Electron transfer photocycloaddition

The thermal Diels-Alder reactions of anthracene with electron-poor olefinic acceptors such as tetracyanoethylene, maleic anhydride, maleimides, etc. have been studied extensively. It is noteworthy that these reactions are often accelerated in the presence of light. Since photoinduced [4 + 2] cycloadditions are symmetry-forbidden according to the Woodward-Hoffman rules, an electron-transfer mechanism has been suggested to reconcile experiment and theory.212 For example, photocycloaddition of anthracene to maleic anhydride and various maleimides occurs in high yield (> 90%) under conditions in which the thermal reaction is completely suppressed (equation 75). 

In the presence of a proper second molecule bimolecular photochemical processes occur. Obviously such reactions can also occur in an intramolecular fashion in bifunctional molecules containing both reactive centres. These reactions are a) hydrogen abstraction by the excited molecule if the second molecule (or reactive centre) is a hydrogen donor RH (1.14) b) photodimerisation (1.15) c) photoaddition or photocycloaddition (1.16) d) electron transfer (1.17), if no bonding takes place between the reactants (or reactive centres). ... 

The photoreactions of mono- and di-cyanonaphthalene derivatives with al-lyltrimethylsilane have been reported by Mizuno and Albini, independently [136,428,433,436], The product ratios of photocycloadducts via exciplexes and the allylated products via electron transfer were dependent on the solvent polarity and additives. The intramolecular photocycloaddition of the reductive allylated products efficiently gives tricyclic compounds in high yields [428,433,436] (Scheme 23). 

These results clearly show that the stereoselectivity in the photocycloaddition of 1-CN and 2-CN with 2- and/or 3-furylmethanol is reasonably explained by the hydrogen-bonding in the excited states. Probably, a partial electron transfer from furylmethanols (F-OH) to the excited singlet states of cyanonaph-thalenes (ArCN) contributes the formation of polar exciplexes [ArCN8 F-OH8+], where ArCN8 interacts with more acidic OH group of F-OH8 to produce hydrogen bonds in the excited states. 

In recent investigations about the intramolecular meto-addition processes, the effects of heteroatoms in the linking chains are obvious. Blakemore and Gilbert reported the photoreaction of 185 linked by the amide group [240], Pho-toinduced intramolecular electron transfer can be inhibited by /V-acetyl or N-car-bomethoxy groups. Fluorescence is then observed from the arene chromophore and intramolecular meta photocycloaddition occurs through high selectivity. 

Synthesis of tetracyclic compounds by the photoreaction of cyanoarenes with benzyhc donors via photoinduced electron transfer is one of the unusual (3 + 3) photocycloaddition reactions (Scheme 110). It is not treated here and we concede to Albini et al. s excellent review [36]. 

A PET reaction between excess phthalimide (in equilibrium with its conjugate base) and an alkene led to a clean phthalimidation of nonactivated double bonds. Here, the singlet excited state of phthalimide acts as the oxidant and a radical ion pair is formed. The olefin cation radical is trapped by the phthalimide anion, and back electron transfer, followed by protonation, affords the photoaddition products [40], Protected phenethylamines are readily accessible in this way. This reaction has been carried out by using NaOH as the base it has been shown that the amounts (usually equimolar with the alkene) must be carefully chosen in order to avoid the undesired competition with [2 + 2] photocycloaddition. 

A [2 + 2] photocycloaddition with two alkenes can also be induced by photochemical electron transfer [16,17]. In such cases, sensitizers are frequently used and the reactions therefore occur under photocatalysis [18]. Under photochemical electron transfer (PET) conditions, the diene 10 yielded in an intramolecular reaction the cyclobutane 11 (Scheme 5.2) [19], such that in this reaction a 12-membered cyclic polyether is built up. The reaction starts with excitation of the sensitizer 1,4-dicyanonaphthalene (DCN) only 0.1 equivalents of the sensitizer are added to the reaction mixture. Electron transfer occurs from the substrate 10 to the excited sensitizer, leading to the radical cation I. This intermediate then undergoes cycli-zation to the radical cation of the cyclobutane (II). Electron transfer from the radical anion of the sensitizer to the intermediate II leads to the final product 11, and regenerates the sensitizer. In some cases, for example the cydodimerization of N-vinylcarbazole, the effidency is particularly high because a chain mechanism is involved [20]. 

Scheme 5.2 [2 + 2] Photocycloaddition induced by photochemical electron transfer.

The [2 + 2]-photocycloaddition of nonconjugated alkenes was described in Chapter 5. This strategy can be used for synthesizing macrocyclic rings by using a long linker between the two alkene moieties (Scheme 9.47). Thus, bicyclic cyclobutanes can be obtained starting from enol ethers and cinnamates by means of electron-transfer sensitizers, such as DCN or DCA [80], and triplet photosensitizers such as benzo-phenone (BP) [81]. 

Bach and coworkers investigated the photocycloaddition of 7V-acyl, 7V-alkyl enamines 125 with benzaldehyde [125]. The 3-amido oxetanes 126 were formed with excellent regioselectivity (analogous to reactions with enolethers—vide supra) and good diastereoselectivity (Sch. 41). Enamines, not deactivated by acylation at the nitrogen atom are poor substrates for Paterno-Buchi reactions due to preferred electron transfer reactivity (formation of the corresponding enamine radical cation and subsequent reactions). 

Alkene+alkene photocycloaddition in the presence of electron transfer sensitizers involves a similar path. For example, see (a) Asaoka S, Ooi M, Jiang P, Wada T, Inoue Y. J Chem Soc, Perkin 2 2000 73 (b) Goez M, Frisch I. J Am Chem Soc. 1995 117 10486. 

At the singlet excited state, ortho and meta photocycloadditions are often competitive processes and physicochemical investigations were carried out to rationalize the modes of cycloaddition of arenes with alkenes. In the context of the study of photochemical electron transfer reactions, it has been proposed that the difference of the redox potentials of the reaction partners might play an important role in this competition [10]. Such a discussion involves the intervention of an exciplex as intermediate. The Rehm-Weller equation [11] was used to quantify the relationship. When an electron transfer process is strongly endergonic (AG>1.5eV), the meta cycloaddition should be favored. When such a process is less endergonic (1 < AG< 1.5 eY), the ortho addition dominate [12]. This means that the... 

Chiral transition metal complexes have been employed in the enantioselective [2 4- 2]-photocycloaddition reaction, in asymmetric electron transfer reactions and photooxidations/reductions. In the enantiodifferentiating step of the latter reaction type the chiral transition metal complex is involved in an electron transfer, i.e., the metal is converted from an excited oxidative state to a more stable one. This... 

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. 

Another mechanism for the 2n -f 27t ) photocycloaddition of alkenes via electron transfer is the reaction that proceeds via a triplet state which is produced by a back-electron transfer from a radical anion of the electron acceptor to a radical cation of the electron donor. The triplet state alkenes generated by this way can undergo the cyclodimerization (Scheme 22). Farid showed that the DCA-sensitized (2n 3- 2n) photocyclodimerization of 1,2-diphenylcyclo-propene-3-carboxylate occurs via the triplet state of the cyclo-propene in acetonitrile [84]. In this photoreaction, two types of the (An -(- 27t) photocycloaddition reactions take place between DCA and the cyclopropene depending upon solvents. One type of the cycloadduct is produced in benzene via exciplex and the other type of the photocycloadduct is produced in... 

TCNE takes place (Scheme 57). Tomioka reported the (3 + 2) photocycloaddition between 1,1,2-triarylcyclopropanes and vinyl ethers in the presence of p-DCB [162]. Mizuno and Otsuji reported the (4 -l- 2) photocycloaddition between 1,2-diarylcyclopropanes and DCA [23]. The 1,4-radical cation produced as an intermediate of the Cope rearrangement of 1,5-dienes via photoinduced electron transfer can be trapped by molecular dioxygen, giving bicyclic dioxanes (Scheme 58) [163]. This photooxygenation takes place in a stereospecific manner. 

This textbook is not a physical organic chemistry textbook The sole purpose of this textbook is to teach students how to come up with reasonable mechanisms for reactions that they have never seen before. As most chemists know, it is usually possible to draw more than one reasonable mechanism for any given reaction. For example, both an Sn2 and a single electron transfer mechanism can be drawn for many substitution reactions, and either a one-step concerted or a two-step radical mechanism can be drawn for [2 + 2] photocycloadditions. In cases like these, my philosophy is that the student should develop a good command of simple and generally sufficient reaction mechanisms before learning the modifications that are necessitated by detailed mechanistic analysis. I try to teach students how to draw reasonable mechanisms by themselves, not to teach them the right mechanisms for various reactions. 

A photocycloaddition of a different type has been reported between indole and substituted cyclohexa-1,3-dienes triaryl-pyrylium tetrafluoroborates are used as sensitisers in this electron-transfer process and the [ 4 + 2] adducts are isolated as their W-acyl derivatives (111). High regioselectivity is observed. 

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