Silylenes Silylsilylenes

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%). 

Whereas the analogous carbenes easily isomerize wherever possible to compounds containing doubly bonded carbon atoms even under the conditions of matrix isolation, silylenes are almost as stable as the corresponding substances with doubly bonded silicon atoms. For example, methyl- and silylsilylene lie just 4 and 8 kcalmol-1 above silaethene and disilene, whereas the difference between ethene and methylcarbene is as high as 70 kcalmol-1 149-151 As a consequence, silylenes are often key intermediates on the way to other highly reactive silicon compounds discussed above. 

Also in 1992 Becerra and Walsh suggested that the increase in the rate of silane loss in the 3 to 30% conversion middle stage of silane pyrolysis is due to the chain process (equation 32) in which the chemically activated silylene insertion product gives rise to a silylsilylene that can consume another silane molecule64. 

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. 

Due to the unique characteristics of Si = Si double bonds as discussed in Section III.A, at least the following three types of mechanisms emerge for the E,Z-isomerization between ( )- and (Z)-ABSi = SiAB (126), as shown in Eqs. (46)-(48) (1) rotation around the Si-Si bond, (2) dissociation to the corresponding silylenes, and (3) 1,2-migration of a substituent to form the corresponding silylsilylene. 

For the Z-to-E isomerization, a mechanism via silylsilylene complex 238 [Eq. (112)] has been proposed. Mechanisms including the following processes are excluded on the basis of the theoretical calculations (1) dissociation-association equilibrium between the corresponding disilene and Fe(CO)4, (2) removal of one CO from 236Z and then Si-Si bond cleavage forming the corresponding bis-silylene... 

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). 

CSiH3+ to be most stable with hydrogen bonded to carbon. Photodecomposition of disilane gives products indicative of three decomposition pathways involving either silylene or silylsilylene formation, or Si—H bond homolysis. ... 

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