Photoinduced radical reactions

As it has already been pointed out, electronic excitation modifies the redox potentials of chemical compounds [32]. Generally, electron transfer is facilitated when electronic excitation is involved. The Rehm-Weller equation (Eq. 29.1) [33] enables an estimation of the exothermicity of a photochemical electron transfer (free enthalpy of electron transfer AG ). Even when the electron transfer at the ground state ( (D+/D)- (A/A )) is endothermic, this may be compensated by the excitation energy E. The attraction of the resulting ions is given by the term w, which is derived from Coulomb s law. It is reduced when reactions are carried out in a polar reaction medium, thus stabilizing the ions with respect to back election transfer. Most frequently, in the case of organic 

SCHEME 29.6 Photochemical electron transfer-mediated S I reactions. The irradiations were carried out 

SCHEME 29.8 Various mechanistic steps in a Witkop reaction. 

Rhaphidophora decursiva Schott (Araceae). It is active against the chloroquine-resistant malaria parasite Plasmodium falciparum. For this reason, such compounds are currently of high mteest in pharmacy. The structural analogue serotobenine has been synthesized using the same strategy. For further synthesis studies, see Ref. [50]. 

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Suginome H., New developments in photoinduced radical reactions in organic synthesis, in Modern Methodology in Organic Synthesis, Shono, T. Ed., Kodansha, Tokyo, 1992, 245. 

Fig. 6. Coupling of polymer chains via (a) photoinduced hydrogen abstraction free-radical reactions and (b) nitrene insertion/addition reactions.

The kinetics of formation and hydrolysis of /-C H OCl have been investigated (262). The chemistry of alkyl hypochlorites, /-C H OCl in particular, has been extensively explored (247). /-Butyl hypochlorite reacts with a variety of olefins via a photoinduced radical chain process to give good yields of aUyflc chlorides (263). Steroid alcohols can be oxidized and chlorinated with /-C H OCl to give good yields of ketosteroids and chlorosteroids (264) (see Steroids). /-Butyl hypochlorite is a more satisfactory reagent than HOCl for /V-chlorination of amines (265). Sulfides are oxidized in excellent yields to sulfoxides without concomitant formation of sulfones (266). 2-Amino-1, 4-quinones are rapidly chlorinated at room temperature chlorination occurs specifically at the position adjacent to the amino group (267). Anhydropenicillin is converted almost quantitatively to its 6-methoxy derivative by /-C H OCl in methanol (268). Reaction of unsaturated hydroperoxides with /-C H OCl provides monocyclic and bicycHc chloroalkyl 1,2-dioxolanes. 

Chlorine molecules are cleaved at high temperatures by photoinduced radical formation. By this means, a gas/liquid reaction can be performed in the side chain of alkyl aromatics quite selectively. The electrophilic ring substitution, instead, is favored using Lewis catalysts in polar solvents at low temperature. 

Substituted cyclopropane systems also undergo nucleophilic addition of suitable solvents (MeOH). For example, the photoinduced ET reaction of 1,2-dimethyl-3-phenylcyclopropane (112, R = Me) with p-dicyanobenzene formed a ring-opened ether by anti-Markovnikov addition. The reaction occurs with essentially complete inversion of configuration at carbon, suggesting a nucleophilic cleavage of a one-electron cyclopropane bond, generating 113. The retention of chirality confirms that the stereochemistry of the parent molecule is unperturbed in the radical cation 112 " ". 

Deprotonation is essential in some cation radical reactions the corresponding examples will be described in Chapter 6. Scheme 1-43 depicts a photoreaction between phenan-threne and triethylamine. This reaction includes photoinduced sequential electron-transfer, proton-transfer, and radical-recombination processes (Lawson et al. 1999). 

Rossi and de Rossi (1983) and Rossi et al. (1999) cited a substantial body of photoinduced ion radical reactions of aromatic substitution. It is interesting here to compare photoinduction and electrochemical stimulation of ion radical reactions. As pointed out in... 

One important discrepancy should be noted between photochemical and chemical ion radical reactions. In the photochemical mode, an oxidized donor and a reduced acceptor remain in the same cage of a solvent and can interact instantly. In the chemical mode, these initial products of electron transfer can come apart and react separately in the bulk solvent. For example, one-electron oxidation of phenylbenzyl sulfide results in formation of the cation radical both in the photoinduced reaction with nitromethane and during treatment with ammoniumyl species. Sulfide cation radicals undergo fragmentation in the chemical process, but they form phenylbenzyl sulfoxide molecules in the photochemical reaction. The sulfoxide is formed at the expense of the oxygen atom donor. The latter comes from the nitromethane anion radical and is directly present in the solvent cage. As for the am-... 

The reactivity of sulfur-centered nucleophiles such as thiourea anion [46] and thioacetate anion [17] in photoinduced SRN1 reaction has been reported as a one-pot, two-step method for the synthesis of several sulfur-aromatic compounds from moderate to good yields. Without isolation, the ArS ions obtained by the aromatic substitution are quenched with Mel to yield ArSMe in a one-pot procedure, together with Ar2S in variable yields, from an SRN1 between ArS- and aryl radicals (Scheme 10.3). 

The nucleophilic photosubstitution reactions of benzylic halides with a nitro group at the ortho- or / ara-position of the aromatic ring such as / -nitrocumyl chloride (26) occur via the S l mechanism. The reactions have been reviewed in References 6-10. The initiation and propagation steps of this photoinduced radical-chain electron transfer process have already been depicted in this review in equations 6 to 9 in Section II. A. Equation 37... 

The photoinduced initiation reaction may have the disadvantage of poor quantum yields, arising from a fast backward ET which annihilates the ion pair before its cage separation. This means a poorly efficient source of radicals. However, if the photochemical ET is the initiation of a very efficient radical chain process, a poor quantum yield in the reaction may turn to an advantage because small extent of production of the intermediates (Ar, ArX- , ArNu ) will disfavour the proposed termination steps of the mechanism. 

Although thermal and photoinduced SET reaction products are frequently identical due to converging reaction pathways [12], there are cases where synthetically useful differences can be observed [38] which can be related to different spin states of the intermediate radical pair. 

However, Kupletskaya and co-workers (100) have shown that the products of photodecomposition of CpFe(CO)2R (R = Ph, 1-naphthyl, 1-azulenyl, or acenaphthyl) in air-saturated solution were those expected from the trapping of Ar radicals by oxygen. These results could be construed as evidence for photoinduced Fe—Ar bond homolysis, but it is more likely that these products arise from a free radical reaction initiated by the oxygen present in solution. A similar free radical-initiated homolysis of an Fe—(tj -CsHs) bond has been postulated for CpFe(CO)2(V CsH5) 101). 

A significant problem in studies on photocatalysis is the definition of positive hole. Positive hole is defined as a defect of an electron (i.e., a positive hole must be included in a substance, while an electron is a real substance). Therefore, not only h produced by photoinduced band-to-band transition in solid materials but also a hydroxyl radical, which is a one-electron deficient hydroxyl anion, can be a positive hole. If this definition is accepted, there should be no difference in the photocatalsrtic oxidation mechanisms between direct hole transfer and surface-adsorbed hydroxyl radical reaction, since it is well known that the surface of a metal oxide is covered with chemically or physically adsorbed water and a positive hole passing through this water layer into a solution may be a hydroxyl radical or its protonated or deprotonated species (Fig. 4). Actually, hydroxyl... 

Cossy J (2001) Photoinduced electron transfer in radical reactions. In Renaud P,... 

This review illustrates the above delineated characteristics of electron-transfer activated reactions by analyzing some representative thermal and photoinduced organometallic reactions. Kinetic studies of thermal reactions, time-resolved spectroscopic studies of photoinduced reactions, and free-energy correlations are presented to underscore the unifying role of ion-radical intermediates [29] in—at first glance—unrelated reactions such as additions, insertions, eliminations, redox reactions, etc. (Photoinduced electron-transfer reactions of metal porphyrin and polypyridine complexes are not included here since they are reviewed separately in Chapters 2.2.16 and 2.2.17, respectively.)... 

Trimethylsilyl)allylation. Photoinduced radical allylation as well as those via ionic pathways (e.g., reaction with acid chlorides ) are useful synthetically. 

Only in the reduced reaction centers, i.e., those in which the iron-sulfur-protein secondary acceptors are chemically pre-reduced and the lifetime ofthe photoinduced radical pair thereby increased to -25 ns, can the difference spectrum of the Chi a-type primary electron acceptor Aq be isolated from the chlorophyll antenna excitation bands. The [Aq -Aq] difference spectrum was thusobtainedwithamajorbleaching band at 693 nm in agreement with the earlier results of Shuvalov et as presented above in Fig. 5... 

Scheme 6.97 presents an example of a photoinduced substitution reaction of iodoben-zene (247) with a good electron donor, such as an acetone enolate (248), affording the product 249 via the Sr-nIAt mechanism in 88% chemical yield.853 The radical anion species (Phi) (250) formed in the first step is short lived and it readily releases the... 

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