Copper- and copper–N-heterocyclic carbene-catalyzed C─H activating carboxylation of terminal alkynes with CO2 at ambient conditions

Dingyi Yu and Yugen Zhang1

Institute of Bioengineering and Nanotechnology, 31 Biopolis Way, The Nanos, Singapore 138669, Singapore

Edited by Jack Halpern, University of Chicago, Chicago, IL, and approved October 4, 2010 (received for review July 26, 2010)

The use of carbon dioxide as a renewable and environmentally or an excess amount of organometallic reagents for transmetalla- friendly source of carbon in organic synthesis is a highly attractive tion processes. An alternative possibility to achieve the catalytic approach, but its real world applications remain a great challenge. synthesis of carboxylic acid from CO2 is by direct C─H bond ac- The major obstacles for commercialization of most current proto- tivation and carboxylation. Very recently, Nolan’s group reported cols are their low catalytic performances, harsh reaction conditions, a gold-catalyzed CO2 carboxylation of C─H bonds of highly and limited substrate scope. It is important to develop new reac- activated arenes and heterocycles (32). Herein, we reported a tions and new protocols for CO2 transformations at mild conditions copper- and copper–N-heterocyclic carbene (NHC)-catalyzed and in cost-efficient ways. Herein, a copper-catalyzed and copper– transformation of CO2 to carboxylic acid through C─H bond ac- N -heterocyclic carbene-cocatalyzed transformation of CO2 to car- tivation and carboxylation of terminal alkynes. Various propiolic boxylic acids via C─H bond activation of terminal alkynes with acids were synthesized in good to excellent yields under ambient or without base additives is reported. Various propiolic acids were conditions. This catalytic system is a simple and economically synthesized in good to excellent yields under ambient conditions viable protocol with great potential in practical applications. without consumption of any organometallic or organic reagent ad- ditives. This system has a wide scope of substrates and functional Results and Discussion CHEMISTRY group tolerances and provides a powerful tool for the synthesis The ubiquity of alkynyl carboxylic acids in a vast array of medic- of highly functionalized propiolic acids. This catalytic system is a inally important compounds as well as its tremendous utility as a simple and economically viable protocol with great potential in synthon in organic synthesis makes them particularly attractive practical applications. targets for pharmaceutical and fine-chemical as well as conduc- tive polymer synthesis (16, 17, 33, 34). A plethora of well-estab- he chemical fixation and transformation of carbon dioxide lished methods for the preparation of alkynyl carboxylic acids includes CO2 insertion into the metal-carbon bond of organome- T(CO2) has attracted much attention in view of environmental, legal, and social issues in the past few decades (1–7). Carbon tallic reagents, the well-known hydrolysis of bromide and related dioxide is an attractive C1 building block in organic synthesis derivatives, and the oxidation of preoxidized substrates, such as because it is an abundant, renewable carbon source and an alcohols or aldehydes (Fig. 1 A–C) (35). Despite the efficiency of environmentally friendly chemical reagent (8–13). The utiliza- these conventional procedures, their major drawback includes tion, as opposed to the storage of CO2, is indeed more attractive severe reaction conditions and restrictions of organometallic especially if the conversion process to useful bulk products is an reagents that dramatically limit the synthesis of a wide scope of economical one. Significant efforts have been devoted toward functionalized propiolic acids. Therefore, a functional group tol- exploring technologies for CO2 transformation, whereby harsh erant and straightforward method for accessing alkynyl carboxylic and severe reaction conditions are one of the major limitations acids (Fig. 1D) is highly desired and will provide opportunities in for their practical applications (1–15). Therefore, the develop- organic, pharmaceutical, and materials synthesis. ment of efficient catalyst systems for CO2 utilization under mild Organocopper reagents are very unique because the metal-car- conditions is highly desired, especially for real world applications. bon bond is of moderate polarity and is ready for CO2 insertion Carboxylic acids are one of the most important types of com- under ambient conditions, and they are also tolerant to most pounds in medicinal chemistry and also in fine-chemicals synth- functional groups (18). Copper catalysts can also catalyze various esis (16, 17). Although there are many well-established protocols C─H and C-halogen activation reactions, and many of them in- for the preparation of carboxylic acids, the direct carboxylation of volve intermediates with a Cu─C bond (36–38). These facts make carbon nucleophiles using CO2 as the electrophile is the most copper catalysts a very promising choice for CO2 transformation, attractive and straightforward method (16, 17). The formation especially with the formation of new C─C bonds. Hay (39) devel- of a stable C─C bond is desired for CO2 fixation and remains oped oxidative homocoupling of terminal alkynes via C─H acti- the most challenging aspect thus far. Typically, this type of reac- vation and copper acetylide intermediate. After that, carbon tion is facilitated by the insertion of CO2 into a metal-carbon dioxide was successfully introduced into the copper–terminal bond (16–26). Widespread use of these methods is limited by alkyne system, and the insertion of CO2 into the copper acetylide the synthesis organometallic reagents as precursors and the intermediate was observed by Inoue’s group (40). However, the restricted substrate scope. copper propynoate intermediate is unstable under the applied reaction conditions (100 °C, 1 atm CO2). Propiolic acid products catalyst RM CO2 R-COOM (M=Li, Mg, Cu, Mn, Sn, Zn, Al, B) Author contributions: Y.Z. designed research; D.Y. and Y.Z. performed research; D.Y. and Y.Z. analyzed data; and D.Y. and Y.Z. wrote the paper. In the past decades, several interesting systems have been The authors declare no conflict of interest. reported for metal-mediated reductive carboxylation of alkynes This article is a PNAS Direct Submission. (27, 28), allenes (29, 30), and alkyenes (31) with CO2 to form 1To whom correspondence should be addressed. E-mail: [email protected]. carboxylic acids or esters. However, most of those systems need This article contains supporting information online at www.pnas.org/lookup/suppl/ either a stoichiometric amount of transition metals as reactants doi:10.1073/pnas.1010962107/-/DCSupplemental.

www.pnas.org/cgi/doi/10.1073/pnas.1010962107 PNAS Early Edition ∣ 1of6 Downloaded by guest on October 2, 2021 Fig. 1. Protocols for synthesis of substituted propiolic acids.

could not be isolated (40). We assumed that the copper propyno- Kinetics studies of this reaction showed that 1b was formed ate intermediate should be stable under milder conditions, such rapidly in the first 4 h, reaching a yield of 70%, before the gradual as room temperature, and the carboxylate end product could be increased to the final yield of 90% in 16 h. The yield remained dissociated from the copper catalyst under normal basic condi- constant around 90% even with further increasing the reaction tions. On the basis of this hypothesis, the initial proof of concept time from 16 to 24 h (Fig. S1). experiment was conducted by using 2 mol% of CuCl, 1.5 mol% of With the optimized reaction conditions of 2.0 mol% CuCl, 0 0 TMEDA (N;N;N ;N -tetramethylethylenediamine) ligand (L1), 1.5 mol% TMEDA, and 120 mol% K2CO3 in DMF for 16 h, and K2CO3 as the base for the carboxylation of 1-ethynylbenzene the substrate scope of the reaction was studied. For aromatic 1a at ambient temperature and atmospheric pressure (Table 1, alkynes with or without electron donation functional groups, entry 1). Remarkably, phenylpropiolic acid 1b was produced in the corresponding alkynyl carboxylic acids were obtained in excellent yield after acid workup. The isolated pure product 80–91% yields (isolated) under standard conditions (Table 1, en- was characterized by NMR and elemental analysis. Phenylpropio- tries 1–11). The catalytic system is not sensitive to the position of lic acid was further converted into the methyl ester and charac- substituents on the benzene ring. The related acid yields of p-, m-, terized by NMR and GC/MS. Interestingly, the catalytic reaction o-substituted 1-ethylbenzene are approximately in the same could be performed without any base additives to give 55% of 1b range. The transformations proceeded smoothly without any side (Tables S1 and S2). Further studies indicated that a catalytic product formation. The initial trials for the carboxylation of the amount of base is important to activate the CuCl catalyst. Mean- alkyl-substituted alkynes were unsatisfactory with a low yield of while, a weak basic environment is necessary to stabilize the acid the corresponding acids (∼20%). The low reactivity of alkyl- product and to ensure that the reaction is completed. In a catalyst substituted alkynes is probably because of the weak acidity of system with basic ligands (Table S2, entries 1–3) or a catalytic the alkyne proton. The conjugation system between the benzene amount of base (Table S2, entry 13), a moderate yield of 1b ring and alkyne C≡C bond in aryl alkynes imposes more negative was obtained when the reaction was conducted in dimethylforma- charge on C1 carbon and makes it a stronger nucleophile than mide (DMF) (a weak Lewis base) but not in THF, CH3CN, or alkyl alkynes. A density functional theory calculation [B3LYP/ DMSO (Table S2). Stoichiometric amounts of base additives 6-31G(d,p) level] indicated that the negative charge on C1 carbon further promote the reaction, and a good yield of 1b could be of 1-ethynylbenzene is −0.534 and that of 1-hexyne is −0.461. obtained in DMF as well as in THF, CH3CN, and DMSO After investigating further, a stronger base Cs2CO3 was used in- (Tables S1 and S3). This result demonstrated a valuable example stead of K2CO3, and the yield of corresponding alkyl-substituted of a base free C─C coupling reaction system. propiolic acids was raised to 80–91% (Table 1, entries 12–16). When the reaction was conducted with 2.0 mol% CuCl catalyst In general, terminal aromatic alkynes with an electron with- in the absence of ligands, the yield of 1b dropped to 50% drawing group are deactivated and often inert to many transfor- (Table S4, entry 8). This result indicates that σ donor ligand can mations (43, 44). With the electron withdrawing group on the increase the catalyst activity (41). Other σ donor ligands, such as phenyl ring, the nucleophilicity of the C1 carbon of alkynes N;N0-dimethylethanediamine, 1,3-dimesitylimidazol-2-ylidene, dropped dramatically. The carboxylation of 4-nitro-1-ethynylben- and 1,8-diazabicyclo[5.4.0]undec-7-ene, also work well for this zene 19a was unsatisfactory with a very low yield of the corre- catalytic system (Table S4). For the carboxylation of 1-ethynyl- sponding acids 19b (0 ∼ 8%) under standard conditions even benzene 1a, when the catalyst loadings was reduced from with a very strong base, such as KOtBu (Table S5). The low yields 2 mol% to 0.5 mol%, good to excellent yields were still obtained (∼2%) were also observed as the reaction temperature was after prolonged reaction time (24 h) (Table S4). The yield of 1b adjusted to 0 °C and 50 °C. This result may be because of the decreased sharply to 8% when the catalyst loading was further low reaction rate at low temperature and instability of the reac- reduced to 0.1 mol%. No reaction was observed for the control tion intermediate at high temperature (Table S5). The key step experiments with K2CO3 or Cs2CO3 as the base and without for this transformation is CO2 insertion into the copper acetylide copper catalyst (Table S4, entries 1 and 14) (42). intermediate. Increasing the nucleophilicity of the carbanionic

2of6 ∣ www.pnas.org/cgi/doi/10.1073/pnas.1010962107 Yu and Zhang Downloaded by guest on October 2, 2021 Table 1. Copper-catalyzed carboxylation of terminal alkynes Table 2. Cu-NHC–catalyzed carboxylation of deactivated terminal with CO2 alkynes with CO2

Base, 2-5 mol% CuCl, Ligand HCl OH N N Poly-NHC R H + CO2 R DMF, R.T., 1 atm O L1 L13 OH Cs2CO3, CuCl, Ligand HCl EW H + CO2 EW Isolated DMF, R.T., 1 atm O yields, %

N N HN N NH P N N N Entry Alkynes Time, h Base L1 L13 N N

1 16 K2CO3 90 95 L1 LL32 4L L5 L6

N 2 18 K2CO3 81 85 N O N N N NN N N NN N

L7 L8 L9 L10 N 3 O 18 K2CO3 86 90 N N N N N N N NNN N N N

4 18 K2CO3 89 92 N N O N N L11 L12 L13 5 F 16 K2CO3 80 82 Ligand CuCl Time Yields Entry Alkyne (mol%) (mol%) h %

6 F 16 K2CO3 85 86 1 L1 (10) 5 24 <5 O2N

7 16 K2CO3 88 86 2 L2 (10) 5 60 2 F 3 L3 (10) 5 60 3

8 Cl 16 K2CO3 84 90 4 L4 (10) 5 60 8 5 L5 (10) 5 60 5 CHEMISTRY 6 L6 (10) 5 60 9

9 Cl 16 K2CO3 86 88 7 L7 (10) 5 60 9 8 L8 (10) 5 48 21 9 L9 (10) 5 48 40 10 16 K2CO3 86 90 10 L10 (10) 5 48 47 Cl 11 L11 (10) 5 48 30 11 24 K2CO3 89 93 12 L12 (20) 5 48 32 S 13 L12 (10) 5 48 25

12 24 Cs2CO3 80 83 14 L13 (10)* 5 48 70 HO 15 L13 (10)* 15 48 18 13 24 Cs2CO3 82 88 16 L13 (10)* 5 36 68 NC OHC

14 H COOC 24 Cs2CO3 82 90 3 17 L13 (10)* 5 24 72 NC 15 n-C4H9 24 Cs2CO3 85 85

16 24 Cs2CO3 83 85 18 L13 (10)* 5 36 73 HO Reaction conditions: for L1, CuCl (2.0 mol%), TMEDA, 1.5 mol%; for L13, 19 L13 (10)* 5 36 79 ð Þ ð ─ Þ P NHC 0.5 NHC Cu 0.5, 5 mol%; alkynes (2.0 mmol), base (2.4 mmol), CO2 (1 atm), DMF (4 mL), room temperature (25 °C). HO

Reaction conditions: alkynes (2.0 mmol), CuCl, Cs2CO3 (2.4 mmol), ligand, intermediate may increase the yield of the carboxylic acid pro- CO2 (1 atm), DMF (4 mL), RT. duct. Various ligands (L1–L12, Table 2) (45) were screened in *10 mol% of NHC. the reaction with 4-nitro-1-ethynylbenzene 19a and the yields of the acid product ranged from 3% to 47%. The catalyst with 36–48 h (Table 2). The longer reaction time may be because the strongest electron donation ligand phenanthroline L10 gave of the heterogeneous reaction behavior in this solid catalyst the highest yield. It is well known that NHCs can activate CO2 system. The mechanism of this unique process is intriguing, and in various catalytic transformations (5, 7). With that, a unique related control experiments were conducted. We used 4-nitro-1- NHC-Cu cocatalyst was designed by using poly-N-heterocyclic ethynylbenzene as a model substrate. No reaction was observed in carbene (PNHC) as both ligand and catalyst. PNHC has a three- a system with PNHC only. When an additional two portions of dimensional network structure, and the carbene units are located CuCl were added into the P1 catalyst reaction system, the yield and fixed in the backbone of the network (Fig. 2) (46, 47). of the desired product dropped dramatically to 18% (Table 2, ð Þ ð ─ Þ P1 P NHC 0.5 NHC Cu 0.5 ( ) catalyst was prepared by the reac- entry 15). This result indicated that free carbene species in the tion of one equivalent of CuCl with two equivalents of PNHC. catalyst play an important role for high activity. Furthermore, ð ─ Þ ð ─ Þ In the structure of this catalyst, only half of the carbene species a reaction intermediate P NHC CO2 0.5 NHC Cu 0.5 was ð Þ ð ─ Þ P1 coordinated with copper, and the other half remained as free car- synthesized by the reaction of P NHC 0.5 NHC Cu 0.5 ( ) with benes (46). The initial experiment was conducted by using 5 mol CO2. This intermediate was directly used to react with a stoichio- %ofP1 and Cs2CO3 as a base for the carboxylation of 4-nitro-1- metric amount of 1-ethynylbenzene (equivalent to NHC─CO2) ethynylbenzene 19a with CO2 at ambient conditions. Remark- under standard conditions without an additional CO2 source. ably, 4-nitro-phenylpropiolic acid 19b was produced in 70% yield A 52% yield of phenylpropiolic acid was obtained in 24 h. With after acid workup. Good yields were also achieved for terminal these experiment results, it is believed that the unique structure aromatic alkynes with different electron withdrawing groups in of P1 catalyst is the key to the high activities. The free carbene

Yu and Zhang PNAS Early Edition ∣ 3of6 Downloaded by guest on October 2, 2021 ─ ð Þ ð ─ Þ Fig. 2. Structures of poly NHC and P NHC 0.5 NHC Cu 0.5 catalyst.

species in the structure are randomly located around the copper process (Fig. 3B). As the temperature was raised from ambient center and act as an organocatalyst to activate CO2. This essential temperature (25 °C) to 60 °C for the reaction of 1a, the yield of 1b step may reduce the activation energy barrier for CO2 insertion. dropped from 90% to 42%. Instead, some homocoupling by-pro- duct c was observed. Under room temperature condition, because A 0.5 eq. P(NHC-CO2)2(NHC-Cu) of the quick insertion of CO2 into intermediate and the absence PhCCH PhCCCOOH (52% of yield) of an oxidant, production of c is prohibited. On heating, inter- Cs2CO3, DMF, r.t., 24 hrs mediate B decomposes to A, and CO2 may also act as an oxidant c P1 in the production of . The same reaction conducted at 0 °C Finally, with this unique catalyst, different terminal alkynes showed lower activity but high selectivity. This observation is well with various functional groups were all successfully converted in agreement with the proposed hypothesis. into the related carboxylic acids in good to excellent yield with In the P1 catalyst system, it is proposed that the copper center carbon dioxide under very mild reaction conditions (2) (Table 1). activates the terminal alkyne with a base to form the copper acet- The most remarkable advantage of this copper or copper–NHC ylide intermediate, whereas the free carbene activates CO2 to catalyst system is its wide scope of substrate and functional groups form NHC carboxylate (Fig. 4). The NHC carboxylate will coor- tolerance. The catalytic system is not sensitive to a variety of func- dinate to a nearby copper center, which will induce the nucleo- tional groups, such as ─COOR, ─OH, ─CHO, ─CN, ─NO2, etc. philic carbanion of the alkyne into attacking the carboxylic It provides a powerful tool for the synthesis of highly functiona- ─ lized propiolic acids. carbon. Following the formation of the new C C bond, the It is known that copper acetylide is the key intermediate for CO2 unit is transferred from the carbene center to the copper copper-catalyzed C─H activation of terminal alkynes and the center, which will be regenerated by metathesis with the alkyne. Cu─C bond is active for CO2 insertion (18, 39–41, 48). A catalytic This system demonstrated an interesting synergistic effect of an cycle for copper-catalyzed carboxylation of terminal alkynes with organocatalyst and an organometallic catalyst in one system. CO2 is proposed as shown in Fig. 3. The copper acetylide inter- In summary, we have successfully developed a process where – mediate A was formed from the reaction of the terminal alkyne copper and copper NHC catalyzed the transformation of CO2 ─ and L2CuCl in the presence of a base. Subsequent CO2 insertion to carboxylic acids through C H bond activation of terminal into the polar Cu─C bond will form propynoate intermediate B, alkynes. Various propiolic acids were synthesized in good to in which it will undergo metathesis with the terminal alkyne under excellent yields under ambient conditions. The most remarkable basic conditions. This step would release propiolic acid and re- advantage of this mild reaction system is its tolerance toward a generate intermediate A (Fig. 3A). However, it must be noted wide substrate scope. This protocol opens up access to a pool of that the copper propynoate intermediate B is not stable at ele- highly functionalized propiolic acids from CO2. In addition, the vated temperatures (40). In this reaction, intermediate B may de- poly─NHC─Cu system demonstrated a concept for the coeffect compose over heat to reform A through a decarboxylation of organo and organometallic catalysts.

Fig. 3. Proposed catalytic cycle (A) and decomposition pathway (B).

4of6 ∣ www.pnas.org/cgi/doi/10.1073/pnas.1010962107 Yu and Zhang Downloaded by guest on October 2, 2021 Fig. 4. Proposed copper-carbene cocatalyzed reaction mechanism.

Materials and Methods General Procedure for Carboxylation of the Terminal Alkynes (1b as an Exam- All solvents were anhydrous and bought from Sigma-Aldrich (99.8%). The ple). CuCl (4.0 mg, 0.04 mmol, 2.0 mol%), TMEDA (3.5 mg, 0.03 mmol, 1.5 mol alkynes were used without purification from commercial suppliers, unless %), and K2CO3 or Cs2CO3 (2.4 mmol) were added to DMF (4 mL) in the reac- otherwise indicated. The carbonates were all dried under vacuum with tion tube (10 mL). CO2 (balloon) and 2 mmol of terminal alkynes (1a, 204 mg) heating before use. Poly─NHC and poly─NHC─Cu were synthesized on were introduced into the reaction mixture under stirring. The reaction mix- the basis of the literature (46). All reactions were performed in oven-dried ture was stirred at room temperature (about 24 °C) for 16 h. After completion CHEMISTRY (140 °C) or flame-dried glassware under an inert atmosphere of dry N2 of the reaction, the reaction mixture was transferred to potassium carbonate or Ar. solution (2 N, 5 mL) and the mixture was stirred for 30 min. The mixture was extracted with dichloromethane (3 × 5 mL), and the aqueous layer was acid- t ¼ 1 Preparation of PðNHCÞ0.5ðNHC─CuÞ0.5. NaO Bu (60 mg, 0.6 mmol) was added ified with concentrated HCl to pH and then extracted with diethyl ether to a DMF (10 ml) suspension of poly-imidazolium (46) (250 mg) in a re- (3 × 5 mL) again. The combined organic layers were dried with anhydrous action flask. The reaction mixture was stirred for 1 h, and then CuCl Na2SO4 and filtered and the solution was concentrated in vacuum, affording (25 mg, 0.25 mmol) was added. The resulting mixture was stirred at pure product 1b. 80 °C for 6 h. The solid product was filtered and dried to obtain a pale ð Þ ð ─ Þ yellow powder P NHC 0.5 NHC Cu 0.5. The catalyst is directly used for re- ACKNOWLEDGMENTS. This work was supported by the Institute of Bioengi- action. The coexistence of a metal center and free carbene was studied neering and Nanotechnology (Biomedical Research Council, Agency for in ref. 46. Science, Technology and Research, Singapore).

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