Silylenes as intermediates

Most recently an example of a mfheninm catalyzed selective addition of primary silanes to alkenes was shown to involve mtheninm silylenes as intermediates (Scheme 5). A remarkable feature of this mechaiusm is, that the insertion of the alkene does not occur either into a metal-hydride or... 

Althoug the mercury-sensitized photolysis of the methylsilanes was consistent with the Si-H bond scission as the primary step as shown in equation (32) to give H and CHjSiHj radicals which combined to give the final products in unit quantum yields, other studies indicated the existence of silylenes as intermediates. The vacuum uv photolysis of CHjSiHj produced SiHMe as shown in equation (33). The major products from the pyrolysis of CHjSiHj are Hj... 

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. 

Investigations of silicon-metal systems are of fundamental interest, since stable coordination compounds with low valent silicon are still rare [64], and furthermore, silicon transition-metal complexes have a high potential for technical applications. For instance, coordination compounds of Ti, Zr, and Hf are effective catalysts for the polymerization of silanes to oligomeric chain-silanes. The mechanism of this polymerization reaction has not yet been fully elucidated, but silylene complexes as intermediates have been the subject of discussion. Polysilanes find wide use in important applications, e.g., as preceramics [65-67] or as photoresists [68-83],... 

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],... 

The mechanism of this reaction is still under discussion however, it seems to be relatively clear that in this case silylene complexes are involved as intermediates. 

The polymerization reaction of silanes with Cp2ZrMe2 as catalyst has also been investigated by several research groups. Some evidence for a reaction mechanism proceeding through silylene complexes as intermediates has been given... 

When silylenes are generated photochemically in hydrocarbon matrices in the presence of electron-pair donors, they may form Lewis acid-base complexes that act as intermediates in the silylene dimerization to disilenes.3233 In a typical example, Mes2Si(SiMe3)2 was photolyzed in 3-meth-ylpentane (3-MP) matrix containing 5% of 2-methyltetrahydrofuran. At 77 K, dimesitylsilylene (Amax 577 nm) was formed. When the matrix was... 

Attempts to produce a germasilene by coupling a germylene and a silylene failed,62 although the germasilene was possibly formed as intermediate (see Scheme 11). 

A major problem in postulating silylenoid metal complexes as intermediates in the redistribution reactions is simply that good model compounds are lacking, and the decomposition mechanisms of silyl transition metal complexes have not been systematically investigated. While there is evidence for transient R2Si species produced by thermal or photochemical means (80-83), there are no known monomeric silylene metal complexes. Several monomeric stannylene and germylene complexes are... 

The chemistry of silicon with unusual coordination numbers has developed in a remarkable progress. While short living silylenes (coordination number 2) were discussed as intermediates in gas phase reactions of suitable precursors already in the seventies, followed by investigations on derivatives with bulky substituents at silicon and thus stable in solution in the early eighties, n- and n-donor stabilized silylenes were synthesized in the solid state already in the late eighties. 

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. 

Thus not only anionic species but neutral silylenes also could be involved as intermediates. In any case, extensive loss of Si-H takes place during the catalyzed process, a fact suggesting that the reaction is more than a simple redistribution. Further studies relating to the mechanism of the process must be carried out. 

Coordinated silylene ligands have been invoked or suggested as intermediates in a number of chemical processes, including Rochow s Direct Process94, catalytic redistribution of silanes39 and various silylene-transfer reactions95-99. Evidence for such species is primarily circumstantial, and M=Si double bonds have never been detected in these systems. Presently there is no conclusive evidence for the involvement of silylene coordination compounds in any transition-metal-mediated silylene-transfer reactions. Some reactions that may involve silylene ligands are discussed below. 

R = Me, H) are catalytically oligomerized by (R3P)2PtCl2 complexes according to equation 45. Generation of an intermediate Pt=SiMe2 silylene as well as product... 

Reactions in which transition-metal complexes extrude silylene ligands from silanes have already been mentioned (equations 32-34). Although the mechanisms of these reactions remain ill-defined, it seems possible that terminal silylene complexes may be involved as intermediates. Such a proposal has been made regarding the formation of the silylene-bridged dicobalt species 16 from Co2(CO)8 and tetramethyldisilane (Scheme l)105. 

Few silaphosphenes (phosphasilenes, silylene phosphines, silylidene phosphanes) have as yet been isolated. They are formed as intermediate species in reactions of type (9.251). First evidence was obtained in 1979 with the reaction (9.252) [49]. A stable compound is formed in Equation 9.251 when large protecting groups are present. 

Covalent chlorinated impurities in MeSi(0Me)2 can be readily removed using Na and substantially reduce its rate of hydrolysis, while 3 amines accelerate the alcoholysis of Ph,SiCl, through silylated quaternary ammonium intermediates. The Bu (Ph)(MeO)Si group provides excellent protection to alcohols, its silyl acetal character providing for ready F cleavage, unlike Bu Ph2Si while analogues of the antibiotic bicyclomycin used bicyclic piperazine-diones as intermediates, and silylene protected dihydroxystyrene... 

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