Disilenes photochemical

When silylenes are generated photochemically in hydrocarbon matrices in the presence of electron-pair donors, they may form Lewis acid-base complexes that act as intermediates in the silylene dimerization to disilenes.3233 In a typical example, Mes2Si(SiMe3)2 was photolyzed in 3-meth-ylpentane (3-MP) matrix containing 5% of 2-methyltetrahydrofuran. At 77 K, dimesitylsilylene (Amax 577 nm) was formed. When the matrix was... 

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... 

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%). 

Routes to the reactive species silenes, silylenes and disilenes, and the photochemical changes which they themselves undergo. 

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. 

The photolysis of trisilanes makes possible the formation of homo- and heteroleptic tetraaryldisilenes as well as 1,2-diary ldisilenes that may have alkyl, silyl or even amino groups as substituents. The heteroleptic disilenes formed in this way are usually obtained as mixtures of the E- and Z-isomers which can be isolated as pure compounds after fractional crystallization and/or can be converted thermally or photochemically to the other isomer. 

In contrast to the acyclic disilenes, very little is yet known about the reactivity of the cyclic members of this class of compounds. The photochemically induced isomerization of the cyclotetrasilene 141c to the bicyclo[1.1.0]butane derivative 140b (Section V.A) has already been mentioned. Similarly to the tetrasilyldisilenes69 the cyclotrisilene 151 also reacts spontaneously with tetrachloromethane even at —70°C to furnish the trans-l, 2-dichlorocyclotrisilane 157 (equation 41)137. 

Kira, Iwamoto and Kabuto reported the isolation and characterization of the first stable cyclic disilene, hexakis(t-butyldimethylsilyl)tetrasilacyclobutene (69, R=t-BuMe2Si), together with its facile photochemical conversion to the corresponding tetrasilabicy-clo[1.1.0]butane 70 and its thermal reversion to 69 (equation 52)130. 

Hexa-substituted cyclotrisilanes have been shown to be excellent precursors of disilenes. Tetraisopropyldisilene (280), for example, can be obtained by photoextrusion of di-isopropylsilylene (281) from hexaisopropylcyclotrisilane (282). Z- and E-substituted disilenes have similarly been obtained on photodecomposition of tri-t-butyl-trimesitylcyclotrisilanes photochemically induced Z,E-isomerization has been reported in this disilene. ... 

A new and potentially valuable photochemical route to tetra-methyldisilene (175) has been reported and involves irradiation of 7,7,8,8-tetramethyl-7,8-disilabicyclo[2.2.0]octa-2,5-diene(176)in an argon matrix at 10 the disilene readily undergoes [ 4 + 2] cycloaddition to benzene to regenerate the precursor. The silane-selenones (177), reactive intermediates with a silicon-selenium double bond, can be photochemically generated and trapped with hexamethylcyclotrisiloxane as shown in Scheme 9. Irradiation of hexamesitylcyclotrisilane (178) in the presence of azobenzene... 

It is evident from the above material that the elimination of bridging silyl groups, as silylenes, disilenes or silenes, is a facile and useful photochemical reaction, but that occasionally interesting and unpredictable behavior may be observed. 

The reaction types known to produce disilenes are summarized in Chart 1 and apply regardless of the ultimate stability of the product. Historically, thermal 4 + 2 cycloreversion of complex l,2-disilacyclohex-4-enes, i.e. a retro-Diels-Alder fragmentation came first8 -14, followed by silylene dimerization15 -17. This early work produced only indirect evidence for the formation of disilenes as reactive intermediates, but in retrospect it is clear that these species were indeed produced. The early history of the subject is discussed in Reference 1. The first directly observable and isolable disilene resulted from the dimerization of photochemically produced dimesitylsilylene18, ushering in a new era in disilene chemistry. These more recent developments are described in Reference 2. 

Substituted disilenes of marginal stability in solution were obtained chiefly in one of the two photochemical processes already mentioned, or by photochemical 4 + 2 cycloreversion. 

Tetraisopropyldisilene was obtained from the disilabicyclo[2.2.2]octadiene 4 by photochemical 4 + 2 cycloreversion, as judged by the formation of its methanol adduct in a trapping experiment48. When 4 was irradiated in the absence of a trapping agent, octaisopropylcyclotetrasilane was isolated along with some reduction product, 1,1,2,2-tetraisopropyldisilane (equation 5). It is quite possible, but not certain, that both of these are formed in reactions of an electronically excited disilene. 

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