Silyl radicals disproportionation

Structures that come to mind for assignment to die observed spectra are those that would be expected from an initial disproportionation of two primary silyl radicals to a silene and a hydrosilane, followed by an addition of another silyl radical to the silene, producing a / silylated silyl radical. Repetition of the process would eventually lead to a highly sterically encumbered and undoubtedly persistent silicon-based radical carrying only silicons in its / positions ... 

In order to document the radical disproportionation reaction, we have used FT-IR spectroscopy to characterize the irradiation products. Upon irradiation of 1 in pentane, the formation of the characteristic peak near 2100 cm-1 due to Si-H stretching vibrations was readily apparent. The IR spectrum obtained in perdeuterated pentane was identical, suggesting that radical processes other than abstraction from the solvent are involved. Furthermore the ESR spectrum obtained in this solvent is identical to that already described. This raises the question whether the initially formed silyl radicals really abstract hydrogen from carbon with the formation of carbon-based radicals as suggested (13), particularly in light of the endothermicity of such a process. 

Silyl Radicals (R3Si). Much is known about the chemistry of silyl radicals. They can be produced from thermolysis, photolysis and electron transfer reactions. With the exception of disproportionation and degradation to silenes, most of the reactions known for the carbon radicals are also known for silyl radicals. 

Me3Si)3Si radicals, respectively [14,16]. While the fate of the reaction between two Et3Si radicals is still not known, the termination products of other silyl radicals have been determined. Pentamethyldisilyl radicals, produced by the reaction of Me3SiSi(H)Me2 with photogenerated t-BuO radicals at room temperature, behave similarly to the Me3Si radical [17]. That is, products due to the combination (Reaction 4.4) and disproportionation (Reaction 4.5) of these radicals were detected in a ratio of > 2.1. 

In contrast, in the excited state the primary cleavage mechanism in silacyclobutanes like 5 involves the breaking of a silicon-carbon bond23. The initially formed silyl radicals 15 and 16 are stabilized by an intramolecular disproportionation reaction giving the silenes 17 and 18 and the homoallylsilane 19.17 and 18 were identified by their trapping products (20, 21) with methanol (equation 5)23. From pyrolysis of Z-5 a different set of products from 1,4-diradical disproportionation is obtained, which can be attributed to predominant cleavage of the carbon-carbon bond23. 

In this connection Ishikawa and coworkers studied the photodegradation of poly(disilanylene)phenylenes 203126, and found that irradiation under the same conditions as in the photolysis of the aryldisilanes results in the formation of another type of nonrearranged silene 204 produced together with silane 205 from homolytic scission of a silicon-silicon bond, followed by disproportionation of the resulting silyl radicals 206 to 204 and 205 (equation 51). 

The much studied photochemistry of aryldisilanes carried out in earlier years has been reviewed51,52. 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 concerted process directly from the photoex-cited aryldisilane (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 recombination52. A very minor process, observed occasionally, is the extrusion of a silylene from the molecule (path e), as shown in Scheme 15. 

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. 

Scheme I. Disproportionation and dimerization of silyl radicals as demonstrated by isotopic substitution and...

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). 

An attractive, although tentative, alternative would be an alkyl-substituted silylsilylene formed from the polymer chain. Two thermodynamically reasonable routes to such intermediates are possible. The first route (equation 4) involves 1,1-elimination to produce the silylsilylene directly. This route has a precedent in organosilane thermal processes (78, 79). The second route (equations 5a and 5b) involves rearrangement from a silene produced by the disproportionation (46, 80, 81) of two silyl radicals caused by bond homolysis. This type of rearrangement has also been described in the literature (82). The postulated silylsilylenes are also attractive intermediates to explain the rebonding of silicon to carbon atoms other than those in the original a positions (CH insertion), which is obvious from the mass spectral analysis of gaseous products from the laser ablation of isotopically labeled poly(di-n-hexylsilane). 

Similarly, the attempted synthesis of diadamantylsilylene by extrusion from a trisilane 267 did not give the wanted product ". The predominant photoreactions are silicon-silicon bond homolysis to give the radicals 268 and 269 (equation 67). Disproportionation of 268 and 269 results in the formation of silene 271 and silane 272. TTie silene is identified by isolation of its head-to-tail dimer 273. In the presence of scavenger reagents like 2,3-dimethylbuta-l,3-diene radical trapping products like 270 could be detected in low yields. Secondary photoprocesses involving the disilane 272 take place. Formation of silyl radicals 269 and 274 with subsequent disproportionation of the radicals explain the formation of diadamantylsilane 275. 

The photochemistry of disilanes has been investigated for several decades. Photolysis yielding two silyl radicals was recognized early on as one of the conmion modes of reaction. Studies of Ph3Si-SiPh2Me or its trideuteriomethyl analog 76, by Boud-jouk and coworkers , indicated the homolysis of the silicon-silicon bond to a pair of radicals which subsequently disproportionated, yielding the silene 77 (equation 11). This provided some of e early evidence for the existence of the silicon-carbon double bond. 

The 1,3-silyl shift in aryl disilanes is suppressed when the aromatic ring is ortho-substituted144. An attempted silylene synthesis from 1,3-dimesitylhexamethyltrisilane 259, however, led to low yields of silylene trapping products (ca 30% generation of Me2S ). The major pathway is the homolytic cleavage of the trisilane, followed by disproportionation of the radicals 260 and 261 to the silene 262 and the disilane 263 (equation 65). 

---