Computational Study on the Palladium-Catalyzed Allenylative

Article pubs.acs.org/Organometallics

Computational Study on the Palladium-Catalyzed Allenylative Dearomatization Reaction Ying Ren, Jianfeng Jia, Ting-Ting Zhang, Hai-Shun Wu,* and Wenxian Liu School of Chemistry and Materials Science, Shanxi Normal University, Linfen 041004, People's Republic of China S Supporting Information *

ABSTRACT: The detailed mechanism of the Pd-catalyzed coupling of naphthalene allyl chloride with allenyltributylstannane, resulting in the dearomatization of the naphthalene group, has been studied using density functional theory (DFT) calculations at the B3LYP level. The catalyst cycle can be divided into three main stages involving oxidative addition, transmetalation, and reductive elimination, none of which contains significantly large barriers. It is found that the oxidative addition takes place through a monophosphine pathway. The transmetalation step is responsible for the formation of the propargylic dearomatized product, due to the orientation of the metal-coordinated allenyl ligand. Reductive elimination of the dearomatized product from the intermediate (η3allylnaphthalene)(η1-allenyl)PdPH3 occurs by coupling of the terminal carbon of the η1-allenyl ligand with the ortho carbon of the η3-naphthalene ligand. Furthermore, it is shown that dichloromethane as solvent does not change the mechanistic picture significantly.

1. INTRODUCTION The synthesis of alicyclic compounds has attracted much interest in recent years, in view of the fact that aliphatic carbocycle moieties frequently appear in the molecules of natural products and bioactive compounds. However, the special stability due to the delocalization of π bonds makes dearomatization of arenes very difficult.1 During the past few decades, many types of dearomatization reactions, such as oxidation,2 reduction,3 photocycloaddition,4 electrophilic addition,5 nucleophilic addition,6 [2,3]-σ-rearrangement,7 and other reactions,8 have been developed for breaking up the conjugated π system. With the development of these methods, the dearomatization reaction of arenes has become a facile and useful tool for the preparation of alicyclic compounds, since the aromatic compounds are stable and widely available.9 Recently, Peng and co-workers10 reported dearomatization reactions of naphthalene allyl chlorides with allenyltributylstannane in the presence of Pd(0) catalyst at room temperature (Scheme 1). The reactions are interesting and unusual because

stannane in the presence of palladium catalyst furnished orthoallenylated products. To better understand the dearomatization reactions promoted by the Pd(0) complex, Peng and coworkers10 proposed a reaction mechanism (Scheme 2) which Scheme 2

Scheme 1

involves oxidative addition, transmetalation, and reductive elimination as found in the Stille coupling catalytic cycle. As shown in Scheme 2, the key feature of the proposed reaction mechanism is the isomerization of intermediate 4, which is formed by transmetalation between 1c and allenyltributyl-

Pd-catalyzed cross-coupling reaction of organic electrophiles with organostannanes, better known as Stille coupling reactions, are widely used, particularly in more demanding synthetic transformations.11,12 Furthermore, they found the reaction of naphthalene allyl chlorides with allenyltributyl© 2012 American Chemical Society

Received: December 16, 2011 Published: February 1, 2012 1168

dx.doi.org/10.1021/om201248t | Organometallics 2012, 31, 1168−1179

Organometallics

Article

Scheme 3

stannane, to the intermediate 6, followed by reductive elimination to form the dearomatized products. However, a comprehensive understanding of the reaction mechanism by experimental methods presents several challenges. The most significant challenge is to isolate or trap the reaction intermediates. Therefore, it has not yet been elucidated why the reaction of naphthalene allyl chloride with allenyltributylstannane offered only propargylic dearomatized products. In the past few years, there have been some theoretical studies using Pd complexes as catalysts for organic transformations. For example, Yamamoto and Pichierri reported DFT studies on the mechanism and chemoselectivity of the Pdcatalyzed allylation of aldehydes.13 Li et al.14 studied the monoligated Pd-catalyzed cross-coupling reactions of aryl chlorides and bromides using the density functional theory method. A computational study on the structure and reactivity of (η1-allyl)Pd complexes was investigated by means of the B3LYP level of density functional theory.15 To the best of our knowledge, no theoretical study has been reported to investigate the mechanism of the Pd-catalyzed dearomatization reaction of naphthalene allyl chloride with allenyltributylstannane. Many mechanistic details of the reaction process remain ambiguous. The structural and energetic details about how these intermediates transform to each other are still unclear. Moreover, many interesting questions arise when we investigate the reaction mechanism. Does η1-allenyl ligand migration from palladium to the ortho carbon of the η3-naphthalene ligand take place easily? Why do we not observe the allenic dearomatized product and Stille coupling product from the reaction of naphthalene allyl chloride with allenyltributylstannane? How does each of the catalytic steps occur? Which step is rate

determining in the whole catalysis? To answer the questions raised above and gain insight into the mechanism of the catalytic cycle, we have studied the mechanism of the Pdcatalyzed dearomatization reaction of naphthalene allyl chloride with allenyltributylstannane using the B3LYP density functional method. The theoretical insights presented in this work are expected to be helpful in understanding such Pd-catalyzed dearomatization reactions and provide helpful information for chemists on similar processes.

2. COMPUTATIONAL DETAILS All calculations were performed with the Gaussian 0316 software package. The geometries of the reactants, transition states, intermediates, and products were fully optimized without any symmetry constraints at the B3LYP level of theory.17−19 Frequency calculations were carried out at the same level of theory for all the stationary points to characterize the transition states (one imaginary frequency) and the equilibrium structures (no imaginary frequency). The effective core potentials of Hay and Wadt with double-ζ valence basis sets (LANL2DZ)20 were used for Pd, P, Cl, and Sn atoms. The 6-311G(d,p) basis set was chosen to describe C and H atoms. Intrinsic reaction coordinate calculations (IRC)21,22 were also performed on transition states to confirm that such structures are indeed connecting two minima. Numerous theoretical studies of Pd-catalyzed reactions at the B3LYP level have been reported in the literature, which confirm that such an exchange-correlation functional is quite suitable to investigate Pd-catalyzed reactions.23,24 To check the reliability of our computational scheme, further single-point B3LYP/SDD25 and PBE1PBE26/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).27 Gibbs free energy corrections at 298 K were determined from harmonic frequencies and added to the total electronic energies 1169


Organometallics

Article

Figure 1. Gibbs free energy profiles for the oxidative addition. Gas-phase Gibbs free energies and solvent-corrected Gibbs free energies (in parentheses) are given in kcal/mol.

Figure 2. Bond distances (in Å) of the key stationary points involved in the oxidative addition.

catalyst, the ligand is considered modeled by PH3.23 This model is sufficient to simulate the present catalytic reaction and to clarify the mechanism of the reactivity and regioselectivity, though the steric and electronic effects may not be fully covered in the model phosphine ligand PH3. We discuss the mechanism of the title reaction in the following sections. 3.1. Oxidative Addition. The catalytic reaction starts with the oxidative addition of naphthalene allyl chloride to Pd(0), as outlined in Scheme 3. Many studies suggest that the true catalyst in this step is either L2Pd0 or LPd0.32,33 Thus, we chose Pd(PH3)2 as the reference point of Gibbs free energies for convenience of comparison. The free energy profiles for this process are reported in Figure 1, and some key structures are shown in Figure 2. The naphthalene allyl chloride oxidative addition might proceed along two different pathways. One pathway involves dissociation of one phosphine ligand from Pd(PH3)2 to

to get the Gibbs free energies. For consideration of the entropy and enthalpy effects, the following discussions are based on the free energies (ΔG) of activation and reaction. Solvent effects were calculated by performing single-point calculations in dichloromethane using the polarized continuum model (PCM)28 with the united atom Hartree−Fock (UAHF)29 radii on the gas-phase-optimized geometries. The solvation free energy was calculated at the B3LYP/6311G(d,p) (LANL2DZ for Pd, P, Cl, and Sn atoms) level and added to the gas-phase free energy to obtain the Gibbs free energy in solution. To estimate the degree of aromaticity of the phenyl ring in naphthalene ligands, nucleus-independent chemical shift (NICS)30 calculations were performed using the GIAO31 method at the same level of theory.

3. RESULTS AND DISCUSSION In this section, the potential energy surface (PES) of the Pdcatalyzed dearomatization reaction of naphthalene allyl chloride with allenyltributylstannane is fully investigated. The detailed mechanism is depicted in Scheme 3. As a model of the Pd 1170


Organometallics

Article

Figure 3. 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.

in Figure 4. Two different pathways are proposed for the transmetalation: cyclic and open. However, in the presence of an electronegative leaving group such as chloride, the transmetalation proceeds preferentially via a cyclic pathway instead of an open pathway.24a Accordingly, the next step is surmised to be the coordination of (allenyl)SnMe3 to the Pd center. SnMe3 is used to model SnBu3, which is commonly used in the experiments. The (allenyl)SnMe3 binds weakly to Pd in 1c to form the π complex 2. The weaker metal coordination causes the adduct between (allenyl)SnMe3 and 1c to not correspond to a local minimum on the PES. The coordination of (allenyl)SnMe3 to the Pd center in 1c is an endergonic process. As shown in Figure 4, the tetracoordinated intermediate 2 exists in two minimum-energy conformers: one where the coordinated C1−C2 bond is opposite the C4−C5 bond (2a) and another where the C2−C1 bond is along the C4−C5 bond (2b). The former is slightly less stable than the latter by 0.6 kcal/mol. These interesting geometrical features provide conditions for the later formation of the propargylic dearomatized product. From 2a,b, the transmetalation proceeds along two pathways, with and without the phosphine ligand pathways. Let us first discuss the absence of the phosphine mechanism proposed by Peng and co-workers. First, the PH3 ligands are dissociated from 2a,b to lead to 3a,b, respectively. In addition, the coordination mode of allylnaphthalene to the Pd center changes from η1 to η3 coordination. The processes 2a → TS(2a/3a) → 3a and 2b → TS(2b/3b) → 3b have low free energy barriers of 6.0 and 10.3 kcal/mol, respectively. Once intermediates 3a,b are formed, the next step should properly correspond to the transmetalation step. The step involves SnMe3 migration from the (allenyl)SnMe3 group to the Cl ligand via a four-membered-ring transition state. It is interesting to note that the two processes (3a → 4 and 3b → 4) have the same transition state TS(3/4). The energy barriers from 3a to 4 and from 3b to 4 are 14.1 and 14.0 kcal/mol. One would raise

generate the monophosphine complex PdPH3, and then PdPH3 and naphthalene allyl chloride undergo oxidative addition via TS(PdPH3/1a) and transform into the tricoordinated complex 1a. The 14-electron unsaturated complex Pd(PH3)2 has a linear geometry with two PH3 ligands staggered with respect to each other and a Pd−P bond length of 2.376 Å. The Pd−P bond decreases to 2.318 Å after losing a PH3 ligand to form monoligated PdPH3, a result attributed to the trans influence of the PH3 ligand. The 12-electron complex PdPH3 can readily combine with one molecule of naphthalene allyl chloride to form the tricoordinated complex 1a via transition state TS(PdPH3/1a). From the free energy profiles in Figure 1, the step is exergonic by 7.2 kcal/mol with an energy barrier of 20.0 kcal/mol as calculated from Pd(PH3)2. The other pathway involves the direct oxidative addition of naphthalene allyl chloride to the bisphosphine complex Pd(PH3)2 via TS(Pd(PH3)2/1b) to form tetracoordinated complex 1b. The corresponding free energy barrier is 27.2 kcal/mol. Then the bisphosphine complex 1b dissociates one PH3 ligand to lead to the monophosphine complex 1a. The process (1b → TS(1b/ 1a) → 1a) is in a fast equilibrium, indicated by the low free energy barrier of 5.9 kcal/mol and the slightly endergonic properties. However, as shown in Figure 1, the bisphosphine complex TS(Pd(PH3)2/1b) is predicted to be higher in free energy than the corresponding monophosphine complex TS(PdPH3/1a) by 7.2 kcal/mol. Therefore, the oxidative addition will take place through the monophosphine pathway. Next, 1a is able to undergo an isomerization reaction via the transition state TS(1a/1c) by overcoming a small barrier of 4.5 kcal/mol to reach 1c. 1c is more stable than 1a by 8.6 kcal/mol, partly due to the strong Pd−η3-allylnaphthalene bonding interaction. 3.2. Transmetalation. Following the oxidative addition, transmetalation takes place. The free energy profiles for this section are depicted in Figure 3, while the structures of several key stationary points involved in transmetalation are illustrated 1171


Organometallics

Article

Figure 4. Bond distances (in Å) of the key stationary points involved in the transmetalation.

to the Pd center from 4 to 8. The lower barrier of 12.4 kcal/ mol indicates that the interconversion can proceed in a facile manner. As displayed in Figure 4, the attacking phosphine ligand occupies the equatorial site of the Pd center in a side-on orientation (η1-PH3) due to the proper orbital interaction and the simultaneous cleavage of the η3-allyl interaction. Clearly, 8 is not particularly stable, likely due to the low stability of the η1allyl bonding interaction. Considering now the presence of the phosphine mechanism, the step corresponds to a direct migration of the SnMe3 group from 2a,b to 8 via a four-membered-ring transition state. Similarly, the processes 2a → 8 and 2b → 8 have the same transition state, TS(2/8). The energy barriers from 2a to 8 and from 2b to 8 are 12.5 and 13.1 kcal/mol, respectively. The

a query why the two processes have the same transition state. On comparison of the structures of 3a,b, the geometric difference between them is trivial; i.e., the Pd−Cl bonds (2.448 and 2.448 Å) and Pd−C3 bonds (2.289 and 2.285 Å) in 3a,b are almost the same. Also, 3a is only slightly more stable than 3b by 0.1 kcal/mol. The main difference between the two complexes is the orientation of the C1−C2 bond coordination mode. These results indicate that the two processes 3a → 4 and 3b → 4 have the same transition state, TS(3/4). This is in line with the experimental findings, where the reaction of naphthalene allyl chloride with allenyltributylstannane offered only a propargylic dearomatized product.10 The above results suggest that the transmetalation is a crucial step for determining the dearomatized product. Next, the PH3 ligand is coordinated 1172


Organometallics

Article

Figure 5. Proposed mechanisms of reductive elimination for producing dearomatized product.

Figure 6. 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.

path with the lowest free energy maximum should be the favored path because the two paths have the same starting point, 1c. Starting with 1c, the highest-energy point of the absence of phosphine pathway is the transition states TS(2a/ 3a) and TS(2b/3b), and their free energies are 5.3 and 9.0 kcal/mol, respectively. On the other hand, the highest point of

reasons are consistent with the finding we discussed above for the processes 3a → 4 and 3b → 4. Thus, we will not discuss the processes 2a → 8 and 2b → 8 in detail. At this stage, we will address the competition of the absence and presence of phosphine ligand pathways in transmetalation. For the two paths in the present study, it is evident that the 1173


Organometallics

Article

Figure 7. Bond distances (in Å) of the key stationary points involved in the reductive elimination along path 1.

those in 4, while the C9−C10 bond is shortened to 1.360 Å. Furthermore, nucleus-independent chemical shift (NICS, a measure of aromaticity) calculations 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. Negative NICS values denote aromaticity, and positive NICS values denote antiaromaticity, while small NICS values indicate nonaromaticity.30 NICS(1) values of −23.6 and −17.0 were calculated for 4 and 5, respectively, indicating that the phenyl ring of 4 has a degree of aromaticity higher than that of 5. The following step is the further conversion of 5 to form 6, in which the phosphine ligand participates in the coordination to the Pd atom. In addition, the coordination mode of the propargyl ligand to the Pd center changes from η3 to η1 coordination. As depicted in Figure 7, in 6 the C1−C2 bond (1.291 Å) is slightly elongated by 0.045 Å, whereas the C2−C3 bond (1.317 Å) is slightly shortened by 0.073 Å. It is indicated that the structure of the propargyl ligand is changed into an allenyl ligand. The corresponding transition state for the coordination of phosphine ligand, TS(5/6), has been located. From the free energy profiles in Figure 6, the step is endergonic by 0.6 kcal/mol with a moderate energy barrier of 13.4 kcal/ mol. Finally, the reductive elimination of the dearomatized product from 6 occurs. The scene is very clear by observing the vivid transition vector corresponding to the imaginary frequency of TS(6/7) (427.8i cm−1). The relative energy of the transition state TS(6/7) correlates well with those of 6 and 7. The process 6 → TS(6/7) → 7 is calculated to be endergonic by 1.8 kcal/mol with an energy barrier of 20.4 kcal/mol. The overall barrier from 4 to 7 is 29.1 kcal/mol. It is clearly seen that the overall barrier is high for the proposed mechanism to be energetically feasible. Path 2. In contrast with path 1 discussed above, a new mechanism other than the proposed mechanism should be

the presence of phosphine pathway is the transition state TS(2/ 8), whose free energy is 11.8 kcal/mol, which is higher than those for TS(2a/3a) and TS(2b/3b) by 6.5 and 2.8 kcal/mol. The differences in energy barrier indicate that the absence of phosphine pathway should be kinetically more favored than the presence of phosphine pathway. Furthermore, 4 is thermodynamically more stable than 8 by 6.7 kcal/mol. It is concluded that the absence of phosphine pathway is the only favored path in transmetalation. These results are consistent with the experimental presumption that transmetalation occurs between 1c and (allenyl)SnMe3 to give 4, rather than 8. 3.3. Reductive Elimination. In this portion of the study, we have investigated four possible pathways for the reductive elimination. To state the reaction mechanism for reductive elimination, four pathways are denoted as path 1, path 2, path 3, and path 4. The proposed mechanisms are illustrated in Figure 5, and the free energy profiles are represented in Figure 6. Path 1. We first discuss the path 1 mechanism, which is in accordance with the proposal by Peng and co-workers. As shown in Scheme 2, the key feature of Peng’s mechanism is the isomerization from (η3-allylnaphthalene)(η3-propargyl)Pd intermediate 4 to (η3-allylnaphthalene)(η1-allenyl)PdPH3 intermediate 6. The optimized structures of the various critical points are shown in Figure 7. Starting with 4, the rearrangement of η3-propargyl to form complex 5 can take place. The transition state TS(4/5) has been located. The free energy profiles in Figure 6 clearly show that the process 4 → TS(4/5) → 5 needs to overcome the free energy barrier of 15.8 kcal/mol. Complex 5 is noticeably more unstable than intermediate 4 by 8.1 kcal/mol. The energy difference is mainly caused by the poorer aromaticity of the η3endo-naphthalene ligand in 5 versus the η3-exo-naphthalene ligand in 4. In comparison with 4, the C7−C12, C7−C8, C8− C9, and C10−C11 bonds in 5 are 1.462, 1.425, 1.428, and 1.433 Å, which are 0.022, 0.039, 0.019, and 0.015 Å longer than 1174


Organometallics

Article

coincidence with the theoretical results found recently by Ariafard and Lin for dearomatization of the benzyl group.34 Moreover, 10 is predicted to be lower than 7 by 6.0 kcal/mol. Therefore, path 2 is more favored both kinetically and thermodynamically. Path 3. As mentioned above, paths 1 and 2 involve the monophosphine complexes. One may ask whether the reaction mechanism could take place via bisphosphine complexes. According to this idea, path 3 is obtained. Similar to path 2, the following discussion starts with 8. The optimized structures of the intermediates and transition states are shown in Figure 9. The bisphosphine complex 11 is formed by coordination of a phosphine ligand. Accompanying the PH3 approach, the η3-

operative. We suppose that reductive elimination takes place from 8. According to this idea, we have examined the path 2 mechanism in detail. The optimized structures are illustrated in Figure 8. First, the isomerization of 8 to 9 has been examined. The process 8 → TS(8/9) → 9 has a low free energy barrier of 8.9


kcal/mol. The isomerization is found to be exergonic, indicating that (η3-allylnaphthalene)(η1-allenyl)PdPH3 intermediate 9 is more stable than (η1-allylnaphthalene)(η3propargyl)PdPH3 intermediate 8 by 6.8 kcal/mol. The following step is to give the corresponding propargylic dearomatized product. The important step of this newly proposed mechanism is the reductive elimination directly from 9 via TS(9/10), in which a coupling of the terminal carbon of the η1-allenyl ligand with the ortho carbon of the η3naphthalene ligand is hypothesized. Similar theoretical studies are formulated that the intramolecular allyl−allyl coupling prefers a pathway via the formation of the C−C bond between the terminal carbon of the allyl ligands in a bis(η1-allyl)metal complex.34,35 Our calculations provide firm support for the hypothesis. Figure 6 shows the energy profile for the 8 → 9 isomerization followed by reductive elimination. 10 is a precursor complex containing the product molecule as a ligand. The energy barrier from 9 to TS(9/10) is 17.1 kcal/mol, which indicates that the step proceeds easily. The overall barrier from 4 to 10 is a moderate, demonstrating the feasibility of the newly proposed mechanism in Scheme 3. Furthermore, the NICS(1) values are −23.8 and −10.3 for 8 and 10, revealing that the phenyl ring in naphthalene ligand of 10 has a degree of aromaticity lower than that of 8. As shown in Figure 6, it is found that the relative stability of 9 plays an important role in the relative stability of the transition state TS(9/10) as well as the complex 10. The overall barrier in path 2 is relatively lower than that in path 1, indicating that the newly proposed path 2 mechanism is more favorable than the path 1 mechanism. This confirmation is in


propargyl coordination mode should switch to the η1-propargyl mode. The transition state TS(8/11) of coordination with two PH3 ligands is also found. Along the proceeding reaction, IRC calculations demonstrate that TS(8/11) converges to the desired 11. The geometry of the transition state is tetrahedral, and the Pd−C3 bond (2.120 Å) in 11 is shorter than that in 8 (2.152 Å). The conversion 8 → TS(8/11) → 11 is an endergonic process. The corresponding free energy barrier is 9.1 kcal/mol. Once 11 is formed, the next step is reductive elimination. The pathway corresponding to a coupling of the terminal carbon of the η1-propargyl ligand with the ortho carbon of the η3-naphthalene ligand is calculated to occur, a result consistent with the finding we discussed above for the reductive elimination from 9. The NICS(1) values are −23.8 and −8.8 for 8 and 12, respectively, demonstrating that the phenyl ring in the naphthalene ligand of 12 has a lower degree of aromaticity than that of 8. The step is exergonic by 8.2 kcal/mol, requiring surpassing an energy barrier of 18.1 kcal/mol. Again, the overall 1175


Organometallics

Article

described above, one can see that path 2 is preferred, because the activation energy barrier of the transition state TS(9/10) is the lowest among the four paths and the four paths have the same starting point 4. It is indicated that the path 2 mechanism should be kinetically more favored than paths 1, 3, and 4. Furthermore, the product of path 3 is an allenic dearomatized product instead of a propargylic dearomatized product, which is not in accordance with experimental observation. Thus, the path 3 mechanism is ruled out. However, complex 10 is thermodynamically more stable than complexes 7 and 14 by 6.0 and 13.8 kcal/mol, respectively. All these results indicate that the path 2 mechanism is the only favored path in reductive elimination leading to the propargylic dearomatized product. 3.4. Comparison with Stille Coupling Reaction. As mentioned in the Introduction, Pd-catalyzed cross-coupling reaction of organic electrophiles with organostannanes, better known as Stille coupling reactions, are widely used. However, Peng and co-workers reported the reaction of naphthalene allyl chlorides with allenyltributylstannane offered only propargylic dearomatized products. The corresponding Stille coupling products were not observed at all. For complete comparison, we have addressed the competition of the dearomatization reaction and Stille coupling reaction. The free energy profiles for this section are depicted in Figure 11, while the structures of stationary points are given in the Supporting Information. In this section, we have explored intermediates 4, 8, and 11 for direct coupling to give the corresponding precursor complexes 15−17, respectively, having the Stille cross-coupling product as a ligand. From the energy profiles in Figure 11, the energy barriers for the direct coupling processes 4 → 15, 8 → 16, and 11 → 17 are calculated to be 35.9, 34.0, and 43.0 kcal/ mol relative to 4, respectively. Consistent with the experimental observation, all the direct coupling energy barriers are much higher than that calculated for the most favorable energy barrier 8 → 10 (17.0 kcal/mol, Figure 6) leading to the formation of the experimentally observed propargylic dearomatized products. In addition, the precursor complexes 15−17 having the Stille coupling product as a ligand are relatively much more stable than the precursor complex 10 having the propargylic dearomatized product. Obviously, the relative stability of these precursor complexes is closely related to the relative stability of the Stille coupling products that act as a ligand. The Stille coupling product is calculated to be more stable than the propargylic dearomatized product by 23.8 kcal/mol. Thus, it is concluded that the formation of propargylic dearomatized products via the process 8 → 10 is kinetically, not thermodynamically, favored over the formation of Stille coupling products. 3.5. Effect of Phosphine Ligands. In most theoretical studies of Pd-catalyzed Stille reactions, the experimentally used phosphine ligands are modeled by PH3. To study the steric effect missed by the small-model calculations, we have performed calculations with the real PPh3 ligand for the Pd(PH3)2 → TS(PdPH3/1a) and Pd(PH3)2 → TS(Pd(PH3)2/ 1b) steps in oxidative addition. In the PH3 model calculations, the activation barriers from Pd(PH3)2 to TS(Pd(PH3)2/1a) and from Pd(PH3)2 to TS(PdPH3/1b) are calculated to be 20.0 and 27.2 kcal/mol, respectively (see Figure 1). In the PPh3 model calculations, the activation barriers from Pd(PPh3)2 to TS(PdPPh3/1a) and from Pd(PPh3)2 to TS(Pd(PPh3)2/1b) are calculated to be 20.7 and 30.1 kcal/mol, respectively. On the basis of these data, it is concluded that the steric bulkiness of the PPh3 ligand does not affect the barriers significantly.

barrier for the reductive elimination going through 11 is high. Furthermore, the product of path 3 is an allenic dearomatized product. This result is not in agreement with experiment, where the reaction of naphthalene allyl chloride with allenyltributylstannane offered only a propargylic dearomatized product. Therefore, the corresponding pathway cannot be responsible for the formation of the propargylic dearomatized product. Path 4. On the basis of the above discussion, there exists another possible reductive elimination pathway via bisphosphine complexes. As illustrated in Figure 5, intermediate 9 first isomerizes to intermediate 6, and then the second PH3 ligand approaches the Pd center of 6. Finally, a conventional reductive elimination might occur, leading to the dearomatized product. Inspired by this idea, the path 4 mechanism is obtained. The optimized structures are illustrated in Figure 10. Following the isomerization of 8, the further isomerization of 9 takes place. From Figure 6, it is found that the process 9 →


TS(9/6) → 6 is endergonic by 8.8 kcal/mol and needs to overcome the free energy barrier of 18.2 kcal/mol. Then the following step should properly correspond to the coordination of PH3 ligand to the palladium center. The coordination mode of the allylnaphthalene group to the Pd center changes from η3 to η1 coordination. This step proceeds via transition state TS(6/13), where the length of the forming Pd−P bond is 2.844 Å. The coordination of the PH3 ligand (6 → TS(6/13) → 13) needs to overcome the low free energy barrier of 9.9 kcal/mol. Subsequently, the reductive elimination of the dearomatized product from 13 takes place. The C3 atom of the η1-allenyl ligand is coupled with the ortho carbon of the η3naphthalene ligand, leading to 14. NICS(1) values of −23.8 and −9.6 were calculated for 8 and 14, respectively, indicating that the phenyl ring of 8 has a degree of aromaticity higher than that of 14. The calculated energy barrier from 13 to TS(13/14) is 22.6 kcal/mol. The overall barrier from 4 to 14 is 38.0 kcal/ mol. Clearly, path 4 is energetically unfeasible. To gain a complete understanding of the mechanistic pathways in reductive elimination, the free energy profiles obtained with the four mechanisms, shown in Figure 6, must be compared. When a multistep reaction reaches the steady state, the overall reaction barrier is approximately determined by the free-energy gap between the highest-energy species and the resting state.33a By comparing the energies of paths 1−4 1176


Organometallics

Article

Figure 11. Gibbs free energy profiles for direct coupling of intermediates 4, 8, and 11 to give the corresponding complexes 15−17, respectively. Gasphase Gibbs free energies and solvent-corrected Gibbs free energies (in parentheses) are given in kcal/mol.

Also, the geometry around the metal center in each of the calculated structures also does not change much (Supporting Information). The results indicate that the PH3 model is sufficient to simulate the present catalytic reaction and to clarify the mechanism of the reactivity and regioselectivity. 3.6. Solvent Effects. As we know, one of the most frequently asked questions in general is the solvent effect in computation, since most of the reactions take place in solvents rather in the ideal gas phase. The experimental reaction10 was carried out in dichloromethane solution at room temperature. Therefore, we have performed a set of calculations to estimate the solvent effects of the dichloromethane solution for the dearomatization reaction. Comparison between gas-phase and medium calculations can provide insight into the effect of solvent on the reaction PES and change in the reaction mechanism. The data are given in parentheses of the corresponding free energy profiles in Figures 1, 3, and 6. From Figures 1, 3, and 6, one can find the solvation free energy for the species involved in oxidative addition is in the range 1.1−6.6 kcal/mol, while the species involved in transmetalation and reductive elimination have a slightly larger solvation free energy of 2.5−8.3 kcal/mol. However, the free energies of activation and reaction in dichloromethane are comparable with the data in the gas phase. The key steps are still the same in solution. For example, in the oxidative addition, the activation free energies are 20.0 (in gas phase) and 23.7 kcal/mol (in solution) in the monophosphine pathway. For the transmetalation, the activation free energies are 5.3 (in gas phase) and 9.9 kcal/mol (in solution) from 2a to 4 and 9.0 (in

gas phase) and 12.8 kcal/mol (in solution) from 2b to 4. In the reductive elimination, the activation free energies are 3.4 (in gas phase) and 2.8 kcal/mol (in solution) in path 2. On the basis of these data, one can see that no disorders of the stationary points on the energy profiles due to solvation can be found. Furthermore, it is remarkable that the final complex, 10, is more stable in solution than in gas phase; therefore, in dichloromethane solution the reaction is more exothermic. It is summarized that there are not important quantitative energetic differences in dichloromethane on one hand, and on the other hand, dichloromethane as solvent does not change the general trends for the reaction potential energy surfaces.

4. CONCLUSIONS The dearomatization reaction of naphthalene allyl chloride with allenyltributylstannane catalyzed by Pd(0) complexes discovered by Peng and co-workers has been theoretically studied using DFT calculations at the B3LYP level. 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. Our calculations gave a number of insights into the experimental observations reported by Peng and co-workers. It is found that the overall catalytic cycle is composed of three major elementary steps: (i) oxidative addition, (ii) transmetalation, and (iii) reductive elimination. The oxidative addition of naphthalene allyl chloride is confirmed to occur through a monophosphine pathway. This step has the highest energy barrier, therefore becoming the rate-determining step in 1177


Organometallics

Article

I. Org. Lett. 2004, 6, 609. (c) Clayden, J.; Kenworthy, M. N.; Helliwell, M. Org. Lett. 2003, 5, 831. (7) Caruana, P. A.; Frontier, A. J. Tetrahedron 2004, 60, 10921. (8) (a) Liu, R.; Zhang, C.; Zhu, Z.; Luo, J.; Zhou, X.; Weng, L. Chem. Eur. J. 2006, 12, 6940. (b) Crich, D.; Sannigrahi, M. Tetrahedron 2002, 58, 3319. (9) (a) Bao, M.; Nakamura, H.; Yamamoto, Y. J. Am. Chem. Soc. 2001, 123, 759. (b) Lu, S. R.; Xu, Z. W.; Bao, M.; Yamamoto, Y. Angew. Chem., Int. Ed. 2008, 47, 4366. (10) Peng, B.; Feng, X. J.; Zhang, X.; Zhang, S.; Bao, M. J. Org. Chem. 2010, 75, 2619. (11) (a) Farina, V. Pure Appl. Chem. 1996, 68, 73. (b) Casado, A. L.; Espinet, P.; Gallego, A. M. J. Am. Chem. Soc. 2000, 122, 11771. (12) (a) Crawforth, C. M.; Fairlamb, I. J. S.; Taylor, R. J. K. Tetrahedron Lett. 2004, 45, 461. (b) Crawforth, C. M.; Burling, S.; Fairlamb, I. J. S.; Kapdi, A. R.; Taylor, R. J. K.; Whitwood, A. C. Tetrahedron 2005, 61, 9736. (13) Pichierri, F.; Yamamoto, Y. J. Org. Chem. 2007, 72, 861. (14) Li, Z.; Fu, Y.; Guo, Q. X.; Liu, L. Organometallics 2008, 27, 4043. (15) Garcia-Iglesias, M.; Bunuel, E.; Cardenas, D. J. Organometallics 2006, 25, 3611. (16) Frisch, M. J.; Trucks, G. W.; Schlegel, H. B.; Scuseria, G. E.; Robb, M. A.; Cheeseman, J. R.; Montgomery, J. A., Jr.; Vreven, T.; Kudin, K. N.; Burant, J. C.; Millam, J. M.; Iyengar, S. S.; Tomasi, J.; Barone, V.; Mennucci, B.; Cossi, M.; Scalmani, G.; Rega, N.; Petersson, G. A.; Nakatsuji, H.; Hada, M.; Ehara, M.; Toyota, K.; Fukuda, R.; Hasegawa, J.; Ishida, M.; Nakajima, T.; Honda, Y.; Kitao, O.; Nakai, H.; Klene, M.; Li, X.; Knox, J. E.; Hratchian, H. P.; Cross, J. B.; Bakken, V.; Adamo, C.; Jaramillo, J.; Gomperts, R.; Stratmann, R. E.; Yazyev, O.; Austin, A. J.; Cammi, R.; Pomelli, C.; Ochterski, J. W.; Ayala, P. Y.; Morokuma, K.; Voth, G. A.; Salvador, P.; Dannenberg, J. J.; Zakrzewski, V. G.; Dapprich, S.; Daniels, A. D.; Strain, M. C.; Farkas, O.; Malick, D. K.; Rabuck, A. D.; Raghavachari, K.; Foresman, J. B.; Ortiz, J. V.; Cui, Q.; Baboul, A. G.; Clifford, S.; Cioslowski, J.; Stefanov, B. B.; Liu, G.; Liashenko, A.; Piskorz, P.; Komaromi, I.; Martin, R. L.; Fox, D. J.; Keith, T.; Al-Laham, M. A.; Peng, C. Y.; Nanayakkara, A.; Challacombe, M.; Gill, P. M. W.; Johnson, B.; Chen, W.; Wong, M. W.; Gonzalez, C.; Pople, J. A. Gaussian 03, Revision D.01; Gaussian, Inc., Wallingford, CT, 2004. (17) Lee, C.; Yang, W.; Parr, R. G. Phys. Rev. B 1988, 37, 785. (18) Becke, A. D. J. Chem. Phys. 1993, 98, 5648. (19) Stephens, P. J.; Devlin, F. J.; Chabalowski, C. F.; Frisch, M. J. J. Phys. Chem. 1994, 98, 11623. (20) (a) Wadt, W. R.; Hay, P. J. J. Chem. Phys. 1985, 82, 284. (b) Hay, P. J.; Wadt, W. R. J. Chem. Phys. 1985, 82, 299. (21) Fukui, K. J. Phys. Chem. 1970, 74, 4161. (22) Fukui, K. Acc. Chem. Res. 1981, 14, 363. (23) Abe, Y.; Kuramoto, K.; Ehara, M.; Nakatsuji, H.; Suginome, M.; Murakami, M.; Ito, Y. Organometallics 2008, 27, 1736. (24) (a) Ariafard, A.; Yates, B. F. J. Am. Chem. Soc. 2009, 131, 13981. (b) Zhang, S. L.; Fu, Y.; Shang, R.; Guo, Q. X.; Liu, L. J. Am. Chem. Soc. 2009, 132, 638. (25) Andrae, D.; Haeussermann, U.; Dolg, M.; Stoll, H.; Preuss, H. Theor. Chim. Acta 1990, 77, 123. (26) Adamo, C.; Barone, V. J. Chem. Phys. 1999, 110, 6158. (27) (a) Ehlers, A. W.; Bohme, M.; Dapprich, S.; Gobbi, A.; Hollwarth, A.; Jonas, V.; Kohler, K. F.; Stegmann, R.; Veldkamp, A.; Frenking, G. Chem. Phys. Lett. 1993, 208, 111. (b) Hollwarth, A.; Bohme, M.; Dapprich, S.; Ehlers, A. W.; Jonas, V.; Kohler, K. F.; Stegmann, R.; Veldkamp, A.; Frenking, G. Chem. Phys. Lett. 1993, 208, 237. (28) (a) Miertus, S.; Scrocco, E.; Tomasi, J. Chem. Phys. 1981, 55, 117. (b) Miertus, S.; Tomasi, J. Chem. Phys. 1982, 65, 239. (29) Barone, V.; Cossi, M.; Tomasi, J. J. Chem. Phys. 1997, 107, 3210. (30) Schleyer, P. v. R.; Maerker, C.; Dransfeld, A.; Jiao, H.; Hommes, N. J. R. v. E. J. Am. Chem. Soc. 1996, 118, 6317. (31) Wolinski, K.; Hinton, J. F.; Pulay, P. J. Am. Chem. Soc. 1990, 112, 8251.

the overall cyclic mechanism. In the transmetalations, starting with 3a,b, both take place via the same four-membered-ring transition state, TS(3/4), to lead to the (η3-allylnaphthalene)(η3-propargyl)Pd intermediate 4. The result suggests that the transmetalation is a crucial step for determining the dearomatized product. This is in line with the experimental findings that propargylic dearomatized product, instead of allenic dearomatized product, is the only product isolated. Furthermore, 4 is a key intermediate along the reaction path because it facilitates the formation of the (η1-allylnaphthalene)(η3-propargyl)PdPH3 intermediate 8. Then, the reductive elimination occurs easily along path 2. The path 2 mechanism involves isomerization of 8 to the intermediate (η 3 allylnaphthalene)(η1-allenyl)PdPH3 (9), followed by reductive elimination through coupling of the terminal carbon of the η1allenyl ligand with the ortho carbon of the η3-naphthalene ligand. Direct coupling between the naphthalene allyl chloride and allenyl groups, which gives the Stille coupling product, is found to be kinetically less favorable. The overall reaction is both exothermic and exergonic. It is noted that dichloromethane as solvent does not change the potential energy surface compared to that found in the gas phase.

■

ASSOCIATED CONTENT

S Supporting Information *

Table S1, giving the single-point calculations, Table S2, giving total electronic energies and zero-point energies as well as thermal corrections to enthalpies and Gibbs free energies for all systems, and tables giving the optimized Cartesian coordinates for all species. This material is available free of charge via the Internet at http://pubs.acs.org.

■

AUTHOR INFORMATION

Corresponding Author

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

■

ACKNOWLEDGMENTS This work was supported by the Natural Science Foundation of China (21031003) and Shanxi Natural Science Foundation (2010011012-1).

■

REFERENCES

(1) (a) Chordia, M. D.; Harman, W. D. J. Am. Chem. Soc. 2000, 122, 2725. (b) Keane, J. M.; Harman, W. D. Organometallics 2005, 24, 1786. (c) Jantunen, K. C.; Scott, B. L.; Hay, P. J.; Gordon, J. C.; Kiplinger, J. L. J. Am. Chem. Soc. 2006, 128, 6322. (d) Monje, P.; Grana, P.; Paleo, M. R.; Sardina, F. J. Org. Lett. 2006, 8, 951. (e) Pigge, F. C.; Coniglio, J. J.; Dalvi, R. J. Am. Chem. Soc. 2006, 128, 3498. (2) (a) Lebrasseur, N.; Fan, G. J.; Oxoby, M.; Looney, M. A.; Quideau, S. Tetrahedron 2005, 61, 1551. (b) Mejorado, L. H.; Hoarau, C.; Pettus, T. R. R. Org. Lett. 2004, 6, 1535. (c) Van De Water, R. W.; Hoarau, C.; Pettus, T. R. R. Tetrahedron Lett. 2003, 44, 5109. (d) Bui, V.; Hansen, T. V.; Stenstrom, Y.; Ribbons, D. W.; Hudlicky, T. J. Chem. Soc., Perkin Trans. 1 2000, 1669. (3) (a) Heller, E.; Lautenschlager, W.; Holzgrabe, U. Tetrahedron Lett. 2005, 46, 1247. (b) Birch, A. J. Pure Appl. Chem. 1996, 68, 553. (4) (a) Kohmoto, S.; Masu, H.; Tatsuno, C.; Kishikawa, K.; Yamamoto, M.; Yamaguchi, K. J. Chem. Soc., Perkin Trans. 1 2000, 4464. (b) Yokoyama, A.; Mizuno, K. Org. Lett. 2000, 2, 3457. (5) Delafuente, D. A.; Myers, W. H.; Sabat, M.; Harman, W. D. Organometallics 2005, 24, 1876. (6) (a) Zhou, L.; Wu, L. Z.; Zhang, L. P.; Tung, C. H. Organometallics 2006, 25, 1707. (b) Clayden, J.; Tumbull, R.; Pinto, 1178


Organometallics

Article

(32) (a) de Meijere, A.; Meyer, F. E. Angew. Chem., Int. Ed. Engl. 1994, 33, 2379. (b) Crisp, G. T. Chem. Soc. Rev. 1998, 27, 427. (c) Beletskaya, I. P.; Cheprakov, A. V. Chem. Rev. 2000, 100, 3009. (33) (a) Christmann, U.; Vilar, R. Angew. Chem., Int. Ed. 2005, 44, 366. (b) Beeby, A.; Bettington, S.; Fairlamb, I. J. S.; Goeta, A. E.; Kapdi, A. R.; Thompson, A. L. New J. Chem. 2004, 28, 600. (c) Li, Z.; Fu, Y.; Zhang, S. L.; Guo, Q. X.; Liu, L. Chem. Asian J. 2010, 5, 1475. (34) Ariafard, A.; Lin., Z. J. Am. Chem. Soc. 2006, 128, 13010. (35) Mendez, M.; Cuerva, J. M.; Gomez-Bengoa, E.; Cardenas, D. J.; Echavarren, A. M. Chem. Eur. J. 2002, 8, 3620.

1179


Computational Study on the Palladium-Catalyzed Allenylative

Recommend Documents