Electron Transfer-Initiated Processes

SrnI Process (Substitution, Radical-Nucleophile, Unimolecular) 

The SrnI process is a chain reaction that has both single-electron transfers and two-electron processes. A single-electron transfer forms a radical anion, which then loses the leaving group, forming the neutral radical that is attacked by a nucleophile to form a new radical anion. This new radical anion serves as the electron source for the initial single electron transfer. Aryl diazonium ions, ArN=N+, may react with nucleophiles this way. 

If R is alkyl, commonly the nucleophile and/or the R group will bear a nitro group so that the single-electron transfer is favorable. If R is aryl, initiation can be by singleelectron transfer from an electrode or an alkali metal. Initiation by light, hv, is also possible. The SrnI process can be stopped with radical inhibitors. Systems that would not have reacted by an SnI or Sn2 process may react by an SrnI mechanism since the R-L bond is considerably weakened in the radical anion compared to the neutral. 

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Alkali metals are obvious examples of electron donors, and indeed polymerization of butadiene or styrene initiated by metallic sodium results from an electron transfer initiation process. This reaction has been, and is still, being studied by many investigators, notably by Ziegler55 and by Russian workers.1 In Ziegler s notation the initiation is represented by the equation... 

The efficacy of various amines in this electron-transfer-initiated process is governed by several factors [51]. While the ionization potential of the amine is obviously important, substituents present on the carbons beta to the nascent radical center also play a role. Thus, it has been shown that dimethylaniline is... 

Srn1 Process, Other Electron Transfer-Initiated Processes... 

First, we examined the efficiency of the initiation process. A solution of buthyllithium was added to a THF solution of 7 at -70°C. The color of the solution turned to red immediately and a strong ESR signal was observed with a well separated hyperfme structure. The observed radical species was identified as the anion radical of 2-butyl-l,l,2,2-tetramethyldisilanyl-substituted biphenyl by computational simulation as well as by comparison with the spectra of a model compound. The anion radical should be a product of a single electron transfer (SET) process from buthyllithium to the monomer. Since no polymeric product was obtained under the above-mentioned conditions, the SET process is an undesired side reaction of the initiation and one of the reasons why more higher molecular weight polymer was observed than expected. ... 

Therefore, it has been considered that the formation of the dimer involves a mechanism different to the simple head-to-head radical coupling of the parent monomer. As suggested by the authors, it is likely that the overall mechanistic sequence is initiated by the radical-anion 472 of compound 469 formed by a single electron transfer (SET) process, which is the first stage of the bromine-lithium exchange (Scheme 68) [128],... 

Reductive Cross-Coupling of Nitrones Recently, reductive coupling of nitrones with various cyclic and acyclic ketones has been carried out electrochem-ically with a tin electrode in 2-propanol (527-529). The reaction mechanism is supposed to include the initial formation of a ketyl radical anion (294), resulting from a single electron transfer (SET) process, with its successive addition to the C=N nitrone bond (Scheme 2.112) (Table 2.9). 

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 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). 

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. 

It was therefore a significant breakthrough when procedures involving electron-transfer initiation began to appear in the 1960s, and today reductive initiation constitutes the most commonly used method of accomplishing the addition of Rpl to olefins and alkynes [228]. Perhaps the first example of such a process was that of Kehoe and Burton in 1966 [229,230]. 

Single Electron Transfer (SET) has an important place in many bimolecular reactions.277 Several types of initiation have been employed. When this transfer is induced by light, which provides sufficient redox potential difference between the two interacting molecules to initiate it, the process is known as the Photoinduced Electron Transfer (PET) process.278,279 Light-induced electron transfer provides an excellent synthetic means to alkylate MSMA. 

Electron transfer (ET) processes can often be classified into three basic types charge separation (CS), charge recombination (CR), and charge shift (CSh) [8,9,28,29], In CS (CR), the initial (final) state is characterized by charge neutral D and A sites, while the final (initial) is dipolar (D+/A ). In CSh processes, an excess charge (positive or negative) is transferred between D and A sites. Equation (3.68) has already introduced the CS case, and examples of CR and CSh ET are displayed, respectively, in Equations (3.69) and (3.70)... 

Nickel catalysis is a very active field in organometallic and organic chemistry (selected reviews [3-7]). Complexes of all oxidation states are active in two-electron transfer processes, such as oxidative addition or reductive elimination as well as in single electron transfer initiating radical reactions. Through these processes, oxidation states from Ni(0) to Ni(III) can be easily accessed under mild conditions. Occasionally, Ni(IV) intermediates were also proposed. Apart from the vast number of Ni(II) complexes, a number of organonickel(I) complexes were characterized by X-ray crystallography and their potency as active species in catalytic cycles tested [8-10]. Either radical or two-electron reactivity was observed. Recently, the structure of some alkylnickel(III) complexes was also structurally elucidated [11]. 

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. 

Addition Reactions.—As the years go by, the importance of electron transfer processes is becoming increasingly apparent, and hardly a month passes without the reinterpretation of a reaction as involving such a process. This has stimulated the publication of review articles such as that by Mattes and Farid on the electron transfer reactions of alkenes, and the more specific reviews by Mariano on the application of electron transfer photochemistry to iminium salts. In this area Mariano and his co-workers have reported further on the electron-transfer-initiated photochemistry of iminium salts (1), and in detail on the spiro-cyclization methodology of iminium salt cyclization. ... 

Irrespective of the electrophone system involved in one-electron transfer-initiated bond dissociation one can easily derive the thermodynamic driving force for a such process by use of thermochemical cycle calculations [15], Such estimates are particularly valuable as experimental numbers, because bond-dissociation data are scarce. 

Electron-transfer Initiation. Initiation of carboanionic growth by aromatic radical anions involves a direct electron transfer to monomer, though in the case of aromatic components with electron affinities greater than naphthalene the process is slow. Perhaps more importantly these can allow a subsequent side-reaction with the growing carboanion (Scheme 13). While the product (15)... 

Historically, the most important application of the electron transfer initiation involved the production of stereoregular diene rubbers by lithium metal initiation. The lithium was used as a fine dispersion with a large surface area to speed up the initiation reaction and the process was carried out in hydrocarbon solvents because polar solvents increase the generally undesired vinyl side chain content of the product polymer.)... 

Amines are generally good electron donors. They readily undergo photoinduced electron transfer (PET) processes, in which amine donates an electron to the reaction partner either in its ground or excited electronic state (entry 10). In contrast, electron-deficient, nitrogen-containing molecules, such as aromatic nitriles, may serve as electron acceptors (entry 11). Many organic metal complexes can also be involved in photochemically initiated redox reactions (Section 6.4.4). 

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... 

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