Computational Mechanistic Study of Palladium(II)-Catalyzed

Article pubs.acs.org/Organometallics

Computational Mechanistic Study of Palladium(II)-Catalyzed Carboxyalkynylation of an Olefin Using an Iodine(III) Oxidant Reagent Alireza Ariafard*,‡,† †

Department of Chemistry, Faculty of Science, Central Tehran Branch, Islamic Azad University, Shahrak Gharb, Tehran, Iran School of Chemistry, University of Tasmania, Private Bag 75, Hobart, TAS 7001, Australia

‡

S Supporting Information *

ABSTRACT: The Pd(II)-catalyzed chemical transformations using an iodine(III) oxidant are mostly believed to proceed via a Pd(IV)/Pd(II) catalytic cycle. The present computational study, however, demonstrates that this statement is not always true, and, in some particular cases, an alternative mechanism could be operative. Herein, the reaction mechanism of the Pd(II)-catalyzed carboxyalkynylation of an olefin using an alkynyl benziodoxolone reagent was elucidated with the aid of density functional theory calculations. The catalytic reaction was found to proceed via a mechanism in which a Pd(II) vinylidene-like complex, not a Pd(IV) complex, plays a leading role. The mechanistic understanding of the carboxyalkynylation reaction may have significant implications in a variety of processes catalyzed by transition metal complexes in the presence of alkynyl benziodoxolones.

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INTRODUCTION Palladium(IV) complexes have been recently proposed as intermediates responsible for C−X bond-forming steps in many catalytic reactions.1,2 The Pd-catalyzed coupling reactions involving the Pd(II)/Pd(IV) mixed-valence intermediates are usually conducted under highly oxidizing conditions. Strong oxidants such as hypervalent iodine(III) reagents are used in order for the formation of the Pd(IV) intermediates to be facilitated during the catalytic reaction. In these reactions, the iodine(III) reagents oxidize Pd(II) to Pd(IV) and concurrently deliver the reacting group to the palladium center. In this context, it was discovered that alkynyl benziodoxolones (EBX) can be used as electrophilic alkynylation reagents in a variety of metal-catalyzed reactions.3 For example, Waser and co-workers demonstrated for the first time that oxyalkynylation of olefins by carboxylic acids and phenols can be catalyzed by palladium using EBX as a reagent and hexafluoroacetylacetonate (hfacac) as a ligand (eq 1).4 In this

to be accessible in this process due to the presence of the iodine(III) oxidant EBX (Scheme 1).3f,4,5 According to Waser’s Scheme 1

mechanism, the catalytic cycle for a carboxylic acid is initiated by carboxypalladation of the olefin to give the Wacker-type intermediate II. Then, the more electron-rich nature of II promotes oxidative addition of the alkynyl benziodoxolon to the Pd(II) center to give the Pd(IV) intermediate III. The Wacker-type catalytic reaction, the alkenes are functionalized using an internal heteroatom nucleophile and an alkynyl electrophile. The Pd(IV)/Pd(II) catalytic cycle was proposed © 2014 American Chemical Society

Received: November 20, 2014 Published: December 9, 2014 7318

dx.doi.org/10.1021/om5011758 | Organometallics 2014, 33, 7318−7324

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reductive elimination eventually gives the final product, and the Pd(II) catalyst is regenerated. In an earlier computational study on gold(I)-catalyzed alkynylation of indole using an iodine(III) oxidant EBX reagent, it was shown that the oxidation of Au(I) to Au(III) by the iodine(III) reagent is an energy-demanding process, resulting in the conclusion that the participation of a gold(III) intermediate in the catalytic reaction is very unlikely.6 This finding stimulated us to investigate whether or not a Pd(IV) intermediate is formed during the course of the oxyalkynylation reaction. Although the feasibility of accessing a Pd(IV) alkynyl complex was confirmed by Canty and co-workers through oxidation of PdII complexes by alkynylidonium salts,7 the computational study conducted here unexpectedly reveals that the Pd(IV)/Pd(II) catalytic cycle is not operative and instead the reaction proceeds via a novel mechanism in which a Pd(II) vinylidene-like complex plays a leading role.

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adducts are calculated at the M06 level to be less stable than 1, suggesting that the precatalyst is likely the catalyst resting state. Scheme 2

COMPUTATIONAL DETAILS

Gaussian 098 was used to fully optimize all the structures reported in this paper at the B3LYP level of density functional theory (DFT)9 in dichloromethane using the CPCM10 solvation model. The effectivecore potential of Hay and Wadt with a double-ξ valence basis set (LANL2DZ)11 was chosen to describe Pd and I. The 6-31G(d) basis set was used for other atoms.12 Polarization functions were also added for Pd (ξf = 1.472) and I (ξf = 0.289).13 This basis set combination will be referred to as BS1. Frequency calculations were carried out at the same level of theory as those for the structural optimization. Transition structures were located using the Berny algorithm. Intrinsic reaction coordinate (IRC)14 calculations were used to confirm the connectivity between transition structures and minima. To further refine the energies obtained from the B3LYP/BS1 calculations, we carried out single-point energy calculations for all of the structures with a larger basis set (BS2) in dichloromethane using the CPCM solvation model at the M0615 levels. BS2 utilizes the triple-ζ valence def2-TZVP basis set16 on all atoms. Effective core potentials including scalar relativistic effects were used for the Pd and I atoms.17 We have used the potential and Gibbs free energies obtained from the M06/BS2//B3LYP/BS1 calculations in dichloromethane throughout the paper unless otherwise stated. Recent computational studies on organometallic reactions have shown that thermodynamic and kinetic results are predicted more accurately if the M06 functional is used instead of B3LYP.15 The use of M06 can also be rationalized based on the fact that this functional estimates the van der Waals interactions more precisely.18 These factors have prompted us to select this functional for all the singlepoint calculations. The atomic orbital populations were calculated on the basis of natural bond orbital (NBO) analyses.19 The translational contribution to the entropy is suppressed upon going from the gas phase to a solvent, leading to an inadequate estimation of the Gibbs free energy changes especially where the number of reactants is different from that of products. Plenty of previous theoretical studies20 used the formulation developed by Whitesides and co-workers21 for obtaining more accurate values of the Gibbs free energy changes. The same formulation was used in this study to correct the entropic contributions. As in a previous study, the TIPS-EBX reagent was modeled using TMS-EBX.6 For organic acids, a dimeric form was considered as the reference point because this form was calculated to be more stable than its corresponding monomeric form by almost ΔG = 2 kcal/mol.

The first step of the reaction is surmised to be the carboxypalladation of the olefin. This process, which gives PdII alkyl complex 8, is calculated to be endergonic by 13.5 kcal/mol (eq 2). The mechanistic details regarding the

formation of the Wacker-type intermediate 8 will be discussed in the subsequent section. We initially focus our attention on addressing the question of whether, starting from 8, the accessibility of a Pd(IV) intermediate is feasible or not. In order for the reaction to proceed through a Pd(IV)/ Pd(II) redox couple, intermediate 8 has to undergo a ligand exchange with TMS-EBX followed by the oxidation of Pd(II) to Pd(IV) by TMS-EBX to yield 10. The oxidative addition process (9 → 10) is calculated to be endergonic by 9.8 kcal/ mol and features an activation barrier as high as 26.0 kcal/mol. Subsequently, the reductive elimination of the product from 10 occurs via transition structure TS10−11 with an activation energy of 8.0 kcal/mol. If a rapid equilibrium between the intermediates 9, 9′, and 12 is assumed, then the carboxyalkynylation reaction occurs with an overall activation barrier of 30.2 kcal/mol (Figure 1). This result suggests that the Pd(IV)/Pd(II) redox couple mechanism is an energydemanding process and less likely to take place under mild conditions (eq 1). Under these circumstances, it appears that there may exist an alternative pathway through which the carboxyalkynylation reaction proceeds. Novel Mechanism for the Oxyalkynylation Reaction. More recently, our theoretical study on the full catalytic cycle of gold-catalyzed alkynylation of indole using TMS-EBX demonstrated that the alkynyl group is easily transferred from iodine(III) to the gold catalyst.6 The results of the present investigation show that the same is true for the palladium catalyst (Figure 2). In order for the alkynyl transfer to occur, the π complex 12 should be initially formed. The relative Gibbs free energy of the transition state for the alkynyl transfer

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RESULTS AND DISCUSSION Waser and co-workers showed that the oxyalkynylation reaction has been achieved using Pd(hfacac)2 as the precatalyst. It is expected that as the alkene and alkyne substrates are introduced, different adducts are formed via opening up of a hfacac ligand from an η2 to an η1 binding mode. All possible 7319


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Figure 1. Energy profile calculated for the alkynylation of the R1 group through formation of a Pd(IV) intermediate. The relative Gibbs and electronic energies (in parentheses) obtained from the M06/BS2//B3LYP/BS1 calculations in dichloromethane are given in kcal/mol. Complex 1 is taken as the reference point.

(TS12−13) is calculated to be only 11.4 kcal/mol.22 After passing through the transition structure, the key intermediate 13 is formed. The structure of 13 can be described by two bonding modes, 13a and 13b in Figure 2. For 13a, we can say that the alkynyl transfer leads to formation of a π complex in which the cationic iodine(III) moiety is stabilized by interaction with one of the Pd-bound alkynyl π orbitals. For the other extreme (13b), we can say that the alkynyl transfer leads to a [1,2]iodine(III) shift process to give a palladium vinylidene complex. The calculations for 13 show a longer distance of 2.851 Å for the I−Ca bond versus 2.295 Å for the I−Cb bond (Figure 3) and a smaller Wiberg bond index of 0.122 for the I−Ca bond versus 0.557 for the I−Cb bond. The Ca−Cb bond in 13 with a Wiberg bond index of 2.241 has more double-bond character. These results suggest that 13b should be the predominant contributor to the bonding of the real structure. The dominance of 13b (a vinylidene form) can be explained in terms of the π-back-donating ability of the palladium metal center. Indeed, the interaction of a higher lying dπ orbital on Pd with the Ca pπ orbital polarizes the Ca−Cb π bond toward Cb, leading to a strengthening of the I−Cb interaction and a weakening of the I−Ca interaction, thereby enhancing the contribution of the vinylidene form to 13. On the basis of the second-order perturbation approach, an energy of 33.8 kcal/ mol is calculated for the charge transfer from the Pd dπ orbital to the Ca pπ orbital in 13. A combination of factors such as the overall neutral charge on the complex, the π-donating character of the η2-O-bound hfacac ligand, and the presence of the

electron-rich alkyl ligand (R1) renders the Pd(II) in 13 a good π-donor metal center. These electronic factors might explain why the Pd(II) vinylidene-like complex 13 with respect to 12 has a moderate stability; Lynam, Fey, and co-workers found in their theoretical study that the electron-donating ligands increase the stability of vinylidene tautomers by promoting metal to vinylidene ligand back-donation.23 Once complex 13 is formed, 1,2-migration of the alkyl group (R1) to the Ca atom24 through three-center transition structure TS13−14 affords the vinyl complex 14. This process, which involves the coupling of two carbon atoms, proceeds with an activation barrier as low as 6.7 kcal/mol. The low activation barrier can be attributed to the high electrophilicity of the vinylidene Ca atom,25 which in turn aids in facilitating the nucleophilic migration of the alkyl group (vide infra). Recently our group reported that an iodine(III) gold(I) vinyl complex is susceptible to a redox reaction through interaction of the Au−C σ orbital with the I−C σ* orbital.6 This process results in the oxidation of the I−Cb bond and the reduction of iodine(III) to iodine(I). Consistent with this earlier study, we found that the Pd(II) vinyl complex 14 is also capable of undergoing the same redox reaction via transition structure TS14−15 with a moderate activation barrier of 16.7 kcal/mol. Due to the interaction of the Pd−Ca σ orbital with the I−Cb σ* orbital, the Pd−Ca, I−Cb, and I−O bonds in TS14−15 are significantly elongated with respect to those in 14 (Figure 3), and eventually the bonds are heterolytically cleaved in the ion pair 15.26 The heterolytic bond cleavage via the electron flow 7320


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Figure 2. Energy profile calculated for the alkynylation of the R1 group through formation of a vinylidene-like intermediate. The relative Gibbs and electronic energies (in parentheses) obtained from the M06/BS2//B3LYP/BS1 calculations in dichloromethane are given in kcal/mol. Complex 1 is taken as the reference point.

Figure 3. Optimized structures with selected structural parameters (bond lengths in Å) for 13, 14, and TS14−15.

palladium remains unchanged at +2 during the completion of the catalytic reaction. These results not only suggest that the reaction does not proceed via a Pd(IV) intermediate but also confirm that, in agreement with the experimental observations, no Pd(0) complex is formed in this process; Waser and coworkers proposed that the oxyalkynylation is less likely to go via a Pd(0) intermediate owing to the tolerance of the reaction to the bromide substituents. Formation of the vinylidene-like intermediate 13 from the πcomplex 12 is an interesting feature from the point of view of an organometallic chemist. While the rearrangement of terminal alkynes to vinylidene complexes is a common process,25,27 it is rarely reported for internal alkynes.28 The present study indicates that an internal alkyne reagent such as 2 is highly prone to rearrangement.

diagram outlined for 14 in Figure 2 results in the formation of a new π bond between the Ca and Cb atoms and the reduction of iodine(III) to iodine(I). From the ion pair 15, the aromatic carboxylate anion can be trapped by the 14e palladium cationic complex to give the product complex 11 at an energy of −47.5 kcal/mol. Subsequently, the organic product (prod) is dissociated from 11 and intermediate 16 is produced. The replacement of the hfacac ligand with the aromatic carboxylate group via a proton exchange reaction finally leads to regeneration of the catalyst 1. As seen from comparison of Figures 1 and 2, all the stationary points of the novel mechanism (Figure 2) lie below those of the redox couple mechanism (Figure 1), suggesting that the novel mechanism is more favorable in kinetic terms. It is also apparent from Figure 2 that the oxidation state of 7321


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Alternative Pathway for Formation of an Iodine(III) Palladium(II) Vinyl Complex. As presented above, formation of an iodine(III) palladium(II) vinyl complex provides an opportunity for oxidation of the C−I bond by the iodine(III) center. The insertion of the alkyne into the Pd−R1 bond in π complex 12 gives an analogous vinyl complex from which the redox chemistry can easily occur. The energy profile for this pathway is presented in Figure 4. The calculations show that

Figure 5. Spatial plots and orbital energies for (a) LUMO+2 of 12 and (b) LUMO of 13.

mechanisms are considered for carboxypalladation of the olefin. Figure 6 shows the possible pathways for this step. TS22−25, TS21−24, and TS20−23 are transition structures of the outersphere mechanism, and TS19−8 shows the transition structure for the inner-sphere mechanism. In the inner-sphere mechanism, the insertion of the alkene into a Pd−O covalent bond produces a Wacker-type intermediate, while in the outer sphere, the intermediate is formed via the nucleophilic attack of

Figure 4. Energy profile calculated for the alkynylation of the R1 group through formation of vinyl complex 17. The relative Gibbs and electronic energies (in parentheses) obtained from the M06/BS2// B3LYP/BS1 calculations in dichloromethane are given in kcal/mol. The complex 1 is taken as the reference point.

although the iodine-centered redox chemistry for this pathway is favorable via TS17−15, the formation of the vinyl complex 17 is energetically more demanding than that of 14. Indeed, the formation of the vinyl complex 17 requires an activation energy of 21.9 kcal/mol (Figure 4), 9.1 kcal/mol higher than the overall barrier for the formation of the vinyl intermediate 14 (ΔG⧧ = 12.8 kcal/mol) (Figure 2). This result implies that the carboxyalkynylation reaction is not likely to proceed through the alkyne insertion pathway and further supports the fact that the pathway leading to formation of the vinyl complex 14 (Figure 2) is the substantial part of the operative mechanism. At the end of this section, it is worth noting that the activation energy for the alkyl migration via TS12−17 (21.9 kcal/ mol, Figure 4) is much higher in energy than that via TS13−14 (6.7 kcal/mol, Figure 2). A molecular orbital rationalization can be used to explain this difference. The major factor that causes the alkyl migration to occur via TS12−17 (Figure 4) is the interaction of the Pd−R1 σ orbital with an alkyne π* orbital. In comparison, transformation 13 → 14 (Figure 2) is mainly caused by the interaction of the Pd−R1 σ orbital with the Pd− Ca π* orbital. Since the alkyne π* orbital in 12 lies much higher in energy than the Pd−Ca π* orbital in 13 (Figure 5), transformation 12 → 17 proceeds with a higher activation barrier. The high electrophilicity of the vinylidene Ca atom in 13 resulting from the low-lying Pd−Ca π* orbital facilitates the migration process via transition structure TS13−14. Carboxypalladation Reaction. Let us now turn our attention to the mechanism leading to the formation of a Waker-type intermediate. There are multiple routes for formation of this intermediate. Both outer- and inner-sphere

Figure 6. Possible reaction pathways for formation of a Wacker-type intermediate. The relative Gibbs and electronic energies (in parentheses) obtained from the M06/BS2//B3LYP/BS1 calculations in dichloromethane are given in kcal/mol. 7322


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the pendant carbonyl oxygen atom into the alkene’s C2 atom. Among the four transition structures shown in Figure 6, TS19−8, with a relative Gibbs energy of 24.1 kcal/mol, is the most stable, suggesting that the carboxypalladation is more likely to proceed via the inner-sphere mechanism.

reduction of the iodine(III) center by two units and formation of the product complex XI (step E). After releasing of the organic product, the aromatic carboxylate ligand is replaced by hfacac via a proton exchange reaction and the catalyst 1 is regenerated (step F). The carboxypalladation of the olefin (step A) through transition structure TS19−18 (Figure 6) was identified as the rate-determining step. It is of interest to note that the oxidation state of palladium remains unchanged at +2 during the completion of the novel catalytic cycle. This result perfectly explains the tolerance of the experimental reaction to the halide substituents and supports the thought that Pd(0) intermediates are not involved in the catalytic cycle. The novel mechanism may have important implications in the catalytic reactions that use the alkynyl benziodoxolones as a reagent and can offer insights into designing future reaction protocols.

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SUMMARY The Pd-catalyzed oxyalkynylation of olefins using an iodine(III) reagent (TIPS-EBX) was proposed by Waser and co-workers to proceed via a Pd(II)−Pd(IV) catalytic cycle due to the presence of the TIPS-EBX oxidant. Contrary to this expectation, the density functional theory calculations revealed that formation of a Pd(IV) intermediate is energetically inaccessible. As shown in Scheme 3, a novel mechanism was found to be

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Scheme 3

ASSOCIATED CONTENT

* Supporting Information S

Text giving the complete ref 8 and a table giving Cartesian coordinates of all optimized structures along with energies. This material is available free of charge via the Internet at http:// pubs.acs.org.

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This study is the result of a research project titled “Theoretical investigation into the reduction mechanism of PtIV to PtII through different pathways with the aim of producing new products”. The Author thanks the Islamic Azad University for providing funding to support this research project. The author also appreciates the Australian National Computational Infrastructure and the University of Tasmania for computing resources. The author thanks Prof. Allan Canty for his valuable advice during the completion of this study.

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REFERENCES

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responsible for the oxyalkynylation reaction. In this novel mechanism, the catalytic cycle for a carboxylic acid starts with the carboxypalladation of the olefin, most likely via an innersphere mechanism, to give the Wacker-type intermediate 8 (step A). 8 then undergoes a substitution reaction with EBX to produce XII (step B). The key intermediate XIII is formed by the alkynyl transfer from iodine(III) to Pd(II) (step C). The structure of XIII can be described by two bonding types, XIIIa and XIIIb. The calculations showed that the vinylidene form (XIIIb) is the predominant contributor to the corresponding bonding. Due to the high electrophilicity of the vinylidene Ca atom in XIIIb, the alkyl group (R1) easily migrates to Ca and generates XIV (step D). The interaction of the Pd−C σ orbital with the I−C σ* orbital in the vinyl complex XIV results in 7323


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