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And Copper–N-Heterocyclic Carbene-Catalyzed C H 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).
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