Disilenes formation

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... 

Lee VY, Takanashi K, Ichinohe M, Sekiguchi A (2003) A chemical trick how to make a digermene from a disilene, formation of 3A-l,2,3,4-disiladigermetene. J Am Chem Soc... 

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 general, an increase in steric demand of the substituents on silicon atoms appears to lead to the formation of more stable disilenes. For example, 3, which has two f-butyl groups, is more stable than 1 while 4, which has two 1-adamantyl groups, is more stable than 3.9 Disilene 4... 

More recently, a new mode of cis-trans isomerization of a disilene has been suggested for the extremely hindered disilene 27. As will be detailed in Section VIII. B, 27 undergoes thermal dissociation into the corresponding silylenes. Monitoring the thermolysis of (Z)-27 at 50°C by H and 29Si NMR reveals a competitive formation of the isomerized ( >27 and benzosilacyclobutene 37, which is most likely formed by intramolecular insertion of silylene 36 into the C—H bond of the o-bis(trimethylsilyl)-methyl group (Scheme 3).22,59 This suggests the possible occurrence of cis-trans isomerization via a dissociation-association mechanism. 

As shown in Scheme 3, disilene 27 undergoes thermal dissociation into silylene 36 under very mild conditions (about 50°C).22,59 This represents the first example of such dissociation. The formation of 36 was confirmed... 

Compounds 71 are precursors to the bicyclobutanes, as shown in Scheme 18 heating of the reaction mixture led to their disappearance, with formation of 70 and 70. At the same time rearrangement of 70 to the exo-exo isomer 70 took place. This appears to be the only reaction of disilenes known to occur with inversion of configuration at silicon. 

Tokitoh et al. have synthesized a sterically hindered disilene TbtMesSiSiMesTbt 210 that is kinetically stable but thermally labile, thus providing the corresponding silylene TbtMesSi 211, whose reaction with isocyanides bearing bulky substituents has resulted in the formation of the first stable silylene isocyanide complexes TbtMesSiCNR (212, R = Tbt 213, R = Mes ) (Scheme 30).327... 

The chalcogenolysis of the disilene 721 with N20, S, Se, and Te results in formation of the three- and four-membered heterocyles 748-752 (Scheme 98). The addition of the isonitrile /-BuNC to the disilene 721 affords... 

Figure 8.2b corresponds to the superimposition of signals from at least two persistent paramagnetic species. The nature of these species is still unclear even if the g factors are consistent only with silyl radicals carrying three silyl groups as in 6. Mechanistic schemes for their formation could be suggested based on the addition of transient silyl radicals to the transient disilenes generated photolytically [14]. 

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... 

Scheme 10.19 Formation of a 1,2-Si202 ring by oxidation of disilenes with oxygen.

Compounds with a Si=Si or Ge=Ge bond (i.e., disilenes and digermenes) can be isolated when the double bond contains bulky substituents. Only a few cycloaddition reactions with diazo compounds are known, and two reaction modes have been observed. One of the paths leads to the formation of disiliranes 184 (242) and digermiranes 185 (243) (Scheme 8.42), probably by ring contraction of an initially formed [3 + 2] cycloaddition product. The other path involves a 1,1-cycloaddition of the diazoalkane to give disilaaziridine 186 (244) and digermaazir-idine 187 (245). This nitrene-like reactivity is rather uncommon although some intramolecular examples are known (see Section 8.6.1). 

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