Electron transfer mechanisms stability

Cobalt trifluoride fluorination corresponds to the electron-transfer mechanism via a radical cation. RF groups attached to the ring enhance the stability of intermediate dienes and monoenes. Perfluoroalkyl pyridines, pyrazines, and pyrimidines were successfully fluorinated but pyridazines eliminated nitrogen. The lack of certain dienes was attributed to the difference in stability of FC=C and RFC=C and steric effects [81JCS(P1)2059]. 

The reaction between OH and phenol lends itself to an analysis of its thermochemistry. On the basis of E7( OH) = 2.3V/NHE and E7(PhO ) = 0.97 V/NHE [42], the formation of PhO and H2O via an electron-transfer mechanism is exothermic byl.33V = 31 kcal mor In spite of this, the reaction proceeds by addition, as outlined in Eq. 24. Again, the propensity of OH to add rather than to oxidize can be understood in terms of the transition state for addition being stabilized by contributions from bond making, in contrast to electron transfer which requires pronounced bond and solvent reorganization which results in a large (entropy-caused) free energy change. 

These reactions occur easily because of the relative stability of the radicals involved.6 The single electron transfer mechanism (SET), which we have met several times (e.g., p. 307) is an important case. 

Cyclic voltammetry, kinetic studies, and DFT calculations using a BP functional and the TZVP basis set showed that the major pathway of the non-regiospeciflc zinc-reduced titanocene-mediated ring opening of epoxides was initiated by a titanium dimer-epoxide compound that reacted in a rate-determining electron transfer mechanism 25 The calculations showed that the transition state is early so the stereoselectivity is determined by steric effects rather than by the stability of intermediate radicals. This was confirmed by studies with more sterically crowded catalysts. 

NQOl is a homodimer with a flavodoxin fold (5). This enzyme does not stabilize the semiquinone state. The obligate two-electron transfer mechanism prevents the generation of quinone radicals and redox cycling, which would result in oxidative stress. The NADPH and quinone substrates occupy the same site, consistent with the observed ping-pong bi-bi mechanism. NQOl is inhibited by many (poly)aromatic compounds including the anticoagulant dicoumarol and the phytoalexin resveratrol (5). 

The electron-transfer mechanism (Eq. 34) also explains the various regioselec-tivities observed for different arenes as the direct result of the symmetry of the arene HOMOs involved [161]. Moreover, the solvent effect on the oxidation products (Eq. 32) is now explicable on the basis of MO considerations. Thus, the ion-radical pair is very short-lived in hexane and collapses at the 9,10-positions where the anthracene HOMO is centered. The 9,10-cycloadduct is subsequently further oxidized to the anthraquinone product. In the more polar dichloromethane, the ion-radical pair is better stabilized and its longer lifetime allows relaxation of the original HOMO ion-radical pair to the subjacent (HOMO-1) ion-radical pair which leads to cycloaddition on the terminal ring (Eq. 32) [161]. 

If the lifetime of the alkyl radical is extremely short, in other words, if a certain given radical is very unstable (i.e., very reactive), the lifetime of a radical pair, consisting of an alkyl and a ketyl radical, after electron transfer and homolysis, would be extremely short. Subsequently, electron transfer and radical combination follow each other, with almost no separation no differentiation can be made between a two-step electron-transfer mechanism and a one-step polar mechanism. For steric reasons, one would expect the following sequence of reactivities in a concerted mechanism tertiary < secondary < primary, whereas the reverse is to be expected for an electron-transfer mechanism, in view of the stability of radicals. 

The raised HOMO also provides an explanation for a single electron transfer mechanism as it allows an electron to be easily transferred to the LUMO of an electrophile to generate a radical pair to further couple to the product. Since the removal of a single electron from one of the two electron pair orbitals on nucleophilic atom leaves behind a stabilized radical, the rate constant for the reaction of a nucleophile with a-effect is likely to be more sensitive to the LUMO energy of the electrophile than the rate constant for a nucleophile with a normal lone pair [60]. This is indeed so in the rates of /V-methylation of a series of /V-phenylhydroxylamines, which are higher than the rates in comparable anilines [61]. 

Dimeric alkenes from stabilized Wittig reagents. In the presence of traces of water, the reaction proceeds by a single electron transfer mechanism. The cation radical dimerizes, and the dimer loses two phosphine molecules. 

The photosystems of green plants and photosynthetic bacteria appear to function with basically the same sort of mechanisms of energy transfer, primary charge separation, electron transfer, charge stabilization, but the molecular constituents of the reaction center are quite diflFerent. Photosystem I contains iron-sulphur proteins as electron acceptors so can be called iron-sulphur (FeS) type reaction center, while photosystem II contains pheophytin as the primary electron acceptor and quinone as the secondary acceptors so it can be called pheophytin-quinone (4>-Q) type . The reaction center of purple bacteria, green nonsulphur bacteria, and PSII are (4>-Q) type. Green sulphur bacteria, heliobacteria, and PSI have (FeS) type reaction centers. ... 

The Indoles have also been shown to form exclplexes (solute-solvent excited-state complexes) with a wide range of electrophilic and nucleophilic compounds (32). Exclplex formation is especially evident In polar solvents. A fluorescence red shift from nonpolar to polar solvents and the concomitant loss of the vibrational structure of the fluorescent spectrum with no corresponding shift In the Indole absorption spectra accounts for this postulated exclplex formation. Hershberger and Lumry (33) suggest that some charge-transfer mechanism stabilizes the excited complex and Is a likely precursor to electron ejection. 

The kinetics of the reduction of hemoglobin by Fe(III) and Cu(II) complexes indicate the presence of both simple outer-sphere and site-specific electron transfer mechanisms. With the Fe(III) chelate oxidants the pathway is dependent on the reduction potential and the stability of the Hb complex, while for Cu(II) oxidations the outer-sphere process occurs at the a subunits, with the site-specific mechanism involving metal binding at the Cys p-93 residue. 

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