(Diarylamino)anthracene Catalyst for Metal-Free Visible-Light

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Strongly Reducing (Diarylamino)anthracene Catalyst for Metal-Free Visible-Light Photocatalytic Fluoroalkylation Naoki Noto, Yuya Tanaka, Takashi Koike, and Munetaka Akita ACS Catal., Just Accepted Manuscript • DOI: 10.1021/acscatal.8b02885 • Publication Date (Web): 30 Aug 2018 Downloaded from http://pubs.acs.org on August 30, 2018

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ACS Catalysis

Strongly Reducing (Diarylamino)anthracene Catalyst for Metal-Free Visible-Light Photocatalytic Fluoroalkylation Naoki Noto, Yuya Tanaka, Takashi Koike,* Munetaka Akita* †‡

†‡

†‡

†‡

†Laboratory for Chemistry and Life Science, Institute of Innovative Research ‡School of Materials and Chemical Technology Tokyo Institute of Technology, R1-27, 4259 Nagatsuta-cho, Midori-ku, Yokohama, 226-8503, Japan.

ABSTRACT: Well-defined 9,10-bis(di(p-tert-butylphenyl)amino)anthracene serves as a photocatalyst for radical fluoroalkylation under visible light irradiation. The diarylamine (Donor)–anthracene (p conjugated system)–diarylamine (Donor) scaffolds are easily accessed by typical palladium-catalyzed cross-coupling protocols of the corresponding halogenated anthracenes with various lithium diarylamides. The anthracene-based photocatalyst exhibits high reducing power, leading to generation of versatile fluoroalkyl radicals such as tri- and di-fluoroethyl and tri- and di-fluoromethyl radicals from the corresponding electron-accepting precursors. Catalyst design strongly influences absorption capability of visible light and stability toward redox stimuli. The detailed mechanistic studies on the metal-free photocatalytic amino-trifluoroethylation of styrene with diphenyl(2,2,2-trifluoroethyl)sulfonium trifluoromethanesulfonate suggest that the reaction proceeds via catalytic radical processes rather than radical chain processes. In addition, static quenching process is involved in the first single-electrontransfer (SET) process from the photocatalyst to the fluoroalkylating reagent. Furthermore, the 1eoxidized cationic radical species of 9,10-bis(di(p-tert-butylphenyl)amino)anthracene, a key active ACS Paragon Plus Environment

ACS Catalysis 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

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catalytic species with long lifetime and a characteristic IVCT (Intervalence Charge Transfer) band in the near IR (NIR) region, is detected. From the viewpoint of elemental strategy initiative and green chemistry, the present noble metal-free organic photocatalytic system provides a pivotal technology to replace ruthenium- and iridium-based metal photocatalysis.

Keywords: photoredox catalysis, organocatalysis, radical reaction, fluoroalkylation, alkene

1. INTRODUCTION In recent years, photoredox catalysis with noble metal polypyridine complexes and organic dyes such as [Ru(bpy) ] , fac-[Ir(ppy) ] (bpy: 2,2’-bipyridine, ppy: 2-pyridylphenyl) and eosin Y, has become a useful 3

2+

3

strategy for synthetic radical chemistry because they catalyze single-electron-transfer (SET) processes under mild reaction conditions: under visible light irradiation at ambient temperature. To design new 1

photoredox reactions, considering redox potentials of three key intermediates is one of the important routines: (i) the photoexcited species, (ii) electron/hole-activated species formed after the first SET processes of the photocatalyst, which can serve either as a 1e-reductant or as a 1e-oxidant in a subsequent process, and (iii) substrates. As a matter of course, development of stronger reducing or oxidizing photocatalysts has attracted a great interest to expand dimensions of photocatalytic reactions. In particular, organic photocatalytic systems, which show strong reducing power, have emerged in the last few years (Figure 1). For example, in 2014, König et al. revealed conPET (consecutive photoinduced electron 2,3

transfer) by a combination of perylene diimides (PDI) and a sacrificial electron donor. Almost at the 2a

same time, the group of Hawker and Fors reported that 10-phenylphenothiazine (PTH) serves as a strong reductant under near UV irradiation.

2b

ACS Paragon Plus Environment

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ACS Catalysis E1/2(PC+/*PC) (V vs. Cp2Fe)a N N

–1.0

N RuII

N

tBu

N

N

N

N

IrIII N N

[Ru(bpy)3]2+: –1.22 V λabs: 452 nm (13)b

tBu

[Ir(ppy)2(dtbbpy)]+: –1.37 V λabs: 415 nm (4.8)b

–1.5 HO2C

Br

N

Br

O

O

IrIII

OH

N

Br Br eosin Y: –1.52 V λabs: 539 nm (60)b

N

fac-[Ir(ppy)3]: –2.14 V λabs: 375 nm (7.2)b

–2.0 O

O

Ar N

N Ar

O

O PDI: –2.14 Vc λabs: 523 nm (86)b

–2.5

pyrene: –2.51 Vd λabs: 334 nm (45)b

Ph N X

X = S, PTH: –2.51 V, λabs: 320 nm (3.2)e X = O, POX: –2.52 V, λabs: 324 nm (7.7)e X = NPh, PAZ: –2.75 V, λabs: 369 nm (6.1)e

Figure 1. Oxidation Potentials of the Photoexcited Catalysts and Absorption Bands in the Ground State.

3

a

Potentials are values reported in the literatures. [E V vs. Cp Fe] = [E V vs. SCE] –0.41 V. Lower-energy b

2

absorption band and its molar extinction coefficient (value in parenthesis: e x 10 M cm ) in MeCN are –3

–1

–1

shown. E (PC/*PC· ). E (PC/PC· ). Lower-energy absorption band and its molar extinction coefficient c

1/2

–

d

–

1/2

e

(value in parenthesis: e x 10 M cm ) in DMA are shown. –3

–1

–1

More recently, the group of Miyake has developed a range of N,N-diphenyldihydrophenazine (PAZ) and 10-phenylphenoxazine (POX) derivatives. Although absorption bands of PAZ and POX are also located 2e,f

in the UV region, their latest works reported that structural modification extends their absorption band to ACS Paragon Plus Environment

ACS Catalysis

visible light region.

2g,h

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

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Furthermore, the group of König showed an interesting concept, SenI-ET

(sensitization-initiated electron transfer), by an assemble of pyrene, [Ru(bpy) ] , and a sacrificial electron 2+

3

donor. However, exploration of new design and concept for organic photocatalysts, which exhibit high 2i

reducing power by themselves under visible light irradiation without a sacrificial electron donor and a cosensitizer, is still a frontline topic to construct versatile noble metal-free photocatalytic system as of now. It is because utilization of lower-energy visible light and noble metal-free catalytic system are useful in terms of energy-saving and safe processes. (a) our previous work Me Me 10 mol % PAH photocat.

+ Ph

CD3CN/D2O Ph rt, 3 h 425 nm blue LEDs via [·CF2H]

S BF4 Me CF2H

Me

PAH photocat:

perylene (λabs: 434 nma) 96% yield

pyrene (λabs: 334 nma) 0% yield

anthracene (λabs: 376 nma) 0% yield

Me 9,10-dimethylanthracene (λabs: 398 nma) 34% yield

1e-reduction Ar

Ar Ar

CF2H

Me

(b) this work

N

NDCOCD3

Ar

RF

N Ar

Ar N

N

Ar

visible light

D–π–D photocatalyst (1)

Ar

+

RF

[D–π–D]·+ 1e-oxidant 1e-oxidation X

RF S CH2CF2X X = F: 3a X = H: 3b

X

S CF3 X = F: 3c X = H: 3d

Me Me

Me F3C I O

S Me CF2H 3e O

CF3SO2Cl 3f 3g

Scheme 1. PAH Photocatalyst for Fluoroalkylation +

R = electron-accepting fluoroalkylating reagents. Absorption band located near visible-light region. a

F


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ACS Catalysis

Recently, we reported that visible-light perylene photocatalysis is useful for catalytic difluoromethylation (Scheme 1(a)). That work suggests that well-designed polyaromatic hydrocarbons 4

(PAHs) can be promising candidates for metal-free photocatalysts with high reducing power. Their 5

extended p-systems enable them to donate a single electron readily, and the resultant radical cations serve as hole-mediators. These results encouraged us to design modulable PAH-based photoredox catalysts, 6

which absorb visible light. Substituted anthracenes have been well-studied in various fields such as organic functional materials

7

and supramolecular chemistry because anthracene is a robust and fluorescent molecule and easy to be 8

functionalized, especially at the 9,10-positions. Among them, we paid our attention to 9,10bis(diarylamino)anthracenes (1) (Scheme 1(b)). The group of Konishi reported that interchromophoric conjugation based on distorted diarylamino units allows efficient internal charge-transfer (CT) transitions, leading to strong absorption bands in the visible light region with high quantum yields F of fluorescence.

9

Meanwhile, the Donor–p conjugated system–Donor (D–p–D) scaffolds often show unique two-stage redox properties triggered by consecutive 1e-oxidation. The group of Ito reported on 1e-oxidation of 10

9,10-bis(di(p-anisyl)amino)anthracene induces formation of the mixed-valence species, i.e., a delocalized cationic radical species, [D–p–D]· . We expected that the photoexcited D–p–D system undergoes SET + 10c

to electron acceptors such as electron-accepting fluoroalkylating reagents ( R ) to generate hole-activated +

F

[D–p–D]· and fluoroalkyl radicals (·R ). Because triarylaminium salts are known as strong 1e-oxidants, +

11

F

the cationic radical species of 1 is also anticipated to show 1e-oxidizing ability, associated with regeneration

of

1

in

the

ground

state

(Scheme

1(b)).

As

mentioned

above,

9,10-

bis(diarylamino)anthracenes (1) are latent organic photoredox catalysts with high reducing power. However, to the best of our knowledge, application of 1 to photocatalysis has not been documented.

12

Herein we will discuss (diarylamino)anthracenes as photoredox catalysts. Fluorinated units containing tri- and di-fluoromethyl groups can deliver unique biological properties to organic molecules. Therefore, development of new catalytic methodologies for introduction of 13

fluoromethyl groups has become an important topic in synthetic organic chemistry. Recently, photoredox 14


ACS Catalysis

Page 6 of 44

catalysis has proven to be a powerful tool for radical fluoromethylation. In particular, fluoromethylative 15

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

difunctionalization of alkenes by photoredox catalysis allows us to access various organofluorine compounds containing fluoromethyl groups. In most of the reactions, a combination of Ru- and Ir-based complexes and electron-accepting fluoroalkylating reagents is still a leading system. However, from the 2g,16

viewpoint of elemental strategy initiative as well as safe and economical synthesis of drugs, use of 17

organic photoredox catalysts instead of noble metal-based catalysts is demanded. To develop versatile 18

metal-free photocatalytic system for fluoroalkylation, visible light organic photocatalysts are required to possess high reducing power in the absence of sacrificial reducing reagents such as tertiary amines because (i) decrease of the number of fluorine atoms as well as (ii) location of fluorine atoms with respect to the assumed radical center causes reduction of oxidation potentials and influences the stability of the resultant fluoroalkylating reagents. In the present article, we will reveal 9,10-bis(diarylamino)anthracene (D–p–D system (1)) serves as a good photocatalyst for catalytic tri- and di-fluoroethylation and tri- and di-fluoromethylation when combined with the corresponding electron-accepting fluoroalkylating reagents. In addition, the detailed mechanistic studies suggest that the present radical trifluoroethylation does not follow (i) radical chain processes but catalytic radical processes involving (ii) static quenching in the first SET process and (iii) [D–p–D]· species. The D–p–D system 1 is promising as an alternative to noble +

metal photocatalysts including the above-mentioned Ru- and Ir-based catalysts in radical fluoroalkylation 19

with various electron-accepting fluoroalkylating reagents via oxidative quenching manner.

2. RESULTS AND DISCUSSION 2.1. Synthesis and Structure of (Diarylamino)anthracene Photocatalysts. Several (diarylamino)anthracenes (1 and 2a) were prepared by the modified procedures of the methods reported previously.

7b–d,

20

Bromoanthracenes (A and B) underwent Pd-catalyzed amination with

diarylamines (C; R = Bu (1a), OMe (1b), H (1c)) to afford the corresponding (diarylamino)anthracenes t

in 73% (1a), 84% (1b), 82% (1c), and 59% (2a) yields (Scheme 2(a)). 7d

7c

7b


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ACS Catalysis (a)

R R

R

R N

Br

N

N H

Br

C 1 mol % Pd2(dba)3 2 mol % RuPhos 1.1-2.5 equiv LiHMDS

A

R R R = tBu (1a; 73%), OMe (1b; 84%), H (1c; 82%) tBu

1,4-dioxane, rt 100 ˚C, 48 h Br

N

B

2a (59%) tBu

(b)

C7’ C18’

C8

N1

C1’ C1

N1’

N1

C7

N1’

C8’

C18

a front view of 1a

a top view of 1a

Scheme 2. Synthesis of (Diarylamino)anthracenes and Molecular Structure of 1a (a) Synthetic scheme for 1 and 2a. (b) ORTEP drawings. The thermal ellipsoids are set at a 50% probability level.

Recrystallization of 9,10-bis(di(p-tert-butylphenyl)amino)anthracene (1a) afforded yellow crystals, which were suitable for single-crystal X-ray structure analysis. The ORTEP views shown in Scheme 2(b) 21

confirm that (i) a torsion angle of C8–N1–C1–C7 is 79.64˚, indicating that the aryl substituents on the amino group are arranged almost perpendicular with respect to the anthracene panel and (ii) the two ptert-butylphenyl groups are twisted around the N1–C8 and N1–C18 bonds. These results are in accord with the previously reported results.

7,9

2.2. Photo- and Electro-chemical Properties of (Diarylamino)anthracene Photocatalysts (1 and 2). 2.2.1. UV-vis absorption spectra. ACS Paragon Plus Environment

ACS Catalysis

Absorption spectra of the obtained (diarylamino)anthracenes (1 and 2a) and anthracene observed in CH Cl are compared in Figure 2. Anthracene does not have an absorption band in the visible light region. 2

2

In contrast, (diarylamino)anthracenes (1 and 2a) exhibit broad and intense absorptions over the visible light region (400–550 nm). Absorptions of bis(diarylamino)anthracenes (1) are broader and stronger than those of mono(diarylamino)anthracene (2a). These results suggest that (diarylamino)anthracenes can efficiently utilize visible light for photoreaction.

10

ε x 10–3 (M–1 cm–1)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 8 of 44

1c

1a 1b

5 2a

0 300

500 400 wavelength (nm)

600

Figure 2. UV-vis Absorption Spectra of 1 and 2a. Measurements were carried out in CH Cl (1.0 x 10 2

–4

2

M) under air at room temperature.

2.2.2. Fluorescence spectra, quantum yields and fluorescence decay. Normalized fluorescence spectra observed in CH Cl at room temperature under N atmosphere are 2

2

2

shown in Figure 3(a). All spectra of (diarylamino)anthracenes (1 and 2a) exhibit broad peaks in contrast to the three sharp spikes of anthracene around 400 nm. The red shift (616 nm) observed for bis(panisylamino)anthracene (1b) is remarkable when compared with the others (1a: 561, 1c: 529, 2a: 549 nm). In addition, fluorescence quantum yield of 1b is even lower (18%) than those of the other anthracene derivatives (1a: 81, 1c: 90, 2: 59%) (Table 1). It should be noted that quantum yields of 1a, 1c and 2a are ACS Paragon Plus Environment

Page 9 of 44

significantly improved compared with that of anthracene (11%). Furthermore, fluorescence decays of anthracene derivatives (1a: 31, 1b: 11, 1c: 23, 2a: 45 ns) are longer than that of anthracene (6 ns) (Figure 3(b)). These results suggest that, upon visible light irradiation, (diarylamino)anthracenes (1 and 2a) generate the excited states, which allow opportunity for electron transfer, while the electron-donating MeO group has a harmful effect on the excited state lifetime. (a) 1c 2a 1a 1b

normalized intensity

1

0.5

0 360

560 wavelength (nm)

760

1

(b)

normalized intensity

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

ACS Catalysis

1b 0.5

1c 1a 2a

0

0

100

time (ns)

200

300

Figure 3. Fluorescence Properties (a) Fluorescence Spectra and (b) Fluorescence Decay of 1 and 2a. Fluorescence spectra (excited at 410 nm for 1a, 1c, and 2a, at 430 nm for 1b, and at 330 nm for anthracene) were measured in CH Cl (1.0 x 10 M) under N . Measurements of fluorescence decay (excited at 355 nm) –5

2

2

2

were carried out for CH Cl solutions (1.0 x 10 M) under N . –4

2

2

2

2.2.3. Cyclic voltammograms and differential pulse voltammograms. ACS Paragon Plus Environment

ACS Catalysis 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

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Irreversible redox waves are observed in the CV traces for anthracene (+0.87 V vs. Cp Fe obtained from 2

DPV), 2a (+0.44 V vs. Cp Fe obtained from DPV), and 1c (+0.40 V vs. Cp Fe obtained from DPV) (Figure 2

2

4). In contrast, 1a (E = +0.32 V vs. Cp Fe) and 1b exhibit reversible wave(s) and, especially, in the DPV 1/2ox

2

and CV charts of 1b, two-stage 1e-redox processes are clearly observed (+0.21 and +0.29 V vs. Cp Fe). 2

As reported previously, two peaks are assigned to the stepwise redox processes associated with neutral 1b, monocationic radical [1b]· and dicationic [1b] species. These results show that the HOMO levels +

2+

10

of (diarylamino)anthracenes (1 and 2a) are higher than that of anthracene, and the 9,10-substituents of the anthracene skeleton and the p-substituent of the arylamino group are essential to gain stabilization against redox stimuli. The selected data obtained by photo- and electro-chemical analysis are summarized in 22

Table 1. Then, reduction power in the excited state (E* ) are estimated from the E values obtained from ox

0,0

the fluorescence spectra and redox potentials (E ) (1a: –1.88 V, 1b: –1.80 V, 1c: –1.94 V, 2a: –1.82 V).

18d

ox

Their reducing power under visible light irradiation are stronger than those of the Ir photocatalyst, [Ir(ppy) (dtbbpy)] , and eosin Y, which is widely used as an organic photocatalyst under visible light +

2

irradiation (Figure 1).

Table 1. Selected Photo- and Electro-chemical Data of 1 and 2a l (nm) (e x 10 M cm )

l (nm)

t (ns)

F (%)

E (V)

E

anthracene 341, 359, 378 (6.7)

382,403, 426

6

11

-

+0.87

-

1a

471 (7.1)

561

31

81

+0.32

+0.33

–1.88

1b

491 (7.1)

616

11

18

-

e

+0.21/+0.29 –1.80

1c

455 (7.8)

529

23

90

-

f

+0.40

–1.94

2a

431 (3.4)

549

45

59

-

f

+0.44

–1.82

a abs

–3

–1

b em

–1

c

d 1/2,ox

c

dpv,ox

d

(V)

E* (V)

ox

d

CH Cl solutions (1 x 10 M), under air at room temperature. CH Cl solutions (1 x 10 M), under N at room temperature. Excited at 410 nm for 1a, 1c, and 2a, at 430 nm for 1b, and at 330 nm for anthracene. CH Cl solutions (1 x 10 M), under N at room temperature. Excited at 355 nm. Ferrocene, Cp Fe, was used as a reference. Two reversible waves were observed. Irreversible wave was observed. a

c

–4

2

2

2

2

b

–5

2

-4

e

2

2

d

2

f


2

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ACS Catalysis

2a

20 µA

1c

+0.32 1a

+0.29 +0.21

1b

–0.5

–1

0 0.5 1 potentials vs. Cp2Fe (V)

1.5

Figure 4. Cyclic Voltammograms of 1 and 2a. Measurements were performed for CH Cl solutions 2

2

([sample] = 1.0 mM, [(NBu )PF ] = 0.10 M) with platinum disk (working electrode) and wire 4

6

electrodes (counter electrode), and a Ag/AgNO reference electrode under N at room temperature. 3

2

The scan rate was 100 mV/s. Ferrocene (Cp Fe) was used as a reference. 2

2.3. Photocatalytic Introduction of Fluorinated Groups. 2.3.1. Photocatalytic trifluoroethylation of alkenes accompanying Ritter amination. So far, we have extensively developed photocatalytic trifluoromethylation of alkenes with electrophilic CF reagents such as Umemoto reagent (3d) and Togni reagent (3g) through redox23

24

3

neutral processes, i.e., without sacrificial reducing or oxidizing reagents. The method allows us 25

to access a variety of vicinally CF -substituted alcohols, amines and ketones. In particular, 3


11

ACS Catalysis 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 12 of 44

sulfonium-based fluoromethylating reagents serve as good fluoromethyl radical precursors when triggered by 1e-reduction by the photoexcited species. Then we paid our attention to development of

a

strategy

for

CF -homologation

26

3

because

diphenyl(2,2,2-trifluoroethyl)sulfonium

trifluoromethansulfonate (3a) is known as a bench-stable and easy-to-handle chemical. However 27

the reagent 3a (–1.60 V vs Cp Fe) requires a reducing photocatalyst stronger than that for the 2

analogous

CF

3

reagent

(3d:

–0.75

V) .

Then,

25a

we

expected

that

the

obtained

(diarylamino)anthracenes (1 and 2a) induce photocatalytic trifluoroethylation of styrene (4a) with 3a under visible light irradiation because the photoexcited species of 1 and 2a would be able to reduce 3a according to their photophysical data mentioned above (Table 1). Based on our previous works, the reaction in a mixed solvent system, dichloromethane and 25b

acetonitrile (4:1), containing a small amount of water, should result in trifluoroethylation accompanying Ritter amination. We commenced the reaction with 9,10-bis(di(p-tertbutylphenyl)amino)anthracene catalyst (1a) (3 mol %) under 425 nm blue LEDs irradiation for 1 h. To our delight, the amino-trifluoroethylated product (5aa-d ) was obtained in a 78% NMR yield 4

as a single regioisomer (entry 1 in Table 2). Next, 9-(di(p-tert-butylphenyl)amino)anthracene (2a) was tested as a photocatalyst. The product (5aa-d ) was obtained but in a lower yield (23%) (entry 4

2). Further exploration of other photocatalysts such as fac-[Ir(ppy) ], perylene, eosin Y, and 3

anthracene (entries 3–6) revealed that 1a was the best photocatalyst among them. According to the absorption spectra (Figure 2 and Table 1), absorption bands of 1 are located beyond 450 nm. Thus, we utilized 470 nm blue light as a light source, resulting in a better yield (86%) (entry 7). While other 9,10-bis(diarylamino)anthracene catalysts (1b and 1c) were also tested, 1a turned out to be the best photocatalyst (entries 8 and 9). In addition, the present reaction did not proceed at all in the dark or in the absence of photocatalyst (entries 10 and 11). These results with respect to 1 and


12

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ACS Catalysis

2a suggest that their stability toward redox stimuli and the large quantum yields might influence photocatalytic activity of the anthracene photocatalysts.

Table 2. Optimization of Photocatalytic Amino-Trifluoroethylation of 4a

a

OTf

S CH2CF3 3a

3 mol % photocat. 1.5 equiv D2O

+ Ph 1.5:1 4a

NDCOCD3 Ph

CD2Cl2/CD3CN rt, 1h LEDs

Entry Photocat.

LEDs

1

1a

425 nm 78

2

2a

425 nm 23

3

fac-[Ir(ppy) ] 425 nm 43

4

perylene

425 nm 63

5

eosin Y

425 nm 0

6

anthracene

380 nm 22

7

1a

470 nm 86

8

1b

470 nm 11

9

1c

470 nm 74

10

-

470 nm 0

11

1a

-

CH2CF3 5aa-d4

NMR yields (%)

b

3

0

See the Supporting Information. Yields were determined by H NMR spectroscopy using SiEt as an internal standard. LED = light-emitting diode. a

b

1

4

The trifluoroethyl source also plays an important role in the present reaction. The reactions with other electron-accepting trifluoroethyl sources such as CF CH I (3a’) and CF CH SO Cl (3a”) were 3

2

3

2

2

examined otherwise under the ideal conditions (Scheme 3). As a result, 4a was converted slightly,


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Page 14 of 44

but 5aa-d was not formed at all. These results indicate that use of 3a as a trifluoroethyl source is 4

crucial for the present CF -homologation. 3

CF3CH2 reagent

+ Ph 1.5:1 4a

3 CF3CH2I (3a’) CF3CH2SO2Cl (3a”)

3 mol % 1a 1.5 equiv D2O CD2Cl2/CD3CN rt, 1h 470 nm blue LEDs

NDCOCD3 Ph CH2CF3 5aa-d4 not observed

Scheme 3. Reactions with Other CF CH Sources 3

2

Next, we investigated the scope of the present amino-trifluoroethylation (Table 3). The reaction could be applied to styrene derivatives with H (4a), Me (4b) and Ph (4c) groups (5aa : 77%, 5ab: 28

74%, 5ac: 73% yield) (entries 1–3). Halogens, F (4d), Cl (4e), Br (4f) (entries 4–6), ester, OAc (4g) (entry 7), Bpin (4h) (entry 8), and TMS (4i) (entry 9) groups are compatible with the reaction. The corresponding products were obtained in good yields (5ad: 73%, 5ae: 70%, 5af: 63%, 5ag: 50%, 5ah: 55%, 5ai: 63%). In addition, more structurally complex estrone derivative (4j) (entry 10) afforded the amino-trifluoroethylated product (5aj) in a 32% yield. These results show that the present photocatalytic system is highly compatible with various functional groups. Furthermore, application to the gram-scale reaction was studied (Scheme 4). The reaction of 4a with 3a in the 29

presence of 1 mol % of 1a gave 1.23 g of the product (5aa) in a 67% yield, although a longer reaction time (60 h) was required. OTf + Ph S 1.2:1 4a CH2CF3 3a

1 mol % 1a 1.5 equiv H2O CH2Cl2/MeCN rt, 60 h 470 nm blue LEDs

NHAc Ph CH2CF3 5aa 1.23 g, 67% yield

Scheme 4. Gram-scale Reaction of 4a with 3a


14

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ACS Catalysis

Table 3. Scope of the Present Photocatalytic Amino-Trifluoroethylation

a

OTf + Ar 1.5:1 4

S CH2CF3 3a

Entry Substrate 4

3 mol % 1a 1.5 equiv H2O CH2Cl2/MeCN rt, 3 h 470 nm blue LEDs

NHAc Ar CH2CF3 5a

Product 5a NHAc

R

CH2CF3

R

1

4a: R = H

5aa : 77%

2

4b: R = Me

5ab: 74%

3

4c: R = Ph

5ac: 73%

4

4d: R = F

5ad: 73%

5

4e: R = Cl

5ae: 70%

6

4f: R = Br

5af: 63%

7

4g: R = OAc

5ag: 50%

8

4h: R = Bpin

5ah: 55%

9

4i: R = TMS

5ai: 63%

b

28

NHAc H

10

H

H

H O

4j

H

CH2CF3

H

Me

O

Me

5aj: 32%

See the Supporting Information. Isolated yields. Reaction time = 6 h. Bpin = pinacol boronic acid ester, TMS = trimethylsilyl. a

b

2.3.2. Photocatalytic oxy-trifluoroethylation of alkenes.


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Page 16 of 44

We extended the reaction system to photocatalytic trifluoroethylation of alkenes accompanying oxy-functionalization (Scheme 5). The reaction of 4c in a mixed solvent system, CH Cl / PrOH i

2

2

(9:1), afforded the oxy-trifluoroethylated product (6ac) in an 86% isolated yield (Scheme 5(a)).

25a

Reactions of 4-phenyl-4-pentenoic acid (4k) and N-[2-(propen-2-yl)phenyl]benzamide (4l) proceeded to give a lactone (6ak: 79%) and a benzoxazine (6al: 52%), respectively, bearing a CF CH group through intramolecular oxy-trifluoroethylation (Scheme 5 (b) and 5(c) ). 3

25c

2

Ph

4c 3 mol % 1a

OiPr

CH2Cl2/iPrOH (9/1) rt, 4 h 470 nm blue LEDs

(a)

CH2CF3

Ph

6ac: 86% yield O

OTf

S CH2CF3 3a (c)

30

OH Ph 4k (b) 3 mol % 1a 1.5 equiv 2,6-di-tbutyl-pyridine CH2Cl2, rt, 2 h 470 nm blue LEDs

O O Ph CH2CF3 6ak: 79% yield

Me NH

4l O Ph 3 mol % 1a 1.5 equiv 2,6-di-tbutyl-pyridine CH2Cl2, rt, 24 h 470 nm blue LEDs

Me O

CH2CF3

N Ph 6al: 52% yield

Scheme 5. Photocatalytic Oxy-Trifluoroethylation For detailed reaction conditions, see the Supporting Information.

2.3.3. Scope of electron-accepting fluoroalkylating reagents.


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ACS Catalysis

electron-accepting fluoroalkylating reagent (3) F

OTf S CH2CF2H 3b Eirr = –1.91 Va

Me Me

F

OTf S CF3 3c Eirr = –0.80 Va

BF4

S Me CF2H 3e Eirr = –1.74 Va

Me

F3C I O O

CF3SO2Cl 3f Eirr = –0.59 Vb

3g Eirr = –1.34 Vb

(a) intramolecular oxy-difluoroethylation

O

O

3 mol % 1a O 1.5 equiv 2,6-di-tbutylpyridine Ph Ph CH2Cl2, rt, 6 h 4k CH2CF2H 1.5:1 470 nm blue LEDs 6bk: 60% yield (b) amino-difluoromethylation 3 mol % 1a NHAc 1.5 equiv H2O + 3e Ph Ph CH2Cl2/MeCN, rt, 3 h 4a 1.5:1 CF2H 470 nm blue LEDs 5ea: 66% yield (c) intramolecular oxy-trifluoromethylation O 3b

+

3c

+

OH

O Ph 1.5:1

3 mol % 1a 1.5 equiv 2,6-di-tbutylpyridine CH2Cl2, rt, 2 h 470 nm blue LEDs

OH 4k

O Ph CF3 6ck: 89% yield

(d) chloro-trifluoromethylation 3f

+ 1.5:1 TsHN

Cl

O

3 mol % 1a O 4m

CF O 3

CH2Cl2, rt, 12 h 470 nm blue LEDs TsHN O 7fm: 68% yield

(e) keto-trifluoromethylation Me 3g

+ 1.5:1 MeO

4n

O 3 mol % 1a

Me

ClCH2CH2Cl/DMSO CF3 rt, 3 h MeO 470 nm blue LEDs 8gn: 75% yield

Scheme 6. Scope of Electron-accepting Fluoroalkylating Reagents For detailed reaction conditions, see the Supporting Information. Potentials (vs. Cp Fe in CH Cl ) were determined by CV traces as described in the Supporting Information. Reported values. a

2

b

2

2

25a,31


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Page 18 of 44

Prospects for successful catalytic performance of 1a prompted us to test various fluoroalkylation with electron-accepting fluoroalkylating reagents (3) (Scheme 6). The CF H-homologating reagent 2

corresponding to 3a, diphenyl(2,2-difluoroethyl)sulfonium trifluoromethansulfonate (3b) , which 32

exhibit an oxidation potential (E = –1.91 V vs Cp Fe) even further lower than 3a, could also be irr

2

applied to the present metal-free photocatalytic reaction with the photocatalyst (1a). The lactone bearing a CF HCH group (6bk) was obtained from the reaction of 4-phenyl-4-pentenoic acid (4k) 2

2

in a 60% yield (Scheme 6(a)). In addition, the sulfonium-based CF H reagent (3e) , which also 4

2

requires a strong reducing photocatalyst (E = –1.74 V), could be also utilized. The aminoirr

difluoromethylation of styrene (4a), which was previously reported by perylene photocatalysis, smoothly proceeded to give the product 5ea in a 66% yield (Scheme 6(b)). NMR experiments (see the Supporting Information) showed 1a is a more promising photocatalyst than perylene (1a: 82% NMR yield, perylene: 52% NMR yield). Furthermore, the present organic photocatalytic system was also applied to various photocatalytic trifluoromethylation with typical electron-accepting CF

3

reagents such as Umemoto reagent II (3c) , CF SO Cl (3f), and Togni reagent (3g). Intramolecular 33

3

31

2

oxy-trifluoromethylation of 4k with 3c (Scheme 6(c)), chloro-trifluoromethylation of 1-hexenyl 34

tosylcarbamate (4m) with 3f (Scheme 6(d)), and keto-trifluoromethylation of trans-anethole (4n) 25d

with 3g (Scheme 6(e)) afforded the corresponding CF -containing products in 89% (6ck), 68% 3

(7fm),

and

75%

(8gn)

yields.

These

results

suggest

that

9,10-bis(di(p-tert-

butylphenyl)amino)anthracene (1a) is a useful organic photoredox catalyst for catalytic fluoroalkylation with various electron-accepting fluoroalkylating reagents under visible light irradiation (l = 470 nm).

2.4. Mechanistic Studies.


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ACS Catalysis

To clarify the reaction mechanism of amino-trifluoroethylation by the photocatalyst (1a) with the electron-accepting CF CH reagent (3a), mechanistic studies were conducted. 3

2

OTf + Ph S 1.5:1 4a CH2CF3 3a

3 mol % 1a 3 equiv TEMPO 1.5 equiv H2O CH2Cl2/CH3CN rt, 3 h 470 nm blue LEDs

Me Me

O N

CH2CF3 Me Me

39% NMR yield

Scheme 7. Radical Trap Experiment For detailed reaction conditions, see the Supporting Information.

2.4.1. Radical trap experiment. A radical trapping agent, TEMPO, was added to the mixture of photocatalytic aminotrifluoroethylation of styrene (4a) with 3a (Scheme 7). As a result, CF CH -OTEMP was formed 3

2

in a 39% NMR yield, and the amino-trifluoroethylated product (5aa) was not formed at all. These results suggest that the present reaction involves radical intermediates.

2.4.2. Stern-Volmer plots.


19

ACS Catalysis

2.5 OTf 2 S CH2CF3 3a

1.5 I0/I

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 20 of 44

1 Ph 4a

0.5

0

0

20

80

40 60 [quencher] (mM)

Figure 5. Stern-Volmer Plots with Respect to 1a. Stern-Volmer plots were carried out with a CH Cl solution of 1a (1.0 x 10 M) excited at 410 nm. 2

–5

2

Fluorescence of 1a was not quenched by styrene (4a) but by the CF CH reagent (3a) (Figure 5), 3

2

suggesting occurrence of single-electron transfer (SET) from the excited 1a to 3a. In addition, fluorescence of the other photocatalysts (1b, 1c and 2a) was also quenched by addition of 3a (see the Supporting Information). The Stern-Volmer constants, K (mM ), of 1 were determined to be –1

SV

1.2 x 10 (1a), 2.8 x 10 (1b), and 1.5 x 10 (1c), indicating that the estimated k values (electron –2

–2

–2

et

transfer rate constant) do not have a correlation with the photocatalytic outcomes.

2.4.3. Consideration on radical chain processes.


20

Page 21 of 44

3a + 4a

NDCOCD3

standard conditionsa 470 nm blue LEDs (ON/OFF)

Ph CH2CF3 5aa-d4

100 ON

80 yield of 5aa-d4

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

ACS Catalysis

OFF 60

OFF

40

OFF

20 0

0

20 time (min)

40

Figure 6. Intermittent Light Irradiation Experiment. The reaction conditions are identical to those a

of entry 7 in Table 2 except for light irradiation.

As mentioned above (Figure 3(b) and Table 1), the lifetime of the excited state of the photocatalyst (1a) was short (31 ns), but the photocatalytic reactions by 1a proceeded in a highly efficient manner. Then, to clarify involvement of radical chain processes, (i) intermittent light irradiation experiment and (ii) determination of the quantum yield of the amino-trifluoroethylation were carried out. As shown in Figure 6, the present photocatalytic reaction proceeded only during visible light irradiation periods. In addition, the quantum yield of the amino-trifluoroethylation of 4a with 3a in the presence of the photocatalyst (1a) was 0.13 (Scheme 8). These results indicate 35

that radical chain processes are not a main component in the present reaction.


21

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OTf

Page 22 of 44

NHAc

3 mol % 1a 1.5 equiv H2O

+ Ph Ph S CH2Cl2/MeCN 1.5:1 4a CH2CF3 CH2CF3 rt, 6 h 0.10 mmol 5aa 3a 436 nm visible light 19% NMR yield 0.15 mmol by fluorescence spectrometer Φ = 0.13 photon –9 –1 flux = 7.02 x 10 einstein s

Scheme 8. Determination of Quantum Yield For detailed reaction conditions, see the Supporting Information.

2.4.4. Consideration on dynamic and static quenching. Photoredox catalysts are usually designed to have long excited lifetime because efficiency of dynamic quenching via collision is enhanced. In general, transition-metal-based photocatalysts have lifetime longer than those of organic photocatalysts due to the intersystem crossing caused by the heavy-atom effect. Some examples of organic photocatalysis show that designed static quenching promotes photoredox reactions. Thus, static quenching between the photocatalyst (1a) 36

and the electron-accepting CF CH reagent (3a) was examined. Measurement of the excited state 3

2

lifetime of 1a in the presence of 3a was carried out (Figure 7(a)). It revealed that plots of I /I 0

(relative fluorescence intensity) and those of t /t (relative lifetime) are not overlapped substantially, 0

indicating that static quenching occurs in the present organic photocatalytic system. But 37

involvement of dynamic quenching cannot be ruled out. The H NMR measurement of a mixture 1

of 1a and 3a (5 equiv) was conducted. As a result, addition of 3a caused broadening of the signals of 1a (Figure 7(b)). It might be due to dynamic behavior via associative interaction between 1a and 3a. Although an absorption spectrum of a mixture of 1a and 3a (5 equiv.) was almost identical to that of 1a (Figure 7(c)), 1a is presumably interacted with 3a in the ground state.


22

Page 23 of 44

(a) 2.5 I0/I

2

I0/I or τ0/τ

1.5 1

τ0/τ

0.5 0

0

20

40 [3a] (mM)

80

60

(b) 1a

1a

1a+3a

8.5 (c)

1a + 3a (5 equiv)

8.0 8.0

7.5

7.0

6.5 6.5

2 1a 1a + 5 equiv 3a

absorbance

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

ACS Catalysis

1

0 300


600

Figure 7. Insight into Static Quenching (a) Plots of Relative Fluorescence Intensities and Lifetime of the Photoexcited 1a with Respect to Concentration of 3a, (b) H NMR Spectra of 1a w/ or w/o 1


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Page 24 of 44

3a, and (c) A UV-vis Spectrum of a Mixture of 1a and 3a (5 equiv). Measurements of fluorescence spectra and decay of fluorescence were performed for a CH Cl solution of 1a (1.5 mM) excited at 2

2

410 and 355 nm, respectively. NMR spectroscopy was conducted for a CD Cl solution ([1a] = 1.5 2

2

mM). UV-vis spectroscopy was carried out for a CH Cl solution ([1a] = 1.5 mM) under air at room 2

2

temperature.

2.4.5. Detection of open-shell [D–p–D]· catalytically active species. +

Visible light irradiation with 470 nm of a mixture of the photocatalyst (1a) and 5 equiv. of the CF CH reagent (3a) for 30 s resulted in a characteristic Vis-NIR spectrum (Figure 8(a)). A broad 3

2

band (1780 nm) was observed in the NIR region, which is assigned to an IVCT (intervalence charge transfer) band derived from the cationic mixed valence radical ([1a]· ) (a transition from +

the delocalized b-HOSO to the delocalized b-LUSO, see the Supporting Information). Chemical oxidation of 1a by tris(p-bromophenyl)aminium hexachloridoantimonate (magic blue) gave an almost identical Vis-NIR spectrum, in which the appearing band was retained for at least 24 h (Figure 8(b)). In addition, TD-DFT calculation

38,39

with respect to [1a]· also supported that an +

absorption around NIR region is derived from the IVCT transition. Furthermore, spin densities are predicted to spread over the D–p–D system, leading to a characteristic low-energy transition (Figure 8(c)). These results strongly support that the catalytically active species [1a]· is generated +

from 1a through the photoinduced SET process and its lifetime is long enough to be detected by Vis-NIR spectroscopy.


24

Page 25 of 44

(a)

1 before irradiation

absorbance

after irradiation with 470 nm visible light (30 s.)

0.5

0 500 (b)


3000

1 predicted transitions by TD-DFT

absorbance

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

ACS Catalysis

1a oxidized by magic blue

0.5

0 500


3000

(c)

tBu

tBu

N

tBu

N

[1a]·+

tBu

Figure 8. Detection of the Cationic Radical Species (a) Vis-NIR Spectra of a Mixture of 1a and 3a (5 equiv), (b) A Vis-NIR Spectrum of 1a Oxidized by Magic Blue (blue line) and Transitions


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Page 26 of 44

Predicted by TD-DFT Calculation (red lines), and (c) Spin Densities of [1a]· Calculated by TD+

DFT. Vis-NIR spectroscopy was carried out for a CH Cl solution ([1a] = 1.5 mM) under N at 2

2

2

room temperature. initial stage in the ground state tBu

tBu

OTf N

tBu

1a

3a

associative SET

*

S CH2CF3

3a

CH2CF3

Ph 3a

– 1e

1a

CH2CF3

4

photoredox [1a] process 1e-oxidant

3a

1a

S CH2CF3 3a

tBu

1a

visible light 1a

+

N

D

Ritter CH2CF3 amination

Ph

Ph

E

NHAc CH2CF3 5

Scheme 9. A Possible Reaction Mechanism 2.4.6. A possible reaction mechanism of the photocatalytic amino-trifluoroethylation. On the basis of the above-mentioned data and previous reports, a possible reaction mechanism 25b

is illustrated in Scheme 9. First of all, a complex ([1a···3a]) is formed from the photocatalyst (1a) and the electron-accepting CF CH reagent (3a) in the ground state. Then, visible light irradiation 3

2

causes excitation of the complex ([1a···3a]), giving a CF CH radical (CF CH ·) and the cationic 3

2

3

2

radical species ([1a]· ) through an associative SET process. The generated CF CH · reacts with +

3

2

alkene (4) to afford the radical intermediate (D), which is oxidized by [1a]· to result in the +

carbocationic intermediate (E). Finally, E undergoes Ritter amination to give amino-


26

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ACS Catalysis

trifluoroethylated product (5). Basically, we think that the other fluoroalkylations by 1a, which are highlighted in this work, also proceed through similar processes.

3. CONCLUSION In conclusion, 9,10-bis(di(p-tert-butylphenyl)amino)anthracene (1a) is found to serve as an efficient noble metal-free photocatalyst for fluoroalkylation of alkenes with various electronaccepting fluoroalkylating reagents. As a result, facile catalytic introduction of CF CH , CF HCH , 3

2

2

2

CF , and CF H groups has become viable. The photocatalyst (1a) features (i) the efficient 3

2

photoexcitation by visible light, (ii) the strong reducing power with no need of a sacrificial reducing reagent, (iii) the static quenching rather than dynamic quenching, and (iv) the long lifetime of the cationic radical species [1a] . The successful catalytic performance of 1a guides ·+

molecular design of a new class of PAH-based photocatalyst. Further development of new organic photocatalysts is currently under way in our laboratory.

AUTHOR INFORMATION Corresponding Authors * [email protected] * [email protected] Funding Sources The authors thank the JSPS (KAKENHI Grants 22350026, 17J07953, JP16H06038 and JP18H04241 in Precisely Designed Catalysts with Customized Scaffolding) and the Naito Foundation.


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Page 28 of 44

Notes The authors declare no competing financial interest.

ASSOCIATED CONTENT Supporting Information The Supporting Information is available free of charge on the ACS Publications website. Full experimental methods including detailed synthetic procedures and characterization data, CV traces for the reagents, DPV traces for the photocatalysts, X-ray crystallographic details, and DFT and TD-DFT calculation details (PDF) Crystallographic data for 1a (CIF) Crystallographic data for 5aa (CIF)

ACKNOWLEDGMENT This work was performed under the Cooperative Research Program of the “Network Joint Research Center for Materials and Devices”. The authors thank Suzukakedai Materials Analysis Division, Technical Department, Tokyo Institute of Technology, for fluorescence decay analysis. The DFT and TD-DFT calculations were performed by using a computer in Research Center for Computational Science, Institute for Molecular Science, Okazaki.

ABBREVIATIONS


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ACS Catalysis

bpy, 2,2’-bipyridine; ppy, 2-pyridylphenyl; SET, single-electron transfer; conPET, consecutive photoinduced electron transfer; PDI, perylene diimides; PTH, 10-phenylphenothiazine; PAZ, N,Ndiphenyl dihydrophenazine; POX, 10-phenylphenoxazine; SenI-ET, sensitization-initiated electron transfer; CT, charge transfer; LED, light-emitting diode; UV, ultraviolet light; vis, visible light; NIR, near infrared absorption; TD-DFT, time-dependent density functional theory; IVCT, intervalence charge transfer; HOSO, the highest occupied spin orbital; LUSO, the lowest unoccupied spin orbital.

REFERENCES (1) (a) Narayanam, J. M. R; Stephenson, C. R. J. Visible Light Photoredox Catalysis: Applications in Organic Synthesis. Chem. Soc. Rev. 2011, 40, 102–113. (b) Xuan, J.; Xiao, W.-J. Visible-Light Photoredox Catalysis. Angew. Chem. Int. Ed. 2012, 51, 6828–6838. (c) Reckenthäler, M.; Griesbeck, A. G. Photoredox Catalysis for Organic Syntheses. Adv. Synth. Catal. 2013, 355, 2727–2744. (d) Xi, Y.; Yi, H.; Lei, A. Synthetic Applications of Photoredox Catalysis with Visible Light. Org. Biomol. Chem. 2013, 11, 2387–2403. (e) Prier, C. K.; Rankic, D. A.; MacMillan, D. W. C. Visible Light Photoredox Catalysis with Transition Metal Complexes: Applications in Organic Synthesis. Chem. Rev. 2013, 113, 5322–5363. (f) Ravelli, D.; Protti, S.; Fagnoni, M. Carbon–Carbon Bond Forming Reactions via Photogenerated Intermediates. Chem. Rev. 2016, 116, 9850–9913. (g) Skubi, K. L.; Blum, T. R.; Yoon, T. P. Dual Catalysis Strategies in Photochemical Synthesis. Chem. Rev. 2016, 116, 10035–10074. (h) Shaw, M. H.; Twilton, J.; MacMillan, D. W. C. Photoredox Catalysis in Organic Chemistry. J. Org. Chem. 2016, 81, 6898– 6926. (i) Matsui, J. K.; Lang, S. B.; Heitz, D. R.; Molander, G. A. Photoredox-Mediated Routes to Radicals: The Value of Catalytic Radical Generation in Synthetic Methods Development. ACS


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2

3

(28) The molecular structure is shown in the Supporting Information. CCDC 1854817 contains the supplementary crystallographic data for 5aa. These data can be obtained free of charge from The Cambridge Crystallographic Data Centre via www.ccdc.cam.ac.uk/data_request/cif. (29) When the catalyst loading was reduced to 1 mol % in the preparative scale reaction, smooth reaction was feasible (74% isolated yield). In contrast, reactions with 1 mol % of fac-Ir(ppy) and 3

perylene were significantly sluggish to give the product 5aa in 25 and 27% isolated yields, respectively. In addition, when the NMR experiment was repeated by addition of substrates after the first run (see the Supporting Information), the yields of the second run were declined (the first run: 86% NMR yield (1 h), the second run: 28% (1 h) and 47% (4 h) NMR yields). These results indicate that 1a is a highly active catalyst, but undergoes decomposition during the reaction. (30) Deng, Q.-H.; Chen, J.-R.; Wei, Q.; Zhao, Q.-Q.; Liu, L.-Q.; Xiao, W.-J. Visible-LightInduced Photocatalytic Oxytrifluoromethylation of N-Allylamides for the Synthesis of CF 3

Containing Oxazolines and Benzoxazines. Chem. Commun. 2015, 51, 3537–3540.


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(31) Nagib, D. A.; MacMillan, D. W. C. Trifluoromethylation of Arenes and Heteroarenes by Means of Photoredox Catalysis. Nature 2011, 480, 224–228. (32) (a) Wang, S.-M.; Song, H.-X.; Wang, X.-Y.; Liu, N.; Qin, H.-L.; Zhang, C.-P. PalladiumCatalyzed Mizoroki–Heck-type Reactions of [Ph SR ][OTf] with Alkenes at Room Temperature. 2

fn

Chem. Commun. 2016, 52, 11893–11896. (b) Song, H.-X.; Wang, S.-M.; Wang, X.-Y.; Han, J.B.; Gao, Y.; Jia, S.-J.; Zhang, C.-P. Solvent-Free Synthesis of Alkyl and Fluoroalkyl Sulfonium Salts from Sulfides and Fluoroalkyl Trifluoromethanesulfonates. J. Fluorine Chem. 2016, 192, 131–140. (33) Umemoto, T.; Zhang, B.; Zhu, T.; Zhou, X.; Zhang, P.; Hu, S.; Li, Y. Powerful, Thermally Stable, One-Pot-Preparable, and Recyclable Electrophilic Trifluoromethylating Agents: 2,8Difluoro- and 2,3,7,8-Tetrafluoro-S-(trifluoromethyl)dibenzothiophenium Salts. J. Org. Chem. 2017, 82, 7708–7719. (34) Oh, S. H.; Malpani, Y. R.; Ha, N.; Jung, Y.-S.; Han, S. B. Vicinal Difunctionalization of Alkenes: Chlorotrifluoromethylation with CF SO Cl by Photoredox Catalysis. Org. Lett. 2014, 16, 3

2

1310–1313. (35) Cismesia, M. A.; Yoon, T. P. Characterizing Chain Processes in Visible Light Photoredox Catalysis. Chem. Sci. 2015, 6, 5426–5434. (36) (a) Arceo, E.; Jurberg, I. D.; Álvarez-Fernández, A.; Melchiorre, P. Photochemical Activity of a Key Donor-Acceptor Complex Can Drive Stereoselective Catalytic a-Alkylation of Aldehydes. Nat. Chem. 2013, 5, 750–756. (b) Nappi, M.; Bergonzini, G.; Melchiorre, P. MetalFree Photochemical Aromatic Perfluoroalkylation of a-Cyano Arylacetates. Angew. Chem. Int. Ed.


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2014, 53, 4921–4925. (c) Cheng, Y.; Yuan, X.; Ma, J.; Yu, S. Direct Aromatic C–H Trifluoromethylation via an Electron-Donor–Acceptor Complex. Chem. Eur. J. 2015, 21, 8355– 8359. (d) Cheng, Y.; Yu, S. Hydrotrifluoromethylation of Unactivated Alkenes and Alkynes Enabled by an Electron-Donor–Acceptor Complex of Togni’s Reagent with a Tertiary Amine. Org. Lett. 2016, 18, 2962–2965. (37) Lima, C. G. S.; Lima, T. de M.; Duarte, M.; Jurberg, I. D.; Paixão, M. W. Organic Synthesis Enabled by Light-Irradiation of EDA Complexes: Theoretical Background and Synthetic Applications. ACS catal. 2016, 6, 1389–1407. (38) Renz, M.; Theilacker, K.; Lambert, C.; Kaupp, M. A Reliable Quantum-Chemical Protocol for the Characterization of Organic Mixed-Valence Compounds. J. Am. Chem. Soc. 2009, 131, 16292–16302. (39) Theoretical calculations were performed by using the Gaussian16 (A03) program package. Gaussian 16, Revision A03, Frisch, M. J.; Trucks, G. W.; Schlegel, H. B.; Scuseria, G. E.; Robb, M. A.; Cheeseman, J. R.; Scalmani, G.; Barone, V.; Petersson, G. A.; Nakatsuji, H.; Li, X.; Caricato, M.; Marenich, A. V.; Bloino, J.; Janesko, B. G.; Gomperts, R.; Mennucci, B.; Hratchian, H. P.; Ortiz, J. V.; Izmaylov, A. F.; Sonnenberg, J. L.; Williams-Young, D.; Ding, F.; Lipparini, F.; Egidi, F.; Goings, J.; Peng, B.; Petrone, A.; Henderson, T.; Ranasinghe, D.; Zakrzewski, V. G.; Gao, J.; Rega, N.; Zheng, G.; Liang, W.; Hada, M.; Ehara, M.; Toyota, K.; Fukuda, R.; Hasegawa, J.; Ishida, M.; Nakajima, T.; Honda, Y.; Kitao, O.; Nakai, H.; Vreven, T.; Throssell, K.; Montgomery, J. A., Jr.; Peralta, J. E.; Ogliaro, F.; Bearpark, M. J.; Heyd, J. J.; Brothers, E. N.; Kudin, K. N.; Staroverov, V. N.; Keith, T. A.; Kobayashi, R.; Normand, J.; Raghavachari, K.; Rendell, A. P.; Burant, J. C.; Iyengar, S. S.; Tomasi, J.; Cossi, M.; Millam, J. M.; Klene, M.;


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Adamo, C.; Cammi, R.; Ochterski, J. W.; Martin, R. L.; Morokuma, K.; Farkas, O.; Foresman, J. B.; Fox, D. J. Gaussian, Inc., Wallingford CT, 2016.


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TOC graphic tBu

tBu

N

tBu

N

RF

tBu

tBu

visible ight tBu

tBu

N

N

+

RF

tBu

RF: CF3CH2, CF2HCH2, CF3, CF2H


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(Diarylamino)anthracene Catalyst for Metal-Free Visible-Light

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