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DOI: 10.1055/a-2293-3370
Visible-Light-Catalyzed Regioselective Arylcarboxylation of Allenes with CO2
Abstract
A visible-light-catalyzed arylcarboxylation of allenes with CO2 was developed using [Ir(ppy)2(dtbbpy)]PF6 (ppy = 2-phenylpyridine; dtbbpy = 4,4′-di-tert-butyl-2,2′-bipyridine) as a photocatalyst to synthesis β-aryl β,γ-unsaturated carboxylic acids. This multicomponent protocol proceeds in an atom-economical way with exclusive regioselectivity. Preliminary mechanistic experiments suggested that allylic carbanion species are the key intermediates.
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As one of the largest classes of organic compounds, carboxylic acids play an important role, not only in organic synthesis, but also in biochemistry and medicinal chemistry, as many drugs and natural products contain a carboxylic acid group.[1] Therefore, it is of great significance to explore an efficient way to introduce carboxylic acid groups into organic compounds. Over the past few decades, many methods have been developed to synthesize carboxylic acids, among which the difunctionalization of unsaturated C–C bonds with CO2 is one of the most direct, efficient, and atom-economical strategies.[2] However, due to the thermodynamic stability and kinetic inertness of CO2, it is challenging to directly utilize CO2 to construct carboxylic acids.
In recent years, visible-light-catalyzed reactions have played an increasingly important role in organic synthesis, and various photocatalytic reactions have emerged.[3] In this context, visible-light-catalyzed difunctionalization of unsaturated C–C bonds with CO2 has attracted significant attention because it allows the simultaneous introduction of a carboxylic acid group and a second functional group, thereby representing a fascinating strategy for rapidly accessing functionalized carboxylic acids.[4] Although well documented for alkenes and alkynes,[5] visible-light-catalyzed difunctionalization of allenes with CO2 has been largely undeveloped, and only a few examples have been reported. In 2021, Hong’s group developed a photoredox catalyzed aminoalkylcarboxylation of aryl allenes with CO2 and N,N-dimethylanilines with moderate to excellent regioselectivity (Scheme [1a]).[6] Later, Yu’s group reported a visible-light-catalyzed dicarboxylation of allenes, with the incorporation of two CO2 molecules (Scheme [1b]).[7] However, both methods suffer from selectivity issues and have poor regioselectivity for some allene substrates, which is a long-standing challenge in the functionalization of allenes. Most recently, our group developed a phosphonocarboxylation of allenes with diarylphosphine oxides and CO2 through visible-light photoredox catalysis, with exclusive regio- and stereoselectivity (Scheme [1c]).[8] Nevertheless, photocatalyzed difunctionalizations of allenes with CO2 are relatively rare and generally display poor selectivity. As a continuation of our work on photocatalyzed selective difunctionalization of unsaturated C–C bonds,[8] [9] we have developed the first visible-light-catalyzed arylcarboxylation of allenes with CO2, delivering β-aryl β,γ-unsaturated carboxylic acids with complete regioselectivity. This strategy involves an efficient and atom-economical multicomponent reaction (Scheme [1d]).


a Reaction conditions: 1a (0.2 mmol), 2a (0.4 mmol, 2 equiv), [Ir(ppy)2(dtbbpy)]PF6 (5 mol%), DABCO (0.1 mmol, 0.5 equiv), K2CO3 (0.5 mmol, 2.5 equiv), HCOONa (0.4 mmol, 2 equiv), DMSO (2 mL), CO2 (1 atm), r.t., 24 h, 20 W blue LED (450 nm).
b Determined by 1H NMR with 1,3,5-trimethoxybenzene as internal standard.
c N.R. = no reaction.
d dF(CF3)ppy = 3,5-difluoro-2-[5-(trifluoromethyl)pyridine.
We began our investigation by employing 1,1-diphenylallene (1a) as the model substrate and iodobenzene (2a) as the arylation reagent with irradiation by a 20 W blue LED in the presence of commercially available [Ir(ppy)2(dtbbpy)]PF6 (ppy = 2-phenylpyridine; dtbbpy = 4,4′-di-tert-butyl-2,2′-bipyridine) as a photocatalyst (PC) under CO2 at atmospheric pressure and ambient temperature for 24 hours (Table [1]). To our delight, the expected reaction proceeded smoothly, delivering the terminal carboxylation product 3a exclusively (Table [1], entry 1). The results of control experiments showed that none of the desired product was obtained in the absence of either light or the photocatalyst (entries 2 and 3). Notably, only a trace amount of the desired product was produced when DABCO, K2CO3, or HCOONa was omitted, indicating that all three are vital to this reaction (entries 4–6), and other alternatives proved to be less efficient (entries 7–10). Moreover, CO2 was also shown to be essential for this transformation (entry 11). Our screening of photocatalysts revealed that [Ir(ppy)2(dtbbpy)]PF6 is the most suitable for the catalytic reaction (entries 12 and 13).


With the optimal reaction conditions in hand, we investigated the scope of the substrates for the reaction, and various allenes and aryl halides were tested. As shown in Scheme [2, 1],1-disubstituted allenes bearing various substituents were subjected to the visible-light-catalyzed arylcarboxylation with CO2. The reaction proceeded efficiently in regioselective manner, producing the terminal carboxylation product exclusively. The main byproducts were allene polymerization products and hydroarylation products. Other byproducts were too complicated to analyze. Besides these, small amounts of the allene (0–10%) were recovered after the reaction. Symmetric 1,1-disubstituted allenes showed good compatibility toward this reaction, generating the corresponding arylcarboxylation product 3a–f in moderate yields. Notably, when the 1-aryl-1-alkylallenes were submitted to the standard reaction condition, high yields of the products were obtained with good selectivity toward the Z-products 3g–l; these are thermodynamically unstable, sterically hindered, and uncommon products for this type of reaction. Unsurprisingly, the reaction showed no Z/E selectivity when asymmetric 1,1-diarylallenes were used as the substrate (3m and 3n). Finally, preliminary studies were conducted to investigate the scope of the aryl iodide, and electron-withdrawing groups (carbonyl and cyano) were found to be tolerated in this reaction (3o and 3p).


Concerning the mechanism, preliminary experiments were conducted to gain further insight into the nature of the allene arylcarboxylation. When five equivalents of a well-known radical scavenger (TEMPO or BHT) were added to the reaction mixture, the desired product was not observed, indicating this transformation might be a radical process (Scheme [3a]). Secondly, we performed isotope labeling studies with D2O under a N2 atmosphere (Scheme [3b]). We found a 78% deuterium incorporation at the terminal position of the hydroarylation product 3a′ when 50 equivalents of D2O were added, indicating the formation of methyl allyl anion intermediate. On the basis of these mechanistic studies and previous works,[5a] we propose a possible mechanism for the reaction of visible-light-catalyzed arylcarboxylation of allenes with CO2 and aryl halides. First, blue light (λ = 450 nm) irradiation promotes the single-electron transfer (SET) between the excited state of [Ir(ppy)2(dtbbpy)]PF6 and DABCO (HAT catalyst) to generate the corresponding DABCO radical cation A, which then traps the hydrogen of the formate salt and provides a CO2 radical anion B. The aryl halide is then reduced by the radical anion B to afford the aryl radical C; this is followed by regioselective radical addition to the allene to afford an allylic radical intermediate D. A second SET between D and the reduced photocatalyst delivers an allylic carbanion intermediate E, which then undergoes nucleophilic addition to CO2 to produce the target arylcarboxylation product.
In conclusion, we have developed the first highly regioselective strategy for the arylcarboxylation of allenes with CO2 through visible-light photoredox catalysis to deliver β-aryl β,γ-unsaturated carboxylic acids,[10] which are widely present in pharmaceuticals. The methodology uses abundant and readily available CO2 as the C1 source and it represents an unprecedented example of a highly regioselective photocatalyzed arylcarboxylation of allenes.
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Conflict of Interest
The authors declare no conflict of interest.
Supporting Information
- Supporting information for this article is available online at https://doi.org/10.1055/a-2293-3370.
- Supporting Information
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References and Notes
- 1a Gooßen LJ, Rodríguez N, Gooßen K. Angew. Chem. Int. Ed. 2008; 47: 3100
- 1b Majumdar N. ACS Catal. 2022; 12: 8291
- 2a Ran C.-K, Liao L.-L, Gao T.-Y, Gui Y.-Y, Yu D.-G. Curr. Opin. Green Sustainable Chem. 2021; 32: 100525
- 2b Ye J.-H, Ju T, Huang H, Liao L.-L, Yu D.-G. Acc. Chem. Res. 2021; 54: 2518
- 3 Romero NA, Nicewicz DA. Chem. Rev. 2016; 116: 10075
- 4a Cao G.-M, Hu X.-L, Liao L.-L, Yan S.-S, Song L, Chruma JJ, Gong L, Yu D.-G. Nat. Commun. 2021; 12: 3306
- 4b Ran C.-K, Xiao H.-Z, Liao L.-L, Ju T, Zhang W, Yu D.-G. Natl. Sci. Open 2023; 2: 20220024 ; DOI:
- 4c Lan J, Lu X, Ren B, Duo F, Niu X, Si J. Org. Biomol. Chem. 2024; 22: 682
- 4d Zhang W, Chen Z, Jiang YX, Liao LL, Wang W, Ye JH, Yu DG. Nat. Commun. 2023; 14: 3529
- 5a Wang H, Gao Y, Zhou C, Li G. J. Am. Chem. Soc. 2020; 142: 8122
- 5b Hou J, Ee A, Cao H, Ong HW, Xu JH, Wu J. Angew. Chem. Int. Ed. 2018; 57: 17220
- 5c Yin Z.-B, Ye J.-H, Zhou W.-J, Zhang Y.-H, Ding L, Gui Y.-Y, Yan S.-S, Li J, Yu D.-G. Org. Lett. 2018; 20: 190
- 5d Ye J.-H, Miao M, Huang H, Yan S.-S, Yin Z.-B, Zhou W.-J, Yu D.-G. Angew. Chem. Int. Ed. 2017; 56: 15416
- 6 Hahm H, Baek D, Kim D, Park S, Ryoo JY, Hong S. Org. Lett. 2021; 23: 3879
- 7 Ju T, Zhou Y.-Q, Cao K.-G, Fu Q, Ye J.-H, Sun G.-Q, Liu X.-F, Chen L, Liao L.-L, Yu D.-G. Nat. Catal. 2021; 4: 304
- 8 Zeng J.-H, Du D.-T, Liu B.-E, Zhang Z.-Q, Zhan Z.-P. J. Org. Chem. 2023; 88: 14789
- 9 Gao C, Zeng J, Zhang X, Liu Y, Zhan Z.-p. Org. Lett. 2023; 25: 3146
- 10 Photocatalyzed Arylcarboxylation of Allenes with CO2: General Procedure An oven-dried 10 mL Schlenk tube equipped with a magnetic stirrer bar was charged with the appropriate allene 1 (0.2 mmol, 1 equiv, if solid), DABCO (0.1 mmol, 0.5 equiv), K2CO3 (0.5 mmol, 2.5 equiv), HCOONa (0.4 mmol, 2 equiv), and Ir[(ppy)2dtbbpy]PF6 (5 mol%). The tube was sealed and degassed by three cycles of vacuum evacuation and subsequent backfilling with CO2. Subsequently, the appropriate allene 1 (0.2 mmol, 1 equiv, if liquid), the appropriate aryl iodide 2 (0.4 mmol, 2 equiv), and anhyd DMSO (2 mL) were added. The tube was placed under a blue LED (λ = 450 nm, 20 W) and irradiated for 24 h at r.t. The mixture was then acidified with 2 N aq HCl (1 mL) and quenched with H2O. It was then extracted with EtOAc (×3) and the combined organic layers were dried (MgSO4) and concentrated under reduced pressure. The crude product was purified by column chromatography [silica gel, hexane–EtOAc (5:1 to 1:1)] or by preparative TLC (hexane–EtOAc, 1:1). 3,4,4-Triphenylbut-3-enoic acid (3a) Prepared by following the general procedure and purified by column chromatography [silica gel, hexane–EtOAc (3:1)] to give a white solid; yield: 27 mg (43%). 1H NMR (400 MHz, CDCl3): δ = 7.38–7.27 (m, 5 H), 7.19–7.10 (m, 5 H), 7.09–6.99 (m, 3 H), 6.96–6.88 (m, 2 H), 3.58 (s, 2 H). 13C NMR (101 MHz, CDCl3): δ = 177.23, 143.75, 142.66, 142.21, 141.36, 131.80, 130.75, 129.80, 129.52, 128.58, 128.10, 127.64, 127.42, 126.87, 126.50, 41.57. HRMS (ESI): m/z [M + H]+ calcd for C22H19O2: 315.1380; found: 315.1382.
For selected examples and reviews of carboxylations using CO2 as a C1 source, see:
Corresponding Authors
Publication History
Received: 12 March 2024
Accepted after revision: 22 March 2024
Accepted Manuscript online:
22 March 2024
Article published online:
09 April 2024
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References and Notes
- 1a Gooßen LJ, Rodríguez N, Gooßen K. Angew. Chem. Int. Ed. 2008; 47: 3100
- 1b Majumdar N. ACS Catal. 2022; 12: 8291
- 2a Ran C.-K, Liao L.-L, Gao T.-Y, Gui Y.-Y, Yu D.-G. Curr. Opin. Green Sustainable Chem. 2021; 32: 100525
- 2b Ye J.-H, Ju T, Huang H, Liao L.-L, Yu D.-G. Acc. Chem. Res. 2021; 54: 2518
- 3 Romero NA, Nicewicz DA. Chem. Rev. 2016; 116: 10075
- 4a Cao G.-M, Hu X.-L, Liao L.-L, Yan S.-S, Song L, Chruma JJ, Gong L, Yu D.-G. Nat. Commun. 2021; 12: 3306
- 4b Ran C.-K, Xiao H.-Z, Liao L.-L, Ju T, Zhang W, Yu D.-G. Natl. Sci. Open 2023; 2: 20220024 ; DOI:
- 4c Lan J, Lu X, Ren B, Duo F, Niu X, Si J. Org. Biomol. Chem. 2024; 22: 682
- 4d Zhang W, Chen Z, Jiang YX, Liao LL, Wang W, Ye JH, Yu DG. Nat. Commun. 2023; 14: 3529
- 5a Wang H, Gao Y, Zhou C, Li G. J. Am. Chem. Soc. 2020; 142: 8122
- 5b Hou J, Ee A, Cao H, Ong HW, Xu JH, Wu J. Angew. Chem. Int. Ed. 2018; 57: 17220
- 5c Yin Z.-B, Ye J.-H, Zhou W.-J, Zhang Y.-H, Ding L, Gui Y.-Y, Yan S.-S, Li J, Yu D.-G. Org. Lett. 2018; 20: 190
- 5d Ye J.-H, Miao M, Huang H, Yan S.-S, Yin Z.-B, Zhou W.-J, Yu D.-G. Angew. Chem. Int. Ed. 2017; 56: 15416
- 6 Hahm H, Baek D, Kim D, Park S, Ryoo JY, Hong S. Org. Lett. 2021; 23: 3879
- 7 Ju T, Zhou Y.-Q, Cao K.-G, Fu Q, Ye J.-H, Sun G.-Q, Liu X.-F, Chen L, Liao L.-L, Yu D.-G. Nat. Catal. 2021; 4: 304
- 8 Zeng J.-H, Du D.-T, Liu B.-E, Zhang Z.-Q, Zhan Z.-P. J. Org. Chem. 2023; 88: 14789
- 9 Gao C, Zeng J, Zhang X, Liu Y, Zhan Z.-p. Org. Lett. 2023; 25: 3146
- 10 Photocatalyzed Arylcarboxylation of Allenes with CO2: General Procedure An oven-dried 10 mL Schlenk tube equipped with a magnetic stirrer bar was charged with the appropriate allene 1 (0.2 mmol, 1 equiv, if solid), DABCO (0.1 mmol, 0.5 equiv), K2CO3 (0.5 mmol, 2.5 equiv), HCOONa (0.4 mmol, 2 equiv), and Ir[(ppy)2dtbbpy]PF6 (5 mol%). The tube was sealed and degassed by three cycles of vacuum evacuation and subsequent backfilling with CO2. Subsequently, the appropriate allene 1 (0.2 mmol, 1 equiv, if liquid), the appropriate aryl iodide 2 (0.4 mmol, 2 equiv), and anhyd DMSO (2 mL) were added. The tube was placed under a blue LED (λ = 450 nm, 20 W) and irradiated for 24 h at r.t. The mixture was then acidified with 2 N aq HCl (1 mL) and quenched with H2O. It was then extracted with EtOAc (×3) and the combined organic layers were dried (MgSO4) and concentrated under reduced pressure. The crude product was purified by column chromatography [silica gel, hexane–EtOAc (5:1 to 1:1)] or by preparative TLC (hexane–EtOAc, 1:1). 3,4,4-Triphenylbut-3-enoic acid (3a) Prepared by following the general procedure and purified by column chromatography [silica gel, hexane–EtOAc (3:1)] to give a white solid; yield: 27 mg (43%). 1H NMR (400 MHz, CDCl3): δ = 7.38–7.27 (m, 5 H), 7.19–7.10 (m, 5 H), 7.09–6.99 (m, 3 H), 6.96–6.88 (m, 2 H), 3.58 (s, 2 H). 13C NMR (101 MHz, CDCl3): δ = 177.23, 143.75, 142.66, 142.21, 141.36, 131.80, 130.75, 129.80, 129.52, 128.58, 128.10, 127.64, 127.42, 126.87, 126.50, 41.57. HRMS (ESI): m/z [M + H]+ calcd for C22H19O2: 315.1380; found: 315.1382.
For selected examples and reviews of carboxylations using CO2 as a C1 source, see:






