Disilanes thermolysis

Keywords Silylene / Disilane / Thermolysis / Cycloaddition / Electronic Stabilization... 

Difluorocarbene generated by the thermolysis of trimethyltnfluoromethylsilane reacts with disilanes by insertion into the silicon-silicon bond [S] (equation 9) Thermolysis of pentafluoroethyltnfluorosilane at 200 °C gives tetrafluoro ethylidene carbene, which reacts with phosphorus trifluonde to give trifluoro vinyltetrafluorophosphorane [9] (equation 10) and with perfluorotnmethylphos-phine to give perfluorodimethyhsopropylphosphine and perfluoro-2-butene [9] (equation 10)... 

Thermolysis. Hexaphenyldisilene does not undergo thermolysis to Ph3Si. This might probably be due to a long Si-Si bond. However, hexamethyldisilane is believed to dissociate homolytically. If true, the end product is an isomer of the starting disilane and formed in quantitative yield. 

Insertion into Si H and Si Si Bonds. Silylenes, generated by thermolysis of cyclotrisilanes, inserted into the Si—Cl or Si—H bonds of monosilane to yield a variety of disilanes, which could be further functionalized. In contrast to carbenes, the insertion of silylenes into C—H bonds has not been observed. However, the insertion into Si—H bonds has been studied extensively. The occurrence of direct insertion has been indicated by formation of nongeminate homocoupling products. ... 

Difluorocarbene generated by the thermolysis of trimethyltnfluoromethylsilane reacts with disilanes by insertion into the silicon-silicon bond [S] (equation 9)... 

Recently, flow thermolysis of disilanes has been used by Heinicke and coworkers to obtain the silylenes Me(Cl)Si, Me(MeO)Si and Me(Me2N)Si, which were trapped with... 

Remarkably facile thermal generation of a silylene has recently been demonstrated for the pentacoordinate alkoxydisilane 92416. Intramolecular N—>-Si coordination in 92 was shown by X-ray crystallography. This disilane underwent thermolysis at 110°C in toluene, or 90 °C in DMF, producing the silylene 93, which was trapped with 2,3-dimethylbutadiene and diphenylacetylene (Scheme 26). The 2 1 adduct with diphenylacetylene was shown to have one nitrogen intramolecularly coordinated to silicon, even though the silicon atom lacks any electronegative substituents. 

Thermolysis of highly hindered l,2-di(adamantoyl)tetrakis(trimethylsilyl)disilane with diphenylacetylene afforded l-silacycloprop-2-ene and l,2-disilacyclobut-3-ene derivatives. The mechanism of the transformation is discussed on the basis of DFT calculations <2006OM3955>. 

Because of the close relationship between silicon and carbon, many attempts have been made to try to synthesize species containing multiple bonds to silicon (Si=C, Si=0, Si=Si, etc.). However, it was not until 1967 that compelling evidence was presented that Si=C might exist in the thermal reaction of 1,1-dimethyl-1-silacyclobutane (equation 90). The first evidence for the existence of Si=Si as transient intermediate was provided in the thermolysis of bridged disilane derivatives (equation 91). Since then, many studies have been published on these unsaturated species, but it was in 1981 that synthesis and characterization of relatively stable crystalline compounds containing Si=C (silene) (equation 92) and Si=Si (disilene) (equation 85) were reported (equations 90-92). 

Thermolysis of hexamethylsilacyclopropane at 60-80°C generates dimethylsilylene and tetra-methylethylene. The half-life of this reaction is greater than nine days at room temperature (75JA2273). Dimethylsilylene can be trapped by various substrates when the thermolysis is conducted in their presence. For example, interception of dimethylsilylene with diethyl silane gave disilane (42) in 48% yield, whereas trapping with 2,3-dimethyl-1,3-butadiene gave 1,1,3,4-tetramethyl-l-silacyclopent-3-ene (43) in 34% yield (Equations (6) and (7)) <75JA7162>. 

Some types of silacyclopropanes are more thermally stable and do not serve as a source of silylene. For instance, thermolysis of dicyclopropylsilacyclopropane (44) in triethylsilane for 17 h gave dimer (45) in 20% yield along with oligomeric material, but yielded no disilane (Equation (8)). 

Tetramethylsilacyclopropene (110) was prepared by flash thermolysis of disilane (109) in the presence of a 10-fold excess of 2-butyne. The product was detected spectroscopically and its methanolysis product isolated (Scheme 39) (76JA3715). 

In addition, cyclotrisilane derivatives can be subjected to photolysis to generate organylsilylenes. For instance, the photolysis of hexakis-t-butylcyclotrisilane (369) in the presence of substituted acetylenes, e.g. (370) affords 1,1,4,4-tetrakis-t-butyl-2,3,5,6-tetraorganyl-l,4-disilacyclohexa-2,5-diene (371) (equation 167)190. Alternatively, thermolysis of disilanes in the presence of alkynes is a viable route to disilacyclohexane derivatives. Thus, thermolysis of l,2-dimethoxy-l,l,2,2-tetramethyldisilane (338) in the presence of diphenylacetylene (370) in a pressure vessel—via reactive intermediates and,... 

While many examples of carbene oxidations have been reported, only four papers on the reaction of silylenes 1 with molecular oxygen have been published. The limited number of experimental studies on the oxygenation of silylenes is mainly due to the lack of suitable precursors.The photolysis of matrix-isolated trisilanes produces silylenes in close proximity to disilenes or other products of the precursor decomposition rather than matrix-isolated silylenes.Gas-phase thermolysis of disilanes and other thermal precursors requires very high temperatures, while the photolysis of diazidosilanes requires short-wavelength UV irradiation. In all of these cases, the yields of silylenes are rather poor. 

Summary Monoaminosilylenes and bis(diethylamino)silylene are formed by thermolysis of amino-substituted disilanes and characterized by trapping with dienes. In the case of MeSiNMe2, cycloaddition reactions were extended to 1,4-diheterodienes, unsaturated ketones and imines allowing an easy synthetic access to functionally substituted unsaturated silicon heterocycles. The syntheses of an isolable, unsymmetric diaminosilylene and of related germylenes and stannylenes are described. The results are presented and discussed in relation to theoretical work on stabilization of donor-substituted silylenes and recent work on isolable diaminosilylenes done by or in cooperation with other groups. 

All known isolable two-coordinated silylenes are cyclic monomers stabilized by bulky substituted amino groups at silicon and in most cases supplemented by a Huckel-type delocalized electron system. They were obtained by reduction of the corresponding dihalogenated derivates [1]. Attempts to synthesize analogous pyrido- ornaphtho-l,3,2A, -diaminosiloles by the same route gave small amounts of the desired pure product or they failed [Id, 2]. This prompted us to investigate (i) the photolysis of bis(trimethylsilyl)diaminosilanes and (ii) the thermolysis of recently reported disilanes [3] and of the trisilanes as alternative methods to prepare stabilized low-coordinated silicon compounds. 

As hinted by the temperature of the pyrolysis shown in Table 1, Atwell and Weyenberg concluded that the more highly alkoxylated compounds have less thermal stability towards degradation/ They also studied the thermolysis of a variety of other hetero-substituted disilanes and determined that the thermal stabilities in terms of the effect of substituent type follow the order/ ... 

A relatively long known access to silylenes is the thermolysis of disilanes , used... 

Another interesting method to obtain cyclotetrasilanes is the [2 + 2]cycloaddition of disilenes. Depending on the nature of R groups disilenes either are stable or undergo a dimerization reaction to a cyclotetrasUane. Tetrakis(trimethylsilyl)disilene, which forms either by thermolysis of methoxytris(trimethylsilyl)silane [104], by treatment of l,2-dipotassiotetrakis(trimethylsilyl)disilane with BrCH2CH2Br [96] or by reaction of the respective fluoride adduct with MgBr2 [105], dimerizes to octakis (trimethylsilyl)cyclotetrasilane (Scheme 6). 

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