Electron-transfer mechanism scheme

The very high limiting quantum yield calculated for the DCA-sensitized photooxygenation of 54 ( = 23.8) [115], together with the cyclic voltammetric spectra (Fig. 2a, b) recorded under inert and/or oxygen atmosphere [146], strongly support the following chain electron-transfer mechanism (Scheme 9). 

Monoamine oxidase, which exists in two distinct forms, referred to as MAO A and MAO B, is one of the enzymes responsible for the degradation of biologically important amines. Compounds that block the catalytic action of MAO A, which is selective for the degradation of norepinephrine and serotinin, have antidepressant effects whereas compounds that inhibit MAO B, which degrades dopamine in the brain, are useful for treating Parkinson s disease [190, 191]. Both MAO A and MAO B contain flavin co-enzyme attached at the 8-a-position to an enzyme-active cysteine residue (54). A one-electron transfer mechanism (Scheme 15) for the oxidations catalyzed by MAO was first proposed by Silverman [192] and Krantz [193,194]. 

A number of reports on transition metal-free formations of benzo [c] cinnolines emerged in 2014. Sythana and collaborators prepared benzo [c] cinnoline N-oxides via a single-electron transfer mechanism (Scheme 11) (14EJ0311). In the presence of potassium tert.-butoxide and an aptotic nonpolar solvent benzo [c] cinnoline N-oxides 18 were obtained in moderate-to-high yields from the reaction of substituted anilines with nitrobenzenes. Both electron-donating substituents and electron-withdrawing halo substituents were well tolerated. When the halo substituents were in the... 

The reduction of copper(ii) can occur either via an outer-sphere electron-transfer mechanism (Scheme 13) or via an inner-sphere process (Scheme 14). Mass spectral... 

Meanwhile, it was found by Asai and colleagues [48] that tetraphenylphosphonium salts having such anions as Cl, Br , and Bp4 work as photoinitiators for radical polymerization. Based on the initiation effects of changing counteranions, they proposed that a one-electron transfer mechanism is reasonable in these initiation reactions. However, in the case of tetraphenylphosphonium tetrafluoroborate, it cannot be ruled out that direct homolysis of the p-phenyl bond gives the phenyl radical as the initiating species since BF4 is not an easily pho-tooxidizable anion [49]. Therefore, it was assumed that a similar photoexcitable moiety exists in both tetraphenyl phosphonium salts and triphenylphosphonium ylide, which can be written as the following resonance hybrid [17] (Scheme 21) ... 

In SiCl4-mediated Mukaiyama-Michael reactions, an electron-transfer mechanism is proposed for the case in which ketene silyl acetals bearing less hindered silyl substituent are used as substrates.342-344 As shown in Scheme 82, ketene silyl acetals having more substituents at the /3-position are much more reactive. 

SCHEME 1 Schematic illustration of the biological process of 02 dismutation into 02 and H202 catalyzed by Cu, Zn-SOD via a cyclic oxidation-reduction electron transfer mechanism. (Reprinted from [98], with permission from Elsevier.)... 

The ESR detection of benzophenone-ketyl radical coupled with the formation of pinacols as byproducts (in Scheme 9) provides the basis for an electron-transfer mechanism between carbonyl acceptors and various Grignard reagents48 (equation 23). 

We emphasize that the critical ion pair stilbene+, CA in the two photoactivation methodologies (i.e., charge-transfer activation as well as chloranil activation) is the same, and the different multiplicities of the ion pairs control only the timescale of reaction sequences.14 Moreover, based on the detailed kinetic analysis of the time-resolved absorption spectra and the effect of solvent polarity (and added salt) on photochemical efficiencies for the oxetane formation, it is readily concluded that the initially formed ion pair undergoes a slow coupling (kc - 108 s-1). Thus competition to form solvent-separated ion pairs as well as back electron transfer limits the quantum yields of oxetane production. Such ion-pair dynamics are readily modulated by choosing a solvent of low polarity for the efficient production of oxetane. Also note that a similar electron-transfer mechanism was demonstrated for the cycloaddition of a variety of diarylacetylenes with a quinone via the [D, A] complex56 (Scheme 12). 

Mechanistic Formulation of Electron Transfer. The Importance of the Work Term. Accordingly, the electron transfer mechanism can be considered in the light of the standard potentials E° for each redox couple, i.e., E x for the oxidation of the donor (D D+ + e ) and E ed for the reduction of the acceptor (A + e" A"). Thus the general reaction scheme for an irreversible process is represented by (20) ... 

Scheme 13 is a mechanism for this reaction that is based upon an electron-transfer process. An electron-transfer mechanism has been proposed in order to explain similar reactions in noncarbohydrate systems.76... 

According to Taube, the inner sphere mechanism can takes place when both oxidizing and reducing agents are substitution inert and when ligand transfer from oxidant to reductant is accompanied by electron transfer. The inner sphere electron transfer mechanism may be represented by the scheme... 

The electron-transfer mechanism for electrophilic aromatic nitration as presented in Scheme 19 is consistent with the CIDNP observation in related systems, in which the life-time of the radical pair [cf. (87)] is of particular concern (Kaptein, 1975 Clemens et al., 1984, 1985 Keumi et al., 1988 Morkovnik, 1988 Olah et al., 1989 Johnston et al., 1991 Ridd, 1991 Rudakov and Lobachev, 1991). As such, other types of experimental evidence for aromatic cation radicals as intermediates in electrophilic aromatic nitration are to be found only when there is significant competition from rate processes on the timescale of r<10 los. For example, the characteristic C-C bond scission of labile cation radicals is observed only during the electrophilic nitration of aromatic donors such as the dianthracenes and bicumene analogues which produce ArH+- with fragmentation rates of kf> 1010s-1 (Kim et al., 1992a,b). 

The addition of hydroxyde ion to nitrosobenzene produces azoxybenzene186. Three techniques (electronic absorption spectroscopy, linear sweep voltammetry and d.c. polarography) have been used to study the equilibrium between nitrosobenzene and hydroxyde ions. The probable reaction pathway to obtain azoxybenzene is indicated by Scheme 4. The importance of the nitroso group in the reduction of nitro derivatives by alkoxide ions, when the electron-transfer mechanism is operating, has been explained187. 

Hantzsch 1,4-dihydropyridines were oxidized quantitatively to give the corresponding pyridine derivatives by irradiation in CCI4. A photo-induced electron-transfer mechanism is involved. The critical step in this mechanism is the fast dechlorination of CCI4 (Scheme A)P... 

Diels-alder adducts at 0°C. This cation radical-vinylcyclobutane rearrangement is non-stereospecific, thus accounting for the formation of a cis-trans mixture of Diels-Alder adducts. Kinetic studies revealed (Scheme 8) that the ionization of these ethers involves an inner-sphere electron-transfer mechanism involving strong covalent (electrophilic) attachment to the substrate via oxygen (oxonium ion) or carbon (carbocation). 

For instance, Kochi and co-workers [89,90] reported the photochemical coupling of various stilbenes and chloranil by specific charge-transfer activation of the precursor donor-acceptor complex (EDA) to form rrans-oxetanes selectively. The primary reaction intermediate is the singlet radical ion pair as revealed by time-resolved spectroscopy and thus establishing the electron-transfer pathway for this typical Paterno-Biichi reaction. This radical ion pair either collapses to a 1,4-biradical species or yields the original EDA complex after back-electron transfer. Because the alternative cycloaddition via specific activation of the carbonyl compound yields the same oxetane regioisomers in identical molar ratios, it can be concluded that a common electron-transfer mechanism is applicable (Scheme 53) [89,90]. 

The photoinduced -elimination of 1,2,3-triazole from 1-(A,A-bisacyl)amino-l,2,3-triazoles (142), itself formed from the photochemical isomerization of triazoles (141), proceeds either via an intra-or intermolecular hydrogen abstraction or electron-transfer mechanism followed by homolytic cleavage of the A,A-bond (path a) or via t -assisted )8-cleavage of the same weak bond (path b). The composition of the products suggests that in all cases a c-type 1,2,3-triazolyl radical (143) is eliminated which is further quenched by hydrogen abstraction as shown in Scheme 24 <93JHC1301>. 

The series of elementary steps which constitute the overall electron transfer mechanisms for outer-sphere and inner-sphere reactions are illustrated in Schemes 1 and 2. 

However, the competition between Srn 1 and polar abstraction mechanisms is complicated in certain reactions by the formation of disulfides which is inhibited by radical and radical anion traps, and requires photolysis [23, 24]. These results implicate a third possibility, the chain SET redox mechanism (Srt2, i.e. substitution, electron transfer, bimolecular), Scheme 10.34. This alternative mechanism occurs when the intermediate radical anion can be intercepted by the thiolate (Equation 10.23) prior to the dissociation required in the SrnI mechanism (Equation 10.17 in Scheme 10.29). It becomes possible when either... 

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