Mechanisms electron transfer induced reactions

This nuance of the original Sr I mechanism may thus occur in quite a number of cases. Nomenclature purists may consider it necessary to find other symbols to name this mechanism and, presumably, to question the adequacy of the 1 in this case. Beyond symbols, if the Sr I mechanism is viewed as an outer sphere electron-transfer-induced nucleophilic substitution , a possible designation of the mechanism under discussion might be dissociative electron-transfer-induced nucleophilic substitution . The original designation of these reactions as nucleophilic reactions proceeding via anion radical intermediates (Komblum, 1975) would still apply to both nuances of the mechanism since, in the present case, RNu is an essential intermediate in the reaction, even if RX is not. 

Various transition metals have been used in redox processes. For example, tandem sequences of cyclization have been initiated from malonate enolates by electron-transfer-induced oxidation with ferricenium ion Cp2pe+ (51) followed by cyclization and either radical or cationic termination (Scheme 41). ° Titanium, in the form of Cp2TiPh, has been used to initiate reductive radical cyclizations to give y- and 5-cyano esters in a 5- or 6-exo manner, respectively (Scheme 42). The Ti(III) reagent coordinates both to the C=0 and CN groups and cyclization proceeds irreversibly without formation of iminyl radical intermediates.The oxidation of benzylic and allylic alcohols in a two-phase system in the presence of r-butyl hydroperoxide, a copper catalyst, and a phase-transfer catalyst has been examined. The reactions were shown to proceed via a heterolytic mechanism however, the oxidations of related active methylene compounds (without the alcohol functionality) were determined to be free-radical processes. 

As exemplified in Figure 2, Type 1 mechanism, electron transfer from L to sens yields two radicals, the substrate radical, L", and the sensitizer radical anion (sens ). In the next step, the lipid radical may induce a chain peroxidation cascade involving propagation reactions -The sensitizer radical anion may also start a sequential one-electron reduction of 2 generating HO in the presence of reduced transition metals. As a result, this may lead to abstraction of a lipid allylic hydrogen with subsequent generation of a carbon-centered lipid radical, L, that is rapidly oxidized to a peroxyl radical (vide supra). 

The corrinoid-mediated reduction of polyhaloethenes has been the subject of a recent study, which reports reaction via homolytic C-halogen bond fission. The elimination of a further halogen radical affords haloalkynes, which lead to acetylene itself.56 The electron transfer-induced reductive cleavage of alkyl phenyl ethers with lithium naphthalenide has been re-examined in a study which showed that it is possible to reverse regioselectivity of the cleavage (i.e. ArOR to ArH or ArOH) by introduction of a positive charge adjacent to the alkyl ether bond.57 A radical intermediate has been detected by ESR spectroscopy in the reduction of imines to amines with formic acid58 which infers reacts takes place via Lukasiewicz s mechanism.59... 

The topic of electron transfer induced geometric isomerization is treated in a separate section, because we wish to emphasize the existence of two fundamentally different mechanisms of isomerization. Although both mechanisms are initiated by an electron transfer step, the key intermediates involved in these isomerizations are fundamentally different. The interaction of acceptor sensitizers with donor olefins leads to the isomerization of alkene radical cations, whereas the reaction of donor sensitizers with acceptor olefins leads, eventually, to the population of alkene triplet states. 

For photo-induced electron transfer (ET) reactions [53], there exist three cases depending on their mechanism (1) non-adiabatic, diabatic, or weak coupling case, (2) adiabatic, or strong coupling case, and (3) charge transfer complex case. This section shall focuses on case (1) to which perturbation theory can be applied. 

In the case of the iron and cobalt porphyrins discussed above, RDox/RDred - - Dox/Dred 0 with cobalt(i) and iron(o) whereas the opposite is true for iron(i). RDox/RDred Dox/Dred s the difference in driving forces between reactions (147) and (145) and also that between reactions (144) and (148). Thus, kc > k c for the isoelectronic Co(l) and Fe(0), which matches the radical character of Co(n) and Fe(i) (Lexa et al., 1981), and kc > k t for Fe(i) hence the occurrence of the chain mechanism is unlikely in all cases. In the case of iron(o) and iron(i), this conclusion falls in line with the observed effect of steric constraints which should not appear in this outer sphere, dissociative electron-transfer-induced chain mechanism. 

Two major mechanisms have to be taken into consideration for the alkylation of Co -corrins. The classical mechanism of a bimolecular nucleophilic substitution reaction at carbon (the Co -corrin acts as a nucleophile) leads to /3-aUcylated Co -corrins with high diastereoselectivity. Secondly, an electron transfer-induced radical process (where the Co -corrin acts as a one-electron reducing agent) may also lead to cobalt alkylation. The observed formation of incomplete a-aUcylated Co -corrins under kinetically controlled conditions has been proposed to occur via this path. The high nucleophilic reactivity of Co -corrins and their diastereoselective nucleophilic reaction on the ( upper ) /3-face are not increased by the nucleotide function on the ( lower ) a-face rather they appear to be an inherent reactivity of the corrin-bound tetracoordinate Co -center. Among the organometallic B12 derivatives prepared to date, neopentylcobalamin, benzylcobalamin, and... 

As already discussed in Section 2.2.1, perfluoroalkyl halides do not act as effective electrophilic perfluoroalkylation reagents, as might be expected by analogy with the reactivity of alkyl halides. Even if some electrophilic perfluoroalkylation reactivity is mimicked with some especially suitable (i. e. easily oxidizable) substrates by an electron-transfer-induced radical mechanism, the practical usefulness of this reaction pathway is limited to very few examples. 

Similar electron transfer-induced oxygenation and [3+2] cycloaddition reactions occur when the EDA complexes of 77a-c and TCNE are stirred under oxygen in the dark at ambient temperature. This result suggests that an electron transfer mechanism is also operable in the thermal [3+2] cycloaddition of 77 and TCNE. ... 

It has been well known since the pioneering work of Bunnett59 that some nucleophilic aromatic substitutions can be catalyzed by single electron transfer. Electrochemistry was shown60,61 to be an efficient technique both for inducing reactions and for determining mechanisms and thermodynamic data concerning equilibria in the overall process. 

Acetylchloride is a trapping agent that allows the reaction to go completion, transforming the product into a less oxidizable compound.The results of other reactions between indole (57) and substituted cyclohexa-1,3-dienes show that the photo-induced Diels-Alder reaction is almost completely regioselective. In the absence of 59 the cycloaddition did not occur the presence of [2+2] adducts was never detected. Experimental data support the mechanism illustrated in Scheme 4.14. The intermediate 57a, originated from bond formation between the indole cation radical and 58, undergoes a back-electron transfer to form the adduct 60 trapped by acetyl chloride. 

The microscopic mechanism of these reactions is closely related to interaction of the reactants with the medium. When the medium is polar (e.g., water), this interaction is primarily of electrostatic nature. The ionic cores of the donor and acceptor located at fixed spatial points in the medium produce an average equilibrium polarization of the medium, which remains unchanged in the course of the reaction and does not affect the process of electron transfer itself. The presence of the transferable electron in the donor induces additional polarization of the solvent around the donor that is, however, different from polarization in the final state where the electron is located in the acceptor. 

The mechanism proposed by the authors is related to the tendency of 02 to capture electrons transferred to the Ti02 conduction band from the excited dye. The dye molecules accept holes and such cationic species react with the negatively charged 02. Such a reaction induces degradation of dye molecules. 

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