Rearrangement silylene

Barton and co-workers" performed flash vacuum pyrolysis (FVP) on trimethyl-silylvinylmethylchlorosilane (30), resulting in the production of trimethylchlorosi-lane (30%), trimethylvinylsilane (11.5%), and most interestingly, ethynylmethyl-silane (34, 11.9%). A proposed mechanism for the synthesis of 34 (Scheme 10) begins with the lo.ss of trimethylchlorosilane to form silylene 31, which can rearrange either to silaallene 32 or to silirene 33, both of which can lead to the isolated ethynylsilane. 

Grobe15 has described the pyrolysis of 1 -methyl-1 -vinyl- and 1,1 -diviny 1-1-silacyclobutanes 166 which led to the formation of methylvinylsilene and divinylsilene, respectively. Under the experimental conditions used, it was suggested that the silenes rearrange to exo-methylene- 1-silacyclo-propanes 167 which extrude methylsilylene or vinylsilylene, respectively. In support of this proposal, when the reactions were carried out in the presence of 2,3-dimethylbutadiene, the anticipated silylenes were trapped as their respective l-silacyclopent-3-enes 168. 

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... 

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). 

The ability to insert in many element-element bonds is an important property of 1 the r -p1 rearrangement of the pentamethylcyclopentadienyl ligands during the reaction is a prerequisite to show a silylene-type reactivity. From a preparative point of view it is worth mentioning that element-silicon bonds which otherwise are difficult to form are easily accessible with the help of 1. In addition, the leaving group character of the pentamethylcyclopentadienyl substituents allows further chemical transformations (vide infra). 

Regarding this proposal, it should be noted that while 1,1-eliminations on Si-Si-C units to generate silylenes are well known thermal processes (54) the photochemical variant seems not to have been described. The rearrangement of silylsilylenes (4) to disilenes is known to be rapid (55), and silyl radical addition at the least hindered site would produce the observed persistent radical. Preliminary evidence for the operation of 1,1-photoelimination processes in the polysilane high polymers has been obtained, in that the exhaustive irradiation at 248 nm of poly(cyclohexylmethylsilane) (PCHMS) produces —10-15% volatile products which contain trialkylsilyl terminal groups. For example, the following products were produced and identified by GC— MS (R=cyclohexyl,R = methyl) H(RR Si)2H (49%), H(RR Si)3H (19%), R2R SiH (2%), R 2RSiRR SiH (5%) and R2R SiRR SiH (7%). 

An attempt to investigate the possible photochemical rearrangement of silacyc-lobutenylidene (132) to give silacyclobutadiene produced seven silylene species in the... 

Scheme 14.18). The silylene-silene rearrangement 27 28 is nearly thermoneutral, with the silene being slightly more stable. The photolysis of a-diazo compounds (30) is the only frequently used reaction path to silenes (31) via a carbene-silene... 

Rearrangement of a silene to a silylene (40) via migration of a Me3Si group has been suggested as a step in the gas-phase silylene to silylene rearrangement. Labeling experiments, however, have indicated an alternative mechanism (Scheme 14.23). ... 

Rearrangements of disilanes to a-silylsilenes are well established and are involved in the exchange of substituents between a silylene center and the adjacent silicon.Pulsed flash pyrolysis of acetylenic disilane (41) gave rise to the acetylenic silene (42), which subsequently rearranged to the cyclic silylene, 1-silacyclopropenylidene (43). Irradiation of the cyclic silylene resulted in the isomerization to the isomeric 42, which itself could be photochemically converted into the allenic silylene (44). Both 42 and 43 also were reported to isomerize on photolysis to the unusual (45), which was characterized spectroscopically (Scheme 14.24). 

Evidence for the generation of a silylene was obtained in the presence of triethylsilane, which afforded 2,3-diphenyltetrasilane (82) in 79% yield. The reactions of some 1,3-diynes, R C=CC=CR (83), with silylenes afforded the silylene adduct. The course of this reaction strongly depends on the nature of the substituents R and R . When R is an alkyl group, the bis(silirene) (84) is formed initially but undergoes rearrangement to a bicyclohexadiene derivative upon longer photolysis (Eq. 9)... 

Not surprising, the insertion of silylene into the S-H bond of hydrogen sulfide shows qualitatively the same features as the reaction with water. However, the complexation energy (36.0 kJ mol-1) and the activation energy (56.1 kJ moL1) of the rearrangement are lower at the MP4SDTQ/6-31G // 3-21G level.14 A compi increases to 55.3 kj mol-1 when calculated at the MP2/CEP-31g(2d,p)//MP2/CEP-31g(2d,p) level.83... 

When 32 is photolyzed in the presence of methyl allyl ether (62),50 51 the generated silylene 13 initially coordinates to the ether oxygen to give 63, w ich subsequently rearranges to allylsilane 64 (Scheme 17), a reaction... 

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