Silyl radicals, silane reactions

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]. 

Knowledge of bond dissociation enthalpies (DH) has always been considered fundamental for understanding kinetics and mechanisms of free radicals. DHs offer an interesting window through which to view stability of radicals. Indeed, based on Reaction (2.1) the bond dissociation enthalpy of silanes D/f(R3Si—H) is related to enthalpy of formation of silyl radicals, A//f (RsSi ), by Equation (2.2). 

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 addition of trisubstituted silanes to carbonyl sulfide has been applied to the synthesis of the corresponding silanethiol derivatives (Reaction 5.40) [78]. In Scheme 5.12 the mechanism is depicted, starting from the silyl radical attack to the sulfur atom of 0=C=S and ejection of carbon monoxide. The resulting silanethiyl radical abstracts hydrogen from the starting silane, to give the silanethiol and to generate fresh silyl radical (see Section 3.4). 

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],... 

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 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],... 

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 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. 

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. 

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). 

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