Coreactant Strategy for the Photoredox Catalytic Generation of

Subscriber access provided by UNIV OF CAMBRIDGE

Article

Coreactant Strategy for the Photoredox Catalytic Generation of Trifluoromethyl Radicals under Low-Energy Photoirradiation Sinheui Kim, Gyurim Park, Eun Jin Cho, and Youngmin You J. Org. Chem., Just Accepted Manuscript • DOI: 10.1021/acs.joc.6b00966 • Publication Date (Web): 03 Jun 2016 Downloaded from http://pubs.acs.org on June 3, 2016

Just Accepted “Just Accepted” manuscripts have been peer-reviewed and accepted for publication. They are posted online prior to technical editing, formatting for publication and author proofing. The American Chemical Society provides “Just Accepted” as a free service to the research community to expedite the dissemination of scientific material as soon as possible after acceptance. “Just Accepted” manuscripts appear in full in PDF format accompanied by an HTML abstract. “Just Accepted” manuscripts have been fully peer reviewed, but should not be considered the official version of record. They are accessible to all readers and citable by the Digital Object Identifier (DOI®). “Just Accepted” is an optional service offered to authors. Therefore, the “Just Accepted” Web site may not include all articles that will be published in the journal. After a manuscript is technically edited and formatted, it will be removed from the “Just Accepted” Web site and published as an ASAP article. Note that technical editing may introduce minor changes to the manuscript text and/or graphics which could affect content, and all legal disclaimers and ethical guidelines that apply to the journal pertain. ACS cannot be held responsible for errors or consequences arising from the use of information contained in these “Just Accepted” manuscripts.

The Journal of Organic Chemistry is published by the American Chemical Society. 1155 Sixteenth Street N.W., Washington, DC 20036 Published by American Chemical Society. Copyright © American Chemical Society. However, no copyright claim is made to original U.S. Government works, or works produced by employees of any Commonwealth realm Crown government in the course of their duties.

Page 1 of 21

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

The Journal of Organic Chemistry

Abstract. Photoredox catalysis has emerged as a valuable alternative to dark-state catalysis. To utilize the full potential of photoredox catalysis, it is imperative to make use of low-energy photons in photoinduced radical processes. We have demonstrated that the use of oxalate as a coreactant provides a useful principle for the photocatalytic production of trifluoromethyl radicals (•CF3) from CF3I upon green or red LED photoirradiation of narrow-bandgap photocatalysts. The photocatalytic cycle involves a radical anion of carbon dioxide (CO2) as a reductant for CF3I, which is generated through photoinduced oxidative decarboxylation of oxalate. Electrochemical and steady-state and transient photophysical investigations were performed to reveal that there are two photoinduced electron-transfer pathways for the oxalate-mediated •CF3 generation.

2

ACS Paragon Plus Environment


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

I.

Page 2 of 21

Introduction

Photoredox catalysis has recently been recognized as a powerful tool for radical-mediated organic transformations.1-20 One particular advantage of the photoredox catalysis is the generation of thermodynamically challenging radical intermediates, such as trifluoromethyl radicals (•CF3), under mild reaction conditions. Transition metal complexes, such as those containing Ir(III),21-25 Ru(II),21,22,24,26-38 and Pt(II),39 are employed to catalyze the reductive cleavage of electrophilic •CF3 precursors upon visible light irradiation. It is envisioned that benefits of the photoredox catalysis can be maximized if the processes are steered by solar photon. However, it is noted that the majority of the reported photoredox catalytic trifluoromethylation reactions have been carried out under blue LED photoirradiation. The necessity of such high-energy photon sources stems from the thermodynamic requirement that the ground-state reduction potentials or excited-state oxidation potentials of the catalysts should be as negative as possible to enable exergonic electron transfer to the •CF3 precursors. This thermodynamic condition can be met for wide-bandgap catalysts, but such catalysts cannot be used in redox catalysis under low-energy photoirradiation. The activation of less-reactive radical precursors by low-energy photons remains a significant challenge, and successful demonstrations have only sporadically been reported.40-44 We propose a coreactant strategy that addresses this difficulty (see Scheme 1). The one-electron oxidation of oxalate (C2O42; Eox = 0.21 V vs SCE, CH3CN) facilitates the fast homolytic cleavage of the CC bond in oxalate, furnishing CO2 and the radical anion of CO2 (CO2) (i.e., C2O42  C2O4 + e, followed by C2O4  CO2 + CO2).26,45,46 The latter product (CO2) possesses a very negative oxidation potential, 2.20 V vs SCE,47-49 so it would be capable of the one-electron reduction of most electrophilic •CF3 sources, including CF3I (Ered = 0.91 V vs SCE, CH3CN).50 Actually, one electron transfer from CO2 to CF3I is predicted to be highly exergonic with a driving force for electron transfer as large as 1.29 eV, which cannot be achieved with conventional visible-light photoredox catalysts. In addition to such high exothermicity, the redox processes mediated by oxalate have the significant advantage that any photoredox catalysts with excited-state reduction potentials more positive than 0.21 3


Page 3 of 21

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


V vs SCE can catalyze the •CF3 generation. Decoupling the two key processes of the photocatalysis (i.e., photon absorption by a photocatalyst and electron transfer from CO2 to a •CF3 precursor) means that a wide range of photocatalysts can be used, and enables •CF3 generation by using low-energy photons. Furthermore, the strategy effectively suppresses hazardous charge recombination between gaseous CO2 and the one electron-reduced species of a •CF3 precursor. Coreactant Strategy

Reductive Pathway

Oxidative Pathway

1) PC* + C2O42

PC

2) CO2 + CF3I

CO2 + CF3 + I

3) PC + CF3I

PC + CF3 + I

1) PC* + CF3I

PC

2) PC

+

C2O42

+ CO2 + CO2

+ CF3 + I

PC + CO2 + CO2 CO2 + CF3 + I

3) CO2 + CF3I

Reductive Pathway Driven by Low-Energy Photon CO2 E ox = -2.20 V vs SCE

CF3I

exergonic electron transfer CF3I

E red = -0.91 V vs SCE

oxidative decarboxylation

O O

O O

CF3 + I

CO2 E ox = 0.21 V vs SCE

low-energy hv photon

exergonic electron transfer photocatalyst (PC) + PF6

F

2+ 2Cl

N F F

Ir

N N

N

N

N

N

Ru

N N

Ir N

N

N

N

F [Ir(dfppy)2(bpy)]PF6 E* red = 1.12 V vs SCE

[Ru(phen)3]Cl2 E* red = 0.81 V vs SCE

f ac-Ir(ppy)3 E* red < 0.10 V vs SCE

Scheme 1. Chemical equations for the coreactant strategy and a schematic representation of the lowenergy photoredox catalytic generation of trifluoromethyl radicals in the presence of an oxalate coreactant. Shown below are the molecular structures for the photoredox catalysts employed in this study.

4



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 4 of 21

The high reducing power of CO2 has been thoroughly investigated and demonstrated to be very useful.51-54 For instance, Bard and co-workers established the unique reduction ability of oxalate for electrochemoluminescence applications.46,47,55,56 Kinetic aspects of electron transfer behaviors of oxalate have also previously been investigated.57,58 In particular, Fukuzumi and co-workers employed oxalate as a two-electron source for the photocatalytic production of H259 and H2O2.60 Building on these studies, we propose a strategy for low-energy photoredox catalytic trifluoromethylation that employs oxalate as a coreactant. We primarily focus on the mechanism of photoinduced electron transfer between the photocatalysts and the oxalate coreactant. Finally, we demonstrate that the coreactant strategy enables the photoredox catalytic generation of •CF3 under photoirradiation with green and red LEDs. Note that the production of •CF3 with low-energy photoirradiation, such as that of red LEDs, is unprecedented. II.

Results and Discussion

We employed tetrabutylammonium oxalate (TBAOX hereafter) as the oxalate source,47,55,56 because it exhibits high solubility in acetonitrile. Three different classes of photoredox catalysts, including a dicationic Ru(II) diimine complex, [Ru(phen)3]Cl2 (phen = 1,10-phenanthroline), a cationic biscyclometalated Ir(III) complex, [Ir(dfppy)2(bpy)]PF6 (dfppy = 2-(2,4-difluorophenyl)pyridinato; bpy = 2,2-bipyridine), and a charge-neutral triscyclometalated Ir(III) complex, fac-Ir(ppy)3 (ppy = 2phenylpyridinato) were selected, because they have varied electrochemical potentials with almost identical metal-to-ligand charge-transfer (MLCT) transition energies (abs(MLCT): [Ru(phen)3]Cl2, 447 nm; [Ir(dfppy)2(bpy)]PF6, 442 nm; fac-Ir(ppy)3, 450 nm; see the Supporting Information (SI), Figure S1). The catalysts exhibited the ground-state and excited-state redox potentials in an increasing order fac-Ir(ppy)3 < [Ru(phen)3]Cl2 < [Ir(dfppy)2(bpy)]PF6 (vide infra). We envisioned this selection of the photoredox catalysts would provide an ideal opportunity to systematically investigate a series of electron-transfer reactions with minimal variation in photon absorption. The ground-state reduction (Ered) and oxidation (Eox) potentials were determined by employing cyclic voltammetry in acetonitrile solutions, and verified with differential pulse voltammetry (SI, Figure S2). 5


Page 5 of 21

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


The excited-state reduction (E*red) and oxidation (E*ox) potentials were calculated by using the relationships, E*red = Ered + ET and E*ox = Eox  ET, where ET is the peak wavelength of the phosphorescence spectrum (SI, Figure S3). Figure 1 depicts a schematic representation of the Eox, Ered, E*ox, and E*red values of the three photocatalysts, along with the Ered value of CF3I and the Eox values of

Figure 1. Comparisons of the electrochemical potentials of the photoexcited states (PC*) and ground states (PC) of the photocatalysts (black), the reduction potential of CF3I (blue), and the oxidation potentials of C2O42 (red) and CO2 (red).

C2O42 and CO2, which suggests that [Ir(dfppy)2(bpy)]PF6 and [Ru(phen)3]Cl2 will be capable of the oxidation of oxalate in their photoexcited states. The driving forces for the photoinduced one-electron oxidation of oxalate (GPC*(red)) were estimated by using the RehmWeller equation GPC*(red) = e[Eox(C2O42)  E*red(PC)] to be 0.91 eV and 0.60 eV for [Ir(dfppy)2(bpy)]PF6 and [Ru(phen)3]Cl2, respectively, where e, Eox(C2O42), and E*red(PC) are the elementary charge of an electron, the Eox value of oxalate, and the E*red value of the photocatalyst (PC), respectively. A negative GPC*(red) value is predicted for fac-Ir(ppy)3, which implies that reductive quenching by TBAOX cannot occur. The oneelectron-oxidized oxalate (C2O4) undergoes CC bond cleavage to yield CO2 and CO2. Subsequent 6



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 6 of 21

electron transfer from CO2 to CF3I with a driving force of 1.29 eV yields •CF3. Finally, the photocatalytic cycle is completed through the neutralization of the one-electron-reduced photocatalysts by electron transfer to CF3I with driving forces of 0.34 eV and 0.36 eV for [Ir(dfppy)2(bpy)]PF6 and [Ru(phen)3]Cl2, respectively. The driving force for each electron transfer (GPC) can be calculated with the relationship GPC = e[Ered(PC)  Ered(CF3I)], where Ered(PC) and Ered(CF3I) are the Ered values of the photocatalyst and CF3I, respectively. This analysis reveals that there are two processes that produce •CF3 (eqs 1 and 2): CO2 + CF3I  CO2 + •CF3 + I (1) PC + CF3I  PC + •CF3 + I (2) The latter process (eq 2) is identical to the reductive quenching pathway of conventional photoredox catalysis.22,23,28,32-34,61 Note that the driving force for •CF3 generation of the former process (eq 1; G = 1.29 eV) is larger than those of the latter processes (eq 2) for [Ru(phen)3]Cl2 (GPC = 0.36 eV) and [Ir(dfppy)2(bpy)]PF6 (GPC = 0.34 eV). An alternative pathway is available for the photoredox catalytic generation of •CF3. Of the three catalysts, [Ru(phen)3]Cl2 and fac-Ir(ppy)3 are capable of oxidative electron transfer to CF3I under photoirradiation with driving forces (GPC*(ox)) of 0.23 eV and 0.73 eV, respectively. Such driving force is estimated by the relationship GPC*(ox) = e[E*ox(PC)  Ered(CF3I)], where E*ox(PC) is the E*ox value of the photocatalyst. This oxidative quenching is not possible for [Ir(dfppy)2(bpy)]PF6 due to the negative driving force (0.16 eV). After oxidative quenching by CF3I, the resulting one-electronoxidized photocatalysts are neutralized by oxalate with driving forces for electron transfer of 0.73 eV and 0.55 eV for [Ru(phen)3]Cl2 and fac-Ir(ppy)3, respectively. This process also generates CO2 which can reductively cleave CF3I into •CF3 and iodide. In analogy with the reductive pathway, there are two processes for •CF3 formation in the presence of [Ru(phen)3]Cl2 and fac-Ir(ppy)3 (eqs 3 and 1): PC* + CF3I  PC+ + •CF3 + I (3) CO2 + CF3I  CO2 + •CF3 + I (1) 7


Page 7 of 21

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


The above thermodynamic considerations predict that the overall reaction can be understood as twoelectron transfer from oxalate to two molecules of CF3I: C2O42 + 2 CF3I  2 CO2 + 2 •CF3 + 2 I. This reaction can be initiated upon photoirradiation through either a reductive quenching pathway ([Ir(dfppy)2(bpy)]PF6 and [Ru(phen)3]Cl2) or an oxidative quenching pathway ([Ru(phen)3]Cl2 and fac-

Figure 2. Monitoring the photoinduced electron transfer processes of the catalysts. Photoluminescence decay traces of 50 M [Ir(dfppy)2(bpy)]+ (a; obs = 500 nm), 50 M [Ru(phen)3]2+ (c and e; obs = 596 nm), and 50 M fac-Ir(ppy)3 (g; obs = 525 nm) upon nanosecond pulsed laser excitation at 377 nm (temporal resolution = 32 ns) with increases in the concentration (050 M) of TBAOX (a and c) or CF3I (e and g). The red curves are nonlinear least-squares fits to monoexponential (a, e, and g) and biexponential (c) decay models. Corresponding pseudo linear plots of the electron transfer rates of [Ir(dfppy)2(bpy)]+ (b), [Ru(phen)3]2+ (d and f), and fac-Ir(ppy)3 (h) as functions of the added concentrations of TBAOX (b and d) or CF3I (f and h). Photoluminescence decay traces for [Ir(dfppy)2(bpy)]+ in the presence of 10 mM CF3I (a) and for fac-Ir(ppy)3 in the presence of 10 mM TBAOX (g) are also shown (red filled triangles).

8



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 21

Ir(ppy)3). To directly probe these quenching pathways, photoluminescence decay traces for the photocatalysts (50 M in Ar-saturated acetonitrile solutions) were monitored by employing timecorrelated single-photon-counting techniques after nanosecond pulsed laser excitation at 377 nm. The excited states of the photocatalysts were long-lived with observed lifetimes (obs) of 1.2 s ([Ir(dfppy)2(bpy)]PF6), 0.44 s ([Ru(phen)3]Cl2), and 1.5 s (fac-Ir(ppy)3). Increasing the concentration of TBAOX in the [Ir(dfppy)2(bpy)]PF6 and [Ru(phen)3]Cl2 solutions leads to decreases in obs (Figures 2a and c), as is consistent with our thermodynamic predictions. Electron-transfer rates were calculated by using the equation, electron transfer rate = 1/obs 1/obs(0), where obs and obs(0) are the photoluminescence lifetimes in the presence and absence of TBAOX, respectively. Pseudo first-order plots of the electron-transfer rates as functions of the concentrations of TBAOX yield the following rate constants for the bimolecular reductive quenching of the catalysts: 5.5  1010 M1 s1 and 2.8  1010 M1 s1 for [Ir(dfppy)2(bpy)]PF6 and [Ru(phen)3]Cl2, respectively (Figures 2b and d). These rate constants approach the diffusion rate constant in acetonitrile at 298 K (~1010 M1 s1), which indicates the occurrence of ultrafast electron transfer. As expected, the photoluminescence decay profile for facIr(ppy)3 is not affected by the presence of TBAOX (Figure 2g). In sharp contrast, the photoexcited state of fac-Ir(ppy)3 is oxidatively quenched by CF3I (Figures 2g and h). The rate constant for oxidative electron transfer was determined to be as large as 2.6  1010 M1 s1. Such oxidative quenching by CF3I is also evident for [Ru(phen)3]Cl2 at a rate constant of 8.6  109 M1 s1 (Figures 2e and f), which is one order of magnitude smaller than the value for reductive quenching by TBAOX. This result indicates the propensity of the reductive quenching pathway in [Ru(phen)3]Cl2, as suggested by the driving forces (i.e., 0.60 eV (reductive quenching) vs 0.23 eV (oxidative quenching)). As expected, [Ir(dfppy)2(bpy)]PF6 does not undergo oxidative quenching by CF3I (Figure 2a). Steady-state photoluminescence quenching experiments also revealed the exclusive occurrence of reductive quenching of [Ir(dfppy)2(bpy)]PF6 by oxalate (SI, Figure S4). Thus, we conclude that the use of [Ir(dfppy)2(bpy)]PF6 selects the reductive quenching pathway in the presence of 9


Page 9 of 21

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


TBAOX, whereas fac-Ir(ppy)3 follows the oxidative quenching pathway in the presence of CF3I. In the case of [Ru(phen)3]2, both quenching routes are available. The rate constants for the initial electron-transfer reactions (keT) can be correlated with their respective driving force values in a Marcus-type plot.62-64 As displayed in Figure 3, the electron transfer is located on the plateau of the Marcus curve of electron transfer. This result indicates that the initial electron transfer is not a rate-determining step in the overall •CF3 generation processes. In addition, the reductive electron transfer from oxalate to the photoexcited catalysts is more favored.

Figure 3. Correlation of the log keT values and driving forces for reductive electron transfer (black stars) and oxidative electron transfer (red diamonds).

The synthetic utility of the oxalate coreactant strategy was tested for Ar-saturated acetonitrile solutions containing 0.25 M 1-dodecene, 0.75 M CF3I, 0.65 M TBAOX, and 1 mol % photocatalyst. 1Dodecene (1) was chosen as the substrate for •CF3 because our previous studies have indicated that the terminal alkene can readily trap •CF3.39 The reaction mixtures were photoirradiated with blue LEDs (4 W) at room temperature for 24 h, then GC MS analyses were performed by using dodecane as an internal standard. Note that the reaction conditions were not optimized, and that further synthetic elaborations are required to obtain the best results. As summarized in Table 1, the photoirradiations yielded a mixture of a trifluoromethylated alkene (2) and an iodotrifluoromethylated product (3). Hydrotrofluoromethylation products (4) were also generated in 210%. This outcome resulted from a common intermediate. As shown in Scheme 2, the addition of •CF3 to 1 generates 1a which can be 10



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 10 of 21

oxidized into a carbocation intermediate 1b. Deprotonation of 1b by oxalate or solvent molecules yields the alkene product 2, while nucleophilic addition of iodide furnishes the iodotrifluoromethylation product 3. Alternatively, 1a can undergo a radical propagation reaction with CF3I to yield 3, as we proposed in our previous studies.39 Finally, hydrogen atom abstraction from solvents produces the hydrotrifluoromethylation product 4.

Table 1. Photoredox Catalytic Generation of Trifluoromethyl Radicals in the Presence of Oxalate Coreactant

11


Page 11 of 21

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


Scheme 2. Plausible mechanism for the syntheses of the trifluoromethylated product 2, iodotrifluoromethylated product 3, and hydrotrifluoromethylated product 4. Control experiments performed in the dark (entry 2) or in the absence of oxalate (entry 3) demonstrate that photoirradiation and the use of an oxalate electron source are essential for the •CF3 generation. Interestingly, the production of •CF3 was observed in a photoirradiated solution devoid of the photocatalyst (entry 4). This result may be due to the photocleavage of oxalate through the weak n* transition.65 However, the strong overlap between the MLCT transition band of [Ir(dfppy)2(bpy)]PF6 and the photochemical action spectrum for trifluomethylation provides convincing evidence that the cleavage of oxalate occurs mainly through photoredox catalysis (Figure 4). Similar results were obtained for [Ru(phen)3]Cl2 (SI, Figure S5). Of the tested photocatalysts, [Ir(dfppy)2(bpy)]PF6 provided the best yield (entry 5). The comparison of this yield with the value obtained for fac-Ir(ppy)3 (entry 7) suggests that reductive quenching of the photocatalyst may be superior to oxidative quenching for the coreactant-mediated •CF3 generation, which is consistent with the kinetic analysis for electron transfer (i.e., Figure 3). Note also that the observation that [Ir(dfppy)2(bpy)]PF6 provides the highest yield is in accord with the conclusion that it exhibits the largest rate constant for reductive quenching by TBAOX. However, although it is tempting to rationalize the reactivates on the basis of the initial electron transfer

12



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 21

processes, further investigations with optimized reaction conditions are required to fully understand the catalysis.

Figure 4. A comparison of the UVvis absorption spectra of [Ir(dfppy)2(bpy)]PF6 (black solid curve) and TBAOX (blue solid curve) with the photoluminescence excitation spectrum (dotted line) and the photochemical action spectrum (filled squares) of [Ir(dfppy)2(bpy)]+, which plots the quantum yields of the photocatalytic generation of •CF3 in the presence of TBAOX.

In the final phase of this study, •CF3 generation was performed in the presence of 5,10,15,20tetraphenyl-21H,23H-porphine zinc (ZnTPP) and 2,3,7,8,12,13,17,18-octaethyl-21H,23H-porphine platinum (PtOEP) as narrow-bandgap photocatalysts. These porphyrin complexes exhibited visible absorption in the orange-red regions due to the presence of their characteristic Q-bands (abs (): ZnTPP, 556 nm (5100 M1 cm1); PtOEP, 533 nm (4900, M1 cm1); see SI, Figure S6). It was envisioned that these photocatalysts would oxidize oxalate because the E*red value was more positive than the Eox value of oxalate.66 Indeed, our photoluminescence quenching experiments revealed that the addition of TBAOX leads to decreases in the photoluminescence intensities of ZnTPP and PtOEP (SI, Figure S6). The recovery of the ground-state catalysts from the one-electron-reduced species is possible through exergonic electron transfer to CF3I (SI, Figure S2). Photoirradiation of the reaction mixtures containing 13


Page 13 of 21

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


ZnTPP and PtOEP employing green (1 W) or red (5 W) LEDs furnished trifluoromethylated products in yields in the range 5375% (see entries 912 in Table 1). The use of green LEDs resulted in higher yields, which is most probably due to the better spectral overlaps with the Q-bands of ZnTPP and PtOEP. The scope of the reaction with PtOEP as a photocatalyst was also examined for a variety of substrates, including alkenes containing hydroxyl (5), epoxide (6), ester (7 and 8), amine (9), thiourea (10), and carbamate (11) groups (Table 2). Product analyses employing

19

F NMR spectroscopy

indicated that the strategy is amenable to the presence of a broad range of functionalities, while yields remained low. Further optimization of the reaction conditions, including polarity of solvent, may improve the catalytic performance. It should be underscored that this is the first demonstration of photoredox catalytic trifluoromethylation performed under low-energy photoirradiation.

Table 2. Substrate Scope of the Coreactant-Mediated Photoredox Catalytic Trifluoromethylation of a Variety of Alkenesa 1.0 mol % PtOEP 2.6 equiv TBAOX 3.0 equiv CF3I

R

I F3C

R

+

F3C

R

CH3CN (0.25 M) green LEDs r.t., 24 h entry 5

92

HO O

6 7

yield (%)b

substrate

O

81 70

O O

8

O 9

67

41

N H S

10

N H

NH2

47

O 11

N H

O

21

a

2.0 mL Ar-saturated CH3CN solutions containing 0.50 mmol substrate, 1.5 mmol CF3I, 0.0050 mmol PtOEP, and 1.3 mmol TBAOX were photoirradiated using green LEDs (1 W) for 24 h at room temperature. bYields determined by employing 19F NMR spectroscopy with 1,4-difluorobenzene as an internal standard. Hydrotrifluoromethylation products might be produced, but they were not distinguishable in our 19F NMR experiments.

14



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 14 of 21

III. Summary and Conclusions To summarize, we have developed a coreactant strategy for the photocatalytic generation of trifluoromethyl radicals. This coreactant strategy involves the reductive and oxidative quenching of photocatalysts by oxalate and CF3I, respectively. In the case of the reductive quenching pathway, catalytic turn-over is completed by the neutralization of the one-electron-reduced catalysts through exergonic electron transfer to CF3I. In the case of the oxidative quenching mechanism, electron donation by oxalate serves to restore the ground-state photocatalyst from the one-electron-oxidized species. In both pathways, the oxidation of oxalate promotes decarboxylative generation of the highly reducing CO2 species, which can then produce •CF3 from CF3I (i.e., CO2 + CF3I  CO2 + •CF3 + I). Overall, the net reaction is two-electron transfer from oxalate to two equivalents of CF3I (i.e., C2O42 + 2 CF3I  2 CO2 + 2 •CF3 + 2 I), irrespective of the quenching mechanism. The process is steered by the photoirradiation of photocatalysts with moderately positive excited-state reduction potentials (E*red > 0.21 V vs SCE). We established the thermodynamic and kinetic parameters of the electron transfer processes in the generation of •CF3 species through electrochemical and transient photoluminescence studies. Our investigation of the mechanism revealed dominance of the reductive quenching pathway over the oxidative quenching pathway, an observation consistent with the thermodynamic prediction made from the RehmWeller terminology. Of importance is decoupling of a reductant and the photonabsorption species, which enables wide versatility in the selection of available photocatalysts. By taking advantage of the alleviated thermodynamic conditions, we successfully demonstrated the production of the thermodynamically challenging •CF3 upon low-energy photoirradiation. Although the synthetic possibilities of this strategy have not yet been fully explored, we conclude that the coreactant strategy would be of great value for a broad range of radical transformations involving -conjugated substrates. IV. Experimental Section Materials and Synthesis. Commercially available chemicals, including fac-Ir(ppy)3, [Ru(phen)3]Cl2, ZnTPP, and PtOEP, were used without further purification. The syntheses of [Ir(dfppy)2(bpy)]PF667 and TBAOX47,55,56 were performed following the methods reported previously. All glassware and magnetic 15


Page 15 of 21

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


stir bars were thoroughly dried in a convection oven. Anhydrous acetonitrile was purchased, and used as received. Reactions were monitored by using thin layer chromatography (TLC). Commercial TLC plates were developed, and the spots were visualized under UV illumination at 254 or 365 nm and stained with phosphomolybdic acid and p-anisaldehyde reagents. 1H NMR spectra were referenced to deuterated solvents. General Procedure for the Photoreactions. An oven-dried resealable tube, equipped with a magnetic stir bar, was charged with 0.50 mmol 1-dodecene, 1.3 mmol TBAOX, 1.0 mol % photocatalyst, and 2.0 mL anhydrous acetonitrile. After deaeration by bubbling with Ar, 1.5 mmol CF3I was injected into the reaction mixture by using a gas-tight syringe. The reaction mixture was placed under LED irradiation at room temperature for 24 h (blue LEDs, ~450 nm and 4 W; green LEDs, ~550 nm and 1 W; red LEDs, ~650 nm and 5 W). Product analysis was performed with GC MS spectrometry and

19

F NMR spectroscopy by using dodecane and 1,4-difluorobenzene as internal standards,

respectively. Spectroscopic Identification Data. 2: colorless oil; 1H NMR (400 MHz, CDCl3) δ 6.38 (dtq, J = 15.9, 6.8, 2.0 Hz, 1H), 5.60 (dqt, J = 15.9, 6.4 (JH-F), 1.6 Hz, 1H), 2.192.10 (m, 2H), 1.471.39 (m, 2H), 1.331.24 (m, 14H), 0.88 (t, J = 6.4 Hz, 3H);

13

C NMR (101 MHz, CDCl3) δ 141.0 (q, J = 6.4 Hz),

123.4 (q, J = 270.0 Hz), 118.6 (q, J = 33.2 Hz), 32.2, 31.7, 29.9, 29.8, 29.7, 29.61, 29.3, 28.3, 23.0, 14.3;

19

F NMR (377 MHz, CDCl3) δ 64.3; FT IR (neat): νmax = 2928, 2857, 1273, 1122 cm1; HR MS

m/z (EI) calc. for C13H23F3 [M+] 236.1752, found 236.1750; Rf 0.95 (hexanes). 3: colorless oil; 1H NMR (400 MHz, CDCl3) δ 4.244.16 (m, 1H), 2.982.70 (m, 2H), 1.851.68 (m, 2H), 1.591.22 (m, 16H), 0.89 (t, J = 7.2 Hz, 3H); 13C NMR (101 MHz, CDCl3) δ 125.8 (q, J = 279.8 Hz), 45.2 (q, J = 28.2 Hz), 39.9, 32.1, 29.8, 29.8, 29.7, 29.6, 29.6, 28.8, 22.9, 22.0 (q, J = 2.7 Hz), 14.3; FT IR (neat): νmax = 2926, 2856, 1256, 1149 cm1; HR MS m/z (EI) calc. for C13H24F3I [M+] 364.0875, found 364.0877; Rf 0.90 (hexanes). Spectroscopic Measurements. The photocatalyst solutions were prepared by dissolution in CH3CN to concentrations of 10 mM, 1 mM, 100 M, and 10 M. The 10 M solutions were used for 16



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 16 of 21

spectroscopic measurements, unless otherwise noted. A 1 cm  1 cm fluorimeter cell with a 14/20 joint that could accommodate a rubber septum was used in the steady-state optical measurements. UVvis absorption spectra were collected at 298 K. Photoluminescence spectra were obtained at room temperature.

The

photocatalyst

solutions

were

excited

at

the

following

wavelengths:

[Ir(dfppy)2(bpy)]PF6, 365 nm; [Ru(phen)3]Cl2, 468 nm; fac-Ir(ppy)3, 380 nm. The solutions were thoroughly degassed by bubbling with Ar prior to performing the measurements. Ar-saturated 50 M solutions (CH3CN) were used to determine the photoluminescence lifetimes. Photoluminescence decay traces were acquired with time-correlated single-photon-counting (TCSPC) techniques. A 377 nm diode laser was used as the excitation source. The photoluminescence signals were obtained with an automated motorized monochromator. Photoluminescence decay profiles were analyzed by using monoor biexponential decay models. Determination of Photochemical Quantum Yields (PCQYs). The quantum yields for trifluoromethylation were determined by performing standard ferrioxalate actinometry. A 0.0060 M K3[Fe(C2O4)3] solution was used as the chemical actinometer. 500 L of this K3[Fe(C2O4)3] solution was transferred to a 1 cm × 1 mm quartz cell, and the solution was photoirradiated with a monochromatized beam at 300, 350, 400, and 450 nm for 30 s. Then the same amount of 1% 1,10phenanthroline in sodium acetate buffer solution (4.09 g CH3COONa dissolved in 18 mL of 0.50 M H2SO4 and 32 mL of distilled water) was added and stored in the dark for 1 h. The absorbance changes at 510 nm were recorded; inserting these values into eq 4 returned light intensities of 1.1  108 einstein s1 at 300 nm, 1.0  108 einstein s1 at 350 nm, 9.7  109 einstein s1 at 400 nm, and 9.8  109 einstein s1 at 450 nm: Light intensity (I0, einstein s1) = (Abs(510 nm)  V)/(  11050 M1 cm1  t) (4). In eq 4, Abs(510 nm), V, , and t are the absorbance change at 510 nm, the volume (L), the quantum yield (1.1) of the ferrioxalate actinometer at 420 nm, and the photoirradiation time (s), respectively. Ar-saturated 2.0 mL acetonitrile solutions containing 0.50 mmol 1-dodecene, 1.3 mmol 17


Page 17 of 21

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


TBAOX, 1.5 mmol CF3I, and 1.0 mol % photocatalyst, were photoirradiated under identical conditions for 1 h. The concentrations of the products resulting from the trapping of trifluoromethyl radicals were quantified with GC MS by using dodecane as an internal standard, and then inserted into eq 5: QYCF3 

[product]  V I  t (5)

In eq 5, [product] is the molar concentration of the products, t (s) is the photoirradiated time, V is the volume of the solution (L), and I0 is the light intensity obtained with eq 4 (einstein s1). Electrochemical Characterization. Cyclic voltammetry and differential pulse voltammetry experiments were carried out by using a three-electrode cell assembly. A Pt wire and a Pt microdisc were used as the counter and working electrodes, respectively. A Ag/AgNO3 couple was used as the pseudo reference electrode. Measurements were carried out in Ar-saturated CH3CN (2.0 mL) by using 0.10 M tetra-n-butylammonium hexafluorophosphate (Bu4NPF6) as the supporting electrolyte at scan rates of 100 mV/s (cyclic voltammetry) and 4.0 mV/s (differential pulse voltammetry). The concentration was 2.0 mM, and a ferrocenium/ferrocene couple was employed as the external reference. Acknowledgments. Y.Y. acknowledges a grant from the Ewha Womans University (1-2015-0447001-1). Supporting Information Available. Figures S1S10, displaying the UVvis absorption spectra, cyclic

and

differential

pulse

voltammograms,

steady-state

photoluminescence

spectra,

photoluminescence quenching experiment results, a comparison of the UVvis absorption spectrum of [Ru(phen)3]2+ and the photochemical action spectrum for trifluoromethylation, spectroscopic data for ZnTPP and PtOEP, and copies of

1

H and

13

C NMR spectra. This material is available at

http://pubs.acs.org. References and Notes (1) (2) (3)

Fukuzumi, S.; Ohkubo, K. Chem. Sci. 2013, 4, 561-574. Fukuzumi, S.; Ohkubo, K. Org. Biomol. Chem. 2014, 12, 6059-6071. Hoffmann, N. Photochem. Photobiol. Sci. 2012, 11, 1613-1641. 18



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 18 of 21

(4) (5) (6) (7) (8) (9) (10) (11) (12) (13) (14) (15) (16) (17) (18) (19) (20) (21)

Hopkinson, M. N.; Sahoo, B.; Li, J.-L.; Glorius, F. Chem. Eur. J. 2014, 20, 3874-3886. Inagaki, A.; Akita, M. Coord. Chem. Rev. 2010, 254, 1220-1239. Ischay, M. A.; Yoon, T. P. Eur. J. Org. Chem. 2012, 3359-3372. Koike, T.; Akita, M. Synlett 2013, 24, 2492-2505. Narayanam, J. M. R.; Stephenson, C. R. J. Chem. Soc. Rev. 2011, 40, 102-113. Nicewicz, D. A.; Nguyen, T. M. ACS Catal. 2014, 4, 355-360. Prier, C. K.; Rankic, D. A.; MacMillan, D. W. C. Chem. Rev. 2013, 113, 5322-5363. Ravelli, D.; Dondi, D.; Fagnoni, M.; Albini, A. Chem. Soc. Rev. 2009, 38, 1999-2011. Ravelli, D.; Fagnoni, M. ChemCatChem 2012, 4, 169-171. Ravelli, D.; Fagnoni, M.; Albini, A. Chem. Soc. Rev. 2013, 42, 97-113. Tucker, J. W.; Stephenson, C. R. J. J. Org. Chem. 2012, 77, 1617-1622. Xi, Y.; Yi, H.; Lei, A. Org. Biomol. Chem. 2013, 11, 2387-2403. Xie, J.; Jin, H.; Xu, P.; Zhu, C. Tetrahedron Lett. 2014, 55, 36-48. Xuan, J.; Xiao, W.-J. Angew. Chem., Int. Ed. 2012, 51, 6828-6838. Yoon, T. P. ACS Catal. 2013, 3, 895-902. Yoon, T. P.; Ischay, M. A.; Du, J. Nat. Chem. 2010, 2, 527-532. Zeitler, K. Angew. Chem., Int. Ed. 2009, 48, 9785-9789. Xu, P.; Xie, J.; Xue, Q.; Pan, C.; Cheng, Y.; Zhu, C. Chem. Eur. J. 2013, 19, 14039-

(22) (23)

Iqbal, N.; Jung, J.; Park, S.; Cho, E. J. Angew. Chem., Int. Ed. 2014, 53, 539-542. Nagib, D. A.; Scott, M. E.; MacMillan, D. W. C. J. Am. Chem. Soc. 2009, 131, 10875-

(24) (25) (26) (27) (28) (29) (30) (31) (32)

Yasu, Y.; Koike, T.; Akita, M. Angew. Chem., Int. Ed. 2012, 51, 9567-9571. Jiang, H.; Cheng, Y.; Zhang, Y.; Yu, S. Eur. J. Org. Chem. 2013, 5485-5492. Kanoufi, F.; Bard, A. J. J. Phys. Chem. B 1999, 103, 10469-10480. Nagib, D. A.; MacMillan, D. W. C. Nature 2011, 480, 224-228. Ye, Y.; Sanford, M. S. J. Am. Chem. Soc. 2012, 134, 9034-9037. Carboni, A.; Dagousset, G.; Magnier, E.; Masson, G. Org. Lett. 2014, 16, 1240-1243. Yasu, Y.; Arai, Y.; Tomita, R.; Koike, T.; Akita, M. Org. Lett. 2014, 16, 780-783. Yasu, Y.; Koike, T.; Akita, M. Chem. Commun. 2013, 49, 2037-2039. Pham, P. V.; Nagib, D. A.; MacMillan, D. W. C. Angew. Chem., Int. Ed. 2011, 50, 6119-

14042. 10877.

6122. (33) Iqbal, N.; Choi, S.; Ko, E.; Cho, E. J. Tetrahedron Lett. 2012, 53, 2005-2008. (34) Iqbal, N.; Choi, S.; Kim, E.; Cho, E. J. J. Org. Chem. 2012, 77, 11383-11387. (35) Mizuta, S.; Verhoog, S.; Engle, K. M.; Khotavivattana, T.; O'Duill, M.; Wheelhouse, K.; Rassias, G.; Médebielle, M.; Gouverneur, V. J. Am. Chem. Soc. 2013, 135, 2505-2508. (36) Mizuta, S.; Engle, K. M.; Verhoog, S.; Galicia-López, O.; O'Duill, M.; Médebielle, M.; Wheelhouse, K.; Rassias, G.; Thompson, A. L.; Gouverneur, V. Org. Lett. 2013, 15, 1250-1253. (37) Mizuta, S.; Verhoog, S.; Wang, X.; Shibata, N.; Gouverneur, V.; Médebielle, M. J. Fluorine Chem. 2013, 155, 124-131. (38) Kim, E.; Choi, S.; Kim, H.; Cho, E. J. Chem. Eur. J. 2013, 19, 6209-6212. (39) Choi, W. J.; Choi, S.; Ohkubo, K.; Fukuzumi, S.; Cho, E. J.; You, Y. Chem. Sci. 2015, 6, 1454-1464. (40) Haering, M.; Perez-Ruiz, R.; Jacobi von Wangelin, A.; Diaz, D. D. Chem. Commun. 2015, 51, 16848-16851. (41) Majek, M.; Faltermeier, U.; Dick, B.; Perez-Ruiz, R.; von Wangelin, A. J. Chem. Eur. J. 2015, 21, 15496-15501. (42) Ghosh, I.; Ghosh, T.; Bardagi, J. I.; Koenig, B. Science 2014, 346, 725-728. (43) Goez, M.; Kerzig, C.; Naumann, R. Angew. Chem., Int. Ed. 2014, 53, 9914-9916. (44) Kerzig, C.; Goez, M. Chem. Sci. 2016, 7, 3862-3868. 19


Page 19 of 21

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


(45) Mulazzani, Q. G.; D'Angelantonio, M.; Venturi, M.; Hoffman, M. Z.; Rodgers, M. A. J. J. Phys. Chem. 1986, 90, 5347-5352. (46) Rubinstein, I.; Bard, A. J. J. Am. Chem. Soc. 1981, 103, 512-516. (47) Chang, M.-M.; Saji, T.; Bard, A. J. J. Am. Chem. Soc. 1977, 99, 5399-5403. (48) Krasna, A. I. Photochem. Photobiol. 1980, 31, 75-82. (49) Pina, F.; Mulazzani, Q. G.; Venturi, M.; Ciano, M.; Balzani, V. Inorg. Chem. 1985, 24, 848-851. (50) Andrieux, C. P.; Gelis, L.; Medebielle, M.; Pinson, J.; Saveant, J. M. J. Am. Chem. Soc. 1990, 112, 3509-3520. (51) Herrmann, J. M.; Mozzanega, M. N.; Pichat, P. J. Photochem. 1983, 22, 333-343. (52) Aguilar, C.; Urzua, U.; Koenig, C.; Vicuna, R. Arch. Biochim. Biophys. 1999, 366, 275282. (53) Opaleye, O.; Rose, R.-S.; Whittaker, M. M.; Woo, E.-J.; Whittaker, J. W.; Pickersgill, R. W. J. Biol. Chem. 2006, 281, 6428-6433. (54) Woo, E.-J.; Dunwell, J. M.; Goodenough, P. W.; Marvier, A. C.; Pickersgill, R. W. Nat. Struct. Biol. 2000, 7, 1036-1040. (55) Moon, H. C.; Lodge, T. P.; Frisbie, C. D. Chem. Mater. 2014, 26, 5358-5364. (56) Qi, H.; Chang, J.; Abdelwahed, S. H.; Thakur, K.; Rathore, R.; Bard, A. J. J. Am. Chem. Soc. 2012, 134, 16265-16274. (57) Yamazaki-Nishida, S.; Kimura, M. Bull. Chem. Soc. Jpn. 1991, 64, 1840-1845. (58) Kimura, M.; Nishida, S. J. Chem. Soc., Dalton Trans. 1985, 355-359. (59) Yamada, Y.; Miyahigashi, T.; Ohkubo, K.; Fukuzumi, S. Phys. Chem. Chem. Phys. 2012, 14, 10564-10571. (60) Yamada, Y.; Nomura, A.; Miyahigashi, T.; Fukuzumi, S. Chem. Commun. 2012, 48, 8329-8331. (61) Wilger, D. J.; Gesmundo, N. J.; Nicewicz, D. A. Chem. Sci. 2013, 4, 3160-3165. (62) Marcus, R. A.; Eyring, H. Annu. Rev. Phys. Chem. 1964, 15, 155-196. (63) Marcus, R. A.; Sutin, N. Biochim. Biophys. Acta, Rev. Bioenerg. 1985, 811, 265-322. (64) Murakami, M.; Ohkubo, K.; Fukuzumi, S. Chem. Eur. J. 2010, 16, 7820-7832. (65) Chang, M. C. Y.; Miller, S. E.; Carpenter, S. D.; Stubbe, J.; Nocera, D. G. J. Org. Chem. 2002, 67, 6820-6822. (66) The excited-state reduction potential of ZnTPP was determined to be 0.86 V vs SCE (SI, Figures S2 and S3). In the case of PtOEP, the reduction potential could not be determined due to the lack of the reduction process within the electrochemical window of the acetonitrile solvent. (67) You, Y.; Lee, S.; Kim, T.; Ohkubo, K.; Chae, W.-S.; Fukuzumi, S.; Jhon, G.-J.; Nam, W.; Lippard, S. J. J. Am. Chem. Soc. 2011, 133, 18328–18342.

20



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 21

For ToC Graphic Only

21


Coreactant Strategy for the Photoredox Catalytic Generation of

Recommend Documents