Protonation acceptor radical anions

One of the solvated electrons is transferred into an antibonding 7t -orbital of the aromatic compound, and a radical anion of type C is formed (Figure 17.82). The alcohol protonates this radical anion in the rate-determining step with high regioselectivity. In the case under scrutiny, and starting from other donor-substituted benzenes as well, the protonation occurs in the ortho position relative to the donor substituent. On the other hand, the protonation of the radical anion intermediate of the Birch reduction of acceptor-substituted benzenes occurs in the para-position relative to the acceptor substituent. 

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 termination step of a propagating chain could involve proton transfer to monomer, combination with the acceptor radical anion, or electron transfer from the acceptor radical anion to the carbonium ion to yield a terminal radical, as below. 

Like the electrohydrodimerization and electrohydrocyclizahon reactions, this process also requires the consumphon of two electrons and two protons. It has been shown to occur via a sequence consisting of electron transfer followed by a ratedetermining protonation of the resulting radical anion, addihon of a second electron to generate a carbanion, cyclization of the carbanion onto the carbonyl acceptor unit and the addition of the second proton [16]. Carbon acids like dimethyl malonate and malononitrile are often used as a proton source. The course of this and other... 

While donor substituents assist in ortho and meta protonation, acceptor substituents direct protonation of the primary anion-radicals to the ipso and para positions. It should be emphasized that water treatment of the naphthalene anion-radical in THF leads to 1,4-dihydronaphthalene. Notably, the same treatment of this anion-radical, but o-bound to rhodium, leads to strikingly different results. In the rhodium-naphthalene compound, an unpaired electron is localized in the naphthalene, but no protonation of the naphthalene part takes places on addition of water. Only evolution of hydrogen was observed (Freeh et al. 2006). Being a-bound to rhodium, naphthalene acts as an electron reservoir. The naphthalene anion-radical part reacts with a proton according to the electron-transfer scheme similar to the anion-radicals of aromatic nitro compounds (see Scheme 1.14). 

The radical-anions from from alkenes with electron withdrawing substituents will add to carbon dioxide [28]. This process involves the alkene radical-anion, which transfers an electron to carbon dioxide for which E° = -2.21 V vs. see [29]. Further reaction then occurs by combination of carbon dioxide and alkene radcal-anions [30]. The carbanion centre formed in this union may either be protonated or react with another molecule of carbon dioxide. If there is a suitable Michael acceptor group present, this carbanion undergoes an intramolecular addition reaction... 

Alkene radical cations may transfer protons to cyanoaromatic radical anions, followed by coupling of the resulting radicals. For example, 1,4-dicyanobenzene and other cyano-aromatic acceptors form substitution products (e.g., 73) with 2,3-dimethylbutene via coupling and loss of... 

Addition to alkenes can be sensitized by both electron-donors and electron-acceptors, and it is most likely that the reactive species is the alkene radical anion or the alkene radical cation, respectively. 1,1-Diphenylethylene can be converted to the Markownikov addition product with methanol (2.48) using the electron-donating sensitizer t-methoxynaphthalene no added proton acid is needed. Using... 

In an analogous way, electron transfer to an acceptor-substituted aromatic like 25 produces a radical anion ot type 26. This is proto-naled in the ipso position to give intermediate 27. A second electron transfer and prolonation leads similarly to product 28. In most cases BuOH proves to be a good source of protons. 

While donor substituents assist the ortho and meta protonation, acceptor substituents direct the protonation of the primary anion radicals to the ipso and para positions. 

In several photochemical electron transfer reactions, addition products are observed between the donor and acceptor molecules. However, the formation of these products does not necessarily involve direct coupling of the radical ion pair. Instead, many of these reactions proceed via proton transfer from the radical cation to the radical anion, followed by coupling of the donor derived radical with an acceptor derived intermediate. For example, 1,4-dicyanobenzene and various other cyanoaromatic acceptors react with 2,3-dimethylbutene to give aromatic substitution products, most likely formed via an addition-elimination sequence [140]. 

In this account, we will focus on the transient analysis of these systems, which has strongly contributed to a deeper understanding of the diverse reaction modes (Patemo-Buchi, proton abstraction, cycloaddition). In general, aromatic ketones were selected as electron acceptors for reasons of suitable excitation and long wavelength absorption of the radical anion intermediates. Among them, fluorenone 3 is particularly well suited since the concentration, solvent, temperature, and cation radius dependence of the absorption spectra of pairs formed with metal cations are already known [29]. Hogen-Esch and Smid [30, 10] pointed out that a differentiation between CIP and SSIP is possible for fluorenone systems. On the other hand, FRI s and SSIP s cannot be differentiated simply by their UV/Vis absorption spectra, whereas for instance conductance measurements may be successful. However, the portion of free radical ions in fluorenyl salt solutions was shown to be less important [9, 31]... 

The extremely rapid rate of formation of bathorhodopsin as compared to isomerization rates observed in model protonated Schiff bases (a factor of 103) suggested the idea that electron transfer between an amino acid residue (e.g., tyrosine or tryptophan) in the protein and the chromophore may catalyse isomerization. Thus, a photoinduced electron transfer leading to a radical anion chromophore, instead of complete cis-trans isomerization, was considered as a plausible alternate mechanism for the primary event [200], This mechanism, however, is difficult to reconcile with the known photoreversibility but thermal irreversibility of the bleaching process. Thermal irreversibility of the light-induced electron transfer would require geometrical separation of donor and acceptor moieties which would then not allow photoreversibility [201]. 

Protonation is another common reaction in this category and reduction of the acceptor via protonation of the radical anion (e.g. by moisture present in the solvent) is often observed as a side process. It may become the main path either under acidic conditions or if the radical cation of the donor is a good proton donor (Scheme 3). As an example, the 1,2-dihydro derivative is the main product from 1,4-dicyanonaphthalene (DON) when irradiated in the presence of a donor and trifluoroacetic acid [33]. Such a reaction is more important when the radical cation is the proton source. Typically, the irradiation of naphthalene with tertiary amines leads to a mixture of 1,4-dihydro- and 1,2,3,4-tetrahydro-naphthalene, as well as tetrahydrobinaphthyl and l(l-diethylamino)ethyl-l,4-... 

Regioselective photocyclization of ethylthiopropyl phenylglyoxalate 793, by an electron-transfer process, produced the 1,5-oxathiocin 773 in 96% yield. The mechanism proposed for such a photocyclization involved excitation of the keto ester 793 to its triplet state (T) upon irradiation (350 nm), and electron transfer between the donor and acceptor occurred in T producing the ketyl radical anion (A). This was deactivated to ground state 793 by back electron transfer or was converted to biradical (B) by proton transfer process. Cyclization occurred between the excited carbonyl group and the carbon a to the sulfur on the remote side (Scheme 156) <1997T7165>. 

A nice extension of this chemistry to sequential anionic/radical/anionic sequence was also provided [5, 9]. Normally after acyl addition and radical cyclization onto a C = C bond, the newly formed carbon radical is reduced to an organosamarium intermediate which is subsequently protonated. However, as depicted in Scheme 5, this organosamarium may be trapped in the presence of a ketone substrate, thus terminating this three-step process. In another demonstration of how such anions may be further exploited, substrates possessing vinyl ethers as the radical acceptor were found to under-... 

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