Atom-transfer substitution

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. 

There is another substitution reaction, not involving transition-metal complexes, namely, reaction of trifluoromethyl bromide with sulphur dioxide anion radicals (165) (Andrieux et al., 1990a) (this is an interesting route 

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Sn2 Substitution versus single outer sphere dissociative electron transfer 98 Atom-transfer substitution 115 Modelling of Sn2 substitution 118... 

Thus, for radicals 19, there is a strong preference for 1,5-hydrogen atom transfer (Table 1.5).111 Although 1,6-transfer is also observed, the preference for 1,5-hydrogen atom transfer over 1,6-transfer is substantial even where the latter pathway would afford a resonance stabilized benzylie radical.111112 No sign of 1,2-, 1,3-, 1,4-, or 1,7-transfer is seen in these cases. Similar requirements for a co-lincar transition state for homolytic substitution on sulfur and oxygen have been postulated. S,6I)... 

Hydrogen-Atom Transfer. Many oxidation and reduction reactions are free-radical substitutions and involve the transfer of a hydrogen atom. For example, one of the two main propagation steps of 14-1 involves abstraction of... 

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. 

The original stabilizer (HBC) was modified as the rapid radiationless deactivation of the stabilizer is (at least partly) due to the intramolecular hydrogen bond, the H-atom was substituted by a methyl group (MBC). This "probe molecule" showed fluorescence and phosphorescence and enabled us to demonstrate the energy transfer to the stabilizer, simply by studying its sensitized luminescence. 

Novel catalytic systems, initially used for atom transfer radical additions in organic chemistry, have been employed in polymer science and referred to as atom transfer radical polymerization, ATRP [62-65]. Among the different systems developed, two have been widely used. The first involves the use of ruthenium catalysts [e.g. RuCl2(PPh3)2] in the presence of CC14 as the initiator and aluminum alkoxides as the activators. The second employs the catalytic system CuX/bpy (X = halogen) in the presence of alkyl halides as the initiators. Bpy is a 4,4/-dialkyl-substituted bipyridine, which acts as the catalyst s ligand. 

Several luminescent ( 650 nm) dioxorhenium(V) systems(25) have been investigated as potential 0-atom transfer agents. The emission quantum yields measured with 436 nm excitation are about 0.03 for trans-ReO (pyridine)u and its isotopically-substituted derivatives in pyridine solution. The excited state lifetimes of these ions vary from 4 to 17 ys. 

Free Radical Substitution and Hydrogen Atom Transfer Reactions... 

FREE RADICAL SUBSTITUTION AND HYDROGEN ATOM TRANSFER REACTIONS... 

The mechanistic proposal of rate-limiting hydrogen atom transfer and radical recombination is based on the observed rate law, the lack of influence of CO pressure, kinetic isotope effects [studied with DMn(CO)s] and CIDNP evidence. In all known cases, exclusive formation of the overall 1,4-addition product has been observed for reaction of butadiene, isoprene and 2,3-dimethyl-l,3-butadiene. The preferred trapping of allyl radicals at the less substituted side by other radicals has actually been so convincing that its observation has been taken as a mechanistic probe78. 

More complicated reactions that combine competition between first- and second-order reactions with ECE-DISP processes are treated in detail in Section 6.2.8. The results of these theoretical treatments are used to analyze the mechanism of carbon dioxide reduction (Section 2.5.4) and the question of Fl-atom transfer vs. electron + proton transfer (Section 2.5.5). A treatment very similar to the latter case has also been used to treat the preparative-scale results in electrochemically triggered SrnI substitution reactions (Section 2.5.6). From this large range of treated reaction schemes and experimental illustrations, one may address with little adaptation any type of reaction scheme that associates electrode electron transfers and homogeneous reactions. 

Enantioselective synthesis of /1-amino acids is important as they are present in various natural products and in many biologically active compounds [26,27]. Several methods exist for the enantioselective synthesis of -substituted /1-amino acids (/l3-amino acids) however, synthesis of a-substituted /1-amino acids (/l2-amino acids) is very limited [28,29]. A report on highly enantioselective hydrogen atom transfer reactions to synthesize /l2-amino acids (Scheme 9) has recently been described [30]. 

Certain redox changes involving atom tranfer can usefully be dealt with applying the ideas which have been developed for nucleophilic substitution. There is, in fact, no sharp distinction between a 2e redox change involving atom transfer and an orthodox nucleophilic substitution. This point is illustrated by the two reactions... 

Aryl substitution on germanium, whether single or multiple, has only a small effect on the rate constants for hydrogen atom transfer, whereas the rate constant increases substantially with substitution of an alkyl group on Ge by a silyl group, much as observed with the silanes. A strong substituent effect also was observed for germane 19. 

Although autoxidation of Ru(sar) + has similar characteristics in acidic solution, in base hydrogen atom transfer from Ru(sar) + to O2 leads to a deprotonated Ru(III) species which is oxidized to relatively stable Ru" (sar-2 H+) + Ref. 175. The strong deviation from linearity for semi-log plots, with a large excess of O2, is removed when Fe(II) is added. This suppresses the step and doubles the rate. Compare Sec. 2.2.1(b). The value of k can be assessed as 1.3 x 10 M s Ref. 176. The behavior of pentacyanoruthenium complexes has been compared with the iron analogs. Substitution in M"(CN)5L" with both M = Fe and Ru is dissociative, with decreased lability for tbe Ru(II) species. Table 8.10. 

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