Single electron transfer fragmentation reactions

Single-electron transfer from a borate anion particle to the excited polymethine cation generates a dye radical and an aLkylphenylbotanyl radical. The aLkylphenylbotanyl radical fragments to form an active alkyl radical. It is the alkyl radical particles that initiate the polymerization reactions (101). 

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). 

Ueda et al. reported a tandem radical addition-cycUzation reaction in aqueous media [184]. This reaction was initiated by single-electron transfer from indium to an alkyl iodide. Fragmentation of the iso-propyl iodide radical anion generated the iso-propyl radical, which triggered the addition/cyclization tandem. Final SET and in situ hydrolysis delivered cyclic sulfonamides in good yield but low stereoselectivity. 

Cyclopropyl ketone 222 (Scheme 55) was prepared to probe the mechanism of the cleavage reaction [39,40]. Isolation of 224 where the cyclopropyl ring is intact (Scheme 56) suggests cleavage proceeds via formation of radical 227 rather than ketyl radical anion 226, formed by single-electron transfer to the ketone carbonyl, as cyclopropylmethyl radical anions are known to undergo facile fragmentation. 

Even though fragmentation of radical anions represents a key step in SrnI reactions (Scheme 76) and in aliphatic nucleophilic substitution reactions (Sn2) proceeding via single electron transfer (Scheme 77), such processes and their mechanistic implications will not be discussed in this section (several reviews are available [271-277]). 

Scheme 2.113 One-pot synthesis of cesium pental

Evidence has been provided for the intermediacy of A-methyl-A-phenylnitre-nium ion in the photolysis of iV-(methylphenylamino)-2,4,6-trimethylpyridinium tetrafluoroborate. Substituted aryltropylium ions have been generated by photolysis of a series of 7-methoxycycloheptatrienes, and found to have lifetimes strongly dependent on the donor capacity of the aryl substituent. Photolysis of a mixture of the two pinacols (96 and 97) in acetonitrile resulted in efficient fragmentation of both compounds, by cleavage of the central C-C bonds. This reaction has a quantum yield of 9 1, which suggest a chain process initiated by single electron-transfer quenching of excited (96) by (97). 

Oxidative Single Electron Transfer (SET) Induced Fragmentation Reactions... 

Yamada, S., Tanaka, T., Akiyama, S., and Ohashi, M., Fragmentation of laudanosine by single electron transfer reactions, J. Chem. Soc., Perkin Trans. 2, 449, 1992. 

For aminophenols, one-electron oxidation and the proton elimination can run together in one stage. This leads to a cation-radical containing O and +NH3 fragments within one and the same molecular carcass (Rhile et al. 2006). Such concerted reactions are classified as proton-coupled electron transfer (Mayer 2004). Proton-coupled electron transfer differs from conventional one-electron redox reaction in the sense that proton motion affects electron transfer. Because the transfers of a proton and an electron proceed in a single step, we can say about the hydrogen-atom transference, (H+ -I- e)=H. It is the fundamental feature of proton-coupled electron-transfer reactions that the proton and electron are transferred simultaneously, but from different places (see Tanko 2006). 

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