Silylenes thermolysis

The trapping of silacyclopentadienes has also been reported recently.115 Using the pyrolysis of 27, or photolysis or pyrolysis of 28, the formation of the silylene 29 was inferred. Further photolysis or thermolysis converted the silylene into silene 30, which could be photochemically isomer-... 

More recently, a new mode of cis-trans isomerization of a disilene has been suggested for the extremely hindered disilene 27. As will be detailed in Section VIII. B, 27 undergoes thermal dissociation into the corresponding silylenes. Monitoring the thermolysis of (Z)-27 at 50°C by H and 29Si NMR reveals a competitive formation of the isomerized ( >27 and benzosilacyclobutene 37, which is most likely formed by intramolecular insertion of silylene 36 into the C—H bond of the o-bis(trimethylsilyl)-methyl group (Scheme 3).22,59 This suggests the possible occurrence of cis-trans isomerization via a dissociation-association mechanism. 

A much explored pathway to simple silenes involves the thermolysis of silacyclobutanes at 400-700°C, the original Gusel nikov-Flowers (155) route. Such temperatures are not readily conducive to the isolation and study of reactive species such as silenes except under special conditions, and flash thermolysis, or low pressure thermolysis, coupled with use of liquid nitrogen or argon traps has frequently been employed if study of the physical properties is desired. Under these high temperature conditions rearrangements of simple silenes to the isomeric silylenes have been observed which can lead to complications in the interpretation of results (53,65). Occasionally phenyl-substituted silacyclobutanes have been photolyzed at 254 nm to yield silenes (113) as has dimethylsilacyclobutane in the vapor phase (147 nm) (162). 

Two indirect routes to silenes, one derived from silylenes and the other from silylcarbenes, are of some generality and importance. Silylenes (e.g., Me3Si—Si—<]) (53) have been derived from the thermolysis of either methoxy or chloro polysilyl compounds. Thermolysis resulted in the elimination of trimethylmethoxy- or trimethylchlorosilane and yielded the silylene, which, based on products of trapping, clearly had rearranged in part to the isomeric silene [Eq. (5)]. Alternatively the silylene Me2Si has... 

Silylated 1,2,3-trisilacyclopentanes (73) under photolysis split off their central Si atom as a silylene (trapped by a scavenger) <85JOM(292)l67>. l,2,3-Trisilacyclopent-4-enes (74) split off only their central silicon atom as silylene by flash vacuum thermolysis (650-720 °C), whereas by photolysis both central and terminal Si atoms may be extruded depending on the nature of the substituents (Ar better than Alk-stabilized silylene) <84CL393>. Pyrolysis of hexamethyl-1,2,4-trisilacyclopentane (75) is unique for Si-compounds since different products arise at normal <76ZAAC(419)249> and low <820M1453> pressure (Scheme 2). 

The reversibility of most silylene addition reactions allows the cycloadduct of a silylene to 1,3-diene to be employed as a sUylene source. Extrusion of a silylene from l-silacyclopent-3-ene (3) has been achieved by thermolysis in the gas phase and also by photolysis in solution (Scheme 14.4). 

Photochemical irradiation of (i-Pr3Si)3SiH (14) with light of 254 nm in either 2,2,4-trimethylpentane or pentane leads to the elimination of f-Pr3SiH and the generation of bis(triisopropylsilyl)silylene (/-Pr3Si)2Si (15). Silylene 15 can also be generated by the thermolysis of the same precursor 14 at 225 °C in 2,2,4-trimethyl-pentane (Scheme 14.11). Reactions of 15 include the precedented insertion into an Si H bond, and additions to the ti bonds of olefins, alkynes, and dienes. 

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. ... 

Belzner64 discovered a mild access to an intramolecularly coordinated silylene by the thermolysis of cyclotrisilanes 105 at moderate temperatures (40-60°C). It was furthermore shown73 that cyclotrisilane 105 and silylene... 

A different decomposition channel is utilized by the silyl-substituted 1-silaallene 617289,29o. In the absence of trapping reagents the transient 617 formed in the thermolysis of 646 at 280 °C undergoes a 1,2-trimethylsilyl shift giving the silylene 648 and finally the 3,5-disilacyclopentene 649 in 25% yield. Alternatively 648 can also be formed from the silacyclopropene 616. The silaindene 650, the formal insertion product of the Si=C bond into the ortho C—H of the phenyl ring, is isolated in 18% yield289. The formation... 

A relatively long known access to silylenes is the thermolysis of disilanes13, used for preparative purposes180 as well as for matrix-isolation studies of silylenes107. In our group this method was recently used in the generation of C2H2Si isomers. Trimethylsilane... 

A silirene may also generate a silylene. In the absence of catalyst, thermolysis of a silirene can give various products resulting from its decomposition into a silylene and an alkyne50. Thus siloles are formed together with 1,2-disilacyclobutanes, 1,4-disilacyclohexadienes and other products. With silirenes bearing bulky substituents on the ring carbon atoms, siloles become the major products (Scheme 9). 

Siliranes are also formed by the reaction of the cyclotrisilane [2-(Me2NCH2)C6H4]6Si3 with terminal and strained internal olefins under mild thermal conditions. The products obtained from the thermolysis of the siliranes thus prepared suggest a thermal equilibrium of the silirane with the cyclotrisilane and the corresponding alkene. This observation provides evidence for an equilibrium between the silylene and the cyclotrisilane and, moreover, proves that free silylenes are involved in the silylene transfer reaction48. 

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... 

Silirane pyrolysis has been employed as a mild route to silylenes since the pioneering work in the 1970s of Seyferth s group on hexamethylsilirane2. An important step forward was made by Boudjouk and coworkers when they found that bulky substituents (such as t-butyl) on silicon increased the stability of the silirane without preventing its thermolysis (equation 35)56. 

Calculations indicate that silacyclopropenes might also be employed as silylene precursors in thermolysis reactions. The enthalpy of dissociation of unsubstituted 1-silacycloprop-2-ene is predicted to be 50.4 kcal mol-1, only ca 7 kcal mol-1 greater than for silirane81. 

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. 

Toward this end, Woerpel and Nevarez examined the possibility of di-tert-butylsilylene transfer from cyclohexene silacyclopropane 58 to imine 169a (Scheme 7.48).123 Thermolysis produced a mixture of silaaziridine 170a and an imine-dimer byproduct (171). The results by Brook and coworkers suggested that if the temperature of silylene transfer were lowered, isolation of 170a without formation of byproduct 171 would be possible. As anticipated, exposure of cyclohexene silacyclopropane 58 to imine 169a in the presence of substoichiometric amounts of silver triflate produced only 170a. This silaazridine could be purified by bulb-to-bulb distillation to afford the product in 80% yield. Copper salts required... 

Summary New silacyclopropanes were synthesized quantitatively under mild thermal conditions by reaction of olefins with cyclotrisilane (cyclo-(Ar2Si)3, Ar = Me2NCH2QH4) 1, which transfers all of its three silylene subunits to terminal and strained internal olefins. Thermolysis of silacyclopropanes 3a und 3b indicated these compounds to be in a thermal equilibrium with cyclotrisilane 1 and die corresponding olefin. Silaindane 13 was synthesized by reaction of 1 with styrene via initially formed 2-phenyl-1-silacyclopropane 3d. Reaction of 1 with conjugated dienes such as 2,3-dimethyl-l,3-butadiene, 1,3-cyclohexadiene or anthracene resulted in the formation of the expected 1,4-cycloaddition products in high yield. 

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