Disilene-silylsilylene rearrangement

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. 

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%). 

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. 

Insertion of hot silicon atoms into silane is believed to first produce silylsilylene, which can rearrange to the parent disilene (equation 11), as judged by the outcome of trapping reactions66. 

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. 

Additional evidence for the disilene-to-silylsilylene rearrangement was obtained when 2,3-benzo-l,4-diphenyl-7,7,8-trimethyl-8-(trimethylsilyl)-7,8-disilabicyclo[2.2.2]octa-2,5-diene (7) was pyrolyzed at 300 °C in the presence of 2,3-dimethylbutadiene to yield, among others, a trapping product, 11, expected for the trapping of the silylene 9 produced from the primary disilene 8 by a 2,1-trimethylsilyl shift52. Trapping products due to the silylene which would result from a 2,1-methyl shift were not detected (see equation 13 and Section II.A.3.b). 

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