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Oxidative Functionalization of Cinnamaldehyde Derivatives: Control of Chemoselectivity by Organophotocatalysis and Dual Organocatalysis Eito Yoshioka, Maika Inoue, Yuka Nagoshi, Ayumi Kobayashi, Rumiko Mizobuchi, Akira Kawashima, Shigeru Kohtani, and Hideto Miyabe J. Org. Chem., Just Accepted Manuscript • DOI: 10.1021/acs.joc.8b01099 • Publication Date (Web): 03 Jul 2018 Downloaded from http://pubs.acs.org on July 4, 2018

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The Journal of Organic Chemistry

Oxidative

Functionalization

of

Cinnamaldehyde

Derivatives:

Control

of

Chemoselectivity

by

Organophotocatalysis and Dual Organocatalysis

Eito Yoshioka, Maika Inoue, Yuka Nagoshi, Ayumi Kobayashi, Rumiko Mizobuchi, Akira Kawashima, Shigeru Kohtani, and Hideto Miyabe* School of Pharmacy, Hyogo University of Health Sciences, Minatojima, Chuo-ku, Kobe, 650-8530, Japan.

Abstract: The catalytic and chemoselective oxidation of cinnamaldehyde derivatives having C=C bond and formyl group was studied by using two organocatalysts. The visible light-induced catalysis using rhodamine 6G as an organophotocatalyst promoted the methoxyhydroxylation of C=C bond in a chemoselective manner. In contrast, the cooperation between rhodamine 6G and N-heterocyclic carbene (NHC) allowed the oxidative esterification of formyl group. TOC

INTRODUCTION Oxidation is one of the most important organic transformations. Increasingly, much effort has been devoted to the evolution of catalytic processes in place of traditional methods using stoichiometric quantities of toxic heavy metal oxidants.1 Particularly, exploring the mild organocatalytic methods is a major challenge in modern organic synthesis.2 In recent years, the visible light-activated ruthenium or iridium complexes have received much attention as photocatalysts.3 Furthermore, the use of organic dyes overcomes the need for precious transition metals and enables the environmentally friendly redox transformations.4- 10 Cinnamaldehyde derivatives are fragrance ingredients having antifungal properties as well as building 1 ACS Paragon Plus Environment

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blocks.11 Our laboratory is interested in developing the organocatalytic protocols for the oxidative functionalization of C=C bond and formyl group of cinnamaldehydes in a chemoselective manner (Figure 1). The chemoselective oxidation of C=C bond was achieved by the organophotocatalysis using rhodamine 6G in MeOH. As described below, the oxidation of 1 led to the formation of acetal product 2 via methoxyhydroxylation intermediate A. In contrast, the chemoselective oxidation of formyl group was accomplished by dual organocatalysis based on the cooperation between N-heterocyclic carbene (NHC) and photocatalyst.12 In the oxidative esterification of 1 leading to 3, the photocatalytic double oxidation of nucleophilic Breslow intermediate B to electrophilic acylium ion equivalent is a key umpolung process.

Figure 1. Chemoselective oxidation of cinnamaldehydes.

RESULTS AND DISCUSSION To study the viability of organic dyes for oxidation processes, our experiments began with the investigation of methoxyhydroxylation of C=C bond (Table 1). The similar functionalization of C=C bond has been studied by using iodobenzene diacetate under base conditions.13 We found that the oxidative functionalization of 1a leading to 2a was promoted by a base such as K2CO3. After a 50 mL round-bottom flask was equipped with 1a (1.0 mmol), BrCCl3 and rhodamine 6G, K2CO3 and MeOH (10 mL), the flask was capped with argon-balloon. Therefore, air (ca 50 mL, ca. 0.45 equivalents of O2) exists in a reaction vessel. In our study, BrCCl3 was used as a quencher toward the photo-activated 2 ACS Paragon Plus Environment

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dye species having reductant property. The reduction potential of BrCCl3 (+0.21 V vs. SCE)14 is more positive than that of O2 (−0.42 V vs. SCE at pH 7 in H2O),15 indicating that electron accepting ability of BrCCl3 is much better than that of O2.

Table 1. Oxidation of carbon-carbon double bond in 1a.a

Entry

Organic dye

quencher

Base

Yield (%)b

1

none

BrCCl3

K2CO3

NR

2

Eosin Y (5 mol%)

BrCCl3

K2CO3

27

3

Fluorescein (5 mol%)

BrCCl3

K2CO3

30

4

Methylene blue (5 mol%)

BrCCl3

K2CO3

65

5

9-Mes-10-Me-acrydinium (5 mol%)

BrCCl3

K2CO3

69

6

Rhodamine B (5 mol%)

BrCCl3

K2CO3

71

7

Rhodamine 6G (5 mol%)

BrCCl3

K2CO3

76

8


BrCCl3

K2CO3

67

9


BrCCl3

K2CO3

60

10

c

Eosin Y (5 mol%)

none

K2CO3

NR

11

c


none

K2CO3

NR

12

d


BrCCl3

K2CO3

54

13


BrCCl3

Na2CO3

8

14


BrCCl3

Cs2CO3

61

15


BrCCl3

K2HPO4

trace

16


BrCCl3

Et3N

NR

17

e


BrCCl3

KOH

89

18

f


BrCCl3

KOH

29 f

3 ACS Paragon Plus Environment


a

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In the presence of dye, the reactions of 1a were carried out with BrCCl3 (3 equiv.) and base (2.5 equiv.) in MeOH with

white LED light irradiation under argon atmosphere containing air (ca. 50 mL) at rt. b Based on isolated yields. 2a was obtained as ca. 5:5-6:4 mixture of syn/anti-isomers. c Reactions were carried out in the absence of BrCCl3. d Reaction was carried in an open-vessel under air.

e

The syn- and anti-isomers were formed in ratio of 59:41. f Reaction was carried out

after degasification to remove O2.

No reaction occurred in the absence of organic dye as a photocatalyst even employing BrCCl3 under white LED light irradiation (entry 1). The oxidizing ability of dyes affected the chemical efficiencies (entries 2−7). In general, the visible light-activated dyes in a lowest excited singlet state (S1) are rapidly converted into a lowest excited triplet state (T1) via a spin-forbidden singlet-triplet intersystem crossing (ISC); thus, the photochemical process occurs from T1. Eosin Y5 (Ered* in T1 [EY–•/EY*]: +0.83 V vs. SCE in CH3CN16) and fluorescein6 (Ered* in T1: +0.77 V vs. SCE in CH3CN16) possessing less positive reduction potentials have shown the low catalytic activities to give the methoxyhydroxylated acetal 2a in 27% and 30% yields, respectively (entries 2 and 3). In contrast, the use of methylene blue7 (Ered* in T1: +1.60 V vs. SCE in CH3CN16) or 9-Mes-10-Me-acrydinium8 (Ered* in T1: +1.45 V vs. SCE in CH3CN17) having more positive potentials led to an enhancement in chemical yields (entries 4 and 5). We recently reported the utility of rhodamine B in the cascade radical reactions.18 In our study on rhodamine B, we proposed that the electron-transfer occurs not from T1 but from S1, because value of quantum yield of ISC is very low (ΦISC = 0.002419) and ISC is slow. As expected from the sufficient positive potential of Ered* in S1 (+1.26 V vs. SCE in CH3CN16), rhodamine B promoted this transformation to give 2a in 71% yield (entry 6). The higher yield of 2a was obtained by rhodamine 6G9 (ΦISC = 0.00220 and Ered* in S1: +1.18 V vs. SCE in CH3CN21) (entry 7). Its catalytic efficiency was confirmed by decreasing the amount of rhodamine 6G (entries 8 and 9). Additionally, this oxidation did not take place in the absence of BrCCl3 (entries 10 and 11). Next, the reaction was carried in an open-vessel under air (entry 12). However, the chemical yield slightly decreased to 54%. Subsequently, the influence of base was examined (entries 13−17). The chemical yield increased to 89% by changing a base from K2CO3 to KOH (entry 17). This reaction proceeded even in the absence of O2, although the chemical yield decreased to 29% yield by degasification to remove O2 (entry 18). 4 ACS Paragon Plus Environment

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Next, the reaction of cinnamaldehyde derivatives 1b−1e was examined (Table 2). Good yields were obtained in the oxidation of 1b or 1c carrying an electron-withdrawing group on aromatic ring (entries 1−3). We again observed the positive effect of KOH as a base (entries 1 and 2). The substrates 1d and 1e were also converted into 2d and 2e in good yields (entries 4 and 5).

Table 2. Oxidative functionalization of cinnamaldehyde derivatives 1b-e.a

Entry

Substrate

Base

Yield (%)b

1

1b (Ar=4-Cl-C6H4)

K2CO3

67

2

1b (Ar=4-Cl-C6H4)

KOH

90

3

1c (Ar=4-F-C6H4)

KOH

72

4

1d (Ar=C6H5)

KOH

89

5

1e (Ar=4-Me-C6H4)

KOH

73

a

In the presence of rhodamine 6G (5 mol%), the reactions of 1b-e were carried out with

BrCCl3 (3 equiv.) and base (2.5 equiv.) in MeOH with white LED light irradiation under argon atmosphere containing air (ca. 50 mL) at rt. b Based on isolated yields (syn:anti = ca. 6:4; See: SI).

Scheme 1. Reactivity of several substrates and further oxidation of 2a.



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To gain further insight, we next explored the reaction of several substrates (Scheme 1). Introducing methyl group at α-position did not affected the reactivity. The desired product 5 was obtained in 99% yield from the sterically hindered substrate 4. Interestingly, the methoxyhydroxylation of polyconjugated substrate 6 proceeded regioselectively to give the δ-methoxyated product 7. Moreover, it is important to note that hydroxylation occurred at α-position. The activation of C=C bond by formyl group was essential to achieve this methoxyhydroxylation. The simple olefin 8 and the acetal 9 of 1a did not work well. These results suggest that acetal formation is the final step and the oxidative process is initiated by Michael addition of MeOH leading to adduct C (Figure 2). Subsequently, the photocatalytic oxidation of anion D, which is momentarily generated in association with KOH, takes place to give the resonance-stabilized radical E. Since the photo-excited rhodamine 6G (Rh6G*) has enough positive reduction potential in S1 (Ered*: +1.18 V21), anion D is easily oxidized to radical E as depicted in photo-redox cycle 1. The reduced Rh6G (Rh6G－•) subsequently reduces BrCCl3 to generate CCl3 radical and Br－, because the reduction potential of rhodamine 6G in the ground state (Ered: −1.14 V vs. SCE in aqueous CH3CN21) is sufficiently negative. In cycle 2, Rh6G* reduces 6 ACS Paragon Plus Environment

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BrCCl3 owing to the sufficient oxidation potential of Rh6G* in S1 (Eox*: −1.09 V vs. SCE in CH3CN22). The oxidized Rh6G (Rh6G+•) possessing enough positive potential in the ground state (Eox: +1.23 V vs. SCE in CH3CN22) oxidizes the anion D. Consequently, radical E can be generated by both cycles 1 and 2.

Figure 2. Possible reaction pathway.

Based on the results described in Table 1, we propose two pathways leading to the acetal product 2 from radical E. The oxygen-free pathway is started by the bromine atom-transfer to radical E from BrCCl3 to give the adduct F.23 The final product 2 would be formed via the substitution reaction of acetal G with HO－ or H2O in a reaction vessel and/or by work-up step, etc. Because this transformation was promoted by air; thus, the oxygen-mediated pathway is also important. The methoxyhydroxylation intermediate I is formed through trapping of E with molecular oxygen O2 followed by the reduction of radical H. Finally, the acetal product 2 is obtained from intermediate I. No 7 ACS Paragon Plus Environment


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reaction of acetal 9 occurred under the standard reaction conditions (Scheme 1); thus, the transformation of aldehyde into acetal occurred after the formation of adduct F or intermediate I. Additionally, both syn-2a and anti-2a were converted to the same ketone 10 (Scheme 1). The inimitable reactivity of NHC has led to the discovery of new organocatalysis.24 In general, the NHC-catalyzed oxidative esterification of cinnamaldehyde can be performed in combination with a stoichiometric amount of external oxidant.25 In the absence of oxidant, the esterification of cinnamaldehyde accompanies the reduction of C=C bond.26 Next, we felt attracted to the cooperative catalysis as a challenging task providing the new oxidative esterification method.27 In the presence of rhodamine 6G (5 mol%) and BrCCl3 (3 equiv.), the oxidative esterification of 1a was examined by using several NHC-precatalysts 11A-11F (entries 1-7 in Table 3). In the absence of NHC-precatalyst, the formation of methyl ester 3a was not detected (entry 1). We found that the use of nucleophilic NHC catalyst,26a,28 generated from N-Mes triazolium 11A and K2CO3, leads to an effective esterification of 1a (entry 2). Next, the effect of oxidant was examined by replacing BrCCl3 to milder oxidants (entries 8 and 9). However, the use of CCl4 or HCCl3 led to lower chemical yields. Among them, BrCCl3 is the best oxidant as a quencher toward the photo-activated dye species.

Table 3. Effects of NHC-catalyst and oxidant on oxidative esterification of 1a.a


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Entry

NHC-precatalyst

Oxidant

Solvent

Yield (%)b

1

none

BrCCl3

MeOH-THF (1:1)

ND

2

Triazolium 11A

BrCCl3

MeOH-THF (1:1)

99

3

Triazolium 11B

BrCCl3

MeOH-THF (1:1)

47

4

Triazolium 11C

BrCCl3

MeOH-THF (1:1)

51

5

Thiazolium 11D

BrCCl3

MeOH-THF (1:1)

ND

6

Thiazolium 11E

BrCCl3

MeOH-THF (1:1)

9

7

Thiazolium 11F

BrCCl3

MeOH-THF (1:1)

14

8

Triazolium 11A

CCl4

MeOH-THF (1:1)

trace

9

Triazolium 11A

HCCl3

MeOH-HCCl3 (1:1)

11

a

In the presence of NHC-precatalyst (10 mol%) and rhodamine 6G (5 mol%), the reactions of 1a were

carried out with oxidant (3 equiv.) and K2CO3 (2.5 equiv.) with white LED light irradiation under argon atmosphere at rt. b Based on isolated yields.

In the presence of BrCCl3, the best combination of dye with triazolium 11A was next examined (Table 4). At first, the reaction was carried out employing BrCCl3 under LED light irradiation in the absence of dye as a photocatalyst (entry 1). In the absence of photocatalyst, the oxidative esterification of 1a did not occur effectively. Although it is reported that polyhalides such as C2Cl6 and BrCCl3 are capable to induce the single electron-transfer (SET) processes of Breslow intermediates under NHC-catalysis without the need of photocatalytic conditions,29 the use of photocatalyst clearly promoted the present reaction. The combination of 11A and fluorescein led to the formation of 9 ACS Paragon Plus Environment


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3a in 68% yield (entry 1). Alizarin red S and rhodamine B worked well (entries 3 and 4). When rhodamine base was used, the ester 3a was obtained in 52% yield (entry 5). Eosin Y promoted the reaction to give 3a in good yields (entries 6 and 7), although chemical yields decreased at 3 mol% or 1 mol% catalyst loading of 11A (entries 8 and 9). The best result was obtained by rhodamine 6G (entries 10−14). The competitive oxidation of C=C bond was not observed under the optimized conditions (entry 11). Even in the presence of lower catalytic amounts of 11A (3 mol%) and rhodamine 6G (1.5 mol%), 3a was obtained in 48% yield (entry 13). The reactivity of product 3a toward methoxyhydroxylation was also examined. In marked contrast to aldehyde 1a (enrty 18 in Table 1), the methoxyhydroxylation of ester 3a did not proceed under the methoxyhydroxylation conditions described in Table 1. The ester 3a was recovered in 60% yield, accompanied with 32% of 4-methoxycinnamic acid due to hydrolysis by coexisting water. Additionally, the use of EtOH-THF as solvent led to lower chemical efficiency. The corresponding ethyl ester was obtained in 65% yield when the reaction carried out at 60 °C.

Table 4. Selective oxidation of formyl group in 1a.a

Entry

Triazolium 11A

Dye

Yield (%)b

1

10 mol%

none

trace

2

10 mol%

Fluorescein (5 mol%)

68

3

10 mol%

Alizarin red S (5 mol%)

83

4

10 mol%

Rhodamine B (5 mol%)

99

5

10 mol%

Rhodamine base (5 mol%)

52

6

10 mol%

Eosin Y (5 mol%)

99

7

5 mol%

Eosin Y (5 mol%)

96

8

3 mol%

Eosin Y (5 mol%)

33

9

1 mol%

Eosin Y (5 mol%)

13

10

10 mol%


99

11

5 mol%


99

12

3 mol%


92


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13

3 mol%

Rhodamine 6G (1.5 mol%)

48

14

1 mol%


30

a

In the presence of triazolium 11A and dye, the reactions of 1a were carried out with

BrCCl3 (3 equiv.) and K2CO3 (2.5 equiv.) in MeOH-THF (1:1, v/v) with white LED light irradiation under argon atmosphere at rt. b Based on isolated yields.

Further investigations were performed (Table 5). The esters 3c-3e were obtained without any problems. The substrate 1f having a methoxy group at o-position of aromatic ring worked well. The substrate 1g bearing furan and 2-naphthaldehyde 12 produced the corresponding esters 3g and 13, allowing facile incorporation of structural variety.

Table 5. Reaction of cinnamaldehydes 1c-g and 2-naphthaldehyde 12.a

a

In the presence of 11A (5 mol%) and rhodamine 6G (5 mol%), the reactions were

carried out with BrCCl3 (3 equiv.) and K2CO3 (2.5 equiv.) in MeOH-THF (1:1, v/v) with white LED light irradiation under argon atmosphere at rt.

The cooperative organocatalysis is initiated by the formation of Breslow intermediate B, which is photocatalytically oxidized to afford the radical J via deprotonation (Figure 3). Particularly, electron-rich Breslow intermediate B is easily oxidized, which is supported by sufficient positive potential of Ered* in S1 of rhodamine 6G (+1.18 V21). The oxidation potential of the Breslow intermediate was reported (Eox: around −0.49 V vs. SCE).30 The oxidation potential of the aza-Breslow intermediate (Eox: +0.49 V vs. SCE in CH2Cl2) was also shown by T. Rovis.31 11 ACS Paragon Plus Environment


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The second photocatalytic oxidation of J leads to acylium ion equivalent K. Finally, cation K reacts with MeOH to give the ester 3 and NHC catalyst.

Figure 3. Cooperative organocatalysis.

CONCLUSION We have developed new organocatalysis for the chemoselective methoxyhydroxylation and oxidative esterification of cinnamaldehydes. The organophotocatalysis using rhodamine 6G promoted the methoxyhydroxylation of C=C bond of cinnamaldehydes under visible light irradiation. In contrast, the oxidative esterification of formyl group was achieved by the dual organocatalysis based on the cooperation between rhodamine 6G and N-heterocyclic carbene (NHC).

EXPERIMENTAL SECTION

General. Melting points were taken on a Yanaco MP-J3 and are uncorrected. Infrared spectra were measured on a JASCO FT/IR-4100. 1H-NMR spectra were measured on a JEOL ECX-400 PSK (400 MHz) or Varian NMRS 600 (600 12 ACS Paragon Plus Environment

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MHz) with CDCl3 as an internal standard (7.26 ppm). 13C-NMR spectra were measured on a JEOL ECX-400 PSK (101 MHz) or Varian NMRS 600 (162 MHz) with CDCl3 as an internal standard (77.0 ppm).

19

F-NMR spectra were

measured on a JEOL ECX-400 PSK (376 MHz) with C6F6 as an internal standard (–162.2 ppm). High-resolution mass spectra were recorded on a time-of-flight (TOF) mass spectrometer by use of Thermo Fisher Scientific Exactive LC/MS spectrometer. For silica gel column chromatography, SiliCycle Inc. SiliaFlash F60 was used. Preparative TLC separations were carried out on precoated silica gel plates (E. Merck 60F254). Products 2d,13a 3a,32 3c,33 3d,34 3e,33,35 3f,33 3g36 and 1337 are known compounds. General procedure for methoxyhydroxylation: A 50 mL round-bottom flask was equipped with crashed KOH (140 mg, 2.5 mmol), undegassing MeOH (10 mL), 1a-e, 4 or 6 (1.0 mmol), bromotrichloromethane (296 µL, 3.0 mmol) and rhodamine 6G (24 mg, 0.050 mmol) at room temperature. After the flask was capped with argon-balloon, the stirring reaction mixture was irradiated with white LED lamp (1000 lm) at room temperature. After being stirred for 5 hours, the reaction mixture was filtrated with folded filter paper which was then wasched with MeOH several times. The filtrate was concentrated under reduced pressure. Purification of the residue by flash silica gel column chromatography (AcOEt:hexanes = 1:4–1:0) afforded the products 1a-e, 5 or 7 as a mixture of syn/anti- isomers. The ratio of products was determined by 1H NMR analysis of the mixture. In the case of 1a-e and 5, second purification of the mixture by preparative TLC afforded the isolated isomers. 2-Hydroxy-3-methoxy-3-(4-methoxyphenyl)propanal dimethylacetal (2a) Following general procedure, the reaction of 1a was carried out. First purification by flash silica gel column chromatography afforded the product 2a (227 mg, 89%) as a mixture of 59:41 syn/anti-isomers. The syn/anti-isomers was separated by second purification by preparative TLC (AcOEt:hexanes = 1:2, 2-fold development). Syn-isomer: Colorless oil. IR (KBr) 3489 (br), 2935, 1611, 1512, 1247 cm-1. 1H NMR (CDCl3) δ 7.27 (2H, d, J = 8.7 Hz), 6.91 (2H, d, J = 8.7 Hz), 4.27 (1H, d, J = 4.6 Hz), 4.21 (1H, d, J = 5.5 Hz), 3.81 (3H, s), 3.68 (1H, m), 3.40 (6H, s), 3.25 (3H, s), 2.52 (1H, br s). 13C NMR (CDCl3) δ 159.4, 130.3, 128.6, 113.8, 103.8, 82.3, 75.1, 56.8, 55.5, 55.2, 54.7. HRMS (ESI+) calcd for C13H20O5Na (M+Na+): 279.1203, Found: 279.1232. Anti-isomer: Colorless oil. IR (KBr) 3476 (br), 2935, 1611, 1512, 1249 cm-1. 1H NMR (CDCl3) δ 7.29 (2H, 13 ACS Paragon Plus Environment


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d, J = 8.7 Hz), 6.90 (2H, d, J = 8.7 Hz), 4.22 (1H, d, J = 5.5 Hz), 4.12 (1H, d, J = 5.5 Hz), 3.94 (1H, m), 3.81 (3H, s), 3.45 (3H, s), 3.38 (3H, s), 3.23 (3H, s), 2.20 (1H, br s). 13C NMR (CDCl3) δ 159.4, 129.5, 129.4, 113.6, 103.7, 82.5, 73.4, 56.5, 55.2, 55.0, 54.7. HRMS (ESI+) calcd for C13H20O5Na (M+Na+): 279.1203, Found: 279.1225. 3-(4-Chlorophenyl)-2-hydroxy-3-methoxypropanal dimethylacetal (2b) Following general procedure, the reaction of 1b was carried out. First purification by flash silica gel column chromatography afforded the product 2b (234 mg, 90%) as a mixture of 61:39 syn/anti-isomers. The syn/anti-isomers was separated by second purification by preparative TLC (AcOEt:hexanes = 1:2, 2-fold development). Syn-isomer: Colorless oil. IR (KBr) 3463 (br), 2935, 1491 cm-1. 1H NMR (CDCl3) δ 7.35 (2H, d, J = 8.7 Hz), 7.30 (2H, d, J = 8.7 Hz), 4.32 (1H, d, J = 4.1 Hz), 4.28 (1H, d, J = 5.5 Hz), 3.64 (1H, m), 3.43 (3H, s), 3.41 (3H, s), 3.27 (3H, s) , 2.45 (1H, br s). 13C NMR (CDCl3) δ 137.1, 133.7, 128.8, 128.6, 103.8, 81.9, 74.9, 57.1, 55.6, 54.6. HRMS (ESI+) calcd for C12H1735ClO4Na (M+Na+): 283.0708, Found: 283.0706. HRMS (ESI+) calcd for C12H1737ClO4Na (M+Na+): 285.0682, Found: 285.0675. Anti-isomer: Colorless oil. IR (KBr) 3465 (br), 2935, 1490 cm-1. 1H NMR (CDCl3) δ 7.36-7.29 (4H, m), 4.24 (1H, d, J = 5.5 Hz), 4.11 (1H, d, J = 5.5 Hz), 3.92 (1H, m), 3.45 (3H, s), 3.37 (3H, s), 3.24 (3H, s), 2.27 (1H, br s). 13C NMR (CDCl3) δ 136.3, 133.8, 129.6, 128.4, 103.5, 82.4, 73.3, 56.8, 55.1, 54.6. HRMS (ESI+) calcd for C12H1735ClO4Na (M+Na+): 283.0708, Found: 283.0705. HRMS (ESI+) calcd for C12H1737ClO4Na (M+Na+): 285.0682, Found: 285.0676. 3-(4-Fluorophenyl)-2-hydroxy-3-methoxypropanal dimethylacetal (2c) Following general procedure, the reaction of 1c was carried out. First purification by flash silica gel column chromatography afforded the product 2c (177 mg, 72%) as a mixture of 61:39 syn/anti-isomers. The syn/anti-isomers was separated by second purification by preparative TLC (AcOEt:hexanes = 1:1). Syn-isomer: Colorless oil. IR (KBr) 3463 (br), 2935, 1604, 1509 cm-1. 1H NMR (CDCl3) δ 7.33 (2H, m), 7.06 (2H, m), 4.32 (1H, d, J = 4.1 Hz), 4.26 (1H, d, J = 5.5 Hz), 3.64 (1H, m), 3.42 (3H, s), 3.41 (3H, s), 3.26 (3H, s), 2.49 (1H, br d, J = 3.7 Hz). 13C NMR (CDCl3) δ 162.5 (d, J = 247 Hz), 134.2 (d, J = 4 Hz), 129.0 (d, J = 8 Hz), 115.3 (d, J = 21 Hz), 103.7, 81.9, 74.9, 57.0, 55.6, 54.6. 19F NMR (CDCl3) δ −115.0 (1F, s). HRMS (ESI+) calcd for C12H17FO4Na (M+Na+): 267.1003, Found: 267.1011. Anti-isomer: Colorless oil. IR (KBr) 3465 (br), 2935, 1604, 1509 cm-1. 1H NMR (CDCl3) δ 7.34 (2H, m), 7.05 (2H, m), 4.25 (1H, d, J = 5.5 Hz), 4.10 (1H, d, J = 5.0 Hz), 3.94 (1H, 14 ACS Paragon Plus Environment

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m), 3.45 (3H, s), 3.37 (3H, s), 3.23 (3H, s), 2.27 (1H, br d, J = 3.6 Hz). 13C NMR (CDCl3) δ 162.2 (d, J = 247 Hz), 133.3 (d, J = 4 Hz), 129.8 (d, J = 8 Hz), 115.1 (d, J = 22 Hz), 103.5, 82.3, 73.2, 56.7, 55.0, 54.6. 19F NMR (CDCl3) δ −115.0 (1F, s). HRMS (ESI+) calcd for C12H17FO4Na (M+Na+): 267.1003, Found: 267.0998. 2-Hydroxy-3-methoxy-3-phenylpropanal dimethylacetal (2d)13a Following general procedure, the reaction of 1d was carried out. First purification by flash silica gel column chromatography afforded the product 2d (201 mg, 89%) as a mixture of 63:37 syn/anti-isomers. The syn/anti-isomers was separated by second purification by preparative TLC (AcOEt:hexanes = 1:2, 2-fold development). Syn-isomer: Colorless oil. IR (KBr) 3465 (br), 2933, 1452, 1197 cm-1. 1H NMR (CDCl3) δ 7.40-7.30 (5H, m), 4.33 (1H, d, J = 4.6 Hz), 4.25 (1H, d, J = 5.5 Hz), 3.70 (1H, m), 3.42 (3H, s), 3.41 (3H, s), 3.28 (3H, s), 2.50 (1H, br d, J = 4.1 Hz). 13C NMR (CDCl3) δ 138.4, 128.4, 128.0, 127.3, 103.7, 82.6, 75.0, 57.1, 55.5, 54.6. HRMS (ESI+) calcd for C12H18O4Na (M+Na+): 249.1097, Found: 249.1094. Anti-isomer: Colorless oil. IR (KBr) 3466 (br), 2932, 1453, 1192 cm-1. 1H NMR (CDCl3) δ 7.37-7.30 (5H, m), 4.28 (1H, d, J = 5.5 Hz), 4.14 (1H, d, J = 5.5 Hz), 3.97 (1H, m), 3.45 (3H, s), 3.38 (3H, s), 3.25 (3H, s), 2.24 (1H, br d, J = 3.7 Hz). 13C NMR (CDCl3) δ 137.5, 128.2 (2C), 128.1, 103.5, 83.0, 73.4, 56.7, 55.0, 54.6. HRMS (ESI+) calcd for C12H18O4Na (M+Na+): 249.1097, Found: 249.1094. 2-Hydroxy-3-methoxy--3-(4-methylphenyl)propanal dimethylacetal (2e) Following general procedure, the reaction of 1e was carried out. First purification by flash silica gel column chromatography afforded the product 2e (176 mg, 73%) as a mixture of 60:40 syn/anti-isomers. The syn/anti-isomers was separated by second purification by preparative TLC (AcOEt:hexanes = 1:2, 2-fold development). Syn-isomer: Colorless oil. IR (KBr) 3459 (br), 2930, 1513, 1449, 1196 cm-1. 1H NMR (CDCl3) δ 7.24 (2H, br d, J = 8.2 Hz), 7.18 (2H, br d, J = 8.2 Hz), 4.30 (1H, d, J = 4.6 Hz), 4.23 (1H, d, J = 5.5 Hz), 3.68 (1H, m), 3.42 (3H, s), 3.41 (3H, s), 3.27 (3H, s), 2.49 (1H, br d, J = 3.7 Hz), 2.36 (3H, s). 13C NMR (CDCl3) δ 137.7, 135.2, 129.1, 127.3, 103.7, 82.5, 75.1, 56.9, 55.5, 54.7, 21.2. HRMS (ESI+) calcd for C13H20O4Na (M+Na+): 263.1254, Found: 263.1255. Anti-isomer: Colorless oil. IR (KBr) 3474 (br), 2929, 1513, 1449, 1191 cm-1. 1H NMR (CDCl3) δ 7.25 (2H, br d, J = 8.2 Hz), 7.18 (2H, br d, J = 8.2 Hz), 4.24 (1H, d, J = 5.5 Hz), 4.14 (1H, d, J = 5.0 Hz), 3.95 (1H, m), 3.45 (3H, s), 3.38 (3H, s), 3.24 (3H, s), 2.35 (3H, s), 2.21 (1H, br d, J = 3.7 Hz). 13C 15 ACS Paragon Plus Environment


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NMR (CDCl3) δ 137.8, 134.4, 128.9, 128.1, 103.6, 82.8, 73.4, 56.6, 55.0, 54.6, 21.2. HRMS (ESI+) calcd for C13H20O4Na (M+Na+): 263.1254, Found: 263.1250. 2-Hydroxy-3-methoxy-2-methyl-3-phenylpropanal dimethylacetal (5) Following general procedure, the reaction of 4 was carried out. First purification by flash silica gel column chromatography afforded the product 5 (237 mg, 99%) as a mixture of 75:25 two isomers. The syn/anti-isomers was separated by second purification by preparative TLC (AcOEt:hexanes = 1:2, 2-fold development), although the stereostructure of each isomers has not been determined. Major isomer: Colorless oil. IR (KBr) 3501 (br), 2937, 1452 cm-1. 1H NMR (CDCl3) δ 7.38-7.30 (5H, m), 4.41 (1H, s), 4.24 (1H, s), 3.60 (3H, s), 3.58 (3H, s), 3.23 (3H, s), 2.28 (1H, s), 0.91 (3H, s). 13C NMR (CDCl3) δ 137.4, 128.8, 127.9 (2C), 108.1, 85.5, 58.2, 58.1, 56.9, 17.6. One carbon peak is missing due to overlapping peak of CDCl3. HRMS (ESI+) calcd for C13H20O4Na (M+Na+): 263.1254, Found: 263.1277. Minor isomer: Colorless oil. IR (KBr) 3500 (br), 2937, 1452 cm-1. 1H NMR (CDCl3) δ 7.38-7.28 (5H, m), 4.21 (1H, s), 3.96 (1H, s), 3.54 (3H, s), 3.42 (3H, s), 3.24 (3H, s), 2.22 (1H, br s), 1.17 (3H, s). 13C NMR (CDCl3) δ 137.4, 128.7, 127.8, 127.7, 107.5, 85.6, 76.7, 58.6, 57.1, 56.8, 18.2. HRMS (ESI+) calcd for C13H20O4Na (M+Na+): 263.1254, Found: 263.1253. (E)-2-Hydroxy-5-methoxyhex-3-enal dimethylacetal (7) Following general procedure, the reaction of 6 was carried out. First purification by flash silica gel column chromatography afforded the product 7 (118 mg, 62%) as a mixture of about 1:1 syn/anti-isomers. The syn/anti-isomers cannot be separated by second purification by preparative TLC (AcOEt:hexanes = 1:2, 2-fold development). Colorless oil. IR (KBr) 3441 (br), 2930, 1450 cm-1. The presence of diastereomers precluded a comprehensive assignment of all proton and carbon resonances. 1H NMR (CDCl3) δ 5.76-5.66 (2H, m), 4.18-4.14 (2H, m), 3.80-3.73 (1H, m), 3.46 (3H, s), 3.433 (3/2H, s), 3.431 (3/2H, s), 3.27 (3H, s), 2.28 (1H, br s), 1.25 (3H, dd, J = 6.4, 1.8 Hz). 13C NMR (CDCl3) δ 134.6, 134.4, 129.1 (2C), 106.5, 106.4, 77.4 (2C), 71.4 (2C), 56.0, 55.9, 55.4 (2C), 55.2, 55.1, 21.2 (2C). HRMS (ESI+) calcd for C9H18O4Na (M+Na+): 213.1097, Found: 213.1103. Procedure for oxidation of anti-2a and syn-2a: To a solution of anti-2a (100 mg, 0.39 mmol) in CH2Cl2 (1.0 mL) were added saturated NaHCO3 solution (1.0 mL), 2-hydroxy-2-azaadamantane (3.0 mg, 0.020 mmol), KBr (4.6 mg, 16 ACS Paragon Plus Environment

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0.039 mmol), tetra-n-butylammonium bromide (6.3 mg, 0.020 mmol) and NaClO.5H2O (96 mg, 0.59 mmol) at room temperature. After being stirred for 4 hours, the reaction mixture was diluted with water and extracted with AcOEt. The organic phase was dried over Na2SO4 and concentrated under reduced pressure. Purification of the residue by preparative TLC (AcOEt:hexanes = 1:2) afforded the product 10 (49 mg, 49%) as colorless oil. The product 10 was obtained in 44% yield (67 mg) as colorless oil from syn-2a (155 mg, 0.60 mmol) by the same procedure. 1,1,3-Trimethoxy-3-(4-methoxyphenyl)propan-2-one (10) IR (KBr) 2939, 1743, 1609, 1512, 1253 cm-1. 1H NMR (CDCl3) δ 7.30 (2H, br d, J = 8.7 Hz), 6.90 (2H, br dt, J = 8.7 Hz), 5.09 (1H, s), 4.57 (1H, s), 3.81 (3H, s), 3.35 (3H, s), 3.32 (3H, s), 3.23 (3H, s). 13C NMR (CDCl3) δ 201.1, 160.0, 129.6, 126.8, 114.2, 100.8, 84.8, 56.9, 55.3, 54.1 (2C). HRMS (ESI+) calcd for C13H18O5Na (M+Na+): 277.1046, Found: 277.1059. General procedure for oxidative esterification: A suspension of powdered K2CO3 (346 mg, 2.5 mmol) in dehydrated THF-MeOH (1:1 v/v, 10 mL) was degassed using three pump-thaw cycles under argon atmosphere at 0 °C. To this suspension were added 1a, 1c-g or 12 (1.0 mmol), bromotrichloromethane (296 µL, 3.0 mmol), triazolium 11A (13.2 mg, 0.050 mmol) and rhodamine 6G (24 mg, 0.050 mmol) at room temperature. The stirring reaction mixture was irradiated with white LED lamp (1000 lm) at room temperature. After being stirred for 3 hours, the reaction mixture was filtrated with folded filter paper which was then wasched with MeOH several times. The filtrate was concentrated under reduced pressure. Purification of the residue by flash silica gel column chromatography (AcOEt:hexanes = 1:10– 1:1) afforded the products 3a, 3c-g or 13. (E)-3-(4-Methoxyphenyl)-2-propenoic acid methyl ester (3a)32 Following general procedure, the reaction of 1a was carried out. Purification by flash silica gel column chromatography afforded the product 3a (190 mg, 99%) as colorless crystals. Mp 85-86 °C (hexane). IR (KBr) 2952, 1716, 1604, 1513, 1255 cm-1. 1H NMR (CDCl3) δ 7.65 (1H, d, J = 16.0 Hz), 7.47 (2H, d, J = 8.7 Hz), 6.90 (2H, d, J = 8.7 Hz), 6.30 (1H, d, J = 16.0 Hz), 3.84 (3H, s), 3.79 (3H, s). 13C NMR (CDCl3) δ 167.8, 161.4, 144.5, 129.7, 127.1, 115.3, 114.3, 55.4, 51.6. HRMS (ESI+) calcd for C11H12O3Na (M+Na+): 215.0679, Found: 215.0689. (E)-3-(4-Fluorophenyl)-2-propenoic acid methyl ester (3c)33 Following general procedure, the reaction of 1c was 17 ACS Paragon Plus Environment


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carried out. Purification by flash silica gel column chromatography afforded the product 3c (154 mg, 85%) as colorless crystals. Mp 40-42 °C (hexane). IR (KBr) 2954, 1705, 1637,1601, 1511, 1436, 1320 cm-1. 1H NMR (CDCl3) δ 7.66 (1H, d, J = 16.0 Hz), 7.51 (2H, m), 7.08 (2H, m), 6.36 (1H, d, J = 16.0 Hz), 3.81 (3H, s). 13C NMR (CDCl3) δ 167.3, 163.9 (d, J = 252 Hz), 143.6, 130.6 (d, J = 3 Hz), 129.9 (d, J = 9 Hz), 117.6 (d, J = 2 Hz), 116.0 (d, J = 22 Hz), 51.7.

19

F

NMR (CDCl3) δ −110.1 (1F, s). HRMS (ESI+) calcd for C10H10FO2 (M+H+): 181.0659, Found: 181.0657. (E)-3-Phenyl-2-propenoic acid methyl ester (3d)34 Following general procedure, the reaction of 1d was carried out. Purification by flash silica gel column chromatography afforded the product 3d (178 mg, 79%) as colorless crystals. Mp 33-35 °C (hexane).. IR (KBr) 2952, 1719, 1637, 1315, 1278 cm-1. 1H NMR (CDCl3) δ 7.70 (1H, d, J = 16.0 Hz), 7.53 (2H, m), 7.39 (3H, m), 6.45 (1H, d, J = 16.0 Hz), 3.81 (3H, s). 13C NMR (CDCl3) δ 167.4, 144.9, 134.4, 130.3, 128.9, 128.1, 117.8, 51.7. HRMS (ESI+) calcd for C10H10O2Na (M+Na+): 185.0573, Found: 185.0571. (E)-3-(4-Methylphenyl)-2-propenoic acid, methyl ester (3e)33,35 Following general procedure, the reaction of 1e was carried out. Purification by flash silica gel column chromatography afforded the product 3e (169 mg, 96%) as colorless crystals. Mp 56-57 °C (hexane). IR (KBr) 2949, 1712, 1634, 1607, 1514, 1436, 1319 cm-1. 1H NMR (CDCl3) δ 7.67 (1H, d, J = 16.0 Hz), 7.42 (2H, br d, J = 7.8 Hz), 7.19 (2H, br d, J = 7.8 Hz), 6.40 (1H, d, J = 16.0 Hz), 3.80 (3H, s), 2.37 (3H, s). 13C NMR (CDCl3) δ 167.6, 144.9, 140.7, 131.6, 129.6, 128.0, 116.7, 51.6, 21.5. HRMS (ESI+) calcd for C11H12O2Na (M+Na+): 199.0730, Found: 199.0732. (E)-3-(2-Methoxyphenyl)-2-propenoic acid methyl ester (3f)33 Following general procedure, the reaction of 1f was carried out. Purification by flash silica gel column chromatography afforded the product 3f (121 mg, 63%) as colorless oil. IR (KBr) 2950, 1714, 1631, 1490, 1437, 1249 cm-1. 1H NMR (CDCl3) δ 8.00 (1H, d, J = 16.0 Hz), 7.50 (1H, dd, J = 7.8, 1.8 Hz), 7.35 (1H, ddd, J = 8.2, 7.3, 1.8 Hz), 6.98-6.90 (2H, m), 6.53 (1H, d, J = 16.0 Hz), 3.89 (3H, s), 3.80 (3H, s). 13C NMR (CDCl3) δ 167.9, 158.3, 140.3, 131.5, 128.9, 123.3, 120.7, 118.3, 111.1, 55.4, 51.6. HRMS (ESI+) calcd for C11H12O3Na (M+Na+): 215.0679, Found: 215.0671. (E)-3-(Furan-2-yl)-2-propenoic acid methyl ester (3g)36 Following general procedure, the reaction of 1g was carried out. Purification by flash silica gel column chromatography afforded the product 3g (79 mg, 52%) as colorless oil. IR 18 ACS Paragon Plus Environment

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(KBr) 2956, 1729, 1439 cm-1. 1H NMR (CDCl3) δ 7.48 (1H, d, J = 1.8 Hz), 7.43 (1H, d, J = 15.8 Hz), 6.61 (1H, d, J = 3.7 Hz), 6.46 (1H, dd, J = 3.7, 1.8 Hz), 6.31 (1H, d, J = 15.8 Hz), 3.78 (3H, s). 13C NMR (CDCl3) δ 167.5, 150.9, 144.7, 131.2, 115.4, 114.8, 112.2, 51.7. HRMS (ESI+) calcd for C8H9O3 (M+H+): 153.0546, Found: 153.0536. (E)-2-Naphthalenecarboxylic acid methyl ester (13)37 Following general procedure, the reaction of 12 was carried out. Purification by flash silica gel column chromatography afforded the product 13 (125 mg, 67%) as colorless crystals. Mp 74-76 °C (hexane). IR (KBr) 3062, 2950, 1712, 1469, 1441, 1355, 1297 cm-1. 1H NMR (CDCl3) δ 8.62 (1H, d, J = 0.6 Hz), 8.06 (1H, dd, J = 8.5, 1.7 Hz), 7.96 (1H, dd, J = 8.2, 0.6 Hz), 7.89-7.87 (2H, m), 7.59 (1H, ddd, J = 7.9, 6.7, 1.2 Hz), 7.54 (1H, ddd, J = 8.2, 6.7, 1.2 Hz), 3.99 (3H, s). 13C NMR (CDCl3) δ 167.2, 135.5, 132.4, 131.0, 129.3, 128.2, 128.1, 127.7, 127.3, 126.6, 125.2, 52.2. HRMS (ESI+) calcd for C12H10O2Na (M+Na+): 209.0573, Found: 209.0594.

ASSOCIATED CONTENT Supporting Information The Supporting Information is available free of charge on the ACS Publications website at DOI: Assignment of syn- and anti-isomers and 1H and 13C NMR spectra (PDF)

AUTHOR INFORMATION Corresponding Author * E-mail: [email protected]

ACKNOWLEDGMENTS This work was supported by JSPS KAKENHI Grant-in-Aid for Scientific Research (C) Grant Number 16K08188 (to H.M.).

Notes and References 19 ACS Paragon Plus Environment


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(a) Modern Oxidation Methods, Bäckvall, J.-E. Ed., Wiley-VCH, Weinheim, 2004. (b) Punniyamurthy, T.; Velusamy, S.; Iqbal, J. Recent Advances in Transition Metal Catalyzed Oxidation of Organic Substrates with Molecular Oxygen. Chem. Rev. 2005, 105, 2329. (c) Matsumoto, T.; Ueno, M.; Wang, N.; Kobayashi, S. Recent Advances in Immobilized Metal Catalysts for Environmentally Benign Oxidation of Alcohols. Chem. Asian J. 2008, 3, 196. (d) Shi, Z.; Zhang, C.; Tang, C.; Jiao, N. Recent Advances in Transition-Metal Catalyzed Reactions Using Molecular Oxygen as the Oxidant. Chem. Soc. Rev. 2012, 41, 3381.

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(a) Vogler, T.; Studer, A. Applications of TEMPO in Synthesis. Synthesis 2008, 1979. (b) Wong, O. A.; Shi, Y. Organocatalytic Oxidation. Asymmetric Epoxidation of Olefins Catalyzed by Chiral Ketones and Iminium Salts. Chem. Rev. 2008, 108, 3958. (c) Ochiai, M.; Miyamoto, K. Catalytic Version of and Reuse in Hypervalent Organo-λ3- and -λ5-iodane Oxidation. Eur. J. Org. Chem. 2008, 4229. (d) Dohi, T.; Kita, Y. Hypervalent Iodine Reagents as a New Entrance to Organocatalysts. Chem. Commun. 2009, 2073. (e) Marin, M. L.; Santos-Juanes, L.; Arques, A.; Amat, A. M.; Miranda, M. A. Organic Photocatalysts for the Oxidation of Pollutants and Model Compounds. Chem. Rev. 2012, 112, 1710; (f) Iwabuchi, Y. Discovery and Exploitation of AZADO: The Highly Active Catalyst for Alcohol Oxidation. Chem. Pharm. Bull. 2013, 61, 1197.

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(a) Tucker, J. W.; Stephenson, C. R. J. Shining Light on Photoredox Catalysis: Theory and Synthetic Applications. J. Org. Chem. 2012, 77, 1617. (b) Ischay, M. A.; Yoon, T. P. Accessing the Synthetic Chemistry of Radical Ions. Eur. J. Org. Chem. 2012, 3359. (c) Shi, L.; Xia, W. Photoredox Functionalization of C-H Bonds Adjacent to a Nitrogen Atom. Chem. Soc. Rev. 2012, 41, 7687. (d) Gurry, M.; Aldabbagh, F. A New Era for Homolytic Aromatic Substitution: Replacing Bu3SnH with Efficient Light-Induced Chain Reactions. Org. Biomol. Chem. 2016, 14, 3849. (e) Kärkäs, M. D.; Porco, Jr., J. A.; Stephenson, C. R. J. Photochemical Approaches to Complex Chemotypes: Applications in Natural Product Synthesis. Chem. Rev. 2016, 116, 9683. (f) Shaw, M. H.; Twilton, J.; MacMillan, D. W. C. Photoredox Catalysis in Organic Chemistry. J. Org. Chem. 2016, 81, 6898. (g) Boubertakh, O.; Goddard, J. P. Construction and Functionalization of Heteroarenes by Use 20 ACS Paragon Plus Environment

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of Photoredox Catalysis. Eur. J. Org. Chem. 2017, 2072. 4.

(a) Ravelli, D.; Fagnoni, M.; Albini, A. Photoorganocatalysis. What for? Chem. Soc. Rev. 2013, 42, 97. (b) Hari, D. P.; König, B. Synthetic Applications of Eosin Y in Photoredox Catalysis. Chem. Commun. 2014, 50, 6688. (c) Fukuzumi, S.; Ohkubo, K. Organic Synthetic Transformations Using Organic Dyes as Photoredox Catalysts. Org. Biomol. Chem. 2014, 12, 6059. (d) Nicewicz, D. A.; Nguyen, T. M. Recent Applications of Organic Dyes as Photoredox Catalysts in Organic Synthesis. ACS Catal. 2014, 4, 355. (e) Romero, N. A.; Nicewicz, D. A. Organic Photoredox Catalysis. Chem. Rev. 2016, 116, 10075.

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For selective examples for eosin Y, see: (a) Hari, D. P.; Schroll, P.; König, B. Metal-Free, Visible-Light-Mediated Direct C–H Arylation of Heteroarenes with Aryl Diazonium Salts. J. Am. Chem. Soc. 2012, 134, 2958. (b) Meng, Q.-Y.; Zhong, J.-J.; Liu, Q.; Gao, X.-W.; Zhang, H.-H.; Lei, T.; Li, Z.-J.; Feng, K.; Chen, B.; Tung, C.-H; Wu, L.-Z. A Cascade Cross-Coupling Hydrogen Evolution Reaction by Visible Light Catalysis. J. Am. Chem. Soc. 2013, 135, 19052. (c) Majek, M.; Jacobi von Wangelin, A. Metal-Free Carbonylations by Photoredox Catalysis. Angew. Chem. Int. Ed. 2015, 54, 2270. (d) Davies, J.; Booth, S. G.; Essafi, S.; Dryfe, R. A. W.; Leonori, D. Visible‐Light‐Mediated Generation of Nitrogen‐Centered Radicals: Metal‐Free Hydroimination and Iminohydroxylation Cyclization Reactions. Angew. Chem. Int. Ed. 2015, 54, 14017. (e) Majek, M.; Jacobi von Wangelin, A. Mechanistic Perspectives on Organic Photoredox Catalysis for Aromatic Substitutions. Acc. Chem. Res. 2016, 49, 2316.

6.

For a selective example for fluorescein, see: Guo, W.; Lu, L.-Q.; Wang, Y.; Wang, Y.-N.; Chen, J.-R.; Xiao, W.-J. Metal‐Free, Room‐Temperature, Radical Alkoxycarbonylation of Aryldiazonium Salts through Visible‐Light Photoredox Catalysis. Angew. Chem. Int. Ed. 2015, 54, 2265.

7.

For selective examples for methylene blue, see: (a) Pitre, S. P.; McTiernan, C. D.; Ismaili, H.; Scaiano, J. C. Metal-Free Photocatalytic Radical Trifluoromethylation Utilizing Methylene Blue and Visible Light Irradiation. ACS Catal. 2014, 4, 2530. (b) Fujiya, A.; Tanaka, M.; Yamaguchi, E.; Tada, N.; Itoh, A. Sequential Photo-oxidative

[3+2]

Cycloaddition/Oxidative

Aromatization


Reactions

for

the

Synthesis

of


Page 22 of 26

Pyrrolo[2,1-a]isoquinolines Using Molecular Oxygen as the Terminal Oxidant. J. Org. Chem. 2016, 81, 7262. 8.

For selective examples for 9-Mes-10-Me-acrydinium, see: (a) Fukuzumi, S.; Kotani, H.; Ohkubo, K.; Ogo, S.; Tkachenko, N. V.; Lemmetyinen, H. Electron-Transfer State of 9-Mesityl-10-methylacridinium Ion with a Much Longer Lifetime and Higher Energy Than That of the Natural Photosynthetic Reaction Center. J. Am. Chem. Soc. 2004, 126, 1600. (b) Hoshino, M.; Uekusa, H.; Tomita, A.; Koshihara, S.; Sato, T.; Nozawa, S.; Adachi, S.; Ohkubo, K.; Kotani, H.; Fukuzumi, S. Determination of the Structural Features of a Long-Lived Electron-Transfer State of 9-Mesityl-10-methylacridinium Ion. J. Am. Chem. Soc. 2012, 134, 4569. (c) Hamilton, D. S.; Nicewicz, D. A. Direct Catalytic Anti-Markovnikov Hydroetherification of Alkenols. J. Am. Chem. Soc. 2012, 134, 18577. (d) Nguyen, T. M.; Manohar, N.; Nicewicz, D. A. anti-Markovnikov Hydroamination of Alkenes Catalyzed by a Two-Component Organic Photoredox System: Direct Access to Phenethylamine Derivatives. Angew. Chem. Int. Ed. 2014, 53, 6198. (e) Perkowski, A. J.; Cruz, C. L.; Nicewicz, D. A. Ambient-Temperature Newman–Kwart Rearrangement Mediated by Organic Photoredox Catalysis. J. Am. Chem. Soc. 2015, 137, 15684. (f) Kato, S.; Saga, Y.; Kojima, M.; Fuse, H.; Matsunaga, S.; Fukatsu, A.; Kondo, M.; Masaoka, S.; Kanai, M. Hybrid Catalysis Enabling Room-Temperature Hydrogen Gas Release from N‑Heterocycles and Tetrahydronaphthalenes. J. Am. Chem. Soc. 2017, 139, 2204.

9.

For selective examples for rhodamine 6G, see: (a) Graml, A.; Ghosh, I.; König, B. Synthesis of Arylated Nucleobases by Visible Light Photoredox Catalysis. J. Org. Chem. 2017, 82, 3552. (b) Meyer, A. U.; Slanina, T.; Heckel, A.; König, B. Lanthanide Ions Coupled with Photoinduced Electron Transfer Generate Strong Reduction Potentials from Visible Light. Chem. Eur. J. 2017, 23, 7900.

10.

For selective examples for other organophotocatalysts, see: (a) Zhu, X.; Xie, X.; Li, P.; Guo, J.; Wang, L. Visible-Light-Induced Direct Thiolation at α-C(sp3)–H of Ethers with Disulfides Using Acridine Red as Photocatalyst. Org. Lett. 2016, 18, 1546. (b) Kalaitzakis, D.; Triantafyllakis, M.; Ioannou, G. I.; Vassilikogiannakis, G. One‐Pot Transformation of Simple Furans into Octahydroindole Scaffolds. Angew. Chem. Int. Ed. 2017, 56, 4020. (c) Qiao, H.; Sun, S.; Yang, F.; Zhu, Y.; Kang, J.; Wu, Y.; Wu, Y. Merging 22 ACS Paragon Plus Environment

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Photoredox Catalysis with Iron(III) Catalysis: C5‐H Bromination and Iodination of 8‐Aminoquinoline Amides in Water. Adv. Synth. Catal. 2017, 359, 1976. 11.

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Cwiertny, D. M.; Arnold, W. A.; Kohn, T.; Rodenburg, L. A.; Roberts, A. L. Reactivity of Alkyl Polyhalides toward Granular Iron: Development of QSARs and Reactivity Cross Correlations for Reductive Dehalogenation. Environ. Sci. Tech. 2010, 44, 7928; adjusted by subtracting 0.24 V from the original value of +0.454 V vs. standard hydrogen electrode (SHE).

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Benniston, A. C.; Harriman, A.; Li, P.; Rostron, J. P.; van Ramesdonk, H. J.; Groeneveld, M. M.; Zhang, H.; 23 ACS Paragon Plus Environment


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