Photoinduced Organic Reactions by Employing Pyrene Catalysts

Abstract

Pyrene is one of the most attractive polycyclic aromatic hydrocarbons (PAHs) in photochemistry. Based on their redox properties, pyrenes have potential as photosensitizers. In this review, we summarize recent developments in pyrene-catalyzed photoinduced organic reactions occurring via energy transfer or single-electron transfer based on the excited state of the pyrene.

1 Introduction

2 Photolysis Involving N–O Bond Cleavage or Decarboxylation

3 (Cyclo)addition Reactions with Styrenes

4 Transformations via Cleavage of C–F, C–I, C–S and C–N Bonds

5 Reactions Based on Sensitization-Initiated Electron Transfer (SenI­-ET)

6 Miscellaneous Transformations

7 Conclusion

Introduction

Pyrene is one of the most attractive polycyclic aromatic hydrocarbons (PAHs) in photochemistry owing to its unique properties,[1] which include its ability to form excimers and exciplexes with various types of compounds, its high fluorescence quantum yields, and the relatively long lifetime (ca. 450 ns)[1c] of its singlet excited states compared with those of other PAHs. By taking advantage of these features, pyrene and its derivatives have been applied in organic electronics, such as organic semiconductors, light-emitting diodes, photovoltaic cells, and field-effect transistors.[1e] Moreover, recent developments in synthetic methods for pyrenes[2] have enabled the introduction of various substituents on the pyrene structure for both subtle and obvious adjustments to their photophysical properties. In addition, the synthesis of pyrene-based metal–organic frameworks (MOFs) and their applications have recently started to gain attention.[3]

Recently, photosensitizers such as transition-metal complexes and organic dyes (i.e., Rh, Ir, acridinium salts, Eosin, etc.) have been explored. Moreover, these sensitizers possess useful photophysical properties related to absorption bands, including activity in the visible-light region, triplet energy, and oxidation/reduction potentials.[4] Pyrene shows the following optoelectronic properties: its singlet and triplet energies are ca. 26100 cm^–1 and ca. 16850 cm^–1, respectively,[5] and its oxidation/reduction potentials are +1.2 V/–2.1 V vs SCE.[6] Its wide range of redox potentials enable electron transfer between excited pyrene and organic compounds; in particular, excited pyrene acts as a strong reductant. Pyrene and its derivatives also have the potential to be used as photocatalysts. The introduction of functional groups to pyrene using well-developed synthetic methods can finely tune the optoelectronic properties of pyrene derivatives. For example, a phenylethynyl pyrene absorbs visible light.[7] In addition, the redox potentials of pyrenes can be adjusted by the introduction of electron-donating or electron-withdrawing group(s) on the pyrene moiety. Recently, visible-light-induced reactions involving pyrene derivatives have been attempted as one of the disadvantages of pyrene is that its excitation requires ultraviolet light. A sensitization-initiated electron-transfer (SenI-ET) process based on energy transfer between pyrene and other visible-light photoredox catalysts can lead to visible-light-induced reactions with pyrene.

This review summarizes the photoinduced reactions in organic synthesis based on pyrene and its derivatives, which involve energy transfer or single-electron transfer (SET) with the excited state of the pyrene catalyst (Scheme [1]). A review on polymerizations involving pyrene-based photoinitiations was recently reported.[8]

Photolysis Involving N–O Bond Cleavage or Decarboxylation

Okubo and co-workers reported the photolysis of N-(1-naphthoyl)-O-(p-toluoyl)-N-phenylhydroxylamine (1) catalyzed by pyrene (Py1) (Scheme [2]).[9] N-Hydroxylamine derivative 1 underwent decomposition in the presence of Py1 as a singlet-sensitizer upon irradiation at 366 nm to give the different products 2–6. The yield of each of these products was not reported. The radical pair, consisting of the amide and acyloxy radicals 1a and 1b, was generated via homolysis of the N–O bond in 1, and subsequent acyl group migration proceeded to yield compounds 2 and 3. Hydrogen atom abstraction by amide radical 1a, acyloxy radical 1b, and the aryl radical generated from decarboxylation of 1b yielded amide 4, 4-methylbenzoic acid (5), and toluene (6), respectively. Experiments on the fluorescence quenching of Py1 by 1 in several solvents showed that the fluorescence based on both the monomer and excimer of Py1 was quenched by 1. These results indicated the following two reaction pathways shown in Scheme [3]: (1) Energy transfer in the exciplex between the excited state of Py1 and 1 caused homolytic cleavage of the N–O bond in 1*. This reaction was succeeded by acyloxy group migration between 1a and 1b to produce products 2 and 3. (2) The radical ion pair between 1^•– and Py1^•+ , generated via single-electron transfer (SET), followed by the decomposition of 1^•– , provided the amide radical 1a and p-tolyl benzoate, which was oxidized to acyloxy radical 1b by Py1^•+ , resulting in products 2 and 3. The authors suggested that the SET process is dominant in this reaction.

Oda and co-workers developed the photoinduced decarboxylation of N-acyloxyphthalimides 7 by using a substoichiometric amount of 1,6-bis(dimethylamino)pyrene (Py2) as the photosensitizer.[10] The reaction of Py2, 7, and ^t BuSH in ⁱ PrOH/water solution upon photoirradiation (>350 nm) provided hydro-decarboxylated product 8, along with phthalimide, carbon dioxide, di-tert-butyldisulfide and recovered Py2 (Scheme [4a]). The large molecular extinction coefficient (log ε = 4.05 at 400 nm) and the low oxidation potential of Py2 (E _1/2 = +0.24 V vs SCE) enabled this efficient photosensitized reaction. This reaction was applicable to primary, secondary, and tertiary carboxylic acid containing phthalimides 7. SET from the singlet excited state of Py2 to 7 initiated the reaction, being supported by the Rehm–Weller equation. Moreover, the decarboxylative radical cyclization of 9 produced cyclopentane derivative 10 with a yield of 84% (Scheme [4b]).

Hamaguchi and co-workers reported that the photolysis of N-phenylglycine (11) induced by Py1 was accelerated in the presence of 1,4-dicyanobenzene (16), which is an electron acceptor (Scheme [5]).[11] Glycine 11 is commonly used as a photoinitiator in photopolymerization and as a sacrificial electron donor in photocatalytic systems. Pyrene-sensitized photolysis of 11 produced N-methylaniline (12), aniline (13), and N-phenylformamide (14) (Scheme [5a]). The yield of each of these products was not reported. First, SET from 11 to Py1* occurred to produce the radical ion pairs of 11^•+ and Py1^•– . The radical cation intermediate 11^•+ was readily deprotonated and decarboxylated, giving the corresponding α-amino methyl radical 15 (Schemes 5a and 6a). Consequently, hydrogen abstraction of 15 caused the formation of 12. In this case, the conversion of 11 was 45%, and considerable consumption of Py1 (88%) was observed. In contrast to the above pyrene radical anion process in the presence of 16, a different reaction mechanism was suggested (Schemes 5b and 6b). SET between the singlet excited state of Py1 and 16 proceeded to give Py1^•+ and 16^•– . The one-electron oxidation of 11 with Py1^•+ was involved in this decomposition, and 16^•– was converted into N-(p-cyanobenzyl)aniline (17) in the presence of 15. To support the proposed mechanism in Scheme [6], the authors demonstrated the direct observation of related radical ion species from the transient absorption spectrum of a mixture of Py1 and 11 in acetonitrile with or without 16. In the absence of 16, the spectrum showed an intense absorption band with a maximum at 493 nm, corresponding to Py1^•– . In the presence of 16, two absorption bands at 445 and 343 nm were observed, which could be assigned to Py1^•+ and 16^•– , respectively.

(Cyclo)addition Reactions with Styrenes

Pérez-Prieto and co-workers reported that the selective [2 + 2] cross-cycloaddition between cyclohexadiene 18 and styrenes 19 was catalyzed by bichromophoric compounds such as pyrene-benzoylthiophene Py3 and pyrene-indole Py4 (Scheme [7]).[12] In general, the reactions between derivatives of 18 and 19 have competing processes, such as [2 + 2] cross-cycloaddition and [4 + 2] cross-cycloaddition, and ­dimerization of olefins or dienes. Nevertheless, selective ­[2 + 2] cross-cycloaddition to give cycloadducts 20 was achieved using Py3, without the competitive formation of dimers. In contrast, fragments of Py3, such as N-acetyl-1-pyrenylmethylamine (Py5) and 2-benzoylthiophene, exclusively resulted in a polymeric mixture. The authors proposed that this selective cross-cycloaddition proceeded via triplet energy transfer from the intramolecular exciplex of Py3 between the singlet excited state of pyrene and the ground state of 2-benzoylthiophene. The proposed mechanism is supported by the fluorescence spectra and the Stern–Volmer plot for the fluorescence quenching of Py3 by cyclohexadiene 18 or styrene 19. Additionally, the experimental results of the reactions under different concentrations of the fluorescence quencher showed that formation of the cycloadducts was predominantly attributed to quenching by cyclohexadiene 18 rather than styrene 19.

The regioselective addition of methanol to diphenylethylene (21) using 1-(N,N-dimethylamino)pyrene (Py6) as the photocatalyst was reported by Wagenknecht and co-workers (Scheme [8]).[13] The reaction was triggered by SET from Py6* to 21, in which the dimethylamino group on Py6 might enhance its strong reducing ability. The addition of Et₃N as an additional electron shuttle improved the yield of product 22. The result suggested that back electron transfer from the diphenylethyl radical 23 to Py6^•+ via Et₃N was crucial because Et₃N smoothly converted Py6^•+ into Py6, thereby avoiding the degradation of Py6 (Scheme [9]). Although the reaction system can be applied to 21, styrene was not suitable, probably because of the small driving force for the initial SET step.

The above-mentioned system requires Et₃N as an electron shuttle. The same research group achieved the nucleophilic addition of methanol to phenylethylene 24 without an electron shuttle by using proline-bearing short peptides Py7 that contain the 1-(N,N-dimethylamino)pyrene (Py6) moiety (Scheme [10]).[14] Peptides with proline linkers of different chain lengths were prepared and tested as photoredox catalysts. Forward and backward electron transfers can be tuned by binding the carboxylic acid or ester groups of 24 with the guanidine or amide moieties of Py7 via hydrogen bonding and/or electrostatic interactions.

Transformations via Cleavage of C–F, C–I, C–S and C–N Bonds

Pyrenes also function as inner-sphere electron-transfer photocatalysts through π–π interactions. Zhang and co-workers developed the hydrodefluorination of polyfluoroarenes FA with a pyrene-based photocatalyst promoted by the ‘π-hole–π’ interaction between pyrene derivatives and FA (Scheme [11]).[15] Photoinduced electron transfer (ET) consisting of an electron donor–acceptor (EDA) complex smoothly proceeds because the process can avoid the high potential energy surface, even though the ET path has a larger underpotential. In this case, the one-electron direct reduction of FA does not readily occur. ET from the excited state of pyrenes to FA is unfavorable because of the relatively large energy gap between the LUMO levels of pyrenes and FA. However, the formation of the EDA complex overcomes this problem.

The authors prepared pyrene derivatives Py8 and Py9, with different steric-sized substituents, and various FAs. The formation constants of each combination were determined by ¹H NMR titrations. Moreover, the authors demonstrated the relationship between the relative initial reaction rate and the size and shape of the photocatalysts Py8 and Py9, which indicated that the efficiency was affected by the steric hindrance of the FA.[15] High steric repulsion between pyrenes and FA led to inefficiency in the formation of the ‘π-hole–π’ complex, and remarkably, decreased the yield of the product 26, suggesting that formation of the EDA complex was crucial to the initial single-electron transfer step. This assumption was also supported by a single crystal X-ray structure analysis of the Py9/hexafluorobenzene (27) pair (Py:27) and the UV/Vis spectra of Py9 and the FAs. The proposed mechanism in Scheme [12] shows that an anionic radical complex intermediate [Py:FA]^•– (at the bottom right in Scheme [12]) was formed through SET from the sacrificial electron donor (i.e., ⁱ Pr₂NEt) to the excited ‘π-hole–π’ complex [Py:FA]*, which was generated from the ground state of ‘π-hole–π’ complex Py:FA upon irradiation with light or a combination of Py* (Py8* or Py9*) and FA. This reaction was applied to various FAs, and the substrates produced mainly mono-hydrodefluorination products. Octafluoronaphthalene provided a mixture of mono- and di-hydrodefluorination products. The reaction system is applicable to the C–F reductive alkylation of 27 with cyclohexene 28 to produce 29 (Scheme [11a]). The yields of 26 are comparable to those of the visible-light-induced hydrodefluorination of FA by Ir(ppy)₃ (Scheme [11b]).[16]

Zhang, Duan, and co-workers applied metal–organic frameworks (MOFs) based on 1,3,6,8-tetrakis(p-benzoic acid)pyrene and zirconium ions (NU-1000) to the photocatalyzed iodoperfluoroalkylation of olefins 30 under visible-light irradiation.[17] This is the first example of NU-1000-catalyzed organic reactions, even though NU-1000 has been applied for singlet oxygen generation[3] (Scheme [13a]). The reaction proceeded via singlet–singlet energy transfer from the excited pyrene moiety to iodoperfluoroalkane 31 and subsequent generation of an iodine radical 35 and a perfluoroalkyl radical 36 by homolytic cleavage of the C–I bond. In the case of aliphatic olefins, the radicals reacted with the olefins to produce products 32 (Scheme [14a]). On the other hand, the reactions of styrenes 33 gave perfluoroalkyl-substituted styrenes 34 through 2,6-lutidine-assisted elimination of HI from an iodofluoroalkylated product 33a (Schemes 13b and 14b). Interestingly, only the Z isomer was obtained, suggesting that triplet–triplet energy transfer (T-TET) was also involved in the reaction process. The E/Z isomers were gradually converted into the Z isomer via excited states (Scheme [14b]).

Orita et al. developed a Julia olefination using a custom-made pyrene photocatalyst (Scheme [15]).[7] Desulfonylation of 37 was efficiently promoted by tetra(arylethynyl)pyrene Py10 under green light irradiation. The tetra(arylethynyl) groups on Py10 contribute to the π-extension and the absorption of green light. SET from ⁱ Pr₂NEt to the excited pyrene Py10* produced Py10^•– , which reduced the sulfonyl compounds. Further protonation and one-electron reduction produced stilbenes 38.

We recently developed a metal-free direct C–N borylation of unreactive aromatic amines using a pyrene catalyst under UV irradiation (Scheme [16]).[18] The borylation of aromatic amines 39 with B₂pin₂ proceeded via C–N bond cleavage in the presence of pyrene (Py1) upon photoirradiation at 365 nm. Notably, borylated products 40 were obtained without any metal catalyst, base, or reagent activating the aromatic C–N bond. Various aromatic amines containing biphenyl amines, π-extended anilines, and naphthyl amines can be used in the reaction. In addition, relatively reactive functional groups, such as fluorine, chlorine, cyclopropyl and silyl groups, remained intact during this reaction.

Reactions Based on Sensitization-Initiated Electron Transfer (SenI-ET)

Photoinduced reductive single-electron transfer of pyrene (Py1) can be applied to various substrates with a high reduction potential (i.e., aryl chlorides) because pyrenes have high potential energy (–2.1 V vs SCE), similar to the transition-metal complex represented for iridium complexes (i.e., [Ir(dFppy)₂bpy]³⁺ ~ –2.1 V vs SCE). However, photocatalysts based on pyrene are usually limited to ultraviolet light irradiation because their absorption band is around 350 nm. In the sensitization-initiated electron transfer (SenI-ET) process, the photosensitizer and redox catalyst are different molecules accessible via energy transfer. Therefore, energy transfer from the visible-light absorbing sensitizer to redox catalysts such as polycyclic aromatics allows access to high reduction potentials, which are inaccessible to direct excitation upon visible light irradiation.

König and co-workers adopted SenI-ET for the activation of C–(pseudo)halogen bonds in aryl chlorides, bromides, and pseudohalides using Ru(bpy)₃ ²⁺ and pyrene (Py1) under visible-light irradiation.[19] The excited state of Py1 is accessible via charge transfer from [Ru(bpy)₃]²⁺*, which is supported by Stern–Volmer plots for the quenching of [Ru(bpy)₃]²⁺ with Py1.[20] The reaction of halogenated-(hetero)aryl equivalents 41 with pyrroles 42 (or electron-rich aromatic rings 43) in the presence of [Ru(bpy)₃]²⁺, Py1, and diisopropylethylamine (DIPEA) under irradiation with 455 nm light gave the C–H-arylated products 44 or 45 (Scheme [17a]). Both [Ru(bpy)₃]²⁺ and Py1 are necessary in this reaction (Scheme [17b]). Various substituted (hetero)aryl bromides and aryl chlorides are good substrates. Notably, the carbon–halogen bond of trapping reagents with a higher reduction potential than Py1^•– is inert under the reaction conditions. Phosphite esters 46, such as P(OEt)₃, can also be used to produce the corresponding phosphonylated products 47 (Scheme [17c]).

There are several controversies regarding the mechanism of the reaction in Scheme [17]. The mechanism first proposed by König’s group postulated that the excited pyrene generated via energy transfer through the excited [Ru(bpy)₃]²⁺ complex was reductively quenched by DIPEA to generate Py1 ^•– and the radical cation of DIPEA. In its ground state, Py1^•– transfers one electron to the (hetero)aryl halide, yielding the (hetero)aryl radical precursor (Het)ArX^•–. Subsequently, the Balzani group further investigated the proposed mechanism, and questioned the single-electron transfer process from DIPEA to a triplet state pyrene (Py1*(T)) via triplet–triplet energy transfer from the triplet state [Ru(bpy)₃]²⁺ complex because this process is endergonic [Py1*(T) + DIPEA → Py1^•– + DIPEA^•+: ~ –1.0 V).[21] As for this process, they alternatively proposed the triplet–triplet annihilation (TTA) process. Regarding this controversial issue, König’s group proposed other mechanisms and discussed their details (Scheme [18]).[6] Subsequently, Moore’s group provided insights into the mechanism of photoredox activity by using a variety of techniques, in particular, nanosecond transient absorption spectroscopy.[22] They estimated the relative contributions of TTA compared to SenI-ET quenching of Py1*(T) in terms of each rate constant (k _TTA and k _SenI-ET). Using a modified Stern–Volmer approach and the concentration-dependent decrease in Py1*(T) lifetime, k _TTA and k _SenI-ET were estimated, and the latter was approximately one order of magnitude faster than the former (k _TTA = 1.34 × 10¹⁰ M^–1s^–1; k _SenI-ET = 2.22 × 10¹¹ M^–1s^–1). Moreover, they evaluated the proportion of Py1*(T) that would be quenched via the TTA compared to SenI-ET pathways. They proposed that the major pathway in the consumption of Py1*(T) would be via SenI-ET, and their proposed mechanism agreed with that put forward by König (Scheme [18]).[6]

Recently, Wenger and co-workers reported mechanistic insights and photocatalytic applications of SenI-ET.[23] They developed a combination of fac-[Ir(ppy)₃]/2,7-di-tert-butylpyrene (Py11)/N,N-dimethylaniline as a SenI-ET system instead of fac-[Ru(bpy)₃]²⁺/Py1/DIPEA. This combination was more suitable for the elucidation of the SenI-ET mechanism because the reaction pathway via sensitized triplet–triplet annihilation upconversion (sTTA-UC) related to SenI-ET was predominant over the side electron-transfer step (i.e., reductive quenching of *[Ru(bpy)₃]²⁺ by DIPEA in Scheme [18]). In addition, the tert-butyl groups on Py11 improve its sTTA-UC properties. The reaction system can be applied to the hydrodehalogenation of aryl halide substrates 48 and 49 (Scheme [19a]). Variation of the reaction conditions shows that the reaction system requires both fac-[Ir(ppy)₃] and Py11 (Scheme [19b]). Pinacol coupling reactions of acetophenone derivative 52 (Scheme [19c]), and detosylation of tosylamide 54 (Scheme [19d]) through ­SenI-ET were also achieved.

Based on spectroscopic characterization and estimation of the efficiency of each elementary step in the possible reaction mechanisms, the main reaction pathway is shown in Scheme [20]. First, triplet–triplet energy transfer (TTET) from ³MLCT-excited fac-[Ir(ppy)₃] to Py11 provides triplet-excited Py11 (Py11*(T)). Subsequently, sensitized triplet–triplet annihilation upconversion (sTTA-UC) with two molecules of Py11*(T) provides singlet-excited Py11 (Py11*(S)). SET from N,N-dimethylaniline, a sacrificial electron donor, to Py11*(S) yields a radical anion (Py11 ^•– ), which has a sufficiently high reduction potential (–2.1 V vs SCE) to promote the one-electron reduction of substrates such as 48 to radical anion species 48 ^•– .

Miscellaneous Transformations

1,6-Bis(dimethylamino)pyrene (Py2) has also been utilized as a catalyst for several other photoinduced reactions. Hasegawa et al. first reported the Py2-catalyzed photoinduced SET reaction of α,β-epoxy ketones.[24] α,β-Epoxy ketone 56a and triethylamine (Et₃N), upon irradiation (λ > 390 nm) in the presence of Py2, gave a mixture of β-hydroxy ketone 57a and β-diketone 58a (Scheme [21a]). It was proposed that the reaction proceeded via SET from the singlet excited state of Py2 to the α,β-epoxy ketone. The selectivity for 57a and 58a depended on the stability of the ion radical pairs and the hydrogen abstraction ability of the amine radical cations.[25] Subsequently, they found that the combination of Py2 and 1,3-dimethyl-2-phenylbenzimidazoline (59) (DMPBI) (a one-electron and hydrogen-transfer reagent) was suitable for the selective transformation of α,β-epoxy ketone 56b into β-hydroxy ketone 57b (Scheme [21b]).[26] A similar reaction system using electron and hydrogen donors 61, 62 or 63 was applied for the radical cyclization of α-bromomethyltetralones 60 (Schemes 22a and 23) and carbon–carbon multiple-bond-tethered ketones 65 (Scheme [22b]).[27]

Metal ions promote pyrene-sensitized photoinduced electron transfer processes. For example, Fukuzumi and co-workers demonstrated the oxygenation of hexamethylbenzene (67) through photoinduced electron transfer from 67 to the singlet excited state of pyrene (Py1*(S)) in the presence of Sc(OTf)₃ (Scheme [24]).[28] The efficient oxygenation of 67 by pyrene-sensitized electron transfer with Sc(OTf)₃ produced pentamethylbenzyl alcohol. UV/Vis and fluorescence spectra indicated that there were no interactions between Sc(OTf)₃ and Py1 or the singlet excited state (Py1*(S)). However, in the presence of Sc(OTf)₃, the fluorescence of pyrene was efficiently quenched by weak electron donors such as 67 because the reduction potential was shifted to the positive side through the formation of [Sc(OTf)₃][Py1^•– ] and [Sc(OTf)₃][Py1^•– ]₂ complexes, which accounts for the dependence on [Sc(OTf)₃] for the fluorescence quenching of Py1*(S) by 67. Based on the correlation between the quantum yields and the concentration of 67, it was proposed that the photocatalytic oxygenation of 67 proceeded via a radical-chain mechanism initiated by the generation of 67 ^•+ through SET between Py1*(S) and 67 in the presence of Sc(OTf)₃ (Scheme [25]).

Conclusion

In conclusion, various photoinduced transformations have been achieved by using designed pyrene catalysts: photolysis of N–O bonds, transformations of α,β-epoxy ketones into β-hydroxy ketones, radical cyclization reactions, addition reactions to styrenes, and deaminative borylation of aromatic amines. In addition, the achievement of the EDA-complex-based hydrodefluorination of polyfluoroarenes and iodoperfluoroalkylation of olefins by pyrene-based MOFs is expected to trigger further investigations on the size, shape, and noncovalent interactions of photocatalysts to enhance the catalytic activity and reaction selectivity. Furthermore, the combination of pyrenes and visible-light photoredox catalysts under visible-light irradiation has resulted in a newly designed photocatalytic system: sensitization-initiated electron transfer (SenI-ET). The application of pyrene derivatives in visible-light-irradiated organic reactions that are not accessible using only ruthenium, iridium, or other organic dye photoredox catalysts might be desirable and attractive in the future.

Conflict of Interest

The authors declare no conflict of interest.

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Corresponding Authors

Publication History

Received: 02 December 2021

Accepted after revision: 13 January 2022

Accepted Manuscript online:
13 January 2022

Article published online:
25 February 2022

© 2022. Thieme. All rights reserved

Georg Thieme Verlag KG
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