Silenes transformations

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... 

Several transformations of silenes upon warming of the matrix have been described20-22. For example, dimerization of 1,1,2-trimethylsilene (12), leading to 1,3-disilacyclobutanes 13, occurs above 45 K20-22, addition of methanol before warming to 20 °C yields 1422 and warming of an 02-doped matrix (0.5-1%) gives silylhydroperoxide 15 and vinyloxysilanol 16 in temperature-dependent relative yields22 (Scheme 3). 

The Wiberg -type silenes like 92, available through salt elimination reactions from 93, react with nonenolisable aldehydes, ketones and the corresponding imino derivatives to give in a first step donor adducts 9459, which are then transformed to the [2 + 2] and [2 + 4] cycloadducts 95 and 96, respectively (equation 21)60-62. These cycloadducts may liberate the silene 92 upon heating and it can be trapped by suitable reagents. 

The further increase in steric bulk is achieved in the potential silene precursors Me2Si(X)—C(Y)(SiMe3)(SiBu-t3)77. For X = F and Y = Na the trisilylmethane 123 transforms into a silene/THF adduct 124-THF in the presence of Me3SiCl (equation 30). Removal of THF destabilizes the silene 124, which can be either trapped or rearranged by a 1,3 H-shift to give 125. 

Silenes of the family Me3SiR1Si=C(OSiMe3)Ad-l 137 undergo a complex silene-to-silene photoisomerization reaction90,94,96. When silenes 137 are generated by photolysis of acylsilanes 138, the isomeric silenes 139 and 140 are formed in a subsequent reaction. The reaction was followed by UV and NMR spectroscopy. The disappearance of 138 cleanly follows first-order kinetics and the overall kinetics were consistent with the transformation 138 -> 137 -> 139. 137 as well as 139 were characterized by NMR spectroscopy and, in addition, the structure of 137 was established by trapping with methanol. The identity of 139 and 140 was confirmed by the isolation of their head-to-tail dimers from which crystals, suitable for X-ray analyses, were isolated (equation 34)90. 

By crystallizing the stable silaethene from diethyl ether/THF we isolated - according to the X-ray structure determination - a tetrahydrofuran adduct (Scheme 6) [16]. This adduct, which could be transformed into the "naked" silaethene, represents the first example of base adducts of silenes which themselves play a central r61e in my following statements. 

Silene Insertion into CH Bonds By thermolyzing complexes of silaneimines with ethyldimethyl amine in solvents, one surprisingly observes the formation of insertion products of the silenes into a-C-H bonds of the donor [3]. For example, the silaneimine tBu2Si=NSiChBu2 slowly transforms in benzene even at room temperature into the mentioned insertion product (Scheme 9) [12]. 

The solution to the dilemma of elimination of LiF was solved by employing the same method Wiberg used to generate silenes, that is, the addition of Me3SiCl220. These transformations are summarized in Scheme 4. 

A similar thermal transformation occurs in 1-phenyl-l-methyl-2-neopentylsilene210. Relative to the retro-ene fragmentation of hydrocarbons, that of silenes is favored thermodynamically since it trades a C=Si bond for a C=C bond. 

A remarkable photochemical silene-to-silene isomerization was observed108 when a solution of the stable l-tert-butyl-l-(trimethylsilyl)-2-(trimethylsiloxy)-2-(l-adamantyl)-silene was irradiated in the UV. The product, 1-methyl-l-(trimethylsiloxy)-2-(terf-butyldimethylsilyl)-2-(l-adamantyl)silene (70), led to two different dimers. The principal one, a head-to-tail dimer, was isolated as a solid, and its structure was identified unambiguously by an X-ray structure determination. When the tert-butyl group was replaced by a phenyl, the photochemical transformation of the starting silene was much more complicated. 

Also, the 2-siloxetane adducts of several substituted cyclopentadienones with the relatively stable silenes l,l-bis(trimethylsilyl)-2-(trimethylsiloxy)-2-phenylsilene and 1,1-bis(trimethylsilyl)-2-(trimethylsiloxy)-2-terf-butylsilene, generated in situ from their dimers by refluxing in dioxan or toluene, were isolated (equation 125)298, while similar reactions with fluorenone and tetraphenyl-a-pyrone yielded products of further transformations of the 2-silaoxetanes. 

The photochemical reactions that the silyldiazoalkanes undergo involve silylcarbene chemistry. In line with reactions described for the more conventional carbenes, a variety of processes are common to both types. Thus irradiation brings about 1,2-alkyl shifts, for example, when 515 is photolysed in alcohols to afford a carbene, which transforms into a silene. Four products 516-519 are obtained in good yield. The formation of 519... 

The complex reaction sequence shown in equation 34 might provide some rationalization. The formation of the silylcarbene 141 is suggested, based on experimental results from related reactions , but there is no evidence for the formation of 141 nor for a silylene intermediate. Thus, the transformation 137 142 might proceed via a dyotropic rearrangement as well. The facile 1,3-methyl shift in 2-trimethylsilylsilenes which interconverts 142 139 is well known from Wiberg -type silenes . 139 (R = i-Bu) is stable in solution at room temperature over days and isomerizes only slowly to 140 (R = t-Bu) which rapidly dimerizes giving a 1,3-disilacyclobutane . 

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