Theoretical Investigation on the Isomerization Reaction of 4-Phenyl

J. Phys. Chem. A 2010, 114, 12893–12899

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Theoretical Investigation on the Isomerization Reaction of 4-Phenyl-hexa-1,5-enyne Catalyzed by Homogeneous Au Catalysts Yuxia Liu,† Dongju Zhang,*,† and Siwei Bi‡ Key Lab of Colloid and Interface Chemistry, Ministry of Education, Institute of Theoretical Chemistry, Shandong UniVersity, Jinan, 250100, P. R. China, and College of Chemistry and Chemical Engineering, Qufu Normal UniVersity, Qufu, 273165, P. R. China ReceiVed: June 9, 2010; ReVised Manuscript ReceiVed: October 8, 2010

By carrying out density functional theory calculations, we have performed a detailed mechanism study for the cycloisomerization reaction of 4-phenyl-hexa-1,5-enyne catalyzed by homogeneous gold to better understand the observed different catalytic activity of several catalysts, including (PPh3)AuBF4, (PPh3)AuCl, AuCl3, and AuCl. In all situations, the reaction is found to involve two major steps: the initial nucleophilic addition of the alkynyl onto the alkene group and the subsequent 1,2-H migration. It is found that the potential energy surface profiles of systems are very different when different catalysts are used. For (PMe3)AuBF4- and (PMe3)AuCl-mediated systems, the nucleophilic addition is the rate-determining step, and the calculated free energy barriers are 15.2 and 41.9 kcal/mol, respectively. In contrast, for AuCl3- and AuCl-mediated systems, the reactions are controlled by the dissociations of catalysts from the product-like intermediates, and the calculated dissociation energies are 18.1 and 21.7 kcal/mol, respectively, which are larger than both the corresponding free energy barriers of the nucleophilic addition and the H-migration processes (8.5 and 7.3 kcal/mol for the AuCl3-mediated reaction, and 16.9 and 11.3 kcal/mol for the AuCl-mediated reaction). These results can rationalize the early experimental observations that the reactant conversion rates are 100, 0, and 50% when using (PPh3)AuBF4, (PPh3)AuCl, and AuCl3 as catalysts, respectively. The present study indicates that both the ligand and counterion of homogeneous Au catalysts importantly influence their catalytic activities, whereas the oxidation state of Au is not a crucial factor controlling the reactivity. 1. Introduction In the past decades, great progress has been made in developing efficient and environmentally benign methods to generate desired target compounds. Gold homogeneous catalysis, as the heart of new developments in green chemistry,1,2 has increasingly obtained widespread concerns. Of the most highlighted of these reactions is gold-catalyzed cycloisomerization of enynes, via the formations of C-H, C-C, and C-heteroatom bonds to construct a variety of carbocyclic and heterocyclic compounds.3 However, in contrast to the catalytic isomerizations of 1,6- and 1,7-enynes,4-9 those of 1,5-enynes have been much less investigated.10,11 Recently, Toste et al. reported the cycloisomerization reaction of 4-phenyl-hexa-1,5-enyne (A) to generate bicyclo[3.1.0]hexenes (B) with different transition metal catalysts, which occurred in dichloromethane solution at room temperature.12 As shown in Scheme 1, the following facts were observed by Toste et al.: (i) Pd(II), Pt(II), and Ag(I) salts are the poor catalysts (entries 1-4); (ii) a Au(I) salt, (PPh3)AuBF4, resulting from mixing AgBF4 and (PPh3)AuCl, possesses unique catalytic activity, giving the complete conversion in only 5 min (entry 5); (iii) when replacing the counterion BF4- in (PPh3)AuBF4 with Cl-, the observed conversion rate is zero (entry 6), that is, (PPh3)AuCl is not an active catalyst for the cycloisomerization reaction, which is in sharp contrast to the 100% conversion rate in the case of (PPh3)AuBF4; and (iv) a Au(III) salt, AuCl3, also * To whom correspondence should be addressed. E-mail: zhangdj@ sdu.edu.cn. Phone: +86-531-88365833. Fax: +86-531-88564464. † Shandong University. ‡ Qufu Normal University.

SCHEME 1: Cycloisomerizations of 4-Phenyl-hexa-1,5-enyne Reported by Toste Groups

promotes the reaction, but with only a conversion rate of 50% (entry 7). These findings indicate that only the gold salts with appropriately selected counterion, ligand, and oxidation state possess satisfactory catalytic activity toward the cycloisomerization shown in Scheme 1. Although these experimental observations are indicative of implementing the cycloisomerization reaction of 1,5-enynes, the intrinsic reasons behind them have not been full understood yet, and the fundamental mechanism involved in the reaction also remains unclear. Theoretical study at the molecular level can provide a direct way to understand these experimental findings and to further lead the researchers to select optimal catalysts for future new reactions. Herein, density functional theory calculations, which provide one of the most direct ways to understand a range of chemical properties and reactions,13-15 are carried out to

10.1021/jp105292s  2010 American Chemical Society Published on Web 11/16/2010

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Figure 1. PES profiles for (a) (PMe3)AuBF4-, (b) (PMe3)AuCl-, (c) AuCl3-, and (d) AuCl-catalyzed cycloisomerization of 4-phenyl-hexa1,5-enyne. The relative free energies are given in kcal/mol.

investigate the detailed mechanism of the catalytic isomerization of 4-phenyl substituted 1,5-enynes. On the basis of the calculated results, we hope to show the influences of the counterion, ligand, and oxidation state of homogeneous Au catalysts on the reactivity. 2. Models and Computational Details Previous studies16,17 indicated that the phenyls have little influence on the reactivity of the catalysts. Therefore, in the present theoretical calculations, the actual catalysts (PPh3)AuBF4

Liu et al. and (PPh3)AuCl are simplified as (PMe3)AuBF4 and (PMe3)AuCl, respectively, that is, the phenyls in the actual catalysts are modeled by methyls to save the computational cost. The calculations presented in this work were performed in the framework of density functional theory (DFT). We used the Becke three-parameter hybrid functional (B3LYP).18-21 For Au atoms, the effective core potentials (ECP) of Hay and Wadt were combined with double-ζ valence basis sets (LanL2DZ),22-24 whereas the standard 6-31G (d, p) basis set was used for C, H, B, F, and Cl atoms. Full geometry optimizations of minima and transition states were performed at the selected level of theory. Frequency calculations at the same level of theory have also been carried out to verify all stationary points as minima (zero imaginary frequencies) or first-order saddle points (one imaginary frequency) and to provide free energies at 298.15 K, which include entropic contributions involved in the vibrations, rotations, and translations of the structures under consideration. The intrinsic reaction coordinate (IRC)25,26 calculations were conducted in both directions (forward and reverse) from the transition states to the corresponding local minima to identify the minimum-energy paths. The solvent effect of dichloromethane was taken into account through a self-consistent reaction field (SCRF) method27,28 based on the polarizable continuum model (PCM).29,30 All the calculations were implemented with the Gaussian 03 software package.31 3. Results and Discussion To analyze the reasons that different homogeneous Au catalysts give rise to significantly distinct conversions (Scheme

Figure 2. Optimized geometries involved in the cycloisomerization reaction of 4-phenyl-hexa-1,5-enyne catalyzed by (PMe3)AuBF4. The hydrogen atoms not participating in the reaction have been omitted for clarity. Bond distances are given in Å.

Catalytic Isomerization of 4-pPhenyl-hexa-1,5-enyne

Figure 3. Optimized geometries involved in the cycloisomerization reaction of 4-phenyl-hexa-1,5-enyne catalyzed by (PMe3)AuCl. The hydrogen atoms not participating in the reaction have been omitted for clarity. Bond distances are given in Å.

1), we have investigated the cycloisomerization of 4-phenylhexa-1,5-enyne catalyzed by three Au(I) catalysts (PMe3)AuBF4, (PMe3)AuCl, and AuCl, and a Au(III) catalyst, AuCl3. The calculated relative free energy profiles for the four systems are collected in Figure 1, and the optimized geometries with selected structural parameters are given in Figures 2-4, respectively, where all hydrogen atoms not participating in the cycloisomerization are omitted for clarity. Note that the calculated relative energy differences between the PCM and gas phase calculations are small. For example, for the (PMe3)AuBF4-mediated reaction, which will be discussed below, the barrier of the rate-determining step is 15.1 kcal/mol in gas phase calculations, whereas it is 15.2 kcal/mol in PCM calculations. This fact indicates that the gas phase models used in the present work are suitable for describing the reactivity of the isomerization reaction. Given the small differences between the PCM and gas phase calculations, our discussion presented hereafter will exclusively focus on the gas phase results. Three possible coordination modes of 4-phenyl-hexa-1,5enyne (A) with Au-catalysts are schematically illustrated in Scheme 2. As reported in previous literatures,14,32 the coordination of Au(I) or (III) catalyst to the double bond of A failed to carry out the desired transformation, leading to a “dead path” (mode I in Scheme 2). On the other hand, due to being isolobal to H+ from stoichiometric chemistry,33-38 the Ph3PAu+ fragment is unable to coordinate with alkyne and alkene simultaneously to form Alder-ene cyclometalation (mode II in Scheme 2).39,40 Thus, the catalytic cycloisomerization reaction of 4-phenyl-hexa1,5-enyne is believed to start from Au-alkyne complexes (mode III in Scheme 2). Therefore, at the reaction entrance, catalysts (PMe3)AuBF4, (PMe3)AuCl, AuCl3, and AuCl coordinate with the C-C triple bond of substrate A, resulting in the generation of the intermediates IMa1, IMb1, IMc1, and IMd1, respectively. From Figure 1, we see that IMc1 and IMd1, the complexes of substrate A with AuCl3 and AuCl, which do not have a (PMe3) ligand, are much more stable than IMa1 and IMb1, the complexes

J. Phys. Chem. A, Vol. 114, No. 49, 2010 12895 of substrate A with (PMe3)AuBF4 and (PMe3)AuCl, which have a (PMe3) ligand. The high energies of IMa1 and IMb1 are mainly ascribed to the steric bulk of PMe3, which makes the anions BF4- and Cl- in (PMe3)AuBF4 and (PMe3)AuCl unable to keep away from the substrate, resulting in the stronger electrostatic repulsions between the anions and the π electrons of the substrate than those in IMc1 and IMd1, especially the latter, where the Cl- anion is well away from the substrate. It should be emphasized that we here mimic the actual ligand PPh3 with the model ligand (PMe3). To evaluate the influence of the ligand on the relative stability of complexes involved in the reactions, as examples, we calculated the relative stabilities of complexes IMa1′ and IMb1′, where the actual ligand rather than the model ligand is used. It is found that these two experimental complexes IMa1′ and IMb1′ lie 2.9 and 19.2 kcal/mol above the corresponding reaction entrances. This situation is not remarkably different from that in IMa1 and IMb1, which lie 0.9 and 19.1 kcal/mol below and above the respective reaction entrances. Thus we believe that the model ligand can approximately mimic the steric effect of the catalysts although it is smaller in size than the actual ligand. Starting from four intermediates IMa1, IMb1, IMc1, and IMd1, we scanned the potential energy surfaces (PESs) along the reaction coordinates. It is found that the cycloisomerization reactions of 4-phenyl-hexa-1,5-enyne catalyzed by four catalysts involve similar mechanisms, which all include two major steps, the initial nucleophilic addition of the alkynyl onto the alkene group and the subsequent 1,2-H migration. However, the PES profiles are very different in four situations, from which we expect to understand the experimental observations. In the following sections, we will discuss the calculated PES profiles in detail one by one. For the (PMe3)AuBF4-mediated reaction, the initial Au(I)alkyne intermediate IMa1 is only more stable by 0.9 kcal/mol than the reactant. The CtC bond length in IMa1 (1.229 Å) is longer than that in the free substrate A (1.208 Å), indicating that (PMe3)AuBF4 catalyst can effectively activate the alkynyl of the substrate. IMa1 evolves into a ring-closed bicyclic intermediate IMa2 via transition state TSa1 with a Gibbs freeenergy barrier of 15.2 kcal/mol. This is a nucleophilic addition process of the alkynyl onto the alkene group. From the optimized structure of TSa1, it can be seen that both the C1-C6 and C2-C6 bonds are being shortened and hence the threemembered C1-C2-C6 ring is being formed. At the same time, the Au(I) center is migrating to C5, generating the Au-carbenoid complex IMa2, in which C5 atom bears high reactivity.14 Subsequently, the reaction proceeds through a direct 1,2-H migration step via transition state TSa2, that is, the H atom on C4 transfers to the reactive C5, to give product-like complex Ba*, which lies 26.0 kcal/mol below the reaction entrance. Calculated free energy barrier for the direct H-shift process is only 7.9 kcal/mol, indicating that this is a kinetically favorable process. Finally, the complex Ba* smoothly dissociates into the product B, [3.1.0] hexane, and catalyst (PMe3)AuBF4, which recycles into the next reaction procedure. From the calculated free energy barriers of the two elementary steps, 15.2 kcal/mol in nucleophilic addition step and 7.9 kcal/mol in the 1,2-H shift step, it is clear that the rate-determining step of the (PMe3)AuBF4catalyzed cycloisomerization reaction is the nucleophilic addition. Similarly, the (PMe3)AuCl-catalyzed reaction first undergoes an intramolecular nucleophilic addition through TSb1 with a whole free energy barrier of 41.9 kcal/mol to yield a closed bicyclic Au-carbenoid IMb2. In the following hydrogen shift step, it seems that intermediate IMb2 should also undertake a direct

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Figure 4. Optimized geometries involved in the cycloisomerization reaction of 4-phenyl-hexa-1,5-enyne catalyzed by AuCl3 and AuCl. The hydrogen atoms not participating in the reaction have been omitted for clarity. Bond distances are given in Å.

1,2-H shift process via a transition state TSb2′′ to make the final product complex B. Unexpectedly, our attempt to locate such a transition state has failed. Alternatively, we have located a counterion Cl- assisted H-shift mechanism, which was also proposed in a recently published work by Li et al.41 for the treatment of bromoallenyl ketone with catalysts AuCl3 and Au(PH3)Cl. As shown in Scheme 3, this mechanism involves two sequential H-migration processes. In the Au-carbenoid complex IMb2, the Au-Cl distance (2.621 Å) is remarkably longer than that (2.348 Å) in (PMe3)AuCl, revealing that the interaction between Au and Cl has been weakened remarkably because of the formation of the Au-C bond in IMb2. In this

case, the chloride anion can bind to the hydrogen at C4 to form a HCl unit in IMb3 via transition state TSb2 with a free energy barrier of only 2.0 kcal/mol. Then, the hydrogen atom in the HCl unit bonds to C5 atom again through the assistance of chloride anion, leading to the more stable complex Bb*, which further decomposes into the bicyclic-hexane derivative B and catalyst (PMe3)AuCl, completing the whole catalytic cycle. The calculated free energy barrier in this step is 9.8 kcal/mol and the dissociation of Bb* is found to be exothermic by 6.5 kcal/ mol. From Figure 1, it is clear that the PES profile involving (PMe3)AuCl lies above that involving (PMe3)AuBF4, and the

Catalytic Isomerization of 4-pPhenyl-hexa-1,5-enyne SCHEME 2: Different Coordination Modes of 4-Phenyl-hexa-1,5-enyne (A) with Au Catalysts

J. Phys. Chem. A, Vol. 114, No. 49, 2010 12897 SCHEME 4: Diagrams of the HOMOs of the Complexes Involved in the Coordination of 4-phenyl-hexa-1,5-enyne (A) with (PMe3)AuBF4 and (PMe3)AuCl, Respectivelya

SCHEME 3: Hydrogen Migration Step for the (PMe3)AuCl-Catalyzed Cycloisomerization of 4-Phenyl-hexa-1,5-enyne

a

free energy required to reach TSb1 is as high as 41.9 kcal/mol, which is in contrast to 14.3 kcal/mol in the situation involving (PMe3)AuBF4. This result is consistent with the experimental finding by Toste groups12 that (PMe3)AuBF4 catalyst achieved the entire transformation in 5 min for the cycloisomerization of 4-phenyl-hexa-1,5-enyne, whereas (PMe3)AuCl did not bring out any [3.1.0]hexene products in 24 h. Our calculations confirm that the counterion of Au(I) catalysts significantly influences its catalytic activity. In the present situation, BF4- anion promotes the reaction whereas Cl- anion blocks the reaction. This fact can be understood by analyzing the electronic interactions between the substrate and catalyst. In Scheme 4, we compared the isodensity surfaces of the highest occupied molecular orbitals (HOMOs) of IMa1 and TSa1 to the corresponding those of IMb1 and TSb1. Clearly, the HOMO in IMa1 keeps the highly delocated characteristic as that in free substrate A, and only a small value is located on catalyst (PMe3)AuBF4. In contrast, the HOMO in IMb1 has a large value on catalyst (PMe3)AuCl with a small one on the substrate. This is mainly ascribed to the relatively weaker polarization (coordinate) capability of Au(I) in (PMe3)AuBF4 to the substrate than that in (PMe3)AuCl, as is indicated by the calculated NBO charges on Au(I), which is 0.45 e in (PMe3)AuBF4 and 0.64 e in (PMe3)AuCl. This fact is also confirmed by the calculated geometrical parameters shown in Figures 1 and 2, in IMa1 one of Au-C distances, 2.551 Å, is much longer than that in IMb1, 2.246 Å. Thus, it is not surprising that the formation of IMa1 is more favorable in energy than that of IMb1, where the HOMO is remarkably destabilized due to the stronger polarization capability of Au(I) in (PMe3)AuCl. Similar situation can also be found in TSa1 and TSb1, that is, TSa1 is expected to be energetically much more favorable than TSb1. These analyses are consistent with the results displayed in Figure 1.

All H atoms have been omitted for clarity.

Next, we turn our attention to the AuCl3-catalyzed reaction. The reaction pathway is almost the same with (PMe3)AuBF4catalyzed system (Figure 1). First, the AuCl3 catalyst coordinates with the substrate A to yield the gold-alkyne intermediate IMc1, and then the intramolecular nucleophilic addition occurs via a TSc1 with an energy barrier of 8.5 kcal/mol to give gold carbenoid complex IMc2. Subsequently, IMc2 transforms into the product-like Bc* through the direct 1,2-H migration with an energy barrier of 7.3 kcal/mol, and finally, Bc* dissociates into the bicyclic product B and the catalyst AuCl3, completing the catalytic cycle. Note that our calculations indicate that the last step, that is, the dissociation of AuCl3 from the compound Bc*, requires an energy of 18.1 kcal/mol, which is much higher than either the barrier of the nucleophilic addition or that of the H-migration step. This situation is in sharp contrast with the reactions catalyzed by (PMe3)AuBF4 and (PMe3)AuCl, which have a PMe3 ligand and their releases from the productlike complexes Ba* and Bb* are exoergic processes. Here, AuCl3 is relatively “naked” due to the lack of ligand and thus has enough room to bind the reactant and product to form stable complexes. However, its binding on the product seems to be too strong to be released on time to work in the next catalytic cycle. In other words, AuCl3 is easily poisoned by the newly formed product, and so it is not an ideal catalyst for the concerned cycloisomerization reaction. As reported by Toste et al.12 (entry 7 in Scheme 1), when AuCl3 is used as a catalyst, the observed conversion rate is only 50%. This medium conversion rate can be understood as follows: at the initial stage of the reaction, the cycloisomerization can smoothly proceed and even faster than the (PMe3)AuBF4-catalyzed reaction since the PES profile of the AuCl3-involved system lies below that of the (PMe3)AuBF4-involved system. However, with the reaction proceeding, the forming products gradually bind to AuCl3 and hence block the reaction. Although the formation of product-like intermediate Bc* is very exothermic, the energies generated from this process would be quickly released into the environments and could not contribute to the dissociation of AuCl3 from Bc*. A general requirement for a catalytic reaction is that the catalyst must be able to appropriately anchor the substrate, but at the same time it must have a relatively weak interaction with the product to maintain its catalytic activity intact in the next

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cycle, at least to a certain degree. For the present three catalysts, the theoretical results indicate that (PPh3)AuBF4 meets the requirement, whereas (PPh3)AuCl and AuCl3 do not satisfy the requirement at the entrance and exit of the reaction, respectively. In addition, regarding the effective active species in homogeneous Au catalytic reactions, there is a conjecture that only Au(I) is a catalytically active species. In this point of view, Au(III) compounds are easily reduced to Au(I) compounds in the reaction mixture due to the high oxidation potential of Au(III).42-45 If this assumption is true for the cycloisomerization of 4-phenyl-hexa-1,5-enyne, what would be the catalytic activity of AuCl? To answer this question, we have further studied the AuCl-catalyzed reaction. The calculated PES profile is very similar to that for AuCl3, as shown in Figure 1. Remarkably, the bindings of AuCl to the reactant and product are even stronger owing to less steric hindrance than those of AuCl3. From Figure 1, the formation of IMd1 is exothermic by 27.1 kcal/mol, and the dissociation of AuCl from the like-product Bd* requires a Gibbs free energy of 21.7 kcal/mol, which is very different from the exothermic disassociation of (PMe3)AuCl from Bb*. Thus, AuCl is also not expected to have a good performance for the present cycloisomerization reaction. These results indicate that the oxidation state of Au is not a crucial factor controlling the reactivity. From the calculated results above, we have rationalized the experimental observations well and shown the mechanism details for the cycloisomerization reaction of 4-phenyl substituted 1,5-enyne. The information gained from the present work is indicative for the design of homogeneous Au catalysts. For the cycloisomerization reaction of 4-phenyl-hexa-1,5-enyne, we confirm that (PPh3)AuBF4, which carries the appropriate ligand and the counterion, is a satisfactory catalyst, where both the PPh3 ligand and the counterion BF4- act as effective “steric blocks” to obstruct the strong binding of Au(I) to the reactant and product and to partly shade the strong polarization of the Au(I) to the reactant, making the π electrons of the reactant remain delocated well during the reaction process. In contrast, in (PPh3)AuCl, Cl- anion is too small in size to effectively shade the strong polarization of the Au(I) to the reactant, making the intermediates and transition states very unstable. For AuCl3 and AuCl, due to the lack of a bulky ligand, the naked Au(III) and Au(I) catalysts stick to the product too strongly to participate in the next catalytic cycle. These theoretical proposals can not only explain the experimental observations well but also provide a clue for designing new homogeneous Au catalysts for the isomerization reaction of enynes. 4. Conclusion The homogeneous gold-catalyzed cycloisomerization reaction of 4-phenyl-hexa-1,5-enyne has been investigated by performing DFT calculations. The calculated results provide deep insight into the observed different catalytic activity of several catalysts, involving (PPh3)AuBF4, (PPh3)AuCl, AuCl3, and AuCl. It is shown that in all cases the catalytic reaction includes two major steps, that is, the nucleophilic addition of alkynyl onto alkene moiety and the subsequent 1,2-H-migration. The potential energy profiles are found to be closely related to the catalyst used. The rate-determining step is the nucleophilic addition for (PMe3)AuBF4- and (PMe3)AuCl-mediated reactions. However, the corresponding free energy barrier of the former (15.2 kcal/ mol) is much lower than the latter (41.9 kcal/mol). This can explain the observed 100% conversion rate for the (PMe3)AuBF4mediated reaction and zero conversion rate for the (PMe3)AuClmediated reaction. However, for AuCl3- and AuCl-mediated

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