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The Gilded Edge in Acetylenic Scaffolding II: A Computational Study of the Transmetalation Processes Involved in Palladium-Catalyzed Cross-Couplings of Gold(I) Acetylides† Mie Højer Larsen* and Mogens Brøndsted Nielsen Center for Exploitation of Solar Energy, Department of Chemistry, University of Copenhagen, Universitetsparken 5, DK-2100 Copenhagen Ø, Denmark S Supporting Information *

ABSTRACT: The palladium-catalyzed cross-coupling reaction between phosphine-gold(I) acetylides and aryl iodides has recently proven as a convenient alternative to the standard Sonogashira reaction, which instead employs terminal alkynes as substrates. This alternative reaction does not require the presence of an amine base, but still, however, requires a copper cocatalyst (CuI). In this theoretical work we have investigated the possible roles that this copper catalyst may play. Three transmetalation pathways can be imagined, proceeding by either (i) transferring the acetylide from gold to copper and thereafter to palladium, (ii) directly transferring the acetylide from gold to palladium, or (iii) directly transferring the acetylide from gold to palladium but aided by a copper coordination to the triple bond. Calculations reveal that the first of these is the most viable reaction pathway, as it involves the initial formation of a very favorable copper/gold acetylide complex. The transmetalations along this pathway run via several equilibria.

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INTRODUCTION Metal-catalyzed cross-coupling reactions have become an essential process in organic synthesis for the formation of carbon−carbon bonds. Among the still growing arsenal of reactions is the palladium and copper cocatalyzed Sonogashira reaction. The reaction was first described in 1975 and enables the cross-coupling of a terminal acetylene with an aryl or vinyl halide (Scheme 1a).1 Over the years modifications of the

presence of gold in equimolar amounts, it has been shown that the copper cocatalyst is necessary in order for the reaction to proceed.4 This is in accordance with joint theoretical and experimental studies on the gold/palladium transmetalation in Stille- and Suzuki-type reactions,6 both of which were found to be thermodynamically unfavorable and having prolonged reaction times compared to the parent palladium-catalyzed reaction. Furthermore, the transmetalation itself has earlier been shown to proceed in near-quantitative yield from gold to copper.7 Conversely, Espinet and co-workers8 have shown that the transmetalation of bulky organic substrates from tin to palladium is eased in the presence of gold(I) complexes. DFT calculations showed that the gold is mediating aryl transfer in the tin/palladium transmetalation, so that the organic moiety first was transferred to gold and then to palladium. Thus, in this case the gold/palladium transmetalation was preferred to the tin/palladium one (Scheme 2a). A similar effect has been obtained experimentally for the Stille reaction by addition of copper salts.9 However, in some cases it has been shown that the kinetic benefit of the added copper(I) salt is simply due to the complexation of excess ligands by copper during the coupling reaction (Scheme 2b).10 Recently, the gold-aided Sonogashira methodology has found use in our group for the synthesis of arylated tetraethynylethenes (TEEs) as well as tetrathiafulvalene (TTF)-fused

Scheme 1. Original and the Modified Sonogashira CrossCoupling Reactiona

a

X = halogen.

traditional cross-coupling have been published, including methods using copper-free conditions.2 One of the more recent modifications is the use of gold(I) acetylides as the coupling partner for the aryl halide (Scheme 1b).3,4 By substitution of the terminal hydrogen with a triphenylphosphine gold(I) fragment a greater stability is achieved for the acetylene moiety. This increased stability is especially advantageous in cross-couplings involving the terminal alkyne of oligoynes or other large conjugated systems.3,5 Despite the © XXXX American Chemical Society

Received: February 17, 2015

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DOI: 10.1021/acs.organomet.5b00135 Organometallics XXXX, XXX, XXX−XXX

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Only the transmetalation processes will be investigated in this study, as the prior oxidative addition of the aryl halide to the Pd(0) catalyst and the final reductive elimination to give the coupling product will be the same for both pathways. The gold(I)-capped phenylacetylene 1 serves as a natural starting point for the considered transmetalation pathways, and thus it is the reference point for all energy comparisons. The ligands on both gold and palladium, PPh3, were modeled by PMe3. A number of reaction complexes involving gold, palladium, and copper were modeled with PPh3 and gave calculated reaction energies showing the same trends as those calculated with the use of the PMe3 ligand (see Table S1). A general trend is evident from the different corrections performed on the gas-phase-determined energies, namely, that the energies of the transition states and reaction complexes benefit from the dispersion correction, whereas the separate species and especially the tricoordinated palladium moieties benefit from the solvation correction. Throughout the text most reported and discussed values are the gas-phase values. However, for the key steps of the reaction the energy effect of both dispersion and solvent correction will be discussed in addition to the values obtained for the gas phase.

Scheme 2. Role of Copper(I) or Gold(I) in Palladium/TinCatalyzed Cross-Coupling Reactionsa

a

[S] denotes a solvent molecule.

radiaannulenes from TTF-bishalides and gold(I) end-capped TEEs.3 A copper(I) salt is needed in this reaction, and CuI was added in catalytic amounts to give the desired macrocycle in decent yield. We became curious as to the role of copper in the coupling; is copper involved in the gold/palladium transmetalation, and how, or is it aiding by coordinating the excess phosphine ligand from palladium?

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RESULTS AND DISCUSSION As stated in the Introduction, there are three possible scenarios for the formation of the transmetalated palladium product. This can happen by a direct transmetalation from gold to palladium; by two subsequent transmetalations from gold to copper and from copper to palladium; or by a direct transmetalation from a copper-complexed gold acetylide. Each of these transmetalations has been computationally investigated in the gas phase and in the experimentally applied solvents THF and TEA (Scheme 3). For the direct gold/palladium transmetalation between acetylide 1 and palladium complex 2, one of the phosphine ligands must leave the coordination sphere of palladium in order to create a vacant coordination site at the metal and for the transmetalation to take place affording complex 3 (Scheme 4). This loss of a ligand can take place either directly from complex 2 or from the π-complex [1;2]. Due to stabilizing interactions of the palladium with the π-system of the triple bond, the latter option is energetically more favorable, as the high-energy intermediate 3 is avoided. Direct loss of the

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COMPUTATIONAL DETAILS DFT calculations were used to investigate the transmetalation pathways discussed above and to evaluate the likelihood of one over the other. The geometry optimizations and transition-state searches were performed on the B3LYP level with the cc-pVDZ basis set.11,12 For copper,13,14 iodine,15 palladium,13,16 and gold,13,14 additional relativistic effective core potentials were used. All obtained energies were generally dispersion-corrected by Grimme’s DFT-D3 correction.17 Solvation energy corrections were calculated on the B3LYP level using the SMD model with tetrahydrofuran (THF) and triethylamine (TEA) as solvents. All calculations were performed with the Gaussian 09 program package.18 The calculated reaction profile was verified for each transition state by following the intrinsic reaction coordinate (IRC).19 The natural bond order (NBO) analyses were performed with the NBO 5.0 program package.20

Scheme 3. Three Possible Routes of Transmetalation: A Direct Route, a Stepwise Route, and a Direct Route with CuI as a Spectator

B


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Organometallics Scheme 4. Two Alternative Routes for the Loss of the Trimethylphoshine Ligand to Form Reaction Complex [1;3]a

a

The listed Gibbs free energies are in kcal/mol and are relative to the energy of the substrates, the parenthesized energies are obtained from D3 corrections, and the bracketed energies are for the solvated species in [THF] and in {TEA}, respectively.

Scheme 5. Transmetalation of Phenylacetylide Directly from Gold to Palladium via Transition State TS1a

a

The DFT-optimized transition-state structure of TS1 is shown on the bottom left. Relevant interatom distances are shown in red on the center structure. The NBO representation (bottom right) shows the energy gained by the indicated interactions (in kcal/mol) in green numbers and in blue the hyperbond distribution for the indicated bonds (in percent). For details on the given energies see Scheme 4.

phosphine ligand costs 15.7 kcal/mol, while loss of the ligand through complex [1;2] costs 8.8 kcal/mol. The gold to palladium transmetalation is achieved through the transition state TS1 from prereaction complex [1;3] as shown in Scheme 5. The activation energy of the direct transmetalation is 19.5 kcal/mol in the gas phase and found to be slightly higher in either of the two examined solvents (THF and TEA). Based on the interatomic distances between carbon and palladium (2.02 Å compared to 2.00 Å in [4;5]) and the NBO characterization of the interaction between the two as a σ-bond, the transition state is located very late along the reaction coordinate toward the product complex [4;5]. Thus, the iodide is almost distanced fully from the palladium (2.89 Å versus 2.94 Å in [4;5]), although the NBO analysis shows strong interactions between the iodide and the palladium moiety. The next steps of the direct transmetalation are the dissociation of the reaction complex [4;5] by loss of the coordinating gold-iodide fragment 5 and complexation of a

phosphine ligand to complete the palladium coordination sphere. The overall formation of the phenylacetylide palladium complex 6 is endothermic by 9.7 kcal/mol in the gas phase. Alternatively, the formation of the phenylacetylide palladium complex 6 can occur in two separate transmetalation steps: a gold to copper transmetalation followed by a copper to palladium transmetalation. Unlike the previously discussed direct transmetalation (via precomplex [1;3]), the formation of the initial dimetallic prereaction complex [1;CuI] is energetically favorable (ΔΔGgas = −33.0 kcal/mol) (Scheme 6). The activation energy necessary to reach transition state TS2 from the prereaction complex was found to be 17.9 kcal/mol in the gas phase, thus making the gold/copper transmetalation the single most energy requiring step of the double-transmetalation process when disregarding the final dissociation of CuI (see below). The calculated activation energy required to reach the transition state when including dispersion correction or solvation is ΔΔGD3 = 20.6 kcal/mol, ΔΔGTHF = 15.7 kcal/ mol, and ΔΔGTEA = 16.8 kcal/mol. Despite the near-equal C


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[5;7] (2.54 Å). Of further notice is the short intermetal distance of 2.52 Å and the stabilizing interaction of 23.0 kcal/ mol directly from copper to gold. This is in contrast to the interactions calculated for the gold/palladium and copper/ palladium systems, where all the interactions are characterized as interactions with the bonding or antibonding orbitals of the palladium−ligand bond. The subsequent loss of the gold moiety, coordination of palladium, and necessary vacation of a coordination site on palladium by loss of a phosphine ligand can occur by two different sequences as illustrated in Scheme 7. The lowest energy pathway was found for the sequence in which palladium coordinates to the copper acetylide and looses a ligand prior to loss of the gold salt 5, via the trimetal complexes [2;5;7] and [3;5;7]. The presence of the trimetallic species [2;5;7] was further verified by taking dispersion (D3) effects into account during the optimization.22 The reoptimization yielded a geometry similar to the one found without the use of D3 during the optimization. However, the three species were tighter coordinated in the reoptimized structure, benefiting from the van der Waal interactions. The second transmetalation, from copper to palladium, occurs through the transition state TS3 and requires little energy to proceed (ΔΔGgas = 2.3 kcal/mol) (Scheme 8). This low activation energy is in accordance with a previously published theoretical study of the traditional Sonogashira reaction.21 Analysis of the transition-state structure shows a transition state nicely centered between the pre- and postreaction complexes. Thus, the NBO analysis predicts strong interactions between the acetylide and both palladium and copper as well as backbonding from the metals toward the πsystem of the acetylide. The interatomic distances from the acetylide carbon and the metals are likewise comparable and intermediary to the distances found for the respective σ- and πcoordinated species in complexes [3;7] and [4;CuI]. However, as noted for the previous two transition states, describing the reaction coordinate position of the iodide in terms of an early or late transition state is difficult. Thus, the copper/iodine distance is practically unaltered from [3;7] through TS3 to [4;CuI].

Scheme 6. Transmetalation of Phenylacetylide from Gold to Copper via Transition State TS2a

a

The DFT-optimized transition-state structure of TS2 is shown below. For further details and color coding see Scheme 5.

activation energies for the transmetalation, it should be noted that the incorporation of TEA as the solvent in general stabilizes the copper-containing complexes to a greater extent than THF does. The transition state can, once again, be characterized as a late transition state. The distance from the acetylide-carbon to the copper is equal to that found for the reaction complex [5;7]. The NBO analysis does not show an actual σ-bond, but it does predict strong interactions between the two fragments. However, the iodide is likewise interacting strongly with the copper, and the interatomic distance is intermediary to the ones found in copper iodide (2.38 Å) and the reaction complex

Scheme 7. Two Alternative Routes for the Loss of the Trimethylphoshine Ligand to Form Reaction Complex [3;7]a

a

For details on the given energies see Scheme 4. D


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coordination to the triple bond seems to enhance the carbon− palladium interaction. Thus, unlike in complex [1;3] both palladium and gold are in close proximity to the terminal carbon of the triple bond. It is from this complex, [1;CuI;3], that the gold moiety is lost as Me3P−Au−I via transition state TS4. The relative energies calculated for this pathway are also reminiscent of the energies found for the simple gold to palladium transmetalation. Only the starting point is shifted −33.0 kcal/mol down in energy because of the initial favorable π-coordination of the copper iodide to the triple bond. Analyzing the transition state in terms of interatom distances and NBO it is seen once again that the copper iodide spectator does not change the geometry nor the complex interactions significantly compared to transition state TS1. Thus, the greatest change of bond lengths is found for the gold−iodine distance, which is elongated by 11 pm to 2.92 Å in TS4. Likewise, the NBO analysis shows strong similarities to the analysis of TS1. The transition state can once again be termed as late when regarding the interatomic distances and the NBO bonding picture. Thus, the bond between the acetylide and the palladium is characterized as a fully formed σ-bond and the iodine is well distanced from the palladium. However, as with TS1 the iodine interacts stronger with the palladium moiety than it does with the gold. Differences include the increased donations from the acetylene and phenyl palladium−carbon bonds toward the phosphine and the appearance of a strong donation in the opposite direction. Furthermore, the NBO analysis shows a strong interaction between the triple bond and the copper as well as a back-donation from the copper into the π-system. Looking at the energy profile of the copper-aided direct transmetalation, the presence of copper lowers the energies of all the reaction complexes to some extent. Although the energy gain is not equal for all involved complexes, the overall energy profile does not change significantly. Thus, the energy requirement for the transmetalation is 22.8 kcal/mol in the gas phase and 27.1 or 25.0 kcal/mol in THF and triethylamine, respectively. As was seen for the parent direct transmetalation, the reactions leading to transition state TS4 consist of two different preequlibria. However, unlike for the parent reaction, the copper-aided direct transmetalation shows no preference for dissociation of palladium complex 2 prior to or after complexation to the acetylene complex. Both of the possible reaction pathways lie lower in energy than prereaction complex [1;CuI;3].

Scheme 8. Transmetalation of Phenylacetylene from Copper to Palladium via Transition State TS3a

a


In accordance with the previously discussed alternative routes for the loss of a trimethylphosphine ligand, the recomplexation of the ligand and the loss of copper iodide from reaction complex [4;CuI] can occur by two separate reaction pathways. The more favorable and exothermic path was found to be keeping copper iodine coordinated until palladium once again is tetracoordinated (Scheme 9). Finally, the favorability of forming the initial dimetallic complex [1;CuI] led us to consider the possibility of exploiting the energy gain of the π-coordination, but without involving copper in the transmetalation itself, thus letting copper iodide act solely as a spectator. Each of the steps of the transmetalation was found to be structurally equal to those already discussed for the direct transmetalation (Schemes 4 and 5), but with the addition of a π-coordinating copper iodide at the carbon−carbon triple bond (Scheme 10). The only exception was the prereaction complex [1;CuI;3], in which the copper

Scheme 9. Two Alternative Routes for the Complexation of the Trimethylphoshine Ligand to Form the Reaction Product 6a

a

For details on the given energies see Scheme 4. E


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Scheme 10. Reaction Route for the Copper-Aided Direct Transmetalation from Gold to Palladium via Transition State TS4a

a


are lower, in particular when including solvation, than those energies required to form TS1 in the direct transmetalation route, 19.5 and 24.2 kcal/mol in the gas phase and THF, respectively. This conversion is followed by conversion into the high-energy species [3;5;7] in steps that are assumed to be under diffusion control. The overall energy required to form [3;5;7] is 20.5 and 24.7 kcal/mol in the gas phase and THF, respectively. These energies are similar to those required to form the TS1 species, but this comparison is somewhat misleading, as it ignores the favorable and assumable fast formation of [1;CuI] from 1 and the catalyst CuI, which is constantly recycled. The final high-energy barrier for dissociation of the CuI from acetylide 6 can presumably be disregarded, as the transfer of copper is likely to happen directly from complex [6;CuI] to 1 or after reductive elimination from [6;CuI], and formation of 6 as an intermediate is thereby avoided. The latter scenario will furthermore pull the many equilibria toward product formation, as the reductive elimination is irreversible. The alternative route (green curve), where [1;CuI] is transformed to [4;CuI;5] via TS4, seems less viable, as formation of TS4 requires 22.8 kcal/mol in the gas phase and 27.1 kcal/mol in THF. Even with the previously discussed higher stabilization by triethylamine for

It was noted above that for the reaction complexes of the gold to copper transmetalation the incorporation of solvation by TEA gives a higher degree of stabilization of the complexes than seen with use of THF. This is also true for the reaction path in which copper acts as a spectator, as the copper iodide generally is sterically available for the solvent and benefits from interaction with the nitrogen of the TEA. Figure 1 shows the full energy profile of the transmetalation processes. The solid line depicts the lower energy pathways via multimetal complexes. Broken lines present the traditionally drawn reaction path of complete complex dissociations prior to the assembly of the next reaction complex. Focusing on the low-energy paths (the solid lines), the direct transmetalation from gold to palladium has an overall activation energy of 19.5 kcal/mol in the gas phase and 24.2 kcal/mol in THF (involving two preequilibria; red curve). For the stepwise transmetalation (blue curve), the initial formation of the gold/ copper complex [1;CuI] results in an energy release of 33.0 kcal/mol in the gas phase and 17.3 kcal/mol in THF. If we first disregard the energy-requiring dissociation of CuI from [6;CuI] in the final step of the stepwise transmetalation route, the initial formation of TS2 is the most energetic step in the sequence of consecutive equilibria toward [6;CuI], requiring 17.9 kcal/mol in the gas phase, but only 15.7 kcal/mol in THF. These values F


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Figure 1. Full reaction profile of the transmetalation processes either directly from gold to palladium (in red) or from gold via copper to palladium (in blue) and the copper-aided direct transmetalation (in green).

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the copper-containing complex the energy requirement is 25.0 kcal/mol.

ASSOCIATED CONTENT

S Supporting Information *

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Computational details, structures, and full Z-matrix for all calculated species. The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/ acs.organomet.5b00135.

CONCLUSION In conclusion, our calculations show that the Sonogashira-type cross-coupling reaction carried out on gold(I) acetylides is most likely to proceed by a copper-mediated stepwise transmetalation. The alternative, namely, a direct transmetalation from gold to palladium, required an activation energy of 19.5 kcal/mol in the gas phase and 24.2 kcal/mol in THF. This transition-state energy would not be influenced by a previous involvement of the copper catalyst in coordinating excess phosphine ligand. Instead, the formation of palladium acetylide complex is achieved through two consecutive transmetalations. When disregarding the final loss of copper iodide, the ratedetermining step is the first transmetalation from gold to copper with an activation energy of 17.9 kcal/mol in the gas phase and 15.7 kcal/mol in THF. Our calculations clearly show that the role of the added copper catalyst is mediating the transmetalations. The high energy requirement found for the dissociation of the final complex makes this the ratedetermining step of the stepwise transmetalation process. However, the subsequent reductive elimination might as well happen from the palladium/copper complex [6;CuI] with concomitant transfer of CuI to the alkyne substrate. Either way, when viewed isolated the rate of the transmetalation should benefit from increased copper loadings. This hypothesis is the subject of an ongoing experimental study.

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AUTHOR INFORMATION

Corresponding Author

*Mie Højer Larsen née Mie Højer Vilhelmsen. E-mail: mhv@ kiku.dk. Notes

The authors declare no competing financial interest. † For the first part of this series see ref 3.

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ACKNOWLEDGMENTS Center for Exploitation of Solar Energy of University of Copenhagen is acknowledged for financial support, and the Danish Center for Scientific Computing for providing computer resources. We thank Dr. Christian R. Parker and Profs. Henrik G. Kjærgaard and Preben Graae Sørensen for helpful discussions.

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REFERENCES

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Cossi, M.; Rega, N.; Millam, J. M.; Klene, M.; Knox, J. E.; Cross, J. B.; Bakken, V.; Adamo, C.; Jaramillo, J.; Gomperts, R.; Stratmann, R. E.; Yazyev, O.; Austin, A. J.; Cammi, R.; Pomelli, C.; Ochterski, J. W.; Martin, R. L.; Morokuma, K.; Zakrzewski, V. G.; Voth, G. A.; Salvador, P.; Dannenberg, J. J.; Dapprich, S.; Daniels, A. D.; Farkas, O.; Foresman, J. B.; Ortiz, J. V.; Cioslowski, J.; Fox, D. J. Gaussian 09, Revision B.01; Gaussian, Inc.: Wallingford, CT, 2010. (19) (a) Fukui, K. Acc. Chem. Res. 1981, 14, 363−368. (b) Gonzalez, C.; Schlegel, H. B. J. Chem. Phys. 1989, 90, 2154−2161. (c) Gonzalez, C.; Schlegel, H. B. J. Phys. Chem. 1990, 94, 5523−5527. (20) Glendening, E. D.; Badenhoop, J. K.; Reed, A. E.; Carpenter, J. E.; Bohmann, J. A.; Morales, C. M.; Weinhold, F. NBO 5.0; Theoretical Chemistry Institute, University of Wisconsin: Madison, WI, 2001; http://nbo.chem.wisc.edu/ (accessed July 10, 2015). (21) Sikk, L.; Tammiku-Taul, J.; Burk, P.; Kotschy, A. J. Mol. Model. 2012, 18, 3025−3033. (22) The geometry optimization with dispersion correction was run with the Gaussian 09, Revision D.01, program package. Same authors as ref 18; Gaussian, Inc.: Wallingford, CT, 2013.

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