Silenes thermal rearrangement

A silene-producing [l,5]-sigmatropic shift of a hydrogen atom in tetramesityldisilene has been invoked76 in an attempt to explain its thermal rearrangement to a silabenzo-cyclobutene (see Section II.B.l.d). 

Evidence for additional silene-to-silylene isomerizations on simple systems is now available. Thus the pyrolysis of a silabicyclo[2.2.2]octadiene precursor for 1-chlorosil-ene229 produces a mixture of matrix-isolated 1-chlorosilene and chloromethylsilylene. In this instance the thermal rearrangement of the silene to the silylene is probably facilitated by the increased exothermicity of the reaction, since halogenated silylenes are particularly stable (equation 95). 

Summary The thermal rearrangement of silylamides into l,l-bis(trimethylsUyl)-2-amino-2-trimethylsilyloxysilenes and their subsequent trapping by 2,3-dimethyl-1,3 -butadiene were investigated both computationally and experimentally, with focus on the geometric and electronic structure of these silenes. 

Barton has reported a wide variety of elegant studies in which various silenes or silylenes have been created, usually thermally, and their subsequent rearrangements investigated in terms of the observed products of trapping (51,53,65,145). It has been clearly established that interconversion between silenes and silylenes, especially where H atoms or Me3Si groups migrate, are facile processes. In some cases, radicals can be the precursors to silenes (65). 

An attractive, although tentative, alternative would be an alkyl-substituted silylsilylene formed from the polymer chain. Two thermodynamically reasonable routes to such intermediates are possible. The first route (equation 4) involves 1,1-elimination to produce the silylsilylene directly. This route has a precedent in organosilane thermal processes (78, 79). The second route (equations 5a and 5b) involves rearrangement from a silene produced by the disproportionation (46, 80, 81) of two silyl radicals caused by bond homolysis. This type of rearrangement has also been described in the literature (82). The postulated silylsilylenes are also attractive intermediates to explain the rebonding of silicon to carbon atoms other than those in the original a positions (CH insertion), which is obvious from the mass spectral analysis of gaseous products from the laser ablation of isotopically labeled poly(di-n-hexylsilane). 

Here we report about 2+2 thermocycloreversions of 1-o-tolyl-l-methyl-1-silacycIobutane (la), 1-m-tolyl-l-methyl-1-silacyclobutane (lb), and 1-p-tolyl-l-methyl-1-silacyclobutane (Ic) resulting in ethylene and corresponding transient 1-o-tolyl-l-methylsilene (2a), l-/w-tolyl-l-methylsilene (2b), and 1-p-tolyl-l-methylsilene (2c) formations, respectively. Silenes 2a-c rearrange thermally yielding appropriate 3,4-benzo-1 -silacyclobut-3-enes 3a, 3b, 3b, and 3c (Scheme 2). 

Several examples in which a C-H bond of a silylene shifts to yield a silene are mentioned in Section III.B.l.b, which discusses the reverse process of thermal silene-to-silylene rearrangement (see in particular References 75 and 229). 

Thermal silylcarbene-to-silene rearrangements have been known for a long time1. The pyrolytic product from trimethylsilyldiazomethane, 1,1,2-trimethylsilene, was trapped in an argon matrix230, and the pyrolysis of bis(trimethylsilyl)diazomethane126 was reported to produce fair amounts of 2,4-bis(trimethylsilyl)hexamethyl-l,3-disilacyclobutane, the expected dimerization product of 2-(trimethylsilyl)-1,1,2-trimethylsilene. A second product was the disilane expected from an ene addition of one... 

Only one of the adducts decomposed thermally to an olefin and the expected trimer of bis(trimethylsilyl)silanone. This was the adduct of 2-mesityl-2-trimethylsiloxy-l,l-bis(trimethylsilyl)silene with fluorenone all others underwent an intramolecular rearrangement and the authors suggest that this is primarily due to steric reasons110. 

Another cycloreversion reaction that leads to a silanimine is the thermal frgamentation of the 2 + 2 adduct of the silene 54 to bis(trimethylsilyl)diimide. It competes with a rearrangement of the adduct to 93 (equation 168). 

When carbene 21 is generated from the diazo precursor photochemically in solution, it reacts with added alcohols by 0,H insertion (equation 55), in contrast to the gas-phase copyrolysis where the silene intermediate is trapped . Similarly, photolysis of 20 in the presence of 2,3-dimethylbutadiene gives mainly vinylcyclopropane 184, while after copyrolysis of 20 and the same diene one finds that most of the vinylcyclopropane rearranged to the cyclopentene, together with the [4-1-2] cycloaddition product of diene and silene 22 °. Furthermore carbene 21, generated photochemically or thermally (at 117-148 °C) in solution, undergoes [1 4-2] cycloaddition to alkynes to give cyclopropenes 185. 

The synthesis of the Wiberg -type silenes is primarily achieved through the metalation of halogenotrisilylmethanes 122 with subsequent mild thermal salt elimination from the metalated species 92-LiX, 97-LrX and 104-LiX . The silenes formed, 92, 97 and 104, rearrange to the silenes 92a, 97a and 104a or are trapped by suitable reagents (equation 29). In the case of R = t-Bu (104a) the silene is metastable and its strnctnre could be determined by X-ray diffraction. ... 

A similar but thermal silene-to-silene rearrangement is reported for the sterically highly crowded silene 143. Upon prolonged heating to 120° /Z-143 isomerizes cleanly to a single isomer of the new silene 144, which was identified by NMR spectroscopy . The silenes 145 and 146 were not detected, although they are regarded as intermediates (equation 35). 

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