Unsaturated disilanes

Keywords unsaturated disilanes, metathesis reactions, inorganic-organic hybrides... 

Table 1. Selected spectroscopic data of the unsaturated disilanes (4 - 6) H, C, Si NMR (measured in C Dj), UV-VIS (measured in THF solutions) and EI-MS.

We have chosen for our metathetical study in the case of unsaturated disilanes the first generation of Grubbs catalysts because of the reduced degree of possible isomerization of the products during the time of polymerization [7]. 

We report here the first ADMET polymerization in the presence of Gmbbs catalyst, starting from the new unsaturated disilane 4 (Scheme 2). 

A cyclic unsaturated disilane, the disilacycloheptene 292, was prepared from acetylene and the disilane 291 with the same catalyst ... 

Ishikawa and coworkers have studied the unique reactivity of strained cyclic disilanes (Equation 9.11) [35]. Transition metals, especially those of Group 10, readily insert into the Si—Si bond of disilacyclobutene 118 and can catalyze the addition of that bond across a variety of unsaturated acceptors. In the case of Ni(0)-catalyzed reactions of 118 with trimethylsilyl alkynes, insertion was found to occur both in a 1,2-and in a 1,1-fashion. The latter of these pathways implies a 1,2-silyl-migration, presumably occurring at the metal center. A nickel vinylidene intermediate was therefore proposed, though efforts to prove its existence were inconclusive. Similar vinylidene intermediates have been proposed by Ishikawa and coworkers to account for migrations observed in related palladium- and platinum-catalyzed reactions [36]. 

Disilanes connected via both the Si-Si bond and an organic or an organo-metallic linkage are activated toward reaction with unsaturated substrates to form cyclic bis(silyl) products. Reactions of 3,4-benzo-l,l,2,2-tetraethyl-1,2-disilacyclobutene with diphenylacetylene or benzaldehyde catalyzed by Ni(PEt3)4 proceed with addition across the multiple bond to form the ring-expanded product.54 A second product is formed in a lesser amount in the case of diphenylacetylene, with insertion into the Si-C bond [Eq. (13)]. 

The 1,4-disilylation of a,/3-unsaturated ketones has been reported by Ito and co-workers.172 Unsymmetrically substituted disilanes, PhCl2SiSiMe3 and Cl3SiSiMe3, undergo reaction in the presence of catalytic amounts of palladium complexes with tertiary alkyl phosphine or bidentate phosphine ligands to yield /3-silyl ketones [Eq. (64)]. 

An interesting reaction of disilane is the transition metal-catalyzed insertion of unsaturated hydrocarbons. The palladium-mediated reaction of cyclotrisilane 26 with phenylacetylene to afford the seven-membered carbosilane 51 (Equation 3) indicates that this general reaction scheme is also applicable to strained cyclic trisilanes <20040M490>. 

The hydroxyl ions again act catalytically, and it has been proposed10 that they do so by preliminary co-ordination to the covalently unsaturated silicon atoms. The complete hydrolysis of disilane in alkaline solution therefore produces seven equivalents of hydrogen and leaves only hydrated silica as a residue ... 

Because of the close relationship between silicon and carbon, many attempts have been made to try to synthesize species containing multiple bonds to silicon (Si=C, Si=0, Si=Si, etc.). However, it was not until 1967 that compelling evidence was presented that Si=C might exist in the thermal reaction of 1,1-dimethyl-1-silacyclobutane (equation 90). The first evidence for the existence of Si=Si as transient intermediate was provided in the thermolysis of bridged disilane derivatives (equation 91). Since then, many studies have been published on these unsaturated species, but it was in 1981 that synthesis and characterization of relatively stable crystalline compounds containing Si=C (silene) (equation 92) and Si=Si (disilene) (equation 85) were reported (equations 90-92). 

The double silylation of unsaturated organic compounds catalyzed by group 10 metals is a convenient synthetic route to disilacyclic compounds. Nickel and platinum complexes, in particular, are excellent catalysts for the transformation of disilanes. Cyclic bis(silyl)metal complexes2,3 have been implicated as key intermediates in the metal-catalyzed double silylation of alkynes, alkenes, and aldehydes however, the intermediates have not been isolated due to their instability. We now describe (i) the isolation of the reactive intermediates cyclic bis(silyl)metal compounds (1) with bulky o-carborane unit 4 (ii) the generation of a new class of heterocyclic compounds (4-5) by the stoichiometric reaction of the intermediates with a variety of substrates such as an alkyne, dione, and nitrile 4 and (iii) the facile double silylation of alkenes and alkynes (10,12-14) catalyzed by the intermediate under mild conditions.5... 

A BINAP-Pd complex brings about enantioselective 1,4-disilylation of a,3-unsaturated ketones with chlorinated disilanes, giving enol silyl ethers in 74-92% ee (eq 21). ... 

Silicon-based polymers form a dimensional hierarchy from disilanes, to crystal silicon, and through polysilanes, ladder polymers, siloxenes, polysilane alloys, clusters, and amorphous silicons and include unsaturated systems, such as polysilenes, hexasilabenzenes, and so on. Their properties depend basically on the network dimensions and can vary from conducting (metallic) and semiconducting to insulating. 

Cathodic addition of trichloromethylbenzene to ketones gives or.yS-unsaturated ketones [139]. Methyl esters of trifluoromethylbenzoic acid can be defluorinated by reduction at lead cathodes in MeOH [140,141]. Fry and Touster [142] have reduced a benzal halide at stainless steel in DMF containing trimethylchlorosilane to synthesize both the (cy-halo-benzyl)silane and the benzal disilane. 

Summary The silylenes MeSiNMe2 and MeSiNH(iPr) can be thermally generated from disilanes. In rapid copyrolytic gas phase reactions between disilanes and trapping reagents the intermediate silylenes and dienes or heterodienes furnish a variety of five-membered unsaturated silicon heterocycles in 35-65 % yield. 

Oxidative addition of Si-Si bonds onto palladium(O) has long been presumed to be involved in a number of palladium-catalyzed bis-silylation reactions of unsaturated carbon compounds. The oxidative addition and its reverse reaction, i.e., reductive elimination, may be in rapid equilibrium, whose direction is influenced by the structure of disilanes and ligands on the palladium atom. In spite of early reports on the formation of bis(organosilyl)palladium(II) complexes [14,15], a well-characterized complex was first synthesized in 1992 by reaction of hydro disilanes with hydridepalladium complex, probably through initial activation of Si-H bond followed by silylene migration (see Sect. 2.3) [16]. Since... 

A variety of catalytic bis-silylation reactions, i.e., addition of Si-Si bonds across multiple bonds, have been reported. Generally the reaction mechanism can be presented as follows (1) formation of bis(organosilyl) transition-metal complexes through activation of Si-Si bonds, (2) insertion of unsaturated organic molecules into the silicon-transition-metal bonds, and (3) reductive elimination of the silicon-element (mostly carbon) bonds giving bis-silylation products. The final step regenerates the active low-valent transition-metal complexes. Not only appropriate choice of transition metal, but also choice of suitable ligand on the transition metal is crucially important for the success of the bis-silylation reaction. In addition, substituents on the silicon atoms of disilane are also of importance. 

The reaction affords triphenylsilylrubidium and -cesium, via the disilane, in situ [4]. Trifurylgermane adds to a,/ -unsaturated carbonyl compounds in the presence of catalytic amount of cesium carbonate (Scheme 2.2) [5]. The Ge-H bond in trifurylgermane releases a proton to form the Ge-Cs compound. 

In reactions that are formally analogous to hydrosilylation, transition-metal complexes catalyze the insertion of unsaturated hydrocarbons into other Si-X (X = C, Si, Sn, etc.) bonds. Palladium catalysts seem to work best in many of these reactions. The work of Kumada and coworkers has already been referred to in connection with metal-catalyzed silylene transfer to alkynes (see equation 47)95"97. Sakurai s group has shown that the cyclic disilane 55 will add to alkynes in the presence of a palladium catalyst (equation 110, see also equation 80). The unstrained disilane Me3SiSiMe3 undergoes a similar reaction... 

The reagents couple with chlorosilanes to form disilanes. They undergo conjugate addition to unsaturated esters. 

We have observed that the disilane R HBrSi-SiBrHR (R = SiHR 2), generated according to Scheme 7.4 from the silane R SiH2Cl (obtained by the action of SiH2Cl2 on NaR and Na via the disilane R K Si-SiH R, reacts with supersilyl sodium NaR in THF at -78 °C to form mainly the disilene R HSi=SiHR (3) besides the cyclotetrasilene R Si H (20). The hydrolysis-and air-sensitive disilene 3, which is in fact the first silicon compound with hydrogen atoms bound to unsaturated silicon atoms that has been isolated as a pure substance ( Si NMR for >Si= d of d at S = 141.32 ppm with = 149.9 and Vjjjj = 0.9 Hz), decomposes slowly at room temperature 3 h) with formation of the colorless isomer 19 (Scheme 7.4), and adds MeOH with formation of the colorless compound R HSi-SiH(OMe)R. Unfortunately, crystals of 3 suitable for an X-ray structure analysis have not yet been obtained. 

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