Halogen interchange

The Union Carbide team also utilized the dilithiocarborane chemistry to produce carboranylenesiloxane polymers by the metal-halogen interchange reaction (Fig. 4) between dilithiocarboranes and dichlorosiloxanes.21 Polymers with molecular weights up to 52,000 were synthesized by this procedure. 

Figure 4 Reaction scheme for the metal-halogen interchange reaction between dilithiocarboranes and dichlorosiloxanes for the synthesis of carboranylenesiloxane polymers. (Adapted from ref. 23.)...

In what appears, initially, to be a closely similar reaction, acid chlorides react with alkyl halides under solidtliquid two-phase conditions using sodium hydrogen carbonate in the presence of sodium iodide and tetra-n-butylammonium bromide [45]. Although the mechanism is not clear, it has been proposed that the acid chloride is initially converted into the carboxylate anion. It is also probable that the halogen interchange between the sodium iodide and the alkyl halides enhances their reactivity. Although the yields are high, the availability of the alkyl halides and alcohols are usually similar and there appears to be little to commend this process over the catalysed reaction of the acid chlorides with the alcohols. 

Ketones. Fluoro ketones have been obtained from fluoro acetoace-tates and sulfuric acid.86 Examples are CF3COCH3 and CHF2COCH3. They have also been obtained by halogen interchange, such as CH2FCOCH3 from CH2ICOCH3 and thallium fluoride.96... 

Hydroxy groups in activated positions are displaced by chlorine using phosphorus chlorides as for the pyridazine (680) (71MI42900). Halogen interchange reactions are to be expected. Bromination of the chloropyrazine (681) can be effected using bromine in chloroform (75JHC451). 

For a literature review dealing with the mechanism of the lithium-halogen interchange, see W. F. Bailey and J. J. Patricia, J. Organomet. Chem., 352, 1 (1988). 

Test of these conclusions is obscured by the simultaneous operation of a metal-halogen interchange (c), which can be interpreted as a car-banion displacement at a halogen atom ... 

That a metal-halogen interchange, (c), does take place under conditions of the Wurtz reaction has been shown by the isolation of amyl iodide in 47% yield from the reaction of amyl sodium with methyl iodide.10... 

Although unproven, the mechanism of the halogen interchange was presumed to involve the transient formation of dihalocarbenes and, e.g., BC13F- (59). 

Ruorides and chlorides, with the exception of some polychlorinated aliphatic and aromatic compounds, are quite resistant to lithium-halogen interchange instead, they tend to promote ortho and a lithiations. ... 

Alkenyl-lithiums, which can be prepared by metal-halogen interchange between an appropriate iodide and t-BuLi at — 78°C, undergo cyclizations on warming to room temperature before quenching with a proton source (equation 117) . This cyclization reaction has been extended for the construction of polycyclic alkanes by sequential polyolefinic cyclizations (tandem cyclizations). A propellane system could be constructed in a single step from his-intramolecular cyclization as shown in equation 118 . 

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