Double carboxylation of o-alkynyl acetophenone with carbon dioxide

Abstract

A copper-catalyzed double carboxylation of o-alkynyl acetophenone using carbon dioxide to afford 1(3H)-isobenzofuranylidene dicarboxylic esters in good yields is described. The reaction proceeds via a carboxylation/intramolecular cyclization/carboxylation sequence. Alkyl-substituted substrates show much higher double carboxylation product selectivities than aryl-substituted substrates.

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The utilization of carbon dioxide in organic synthesis has gained significant attention since it provides the opportunity to convert this greenhouse gas into value-added fine chemicals.¹ In the last few decades, various methodologies using carbon dioxide as a carboxylative reagent have been developed, offering attractive and powerful access to synthetically important carboxylic acids and derivatives.^2–9 Generally, one single carbon dioxide molecule is incorporated into a product in these carboxylation reactions. Double carboxylation of a substrate using carbon dioxide is rarely reported, and electrochemical conditions,¹⁰ stoichiometric metal complexes¹¹ or specific substrates¹² are used to enable the second carbon dioxide insertion. Most recently, Tsuji et al. disclosed a nickel-catalyzed reductive double carboxylation of internal alkynes with carbon dioxide to produce maleic anhydrides.¹³ Herein, we describe a CuI-catalyzed double carboxylation of o-alkynyl acetophenone using carbon dioxide to give 1(3H)-isobenzofuranylidene dicarboxylic esters in good yields.

Yamada¹⁴ and our group¹⁵ previously found a silver-catalyzed sequential carboxylation/intramolecular cyclization reaction of o-alkynyl acetophenone with carbon dioxide, in which β-ketocarboxylate coming from the α-carboxylation of the carbonyl group¹⁶ initiated a nucleophilic attack towards the silver(I)-activated alkyne moiety to produce the 1(3H)-isobenzofuranylidene acetate product (Scheme 1, a). It is noteworthy that a new metal–carbon bond might be formed during the above cyclization reaction.¹⁷ We envisioned that with a judicious choice of catalytic and reaction systems, the second carbon dioxide molecule would insert into this newly-formed metal–carbon bond,¹⁸ and thus a double carboxylation product would be obtained via a carboxylation/intramolecular cyclization/carboxylation sequence (Scheme 1, a). Indeed, Ma once reported a copper-catalyzed carboxylation of a vinylic metallic intermediate generated by cyclic anti-azametallation of 2-alkynylaniline using carbon dioxide in the presence of dimethylzinc and cesium fluoride.^18a Encouraged by the above mechanistic and experimental investigations, we chose the model reaction of 2-(phenylethynyl)phenylethanone (1a) with carbon dioxide as used previously¹⁵ to identify the optimal catalytic system for the double carboxylation reaction.

Scheme 1 Reaction of o-alkynyl acetophenone with carbon dioxide.

As shown in Table 1, a combination of 10 mol% of CuI and 4 equiv. of DBU in DMF at 60 °C gave 86% yield of the mono-carboxylation product 3a, and no double carboxylation product 2a was observed (entry 1). When cesium carbonate was used as a base, although 3a was still the major product, 24% yield of 2a was isolated (entry 2). The structure of product 2a was determined unambiguously by single crystal X-ray diffraction (Fig. 1).¹⁹ The configuration of two double bonds in 2a was also confirmed by NOE studies. The reactions in DMAc and NMP also gave a much higher yield of 3a than that of 2a (entries 3 and 4). It's delightedly found that when DMSO was used as a solvent, 54% yield of double carboxylation product 2a was obtained as the major product (entry 5). A decrease in cesium carbonate's quantity from 4 equiv. to 2 equiv. obviously switches the product selectivity (entry 6). Cesium fluoride proved to be a less efficient base (entry 7), and potassium carbonate or tert-butoxide was not the suitable base for this reaction (entries 8 and 9). The reaction at a decreased temperature (25 °C) resulted in a low conversion and a lower yield of 2a was obtained (entry 10). Increasing the reaction temperature to 100 °C didn't enhance the yield of 2a (entry 11). Both introduction of a nitrogen ligand to the catalytic system and the use of the N-heterocyclic carbene copper(I) complex led to the major formation of the mono-carboxylation product 3a (entries 12–14).

Fig. 1 ORTEP plot of 2a shown with ellipsoids at the 30% probability level; hydrogen atoms were omitted for clarity.

Table 1 Reaction of 2-(phenylethynyl)phenylethanone with carbon dioxide^a

Entry	Cat. (mol%)	Base	T/°C	2a /%	3a /%
a Reaction conditions: 1a (0.2 mmol), base (0.8 mmol), 2 mL DMSO as the solvent, 2.0 MPa of carbon dioxide, 24 h; then MeI (1.0 mmol), 30 °C, 3 h. b Isolated yield. c DMF as the solvent. d DMAc as the solvent. e NMP as the solvent. f 2.0 equiv. cesium carbonate. g Cu(CH3CN)4PF6 as the catalyst. h 0.1 MPa of carbon dioxide. i In the absence of carbon dioxide. L1: 1,10-phenanthroline. L2: 5-nitro-1,10-phenanthroline.
1^c	CuI (10)	DBU	60	—	86
2^c	CuI (10)	Cs2CO3	60	24	52
3^d	CuI (10)	Cs2CO3	60	16	63
4^e	CuI (10)	Cs2CO3	60	22	56
5	CuI (10)	Cs2CO3	60	54	32
6^f	CuI (10)	Cs2CO3	60	24	58
7	CuI (10)	CsF	60	33	10
8	CuI (10)	K2CO3	60	—	—
9	CuI (10)	KO^tBu	60	—	—
10	CuI (10)	Cs2CO3	25	28	7
11	CuI (10)	Cs2CO3	100	54	28
12	CuI + L1 (10)	Cs2CO3	60	24	74
13	CuI + L2 (10)	Cs2CO3	60	22	68
14	IPrCuCl (10)	Cs2CO3	60	40	59
15	CuBr (10)	Cs2CO3	60	33	45
16	CuCl (10)	Cs2CO3	60	37	55
17^g	CuPF6 (10)	Cs2CO3	60	28	43
18	CuCl2 (10)	Cs2CO3	60	41	28
19	PdCl2 (10)	Cs2CO3	60	37	11
20	AgI (10)	Cs2CO3	60	19	59
21	CuI (2)	Cs2CO3	60	51	35
22^h	CuI (10)	Cs2CO3	60	36	39
23	—	Cs2CO3	60	16	3
24	CuI (10)	—	60	—	—
25^i	CuI (10)	Cs2CO3	60	—	—

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Other copper(I) salts such as CuBr, CuCl, and Cu(CH₃CN)₄PF₆ show inferior double product selectivities than CuI (entries 15–17). CuCl₂ and PdCl₂ can also catalyze the reaction to afford the major double carboxylation product (entries 18 and 19). The silver(I) catalyst previously used in our mono-carboxylation reaction¹⁵ shows a very low double carboxylation product selectivity (entry 20). When the catalyst loading decreased from 10 mol% to 2 mol%, a slightly lower yield of 2a was formed (entry 21). The product selectivity is sensitive to CO₂ pressure and the reaction using atmospheric carbon dioxide gives both 36% yield of 2a and 39% yield of 3a (entry 22). The reaction in the absence of CuI proceeds very slowly, which clearly demonstrates the catalytic role of the copper salt (entry 23). The reaction without a base provides no product, which implies the essential role of the base to deprotonate the α-H of the carbonyl group and then trigger the following carboxylation reaction (entry 24). No carboxylation product was detected when the reaction was carried out in the absence of carbon dioxide, showing that the CO₂ moiety in the carboxylation product comes from carbon dioxide, not from cesium carbonate (entry 25).

The substrate scope of the copper-catalyzed double carboxylation of o-alkynyl acetophenones was then investigated using 10 mol% CuI as the catalyst, 4 equiv. of cesium carbonate as the base in DMSO at 60 °C (Table 2). A variety of aryl-substituted o-alkynyl acetophenones conducted the double carboxylation reaction smoothly to give 1(3H)-isobenzofuranylidene dicarboxylic esters in moderate to good yields (2a–2g). The electron-deficient phenylethynyl substrates (1e, 1f) show higher double carboxylation product selectivities than the electron-rich phenylethynyl substrates (1b–1d, 1g).

Table 2 Substrate scope^a,b

a Reaction conditions: 1 (0.2 mmol), CuI (0.02 mmol), Cs₂CO₃ (0.8 mmol), 2 mL DMSO as the solvent, 2.0 MPa of carbon dioxide, 60 °C, 24 h; then MeI (1.0 mmol), 30 °C, 3 h. b Isolated yields (2/3) are given. c Compound 4 was isolated as the main product. d 1q was used as a substrate.

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Alkyl-substituted o-alkynyl acetophenones show much higher double carboxylation product selectivities than aryl-substituted substrates. When 1h was used as the substrate, 91% yield of the double carboxylation product 2h was isolated and only a trace mono-carboxylation product 3h was observed in mass spectroscopy. The steric factor has a profound effect on the product selectivity, and the bulky substitutes on the alkyne moiety lead to the decreased selectivity of the double carboxylation product (2k, 2m), presumably due to the hindrance of the insertion of the second carbon dioxide. When the tert-butyl substituted substrate 1n was used, only traces of carboxylative cyclization products were observed in mass spectroscopy. Instead, compound 4 was isolated as the major product. When 1q containing a chloride group to intramolecularly trap the carboxylate intermediate was used as a substrate, a lactone product 5 was formed exclusively in good yield.

Although the detailed mechanism is not clear, a possible pathway (Scheme 2) could initially involve a base-promoted carboxylation of o-alkynyl acetophenone with carbon dioxide.^14–16 Then the copper(I)-catalyzed intramolecular cyclization would afford a vinylic copper intermediate A.¹⁸ The following insertion of the second carbon dioxide into the newly-formed carbon–copper bond would finally furnish the double carboxylation product 2. Meanwhile, the competitive protonation of the vinylic copper intermediate would irreversibly give the mono-carboxylation product 3. When the standard reaction was carried out in the presence of 1 equiv. of D₂O, the yields of the 2a/3a evidently changed from 54/28% to 26/45% (Scheme 3, a). The mono-carboxylation product 3a′ cannot convert into the double carboxylation product under the standard reaction conditions (Scheme 3, b). When a mixture of 2a′ and 3a′ (molar ratio: 2a′/3a′ = 3.8 : 1) were subjected to the standard catalytic system, a decreased molar ratio of 2a/3a was obtained (Scheme 3, c), which demonstrated that the second carboxylation might be reversible. Therefore, the relatively high carbon dioxide pressure and large excess of the base are beneficial to the formation of the double carboxylation product.

Scheme 2 Possible mechanism.

Scheme 3 Control experiments. Standard conditions: CuI (10 mol%), Cs₂CO₃ (4 equiv.), DMSO (0.1 M), 2.0 MPa of carbon dioxide, 60 °C, 24 h; then MeI (5 equiv.), 30 °C, 3 h.

In summary, we have developed a copper-catalyzed double carboxylation of o-alkynyl acetophenone using carbon dioxide as a carboxylative reagent to give 1(3H)-isobenzofuranylidene dicarboxylic esters in moderate to good yields. This sequential reaction is triggered by α-carboxylation of the carbonyl group with carbon dioxide and proceeds via a carboxylation/intramolecular cyclization/second carboxylation sequence. The substitute in the alkyne moiety has a profound effect on the reaction product and alkyl-substituted substrates show much higher double carboxylation product selectivities than aryl-substituted substrates. Further mechanistic studies are currently under investigation in our laboratory.

Acknowledgements

This work was supported by the National Natural Science Foundation of China (21172026), the Fundamental Research Funds for the Central Universities (DUT15LAB21), and the Program for Changjiang Scholars and Innovative Research Team in University (IRT13008). X.-B. Lu gratefully acknowledges the Chang Jiang Scholars Program (no. T2011056) from Ministry of Education, People's Republic of China.

Notes and references

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Footnote

† Electronic supplementary information (ESI) available: Experimental details and data. CCDC 1406654. For ESI and crystallographic data in CIF or other electronic format see DOI: 10.1039/c5qo00374a

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