Silyl radicals recombination

Silyl radicals diffuse toward the wafer surface where recombination reactions occur ... 

Encounters between silyl radicals in solution or in the gas phase usually result in recombination and disproportionation (45, 46). Disproportionation results in the production of silanes and highly reactive silenes. The disproportionation reaction is thermodynamically favorable because of the formation of a silicon-carbon double bond, which, although subsequently chemically reactive, is worth —39 kcal/mol (44). For pentamethyldisilanyl radicals, disproportionation is kinetically competitive with radical dimerization (46). In an earlier study, Boudjouk and co-workers (47) demonstrated conclusively by isotopic substitution and trapping that the silyl radicals generated by photolysis undergo disproportionation, as well as, presumably, dimerization (Scheme I). In deuterated methanol, the silanes produced were predominantly undeuterated, whereas methoxymethyldiphenylsilane was extensively deuterated in the a position. The results of these experiments strongly implicated the substituted silene produced by disproportionation. 

Disproportionation of Silyl Radicals. Faced with the task of generating a series of sterically hindered polysilyl substituted radicals upon irradiation, the authors have proposed a tentative reaction scheme involving silyl radical formation, disproportionation to silanes and silenes, and readdition of silyl radicals to the silenes. The disproportionation of silyl radicals is a well-established process that is kinetically competitive with recombination (46). Repetition of this process would lead eventually to highly sterically encumbered and undoubtedly persistent silicon-based radicals carrying only silicons in the a positions. Although such a scheme would explain much of the data in this obviously very complex process, it is very tentative, and other possible routes to and structures for the persistent silyl radicals have not been ruled out (58). 

Several dimerization rates of alkyl-substituted silyl radicals were measured earlier [8]. However, the dimerization rates of silyl-substituted silicon-centered radicals have not previously been determined. In this study we have measured, using EPR spectroscopy, the rate constants for the recombination of four silyl radicals (lb, 2b, 3b, and 4b), to produce the corresponding disilane dimers of type a (i.e. la, 2a, 3a, and 4a respectively). This dimerization reaction is shown as the backward reaction of Eq. 2 in Scheme 1. Radicals lb, 2b, and 3b were generated photochemically fiom the corresponding disilane dimers of type a (Scheme 1, Eq. 2), while radical 4b was generated photochemically from the corresponding silylmercury compound 4c (Scheme 1, Eq. 1). 

The fact that irradiation of disilanes la - 3a did not produce detectable products other than the corresponding silyl radicals indicates that the radicals produced photochemically undergo clean recombination to produce the starting silane, as shown in Scheme 5 path (b). An exception is the photochemical reaction of silyl mercurial compounds 4c, which produce the hydrosilane 6 [Scheme 5, path (c)], while the corresponding dimer 4a [Scheme 5, path (b)] is not detected. Apparently, for radical 4b, hydrogen abstraction to produce silane 6 occurs significantly faster than its dimerization to 4a. 

Trisylbenzene [tris(trimethylsilyl)methylbenzene] when photolyzed is highly susceptible to homolysis (ESR spectroscopy demonstrated the presence of radicals) the radicals recombined by attack of the silyl radical on the aromatic ring as the major pathway16 (equation 5). 

The much studied photochemistry of aryidisilanes carried out in earlier years has been reviewed . Cleavage of the silicon-silicon bond of the disilyl moiety is always involved, but various other reactions have been observed depending on the structure of the disilane and the conditions employed. Thus cleavage to a pair of silyl radicals, path a of Scheme 15, is normally observed, and their subsequent disproportionation to a silene and silane, path b, is often observed. There is evidence that the formation of this latter pair of compounds may also occur by a concated process directly from the photoex-cited aiyldisilane (path c). Probably the most common photoreaction is a 1,3-silyl shift onto the aromatic ring to form a silatriene, 105, path d, which may proceed via radical recombination. A very minor process, obsa-ved occasionally, is the extrusion of a silylene from the molecule (path e), as shown in Scheme 15. 

Treatment of O-silyl enols with silver oxide leads to radical coupling via silver enolates. If the carbon atom bears no substituents, two such r -synthons recombine to symmetrical 1,4-dicarbonyl compounds in good vield (Y. Ito, 1975). 

When all the mechanistic evidence is taken into consideration, the following reaction sequence appears to best satisfy the data. The silane undergoes reversible complexation (A) with the ozone, the complex being present in only small concentrations. The rate step then involves electrophilic attack on the hydridic hydrogen, passing through a five-center transition state. This may decompose to either a silyl hydrotrioxide (Bi) or directly to the radical pair (B2). The silyl hydrotrioxide, if present, must decompose rapidly to the radical pair (C). This radical pair then recombines with retention of configuration to afford the ultimate product, the silanol (D). 

Benzylsilanes are relatively inert to photolysis although Sakurai and coworkers detected a small amount of dissociation to silyl and benzyl radicals, which subsequently recombined to give o-sily toluenes14 (equation 2). 

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