Disilenes photolysis

The method employed for the first synthesis of stable disilene l,1 the photolysis of linear trisilanes, has been the most widely used synthetic method for disilenes. However, photolysis of cyclic trisilanes and dehalo-genation of dihalosilanes and 1,2-dihalodisilanes are also good routes in some cases (Table I). 

The disilene synthesis by the photolysis of linear trisilanes proceeds via initial formation of a silylene followed by its dimerization (Eq. 1). Disilene 1 has now become a common organometallic reagent. A detailed synthetic procedure employing photolysis of the corresponding linear trisilane is described in Inorganic Syntheses (Eq. 2).6... 

In the photolysis of trisilanes RR Si(SiMe3)2 (R = R = Mes R = Mes, R = Tip R = R = Tip), the rate of disilene formation was predicted to increase in this order because the Si—Si—Si bond angles decrease in the same order, as revealed by X-ray structural analyses, thus facilitating the formation of Me3SiSiMe3. Although the predicted trend in the rates of photolysis was found, the differences were small.12... 

The first synthesis of a disilene by photolysis of the corresponding cyclic trisilane was reported by Masamune et al. in 1982.14 This method has been adapted for the synthesis of a variety of stable and marginally stable disilenes (Eq. 6).15-20 More recently, Kira et al. synthesized the first example of a stable tetrakis(trialkylsilyl)disilene 22 by this method.21... 

Disilenes are very reactive toward a variety of unsaturated bonds, such as C=C, C=C, C=N, C=N, C=0, C=S, N=N, and N=0, giving interesting four-membered ring compounds that are otherwise difficult to synthesize.3b Reactions using stable tetraaryl- or dialkyldiaryldisilenes and marginally stable tetra-t-butyldisilene (21), which is generated by photolysis of the corresponding cyclotrisilane, are summarized in Schemes 10 and 11, respectively. Some of them merit further comments. 

The cyclotrisilane is one of a few cyclotrisilanes and is, upon photolysis, a useful source of silylenes and disilenes. 

Co-photolysis of 2,5-di(trifluoromethyl)-l,3,4-oxadiazole with cyclotrisilane, which under these conditions decomposes to afford tetra-(/-butyl)disilene and di-(/-butyl)silylene, provides dihydrodioxadiazadisilocine and trihydro-oxazatrisiline derivatives (Scheme 13) <1996JOM355>. 

The photochemical cleavage of Si-Si bonds of cyclotetrasilanes has been reported to generate several reactive intermediates. For example, Nagai and co-workers reported that silylene and cyclotrisilane are generated during the photolysis of a cyclotetrasilane with a folded structure.73 Shizuka, Nagai, West, and co-workers reported that the photolysis of planar cyclotetrasilanes gives two molecules of disilene.74... 

Scheme 38 Photolysis of ladder polysilane and trapping products of extruded cyclic disilene (substituents /-Pr, except as noted)308...

The photolysis of cyclic polysilanes results in ring contraction with concomitant extrusion of a silylene fragment. Although the formation of two reactive intermediates potentially complicates mechanisms for product formation, it has provided a useful method for the synthesis of both unstable and stable disilenes... 

Phenyltrimethyldisilene (15) and (E)- and (Z)-l,2-dimethyl-l,2-diphenyldisilene (16) were also observed as transient absorption spectra by laser flash photolysis of the precursors in methylcyclohexanes28. The absorption band at 380 nm, assigned to the disilene 15, reached maximum intensity at ca 10 ns after the excitation and then started to decrease. The half-life assigned to 15 was 700 ns. The logarithm of the decay profile of the transient absorption at 380 nm versus time shows a very good linear relationship, indicating that the decay of the transient absorption fits first-order kinetics. This result shows that intramolecular isomerization or proton abstraction from the solvent is the origin for the decay of the disilene 15, which survives in solution only for several nanoseconds. 

Alkoxy- and amino-substituted disilenes (17, 18 and 19) are produced by photolysis of the corresponding masked disilenes (20, 21 and 22)29. Thus, when a degassed 3-methylpentane (3-MP) matrix of 20 was irradiated at 77 K, the corresponding alkoxy-substituted disilene 17 was produced (equation 5) as indicated by its broad band at 373 nm. 

Generation of various phenyl-substituted disilenes by the photolysis of the masked disilenes, 7,8-disilabicyclo[2.2.2]octadiene derivatives, is quite useful, especially for unsymmetrically substituted disilenes. Investigation of the regiochemistry as well as the diastereochemistry of alcohol addition to phenyltrimethyldisilene was made possible for the first time by using this method27. 

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. 

Subsequent studies by Sakurai and coworkers of the photolysis of disilyl-bridged aromatic compound 78, indicated that two interesting (not necessarily independent) processes were occurring, namely elimination of the disilyl group as a disilene, 79, another reactive intermediate of silicon chemistry, and secondly, Si-C homolysis to a diradical 80, which overall underwent a 1,2-silyl shift yielding the isomeric disilyl-bridged compound 8141-42 (Scheme 11). 

The photolysis of silicon-bridged dihydroaromatic compounds has been utilized to prepare reactive organosilicon species such as disilenes. Thus, photolysis of the... 

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. 

Peralkylcyclodisilagermanes and peralkylgermadisilaoxetanes on photolysis at 254 nm were observed to yield predominantly the corresponding disilene and disiloxirane, respectively, in addition to the dialkylgermylene85. 

Other recent examples89-92 of the photolysis of trisilanes to prepare silylenes as precursors to disilenes will be found in Section VII. 

Cyclotrisilanes are easily converted to a mixture of silylene and disilene. Reactions of the silylene (l-Bu)2Si and the disilene l-Bu2Si=Si(Bu-l)2 produced by photolysis of the cyclotrisilane 293 with cyclopentadiene, furan, and thiophene have been reported recently152, as were their reactions with 2,2 -bipyridyl, pyridine-aldimine and ketoimines153. 

Among the limited studies of the photochemistry of disilenes, it has been reported that photolysis of tetramesityldisilene, 328, at 254 nm in pentane gave rise to the dihydrido compound 329 (equation 27), probably as a result of the radical character of the excited... 

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. 

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