Disilenes Tetrasilabutadiene

Keywords Disilenes / Tetrasilabutadiene / Oxidation Reactions / Hydrolysis Reactions... 

Keywords silylenes, disilenes, tetrasilabutadiene, multiple bonds, addition reactions, cycloadditions, germanium compounds... 

The reaction of tetrasilabutadiene 64 with Cl2 gives 1,2,3,4-tetrachlorotetrasilane 182 [Eq. (86)].125 As chlorine gas is added, the solution of 64 turns from dark red to blue-black to orange, and finally to colorless. On this basis, the addition is proposed to proceed stepwise via the initial formation of a charge-transfer complex between 64 and Cl2 to give 183, which is converted to disilene 184 and then to 182 by the addition of a second molecule of Cl2. 

Thus, in 1997 Weidenbruch and coworkers have reported the synthesis and isolation of the first stable conjugated Si-Si double-bond compound, i.e. hexatipyltetrasilabuta-1,3-diene 9479. Tetrasilabutadiene 94 was prepared through a rather unique synthetic route starting from the corresponding tetraaryl-substituted disilene 95 via the mono-lithiated disilene 96 as shown in Scheme 35. 

Compound 828 and two other disilenes 36 and 37, formed by the reaction of the tetrasilabutadiene (see later) with quinones in the presence of water43,47, also belong to this group. 

The molecular structures of eleven acyclic disilenes are described in detail in two recent review articles covering the literature up to about the middle of 19956,49a. The number of known compounds has doubled in the subsequent few years49b. These new, structurally characterized disilenes include not only a tetrasilabutadiene and the first molecules with an endocyclic Si=Si double bond, described in separate sections, but also some acyclic disilenes and, in particular, the unusual compound 8. Thus, another brief survey of all the known molecular structures of disilenes reported through the end of 1999 seems to be justified. 

Molecules with endocyclic Si=Si double bonds, like the tetrasilabutadiene 139, have also only recently become accessible and their chemistry has not yet been reviewed. In the less than four years since 1996 two three-membered, two four-membered and three five-membered ring compounds containing an Si=Si double bond have been prepared and, in most cases, their structures have been characterized by X-ray crystallography. Although two synthetic routes dominate in the synthesis of acyclic disilenes, the cyclic disilenes have mostly been prepared by the special methods summarized below. 

The situation is different for the molecules containing endocyclic Si=Si double bonds which just recently became accessible and for the as yet sole known tetrasilabutadiene, the chemistry of which still remains mostly unknown. Not only are further representatives of these classes of compounds to be expected but also novel modes of reactions that have as yet not been observed for the acyclic disilenes. Another interesting question is whether it will be possible to prepare molecules with an extended system of conjugated double bonds. 

In the following text, results concerning the syntheses and reactions of disilenes of types 1-4 are presented. In addition, dehalogenations of the disilenes 2 and 4, and dehydrobromination of the disilane R HBrSi-SiBrHR, which possibly lead to disilynes of types 5-7, will be described (in some cases, these reactions obviously proceed via cyclotrisilenes as well as cyclotetrasilenes and even tetrasilabutadienes). The disilenes 1 (R = Ph), 3, and 4 and, (obviously) the disilyne 7, could be obtained under normal conditions due to an adequate steric shielding of the reactive >Si=Si< and -Si Si- entities with bulky silicon-bound groups. 

Recently, we obtained the first and, as yet, only tetrasilabuta-1,3-diene 6 from the tetraaryldisilene as follows. The disilene was treated with excess lithium to give the putative disilenyllithium compound 4. In the second step of the reaction sequence, mesityl bromide was added in the expectation that the bulk of this aryl group and the poor solubility of mesityllithium would favor halogenation over the competing transarylation. In fact, the bromodisilene 5 does appear to be formed smoothly but, like 4, has not yet been unambiguously identified. Intermolecular cleavage of lithium bromide from the two intermediates 4 and 5 then furnished the tetrasilabutadiene 6 in up to 60% yield. 

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