Metal Wurtz coupling

The derivatives of structural type (VII) may also be prepared by organo-metallic Wurtz coupling reactions as was the case for smaller ring sizes. 

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

In addition to sodium, other metals have found application for the Wurtz coupling reaction, e.g. zinc, iron, copper, lithium, magnesium. The use of ultrasound can have positive effect on reactivity as well as rate and yield of this two-phase reaction aryl halides can then even undergo an aryl-aryl coupling reaction to yield biaryls. ... 

Polysilanes are polymers in which there is catenation of silicon, that is, where silicon atoms are bonded to each other in a continuous manner. Synthesis of polysilanes involves the Wurtz coupling of diorganodichlorosilanes with sodium metal (Eq. 2-239) [Baldus and Jansen, 1997 Corriu, 2000 Manners, 1996 Miller and Michl, 1989 West, 1986 West and Maxka,... 

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. 

The symmetrical Wurtz coupling has been found to give particularly good yields in the case of the higher alkane homologues. Selected illustrative examples (hexane, octane, decane and dodecane) are to be found in Expt 5.7. A related reaction is to be found in the synthesis of hexa- 1,5-diene (Expt 5.7, cognate preparation), which illustrates the symmetrical coupling of two molecules of allyl iodide by the action of sodium metal. 

Some oligomers were also isolated along with the polysilole when 1.2 equiv of the metal was used at room temperature for 1 day. Tamao and collaborators had earlier reported the Wurtz coupling synthesis of polysiloles.28d... 

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... 

Several comprehensive reviews on the preparation, structure, properties, photochemistry, and redistribution of polysilanes prepared by the Wurtz coupling method have been published.1516 31 33 There are also several excellent reviews on the transition-metal-complex-catalyzed dehydrocoupling of hydrosilanes to polysilanes.34 36 The present section reviews the more recent work on the dehydropolymerization of hydrosilanes to polysilanes, catalyzed by transition-metal complexes under homogeneous conditions. Extensions of the methodology to the dehydropolymerization of hydrogermanes, hydrostannanes, and hydrophosphanes to the corresponding polymers are also reviewed. The literature is covered up to early 1997. 

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). 

A variation of the Wurtz coupling uses other metals to mediate or facilitate the coupling. In certain cases, such variations can be synthetically useful. 

The mechanism of the Wurtz coupling is not well understood, and the currently accepted mechanism involves two steps 1) formation of a carbanionic organosodium compound via metal-halogen exchange and 2) the displacement of the halide ion by the organosodium species in an Sn2 reaction. Alternatively, a radical process can also be envisioned, although to date there has been no experimental evidence to support this assumption. 

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