Chlorine transfer reactions

Pattison, D. I. and Davies, M. J. (2006). Evidence for rapid inter- and intramolecular chlorine transfer reactions of histamine and carnosine chloramines Implications for the prevention of hypochlorous-acid-mediated damage. Biochemistry 45, 8152-8162. 

Chlorine transfer reactions. The rate constants and Arrhenius parameters of chlorine transfer reactions, given in Table II, were determined by competition studies in solutions that contained another compound, SX, in addition to the chloro-methane (CIM) or chloroethane (EtCl2) under investigation. The reaction of SX with cyclohexyl radicals C eaction 38) served as a reference. 

In general, chlorine transfer reactions of silyl radicals were found to be much faster and less selective than the analogous... 

A number of chemiluminescent reactions have been studied by producing key reactants through pulsed electric discharge, by microwave dissociation, or by observing the reactions of atoms and free radicals produced in the inner cone of a laminar flame as they diffuse into the flame s cool outer cone (182,183). These are either combination reactions or atom-transfer reactions involving transfer of chlorine (184) or oxygen atoms (181,185—187), the latter giving excited oxides. 

Kattenberg and coworkers54 studied the chlorination of a-lithiated sulfones with hexachloroethane. These compounds may react as nucleophiles in a nucleophilic substitution on halogen (path a, Scheme 5) or in an electron transfer reaction (path b, Scheme 5) leading to the radical anions. The absence of proof for radical intermediates (in particular, no sulfone dimers detected) is interpreted by these authors in favour of a SN substitution on X. 

In the Falconbridge process, copper-containing nickel mattes produced by preliminary pyro-metallurgical processing of ore concentrates (see Figure 2) are oxidized by chlorine in reactions in which Cu11 is responsible for electron transfer to the metal. 

Reductive dechlorination or reductive hydrogenolysis is a common transformation of 1- and 2-carbon chlorinated aliphatics under methanogenic conditions [373,374]. 1,1,1-Trichloroethane (l,l,l-TCA),for example,is converted to 1,1-dichloroethane (1,1-DCA) [375], and Perchloroethylene (PCE) is successively converted to TCE, cDCE, VC, and ethane [274]. Each reductive dechlorination is a two-electron transfer reaction. 

These Ionic reactions or electron transfer reactions are not what generally occur in the structure of both natural and synthetic polymers. In polymers it is the covalent bond that dominates, and in a covalently bonded structure there is no transfer of electrons from one atom to another. Instead the electrons are shared between the adjacent atoms In the molecule. The commercial polymeric materials that will be covered In this text will generally be based on seven atomic species silicon, hydrogen, chlorine, carbon, oxygen, nitrogen, and sulfur. Figure 2.4 shows these atoms with the number of outer valance electrons. 

Chloroadamantanes (149) and (150) reacted with CH2COPh to afford the monosubstitution products (151) and (152) as intermediates, the intramolecular electron-transfer reaction of the radical anion intermediate being a slow process. Product (151) with chlorine in the 1-position reacted further to give (153), whereas (152) with chlorine in the 2-position is unreactive, showing that the 1-position is the more reactive. 1,2-Diiodoadamantane (154) reacted with CH2NO2 to give the monosubstitution products (155) and (156). This implies that the intramolecular electron-transfer reaction of the radical anion is a slow process. The fact that (155) was formed as major product and (156) was the minor product shows that, when (154) accepts an electron, fragmentation occurs faster at the 1-position than the 2-position. 

Some organic reactions can be accomplished by using two-layer systems in which phase-transfer catalysts play an important role (34). The phase-transfer reaction proceeds via ion pairs, and asymmetric induction is expected to emerge when chiral quaternary ammonium salts are used. The ion-pair interaction, however, is usually not strong enough to control the absolute stereochemistry of the reaction (35). Numerous trials have resulted in low or only moderate stereoselectivity, probably because of the loose orientation of the ion-paired intermediates or transition states. These reactions include, but are not limited to, carbene addition to alkenes, reaction of sulfur ylides and aldehydes, nucleophilic substitution of secondary alkyl halides, Darzens reaction, chlorination... 

Polar effects can also be important in atom transfer reactions. 4 In an oft-cited example (Scheme 13), the methyl radical attacks the weaker of the C—H bonds of propionic acid, probably more for reasons of bond strength than polar effects. However, the highly electrophilic chlorine radical attacks the stronger of the C—H bonds to avoid unfavorable polar interactions. As expected, the hydroxy hydrogen remains intact in both reactions. 

Libby RD, Beachy TM, Phipps AK (1996) Quantitating Direct Chlorine Transfer from Enzyme to Substrate in Chloroperoxidase-Catalyzed Reactions. J Biol Chem 271 21820... 

Under this type of catalysis we may classify those reactions in which reversible electron transfer takes place. The electron to be transferred may be carried by an atom, and in such a case we speak about hydrogen or chlorine transfer and the like. While in the previous case the reaction was characterized by the development of a positive charge at the reactive centre, and we could speak about carbonium ions, in the present case the reaction centre is characterized by the presence of an unpaired spin, and we may speak about free radical reactions, taking into account the same precautions as previously. 

These redox reactions, in which oxygen transfer occurs, involve changes of two units in the oxidation numbers of reactant and product. One-electron redox reactions may occur with the transfer of a halogen. The reaction between Cr+2 and Fe 3, for example, is strongly catalyzed by added chloride ion and when chloride is added to the reaction mixture, the resultant Cr(III) is present as (BUO CrC 2. It might be suggested that Cl becomes attached to Cr+a after the redox has occurred, but this cannot be. In the first place, independent experiments show that under these conditions the reaction between Cr+3 and Cl is very slow second, chloride attachment after the redox has occurred would not explain the catalytic role of chloride. It is more likely that the reaction occurs via a chloride-bridge transition state, and that the redox is accomplished by a chlorine transfer ... 

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