Nucleophilic displacement of halogen

The nucleophilic displacement of halogens by pentafluorophenoxide ion resulted in the formation of the corresponding esters [31] (equation 29) (Table 12). Reactions of trifluoromethanesulfinyl fluoride with fluoro alcohols in the presence of sodium fluoride or cesium fluoride are used to prepare sulfmates [32] (equation 30) (Table 12). 

An interesting variation on nucleophilic displacement of halogens includes the use of N-sulfinylamide 180 to generate optically pure 2-(l-hydroxybenzyl)piperidine 181 <06S687>. 

An effort to improve the efficiency of the nucleophilic displacement of halogens in a cyclocondensation reaction of primary amines with alkyl dihalides for the synthesis of N-phenylpiperidine through the use of microwave irradiation (80100 W, 120 °C, 20 min, 96% yield) was reported this year <06JOC135>. 

Nucleophilic displacement of halogen by amines is an important method of introducing amino groups into the anthraquinone ring system. In the Ullmann reaction the displacement is catalysed by metallic copper or by copper ions so that relatively mild conditions can be used. Mechanistic studies suggest that copper(I) ions exert a catalytic effect via complex formation. Derivatives of 1,4-diaminoanthraquinone are of considerable industrial significance. Many compounds are prepared from the reduced form of quinizarin (6.6). 

Intramolecular nucleophilic displacements of halogen from u> -halohexylamines is an obvious route to saturated azepines and was the route by which hexamethyleneimine was first prepared. The method has since been superseded as a route to simple hexamethyl-eneimines but is still used for highly substituted derivatives (67MI51600). However, high dilution techniques are necessary in order to avoid polymerization intermolecular N-alkylations are minimized by using amine protecting groups, e.g. Af-tosyl. 

The nucleophilic displacement of halogens by fluorine can be successfully carried out by reaction with potassium fluoride under several conditions. Primary and secondary alkyl halides 1 are converted to the corresponding fluorides 2 by heating with a saturated aqueous solution of potassium fluoride in the presence of catalytic amounts of hexadecyltributylphosphonium bromide.56 Small amounts of the corresponding alcohol and the olefinic elimination product are formed as side products. While neopentyl bromide does not react at all, polyhaloalkanes give only products of elimination or hydrolysis. Chlorocyclohexane gives only the elimination product.56... 

The nucleophilic displacement of halogens in aromatic compounds by fluorine is aided by utilizing an appropriate catalyst. Polymer-supported aminopyridinium salts have been found to be versatile catalysts for the synthesis of aryl fluorides. The advantage of the catalyst is that it can be recycled and used again. l-Chloro-4-nitrobenzene (3) is converted to l-fluoro-4-nitrobenzene (4) in 71 % isolated yield using this method. The catalyst used has the structure 5.91... 

Nucleophilic Displacement of Halogens at Saturated Carbon Atoms Box 13.1 The Concept of Hard and Soft Lewis Acids and Bases (HSAB) Illustrative Example 13.2 Some More Reactions Involving Methyl Bromide Illustrative Example 13.3 1,2-Dibromoethane in the Hypolimnion of the Lower Mystic Lake, Massachusetts Polyhalogenated Alkanes — Elimination Mechanisms... 

Nucleophilic Displacement of Halogens at Saturated Carbon Atoms... 

As has already been mentioned, nucleophilic displacement of halogen atoms in position 2 and 4 occurs without difficulty. Reduction of 2-chloro- and 2,4-dichloro-thieno[2,3-d]pyrimidines with sodium borohydride gives the corresponding 2-chloro-3,4-dihydro derivatives (Scheme 100) (80CPB3172, 81JMC376,81JHC67). 

This involves the direct nucleophilic displacement of halogen in an alkyl halide by an alkoxide ion (the Williamson synthesis) (Expt 5.72), and the method is particularly useful for the preparation of mixed ethers. For an unsymmetrical ether [e.g. t-butyl ethyl ether (7)], the disconnection approach suggests two feasible routes. 

This method cannot be applied to polynitro amines, since these are so weakly basic that they can be diazotised only under special conditions in strongly acidic media (Section 6.7.1, p. 922). In such cases use may be made of the susceptibility to nucleophilic displacement of halogen when activated by ortho and para nitro groups. Thus the valuable reagent 2,4-dinitrophenylhydrazine (Expt 6.93) is readily prepared by reacting l-chloro-2,4-dinitrobenzene with hydrazine. Reaction with ammonia similarly gives 2,4-dinitroaniline (cognate preparation in Expt 6.93). 

Bordwell, F. G. Cooper, G. D. The effect of the sulfonyl group on the nucleophilic displacement of halogen in a-halo sulfones and related substances./. Am. Chem. Soc. 1951, 73, 5184-5186. 

The use of these cathodically generated anions Rm for nucleophilic displacement of halogen in Rm X offers an elegant synthesis for unsymmetric bimetallic species, e.g.,... 

HF amine complexes are also employed for nucleophilic displacement of halogens and hydroxyls. Olah and coworkers used the reaction with and without the help of NCVBF for preparing fluoroadamantanes and fluorodiadamantanes28. They also found that an electrophilic salt would extract an hydride a to a C—Br moiety, followed by F attack on the resulting carbocation to form vicinal fluorobromo alkanes (equation 15)29. 

As in synthesis of 7r-bonded divalent compounds, the principal method of synthesis is nucleophilic displacement of halogen from the divalent metal halide as shown in Eq. (34). 

This substitution does not occur with cyanide as the nucleophile, nor with a carboxyl group in the place of the nitro group.405 Nucleophilic displacement of halogen by thiophenol occurs in 1-substituted 2-halogenobenzimidazoles.403... 

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