Trimethylsilyl radicals

Matrix IR spectra of various silenes are important analytical features and allow detection of these intermediates in very complex reaction mixtures. Thus, the vibrational frequencies of Me2Si=CH2 were used in the study of the pyrolysis mechanism of allyltrimethylsilane [120] (Mal tsev et al., 1983). It was found that two pathways occur simultaneously for this reaction (Scheme 6). On the one hand, thermal destruction of the silane [120] results in formation of propylene and silene [117] (retroene reaction) on the other hand, homolytic cleavage of the Si—C bond leads to the generation of free allyl and trimethylsilyl radicals. While both the silene [117] and allyl radical [115] were stabilized and detected in the argon matrix, the radical SiMc3 was unstable under the pyrolysis conditions and decomposed to form low-molecular products. 

Corresponds to the C—H bond dissociation energy in the trimethylsilyl radical, implying a K bond strength of 192-193 kJ/mol. 

The trimethylsilyl radical produced either rapidly dimerizes or reacts with solvent so that very clean ESR spectra of the radical anion, with minimum interactions with the counterion, can be obtained (116). Further reduction to dianions is very slow, and exhaustive reduction to anion radicals minimizes problems associated with exchange between anion radicals and unreduced substrate (115). It now appears that the solvent HMPA greatly facilitates the one-electron reduction, not only for trimethylsilylsodium, but also for organolithium and magnesium reagents (110). It was found that 0.1F solutions of methyl-, n-butyl-, or f-butyllithium or benzylmagnesium chloride will quantitatively reduce biphenyl to its radical anion in less than 10 minutes (110). 

The exceptionally stoically demanding trimethylsilyl radical selectively removes an equatorial chlorine atom. Attack on axial chlorine atoms is severely limited here by the concave molecular geometry. Tnalkyi- and triarylstannane were also tried, but they failed to distinguish between the chlorines and instead produced a 1 1 epi-meric mixture at C-7. 

Similarly, for the purpose of studying silyl radicals in the gas phase in the presence of a spin trap, it has been established that the trimethylsilyl radical added to the double bond of a nitrone to yield the a-trimethylsilylated hydroxylamine.204 Other studies have been... 

The generation of di- and trimethylsilyl radicals by the reactions of hydrogen atoms with di- and trimethylsilane and their subsequent reactions have been described23. Three main products were observed in these reactions 1,1,2,2-tetramethyldisilane, pentamethyldisilane and hexamethyldisilane which are formed by both radical coupling and disproportionation processes. The authors describe a detailed kinetic analysis of the various reactions of interest but have difficulty in providing a mechanistic description that fully fits their observed data23. 

The structure of 5 was confirmed by metathesis of dimethyldiacetoxysilane with perfluoropinacol 108). The reactions according to Eqs. (8) and (9) proceed via the alkali salts of perfluoropinacolate dianion, which in the case of sodium can be obtained as a pure white powder 153). Compound 4 is also accessible by reaction of HFA and bis(trimethylsilyl)mercury under mild conditions 153). As the reaction rate is increased by UV radiation, formation of trimethylsilyl radicals is assumed to be the first step. 

Stannyl ketyl radicals were generated from an amide carbonyl group and when incorporated in cinnamic enamides, indolizidinone rings can be assembled [37]. Du and Curran have shown that a-thioaminoalkyl radicals such as 24 resulting from the addition of a tris(trimethylsilyl)radical onto N-aryl thiocarbamates, thioamides or thioureas 22, can be exploited in the context of the synthesis of carbocyclic and heterocyclic fused quinolines 23 (Scheme 8) [38]. 

When pentamethyldisilanyl radicals were produced by hydrogen atom abstraction in a room-temperature solution, they also underwent disproportionation and recombination reactions238. No evidence was found for fragmentation to dimethylsilylene and a trimethylsilyl radical. 

If the chemical model is relevant, P450j may be inactivated by reaction of the enzyme with the trimethylsilyl radical or, less likely, from oxidation of the ethylene produced in the initial catal54ic turnover. An interesting variant of a mechanism-based inhibitor is provided by (20S)-22-nor-22-thiacholesterol, in which the sulfur that replaces the carbon at position 22 is oxidized by P4503, to a sulfoxide that is a potent but not an irreversible inhibitor of the enzyme ... 

A prototypical oligosilane, hexamethyldisilane, reacts upon irradiation of 147 nm light without any trapping reagent to give trimethylsilyl radicals . Recombination and redistribution reactions of the trimethylsilyl radicals give back the starting hexamethyldisilane as well as 1,1-dimethylsilaethene plus trimethylsilane, respectively (equation 3). 

It is likely that it is the trimethylsilyl radical which attacks the hydrogen molecule in preference to the phenyl radical which is stabilised by delocalisation of the odd electron into the ir-orbitals of the ring. 

The fact that xylene is the main product indicates clearly that the Si-CH2 bond breaks first. This would also follow from bond energy considerations. Thus, the mechanism for the formation of benzyltrimethylsilane must be due to reaction of toluene with trimethylsilyl radicals, rather than by the shearing off of a methyl radical with subsequent replacement by a hydrogen radical. 

The other silane formed, namely the methyibenzyitrimethylsilane is likely to be the ortho-isomer. Evidence for this is that it has a longer retention time than the para-isomer, the retention time of which is known. It would be expected to have a higher boiling point than the para-isomer (cf. o- and p-tolyltrimethylsilanes). This compound is probably formed by attachment by a trimethylsilyl radical on an o-xylene molecule. 

Phenyltrimethylsilane is also formed in small amounts. It is probably formed by combination of a trimethylsilyl radical with a phenyl radical formed from the degradation of a toluene molecule. This is offered as an explanation for the abscence of benzene in the products. 

This is initiated by Hg 6( Pi) sensitized dissociation of trimethylsilane to give trimethylsilyl radicals and hydrogen atoms the latter abstract hydrogen atoms from trimethylsilane to produce molecular hydrogen. The trimethylsilyl radicals enter a free-radical chain propagating sequence leading to the production of the 1 1 and 2 1 adducts MeaSi(CF -CFj) H, where jc = 1 or 2, and the kinetics of the formation of these compounds are consistent with the proposed chain mechanism, which involves termination predominantly by tetrafluoro-2-trimethyIsilylethyl radical combination. 

Preliminary work on the mercury-photosensitized reaction of trimethylsilane with vinylidene fluoride has indicated that hydrogen is produced with the quantum yield to be expected by analogy with the trimethylsilane-tetra-fluoroethylene reaction, and that hexamethyldisilane is not formed, so that initiation is proceeding as before to give trimethylsilyl radicals, which add to the olefin. Since no appreciable yields of 1 1 or 1 2 adduct were obtained, however, it appears that the MesSi-CHs CFj- radicals which are produced react by termination rather than by hydrogen abstraction. 

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