How to Control Inversion vs Retention Transmetalation between PdII

Article pubs.acs.org/JACS

How to Control Inversion vs Retention Transmetalation between PdII−Phenyl and CuI−Alkyl Complexes: Theoretical Insight Hong Zheng,† Kazuhiko Semba,‡ Yoshiaki Nakao,‡ and Shigeyoshi Sakaki*,† †

Fukui Institute for Fundamental Chemistry, Kyoto University, Takano-Nishi-hiraki-cho 34-4, Sakyo-ku, Kyoto 606-8103, Japan Department of Material Chemistry, Graduate School of Engineering, Kyoto University, Katsura, Nishikyo-ku, Kyoto 615-8510, Japan

‡

S Supporting Information *

ABSTRACT: Transmetalation between Pd(Br)(PhA)(PCyp3)2 (Ph = phenyl, Cyp = cyclopentyl) and Cu(CaHMePhB)(NHC) (NHC = 1,3-bis(2,6-diisopropylphenyl)-imidazolidin-2-ylidene) is an important elementary step in recently reported catalytic cross-coupling reaction by Pd/Cu cooperative system. DFT study discloses that the transmetalation occurs with inversion of the stereochemistry of the CaHMePhB group. In its transition state, the CaHMePhB group has almost planar structure around the Ca atom. That planar geometry is stabilized by conjugation between the π* orbital of the PhB and the 2p orbital of the Ca. Another important factor is activation entropy (ΔS°‡); retention transmetalation occurs through Br-bridging transition state, which is less flexible than that of the inversion transmetalation because of the Br-bridging structure, leading to a smaller activation entropy in the retention transition state than in the inversion transition state. For CaHMeEt group, transmetalation occurs in a retention manner. In the planar CaHMeEt group of the inversion transition state, the Ca 2p orbital cannot find a conjugation partner because of the absence of π-electron system in the CaHMeEt. Transmetalation of CaHMe(CHCH2) occurs in a retention manner because the vinyl π* is less effective for the conjugation with the Ca 2p because of its higher orbital energy than the Ph π*. The introduction of electronwithdrawing substituent on the PhB is favorable for inversion transmetalation. These results suggest that the stereochemistry of the Ca atom in transmetalation can be controlled by electronic effect of the CaHMeR (R = phenyl, vinyl, or alkyl) and sizes of the substituent and ligand.

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INTRODUCTION Transition-metal-catalyzed cross-coupling reaction is an important reaction for C−C bond formation.1−4 Catalytic cycle of conventional cross-coupling reaction is well-known to consist of oxidative addition of aryl halide (ArX) to palladium(0) complex, transmetalation between palladium(II) aryl complex and organometallic reagent (R−M), and reductive elimination of product (Ar−R) from palladium(II) complex. For better understanding of the cross-coupling reaction, a theoretical study of each elementary step is necessary. The oxidative addition and reductive elimination have been investigated theoretically, as reviewed recently.5,6 Furthermore, the transmetalation of palladium(II) phenyl complex with organoborane,7−9 diborane,10 organosilicon,11 organogold,12,13 and organozinc13,14 compounds has been investigated theoretically by several groups to elucidate reaction mechanisms and electronic processes. However, one issue remains to be investigated: stereochemistry in transmetalation, which is recognized as an important issue recently because it directly relates to stereospecific cross-coupling reactions. Actually, stereochemistry in transmetalation has been investigated in several pioneering works. In the Hiyama cross-coupling reaction with trifluoro(benzyl)silanes, inversion of stereochemistry was successful by tuning the reaction temperature and solvent.15 In some cross© 2017 American Chemical Society

coupling reactions with organoboron reagents, intramolecular coordination of carbonyl group to boron was found to induce stereoinversive transmetalation.16−18 Recently, Biscoe and coworkers suggested that the use of optically active alkylboron compound is crucially important for inversion cross-coupling reaction between secondary alkylboron nucleophile and aryl chloride.19 Brown and co-workers reported arylboration of alkenes using Pd/Cu synergistic catalyst.20,21 They succeeded in conducting syn- and antiselective reactions.21 Moreover, enantioselectivity in the Stille reaction was reviewed by Espinet and co-workers.22 Nakao and co-workers23,24 and Buchwald and co-workers25,26 have independently reported the reductive crosscoupling reaction between 1-arylalkenes and aryl bromides using Pd/Cu cooperative catalyst. According to several mechanistic studies, Nakao and co-workers proposed a catalytic cycle as shown in Scheme 1.24 Oxidative addition of aryl bromide 3 to palladium(0) complex 5 occurs in step a of the Pd-cycle. Hydrocupration of arylalkene 2 occurs in step f of the Cu-cycle. Transmetalation between CuI-alkyl 1 and PdII-aryl complex 6 occurs in step b. A stoichiometric reaction between 1 and 6 revealed that transmetalation occurred in an inversion Received: April 28, 2017 Published: September 18, 2017 14065

DOI: 10.1021/jacs.7b04383 J. Am. Chem. Soc. 2017, 139, 14065−14076

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Journal of the American Chemical Society Scheme 1. Proposed Reaction Mechanism of Cross-Coupling Reaction between Arylalkene and Aryl Bromide24

because this transmetalation is involved as a crucially important step in the reductive cross-coupling reaction by the Pd/Cu cooperative catalyst.23,24 Our purposes are to elucidate the reason that inversion transmetalation occurs more easily than retention transmetalation, to ascertain cases in which transmetalation occurs in a retention manner, and to identify important factors affecting stereochemistry in transmetalation. Through these investigations, we hope to find ideas leading to control of the stereochemistry of this transmetalation, which are expected to be useful also for other transmetalations.

manner of the alkyl group (Scheme 2 shows inversion and retention reactions). Although preliminary DFT calculations for Scheme 2. Inversion and Retention Transmetalations between PdII(Br)(Ph)(PR3)2 and CuI(CHMePh)(NHC)

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this transmetalation have been done, no systematic survey has been made to date. Because of the versatility of the Pd/Cu cooperative catalysis in cross-coupling reactions,17,18,20−33 it is highly desirable to elucidate transmetalation between PdII-aryl and CuI-alkyl complexes and to use that knowledge to control its stereochemistry. Here, we theoretically investigated the stereochemistry in transmetalation between PdII(Br)(Ph)(PCyp3)2 (Cyp = cyclopentyl) and CuI(CHMePh)(NHC) (NHC = 1,3-bis(2,6diisopropylphenyl)-imidazolidin-2-ylidene) using DFT method,

COMPUTATIONAL DETAILS AND MODELS

For this study, we used real phosphine (PCyp3; tris-cyclopentylphosphine) and NHC (1,3-bis(2,6-diisopropylphenyl)-imidazolidin-2-ylidene) ligands,24 as shown in Scheme 1. Although various styrenes 2 and aryl bromides 3 were used in experiment, phenyl bromide and unsubstituted styrene were used as representative substrates in calculations. The oxidative addition of phenyl bromide 3 to Pd0(PCyp3)2 5 engenders the formation of PdII(Br)(PhA)(PCyp3)2 6. The hydrocupuration of styrene 2 with CuIH(NHC) 10 engenders the formation of CuI(CaHMePhB)(NHC) 1, where the alkyl carbon

Figure 1. Geometry changes in inversion transmetalation of the CaHMePhB group. Bond distance is in angstrom. BA represents the sum of three bond angles (∠Ph−Ca−H, ∠Ph−Ca−Me, ∠H−Ca−Me) around the Ca atom of the CaHMePhB group. 14066


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Journal of the American Chemical Society participating in the transmetalation is designated as Ca hereinafter. We investigated transmetalation between 1 and 6 after dissociation of one phosphine ligand from the Pd center because phosphine dissociation was experimentally suggested and theoretically explained in our recent study.24 Geometry optimizations were conducted using DFT method with the ωB97XD functional34 and a smaller basis set system BS-I. We selected this method after comparing optimized geometries by the ωB97XD, M06, and B3LYP-D3 functionals (Figure S1 and pages S5− S6 in Supporting Information). In BS-I, the LANL2DZ basis sets with the Los Alamos effective core potentials (ECPs)35,36 were used for Pd, Cu, and Br, where one d polarization function was added to Br.37 The usual 6-31G(d) basis sets were used for other atoms.38−41 The potential energies were re-evaluated with the M06 functional,42 using a larger basis set system BS-II. In BS-II, (311111/22111/411/11)43,44 basis sets by the Stuttgart−Dresden−Bonn group with the ECPs were used for Pd and Cu. The 6-311G(d) basis sets were employed for other atoms,45−47 where a set of diffuse functions was added to the Ca and Br atoms. The solvation effect (toluene) was evaluated with a polarizable continuum model (PCM).48 In summary, the M06(with PCM)/BS-II//ωB97XD/BS-I was used for evaluating potential energy. The choice of this method is explained in pages S5−S6 (analysis of Figure S1) of Supporting Information. The ωB97XD/BS-I was used for evaluating thermal correction and entropy. The Gibbs free energy was evaluated at 298 K here. NBO population analysis was made using M06/BS-II calculations. All these DFT calculations were conducted using the Gaussian 09 program.49 The translational entropy in solution was corrected using the method proposed by Whitesides et al.50

reasonable because the steric repulsion between the PhA and PhB groups decreases as the Pd−Ca−Cu angle decreases from 180° to 135°, as shown in TSa1 of Figure 1. After transmetalation, an intermediate PdII(Br)(PhA)(CaHMePhB)(PCyp3)−CuI(NHC) Pa1 is formed, in which the Pd−Ca distance is 2.151 Å and the sum of the bond angles around the Ca decreases to 338°, indicating that the Pd−CaHMePhB bond is formed completely and the C a atom has sp 3 hybridization. In addition, the Cu−Ca distance is 2.558 Å. Moreover, the Cu−C(PhB) distances are 2.073 and 2.309 Å, indicating that the Cu atom coordinates with the PhB in the CaHMePhB. In Pa1, the PdII(Br)(PhA)(CaHMePhB)(PCyp3) moiety has one negative charge in a formal sense because the Br, PhA, and CaHMePhB ligands are considered anion in a formal sense. By contrast, the CuI(NHC) moiety has one positive charge in a formal sense, because the NHC is neutral. Consequently, Pa1 is regarded as an ion-pair adduct with the Cu-PhB bonding interaction. Figure 2 shows the Gibbs free energy profile. One phosphine dissociation from ADa1 affords ADa2 with substantially large

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RESULTS AND DISCUSSION Geometry and Energy Changes in Transmetalation between PdII(Br)(PhA)(PCyp3)2 and CuI(CaHMePhB)(NHC). PdII(Br)(PhA)(PCyp3)2 and CuI(CHMePhB)(NHC) form a weak contact adduct ADa1, as shown in Figure 1. This ADa1 is 0.9 kcal/mol (in Gibbs free energy) below the sum of the two reactants. The experimental work revealed that threecoordinated PdII complex PdII(Br)(Ph)(PCyp3) participates in the transmetalation after dissociation of one PCyp3 ligand.24 This dissociation occurs because PCyp3 is so bulky that the Ca atom can not approach the Pd center without the phosphine dissociation. The PCyp3 dissociation engenders the formation of Pd(Br)(PhA)(PCyp3)−CuI(CaHMePhB)(NHC) ADa2, which has a larger Pd−Ca−Cu angle of 127° than in ADa1 (94°). Although the rather short Pd−Ca distance (2.777 Å) seems favorable for transmetalation, the Cu−Br distance is very long (4.154 Å), suggesting that the Pd−Br−Cu bridge formation is difficult. In a transition state TSa1, the CaHMePhB group is moving from the Cu center to the Pd center. It is noteworthy that the Pd− Ca−Cu angle increases to 135° and that the CaHMePhB group becomes almost planar in TSa1. Actually, the sum of the bond angles around the Ca atom is 345° in ADa2 but 358° in TSa1. The Pd−Ca distance decreases considerably by 0.248 Å but the Cu−Ca distance changes little going from ADa2 to TSa1. These geometry changes show that the main events in TSa1 are Pd−Ca bond formation and inversion of the CaHMePh group. The large Pd−Ca−Cu angle (135°) and the almost planar CaHMePh group in TSa1 suggest that the Ca atom in TSa1 is understood to have a distorted trigonal bipyramidal structure by counting the Pd−Ca and Cu−Ca bonding interactions and this transmetalation is similar to SE2(back) reaction, as discussed below. This TSa1 is fundamentally equivalent to the experimentally proposed structure by Brown et al.20 Although the Pd−Ca−Cu angle is not 180°, which is expected for the transition state of the SE2(back) reaction, this angle is

Figure 2. Gibbs free energy changes (in kcal/mol) in transmetalation between PdII(Br)(PhA)(PCyp3)2 and CuI(CaHMePhB)(NHC). The Gibbs free energy and potential energy changes are presented without and with parentheses.

destabilization energy. The step going from ADa2 to TSa1 occurs with nearly no energy loss,51 suggesting that the dissociation of PCyp3 immediately induces transmetalation to afford Pa1. Pa1 is slightly more stable than TSa1. Actually, this unexpectedly small stabilization of Pa1 arises from absence of the Cu−Br bond, as explained above. The Cu−Br bond is formed through a transition state TSa2 to afford a stable product complex Pd II (Ph A )(C a HMePh B )(PCyp 3 )(μ-Br)CuI(NHC) Pa2 with a normal CuI−Br bond (2.305 Å). The Pd−Br distance (2.775 Å) is moderately longer than in Pa1, suggesting that the Br interacts with the Pd, too. The Cu interaction with the PhB of the CaHMePhB is broken in Pa2.52 Another reactant complex ADa3 was optimized after the dissociation of one PCyp3 ligand, as shown in Figure 3. In ADa3, the Br anion bridges the Pd and the Cu. The Pd−Ca distance (2.397 Å) is much shorter than in ADa2. Consistent with the short Pd−Ca distance, the Cu−Ca distance is considerably elongated by 0.230 Å compared to that of Cu(CaHMePhB)(NHC). These geometrical features indicate that interaction is strongly formed between Pd(PhA)(PCyp3) and Cu(CaHMePhB)(NHC) through the Br bridge. The CaHMePhB group is moving from the Cu to the Pd through a transition state TSa3 to afford an adduct Pd(PhA)(CaHMePhB)(PCyp3)(μ-Br)Cu(NHC) Pa3. It is noteworthy that Pa3 is fundamentally the same as Pa2 but the 14067


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Journal of the American Chemical Society

Figure 3. Geometry changes in retention transmetalation of the CaHMePhB group followed by the reductive elimination of Ph2CHMe. Bond distance is in angstrom. BA represents the sum of three bond angles (∠Ph−Ca−H, ∠Ph−Ca−Me, ∠H−Ca−Me) around the Ca atom of the CaHMePh group. Rotation of NHC was checked, as shown in Figure S3 in Supporting Information.

conformation of the CaHMePhB is slightly different. In TSa3, the Cu−Ca bond increases to 2.483 Å but the Pd−Ca distance decreases to 2.255 Å. The Pd−Br distance changes little but the Cu−Br distance decreases to 2.494 Å. These geometrical features indicate that the CaHMePhB group is moving from the Cu toward the Pd and that the Br is moving simultaneously from the Pd toward the Cu. The sum of the bond angles around Ca changes little, as going from ADa3 to Pa3, showing clearly that transmetalation occurs while maintaining the sp3 hybridization of the Ca atom. Moreover, the stereochemistry around the Ca atom does not change. Consequesntly, this is the transition state for retention transmetalation. TSa3 is 3.2 kcal/mol lower than TSa1 in potential energy but 3.1 kcal/mol higher than TSa1 in Gibbs free energy (Figure 2). The Br bridges the Pd and the Cu atoms in TSa3, but such Br bridging is absent in TSa1. Therefore, TSa3 is more stable than TSa1 in terms of potential energy. However, TSa1 becomes more stable than TSa3 in Gibbs free energy because the vibrational activation entropy is less in TSa3 than in TSa1 because of the less-flexible geometry of TSa3 than that of TSa1, as discussed below. We investigated an inversion transition state with a bridging Br atom but it is unstable, as shown in Supporting Information Figure S2. After transmetalation, one PCyp3 ligand is bound with the Pd center of Pa2 to afford a four-coordinated palladium(II) complex PdII(PhA)(CaHMePhB)(PCyp3)2 7a (Figure 3). For this study, we used the same numbering as that shown in Scheme 1. Reductive elimination occurs from 7a through a transition state TSa4 to afford the Pd0 complex Pd(PCyp3)2 and a product Ph2CaHMe. This TSa4 is nonplanar (the dihedral angle between PdP2 and PdCaPhB planes = 50°) to avoid steric repulsion between the PCyp3 and CaHMePhB groups.53 The Gibbs activation free energy (ΔG°‡) is 19.7 kcal/mol relative to ADa1 (Figure 2), which is smaller than those for TSa1 and TSa2. The reductive elimination is not rate-

determining because the PdII complex easily undergoes reductive elimination. Above results suggest that the Br bridging is favorable for retention transmetalation in potential energy but the less flexible structure of the transition state is not favorable for the retention transmetalation because of the small activation entropy. Based on these results, it is concluded that (i) the inversion transition state with a planar CaHMePhB group is not favorable in potential energy but favorable in the Gibbs free energy, (ii) the Br-bridging retention transition state is favorable in the potential energy but not favorable in Gibbs free energy because of the less flexible geometry, as discussed below, and (iii) the use of strongly donating bridging ligand is favorable for retention transmetalation because it stabilizes the bridging structure. Electronic Process of Transmetalation. In inversion transmetalation, the negative charge of the CaHMePhB changes little from ADa2 to TSa1 but then decreases considerably (less negative) from TSa1 to Pa1, as shown in Figure 4a. This change derives mainly from the change in the Ca atomic charge

Figure 4. Important changes in natural charge by inversion and retention transmetalations between PdII(Br)(PhA)(PCyp3) and CuI(CaHMePhB)(NHC). 14068


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Scheme 3. Electron Density Flow in Inversion Transmetalation (a) and Elecron Density Flow in the SE2(back) Reaction (b)a

a

M, Yδ− and Zδ+ correspond to the Cu(NHC), Br, and the PdII(PhA)(PCyp3), respectively.

of direction weakens the CT from the CaHMePhB to the Cu, but it enhances the CT from the CaHMePhB to the Pd, as shown in Scheme 4, leading to the changes in the Pd, Cu, and

which becomes moderately more negative from ADa2 to TSa1 but then becomes markedly less negative from TSa1 to Pa1. The Cu atomic charge becomes more positive from ADa2 to Pa1, although the Pd atomic charge slightly decreases (less positive) and the positive charge of the PCyp3 decreases considerably, suggesting that PCyp3 serves an electron reservoir. These changes in natural charge reveal the electronic process. Because the PdII is electrophilic, it wants to interact with the electron-rich sp3 orbital of the Ca atom. However, this sp3 orbital cannot interact well with the PdII because it extends toward the Cu atom in ADa2. To form interaction with the PdII, the Ca sp3 orbital must change its direction toward the Pd, but such a direction change is difficult because the Pd exists at the back side of the sp3 orbital in ADa2. Alternatively, the tetrahedral-like structure of the CaHMePhB group becomes planar. In such planar geometry, the σ-donation from the CaHMePhB to the Cu becomes weaker because the sp3 feature disappears and the Ca 2p orbital interacts with the Cu, leading to the moderately more positive charge of the Cu and the moderately more negative charge of the Ca. Simultaneously, the electrophilic attack of the PdII to the Ca 2p orbital starts to occur around TSa1 to induce charge transfer (CT) from the Ca 2p orbital to the PdII, leading to the less negatively charged Ca atom and the less positively charged PdII atom from TSa1 to Pa1. The CaHMePhB with strong trans-influence starts to coordinate with the Pd at the position trans to the PCyp3 and the Pd receives CT from the Ca as going from TSa1 to Pa1. Therefore, donation from the PCyp3 to the Pd is suppressed and the positive charge of the PCyp3 decreases from ADa2 to Pa1. The increase in CT from the Ca to the Pd is compensated by the decrease in CT from the PCyp3 to the Pd. Therefore, the Pd positive charge moderately decreases. The electron flows discussed above are presented in Scheme 3a. In the SE2(back) reaction between Yδ−−Zδ+ and M-alkyl, the Zδ+ attacks the sp3 carbon in electrophilic manner from the back-side of the Malkyl bond to induce the dissociation of M from the alkyl group, releasing the electron density, as shown in Scheme 3b. The geometry around the Ca atom in TSa1 resembles the transition state of the SE2(back) reaction. The electron flow is fundamentally the same in inversion transmetalation and the SE2(back) reaction. In retention transmetalation, the Pd atomic charge becomes negative from ADa3 to ADa2 but the Cu charge becomes more positive (Figure 4b). The negative charge of the CaHMePhB decreases to a similar degree to the change in the Pd atomic charge. In ADa3, the Cu−Ca−H angle (141°) is larger than the normal tetrahedral angle (109.5°), indicating that the Ca sp3 orbital starts to change its direction toward the Pd. This change

Scheme 4. Electron Density Flow in Retention Transmetalation

CaHMePhB charges described above. The Br negative charge is smaller in ADa3 than in ADa2 because the Br interacts only with the Cu in ADa2 but it interacts with both of the PdII and the CuI in ADa3. From ADa3 to Pa3 through TSa3, the CaHMePhB charge becomes less negative, indicating that the CT from the CaHMePhB to the PdII becomes stronger. In addition, the CaHMePhB starts to coordinate with the Pd at the position trans to PCyp3 around TSa3. Both suppress the CT from the PCyp3 to the Pd, leading to a decrease in the positive charge of PCyp3. However, the PdII atomic charge becomes slightly more negative at TSa3. It subsequently becomes positive at Pa3, although the CuI atomic charge becomes slightly more positive then becomes less positive thereafter. These results are reasonable because the Pd−Br bond breaking and the Cu−Br bond formation enhance CT from the Br to the Cu and weaken CT from the Br to the Pd as going from ADa3 to Pa3. These charge changes can be reasonably understood based on changes in the interactions, as summarized in Scheme 4. This transmetalation can be viewed as bond-rearrangement: the nucleophilic CaHMePhB dissociates from the CuI and approaches the electron-deficient PdII in a nucleophilic manner, maintaining its sp3 orbital. Simultaneously, the Br atom approaches the CuI because the CuI becomes electron-deficient as a result of the Cu−Ca bond breaking, to induce the Pd−Br bond elongation. It is noteworthy that the Ca becomes more negative in TSa1, but it becomes less negative in TSa3. Moreover, the Br atomic charge changes little in TSa1 but becomes less negative in ADa3 and TSa3 compared to ADa2. The more negative Ca atomic charge at TSa1 suggests that the introduction of electron-withdrawing substituent on the Ca is favorable for inversion transition state. The less negative Br atomic charge in ADa3 compared to that in ADa2 arises from electron-donation from the Br to both of the Pd and the Cu, suggesting that the strongly donating bridging ligand is favorable for retention 14069


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Figure 5. Geometry and energy changes in transmetalation between PdII(Br)(PhA)(PCyp3) and CuI(CaHMeEt)(NHC). Bond distance is in angstrom. BA represents the sum of three bond angles (∠Et−Ca−H, ∠Et−Ca−Me, ∠H−Ca−Me) around the Ca atom of the CaHMeEt. The Gibbs free energy and potential energy changes (in kcal/mol) are presented without and with parentheses, respectively.

Figure 6. Geometry and energy changes (in kcal/mol) in transmetalation between PdII(Br)(PhA)(PCyp3)2 and CuI{CaHMe(CHCH2)}(NHC). Bond distance is in angstrom. BA represents the sum of three bond angles (∠(CH2CH)−Ca−H, ∠(CH2CH)−Ca−Me, ∠H−Ca−Me) around the Ca atom of the CaHMe(CHCH2) group. Figure S6a and S6b in Supporting Information present details in bond distance and bond angle. The Gibbs free energy and potential energy changes (in kcal/mol) are presented without and with parentheses, respectively.

planar CaHMePhB structure. The presence of PhB facilitates inversion transmetalation if the understanding above is correct. To elucidate whether the presence of PhB is important or not, we investigated transmetalation of a CaHMeEt group between PdII(Br)(PhA)(PCyp3)2 and CuI(CaHMeEt)(NHC). As shown in Figure 5, adducts ADb1 and ADb2 are formed like ADa1 and ADa2. Then, transmetalation occurs through a transition state TSb1 in an inversion manner to afford PdII(PhA)(CaHMeEt)(PCyp3)(μ-Br)CuI(NHC) Pb1. After coordination of PCyp3, reductive elimination occurs to afford product Prd2 Pd0(PCyp3)2 + PhEtCaHMe. An explanation of geometry and

transition state because such ligands are good for bridging Pd and Cu. Transmetalations of Cu I (C a HMeEt)(NHC) and CuI{CaHMe(CHCH2)}(NHC) with PdII(Br)(PhA)(PCyp3)2. It is important to ascertain the cases in which inversion transmetalation occurs. As discussed above, the CaHMePhB group has an almost planar structure in TSa1. In such geometry, three sp2 orbitals of the Ca atom form three covalent bonds with H, Me, and PhB groups. One remaining 2p orbital interacts with the CuI and PdII atoms. It is likely that the Ca 2p orbital can conjugate with the PhB π* orbital to stabilize the 14070


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Table 1. Activation Barrier in Potential Energy (Ea, in kcal/mol), Gibbs Activation Free Energy (ΔG°‡, in kcal/mol), Difference in Zero-Point-Energy between TS and AD (ΔZPE in kcal/mol; TS = TSa1, TSb1, etc.; AD = ADa1, ADb1, etc.), and Activation Entropy (Δ(TΔS°‡), in kcal/mol) of TSa1, TSa3, TSb1, TSb3, TSc1, and TSc3 at 298 K TSa1 TSa3 TSb1 TSb3 TSc1 TSc3

Ea

ΔG°‡

ΔZPE°‡

Δ(ΤΔS°‡Tr)a

Δ(ΤΔS°‡Rot)b

Δ(ΤΔS°‡Vib)c

39.9 36.7 42.7 35.7g 40.0 28.6

21.2 (18.0)d 24.2 (21.5)d 31.7 − 24.3 19.9

4.9e 2.4e 2.8e − 3.7e −1.5e

5.7f (7.1)d 5.7f (7.1)d 5.6f − 5.6f 5.6f

9.5f (11.9)d 9.3f (11.9)d 9.5f − 9.5f 9.4f

−1.5f (−1.9)d −5.9f (−7.4)d −8.4f − −3.8f −9.3f

Translational entropy corrected by the method of Whitesides et al.50 bRotational entropy. cVibrational entropy. dThe value for 373 K. eΔZPE°‡ = ZPE(TSa, TSb, or TSc) + ZPE(PCyp3) − ZPE(ADa1, ADb1, or ADc1). fΔ(TΔS°‡) = TΔS(TSa, TSb, or TSc) + TΔS(PCyp3) − TΔS(ADa1, ADb1, or ADc1). gThe retention transmetalation starting from ADb2 occurs with nearly no barrier, as shown in the Supporting Information Table S1 and Figure S4. This is an approximate value which was obtained for the geometry at a3 in Figure S5(A) in Supporting Information presented by the potential energy surface against the direction change of the CaHMeEt group. a

and vibrational activation entropies (ΔS°‡Tr, ΔS°‡Rot, and ΔS°‡Vib, respectively). Although ΔZPE, ΤΔS°‡Tr, and ΤΔS°‡Rot differ little between TSa1 and TSa3, the ΤΔS°‡Vib value is somewhat larger in TSa1 than in TSa3. As a result, the ΔG°‡ becomes smaller for TSa1 than for TSa3; the difference in ΔG°‡ between TSa1 and TSa3 is larger at 373 K than at 298 K, as expected (these values at 373 K are presented in parentheses of Table 1). The larger ΤΔS°‡Vib of TSa1 arises from the presence of smaller vibrational frequencies in the transition state. In other words, TSa3 has a tighter structure than TSa1 because the large Pd−Ca−Cu angle of TSa1 allows easy bending vibration and rotational−vibrational motions but the Br-bridging structure of TSa3 suppresses these vibrations (i.e., TSa3 is tighter than TSa1). Also, it is likely that the presence of bulky PCyp3 and NHC contributes to the congested structure of TSa3 (Figure 3). This comparison cannot be made between TSb1 and TSb3 because TSb3 could not be fully optimized, as described above.55 Only the comparison between TSc1 and TSc3 was made here. It shows that the vibrational activation entropy is larger in TSc1 than in TSc3 similar to TSa1 and TSa3. However, the activation barrier (Ea) in potential energy is much larger in TSc1 than in TSc3. Therefore, the larger ΔS°‡Vib cannot overcome the larger Ea value of TSc1. Hereinafter, the term “activation barrier (Ea)” is defined as the potential energy difference between the transition state and reactant adduct. The difference in the activation barrier between retention and inversion transmetalations is also an important factor, as shown above. Because significant differences were found between the CaHMePhB and the CaHMeEt, we first compare the transmetalation of the CaHMePhB with that of the CaHMeEt, using deformation/interaction analysis.56−60 The transition state structure was separated into three moieties, as shown in Scheme 5. The deformation energy (DE) of each fragment was defined as the energy change of the fragment when going from reactant to transition state and the interaction energy (INT) as stabilization energy by interaction between the two fragments with distorted geometries involved in the transition state: DE(A) = Et(A in transition state) − Et(A in reactant) and INT(A−B) = Et(A−B in transition state) − Et(A in transition state) − Et(B in transition state). As shown in Table 2, INT(A−C) is considerably larger in TSa3 than in TSa1, because the Br is bridging the Pd and the Cu in TSa3 but not in TSa1. Therefore, the sum of INT terms is more negative (more attractive) in TSa3 than in TSa1, but all DE terms are more favorable (less repulsive) for TSa1 than for TSa3. It is

energy changes is presented in the Supporting Information on page S9. It is noteworthy that ΔG°‡ for TSb1 is 31.7 kcal/mol (relative to ADb1), which is larger than that for TSa1. We tried to locate a transition state for retention transmetalation starting from ADb2, but we were unable to do so. With a tiny orientation change of the CaHMeEt group toward the Pd atom and a tiny decrease in the Pd−Cu distance, the reaction system becomes more stable in terms of potential energy and reaches a product (Pb2)54 of transmetalation; in other words, no barrier was found. Details are presented in Supporting Information pages S10−14 (Table S1 and Figures S4 and S5). From these results, we inferred that, in the case of the CaHMeEt group, retention transmetalation occurs with nearly no barrier from ADb2. Its transition state is close to ADb2 but inversion transmetalation becomes difficult. The results presented above suggest that conjugation between the PhB π* and the Ca 2p orbitals is crucially important for the inversion transmetalation of the CaHMePhB. Therefore, we investigated transmetalation of CaHMe(CH CH2), which has small π-electron system. Figure 6 shows that inversion transmetalation occurs via ADc1 and TSc1 to afford Pd(Br)(PhA){CaHMe(CHCH2)}(PCyp3)-Cu(NHC) Pc1 in which the Cu does not interact with the Br. The Br-bridging intermediate Pc2 is formed via TSc2. The ΔG°‡ for TSc1 is 24.3 kcal/mol relative to the reactant ADc1. An explanation of geometry changes is presented on Supporting Information pages S15−S19 and in Figures S6a and S7. Starting from ADc3, retention transmetalation occurs via a transition state TSc3 to afford the Br-bridged adduct Pd(PhA){CaHMe(CHCH2)}(PCyp3)(μ-Br)Cu(NHC) Pc4. The ΔG°‡ for TSc3 is 19.9 kcal/mol, which is much smaller than that for TSc1. The details in geometry changes are depicted in Supporting Information Figure S6b. The results discussed above show that (i) the presence of phenyl group on the Ca atom is crucially important for inversion transmetalation, (ii) vinyl is insufficient for inversion transmetalation, but (iii) alkyl group on Ca facilitates retention transmetalation. Difference in Potential Energy and Activation Entropy between Inversion and Retention Transmetalations. In transmetalation of the CaHMePhB group, the Br is bridging the Pd and the Cu in TSa3 but does not in TSa1. As a result, TSa3 is more stable than TSa1 in terms of potential energy, as shown in Table 1. However, the ΔG°‡ for TSa1 is smaller than that for TSa3. To ascertain the reason, we evaluated zero-point energy change (ΔZPE) from ADa1 to TSa1, translational, rotational, 14071


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electron system. When the Ca has no π substituent, the DE(B) term is more positive (larger destabilization) in the inversion transition state than in the retention transition state, which facilitates the retention transmetalation. Important Factors for Stabilizing Planar CaHMeR Group (R = Phenyl, Ethyl, or Vinyl). As discussed above, the DE(B) term is an important factor for facilitating inversion transmetalation. The DE(B) term arises from distortion of the CaHMeR group. In the inversion transmetalation, the geometry around the Ca becomes almost planar. Such planar geometries of the CaHMePhB and CaHMe(CHCH2) are more stable than the tetrahedral geometry, as shown in Table 2, which engenders a negative DE(B) term in the inversion transmetalation. However, the planar geometry of the CaHMeEt is unstable, which engenders the large (positive) DE(B) term in inversion transmetalation. Going from the tetrahedral structure to the planar one, the sp3 orbital of the Ca atom changes to the 2p orbital at higher energy and the steric repulsion of substituents on the Ca becomes smaller, suggesting that these two factors are crucially important for determining the DE(B) term. The DE(B) is small if the Ca 2p orbital energy is not very unstable in the planar CaHMeR (R = Ph, Et, or CHCH2) group. As shown in Figure 7A, the π* orbital of the phenyl group is a conjugation partner of the Ca 2p orbital of the planar CaHMePhB to lower the 2p orbital energy. In the CaHMeEt, the 2p orbital is not stabilized in energy at all because of the absence of the conjugation partner in the ethyl group (Figure 7B). In the CaHMe(CHCH2), the vinyl π* orbital is a conjugation partner of the 2p orbital. However, its π* energy is higher than the Ph π* (Table 3), leading to the formation of weaker conjugation. Actually, the 2p orbital of the planar CaHMeR becomes higher in energy than the sp3 of the tetrahedral CaHMeR and the destabilization energy increases in the order of R = phenyl (0.29 eV) < vinyl (0.40 eV) < ethyl (0.57 eV), as shown in Figure 7. This order is consistent with the discussion of the conjugation with the Ca 2p orbital presented above. Another factor is steric effect of the substituents on the Ca atom. To stabilize (or less destabilize) the planar CaHMeR structure, a bulky substituent is favorable because the steric repulsion in the planar CaHMeR is less than in the tetrahedral structure. Because the phenyl group is larger than the vinyl, the DE(B) term of the CaHMePhB is more negative than that of the CaHMe(CHCH2) because of both of electronic and steric effects, which favors inversion transmetalation for the CaHMePhB compared to the CaHMe(CHCH2). In the case

Scheme 5. How to Separate Reaction System into Three Fragments

noteworthy that the greatest difference is found in the DE(B), which is the deformation energy of the CaHMePhB group. The energy is negative in TSa1, but considerably positive in TSa3, indicating that the CaHMePhB tends to take a planar geometry that is favorable for inversion transmetalation. For the analysis of TSb3 (Table 2), an approximate geometry was used because TSb3 could not be fully optimized. Because the potential energy starts to decrease with a tiny direction change of the CaHMeEt, as described above, the approximate geometry was taken to be the same as that immediately before the energy decrease, which is structure a3 in Figures S4 and S5 and page S14 in Supporting Information.55 The sum of INT terms is similar between TSb1 and TSb3 because the Br is bridging the Pd and the Cu in both transition states. A significant difference was found in DE(B) again. In contrast to TSa1 and TSa3, this DE(B) value is significantly larger in TSb1 than in TSb3, indicating that the CaHMeEt group tends to take a tetrahedral structure and that its planar structure is considerably unstable. In transmetalation of the CaHMe(CHCH2), the sum of INT terms is more negative in TSc3 than in TSc1 as TSa1 and TSa3, but the difference in the sum between TSc1 and TSc3 (32.3 kcal/mol) is significantly larger than that (26.6 kcal/mol) between TSa1 and TSa3. Nevertheless, the difference (18.7 kcal/mol) in the sum of DE terms between TSc1 and TSc3 is smaller than that (22.5 kcal/mol) between TSa1 and TSa3. Therefore, the DE(B) term contributes to the stabilization of TSc1 but it is insufficient to overcome the less-negative values of INTs in TSc1. The results presented above show that the INT(A-C) term is more favorable for retention transmetalation than for inversion transmetalation,61 but the DE(B) term is more negative (larger stabilization) in the inversion transmetalation than in the retention transmetalation when the Ca has a substituent with π

Table 2. Deformation energies (DE, in kcal/mol) and interaction energies (INT, in kcal/mol) of TSa1, TSa3, TSb1, TSb3a, TSc1, and TSc3 TSa1 TSa3 TSb1 TSb3a TSc1 TSc3

DE(A)

DE(B)

DE(C)

INT(A−B)

INT(B−C)

INT(A−C)

INT(MIX)b

ΣINT

3.7 9.3 5.4 2.0 3.4 9.8

−3.0 8.4 8.6 0.5 −1.2 6.4

−0.4 5.1 −0.2 0.4 −0.5 4.2

−49.1 −73.2 −76.2 −29.9 −55.0 −73.7

−155.0 −134.5 −162.3 −183.1 −158.1 −138.6

−14.4 −51.8 −24.2 −24.2 −14.7 −52.7

32.9 47.3 51.1 29.7 40.1 45.0

−185.6 −212.2 −211.6 −207.5 −187.7 −220.0

a

The retention transmetalation starting from ADb2 occurs with nearly no barrier, as shown by the Supporting Information page S10, Table S1, and Figure S4. TSb3 is an approximate transition state which was obtained by the potential energy surface against the direction change of the CaHMeEt group (the geometry at a3; see Table S1, Figures S4 and S5, and footnote 55). bINT(MIX) = [E(TS) − E(A) − E(B) − E(C)] − [INT(A−B) + INT(A−C) + INT(B−C)]. 14072


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Figure 7. Orbital interaction between the π* orbital of the phenyl/vinyl groups and the 2p orbital of the Ca in transition state of inversion transmetalation of CaHMePhB (A), CaHMeEt (B), and CaHMe(CHCH2) (C). M06/BS-II was used.

Table 3. Difference in Activation Barrier (ΔEa, in kcal/mol)a between Inversion and Retention Transition States in the Transmetalation of CaHMe(C6H4-Xpara) (X = H, CF3, OMe, CH3, NO2) and CaHMe(CHCH2), Difference in Deformation Energy (ΔDE(B)b, in kcal/mol) between the Inversion and Retention TSs,c and π* MO Energies (in eV) of X-Substituted Phenyl (C6H5X) and Vinyl (CH2CH2) Groupsd

ΔEa ΔDE(B) π* MO

 CH CH2

−C6H4− OMep

−C6H4− CH3p

9.4 −7.6 0.35

3.5 −10.5 0.32

3.2 −11.2 0.00

−Ph

− C6H4− CF3p

−C6H4− NO2p

3.2 −11.4 −0.08

0.4 −14.2 −0.91

−0.8 −17.7 −2.49

CaHMe(C2H5) (Table 2), suggesting that the bulky isopropyl group reduces the destabilization of the planar geometry of the CaHMe(iPr), which was expected above. However, the DE(B) term is still larger for the transition state of the inversion transmetalation than that for the retention transmetalation. In addition, we investigated the transmetalation of CaHMe2 group to get better understanding of the steric effect. Inversion transmetalation occurs with similar energy changes to that of the CaHMeEt (Figure S9 in Supporting Information). However, transition state for retention transmetalation could not be obtained because this transmetalation occurs with nearly no barrier, as shown in Supporting Information Table S3 and Figure S10, like that of the CaHMeEt. The results of deformation/interaction analysis are similar to those of the transmetalation of CaHMeEt (Table S5 in Supporting Information), where an approximate geometry for the retention transition state was used (see Supporting Information Table S3 and page S22 to S23). These results suggest that normal-alkyl group does not induce significantly different steric effect from that for the CaHMeEt. In conclusion, the electronic effect (π* orbital energy of substituent on the Ca) and steric effect of the substituents on the Ca are important but the electronic effect is more important than the steric one for stabilizing the planar CaHMeR geometry. Substituent and Phosphine Effects on Inversion vs Retention Transmetalation. Orbital conjugation between the PhB π* and the Ca 2p orbitals depends on the π* orbital energy. The best way to control the π* orbital energy is to introduce an appropriate substituent into the phenyl group. Here, electron-withdrawing and electron-releasing substituents (CF3, NO2, OMe, and CH3) were introduced to the PhB group at the para position to the Ca to avoid the change in steric effect.

a ΔEa = Ea(inversion) − Ea(retention). bΔDE(B) = DE(B; inversion) − DE(B; retention). cThe geometry of the [CaHMe(CHCH2)]− fragment was taken to be the same as the corresponding moiety in RcB1, TSc1 and TSc3 (Scheme 5). dTo avoid the orbital conjugation with sp3 orbital of the Ca atom, C6H5X and CH2CH2 were calculated by replacing the Ca with H, using the same geometry as that in TSa1 and TSc1.

of the CaHMeEt, the conjugation partner for the Ca 2p is absent. Only the steric factor participates in decreasing the DE(B) term. Consequently, the DE(B) term becomes larger (positive), which is favorable for retention transmetalation. To elucidate the importance of the steric effect further, we investigated the transmetalation of CaHMe(iPr) and found that inversion transmetalation is difficult but retention transmetalation easily occurs. Details of this transmetalation are shown in Supporting Information Figure S8. The DE(B) terms are 7.5 and 1.0 kcal/mol, respectively, for the inversion and retention transition states (Table S5 in Supporting Information). This DE(B) value for inversion transmetalation is smaller than that (8.6 kcal/mol) for inversion transmetalation of the 14073


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Journal of the American Chemical Society The difference in Ea, named ΔEa, is defined as Ea(inversion) − Ea(retention). The ΔEa decreases in the order of X = OMe > CH3 ∼ H > CF3 > NO2, as shown in Table 3, indicating that the inversion transmetalation becomes easier in the order of OMe < CH3 ∼ H < CF3 < NO2. The decreasing order of ΔEa is almost parallel to that of ΔDE(B) OMe > CH3 ∼ H > CF3 > NO2, where ΔDE(B) is defined as DE(B; inversion) − DE(B; retention). In addition, the π* orbital energy becomes lower in the same order of OMe > CH3 ∼ H > CF3 > NO2. These results demonstrate clearly that the presence of electronwithdrawing substituent on the PhB is favorable for the inversion transmetalation. Here, it should be described that the yield of the product was found experimentally to be very small when the electronwithdrawing Cl was introduced to the phenyl group of styrene.20 To investigate this experimental finding, we calculated the transmetalation of Cu{CaHMe(C6H4Clp)}(NHC) with Pd(Br)(PhA)(PCyp3)2, which revealed that, as expected, inversion transmetalation occurs easily but reductive elimination requires larger activation energy to give rise to very small yield. Details of this transmetalation and reductive elimination are presented in Supporting Information Figures S11 and S12. The results show that the use of electronwithdrawing substituent on the PhB facilitates the inversion transmetalation by lowering the PhB π* energy. However, the use of too strongly electron-withdrawing substituent is not recommended because it suppresses reductive elimination. It would be interesting to know if the σ-electron-withdrawing group facilitates inversion transmetalation. However, the retention transmetalation of Cu{CaHMe(CF3)}(NHC) with Pd(Br)(PhA)(PCyp3)2 occurs more easily than the inversion transmetalation. Details of this transmetalation are presented in Supporting Information Figures S13 and S14, and Table S4. Although the transition state for the retention transmetalation could not be optimized, a tiny direction change of the CaHMe(CF3) group toward the Pd in a reactant adduct Pd(Br)(PhA)(PCyp3)-Cu(CaHMe(CF3))(NHC) induces a markedly large energy decrease, suggesting that retention transmetalation occurs with nearly no barrier from the adduct like that in CaHMe(C2H5). The ΔG°‡ for the inversion transition state is 33.8 kcal/mol, larger than that (31.7 kcal/ mol) for the inversion transmetalation of the CaHMe(C2H5), indicating that the σ-electron-withdrawing alkyl group is ineffective for inversion transmetalation. At the end of this section, we wish to describe whether the use of PCyp3 is crucially important or not for inversion transmetalation. Because the use of XPhos was reported from experiment to induce retention transmetalation between PdII and CuI complexes,20 transmetalation between Pd(Br)(PhA)(XPhos) and Cu(CaHMePhB)(NHC) was investigated here. In this combination, inversion transmetalation occurs more easily than retention transmetalation, but the difference in ΔG°‡ between two transmetalations is moderately smaller than that with Pd(Br)(PhA)(PCyp3), as shown in Supporting Information Figure S15(A). Because SIMes (1,3-dimesitylimidazolidin2-ylidene) was used as a carbene ligand for the CuI complex in the experiment,20 Cu(CaHMePhB)(SIMes) was used for calculation instead of Cu(C aHMePhB )(NHC). In such combination, retention transmetalation occurs via a lower energy transition state than inversion transmetalation (Figure S15(B)), which is consistent with the experimentally observed result.20 This significant effect of carbene ligand arises from steric repulsion. As shown in Figure 3, the iPr group of NHC

exists close to the bridging Br to induce steric repulsion with the Br in TSa3 for retention transition state. In the case of the SIMes ligand, however, the Me group of SIMes is distant from the Br so as not to induce large steric repulsion with the Br because of its smaller size than iPr, as shown in Figure S15(B). Therefore, the transition state for retention transmetalation becomes more stable in the case of SIMes ligand. These results reflect the particular importance of using bulky carbene ligand to achieve inversion transmetalation.

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CONCLUSIONS Transmetalation between Pd II (Br)(Ph A )(PCyp 3 ) 2 and CuI(CaHMeR)(NHC) (R = phenyl (PhB), methyl (Me), ethyl (Et), isopropyl (iPr), trifluoromethyl (CF3), and vinyl (CHCH2)) was theoretically investigated. The computational results show that inversion transmetalation occurs more easily than retention transmetalation in the case of R = phenyl. In the inversion transition state, the CaHMePhB group has almost planar geometry around the Ca atom, in which the Ca 2p orbital interacts with both of the Pd and Cu atoms. This transition state resembles that of the SE2(back) reaction. In the transition state with a planar CaHMePhB group, the Ca 2p orbital conjugates with the PhB π* to stabilize the planar geometry of the CaHMePhB group. The retention transmetalation occurs with a smaller activation barrier (in potential energy) than the inversion transmetalation because the Br bridges the Pd and Cu in retention transition state. However, the Gibbs activation free energy is greater than that of the inversion transmetalation because the retention transition state with Br-bridging structure is tighter than the inversion transition state and the activation entropy is less in retention transition state than in inversion transition state. In the case of R = methyl, ethyl, isopropyl, and CF3, retention transmetalation occurs more easily than inversion transmetalation. Orbital conjugation is absent between the alkyl group and the Ca 2p orbital in the planar CaHMeR (R = alkyl). Therefore, the planar CaHMeR geometry for inversion transition state is not stabilized well. In the case of R = vinyl, retention transmetalation occurs more readily than inversion transmetalation despite of the presence of the vinyl π* orbital because the vinyl π* orbital exists at higher energy than the phenyl π* and the conjugation with the Ca 2p orbital is not large. In addition, the vinyl is smaller than the phenyl. The importance of the π* orbital energy of the PhB is shown clearly by the transmetalation of Cu{CaHMe(C6H4Xp)}(NHC) (X = OMe, Me, H, CF3, or NO2). The π* orbital energy becomes lower in the order of OMe > Me ∼ H > CF3 > NO2, and the energy difference ΔEa (= Ea(inversion TS) − Ea(retention TS)) decreases in the same order of OMe > Me ∼ H > CF3 > NO2. It is concluded that the electronwithdrawing substituent is favorable for inversion transmetalation because the Ph π* orbital at low energy forms good conjugation with the Ca 2p orbital to stabilize the planar CaHMe(C6H4Xp). However, the use of overly strong electronwithdrawing substituent is not recommended for presenting good yield because it suppresses the reductive elimination. Summarizing the results presented above, we wish to propose three factors that facilitate inversion transmetalation: One is to provide a good π conjugation partner with the Ca 2p orbital. The second is not to use a ligand that strongly bridges Pd and Cu. The last is to use a bulky substituent and/or ligand that increases the tightness of the bridging transition state and/ or decreases the destabilization energy of the planar CaHMeR 14074


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group relative to the tetrahedral structure. The combination of phosphine and carbene ligands is also important.

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/jacs.7b04383. Full citation of ref 49, Figures S1−S15, Tables S1−S7 (PDF) Cartesian coordinates of important species calculated here (PDF)

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AUTHOR INFORMATION

Corresponding Author

*[email protected] ORCID

Yoshiaki Nakao: 0000-0003-4864-3761 Shigeyoshi Sakaki: 0000-0002-1783-3282 Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work is financially supported by the Grants-in Aid for Scientific Research (B) (JP15H03770) and Scientific Research (C) (JP17KT0098) and Japan Science and Technology Cooperation (CREST “Establishment of Molecular Technology towards the Creation of New Functions” Area). We are also thankful to the computational facility at the Institute of Molecular Science, Okazaki, Japan.

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REFERENCES

(1) Palladium-Catalyzed Reactions Involving Reductive Elimination: Handbook of Organopalladium Chemistry for Organic Synthesis; Negishi, E., Ed.; Wiley: New York, 2002; Vol. 1, pp 215−994. (2) Metal-Catalyzed Cross-Coupling Reactions, 2nd ed.; deMeijere, A., Diederich, F., Eds.; Wiley-VCH: Weinheim, 2004. (3) Trost, B. M.; Ball, Z. T. Synthesis 2005, 2005, 853−887. (4) Modern Reduction Methods; Andersson, P. G., Munslow, I. J., Eds.; Wiley-VCH: Weinheim, 2008. (5) Sakaki, S. Practical Aspects of Computational Chemistry II; Leszczynski, J., Shukla, M. K., Eds.; Springer: Dordrecht, 2012; pp 391−470. (6) Guan, W.; Sayyed, F. B.; Zeng, G.; Sakaki, S. Inorg. Chem. 2014, 53, 6444−6458. (7) Braga, A. A. C.; Morgon, N. H.; Ujaque, G.; Maseras, F. J. Am. Chem. Soc. 2005, 127, 9298−9307. (8) Garcia-Melchor, M.; Braga, A. A. C.; Lledos, A.; Ujaque, G.; Maseras, F. Acc. Chem. Res. 2013, 46, 2626−2634. (9) Ortuno, M. A.; Lledos, A.; Maseras, F.; Ujaque, G. ChemCatChem 2014, 6, 3132−3138. (10) Sumimoto, M.; Iwane, N.; Takahama, T.; Sakaki, S. J. Am. Chem. Soc. 2004, 126, 10457−10471. (11) Sugiyama, A.; Ohnishi, Y.; Nakaoka, M.; Nakao, Y.; Sato, H.; Sakaki, S.; Nakao, Y.; Hiyama, T. J. Am. Chem. Soc. 2008, 130, 12975− 12985. (12) Prez-Temprano, M. H.; Casares, J. A.; de Lera, A. R.; Á lvarez, R.; Espinet, P. Angew. Chem., Int. Ed. 2012, 51, 4917−4920. (13) delPozo, J.; Gioria, E.; Casares, J. A.; Á lvarez, R.; Espinet, P. Organometallics 2015, 34, 3120−3128. (14) delPozo, J.; Salas, G.; Á lvarez, R.; Casares, J. A.; Espinet, P. Organometallics 2016, 35, 3604−3611. (15) Hatanaka, Y.; Hiyama, T. J. Am. Chem. Soc. 1990, 112, 7793− 7794. 14075


Article

Journal of the American Chemical Society (51) When BS-I was employed, TSa1 is moderately less stable than ADa2. However, TSa1 becomes slightly more stable when BS-II was used. These results clearly show that the transmetalation from ADa2 occurs with nearly no barrier. (52) To check if the Cu interacts with the PhB group in the product adduct, another geometry optimization was carried out starting from the structure in which the Cu interacts with the PhB without the Pd-Br interaction. The optimized geometry was the same as Pa2, indicating that the Br-bridging structure is very stable and does not allow the Cu interaction with the PhB due to the strained geometry. (53) Sakaki, S.; Mizoe, N.; Musashi, Y.; Biswas, B.; Sugimoto, M. J. Phys. Chem. A 1998, 102, 8027−8036. (54) Pb2 is essentially the same as Pb1, while the conformation of the CaHMeEt is slightly different between them. (55) Potential energy surface was calculated against the orientation of the CaHMeEt group and the Pd−Cu distance (Figure S4). The geometry a3 (Figure S4(A)), immediately before the potential energy against the CaHMePhB orientation change starts to decrease, was taken as the approximate geometry for transition state. The potential energy surface against the Pd−Cu distance changes little around geometries b2 and b3, but then decreases monotonously (Figure S4(B)). The geometry a3 is intermediate between b2 and b3, suggesting that the approximate geometry a3 is reasonable if we take the potential energy surface against the Pd−Cu distance. (56) Ess, D. H.; Houk, K. N. J. Am. Chem. Soc. 2007, 129, 10646− 10647. (57) Ess, D. H.; Houk, K. N. J. Am. Chem. Soc. 2008, 130, 10187− 106198. (58) Schoenebeck, F.; Houk, K. N. J. Am. Chem. Soc. 2010, 132, 2496−2497. (59) Frernandez, I.; Bickelhaupt, F. M. Chem. Soc. Rev. 2014, 43, 4953−4967. (60) Levandowski, B. J.; Houk, K. N. J. Am. Chem. Soc. 2016, 138, 16731−16736. (61) This is true for the comparison between the retention transition state with the Br-bridging structure and the inversion one with the large Pd−Ca−Cu angle. However, the comparison between TSb1 and TSb3 does not fit to this conclusion because both transition states have the Br-bridging structure. In this comparison, not the INT terms but the DE terms are important for the difference.

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How to Control Inversion vs Retention Transmetalation between PdII

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