Electron transfer donor radical cations

Electron-transfer oxidation of organic compounds involves multiple steps with transient radicals as key reactive intermediates.14 The electron-transfer oxidation of a neutral, diamagnetic organic donor (RH), having an even number of electrons, produces a radical cation, as shown in Eq. (7). 

However, the initial step of the electron transfer reaction strongly depends on the solvent polarity. By changing the solvent to less polar or nonpolar solvents like benzene or nonaromatic hydrocarbons the transient absorptions of 3C 0, G)0 and donor radical cation appear immediately after the laser pulse. The decay of all the absorptions is also completed at the same time. The fast appearance and the fast decay of the Go and donor radical cation absorption suggest that there is an interaction between fullerene and donor in less polar and nonpolar solvents before laser irradiation [120,125,133-139],... 

Time profiles of the formation of fullerene radical anions in polar solvents as well as the decay of 3C o obey pseudo first-order kinetics due to high concentrations of the donor molecule [120,125,127,146,159], By changing to nonpolar solvents the rise kinetics of Go changes to second-order as well as the decay kinetics for 3C o [120,125,133,148], The analysis of the decay kinetics of the fullerene radical anions confirm this suggestion as well. In the case of polar solvents, the decay of the radical ion absorptions obey second-order kinetics, while changing to nonpolar solvents the decay obey first-order kinetics [120,125,127,133,147]. This can be explained by radical ion pairs of the C o and the donor radical cation in less polar and nonpolar solvents, which do not dissociate. The back-electron transfer takes place within the ion pair. This is also the reason for the fast back-electron transfer in comparison to the slower back-electron transfer in polar solvents, where the radical ions are solvated as free ions or solvent-separated ion pairs [120,125,147]. However, back-electron transfer is suppressed when using mixtures of fullerene and borates as donors in o-dichlorobenzene (less polar solvent), since the borate radicals immediately dissociate into Ph3B and Bu /Ph" [Eq. (2)][156],... 

The first-order rate constant can be evaluated from the decay curves of 3C o and the rise curves of Qo and the donor radical cation [125,154], The observed electron transfer rate constants for C6o are usually in the order of 109-1010 dm3 mol-1 s-1 and thus near the diffusion controlled limit which depends on the solvent (e.g., diffusion controlled limit in benzonitrile -5.6 X 109 M-1 s-1) [120,125,127,141,154-156],... 

Photosensitized electron transfer reactions conducted in the presence of molecular oxygen occasionally yield oxygenated products. The mechanism proposed to account for many of these reactions [145-147] is initiated by electron transfer to the photo-excited acceptor. Subsequently, a secondary electron transfer from the acceptor anion to oxygen forms a superoxide anion, which couples with the donor radical cation. The key step, Eq. (18), is supported by spectroscopic evidence. The absorption [148] and ESR spectra [146] of trans-stilbene radical cation and 9-cyanophenanthrene radical anion have been observed upon optical irradiation and the anion spectrum was found to decay rapidly in the presence of oxygen. 

One strategy to overcome the regioselectivity problem and to enhance the efficiency for the photocyclization is the use of a suitable leaving group in a-position to the donor (Sch. 31). The intermediate donor radical-cation formed after electron transfer subsequently undergoes... 

As mentioned above, triplet Cgo is readily photoreduced by amines and other donors to Cgo radical anion and the donor radical cations [64], We expected this reaction to lead to adducts with covalent bonds. Such adducts are formed with some amines in ground state chemistry [33, 60, 83], but the photochemical process should be more selective and easily controlled, since only one-electron reduction is possible in the photochemical process. C o in the Si state has been suggested to produce an exciplex with triethylamine which seems to react with ground-state Cgo to give a stable product [117]. The reduction potential of the triplet is high enough that electron-transfer from many donors such as electron-rich aromatics and alkenes should be possible. 

The unrestricted and free electron transfer (FET) from donor molecules to solvent radical cations of alkanes and alkyl chlorides has been studied by electron pulse radiolysis in the nanosecond time range. In the presence of arenes with hetero-atom-centered substituents, such as phenols, aromatic amines, benzylsilanes, and aromatic sulfides as electron donors, this electron transfer leads to the practically simultaneous formation of two distinguishable products, namely donor radical cations and fragment radicals, in comparable amounts. 

At low free energy of the electron transfer reaction, the mechanism changes. Instead of prompt electron jumps in each collision (see above), an encounter complex appears which delays the electron transition and controls the energetics of the process. This results in the formation of only one uniform and metastable donor radical cation. 

The carbon-carbon bond formation via photoinduced electron transfer has recently attracted considerable attention from both synthetic and mechanistic viewpoints [240-243]. In order to achieve efficient C-C bond formation via photoinduced electron transfer, the choice of an appropriate electron donor is essential. Most importantly, the donor should be sufficiently strong to attain efficient photoinduced electron transfer. Furthermore, the bond cleavage in the donor radical cation produced in the photoinduced electron transfer should occur rapidly in competition with the fast back electron transfer. Organosilanes that have been frequently used as key reagents for many synthetically important transformations [244-247] have been reported to act as good electron donors in photoinduced electron-transfer reactions [248, 249]. The one-electron oxidation potentials of ketene silyl acetals (e.g., E°o relative to the SCE = 0.90 V for Me2C=C(OMe)OSiMe3) [248] are sufficiently low to render the efficient photoinduced electron transfer to Ceo [22], which, after the addition of ketene silyl acetals, yields the fullerene with an ester functionality (Eq. 15) [250, 251]. 

Type III A non-absorbing molecule transports the electron or the hole after the first electron-transfer step to the target species and thus enable separation of the originally formed acceptor radical anion-donor radical cation pair. This mediator is often used in catalytic amounts but can also be consumed during the PET reaction. In many of these cases, mediators serve as the terminal proton-hydrogen donors. 

The Gibbs free energy ofphotoinduced electron transfer, AetG°, in an excited encounter complex (D -A) can be estimated from Equation 5.1, where Zs°(D + /D) is the standard electrode potential of the donor radical cation, E° A/A ) that of the acceptor A and A/i0 o is the 0 0 excitation energy of the excited molecule (D or A ) that participates in the reaction. 

Donor/Acceptor Systems Upon excitation, a monomer donor (e.g., styrene) undergoes an electron transfer with a monomer acceptor (e.g., maleic anhydride). Then, the donor radical cation and the acceptor radical anion can recombine to form a biradical a recent review was provided in Ref. [193]. 

In photosystem-II, two such amino acids, tyrosine (Tyr-160 on D -subunit and Tyr-161 on D -subunit) seem to actively participate in the electron transfer sequence between the watersplitting site and the primary donor radical cation, P680+ [9]. The homologous Tyr-M162 in reaction centers from Rb, sphaeroides is located between P and the putative binding site for the cytochrome-C2 (acting as electron donor to P+) its replacement results in a decreased rate of electron transfer (Gray et al., unpublished results). 

The radical cation of 1 (T ) is produced by a photo-induced electron transfer reaction with an excited electron acceptor, chloranil. The major product observed in the CIDNP spectrum is the regenerated electron donor, 1. The parameters for Kaptein s net effect rule in this case are that the RP is from a triplet precursor (p. is +), the recombination product is that which is under consideration (e is +) and Ag is negative. This leaves the sign of the hyperfine coupling constant as the only unknown in the expression for the polarization phase. Roth et aJ [10] used the phase and intensity of each signal to detemiine the relative signs and magnitudes of the... 

When equimolar quantities of 80a and its dication 110 are combined in acetonitrile, single electron transfer occurs and the coproportionation product was obtained (95TL2741).Tliis deeply red-colored, air-sensitive radical cation 111 showed a strong ESR signal (g = 2.0034). On the other hand, the excellent electron donor 80a could be prepared by electrolytic reduction starting from 110. It was necessary to carry out the reduction with scrupulous exclusion of oxygen. Tlius, the electrolysis of 110 at -1.10 V initially gave rise to an intense red color, which was presumably due to the formation of 111. Upon further reduction, the red color faded and the tetraaza-fulvalene 80a was isolated at a 62% yield (Scheme 45). 

Elegant evidence that free electrons can be transferred from an organic donor to a diazonium ion was found by Becker et al. (1975, 1977a see also Becker, 1978). These authors observed that diazonium salts quench the fluorescence of pyrene (and other arenes) at a rate k = 2.5 x 1010 m-1 s-1. The pyrene radical cation and the aryldiazenyl radical would appear to be the likely products of electron transfer. However, pyrene is a weak nucleophile the concentration of its covalent product with the diazonium ion is estimated to lie below 0.019o at equilibrium. If electron transfer were to proceed via this proposed intermediate present in such a low concentration, then the measured rate constant could not be so large. Nevertheless, dynamic fluorescence quenching in the excited state of the electron donor-acceptor complex preferred at equilibrium would fit the facts. Evidence supporting a diffusion-controlled electron transfer (k = 1.8 x 1010 to 2.5 X 1010 s-1) was provided by pulse radiolysis. 

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