Photosensitization and Photocatalysis - Perspectives in Organic

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Photosensitization and Photocatalysis - Perspectives in Organic Synthesis Clément Michelin, and Norbert Hoffmann ACS Catal., Just Accepted Manuscript • DOI: 10.1021/acscatal.8b03050 • Publication Date (Web): 19 Nov 2018 Downloaded from http://pubs.acs.org on November 19, 2018

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Photosensitization and Photocatalysis – Perspectives in Organic Synthesis Clément Michelin, Norbert Hoffmann* CNRS, Université de Reims Champagne-Ardenne, ICMR, Equipe de Photochimie, UFR Sciences, B.P. 1039, 51687 Reims, France; e-mail: [email protected]

Abstract Photochemical sensitization and photocatalysis have very similar definitions and are closely related. Each of the two terms are preferentially used in different scientific communities. Three types of processes are discussed: (1) sensitization involving energy transfer, (2) photocatalysis in which hydrogen abstraction plays a key role and (3) photoredox catalysis in which electron transfer is involved. The processes are discussed in connection with [2+2] photocycloadditions and C-H activation, which are of particular interest for organic synthesis.

Key words Photosensitization, Photocatalysis, Energy transfer, Electron transfer, Hydrogen atom transfer, [2+2] Photocycloaddition, C-H activation.

Graphical Abstract

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2 Introduction A sustainable transformation of matter is a key challenge in chemistry. A series of requirements and objectives for the research in this domain has been defined1 among them the need to develop catalytic methods and to prevent waste. The replacement of stoichiometric chemical reagents by catalysis is an efficient strategy to minimize the formation of side or by-products. In photochemical reactions when applied to synthesis, light plays the role of a reagent which induces a transformation of a chemical compound as it is more commonly done by chemical reagents. In this context, the photon is considered as a traceless reagent.2 This effect is explained by the fact that electronic excitation by light absorption causes a particular reactivity which is often complementary to that one of the ground state.3,4 In this regard combination of catalysis with photochemical reactions should generate synergistic effects for sustainable transformation of matter.5 More precisely, Ryoji Noyori recently recommended to "develop (1) a 'photo-synthetic' catalyst that facilitates a thermally unachievable, energetically uphill reaction, and (2) a 'single-step cascade synthesis' using multiple components."6 Especially in point (1) the use of catalysts is recommended which are activated by light absorption. The present article focusses on the interest of photosensitization and photocatalysis for synthetic organic chemistry. Two reaction types, photocycloadditions and C-H activation, have been chosen.

Definitions In organic photochemistry, sensitization is often applied, when excitation by direct light absorption is difficult due to a low absorption coefficient or when competing processes such as fluorescence take place. Sensitization enables transfer of electronic excitation energy at a lower level which also increase chemical selectivity. In such cases, sensitizers absorb light and transfer their excitation energy to a substrate. The role of the sensitizer in such processes is very close to that one of a catalyst. The IUPAC gold book suggest the following definition for photosensitization:


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3 The process by which a photochemical or photophysical alteration occurs in one molecular entity as a result of initial absorption of radiation by another molecular entity called a photosensitizer. In mechanistic photochemistry the term is limited to cases in which the photosensitizer is not consumed in the reaction.7 This definition is very close to that one of catalysis or more precisely photocatalysis: Change in the rate of a chemical reaction or its initiation under the action of ultraviolet, visible or infrared radiation in the presence of a substance—the photocatalyst—that absorbs light and is involved in the chemical transformation of the reaction partners.8 Alternatively to electronic excitation by light absorption, sensitization is often an efficient method to induce a photochemical reaction (Scheme 1). In particular, triplet sensitization is often used in synthetic organic photochemistry. Thus, sensitization is the classical form of photocatalysis. In this regard different modes of photosensitization are most frequently observed.9 (1) A photochemical reaction can be induced by energy transfer from the sensitizer to the substrate (Scheme 1, eq. 1). (2) Photochemical electron transfer is often observed. Either the photoredox catalyst is oxidized or reduced (Scheme 1, eq. 2 and 3 respectively). In these cases, the catalyst is regenerated by electron transfer involving a sacrificial electron donor or acceptor. Electron transfer may also occur from a charged reaction intermediate. In the latter case, the use of an additional reagent (sacrificial electron donor or acceptor) is avoided. This activation plays a central role in many photoredox catalytic reactions as they have been developed mainly with UV-light in the past and with visible light in the present. In this case of electron transfer photosensitization, the definition of sensitization is also very close to that one of catalysis. Photochemical process in which a reaction of a non-absorbing substrate is induced by electron transfer (not energy transfer) with an excited light-absorbing sensitizer. The overall process must be such that the sensitizer is recycled. Depending on the action of the excited sensitizer as electron donor or acceptor the sensitization is called reductive or oxidative. 10 (3) Detailed investigations are currently performed on photochemical induced hydrogen atom transfer as initiating step in photocatalytic reaction of synthetic interest (Scheme 1, eq. 4).11,12 ACS Paragon Plus Environment

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4 The different kinds of sensitization are not only closely linked by their definitions. Discussed on a very fundamental level, often strong relationships between the two forms can be detected as it was shown, for example for electron transfer on one hand and excitation via energy transfer on the other.13 From the point of view of application, these two processes are sometimes observed for the same sensitizer or photocatalyst. In such cases, the exer or endergonicity of an electron transfer involving the electronically excited photocatalyst can be estimated with the Rehm-Weller equation.14 In this relationship, the redoxpotentials of the excited state of the catalyst (calculated from electrochemical measurements and the excitation energy) and the substrate or sacrificial electron donor or acceptor are taken into account. The exer or endergoncity of an energy transfer can be estimated by comparison of the excitation energy of both species, the sensitizer as energy donor and the substrate as energy acceptor. As an example, such a detailed discussion of both mechanisms has been performed for [2+2] photocycloaddition which is sensitized by an iridium complex.15


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5 Scheme 1. Most frequent elementary steps of photosensitization or photocatalysis as applied to organic synthesis. The photochemically excited sensitizer (sens) or catalyst (cat) reacts with a substrate (S) which induces chemical reactions. When energy transfer is involved, most frequently, the process is called sensitization. When electron or hydrogen transfer is involved, the process is often called photocatalysis or photoredox catalysis.

Cycloadditions Photocycloadditions are the most frequently applied photochemical reactions applied to the synthesis of complex compounds.16,17,18,19 Especially in reactions of α,β-unsaturated lactones or lactames with alkenes, photochemical sensitization improves yields and facilitates the transformations. Direct excitation often occurs at low wavelengths (λ≈250nm) which causes side reactions. In such cases, particular conditions such as low reaction temperatures are necessary for a successful transformation. However since the triplet energy of such compounds is much lower, triplet sensitization leads to high yields, even at room temperature. An impressive example involving asymmetric catalysis is depicted in Scheme 2.20 The lactame 1 is linked to the chiral sensitizer 2 via hydrogen bonds (3). The xanthone moiety is capable of absorbing visible light in the 400 nm wavelength domain while the absorption maximum of 1 is in the range of λ = 330 nm. Under the applied irradiation conditions, almost only the xanthone chromophore is excited. In the complex 3, it transfers its triplet energy (264 kJ mol-1) to the lactame, which undergoes enantioselective [2+2] photocycloaddition. The cyclobutane derivative 4 is obtained in high yield and excellent enantioselectivity. This method of asymmetric catalysis has been applied to many other photochemical reactions.21 In similar reactions, triplet energy transfer occurred from a homo-chiral iridium complex (Scheme 3).15 The chiral sensitizer was attached to a pyrazolylpyridin moiety which bonded the lactame substrate.


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Scheme 2. Intramolecular [2+2] photocycloaddition sensitized by a hydrogen bonded chiral sensitizer.

Scheme 3. Intramolecular [2+2] photocycloaddition sensitized by an iridium complex.

[2+2] Photocycloaddition with non-cyclic α,β-unsaturated carbonyl or carboxyl compounds is often difficult since competitive cis/trans isomerization is very efficient. Such reactions can be carried out using photoredox catalysis or photochemical electron transfer sensitization. Compounds such as 5 (Scheme 4) do not undergo intramolecular [2+2] photocycloaddition when they absorb light under conventional photochemical conditions without sensitization. Using photoredox catalysis with


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7 [Ru(bipy)3]Cl2 such transformations were successfully carried out under irradiation with visible light.22 The reaction starts with electronic excitation of the ruthenium photocatalyst. At the excited state, the species is easily reduced via electron transfer from a tertiary amine 6. The resulting [Ru(bipy)3]+ complex possessing a high reduction potential transfers an electron onto one of the α,β-unsaturated ketone functions of 5. However, this transfer is only exergonic when the ketone is complexed, for example by a lithium ion (7). In this way the photocatalyst is regenerated. The resulting radical species 8 undergoes cyclization leading to the electron poor radical intermediate 9. Fast addition of this radical to the enolate yields the ketyl radical 10. Oxidation by either by the radical cation the Ru(II) complex or the radical cation 12 of the tertiary amine yields the final cyclobutane derivative 11. Similar intermolecular transformations have been carried out with two different enone reaction partners.23 Using asymmetric co-catalysis, the corresponding cyclobutanes have been obtained with high enantioselectivities. In the present case, a Ru(I) species transfers an electron onto the substrate. In a similar reaction, styrene moieties undergoes [2+2] photocycloaddition with the same photoredox catalyst. In these reactions, a Ru(III) intermediate is generated via photochemical electron transfer.24 This oxidized ruthenium species abstracts an electron from the substrate and the resulting styrene radical cation undergoes cyclization.


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Scheme 4. Intramolecular [2+2] photocycloaddition involving photochemical electron transfer sensitization.

C-H Activation involving hydrogen abstraction In organic synthesis the activation of particularly stable C-H bonds is a particular challenge. Although known for a long time25 a lot of new reactions of this type are continuously reported.26 Many of such transformations are successfully carried out using photochemical or photocatalytic reactions. Acetone is a very efficient triplet sensitizer. The high triplet energy of 79.4 kcal mol-1 allows triplet sensitization of numerous chromophores. In many reactions, it is used as solvent or in high concentrations. Thus the optical density is increased and significant absorption of light at around


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9 λ ≈ 300 nm is still possible although λmax ≈ 270nm.27 In such reactions, the regeneration of the sensitizer occurs easily and no significant side reaction of acetone is observed. Furanones like 13 and 14 are typical examples (Scheme 5). When irradiated in the presence of acetone, compounds 15a,b and 16 are formed.28,29 Under the reaction conditions, acetone absorbs most of light and is excited to its nπ* singlet state. After intersystem crossing (ISC) to the corresponding triplet state, efficient triplet energy transfer to the furanones 13 or 14 occurs leading to the ππ* triplet state 17. The relaxed structure is characterized by a high spin density in the β position.30 For this reason, a hydrogen atom is transferred from 1’ or the 5’ position of the glucosyl moiety to the β position of the furanone moiety leading to diradical intermediates 18 and 19 depending on the relative configuration of the substrate. Radical combination leads to the final products 15a,b and 16 respectively. In this reaction, a C-C bond is formed in the α position of an α,β-unsaturated carboxyl compound. This unusual regioselectivity was explained by the mechanism of hydrogen transfer. In this case, both particles, the proton and the electron are transferred simultaneously.11 Similar reactions have been carried out with analogous imine derivatives leading to complex polycyclic nitrogen containing compounds (Scheme 6).31 It must be pointed out that the in these reactions, the hydrogen transfer is not part of the photocatalytic process. Hydrogen transfer as part of the photocatalysis is discussed below. The reaction depicted in Scheme 5 has also been carried via direct light absorption of the furanones with similar results.29 However, these reaction conditions must be carefully chosen in order to minimize side product formation. Especially, the reaction temperature is low. For these reasons, acetone sensitization is often applied when the reaction is applied to synthesis.


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Scheme 5. Intramolecular addition of an acetal to a photochemically excited α,β-unsaturated lactone involving a hydrogen transfer in one step.


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11 Scheme 6. Photochemically induced cyclization reaction involving a hydrogen transfer step in a non catalytical reaction.

Instead of energy transfer, a photochemical sensitizer can also induce a reaction via hydrogen abstraction. Aromatic ketones such as benzophenone or its derivatives are often used in this regard.32 Polyoxometallates such as the decatungstate anion 20 are also efficient in this regard (Scheme 7).33 In contrast to aromatic ketones which are excited by UV light absorption, These compounds are electronically excited by absorption of sun light.34 Often, it is used as its tetrabutyl ammonium salt (TBADT). After photochemical excitation, this compound abstracts selectively a hydrogen atom from donor molecule such as 21. The resulting radical 22 is added to alkenes such as dimethylmaleate 23 leading to the electron deficient radical 24.35 Hydrogen atom transfer from the sensitizer leads to the final product 25. The regioselectivity is mainly influenced by the steric hindrance due to the bulkiness of the catalyst. In this case, the formation of the radical intermediate is only observed at the alkyl side chain. But, electronic effects also contribute significantly to the selectivity.36,37 This is shown for the transformation of cyclopentanone 26.38 When irradiated under the same conditions, hydrogen abstraction occurs in the β position leading to the intermediate 27. Addition of the vinylsulfone 28 leads to the final product 29 in good yield. Similar reactions have been carried out with inorganic semi-conductors such as TiO2.39


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Scheme 7. Photochemical C-H activation using tetrabutyl ammonium decatungstate (TBADT) as photocatalyst.

Aromatic ketones when electronically excited are very efficient sensitizer to induce photochemically transformations by hydrogen atom transfer.40 Fluorenone 30 has been used to abstract a hydrogen atom in the benzylic position (33) (Scheme 8). In combination with fluorination reagent 31, this reaction step was applied to the synthesis of mono fluorinated compounds such 33.41 Compounds 34, 35, 36 and 37 are typical examples. Using xanthone 38 as sensitizer in combination with 39, corresponding difluorinated products such as 40, 41 or 42 are obtained (Scheme 9). The following mechanism has been discussed (Scheme 10). After photochemical excitation of the aromatic ketone 45, hydrogen abstraction occurs and a ketyl radical 46 as well as a benzyl radical 47 are formed. The latter abstracts a fluorine atom from 48 which leads to the final product 49. The resulting radical cation 50 is able to abstract a hydrogen atom form the ketyl radical 46 which regenerates the sensitizer. Most probably this hydrogen atom transfer occurs in two steps (compare ref. 11). An electron is transferred first leading to highly stabilized carboxonium ion and a tertiary amine. The


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13 proton is then transferred from the carboxonium ion to the amine. The same reaction has been carried out in a continuous-flow photoreactor.42 Among other transformations, ibuprofen methyl ester (43) and celestolide (44) have been mono-fluorinated (Scheme 9).

Scheme 8. Monofluorination aromatic ketones as sensitizer.

Scheme 9. Difluorination aromatic ketones as sensitizer.


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Scheme 10. Proposed mechanism of monofluorination aromatic ketones as sensitizer.

C-H Activation involving photochemical electron transfer Photoredox catalysis is a combination between photochemical reactions, redox processes and catalysis. A lot of such reactions have been carried out in the past using UV light.43 Now, this field of research enjoys a surge of popularity and many transformations are carried out with visible light. This can probably be explained by the fact that it allows the access to radical chemistry by green methods while these compounds have historically been generated using toxic reagents, such as organotin compounds.44 Many photoredox catalysts have been described. For a precise transformation, they are generally chosen for different characteristics such as the absorption wavelength, the excited state lifetime and its reductive/oxidative potential. In this field, some transition metal complexes have attracted a special attention and more particularly ruthenium and iridium complexes.45 Nevertheless, some less expensive organic catalysts are also investigated.46,47 Most of the time, photoredox catalysis is based on the ability of an excited catalyst to engage in single electron transfer with an organic substrate. This mechanism is generally equilibrated with the Back Electron Transfer (BET) which is, therefore, a key step to avoid (Scheme 11). At this stage, depending on the reaction, the catalyst will take or give an electron to a quencher or substrate. This


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15 step generates reactive species that will react with the previously formed radical by different mechanisms to give the product. The regenerated catalyst is excited by light and can react via another SET (Scheme 1, eq. 2 and 3). In some examples, the quencher is replaced by a second catalytic cycle. This one can be organic or organometallic.48 Numerous photoredox catalytic reactions have been described. Among those are reductive dehalogenations, cycloadditions or additions on alkene.

Scheme 11. After single electron transfer between the electronically excited catalyst (cat)* and the substrate S, chemical reactions may occur or a back electron transfer can take place. The latter process yields the photocatalyst at its ground state and the unchanged substrate.

Typical substrates for such reactions are amines. Photochemical electron transfer from these substrates to electronically excited species is very easy and can be used for the synthesis of a variety of nitrogen containing compounds.49 P. Melchiorre et al. have described the first example of enantioselective 1,4-addition on cyclic conjugated ketones by iminium ion trapping of amine derived radicals (Scheme 12). 50 In the presence of the organocatalyst 51 and the photoredox catalyst 53, N,N-dimethylaniline 54 was coupled with 3-methyl-cyclohexenone 55. The reaction starts with the excitation of the IrIII catalyst [Ir(dF(CF3)ppy)2(dtbbpy)]+. After electron transfer from 54 and deprotonation of the resulting amine radical cation, the α-aminoalkyl radical 56 is formed. Addition to the iminium species 57, which is generated from the substrate 55 and the chiral organo catalyst


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16 51, leads to 58. An intramolecular electron transfer has been proposed yielding the imine intermediate 59 which is reduced by the IrII species. In this reaction step, the photocatalyst is regenerated. Hydrolysis of the resulting neutral imine (regeneration of the organo catalyst 51) leads to the final product 60 in high yield and good enantioselectivity. The same reaction was also performed using the much cheaper organic photoredox catalyst 61 under the same conditions with almost the same results. Similar reactions with this photocatalyst and other aromatic ketones have previously been carried out.51,52

Scheme 12. Combined asymmetric organo and photoredoxcatalysis applied to the addition of N,Ndimethylaniline 54 was added with 3-methyl-cyclohexenone 55. ACS Paragon Plus Environment

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When carbonyl compounds are used as photosensitizer to produce α-aminoalkyl radicals, ketyl radicals are concomitantly generated. A coupling between these two species may be observed leading to 1,2-amino alcohols.53 Recently, such a reaction has been carried out using asymmetric catalysis (Scheme 13).54 The substrate 62 is complexed to the homo-chiral iridium catalyst (65). After photochemical excitation, an electron transfer from the tertiary amine 63 to the complex takes place and a radical ion pair 66, 67 is generated. Proton transfer leads to neutral the α-aminoalkyl radical 68 and the complexed ketyl radical 69. Radical combination leads to the final product still complexed to the iridium atom (70). In this step, chirality is induced since the combination step occurs in the homochiral ligand sphere. Ligand exchange yields the free final product and generates the complex between the iridium photochatalyst and starting compound 62. It must be pointed out that in this case the catalyst-substrate complex is photochemical excited which is in contrast to the previous example where light absorption of the photocatalyst induces the transformation. The replacement of one acetonitrile ligand by a substrate molecule alters the photochemical reactivity. In this regard the example described in Scheme 13 is different from that one discussed in Scheme 4 or for the photoredox catalytic steps depicted in Scheme 12. In conceptually related approaches, substrates or reaction intermediates form charge transfer complexes which then absorb light and induce the photocatalytic reaction (Figure 1).55,56 Furthermore, it should be pointed out that the excited iridium substrate complex 65 (Scheme 13) reacts via electron transfer while the excited iridium substrate complex in Scheme 3 reacts via energy transfer to the lactame substrate attached to a pyrazolylpyridin ligand via hydrogen bonds (also compare to the reaction in Scheme 2). Using photoredox catalysis, α-aminoalkyl radicals such as 68 have also been added to N-aryl aldimines57 or to N-methanesulfonylaldimines (Scheme 14).58 In the latter case, the imine and the corresponding radical anion are bonded to a chiral (arylamino)-phosphonium ion. In most cases, ee values >90% were observed for the formation of the diamino products.


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Scheme 13. Asymmetric photoredox catalytic addition of a tertiary amine to 2,2,2-trifluoro(pyridin-2yl)ketone 62.

Figure 1. Chrage transfer complex of a substrate and an enamine intermediate. Light absorption induces an electron transfer which leads to the alkylation reaction.


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Scheme 14. Asymmetric C-C coupling of tertiary amine to an imine derivative. 58

Conclusion Photochemical sensitization and photocatalysis are closely related processes. Often both terms are used for a same process depending on the scientific community. They are involved in many photochemical reactions of high interest for application to organic synthesis. A variety of elementary reaction steps such as energy transfer or electron and hydrogen transfer appertain to these processes. To investigate particular reactions in connection with organic synthesis, it is more appropriate to discuss these elementary steps than to range them into the two categories of photochemical sensitization and photocatalysis. The discussion of these steps enables the understanding of reactivity and various kinds of selectivity such as chemo regio or stereoselectivity. On the other hand, when studying organic photochemical reactions it is very beneficial to consider information from the domain of photo-physical chemistry59 where the term "photochemical sensitization" play an important role. The combination of typical photochemical processes with catalysis thus opens new perspectives in organic synthesis. A particular important topic is C-H activation. But also the scope of more classical reactions such as the [2+2] photocycloaddition is considerably extended when sensitization or photocatalytic


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20 conditions are applied. Currently, photoredox processes, especially with visible light are intensively investigated. Some of them are carried out in a stereoselective way using asymmetric catalysis. Many of such reactions fulfill requirements of sustainable chemistry. In order to further optimize them in this sense, they are carried out in micro and continuous flow reactors.32,60 Some of these reactions are also be carried out with sun light.61

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