Design, Synthesis, and Application of Highly Reducing Organic Visible

Letter Cite This: Org. Lett. XXXX, XXX, XXX−XXX

pubs.acs.org/OrgLett

Design, Synthesis, and Application of Highly Reducing Organic Visible-Light Photocatalysts Dan Liu, Meng-Jie Jiao, Zhi-Tao Feng, Xing-Zhi Wang, Guo-Qiang Xu, and Peng-Fei Xu* State Key Laboratory of Applied Organic Chemistry, College of Chemistry and Chemical Engineering, Lanzhou University, Lanzhou 730000, China

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S Supporting Information *

ABSTRACT: A series of inexpensive, easily tunable, and highly reducing organic photocatalysts (PCs) that exhibit strong absorption in the visible region are described. Timedependent density functional theory calculations were applied to corroborate the experimental observations and explain the relationship between the structure and property. The photocatalytic efficiency of these PCs was demonstrated by catalyzing several challenging reactions and the radical cascade cyclization of 1,6-diyne with superior photocatalytic efficiency to other organic photocatalysts.

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light more efficiently and, therefore, are more efficient catalysts. Considering all of those facts, we aimed to develop highly reducing PCs with strong absorption in the visible region to expand the toolbox for photosynthesis further. We posited that the extension of conjugation would reduce the energy gap between the HOMO and LUMO, which would render PCs photoexcitable using visible light. Initially, cat. 115 and cat. 2,16 equipped with the extensive conjugation structure of PTH, were synthesized, but little change in the absorption property was observed (for cat. 1, λmax, abs = 371 nm; for cat. 2, λmax, abs = 376 nm) (Figure 1a). We then hypothesized that the replacement of naphthalene with quinoxaline might stabilize the LUMO engaged in photon absorption, allowing low energy visible light to photoexcitate the PCs. As a consequence, it was found that when two nitrogen atoms were introduced into cat. 1, and the λmax, abs red-shifted to 419 nm (cat. 3)17 (see Supporting Information (SI), Figure S1), proving cat. 3 to be a better visible light absorber than PTH.11 DFT calculations were employed to interpret the orbitals related to the photoexcitation for cat. 1 and cat. 3 (Figure 1b). As expected, the HOMOs of cat. 1 and cat. 3 were distributed in the whole molecule, while the LUMOs were mainly localized on the acceptor parts of the molecules (naphthalene or quinoxaline). Therefore, the replacement of naphthalene with more electronwithdrawing quinoxaline exerted a greater stabilization effect on the LUMOs of PCs than the destabilization effect on the HOMOs, with the net effect of λmax, abs red-shifted into the visible regime (see Table S5). In addition to the red-shifting of the λmax, abs, this replacement also enhanced the εmax of cat. 1

isible light as a safe, inexpensive, abundant, and sustainable energy source can provide sufficient chemical potential to facilitate many chemical reactions.1 Biosynthesis uses the visible portion of sunlight to provide energy for water oxidation. It is highly desired that many more chemicals can be prepared in the same way as the photosynthesis process; however, the incapability of most organic molecules to absorb visible light presents a major obstacle.2 Although the discovery of ruthenium,3 iridium,4 and some earth-abundant transitionmetal-based complexes (Cu,5 Cr,6 Fe7 complexes) promoted the development of visible-light-photocatalyzed organic synthesis in the past decade,8 some challenging hurdles to remove transition-metal photocatalysts from the final products have impeded their further adaptation in organic synthesis. To overcome these limitations, organic dyes9 have attracted considerable attention as environmentally friendly photocatalysts (PCs) over transition-metal complexes. Although extensive efforts have been devoted to the development of organic PCs,10 visible-light organic PCs with strong reduction potentials are still very rare. Some fluorescent dyes (e.g., PTH (10-phenylphenothiazine)11 and perylene12) have also been employed as PCs for organic reactions, while their weak absorption in visible light limits their synthetic utility. Recently, Miyake and co-workers also developed series of highly reducing PCs for the photocatalyzed polymer synthesis via O-ATRP (organocatalyzed atom transfer radical polymerization), with most maximum absorption wavelengths (λmax, abs) of these PCs being less than 400 nm.13 Most PC design researchers believe that PCs absorbing visible light rather than UV light are preferred because UV light can lead to undesirable decomposition reactions.14 Meanwhile, PCs possessing higher molar extinction coefficients (ε) can absorb © XXXX American Chemical Society

Received: July 30, 2018

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DOI: 10.1021/acs.orglett.8b02420 Org. Lett. XXXX, XXX, XXX−XXX

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state reductive potential. The replacement of the sulfur atom with the nitrogen atom, which is a stronger electron-donating conjugate atom (cat. 5, ES11/2* = −2.31 V) (Figure 3) resulting

Figure 3. Structures and photoredox potentials (vs SCE; all potentials mentioned hereafter are against SCE).

in a higher HOMO energy, could make up the loss. Although strong single-electron reductants such as cat. 5 are necessary for some reactions, their weak positive ground-state oxidation potentials will limit their application in some other transformations (for cat. 5, Eox1/2 = 0.26 V vs SCE, Figure 3). To overcome these limitations, we modified cat. 5 with Nsubstituents to obtain cat. 6−cat. 9, which are easier to be reduced in the ground state. For those PCs with similar ES1 (Table S4), modification of cat. 5 with an ethyl or Ph group decreased the ES11/2* to −2.02 V and −1.89 V for cat. 6 and cat. 7, respectively, making them less reductive in the excited state. Similar results were observed for other PCs, which could be explained by the N-substituents acting as electronwithdrawing groups decreasing the HOMO energy (Tables S4 and S5), rendering them less reactive in the excited state. Furthermore, the PCs modified with electron-donating groups exhibited the opposite effect. For example, cat. 7, modified with electron-donating groups (OMe, cat. 8), had a stronger reducing ability than that with withdrawing groups (CF3, cat. 9) in their excited states (for cat. 8, ES11/2* = −1.91 V; cat. 9, ES11/2* = −1.79 V vs SCE). In addition, we computationally calculated the triplet energies and the excited-state reduction potentials. All of the computationally calculated triplet energies for these PCs exceeded 2.0 eV, and the triplet-excited-state reduction potentials basically followed the same trend as the singlet-excited-state reduction potentials. Notably, the tripletexcited-state potential of cat. 6 was close to that of facIr(ppy)3,8 one of the most powerful reducing transition metal PCs. Moreover, compared with PCs without N-substituents, PCs with N-substituents exhibited reversible oxidation peaks, which indicated that the N-aryl substituents improved the stability of the radical cation species (PC•+) generated from the oxidation of PCs.16 Recent applications of strongly reductive PCs include the reduction of aryl iodides,20 aryl bromides,21 and aryl chlorides,22 which possess negative reduction potentials and high bond dissociation energies. With these PCs developed, we expected them to catalyze these challenging reactions. Cat. 7 was successfully used to realize the intramolecular 1,5-H transfer reaction of aryl iodide,23 in which a highly reductive photocatalyst was necessary (Scheme 1a). Notably, cat. 5

Figure 1. (a) UV−vis light absorption of cat. 1 and cat. 2. (b) DFT calculation of LUMO (left) and HOMO (right) localizations in cat. 1 and cat. 3.

from 5671 to 11501 M−1 cm−1 of cat. 3. Replacing the sulfur atom with a nitrogen atom further enhanced the εmax (cat. 5,18 εmax = 22274 M−1 cm−1) without appreciable change of λmax, abs (see Figure S2). Furthermore, these N-substituent groups did not appreciably perturb the λmax,abs, and all of those modified PCs possessed excellent εmax’s typically greater than 10000 M−1 cm−1, as demonstrated by the UV−vis absorption of cat. 6− cat. 9 (Figure 2), which suggested that N-substituent groups

Figure 2. UV−vis light absorption of cat. 5−cat. 9.

had little influence on the energy gap of HOMO and LUMO, as supported by the DFT-calculated Eg of 5.64, 5.68, 5.68, and 5.70 eV for cat. 6, cat. 7, cat. 8, and cat. 9, respectively (Table S5). Compared with structurally similar N,N-diaryldihydrophenazines, reported by Miyake and co-workers, cat. 5−cat. 9 typically absorb lower energy visible light to access excited states. The excited-state potentials were calculated with the equation E1/2* = Eox1/2 − E0,0 (the energy of the excited state), which was important for designing organic reactions.19 The singlet energy (ES1) was determined with the fluorescence emission. Compared with cat. 1, the absorption profile of cat. 3 red-shifted to the visible regime with sacrificing the excitedB

DOI: 10.1021/acs.orglett.8b02420 Org. Lett. XXXX, XXX, XXX−XXX

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Organic Letters Table 1. Optimization of the Reaction Conditionsa

Scheme 1. Application of the PCs in Some Challenging Reactions

entry

PC

1c 2c 3 4 5 6 7 8 9 10 11d 12 13e

cat. 1 cat. 2 cat. 3 cat. 4 cat. 5 cat. 6 cat. 7 cat. 8 cat. 9 cat. 8 EY 4CzIPN cat. 8

base

yieldb (%)

KHCO3 KHCO3 KHCO3 KHCO3

23 18 16 41 51 56 58 60 58 77 38 40 80

a Reaction conditions: 1a (0.1 mmol), 2a (0.2 mmol), ox. 1 (0.2 mmol) cat. (1 mol %) in dry DMF under blue LED at 25 °C for 12 h. b Isolated yield. cWhite LED. dGreen LED. e48 h.

could be applied to activate the more challenging C−Br bonds and C−Cl bonds with 3 mol % catalyst loading at room temperature (Scheme 1b,c), which was achieved by König and co-workers using 5−10 mol % perylene diimides (PDI), a class of fluorescent dyes, at 40 °C.22 Meanwhile, cat. 8 could also act as a PC to generate the acyl radical with slightly inferior photocatalytic efficiency (Scheme 1d).24 Organophosphorus compounds have attracted researchers’ attention due to their potential as pharmaceutical agents.25 To explore the photocatalytic activities of PCs further, we designed a visible-light-mediated construction of organophosphorus fluorene derivatives through the phosphorus radical-mediated cascade cyclization reaction of 1,6-diynes, which had been developed as highly versatile substrates for the construction of the carbo- and heterocycles in a sustainable and atom-economical fashion.26 Initially, the radical cascade cyclization reaction was performed using cat. 1 and cat. 2 as catalysts under white LED irradiation. The fluorene derivative was obtained in poor yields (Table 1, entries 1 and 2), which was due primarily to the catalyst’s poor visible-light absorption. To investigate further how the catalyst structure would influence the catalyst performance, we tested the good visible light absorbers cat. 3 and cat. 4 in the reaction, which featured similar absorption properties (with λmax, abs = 420 and 412 nm, respectively, and εmax = 11501 and 10646 M−1 cm−1, respectively). Cat. 3 exhibited inferior performance to that of cat. 4 (Table 1, entries 3 and 4), demonstrating the significance of the Nsubstituent in the PCs, which may reduce the PC decomposition. In addition, cat. 6−cat. 9 performed even slightly better than cat. 5, which further corroborated that the N-substituents were essential for the PCs. It was also found that the addition of KHCO3 significantly increased the reaction yield. Furthermore, we also applied the common organic PCs (e.g., EY, RB, DCA, PDI, etc.; see Table S1) in this transformation, which were able to catalyze this reaction albeit with low to moderate yields, which confirmed the superior photocatalytic efficiency of cat. 5−cat. 9. Control experiments confirm that the light, oxidant and PC are essential for this transformation. To study the generality of this highly reducing PC in this reaction, we extended the substrate scope of 1,6-diynes. It was

found that 1,6-diynes bearing aromatic rings substituted with either electron-donating or electron-withdrawing as well as multiple substituents delivered the desired products in good to excellent yields (51−93%) (3a−k, Scheme 2). It should be Scheme 2. Substrate Scope of 1,6-Diynesa,b

a Reaction conditions: 1 (0.1 mmol), 2a (0.2 mmol), ox. 1 (0.2 mmol), cat. 8 (1 mol %) and KHCO3 (0.15 mmol) in dry DMF under blue LED at 25 °C for 48 h. bIsolated yield.

noted that the electron-donating groups on the phenyl group of the 1,6-diynes appeared to play a positive role in this cascade reaction (e.g., 3p/3p′ = 2:1). We next evaluated 1,6diynes with different P-centered radicals in this photoredoxmediated cascade cyclization, as shown in Scheme 3. As can be seen, substrates possessing geminal substituents proved to be C

DOI: 10.1021/acs.orglett.8b02420 Org. Lett. XXXX, XXX, XXX−XXX

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Figure S5), which indicated that oxidant is the most likely quencher for the excited state of cat. 8. In addition, the quantum yield of the reaction revealed that the reaction contains a radical-chain propagation process. According to the analysis above, a plausible mechanism for this radical addition tandem cyclization was proposed (Figure S9).28 In summary, we have developed a series of organic dyes as inexpensive, easily tunable, highly reducing, metal-free photocatalysts which exhibited good absorption of visible light and excellent redox properties. These PCs successfully catalyzed several challenging reactions in which highly reducing PCs were necessary, and the cascade cyclization of 1,6-diynes shows superior photocatalytic efficiency compared to common organic PCs.

Scheme 3. Substrate Scope of 1,6-Diynes and Phosphoryl Radical Precursorsa,b

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ASSOCIATED CONTENT

S Supporting Information * a

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.orglett.8b02420.

Reaction conditions: 1 (0.1 mmol), 2 (0.2 mmol), ox. 1 (0.2 mmol), cat. 8 (1 mol %) and KHCO3 (0.15 mmol) in dry DMF under blue LED at 25 °C for 48 h. bIsolated yield. c43 h. d45 h.

Experimental procedures and spectroscopic data for all new compounds; electrochemical, UV−vis spectrometry, and fluorescence studies; DFT calculations (PDF)

highly effective for the cyclization reaction (4e−i vs 4a,b), which could be attributed to Thorpe−Ingold effect. To demonstrate the efficiency of this PC further, we scaled up the production of 3a to 2.8 mmol and obtained the product in 60% yield. Compared with our previous work,27 this protocol was an effective method to construct 3-phosphorylated coumarins, which was obtained in 90% yield even with lower catalyst loading. Furthermore, this protocol could also be applied to the synthesis of diphenyl(10-phenylindeno[2,1a]inden-5-yl)phosphine oxide 8a in a yield of 40% (Scheme 4).

Accession Codes

CCDC 1827446 contains the supplementary crystallographic data for this paper. These data can be obtained free of charge via www.ccdc.cam.ac.uk/data_request/cif, or by emailing [email protected], or by contacting The Cambridge Crystallographic Data Centre, 12 Union Road, Cambridge CB2 1EZ, UK; fax: +44 1223 336033.

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Scheme 4. Further Application of the Protocol

AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. ORCID

Peng-Fei Xu: 0000-0002-5746-758X Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS We are grateful to the NSFC (21632003 and 21572087), the Key Program of Gansu Province (17ZD2GC011), and the “111” Pro-gram from the MOE of PR China for financial support.

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To investigate the mechanism of the photoredox radical cascade cyclization, several control experiments were performed (Scheme 5). First, the reaction was conducted in the presence of TEMPO, a radical quencher, and the reaction was completely inhibited. Meanwhile, we also performed a series of Stern−Volmer fluorescence quenching experiments (see

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Scheme 5. Control Experiment

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DOI: 10.1021/acs.orglett.8b02420 Org. Lett. XXXX, XXX, XXX−XXX