DFT Studies on the Silver-Catalyzed Carboxylation of Terminal

Article pubs.acs.org/Organometallics

DFT Studies on the Silver-Catalyzed Carboxylation of Terminal Alkynes with CO2: An Insight into the Catalytically Active Species Chuang Liu,† Yi Luo,*,†,‡ Wenzheng Zhang,*,† Jingping Qu,†,‡ and Xiaobing Lu† †

State Key Laboratory of Fine Chemicals, ‡School of Pharmaceutical Science and Technology, Dalian University of Technology, Dalian 116024, People’s Republic of China S Supporting Information *

ABSTRACT: DFT calculations on the Ag-catalyzed carboxylation of phenyl acetylene with CO2 indicate that the true catalytically active species is a CsCO3−-coordinated Ag complex rather than neutral PhCCAg conventionally considered for such a process. The energy barrier for the insertion of CO2 into the C−Ag bond of PhCCAg (28.8 kcal/mol) is higher than that of PhCCAgI− and PhCCAgCsCO3− anions (19.0 and 23.6 kcal/mol, respectively). Such an anion as a key intermediate is the predominant feature of the carboxylation process. The electronic effect plays a crucial role in stabilizing such transition states. In addition, the presence of an organic ligand slightly hampers generation of the active species and, therefore, reduced the yield of the final carboxylation product, which was observed experimentally.

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INTRODUCTION The catalytic conversion of CO2 into useful chemicals is one of a chemist’s long-standing interests and goals, since CO2 is a nontoxic, cheap, and renewable C1 source.1 Direct carboxylation of organic compounds, such as terminal alkynes with CO2 to produce carboxylic acids and derivatives, is one of the most promising reactions.2,3 Although CO2 can directly carboxylate highly reactive Grignard and organolithium reagents, these reactions are not compatible with sensitive functional groups. Transition metal, especially copper- and gold-catalyzed direct carboxylation of relatively active C−H bonds with CO2, shows good catalytic efficiency and broad substrate scope.4 However, those catalytic reactions ineluctably use expensive or complex organic ligands to guarantee selectivity and catalytic efficiency. Therefore, developing a simple, efficient, and relatively cheap catalyst is highly desirable. Some of us had recently reported that the terminal alkynes can be carboxylated with CO2 in the presence of the catalyst AgI in DMF under ligand-free conditions (eq 1).5 On the basis

of some control reactions, a reaction mechanism for the Agcatalyzed carboxylation was proposed (Scheme 1). The Scheme 1

terminal alkyne first coordinated to the silver(I) salt, and it was then deprotonated by Cs2CO3 to afford silver(I) acetylide, RCCAg. The insertion of CO2 into the C−Ag bond of the newly formed RCCAg gave a silver propiolate intermediate, RCC−COOAg, which subsequently reacted with another molecule of terminal alkyne and Cs2CO3 to regenerate RC Received: January 24, 2014 Published: June 9, 2014

© 2014 American Chemical Society

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model (CPCM). DMF was employed as a solvent (according to the experimental conditions) in the CPCM calculations. Unless otherwise specified, the energy reported is the free energy in solution, including gas-phase free energy correction. To check the reliability of basis sets used and the solvation effect on the geometries, the def2-TZVPD basis set containing a diffusion function has been used for all atoms to optimize some important structures in solution, and no significant change in relative energies has been found (see Figure 1 and Figure S2, Supporting Information).

CAg. The silver(I) acetylide RCCAg is generally proposed as the catalytically active species or a key intermediate in the silver(I)-catalyzed alkynylation reaction6 and silver-cocatalyzed Sonogashira reaction.7 It was found in this reaction that the yield was diminished when the reaction was conducted in the presence of a ligand, such as PPh3 or PCy3. This is in contrast to the conventional situation that organic ligands generally improve the catalytic efficiency in transition-metal-mediated reactions. The ligandfree condition for this catalytic process drove us to reconsider the detailed reaction mechanism including the effect of an organic ligand. Theoretical calculation as a powerful tool has been extensively utilized to investigate some CO2-based reactions mediated/catalyzed by transition-metal complexes.8,9 These studies provided valuable information for a better understanding of related mechanisms and development of a new strategy for CO2 conversion. During our computational studies on rare earth metal complexes,10 we have also become interested in the mechanism of reactions mediated/catalyzed by a transition-metal complex,11 including the Salen-Co-catalyzed CO2/epoxide copolymerization.11e These computational results encouraged us to investigate the mechanism of the reaction shown in eq 1 through DFT calculations. In the present work, the substrate of phenyl acetylene was used for our calculations. It has been unexpectedly found that the catalytically active species is the IAgCsCO3 anion species rather than PhCCAg or AgCO3− anion. In general, a ligand-promoted carboxylation of terminal alkynes with CO2 catalyzed by silver first generates the complex A (Chart 1) with a carbon−metal bond and the organic ligand

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RESULTS AND DISCUSSION The computed carboxylation pathways are shown in Figure 1. As shown in this figure, it is energetically favorable for a CsCO3− anion17 and AgI to form complex 1 (exergonic by −18.4 kcal/mol). Along with Path I, complex 1 goes through TS(1-2), leading to 2 and then to anionic 3 with the release of newly formed CsI. The transition state TS(1-2) with an energy barrier of 23.2 kcal/mol shows a character of concerted cleavage of the Ag−I bond and formation of the Cs−I bond. The conversion of 1 to 3 is endergonic by 16.3 kcal/mol. An approach of phenylacetylene toward 3 results in C−H bond activation to generate a HCO3− anion and B, PhCCAg, a species previously proposed as the catalytically active species for such a carboxylation. A relative energy barrier of 7.0 kcal/ mol is required for the formation of B through a transition state TS(4-B). The B is 4.1 kcal/mol higher than 1. The further carboxylation of B with CO2 goes through TS(B-5) with an energy barrier of 28.8 kcal/mol,18 leading to 5, a silver propiolate intermediate (Figure 1). The reaction of 5 with a CsCO3− anion regenerates 3, with release of cesium propiolate, a precursor leading to the final carboxylation product. Alternatively, the interaction of CsCO3− anion with 5 might also lead to more stable anionic complex 6, a CsCO3− anion coordinated silver phenylpropiolate. On the other hand, along with Path II (Figure 1), the direct reaction of 1 with substrate phenylacetylene yields an intermediate C, an I−-coordinated silver(I) acetylide. This exergonic (by 11.5 kcal/mol) step is accompanied by release of a molecule of CsHCO3 and requires a relative energy barrier of 14.0 kcal/mol to achieve C−H bond activation. The subsequent carboxylation of the C with CO2 goes through TS(C-7) to give 7. This carboxylation event requires an energy barrier of 19.0 kcal/mol. It is noted that one of the Cs+ ions could serve as a counterion and finally participate in the formation of PhCCOOAg. The Cs+-involved analogue of TS(C-7) (Figure S3, Supporting Information) has an energy barrier of 21.7 kcal/mol, which is compared with that for TS(C7) (19.0 kcal/mol). 7 further reacts with a CsCO3− anion to regenerate 1 and simultaneously to liberate a PhCCCOO− anion, which smoothly converts to the final carboxylation product in the presence of acid. The regenerated 1 is involved in the catalytic cycle. That is, the anion 1 serves as the catalytically active species in the pathway II (Figure 1, Scheme 2). This catalytic cycle features a key intermediate C, a PhC CAgI anion. It is noteworthy that attempts to find the pathway leading to B (PhCCAg), starting with the coordination complex of AgI and PhCCH (as previously proposed in Scheme 1), were fruitless, but the C was formed instead (see Figure S1 in the Supporting Information). With a comparison of Path I and Path II shown in Figure 1, it is obvious that the stationary points involved in Path II are significantly lower in energy than that in Path I and the carboxylation is the rate-determining step in each pathway. Moreover, the energy barrier for carboxylation in Path II is

Chart 1

L, whereas, for the ligand-free reaction, the complex B could be generated instead at the initial stage. With the aid of theoretical calculations, we have found in this study that the iodine anion acts as an inorganic anion ligand that promotes the CO2 inserting into the alkyne−silver bond (C in Chart 1). The effect of an organic ligand has also been computationally investigated. These theoretical results may shed new light on the development of a new catalytic system for carboxylation of alkyne with CO2.

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COMPUTATIONAL DETAILS

All calculations were performed with the Gaussian 09 suite of the programs.12 The DFT method of B3PW9113 was used for geometry optimizations and subsequent frequency calculations. The 6-31G(d) all-election basis set was used for the C, H, and O atoms, and the LanL2DZ14 basis set together with the associated effective core potential (ECP) was used for Ag and Cs atoms, and the LanL2DZdp15 basis set and ECP was used for the I atom. Frequency calculations were performed to identify the geometrically optimized stationary points as minima (zero imaginary frequency) or transition states (one imaginary frequency) and to obtain the thermodynamic data. Intrinsic reaction coordinates (IRCs) were carried out to identify the transition states (TS) connecting two relevant minima. The single-point calculations were performed with the 6-311+G** basis set for the C, H, and O atoms, and the SDD16 basis set as well as associated ECP for Ag, Cs, and I atoms. In these single-point calculations, solvation effects were considered with the conductor polarizable continuum 2985

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Figure 1. Computed carboxylation pathways through PhCCAg (Path I) or PhCCAgI anion (Path II). The energy (in kcal/mol) given includes that of all species involved in the corresponding reaction.

Scheme 2. Catalytic Cycles of Path I and Path II Shown in Figure 1

lower than that in Path I by 9.8 kcal/mol. For this reason, the regeneration of B (PhCCAg) from 5 (PhCCOOAg), like that in Scheme 1, was not further considered in our calculations. In addition, the formation of 3 as the catalytically active species in Path I is energetically unfavorable in comparison with the active species 1 in Path II. These results suggest that Path II is more favorable than Path I. That is, B,

silver(I) phenylacetylide, as either the catalytically active species proposed previously (Scheme 1) or a key intermediate could not be involved in such a catalytic carboxylation process. As mentioned above, CO2 insertion is the rate-determining step during the catalytic reaction. This led us to further analyze the important stationary points involved in this step, with the purpose of clarifying the reason why the carboxylation of B is 2986

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Table 1. Computed Frontier Molecular Orbital Energies (a.u.), WBI, and BDE (kcal/mol) of C−Ag Bond LUMO

HOMO

WBI of C−Ag bond

BDE of C−Ag bond

−0.217 95 −0.097 62

0.65 0.57

184.5 79.4

−0.008 78

CO2 PhCC−Ag (B) PhCC−AgI (C)

metal to the carbon atom. The interatomic distances between the C(CO2) and C(alkyne) are 1.96 Å in TS(B-5) and 1.72 Å in TS(C-7). The shorter distance in the latter suggests a stronger interaction between CO2 and the silver acetylide moiety, which is in line with the results of energy decomposition analysis. From the electronic point of view, the unsigned natural charge shown in TS(C-7) is smaller than that on the corresponding atoms in TS(B-5), suggesting that the iodine anion in TS(C-7) dispersed the charges on the central atoms and, therefore, stabilized this structure. The result of the carboxylation mechanism (Path II in Figure 1) raised a question about the catalytically active species for the analogous carboxylation reaction where silver phenylacetylide (PhCCAg, B) or silver phenylpropiolate (PhCCCOOAg, 5) was experimentally used instead of AgI.5 This situation stimulated us to further investigate the PhCCAg (B) involved reaction in the absence of AgI. Figure 3 shows the computed energy profile for this reaction. As shown in this figure, B interacts with a CsCO3− anion to form complex 8 (see also Figure 1). The subsequent insertion of CO2 to the C−Ag bond of 8 needs to overcome a transition state TS(8-6) with an energy barrier of 23.6 kcal/mol, leading to 6, which can be also formed by the coordination of CsCO3− to 5 (see Figure 1). This energy barrier (23.6 kcal/mol) is lower than that for the insertion of CO2 into the C−Ag bond of PhCC−Ag (28.8 kcal/mol; see Path I in Figure 1), suggesting that the carboxylation step shown in Scheme 1 is less favorable in comparison with that shown in Figure 3. The reaction of 6 with CsCO3− yields anionic 9 and PhCCCOO−. The latter should lead to the final carboxylation product in the presence of acid. 9 is capable of reacting with another molecule of PhCCH to regenerate 8, which serves as the catalytically active species for this cycle. The regeneration of 8 from 9 has an energy barrier of 11.9 kcal/mol (see TS(9-8) in Figure 3) and is exergonic by 19.1 kcal/mol. As to the 5 involved carboxylation observed experimentally,5 it is reasonably considered that 5 first converted to 6 via coordination of CsCO3− (exergonic by 13.4 kcal/mol; see Figure 1) and then transformed to active species 8 (Figure 3). Consequently, in the silver phenylacetylide or silver phenylpropiolate initiated carboxylation, the

kinetically less favorable than that of C. In such a carboxylation, CO2 as an electrophilic reagent is capable of providing its LUMO to accept the electrons from the HOMO of nucleophilic B or C. As shown in Table 1, the computed frontier molecular orbital energies indicate that the coordination of an I− anion increased the HOMO energy level and made the HOMO of C closer in energy to the LUMO of CO2, resulting in higher reactivity of C toward CO2. This adds a better understanding to the result that the reaction of CO2 with C is easier to occur than that with B. In addition, both the Wiberg bond index (WBI) and bond dissociation energy (BDE) of the C−Ag bond in B (0.65, 184.5 kcal/mol) are larger than those in C (0.57, 79.4 kcal/mol). This illustrates that the cleavage of the C−Ag bond that occurred in the carboxylation step is more difficult for B than that for C. To get deeper insights into the stabilities of the two transition states, viz., TS(B-5) and TS(C-7), energy decomposition analyses were carried out. Both structures can be divided into two fragments: CO2 and silver acetylide moieties. As shown in Table 2, although the sum of deformation energies Table 2. Energy Decomposition Analyses (in kcal/mol) for TS(B-5) and TS(C-7) TS(B-5) TS(C-7) a

ΔEdef

ΔEint

ΔEa

32.1 45.3

−12.2 −30.7

19.9 14.6

ΔE = ΔEdef + ΔEint.

(ΔEdef) for TS(C-7) (45.3 kcal/mol) is larger than that for TS(B-5) (32.1 kcal/mol), the interaction energy (ΔEint) between the two fragments in TS(C-7) (−30.7 kcal/mol) is significantly more negative than that in TS(B-5) (−12.2 kcal/ mol) and makes TS(C-7) lower in energy and thus more stable compared with TS(B-5). This result suggests that the electronic factor plays an important role in the stability of TS(C-7). The TS(B-5) and TS(C-7) are also structurally different (Figure 2). The former is a four-center transition state that adopts the π coordination of one CO bond to the metal− carbon bond, whereas the latter adopts the σ bonding of the

Figure 2. Optimized structures of TS(B-5) and TS(C-7). The numbers with and without parentheses denote the natural charge and interatomic distance (in Å), respectively. The unsaturated bonds are not shown. 2987

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Figure 3. Computed carboxylation pathway initiated by PhCCAg (B). The energy given includes that of all species involved in the corresponding reaction.

catalytically active species could be the anion 8, CsCO3−coordinated PhCCAg. Having the structure of active species 1 (Path II in Figure 1) in mind, the catalytically active species of such a carboxylation process is a Ag compound bearing an anionic ligand CsCO3−, such as 1 in Figure 1 and 8 in Figure 3. These results suggest that the reagent Cs2CO3 not only functions as a base to abstract hydrogen from the substrate PhCCH, as conventionally considered, but also plays a crucial role in formation of the catalytically active species in such a carboxylation process. The latter has been found, for the first time, in this study. As shown in Figures 1 and 3, the energy barriers for the carboxylation of B (Path I in Figure 1), C (Path II in Figure 1), and 8 (Figure 3) are 28.8, 19.0, and 23.6 kcal/mol, respectively. This suggests that B is the most kinetically difficult for carboxylation with CO2 among these three key intermediates. To corroborate this result, various density functionals, viz., TPSSTPSS, MPW1PW91, and MPW1K, were utilized to calculate the carboxylation energy barriers. The MPW1K functional developed by Truhlar’s group was suggested to be an efficient method for predicting barrier heights and was comparable to multireference methods.19 The results indicate the same trend in the energy barrier for carboxylation of these three species (see Table S1 in the Supporting Information). The experimental finding that an addition of an organic ligand has a negative effect on catalyst activity5 stimulated us to investigate the effect of an organic ligand, such as PCy3. As shown in eq 2, the computed ΔH for the replacement of PCy3

active species 1 and, therefore, reduced the yields of carboxylation product. This also indirectly supports the result of proposed active species along with Path II (Figure 1). In summary, the catalytically active species in a silvercatalyzed carboxylation of terminal alkyne with CO2 in the presence of Cs2CO3 has been computationally found to be a silver compound bearing a CsCO3− anionic ligand rather than the conventionally considered silver propiolate. The Cs2CO3 not only functions as a base to abstract hydrogen from the terminal alkyne but also participates in generation of the catalytically active species. The electronic effect plays an important role in stabilizing the CO2 insertion transition state.

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ASSOCIATED CONTENT

S Supporting Information *

The energy barriers for CO2 insertion computed by various DFT methods, calculated pathway according to Scheme 1, B3PW91/def2-TZVPD energy profiles for generation of 3 and C, neutral species IAg(Cs2CO3)-involved pathways for generation of C and B, optimized Cartesian coordinates of stationary points. This material is available free of charge via the Internet at http://pubs.acs.org.

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AUTHOR INFORMATION

Corresponding Authors

*E-mail: [email protected] (Y.L.). *E-mail: [email protected] (W.Z.). Notes

The authors declare no competing financial interests.

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with CsCO3− is +3.65 kcal/mol (free energy of +2.76 kcal/ mol). Such a small, but positive, value suggests that eq 2 is a thermodynamic equilibrium process and that the presence of an organic ligand slightly hampered the generation of catalytically

ACKNOWLEDGMENTS

This work was supported by the National Natural Science Foundation of China (No. 21028001, 21174023). Y.L. thanks the Fundamental Research Funds for the Central Universities (DUT13ZD103). The authors also thank the Network and 2988

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Information Center of Dalian University of Technology for part of the computational resources.

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