Wurtz-type condensation

The currently accepted mechanism of the alkali metal-mediated Wurtz-type condensation of dichlorosilanes is essentially that outlined in COMC II (1995) (chapter Organopolysilanes, p 98) which derived from studies by Gautier and Worsfold,42 and the groups of Matyjaszewski43 and Jones,22,44,45 a modified polymerization scheme of which is included here. The mechanism was deduced from careful observations on the progress of polymerizations in different solvents (such as those which better stabilize anions and those which do not), at different temperatures,44 with additives, and with different alkali metal reductants. Silyl anions, silyl anion radicals,42 and silyl radicals28,46,47 are believed to be involved, as shown in Scheme 3. 

A linear phenylene-ethylene copolymer is obtained by a Wurtz-type condensation of 1,2-dibromoethane and 1,4-dibromobenzene. A cross-linked variation can be obtained on addition of 1,3,5-tribromobenzene to the reaction mixture. Equation 119 illustrates the reductive scission of a cyclic ether, catalyzed by such polymers in the presence of lithium, followed by quenching with cyclohexanone . 

Dehalogenation.1 Cuprous chloride dissolves in DMSO to give a colorless clear solution, which reacts with certain benzylic and allylic halides to give products of Wurtz-type condensation. 

The usual synthesis of polysilanes is by Wurtz-type condensation of diorganodichloro-silanes with finely-divided sodium metal in an inert diluent. Either homopolymers or copolymers can be made (equation 44). The organic groups can be widely varied, to... 

Wurtz-Type Condensation - The Most Versatile Method... 

In contrast to growing interest with polysilanes, their preparation method is still limited. They have been traditionally prepared by the Wurtz-type condensation of dichlorosilanes with alkali metal (Kipping method). This method, however, requires drastic reaction conditions and, hence, is often limited in the type of substituents that are allowed to be located on the dichlorosilane monomer and also has a disadvantage in controlling the unit structure in the copolymerization. Several modified or alternative methods have been... 

Multiblock polysilane copolymers can be made by modified Wurtz-type condensation reactions of dichlorosilanes [34] however, it is difficult to obtain simple di-block polysilane copolymers. The electroreductive method in this study provides a new procedure to synthesize well-controlled di-block polysilane copolymers. It is possible to control the sequence length of each block by tuning supplied electricity and monomer concentrations. 

The chemical and physical properties of polysilanes are strongly influenced by substituents attached to the polymer backbone. In this respect, heteroatom-substituted polysilanes should be very much intriguing on their properties. However, heteroatom-functional substituents such as amino and alkoxy groups on silicon cannot survive under the vigorous synthetic conditions of polysilanes by the conventional Wurtz-type condensation of dichlorosilanes. Therefore, it is difficult to prepare heteroatom-functional polysilanes. We have recently found that amino-substituted masked disilenes can be prepared and polymerized successfully to unprecedented amino-substituted polysilane of the completely alternative structure, poly[l,l,2-trimethyl-2-(dialkylamino)disilene]. 

Among a variety of new preparative methods for creating Ae Si-Si bond such as transition metal catalysed dehydrocoupling of hydrosilanes [12-14a], ring opening polymerization [11,12], polymerization of masked disilenes [14b], the most practice one is still the Wurtz type condensation of organomonochloro- or dichlorosilanes which requires a stoichiometric amount of an alkali metal and high reaction temperatures in the cases of Na and K [3-9, 12] (Equations 1 and 2). Lower temperature variants have yet recently been reported [14c]. 

Another condensation method was investigated by Carothers and co-workers and reported in 1930. They reacted decamethylene dibromide with sodium in a Wurtz-type reaction but found it difficult to obtain polymers with molecular weights above 1300. 

On the other hand, to accelerate alcoholysis of NaH and KH in solutions of benzene or THF [22.1] - [2.22] - ctyptands are used (which bind alkaline metal into a rather stable chelate complex) [1004], Quite stable, volatile per-fluorotert-butoxides were first obtained in reactions of LiH or NaH with (CF3)3COH they distill at atmospheric pressure at 218 and 232°C, respectively [467] (the application of metals would presumably lead in this case to condensation of Wurtz type). Li and Na hydrides are used as cheaper than metal raw materials for production of the corresponding metal alkoxides. In particular it has been suggested that the equipment used in production of MH could be cleaned from its residue by the mixture ofEtOH and the aromatic hydrocarbon (40 to 60% by volume). After hydrogen evolution is completed the solvent is eliminated under vacuum at < 90°C the residue is MOEt with the content of the main product > 98% [342],... 

The use of polysilanes as photoinitiators of radical polymerization was one of the hrst means whereby they were incorporated within block copolymer structures [38 0], albeit in an uncontrolled fashion. However the resulting block copolymer structures were poorly defined and interest in them principally lay in their application as compatibilisers for polystyrene (PS) and polymethylphenylsilane blends PMPS. The earliest synthetic strategies for relatively well-defined copolymers based on polysilanes exploited the condensation of the chain ends of polysilanes prepared by Wurtz-type syntheses with those of a second prepolymer that was to constitute the other component block. Typically, a mixture of AB and ABA block copolymers in which the A block was polystyrene (PS) and the B block was polymethylphenylsilane (PMPS) was prepared by reaction of anionically active chains ends of polystyrene (e.g. polystyryl lithium) with Si-X (X=Br, Cl) chain ends of a,co-dihalo-polymethylphenylsilane an example of which is shown in Fig. 2 [43,44,45]. Similar strategies were subsequently used to prepare an AB/ABA copolymer mixture in which the A block was poly(methyl methacrylate) (PMMA) [46] and also a multi- block copolymer of PMPS and polyisoprene (PI) [47]. 

Summary Reacticm of a-heteroatom-substituted oligosilanes with potassium alkoxides is of interest to obtain insight into the mechanism of the Wurtz type polymerization of halosilanes. It also provides access to building blocks with unique reactivity. Reactions with fluoro- and methoxy-substituted silanes exhibited initial formation of silylenoid species which can undergo self-condensation. This property is less pronounced with the alkoxysilanes, which allowed for the isolation and structural characterization of an a-methoxy-silyl potassium compound (3). 

A further method often used for preparation of organosilicon compounds is a Wurtz-type synthesis in which a silicon tetrahalide is condensed with an alkyl or aryl halide by means of metallic sodium in ether or a hydrocarbon solvent 313... 

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

The preparation of succinic esters by anion coupling with polyhalides has been described/ The scope of the Wurtz coupling of a-halogenoesters to give alkyl-substituted succinic esters has been investigated optimum conditions utilize a-bromoesters and zinc in THF, the presence of copper(n) salts being mandatory to avoid Dieckmann-type condensation to form p-ketoesters. 

Evidently, the types of compound compatible with this mixture are not many, but it has been used successfully for various 1,3-dioxacycloalkanes and hydrocarbons. It must not be used with halogen compounds although chloro- and bromo-alkanes can be dried safely over sodium films, provided that they are initially at least Grignard dry and free of oxygen. In the author s laboratory specimens of CHjClj and MeBr which had been over sodium films under vacuum for several years were analysed and found to contain only traces of the Wurtz-condensation products. 

Kolbe found Gerhardt s four types too restricted to accommodate all inorganic and organic compounds, but Wurtz claimed that they could, and could also be derived from one type as condensed hydrogen types, and... 

Wurtz, in reply to Kolbe s criticism of Gerhardt s types, thought these could be reduced to three, corresponding with condensed hydrogen types ... 

The nineteenth century was a time when chemists were exploring chemical reactivity with no knowledge of mechanism and with a limited ability to identify products when compared to today s methodology (see Chapter 1, Section 1.1). Nonetheless, many reactions were discovered that are used today, and the structures of the products were accurately determined. L. Chiozza in 1856 (Italy) and Adolphe Wurtz (France 1817-1884) in 1872 reported independent experiments in which an aldehyde was treated with an alkoxide base in an alcohol solvent heated to reflux (heated at the boiling point of the alcohol). When this reaction was cooled to room temperature and treated with dilute aqueous acid at low temperatures, a P-hydroxy aldehyde (known generically as an aldol) was isolated. Because of the type of product formed, this reaction has come to be called the aldol condensation. 

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