Electron-transfer processes, mechanistic

Platinum.—The Pt—C bond of [Pt(CHDPh)Cl(PPhg)2] is cleaved by /n-chloro-perbenzoic acid to give [ H]benzyl m-chlorobenzoate and [ H]benzyl alcohol with retention of stereochemistry at carbon. This can be accounted for by an addition-elimination mechanism, but provides no support for the involvement of an electron-transfer process. Mechanistic studies of the electrophilic cleavage of Pt—aryl bonds,... 

The Nenitzescu process is presumed to involve an internal oxidation-reduction sequence. Since electron transfer processes, characterized by deep burgundy colored reaction mixtures, may be an important mechanistic aspect, the outcome should be sensitive to the reaction medium. Many solvents have been employed in the Nenitzescu reaction including acetone, methanol, ethanol, benzene, methylene chloride, chloroform, and ethylene chloride however, acetic acid and nitromethane are the most effective solvents for the process. The utility of acetic acid is likely the result of its ability to isomerize the olefinic intermediate (9) to the isomeric (10) capable of providing 5-hydroxyindole derivatives. The reaction of benzoquinone 4 with ethyl 3-aminocinnamate 35 illustrates this effect. ... 

Mechanistic studies also indicate that 4-nitroveratrole (equation 20) and 4,5-dinitroveratrole (equation 21) undergo both singlet and triplet nucleophilic aromatic substitution with ethyl glycinate23. An electron transfer process competes against the nucleophilic aromatic photosubstitution for singlet excited 4-nitroveratrole, causing a decreased product yield in equation 20. 

Examination of the behaviour of a dilute solution of the substrate at a small electrode is a preliminary step towards electrochemical transformation of an organic compound. The electrode potential is swept in a linear fashion and the current recorded. This experiment shows the potential range where the substrate is electroactive and information about the mechanism of the electrochemical process can be deduced from the shape of the voltammetric response curve [44]. Substrate concentrations of the order of 10 molar are used with electrodes of area 0.2 cm or less and a supporting electrolyte concentration around 0.1 molar. As the electrode potential is swept through the electroactive region, a current response of the order of microamperes is seen. The response rises and eventually reaches a maximum value. At such low substrate concentration, the rate of the surface electron transfer process eventually becomes limited by the rate of diffusion of substrate towards the electrode. The counter electrode is placed in the same reaction vessel. At these low concentrations, products formed at the counter electrode do not interfere with the working electrode process. The potential of the working electrode is controlled relative to a reference electrode. For most work, even in aprotic solvents, the reference electrode is the aqueous saturated calomel electrode. Quoted reaction potentials then include the liquid junction potential. A reference electrode, which uses the same solvent as the main electrochemical cell, is used when mechanistic conclusions are to be drawn from the experimental results. 

Steric constraints dictate that reactions of organohalides catalysed by square planar nickel complexes cannot involve a cw-dialkyl or diaryl Ni(iii) intermediate. The mechanistic aspects of these reactions have been studied using a macrocyclic tetraaza-ligand [209] while quantitative studies on primary alkyl halides used Ni(n)(salen) as catalyst source [210]. One-electron reduction affords Ni(l)(salen) which is involved in the catalytic cycle. Nickel(l) interacts with alkyl halides by an outer sphere single electron transfer process to give alkyl radicals and Ni(ii). The radicals take part in bimolecular reactions of dimerization and disproportionation, react with added species or react with Ni(t) to form the alkylnickel(n)(salen). Alkanes are also fonned by protolysis of the alkylNi(ii). 

Many synthetically important substitutions of aromatic compounds are effected by nucleophilic reagents. There are several general mechanisms for substitution by nucleophiles. Unlike nucleophilic substitution at saturated carbon, aromatic nucleophilic substitution does not occur by a single-step mechanism. The broad mechanistic classes that can be recognized include addition-elimination, elimination-addition, metal-catalyzed, and radical or electron-transfer processes. (See Sections 10.5, 10.6, 12.8, and 12.9, Part A to review these mechanisms.)... 

Intensive studies in the field of mechanistic CL by several research groups have resulted in the description of a large variety of peroxides which, in the presence of appropriate activators, show decomposition in an activated CL process and might involve the CIEEL mechanism . Even before the formulation of the CIEEL mechanism, Rauhut s research group obtained evidence of the involvement of electron-transfer processes in the excitation step of the peroxyoxalate CL. Results obtained in the activated CL of diphe-noyl peroxide (4) led to the formulation of this chemiexcitation mechanism , and several 1,2-dioxetanones (a-peroxylactones), such as 3,3-dimethyl-l,2-dioxetanone (9) and the first a-peroxylactone synthesized, 3-ierr-butyl-l,2-dioxetanone (14), have been shown to possess similar CL properties, compatible with the CIEEL mechanism Furthermore, the CL properties of secondary peroxyesters, such as 1-phenethylperoxy acetate (15) , peroxylates (16) , o-xylylene peroxide (17) , malonyl peroxides... 

Of course, in inner sphere bridged electron transfer reactions, one of the steps in the mechanistic sequence is the substitution of one of the complexes into the coordination shell of the second. Thus, if the electron transfer process that follows is fast enough, the substitutional step may become rate determining. I am not sure that there is any clear cut evidence that this is the case for any systems actually examined. I did cite a case of a bridged electron transfer reaction that proceeded with a rate constant of 109—i.e.,... 

The fundamental mechanistic difference between the SN2 and electron transfer processes is whether after the single electron shift takes place, an intermediate is formed or not. Any factor capable of delaying N—R coupling (77) after the single electron shift may lead to the actual generation of radical intermediates. Let us explore what these factors may be. 

Last but not least, it should be noted that the description of ECL processes as a simple superposition of the two or three electron transfer channels is somewhat oversimplified from the mechanistic point of view. In real cases, the electron transfer processes are preceded and followed by the diffusion of reactants from and electron transfer products into the bulk solution, respectively. Moreover, ECL reactants and products are species with distinctly different spin multiplicities, which causes an additional kinetic complication because of spin conservation rules. Correspondingly, the spin up-conversion processes (e.g., between two forms of an activated complex 1 [A- D + ] 3 [A- D + ]) cannot be a priori excluded from the kinetic con-... 

Some comments need to be made concerning the data in Table 1. For some couples an extensive set of additional data are available in a variety of media, e.g. Fe3+/2+. For others, data are available for a series of structurally related analogs, e.g. Fe(C5H5)2+/0. For couples like Cr(bipy)3+/° (5, Table 1) the electron transfer process is ligand n (bipy) rather than metal based and in clusters like those in couples 19 and 26 (Table 1) the redox levels are almost surely delocalized over the cluster unit. The inclusion of some of the entries listed as inner-sphere cases is not based on product studies but, rather, mechanistic details have been inferred from rate comparisons, as illustrated in a later section of the chapter. 

Despite these apparent difficulties, there are now a number of examples for photoinduced electron transfer reactions that are significantly catalyzed. It is the purpose of this chapter to present fundamental concepts and the application of catalysis of photoinduced electron transfer reactions. The photochemical redox reactions, which would otherwise be unlikely to occur, are made possible to proceed efficiently by the catalysis on the photoinduced electron transfer steps. First, the fundamental concepts of catalysis on photoinduced electron transfer are presented. Subsequently, the mechanistic viability is described by showing a number of examples of photochemical reactions that involve catalyzed electron transfer processes as the ratedetermining steps. 

The fact that electrochemical processes are tied to electron transfer processes makes electrochemical methods generally less applicable for kinetics and mechanism studies than, for instance, spectroscopic methods. On the other hand, if the reaction under scrutiny involves a radical or radical-like species, electrochemical methods are invaluable tools that often provide a wealth of mechanistic detail. A major advantage of electrochemical methods for kinetics and mechanism studies is that intermediates (radical ions, radicals, etc.) may be formed and their chemical reactions studied at the same electrode in the same operation. 

Aminyl radicals can also be generated from amide bases and organic oxidants via an electron transfer process. The utility of /V-lithio-Af-butyl-5-methyl-l-hex-4-enamine (10) as a mechanistic probe for such a process was studied (Scheme 2) (88JA6528). The formation of cyclic pyrrolidine... 

Gassman, P.G. and Bottorff, K.J. (1987) Photoinduced lactonization. a useful but mechanistically complex single electron transfer process. Journal of the American Chemical Society, 109, 7547-7548. 

The by far most widespread mechanism by which an N-NDR is hidden is the adsorption of a species that inhibits the main electron-transfer process. The species might be dissolved in the electrolyte, e.g., it might be the anion of the supporting electrolyte, or it is formed in a side reaction path, as it is the case in nearly all oxidation reactions of small organic molecules. Before we introduce specific examples of this type of HN-NDR oscillators, it is useful to study the dynamics of a prototype model. This will then help us to identify the essential mechanistic steps in real systems whose quantitative description requires more variables such that the basic feedback loops are not as obvious. 

Key-step in the mechanistic scenario is a primary electron transfer process involving a sacrificial electron donor as exemplary shown for the triphenylphosphine case in Sch. 28. The 9,10-dicyanoanthracene radical anion (DCA -) thus generated undergoes a secondary thermal electron transfer to the unsaturated ketone. The resulting carbon-centered radical or radical anionic intermediate, subsequently cyclizes stereoselectively with a proximate olefin. The observed 1,2- 77 -stereochemistry of the C-C bond formation step contrasts with the commonly observed -stereoselectivity of 5-hexenyl radical cyclizations. As sacrificial electron donors, the... 

Photochemical methods offer a convenient tool to study intra- and interprotein ET because of their time resolution and selectivity. Various mechanistic and design approaches based on photochemistry of metal complexes have been undertaken. Most of the studies on protein electron transfer processes have been done for hae-moproteins using among others ruthenium complex as a photosensitizer, modified haemoproteins in which haem iron is substituted by another metal (mainly Zn), and CO-bonded haem proteins [6,7],... 

The data in Figure 7.13 show reductive-dissolution kinetics of various Mn-oxide minerals as discussed above. These data obey pseudo first-order reaction kinetics and the various manganese-oxides exhibit different stability. Mechanistic interpretation of the pseudo first-order plots is difficult because reductive dissolution is a complex process. It involves many elementary reactions, including formation of a Mn-oxide-H202 complex, a surface electron-transfer process, and a dissolution process. Therefore, the fact that such reactions appear to obey pseudo first-order reaction kinetics reveals little about the mechanisms of the process. In nature, reductive dissolution of manganese is most likely catalyzed by microbes and may need a few minutes to hours to reach completion. The abiotic reductive-dissolution data presented in Figure 7.13 may have relative meaning with respect to nature, but this would need experimental verification. 

Fig. 5.6 Didier Astruc (bom 1946 in Versailles) studied chemistry at the University of Rennes, where he received his Ph.D. with Professor Rene Dabard in 1975. He then moved to MIT as a NATO Postdoctoral Fellow, where he worked with the 2005 Nobel laureate Richard R. Schrock. After being a Lecturer and Master Lecturer at the University Institute for Technology of Saint-Nazaire, he worked for the CNRS at Rennes where he became Maitre de Recherche in 1982. Since 1983 he is Professor of Chemistry at the University of Bordeaux I and has been promoted to the exceptional class of university professors in 1996. His research interests comprise preparative and mechanistic organometallic chemistry, catalysis, and electron transfer processes. More recently, he has developed the synthesis and supramolecular electronics of organometallic dendrimers. He is the author of Electron Transfer and Radical Processes in Transition-Metal Chemistry and of the standard textbook Organometallic Chemistry and Catalysis . A recipient of several major research awards, Didier is also a senior member of the Institut Universitaire de France, a member of the Academia Europeae, London, and the German Academy Leopoldina, and a Fellow of the Royal Society of Chemistry (photo by courtesy from D. A.)...

---