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DOI: 10.1055/a-2126-1897
Small Aromatics Bearing Two Diarylamino Termini: Highly Reducing Organic Photocatalysts
The works highlighted in the present account were supported by JSPS (KAKENHI Grants 16H06038 and 21H01928), MEXT [KAKENHI Grant 21H05209 in Digitalization-Driven Transformative Organic Synthesis (Digi-TOS)], and JST CREST (Grant JPMJCR18R4).
Dedicated to Professor Dennis Curran on the occasion of his 70th birthday.
Abstract
Small aromatics such as anthracene, naphthalene, or benzene bearing two diarylamino termini function as highly reducing organic photocatalysts (OPCs). In particular, the small aromatic core remarkably enhances the reducing power of the catalyst in the excited state. An appropriate combination of an OPC and an electron-accepting fluoroalkylating reagent is the key to successful radical fluoroalkylation. The basic design of the photocatalyst and the photocatalytic fluoroalkylation of olefins are discussed.
1 Introduction
2 Basic Catalyst Design and Photo- and Electrochemical Properties
3 Photocatalytic Reactions of 9,10-Bis(diphenylamino)anthracene Derivatives
4 Photocatalytic Reactions of 1,4-Bis(diphenylamino)naphthalene Derivatives
5 Photocatalytic Reactions of 1,4-Bis(diphenylamino)benzene
6 Summary and Outlook
#
Key words
photoredox catalysis - organocatalysis - radical reaction - fluoroalkylation - electron transfer - photoreactionBiographical Sketch
Takashi Koike received his doctoral degree under the supervision of Professor Takao Ikariya at the Tokyo Institute of Technology in 2005. After his graduate career, he worked at the California Institute of Technology alongside Professor Robert H. Grubbs as a postdoctoral research scholar. In 2007, he returned to the Tokyo Institute of Technology as an assistant professor. In 2021, he was appointed as an associate professor (principal investigator) at the Nippon Institute of Technology. Recently, his efforts have been focused on the development of new organic photocatalysts and the synthesis of organofluorine compounds by photocatalysis.
Introduction
Recently, photoredox catalysis has become a powerful strategy for realizing elusive molecular transformations under mild reaction conditions.[2] Since the seminal 2008 report by MacMillan and Nicewicz,[3] we have witnessed many useful photochemical radical reactions catalyzed by Ru-, Ir-, or Pt-based photoredox catalysts such as [Ru(bpy)3]2+, fac-[Ir(ppy)3], and Pt(acac)(ppy) (bpy = 2,2′-bipyridine; ppy = 2-pyridylphenyl; acac = acetylacetonato). However, the use of noble-transition-metal catalysts has led to issues pertaining to sustainability, toxicity, and cost. Thus, noble-metal-free reaction systems are essential for the development of sustainable synthetic strategies.[4] Organic photosensitizers, such as rose bengal and benzophenone, are widely used in metal-free photochemical reactions.[5] In the past decade, remarkable progress has been made in noble-metal-free photoredox reactions as newly designed organic photocatalysts (OPCs) with excellent redox abilities, unachievable with metal-based complexes, have emerged.[6] In particular, we have focused on developing highly reducing OPCs because the development of one-electron-injection methods related to the replacement of dangerous and sensitive alkali metals is an important topic in modern organic synthesis. More recently, consecutive photoinduced electron transfer and the merging of electrolysis and photocatalysis have been regarded as advanced photoredox processes for realizing a high reduction power.[7] However, direct one-electron-transfer from the excited OPC (*OPC) with a high reduction power (E ox*) to electron-accepting reagents (i.e., the oxidative quenching pathway) is still useful for radical reactions.
Since 2018, we have studied small aromatics bearing two diarylamino termini as OPCs that are highly reducing compared with Ru- and Ir-based photocatalysts.[8] The reduction powers (E ox*) of the photocatalysts in the excited state and the redox potentials (E red) of the electron-accepting fluoroalkyl radical precursors that are highlighted in the current account are shown in Figure [1]. The data suggest that our OPC systems can be applied to the one-electron-injection of hard-to-reduce chemicals, which noble-metal-based photocatalysts cannot achieve.
Herein, we discuss (i) basic catalyst design: two diarylamino groups bridged by anthracene (BDA), naphthalene (BDN), or benzene (BDB) moieties, and their photo- and electrochemical properties(Section 2) and (ii) their applications to regioselective radical fluoroalkylation reactions of olefins with electron-accepting fluoroalkyl radical sources 1 (RF–L) (Sections 3–5). Our concept of the radical functionalization of olefins based on oxidative quenching without sacrificial redox agents is shown in Scheme [1].[9]
First, photoirradiation induces excitation of the OPC to the excited species *OPC; this undergoes single-electron transfer (SET) to RF–L (1) to generate the corresponding fluoroalkyl radical species (∙RF) and the oxidized catalyst (OPC+). The generated radical ∙RF then reacts with olefin 2 to yield radical intermediate A, which is oxidized by OPC+. Finally, the resulting carbocationic intermediate B is attacked by an appropriate nucleophile Nu to afford a radical–polar crossover-type product. In the case where olefin 2 is an alkenyl acetate (X = OAc), hydrolysis of B produces the corresponding carbonyl compound. In the present study, we address redox-neutral photocatalytic reaction systems in which the excited OPCs function as strong one-electron-reductants.
# 2
Basic Catalyst Design and Photo- and Electrochemical Properties
Simple polyaromatic hydrocarbons, such as perylene derivatives, can act as OPCs.[10] However, redox stimuli induce degradation and polymerization of simple aromatics, i.e., photoredox cycles damage the aromatic moieties. Therefore, to improve the robustness of OPCs, we focused on triarylamine scaffolds as redox-active moieties. In addition, catalysts composed of a small aromatic bridge and two triarylamine scaffolds were expected to form radical cationic species by one-electron-oxidation (photoinduced SET) that would be significantly stabilized owing to the delocalization of the spin center over the whole molecule (Scheme [2]).[11] The radical cationic species correspond to OPC+ mentioned in Scheme [1].
To date, we have studied systems consisting of two diarylamino groups bridged by an anthracene-9,10-diyl (BDA), naphthalene-1,4-diyl (BDN), or benzene-1,4-diyl (BDB) system.[8] Selected photo- and electrochemical data for BDA, 4tBu-BDA, BDN, 2tBu-BDN, and BDB are summarized in Table [1].
|
BDA |
4tBu-BDA[8a] |
BDN[8b] |
2tBu-BDN |
BDB[8f] |
|
|
E ox (V vs. [Cp2Fe])b |
+0.40 |
+0.33 |
+0.36 |
+0.32 |
+0.16 |
|
λmax (nm)c |
520 |
540 |
432 |
442 |
383 |
|
λem (nm) |
529 |
561 |
449 |
458 |
404 |
|
τ (ns) |
23 |
31 |
10 |
10 |
3 |
|
Φd |
90 |
0.81 |
0.94 |
>0.99 |
0.32 |
|
E ox* (V vs. [Cp2Fe])e |
–1.94 |
–1.88 |
–2.40 |
–2.39 |
–2.91 |
a Data for BDA and 4tBu-BDA were collected in CH2Cl2. Data for BDN, 2tBu-BDN, and BDB were collected in acetone.
b The electrochemical measurements were performed in the above-mentioned solvents ([sample] = 1.0 mM, [N(Bu)4PF6] = 0.10 M) with a Pt disk working electrode, a Pt wire counter-electrode, and a Ag/AgNO3 reference electrode at rt.
c Ends of the absorption bands.
d Fluorescence quantum yield.
e Eox * = Eox – E 0,0, the oxidation potential in the ground state (E ox) was measured by differential pulse voltammetry (DPV) or cyclic voltammetry (CV), and the excitation energy (E 0,0) was obtained from the emission wavelength (λem) at room temperature. The values are therefore likely to have been underestimated.
The cyclic voltammetry (CV) traces show almost reversible wave(s), except for BDA, suggesting that the catalysts are somewhat stable during the redox cycles. In particular, two redox waves are observed for the BDNs and BDB, indicating that the corresponding radical cationic species (OPC+) have a relatively long lifetime. A characteristic absorption band of [4tBu-BDA]∙+ was observed in the near-infrared region (1780 nm). The UV-vis spectra showed absorption bands extending into the visible-light region in the order BDB, BDN, BDA (λmax in Table [1]). Thus, the wavelengths of the LEDs used for the reactions were different (see below). All the emission spectra related to the excitation energy (E 0,0) showed one signal, and a smaller aromatic core led to a blue shift (λem in Table [1]). Based on these photo- and electrochemical data,[6d] [12] the reduction powers (E ox*) in the excited state were evaluated to be considerably stronger than those of Ru-, Ir-, and Pt-based photoredox catalysts, as mentioned in Section 1. Sections 3–5 below describe the applications of the BDA, BDN, and BDB photocatalysts, respectively.
Organofluorine compounds are frequently found in pharmaceutical, agrochemical, and functional organic materials. Consequently, the development of selective and efficient methods for synthesizing fluorine-containing molecules has become a hot topic in synthetic organic chemistry.[13] Since 2012, we have extensively developed radical tri- and difluoromethylations of olefins through redox-neutral oxidative quenching processes (Scheme [1]) that use Ru- or Ir-based photoredox catalysts.[14] Indeed, this concept is versatile for the radical fluoroalkylative functionalization of olefins. In this account, the appropriate choice of OPCs and electron-accepting fluoroalkyl radical sources (1: RF–L) for the synthesis of a variety of organofluorine compounds is discussed.
# 3
Photocatalytic Reactions of 9,10-Bis(diphenylamino)anthracene Derivatives
We previously studied photocatalytic trifluoromethylations of alkenes with electrophilic CF3 reagents such as the Umemoto’s reagent (1a).[14] [15] This method allowed us to access various vicinally CF3-substituted alcohols, amines, and ketones. Sulfonium-based fluoroalkylating reagents can serve as good electron-accepting fluoroalkyl radical sources when triggered by one-electron-injection. Diphenyl(2,2,2-trifluoroethyl)sulfonium trifluoromethansulfonate (1b) is a bench-stable and easy-to-handle chemical.[16] While we expected the development of CF3-homologation,[17] 1b (E red = –1.60 V vs. Cp2Fe) requires a stronger reducing photocatalyst than that for the analogous CF3 reagent (1a: –0.75 V). The 9,10-bis(diphenylamino)anthracene derivatives BDA and 4tBu-BDA can induce photocatalytic trifluoroethylation of styrene (2a) with 1b because the photoexcited species of the BDAs (E ox* ≈ –1.9 V) are able to reduce 1b, according to their redox potentials (Figure [1]).
In fact, the reaction of 2a with 1b in the presence of 4tBu-BDA (3 mol%) in the mixed solvent system CH2Cl2–MeCN (4:1) containing a small amount of H2O under visible-light irradiation (470 nm, 3 h) induced trifluoroethylation together with Ritter amination (Scheme [3]).[8a] Product 3ba was obtained in 77% isolated yield. The reaction follows the basic reaction design (Scheme [1]) through formation of radical (A1) and carbocationic (B1) intermediates, followed by nucleophilic attack by MeCN. Notably, the Ir photocatalyst fac-[Ir(ppy)3] and BDA gave lower yields. Thus, the aminotrifluoroethylation of other aromatic alkenes by using 4tBu-BDA was studied. The reaction can be applied to styrene derivatives with various functional groups, such as halo (2b), ester (2c), or Bpin (2d) groups. The corresponding products were obtained in good yields (10 examples; 32–77% yield). In addition, the structurally more complex estrone derivative 2e afforded the aminotrifluoroethylated product 3be in 32% yield. These results indicated that the present photocatalytic reaction system is highly regioselective and is compatible with various functional groups. Furthermore, application to a gram-scale reaction is feasible.
Next, the reaction system was extended to the photocatalytic trifluoroethylation of alkenes, accompanied by oxy-functionalization of carbocationic intermediates B (Scheme [4]). The reaction of 4-vinylbiphenyl (2f) in the mixed solvent system CH2Cl2–iPrOH (9:1) afforded the oxytrifluoroethylated product 4bf in 86% isolated yield (Scheme [4a]). The reactions of 4-phenylpent-4-enoic acid (2g) or N-[2-(propen-2-yl)phenyl]benzamide (2h) afforded lactone 4bg (79% yield) and benzoxazine 4bh bearing a CF3CH2 group (52% yield), respectively, through intramolecular oxytrifluoroethylation (Schemes 4b and 4c).[8a]
Furthermore, the present organic photocatalyst system allowed us to realize other radical fluoroalkylation reactions by using electron-accepting fluoroalkylating reagents such as the CF2H-homologating reagent corresponding to 1b, diphenyl-(2,2-difluoroethyl)sulfonium trifluoromethansulfonate (1c; E red = –1.91 V vs. Cp2Fe),[19] or the sulfonium-based CF2H reagent 1e (E red = –1.74 V)[10d] (Scheme [5]). Furthermore, typical electron-accepting CF3 reagents, such as Umemoto’s reagent II (1d),[20] CF3SO2Cl (1f), and Togni’s reagent (1g),[21] are applicable (Scheme [5]). These results show that BDAs can serve as useful OPCs for various radical fluoroalkylation reactions with appropriate fluoroalkylating reagents that are relatively susceptible to one-electron-injection.
# 4
Photocatalytic Reactions of 1,4-Bis(diphenylamino)naphthalene Derivatives
As mentioned in Section 3, sulfonium-based fluoroalkylating reagents are good sources of fluoroalkyl radicals. However, depending on their structure, some fluoroalkylating reagents induce nonselective radical generation, i.e., generation of fluoroalkyl and aryl radicals, resulting in nonselective reactions.[8b] In contrast, sulfoximine-based fluoroalkylating reagents are selective fluoroalkyl radical sources when triggered by one-electron-injection (Scheme [6]).[22]
In particular, various α-monofluoromethylated sulfoximines are readily synthesized.[23] Thus, fluorinated sulfoximines can be applied to radical monofluoromethylation, i.e., fluorinative homologation,[24] and α-monofluoroalkylation reactions. However, the reduction of charge-neutral sulfoximines is more difficult than that of cationic sulfonium reagents. In fact, BDN, which has sufficiently high reducing power in the excited state, induces efficient radical monofluoromethylation of alkenes with bench-stable N-tosyl-S-monofluoromethyl-S-phenylsulfoximine (1h).
The reaction of aromatic alkene 2f with 1h in the presence of 0.5 mol% BDN in acetone–H2O (9:1) under visible-light irradiation (425 nm) afforded the γ-fluoro alcohol product 5hf in 70% isolated yield in a regioselective manner (Scheme [7]).[8b] The reaction follows the basic reaction design (Scheme [1]) through formation of radical (A2) and carbocationic (B2) intermediates, and subsequent nucleophilic attack by H2O. Notably, fac-[Ir(ppy)3] and BDA exhibited lower activities. The present reaction system shows a broad scope and is regiospecific toward terminal and internal alkenes 2 (35 examples; 30–84% yield). Furthermore, the present reaction system was applied to the reaction of a structurally complex bioactive vinylestrone (2e), bexarotene (2k), and flavonoid derivatives (2l), potentially connected to late-stage functionalization.[25]
The reaction system was then extended to photocatalytic α-monofluoroalkylation by using the corresponding α-fluorinated sulfoximines. In the reaction of alkenyl acetates, 2tBu-BDN was found to be a better catalyst. The reaction of alkenyl acetate 2m with N-tosyl-S-(1-fluoroethyl)-S-phenylsulfoximine (1i) in the presence of 2tBu-BDN (5 mol%) in acetone–H2O (19:1) produced the β-monofluoro ketone 6im in 76% isolated yield (Scheme [8]).[8e] The reaction follows the basic reaction design (Scheme [1]) through formation of radical (A3) and carbocationic (B3) intermediates, and subsequent hydrolysis. Other structurally diverse α-monofluoroalkyl sulfoximines have also been used for the modular synthesis of valuable β-monofluoro ketones. Notably, vinyl (1j), alkynyl (1k and 1l), and heteroaromatic units (1m and 1n), which are potentially susceptible to radical addition, do not deteriorate under these reaction conditions.
The 1,4-bis(diphenylamino)naphthalene derivatives serve as highly reducing photocatalysts in the excited state; in one-electron-injection to difficult-to-reduce sulfoximines, they are more useful than BDA or the commonly used noble-metal-based photocatalyst fac-[Ir(ppy)3]. Thus, versatile radical α-monofluoroalkylation reactions are feasible under operationally simple conditions, leading to the synthesis of various fluorine-containing molecules.
# 5
Photocatalytic Reactions of 1,4-Bis(diphenylamino)benzene
The development and selection of fluoroalkyl sources are vital for synthesis of organofluorine compounds and for catalytic fluoroalkylation. Recently, we have focused on fluorinated alkyl benzoates as electron-accepting fluoroalkyl radical sources. In particular, 1,1-dialkyl-2,2,2-trifluoroethyl benzoates are readily obtained by the trifluoromethylation of carbonyl compounds[26] followed by benzoylation with acid chloride derivatives (Scheme [9]).
In general, fluorinated alkyl benzoates are bench-stable chemicals that are difficult to reduce (for example, 1o: E red = –2.68 V vs. Cp2Fe). In our reaction design, we assumed that photoredox catalysis induces a Markó–Lam-type activation[27] [28] of alkyl benzoates. The reduction power of the excited state of BDB was estimated to be sufficiently strong to reduce simple alkyl benzoates (1r and 1s) and fluorinated alkyl benzoates (1o–q) (Scheme [10]). In fact, a high catalytic activity of BDB has been demonstrated for the hydroxy-dimethyltrifluoroethylation of aromatic alkene 2f with 2,2,2-trifluoro-1,1-dimethylethyl benzoate (1o) under reaction conditions similar to those of the hydroxy-monofluoromethylation mentioned in Section 3, except for the use of 365 nm LEDs, affording 7of in 70% isolated yield (Scheme [10]).[8f] The reaction follows the basic reaction design (Scheme [1]) through formation of radical (A4) and carbocationic (B4) intermediates, and subsequent nucleophilic attack by H2O.
Other photocatalysts such as fac-[Ir(ppy)3], BDA, and BDN exhibited lower activities. Remarkably, the reaction could be applied to fluorinated and nonfluorinated tertiary-alkyl benzoates (8 examples; 40–75% yield). The high reducing power of BDB is the key to this successful transformation. However, in addition to one-electron-injection into alkyl benzoates, the restriction of back electron transfer and efficient homolytic C–O bond cleavage of the ester functionality in the resultant anionic radical species are critical factors for radical generation. Further studies on the control of these processes and additional mechanistic studies will open new avenues for radical reactions triggered by strong one-electron injectors.
# 6
Summary and Outlook
We have discussed useful structural motifs for highly reducing OPCs, i.e., two diarylamine units bridged by a small aromatic core such as anthracene, naphthalene, or benzene. A smaller aromatic core dramatically enhances the reducing power in the excited state. Modification of the terminal Ar2N units described in the present account influenced the stability and solubility of the catalyst, resulting in differences in catalytic activity. In addition, the application of the catalysts in radical fluoroalkylations has been demonstrated, suggesting that the use of our OPCs is a promising strategy for sustainable organic synthesis. Although a long lifetime of the excited species often seems to be an important factor in the design of photoredox catalysts, the present account indicates that a long lifetime (close to a microsecond) of the excited catalyst, such as those of Ru-, Ir-, and Pt-based complexes, is not necessary. Further advancements in time-resolved spectroscopic techniques will shed light on reaction mechanisms involving such short-lived excited species. In addition, improvements in the robustness, recovery, and reuse of OPCs through sophisticated molecular designs are future challenges.
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Conflict of Interest
The authors declare no conflict of interest.
Acknowledgment
I would like to express my gratitude to my former co-workers, particularly Professor Munetaka Akita, and graduate students at the Tokyo Institute of Technology. I would also like to thank Editage (www.editage.com) for English language editing.
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For selected reviews on organic photocatalysts, see:
For selected reports on photocatalysis by polyaromatic hydrocarbons, see:
For selected reports on CF3-homologation, see:
For selected reports on the synthesis of fluorinated sulfoximines, see:
For selected reviews on late-stage functionalization, see:
Corresponding Author
Publication History
Received: 31 May 2023
Accepted after revision: 10 July 2023
Accepted Manuscript online:
10 July 2023
Article published online:
22 August 2023
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References
- 1 The works highlighted in the present account were mainly carried out at the author’s previous affiliation, the Laboratory for Chemistry and Life Science, Institute of Innovative Research, Tokyo Institute of Technology.
- 2 Visible Light Photocatalysis in Organic Chemistry . Stephenson CR. J, Yoon TP, MacMillan DW. C. Wiley-VCH; Weinheim: 2018
- 3 Nicewicz DA, MacMillan DW. C. Science 2008; 322: 77
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For selected reviews on organic photocatalysts, see:
For selected reports on photocatalysis by polyaromatic hydrocarbons, see:
For selected reports on CF3-homologation, see:
For selected reports on the synthesis of fluorinated sulfoximines, see:
For selected reviews on late-stage functionalization, see: