Theoretical Study on the Mechanism of Palladium-Catalyzed

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Theoretical Study on the Mechanism of Palladium-Catalyzed Dearomatization Reaction of Chloromethylnaphthalene Ying Ren, Jianfeng Jia,* Wenxian Liu, and Hai-Shun Wu School of Chemistry and Materials Science, Shanxi Normal University, Linfen, 041004, People’ Republic of China S Supporting Information *

ABSTRACT: The Pd-catalyzed dearomatization of chloromethylnaphthalene with allenyltributylstannane has been investigated at the B3LYP density functional level of theory. The calculations indicate that the monophosphine complex is catalytically more active than the bisphosphine complex for oxidative addition. The transmetalation step is a crucial step for determining the dearomatized products due to the formation of two stable bis-π-complexes. It is found that reductive elimination occurs by coupling the terminal carbons of the η1-propargyl ligand and η1-allenyl ligand with the para-carbons of the η3methylnaphthalene ligands in η3-methylnaphthalene-η1-propargyl-Pd(PH3) and η3-methylnaphthalene-η1-allenyl-Pd(PH3) to form the corresponding allenylated and propargylated dearomatization products. For comparison, various C−C coupling pathways in reductive elimination have also been studied.

1. INTRODUCTION The asymmetric dearomatization of aromatic molecules has received considerable interest for the preparation of highly functionalized, stereodefined alicyclic compounds. Previously, it was believed that dearomatization of arenes was not an easy task due to the loss of aromatic stabilization. Thus, many types of dearomatization reactions, such as the asymmetric Birch reduction,1 enzymatic dihydroxylation,2 photochemical cycloaddition,3 nucleophilic addition,4 electrophilic addition,5 and so on,6 have seen widespread application for breaking up the conjugated π-system. In addition, transition-metal-mediated dearomatization methodologies have also shown promise.7 The complexation of the aromatic system to transition metals leads to activation of arenes and thus facilitates the electrophilic addition of [M(η2-arene)] (M = Os, Re, Mo, and W) complexes and the nucleophilic addition of [M(η2-arene)] (M = Cr, Mn, Fe, and Ru) complexes.8 Recently, Peng et al.8 discovered a new and efficient dearomatization reaction between chloromethylnaphthalenes and allenyltributylstannane in the presence of Pd(0) complexes at room temperature to give the corresponding propargylated and allenylated carbocycles (Scheme 1). This reaction is unusual because the Pd-catalyzed cross-coupling reaction of organic electrophiles (RX) with organostannanes (e.g., R′SnR3), better known as the Stille cross-coupling reaction, usually gives the products R−R′, which are derived by coupling the R and R′ groups directly without changing their individual configurations.9,10 However, Peng et al. found that the reaction of chloromethylnaphthalenes with allenyltributylstannane gave para propargylated and allenylated dearomatization products. © 2012 American Chemical Society

Scheme 1

The corresponding Stille cross-coupling products were not observed at all. On the basis of their experimental findings, the general mechanism of the reaction proposed by Peng and coworkers consists of three successive fundamental steps (oxidative addition, transmetalation, and reductive elimination), as found in the Stille cross-coupling catalytic cycle in Scheme 2. To our knowledge, although the dearomatization reaction of chloromethylnaphthalenes with allenyltributylstannane catalyzed by Pd complexes has been successfully achieved, the details of the reaction mechanism are still ambiguous. Furthermore, there are several significant questions in the dearomatization reaction of chloromethylnaphthalenes with allenyltributylstannane. Why does the 1,3-rearrangement of the 1,3-allenyl ligand migrate from palladium to the para-carbon of the η3-methylnaphthalene ligand? Why does the reaction proceed to give the two corresponding kinds of products (propargylated and allenylated dearomatization products)? Answers to these questions are of obvious value to the Received: August 17, 2012 Published: December 31, 2012 52

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Scheme 2

atoms were described using the LANL2DZ basis set including a double-ζ valence basis set with the Hay and Wadt effective core potential.20,21 The 6-311G(d,p)22 basis set was chosen to describe C and H atoms. The B3LYP method used in this work is a reasonable method for obtaining reliable geometries.23,24 Gibbs free-energy corrections at 298 K were determined from harmonic frequencies and added to the total electronic energies to get the Gibbs free energies. To check the reliability of our computational scheme, further single-point wB97XD25/LANL2DZ+p calculations were performed on all the stationary points optimized at the B3LYP/LANL2DZ level (see Table S1 in the Supporting Information). Polarization functions were added for Cl (ζd = 0.640), P (ζd = 0.387), Sn (ζd = 0.180), and Pd (ζf = 1.472).13 Phosphines were modeled by PH3.26−28 The effect of phosphine ligands was calculated for the key steps in the dearomatization reaction (see Supporting Information). For interpretation purposes, natural charges were analyzed with the natural bond orbital (NBO) program.29 To estimate the degree of aromaticity of the benzyl ligand in methylnaphthalene, nucleus-independent chemical shift (NICS, a measure of aromaticity)30 calculations were performed using the GIAO31 method at the B3LYP level. For evaluating the solvent effect, the single-point self-consistent reaction field calculations on the gas-phase-optimized structures in dichloromethane were performed by means of the polarized continuum model32,33 with the united atom Hartree−Fock34 radii on the optimized structures. The solvation free energy was calculated at B3LYP/6-311G(d,p) (LANL2DZ for Pd, Sn, Cl, and P atoms) level and added to the gas-phase free energy to obtain the Gibbs free energy in solution. The data are given in parentheses of the corresponding free energy profiles throughout the study.

continuing study of the mechanism of the dearomatization reaction and, therefore, constitute the focal points of our present work. It is noted that there were some theoretical studies using Pd complexes as catalysts for organic transformations. For example, the mechanism of the Pd-catalyzed transmetalation step in the Stille cross-coupling reaction was investigated by means of the B3LYP level of density functional theory.11 Li et al.12 employed the DFT methods to study the mechanism of oxidative addition of PhCl and PhBr to [Pd(PR3)2]. Ariafard13 and Yates studied the steric effects of the phosphine ligands on the mechanism of the Stille crosscoupling reaction using the B3LYP density functional method. Evidently, the above mechanistic studies have provided important insights into the Stille cross-coupling reaction in the presence of Pd catalysts. Nonetheless, there were few theoretical studies using Pd complexes as catalysts for the dearomatization reaction. More recently, we have studied the mechanistic details of the dearomatization reaction of naphthalene allyl chlorides with allenyltributylstannane by Pd(0) complexes at the B3LYP level of theory.14 In this work, a detailed DFT study on the Pd-catalyzed dearomatization reaction of chloromethylnaphthalene with allenyltributylstannane is performed. The complete mechanistic pictures are presented, and the regioselectivity is emphasized and clarified. To take the effects of entropy and enthalpy into account, we use the free energies (ΔG) of activation and reaction to analyze the reaction mechanism. We hope that the results presented in this article may have valuable implications for the development of new, more effective catalysts for dearomatization of naturally abundant aromatic compounds.

3. RESULTS AND DISCUSSION 3.1. Reaction Mechanism. We investigated the detailed mechanism for the dearomatization of the methylnaphthalene group by discussing a series of reasonable structures of intermediates and transition states. The reaction is conventionally grouped into three processes, as shown in Scheme 2: (I) oxidative addition, (II) transmetalation, and (III) reductive elimination. In oxidative addition, it is found that monophosphine PdPH3 is catalytically more active than bisphosphine Pd(PH3)2. This conclusion is similar to that of naphthalene allyl chloride.14 The relative details of the mechanism for oxidative addition are presented in the Supporting Information. Therefore, we discuss the detailed mechanism of the dearomatization

2. COMPUTATIONAL DETAILS All calculations were performed at the DFT level, by means of the hybrid B3LYP15,16 functional as implemented in the Gaussian suite of programs.17 The harmonic vibrational frequencies were also calculated at the same level to characterize the nature of the stationary points as true minima with no imaginary frequency or transition states with only one imaginary frequency. Especially, the lone imaginary frequency of each transition state displayed the desired displacement orientation, and the validity of each reaction path was further examined by the intrinsic reaction coordinate calculations.18,19 The Pd, Sn, Cl, and P 53

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and 4a, respectively. Furthermore, the low rotation barriers of 2.7 and 3.3 kcal/mol indicate that 4b and 4c can facilely convert to 4a and 4d, respectively (see Figure S3 in the Supporting Information). Hereby, the following discussion is started with 4a and 4d. From 4a and 4d, it should be noted that there exist two possible mechanisms for the transmetalation, which can be divided into two sets: the three-step mechanism and the onestep mechanism. Let us first discuss the three-step mechanism. Initially, intermediates 4a and 4d, losing a phosphine ligand, convert to complexes 5a and 5d, respectively. Simultaneously, the coordination mode of the methylnaphthalene group to the Pd center changes from η1- to η3-coordination. From the free energy profiles in Figure 2, it can be found that these processes are slightly exergonic by 4.0 and 3.5 kcal/mol and need to overcome free energy barriers of 7.4 and 11.1 kcal/mol, respectively. On the basis of the conformation of the complexes 5a and 5d, the subsequent step is the transmetalation. This step involves a large structural rearrangement involving migration of the SnMe3 group from the π-bonded (allenyl)SnMe3 group to the Cl ligand via the four-membered-ring transition state. In addition, the allenyl group switches from η2- to η3-coordination. The transmetalation steps (5a → 6b and 5d → 6a) are predicted to be exergonic by 2.4 and 1.7 kcal/mol, with the moderate free energy barriers of 13.5 and 19.8 kcal/mol, respectively. 6a and 6b are key intermediates along the reaction path because they are responsible for the formation of propargylic and allenic dearomatized products. The above results suggest that the transmetalation is another crucial step for determining the observed regioselectivity. The following step is the coordination of the PH3 ligand. As a result, 11a and 11b are formed. The attacking phosphine ligand occupies the equatorial site of the Pd center in a side-on orientation (η1PH 3 ) due to the proper orbital interaction and the simultaneous break of the η3-methylnaphthalene interaction. The free energy barriers for the interconversion processes are relatively small. In contrast to the three-step mechanism discussed above, we have also located the one-step mechanism for the transmetalation. The one-step mechanism corresponds to the direct migration of the SnMe3 group via the four-membered-ring transition state. Starting with 4a and 4d, these steps go via the transition states TS(4a/11b) and TS(4d/11a), leading to the intermediates 11b and 11a, respectively. As shown in the free energy profiles of Figure 2, the steps 4a → 11b and 4d → 11a are exergonic by 5.6 and 7.2 kcal/mol and have free energy barriers of 12.9 and 23.7 kcal/mol, respectively. According to the above discussions, we will address the competition of the three-step mechanism and the one-step mechanism. Summarizing the data in Figures 1 and 2, we can map out the completed energy surface for the transmetalation processes. From 4a and 4d, the highest energy points of the three-step mechanisms are the transition states TS(5a/6b) and TS(5d/6a), and their free energies are 11.6 and 17.4 kcal/mol, respectively. However, the highest energy points of the onestep mechanisms are the transition states TS(4a/11b) and TS(4d/11a), and their free energies are 15.0 and 24.8 kcal/ mol, respectively. Clearly, TS(4a/11b) and TS(4d/11a) have higher free energies than the corresponding TS(5a/6b) and TS(5d/6a) by 3.4 and 7.4 kcal/mol, respectively. The data indicate that the transmetalation via the three-step mechanism should be more favored than the one-step mechanism. Furthermore, what is the driving force for these energy

reaction of chloromethylnaphthalene with allenyltributylstannane in the following section. 3.1.1. Transmetalation. The free energy profiles for the transmetalation are reported in Figures 1 and 2, while some key

Figure 1. Coordination of (allenyl)SnMe3 to different π-complexes. The relative free energies are given in kcal/mol.

structures are shown in Figure 3, and the others are given in the Supporting Information. Recently theoretical studies provided more details regarding the mechanism of transmetalation, which involves formation of a π-complex and then transmetalation in the π-complex proceeding via a cyclic tetracoordinated transition state, TS.35 Accordingly, the next step is the coordination of (allenyl)SnMe3 to Pd. SnMe3 is used to model SnBu3, which is commonly used in the experiments.13,36 The intermediates 2 and 3 are formed via oxidative addition, which having a vacant site are expected to be the active reactant species for the transmetalation reaction. The (allenyl)SnMe3 binds weakly to the Pd center to form the corresponding π-complex η1-methylnaphthalene-η1-(allenyl)SnMe3-Pd(PH3)(Cl). The weaker metal coordination causes the adducts between (allenyl)SnMe3 and 2 and 3 to not correspond to local minimum on the potential energy surface. Here, we are interested in the structure and stability of the different isomers. Thus four structures of 4 have been calculated because of the orientation and site of the Pdcoordinated (allenyl)SnMe3 ligand. As shown in Figure 1, isomer 4d is the most stable, with the (allenyl)SnMe3 ligand trans to the methylnaphthalene ligand (1.1 kcal/mol), closely followed by 4a, with the (allenyl)SnMe3 ligand trans to the PH3 ligand (2.1 kcal/mol), whereas 4c and 4b (2.8 and 3.5 kcal/mol for 4c and 4b, respectively) are higher in energy because the orientation of the (allenyl)SnMe3 ligand is opposite that of 4d 54

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Figure 2. Gibbs free energy profiles for the transmetalation. Gas-phase Gibbs free energies and solvent-corrected Gibbs free energies (in parentheses) are given in kcal/mol.

Figure 3. Calculated structures for the critical species involved in the transmetalation. Bond lengths are given in angstroms.

differences? Comparing the structures of the corresponding transition states in Figure 3, it is found that the main difference between them can be ascribed to the absence and presence of phosphine ligands. This indicates that the phosphine ligand has

a stronger steric effect in the transmetalation, making the migration of the SnMe3 group difficult. Thus TS(4a/11b) and TS(4d/11a) are less favored energetically than TS(5a/6b) and TS(5d/6a). In addition, TS(5d/6a) is predicted to be higher in 55

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Figure 4. Gibbs free energy profiles for the reductive elimination. Gas-phase Gibbs free energies and solvent-corrected Gibbs free energies (in parentheses) are given in kcal/mol.

difference provides an opportunity for the optimization or manipulation of reaction conditions for the desired rate and products. Thus, it is concluded that the processes 4a → TS(4a/ 5a) → 5a → TS(5a/6b) → 6b and 4d → TS(4d/5d) → 5d →

free energy than TS(5a/6b) by 5.8 kcal/mol on one hand, and on the other hand, the allenic product via the intermediate 6a is thermodynamically more stable than the propargylic product via the intermediate 6b by 5.3 kcal/mol. This thermodynamic 56

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Figure 5. Calculated structures for the critical species involved in the reductive elimination. Bond lengths are given in angstroms.

TS(5d/6a) → 6a are both feasible in the reaction. These results are in agreement with the experimental observations, where the dearomatization reaction of chloromethylnaphthalene proceeds to give the corresponding propargylated and allenylated dearomatization products.8 This conclusion differs from that of naphthalene allyl chloride, in which the reaction offered only the propargylated dearomatization product.14 3.1.2. Reductive Elimination. By performing extensive potential energy surface scans, it is found that there exist four possible pathways, 1−4, for the reductive elimination in the presence of a Pd catalyst. The free energy profiles for these pathways are shown in Figure 4, while the structures of the

critical points associated with this section are illustrated in Figure 5. Path 1. Initially, the path 1 mechanism is postulated (see Scheme 2), which is in accordance with that proposed by Peng et al.8 From 6a and 6b, the mechanism may follow two different competitive pathways, path 1a and path 1b. First, the rearrangement from 6a and 6b to 7a and 7b has been examined. From the energy aspect, 7a and 7b are more noticeably unstable than 6a and 6b by 11.5 and 11.7 kcal/mol, respectively. These energy differences are mainly caused by the poorer aromaticity of 7a and 7b versus 6a and 6b due to the rearrangement of the η3-propargyl ligand. NICS26 calculations 57

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product (path 3b). These scenarios are very clear by observing the vivid transition vectors corresponding to the imaginary frequencies of TS(12a/13a) (292.1i cm−1) and TS(12b/13b) (357.8i cm−1). The reductive elimination processes from 12a and 12b have barriers of 10.9 and 11.6 kcal/mol, respectively. The overall barriers from 11a and 11b to 13a and 13b (18.4 and 14.1 kcal/mol) are moderate, demonstrating the feasibility of the newly proposed mechanism. It can be seen that intermediates and transition states are always quite similar and low in path 3a and path 3b. The minor differences between them indicate that both path 3a and path 3b possibly exist in reaction. The above results suggest that the path 3 mechanism proceeds smoothly to give the proparglated and allenylated dearomatization products. Path 4. As mentioned above, paths 1, 2, and 3 proceed with and without the monophosphine ligands. One may ask whether the reaction mechanism could take place via a bisphosphine complex (Pd(PH3)2). Inspired by this idea, the path 4 mechanism is obtained. In the path 4 mechanism, we choose 11a and 11b as the starting points of the free energy profiles. Starting from the intermediates 11a and 11b, the step involves attack of the PH3 ligand to the Pd center. The two transition states TS(11a/14a) and TS(11b/14b) have been located, where the Pd−C bonds are breaking and the Pd−P bonds are forming. Meanwhile, the propargyl group to the Pd center changes from η3- to η1-coordination. Once 14a and 14b are formed, the following step is the reductive elimination. The pathways corresponding to the direct coupling of the terminal carbons of the η1-propargyl ligand and η1-allenyl ligand with the para-carbons of the η3-methylnaphthalene ligands are calculated, a result consistent with the finding we discussed above for the reductive elimination from 12a and 12b. 15a and 15b are precursor complexes containing the product molecule as a ligand and having two phosphine ligands. The overall barriers for the reductive elimination processes going through 11a and 11b to give precursor complexes 15a and 15b are predicted high by 30.9 and 25.9 kcal/mol, respectively. Therefore, the corresponding pathways (path 4a and path 4b) cannot be responsible for the formation of the dearomatized products. 3.2. Comparison with Pathways Leading to Other Dearomatized Products. From 12a and 12b, it is worth noting that there exist four other possible reductive elimination pathways, apart from path 3a and path 3b discussed above. These pathways correspond to coupling of the terminal carbons of the η1-propargyl ligand and η1-allenyl ligand with the orthoand meta-carbons of the η3-methylnaphthalene ligands, leading to the formation of the other dearomatized products. In this section, we have explored these pathways in detail in order to understand the regioselectivity in reductive elimination. Figure 6 shows the free energy profiles calculated for the four possible reductive elimination pathways. The structures of key stationary points are given in the Supporting Information. The pathways 12a → TS(12a/13a1) → 13a1 and 12b → TS(12b/13b1) → 13b1 have larger free energy barriers of 15.7 and 16.2 kcal/mol, respectively. Similarly, the processes 12a → TS(12a/13a2) → 13a2 and 12b → TS(12b/13b2) → 13b2 can be accomplished by overcoming the high free energy barriers of 33.4 and 39.0 kcal/mol, respectively. Combining the results in Figures 4 and 6, the transition states TS(12a/13a1), TS(12b/ 13b1), TS(12a/13a2), and TS(12b/13b2) are higher in free energy than TS(12a/13a) and TS(12b/13b), supporting the conclusion that the most favorable mechanism for the reductive elimination is the coupling of the terminal carbons of the η1-

were performed to support the claim above. NICS(1) values were calculated at a point 1.0 Å away from the center of the phenyl ring, in a direction perpendicular to the plane of the ring. The NICS(1) values of −16.4, −16.7, −10.2, and −9.1 were calculated for 6a, 6b, 7a, and 7b, respectively, indicating that the phenyl rings in 6a and 6b have a higher degree of aromaticity than those of 7a and 7b. Once 7a and 7b are formed, the next step should properly correspond to the coordination of the PH3 ligand to the palladium center. In addition, the coordination mode of the propargyl group to the Pd center changes from η3- to η1-coordination. These steps proceed via the transition states TS(7a/8a) and TS(7b/8b), where the lengths of the forming Pd−P bonds are 2.440 and 2.431 Å, respectively. Subsequently, the reductive elimination of the dearomatized products from 8a and 8b takes place. The terminal carbon atom of the η1-propargyl ligand is coupled with the para-carbon of the η3-methylnaphthalene ligand, forming the allenylated dearomatized product 9a (path 1a), while the coupling of the terminal carbon of the η1-allenyl ligand leads to the proparglated dearomatized product 9b (path 1b). As shown in Figure 4, the processes have high free energy barriers of 31.5 and 36.6 kcal/mol, respectively. The overall barriers from 6a and 6b to 9a and 9b are 45.9 and 46.4 kcal/mol, which indicates that path 1 is not the main mechanism in the dearomatization reaction. Path 2. Bao and Yamamoto proposed a slightly different mechanism.37 We have evaluated the path 2 mechanism using intermediates 6a and 6b as starting points. As shown in Figure 4, path 2 is an analogue of path 1. The main difference between the two mechanisms is the reductive elimination directly from 7a and 7b via TS(7a/10a) and TS(7b/10b) to dearomatization products instead of the coordination of the PH3 ligand to the palladium center. For simplification, here we only draw the free energy profiles along path 2; the relevant details of geometries, energies, and mechanism are not involved. The processes from 6a to 10a in path 2a and from 6b to 10b in path 2b are endergonic by 10.7 and 3.8 kcal/mol and have higher free energy barriers of 47.6 and 47.6 kcal/mol, respectively. These data indicate that path 2a and path 2b are less favorable. Thus, path 2a and path 2b may not be preferred in the reaction, although they are competitive with path 1a and path 1b in the present case. Path 3. Ariafard and Lin proposed a new possible mechanism.38 Thus, we have considered two different competitive paths, depending on whether the calculation starts from 11a (path 3a) or 11b (path 3b). The first step is the isomerization of 11a and 11b to form the more unstable conformations 12a and 12b, respectively. The methylnaphthalene group to the Pd center changes from η1- to η3coordination, while the propargyl group to the Pd center switches from η3- to η1-coordination. The free energy profiles in Figure 4 clearly show that the isomerization processes are endergonic by 7.5 and 2.5 kcal/mol and need to overcome the moderate free energy barriers of 15.8 and 11.2 kcal/mol for path 3a and path 3b, respectively. Following the isomerization, reductive elimination takes place. The important step of this newly proposed mechanism is the reductive elimination directly from 12a and 12b via TS(12a/13a) and TS(12b/13b) in which the coupling between the terminal carbons of the η1propargyl ligand and η1-allenyl ligand and the para-carbons of the η3-methylnaphthalene ligands occurs. The former eventually leads to the allenylated dearomatized product (path 3a), while the latter eventually yields the proparglated dearomatized 58

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whereas the detailed structures of the key stationary points are illustrated in the Supporting Information. In this section, we have studied intermediates 6, 11, and 14 for direct coupling to give the corresponding precursor complexes 16−18, respectively. From the free energy profiles in Figure 7, processes 6a → 16a and 6b → 16b need to overcome free energy barriers of 32.1 and 33.8 kcal/mol; processes 11a → 17a and 11b → 17b need to overcome free energy barriers of 31.6 and 26.8 kcal/mol; processes 14a → 18a and 14b → 18b need to overcome free energy barriers of 29.8 and 21.6 kcal/mol. It is evident that all the direct coupling energy barriers are much higher than that calculated for the most favorable energy barriers 11a → 13a and 11b → 13b (18.4 and 14.1 kcal/mol), leading to the formation of the experimentally observed coupling products. In addition, the precursor complexes 16a, 16b, 17a, 17b, 18a, and 18b are relatively much more stable than the precursor complexes 13a and 13b. Thus, it is concluded that the reductive elimination processes 11a → 13a and 11b → 13b are kinetically, not thermodynamically, favored over the reductive processes 6a → 16a, 6b → 16b, 11a → 13a, 11b → 13b, 14a → 18a, and 14b → 18b. To understand why the transition states TS(12a/13a) and TS(12b/13b) are much more stable when compared with the directly coupling transition states TS(11a/17a) and TS(11b/ 17b), we have carried out an NBO analysis on the transition states TS(12a/13a), TS(12b/13b), TS(11a/17a), and TS(11b/17b). The results show that the coupling terminal carbons of the η1-propargyl ligand and η1-allenyl ligand in the transition states TS(12a/13a), TS(12b/13b), TS(11a/17a), and TS(11b/17b) are negative. The para-carbons of the η3methylnaphthalene ligands carry positive charges in TS(12a/ 13a) and TS(12b/13b). However, the direct coupling carbons of the η3-methylnaphthalene ligands carry more negative charges in TS(11a/17a) and TS(11b/17b). Thus, the terminal carbons of the η1-propargyl ligand and η1-allenyl ligand are favorable for coupling with the para-carbons of the η3methylnaphthalene ligands via electrostatic interactions. Furthermore, the methylnaphthalenes and allenyl ligands maintain strong coordination with the Pd center in TS(12a/13a) and TS(12b/13b). However, the methylnaphthalenes and allenyl ligands do not effectively bind to the Pd metal center in the directly coupling transition states (TS(11a/17a) and TS(11b/ 17b)). In other words, the reductive elimination via TS(12a/ 13a) and TS(12b/13b) requires a minimum disruption in the metal−ligand bonding on going from 12a to TS(12a/13a) and from 12b to TS(12b/13b), while the elimination via the direct coupling transition states requires drastic change in the metal− ligand bonding on going from 11a to TS(11a/17a) and from 11b to TS(11b/17b).

Figure 6. Gibbs free energy profiles for four other possible reductive elimination pathways from 12. Gas-phase Gibbs free energies and solvent-corrected Gibbs free energies (in parentheses) are given in kcal/mol.

propargyl ligand and η1-allenyl ligand with the para-carbons of the η3-methylnaphthalene ligands. In addition, we believe that the terminal carbons of the η1-propargyl ligand and η1-allenyl ligand are the nucleophilic atoms and the coupling paracarbons of the η3-methylnaphthalene ligands act as electrophiles. To further support the claim above, we have performed NBO charge analysis on 12a and 12b. The coupling terminal carbons of the η1-propargyl ligand and η1-allenyl ligand carry negative charge (−0.334e and −0.519e for 12a and 12b, respectively), and the coupling para-carbons (−0.005e and 0.002e for 12a and 12b, respectively) of the η3-methylnaphthalene ligands carry more positive charge than the orthocarbons (−0.276e and −0.268e for 12a and 12b, respectively) and the meta-carbons (−0.175e and −0.169e for 12a and 12b, respectively) of η3-methylnaphthalene ligands. Therefore, electrostatic interactions between the negatively charged terminal carbons of the η1-propargyl ligand and the η1-allenyl ligand and the positively charged para-carbons of the η3methylnaphthalene ligands are dominant for stabilizing TS(12a/13a) and TS(12b/13b). This is in line with the experimental findings, where 13a and 13b are the only products isolated instead of 13a1, 13a2, 13b1, and 13b2. This section strongly differs from that of naphthalene allyl chloride,14 in which the reaction of naphthalene allyl chloride with allenyltributylstannane gave the ortho dearomatization product. The above results indicate that the electronic factors are responsible for the selectivity for the methylnaphthalene position over the other positions in this reaction. 3.3. Comparison of Dearomatization Reaction and Stille Coupling Reaction. To study why the usually expected Stille coupling products were not observed in the reaction of chloromethylnaphthalene and allenyltributystannane, we have calculated the reaction barriers for pathways directly coupling the allenyl and methylnaphthalene groups. The free energy profiles associated with this section are shown in Figure 7,

4. CONCLUSIONS Summarizing the results of each individual step discussed above, the complete free energy surface for the Pd-catalyzed dearomatization reaction of chloromethylnaphthalene with allenyltributylstannane is constructed in Figure 8, and the detailed mechanism is illustrated in Scheme 3. The origin of the observed regioselectivity is also clarified. All species involved in the catalytic cycle have been fully characterized to be energy minimum structures for the intermediates or saddle point structures for the transition states. Due to the difference of the substituent in the naphthalene group, some crucial reaction behaviors of chloromethylnaph59

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Figure 7. Gibbs free energy profiles for direct coupling of intermediates 6, 11, and 14 to give the corresponding complexes 16−18, respectively. Gasphase Gibbs free energies and solvent-corrected Gibbs free energies (in parentheses) are given in kcal/mol.

Figure 8. Gibbs free energy profiles for the whole catalytic cycle. Gas-phase Gibbs free energies and solvent-corrected Gibbs free energies (in parentheses) are given in kcal/mol.

60

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Scheme 3

elimination, the path 3 mechanism is more favored both kinetically and thermodynamically than paths 1, 2, and 4. The preferred path involves isomerization of 11a and 11b to the intermediates η 3 -methylnaphthalene-η 1 -propargylPd(PH 3 ) (12a) and η3-methylnaphthalene-η1-allenylPd(PH3) (12b), followed by reductive elimination through coupling of the terminal carbons of the η1-propargyl ligand and η1-allenyl ligand with the para-carbons of the η3-methylnaphthalene ligands. Furthermore, reductive elimination starting from the intermediates 12a and 12b through coupling of the terminal carbons of the η1-propargyl ligand and η1-allenyl ligand with the ortho- or the meta-carbons of the η3-methylnaphthalene ligands, which gives different dearomatized products, is also studied. These couplings are found to be less favored than the coupling between the terminal carbons of the η1-propargyl ligand and η1allenyl ligand with the para-carbons of the η3-methylnaphthalene ligands. Direct coupling between the chloromethylnaphthalene and allenyl group is also found to be less favorable. These results should also have valuable important implications for other transition-metal-catalyzed carbon−carbon coupling reactions.

thalene dearomatization strongly differ from those of naphthalene allyl chloride. The dearomatization of chloromethylnaphthalene can give two kinds of para products (propargylated and allenylated dearomatization products). The analysis of the oxidative addition shows that the 12electron monophosphine species PdPH3 is the most active species. The oxidative addition has the largest free energy barrier and should be the rate-determining step in the overall reaction. The activation barrier of 21.0 kcal/mol is moderate, consistent with the reaction temperature. By comparing the energetics of the three-step and the one-step mechanisms in transmetaltion, one can see that the three-step mechanism is feasible. As discussed above, the three-step mechanism occurs in three steps. The first step corresponds to dissociaton of one phosphine ligand of 4a and 4d, which are formed by coordination between 2 and 3 and (allenyl)SnMe3, to the intermediates 5a and 5d, respectively. From 5a and 5d, the transmetalation step relates to the transfer of the SnMe3 group from the (allenyl)SnMe3 to the Cl ligand, leading to the two stable intermediates η3-methylnaphthalene-η3-propargyl-Pd (6a and 6b). 6a and 6b are key intermediates along the reaction path because they are responsible for the formation of propargylic and allenic dearomatized products. It is worth noting that the allenic product via the intermediate 6a is thermodynamically more stable than the propargylic product via the intermediate 6b by 5.3 kcal/mol. The thermodynamic difference between them provides an opportunity for the variation of the reaction conditions for the desired products. Then, the phosphine ligand recoordinates to the Pd center of 6a and 6b to afford 11a and 11b, respectively. For the reductive

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ASSOCIATED CONTENT

S Supporting Information *

Discussion on the oxidative addition, solvent effects, and ligand effects; Figures S1−S5, Tables S1 and S2 listing the single-point calculations, Table S3 giving total electronic energies and zeropoint energies as well as thermal corrections to enthalpies and Gibbs free energies for all systems, and the optimized Cartesian 61

dx.doi.org/10.1021/om300797n | Organometallics 2013, 32, 52−62

Organometallics

Article

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coordinates for all species; complete ref 17. This material is available free of charge via the Internet at http://pubs.acs.org.

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AUTHOR INFORMATION

Corresponding Author

*Fax and Tel: Int. code +86 0357 2052468. E-mail: jiajf@dns. sxnu.edu.cn. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS We are grateful to the Natural Science Foundation of China (21031003) and Shanxi Natural Science Foundation (2010011012-2).

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