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%). 

Disilene and its isomer silylsilylene were neither available by standard vacuum flash pyrolysis of precursors 59-63, nor by the more elaborate method of pulsed flash pyrolysis of 60-63, a pulsed discharge in mixtures of argon and mono- and disilane74 or by the matrix photolysis of educts 59-66 using various light sources (Hg lamps, excimer laser)69,70,72, the microwave discharge in disilane 66 or the cocondensation of silicon atoms with SiFLt. 

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... 

Transition states and barrier heights for the silylsilylene (61)-disilene (62) isomerization via 1,2-silyl and 1,2-methyl migrations were investigated by ab initio MO calculations at the 6-31G level (equation 49). The 1,2-silyl migration of disilanylsilylene... 

An attractive, although tentative, alternative would be an alkyl-substituted silylsilylene formed from the polymer chain. Two thermodynamically reasonable routes to such intermediates are possible. The first route (equation 4) involves 1,1-elimination to produce the silylsilylene directly. This route has a precedent in organosilane thermal processes (78, 79). The second route (equations 5a and 5b) involves rearrangement from a silene produced by the disproportionation (46, 80, 81) of two silyl radicals caused by bond homolysis. This type of rearrangement has also been described in the literature (82). The postulated silylsilylenes are also attractive intermediates to explain the rebonding of silicon to carbon atoms other than those in the original a positions (CH insertion), which is obvious from the mass spectral analysis of gaseous products from the laser ablation of isotopically labeled poly(di-n-hexylsilane). 

The reported values of quantum yields for each of the above processes are 0.61, 0.18, 0.21, respectively. Of course these quantum yields are not necessarily applicable if excitation energy is different. For example, according to the recent theoretical thermochemical data given by Ho and co-workers (14,15) the reaction enthalpy for the formation of a silylsilylene and two hydrogen atoms from disilane (decomposition (b)) is about 166 kcal/mol, which is equivalent to the photon energy at 172 nm. On the other hand, the dissociative excitation of disilane can be effected with even longer wavelength of up to... 

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. 

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