Electron-transfer initiation mechanism

The outer-sphere electron-transfer initiation mechanism cannot account for the observed kinetics, the half-reaction time being more than 100 times greater than that observed. The chain process considerably enhances the global rate of the reaction (without a chain process, the half-reaction time would be three centuries). 

Scheme 2.96 The reactivity of some oxidizable nucleophiles toward perfluoroalkyl halides suggests an electron-transfer initiated mechanism [5].

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]. 

Hydroperoxides are more widely used as initiators in low temperature appHcations (at or below room temperature) where transition-metal (M) salts are employed as activators. The activation reaction involves electron-transfer (redox) mechanisms ... 

The amide functionality plays an important role in the physical and chemical properties of proteins and peptides, especially in their ability to be involved in the photoinduced electron transfer process. Polyamides and proteins are known to take part in the biological electron transport mechanism for oxidation-reduction and photosynthesis processes. Therefore studies of the photochemistry of proteins or peptides are very important. Irradiation (at 254 nm) of the simplest dipeptide, glycylglycine, in aqueous solution affords carbon dioxide, ammonia and acetamide in relatively high yields and quantum yield (0.44)202 (equation 147). The reaction mechanism is thought to involve an electron transfer process. The isolation of intermediates such as IV-hydroxymethylacetamide and 7V-glycylglycyl-methyl acetamide confirmed the electron-transfer initiated free radical processes203 (equation 148). 

The solution of the riddle posed by Kornblum s dark Sj l reaction is as follows. The nucleophile does work as a single electron-transfer initiator of the chain process. However, the mechanism of initiation does not consist of a mere outer-sphere electron transfer from the nucleophile to form the anion-radical of the substrate. Rather, it involves a dissociative process in which electron transfer and bond breaking are concerted (Costentin and Saveant 2000). Scheme b at the beginning of Section 7.8 illustrates the concerted mechanism. 

Electron-transfer initiation from other radical-anions, such as those formed by reaction of sodium with nonenolizable ketones, azomthines, nitriles, azo and azoxy compounds, has also been studied. In addition to radical-anions, initiation by electron transfer has been observed when one uses certain alkali metals in liquid ammonia. Polymerizations initiated by alkali metals in liquid ammonia proceed by two different mechanisms. In some systems, such as the polymerizations of styrene and methacrylonitrile by potassium, the initiation is due to amide ion formed in the system [Overberger et al., I960]. Such polymerizations are analogous to those initiated by alkali amides. Polymerization in other systems cannot be due to amide ion. Thus, polymerization of methacrylonitrile by lithium in liquid ammonia proceeds at a much faster rate than that initiated by lithium amide in liquid ammonia [Overberger et al., 1959]. The mechanism of polymerization is considered to involve the formation of a solvated electron ... 

Nevertheless, there are two highly efficient CL systems which are believed to involve the CIEEL mechanism in the chemiexcitation step, i.e. the peroxyoxalate reaction and the electron transfer initiated decomposition of properly substituted 1,2-dioxetanes (Table 1)17,26 We have recently confirmed the high quantum yields of the peroxyoxalate system and obtained experimental evidence for the validity of the CIEEL hypothesis as the excitation mechanism in this reaction. The catalyzed decomposition of protected phenoxyl-substituted 1,2-dioxetanes is believed to be initiated by an intramolecular electron transfer, analogously to the intermolecular CIEEL mechanism. Therefore, these two highly efficient systems demonstrate the feasibility of efficient excited-state formation by subsequent electron transfer, chemical transformation (cleavage) and back-electron transfer steps, as proposed in the CIEEL hypothesis. 

A single electron transfer (SET) mechanism involving initial transfer of an electron from the FAD cofactor has been proposed for MAO-catalyzed oxidations [10]. An alternative polar nucleophilic mechanism has also received support [11]. 

In general, any of several possibile mechanisms may be operative for complex reactions. For example, in the oxidation of isopropanol by RuIV the key redox step could involve initial outer-sphere electron transfer, initial H-atom transfer, or even two-electron hydride transfer. The hydride mechanism, which has been proposed to be the actual low-energy pathway in water at 25 °C, is illustrated in reaction (3).2... 

It is the process forming the reactive centers from which macromolecules evolve. It may result from two different mechanisms a nucleophilic attack of the monomer by an organometallic initiator (butyllithium, cumylpotassium, benzylsodium, etc.), or the transfer to the monomer of the counterion and the extra electron of an electron-transfer initiator (lithium or sodium naphtalene, biphenyl.) later the ion-radical monomer having an extra electron in its lowest antibonding n orbital becomes a dicarbanion by dimerization of two activated monomer molecules. The use of a monofunctional initiator leads to a biblock copolymer AB, while that of a bifunctional initiator leads to a triblock copolymer ABA. 

Irradiation of NADH model compounds in the presence of benzyl bromide or p-cyanobenzyl bromide in acetonitrile brings about reduction of the benzyl halides to the corresponding toluene compounds114. Like the S l substitution reaction, this photoreduction also occurs via an electron-transfer chain mechanism. Unlike in that case, though, here an electron transfer from the excited state of the NADH compound is solely responsible for the initiation step. In the propagation, the benzyl radical produced by C—Br bond cleavage in the radical anion abstracts hydrogen from the NADH compound. This yields a radical intermediate, from which electron transfer to benzyl bromide occurs readily (equations 39-42). 

This step is analogous to the single-electron-transfer (SET) mechanism that has been proposed for many nucleophilic reactions, that is, an initial transfer of an electron followed by the collapse of a radical pair (equation 141). 

Akbulut et al." polymerised styrene by a direct electron transfer initiation carried out at the anodic peak potential of the monomer. These authors claimed the following mechanism involving styrene adsorbed on the anode surface ... 

Mechanisms of Electron-Transfer Initiation of Polymerization A Quantitative Study... 

Effect of Solvents and Reaction Conditions Synthesis Capabilities Block Copolymers Functional End-Group Polymers Initiation Processes in Anionic Polymerization Initiation by Electron Transfer Initiation by Nucleophilic Attack Mechanism and Kinetics of Homogeneous Anionic Polymerization Polar Media Nonpolar Media... 

Initiation by Electron Transfer. This mechanism of initiation operates in polymerizations by alkali metals or their complexes and was best elucidated in the case of the sodium naphthalene complex (13). which was used to form the well-known "living" polymers. In these complexes, the naphthalene is a radical anion (35) formed by transfer of an electron from sodium in the presence of a highly solvating solvent such as H -furan ... 

Substitution reactions by anions at carbon are also known to occur by initial electron transfer. The mechanism of such transformations was first characterized by Russell and Danen (14) and Kornblum et al. (15), and Bunnett (16) significantly developed its applications and named it the SRN1 reaction an... 

Reoxidation of the cosubstrate at an appropriate electrode surface will lead to the generation of a current that is proportional to the concentration of the substrate, hence the coenzyme can be used as a kind of mediator. The formal potential of the NADH/NAD couple is - 560 mV vs. SCE (KCl-saturated calomel electrode) at pH 7, but for the oxidation of reduced nicotinamide adenine dinucleotide (NADH) at unmodified platinum electrodes potentials >750 mV vs. SCE have to be applied [142] and on carbon electrodes potentials of 550-700 mV vs. SCE [143]. Under these conditions the oxidation proceeds via radical intermediates facilitating dimerization of the coenzyme and forming side-products. In the anodic oxidation of NADH the initial step is an irreversible heterogeneous electron transfer. The resulting cation radical NADH + looses a proton in a first-order reaction to form the neutral radical NAD, which may participate in a second electron transfer (ECE mechanism) or may react with NADH (disproportionation) to yield NAD [144]. The irreversibility of the first electron transfer seems to be the reason for the high overpotential required in comparison with the enzymatically determined oxidation potential. 

Szwarc M. Mechanisms of electron-transfer initiation of polymerization a quantitative study. In Bailey FE Jr editor. Initiation of Polymerization. ACS Symposium Series. Volume 212. Washington (DC) American Chemical Society 1983. p 419. 

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