Photoinduced Electron Transfer-Promoted Reactions Using Exciplex-Type Organic Photoredox Catalyst Directly Linking Donor and Acceptor Arenes

Directly linked donor and acceptor arenes, such as phenanthrene/naphthalene/biphenyl and 1,3-dicyanobenzene were found to work as photoredox catalysts in the photoreactions of indene, 2,3-dimethyl-2-butene, and 4-methoxyphenylacetic acid. The new stable organic photocatalyst forms an intramolecular exciplex (excited complex) when irradiated in a polar solvent and shows redox catalyst activity, even at low concentrations. To the best of our knowledge, this is the first example of an intramolecular exciplex working as a redox catalyst.


Results and Discussion
A variety of photocatalysts 1 were prepared from 1,3-DCB and a selection of arylboronic acids 3 (Scheme 2). Initial iodination of 1,3-DCB by CH 3 Cu(TMP)(CN)Li 2 and I 2 yielded 1,3-dicyano-2-iodobenzene 2 in a 99% yield [19]. Suzuki-Miyaura coupling [20] of 9-phenanthrylboronic acid 3a with 2 in the presence of Pd(dba) 3 and (4-biphenyl)dicyclohexylphosphine (PCy 2 Bp) produced photocatalyst 1a in moderate yield (52%). Similar coupling reactions of 1-naphthylboronic acid 3b, 2-naphthylboronic acid 3c, 4-biphenylboronic acid 3d, and 2-biphenylboronic acid 3e provided photocatalysts 1b-e in moderate yields. Next, the UV absorption spectra of photocatalysts 1a-e were measured in aqueous acetonitrile solution (CH3CN/H2O = 9:1, v/v) and are shown in Figure 1. The absorption spectrum of 1a (red) overlapped with that of Phen (blue) and 1,3-DCB (green), and showed an extended absorption range compared to that of Phen, indicating the existence of a π-π interaction between the Phen and DCB moieties in the ground state of 1a [21][22][23]. A similar trend of extended absorption ranges was observed for other catalysts 1b-e, bearing naphthyl and biphenyl units. These results indicated that the rotation of the aryl-aryl σ-bonds in 1 is not completely restricted by the cyano substituents and that π-π interactions exist between the donor and acceptor arene units within 1. In fact, the UV absorption spectrum of the less sterically hindered 1d showed stronger absorption at long wavelengths, as compared to that of the bulkier 1a and 1e. This could be attributed to the stronger intramolecular interaction between the aromatic rings, due to the smooth rotation of the aryl-aryl σ-bonds [9,[21][22][23]. Solvent polarity did not have any notable effect on the absorption spectra of 1a ( Figure 2). In contrast, the use of more polar solvents, such as CH3CN and DMF, caused broadening of the observed fluorescence spectra, which is characteristic of an intramolecular exciplex ( Figure 2). Compared to the fluorescence of Phen, the quenching fluorescence of the phenanthryl part by the DCB part in 1a and the intramolecular exciplex fluorescence were obtained in an aqueous acetonitrile solution (Figure 3). A similar quenching and appearance of fluorescence in 1b-e was observed, indicating that intramolecular exciplexes were formed ( Figure 3). In addition, the exciplex fluorescence of 1a may be quenched by indene 4, 2,3-dimethyl-2-butene 5, and (4-methoxyphenyl)acetic acid 6, for which the Stern-Volmer constants kqτ were estimated to be 1.2 × 10 −1 , 2.1 × 10 −2 , and 8.5 × 10 −1 /M, respectively ( Figure 4). These results indicated that the donor and acceptor arene parts in 1 interacted, both in the ground and excited states, through the rotation of the aryl-aryl σ-bond and formed intramolecular exciplexes in a polar solvent, which were quenched by 4, 5, and 6. Next, the UV absorption spectra of photocatalysts 1a-e were measured in aqueous acetonitrile solution (CH 3 CN/H 2 O = 9:1, v/v) and are shown in Figure 1. The absorption spectrum of 1a (red) overlapped with that of Phen (blue) and 1,3-DCB (green), and showed an extended absorption range compared to that of Phen, indicating the existence of a π-π interaction between the Phen and DCB moieties in the ground state of 1a [21][22][23]. A similar trend of extended absorption ranges was observed for other catalysts 1b-e, bearing naphthyl and biphenyl units. These results indicated that the rotation of the aryl-aryl σ-bonds in 1 is not completely restricted by the cyano substituents and that π-π interactions exist between the donor and acceptor arene units within 1. In fact, the UV absorption spectrum of the less sterically hindered 1d showed stronger absorption at long wavelengths, as compared to that of the bulkier 1a and 1e. This could be attributed to the stronger intramolecular interaction between the aromatic rings, due to the smooth rotation of the aryl-aryl σ-bonds [9,[21][22][23]. Solvent polarity did not have any notable effect on the absorption spectra of 1a ( Figure 2). In contrast, the use of more polar solvents, such as CH 3 CN and DMF, caused broadening of the observed fluorescence spectra, which is characteristic of an intramolecular exciplex ( Figure 2). Compared to the fluorescence of Phen, the quenching fluorescence of the phenanthryl part by the DCB part in 1a and the intramolecular exciplex fluorescence were obtained in an aqueous acetonitrile solution ( Figure 3). A similar quenching and appearance of fluorescence in 1b-e was observed, indicating that intramolecular exciplexes were formed ( Figure 3). In addition, the exciplex fluorescence of 1a may be quenched by indene 4, 2,3-dimethyl-2-butene 5, and (4-methoxyphenyl)acetic acid 6, for which the Stern-Volmer constants kqτ were estimated to be 1.2 × 10 −1 , 2.1 × 10 −2 , and 8.5 × 10 −1 /M, respectively ( Figure 4). These results indicated that the donor and acceptor arene parts in 1 interacted, both in the ground and excited states, through the rotation of the aryl-aryl σ-bond and formed intramolecular exciplexes in a polar solvent, which were quenched by 4, 5, and 6. Molecules 2019, 24, x FOR PEER REVIEW 4 of 12     We attempted to evaluate the photochemical reactivity of photocatalyst 1 using indene 4 [16] ( Table 1), 2,3-dimethyl-2-butene 5 [18] (Scheme 3), and (4-methoxyphenyl)acetic acid 6 [10][11][12][13][14][15] (Scheme 4) as substrates. Reactions were carried out in an aqueous acetonitrile solution (CH3CN/H2O = 9:1, v/v), under an argon atmosphere, under irradiation by a 100 W high-pressure mercury lamp in Pyrex vessels (18 × 180 mm) at room temperature. The irradiation of 4 (20 mM) for 10 h, in the presence of 1a as a photoredox catalyst, exclusively afforded alcohol 7 in a 49% yield through the formation of a radical cation of 4 via PET between the excited state of 1a and 4 [16] (Entry 1, Table 1), along with the unidentified products. The quantum yield of this photoinduced reaction of 4 using 1a is about 0.01. In the absence of 1a, the photoreaction of 4 under the same conditions yielded only the [2 + 2] cycloadduct 8 through dimerization between a molecule of 4 in the excited state with another in the ground state (Entry 2). Using a combination of Phen and 1,3-DCB as photocatalysts, the alcohol 7 was yielded (50%), as observed in Entry 1, along with a small amount of 8 (6%) (Entry 3), indicating that the efficiency of 1a in the photoreaction of 4 is comparable to that of the Phen/1,3-DCB system. This suggests that the appearance of the longer absorption of 3a prevented dimerization of 4. The use of only Phen or 1,3-DCB as the photocatalyst in the photoreaction did not work well (Entries 4 and 5). Decreasing the concentration of 1a led to a decrease in the yield of 7 and an increase in the yield of 8 (Entries 6-9) because of the gradually diminished absorption of 1a. It should be noted, however, that 7 was obtained even when using a very low concentration of 1a (0.1 mM, Entry 9). When the intermolecular Phen/1,3-DCB system was subjected to the same low concentration conditions (0.1 mM) only dimer 8 was observed (Entry 10). Thus, even at low concentrations, 1a works as a more efficient photoredox catalyst than the Phen/1,3-DCB system because of the intramolecular system. The use of 1b,c bearing a naphthyl group also yielded alcohol 7 in similar yields to those observed for 1a and the NP/1,3-DCB system (Entries 11-13). Interestingly, similar photoreactions of 1d and 1e, bearing biphenyl groups, afforded 7 along with 8 (Entries 15 and 16), whereas the intermolecular BP/1,3-DCB system exclusively gave 8 because it does not absorb at 313 nm (Entry 14). Direct linking of BP and 1,3-DCB through σ-bonding led to longer absorption, which lead to them working as a photoredox catalyst by 313 nm light-irradiation.  We attempted to evaluate the photochemical reactivity of photocatalyst 1 using indene 4 [16] ( Table 1) Table 1), along with the unidentified products. The quantum yield of this photoinduced reaction of 4 using 1a is about 0.01. In the absence of 1a, the photoreaction of 4 under the same conditions yielded only the [2 + 2] cycloadduct 8 through dimerization between a molecule of 4 in the excited state with another in the ground state (Entry 2). Using a combination of Phen and 1,3-DCB as photocatalysts, the alcohol 7 was yielded (50%), as observed in Entry 1, along with a small amount of 8 (6%) (Entry 3), indicating that the efficiency of 1a in the photoreaction of 4 is comparable to that of the Phen/1,3-DCB system. This suggests that the appearance of the longer absorption of 3a prevented dimerization of 4. The use of only Phen or 1,3-DCB as the photocatalyst in the photoreaction did not work well (Entries 4 and 5). Decreasing the concentration of 1a led to a decrease in the yield of 7 and an increase in the yield of 8 (Entries 6-9) because of the gradually diminished absorption of 1a. It should be noted, however, that 7 was obtained even when using a very low concentration of 1a (0.1 mM, Entry 9). When the intermolecular Phen/1,3-DCB system was subjected to the same low concentration conditions (0.1 mM) only dimer 8 was observed (Entry 10). Thus, even at low concentrations, 1a works as a more efficient photoredox catalyst than the Phen/1,3-DCB system because of the intramolecular system. The use of 1b,c bearing a naphthyl group also yielded alcohol 7 in similar yields to those observed for 1a and the NP/1,3-DCB system (Entries 11-13). Interestingly, similar photoreactions of 1d and 1e, bearing biphenyl groups, afforded 7 along with 8 (Entries 15 and 16), whereas the intermolecular BP/1,3-DCB system exclusively gave 8 because it does not absorb at 313 nm (Entry 14). Direct linking of BP and 1,3-DCB through σ-bonding led to longer absorption, which lead to them working as a photoredox catalyst by 313 nm light-irradiation. (Entries 11-13). Interestingly, similar photoreactions of 1d and 1e, bearing biphenyl groups, afforded 7 along with 8 (Entries 15 and 16), whereas the intermolecular BP/1,3-DCB system exclusively gave 8 because it does not absorb at 313 nm (Entry 14). Direct linking of BP and 1,3-DCB through σ-bonding led to longer absorption, which lead to them working as a photoredox catalyst by 313 nm light-irradiation. Recently, photoinduced decarboxylation of carboxylic acids using the Phen/DCB system has been established as a powerful method for the generation of alkyl radicals that can react with a variety of radical acceptors to provide the respective products in high yields [10][11][12][13][14][15]. However, photoinduced decarboxylation of carboxylic acids, such as N-Boc valine (Boc = t-butoxycarbonyl) using 1, were unsuccessful and the starting carboxylic acids were almost recovered. The use of (4-methoxyphenyl)acetic acid 6 (20 mM) as the substrate, which had a more easily oxidizable aromatic ring, with 1a (10 mM) in the presence of NaOH (20 mM) yielded dimer 11. This reaction may proceed through photoinduced decarboxylation, following the formation of a radical cation localized in the aromatic part of 6, via PET with 1a. Thus, PET-promoted decarboxylation using 1 required substrates bearing oxidizable aromatic rings.
Based on these results, a plausible mechanism is proposed, as shown in Scheme 5. First, the excited state of 1 formed by absorption at 313 nm leads to the formation of an intramolecular exciplex in a polar solvent. Even though substrates, such as 4-6 have higher oxidation potentials than those of the corresponding radical cations of the arene moieties in 1a-e (4: +1.66 V vs. SCE in acetonitrile [24], 5: +2.08 V [18], 6: methoxybenzene +1.81 V [24], Phen: +1.50 V [13], NP: +1.70 V [13], BP: +1.85 V [18]), PET occurs between the exciplex and 4-6 proceeds to form both the corresponding radical cations of 4-6 and the radical anion of 1. This may occur due to the formation of a π-complex between the exciplex and substrates 4-6, which is considered to promote electron transfer [18]. These radical cations react with H 2 O and an alkene or induce decarboxylation to generate the corresponding radicals. Back electron transfer (BET) from the radical anions of 1 to the thusly formed substrate radicals generate anions, followed by protonation, to yield the electronically neutral products 7 and 10. In the case of 6, the radical generated via decarboxylation dimerizes to give 11 and BET from the generated radical anion of 1 does not occur, indicating that 1 does not work as a catalyst in this photodecarboxylation. In general, the intramolecular exciplex is considered an important intermediate to cycloaddition [25,26]. However, in this case, the intramolecular exciplex of 1 can work as a redox catalyst for substrates containing alkene or aromatic ring functional groups.
The photoreaction of 2,3-dimethyl-2-butene 5 (20 mM) with acrylonitrile 9 (20 mM) in the presence of 1a (5 mM) for 5 h in aqueous acetonitrile solution (CH3CN/H2O = 9:1, v/v) provided adduct 10 in moderate yield (47%) through the formation of a radical cation of 5 [18] (Scheme 3), along with oligomeric products. Catalysts 1b-e also worked for the photoreaction of 5 with 9 to furnish 10, although the BP/1,3-DCB system did not act as a photoredox catalyst under these photochemical conditions. Recently, photoinduced decarboxylation of carboxylic acids using the Phen/DCB system has been established as a powerful method for the generation of alkyl radicals that can react with a variety of radical acceptors to provide the respective products in high yields [10][11][12][13][14][15]. However, photoinduced decarboxylation of carboxylic acids, such as N-Boc valine (Boc = t-butoxycarbonyl) using 1, were unsuccessful and the starting carboxylic acids were almost recovered. The use of (4-methoxyphenyl)acetic acid 6 (20 mM) as the substrate, which had a more easily oxidizable The photoreaction of 2,3-dimethyl-2-butene 5 (20 mM) with acrylonitrile 9 (20 mM) in the presence of 1a (5 mM) for 5 h in aqueous acetonitrile solution (CH3CN/H2O = 9:1, v/v) provided adduct 10 in moderate yield (47%) through the formation of a radical cation of 5 [18] (Scheme 3), along with oligomeric products. Catalysts 1b-e also worked for the photoreaction of 5 with 9 to furnish 10, although the BP/1,3-DCB system did not act as a photoredox catalyst under these photochemical conditions. Recently, photoinduced decarboxylation of carboxylic acids using the Phen/DCB system has been established as a powerful method for the generation of alkyl radicals that can react with a variety of radical acceptors to provide the respective products in high yields [10][11][12][13][14][15]. However, photoinduced decarboxylation of carboxylic acids, such as N-Boc valine (Boc = t-butoxycarbonyl) using 1, were unsuccessful and the starting carboxylic acids were almost recovered. The use of (4-methoxyphenyl)acetic acid 6 (20 mM) as the substrate, which had a more easily oxidizable aromatic ring, with 1a (10 mM) in the presence of NaOH (20 mM) yielded dimer 11. This reaction may proceed through photoinduced decarboxylation, following the formation of a radical cation localized in the aromatic part of 6, via PET with 1a. Thus, PET-promoted decarboxylation using 1 required substrates bearing oxidizable aromatic rings. Based on these results, a plausible mechanism is proposed, as shown in Scheme 5. First, the excited state of 1 formed by absorption at 313 nm leads to the formation of an intramolecular exciplex in a polar solvent. Even though substrates, such as 4-6 have higher oxidation potentials than those of the corresponding radical cations of the arene moieties in 1a-e (4: +1.66 V vs. SCE in acetonitrile [24], 5: +2.08 V [18], 6: methoxybenzene +1.81 V [24], Phen: +1.50 V [13], NP: +1.70 V [13], BP: +1.85 V [18]), PET occurs between the exciplex and 4-6 proceeds to form both the corresponding radical cations of 4-6 and the radical anion of 1. This may occur due to the formation of a π-complex between the exciplex and substrates 4-6, which is considered to promote electron transfer [18]. These radical cations react with H2O and an alkene or induce decarboxylation to generate the corresponding radicals. Back electron transfer (BET) from the radical anions of 1 to the thusly formed substrate radicals generate anions, followed by protonation, to yield the electronically neutral products 7 and 10. In the case of 6, the radical generated via decarboxylation dimerizes to give 11 and BET from the generated radical anion of 1 does not occur, indicating that 1 does not work as a catalyst in this photodecarboxylation. In general, the intramolecular exciplex is considered an important intermediate to cycloaddition [25,26]. However, in this case, the intramolecular exciplex of 1 can work as a redox catalyst for substrates containing alkene or aromatic ring functional groups.

Materials and Methods
General Information: All reagents and solvents were used as received from commercial suppliers. IR spectra were recorded on an FT-IR spectrometer. 1 H NMR spectra were recorded in CDCl3 containing tetramethylsilane as an internal standard and were acquired on a 500 MHz Scheme 5. The proposed mechanism of 4-6 using 1.

Materials and Methods
General Information: All reagents and solvents were used as received from commercial suppliers. IR spectra were recorded on an FT-IR spectrometer. 1 H NMR spectra were recorded in CDCl 3 containing tetramethylsilane as an internal standard and were acquired on a 500 MHz spectrometer. 13 C{ 1 H} NMR spectra were acquired on a 125 MHz spectrometer. High-resolution mass spectra were obtained using a double-focusing magnetic sector mass spectrometer coupled with FAB. The UV-light source was a Riko UV-100HA high-pressure (100 W) mercury arc. Pyrex vessels (18 mm × 180 mm) were directly attached to the light source (λ > 280 nm, Photocatalyst 1 mainly absorbs at 313 nm light). Column chromatography was performed on Wakogel C-300, particle size 45-75 µm.

General procedure for synthesis of photocatalyst 1
To a solution of 2,2,6,6-tetramethylpiperidine (0.68 mL, 4.0 mmol) in 3 mL of anhydrous THF, n-BuLi (2.44 M n-hexane solution, 1.64 mL, 4.0 mmol) was added at −78 • C under an Ar atmosphere. The mixture was stirred for 30 min at 0 • C. The resulting solution was added to a suspension of copper cyanide (183 mg, 2.0 mmol) in 5 mL of anhydrous THF at −78 • C under an Ar atmosphere. The mixture was stirred at 0 • C for 30 min to give a solution of (TMP) 2 CuCNLi 2 (2.0 mmol). 1,3-Dicyanobenzene (0.105 g, 1.0 mmol) in anhydrous THF (5 mL) was then added to the resulting mixture at −78 • C under an Ar atmosphere, and the mixture was stirred for 3 h at 0 • C. I 2 (1.79 g 7.0 mmol) in anhydrous THF (5 mL) was slowly added to the mixture at −78 • C, and the resulting mixture was stirred at room temperature for 16 h before quenching with saturated aq. NH 4 Cl and NaHS 2 O 3 . The resultant reaction mixture was extracted with AcOEt and dried over Na 2 SO 4 , and volatiles were removed by evaporation. The crude product was purified by silica-gel column chromatography using hexane/EtOAc = 9:1 as the eluent to yield 1,3-dicyano-2-iodobenzene 2 as a white solid. Compound 2 has been previously reported [19]. 1 H and 13 C{ 1 H} NMR spectra for all compounds 1, 2, 7, 8, 9, 10, and 11 can be found in the Supplementary Materials. A solution of 2 (0.10 g, 0.39 mmol), arylboronic acid 3 (0.47 mmol), K 3 PO 4 (0.020 g, 0.094 mmol), Pd 2 (dba) 3 (0.014 mg, 1.7 mol %), and (4-biphenyl)dicyclohexylphosphine (PCy 2 Bp) (0.014 mg, 3.4 mol%) in toluene (6 mL) was refluxed at 130 • C for 24 h. After the starting material 2 was consumed, the solvent was removed by evaporation in vacuo. The crude product was purified by silica-gel column chromatography using hexane/EtOAc = 9:1 as the eluent to yield photocatalyst 1 as a white solid.  13  General procedure for the photoreaction of 4, 5, and 6 with 1 An aqueous CH 3 CN solution (CH 3 CN 45 mL, H 2 O 5 mL) of 1 (10 mM), 4 (20 mM), NaOH (20 mM) in Pyrex vessels (18 mm x 180 mm) was purged with Ar for 10 min. The mixture was irradiated with a 100 W high-pressure mercury lamp for 10 h and then the solvent was removed under reduced pressure. The crude product was purified by silica-gel column chromatography using hexane/EtOAc = 9:1 as the eluent to yield alcohol 7 along with dimer 8. Similar photoreactions of 5 and 6 were carried out under the similar conditions to yield 10 and 11.

Conclusions
This study has led to the development of PET-promoted reactions using a new class of organic photoredox catalysts 1 wherein donor and acceptor arenes are directly linked. The intramolecular exciplex of 1 functions as a photoredox catalyst for the photoreactions of 4, 5, and 6 in an aqueous acetonitrile solution, even at low concentrations. Further investigations on the application of catalyst 1 to other PET-promoted reactions and the elucidation of the detailed mechanism are underway. Funding: This work was partially supported by the Japan Society for the Promotion of Science (JSPS), Grant-in-Aid no. 17K05779, for scientific research.

Conflicts of Interest:
The authors declare no conflict of interest.