Replacement of halogen

Metal alkoxides may achieve complete replacement of halogen. 

The introduction of tritium into molecules is most commonly achieved by reductive methods, including catalytic reduction by tritium gas, PH2], of olefins, catalytic reductive replacement of halogen (Cl, Br, or I) by H2, and metal pH] hydride reduction of carbonyl compounds, eg, ketones (qv) and some esters, to tritium-labeled alcohols (5). The use of tritium-labeled building blocks, eg, pH] methyl iodide and pH]-acetic anhydride, is an alternative route to the preparation of high specific activity, tritium-labeled compounds. The use of these techniques for the synthesis of radiolabeled receptor ligands, ie, dmgs and dmg analogues, has been described ia detail ia the Hterature (6,7). 

POCls-catalyzed formylation, so POBrs must be used instead. Another example is the replacement of halogen by the nitro group in the nitration of halogenated 2-acetamidothiophenes. ... 

Though the great majority of antiinflammatory agents contain some form of acidic proton, occasional compounds devoid of such a function do show that activity. Thus the nonacidic pyrazolylpyrimidine epi razole (47) is described as a nonsteroid antiinflammatory agent. Reaction of pyrimidinone 42 with phosphorus oxychloride leads to the chloro derivative Replacement of halogen with hydrazine gives... 

With the exception of the reactions of trifluoromethyl radicals with sulfur vapor, which is really a separate class of reactions, if the power supplied to the load coil surrounding the reactor (see Fig. 2) was maintained at, or near, the minimum amount needed to support the discharge, in only two cases were compounds found that clearly resulted from reactions other than replacement of halogen by trifluoromethyl. The reaction of tellurium tetrabromide (or the chloride) gave, in addition to the products just reported, very small proportions of such species as BrCF2TeCF2Br and (C2F5)2Te, which were isolated in yields of... 

The replacement of halogen by alkyl or aryl groups is included in this section. For the conversion of RX RH (X = halogen) see Section 160 (Hydrides from Halides and Sulfonates). 

Table 5.6. Reaction Conditions for Reductive Replacement of Halogen and Sulfonate... 

Reduction by hydrogen atom donors involves free radical intermediates and usually proceeds by chain mechanisms. Tri-n-butylstannane is the most prominent example of this type of reducing agent. Other synthetically useful hydrogen atom donors include hypophosphorous acid, dialkyl phosphites, and tris-(trimethylsilyl)silane. The processes that have found most synthetic application are reductive replacement of halogen and various types of thiono esters. 

After 20 years pharmacological interest in nitrofurans is as strong as ever, and still prompts studies including nucleophilic substitutions. Kinetic studies of the replacement of halogen by dimethylamine in 5-halo-2-nitrofurans yield second order rate constants and disclose spectroscopic (IR, UV, and... 

Aromatic amination by replacement of halogens with amines (example... 

Similar results for the replacement of halogen on an olefinic linkage by phosphorus have been accomplished using dialkyl phosphites with palladium(O) catalysts.4179 Another reaction involving replacement of a vinylic halide by phosphorus utilizes palladium catalysis with a trimethylsilyl-substituted phosphine (Figure 6.19).80... 

Replacement of halogen in dimeric iminoboranes containing tetracoordin-ated boron is not so readily accomplished. For example, bis[(dichloromethyl-eneamino)dichloroborane]reacts with C4H9MgCl in nonetheric solvents to yield dimeric (dichloromethyleneamino)butylchloroborane, whereas C4H9Ii replaces both boron-bonded chlorine atoms by butyl groups 25). 

There are reasons to explain the catalytic activity of copper(ll) in terms of cation-radical mechanism. This mechanism is confirmed by the unusual direction of Meerwein reaction in some cases, for example, when the replacement of halogen by an aryl radical occurs in the reaction of halo-styrenes with aryldiazonium salts (Obushak et al. 1991). A cation-radical in the system [olefin-Cu(II)] has been detected by UV spectroscopy (Obushak et al. 1991). In the cases of cis isomers of benzylidenacetone (Allard and Levisalles 1972) and maleic esters (Isaev et al. 1972), the unreacted... 

For the replacement of halogen by hydrogen - hydrogenolysis of the carbon-halogen bond - many reduction methods are available. Since breaking of carbon-halogen bonds is involved, the ease of hydrogenolysis decreases in... 

Alkyl chlorides are with a few exceptions not reduced by mild catalytic hydrogenation over platinum [502], rhodium [40] and nickel [63], even in the presence of alkali. Metal hydrides and complex hydrides are used more successfully various lithium aluminum hydrides [506, 507], lithium copper hydrides [501], sodium borohydride [504, 505], and especially different tin hydrides (stannanes) [503,508,509,510] are the reagents of choice for selective replacement of halogen in the presence of other functional groups. In some cases the reduction is stereoselective. Both cis- and rrunj-9-chlorodecaIin, on reductions with triphenylstannane or dibutylstannane, gave predominantly trani-decalin [509]. 

Replacement of halogen in compounds with functional groups is discussed in the appropriate chapters. 

Halogenated nitro compoands may suffer replacement of halogen during some reductions thus 9-bromo-lO-nitrophenanthrene reduced with zinc and acid, with stannous chloride, with ammonium sulfide, or with hydrazine in the presence of palladium gave 9-aminophenanthrene (87.5% yield). Reduction to 9-bromo-lO-aminophenanthrene was accomplished with iron in dilute acetic acid (yield 35%) [278]. 

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