Homogeneous Gold Catalysis: Hydration of 1,2-Diphenylacetylene

Article pubs.acs.org/Organometallics

Homogeneous Gold Catalysis: Hydration of 1,2-Diphenylacetylene with Methanol in Aqueous Media. A Theoretical Viewpoint Gloria Mazzone, Nino Russo, and Emilia Sicilia* Dipartimento di Chimica and Centro di Calcolo ad Alte Prestazioni per Elaborazioni Parallele e Distribuite-Centro d’Eccellenza MIUR, Università della Calabria, I-87036 Arcavacata di Rende, Italy S Supporting Information *

ABSTRACT: Hydration of alkynes to the corresponding ketones can be afforded in high yields at room temperature by using gold(I) phosphine complexes as catalyst, with no acidic cocatalysts required. A detailed DFT computational study of the nucleophilic attack of methanol to 1,2diphenylacetylene assisted by [(Ph3P)Au]+ catalyst has been carried out to shed light on the mechanistic aspects of such a process. The effect of the presence of an additional molecule of water that assists the reaction has been investigated. Calculations suggest that the rate-determining step of the whole process is the addition of a second nucleophile molecule to the formed enol ether to yield the final ketone product, along the pathway that describes the second part of the reaction. Comparison with an analogous study for the nucleophilic attack of water shows that, according to experimental findings, addition to diphenylacetylene of MeOH is faster than that of H2O.

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catalyst leads to the formation of a linear Au-η2-alkyne complex that is then attacked in a syn fashion by a molecule of methanol at the side of the coordinated metal. Similar results were obtained a few years later by Tanaka et al.,3 Laguna,6 and others,7 which have shown that alkyne hydration can be performed by organometallic gold(I) complexes, especially in combination with strong acids with heating. More recently, in a series of experiments on gold-catalyzed ketone formation from a wide range of alkynes, Leyva and Corma have demonstrated that water-soluble phosphine ligands combined with a soft noncoordinating anion would make the Au(I) catalytic center sufficiently acidic to achieve the nucleophile addition to alkynes without additives.8 Catalysts such as R3PAuX (PR3 = PPh3, SPhos, PtBu3; X = Cl, OTf, NTf2) have been formed in situ by treatment of the corresponding chloride complex, R3PAuCl, with a silver salt. Moreover, with the purpose of shedding light on the proposed mechanistic hypotheses3,5,6 for this reaction the authors have carried out a series of kinetic experiments using H2O, MeOH, or both as nucleophiles, 1,2-diphenylacetylene as substrate, and AuPR3X (PR3 = PPh3, SPhos, PtBu3; X = Cl, OTf, NTf2) as catalyst with different amounts of water and solvents. The mechanism proposed by the authors on the basis of experimental findings is summarized in Scheme 1. The first steps of the reaction are coordination of the triple bond to the Au(I) complex with formation of the Au-π-alkyne complex I

INTRODUCTION Gold, a noble metal, was long considered useless as a catalyst; however, in the last 15 years homogeneous Au(I) and Au(III) complexes have emerged as highly competent and selective catalysts to act as soft carbophilic Lewis acids toward C−C multiple bonds.1 Hence, an understanding of the mechanistic and structural aspects of the reactions mediated by gold complexes has become the subject of an ever-increasing number of publications.2 A surprising efficiency of Au(I) complexes has been detected, especially toward the addition of methanol to alkynes under mild conditions. This new synthetic strategy has provided an alternative to mercury salts, used for the industrial production of ketones from alkynes3 until the discovery of their toxicity. Linear dicoordinated gold compounds, upon activation by a silver(I) salt, generate a monoligated cationic catalyst which requires strong electronic and steric stabilization from its ancillary ligand. To date, cationic Au(I)-phosphine complexes have been identified as the best gold-based catalysts for selective activation of the triple bond in simple alkynes in aqueous media.4 The mechanism by which this reaction occurs has been studied for the first time by Teles et al. at the end of the 1990s. In their investigation of methanol addition to alkynes, the authors have detected an efficient catalytic activity of coordinatively unsaturated Au(I) species, of the type R3PAu+, generated in situ by the protonolysis of R3PAuCH3 and release of CH4.5 They have shown how in the presence of water ketones are the only products. Furthermore, with the aid of ab initio calculations, the authors have suggested an associative mechanism in which the first interaction between alkyne and © 2012 American Chemical Society

Received: December 13, 2011 Published: March 16, 2012 3074

dx.doi.org/10.1021/om2012369 | Organometallics 2012, 31, 3074−3080

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With the aim of providing a general framework for the mechanism of alkyne hydration, we report here the DFT outcomes of the intermolecular addition of MeOH to 1,2diphenylacetylene, the alkyne used in the experiments, catalyzed by the same gold(I) complex used by us for the addition of water molecules, [(Ph3P)Au]+.9 According to our previous results, this complex is sufficiently representative of the catalysts used in the experiments. Indeed, with respect to other computationally cheaper ligands it is sterically hindered and allows us to perform calculations without imposing any structural constraints or computational artifices. As has been formerly pointed out,12 however, if the role of conformational diversity is not taken into consideration, the use of larger model ligands such as PPh3 can produce less accurate results than the use of smaller, but conformationally simpler, ligands. Therefore, the influence of the conformational diversity on the process under examination has been estimated. The principal differences between water and methanol nucleophiles, in terms of intermediates and transition states intercepted along the reaction paths, have been highlighted in order to compare their relative abilities to add at activated gold(I)-alkyne complexes. Moreover, interesting findings have been obtained by taking into consideration how one explicit solvent water molecule comes into play in the hydration process. Calculations show that the water molecule assists the first nucleophile addition and lowers the activation barriers.

Scheme 1. Schematic Representation of Au(I)-Catalyzed Hydration of 1,2-Diphenylacetylene

and subsequent attack of the R′OH (R′ = H, CH3) nucleophile. R′OH weakly coordinated to the gold atom (II) adds to the alkyne, forming the intermediate III, which by protodeauration affords the E isomer of the enolic product IV together with the release of the catalyst. Formation of the Z from the E isomer can occur by rotation around the C−C double bond5 or in the next step of the overall process: that is, addition of a second molecule of R′OH to form the corresponding ketal V. Elimination of a R′OH molecule from V should explain the formation of both Z and E isomers, whereas subsequent reaction of ketal V with H2O leads to the formation of the ketone final product. Very recently, with the support of experimental observations and the proposed reaction mechanism, we have investigated, with the aid of density functional theory (DFT) calculations, the mechanistic details of the whole 1,2-diphenylacetylene hydration process catalyzed by the gold(I) [(Ph3P)Au]+ complex using pure water as nucleophile.9 Both possibilities for attack of water at the alkyne, from the same side of the coordinated catalyst (inner-sphere mechanism) or from the opposite side (outer-sphere mechanism), have been explored, and the first one has been found to be the most favorable. The most likely reaction pathway involves the first molecule addition with gold acting as a proton shuttle to transfer the migrating hydrogen in a cis position with respect to the OH group; from the E isomer of the enol coordinated to the catalyst the Z isomer could be formed by rotation around the C−C bond. Moreover, we have shown how the presence of the catalyst also in the second part of the process, that is the second water molecule addition, lowers the energy barrier of the step that controls the overall reaction. Few studies reported recently in the literature, about nucleophilic attack at an activated C−C multiple bond, show how the presence of a proton transfer agent promotes the overall process, as these species act as proton shuttles in the proton transfer step.10,11 Hashmi and co-workers have recently investigated the role played by solvent molecules on the mechanism of triple bond activation to nucleophilic attack catalyzed by both Au(I) and Au(III) complexes, using propyne as substrate.10 In the addition of both water and methanol, catalyzed by a (PH3)AuI complex, the authors highlighted how a solvent cage acts as a proton acceptor, accelerating both addition and proton migration steps.10a In the case of the AuCl3 catalyst, only one water molecule has been used to stabilize, through hydrogen bonds, the transition states lowering the energy barriers computed for the water addition.10b

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COMPUTATIONAL DETAILS

All the electronic structure calculations involved in the hydration process of 1,2-diphenylactelyne with methanol catalyzed by [(Ph3P)Au]+ complex have been performed using the Gaussian 03 program package13 at the DF level by employing the hybrid exchange functional by Becke (B3) in combination with the Lee, Yang, and Parr (LYP) correlation functional.14,15 Numerous theoretical studies of Au-catalyzed reactions at the B3LYP level have been reported in the literature, which confirm that this functional is quite suitable to investigate Au-catalyzed reactions.16 In order to reduce the computational cost, all equilibrium structures and transition states have been fully optimized using the standard 631G basis set of Pople for carbon and hydrogen atoms of phenyl rings together with the 6-31G** set for the rest of the atoms except the gold atom, for which the relativistic compact Stuttgart/Dresden effective core potential17 has been used in conjunction with its split valence basis set. The errors that could be introduced by the conformational diversity have been estimated by carrying out scan calculations on the geometry of complex I varying torsional angles obtained by unconstrained optimization. The results show that other local minima should be located on the potential energy hypersurface. Nevertheless, most intercepted stable local minima are less stable by about 0.5 kcal/mol than the global minimum. When specific weak interactions exist, such as in complex II, the energy difference oscillates between 1 and 2 kcal/ mol in favor of the conformer indicated by us as the most stable. Conformational complexity, therefore, does not play a significant role in the case under examination. Frequency calculations at the same level of theory have been also performed to identify all stationary points as minima (zero imaginary frequencies) or transition states (one imaginary frequency). Aside from this local criterion, minima connected through the transition states have been identified by performing the calculation of the intrinsic reaction coordinate (IRC), defined as the minimum energy reaction pathway (MERP) in mass-weighted Cartesian coordinates between the transition state of a reaction and its reactants and products. The Gonzalez−Schlegel method implemented in Gaussian 03 for following the IRC has been used.18,19 Final energies have been calculated by performing single-point calculations on the optimized geometries at the same level of theory 3075


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and employing 6-311+G** standard basis sets for H, C, O, and P atoms. Since preliminary calculations have clearly shown geometry relaxation effects to be insignificant, only single-point calculations on optimized structures have been carried out to correct the corresponding free energy for the gas phase by the solvation energy. Reaction Gibbs free energies in solution, ΔGsol, have been calculated for each process as the sum of two contributions: a gas-phase reaction free energy, ΔGgas, and a solvation reaction free energy term calculated with the continuum approach, ΔGsolv. The water (ε = 78.4) environment has been modeled using the conductor polarized continuum model (CPCM)20 as implemented in Gaussian 03, and the UAHF set of radii has been selected to build up the cavity. Moreover, since gas-phase free energies for each reaction have been computed with each species at 1 atm pressure, there is one additional thermodynamic correction that needs to be included. Computation of entropies at higher pressures can be a simple way to model translational degrees of freedom in the solvent. Free energies have been evaluated at a pressure of 1354 atm to mimic a condensed phase.21 This procedure reduces the translational entropy contribution to TΔS by 4.3 kcal/mol per particle at 298 K. Basis set superposition error (BSSE) upon formation of minima and transition states has been estimated using a counterpoise correction.22 Structures of transition states have been split into three fragments corresponding to initial reactants. It has been found that 2.4−3.1 kcal/ mol of apparent interaction energy is due to BSSE, but this contribution is approximately the same for all stationary points and the BSSE effect largely cancels out in the calculation of activation barriers and energy differences.

Figure 2. Calculated energy profile for the addition of the first methanol molecule to 1,2-diphenylacetylene catalyzed by the [(Ph3P)Au]+ complex and assisted by a water solvent molecule. Relative Gibbs free energies (in kcal/mol) in solution at a pressure of 1354 atm are reported.

Table 1. Activation Barriers (in kcal/mol) Corresponding to the Transition State Structures of the First Nucleophile (H2O and MeOH Not Assisted and Assisted by a Water Molecule) Molecule Additiona TS1x TS2x TS3x

3. RESULTS AND DISCUSSION First Methanol Molecule Addition. Calculated energy profiles for the first methanol molecule addition on 1,2diphenylacetylene and that assisted by a water solvent molecule are reported in Figures 1 and 2, respectively. Relative ZPE

waterb

methanol

methanol/water

22.4 15.4 21.1

17.7 12.4 18.5

17.9 8.8 18.8

a

The x subscript has been introduced to identify the nucleophile molecule (x = w, m, m/w). bReference 9.

also a comparison with the previous investigation of the same reaction with water as nucleophile9 is reported. Unless otherwise noted, in what follows, the discussed energies are the relative free energies in solution at a pressure of 1354 atm calculated with respect to the intermediate II. The energy profile reported in Figure 1 shows that the first step of the whole catalyzed hydration process involves the preliminary intermediate I, where the C−C triple bond interacts with the catalyst, establishing a weak bond with the gold atom. The coordination of the alkyne to the gold atom enhances the electrophilicity of the triple bond and favors the attack of a nucleophile. The precoordination of a methanol molecule to the activated complex I, to form IIm, is only slightly exothermic with respect to separated reactants. In this intermediate the methanol molecule approaches the metal catalyst, establishing a weak interaction with it. The distance between the oxygen atom of methanol and gold atom is 3.187 Å. Nevertheless, the presence of the nucleophile introduces some differences in Au-π-alkyne complex structure; the lengths of the two Au−C bonds become 2.293 and 2.360 Å for C1 and C2, respectively, while the angle between phosphorus, gold, and the next C2 carbon of the triple bond changes from 164.3 to 158.7°. The reaction proceeds with the formation of intermediate IIIm by attack of methanol at one of the diphenylacetylene carbon atoms. Concomitantly, gold changes its coordination, forming a direct bond with the other carbon atom that lies 2.092 Å from it. The activation energy required to form the intermediate IIIm is 17.7 kcal/mol, and the formed IIIm is 4.2 kcal/mol more stable than the transition state leading to it. The further reaction toward formation of the E isomer of the enol ether coordinated to the Au(I) complex

Figure 1. Calculated energy profile for the addition of the first methanol molecule to 1,2-diphenylacetylene catalyzed by the [(Ph3P)Au]+ complex. Relative Gibbs free energies (in kcal/mol) in solution at a pressure of 1354 atm are reported.

corrected electronic energies (ΔE), enthalpies (ΔH), and Gibbs free energies (ΔG) at 298 K as well as free energies in water (ΔGsol) have been calculated. All the optimized geometries for reactants, intermediates, transition states, and products located along both the potential energy surfaces are schematically depicted in the Supporting Information (Figures S1 and S2, respectively). The activation barriers corresponding to the transition state structures involved in the addition of the first nucleophile molecule are fully shown in Table 1, in which 3076


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Figure 3. Calculated energy profiles for the addition of methanol (a) or water (b) to both enol ether E (solid line) and Z (dashed line) isomers coordinated to the [(Ph3P)Au]+ complex. Relative Gibbs free energies (in kcal/mol) in solution at a pressure of 1354 atm are reported.

outer-sphere attack assisted by an additional water molecule. Along both pathways are depicted the geometrical structures of all the intercepted stationary points. The nucleophilic attack is calculated to be the rate-limiting step of the first molecule addition (see Figure 1). Consequently, as anticipated by Leyva and Corma8 and according to data reported in Table 1, the energetics for the formation of the enolic species by coordination of MeOH to the gold-alkyne complex I is more favorable with respect to that computed for water addition.9 From the reaction path computed for the methanol addition assisted by one explicit solvent molecule shown in Figure 2, it is evident that the presence of a water molecule does not affect the reaction mechanism, which remains the same as that illustrated for the nucleophilic addition in the absence of a proton transfer agent. The solvent water molecule acts only on the energetics of the reaction, affecting the height of the energy barriers (see Table 1), and the stability of the stationary points intercepted along the water-assisted reaction pathway. In the first part of the reaction the H-bonded adduct of methanol and water molecules precoordinates to the Au-π-alkyne complex to form the intermediate IIm/w. The height of the energy barrier relative to the attack of MeOH in TS1m/w is not influenced by the presence of a solvent molecule (17.9 vs 17.7 kcal/mol). On the contrary, intermediate IIIm/w is stabilized by the formation of a hydrogen bond between the water oxygen atom and the hydrogen of the methanol OH group. The E isomer of the enol ether coordinated to the Au(I) complex, intermediate IVm/w, is formed with a relative energy of 9.8 kcal/mol. Formation of such an intermediate involves a water-assisted hydrogen migration from the oxygen to the terminal carbon. In the transition state, TS2m/w, the solvent molecule works as a bridge transferring one of its H atoms to terminal carbon and simultaneously abstracting the methanol hydrogen atom. As a consequence, the energy barrier is lower by 3.6 kcal/mol than for the nonassisted hydrogen transfer. Also in the water-assisted pathway, the catalyst activates the triple bond and at the same time acts as a directing agent that guides the reactants into the right position to react. The presence of an explicit solvent molecule seems to be not beneficial in the last part of the first addition pathway. Both E and Z isomers, coordinated or not to the catalyst, as well as the

involves an Au-assisted hydrogen migration from oxygen to the terminal carbon atom to finally afford the product complex IVm with a relative energy of −15.0 kcal/mol. The rearrangement, through the TS2m transition state with a barrier of 12.4 kcal/ mol, of IIIm to IVm enol ether product proceeds in a concerted manner. The gold atom assists the hydrogen migration without forming an intermediate complex in which the hydrogen is directly coordinated to it. Such an intermediate, instead, is formed when water is employed as nucleophile.9 The IRC calculation performed on TS2m confirms that this TS connects IIIm and IVm intermediates, while the vibrational normal mode associated with the imaginary frequency clearly corresponds to the role described above played by gold. The O−H and H−C2 bond lengths are 1.550 and 1.721 Å, respectively, whereas the fact that the H atom lies 1.683 Å from the gold atom means that, even if no hydride intermediate is formed, gold assists the H migration. The assistance of the catalyst in the Z isomer formation significantly lowers the isomerization barrier for the rotation around the double bond in the absence of catalyst, which has been both experimentally and theoretically estimated to be approximately 65 kcal/mol.23 Therefore, the Z isomer is formed exclusively by isomerization of the IVm complex with an activation energy of 18.5 kcal/mol. All the attempts to intercept minima and transition states for the direct transfer of the hydrogen atom from oxygen to the position trans to the methoxy group have failed. The Z isomer coordinated to the gold complex Vm lies at −17.9 kcal/mol, and it is stabilized by 2.9 kcal/mol with respect to the E isomer. The catalytic cycle could be, then, closed either by the Z isomer of the enol ether release that is calculated to be endothermic by 10.7 kcal/mol or by the loss of the E isomer that requires 10.9 kcal/mol to occur. As recently demonstrated by us, the nucleophilic attack from the opposite side with respect to the coordinated catalyst is energetically unfavorable in comparison to the usual attack of the nucleophile on the same side of the gold catalyst as described in previous paragraphs. Nevertheless, the pathway for the outer-sphere attack of MeOH to the alkyne has also been explored. Figure S3 in the Supporting Information shows the calculated energy profiles for methanol attack from the opposite side with respect to the coordinated catalyst and for 3077


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the hydrogen atoms to Au. The barrier for this rearrangement (TS5aw) is 43.7 kcal/mol. The barrier for the next hydrogen shift from Au to the carbon atom (TS6aw) to form the corresponding hemiketal VIIIw is, instead, very low. Along the Z isomer pathway the intermediate VIIIw formation takes place in one step (TS4bw). The hydrogen atom is transferred directly from oxygen to carbon with a barrier of 45.3 kcal/mol. By inspection of Figure 3 it appears that the reverse barriers that hamper the Z isomer formation by elimination of either a methanol or water molecule from corresponding VIIm and VIIIw intermediates (37.8 and 44.9 kcal/mol, respectively) are higher than that calculated for the E to Z isomerization (first MeOH molecule addition). Thus, comparable to what is observed for the water addition to the activated triple bond,9 even when the nucleophile is methanol, formation of the Z isomer is likely to occur by direct isomerization from the E isomer. Since the second nucleophile addition is the step that controls the reaction rate of the overall process, the data reported in Table 2 show that the addition of methanol to the

transition state TS3m/w corresponding to the catalyst assisted isomerization of E-enol ether to the Z isomer are less stable by about 5 kcal/mol than the analogous stationary points along the nonassisted pathway. Due to the migration of the hydrogen of the OH group, the hydrogen bond between the water oxygen and the methanol hydrogen, formed in IIIm/w, is substituted by the less strong hydrogen bond between a water hydrogen and the oxygen atom of the methoxy group. Second Nucleophile Molecule Addition. Although in previous theoretical works the addition of a second nucleophile molecule has not been taken into account to describe the reaction mechanism that leads to ketone production,3,5,6,10 our recent work on the hydration process of 1,2-diphenylacetylene with water has shown how the rate-determining step of the overall process is the addition of a second nucleophile molecule. We have found that the intervention of the catalyst also in the second part of the reaction lowers the activation barriers that must be overcome to form the gem-diol species.9 Therefore, the addition assisted by the gold catalyst of a second nucleophile molecule to both E and Z enol ether isomers has been investigated. With the aim of exploring all the reaction pathways considered conceivable on the basis of the experimental findings,8 the second molecule addition has been investigated using both MeOH and H2O as nucleophiles. Calculated energy profiles for the reaction pathways corresponding to the addition of a second methanol molecule and a water molecule are reported in parts a and b of Figure 3, respectively. Gibbs free energies in water (ΔGsol) at a pressure of 1354 atm, calculated with respect to IIm, have been reported. Fully optimized structures of stationary points intercepted along the catalystassisted reaction pathways can be found in Figures S4 and S5 in the Supporting Information. The last step of the overall reaction, ketone formation, is not reported in the Figure 3 because it requires a brief discussion; it will be examined later. Along the second addition pathway both adducts, that are E and Z isomers coordinated to the [(Ph3P)Au]+ complex, can enter the subsequent catalytic cycle for the addition of a second methanol molecule. Indeed, the possibility that the isomerization from E to Z isomers occurs depends on how rapidly the final ketone product is formed. For that reason we have investigated the pathway for the addition of a second nucleophile molecule starting from both IVm (solid line in Figure 3) and Vm (dashed line in Figure 3) intermediates. The calculated energy profiles in Figure 3 show that the first step of the process is the nucleophile loose coordination. In Figure 3a the loose coordination of methanol is calculated to be exothermic, by 5.3 and 8.6 kcal/mol, with respect to the IIm intermediate along the E and Z enol ether isomer pathways, respectively. The subsequent nucleophilic attack at the C1 atom takes place in a concerted way and leads directly to dimethyl ketal VIIm formation. The barrier that is necessary to overcome is 44.2 and 47.2 kcal/mol for E and Z isomers, respectively. From an examination of Figure 3b the intermediates VIaw and VIbw, corresponding to the loose coordination of water, are calculated to be more stable by 8.1 and 10.8 kcal/mol than intermediate IIm along the E and Z enol ether isomer pathways, respectively. Along the path involving the E isomer the reaction proceeds in two steps by a 1,3-hydrogen migration with gold acting as a proton shuttle. The VIIaw intermediate, which lies 35.4 kcal/mol above the energy of the IIm intermediate, is formed by oxygen coordination to C1 and migration of one of

Table 2. Activation Barriers (in kcal/mol) Corresponding to the Addition Step in the Second Nucleophile Molecule Additiona pathway a pathway b

waterb

methanol

water

48.8 54.7

44.2 47.2

43.7 45.3

a

The symbols a and b have been used for addition to the E and Z isomers of enolic species, respectively. bReference 9.

enol ether intermediate is energetically favored compared with the water addition to enol9 but slightly disfavored with respect to water addition to the enol ether intermediates. Despite numerous attempts, we have been unable to complete the pathway for the addition of a second methanol molecule. It is, indeed, truncated at the formation of the dimethyl ketal species VIIm. As shown in Scheme1, the last step of the reaction, that is the demethoxylation of the dimethyl ketal intermediate, requires the intervention of a water molecule. We have tried to intercept, but without success, a plausible transition state for the release of a methanol molecule and formation of an Au-hemiketal complex. Instead, the replacement of methanol with water as nucleophile in the second part of the process represents an alternative route that leads directly to hemiketal intermediate formation. Ketone product, indeed, can be formed from intermediate VIIIw by a direct hydrogen shift from the OH group to the methoxy group. Moreover, an additional water molecule can assist the process, transferring one of its H atoms to the methoxy group and abstracting the H atom of the OH group, favoring the release of a methanol molecule and thus the ketone formation. Both transition states for envisaged rearrangements have been intercepted. The energetics of the final step of the process leading to the release of the formed ketone is reported in Figure 4 for the direct (a) and water-assisted (b) proton transfer. Geometrical structures of involved stationary points are included in Figure S5 in the Supporting Information. The elimination of a methanol molecule directly from the intermediate VIIIw occurs through the TS6w transition state, whose formation is endothermic by 15.2 kcal/mol with respect to the reference energy of the intermediate IIm, and the corresponding activation barrier amounts to 25.6 kcal/mol. As a 3078


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Figure 4. Calculated energy profiles for the methanol elimination step, leading to ketone formation, not assisted (a) or assisted (b) by a water molecule. Relative Gibbs free energies (in kcal/mol) in solution at a pressure of 1354 atm are reported.

intermediate has been located, IRC calculations and visualization of the imaginary frequency associated with the hydrogen migration transition state confirm the role played by gold. The use of methanol as nucleophile, compared to water, results in lower activation barriers for the first addition step, especially if a water solvent molecule assists the overall process. The second nucleophile molecule addition can lead to the formation of the ketone final product only when water is the nucleophile. Indeed, the reaction of both enol ether isomers with methanol affords a ketal intermediate, which does not undergo any attack by a nucleophile molecule to form the ketone product. In contrast, by addition of water a hemiketal intermediate coordinated to gold is formed that can easily lead to the formation of the π carbonyl bond directly or with the assistance of an additional water molecule. Our computational results are consistent with the experimental observation of Leyva and Corma for gold(I)-catalyzed hydration of alkynes and provide a general framework for the nucleophilic addition to triple bond catalyzed by Au(I) complexes.

result of the hydrogen migration from the OH group oxygen to that of the OMe group a methanol molecule is eliminated and the carbonyl bond is formed. The corresponding minimum IXw is stabilized by 8.8 kcal/mol with respect to the preceding species. The energetics along the water-assisted pathway is calculated with respect to the intermediate formed by coordination of a water molecule to the IIm intermediate. The TS6′w transition state lies at 7.8 kcal/mol above the reference energy and corresponds to a barrier of 16.9 kcal/mol. The calculated imaginary frequency is associated with the movement of the external water molecule that approaches the complex and induces formation of the product by abstracting a hydrogen atom from the OH group and simultaneously transferring a hydrogen atom to the OMe group. The formation of the adduct IX′w with one methanol and one water molecule directly interacting is calculated to be slightly less exothermic than the formation of the IXw adduct with one methanol molecule: that is, 16.4 versus 19.2 kcal/mol. As can be clearly inferred from the comparison between the two pathways reported in Figure 4, the process is calculated to be more favorable if the participation of an additional water molecule is taken into consideration. The last step of the reaction, release of the product and regeneration of the catalyst, is endothermic by 7.6 and 8.2 kcal/ mol from intermediates IXw and IX′w, respectively. With respect to separated starting reactants the whole catalytic process is exothermic by 10.2 kcal/mol.

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ASSOCIATED CONTENT

S Supporting Information *

Figures and tables giving fully optimized structures of stationary points intercepted along the pathway for the first methanol molecule addition to the Au-η2-alkyne complex assisted or not by a water solvent molecule, s calculated energy profile for the outer-sphere attack of MeOH sy the Au-η2-alkyne complex assisted or not by a water solvent molecule, fully optimized structures of stationary points intercepted along the pathway for the second nucleophile (MeOH or H2O) molecule addition to both E and Z enol ether isomers, and Cartesian coordinates and absolute ZPE corrected electronic energies (hartree) for all optimized stationary points. This material is available free of charge via the Internet at http://pubs.acs.org.

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CONCLUSION A mechanistic investigation of the whole hydration process of 1,2-diphenylacetylene by methanol catalyzed by the [(Ph3P)Au]+ complex has been theoretically performed. The main differences due to the use of methanol as nucleophile instead of water have been highlighted, and the most reliable pathway has been described. The mechanistic hypotheses regarding the hydration of alkynes to the corresponding ketones catalyzed by gold(I) phosphine complexes have been confirmed. Our calculations show that the first molecule addition occurs with gold acting as a proton shuttle. Even if no gold hydride

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AUTHOR INFORMATION

Corresponding Author

*Fax: +39-0984-492044. E-mail: [email protected]. 3079


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Notes

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 B.05; Gaussian, Inc., Wallingford, CT, 2004. (14) Becke, A. D. J. Chem. Phys. 1993, 98, 5648−5652. (15) Stephens, P. J.; Devlin, F. J.; Chabalowski, C. F.; Frisch, M. J. J. Phys. Chem. 1994, 98, 11623−11627. (16) (a) Roithová, J.; Hrušak, J.; Schröder, D.; Schwarz, H. Inorg. Chim. Acta 2005, 358, 4287−4292. (b) Nieto-Oberhuber, C.; López, S.; Jiménez-Núñez, E.; Echavarren, A. M. Chem. Eur. J. 2006, 12, 5916−5923. (c) Shi, F.-Q.; Li, X.; Xia, Y.; Zhang, L.; Yu, Z.-X. J. Am. Chem. Soc. 2007, 129, 15503−15512. (d) Kovács, G.; Ujaque, G.; Lledós, A. J. Am. Chem. Soc. 2008, 130, 853−864. (17) Andrae, D.; Häussermann, U.; Dolg, M.; Stoll, H.; Preuss, H. Theor. Chim. Acta 1990, 77, 123−141. (18) Fukui, K. J. Phys. Chem. 1970, 74, 4161−4163. (19) Gonzalez, C.; Schlegel, H. B. J. Chem. Phys. 1989, 90, 2154− 2161. (20) Cossi, M.; Rega, N.; Scalmani, G.; Barone, V. J. Comput. Chem. 2003, 24, 669−681. (21) R Martin, R. L.; Hay, P. J.; Pratt, L. R. J. Phys. Chem. A 1998, 102, 3565−3573. (22) (a) Boys, S. F.; Bernardi, F. Mol. Phys. 1970, 19, 553−566. (b) Simon, S.; Duran, M.; Dannenberg, J. J. J. Chem. Phys. 1996, 105, 11024−11029. (23) (a) Barrows, S. E.; Eberlein, T. H. J. Chem. Educ. 2005, 82, 1329−1333. (b) Douglas, J. E.; Rabinovitch, B. S.; Looney, F. S. J. Chem. Phys. 1955, 23, 315−323.

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS CASPUR and Università della Calabria are gratefully acknowledged. REFERENCES

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