Silyl radical with silanes

Allyloxysilanes (14) undergo radical chain cyclization in the presence of di-tert-butyl hyponitrite as radical initiator and thiol as a catalyst at ca 60 °C (Reaction 6.3) [5]. The thiol promotes the overall abstraction from the Si—H moiety as shown in Scheme 6.4 and the silyl radical undergoes a rapid 5-endo-trig cyclization. Indeed, EPR studies on the reaction of t-BuO radical with silanes 14 detected only spectra from the cyclized radicals even at — 100°C, which implies that the rate constants for cyclization are > 10 s at this temperature. 

The action of a catalytic amount of triethylborane on tris(trimethylsilyl)silane induces the formation of tris(trimethylsilyl)silyl radicals, which promote the ring-closure of 1,6-heptadiene to a mixture of the cis- and rirms-cyclopcntanc derivatives 115, together with a small amount of the silicon heterocycle 116 (equation 61)68. 

The reaction of thermally and photochemically generated tert-butoxyl radicals with trisubstituted silanes [Eqs. (6) and (7)] has been used extensively for the generation of silyl radicals in ESR studies, in time-resolved optical techniques, and in organic synthesis. Absolute rate constants for reaction (7) were measured directly by LFP techniques,56,62,63 whereas the gas phase kinetic values for reactions of Me3SiH were obtained by competition with decomposition of the tert-butoxyl radical.64,65... 

Several methods for generating of silyl radicals exist using direct interaction of silanes with light (Reaction 1.2). However, none of them is of general applicability, being limited to some specific application [3]. 

Table 2.5 Electron affinities of silyl radicals together with gas-phase acidities and bond dissociation enthalpies of silanes"...

The hydrogen abstraction from the Si—H moiety of silanes is fundamentally important not only because it is the method of choice for studying spectroscopically the silyl radicals but also because it is associated with the reduction of organic molecules, process stabilizers and organic modification of silicon surfaces. 

Cyclohexyl xanthate has been used as a model compound for mechanistic studies [43]. From laser flash photolysis experiments the absolute rate constant of the reaction with (TMS)3Si has been measured (see Table 4.3). From a competition experiment between cyclohexyl xanthate and -octyl bromide, xanthate was ca 2 times more reactive than the primary alkyl bromide instead of ca 50 as expected from the rate constants reported in Tables 4.1 and 4.3. This result suggests that the addition of silyl radical to thiocarbonyl moiety is reversible. The mechanism of xanthate reduction is depicted in Scheme 4.3 (TMS)3Si radicals, initially generated by small amounts of AIBN, attack the thiocarbonyl moiety to form in a reversible manner a radical intermediate that undergoes (3-scission to form alkyl radicals. Hydrogen abstraction from the silane gives the alkane and (TMS)3Si radical, thus completing the cycle of this chain reaction. 

The addition of silyl radicals to double bonds in benzene or substituted benzenes (Reaction 5.2) is the key step in the mechanism of homolytic aromatic substitution with silanes [8,9]. The intermediate cyclohexadienyl radical 2 has been detected by both EPR and optical techniques [21,22]. Similar cyclohex-adienyl-type intermediates have also been detected with heteroaromatics like furan and thiophene [23]. 

The endo-mode of cyclization is found to be the preferred path also in the lower homologues. Reaction (6.2) shows the reactions of two silanes (8) with thermally generated t-BuO radicals to afford the five-membered ring in low yields via a 5-endo-trig cyclization [1], EPR spectra recorded from these two silanes with photogenerated t-BuO radicals are assigned to secondary alkyl radical intermediates formed by an intermolecular addition involving the expected silyl radical and the parent silane [2],... 

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],... 

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). 

Tris(trimethylsilyl)silyl radical is relatively stable and can therefore serve as a radical leaving group. This reaction has been extended to the radical-initiated allylation of organic halides202 203. Thus, thermolyses of bromides a to a carbonyl substituent 144 or of simple iodides with allyltris(silyl)silane in the presence of a radical initiator gives the corresponding allylation products (equation 112). 

Several of the originally planned chapters, on comparison of silicon compounds with their higher group 14 congeners, interplay between theory and experiment in organisilicon chemistry, silyl radicals, recent advances in the chemistry of silicon-phosphorous,-arsenic,-antimony and -bismuth compounds, and the chemistry of poly silanes, regrettably did not materialize. We hope to include these important chapters in a future complementary volume. The current pace of research in silicon chemistry will certainly soon require the publication of an additional updated volume. 

Silyl radicals have also been observed during /-irradiation of solid polysilanes. Tagawa and coworkers examined the EPR spectrum observed upon irradiation of solid poly-(dimethylsilane) and concluded that the spectrum corresponded to silyl radicals generated by homolysis of the silicon skeleton in the polysilane (equation 5)18. Indeed, the EPR spectrum of the poly(dimethylsilane) radical (13), with hyperfine splitting constants H) and a(y-1 H) of 0.813 and 0.046 mT respectively, corresponded remarkably well to that published for the dimethyl(trimethylsilyl)silyl radical [a( -1H) = 0.821 mT a(/-1H) = 0.047 mT]1. Radical (13) appears to be very stable in solid poly(dimethyl-silane), since the EPR signal was strong and clearly observable at room temperature. 

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