Photoaddition and Coupling

This review article deals with addition and cycloaddition reactions of organic compounds via photoinduced electron transfer. Various reactive species such as exdplex, triplex, radical ion pair and free radical ions are generated via photoinduced electron transfer reactions. These reactive species have their characteristic reactivities and discrimination among these species provides selective photoreactions. The solvent and salt effects and also the effects of electron transfer sensitizers on photoinduced electron transfer reactions can be applied to the selective generation of the reactive species. Examples and mechanistic features of photoaddition and photocycloaddition reactions that proceed via the following steps are given reactions of radical cations with nucleophiles reactions of radical anions with electrophiles reactions of radical cations and radical anions with neutral radicals radical-radical coupling reactions addition and cycloaddition reactions via triplexes three-component addition reactions. 

Studies of SET-induced excited-state reactions of phthalimides with a-trialkylsilyl substituted ethers, thioethers, and amines have shown that cation radical desilylation is the foundation of interesting photochemical reactions that proceed by pathways involving the generation and coupling of radical intermediates. In an early effort, we observed that simple, a-silyl-substituted ethers, thioethers, amines, and amides undergo efficient photoadditions to phthalimide and its N-methyl derivative. One example in this series of closely related reactions is the photoaddition of silylmethylpropylthioether 4 to N-methylphthalimide, which produces the product 5 (Scheme 4). In the mechanistic route for this process, thermodynamically/kinetically driven SET from 4 to the excited phthalimide leads to formation of the ion radical pair 6. Solvent (MeOH)-promoted desilylation of the cation radical and protonation of the phthalimide anion radical then provides radical pair 7, the direct precursor of adduct 5. 

Photoinduced electron-transfer reactions generate the radical ion species from the electron-donating molecule to the electron-accepting molecules. The radical cations of aromatic compounds are favorably attacked by nucleophiles [Eq. (5)]. On the contrary, the radical anions of aromatic compounds react with electrophiles [Eq. (6)] or carbon radical species generated from the radical cations [Eq. (7)]. In some cases, the coupling reactions between the radical cations and the radical anions directly take place [Eq. (8)] or the proton transfer from the radical cation to the radical anion followed by the radical coupling occurs as a major pathway. In this section, we will mainly deal with the intermolecular and intramolecular photoaddition to the aromatic rings via photoinduced electron transfer. 

Photoaddition of. V-melhylaniline to phenantherene ring via C—O cleavage also occurs to give N-, <>-, and p-(phenanthrylmethyl)-substituted /V-mcthylani-lines, as shown in Scheme 115 [390], It is explained in terms of one-electron transfer from iV-methylanilinc to the phenanthrene derivatives, followed by proton transfer and the radical cross-coupling. 

One of the most useful and widely used applications in the synthesis of natural product derivatives relies on the efficient photoaddition of RS-H onto a double bond (a reaction known as thiol-ene coupling) [55], The reaction exploits the weakness of the S—H bond that can be cleaved homolytically under irradiation (atca. 254nm). The electrophilic sulfur-centered radical attacks a nucleophilic double bond, thus starting a radical chain reaction. 

The three-component photoaddition reactions of this type have also been reported by several groups [29,40-42,173-178], These examples are shown in Schemes 66,67 and 68. These photoreactions involve, in most cases, a coupling... 

One of the key steps used in a new synthesis of the bis(tetrahydrofuran) moiety of Asteltoxin (94) is the photoaddition of the propanal (95) to 3,4-dimethylfuran, yielding the adduct (96). This cycloaddition is a common outcome of the irradiation of aldehydes or ketones with furans. An analogous adduct (97) results from the photoreaction of butyl glyoxalate with 2-methylfuran. Two other products [(98) and (99)] are also formed, the first of which is presumably the result of ring opening of the isomeric oxetane (100), while (99) is produced by a hydrogen abstraction radical coupling pathway. 

Addition Reactions.- The photoelectron transfer process of the iminium salt (38) with the 3-butenoate anion results in the formation of the allylated product (39). The reaction involves decarboxylation of the 3-butenoate followed by a radical coupling reaction. The photoaddition of halogenated alkenes to the tetraraza phenanthrene (40) yields products (41) of (2+2)-addition. The Eu(III)/Eu(II) photoredox system has been studied with regards to its reactivity toweu ds a-methylstyrene. Irradiation of the system at > 280 nm in methanol yielded the products (42) and(43). ... 

Contrary to previous reports, acridizinium bromide yields all four isomeric [4+4]photodimerisation products both in solution and in the solid state. 2-Cyano-6,6-dimethyl-2-cyclohexenone undergoes photoaddition to 2,3-dimethyl-2-butene to give tricyclic[c,d]fused isoxazole (146) in 92% yield, possibly involving the triplet biradical (144) and the nitrene (145). With 2-methyI-2-butene and 2-methylpropene, isoxazoles (147) and (148) respectively are formed, though products from competitive formation of triplet biradical (149) are also found.The crystalline 1 2 adducts of 1,2,4,5-tetracyanobenzene with benzyl cyanide yield the coupling product (150) on irradiation this involves electron and proton transfer, radical coupling and tautomerisation steps. In solution (150) is not formed. 

A second example is found in the photoaddition of allylic or benzylic silanes to iminium salts reported by Mariano and his co-woikers. - PET reactions of allylic or benzylic silanes with iminium salts generate two kinds of neutral radicals, allyl or benzyl radicals and a-amino radicals. The C-C bond-forming reaction between two different radicals occurs by cross coupling (Scheme 4). 

In contrast to cyclopentenones and cyclohexenones, medium-ring vinylogous esters are not suitable for photocycloaddition reaction unless the olfin coupling partner is substituted. For example, when ( -3-(but-3-enyloxy)cyclooct-2-enone 78a is irradiated under a variety of conditions, no intramolecular cycloadduction occurs. However, upon substitution of a vinyl or phenyl on the olefin, the cycloaddition proceeds efficiently to give diastereomeric mixtures 79b/c, and 80b/c, respectively. The dramatically enhanced yields and rates of the photoaddition reactions upon olefin substitution result from the stabilization of the 1,4-biradical by a vinyl or phenyl. A mixture of diastereomers is formed presumable because the rotational relaxation of the intermediate 1,4-diradical is faster than [2 + 2] ring closure. 

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