Silyl radical with oxygen

The reactions are radical chain processes (Scheme 3) and, therefore, the initial silyl radicals are generated by some initiation. The most popular thermal initiator is azobisisobutyronitrile (AIBN), with a half-life of 1 h at 81 °C. Other azocompounds are used from time to time depending on the reaction conditions. EtsB in the presence of very small amounts of oxygen is an excellent initiator for lower temperature reactions (down to —78°C). The procedures and examples for reductive removal of functional groups by (TMSlsSiH are numerous and have recently been summarized in the book Organosilanes in Radical Chemistry. ... 

The main route for the photodegradation of the polymers 10 and 11 in the solid films in air consists of radical scission of the silicon-silicon bonds, analogous to that observed in solution, but in the films, the resulting silyl radicals react with oxygen to give... 

The reaction of atoms, radicals or excited triplet states of some molecules with silicon hydrides is the most important way for generating silyl radicals [1,2]. Indeed, Reaction (1.1) in solution has been used for different applications. Usually radicals X are centred at carbon, nitrogen, oxygen, or sulfur atoms... 

The reaction sequence shown in Scheme 8.4 is in accord with all the experimental observations and involves at least two consecutive unimolecular steps [15,18]. Silyl radical 11 adds to molecular oxygen to form the peroxyl radical 12. 

Whatever the initial step of formation of surface silyl radicals, the mechanism for the oxidation of silicon surfaces by O2 is expected to be similar to the proposed Scheme 8.10. This proposal is also in agreement with the various spectroscopic measurements that provided evidence for a peroxyl radical species on the surface of silicon [53] during thermal oxidation (see also references cited in [50]). The reaction being a surface radical chain oxidation, it is obvious that temperature, efficiency of radical initiation, surface precursor and oxygen concentration will play important roles in the acceleration of the surface oxidation and outcome of oxidation. 

When a thin film prepared from poly[(tetraethyldisilanylene)bis(2,5-thienylene)] was irradiated in air with a 6-W low pressure mercury lamp bearing a Vycor filter, the absorption maximum near 340 nm disappeared within 40 min. Poly[(tetraethyldisilanylene)(2,5-thienylene)] also exhibited a rapid UV change when its thin film was irradiated. IR spectra of the resulting films reveal strong absorption bands due to Si-O-H and Si-O-Si bonds at 3300 and 1100 cm [. The formation of the Si-O-H and Si-O-Si bonds can be best explained by the reaction of the silyl radicals generated by homolytic scission of the silicon-silicon bonds in the polymer backbone with oxygen in air. The other polymers are also photoactive,... 

However, because a reverse reaction of oxygen addition to a silyl radical proceeds with a high rate, one can state that the formation of the main products of transformation is related to the first process. 

An example of the latter possibility is the ring closure of formyl bromide (158) with PhSiH3 instead of Bu3SnH, where silyl radical has strong affinity to an oxygen-centered radical, to form cyclohexyl silyl ether (159) via 6-exo-trig manner (eq. 3.58). 

PhSeSiPh2(f-Bu) afforded a silyl radical that can abstract a bromine atom from 152 to give the radical 153. Cyclization affords radical 154, which react with diphenyl diselenide to give 155. Diphenyl diselenide results from the oxidation of the phenylselenolate anion presumably by the oxygen present in the solvent. 

Beside the above-mentioned nonradically occurring reactions in condensed phase, one observes many radical reactions of silyl- and germyl-diazenes and -organyldiazenes. Examples have already been discussed in Section IV. More reactions of this type are exemplified by oxidation with oxygen as well as by reactions with nitric oxide and halohydrocarbons (Section VI,B,C,D). The reactivity of BSD has been investigated quite exhaustively. Reactions of this diazene therefore become the focal point for subsequent sections of this chapter. 

Low-temperature autoxidation of 3,4-dihydro-2//-pyrazoles (e.g. 10, R = Me) provides unstable a-azo hydroperoxides (Scheme 30) [61a,bj 3,4-dihydro-2//-pyrazoles that are inert towards autoxidation (cf. 10, R = Ph) can be converted to hydroperoxides by reaction with singlet oxygen (photooxygenation) at 0- 5 °C [61b], a-Azo hydroperoxides readily decompose at room temperature to -keto radicals which can be entrapped with oxygen to furnish 3-hydroxy-1,2-dioxolanes (e.g. 11 —> 12, Scheme 30) [61cj. The preparation and thermal decomposition of several ether, silyl ether and ester derivatives of 3-hydroxy-1,2-dioxolanes have been reported [62]. 

Disubstituted nitroxides constitute a well-known class of rather stable free radicals. N,N-disubstituted anions of organometallic hydroxylamines have just one additional electron more than the corresponding nitroxide. Oxidation of the rearranged anions of silyl or germylhydroxyl amines, electrolytically or with oxygen, produces solutions of organometal nitroxides (30). Examples are shown in Eqs. (19)-(21). These nitroxides are stable in dilute solution for several days at room temperature. 

In the silsesquioxanes, it is oxygen that couples the two radical functions, as shown schematically in the model compound 9. Quantum chemical calculations on 4 and 9 substantiate the qualitative assertions (Table 30.1). The singlet state of the C2 conformation is substantially lower in energy (117.7 kJ/mol) than the planar structure (C2V geometry), in agreement with the known strong pyramidalization forces in silyl radicals. 

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