PdIV Species Mediation in PdII-Catalyzed Direct Alkylation of Arenes

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Article IV

II

Pd Species Mediation in Pd-catalyzed Direct Alkylation of Arenes with Oxiranes: A DFT Study LIAN BING, Lei Zhang, Shi-Jun Li, Lu-Lu Zhang, and De-Cai Fang J. Org. Chem., Just Accepted Manuscript • DOI: 10.1021/acs.joc.7b03236 • Publication Date (Web): 27 Feb 2018 Downloaded from http://pubs.acs.org on February 28, 2018

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The Journal of Organic Chemistry

PdIV Species Mediation in PdII-Catalyzed Direct Alkylation of Arenes with Oxiranes: A DFT Study Bing Liana, Lei Zhanga,b, Shi-Jun Lia, Lu-Lu Zhanga, and De-Cai Fang*a a

College of Chemistry, Beijing Normal University, Beijing 100875, China

b

School of Science, Tianjin Chengjian University, Tianjin 300384, China *E-mail for D.-C. F.: [email protected]

Abstract: The reaction mechanisms of Pd(OAc)2-catalyzed dehydrogenative alkylation of 2-phenylpyridine with oxirane were investigated using DFT calculations. The most plausible reaction pathway was confirmed as a PdII/IV/II catalytic cycle consisting of four processes: C−H activation, ring-opening oxidative addition of oxirane, reductive elimination, and recovery of the catalyst. According to the B2PLYP/DGDZVP computational data, the oxidative addition of oxirane for converting PdII to PdIV was 1

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assigned to be the rate-determining step with a free-energy barrier of 28.1 kcal·mol-1. For comparison, we also studied the alternative PdII-only pathway without a change of oxidation state and found that it was hindered kinetically by a high free-energy barrier of 75.1 kcal·mol-1 occurring for the ring-opening migratory insertion of oxirane. In addition, the small-ring strain of oxirane should be responsible for the feasible C−O bond-cleavage and subsequent PdII→PdIV conversion, since the designed 4-, 5- and 6-member-ring reagents did not display such an oxidative addition reactivity. Lastly, an extended reactivity order among oxirane, PhI, PhBr and PhCl toward oxidative addition onto PdII to form PdIV was proposed in this article based on the computed kinetic parameters.

Introduction Over the past decades, carbon-carbon and carbon-hetero cross-couplings via transition-metal-catalyzed C−H activation have emerged as powerful and straightforward synthetic methodologies in modern organometallic chemistry.1 Diverse transition-metal catalysts have been designed to activate the target C−H bond on the reactant,2 among which palladium acetate represented the most widely used catalyst within the field of C−H activation chemistry.3 Most of the previous experimental4 and computational studies5,6 focused on the PdII/0-catalyzed C−H functionalizations. However, the mediation of high-oxidation-state palladium in cross-coupling reactions has become a hot research topic in recent years, with

main

attention

paid

to

the

development 2


of

PdII/IV-mediated

C−H

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functionalizations.7,8 The PdII → PdIV conversion should be responsible for the occurrence of PdIV-species in catalytic cycles, in which oxidative insertion of PdII into an easily broken bond and direct oxidation of PdII using a powerful oxidant have been the two most common methods for the formation of PdIV.9 Active single bonds, such as C−I, C−Br, I−O, I−Cl and O−O, are capable of oxidatively adding onto PdII to form PdIV, while the C−O bond is generally inert in this aspect. Additionally, powerful oxidants, such as PhI(OAc)2, PhICl2, IOAc and K2S2O8, have been widely applied to oxidation of PdII to PdIV,10,11,12 possibly via the electron transfer and radical generation mechanisms. In 2015, Motomu Kanai and co-workers13 reported a novel synthetic method for Pd(OAc)2-catalyzed

C−C

cross-coupling

between

2-phenylpyridine

(1a)

and

2-(phenoxymethyl)oxirane (2a) via C−H activation and oxirane-opening (see Scheme 1).

Scheme 1. Pd(OAc)2-catalyzed C−H activation and SN2-type oxirane-opening reaction, characterized in this work using DFT method.

Experimental observations showed that 1a reacting with 2a in the mixed solvent of hexafluoroisopropanol (HFIP) and AcOH generated the desired C−C cross-coupling product 3a in excellent yields (99%). Notably, this synthetic procedure performed very 3



well even at room temperature (25oC). These authors assumed that the occurrence of PdIV-intermediate, in situ generated from the oxidative addition of oxirane with a PdII-intermediate, should be responsible for the high reaction efficiency observed.13 It indeed raised a question whether the high-strain C−O bond of oxirane was able to facilitate the conversion of PdII to PdIV, considering that the normal C−O bond in acyclic compounds had difficulty in undergoing oxidative addition with PdII-precursors. Owing to the increasing importance of PdIV-catalysis in modern organometallic chemistry, 14 density functional theory (DFT) 15 studies have been conducted to locate the most favorable mechanism dominating the reaction in Scheme 1. A complete comparison between PdII/IV/II and PdII-only pathways has been made to demonstrate the indispensable role of the transient PdIV-intermediate. In addition, an extended reactivity order among a series of common reagents, including oxirane (and its homologues), PhI, PhBr and PhCl, has been proposed in this article.

Results and Discussion The left part of Scheme 2 shows the most favorable reaction pathway for the cross-coupling of 2-phenylpyridine (1a) with oxirane (2a), which presents a novel PdII/IV/II catalytic cycle consisting of four sequential steps: C−H activation, oxidative addition of 2a, reductive elimination, and recovery of the catalyst. For comparison, the inefficiency of the PdII-only pathway (see the right part of Scheme 2) will be discussed in the following. 4


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Scheme 2. Possible catalytic cycles for the cross-coupling between 2-phenylpyridine and oxirane: PdII/IV/II pathway (left) and PdII-only pathway (right). (CHA: C−H activation; OA: oxidative addition; RE: reductive elimination; RC: recovery of the catalyst; MI: migratory insertion)

1 Plausible Reaction Mechanism — PdII/IV/II Catalytic Cycle 1.1 C−H Activation The whole reaction mechanism of the PdII/IV/II catalytic cycle is depicted in Figure 1. C−H activation of 1a with Pd(OAc)2 should proceed with the assistance of the directing group,16 and the detailed reaction mechanism is characterized as a three-step pathway involving three transition states and three intermediates (see Figure 1a).

5



3

Figure 1. Proposed mechanisms of the PdII/IV/II pathway, along with the relative free-energies (kcal·mol-1) determined at the B3LYP/DGDZVP (in blue) and B2PLYP/DGDZVP (in green) levels of theory in acetic acid.

Initially, one of the O-arms of catalytic Pd(OAc)2 is substituted by the pyridine nitrogen to generate the substrate-catalyst encounter complex INT-1. This is followed by the intramolecular ligand exchange INT-1 → INT-2, through which a second O-arm of 6


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the original catalyst is replaced by a loose Pd…C interaction via the transition state TS-2. Afterward, the hydrogen on the target carbon could be abstracted by an adjacent acetate oxygen via the proton-transfer transition state TS-3, subsequently leading to the five-membered palladacyclic structure INT-3, a stable PdII-compound. Figure 1 provides the relative free-energies calculated at the B3LYP/DGDZVP (in blue) and B2PLYP/DGDZVP (in green) levels of theory. Since the high-level B2PLYP method is more likely to generate reasonable kinetic parameters than the relatively cheaper B3LYP method17, the following discussion is based on this high-level method, except noted elsewhere. The complexation between 1a and Pd(OAc)2 to form INT-1 is strongly favored in the free-energy change (i.e., ∆Gb2plyp = −9.6 kcal·mol-1) and has a low free-energy barrier of 8.5 kcal·mol-1, which can be ascribed to the stronger coordination ability of the pyridine nitrogen in comparison to that of the acetate oxygen. If one assumes the initial resting state of the catalyst to be the dimer form [Pd(OAc)2]2, the dissociation of [Pd(OAc)2]2 in the presence of 1a into two molecules of INT-1 could be spontaneous with the free-energy change of −3.6 kcal·mol-1 (i.e., [Pd(OAc)2]2 + 2 1a → 2 INT-1, ∆G = −3.6 kcal·mol-1). The following intramolecular ligand exchange, INT-1 → INT-2, is free-energy endothermal by 9.5 kcal·mol-1, since the Pd…C interaction is weaker than the Pd−O coordination. The proton-transfer transition state TS-3 is computed to be 14.3 kcal·mol-1 in free-energy above the initial materials, and the overall C−H activation reaction Pd(OAc)2 + 1a → INT-3 releases a total free-energy of 18.0 kcal·mol-1. 7



According to the steady-state approximation,18 the total free-energy barrier of C−H activation should be 23.9 kcal·mol-1, the free-energy difference between TS-3 and INT-1 determined at the B2PLYP/DGDZVP level of theory.

1.2 Oxidative Addition of Oxirane The subsequent oxidative addition of 2a with INT-3 is a apparently ring-opening addition process that involves the cleavage of a C−O bond and the formation of a Pd−C bond and a Pd−O bond, concurrently changing the oxidation state of palladium from +2 to +4 and the coordination number of palladium from 4 to 6. It is also a multi-step reaction pathway, as shown in Figure 1b. The first elementary step is the complexation of 2a with INT-3, which proceeds via a concerted ligand exchange resulting from the nucleophilic attack of the oxirane oxygen. INT-4 is the real precursor of the oxidative addition step (INT-4 → INT-5), as determined by IRC calculations. The optimized molecular geometry of TS-5 indicates a three-center transition state for the concerted addition of O1 and C2 onto the PdII-center at the price of breaking the C2−O1 bond. This step would increase the oxidation state of palladium from +2 to +4 and hence it could be facilitated by an acetic acid molecule, in that coordination of the acetic acid O3 atom to the central palladium can help to satisty the hexa-coordinate preference of PdIV. The formed PdIV-intermediate INT-5 has a typical hexa-coordinate octahedral geometry, containing two bidentate ligands of 1a and 2a moieties, a κ1-acetate ligand and a neutral acetic acid ligand. Then the proton of the acetic acid ligand can shift to the O1 atom via the transition state TS-6, 8


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which would convert the covalent Pd−O1 bond in INT-5 to the dative Pd−O1 interaction in INT-6. Note that INT-5 does not connect to INT-6 computationally, since several low-lying conformational change transition states exist between them; details are available in the Supporting Information. Finally, an intramolecular ligand substitution can readily cleave the loose Pd−O1 interaction to form INT-7, the most plausible PdIV-intermediate. The free-energy barrier of the oxdative addition process is estimated to be ca. 28.1 kcal·mol-1 (B2PLYP/DGDZVP) based on the difference between TS-5 and INT-3 + 2a, which seems to be slightly larger than that estimated from the experimental temperature of 25 oC. If one assumes the half-life time to be 24 h, the free-energy barrier should be 24.2 kcal·mol-1 at room temperature, which is close to that predicted by the CCSD(T)/DGDZVP single-point calculations (∆G ≠ = 25.8 kcal·mol-1), whereas the B3LYP method gives a much higher and thus unreasonable barrier (∆G ≠ = 34.1 kcal·mol-1). The overall transformation of INT-3 + 2a → INT-7 has to absorb free-energy by 9.8 kcal·mol-1, meaning that this PdII → PdIV conversion is moderately disfavored from thermodynamics.

1.3 Reductive Elimination and Recovery of the Catalyst The reductive elimination from INT-7 to INT-8, as shown in Figure 1c, has to pass over a three-center transition state TS-8. This step could recover the oxidation state of palladium from +4 to +2, concomitant with the change of the coordination number of 9



palladium from 6 to 4. The formed product 3a is still stuck on the palladium center in INT-8 using both coordination bonds of Pd−N and Pd…C. Two additional intramolecular ligand substitution steps, via the transition state TS-9 and the intermediate INT-9, are therefore necessary to liberate the product 3a and regenerate the catalyst Pd(OAc)2. Free-energies data demonstrate that the reductive elimination step has a low free-energy barrier of 14.7 kcal·mol-1 and is strongly exothermic (∆G = −25.9 kcal·mol-1). The next step needed for the recovery of the catalyst involves a free-energy barrier of merely 4.6 kcal·mol-1, indicating that 3a can easily be removed from the complex.

1.4 Entire Free-energy Profile The entire free-energy profile of the PdII/IV/II pathway is provided in Figure 2, in which the initial point is designated as the common zero reference. According to Kanai’s kinetic isotope effect experiments13, C-H activation is a rate-determining step. Our B2PLYP calculation indicates that TS-3 is the highest-energy stationary point on the whole potential energy surface. However, INT-3 is calculated to be 18.0 kcal·mol-1 lower than the initial points, leading to that the oxidative addition is the highest free-energy barrier among the all of elementary steps, slightly different from the experimental evidence. Such inconsistence might originate from the calculation method for the process of oxidation change from PdII to PdIV. The reductive elimination INT-7 → INT-8 need to overcome 14.7 kcal·mol-1, which should be accessible at room temperature. The most stable PdIV-intermediate on the 10


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potential energy surface is INT-7, which lies at 8.4 kcal·mol-1 in free-energy above the PdII-complex INT-4 but lies at 8.2 kcal·mol-1 below the initial point. The overall cross-coupling reaction of 1a + 2a → INT-9 has a total free-energy change of −40.1 kcal·mol-1, validating the thermodynamic accessibility for such a reaction.

Figure 2. The whole free-energy (in kcal·mol-1) profile of the plausible PdII/IV/II pathway, determined at the B3LYP/DGDZVP and B2PLYP/DGDZVP levels of theory in acetic acid.

2 Comparison of PdII/IV/II and PdII-only Mechanisms Computationally, the desired C−C cross-coupling between 1a and 2a could also take place immediately from INT-4, a PdII-complex formed from 2a and INT-3 (see the red path in Figure 3). Such a reaction route has to surmount a four-center transition state TS-PdII for the C−C bond-formation and the simultaneous C−O bond-cleavage. This 11



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reaction path leads to the formation of the PdII-intermediate INT-PdII without a change of oxidation state, and hence it matches the PdII-only pathway shown in Scheme 2. From the geometry of TS-PdII, this reaction path can be considered as a [2σ + 2σ] cycloaddition or migratory insertion mechanism between two σ-bonds. However, free-energy calculations demonstrate that this PdII-only pathway can surely be precluded from chemical kinetics, since the transition state TS-PdII is at least 75.1 kcal·mol-1 higher-lying than its precursor INT-3. Therefore, migration insertion between two σ-bonds is strongly disfavored according to our calculations, which is in sharp contrast to that participated by an olefinic π-bond in the Heck reaction.19 These data strongly support the PdIV-mediated pathway to be more probable than the PdII-only pathway. 45.6 57.1 TS-PdII

10.2 10.3 INT-PdII O

O O

N

HOAc

N

Pd

O

Pd O

O OPh

G =

2a INT-3 20.5 18.0

66.1 75.1

OPh

Inaccessible! PdII-only Path

PdII

PdII/IV/II Path Favorble!

PdIV

3a

INT-4 Formation of PdIV

34.1 G = 28.1 N

O Pd

N

O OAc Pd O OH OPh

0.4 8.2

INT-7 13.2 G = 14.7

O O O OH OPh

INT-8

O Pd

O OPh

TS-PdIV (TS-8) 12.8 6.5

O

O N

OH

G(B3LYP/DGDZVP) G(B2PLYP/DGDZVP)

27.5 34.1

Figure 3. Comparison of the PdII-only path (in red) and PdII/IV/II path (in blue), along with the relative free-energies (kcal·mol-1) determined at the B3LYP/DGDZVP (in blue) and 12


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B2PLYP/DGDZVP (in green) levels of theory in acetic acid.

3 Reactivities of Oxirane and Other Related Reagents The previous discussion has excluded the PdII-only pathway as an accessible pathway, and hence understanding the structure factors favoring the conversion of PdII to PdIV is of great value. Oxirane reacting favorably with PdII to form PdIV is due to a ring-opening oxidative addition process. In this section, we will explore the origin of the enhanced reactivity of oxirane, which is attributed to the strain effect of the small ring according to the following data. More importantly, the reactivity of oxirane will be compared to those of iodobenzene (PhI), bromobenzene (PhBr) and chlorobenzene (PhCl), because aryl halides have played a vital role in the formation of PdIV-intermediates.20

3.1 Strain Effect of Oxirane To probe the origin of the enhanced reactivity of oxirane, four oxygen-containing cyclic systems with different ring-sizes (3-, 4-, 5- and 6-member) have been designed, and their main kinetic and thermodynamic parameters are provided in Figure 4. The free-energy barriers for the 3-, 4-, 5- and 6-member-ring systems (n = 0, 1, 2 and 3) are computed to be 28.1, 37.7, 45.3 and 53.1 kcal·mol-1, respectively, which are in good agreement with the experimental observation that only the 3-membered oxirane could undergo oxidative addition reaction. Besides, the thermodynamic factor becomes less favorable as the ring-size increases; for example, the reaction free-energy changes for the 13



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n = 2 and n = 3 systems are computed to be 33.4 and 33.3 kcal·mol-1, respectively, strongly negating the thermodynamic spontaneity. Since the ring strain energy should be closely related to the bond-angle of ∠OCC (∠α), Figure 4 provides the changes of this bond-angle during the transformation R-PdII → TS → P-PdIV. Obviously, the changes of ∠ α are quite evident for the n = 0 system, ranging from 60o to 102o via 85o, which indicates substantial strain energies to be released during the transformation. For the n =1, n=2 and n = 3 systems, however, the changes are not so evident, especially for the last system in which the bond-angle ∠α varies very little. O N

O O

HO

Pd

OH

O N

OAc Pd

n

n=0

∠ ∠ ∠

(TS) (P-Pd

Ec-o = G = G =

Figure

4.

IV )

OPh

n

TS for PdII to PdIV

R-PdII

II (R-Pd )

O

O

OPh n = 0 (2a), 1, 2 and 3

OAc Pd

O n

OH

O N

OPh

PdIV-intermediate P-PdIV

TS n=2

n=3

60°

n=1 92° 101° 110°

106° 108° 112°

111°

85° 102° 56.4 28.1 10.3

59.9 37.7 11.3

101.3 45.3 33.4

111.1 53.1 33.3

Designed

ring-opening

oxidative

addition

108° 112°

processes

of

different

oxygen-containing rings with the shared PdII-intermediate (INT-3), along with the thermodynamic and kinetic parameters (kcal·mol-1) determined at the B2PLYP/DGDZVP level of theory in acetic acid. The calculated bond-angles (in degree) of ∠OCC (∠α) and bond dissociation energies (in kcal·mol-1) of C−O are given in the box.

14


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3.2 Reactivity Order for a Series of Oxidative Addition Reactions The carbon−halogen (C−X) bonds represent a typical type of reagents that have been widely used in oxidative addition reactions of PdII to PdIV, in which iodobenzene (PhI) and bromobenzene (PhBr) are high-efficiency while in most cases chlorobenzene is inert due to the high bond-strength of C−Cl. Figure 5 provides the oxidative addition reactions of PhI, PhBr and PhCl with the shared PdII-intermediate INT-3. The results indicate that the reaction becomes more challenging both kinetically and thermodynamically as the radius of halogen decreases (i.e., I → Br → Cl). The free-energy barriers are calculated to be 29.1 kcal·mol-1 for PhI, 34.7 kcal·mol-1 for PhBr and 41.0 kcal·mol-1 for PhCl, respectively, being parallel with the variations of the C−X bond dissociation energies. Therefore, the reactivity order toward oxidative addition with PdII is proposed to be 2a ≈ PhI > PhBr > PhCl.

Figure 5. Oxidative addition reactions of PhCl, PhBr and PhI with the shared PdII-intermediate (INT-3), along with the B2PLYP/DGDZVP calculated free-energy barriers and bond dissociation energies of C−X in kcal·mol-1.

15



COMPUTATIONAL DETAILS All of the theoretical computations were performed using the Gaussian 09 program package.21 The Becke-3-Lee-Yang-Parr (B3LYP)22 density functional method and standard double-ζ valence polarized all electron DGDZVP basis set23 were employed to fully optimize the reactant, intermediate, transition state and product geometries. The solvent effect of acetic acid (ε = 6.25) was simulated by the self-consistent reaction field (SCRF) polarizable continuum model (PCM),24 with our IDSRCF radii25 to define the molecular cavity. The optimized stationary point structures were subsequently characterized by frequency analyses at the same level of theory, from which zero-point energies and relative free-energies were obtained, in addition to conforming all the stationary points to be minimal or first-order saddle points on the potential energy surfaces. Intrinsic reaction coordinate (IRC)26 calculations with the Hessian-based predictor-corrector integrator (HPC)27 were also conducted to test the connectivity of some suspected transition state structures. The solution translational entropy correction was carried out using our THERMO program28 to get more accurate relative free-energies in solution. Due to the significant electron-correlation effect occurring in some loose transition states, the medium-level B3LYP calculations might not be so reliable for generating reasonable free-energy data. Therefore, single point calculations using the high-level B2PLYP29 density functional method were performed under the ORCA 04 program;30 this density functional method could compute more electron-correlation energies, including the 16


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possible dispersion contribution between different fragments, based on the second order perturbation treatment. In addition, we have performed the CCSD(T)31 single point calculations to validate the quality of the B2PLYP results (see SI for details).

Conclusion Density functional theory determinations have been carried out to qualitatively characterize the thermodynamic and kinetic aspects of Pd(OAc)2‐catalyzed alkylation of arenes with oxiranes. The following conclusions have been reached: (1) The PdII/IV/II catalytic cycle mediated by PdIV-species was confirmed to be the most plausible reaction pathway, in which the oxidative addition of PdII → PdIV preceded the C−C reductive elimination step and had the rate-controlling free-energy barrier of 28.1 kcal·mol-1. (2) The alternative PdII-only catalytic cycle was kinetically inaccessible, due to the presence of a high free-energy barrier of 75.1 kcal·mol-1. (3) Oxirane was an efficient reagent for converting PdII to PdIV mainly due to the strain effect of the small ring, since the designed 4-, 5- and 6-membered cyclic compounds did not display such a chemical behavior from our calculations. (4) A reactivity order of oxirane ≈ PhI > PhBr > PhCl toward oxidative addition with PdII to form PdIV was proposed in this article.

Acknowledgements: This work was supported by the National Natural Science 17



Foundation of China (21773010).

Supporting Information: The optimized geometric parameters for all stationary points; vibrational frequencies; electronic energies, zero-point energies and total free energies; complementary mechanistic characterizations and computational methods. This material is available free of charge via the Internet at http://pubs.acs.org.

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