Fragmentation acceptor radical anions

Photoinduced single-electron transfer followed by fragmentation of the radical cation is an efficient method for generating carbon-centered radicals under exceptionally mild conditions. The fate of the thus formed radicals depends primarily on their interaction with the acceptor radical anions. Typically observed reactions are either back-electron transfer or radical coupling, but from the synthetic point of view, another most intriguing possibility is the trapping of the radical with suitable substrates such as olefins (Scheme 16). 

In summary, the electron transfer reactions of amines offer an interesting variety of pathways, involving free radical or ionic intermediates and leading to two-electron oxidation as well as coupling and fragmentation reactions. In many systems, however, the most important reaction involves electron return from the acceptor radical anion, i.e. quenching of an excited state without net chemical change. 

II. Photoinduced electron transfer reactions and subsequent fragmentation In electron transfer reactions, the photoexcited molecule, termed the sensitizer for the convenience, can act as either electron donor or electron acceptor according to the nature of the sensitizer and coinitiator. Fragmentation yields radical anions and radical cations, which are often not directly acting as initiating... 

In a similar vein, various electron acceptors yielding anion radicals that undergo rapid unimolecular decomposition also facilitate the efficacy of Scheme 1 by effectively obviating the back-electron transfer. For example, the nitration of enol silyl ether with tetranitromethane (TNM) occurs rapidly (despite an unfavorable redox equilibrium)78 owing to the fast mesolytic fragmentation of the TNM anion radical79 (Scheme 15). 

The attachment of an electron to an organic acceptor generates an umpolung anion radical that undergoes a variety of rapid unimolecular decompositions such as fragmentation, cyclization, rearrangement, etc., as well as bimolecular reactions with acids, electrophiles, electron acceptors, radicals, etc., as demonstrated by the following examples.135"137... 

An useful alternative to the already known retropinacol reactions is presented by Liu and co-workers [7], This works demonstrates that pinacols bearing (dimethylamino)phenyl substiments can be subjected to fast oxidative fragmentation via photoinduced electron transfer with chloroform as the electron acceptor in yields up to 80%. The extremely fast dechlorination of the chloroform radical anion inhibits back-electron transfer and thus leads to effective fragmentation of the pinacol radical cation (Scheme 8). 

Alkyl Nicotinamides. Benzyl nicotinamide can be used as a photochemical model for biological reductions. Upon photoexcitation, electron transfer causes reduction of a variety of acceptors. The radical anionic intermediates thus formed can fragment or participate in further chemical reduction. Several examples of observable chemistry initiated in this way include eqs. 69 (208), 70 (209), and 71 (210) ... 

Other examples of the use of electron acceptors whose ion radicals are unstable with respect to fragmentation to an anion and a radical capable of initiation of polymerization were provided by Eaton (63,101,102). It was shown that -nitrobenzyl halides could be used in dye-sensitized compositions of semiconductor pigments such as Ti02 and CdS to induce polymerization of vinyl monomers using visible light. The sequence of events is outlined in eqs. 46-49 and Scheme 6 ... 

The Srn 1 reaction has been applied to heterocycles. Among five-membered ring compounds, halothiophenes have been the most studied they have been shown to be susceptible to both electron-stimulated and photostimulated reactions, and have been converted to the corresponding acetonitriles [150], acetones [151], and phenyl-sulfides [ 152] in low to medium yields. In the study with the benzenethiolate anion it has been shown that the yield is low because of fragmentation of the adduct radical anion it can be increased by adding an electron acceptor, e.g. benzonitrile which prevents decomposition. Further applications include the thermally activated SrnI reaction between 3-iodobenzothiophene and enolates [153] and the photo-stimulated reaction of 3-halo-2-aminobenzothiophenes [154]. 

As an example, photochemical excitation of donor-acceptor complexes may be considered. Irradiaiion into the CT band of the anthracene-tetracyano-ethylene complex leads directly to the radical ion pair, the components of which are identifiable from their UV-visible spectra. The transient absorptions decay in 60 ps after excitation, as the radical ion pairs undergo rapid back electron transfer to afford the original donor-acceptor complex (Hilinski et al., 1984). With tetranitromethane as acceptor, however, an addition product is obtained in both high quantum and chemical yield. This is due to the fact that the tetranitromethane radical anion undergoes spontaneous fragmentation lo a NO, radical and a trinitromethyl anion, which is not able to reduce the anthracene radical cation (Masnovi et al., 1985) ... 

As already indicated, carbonyl compounds such as ketones, aldehydes, enones, and quinones possess the property to act as effective electron acceptors in the excited state for generating radical anions in the presence of electron-donating partners such as alkenes, aromatics, ruthenium complexes, amines, and alcohols. We will not consider the reactivity of enones and quinones, but we will focus our attention on the behavior of the radical anions formed from ketones and aldehydes. Four different processes can occur from these radical anions including coupling of two radical anions and/or coupling of the radical anion with the radical cation formed from the donor, abstraction of hydrogen from the reaction media to produce alcohols, cyclization, in the case of ce-unsaturated radical anions, and fragmentation when a C -X bond (X=0, C) is present (Scheme 18). 

Heteroarenes have been photochemically functionalized by PET reactions forming new C—C bonds both in an inter- and intramolecular fashion via a similar mechanism [46]. The heteroarenes could serve both as electron donor (e.g. pyrroles or indoles) or electron acceptor (e.g. cyanopyridines or cyanopyrazines). Again, fragmentation of the radical cation, coupled with the radical anion and loss of the anion, led to overall ipso-substitution. In addition to the cyano group, halides could also function as leaving groups, such that in some cases an attack at an unsubstituted position took place [46],... 

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