Alkali Wurtz coupling

The main method used to synthesise polysilanes 10.7 involves the thermally induced Wurtz coupling reaction of organodichlorosilanes with alkali metals [eqn (10.7)]. Although improvements in this process have been reported e.g. by the use of ultrasound), the harsh conditions for this reaction tend to limit the side groups that can be successfully introduced to non-functionalised alkyl and aryl units and makes scale-up unattractive. 

WurtZ-Type Coupling of Dihalosilanes. Several approaches have been developed for the synthesis of polysilanes. However, the most only utilized method is based on the Wurtz-type alkali metal coupling of dichlorosilanes. Both homo- and copolymers can be prepared this way (eq. 

As described in Section II.E, polystannanes, which can be prepared by the Wurtz coupling of chlorostannanes with alkali metals and by the dehydrocoupling of hydrostannanes, exhibit intriguing physical properties. The Sita group reported a peculiar polymerization of -Bu2SnH2 with a RhH(CO)(PPh3)3 catalyst [Eqs. (21a) and (21b)].63... 

The use of cesium compounds in C—C bond formation reactions has so far been limited to a few cases. Vogtle and KiUener [21] observed that the use of cesium metal in the Mtiller-Rdscheisen procedure [22] of the Wurtz coupling reaction of p-xylylene dibromide, intended to yield [2 ]paracyclophanes 1, leads to the [23]paracyclophane lb with the highest yield compared to other alkali metals. The yields of the other cyclic compounds Ic-e thereby are much lower compared to the application of sodium metal (Fig. 1) [23]. 

This route is useful for the synthesis of organolithium compounds, but is often hindered with the more reactive and heavier alkali metals because of the tendency toward Wurtz coupling (equation 2). 

Further evidence of monomer adsorption on a metal surface is provided by the extensive studies of Richards and his co-workers (14). As is well known, alkyl bromides in tetrahydrofuran vigorously react with alkali metals, say lithium, yielding the Wurtz coupling products. The violent reaction slows down on addition of aromatic monomers like styrene, and the nature of... 

When only one unit can be changed between the metal oxidation state, coordination number, and electron accounts, two metal atoms are required to form R-M species (Equation (2)). Equation (2) is also favorable for lithium metal, but the remaining alkali metals are limited due to the possibility of Wurtz coupling. 

III. COUPLING REACTIONS A. Alkali Metals (Wurtz Coupling)... 

The most common route to polysilane-high polymers is by the Wurtz-type alkali metal-mediated polycondensation of dichlorosilanes. However, , u-dihalooligosi lanes can also be coupled in a similar manner. Wurtz-type polymerization of BrSiMe2SiR2SiMe2Br led to bimodal molecular weight distributions of alternating co-polymers of the type (SiMe2SiR2SiMe2) (R = hexyl or Bu),17 as in Equation (1). 

Polysilylenes are usually prepared by the Wurtz-type reductive coupling of dichlorosilanes with alkali metals. 

Wurtz-Fittig reactions are C-C coupling reactions of alkyl halides with alkali metals. This synthetic procedure is often applied for Si-Si coupling of silicon halides. See (a) A. Wurtz, Ann. Chim. Phys. 44, 275 (1855). (b) B. Tollens, R. 

In spite of its many drawbacks (as will be evident during the course of this discussion), the alkali metal-assisted Wurtz-type coupling reaction remains by far the most widely used procedure for the preparation of polysilanes... 

The Wurtz and Ullmann couplings of organic halides are usually carried out with alkali metals in ethereal solvents, or copper in a high-boiling solvent, commonly DMF. Surface phenomena occurring on the metal and the existence of radical intermediates were evidenced in the reaction of iodobenzene with copper.194... 

Polysilanes are synthesized by following five methods (1) reductive condensation of dichlorosilanes with alkali or alkali earth metals (Wurtz-type coupling), (2)... 

ABSTRACT. Polysilanes, (-SiRR -)n, represent a class of inorganic polymers that have unusual chemical properties and a number of potential applications. Currently the most practical synthesis is the Wurtz-type coupling of a dihalosilane with an alkali metal, which suffers from a number of limitations that discourage commercial development. A coordination polymerization route to polysilanes based on a transition metal catalyst offers a number of potential advantages. Both late and early metal dehydrogenative coupling catalysts have been reported, but the best to date appear to be based on titanocene and zirconocene derivatives. Our studies with transition metal silicon complexes have uncovered a number of observations that are relevant to this reaction chemistry, and hopefully important with respect to development of better catalysts. We have determined that many early transition metal silyl complexes are active catalysts for polysilane synthesis from monosilanes. A number of structure-reactivity correlations have been established, and reactivity studies have implicated a new metal-mediated polymerization mechanism. This mechanism, based on step growth of the polymer, has been tested in a number of ways. All proposed intermediates have now been observed in model reactions. 

The synthetic techniques used to make Si-Si bonds have changed little since the earliest developments. By far the most widely used methods involve variations on the Wurtz-type coupling where chlorosUanes are reductively coupled using an alkali metal (Eq. (14.4)) [35,40). Various metals have been employed under a variety of conditions with Na and Li remaining the most popular. Other techniques include reaction of metallosilanes with halosilanes (e.g., Eq. (14.5)) [41], photolysis of hydrosilanes [42], and silylene insertion reactions (e.g., Eq. (14.6)). 

Silacycles are coupling products of the reaction of dichlorodiorganosilanes with alkali metals [42]. This resembles the Wurtz-type coupling reaction of chlorosilanes in educts and reaction conditions and thus tolerates the same narrow range of functional groups. For this reason, it is mainly the methyl and phenyl substituted silanes which have been investigated. 

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