Oxidative Dimerization of

Article pubs.acs.org/Organometallics

Computational Study on Cycloisomerization/Oxidative Dimerization of Aryl Propargyl Ethers Catalyzed by Gold Nanoclusters: Mechanism and Selectivity Dianyong Tang,* Zhongzhu Chen, Jin Zhang,* Ying Tang, and Zhigang Xu Research Institute for New Materials Technology, Chongqing Key Laboratory of Environmental Materials and Remediation Technologies, and Drug Discovery Center of Innovation, Chongqing University of Arts and Sciences, Chongqing 402160, China S Supporting Information *

ABSTRACT: A theoretical analysis of the cycloisomerization and oxidative dimerization of phenyl propargyl ether catalyzed by the Au38 cluster is performed by means of density functional theory. The role of the cationic gold species is also clarified. The substituent effect at the para site of phenyl is studied to explore the selectivity of cycloisomerization and oxidative dimerization. Phenyl propargyl ether preferred to adsorb at the T1 site of the Au38 surface with an adsorption energy of −10.61 kcal/mol. The 6-endo pathway to give 2Hchromene is the most feasible pathway, with an energy barrier of 20.50 kcal/mol in dichloroethane solvent. The energy barriers of the 5-exo and oxidative dimerization pathways in dichloroethane solvent are 25.81 and 30.14 kcal/mol, respectively. 2H-Chromene is the main product of the cycloisomerization of phenyl propargyl ether catalyzed by the gold cluster. The presence of the cationic gold species can increase the yield of dimeric 2H-chromene, which is in agreement with experiment results. The binding strength between the active sites and 2H-chromen-3-yl is crucial for oxidative dimerization. Substituents at the para site of phenyl have only a slight influence on the 6-endo pathway, except for the methoxyl group. The differences in the energy barriers between cycloisomerization and oxidative dimerization are in agreement with the ratio of 2Hchromenes and 2H,2′H-4,4′-bichromenes obtained in experiments. The selectivity for the 6-endo/dimeric pathways is sensitive to the substituents of the substrates and the electronic prosperities of the active site of the catalysts. Our theoretical results are in agreement with the product distribution and phenomena in experiments.

1. INTRODUCTION Intramolecular hydroarylation of alkynes catalyzed by transition metal complexes through the addition of an aryl C−H bond across a π-bond has attracted considerable attention because it is a valuable synthetic alternative to the Heck and crosscoupling processes for the synthesis of alkenyl arenes.1−3 Since Pd(II)-catalyzed intramolecular hydroarylation of alkynes (Scheme 1) was first reported by Fujiwara and co-workers,4,5 a wide range of transition metal-, Lewis acid- (e.g., GaCl3, InCl3), and Brønsted acid-catalyzed intramolecular hydroarylation of alkyne reactions have been reported. The intramolecular hydroarylation of alkynes catalyzed by transition metal complexes such as Ru(II),6 Pt(II),6,7 Hg(II),8 and PtCl49 to give dihydronaphthalene and naphthalene derivatives (Scheme 1), PtCl2-catalyzed reaction producing phenanthrenes (Scheme 1),10 Pd(II)-,4,5,11,12 PtCl4-,13 and Fe(III)-catalyzed14 cyclization of arene-alkynes to 2H-chromene and dihydroquinoline derivatives (Scheme 1), and Pd(II)-mediated fluorene synthesis via the palladium-catalyzed 5-exo-dig annulation of oalkynylbiaryls (Scheme 1)15−17 have been intensively studied. Lewis acids, such as GaCl3 and InCl3, and the Brønsted acid HNTf2 have been used to catalyze the cyclization of arenealkynes (Scheme 1).18−21 Gold(I) complexes are active for the © XXXX American Chemical Society

intramolecular hydroarylation of terminal alkynes to synthesize coumarins, benzofurans, and dihydroquinolines (Scheme 2).22,23 Au(III) catalyzes the domino cyclization and oxidative coupling of phenyl propiolate (Scheme 2).24,25 Recently, the heterogeneous gold-based catalyst Au/TiO2 has been found to activate C−C triple bonds and effectively catalyze the selective cycloisomerization of aryl propargyl ethers to the corresponding 2H-chromenes by Stratakis et al. 2H,2′H-3,3′-Bichromenes resulting from a catalytic oxidative dimerization pathway are also formed as byproducts (Scheme 2).26 A theoretical study of the cyclization mechanism of arylalkynes showed the kinetic and thermodynamic preference for 6-endo-dig cyclization in Pt(II)-catalyzed processes.27 Although Friedel−Crafts and cyclopropanation processes via metal cyclopropyl carbenes show very similar energy barriers, platinum cyclopropyl carbenes are the most stable stationary point.27 A density functional theory (DFT) study by Soriano and Marco-Contelles indicated that the mechanism of intramolecular hydroarylation of aryl alkynes catalyzed by metal complexes may proceed via an endo and/or exo-dig cyclization Received: September 17, 2014

A

dx.doi.org/10.1021/om5009499 | Organometallics XXXX, XXX, XXX−XXX

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spectroscopy (XPS),32 can be titrated by Fourier transforminfrared spectroscopy (FT-IR) using CO as a probe,33 and are likely to play a key role in the supported gold nanoparticlecatalyzed processes. To our knowledge, the detailed reaction mechanism of cycloisomerization/oxidative dimerization of aryl propargyl ethers catalyzed by gold clusters has not been determined, especially the selectivity for the 2H-chromenes, benzofuran, and dimeric 2H-chromene products. Therefore, it is difficult to evaluate the potential effects of neutral and cationic gold species, which hinders the theoretical design of new effective catalysts to obtain a single selective product. In this study, a comprehensive computational study was carried out to explore the mechanism and chemoselectivity of cycloisomerization/ oxidative dimerization of aryl propargyl ethers catalyzed by nanosized Au clusters. First, the full catalytic cycle of cycloisomerization/oxidative dimerization of aryl propargyl ethers catalyzed by Au38 clusters was investigated. Then, the critical role of the cationic gold species was clarified, and the control factor for selectivity is proposed. Finally, fluoro, chloro, bromo, methyl, and methoxyl substituents at the para position of the aryl segment were investigated. It should be noted that our efforts focused on the free gold clusters in the gas phase and DCE solvent. Even with the same structures, the catalytic activities were likely different from the gas phase when supported on carriers (such as TiO2). At the same time, the gold−carrier interface may act as the active site to promote the reaction. The support and interface effects are the subject of ongoing work.

Scheme 1. Intramolecular Hydroarylation of Alkynes

Scheme 2. Gold-Catalyzed Intramolecular Hydroarylation of Alkynes

2. COMPUTATIONAL DETAILS The full possible pathway for cycloisomerization/oxidative dimerization of phenyl propargyl ether to 2H-chromene, benzofuran, and 2H,2′H-4,4′-bichromene was investigated using 38-atom Au clusters as a catalyst (Figure 1). The 38-atom Au clusters were chosen because

pathway, which depends on the catalyst and on the reactant structure.28 They also concluded that endo/exo selectivity greatly depends on the precursor substituents, where structural and electronic factors play a critical role. Recently, it was reported that Au/TiO2 could catalyze the selective isomerization of epoxides into allylic alcohols.29 This transformation process requires the cooperation of an acidic and a basic catalytic site. Apparently, the Lewis acidic sites are provided by cationic Au species (Au(I) and/or Au(III)) stabilized by titanium dioxide. Simultaneously, gold nanoparticles supported on nanocrystalline CeO2 are active and highly selective for the Sonogashira cross-coupling reaction. From DFT calculations and kinetic measurements, the selectivity of the Sonogashira cross-coupling reaction is governed by the Au0/Auδ+ ratio of Au/CeO2.30,31 The cationic gold species have been characterized by X-ray photoelectron

Figure 1. Models of the catalysts and the reaction pathway. they have been shown to be the size threshold in terms of adsorption, electronic structure, and energy barriers for C−C coupling, epoxidation, and CO oxidation catalysis.30,31,34−36 Au38O, Au38O2, and Au38O16 model clusters were chosen to mimic cationic gold species to clarify the catalytic role of cationic gold species.30,31,36 The 1996 gradient-corrected correlation functional of Perdew, Burke, and Ernzerhof with a DFT-based semicore pseudopotential and the double numerical plus polarization basis was used to obtain all of the results presented in the present paper.37,38 A finite-temperature Fermi function smearing technique with a 0.05 eV width was used to increase convergence, but all energies presented below were extrapolated to zero smearing (0 K). The transition states were B


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Scheme 3. Possible Pathways of Cycloisomerization and Oxidative Dimerization of Phenyl Propargyl Ether

calculated using the linear and quadratic synchronous transit method and confirmed by nudged elastic band calculations and relaxed potential energy surface scans.39,40 The solvent effect of dichloroethane (DCE, dielectric constant: 10.125) was evaluated by the conductor-like screening model (COSMO) solvation procedure.41−43 All calculations were performed using the DMol3 software package.38,44 The overall quality for the DMol3 calculation was set to fine. For the Au38 system, all of the atoms in the system were fully relaxed. In the case of the partially oxidized gold clusters (Au38O, Au38O2, and Au38O16), only the positions of the substrates were allowed to relax during structural optimization to avoid undesired deformation of the cluster. The efficiency of a catalytic cycle can be evaluated by the turnover frequency (TOF). The energetic span model proposed by Shaik and co-workers can predict the TOF from theoretically obtained free energy profiles.45−48 According to this model, the TOF-determined transition state (TDTS) and intermediate (TDI) can be determined by evaluating the degree of TOF control (XTOF,i), shown in eq 1:

X TOF,i =

1 ∂TOF TOF ∂E i

3. RESULTS AND DISCUSSION 3.1. Cycloisomerization of Phenyl Propargyl Ether Catalyzed by the Au38 Cluster. First, we discuss the cycloisomerization of phenyl propargyl ether to 2H-chromene and benzofuran catalyzed by the Au38 cluster. According to the product distribution and previous theoretical studies of the mechanism of cycloisomerization of aryl propargyl ethers catalyzed by PtCl2 and AuCl,27,28 the possible reaction routes were determined and are shown in Scheme 3. The initial step of the cycloisomerization process is the adsorption of phenyl propargyl ether on the surface of the Au38 cluster. All observed products in the experiments are formed through alkyne activation; hence, only the adsorption of the alkyne group to the Au38 clusters was investigated. The successive steps of cycloisomerization initializing from the adsorption of phenyl propargyl ether on the bridge sites of the Au38 cluster are unfeasible because of the strong activation of the C−C triple bond on the bridge sites (details are supplied in the Supporting Information). Hence, only the adsorption of phenyl propargyl ether on the top sites was investigated. As shown in Figure 1, there are only two different adsorption sites on the surface of Au38 clusters: T1 and T2. The calculated adsorption energies of phenyl propargyl ether at the T1 and T2 sites are −11.33 and −4.73 kcal/mol, respectively. Thus, adsorption at the T1 site is more stable than that at the T2 site. Therefore, all of the possible reaction routes originate from adsorption of the substrate at the T1 site. The Mayer bond orders of the Au−C1 and Au−C2 bonds in IM1 are 0.50 and 0.45, respectively. The C1−C2 bond order is 2.07 in IM1, which is less than that of free phenyl propargyl ether (2.64). Hence, the adsorption process results in the activation of the C−C triple bond. To illustrate the bonding interaction between the substrate (phenyl propargyl ether) and the catalyst (Au38), the contours in the C1−C2−Au plane of the Laplacian of electron density [∇2ρ(r)],49,50 the electron localization function

(1)

where Ei is the energy of a transition state or intermediate. The larger the XTOF value, the higher the influence of the corresponding state (transition state or intermediate) on the TOF. The TOF can be calculated by eq 2: TOF =

kBT −δE / RT e h

(2)

where δE is the energetic span, which can be defined as eq 3:

⎧ E TDTS − E TDI if TDTS appears after TDI δE = ⎨ ⎩ E TDTS − E TDI + ΔEr if TDTS appears before TDI ⎪

⎪

(3) This model was used to calculate the TOFs for the energy profiles obtained for the different mechanisms. All of the kinetic information was obtained by applying the energetic span model using the AUTOF program.47 C


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(ELF),51 and the localized orbital locator (LOL)52 of IM1, and induced charge density [Δρ(r)] due to the adsorption of phenyl propargyl ether are shown in Figure 2. The ELF, LOL,

9.01 kcal/mol in the gas phase and DCE solvent, respectively. The C2 atom in IM2-s tilts toward the surface of the gold cluster, resulting in a Au−C2 bond order of 0.86, which indicates formation of a single covalent bond between C2 and Au. The C1−C2 bond is predicted to be a double bond (C1− C2 bond order of 1.73). Subsequently, one hydrogen atom transfers from the C3 atom to the C1 atom to form the carbene−gold complex IM3-s through TS2/3-s. The bond lengths (Figure 4) and Mayer bond orders of the C1−H and C3−H bonds in TS2/3-s (0.36 and 0.52 for the C1−H and C3−H bonds, respectively) indicate that TS2/3-s is an early transition state. This step is exothermic by about 11.82 (8.56) kcal/mol with an energy barrier of 22.28 (17.50) kcal/mol in the gas phase (DCE solvent). The Au−C2 bond order increases from about 0.86 in IM2-s to about 0.95 in TS2/3-s, and finally to about 1.14 in IM3-s. The high bond order of Au− C2 indicates the increased double-bond character between C2 and Au.58 The C1−C2 bond changes from a double bond in IM2-s to a single bond in IM3-s (C1−C2 bond order of 0.97) because of electron transfer. Next, the hydrogen atom attached to the C1 atom transfers to the C2 atom to form the product− catalyst complex IM4-s via TS3/4-s. The variations of the related bond lengths indicate that TS3/4-s is also an early transition state (Figure 4). This step is predicted to be greatly exothermic by about 30.55 and 31.81 kcal/mol in the gas phase and DCE solvent, respectively. The activation energies are 14.65 and 6.94 kcal/mol in the gas phase and DCE, respectively. Finally, the product 2H-chromene desorbs with regeneration of the Au38 cluster, which is endothermic by about 11.22 and 12.56 kcal/mol in the gas phase and DCE solvent, respectively. The complete pathway is exothermic by 44.76 and 43.07 kcal/mol in the gas phase and DCE solvent, respectively. The rate-limiting step of the whole pathway is the first hydrogen transfer step, i.e., IM2-s → TS2/3-s → IM3-s, with energy barriers of 22.28 and 17.50 kcal/mol in the gas phase and DCE solvent, respectively. It should be noted that the coordination of solvent molecules to the surface of the gold cluster may decrease energy barriers of the hydrogen transfer step. This effect of solvent could not be seen with the continuum solvent model. However, such calculations are beyond the scope of this work. It should be noted that IM1 could isomerize to a vinylidene− gold intermediate and form the final product 2H-chromene (Scheme 3). The optimized structures and related parameters of IM2-ene and TS1/2-ene are shown in Figure S1. The activation energy for the formation of the vinylidene−gold intermediate is 31.37 (26.53) kcal/mol with an endothermicity of 9.18 (9.50) kcal/mol in the gas phase (DCE solvent), which indicates that the formation of the vinylidene−gold intermediate IM2-ene is unfeasible both kinetically and thermodynamically. Therefore, the subsequent steps to give 2Hchromene were not investigated. The energy profile, optimized structures, and related parameters for the 5-exo cyclization pathway are shown in Figures 3 and 5. The C2 atom could attack the phenyl group in IM1 (5-exo cyclization) to form the five-membered cyclic intermediate IM2-f through TS1/2-f. The intermediate IM2-f is comparable with IM1 in energy in both the gas phase and DCE solvent. The calculated activation energy is 13.02 (12.88) kcal/mol in the gas phase (DCE solvent). The variations of related bonds are similar to those for IM1 → IM2-s. The hydrogen atom attached to the C3 atom then transfers to the C2 atom to give IM3-f. This step is exothermic by 11.65 (8.93)

Figure 2. Contours in the C1−C2−Au plane of the Laplacian of the electron density [∇2ρ(r)], the electron localization function (ELF), and the localized orbital locator (LOL) of IM1. The contours of the substrate in the C1−C2−H plane of ∇2ρ(r), ELF, and LOL are shown in a red frame in the top right corner. The induced charge density [Δρ(r)] for the adsorption of phenyl propargyl ether. The isosurface of Δρ(r) is shown in the bottom left corner (cutoff = 0.02). The red areas indicate charge accumulation, while the blue areas indicate charge depletion.

and ∇2ρ(r) calculations were performed with the Multiwfn code.53,54 The wave functions for the ELF, LOL, and ∇2ρ(r) calculations were obtained at the TPSSh/def2-SVP level of theory with the G09 program.55−57 The contours of the substrate in the C1−C2−H plane of ∇2ρ(r), the ELF, and the LOL are shown in a red frame at the top right corner of Figure 2 for reference. The isosurface of Δρ(r) is shown in the bottom left corner. The ∇2ρ(r) of Au and the C1C2 bonding region is positive, indicating a close shell interaction, i.e., a coordination bond. The coordination bonding interaction between Au and C1C2 is more clearly shown in the ELF and LOL. The induced charge density of substrate adsorption on the Au38 surface indicates charge redistribution, as shown in Figure 2. This figure shows charge transfer from the π orbital of C1C2 to the d orbital of gold and back-donation from the d orbital of gold to the π* orbital of the C1C2 bond. The charge accumulation is not strongly localized. However, the shared charge can be an indication of a coordination bond between the C1C2 bond and gold. The first pathway is 6-endo cyclization, that is, the attack of the terminal carbon atom (C1, Figure 3) of the phenyl group to form the six-membered cycle intermediate IM2-s. Because of the breakdown of the aromaticity of phenyl, the variation of the substrate fragment should result in a large energy barrier and endothermicity. However, the strong binding between the substrate and the Au38 fragments in TS1/2-s and IM2-s lowers the energy barrier and makes this step slightly exothermic. As expected, this step is predicted to be slightly exothermic by about 2.28 and 4.64 kcal/mol with energy barriers of 10.55 and D


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Figure 3. Energy profiles of the 6-endo and 5-exo cyclization pathways.

from the C2 atom to the C1 atom via TS3/4-f to produce the benzofuran−catalyst complex IM4-f with an energy barrier of 9.19 (6.65) kcal/mol in the gas phase (DCE solvent). Finally, benzofuran detaches from the Au38 cluster with an endothermic energy of 13.37 (19.18) kcal/mol in the gas phase (DCE solvent). The complete pathway is exothermic by 43.92 (37.90) kcal/mol in the gas phase (DCE solvent). The recently designed energetic span model by Shaik and coworkers45−48 has been used to determine the TOFs for competitive reaction routines. This is a more reliable way of comparing the different mechanisms because it takes into account not only the principal rate-determining transition state but also the other potentially rate-influencing transition states and intermediates during the catalysis process.45 The TDI and TDTS of the 6-endo pathway are IM2-s and TS2/3-s, respectively, with an energetic span (δE) of 20.50 kcal/mol in DCE solvent, while the TDI and TDTS of the 5-exo pathway are IM4-f and TS2/3-f, respectively, with a δE of 25.81 kcal/ mol in DCE solvent (Table 1). The relative TOF of 6-endo/5exo is 2385, which indicates that the formation of 2H-chromene is kinetically dominant. The energy profile in Figure 3 shows that the 6-endo pathway is more thermodynamically favorable than the 5-exo pathway. Therefore, the main products of the cycloisomerization of phenyl propargyl ethers are 2H-chromenes, which is in agreement with the product distributions obtained in experiments (2H-chromenes, 83−100%; benzofurans, 0−5%).26 3.2. Oxidative Dimerization of Phenyl Propargyl Ether Catalyzed by the Au38 Cluster. In this section, the formation

Figure 4. Optimized structures and related parameters of the 6-endo pathway.

kcal/mol with an activation energy of 19.05 (17.88) kcal/mol in the gas phase (DCE solvent). The hydrogen then migrates E


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steps are predicted to be slightly endothermic (1−3 kcal/mol), with activation energies of 15.58 (13.53) and 13.46 (11.00) kcal/mol in the gas phase (DCE solvent) for IM6-fs → IM7-fs and IM6-ss → IM7-ss, respectively. The bonding variations for the cyclization of phenyl propargyl ether in IM6 are similar to those for IM1 → IM2-s. Next, the hydrogen atom attached to the C3′ atom in IM7-fs and IM7-ss detaches from the C3′ atom to form IM8-fs and IM8-ss with exothermic energies of 3.56 (59.58) and 2.74 (58.60) kcal/mol in the gas phase (DCE solvent), respectively. Finally, the coupling of the two 2Hchromen-3-yl fragments in IM8-fs and IM8-ss produces dimeric 2H-chromene and Au382− via TS8-fs and TS8-ss, respectively. The oxidation of Au382− by air recovers the catalyst. The calculated activation energies are 31.82 (30.14) and 34.14 (32.48) kcal/mol in the gas phase (DCE solvent) for TS8-fs and TS8-ss, respectively. The Au−C2 and Au−C2′ bonds lengthen from about 2.06 Å in IM8 to about 2.15 Å in TS8, and the lengths of C2−C2′ bonds formed in TS8-fs and TS8-ss are 2.27 and 2.18 Å, respectively. The bond distances of the transition states are reasonable for the formation of the C2−C2′ bond and the cleavage of the Au−C2 and Au−C2′ bonds. The reaction channel for oxidative dimerization at the four−six bridge sites is slightly less favorable than that of the six−six bridge sites both kinetically and thermodynamically. The rate-limiting step for oxidative dimerization is the coupling of two 2H-chromen-3-yl fragments. The complete reaction is highly exothermic by about 144 kcal/mol. The predicted TOF is 4.47 × 10−7 s−1 at 343 K in the DCE solvent, indicating that the formation of dimeric 2H-chromene is unfavorable. Comparing the cycloisomerization and oxidative dimerization pathways, the cycloisomerization of phenyl propargyl ether to give 2H-chromene is the most feasible pathway, with an energy barrier of 20.50 kcal/mol in DCE solvent. Hence, 2Hchromene is the main product of cycloisomerization of phenyl propargyl ether catalyzed by gold clusters, which is in agreement with the product distribution from experiments.26 3.3. Catalytic Activity of Cationic Gold Species. Only the pathway for the formation of 2H-chromene at the lowcoordinated Auδ+ site of Au38O was investigated. The coupling of two 2H-chromen-3-yl fragments at the various sites on Au38O2 and Au38O16 was investigated to clarify the influence on the selectivity of the product because this step is the ratelimiting step. The energy profiles, optimized structures, and related parameters of the Au38O system are shown in Figures S4−S6. The adsorption energy of the substrate at the Auδ+ site of Au38O is −4.38 (−2.35) kcal/mol in the gas phase (DCE solvent), which is significantly lower than that of Au38 because of the absence of unoccupied d orbitals. The activation energy of the cyclization step is 9.25 (8.82) kcal/mol in the gas phase (DCE solvent), which is comparable with that of Au38. The two hydrogen transfer steps are exothermic by about 20 kcal/mol, with energy barriers of 22.47 (20.38) (IM2−O → IM3−O) and 27.34 (24.19) (IM3−O → IM4−O) kcal/mol in the gas phase (DCE solvent). The high energy barrier for IM3−O → IM4−O can be attributed to the stable μ2-carbene coordination in IM3−O. The rate-limiting step for the cyclization of phenyl propargyl ether to 2H-chromene is the hydrogen transfer from the C1 atom to the C2 atom to produce the adsorption state of the product (IM4−O) with an energy barrier of 27.34 (24.19) in the gas phase (DCE solvent). The formation of the adsorption state of 2H-chromen-3-yl (IM5−O) from IM2−O is −47.69 (−64.69) kcal/mol in the gas phase (DCE solvent).

Figure 5. Optimized structures and related parameters of the 5-exo pathway.

Table 1. TDI, TDTS, and TOF Values for the 6-endo, 5-exo, and Dimeric Pathwaysa

a

path

TDI

TDTS

TOF (s−1)

6-endo 5-exo dimeric

IM2-s IM4-f IM8-fs

TS2/3-s TS2/3-f TS8-fs

6.2 × 10−1 2.6 × 10−4 4.47 × 10−7

T = 343 K.

pathways of dimeric 2H-chromenes are presented. One proton detaches from IM2-s to produce IM5 with an exothermic energy of 48.96 (65.44) kcal/mol in the gas phase (DCE solvent). Then, the second phenyl propargyl ether molecule adsorbs to the neighboring low-coordinated Au atoms of the Au38 cluster of IM5 to cyclize the second propargyl ether, and the diorganogold intermediate undergoes intramolecular C−C coupling to form the dimeric products with the regeneration of the catalyst. There are two pathways to produce the final product. The first pathway is the adsorption of two phenyl propargyl ether molecules at the four−six bridge site (denoted as fs), and the other pathway is the adsorption of two phenyl propargyl ether molecules at the six−six bridge site (denoted as ss). The energy profiles are shown in Figure 6, and the optimized structures and related parameters are supplied in Figures S2 and S3 (Supporting Information). The adsorption of the second phenyl propargyl ether molecule on IM5 forms IM6-fs and IM6-ss, which have exothermic energies of 12.94 (13.18) and 12.08 (10.13) kcal/ mol in the gas phase (DCE solvent), respectively. The adsorption energies are comparable with the adsorption energies of the first phenyl propargyl ether molecule. Then, cyclization of phenyl propargyl ether in IM6 gives IM7. These F


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Figure 6. Energy profiles of the oxidative dimerization pathways.

2H-chromene (Figure 1). The optimized structures and related parameters are shown in Figures S7 and S8. The Au−C bond lengths are about 2.08−2.10 Å in the adsorption states of the two 2H-chromen-3-yl fragments, indicating similar Au−C bond strengths. The energy barriers for the coupling of two 2Hchromen-3-yl fragments at the Au+−Auδ+ and Auδ+−Auδ+ sites are 40.56 (38.26) and 41.97 (40.34) kcal/mol in the gas phase (DCE solvent), respectively. The high energy barriers are caused by the weak interactions between two 2H-chromen-3-yl fragments in the transition states. In contrast, the coupling occurring at Au+−Au0 only needs to overcome a relatively low energy barrier of 20.82 (20.29) kcal/mol in the gas phase (DCE solvent). Therefore, the coupling of two 2H-chromen-3-yl fragments on the Au38O2 cluster is kinetically favorable. The coupling of two 2H-chromen-3-yl fragments at the Au+−Au+ and Au+−Auδ+ sites of Au38O16 has to overcome energy barriers of 20.94 (17.82) and 23.44 (20.31) kcal/mol in the gas phase (DCE solvent), respectively. Thus, the Au+−Au+ and Au+−Au0 sites showed the highest catalytic activity for the coupling of two 2H-chromen-3-yl fragments, and the cationic gold species present in Au/TiO2 have good catalytic activity for the oxidative dimerization of 2H-chromen-3-yl to dimeric 2Hchromene, which is in agreement with experimental observations.26 The amount of dimeric 2H-chromene drastically decreased to 3−4% by excluding air from the reaction medium in experiments. In addition, the relative amount of dimeric 2Hchromene increased to 25% by occasionally blowing air into the reaction mixture.26 To understand the intrinsic role of the cationic gold species in the coupling of two 2H-chromen-3-yl fragments, energy decomposition analysis (EDA)59−61 of the energy barriers for the coupling step was performed. The energy barriers were decomposed into the deformation energy of catalyst fragments (ΔEd(Ca)), the deformation energies of two 2H-chromen-3-yl

To investigate the effect of cationic gold species on the coupling of two 2H-chromen-3-yl fragments, the adsorption and cyclization of the second phenyl propargyl ether at the Auδ+ and Au0 sites were considered. The adsorption energies of phenyl propargyl ether at the Auδ+ and Au0 sites are −4.80 (−1.89) and −9.86 (−7.24) kcal/mol in the gas phase (DCE solvent), respectively. Clearly, the adsorption of phenyl propargyl ether at the Auδ+ site is weaker than at the Au0 site. The energy barriers for the cyclization step at the Auδ+ and Au0 sites are 12.33 (10.38) and 12.83 (11.33) kcal/mol in the gas phase (DCE solvent), respectively. Hence, the charge of the reaction site has a slight influence on the cyclization step of phenyl propargyl ether. The coupling of two 2H-chromen-3-yl fragments at two Auδ+ sites needs to overcome a 34.88 (33.99) kcal/mol energy barrier in the gas phase (DCE solvent). The high energy barrier is related to the weak interaction between 2H-chromen-3-yl and Au38O fragments in the transition state. The coupling of two 2H-chromen-3-yl fragments at the Auδ+ and Au0 sites has a low activation energy of 27.42 (26.24) kcal/ mol in the gas phase (DCE solvent), indicating a stable transition state with a strong interaction between two 2Hchromen-3-yl fragments. Therefore, the coupling of two 2Hchromen-3-yl fragments at the Auδ+ and Au0 sites is easier than at two Auδ+ sites. The cycloisomerization of phenyl propargyl ether to 2H-chromene is comparable with oxidative dimerization to dimeric 2H-chromene in the gas phase, while the formation of 2H-chromene is more favorable than the formation of dimeric 2H-chromene in DCE solvent. In summary, the presence of cationic gold species in the reaction system is favorable for producing dimeric 2H-chromene. Furthermore, the coupling of two 2H-chromen-3-yl fragments at Au+−Auδ+, Au+−Au0, and Auδ+−Auδ+ of Au38O2 and at Au+−Au+ and Au+−Auδ+ of Au38O16 was investigated to clarify the effect of the cationic gold species on forming dimeric G


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Table 2. Energy Barriers (ΔrEa) and Their Energy Components for the Coupling of Two 2H-Chromen-3-yl Fragments (units: kcal/mol) catalyst

site

ΔEd(Ca)

ΔEd(2Ch)

ΔEB(2Ch)

ΔEB(Ca−2Ch)

ΔrEa

Au38 Au38O Au38O Au38O2 Au38O2 Au38O2 Au38O16 Au38O16

Au0−Au0 Auδ+−Auδ+ Auδ+−Au0 Au+−Auδ+ Au+−Au0 Auδ+−Auδ+ Au+−Au+ Au+−Auδ+

−6.32 0.00 0.00 0.00 0.00 0.00 0.00 0.00

3.25 9.97 4.80 5.13 9.40 0.66 9.97 1.11

−30.53 −24.57 −30.81 −18.35 −18.74 −23.50 −18.54 −23.63

68.57 59.47 58.23 58.91 39.56 65.47 42.03 49.41

31.72 34.90 27.42 40.56 20.82 41.97 23.49 25.77

fragments (ΔEd(2Ch)), the change of the interaction energies between two 2H-chromen-3-yl fragments (ΔEB(2Ch)), and the change of the interaction energy between the catalyst and two 2H-chromen-3-yl fragments (ΔEB(Ca−2Ch)).The details of the EDA are given in the Supporting Information. The results of the EDA are shown in Table 2. The binding strengths between the various gold sites and the 2H-chromen-3-yl fragment of the intermediates are as follows: Au0 > Auδ+ > Au+. The deformation of the Au38 fragment is slightly favorable for the coupling process. The deformation energies of the catalyst fragments for the Au38O, Au38O2, and Au38O16 systems are zero because of the fixed structures of the catalyst fragments during the calculations. As shown in Table 2, the largest portion of the energy barriers comes from the weakening of the two Au−C bonds. The high energy barriers for the Au38 and A38O system can be attributed to the strong interaction between Au atoms (Au0 and Auδ+) and 2H-chromen-3-yl fragments in the intermediates. The strongest C−C bond formed in the transition states accounts for only half of the weakening Au− C bond. The lower energy barriers are controlled by the small weakening Au−C bond from the intermediate to the transition state and the small interaction between two 2H-chromen-3-yl fragments in the transition state. The formation of a new C−C bond can effectively decrease the energy barriers. The distortion energies of the two 2H-chromen-3-yl fragments are 0.66−9.97 kcal/mol. Hence, the distortion of the two 2Hchromen-3-yl fragments do not favor the coupling step. Therefore, the binding strength between the active site and the 2H-chromen-3-yl fragment is the key factor for the formation of the dimeric product. 3.4. Substituent Effect. To verify the selectivity of cycloisomerization/oxidative dimerization, the effect of various substituents at the para position of the phenyl group was investigated. The substituents considered were F (Sub-F), Cl (Sub-Cl), Br (Sub-Br), methyl (Sub-Me), and methoxyl (SubOMe). The structures of these substrates are shown in Scheme 4. The energy profiles, optimized structures, and related parameters of cycloisomerization and oxidative dimerization of these para-substituted substrates catalyzed by the Au38 cluster are shown in Figures S9−S16. The activation energies of the rate-limiting steps are shown in Table 3. In Table 3, the activation energies of the rate-limiting step for cycloisomerization and oxidative dimerization pathways are denoted as ΔrEa,ci and ΔrEa,od, respectively. The structural parameters of the intermediates and transition states involved in the cycloisomerization and oxidative dimerization of p- fluoro/chloro/bromo/methyl/methoxylphenyl propargyl ether are similar to those of phenyl propargyl ether. The rate-determining steps for cycloisomerization and oxidative dimerization of these substrates are the first hydrogen

Scheme 4. Substrates Used to Investigate Substituent Effects

Table 3. Energy Barriers of the Rate-Limiting Steps for the 6-endo Cyclization (ΔrEa,ci) and Oxidative Dimerization (ΔrEa,od) Pathways and the Ratio of 2H-Chromene and Dimeric 2H-Chromene (ci:od) (units: kcal/mol) substrate

ΔrEa,ci

ΔrEa,od

ci:oda

Sub Sub-F Sub-Cl Sub-Br Sub-Me Sub-OMe

22.29(20.50) 23.00(20.94) 22.67(20.55) 22.57(20.57) 22.68(21.01) 18.66(17.10)

31.82(30.14) 31.67(30.19) 27.47(26.41) 37.32(36.00) 36.25(34.69) 35.97(34.68)

83:17 88:12 94:6 82:18 93:7

a

The ratio of 2H-chromene and dimeric 2H-chromene was taken from ref 26.

transfer step (IM2 → IM3) and C−C coupling step (IM8 → Prd + Au382−), respectively. The activation energies for cycloisomerization of p-fluoro/chloro/bromo/methylphenyl propargyl ether are about 20.50−21.00 kcal/mol in DCE solvent, which is close to that of phenyl propargyl ether. The cycloisomerization of Sub-OMe was more favorable than that of phenyl propargyl ether, Sub-F, Sub-Cl, Sub-Br, and Sub-Me because of a low energy barrier of 17.10 kcal/mol in DCE solvent. The energy barrier of the C−C coupling step for Sub-F is comparable with that of phenyl propargyl ether. The differences in the energy barriers between cycloisomerization and oxidative dimerization are in agreement with the ratio of 2H-chromenes and 2H,2′H-4, 4′-bichromenes (Table 3). On the basis of our calculations, it is predicted that the pchlorophenyl propargyl ether substrate will produce more dimeric 2H-chromene than the other substrates. Thus, the 6endo/dimeric selectivity is sensitive to the para substituents of the substrates. H


Organometallics

4. CONCLUSIONS In summary, a detailed theoretical analysis of the cycloisomerization and oxidative dimerization of phenyl propargyl ether catalyzed by a Au38 cluster was performed. The reaction pathways to give 2H-chromenes, benzofurans, and bichromenes were investigated in detail. The phenyl propargyl ether prefers to adsorb at the T1 site of the Au38 surface with an adsorption energy of −10.61 kcal/mol. The Laplacian of electron density, LOL, ELF, and induced charge density show charge transfer from the π orbital of C1C2 to the d orbital of gold and backdonation from the d orbital of gold to the π* orbital of C1 C2. There are three possible reaction pathways to produce three different products. The 6-endo pathway to give 2Hchromene is the most favorable pathway, with an energy barrier of 20.50 kcal/mol in DCE solvent. Hence, 2H-chromene is the main product of cycloisomerization of phenyl propargyl ether catalyzed by the gold cluster. The energy barriers for the 5-exo and oxidative dimerization pathways are 25.81 and 30.14 kcal/ mol in DCE solvent, respectively. Because the reaction occurs at 343 K, benzofuran may be observed in experiments as a side product. The energy barriers for the formation of dimeric 2Hchromene decrease to about 20 kcal/mol with cationic gold species as catalysts. The presence of the cationic gold species could increase the yield of dimeric 2H-chromene, which is in agreement with experiment results. An energy decomposition analysis indicated that the binding strength between the active sites and the 2H-chromen-3-yl fragment was crucial for oxidative dimerization. The substituent effect at the para site of phenyl was investigated to clarify the selectivity of cycloisomerization and oxidative dimerization. Substituents at the para site of phenyl had only a slight influence on the 6-endo pathway, except for the methoxyl substituent. The methoxyl substituent lowered the energy barrier of the 6-endo pathway and enhanced the activation energy of the oxidative dimerization pathway. The differences in the energy barriers between cycloisomerization and oxidative dimerization are in agreement with the ratio of 2H-chromenes and 2H,2′H-4,4′bichromenes in experiments. The 6-endo/dimeric selectivity was sensitive to the substituent of the substrates and the electronic prosperities of the active site of the catalyst. Our theoretical results are in agreement with the product distribution obtained from experiments.

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ACKNOWLEDGMENTS

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REFERENCES

This work was supported by the Program for Innovation Team Building at the Institution of Higher Education in Chongqing (Grant No. 201042) and the Chongqing Science & Technology Commission, China (Grant No. CSTC2013JCYJA50028). The calculations were performed at the National Supercomputing Center in Shenzhen (Shenzhen Cloud Computing Center).

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ASSOCIATED CONTENT

S Supporting Information *

Additional computational results, details for energy decomposition analysis, EDA data, and the complete reference for Gaussian 09. A text file of all computed molecule Cartesian coordinates in .xyz format for convenient visualization. This material is available free of charge via the Internet at http:// pubs.acs.org.

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AUTHOR INFORMATION

Corresponding Authors

*Tel: +86-23-61162836. E-mail: [email protected] (D.-Y. Tang). *Tel: +86-23-49891903. E-mail: [email protected] (J. Zhang). Notes

The authors declare no competing financial interest. I


Organometallics

Article

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Oxidative Dimerization of

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