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DOI: 10.1039/D3GC01299A (Communication) Green Chem., 2023, 25, 6194-6199

Photocatalytic defluorocarboxylation using formate salts as both a reductant and a carbon dioxide source†

Shi-Yun Min a, He-Xin Song a, Si-Shun Yan *bc, Rong Yuan a, Jian-Heng Ye b, Bi-Qin Wang a, Yong-Yuan Gui *a and Da-Gang Yu *b
aCollege of Chemistry and Materials Science, Sichuan Normal University, Chengdu 610068, P. R. China. E-mail: yygui@sicnu.edu.cn
bKey Laboratory of Green Chemistry & Technology of Ministry of Education, College of Chemistry, Sichuan University, Chengdu 610064, P. R. China. E-mail: si-shun.yan@catalysis.de; dgyu@scu.edu.cn
cLeibniz Institute for Catalysis e.V., Albert-Einstein-Strasse 29a, Rostock 18059, Germany

Received 21st April 2023 , Accepted 21st July 2023

First published on 21st July 2023

Abstract

Herein, we report a photocatalytic defluorocarboxylation of benzylic C(sp3)–F bonds using formate salts as both a reductant and a carbon dioxide source. A variety of benzyl fluorides, bearing primary or secondary C(sp3)–F bonds, undergo defluorinative carboxylation smoothly with HCOOK. This transition metal-free strategy provides a mild, efficient, and sustainable approach for accessing a series of valuable aryl acetic acids, such as flurbiprofen. This protocol also features low catalyst loading, mild reaction conditions, good functional group tolerance, and ready scalability and sustainability. Mechanistic investigations indicate that a carbon dioxide radical anion (CO2˙−) is generated via hydrogen atom transfer (HAT) and acts as a strong reductant to promote single electron reduction of benzyl fluorides. The following nucleophilic attack of carbanions on CO2, both of which are in situ generated intermediates, delivers the desired products, demonstrating the high efficiency of the CO2 capture process.

The development of green and sustainable methods for accessing high-value-added chemicals from widely available and inexpensive raw materials not only conforms to the principles of green chemistry but also meets the strategic requirements of sustainable development.1 In this context, formic acid and its formate salts, which are commercial chemicals with abundant reserves, have the advantages of non-toxicity, biodegradability, and convenient storage and transportation.2 As they can be easily available through the hydrogenation of CO2 with H2, they are both an ideal green C1 source and promising reductants for laboratory synthesis and industrial production. The past several decades have witnessed great advances in the transition metal-catalyzed transformations of formic acid and formate salts via the CO or CO2 intermediate (Scheme 1a).3–5 An array of carbonylation and related reactions has thus been developed to synthesize carboxylic acids and their derivatives. In general, these processes usually employ noble transition metals such as palladium, rhodium and iridium as catalysts. On the other hand, excess acids, anhydrides, or dehydration reagents are usually needed to generate more reactive mixed anhydrides to promote CO formation. However, the use of noble transition metals and additives presents an obstacle to the widespread use of these systems because of their high cost and low atom economy. Therefore, the development of green and sustainable methods for using formate salts directly, without the need for noble transition metals and excess activators, would meet the strong synthetic demand for green chemistry and sustainable development.
image file: d3gc01299a-s1.tif
Scheme 1 Catalytic transformations of formic acid and its formate salts.

Visible-light photoredox catalysis has emerged as an enabling paradigm to accomplish novel chemical transformations via unique single electron transfer (SET) pathways in the past decade.6 Very recently, the photocatalytic in situ generation of CO2˙− from formate salts via a HAT process has emerged as an efficient strategy to provide powerful reductants in organic synthesis, due to the low bond dissociation energy (BDE) of the C–H bond in HCOOM and a more negative reduction potential (image file: d3gc01299a-t1.tifvs. SCE) of CO2˙−.7 Many challenging reduction events have been realized through the single electron reduction of inert C–X bonds by CO2˙−,8 with successful examples, especially for the challenging C–F bond activation and functionalizations under mild conditions. For example, the groups of Jui,9 Wickens and Yeung,10 Molander,11 Glorius,12 Shang,13 and Zhu and Guo14 have developed elegant dehalogenative alkylations of C–Cl or C–F bonds via in situ generation of CO2˙− from formate salts (Scheme 1b). Although great progress has been made, these reports have mainly focused on the dehalogenative alkylation reactions, with CO2 released as an innocent byproduct in the reaction system.15 If the CO2 released in the reaction system was reutilized to generate highly valuable carboxylic acids and their derivatives, it would show great advantages over the traditional reductive carboxylations of organic halides with CO2,16 which have the problems of handling the CO2 gas and usage of stoichiometric amounts of metallic reductants (e.g. Zn, Mn, etc.) or organic sacrificial electron donors (e.g. iPr2NEt, BI(OH)H, etc.). Recently, the group of Fu has reported an elegant dual nickel/photoredox-catalyzed dehalocarboxylation of aryl halides using formate as both the CO2 source and the reductant (Scheme 1c).17

On the other hand, the efficient activation and selective functionalization of C–F bonds under mild conditions can serve as an important method for organic synthesis including the modification of pharmaceuticals and agrochemicals and the synthesis of value-added chemicals.18 However, they are highly challenging owing to their high energy and low activity. In continuation of our interest in carboxylation with CO2 and our former work about C–F bond transformations,19 we hope to develop a new photocatalytic system to realize the challenging reductive carboxylation of C(sp3)–F bonds through recycling CO2 generated in the reaction system (Scheme 1d). Specifically, the formate salt could be converted into CO2˙−via a HAT process. As CO2˙− is a highly reductive species, it might be able to promote the reductive cleavage of inert C(sp3)–F bonds to generate a carbon radical intermediate and release a CO2 molecule. After photocatalytic SET reduction of the carbon radical to a carbanion, its nucleophilic attack on the generated CO2 would deliver the desired product of defluorocarboxylation. Herein, we report our success in realizing the photocatalytic defluorocarboxylation of benzylic C(sp3)–F bonds with HCOOK. Notably, the use of HCOOK as both the CO2 source and the reductant is the key to realize such a sustainable carboxylation of C(sp3)–F bonds.

Results and discussion

We initiated the project by investigating defluorocarboxylation of 4-(fluoromethyl)-1,1′-biphenyl 1a under an N2 atmosphere and visible light irradiation at room temperature (r.t.). After systematic optimization of the reaction conditions (Table 1), we obtained the desired product 2-([1,1′-biphenyl]-4-yl)acetic acid 2a in 87% yield using 2,4,5,6-tetrakis(diphenylamino)isophthalonitrile (4DPAIPN) as a photocatalyst, commercially available HCOOK as both the reductant and carboxylic source, methyl 2-mercaptobenzoate as a HAT reagent, Na2CO3 as a base and DMSO as solvent (Table 1, entry 1). Switching to several structurally similar organic photosensitizers and Ir(III) complexes resulted in a dramatic decrease in reaction efficiency (entries 2–5). With hydrosilanes (such as Et3SiH, PMHS and PhMe2SiH) instead of thiols, the yield of 2a dramatically decreased to less than 30% yield, demonstrating the crucial role of the HAT reagent (Table 1, entry 6). The choice of the HAT reagent was also important for the reaction. Electron-deficient thiols such as methyl 2-mercaptobenzoate (T1) showed much higher efficiencies than electron-neutral or electron-rich thiols (entry 1 vs. entries 7–9) since electron-deficient thiols were ready to undergo deprotonations to generate the corresponding thiophenolate under basic conditions, which initiated the photoredox catalytic cycle through reductive quenching of the excited photocatalyst. Other solvents, such as acetonitrile and amides, were also tested (Table 1, entry 10). CH3CN was not a good solvent for this reaction, mainly due to the poor solubility of formate salts and bases in CH3CN. DMSO, besides acting as a solvent, may also oxidize thiols to generate disulfide, which forms thiyl radicals under visible light conditions, thus facilitating the efficiency of the entire catalytic cycle.20 Other alkali metal formates such as HCOOLi, HCOONa, and HCOOCs also promoted this transformation with comparable efficiency (Table 1, entry 11). Other bases, like KOH and NaOH or even common organic bases like Et3N and DMAP were also successful in performing the reactions, affording the desired products in moderate to good yields, except for the cases of pyridine and morpholine (Table 1, entries 12 and 13). Variations in the amounts of HAT catalysts and bases led to slightly reduced yields of product 2a (Table 1, entries 15 and 16). We also performed the standard reaction under an air or O2 atmosphere, and the desired defluorinative carboxylation product 2a was obtained in 80% or 76% yield, demonstrating that the reaction was also amenable to the oxygen atmosphere (Table 1, entry 17). Besides benzylic fluorides, benzyl chlorides and bromides work appropriately under the present conditions, with 4-(chloromethyl)-1,1′-biphenyl and 4-(bromomethyl)-1,1′-biphenyl as substrates, and the desired products 2a were obtained in 76% and 42% yields, respectively (Table 1, entries 18 and 19).21 Finally, control experiments revealed that the visible-light irradiation photocatalyst and formate were all essential for the reaction (Table 1, entry 21).
Table 1 Screening of the reaction conditionsa

image file: d3gc01299a-u1.tif
Entry Variation from the above conditions Yield of 2a[thin space (1/6-em)]b (%)  
a [thin space (1/6-em)]1a (0.2 mmol), 4DPAIPN (0.1 mol%), methyl 2-mercaptobenzoate (10 mol%), HCOOK (0.24 mmol), Na2CO3 (0.2 mmol), DMSO (0.1 M), irradiation using 30 W blue LEDs at r.t. under N2 (1 atm) for 4 h, 10 mL Schlenk tube. b Isolated yields. 3DPAFIPN = 2,4,6-tris(diphenylamino)-5-fluoroisophthalonitrile; 3DPA2FBN = 2,4,6-tris(diphenylamino)-3,5-difluorobenzonitrile; 4CzIPN = 2,4,5,6-tetra(9H-carbazol-9-yl)isophthalonitrile; ppy = 2-phenylpyridine; Phen = 1,10-phenanthroline, DMF = N,N-dimethylformamide, NMP = N-methyl-2-pyrrolidone.
1 None 87 image file: d3gc01299a-u2.tif
2 3DPAFIPN instead of 4DPAIPN 40
3 3DPA2FBN instead of 4DPAIPN 22
4 4CzIPN instead of 4DPAIPN 20
5 Ir[dF(CF3)ppy]2(Phen)PF6 instead of 4DPAIPN 15
6 Et3SiH/PMHS/PhMe2SiH instead of T1 24/24/27
7 CySH instead of T1 38
8 Methyl thioglycolate instead of T1 48
9 4-Methooxybenzenethiol instead of T1 65
10 CH3CN/NMP/DMF instead of DMSO Trace/40/38
11 HCO2Li/HCO2Na/HCO2Cs instead of HCO2K 82/77/70
12 NaOH/KOH instead of Na2CO3 65/63
13 Et3N/pyridine/DMAP/Morpholine instead of Na2CO3 73/47/67/28
14 Li2CO3/K2CO3/Cs2CO3 instead of Na2CO3 73/75/80
15 0.5/1.5 eq. Na2CO3 instead of 1.0 eq. Na2CO3 77/74
16 5 mol% T1 instead of 10 mol% T1 79
17 Under an air/O2 atmosphere 80/76
18 4-(Chloromethyl)-1,1′-biphenyl instead of 1a 76
19 4-(Bromomethyl)-1,1′-biphenyl instead of 1a 42
20 w/o T1, or Na2CO3 30/73
21 w/o LEDs, or 4DPAIPN, or HCO2K Trace

After establishing the optimal reaction conditions for defluorocarboxylation (Table 1, entry 1), we subsequently investigated the scope of benzyl fluorides (Table 2). A broad range of substituted benzyl fluorides bearing either electron-withdrawing or electron-donating groups gave the corresponding products (2f–2t) in good yields. Some synthetically valuable functional groups, such as the aryl C–F bond (2b–2d), Bpin (2f), nitrile (2d, 2m, and 2n), ester (2k, 2l and 2p–2s), ether (2j), sulfone (2o), and ketone (2t), were all tolerated, providing opportunities for their downstream transformations. As illustrated in Table 2, substrates containing different heteroarenes, such as thiophene (2u), carbazole (2v), benzofuran (2w), and benzothiophene (2x), which are prevalent structural elements in pharmaceuticals, were transformed into the corresponding heteroaryl acetic acid products in moderate-to-good yields. Of particular note, the present method is well suited to synthesize an array of aryl acetic acids bearing complex or bioactive alkyl fragments derived from drugs and natural products. For example, benzylic fluorides bearing menthol and geraniol were viable substrates to generate the desired products 2y and 2z in good yields. To our delight, secondary benzylic fluorides were also amenable to this transformation, providing α-substituted aryl acetic acids efficiently (2aa and 2ab).

Table 2 Substrate scope of benzylic fluoridesa
a Standard conditions: benzylic fluoride (0.2 mmol), HCO2K (0.24 mmol), Na2CO3 (0.2 mmol), 4DPAIPN (0.1 mol%), DMSO (2 mL), N2 atmosphere, 30 W blue LEDs, rt, 4 h. b 4DPAIPN (2.0 mol%), 96 h. c 4DPAIPN (2.0 mol%), 24 h.
image file: d3gc01299a-u3.tif

To showcase the scalability of this transformation, a gram-scale reaction was performed with benzylic fluoride (1a, 10.0 mmol) and HCO2K (Scheme 2). Gratifyingly, a satisfactory isolated yield (80%, 1.7 g) of the corresponding aryl acetic acid 2a was obtained without modification of the standard conditions.


image file: d3gc01299a-s2.tif
Scheme 2 Gram-scale synthesis of aryl acetic acid 2a.

To gain more insights into the mechanism, a series of control experiments were conducted. Firstly, the addition of the radical scavenger 2,2,6,6-tetramethyl-1-piperdiny-1-oxy (TEMPO) to the standard reaction conditions fully suppressed the reaction with no detection of 2a. The adduct of the benzylic radical with TEMPO was detected by high-resolution mass spectrometry (HRMS), which indicated the involvement of the benzylic radical intermediate in the reaction (Fig. 1a). Next, we conducted the deuterium-labeling experiment (Fig. 1b). Under the standard conditions, the deuterium incorporation ratio was up to 90% when D2O was used as an additive, suggesting that a benzylic carbanion intermediate might exist in the reaction process. Finally, the 13C-labeling experiment was conducted under a nitrogen atmosphere with 13C-labeled sodium formate as the carboxylic source (Fig. 1c). Defluorocarboxylation afforded 13C-labeled 2a in a good yield with excellent 13C incorporation (>99% 13C incorporation), which shows great potential in generating a carbon-labeled version of potentially simple carboxylate prodrug derivatives for subsequent absorption, distribution, metabolism, and excretion (ADME) studies.22 This result demonstrated that the formate salt acted as the carboxylic source in this defluorocarboxylation reaction. Finally, Stern–Volmer luminescence studies were conducted (Fig. 1d). The initial results indicated that 1a, sodium carbonate, and potassium formate had no obvious quenching of the excited 4DPAIPN, suggesting that the excited photocatalyst may not likely react with benzylfluorine via an ET (electron transfer) mechanism to produce a radical anion that cleavages to a fluorine anion and a benzyl radical.23 Both methyl 2-mercaptobenzoate and its sodium salt quenched the excited 4DPAIPN under an N2 atmosphere, with the latter showing much higher quenching efficiencies, demonstrating that sodium methyl 2-mercaptobenzoate initiated the photoredox catalytic cycle through reductive quenching of the excited photocatalyst.


image file: d3gc01299a-f1.tif
Fig. 1 Mechanistic investigations.

Based on these experimental results and previous work, we proposed the catalytic cycle as illustrated in Fig. 2. Under the reaction conditions, a mixture of the base and methyl 2-mercaptobenzoate could generate sodium methyl 2-mercaptobenzoate. The latter will undergo a SET process with an excited photocatalyst, providing the reduced PC and a thiyl radical species, which might subsequently undergo HAT with potassium formate to deliver CO2˙− and regenerate 2-mercaptobenzoate. The generated CO2˙− is able to reduce benzylic fluoride through a SET process. The resulting carbon radicals further undergo a SET-reduction process with the reduced PC to give carbanions, which could further react with in situ generated CO2 to give the desired carboxylic acid after the acid workup.


image file: d3gc01299a-f2.tif
Fig. 2 Proposed mechanism.

Conclusions

In summary, we have developed an effective method for the defluorocarboxylation of benzylic C(sp3)–F bonds with HCOOK, which act as both a reductant and a carboxylic source. A variety of benzyl fluorides, bearing primary and secondary C(sp3)–F bonds, undergo defluorinative carboxylation. This transition metal-free strategy provides a mild, efficient, and sustainable approach for accessing a series of valuable aryl acetic acids, such as flurbiprofen. This protocol also features low catalyst loading, mild reaction conditions, good functional group tolerance, and ready scalability and sustainability. Mechanistic investigations validate the importance of the dual role of potassium formate in this transformation and the high efficiency of the CO2 capture process.

Conflicts of interest

There are no conflicts to declare.

Acknowledgements

We gratefully acknowledge the financial support from the National Natural Science Foundation of China (22101192 and 22225106), Sichuan Normal University (024-341914001), the Fundamental Research Funds from Sichuan University (2020SCUNL102), and the Fundamental Research Funds for the Central Universities for the financial support.

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  21. The main side reaction with 4-(bromomethyl)-1,1′-biphenyl is the protonation reaction of the starting material, probably due to the high reactivity of benzyl bromide.
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  23. For the reduction of 1a (Ered = −2.25 V vs. SCE for 1a), see ref. 19. (a) could be predicted to be endothermic by about 0.75 V based on the typical reduction potential of 4DPAIPN (E1/2(PC/PC˙−) = −1.5 V vs. SCE), thus it may be not likely that the reduced photocatalyst can react with benzylic fluoride via an ET mechanism to produce a radical anion that cleaves a fluorine anion and benzyl radical.

Footnote

† Electronic supplementary information (ESI) available. See DOI: https://doi.org/10.1039/d3gc01299a
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