Processes halogen-exchanging

Thus, from the series of reactions presented In this paper it appears that a major reaction pathway for chloror-cyclophosphazenes with certain organometallic reagents, involves a metal-halogen exchange process. Subsequent chemistry however appears to be extremely dependent on the metal involved In the organometallic reagent. 

An interesting activation of Cul by alumina and by charcoal has been reported106. These reagents are applied for the iodination of aryl chlorides and bromides to the corresponding iodides via a halogen exchange process. The presence of the support is found to be essential and no reaction is detected in its absence. 

Direct evidence for a combination of catalytic fluorination and chlorination [4] was obtained from radiotracer studies in which fluorinated chromia catalysts were labelled with the short-lived (t /2 = 110 min) / + emitting isotope fluorine-18 [11]. Using this isotope it was possible to probe the interactions between HF and various fluorinated chromia catalysts more directly than had been possible hitherto. Three types of surface F-containing species were differentiated, weakly adsorbed HF which was easily removed by an inert gas flow, non-labile F, believed to be bound directly to surface Crin, and catalytically active F which could be incorporated into the organic products [12]. The controversy between dismutation (concerted F-for-Cl and Cl-for-F transfers) and non-concerted halogen exchange processes has been resolved more recently and the evidence is described later in the chapter. What is clear from this early work however, is the importance of aluminium and chromium(III) oxides as catalyst precursors. Fluorination of the surfaces of these oxides is slow (cf [12]) and although there are many references to alu-... 

In addition to the liquid-phase processes discussed above, vapor-phase halogen-exchange processes have also been developed. A variety of metal... 

Evidence has been presented for the mechanism of the halogen-exchange processes in the systems Me3BX3 + BY3, where X and Y are halogens.159... 

Lithio-benzothiophenes can be generated, and reacted with electrophiles, if the temperature is kept low. ° Direct deprotonation of benzothiophenes follows the usual pattern for flve-membered heterocycles and takes place adjacent to the heteroatom,"" and in concord with this pattern, metal-halogen exchange processes favour a 2- over a 3-halogen the sequence below shows how this can be utilised to develop substituted benzothiophenes. 2-Lithiated reagents react with electrophiles for example with p-toluenesulfonyl cyanide, 2-cyano derivatives are produced and similarly, 2-trimethylstannylbenzofuran and -benzothiophene and benzofuran-2- and benzothiophene-2-boronic acids can be prepared. 

The intrinsic tendency to lithiate at C-2, then C-5, taken with metal-halogen exchange processes for the... 

The regioselectivity of metal-halogen exchange processes mirrors the C-H metallations discussed above thus 2,4-dibromothiazole reacts at C-2 hrstly/ ... 

L-Proline (LI) was also identified as an effective additive to promote the reaction of aryl halides and CuCN at relatively low temperatures (80-120 °C), affording the corresponding arylnitriles in good yields (Scheme 9.10) [73], In this case, aryl bromides could be converted directly into arylnitriles, without the requirement of the extra halogen-exchanging process. 

A further rapid bimolecular reaction of HOSCN then probably takes place, although further oxidation of this intermediate by I2 cannot be eliminated. In the reaction of iodine monochloride with alcohols, the slow step is the attack of I+ on the alcohol to yield ROI. There is no evidence for any chlorinated product. An n,m.r. study of the exchange between Cl and Cl a in aqueous solution has been reported and comparisons have been made with other halogen exchange processes. The reduction of perchlorate to chloride by molecular hydrogen in the presence of tungsten carbide catalysts has also been described. ... 

Peterson Oleflnation Reactions. McNulty and Das have shown that the treatment of chlorobis(trimethylsilyl)methane (1) with 5-BuLi at low temperatures favors lithium-halogen exchange processes over deprotonation pathways (eq 1)The resulting lithi-ated derivative 2 undergoes Peterson oleflnation reactions with a... 

B., Byrne, D., Lee, H., Yee, N.K. and Senanayake, C.H. (2009) Formation of 2-trifluoromethylphenyl Grignard reagent via magnesium-halogen exchange process safety evaluation and concentration effect. Org. Process Res. Dev., 13, 1426-1430. 

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