Replacement reactions halogens

Hydrogen substitution in CgHsF, CgHjCl, and CgHsBr by AtCl and AtBr is less efficient than the competing halogen-replacement reactions in these systems (vide supra) the total radiochemical yields are only a few percent and are poorly reproducible (43, 99,104). 

The final compounds were s)mthesized according to Scheme 1. Dipyridines 1 and 2 were prepared from commerdally available diamines and 4-bromopyridine in the presence of phenol under argon atmosphere, as previously described [11]. A reaction involving phenol as proton donor, solvating agent, and source of phenoxide ion is envisaged, as outlined in Scheme 1 [16]. As a reaction medium, phenol reduces both the reaction time and temperature of the halogen-replacement reactions. 

Sheme 1. As a reaction medium, phenol reduces reaction time and temperature of halogen-replacement reactions, by... 

The replacement of fluorine, chlorine or bromine from the corresponding halogenocyclophosphazenes by another substituent (a nucleophile) is an important aspect of the chemistry of these ring systems [2, 7, 12]. First, this route provides a way of preparing cyclophosphazenes with substituents involving exocyclic P-N, P-0, P-S, P-C, P-H and P-M linkages. Thus, in a way it may be said that the halogen replacement reactions are the most... 

Polycarbophosphazenes possess a backbone of phosphorus, nitrogen, and carbon atoms and can be regarded as derivatives of classical polyphosphazenes (1) in which every third phosphorus atom is replaced by carbon. The first examples of these materials were discovered in 1989 (88). Thermal ROP of a cyclic carbophos-phazene was used to prepare the chlorinated polymeric species (23), which im-dergoes halogen replacement reactions with nucleophiles such as aryloxides and aniline to yield hydrolytically stable poly(aryloxycarbophosphazenes) (24) (Afw = ca 10 , Mn = 10 ) (eq. 23) (88-91). The polymer backbone in these materials was found to be less flexible than in classical polyphosphazenes. For example, the halo-genated polymer (23) possesses a Tg of —21°C compared to a value of —66°C for poly(dichlorophosphazene) (2). 

Piperazinothiazoies (2) were obtained by such a replacement reaction, Cu powder being used as catalyst (25. 26). 2-Piperidinothiazoles are obtained in a similar way (Scheme 2) (27). This catalytic reaction has been postulated in the case of benzene derivatives as a nucleophilic substitution on the copper-complexed halide in which the halogen possesses a positive character by coordination (29). For heterocyclic compounds the coordination probably occurs on the ring nitrogen. 

Sn2 and SN2 Reactions with Halides and Sulfonates. Corey and Posner discovered that lithium dimethylcuprate can replace iodine or bromine by methyl in a wide variety of compounds, including aryl, alkenyl, and alkyl derivatives. This halogen displacement reaction is more general and gives higher yields than displacements with... 

Substitution of P-substituents other than halogens has been reported for P-ethoxy-l,3,2-diazaphospholene and l,3,2-diazaphospholene-2-oxide which react with trichlorosilane to yield the corresponding P-chloro-substituted heterocycles [49,50]. This reaction reflects a typical behavior of phosphine derivatives undergoing halogen replacement similar to the previously discussed transformations. 

For example, in the instance of 9-chloroacridine, the attachment of the halogen (leaving group) at a suitably electrophilic carbon site allows the occurrence of a replacement reaction, presumably occurring via an addition-elimination procedure for phosphorus attachment, followed by the common nucleophilic displacement (ester cleavage) of the Michaelis-Arbuzov process (Figure 6.1).4... 

No synthesis of oligosilsesquioxanes with X = OH, OR, Hal has been reported. The preparation of such compounds seems possible from the corresponding oligomers by replacement reactions or halogenation. 

The stability of dialkylimidazolium cation-containing ionic liquids can be a problem even at moderate temperatures in the presence of some reagents or catalysts. For example, when CsF and KF were used in the ionic liquid [BMIM]PFg to perform a halogen exchange reaction in an attempt to replace Br from bromo-carbons with F , it was found that alkyl elimination from the [BMIM] cation took place, forming methyl imidazole, 1-butene, 1-fluorobutane, and other unidentified products at 150°C overnight 69). The fluoride ion acted as a base that promotes elimination or substitution processes. 

We have therefore deduced from the formula for the heat of formation of ionic compounds the important rule that the halogens replace one another in the order F2, Cl2, Br2, I2. There are ho known exceptions to this rule. Fluorine displaces chlorine, bromine and iodine from all chlorides, bromides and iodides, while chlorine and bromine displace iodine from all iodides. It is to be expected that the same substitution reactions can take place with the chal-cogens 02, S, Se and Te, and, since the heat of formation of the oxides is, as a rule, greater than that of the sulphides, the reactions of the type... 

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