Tetrasilabutadienes

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. 

The treatment of hexakis(2,4,6-triisopropylphenyl)tetrasilabutadiene with maleic anhydride furnished the 2,9-dioxa-5,6,7,8-tetrasilatetracyclodecan-3-one derivative 29 (Scheme 19). The reaction pathway involves a [2+2] cycloaddition of one of the Si=Si bonds of tetrasilabutadiene to the highly reactive C=0 group, followed by a second cycloaddition of the remaining Si=Si bond across the C=C double bond to complete the formation of the final product 29 <20010M2451>. 

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. 

Weidenbruch and coworkers have recently succeeded in extending their chemistry of tetrasilabutadiene (vide supra) to its heavier congener, i.e. hexaaryltetragermabuta-1,3-diene 1008°. In this case, tetragermabutadiene 100 was prepared from digermene 4 via 99 by the synthetic method similar to that of its silicon analogue (Scheme 37)79. 

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. 

Three additional rings systems containing endocyclic Si=Si double bonds were obtained from the tetrasilabutadiene 139. Since Diels-Alder products of this compound have as yet not been synthesized, probably because of the steric overcrowding and the large 1, 4-separation of the terminal silicon atoms, it was surprising to find that the action of sulfur on 139 resulted in a formal [4 + 1] cycloaddition to furnish the thiatetrasilacyclopentene 154 in high yield (equation 39)142. 

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. 

Reactions. Experimental results of reactions of (Tip)2 Si= Si(Tip) Si(Tip)=Si(Tip)2 are summarized in Scheme 40. Treatment of the tetrasilabutadiene with a small amount of water gave an analog of tetrahydrolhran, an oxatetrasilolane derivative, probably via the 1,2-addition and the following rearrangement. Addition of an excess amount of water resulted in the formation of the tetrasila-l,4-diol derivative, which showed no tendency to ehminate water with the formation of the oxatetrasilolane derivative. 

Tetrasilabutadiene, trisilacyclopropene (cyclotrisilene), and tetrasilacyclobutene (cyclotetrasilene) are isolated and the structures are determined by X-ray crystallography. ... 

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. 

The red cyclotetrasilene 20 (puckered Si ring 2.36 A may arise via an intermediate disilyne R Si SiR (6), which may then transform via the tetrasilabutadiene R HSi=SiR -SiR =SiHR (6j ) into 20. Thus, the intermediate must be formed by a migration of two R groups associated with a relief of the steric crowding of 6, and must then react further by an electrocyclic process in a conrotatory sense with formation of 20. 

The authors group addressed the question as to whether molecules with two or more neighboring Si=Si double bonds can be synthesized. This chapter details the isolation of a tetrasilabutadiene and a tetragermabutadiene, as well as the formation of stable or intermediate compounds with conjugated Si=C and Ge=C double bonds. 

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. 

Addition Reactions. While cycloadditions to 6 are still exceedingly rare, reactions of 6 with small molecules, as summarized in Scheme 8.1, were more successful. Treatment of the tetrasilabutadiene with small amounts of water led, via the 1,2-addition product 16 to the rearranged oxatetrasilacyclopentane 17, an analogue of tetrahydrofuran. With an excess of water the tetrasilane-l,4-diol 18 was obtained, which showed no tendency to eliminate water with the formation of 17. ... 

Keywords Conjugation / Density Functional Calculations / Tetrasilacyclobutene / Tetrasilabutadiene / Trisilacyclopropene... 

The recent synthesis of tetrasilabutadiene 1 [1] and of tetrasilacyclobutene 2 [2, 3] in the groups of Weidenbruch and Kira, respectively, has renewed the interest in Si4H6 isomers, their properties and their interconversions. The bicyclic compound 3 [4] has attracted considerable interest in the past, both theoretically and experimentally, due to the possibility of bond-stretching isomers [5,6]. More recently the groups of Kira [7] and Sekiguchi [8] were also succesful in the synthesis of silyl-substituted trisilacyclopropenes 4. 

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