Disilenes rearrangement

A ring expansion by a silylsilylene-to-disilene rearrangement has been considered as one of two probable mechanisms in the pyrolysis of a bi(7-silanorbornadien-7-yl), which acts as a synthon for dimethyldisilyne64. The process is discussed in Section X.A. 

The silylsilylene-to-disilene rearrangement has also been under discussion for some time in connection with the pyrolysis of trisilane1. The pyrolysis of 1,1,1,3,3,3-hexa-methyltrisilane has been investigated recently69 it also involves a silylsilylene-to-disilene rearrangement and will be discussed in more detail in Section II.B.l.b. 

Thermal 1,2-diaryl rearrangement in disilenes was first demonstrated for tetraaryldisilene 13, which was found to give a mixture of (Z)- and (E)-S.64 More recently, kinetic data, as well as the scope and limitation of the 1,2-rearrangement, have been reported (Scheme 8).11,12 Similar rearrangement was also observed for 2,6-dimethyl-4-t-butylphenyl derivative 14 to give a mixture of (Z)- and ( )-6, but not for disilenes Mes(R) Si=SiMes(R) (3 R = /-Bu 7 R = N(SiMe3)2 8 R = Tip). These re-... 

Compounds 71 are precursors to the bicyclobutanes, as shown in Scheme 18 heating of the reaction mixture led to their disappearance, with formation of 70 and 70. At the same time rearrangement of 70 to the exo-exo isomer 70 took place. This appears to be the only reaction of disilenes known to occur with inversion of configuration at silicon. 

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%). 

West and coworkers studied the photolysis of several adducts of disilenes with ketones, i.e. 1,2-disiloxetanes83. Based on the products obtained when the photolysis was carried out in ethanol as a trapping agent, it appears that the heterocyclic disiloxetane 179 decomposed to the silanone 180 and the silene 181, each trapped by ethanol to give the adducts 182 and 183, respectively (Scheme 29). In the absence of a trapping agent the silene photochemically rearranged to 184. A related 1,2-disilathietane 185 showed similar behavior (Scheme 29184. 

In other studies, the photochemical rearrangements of disilenes with the general structures A2Si=SiB2 — ABSi=SiAB were observed165. This was believed to be a dyotropic process. 

Kira and coworkers recently described the preparation of the first cyclic disilene, 396210. On photolysis at 420 nm 1,2-silyl rearrangement occurred to yield the bicyclic[1.1.0] system, 397 (equation 52), which on standing in the dark reverted to the relatively stabler disilene. 

The first 1,2,4-thiadisiletane results from the reaction of carbon disulphide and the hindered silylene [2,4,6- (Me3Si)2CH 3C6H2]MesSi (TbtMesSi ) formed from the Z-disilene precursor. The mechanism is thought to involve a skeletal rearrangement of the 3,3 -spirobi(l,2-thiasilirane) intermediate formed by silylene addition to each carbon-sulphur double bond (equation 31)64. 

Rearrangements of disilenes to a-silylsilylenes are, however, well established (equation 52)5 and are involved in the exchange of substituents between a silylene center and an adjacent silicon, a process that has been called a transposition (equation 53)106. 

An ABSi = SiAC-type disilene is synthesized by the reduction of dibromosilane 81 with KCs.16 The facile disilanylsilylene-silyldisilene rearrangement of silylene 82 formed by the reduction is proposed to give disilene 32 ... 

Migration of a substituent on a disilene giving the corresponding silylene [pathway (3)] should be considered as a pathway for the /AZ-isomerization but occurs usually with much higher activation energies than pathways (1) and/or (2). The pathway (3) and related dyotropic rearrangement are discussed in detail in Section IV.A.3. 

Neither water nor methanol reacts with the disilene moiety of 3,4-di-iodocyclotetrasilene 52 but rather with the silicon-bound iodine atom to form tricyclic compound 224 via skeletal rearrangement or 3,4-dimethoxycyclotetrasilene 225 [Eq. (107)].25 The rapid conversion of 52 into 224 or 225 may be explained by the formation of the ionic intermediate 226 via dissociative activation (SN1 mechanism) that is facilitated by bulky t-Bu3Si groups. In favor of this mechanism,... 

The Si=Si jr-bond is cleaved in practically all reactions of disilenes while the cr-bond remains intact. Even the action of the very reactive oxygen on the Si=Si bond initially results in the formation of the 3,4-disiladioxetanes 119 which, however, rapidly rearrange to give the cyclodisiloxanes 121. The reactions of the disilenes 9, 10 and 14 [R = Mes, 2, 6-Me2C6H3, 4-f-Bu-2,6-Me2CgH2 (Dmt)] with white phosphorus follow a different course and proceed with cleavage of both Si=Si bonds to furnish the bicyclo[1.1.0]butane derivatives 127-129 (equation 30)126 127. 

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