Disilenes cleavage

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... 

As a result of several decades of research it is now known that a polysilane of three or more contiguous silicon atoms is susceptible to reaction by one or more of several pathways when photolyzed, each associated with cleavage of a silicon-silicon bond. The two most common processes observed are the homolysis of a silicon-silicon bond to yield a pair of silyl radicals, and the elimination of a silicon atom from the chain in the form of a silylene. As discussed in Section VII, the use of trisilanes, particularly where the central silicon atom bears aryl groups, has become an important route for the preparation of a wide variety of diarylsilylenes, A Si , many of which have been captured in glasses at low temperature, or have been allowed to dimerize to disilenes by warming. 

The catalysed cleavage of cyclotrisilanes (or trisiliranes) gives a disilene and a silylene. The latter can be trapped by various multiple-bonded compounds, in particular by 1-phenylacetylene leading to the silole 36 (equation ll)51. 

In 1997, Weidenbruch et al. successfully synthesized the first silicon analog of butadiene, hexaaryltetrasila-1,3-diene 64.17 Treatment of disilene 2 with lithium in DME at rt followed by the addition of 0.5 equivalent of mesityl bromide at —30 °C gives 64 as reddish brown crystals [Eq. (29)]. In a proposed mechanism, a half of the lithiodisilene 68, which is formed by the reductive Si-Ar bond cleavage [Eq. (8)], is... 

The reactions of cyclic disilenes with haloalkanes proceed in a similar way. Cyclotrisilene 48 reacts with carbon tetrachloride without cleavage of endocyclic Si-Si bonds to give trans-1,2-dichlorocyclotrisilane 179 stereo specifically [Eq. (84)].24 The a h -stereochemistry is in good accord with the radical mechanism of the reactions as stated above. Similarly, cyclic disilenes 49 and 50 react with carbon tetrachloride and 1,2-dibromoethane giving the corresponding trau.v-1,2-dihalodi-silanes 180 [Eq. (85)].129 Methyl iodide adds to cyclic disilene 50 in an a tz-addition manner to give 181.104... 

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... 

Thermolysis of compound 29 at 150-180 °G in the absence of trapping agents presumably proceeds via the competitive cleavage of the Si-Si and C-C bonds, resulting in simultaneous transient formation of the silene 30 and the disilene 31, which undergo fast cycloaddition to produce trisilacyclobutane 34 (Equation 5) <20020M2049>. 

An alternative method for the preparation of disilenes involves the photolysis of cyclo-trisilanes18 in which a disilene and a silylene are formed simultaneously through cleavage of two Si—Si bonds. The latter species undergoes dimerization to furnish the corresponding disilene (equation 2). 

The Si=Si jr-bond is cleaved in practically all reactions of disilenes while the cr-bond remains intact. Even the action of the very reactive oxygen on the Si=Si bond initially results in the formation of the 3,4-disiladioxetanes 119 which, however, rapidly rearrange to give the cyclodisiloxanes 121. The reactions of the disilenes 9, 10 and 14 [R = Mes, 2, 6-Me2C6H3, 4-f-Bu-2,6-Me2CgH2 (Dmt)] with white phosphorus follow a different course and proceed with cleavage of both Si=Si bonds to furnish the bicyclo[1.1.0]butane derivatives 127-129 (equation 30)126 127. 

The first and as yet only molecule with conjugated Si=Si double bonds is the tetrasilabuta-1,3-diene (tetrasila-1,3-diene) 139, which was prepared as follows. The disilene 13 is treated with excess lithium to give the putative disilenyllithium species 137. In the second step of the sequence mesityl bromide was added in the expectation that the bulk of the aryl group and the poor solubility of mesityllithium would favor halogenation over the competing transarylation. Indeed, the bromodisilene 138 does appear to be formed smoothly but, like 137, it has not yet been positively identified. Intermolecular cleavage of lithium bromide from the two intermediates then furnishes the tetrasilabuta-1,3-diene 139 in 60% yield (equation 32)130. 

The evidence that ,Z-isomerization of 92-95 proceeds by Si=Si bond rotation and not a mechanism involving silylene intermediates, produced by cleavage of the Si=Si bond followed by recombination, rests upon the fact that no trapping products consistent with the intermediacy of the corresponding diarylsilylenes could be detected upon heating the disilenes in the presence of known silylene traps such as methanol, triethylsilane or 2,3-dimethyl-l,3-butadiene. In fact, one tetraaryldisilene has been shown to isomerize by this mechanism, the 1,2-dimesityl-l,2-bis(2,4,6-tris[bis(trimethylsilyl)methyl]phenyl derivatives (E)- and (Z)-97a (equation 70)142,143. Arrhenius parameters for the thermal dissociation of (E)- and (Z)-97a to diarylsilylene 98 are listed in equation 70. 

Hexa-/butylcyclotrisilane 1 [7] which, upon photolysis, furnishes di-rbutylsilylene 2 and tetra-/butyldisilene 3 by concomitant cleavage of two Si-Si bonds [8] reacts with or/ho-methylstyrene to produce not only the silirane 4 but also the 1,2-disiletane 5 as the first example of a [2+2]-cycloadduct of a disilene to an alkene [9],... 

Under slightly different conditions both disilene extrusion and rearrangement were observed. Evidence was presented that the rearrangement occurred as the result of radical cleavage of a carbon-silicon bond, followed by a 1,2-silyl migration in the resulting allylsilane system87 (equation 54). 

Scheme 7. Enthalpies (kcal mor ) of formation and Si-Si bond cleavage reactions of some coordinated disilenes, calculated at the B3LYP/6-31 l+G //B3LYP/6-31 l+G level values in parentheses refer to the reaction enthalpy of the formation of the coordinated disilene from disilene and 1 or 2 molecules of ammonia bond lengths in pm.

The cleavage of the Si-Si bond of a 1,2-dicoordinated disilene is appreciably less endothermic than that of disilene itself, or a mono- or geminally dieoordinated disilene. Based on these thermodynamic parameters, a plausible meehanism of the experimentally observed formation of cyclotrisilanes from dichlorodisilanes 6 and 7 could involve a vicinally dieoordinated disilene. 

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