Initiation mechanisms, radical reactions electron transfer

The usually considered monomolecular mechanism of substitution implies that one-electron reduction activates a substrate sufficiently so that it could dissociate with no further assistance from a nucleophile. The next steps of the reaction consist of transformations of the resultant radical. However, in substrates having sp3 carbon as a reaction center, the influence of the leaving group has been fixed (Russell Mudryk 1982a, 1982b). This led to the formulation of the SRN2 bimolecular mechanism of radical-nucleophilic substitution. In this mechanism, the initial products of single-electron transfer are combined to form the... 

The nucleophilic photosubstitution reactions of benzylic halides with a nitro group at the ortho- or / ara-position of the aromatic ring such as / -nitrocumyl chloride (26) occur via the S l mechanism. The reactions have been reviewed in References 6-10. The initiation and propagation steps of this photoinduced radical-chain electron transfer process have already been depicted in this review in equations 6 to 9 in Section II. A. Equation 37... 

The initially formed chlorides PyX ionize to a pyridinium halide while PyR represents mixed 1,4- and 1,2-dihydropyridines.233,234 4-Nitrobenzyl halides react by a different mechanism the initial step here is electron transfer from 66 to the halide to give a nitro anion-radical which subsequently loses halide. Since this reaction involves ionic intermediates it is much more susceptible to solvent effects than the atom transfer reaction.235,236... 

Another, related type of reaction is the halodifluoromethylation of nucleophiles by dihalodifluoromethanes (e.g. CF2Br2) [9]. This reaction is always initiated by a single electron transfer from the nucleophile to the CF2XY species (X and Y denote halogens other than fluorine). The subsequent fate of the resulting radical ion pair depends on the ability of the nucleophile to form a stabilized radical, and also on the choice of solvent [10]. For phenoxides [4a, 5, 11] and thiophenoxides [4c, Ila] a reaction pathway via difluorocarbene is usually preferred whereas enamines and ynamines are halodifluoromethylated by a radical chain mechanism (see also Section 2.2.1) [12] (Scheme 2.169). 

Single electron transfer generates radicals and although this mechanism is now more common than once thought in non-biological redox reactions, its prevalence in enzyme-catalysed reactions is limited to coenzymes with quinoid-type structures e.g. flavins, coenzyme Q, vitamins C, E and K and to enzymes containing transition metals. Of course, there is a growing interest in metabolic disorders initiated by radical reactions. Reduction by 2-electron transfer can take place by either (a) hydride, H, transfer or (b) discrete electron, e , and proton, H", addition. 

The excited state of ketones can thus initiate free-radical reactions, and this is probably the mechanism for many examples of enhanced photodecomposition of environmental pollutants sensitized by acetone or other simple carbonyl compounds. A good example of such reactions is the acetone-promoted photooxidation of atrazine (24) and related triazine herbicides described by Burkhard and Guth (1976). In water, atrazine absorbs almost no solar UV and was accordingly quite stable to photolysis, but in the presence of large amounts of acetone (about 0.13 M), its half-life was decreased to about 5 hr. The produets were N-dealkylation products and ring-hydroxylated triazines. Similar products were also identified in riboflavin-sensitized photooxidation of triazines (Rejto et al., 1983). Presumably, a principal mechanism of photodecomposition would be H-abstraction from the N-alkyl substituents of atrazine, perhaps in conjunction with electron transfer from the unshmed pairs of the nitrogen atoms. 

Furthermore, Medebielle and Saveant have uncovered elegant examples of electrochemicaUy induced nucleophilic substitution reactions of perfluoroallqrl halides. The reaction mechanism is a slightly modified version of the classical SRN, pathway in which initiation occurs by dissociative electron transfer, and the route does not involve the intermediacy of the anion radical of the substrate as shown in Scheme 6. ... 

The true nature of homogeneous anionic polymerization only became apparent through studies of the soluble aromatic complexes of alkali metals, such as sodium naphthalene. These species are known to be radical anions [154-158], with one unpaired electron stabilized by resonance and a high solvation energy, and are therefore chemically equivalent to a soluble sodium. They initiate polymerization by an electron transfer process [145,148], just as in the case of the metal itself, except that the reaction is homogeneous and therefore involves a much higher concentration of initiator. The mechanism... 

Under the proposed mechanism (Scheme 2.3), the reaction initiated by pho-toinduceed electron transfer (PET) from an arene to generate an arene cation radical, which was attacked by an amine to give cr-adduct, followed by deprotonation and oxidative aromatization to afford the desired arene. In the presence of TEMPO, dioxygen served as a terminal oxidant and played a role in both the regeneration of the photoredox catalyst and the aromatization. 

The key feature of this mechanism is that electron transfer between the photosensitizer and the dialkylphenacylsulfonium salt results in the generation of a cation-radical species which ultimately initiates polymerization. The co-product of the reaction is a resonance-stabilized sulfur radical species which eventually undergoes fragmentation as shown in Eq. (81). 

These reactions are relevant to this review in that they may be photoinduced, but as radical chain processes many of them can be initiated in other ways. Radical chain dehalogenation of aryl halides has been reviewed previously.The chain propagation sequence for the deiodination of aryl iodides with CH5O-/CH3OH is shown in Scheme 1. The species (Arl) is very short lived and deiodinates efficiently. Reactivity diminishes in the order Arl > ArBr > ArCl and is enhanced in cases where there is relief of steric strain. Photochemical dehalogenations of aryl halides with AIH4 and BH4 have been proposed to follow similar radical chain mechanisms. Evidence for electron transfer from BH4 to ArCl has been present by Freeman and Ramnath. ... 

The reaction of perfluoroalkyl iodides with alkenes affords the perfluoro-alkylated alkyl iodides 931. Q.a-Difluoro-functionalized phosphonates are prepared by the addition of the iododifluoromethylphosphonate (932) at room temperature[778], A one-electron transfer-initiated radical mechanism has been proposed for the addition reaction. Addition to alkynes affords 1-perfluoro-alkyl-2-iodoalkenes (933)[779-781]. The fluorine-containing oxirane 934 is obtained by the reaction of allyl aicohol[782]. Under a CO atmosphere, the carbocarbonylation of the alkenol 935 and the alkynol 937 takes place with perfluoroalkyl iodides to give the fluorine-containing lactones 936 and 938[783]. 

The initiating radicals are assumed to be SCN, ONO or N3 free radicals. Tris oxalate-ferrate-amine anion salt complexes have been studied as photoinitiators (A = 436 nm) of acrylamide polymer [48]. In this initiating system it is proposed that the CO2 radical anion found in the primary photolytic process reacts with iodonium salt (usually diphenyl iodonium chloride salt) by an electron transfer mechanism to give photoactive initiating phenyl radicals by the following reaction machanism ... 

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