Silyl radicals rearrangement

Regarding this proposal, it should be noted that while 1,1-eliminations on Si-Si-C units to generate silylenes are well known thermal processes (54) the photochemical variant seems not to have been described. The rearrangement of silylsilylenes (4) to disilenes is known to be rapid (55), and silyl radical addition at the least hindered site would produce the observed persistent radical. Preliminary evidence for the operation of 1,1-photoelimination processes in the polysilane high polymers has been obtained, in that the exhaustive irradiation at 248 nm of poly(cyclohexylmethylsilane) (PCHMS) produces —10-15% volatile products which contain trialkylsilyl terminal groups. For example, the following products were produced and identified by GC— MS (R=cyclohexyl,R = methyl) H(RR Si)2H (49%), H(RR Si)3H (19%), R2R SiH (2%), R 2RSiRR SiH (5%) and R2R SiRR SiH (7%). 

The addition of silyl radicals to various 2,6-disubstituted quinones takes place at two different sites, i.e., at the less hindered C=0 and at the C=C double bond, the former being ca 4 times slower (Scheme 5.5) [48]. However, kinetic studies showed that radical adducts 27 are prone to rearrange to the thermodynamically more stable isomers 28 via a four-membered transition... 

It is worth mentioning that in a few cases the (3-elimination of the silyl radical from the a-silyl alkoxyl radical (47) with the formation of corresponding carbonyl derivative was observed [63,64]. Evidently the fate of a-silyl alkoxyl radical depends on the method of radical generation and/or the nature of the substrate. Two examples that delineate the potentialities of this rearrangements are reported in Reactions (5.33) and (5.34). In the former, the 5-exo cyclization of secondary alkyl radical on the carbonyl moiety followed by the radical Brook rearrangement afforded the cyclopentyl silyl ether [65], whereas Reaction (5.34) shows the treatment of an a-silyl alcohol with lead tetracetate to afford the mixed acetyl silyl acetal under mild conditions [63]. 

The intramolecular addition of silyl radicals to aromatic rings has also attracted some attention. Early work on the silyl radical obtained by the reaction of silanes 41 with thermally generated t-BuO radicals at 135 °C showed the formation of rearranged products only for = 3 or 4, whereas for = 1, 2, 5, and 6 no rearrangement took place [20],... 

Silyl substituted carbon-centred radicals, which are produced when adding RsSi to unsaturated bonds can participate in consecutive reactions other than cyclization. A simple example is given in Reaction (7.66) where the adduct of silyl radical to (3-pinene rearranged by opening the four-membered ring prior to H atom transfer [33,77],... 

This silylperoxyl radical undergoes an unusual rearrangement to 13 followed by a 1,2-shift of the MesSi group to give 14. Hydrogen abstraction from the silane by radical 14 gives the desired product and another silyl radical 11, thus completing the cycle of this chain reaction. 

The rearrangement 12 13 is not straightforward and two alternatives routes have been suggested. One of them involves a 1,3 transfer from silicon to oxygen to give the silyl radical 15 followed by Sui reaction on the peroxide moiety [15]. The other indicated by PM3 calculations is a dioxirane-like three-membered intermediate 16 having a pentacoordinated central silicon followed by a 1,2 silyl migration to afford radical 13 [19]. 

Compounds with two or more silicon atoms directly attached to one another, subdivided into sections based first on the number of silicon atoms and then on the carbon functionality attached to the silicon atoms. Frequently, but not exclusively, the main photochemical behavior involves homolysis of a silicon-silicon bond yielding silyl radicals, but in some cases silylenes result directly from the photochemistry. The resulting compounds are frequently the products of a molecular rearrangement. 

Conlin and coworkers photolyzed vinyltris(trimethylsilyl)silane 188 in the presence of a variety of trapping reagents such as butadiene, substituted butadienes or silanes and observed products derived from intermediate silenes 189 (formed by rearrangement) or from silylenes 190 resulting from elimination of hexamethyldisilane93. In some cases complex mixtures of products which could have been derived from intermediate silyl radicals were also observed. The reaction products formed from the silene and the silylene in the presence of butadiene, 191 and 192 respectively, are shown in Scheme 32. 

The photochemistry of trisilanes 16 and 19 has been investigated in some detail (Schemes 1 and 2)21. Upon irradiation of compound 16 only the Si—SiMe2Ph bond is broken and the initially formed silyl radical 17 undergoes a rearrangement to the more stable silacyclobutenyl radical 18 whose EPR spectrum has been recorded (Scheme l)21a. Irradiation of trisilane 19 with a medium pressure mercury lamp resulted in the formation of hexamethyldisilane, 2-(trimethylsilyl)thiophene and 20, with 20 dominating (Scheme 2)21b. In the presence of carbon tetrachloride, a significant yield (19.2%) of the... 

Ishikawa and co-workers (50, 51) have also reported the photodegradation of backbone disilane polymers (Scheme III). In this case, as in the previous example with pendant silyl substituents, silicon-silicon bond homolysis to produce silyl radicals was the predominant process. When irradiated in toluene, the photodegraded polymer showed a substantial -SiH band at 2150 cm in the IR spectrum. Similarly, NMR spectroscopic examination of the irradiated polymer showed evidence of silyl radical substitution into the solvent toluene. Irradiation of the polymeric disilane in deuterated methanol produced no bands due to Si-D in the IR spectrum and resulted in the incorporation of the elements of methanol into the chain ends (as revealed by NMR spectroscopy). For the phenyl-substituted polymer, the NMR evidence indicated that <5% of rearranged cyclohexadiene derivatives were formed. 

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). 

Only two chapters among those planned did not materialize, namely that on Silyl radicals, silylenes and rearrangements and that on Silicon-nitrogen bonds but at least part of these topics is covered in various sections of other chapters. 

The facts that thiols are good H-atom donors toward alkyl radicals and that silyl radicals are among the most reactive known species for abstraction and addition reactions suggest that any class of compounds which allows the transformation of a thiyl to a silyl radical via a fast intramolecular rearrangement will potentially be a good radical-based reducing agent. The silanethiols 11 and 12 are found to have this property [84, 85]. The reductions of bromides, iodides and isocyanides by thiol 12 are demonstrated to follow the expected mechanism [85]. 

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