Electron transfer radical addition

KEYWORDS carbon dioxide radical anion vacuum ultraviolet irradiation electron transfer radical addition phenol toluene benzene chlorobenzene benzaldehyde benzoic acid... 

Most of the free-radical mechanisms discussed thus far have involved some combination of homolytic bond dissociation, atom abstraction, and addition steps. In this section, we will discuss reactions that include discrete electron-transfer steps. Addition to or removal of one electron fi om a diamagnetic organic molecule generates a radical. Organic reactions that involve electron-transfer steps are often mediated by transition-metal ions. Many transition-metal ions have two or more relatively stable oxidation states differing by one electron. Transition-metal ions therefore firequently participate in electron-transfer processes. 

Photochemical addition of ammonia and primary amines to aryl olefins (equation 42) can be effected by irradiation in the presence of an electron acceptor such as dicyanoben-zene (DCNB)103-106. The proposed mechanism for the sensitised addition to the stilbene system is shown in Scheme 7. Electron transfer quenching of DCNB by t-S (or vice versa) yields the t-S cation radical (t-S)+ Nucleophilic addition of ammonia or the primary amine to (t-S)+ followed by proton and electron transfer steps yields the adduct and regenerates the electron transfer sensitizer. The reaction is a variation of the electron-transfer sensitized addition of nucleophiles to terminal arylolefins107,108. 

The analogy between electron-transfer via addition/elimination (Eq. 2b,c) or abstraction/elimination (Eq. 2a, c) and classical solvolysis involving closed-shell molecules (nonradicals) is seen by comparing Scheme 1 with Scheme 3, in which XY, the precursor of the ions X and Y , is formally derived from the two radicals X and Y". Analogous to Scheme 1, on the way to the ionic products that result from the interaction between X and Y there are two possibilities if XY denotes a transition state, the reaction (Eq. 3a, a ) is a case of outer-sphere electron transfer. If, however, a covalent bond is formed between X and Y, the path (Eq. 3b, b ) is an example of inner- sphere electron transfer. Obviously, part b of the scheme describes the classical area of S l solvolysis reactions (assuming either X or Y to be equal to C) [9, 10]. If a second reaction partner for C (other than the solvent) is allowed for (the (partial) ions then represent transition states), then Eq. 3b also covers Sn2 reactions. If looked upon from the point of view of radical-radical reactivity, Eqs. 3a and b show well-known reactions radical disproportionation in Eq. 3a,a and combination in Eq. 3b. 

FIGURE 27. The polar addition mechanism and electron transfer-radical coupling sequence in the addition of lithium pinacolone enolate to benzaldehyde. Reprinted with permission from Reference 28. Copyright 1997 American Chemical Society... 

Two possible reaction pathways have been proposed421 for the Barbler-type carbonyl addition (equation 220) the polar (PL) route and the electron transfer-radical coupling (ET-RC) sequences with rate-determining ET or rate-determining RC (equation 221). 

Fang X, Mark G, von Sonntag C (1996) OH-Radical formation by ultrasound in aqueous solutions, part I. The chemistry underlying the terephthalate dosimeter. Ultrason Sonochem 3 57-63 Fang X, Schuchmann H-P, von Sonntag C (2000) The reaction of the OH radical with pentafluoro-, pentachloro-, pentabromo- and 2,4,6-triiodophenol in water electron transfer vs. addition to the ring. J Chem Soc Perkin Trans 2 1391-1398... 

Jagannadham V, Steenken S (1988b) One-electron reduction of nitrobenzenes by OH and H radical adducts to 6-methyluracil and 6-methylisocytosine via electron transfer and addition elimina-... 

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]. 

The ZnPc-Cgo dyad 110 has a low fluorescence quantum yield (f = 0.01) in toluene due to the intramolecular electron-transfer deactivation. Addition of 108 leads to a further reduction in the fluorescence intensity (4>f = 0.002) and a slight blue shift of the emission maximum, suggesting the formation of the ensemble 108-110, in which the two components have strong excitonic coupling. The addition of 108 (i.e. the hetero-association) also stabilizes the radical ion pair ZnPc+-C6o, increasing the lifetime from 130 ns (for 110) to 475 ns (for 108-110) (measured in tetrahydrofuran). 

Keywords Addition Catalysis Cross-coupling Cyclization Electron transfer Radicals Transition metals... 

Keywords Catalysis Transition metals Radicals Electron transfer Crosscoupling Addition Cyclization... 

Electron transfer from copper or copper salts to alkyl halides has been used to initiate atom transfer radical additions. One modification of this process involves catalytic amounts of copper powder and fluorinated alkyl iodides the radicals so generated may react in either inter- or intramolecular fashion with alkenes (equation 13)19. Alternatively, a-chloroesters with remote alkene functions undergo cyclization in the presence of cat-... 

During a microsecond time frame, which is the typical paramagnetic relaxation time of free radicals, polarized free radicals can participate in addition to multiple bonds, to dioxygen, in hydrogen (electron) transfer, in addition to polyradicals, etc. Products of these reactions are polarized in most cases, and they demonstrate TR ESR signals. 

C-C bond dissociation energies are not the only molecular properties affected by electron transfer. In addition, radical cations exhibit a number of unique characteristics flexible structures [23], a low sensitivity towards steric effects [24], low activation barriers for inter- and intramolecular reactions [25-28], high acidities [29-32] and the inversion of the thermochemical stability order for certain tautomeric systems. Examples in the recent literature demonstrate that it is worthwhile thinking about how the changed molecular properties can be used for the design of new reactions that complement the thermal and photochemical reactivity patterns [26,33-35]. 

Intermolecular alkyl radical addition to imine derivatives was studied in aqueous media using indium as a single electron transfer radical initiator. The one-pot reaction based on radical addition to glyoxylic hydrazone provided a convenient method for preparing the a-amino acids. Indium-mediated radical addition to an electron-deficient C=C bond also proceeded effectively to provide a new carbon-carbon bond-forming method in aqueous media (Scheme 7.11).15... 

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