Mechanism of Pd-Catalyzed ArâAr Bond Formation Involving Ligand

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Mechanism of Pd-Catalyzed Ar−Ar Bond Formation Involving Ligand-Directed C−H Arylation and Diaryliodonium Oxidants: Computational Studies of Orthopalladation at Binuclear Pd(II) Centers, Oxidation To Form Binuclear Palladium(III) Species, and Ar···Ar Reductive Coupling Allan J. Canty,*,† Alireza Ariafard,†,‡ Melanie S. Sanford,*,§ and Brian F. Yates*,† †

School of Chemistry, University of Tasmania, Private Bag 75, Hobart, Tasmania 7001, Australia Department of Chemistry, Faculty of Science, Central Tehran Branch, Islamic Azad University, Shahrak Gharb, Tehran, Iran § Department of Chemistry, University of Michigan, 930 North University Avenue, Ann Arbor, Michigan 48109, United States ‡

S Supporting Information *

ABSTRACT: A computational analysis of the Pd-catalyzed coupling of 3-methyl-2phenylpyridine (mppH) with [Ph2I]BF4 to form mppPh is supportive of a prior synthetic and kinetic study implicating binuclear palladium species in a rate-limiting oxidation step. The Pd(OAc)2 precatalyst forms the “clamshell” orthopalladated complex [Pd(mpp)(μ-OAc)]2 (8) as the active catalyst, which is oxidized by [Ph2I]+ in a reaction having the highest energy requirement of all steps in the catalytic cycle. In the oxidation reaction, involving formal transfer of Ph+, the electrophilic iodine center interacts initially with a bridging acetate oxygen atom of [Pd(mpp)(μ-OAc)]2 (8), “Pd−O···IPh2”, which transforms to a transition structure with retention of the O···I interaction and formation of a “Pd(μ-Ph-η1)I” bridge in a four-membered ring, “Pd···Ph···I(Ph)···O−Pd”, followed by elimination of PhI with formation of a binuclear Pd(III) cation containing a Pd−Pd bond, [Ph(mpp)Pd(μ-OAc)2Pd(mpp)]+ (14). Cation 14 undergoes mpp···Ph coupling at one Pd center to form the binuclear Pd(II) cation [(mppPh-N)Pd(μOAc)2Pd(mpp)]+ (Da). Cation Da may fragment to release mppPh and mononuclear palladium species, followed by orthopalladation at a mononuclear center. However, in an environment of very low acetate concentration and high nitrogendonor concentration, it is considered far more likely that Da undergoes ligand exchange with release of mppPh and formation of [(mppH-N)Pd(μ-OAc)2Pd(mpp)]+ (I). Computation shows a low-energy pathway for orthopalladation at cation I that involves nitrogen-donor reagents mppH and mppPh acting as bases to remove a proton as [HN-donor]+. This orthopalladation would complete the cycle and regenerate the catalyst, [Pd(mpp)(μ-OAc)]2 (8). A Hammett plot obtained from a computational analysis of the reaction of [(p-X-C6H4)(Mes)I]BF4 (X = H, Me, OMe, F, Cl, COMe, CF3) has a reaction constant (ρ) of 1.8, which compares well with the experimental result (ρ = 1.7 ± 0.2). Consistent with this, the analysis reveals the dominant role of the interaction energy for palladium- and iodine-containing fragments in the transition structure.

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INTRODUCTION Diorganoiodine(III) reagents are widely used in Pd0/PdII mediated catalysis in which the reagents, typically [RPhI]+ (R = aryl, alkenyl, alkynyl) oxidatively transfer “R+” to Pd0 to form RPdII species with concurrent release of PhI.1,2 There is also an emerging role for diorganoiodine(III) reagents in stoichiometric organic synthesis involving detected Pd(IV) intermediates.3,4 In these latter reactions, Pd(III) and/or Pd(IV) species have been proposed4,5a−f as intermediates or considered as alternatives to Pd(0) and Pd(II)5g (Scheme 1). The involvement of oxidation states greater than II for palladium in catalysis is supported by several studies of stoichiometric reactions. Prominent among these is the synthesis of benzofurans using [alkenyl(phenyl)I]+ reagents where an alkenylpalladium(IV) intermediate has been detected by NMR spectroscopy.3 Also, for the reaction of [Pd(bzq)(μ© XXXX American Chemical Society

Scheme 1. Reaction of Diorganoiodine(III) Reagents with Palladium Species Resulting in Transfer of “Ph+” and Release of Iodoarene

OAc)]2 (bzq = benzo[h]quinolinyl) (1) with “CF3+” reagents, such as 2, the complex PdIV(CF3)(bzq)(OAc)2(OH2) (3) has been isolated and found to decompose in a range of solvents to form bzq-CF3 (4) (Scheme 2).4 Palladium(IV) species formed upon “R+” transfer have also been detected by NMR in the stoichiometric reaction of [Me3SiCC(Ph)I]+ with Pd(II) reagents, such as Pd(pincer-N,C,N)(OAc) (pincer = [2,6Received: October 28, 2012

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dx.doi.org/10.1021/om301013w | Organometallics XXXX, XXX, XXX−XXX

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Kinetic and other evidence is supportive of orthopalladation of mppH to give [Pd(mpp)(μ-OAc)]2 (8), followed by the formation of a higher oxidation state species upon transfer of “Ph+” from [Ph(Mes)I]+ (11a). Subsequent reductive elimination is then proposed to result in release of the product, mppPh (9).5c NMR and kinetic data demonstrate the presence of two resting states: a monomeric Pd(II) complex (12) and a complex (13a) formed by binding of mppH to [Ph(Mes)I]+.5c The protocol in Scheme 3 has been extended to include a wide range of substrates capable of undergoing cyclopalladation and shown to tolerate the presence of enolizable ketones, aldehydes, esters, amides, benzylic hydrogens, and aryl-halogen bonds in the substrate.5c The overall mechanism in Scheme 3 is supported by several pieces of data: (i) independent observation of orthopalladation to form [Pd(mpp)(μ-OAc)]2 (8) in the absence of [Ph(Mes)I]BF4, (ii) the stoichiometric reaction of 8 with [Ph(Mes)I]BF4 to form mppPh, (iii) kinetic studies indicating that a binuclear palladium species is involved in the turnover-limiting step of catalysis, (iv) a Hammett plot consistent with oxidation as the turnover-limiting step, and (v) the observation of an intramolecular, but not an intermolecular, kinetic isotope effect consistent with C−H activation occurring during the catalytic cycle, but after the turnover-limiting step.5c Potential binuclear intermediates that are consistent with the experimental evidence include palladium(III) species (14, 15) and species containing both Pd(IV) and Pd(II) centers (16, 17).

Scheme 2. Formation of Pd(IV) Complexes (3, 6, 7) on the Reaction of Pd(II) Complexes with Iodonium Reagents. Reductive Elimination from 3 Is Also Illustrated, Together with the Stoichiometric Reaction of 8 with [Ph2I]BF4 to give 9

(dimethylaminomethyl)phenyl] − ) 6a,b and PdMe(p-Tol)(dmpy) (dmpy = 1,2-(bisdimethylphosphino)ethane),6c as well as in the reaction of [Ph2I]+ with substrates, such as PdMe2(bipy) (bipy = 2,2′-bipyridine) (5), to give isomers 6 and 7.7 In a closely related stoichiometric transformation where an intermediate has not been detected, the binuclear complex 8 reacts with [Ph2I]+ to afford 9 in 90% yield.5a Evidence has been presented for the involvement of Pd(IV) intermediates in reactions catalyzed by pincer complexes, such as Pd(pincer-N,C,N)(OAc).8 Additionally, experimental studies implicate the involvement of binuclear Pd(III) and/or Pd(IV) intermediates in reactions of 3-methyl-2-phenylpyridine (mppH) (10) facilitated by Pd(II) acetate (Scheme 3).5c Scheme 3. Proposal for Catalysis Involving Ar···Ar Coupling in the Presence of 5 mol % Pd(OAc)2, Based on Synthetic and Kinetic Studies, Illustrated for [Ph(Mes)I]+ as Oxidant5c

We report here a computational evaluation of feasible mechanisms for the stoichiometric reaction of [Pd(mpp)(μOAc)]2 (8) with [Ph2I]BF4 (Scheme 2), and extension to include all steps in the catalytic cycle of Scheme 3. In examining catalysis, potential mononuclear species as intermediates have been considered in addition to the binuclear species implicated from kinetic studies. The resting state detected by NMR (12) has been examined, together with Pd(mpp)(K2-OAc) (18) as a species that may be formed on the orthopalladation pathway to 8 or upon fragmentation of 8. In addition to the Pd(II) dimer 8, an intermediate with the molecular formula of 8, but with separated Pd(II) centers, has been examined (19). All of the results from this computational study are supportive of direct arylation of 8 to form the binuclear Pd(III) cation [Ph(mpp)Pd(μ-OAc)2Pd(mpp)]+ (14), leading us to confine studies of C···C coupling to processes from binuclear species. We have also explored the potential for fragmentation of binuclear Pd(III) species to form mononuclear Pd(II) and Pd(IV) species, as demonstrated to be a likely pathway for the reaction B


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of 1 with “CF3+” to form 3 (Scheme 2).5d Potential pathways for orthopalladation have also been examined.

pathway. Several of the reaction schemes investigated exhibit two possible pathways, and the lower-energy pathways are indicated in red in energy profiles. Energies ΔG (ΔH) in kJ mol−1 are presented, although discussion is confined to considerations of ΔG. Preliminary Considerations for the Pd-Catalyzed Reaction of mppH with Diaryliodonium(III) Tetrafluoroborates. The palladium acetate precatalyst is anticipated to be present as Pd3(OAc)6 in acetic acid,14 without dissociation of acetate ligands,14c and it is expected that the precatalyst contains a nitrite impurity Pd3(OAc)5(NO2).14c The precatalyst is present in a mole ratio of 1:20 relative to each organic reagent, and the initial orthopalladation reaction by palladium acetate depletes the amount of acetate present initially, as deprotonation of mppH gives acetic acid and a 1:1 ratio of Pd/ OAc in the orthopalladation product (eq 1). This 1:1 ratio is retained in the resting state Pd(mpp)(OAc)(mppH) (12) as well as in the other intermediates formed throughout the catalytic cycle. We have assumed that the acetate ion concentration is low relative to other species in the catalytic cycle. We believe that this is a reasonable assumption based on the energies obtained by computation for the dissociation of acetic acid (eq 2). We have employed the hydrogen-bonded ion [AcO·H·OAc]− as the model for dissolved acetate, as it occurs 30.0 kJ mol−1 lower than “OAc− + 1/2[AcOH]2”. The weak base [BF4]− and nitrogen-donor ligands are present in high concentration; however, these species are considered to be inactive in generating acetate ion by reaction with acetic acid in the nonpolar environment, as illustrated in eqs 3−5.

The results reported here (i) are consistent with the role of binuclear 8 to form [Ph(mpp)Pd(μ-OAc)2Pd(mpp)]+ (14), which decomposes via mpp···Ph coupling; (ii) support the feasibility of the complete catalytic cycle in Scheme 3, including occurrence of orthopalladation at a binuclear Pd(II) species where abstraction of H+ from the coordinated nitrogen donor is facilitated by interaction with a base (mppH and mppPh); and (iii) indicate that fragmentation of binuclear Pd(III) species to form mononuclear Pd(IV) and Pd(II) species in the presence of additional acetate ion is unlikely for this system but needs to be considered as a competing mechanism in related transformations.

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EXPERIMENTAL SECTION

Computational Details. Computation was carried out with identical procedures to our recent studies of binuclear arylpalladium chemistry in order to allow a more direct comparison of results with previous reports.9 Dichloromethane was used as a solvent for single point calculation in the earlier studies to model synthetic studies in this solvent. In the present case, acetic acid was used as a solvent for catalysis,5a,c but we used dichloromethane for computation to retain the ability to more directly compare structures with previous work and also in view of similar dielectric constants for dichloromethane (4.7113) and acetic acid (6.2528). Gaussian 0910a was used at the M0610b−d level of density functional theory (DFT) in view of the ability of the M06 functional to examine species in which metal···metal and noncovalent interactions between proximal aromatic groups may be present.10−12 The effective core potential of Hay and Wadt with a triple-ξ valence basis set (LAN2TZ) was used to describe Pd,10e−g the 6-31G(d) basis set was used for other atoms,10h and a polarization function of ξf = 1.472 was also added to Pd10i to form basis set BS1. Frequency calculations were carried out at the BS1 level. To further refine energies obtained from the M06/BS1 calculations, we carried out single-point energy calculations for all structures with a larger basis set (BS2) utilizing the quadruple-ξ valence def2-QZVP10j basis set on Pd along with the corresponding ECP and the 6-311+G(2d,p) basis set on other atoms. Energies in dichloromethane were calculated using BS2 on gas phase optimized geometries with the CPCM solvation model.10k To estimate the corresponding Gibbs free energies in dichloromethane (ΔG), entropy corrections were calculated at the gas phase M06/BS1 level and added to the solvent potential energies,10l adjusted by the method proposed by Okuno.13 Basis set superposition errors (BSSEs, counterpoise method)10a for the acetate ligand were evaluated, and natural population analyses (NPA) for selected species were performed in conjunction with natural bond order analyses (NBO3).10a All transition structures contained one imaginary frequency, exhibiting atom displacements consistent with the anticipated reaction pathway. The nature of transition structures was confirmed by intrinsic reaction coordinate (IRC) searches, vibrational frequency calculations, and potential energy surface scans. Conformational effects were encountered throughout this study, and the general approach to this phenomenon is described in the Supporting Information. Structures were explored using intrinsic reaction coordinate (IRC) analyses for transition structures and potential energy scans varying dihedral angles, for example, N−Pd−Pd−N angles, and C(bzq)−Pd−O−C for axial acetate groups.

2mppH + 2Pd(OAc)2 → [Pd(mpp)(μ‐OAc)]2 + [AcOH]2

(1)

[AcOH]2 → [AcOH 2]+ + [OAc]− ΔG = 241.2 kJ mol−1

(2)

mppH + [AcOH]2 → [mppH2]+ + [AcO· H· OAc]− ΔG = 75.9 kJ mol−1

(3)

mppPh + [AcOH]2 → [mppPhH]+ + [AcO·H ·OAc]− ΔG = 69.0 kJ mol−1

(4)

2[BF4 ]− + [AcOH]2 → [H(BF4)2 ]− + [AcO· H· OAc]− ΔG = 161.9 kJ mol−1

(5)

Although acetate is likely to be present only at very low concentration, we have explored potential roles for [OAc]− in this study in view of the demonstrated role of this ion in promoting fragmentation of binuclear palladium(III) species,5d and the frequent presence of acetate in other areas of palladium catalysis. The oxidation reaction is discussed first, followed by considerations of reductive elimination to form mppPh, the potential role of fragmentation, and an exploration of orthopalladation mechanisms. Oxidation of 8 by [Ph2I]BF4. Although the mechanistic studies implicate oxidation of [Pd(mpp)(μ-OAc)2]2 (8) or a related binuclear species without a Pd···Pd interaction (19) in the turnover-limiting step, we have also examined two mononuclear species for comparison. The first is the monomer Pd(mpp)(K2-OAc) (18), which may be on the orthopallada-

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RESULTS AND DISCUSSION Detailed discussion of geometries and bonding is limited to those species considered to be the most relevant in the catalytic C


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a model for undetected intermediates in Heck coupling.8b The transition structure identified for this reaction contains a “Pd(μPh-Cipso)I” bridge, and there is a weak “I···O3SCF3” interaction opposite Cipso.8b Thus, transition structures containing the analogous μ-Ph bridging moiety are also explored in the current work. Transition structures with the “Pd···Cipso···I” motif were trialed, followed by successful IRC calculations, illustrated for the binuclear reagents in Figure 1. Two pathways were found for the direct reaction of the dimer [Pd(mpp)(μ-OAc)]2 (8), for which the transition structures are 86.6 and 100.9 kJ mol−1 above the pair of reagents (8 + 20). The transition structures show an O···I interaction for the low energy pathway, TS_Ca (2.734 Å; ∑(van der Waals radii) 3.5 Å16), and a weak I···N interaction for the higher-energy pathway, TS_Cb (3.170 Å). The IRC calculation for TS_Ca leads back to intermediate Ca retaining the O···I interaction (2.674 Å), and for TS_Cb leads back to intermediate Cb adopting a Pd···I interaction (∑(vdW) 3.61 Å). IRC calculations for both TS_Ca and TS_Cb lead forward to the same binuclear cation, [Ph(mpp)(μ-OAc)2Pd(mpp)]+ (14). For the pathway from binuclear 19, with well-separated palladium centers, energies of the transition structures TS_Aa and TS_Ab are 71.3−95.2 kJ mol−1 higher than those commencing from 8 (Figure 1a), and thus these pathways are unfavorable. The computed structures show some features related to those for the direct reaction from 8, for example, O···I interactions for Aa and TS_Aa. Intermediate Ab has a distorted square-planar geometry for iodine(III), and has the geometry expected for an additional weak interaction by Tshaped Ph2IX molecules,17 and product Ba has Pd···Pd 4.639 Å. The mononuclear reagents “Pd(mpp)(K2-OAc)” (18) and resting state anti-12 lead to pairs of transition structures with environments at palladium similar to those shown in Figure 1. These transition structures occur at significantly higher energy than the binuclear transition structure TS_Ca (+85.6 and 54.8 kJ mol−1, respectively) (Figure 1b). Thus, the mononuclear systems were deemed unlikely and are not discussed further (Supporting Information). We have also examined the effect of additional acetate coordination to the cationic transition structures for binuclear species shown in Figure 1a,b. The transition structures were found to be higher than that for TS_Ca by 83.6 and 52.7 kJ mol−1, respectively, and thus these systems are not discussed further (Supporting Information). On the basis of all of these calculations, it is considered highly likely that the oxidation reaction in catalysis occurs directly at the binuclear species [Pd(mpp)(μ-OAc)]2 (8) via intermediate Ca and transition structure TS_Ca. For the stoichiometric reaction of 8 with [Ph2I]BF4 (8 → 14) in the absence of mppH, the transition structure occurs ΔG (ΔH) 73.5 (22.9) kJ mol−1 relative to the reference “8 + [Ph2I]+”. In view of the significantly lower energy requirement to attain TS_Ca compared to all other transition structures examined, only this pathway and structures Ca and TS_Ca (Figure 2) were considered in more detail. Additional details of the transition structure are discussed in the following section. The intermediate Ca has an O···I contact involving an oxygen atom of a bridging acetate group (2.674 Å, ∑vdW 3.5 Å).16 This may be considered as a weak donor−acceptor interaction, noting also the polar nature of the interaction in Ca (charge on oxygen, −0.767; charge on iodine, +1.14 (natural population analysis data)). Formation of intermediate Ca can be

tion pathway to 8 or present as an intermediate by dissociation of 8. The second is the resting state detected by NMR studies, Pd(mpp)(OAc)(mppH) (12). Both species have the 1:1 Pd/ OAc ratio that is expected following initial orthopalladation at the palladium acetate precatalyst. The reagent [Ph2I]BF4 was chosen as a model for [Ph(Mes)I]BF4, as it also has been shown to react with 8 in an identical manner.5a,c Computation indicates that separation of the ions [Ph2I]+ and [BF4]− in CH2Cl2 solution is thermoneutral with “Ph2I···FBF3” containing the weakly interacting tetrafluoroborate ion (I···F = 2.423 Å) (separated ions lower by 0.4 kJ mol−1). Thus, [Ph2I]+ was adopted as the reference for the iodine(III) reagent in the direct stoichiometric reaction with [Pd(mpp)(μ-OAc)]2 (8) (Scheme 2). For the catalytic reaction, involving 5 mol % Pd(OAc)2 in the presence of mppH, NMR and kinetic studies led to assignment of the resting state for the oxidant as [Ph(Mes)I(mppH)]+ (13a). The reagent used here as a model for computation, [Ph2I]BF4, is known to form a 1:1 complex with pyridine (Keq = 20.2 M−1 at 24 °C in CH2Cl2).15 Our calculations confirm that the cation [Ph2I(mppH)]+ (20) lies 13.1 kJ mol−1 below “[IPh2]+ + mppH” in CH2Cl2. Computation of the equilibrium forming the resting state 12 (Scheme 3), including consideration of isomers anti-12 (acetate and mppH groups oriented above and below the palladium coordination square plane) and syn-12 (both oriented above), indicate that both of these species are higher in energy than [Pd(mpp)(μ-OAc)2]2 (8) and that the anti isomer is favored (anti-12 18.1 and syn-12 30.1 kJ mol−1 relative to 8 + mppH). Although 12 lies at a higher energy than 8, its detection by NMR during catalysis presumably results from the high relative concentration of ligand in the kinetic studies carried out under initial rate conditions. At later stages in the reaction, ligated mppH is likely replaced by mppPh, forming anti-[Pd(mpp)(mppPh)(OAc)] (21). However, it is important to note that computation provides an energy value well above the analogous mppH resting state anti-12 (40.6 kJ mol−1 with respect to 8). The monomer Pd(mpp)(K2-OAc) (18) was found to be ΔG 51.1 kJ mol−1 higher in energy than the dimeric form [Pd(mpp)(μ-OAc)]2 (8).

In examining possible roles for reactants of the same stoichiometry as 8, but without a Pd···Pd interaction (19), we evaluated conformers of the eight-membered ring “(mpp)Pd(μOAc)2Pd(mpp)” with well-separated palladium centers. Two conformers were identified, both with energies significantly above 8 (61.6, 70.1 kJ mol−1). Thus, the lowest energy conformer, 19 (with a Pd···Pd distance of 4.556 Å), was utilized for studies of the oxidation reaction. Notably, for all of these computations of potential reagent and intermediate species, “[Pd(mpp)(μ-OAc)]2 (8) + [Ph2I(mppH)]+ (20)” is adopted as the reference in energy schemes. Previous computational studies of the interaction of diaryliodine(III) species with metal centers appear to be limited to a single investigation of the reaction between diphenyliodonium(III) triflate and PdII(pincer-N,C,N)(OAc) as D


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Figure 2. Structure of the intermediate Ca formed on reaction of [Pd(mpp)(μ-OAc)]2 (8) with [Ph2I(mpp)]+, and the transition structure TS_Ca formed from Ca on the pathway to [Ph(mpp)Pd(μ-OAc)2Pd(mpp)]+ (14).

shortest distance from Pd to the phenyl group immediately above the binuclear fragment is 3.148 Å for Pd···Cipso, indicating absence of an interaction on comparison with other species, for example, Da in Figure 5 exhibiting η2coordination with Pd···C 2.385 and 2.476 Å. The weak Pd···Pd bonding interaction present in binuclear orthopalladated arylpyridines adopting the “clamshell” configuration (estimated bond order ∼ 0.11)18 appears to be retained in this system during the oxidation process. The Pd···Pd distance when calculated with the M06 functional is found to be 2.907 Å for 8, decreasing through Ca (2.876 Å) and TS_Ca (2.832 Å) to form the Pd(III) complex [Ph(mpp)Pd(μOAc)2Pd(mpp)]+ (17) (2.766 Å). Reactions of Cyclopalladated Species with [(p-XC6H4)(Mes)I]BF4 and Considerations of Bonding Interactions in the Transition Structure. Experimental kinetic studies of initial rates for catalysis employed the aryl(mesityl)iodine(III) reagents [(p-XC 6 H 4)(Mes)I]BF 4 .5c We have thus also computationally examined the application of the two pathways shown in Figure 1b for the direct reaction of 8 with the unsymmetrical iodine(III) reagent [Ph(Mes)I(mppH)]+ (13a). Phenyl group transfer via transition states analogous to TS_Ca and TS_Cb was found, with ΔG⧧ 93.2 kJ mol−1 and ΔG⧧ 115.6 kJ mol−1, respectively, above “8 + [Ph(Mes)I(mppH)]+ (13a)”. Thus, a preference for the route involving O···I interactions is observed, as found for [Ph2I]+ (TS_Ca). The kinetic studies of the catalytic reaction of 8 allowed a Hammett plot analysis for a series of iodonium reagents [(pXC6H4)(Mes)I]BF4.5c A Hammett plot of initial rates, for various X compared with X = H, afforded a value of 1.7 ± 0.2,5c indicating acceleration by electron-withdrawing groups.19 Our computation was extended to all of the iodonium reagents examined experimentally, allowing construction of a Hammett plot by using computed ΔG⧧ data to calculate log[(kX/kH)]

Figure 1. Energy profile for the reaction of the diphenyliodonium cation with binuclear Pd(II) species (a) containing well-separated Pd(II) centers (19 → Aa, Ab → transition structures TS_Aa, TS_Ab → Ba, Bb), and (b) [Pd(mpp)(μ-OAc)]2 (8 → Ca, Cb → transition structures TS_Ca, TS_Cb → 14). The distance between Pd centers in 8 is not to scale with other structures. Energies ΔG (ΔH) in kJ mol−1, relative to “[Pd(mpp)(μ-OAc)]2 (8) + [Ph2I(mppH)] (20)”.

satisfactorily viewed in an essentially identical manner to that described by Hoffmann and co-workers for anions, such as chloride, as a donor.17 They note that the LUMO and LUMO +1 orbitals of [Ph2I]+ have shapes that are suitable for overlap with an X− σ orbital, leading to partial occupation of these orbitals and formation of a weak I···X interaction. For Ca, the E


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values. This approach requires determination of the relative energies of isomers for the cations [(p-XC6H4)(Mes)I(mppH)]+ (X = H (13a), Me (13b), OMe (13c), F (13d), Cl (13e), COMe (13f), CF3 (13g)) in order that ΔG⧧ values are in relation to the isomer of lower energy. Computation indicates essentially identical energies (0.4−6.5 kJ mol−1 favoring mppH cis to Mes except for X = H and COMe). As found for the [Ph2I]+ system, the cations [(p-XC6H4)(Mes)I(mppH)]+ were found to be of lower energy than [(pXC6H4)(Mes)I]+. Computation of the transition structures (TS_Mesa_X) was carried out in a similar manner to that for TS_Ca, and with X = OMe and COMe, alternative orientations of the methoxy and acetyl groups in the transition structures were explored. These groups were found to align coplanar with the aryl group, where alternative orientations are essentially thermoneutral, favoring the methyl group oriented above the phenyl plane of mpp (by 1.2 kJ mol−1 for X = COMe, 6.6 kJ mol−1 for X = OMe). A Hammett plot of the resulting computational data has a ρ value of 1.8 (Figure 3), comparing well with the experimental data (ρ = 1.7 ± 0.2).5c Figure 4. Aspects of formation of the transition structures [{Pd2(mpp)2(OAc)2}{(p-XC6H4)(Mes)I}]+ (TS_Mesa_X) illustrating (a) deformation energy (Edef) and electronic energy (Eint) contributions to energies of activation (Eact), and (b) energies of activation versus electronic energy contribution.

Table 1. ΔG⧧ and the Contribution of Deformation Energies (Edef = Edef (8a−8g) + Edef (13a−13g)) and Electronic Energy (Eint) to Energies of Activation (Eact = Edef − Eint) for Formation of Transition Structures [{Pd2(mpp)2(OAc)2}{(p-XC6H4)(Mes)I}]+ (TS_Mesa_X), where X Are Listed in Order of Decreasing σ+a Figure 3. Hammett plot for oxidation of [Pd(mpp)(μ-OAc)]2 (8) by [(p-XC6H4)(Mes)I]+ (13a−13g), for which ΔG⧧ values are obtained from the energy of transition structures for TS_Mesa_X relative to “8 + [(p-XC6H4)(Mes)I(mppH)]+ (13a−13g)”.

The agreement between the linear free energy relationships obtained from experimental and computational data provides assurance that a more detailed analysis of computational data is justified. We have undertaken an activation-strain analysis,20 examining the role of the energy of deformation (Edef) of [Pd(mpp)(μ-OAc)]2 (8) and of [(p-XC6H4)(Mes)I]+ (13a− 13g) when generating fragments within the transition structures TS-Mesa_X, as well as the role of the compensating interaction energy (Eint) between the deformed fragments in contributing to the overall Eact (Figure 4a). Values of Eact were found to follow the same trend as for ΔG⧧ (Table 1). Deformation energies are significantly lower for 8 (Edef 6.8−8.9 kJ mol−1) in forming the transition structures than for deformation of the iodonium species (Edef 121.1−128.4 kJ mol−1). Substituents giving low (favorable) Edef (X = COMe, CF3) give low (favorable) Eact values (Table 1). There is a good correlation between Eint and Eact (R2 = 0.896) (Figure 4b), in which lower energies of activation are associated with higher (favorable) interaction energies for the more electron-withdrawing substituents. These results are consistent with the Hammett plots, as well as with observation of a lowering in the energies of the LUMO and LUMO+1 with the more electronegative substituents (Table 2).

a

X

ΔG⧧

Eact

Edef (8)

Edef (16)

Edef

Eint

OMe Me F H Cl COMe CF3

103.6 99.0 97.8 93.2 96.4 95.0 85.8

32.0 25.3 19.9 22.1 18.9 9.6 8.4

8.9 7.7 8.5 6.8 8.5 7.5 7.4

128.4 122.4 125.0 125.6 125.8 121.1 121.5

137.3 130.1 133.5 132.4 134.3 128.6 128.9

105.3 104.8 113.6 110.3 115.4 119.0 120.5

Energies in kJ mol−1.

Table 2. Energies of the LUMO and LUMO+1 in [(pXC6H4)(Mes)I]+ (13a−13g), where X Are Listed in Order of Decreasing σ+a

a

X

LUMO

LUMO+1

OMe Me F H Cl COMe CF3

−5.36 −5.51 −5.88 −5.62 −5.73 −5.77 −5.87

−4.83 −4.91 −4.88 −5.01 −5.15 −5.24 −5.27

Energies in electronvolts (eV).

Computational Studies of Reductive Elimination To Form 3-Methyl-2-(3′-phenyl)phenylpyridine (mppPh) (9). Experimental studies of the catalytic reaction provide strong evidence for the role of a binuclear palladium species in F


Organometallics

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Figure 5. Energy profile and structures of species for C···C coupling from the cation [Ph(mpp)Pd(μ-OAc)2Pd(mpp)]+ (14). Energies ΔG (ΔH) in kJ mol−1 are relative to 14.

mol−1 higher than TS_14, and thus, they are omitted from further discussion (Supporting Information). Although free acetate ions are not considered to be readily available under the catalytic conditions (vide supra), computation of transition structures for reductive elimination from species formed on coordination of acetate was explored. The addition of acetate to TS_14 leads to a transition structure “Ph(mpp)(μ-OAc)2Pd(mpp)(OAc)” that is 34.1 kJ mol−1 higher than its precursor (15), and 50.3 kJ mol−1 higher than TS_14. In a similar fashion, modeling of another two binuclear species with bridging acetate groups and nonbonding Pd···Pd distances (3.905, 4.876 Å) led to transition states that are 38.9 and 30.3 kJ mol−1 higher than their precursors, and 36.3 and 57.4 kJ mol−1, respectively, above TS_14 (Supporting Information). Consideration of a Role for Unreacted Reagent (mppH) and the Product (mppPh) as Nitrogen-Donor Ligands in Binuclear Species. It has been determined experimentally that mppH acts as a neutral ligand to form the catalyst resting state, Pd(mpp)(OAc)(mppH) (12). The catalytic reaction (Scheme 3) has an ∼20-fold excess of nitrogen donor to palladium, in the form of reagent and product, and thus we have also explored the possible involvement of mppH as a ligand for 14 during reductive elimination. Optimizations proceeded satisfactorily (Supporting Information) to give structures with weak axial Pd···N(mppH) interactions (2.729−3.039 Å). The observed bond distances are considerably longer than those found for anti-[Pd(mpp)(OAc)(mppH)]+ (anti-12) (2.097 Å), presumably due to steric interactions encounterd upon coordination of 2-substituted pyridines to an octahedral center. The transition structure is 34.4 kJ mol−1 higher than its precursor, and 62.2 kJ mol−1 higher than 14. Thus, the involvement of coordinating reagent and product nitrogen donors during the reductive elimination step can be effectively discounted. Exploration of Fragmentation of Binuclear Species Ph(mpp)Pd(μ-OAc)2Pd(mpp)(OAc) (15) and [Ph(mpp)Pd(μ-OAc)2Pd(mpp)]+ (14). We recently reported that the trifluoromethyl/benzo[h]quinoline complex CF3(bzq)Pd(μOAc)2Pd(bzq)(OAc), an analogue of 15, undergoes fragmentation to form monomeric Pd(IV) and Pd(II) products.5d The binuclear complex was generated by the reaction of [Pd(CF3)(bzq)(μ-OAc)]2 with acetic acid, releasing CF3H and an acetate

the rate-limiting step. Similarly, the current computational studies are supportive of [Pd(mpp)(μ-OAc)]2 (8) as the key species in oxidation to form [Ph(mpp)(μ-OAc)2Pd(mpp)]+ (14). This reaction is believed to be followed by a process(es) for mpp···Ph coupling. Thus, direct reductive elimination from 14 was next evaluated. In addition, analogous C−C bondforming processes from related binuclear species lacking a Pd···Pd interaction were examined. Finally, the possible role of acetate, mppH, and mppPh as additional ligands during reductive elimination was evaluated in detail. Consideration of Reductive Elimination from [Ph(mpp)Pd(μ-OAc)2Pd(mpp)]+ (14) and Related Binuclear Species. Earlier computational studies9b of reductive elimination from the benzo[h]quinoline analogue of 14 were used to construct a trial transition structure for the present system, leading to successful optimization and IRC analysis (Figure 5). The structure for 14 obtained from the IRC analysis is identical to that obtained from modeling the oxidation of 8 (Figure 1b). For 14 and related species mentioned below, the phenyl ring orientation is approximately normal to the Pd−Cmpp bond, as anticipated for aryl···aryl coupling. The transition structure (TS_14) is almost identical to that obtained for the related benzo[h]quinoline analogue and is 29.4 kJ mol−1 above 14, compared with 24.9 kJ mol−1 for the benzo[h]quinoline system. The IRC analysis leads to product Da. Computation for an alternative product structure (Db), which is directly related to that formed from coupling in the benzo[h]quinoline system, provides an energy for Db that is similar to that for Da (Figure 5). The Pd···Pd distance increases in the sequence 14 → TS_14 → Da and Db, as found for the benzo[h]quinoline system for which a natural bond order analysis is consistent with depopulation of the dx2-y2 orbital at the PhPd center and population of the dz2 orbitals at both Pd centers during reductive elimination.9 The reductive elimination step 14 → TS_14 has an energy requirement (29.4 kJ mol−1) that is lower than the energy required to attain the transition structure TS_Ca for oxidation of 8 (86.6 kJ mol−1) (Figure 1b). Modeling of reductive elimination from two binuclear cations with bridging acetate groups, but lacking a Pd···Pd interaction (3.661, 4.677 Å), provided reaction profiles in which the transition structures are at energies only 2.1 and 3.3 kJ mol−1 above their respective precursors. However, the precursor cations are 62.6 and 98.0 kJ G


Organometallics

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Figure 6. High-energy fragmentation process for the neutral species 15. Energies ΔG (ΔH) in kJ mol−1 are relative to [Ph(mpp)Pd(μOAc)2Pd(mpp)]+ (14).

kJ mol−1 (BS1) at a distance of 4.5 Å. This indicates that fragmentation of the binuclear cation is very unlikely. Interestingly, an inflection at ∼3.7 Å allowed optimization of a high-energy species (60.5 kJ mol−1 higher than 14) (Supporting Information). Exploration of Mechanism for the Orthopalladation Step. The kinetic studies indicate that a binuclear palladium reagent is involved in the rate-limiting step of the catalytic reaction.5c The computational studies detailed above are supportive of this, in particular, indicating that direct oxidation of [Pd(mpp)(μ-OAc)]2 (8) proceeds via a significantly lower energy transition state than several alternative pathways examined. In addition, there is excellent agreement between experimental and computationally determined Hammett plots as X is varied in the reagents of the general structure [(pXC6H4)(Mes)I]BF4. Finally, computational data clearly indicate that C···C reductive elimination requires less energy than that required for the oxidation process. We have explored several orthopalladation mechanisms that appear to be feasible, seeking to determine whether a pathway can be found computationally that is more facile than that for the oxidation step. We were particularly attracted to exploring the possibility of orthopalladation occurring at binuclear Pd(II) species that may be formed subsequent to the final cation computed for reductive elimination [(mppPh-N)Pd(μ-OAc)2Pd(mpp)]+ (Da, Db in Figure 5). Similar proposals have been discussed recently.21 Dissociation of the organic product mppPh from Da would lead to a cation “[Pd(μ-OAc)2Pd(mpp)]+” with strong acetate bridging, and it seems feasible that this species may react with mppH to form [(mppHN)Pd(μ-OAc)2Pd(mpp)]+ (I) rather than dissociate to form fragments, such as “[Pd(OAc)]+” and Pd(mpp)(K2-OAc) (18). Computation indicates that the exchange reaction by Da to form is exergonic by 10.6 kJ mol−1 and, by related Db to form I is by 7.9 kJ mol−1.

ion for coordination.5d Computation indicates that the fragmentation pathway has a lower energy requirement than that for bzq···CF3 coupling from both CF3(bzq)Pd(μ-OAc)2Pd(bzq)(OAc) and the cation [CF3(bzq)Pd(μ-OAc)2Pd(bzq)]+.5d Although free acetate ion is unlikely to be present at an appreciable concentration under the catalytic conditions used in the current phenyl system, a search for a fragmentation pathway from Ph(mpp)(μ-OAc)2Pd(mpp)(OAc) (15) was undertaken via a scan of the potential energy surface as a function of increasing Pd···Pd distance in increments of 0.1 Å. A series of structures, including two transition structures, were identified (Figure 6), for which optimizations led to a sequence remarkably similar to that for fragmentation of CF3(bzq)Pd(μ-OAc)2Pd(bzq)(OAc). Initially, conversion of 15 (Pda − Pdb 2.714 Å) through transition structure TS_15 (Pda···Pdb 2.884 Å) leads to 15′ (Pda···Pdb 2.860 Å), an isomer of 15. The isomer has only one bridging acetate, with one monodentate acetate at Pda and a semibidentate acetate at Pdb (Pdb···O 2.843 Å). Increasing the Pda···Pdb distance further led to detection of a transition structure (TS_15′) exhibiting well-separated Pd centers (3.221 Å) with weak Pd···O interactions at each center (2.595 Å for Pda, 2.886 Å for Pdb). Transition structure TS_15′ leads to E (Pda···Pdb 3.567 Å), and then F (Pda···Pdb 4.023 Å, Pdb···O 2.923 Å), which may be regarded as a weak adduct of the Pd(IV) and Pd(II) fragments G and 18. The highest-energy structure in the fragmentation sequence (TS_15) (79.7 kJ mol−1 above 14) is considerably higher than the transition structure for mpp···Ph coupling from cation 14 (29.4 kJ mol−1 above 14), indicating that the fragmentation process is very unlikely for this system. However, demonstration of a fragmentation sequence for CF3,5d as well as the feasibility (computationally) for the phenyl system, suggests that this pathway may be relevant for consideration in other catalytic systems. In view of this, the fragmentation calculations were extended beyond the Pd(IV) and Pd(II) fragments to include anticipated dimerization of 18 to give 8 and coordination of water to G to form the phenyl analogue (H) of the CF3 product (3). Structure H has the coordinated water molecule hydrogen-bonded to both acetate groups, as found in the crystal structure of 3.4 For cation 14, exploration of potential fragmentation via the same procedure led to steadily increasing energy to values > 75 H


Organometallics

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Figure 7. Energy profile for orthopalladation of binuclear [(mppH)Pd(μ-OAc)2Pd(mpp)]+ (I), using the external bases (a) acetate, and (b) nitrogen-donor reagent mppH and product mppPh; and (c) for orthopalladation of mononuclear Pd(mppH)(OAc)2 (21), together with structures of precursor complexes and transition structures. Structures for La, TS_La, and Ma are similar to those for Lb, TS_Lb, and Mb. Energies ΔG (ΔH) in kJ mol−1 are relative to [(mppH)Pd(μ-OAc)2Pd(mpp)]+ (I) for (a) and (b), and relative to 21 for (c).

unreacted Pd(OAc)2 is retained until completion of reaction. In the current case, where the Pd(OAc)2 precatalyst is present at 5 mol %, the Pd/OAc ratio is reduced to 1:1 after orthopalladation of 5% of the mppH reagent. This ratio propagates through all subsequent species identified in the catalytic cycle, thus reducing the availability of acetate to act as a base for orthopalladation or to act as a ligand for Da. For structure I, it is unlikely that the bridging acetates could assume the role of base to remove a proton from coordinated mppH, as such a route would appear to require dissociation of a Pd−OAc bond. We have instead explored the possibility that an external base could be involved. We have examined acetate ion, mppH, and mppPh as bases (Figure 7a,b), together with the

Modeling of orthopalladation routes from I was based on approaches used for investigating mononuclear orthopalladation.22 Computational studies of orthopalladation of nitrogendonor reagents at mononuclear Pd(II) centers indicate an ambiphilic mechanism, in which electrophilic interaction of Pd(II) with the ortho-CH group is supported by concomitant interaction of the ortho-CH group with a base to enable removal of the hydrogen atom as a proton bonded to the base.22a Acetate is the most widely studied base in computational investigations of orthopalladation,22d although a role for “AcOH” as the base to release [AcOH2]+ has also been explored.22e,g Orthopalladation is normally carried out in a stoichiometric manner, and thus, the 1:2 ratio of Pd/OAc for I


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Scheme 4. Catalytic Cycle for Ar1···Ar2 Coupling Involving Pd-Catalyzed Ligand-Directed C−H Arylation (Ar1−H) with Diaryliodonium Cations [Ar22I]+, Based on Agreement between Synthetic, Kinetic, and Computational Studies for the Reaction of 3-Methyl-2-phenylpyridine (mppH) with [(p-XC6H4)(Mes)I]BF4a

a The Pd(OAc)2 precatalyst forms the catalyst 8, and steps in the sequence are simplified from figures that show additional intermediates for steps 1 and 4 (step 1, Figure 1b; step 2, Figure 4; step 4, Figure 7).

much weaker bases “AcOH” and [BF4]− in view of the high concentrations of [AcOH]2 and [BF4]− that are present relative to [Pd]. Transition structures were identified for all five bases, thereby enabling the identification of precursor species and orthopalladation products from IRC calculations. The activation energies for “HOAc” (91.4 kJ mol−1) and [BF4]− (78.2 kJ mol−1) as bases are similar to that required for the oxidation reaction (86.6 kJ mol−1, Figure 1b), and as these processes for orthopalladation are calculated to be overall endergonic (88.5 and 50.0 kJ mol−1, respectively), they are not discussed further (Supporting Information). Orthopalladation at a mononuclear precursor complex involving acetate as an internal base was explored also for Pd(mppH)(OAc)2 (21) with guidance from reported computation for related arylimines,22b,c rigid benzo[h]quinoline,22f flexible N,N-dimethylbenzylamine, and22a,d for aliphatic C−H cyclopalladation of oxazolines.21c The transition structure TS_21 was readily identified (Figure 7c) and was found to be very similar to those reported for previous systems. Overall, it is 51.3 kJ mol−1 higher in energy than its precursor complex. The computation results presented in Figure 7 provide four feasible orthopalladation pathways for which the energy requirement (23.9−51.3 kJ mol−1) is significantly lower than that for the oxidation reaction at binuclear [Pd(mpp)(μOAc)]2 (8) (86.6 kJ mol−1). The results are supportive of orthopalladation at a binuclear center, rather than via fragmentation of Da or I leading to mononuclear orthopalladation for the following reasons: (i) exchange of organic product mppPh in Da with reagent mppH to give I is exergonic; (ii) acetate bridging in Da and I is unlikely to allow fragmentation to “[Pd(OAc)(N-donor)]+” and Pd(mpp)(K2OAc); (iii) reaction of an additional donor group with I to assist fragmentation is anticipated to lead to orthopalladation, for acetate (Figure 7a), and for mppH and mppPh (Figure 7b) with similar low activation energy (∼24 kJ mol−1); and (iv) the concentration of nitrogen-donor bases (∑([mppH] + [mppPh])) is high relative to [Pd] throughout catalysis.

There are several features of the species in the orthopalladation pathways that are of particular interest. Structure J has a typical Pd−O bond distance for the additional acetate group (2.050 Å), a weak Pd···O interaction for the bridging acetate group trans to the C−H bond to be activated (2.735 Å), and a Pd···Pd distance of 2.688 Å, shorter than in all other species mentioned throughout this study. For example, it is instructive to compare J to La (2.926 Å), TS_La (2.912 Å), and Ma (2.864 Å) for mppH as the base, as well as to 8 (2.907 Å), which has a Pd···Pd order estimated as ∼0.11.18 There appears to be a significant Pd−Pd bonding interaction in J, with pseudo-square-planar coordination at one Pd center (“PdNO2Pd”, top) and square-pyramidal at the other Pd center (“[PdCNO2Pd”, bottom). The structure may be interpreted in terms of resonance involving separated Pd(II) centers and Pd(I)/Pd(III), that is, “Pd(II) Pd(II) ↔ Pd(I) → Pd(III)”, where the formal oxidation state Pd(I) refers to the squareplanar center. The subsequent transition structure (TS_J) exhibits a geometry for C−H activation that is similar to the mononuclear transition structure (TS_21), closely related to previous studies of orthopalladation at mononuclear centers, in particular, in two respects. First, both TS_J and TS_21 have approximate square-planar motifs “PdO2N(CH)”. Second, the hydrogen atoms interacting with nitrogen in La and Lb are slightly removed from the phenyl plane, and the “PdCHN” moieties in TS_La and TS_Lb have geometries similar to the acetate analogue (TS_J). The weak H···O interactions in TS_La (2.297 Å) and TS_Lb (2.312 Å) illustrate that the Ndonor bases act in a bimodal fashion (N···H and H···O) related to that of acetate in TS_J (O···H and O···Pd). The departing [mppH2]+ and [mppPhH]+ ions in the products Ma and Mb are weakly linked to a bridging acetate oxygen atom (H···O 2.153 and 2.270 Å). Weak Pd···HNAr+ interactions in Ma (2.267 Å) and Mb (2.334 Å) are similar to those in a related Pt(II) complex exhibiting hydrogen-bonding, Pt···HNMe2R+ 2.11(5) Å,23 although, in the current cases, longer and further removed from linearity (156.1° and 144.2°). J


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The mechanism of the oxidation of [Pd(mpp)(μ-OAc)]2 by [Ar2I]+ reagents involving [Ar]+ transfer provides a model that is anticipated to be applicable for a wide range of organometallic substrates. Of particular interest is the favored interaction of iodine with a donor atom coordinated to palladium, that is, “Pd−O···IPh2” preferred relative to “Pd···IPh2” in the initial intermediate, followed by formation of a four-membered transition state in which the O···I interaction is retained, “Pd···(μ-Ph)···I(Ph)···O−”. In summary, we report computational results for a catalytic system for Ar···Ar coupling that are consistent with synthetic and kinetic studies of the system. These studies illustrate the role of a binuclear Pd(II) complex as the catalyst in a cycle based entirely on binuclear species, including for the orthopalladation and Ar···Ar coupling steps. These calculations also elucidate reaction mechanisms for transfer of [Ar]+ groups from [Ar2I]+ to binuclear Pd(II) centers to form a binuclear Pd(III) cation (formulated as Pd(III)−Pd(III) ↔ Pd(II) → Pd(IV))9 for which the initial contact involves an acetate oxygen atom interacting with iodine in a weak donor−acceptor manner (“Pd−Oδ−···Iδ+Ar2”), followed by a four-centered transition state (“Pd···(μ-Ph)···I(Ph)···O−”). We anticipate that this study will prove useful in further elucidation of mechanisms of other Pd-catalyzed reactions involving iodine(III) reagents, and of reactions where orthopalladation occurs under conditions where binuclear intermediates are favored.

The nitrogen-donor route is clearly favored because of the low availability of acetate after completion of initial orthopalladation by the 5 mol % palladium acetate. After this initial event, the Pd/OAc ratio is 1:1 (eq 1) and all species in the cycle have a 1:1 ratio of Pd/OAc. In contrast, there is a large excess of available N-donor relative to [Pd] throughout catalysis.

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CONCLUSION This paper provides detailed information about the mechanism of oxidation of palladium complexes by diaryliodine(III) reagents as well as about orthopalladation at binuclear palladium centers. Together with studies of C···C reductive elimination from binuclear palladium centers, this paper presents a complete feasible mechanistic picture for a palladium-catalyzed ligand-directed C−H arylation with diaryliodonium reagents. Computational studies indicate the pathway shown in Scheme 4 for catalysis, and also for the stoichiometric reaction of 8 with [Ph2I]BF4 for which step 1 of the cycle has [Ph2I]+ as the oxidant. The Pd(OAc)2 precatalyst most likely participates in orthopalladation at a mononuclear palladium center to form the active catalyst [Pd(mpp)(μ-OAc)]2 (8). Subsequent orthopalladation most likely occurs predominantly at a binuclear center within the catalytic cycle (step 4). For this system, orthopalladation facilitated by the N-donor bases is favored, but if additional acetate were present, orthopalladation involving acetate as a base would be competitive. Oxidation (step 1) involves formation of a weak O···I donor−acceptor interaction from a bridging oxygen atom trans to nitrogen in 8 to the iodine atom of [Ph2I]+, forming an adduct with T-shaped geometry at iodine (Ca, Figure 1). The O···I interaction is retained upon conversion of Ca to the transition structure, which contains a four-membered ring (TS_Ca, Scheme 4). Reductive C···C coupling from 14 (via TS_14, step 2) is followed by ligand exchange (step 3) to liberate the product and form an mppH complex (I) that undergoes orthopalladation employing N-donor bases to regenerate catalyst 8 (step 4). The computational studies show that the energy required to achieve transition structures for orthopalladation (both mononuclear and binuclear mechanisms), and for C···C coupling, is lower than that for the oxidation reaction. In addition, the calculations demonstrate that the lowest-energy pathway for oxidation involves direct interaction between the iodonium reagent and the dimer [Pd(mpp)(μ-OAc)]2 (8). All of these results are consistent with kinetic studies showing that the rate-limiting step of catalysis involves oxidation of a binuclear palladium species.5c A computational analysis of the rate-limiting oxidation step with [(p-XC6H4)(Mes)I]BF4 provides a Hammett plot with a ρ value within error of that obtained experimentally. The rate of reaction increases with more electron-withdrawing groups X, and the interaction energy between the components {Pd(mpp)(μ-OAc)2} and {(p-XC6H4)(Mes)I} in the transition structures (for example, TS_Ca in Scheme 4) is a dominant contributor in the reaction. A fragmentation process similar to that which occurs for CF3(bzq)Pd(μ-OAc)2Pd(bzq)(OAc) (Figure 4)5d is unlikely to occur in the aryl/mpp system examined here, as it involves high-energy transition structures. However, we believe that such fragmentation pathways need to be considered for the application of other iodonium reagents in which different types of [organyl]+ groups are transferred.

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ASSOCIATED CONTENT

S Supporting Information *

Complete ref 10, computation for reactions of mononuclear and binuclear palladium species with iodonium reagents, mpp···Ph coupling for a range of binuclear species, computation for fragmentation of 14, and computation for orthopalladation at binuclear centers using AcOH and [BF4]− as bases. Energy parameters and Cartesian coordinates of all optimized structures. 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] (A.J.C.), Brian.Yates@utas. edu.au (B.F.Y.), [email protected] (M.S.S.). Fax: (613) 6226-2858. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS We thank the Australian Research Council and the U.S. National Science Foundation (1111563) for financial support, and the Australian National Computational Infrastructure and the University of Tasmania for computing resources.

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REFERENCES

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