Mechanism and Substrate-Dependent Rate-Determining Step in

Nov 7, 2013 - elimination step can become the rate-determining step for the full catalytic ... with the decarboxylation step being rate determining, i...
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Mechanism and Substrate-Dependent Rate-Determining Step in Palladium-Catalyzed Intramolecular Decarboxylative Coupling of Arenecarboxylic Acids with Aryl Bromides: A DFT Study Hujun Xie,*,† Furong Lin,† Qunfang Lei,‡ and Wenjun Fang‡ †

Department of Applied Chemistry, Zhejiang Gongshang University, Hangzhou 310035, People’s Republic of China Department of Chemistry, Zhejiang University,Hangzhou 310027, People’s Republic of China

‡

S Supporting Information *

ABSTRACT: The mechanism of palladium-catalyzed intramolecular decarboxylative coupling of arenecarboxylic acids with aryl bromides has been studied computationally with the aid of density functional theory. Full free-energy profiles have been computed for all ether- and amine-containing substituted substrates. The calculations indicate that the rate-determining step is indeed substrate dependent, as reflected in free energy profiles; the oxidative addition, decarboxylation, or reductive elimination step can become the rate-determining step for the full catalytic cycle due to the different substituents on the substrates. In addition, we also demonstrate the preference of NCH3- over NH-containing amine substrates for the decarboxylation process. The calculations are in good agreement with the experimental observations.

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determining step for the whole catalytic cycle. Fu and Liu13 have theoretically explored the detailed mechanism for intermolecular decarboxylative coupling reactions of olefins with arenecarboxylic acids. The calculations showed that the mechanism includes four steps, decarboxylation, olefin insertion, β-hydride elimination, and catalyst regeneration, with the decarboxylation step being rate determining, in which Pd mediated the loss of CO2 from the aromatic carboxylic acid to form the Pd−aryl complex. Fu and Chruma14 have reported the mechanism for the palladium-catalyzed decarboxylative allylation of α-imino esters. The results showed the catalytic cycle consists of three steps, oxidative addition, decarboxylation, and reductive allylation. Decarboxylation involving a solvent-exposed α-imino carboxylate anion is the ratedetermining step. Recently, Skold et al.15 have theoretically and experimentally investigated the palladium(II)-catalyzed decarboxylative addition of arenecarboxylic acid to nitrile. The DFT calculations showed that carbopalladation is the ratedetermining step for the whole catalytic cycle. However, the intramolecular decarboxylative arylation of benzoic acids with aryl halides via Pd catalysts has not been well explored. Steglich et al.16 have reported an intramolecular Heck-like coupling of aryl bromide with tetrasubstituted pyrrolecarboxylic acid for the synthesis of lamellarin L/G using a stoichiometric amount of palladium catalyst. Forgione et al.17 have experimentally studied the intramolecular Pd-

INTRODUCTION The synthesis of the biaryl moiety has been the subject of tremendous research activity over the past few years, due to its being an important structural motif in a variety of biologically active compounds and functional molecules.1 The conventional syntheses of biaryl compounds are involved in transition-metalcatalyzed cross-coupling reactions using organometallic reagents as coupling partners,2 such as Suzuki−Miyaura coupling,3 the Negishi reaction,4 the Heck reaction,5 the Scholl reaction,6 the Gomberg−Bachmann reaction,7 and Ullmanntype couplings.8 However, the drawbacks of these reactions are still the harsh reaction conditions, the sensitivity of the organometallic reagents, and low yields of the unsymmetrically substituted biaryls. Transition-metal-catalyzed decarboxylation using aromatic carboxylic acids has emerged as an exciting strategy for the formation of biaryls via the liberation of CO2.9 In contrast to the organometallic reagents, the metal salts of aromatic carboxylic acids are stable against air and water and are easily available at low cost. The first example by Nilsson et al.10 has shown that the decarboxylative biaryl coupling reaction of aromatic acids with ArI can take place by copper catalysts. Progress has been made by Goossen et al.11 for the bimetallic Cu/Pd system, in which not only copper but also palladium is used for intermolecular decarboxylative coupling of benzoic acids with aryl halides. The Pd catalyst itself has also been developed further by Myers et al.12 involving the palladium-catalyzed decarboxylative Hecktype coupling of olefins with arenecarboxylic acids, and the kinetic experiments indicated that decarboxylation is the rate© 2013 American Chemical Society

Received: May 31, 2013 Published: November 7, 2013 6957

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Preliminary experimental results allowed Shen and coworkers19 to propose a mechanism (Scheme 1) which consists

catalyzed cross-coupling reaction using heteroaromatic carboxylic acids as coupling partners. The reaction needs short reaction times providing facile synthesis of aryl-substituted heteroaromatics, but this reaction can only tolerate two heteroaromatic acids. Ohe et al. have studied the palladiumcatalyzed intramolecular aziridination from 4H-isoxazol-5-one to form N-fused bicyclic aziridine.18 Preliminary experimental results allowed Ohe and co-workers to propose a mechanism, which includes oxidative addition, decarboxylation, cycloaddition, and reductive elimination. Recently, Shen et al.19 have investigated a practical and efficient protocol for the palladium-catalyzed intramolecular arylation of arenecarboxylic acids with aryl halides through a decarboxylative cross-coupling pathway under mild conditions. The method has a tolerance of a variety of substrates and is complementary to that of the intermolecular decarboxylative coupling reaction under Pd/Cucocatalyzed conditions. The experiments also showed that the NCH3-substituted substrate can take part in the decarboxylative coupling reaction, while the reaction cannot take place for the NH-substituted substrate. Despite important contributions from experimental results, detailed decarboxylation coupling reaction mechanisms are still elusive, and the experiments also cannot explain why the reaction will not occur for the NH-substituted substrate. Herein, the mechanism for the palladium-catalyzed intramolecular decarboxylation coupling reaction between arenecarboxylic acids and aryl bromides has been investigated with the aid of density functional theory (DFT), and a broad range of substrates has also been explored and addressed in combination with experimental findings.

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Scheme 1. Proposed Mechanism for the Decarboxylative Biaryl Synthesis

of oxidative addition, decarboxylation, and reductive elimination to account for the Pd-catalyzed intramolecular decarboxylative coupling of arenecarboxylic acids with aryl bromides. Herein, we have performed DFT calculations to explore the mechanism in detail, and the effect of different substrates (2-[(2-bromobenzyl)oxy]benzoic acid derivatives) on the reaction mechanism has also been demonstrated. According to the experimental results, the reaction can tolerate a variety of substrates (Scheme 2). The substrate 2-[(2-

COMPUTATIONAL DETAILS

Scheme 2

All geometries of the reactants, intermediates, transition states, and products were fully optimized by means of DFT calculations using the hybrid Becke3LYP (B3LYP) method.20 The reliability of the chosen method has been confirmed by our previous work21 and other theoretical studies for the investigation of Pd-catalyzed reaction mechanisms.22 The 6-31g(d) basis set was used for the C, O, N, and H atoms, while the effective core potentials (ECPs) of Hay and Wadt with double-ζ valence basis set (LanL2DZ)23 were chosen to describe the Pd, Br, P, and K atoms. In addition, polarization functions were added for Pd(ζf) = 1.472,24 Br(ζd) = 0.389, P(ζd) = 0.340, and K(ζd) = 0.200.25 Frequency analyses have been performed to obtain the zeropoint energies (ZPE) and identify all of the stationary point as minima (zero imaginary frequency) or transition states (one imaginary frequency) on the potential energy surfaces (PES). Intrinsic reaction coordinate (IRC) calculations were also calculated for the transition states to confirm that such structures indeed connect two relevant minima.26 All calculations were performed with the Gaussian09 software package.27 To consider solvent effects, a continuum medium was employed to do single-point energy calculations for all of the optimized species, using UAHF radii on the conductor-like polarizable continuum model (CPCM).28 N-Methylpyrrolidone (NMP) with a dielectric constant (ε) of 32.0 was used as the solvent, in accord with the experimental reaction conditions.

bromobenzyl)oxy]benzoic acid derivatives have two aromatic rings. Ring A is linked to the carboxylic group, and the meta site of the carboxylic group (R1) can be substituted by an OMe, Cl, or NO2 group; ring B is linked to the bromide group, and the para site of the bromide group (R2) can be replaced by an OMe or F group. The two rings are linked by an OCH2 group to form ether-containing substrates with the O atom near ring A. In addition, the ether oxygen (R3) can switch to an NH or NCH3 group to form amine-containing substrates. In the present calculations, we will consider all substrates affecting the mechanism for palladium-catalyzed intramolecular decarboxylic coupling reactions. Mechanism for Pd(0)-Catalyzed Intramolecular Decarboxylative Coupling of Ether-Containing Substrates. For the Pd(0)-catalyzed intramolecular decarboxylative coupling of arenecarboxylic acid with aryl bromide, the catalytic cycle was proposed to contain three steps. (1) Ligand substitution of the substrate for phosphine gives an η2-phenyl intermediate, followed by oxidative addition (OA) of the C−Br bond to give an arylpalladium intermediate together with loss of one KBr and recoordination of a phosphine ligand. (2) A decarboxylation step generates a seven-membered palladacycle

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RESULTS AND DISCUSSION On the basis of the recent experimental studies, the active catalyst is initially generated from a palladium complex and 2 equiv of phosphine under the reaction conditions. In addition, in order to reduce the computational cost, the triphenylphosphine (PPh3) ligand was replaced by the trimethylphosphine (PMe3) ligand. Thus, we used Me3P−Pd−PMe3 as the model catalyst in the following calculations. 6958

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Figure 1. Free energy profiles calculated for the oxidative addition and decarboxylation steps. The solvation-corrected relative free energies are given in kcal/mol.

Figure 2. Free energy profiles calculated for the reductive elimination step. The solvation-corrected relative free energies are given in kcal/mol.

intermediate. (3) Reductive elimination gives the final product and regenerates the active species Pd(PMe3)2. We first consider the decarboxylative coupling of arenecarboxylic acids with aryl bromide. The free energy profiles for the reaction are depicted in Figures 1 and 2. The corresponding optimized structures are presented in Figures 3 and 4. The reaction starts with the oxidative addition process. Figure 1 shows two possible pathways leading to the oxidative addition

product. Path a is a direct oxidative addition of organic bromide substrate to the model catalyst 1 containing two phosphine ligands. Path b considers the oxidative addition involving only one phosphine ligand. As shown in Figure 1, in path a oxidative addition occurs directly via the transition state TS2_A with a barrier of 29.1 kcal/mol. The C−Br, Pd−Br, and C−Pd bond lengths in TS2_A were calculated to be 2.175, 2.750, and 2.257 Å, respectively (Figure 3). In path b, the CC double bond of 6959

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Figure 3. Optimized structures for selected species involved in the oxidative addition and decarboxylation steps shown in Figure 1.

ligand to form the η2 complex 3_A. The ligand substitution process is endergonic by 13.0 kcal/mol, because the coordination of PMe3 with Pd is much stronger than the Pd−aryl binding in complex 3_A. The following oxidative addition of aryl bromide occurs via the transition state TS4_A to form complex 5_A, accompanied by the loss of KBr. The overall barrier is predicted to be 23.8 kcal/mol (Figure 1) from 1_A to TS4_A, and the Pd−Br, Pd−C, and C−Br distances in TS4_A are calculated to be 2.993, 2.147, and 2.167 Å, respectively (Figure 3). The oxidative addition product 5_A is thermodynamically favored with a greater exoergicity of 17.5 kcal/mol from 1_A to 5_A. Taking into account the results shown in Figure 1, we conclude that path b is more favorable than path a for the oxidation addition process. From precursor complex 5_A, the decarboxylation step proceeds via the transition state (TS6_A) to give complex 7_A with a relatively high barrier of 28.0 kcal/mol, which is the ratedetermining step for the full catalytic cycle. The Pd−O and C− C distances in TS6_A are calculated to be 2.213 and 2.032 Å, respectively. After the C−C bond breaks, one oxygen atom of the CO2 group is weakly coordinated to the palladium center in complex 7_A with a Pd−O distance of 2.518 Å. Complex 7_A readily liberates the CO2 molecule to yield the complex 8_A and recoordinates a PMe3 ligand to generate the stable complex 9_A.

Figure 4. Optimized structures for selected species involved in the reductive elimination step shown in Figure 2.

ring B in the substrate is initially coordinated to the Pd(0) catalyst, accompanied by the loss of one phosphine (PMe3) 6960

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Figure 5. Free energy profiles calculated for the decarboxylative biaryl synthesis from NO2 substituted on ring A of the substrates. The solvationcorrected relative free energies are given in kcal/mol.

decarboxylation of the OMe-substituted substrate is slightly decreased with a value of 27.8 kcal/mol (Figure S1), while the barrier for decarboxylation of the Cl-substituted substrate is slightly increased with a value of 29.0 kcal/mol. These results showed that both substituents on ring A of the substrates have a minor effect on the barrier for the decarboxylation process, which is in good agreement with the experimental observations that both substrates can form the final product with high yields (92% for Cl; 88% for OMe).19 In addition, it is interesting to note that when R1 is substituted by an NO2 group (meta site relative to the carboxylic group of ring A), the reaction cannot take place under experimental reaction conditions.19 Thus, we calculated the free energy profile for the decarboxylative biaryl synthesis from NO2 substituted on ring A of the substrate (Figure 5). The key structures are described in Figure 6. The calculation results show that reductive elimination is the ratedetermining step for the whole catalytic cycle with a barrier of 44.3 kcal/mol from 8_F to TS9_F, indicating that the reaction is kinetically unfavorable. The high barrier may explain why the final product was not observed in the experiments.19 Finally, we consider the Pd(0)-catalyzed intramolecular decarboxylative coupling of arenecarboxylic acid with two para site substituted (para site relative to the bromide group of ring B shown in Scheme 2) aryl bromides for ether-containing substrates. Figure 7 shows the free energy profiles for the decarboxylative biaryl synthesis from F and OMe substituents on ring B of the substrates. Key structures and transition states with selected structural parameters are depicted in Figures 8 (Fcontaining substrate) and 9 (OMe-containing substrate). According to the calculations, the free energy profiles of the two substrates show different characteristics (Figure 7). In the case of the OMe-substituted substrate, decarboxylation is the rate-determining step for the whole catalytic cycle with a free

From 9_A, reductive elimination can proceed via two possible pathways, a dissociative path (path c) and an associative path (path d). Reductive elimination from palladium complexes via dissociative and associative mechanisms has been proposed in previous calculations by Alvarez and co-workers.29 Figure 2 shows the free energy profiles calculated for the reductive elimination process. Selected optimized structures are presented in Figure 4. The results showed that an associative path (path d) via diphosphine ligands is more favorable. In path d, reductive elimination for the intramolecular C(sp2)−C(sp2) bond coupling occurs directly via the transition state TS12_A containing diphosphine ligands. A free energy barrier of 5.8 kcal/mol was calculated for this path, leading to the formation of the reductive elimination product 13_A. The C−C distance in TS12_A was calculated to be 1.988 Å (Figure 4). Finally, the product is released and regenerates the palladium diphosphine complex for the next catalytic cycle. On the basis of the calculations, the whole reaction is significantly exergonic by 59.1 kcal/mol. We also consider the Pd(0)-catalyzed intramolecular decarboxylative coupling of three meta site substituted (meta sites relative to the carboxylic group of ring A shown in Scheme 2) arenecarboxylic acids with aryl bromide for ether-containing substrates. The free energy profiles for the decarboxylative biaryl synthesis from the Cl- and OMe-substituted arenecarboxylic acids with aryl bromide are shown in Figure S1 (see the Supporting Information). Key structures and transition states with selected structural parameters are described in Figures S2 and S3 (see the Supporting Information). On the basis of the calculations, the decarboxylation process of both substrates is the rate-determining step for the whole catalytic cycle (Figure S1). In contrast to the barrier (28.0 kcal/mol) for the decarboxylation process shown in Figure 1, the barrier for 6961

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experimental findings that both substrates can form the final biaryl product in high yields (85% for F; 96% for OMe).19 On the basis of the calculations, the substituents have significant effects on the rate-determining step of the catalytic cycle for ether-containing substrates. In the case of the meta site substituents on ring A, decarboxylation is the ratedetermining step for the Cl- and OMe-substituted substrates, while reductive elimination is the rate-determining step for the NO2-substituted substrate. In the case of the para site substituents on ring B, decarboxylation is also the ratedetermining step for the OMe-substituted substrate, while oxidative addition becomes the rate-determining step for the Fsubstituted substrate. Mechanism for Pd(0)-Catalyzed Intramolecular Decarboxylative Coupling of Amine-Containing Substrates. We now consider the mechanism for Pd(0)-catalyzed intramolecular decarboxylative coupling of amine-containing substrates. The free energy profiles for the decarboxylative biaryl synthesis from the amine-containing substrates are shown in Figures 10 (NH substituent) and 11 (NCH3 substituent). Key structures and transition states with selected structural parameters are presented in Figures 12 (NH substituent) and 13 (NCH3 substituent). The calculation results showed that the catalytic mechanism is found to include three fundamental steps, oxidative addition, decarboxylation ,and reductive elimination, to account for the Pd-catalyzed intramolecular decarboxylative coupling of amine-containing substrates. For the NH-substituted substrate, the reaction is initiated by ligand substitution of the NH-containing substrate for phosphine to form the η2-phenyl intermediate 2_D, and the process is endergonic by 16.5 kcal/mol. This is followed by oxidative addition of the C−Br bond, together with the loss of KBr, and coordination of the carboxylic group to give the fourcoordinated palladacyclic intermediate 4_D. An overall barrier (TS3_D) for the oxidative addition step is equal to 34.0 kcal/ mol (Figure 10), and the Pd−Br, Pd−C, and C−Br distances in TS3_D are calculated to be 3.126, 2.394, and 2.032 Å, respectively (Figure 12). A further decarboxylation step

Figure 6. Optimized structures for selected species involved in the decarboxylative biaryl synthesis from NO2 substituted on ring A of the substrates shown in Figure 5.

energy barrier of 28.0 kcal/mol, while in the case of the Fsubstituted substrate, it is interesting that oxidative addition is found to be the rate-determining step for the whole catalytic cycle, and the overall barrier is calculated to be 27.7 kcal/mol, slightly higher than the barrier for the decarboxylation step (25.7 kcal/mol). These results are different from those of previous calculations,13,14 in which decarboxylation is the ratedetermining step for both the Pd-catalyzed decarboxylative allylation of α-imine esters and Pd-catalyzed decarboxylative coupling of carboxylic acids with olefins. The low barriers of both substrates calculated are in good agreement with the

Figure 7. Free energy profile calculated for the decarboxylative biaryl synthesis from F and OMe substituted on ring B of the substrates. The solvation-corrected relative free energies are given in kcal/mol. 6962

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Figure 8. Optimized structures for selected species involved in the decarboxylative biaryl synthesis from F substituted on ring B of the substrates shown in Figure 7.

intermediate 2_E; the process is endergonic by only 7.5 kcal/ mol (Figure 11). This is followed by oxidative addition of the aryl bromide to Pd(0), together with the loss of KBr, and coordination of the carboxylic group to form the fivemembered palladacyclic intermediate 4_E. The overall barrier (TS3_E) for the oxidative addition step is equal to 25.8 kcal/ mol, and the Pd−Br, Pd−C, and C−Br distances in TS3_E are calculated to be 2.809, 2.218, and 2.074 Å, respectively (Figure 13). On the basis of the present calculations, it is interesting to note that oxidative addition is the rate-determining step, which is different from the case for the NH-substituted substrate, involving decarboxylation as the rate-determining step. Decarboxylation then occurs to produce the four-coordinated complex 6_E, which can easily lose the weakly coordinated CO2 and recoordinate a phosphine ligand to produce the complex 8_E. The barrier (TS5_E) for the decarboxylation process is predicted to be only 15.5 kcal/mol. In addition, it was found that the structure of TS5_E (Figure 13) is obviously different from that of the transition state TS5_D (Figure 12) for the NH-substituted substrate. In the Pd−C−C−O fourmembered-ring transition state TS5_E, the nitrogen atom of the NCH3 group is not coordinated to the palladium center due to the steric hindrance between the methyl group of NCH3 with the phosphine ligand and carboxylic group. Thus, the oxygen atom of the CO2 group can still coordinate to the palladium center, while TS5_D is related to the Pd−C−C three-membered ring transition state and the Pd center has a coordination number of 5 on insertion into the C−CO2 bond.

generates the four-coordinated complex 6_D, which can easily liberate the weakly coordinated CO2 and recoordinate a phosphine ligand to produce the complex 8_D. The barrier (TS5_D) for the decarboxylation process is predicted to be 38.3 kcal/mol, which is the rate-determining step for the whole catalytic cycle and is kinetically unfavorable. As Figure 12 shows, it should be noted that the structure of TS5_D is significantly different from the transition states for the ring A and ring B substituted ether substrates. In this transition state TS5_D, the O-coordinated site of CO2 group to palladium is substituted by the NH group; thus, the CO2 group is weakly coordinated to the palladium center by its carbon atom, and the predicted Pd−C distance in TS5_D is about 2.830 Å. As Scheme 3 shows, according to the electron flow mechanism, the NH group coordinated to the palladium center disfavors the decarboxylation process with a significantly higher barrier. The calculated results are consistent with the experimental observations that, when the ether oxygen is switched for an NH group, the substrate will not form the desired coupling product.19 Subsequently, reductive elimination gives the final product and regenerates the active species Pd(PMe3)2 for the next catalytic cycle. The barrier (TS9_D) for the reductive elimination step is only 5.7 kcal/mol, and the C−C distance in TS9_D is about 1.995 Å. On the basis of the calculations, the whole reaction is significantly exergonic by 52.0 kcal/mol. For the NCH3-substituted substrate, from complex 1, the reaction proceeds through the ligand substitution of the NCH3containing substrate for phosphine to give the η2-phenyl 6963

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Figure 9. Optimized structures for selected species involved in the decarboxylative biaryl synthesis from OMe substituted on ring B of the substrates shown in Figure 7.

Figure 10. Free energy profile calculated for the decarboxylative biaryl synthesis from amine-containing (NH) substrates. The solvation-corrected relative free energies are given in kcal/mol.

Previous calculations by Fu and Liu13,30 indicated that the barrier of the Pd−C−C−O four-membered-ring transition state

is lower than that of the Pd−C−C three-membered-ring transition state. As Scheme 3 shows, the electron flow 6964

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Figure 11. Free energy profile calculated for the decarboxylative biaryl synthesis from amine-containing (NCH3) substrates. The solvation-corrected relative free energies are given in kcal/mol.

Figure 12. Optimized structures for selected species involved in the decarboxylative biaryl synthesis from amine-containing (NH) substrates shown in Figure 10.

mechanism of TS5_E also facilitates the decarboxylation step and greatly decreases the barrier.31 These calculations are in accordance with the experimental results that only the NCH3containing substrate can form the final biaryl product with high yields.19 Subsequently, reductive elimination gives the final product and regenerates the active species Pd(PMe3)2 for the next catalytic cycle. The barrier (TS9_E) for the reductive elimination step is only 8.9 kcal/mol, and the C−C distance in

TS9_E is about 2.015 Å. On the basis of the calculations, the whole reaction is significantly exergonic by 63.6 kcal/mol. For the free energy profiles of the NCH3-containing substrate, we also consider adding the dispersion correction to the B3LYP, B3PW91,32 and X3LYP33 functionals, and the free energy profiles are shown in Figure S4 (see the Supporting Information). The calculation results indicated that the dispersion correction and different functionals have only a 6965

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Figure 13. Optimized structures for selected species involved in the decarboxylative biaryl synthesis from amine-containing (NCH3) substrates shown in Figure 11.

been investigated via density functional theory. The full catalytic cycle passes through oxidative addition of the aryl halide, decarboxylation, and a novel C(sp2)−C(sp2) reductive elimination and regenerates the catalyst. On the basis of the present calculations, the substituents have significant effects on the rate-determining step of the catalytic cycle. For the meta site substituents on the ring A of ethercontaining substrates, decarboxylation is the rate-determining step for the Cl- and OMe-substituted substrates, and reductive elimination is the rate-determining step for the NO2-substituted substrate. For the para site substituents on ring B of the ethercontaining substrates, decarboxylation is also the ratedetermining step for the OMe-substituted substrate, while oxidative addition becomes the rate-determining step for the Fsubstituted substrates. For the amine-containing substrates, decarboxylation is the rate-determining step for the NHsubstituted substrate with a significantly high barrier due to coordination of the NH group to the palladium center disfavoring the decarboxylation process. For the NCH3containing substrates, oxidative addition is the rate-determining step. In this transition state of the decarboxylation step, the Ocoordinated site of the CO2 group is not substituted by an NCH3 group due to the steric repulsion between the methyl group of NCH3 and the phosphine ligand and carboxylic group, which facilitates the palladium center assisting the decarboxylation process and obviously decreases the barrier. Our calculations are in good agreement with the experimental results.

Scheme 3. Electron Flow Mechanism of the Decarboxylation Step for the NH- and NCH3-Containing Substrates

slight effect on the free energy profile calculated for the decarboxylative biaryl synthesis from amine-containing (NCH3) substrates. On the basis of the calculations, the free energy profiles for NH- and NCH3-containing substrates exhibit significant differences (Figures 10 and 11). In the case of an NHsubstituted substrate, decarboxylation is the rate-determining step for the full catalytic cycle with a barrier of 38.3 kcal/mol, while in the case of an NCH3-substituted substrate, oxidative addition is the rate-determining step for the full catalytic cycle with a barrier of 25.8 kcal/mol.

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CONCLUSIONS The mechanism for Pd(0)-catalyzed intramolecular decarboxylative coupling of arenecarboxylic acids with aryl bromides has 6966

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ASSOCIATED CONTENT

S Supporting Information *

Figures giving additional free energy profiles and structures and tables giving Cartesian coordinates of the optimized structures used in the theoretical calculations. This material is available free of charge via the Internet at http://pubs.acs.org.

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AUTHOR INFORMATION

Corresponding Author

*: [email protected]. Fax: +86−571−2183047. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS We acknowledge financial support from the National Science Foundation of China (21203166, 91127010, and 21273201), the Natural Science Foundation of Zhejiang Province (Y4100620 and LY12B04003), and the China Postdoctoral Science Foundation (20110491774). We thank the State Key Laboratory of Physical Chemistry of Solid Surfaces (Xiamen University) for providing computational resources.

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