene type radical anions have been often postulated as reactive intermediates. An tetratrimethylsilyl substituted silylene (16) was reduced with several alkali metals in dimethoxyethane to yield a persistent silylene radical anion.169... 

The laser flash photolysis technique allows the observation of rapid reactions and determines the lifetime of reactive intermediates. Thus, it is not surprising that this method was used extensively to obtain information about the formation, stability, and reactivity of silylene-Lewis base complexes. 

The photochemistry of organosilicon compounds has been extensively studied, since many types of interesting reactive intermediates such as silylenes, silenes, disilenes and silyl... 

The inversion in selectivity between the insertion into an H—Si bond favored by the intermediate in the dehalogenation and the addition to a triple bond favored by the intermediate formed upon silirane pyrolysis suggests that different reactive intermediates are formed. Since free silylene is implicated in the silirane pyrolysis, the lithium-induced deiodination probably involves a silylenoid such as a complex of the silylene with Lil or with THF. For a further discussion of this possibility, see Section II.E. 

Reaction of 177-phosphirene with silylene 39 results in 2,3-clihydro-l,3-phosphasilete or l 2-phosphasiletene via a phospha-4-silabicyclo[1.1.0]butane reactive intermediate (Scheme 35) <2004AGE3474>. 

In addition to participating in [2 + l]-cycloaddition reactions, divalent reactive intermediates can form ylides in the presence of carbonyl or other Lewis basic functionalities.108 These ylides participate in cycloaddition or other pericyclic reactions to furnish products with dramatically increased complexity. While carbenes (or metal carbenoids) are well known to participate in these pericyclic reactions, silylenes, in contrast, were reported to react with aldehydes or ketones to form cyclic siloxanes109,110 or enoxysilanes.111,112 Reaction of silylene with an a,p-unsaturated ester was known to produce an oxasilacyclopentene.109,113,114 By forming a silver silylenoid reactive intermediate, Woerpel and coworkers enabled involvement of divalent silylenes in pericyclic reactions involving silacarbonyl ylides115 to afford synthetically useful products.82,116,117... 

The nitrogen analog of oxasilacyclopropane, azasilacyclopropane (or silaziridine), has been postulated as a potential reactive intermediate in photochemical- or thermal processes involving silylenes. These strained A-heterocycles can be accessed from the... 

Divalent state stabilization energies are not easy to come by, as they require knowledge of both the first and second BDE, and the reactive intermediates MR2 are not trivially characterized. Quantum mechanical studies are certainly ahead of experiment in this area, and we can combine the results of two separate studies, one by Coolidge and Borden (109) and the other by Luke et al. (88), to assemble a small list of DSSEs for monosubstituted carbenes and silylenes. Specifically, Coolidge and Borden determined the effects of substituents, X, on the stability of methyl and silyl radicals through determination of the heat of reaction... 

The field of silicon chemistry has enjoyed a very fast development in the last two decades with many novel significant discoveries being made [1]. Of particular interest in the context of this paper is the synthesis and characterization of a variety of reactive intermediates such as silylenes [2] and compounds with multiple bonds to silicon [3]. These exciting developments were occurring at the time when theory, in particular ab initio molecular orbital theory, was reaching "maturity" i.e. at the time when these methods could be used routinely to calculate reliably the properties of a variety of molecules, including silicon compounds [4]. 

Summary The formation of reactive intermediates via dehalogenation of chlorosilanes was investigated by using lithium powder and sonication. Whereas in the absence of a diene substrate mainly polysilanes are obtained, reactions with, e.g., dimethylbutadiene, yield the corresponding cycloaddition products, indicating silylenes and silaethenes as intermediates. 

The presence of very reactive intermediates is obvious from the formation of a variety of non identified by-products. However, the reactions with DMB as the diene substrate provide evidence for the formation of silylene and silaethene intermediates. Further investigations are necessary to elucidate the reaction pathways. 

Two-coordinate X2Si species are called silylenes (by analogy with carbenes) and are best described as reactive intermediates which may be trapped and observed spectroscopically at low temperatures in an inert matrix, but most of which have short lifetimes under normal conditions. The structures and electroiuc coirfigurations of X2Si (X = H, F,... 

A mixture of reactive intermediates, including l,l-dimethyl-3,3-bis(trimethylsilyl)-Tsilaallene and dimethylsilylene, along with l,l-dimethyl-2,3-bis(trimethylsilyl)-l-silacyclopropene 86 were formed and detected from the direct irradiation of [(trimethylsilyl)ethynyl]pentamethyldisilane in hydrocarbon solution (Equation 21). These species were detected and identified using laser flash photolysis. They were trapped as their methanol adducts in steady-state irradiation experiments. Steady-state irradiation in the presence of methanol affords MeOH-addition products which are consistent with the formation of the silaallene, silacyclopropene, and silylene along with bis(trimethylsilyl) acetylene as the major product <1997JA466>. 

Gaspar, P. P. In Reactive Intermediates Jones, M. Moss, R., Eds. Wiley New York. Keeping up with silylene chemistry is made easy by these critical comprehensive reviews every two or three years by Gaspar. 

Reactive Intermediates in Photolysis. Silylenes and Silyl Radicals, The results of exhaustive irradiation of substituted high-molecular-weight polysilanes in solution suggest that at least two reactive intermediates (i.e., silylenes and silyl radicals) are produced. On the basis of this information, a tentative reaction scheme (Scheme V) is proposed. 

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