Silenes oxidation

A recent study of the photolysis of simple diazoalkanes 314 or diazirines 315, compounds known to lead to the formation of silenes under inert conditions, led, in oxygen-doped argon matrices, via the silene 316 to the siladioxirane 317. While previously postulated as an intermediate in silene oxidations, this is important experimental evidence for this intermediate. Continued photolysis of the system led to a compound identified as the silanone-formaldehyde complex 318, which on further irradiation led to the silanol-aldehyde 319. The latter compound itself underwent further photochemical oxidation leading to the silanediol 320160. The reactions are summarized in Scheme 58. Detailed infrared studies, including the use of isotopes, and calculations, were used to establish the structures of the compounds. 

The products of the thermolysis of 3-phenyl-5-(arylamino)-l,2,4-oxadiazoles and thiazoles have been accounted for by a radical mechanism.266 Flash vacuum pyrolysis of 1,3-dithiolane-1-oxides has led to thiocarbonyl compounds, but the transformation is not general.267 hi an ongoing study of silacyclobutane pyrolysis, CASSF(4,4), MR-CI and CASSCF(4,4)+MP2 calculations using the 3-21G and 6-31G basis sets have modelled the reaction between silenes and ethylene, suggesting a cyclic transition state from which silacyclobutane or a trcins-biradical are formed.268 An AMI study of the thermolysis of 1,3,3-trinitroazacyclobutane and its derivatives has identified gem-dinitro C—N bond homolysis as the initial reaction.269 Similar AMI analysis has determined the activation energy of die formation of NCh from methyl nitrate.270 Thermal decomposition of nitromethane in a shock tube (1050-1400 K, 0.2-40 atm) was studied spectrophotometrically, allowing determination of rate constants.271... 

Phenyl(triphenylsilyl)carbene has also been trapped without the interference of a silylcarbene-to-silene rearrangement84. It undergoes 0,H insertion with alcohols and is oxidized to the ketone by DMSO the latter reaction is likely to include an S-oxide ylide (equation 56). 

All known silenes react violently with molecular oxygen with the exception of the stable 1-silaallenes which are air-stable. Along with carbonyl compounds the products of the oxidation of silenes are cyclic siloxanes from the corresponding silanone, depending on the reaction conditions3,8,28. For example, when the stable adamantylsilene 150 is... 

The same authors investigated the photochemical and thermal oxidation of 2-silapropene 12, silaisobutene 7 and 1,1,2-trimethylsilene 13 in O2-doped argon matrices (equation 5)28. These silenes are easily photooxidized in matrices containing more than 1% 62, but only trimethylsilene 13 exhibits thermal reactivity toward oxygen at temperatures as low as 20-40 K. 

The photochemical reactivity goes up with increasing number of methyl groups at the double bond and decreasing ionization potential28. Key intermediates in both the photochemical and the thermal oxidation of silenes 12, 7 and 13 are the siladioxetanes 14. These species are labile even in low-temperature matrices and could not be identified spectroscopically. Evidence for their formation comes from the observed oxidation products such as complexes 15 between silanones and formaldehyde and formylsilanols 16. 

An additional oxidation pathway is observed for 13 with a methyl group at the C-atom of the silene moiety. Here, the primary adduct of 13 and 302 under photo lytic conditions, i.e. triplet diradical 17, can either ring-close to give the corresponding dioxetane (the dioxetane is not stable under the photolytic conditions required to induce the photooxidation, and therefore cannot be observed) leading to products derived from it, or produce dimethylvinylsilyl hydroperoxide (18) via H-abstraction from a methyl group (equation 6). 

De Vos et al. (1989) suggest that the copper-induced damage to the permeability barrier in roots of Silene cucubalus is caused by a direct metal action on both membrane lipids and thiols. They propose that the first damaging effects of copper ions is the oxidation and cross-linking of membrane protein sulphydryls. However, they also adjudge an important role to the copper induced membrane lipid peroxidation, possibly due to direct free radical formation in the membrane this effect could be enhanced by a depletion of thiols such as glutathione which results in a concomitant decrease of the cellular defence system against free radicals. 

Absolute kinetic data have been reported for four of the characteristic bimolecular reactions of disilenes 1,2-addition of alcohols and phenols (equation 72), [2 + 2]-cycloaddition of ketones (equation 73), [2 +4]-cycloaddition of aliphatic dienes (equation 74) and oxidation with molecular oxygen (equation 75). As with silenes, the addition of alcohols has been studied in greatest detail. 

Oxidation of Silenes and Silylenes Matrix Isolation of Unusual Silicon Species... 

Keywords Matrix Isolation / Oxidation / Silene / Silylene... 

All these observations can now be summarized in a general mechanism for the oxidation of the silenes 5 (Scheme 8) ... 

The first step of the oxidation of silenes 5 is the formation of charge transfer complexes, which in the case of the electron-rich silene 5d thermally produce diradicals 6, while 5b and c require photochemical activation. 

Hydrogenation of a ruthenium silene complex also involves oxidative addition and reductive elimination pathways , giving two hydride-containing products ... 

De Vos ChHR, Vonk MJ, Vooijs R and Schat H (1992) Glutathione depletion due to copper-induced phytochelatin synthesis causes oxidative stress in Silene cucubalus. Plant Physiol 98 853-858. 

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