Halogen atom transfer reactions

Halogen atom transfer reactions are relatively uncommon for alkyl halides and Ru(bpj)3. This lack of reactivity is primarily due to the excited state Ru(bpj)3 being an outer-sphere redox reagent, and for alkyl halides both the oxidation and reduction potentials have values that make such electron transfer reactions unfavorable. These considerations are particularly valid for chlorocarbons, but for bromocarbons or iodocarbons it is possible that selective photoreactions with Ru(bpj)3 may be observed. 

PHOTOCHEMICAL APPLICATIONS OF CHEMICALLY MODIFIED Ru(bpy)f-TYPE COMPLEXES 

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Halogen atom transfer reactions involve homolysis ofaC-XoraX-X bond in a neutral molecule and transfer of both radical components to unsaturated functional groups. There is atom economy in such processes and they provide functionality for further transformations [73]. 

Ruthenium complexes are capable of catalyzing halogen atom transfer reactions to olefins. This has been illustrated in the enantioselective atom transfer reactions of alkane and arene-sulfonyl chlorides and bro-motrichloromethanes to olefins using chiral ruthenium complexes. Moderate ee s up to 40% can be achieved for these transformations [74-77]. These specific reactions are believed to follow a radical redox transfer chain process. 

Knuehl BP T, Kajiwara A, Fischer H, Matyjaszewski, K. Characterization of Cu(II) bipyridine complexes in halogen atom transfer reactions by electron spin resonance. Macromolecules 2003 36 8291-8296. 

The reactivity shown in Scheme 3 results from the low bond dissociation energy (BDE) of the P-H bond [11] k=l.2 10 M s for the H-transfer from R02P(0)H to a primary C-centered radical) and the fast halogen-atom transfer from a C-halogen bond to a phosphonyl radical [9,12] (fc=4 10 M s for f-Bu-Br and k=83 10 M s for Cl3C-Br). Piettre et al. [13] pointed out that these chain reactions were even more efficient when dialkylthiophosphites and the corresponding dialkylphosphinothioyl radicals were involved. 

Radicals for addition reactions can be generated by halogen atom abstraction by stannyl radicals. The chain mechanism for alkylation of alkyl halides by reaction with a substituted alkene is outlined below. There are three reactions in the propagation cycle of this chain mechanism addition, hydrogen atom abstraction, and halogen atom transfer. 

Entries 7 and 8 illustrate conversion of diazonium salts to phenols. Entries 9 and 10 use the traditional conditions for the Sandmeyer reaction. Entry 11 is a Sandmeyer reaction under in situ diazotization conditions, whereas Entry 12 involves halogen atom transfer from solvent. Entry 13 is an example of formation of an aryl iodide. Entries 14 and 15 are Schiemann reactions. The reaction in Entry 16 was used to introduce a chlorine substituent on vancomycin. Of several procedures investigated, the CuCl-CuCl2 catalysis of chlorine atom transfer form CC14 proved to be the best. The diazonium salt was isolated as the tetrafluoroborate after in situ diazotization. Entries 17 and 18 show procedures for introducing cyano and azido groups, respectively. 

An investigation of the competing halogen transfer from BrCCl3 and CCl45, 79 has shown that steric effects are also of importance in atom transfer reactions to alkyl and aryl radicals. Giese80 investigated very carefully the temperature depen-... 

The possibility that substitution results from halogen-atom transfer to the nucleophile, thus generating an alkyl radical that could then couple with its reduced or oxidized form, has been mentioned earlier in the reaction of iron(i) and iron(o) porphyrins with aliphatic halides. This mechanism has been extensively investigated in two cases, namely the oxidative addition of various aliphatic and benzylic halides to cobalt(n) and chromiumfn) complexes. 

The reactions of several Co(ii) complexes have been examined (Halpern, 1974), namely, pentacyanocobaltate(n) (Chock and Halpern, 1969 Halpern and Maher, 1964, 1965 Kwiatek and Seyler, 1965,1968 Kwiatek, 1967), bis-(glyoximato)cobalt(il) (Schneider et al., 1969), cobalt(li) Schiff s base (Marzilli et al., 1970, 1971) and bis(dioximato)cobalt(ii) (Halpern and Phelan, 1972) complexes. A halogen-atom-transfer mechanism has been proposed for most halides (158, 159), with the exception of the reaction of cobalt(ii) Schiflf s... 

The fact that the anion radical is an intermediate in this case falls in line with the observation that it is also an intermediate in the reduction of the same substrates by homogeneous or heterogeneous outer sphere electron donors and also that nitrobenzyl halides are quite easy to reduce (see Section 2, p. 66). In the other cases, the generation of the R radical has been assumed to proceed by halogen-atom transfer (158). It should, however, be noted that an outer sphere, dissociative electron-transfer reaction (163) would also... 

Similar investigations have been carried out and similar conclusions reached with the reaction of chromium(ii) complexes with alkyl halides (Castro, 1963 Kochi and Davis, 1964 Kochi and Mocadlo, 1966 Kray and Castro, 1964). The main argument in favour of the halogen-atom-transfer mechanism in this case was the order of reactivity of the halides tertiary > secondary > primary. 

Addition of excess CH3I to a solution of [Ni (tmc)]+ results in the rapid loss of the absorption (A = 360 nm, e = 4 x 103 M-1 cm-1) and appearance of a less intense band at A = 346 nm. A subsequent slower reaction gives rise to the weaker absorbance profile of [Ni"(tmc)]2+. The data are interpreted in terms of the formation of an organo-nickel(II) species followed by a slower hydrolysis with breaking of the Ni-C bond. Kinetic studies under conditions of excess alkyl halide show a dependence according to the equation — d[Ni1(tmc)+]/cft = 2 [Ni(I)][RX]. The data have been interpreted in terms of a ratedetermining one-electron transfer from the nickel(I) species to RX, either by outer-sphere electron transfer or by halogen atom transfer, to yield the alkyl radical R. This reactive intermediate reacts rapidly with a second nickel(I) species ... 

Alcohols, water and even acetic acid are useful solvents for some radical reactions. However, they cannot be employed with organometallics that are basic. Conversely, halocarbon solvents are popular for many types of ionic reactions but they are not generally useful for radical reactions. Chloroform and carbon tetrachloride (and to a lesser extent, dichloromethane) can interfere by donating either hydrogen or halogen atoms to intermediate radicals and they are used only in atom transfer reactions where the solvent is also a reagent. 

Atom transfer radical polymerization (ATRP) [52-55]. Active species are produced by a reversible redox reaction, catalyzed by a transition metal/ligand complex (Mtn-Y/Lx). This catalyst is oxidized via the halogen atom transfer from the dormant species (Pn-X) to form an active species (Pn ) and the complex at a higher oxidation state (X-Mtn+1-Y/Lx). 

We saw in the previous section how acid/base reactions can be viewed as ion-transfer processes, the ions in question being usually proton, halide or oxide, without any changes in oxidation state. Redox reactions may often be seen as atom-transfer reactions, in which H, O, halogen etc. are transferred from one ion/molecule to another, with concomitant changes in oxidation state, e.g. ... 

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