Mechanism of the SuzukiâMiyaura Cross-Coupling Reaction Mediated

Article pubs.acs.org/Organometallics

Mechanism of the Suzuki−Miyaura Cross-Coupling Reaction Mediated by [Pd(NHC)(allyl)Cl] Precatalysts Giulia Magi Meconi,†,‡ Sai Vikrama Chaitanya Vummaleti,§ Jesús Antonio Luque-Urrutia,† Paola Belanzoni,‡ Steven P. Nolan,⊥ Heiko Jacobsen,∥ Luigi Cavallo,§ Miquel Solà,† and Albert Poater*,† †

Institut de Química Computacional i Catàlisi and Departament de Química, Universitat de Girona, c/Maria Aurèlia Capmany 6, 17003 Girona, Catalonia, Spain ‡ Dipartimento di Chimica, Biologia e Biotecnologie and Istituto di Scienze e Tecnologie Molecolari del CNR (CNR-ISTM), Università di Perugia, Via Elce di Sotto, 8, 06123 Perugia, Italy § King Abdullah University of Science & Technology, KAUST Catalysis Center (KCC), 23955-6900 Thuwal, Saudi Arabia ⊥ Department of Inorganic and Physical Chemistry, Universiteit Gent, Krijgslaan 281, S-3, B-9000 Ghent, Belgium ∥ KemKom, 1215 Ursulines Avenue, New Orleans, Louisiana 70116, United States S Supporting Information *

ABSTRACT: Density functional theory calculations have been used to investigate the activation mechanism for the precatalyst series [Pd]-X-1−4 derived from [Pd(IPr)(Rallyl)X] species by substitutions at the terminal position of the allyl moiety ([Pd] = Pd(IPr); R = H (1), Me (2), gem-Me2 (3), Ph (4), X = Cl, Br). Next, we have investigated the Suzuki−Miyaura cross-coupling reaction for the active catalyst species IPr-Pd(0) using 4-chlorotoluene and phenylboronic acid as substrates and isopropyl alcohol as a solvent. Our theoretical findings predict an upper barrier trend, corresponding to the activation mechanism for the [Pd]-Cl-1−4 series, in good agreement with the experiments. They indeed provide a quantitative explanation of the low yield (12%) displayed by [Pd]Cl-1 species (ΔG⧧ ≈ 30.0 kcal/mol) and of the high yields (≈90%) observed in the case of [Pd]-Cl-2−4 complexes (ΔG⧧ ≈ 20.0 kcal/mol). Additionally, the studied Suzuki−Miyaura reaction involving the IPr-Pd(0) species is calculated to be thermodynamically favorable and kinetically facile. Similar investigations for the [Pd]-Br-1−4 series, derived from [Pd(IPr)(Rallyl)Br], indicate that the oxidative addition step for IPr-Pd(0)-mediated catalysis with 4-bromotoluene is kinetically more favored than that with 4-chlorotoluene. Finally, we have explored the potential of Ni-based complexes [Ni((IPr)(R-allyl)X] (X = Cl, Br) as Suzuki−Miyaura reaction catalysts. Apart from a less endergonic reaction energy profile for both precatalyst activation and catalytic cycle, a steep increase in the predicted upper energy barriers (by 2.0−15.0 kcal/mol) is calculated in the activation mechanism for the [Ni]-X-1−4 series compared to the [Pd]-X-1−4 series. Overall, these results suggest that Ni-based precatalysts are expected to be less active than the Pd-based precatalysts for the studied Suzuki−Miyaura reaction.

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INTRODUCTION Transition-metal-catalyzed Suzuki−Miyaura cross-coupling reactions are among the most useful processes in modern organic synthesis given their broad scope and selectivity under mild conditions.1 In this context, palladium is the most studied metal,2 facilitating the formation of biaryl compounds found in natural products, as well as reagents and liquid crystals bearing chirality.3 Among its features, palladium is able to manage perfectly both oxidation states, 0 and 2+,4 required for the Suzuki−Miyaura reaction,5 especially stable with just one ligand in its coordination sphere.6,7 In recent years, substantial research has been carried out on N-heterocyclic carbene (NHC) ligands,8 and the replacement of tertiary phosphine9,10 by NHC ligands has had notable success in the development of catalysts in the context of palladium chemistry.11,12 These twoelectron donor ligands combine strong σ-donating properties © XXXX American Chemical Society

with a bulky steric pattern, which allow stabilization of the metal center and an increase in its catalytic activity.13 Consequently, the number of well-defined NHC-bearing palladium(II) complexes is growing, and their use in coupling reactions is witnessing growing interest.14,15 This family of palladium complexes exhibits high stability, allowing for indefinite storage and easy handling. The use of distinct complexes permits strict control of the [Pd]/ligand ratio (1:1), eliminating the need for the use of excess of costly ligands. Furthermore, it partially removes the “black box” character often associated with cross-coupling chemistry and NHC-[Pd] catalyst formation: a large number of [L-Pd]-based protocols have been developed for their preparation, but many are Received: February 14, 2017

A

DOI: 10.1021/acs.organomet.7b00114 Organometallics XXXX, XXX, XXX−XXX

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Organometallics conducted using an inert atmosphere and often require the use of a glovebox, significantly limiting the broader application of these techniques.16 To this end, numerous studies have been carried out toward the development of an idealized practical protocol that should be efficient at room temperature.17 Complexes of type [Pd(NHC)(R-allyl)Cl] have shown to be very active in the Suzuki−Miyaura coupling reaction.18 Their good activity results from the fact that they offer easy access to the monoligated species.19 [Pd(NHC)(R-allyl)Cl] catalysts are isolable complexes for which there is total control over the [Pd]/ligand ratio (1:1). Additionally, these compounds can be synthesized without the use of a glovebox, and after their preparation, they are indefinitely air- and moisture-stable. In 2002, Nolan and co-workers studied the catalytic performance of a precatalyst series [Pd]-Cl-1−4 (see Scheme 1a), derived from

Scheme 2. Two Proposed Precatalyst Activation Mechanisms (A and B) for the Considered Precatalyst Series [Pd]-Cl-1−4

Scheme 1. (a) Effect of Substitution at the Terminal Position of an Allyl Moiety on the Performance of Precatalyst Series [Pd]-Cl-1−4 in the Suzuki−Miyaura Cross-Coupling Reaction* and (b) Chemical Structures of [Pd]-Cl-1−4 Series

of the OtBu group to the allyl moiety to yield the precatalyst and the allyl(OtBu) species (mechanism B). We believe that understanding the details of the precatalyst activation mechanism proposed by Nolan et al. and a more in-depth understanding of the entire Suzuki−Miyaura reaction mechanism could help the future development of catalysts facilitating this critical reaction.24,25 In particular, a recent study by Hazari and co-workers has shown that the activation of the precatalyst can be improved employing the [Pd(η3-1-tBu-indenyl)(μ-Cl)2]2 scaffold, which avoids the formation of inactive PdI dimers,26 found when [Pd(η3-cinnamyl)(μ-Cl)2]2 is used.27 To summarize, in the current study, we perform density functional theory (DFT) calculations to investigate in detail the activation pathways in Scheme 2 for the [Pd]-Cl-1−4 series,28 followed by a complete description of the Suzuki−Miyaura reaction in Scheme 1a for catalytically active IPr-Pd(0) species.20,29 Additionally, to understand the effect of the halogen substituent, we extend the same investigations to the [Pd]-Br-1−4 series, derived from [Pd(IPr)(R-allyl)Br]. Finally, we explore the role of the metal by further extending similar investigations to Ni-based precatalyst series,30 [Ni]-X-1−4, derived from [Ni(IPr)(R-allyl)X] (X = Cl and Br).

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*

With 4-chlorotoluene and phenylboronic acid as substrates using isopropyl alcohol (iPrOH) solvent, KOtBu = potassium tert-butoxide. a Label used to define each precatalyst in the [Pd]-Cl-1−4 series considered for the present study (IPr is the NHC ligand N,N′-bis[2,6(diisopropyl)phenyl]imidazol-2-ylidene)). bData from ref 21.

COMPUTATIONAL DETAILS

All DFT static calculations were performed at the GGA level with the Gaussian09 set of programs,31 using the BP86 functional of Becke and Perdew.32 The electronic configuration of the molecular systems was described with the standard split-valence basis set with a polarization function of Ahlrichs and co-workers for H, C, B, N, O, Cl, and Br (SVP keyword in Gaussian)33 and Def2-QZVPP for K.34 For Pd and Ni, we used the quasi-relativistic Stuttgart/Dresden effective core potential, with an associated valence basis set (standard SDD keywords in Gaussian09).35 Geometry optimizations were carried out without symmetry constraints, and the characterization of the stationary points was performed by analytical frequency calculations. These frequencies were used to calculate unscaled zero-point energies (ZPEs) as well as thermal corrections and entropy effects at 298 K and 1 atm by using the standard statistical mechanics relationships for an ideal gas. Moreover, we also included the D3 Grimme pairwise scheme to account for dispersion corrections in the geometry optimizations.36 Energies were obtained via single-point calculations on the BP86optimized geometries using the M06 functional.37 In these single-point energy calculations, H, C, B, N, O, Cl, and Br were described by using the Def2-TZVP basis set that includes polarization and diffuse functions,38 Def2-QZVPP for K, whereas for the metals (Pd, Ni), the SDD basis set has been employed. On top of the M06/Def2TZVP∼sdd//BP86-D3/SVP∼sdd energies, we added the ZPEs’

[Pd(IPr)(R-allyl)Cl] by substitutions at the terminal position of the allyl moiety ([Pd] = Pd(IPr); R = H (1), Me (2), gemMe2 (3), Ph (4)), for the Suzuki−Miyaura cross-coupling reaction of 4-chlorotoluene and phenylboronic acid using isopropyl alcohol as solvent at room temperature.20 Nolan demonstrated that substitution at the terminal position of the allyl scaffold results in a more facile precatalyst activation, which translates into the generation of the active catalytic species in higher concentration and therefore to higher catalytic activity.21 The catalyst preactivation can occur through two possible reaction pathways (see Scheme 2). First, mechanism A, which consists of a nucleophilic intermolecular attack of the tertbutoxide (OtBu−) to generate in a single step the precatalyst, KCl, and allyl(OtBu).22 Second, as demonstrated by Melvin et al.,23 a two-step mechanism with an initial Cl− by OtBu− substitution, followed by the nucleophilic intramolecular attack B


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Organometallics thermal and entropy corrections obtained at the BP86-D3/SVP∼sdd level. In addition, to calculate the reported Gibbs energies, we included solvent effects of an iPrOH solution estimated with the polarizable continuous solvation model PCM as implemented in Gaussian09.39

for the process could not be found, leading directly to I3-[Pd]Cl. For the two-step reaction mechanism (B in Scheme 2), the reaction starts from the precatalyst species [Pd]-Cl. Substitution of Cl in [Pd]-Cl by a molecule of KOtBu leads to the formation of intermediate I1-[Pd]-Cl and KCl as the byproduct. On the basis of previous computational studies, we know that this step is not fundamental; kinetically, it demands roughly 15 kcal/mol,22 and moreover, the basic character of KOtBu could deprotonate the isopropyl alcohol solvent first, and consequently, the base could be iPrO−. Despite other studies suggesting the presence of dimeric species [Pd2(μ-allyl)(μ-Cl)(NHC)2],14 this substitution reaction is predicted to be spontaneous for the [Pd]-Cl-1−4 series, and the overall thermodynamic stability of I1-[Pd]-Cl intermediates follows the order I1-[Pd]-Cl-3 (+4.3 kcal/mol) < I1-[Pd]-Cl-2 (+3.2 kcal/mol) < I1-[Pd]-Cl-1 (+0.6 kcal/ mol) < I1-[Pd]-Cl-4 (−1.2 kcal/mol). Despite OtBu− being a better nucleophile than Cl−, this step is not exergonic for all species, Additionally, the considered simple modification on the allyl scaffold has a significant effect on the stability of I1-[Pd]Cl intermediates relative to [Pd]-Cl. For instance, more electron-donating R1 and R2 substituents destabilize the intermediate I1-[Pd]-Cl. Therefore, intermediate I1-[Pd]-Cl3 (with two methyl substituents on C3) is the least stable, followed by the I1-[Pd]-Cl-2 (with one methyl group), then I1-[Pd]-Cl-1 (with H atoms) and finally I1-[Pd]-Cl-4 (with a benzyl group). This will have signif icant ramif ications on catalyst design ef forts. The next step corresponds to the structural rearrangement of allyl scaffold in I1-[Pd]-Cl intermediates, leading to the formation of less stable I2-[Pd]-Cl intermediates. The order in thermodynamic stability of I2-[Pd]-Cl from the starting reactants is as follows: I2-[Pd]-Cl-1−3 are nearly isoenergetic and placed around 8.0 kcal/mol above [Pd]-Cl, whereas I2-[Pd]-Cl-4 is still above [Pd]-Cl but by only 1.4 kcal/mol. These results show that the phenyl group in the allyl moiety is particularly efficient for the stabilization of I1-[Pd]-Cl and I2-[Pd]-Cl. On the other hand, the predicted barrier for this step (TS1-[Pd]-Cl) ranges from 8.2 to 12.6 kcal/mol with the lowest barrier calculated for [Pd]-Cl-3 and the highest barrier for [Pd]-Cl-1. A point worth mentioning here is that intermediate I2-[Pd]Cl is thermodynamically unfavored with respect to I1-[Pd]-Cl, and in particular, I2-[Pd]-Cl-3 is only 0.2 kcal/mol more stable than TS1-[Pd]-Cl-3, which reinforces the idea that even though this step is kinetically accessible, it is thermodynamically unfavored. The final step should correspond to the formation of the active catalyst species IPr-Pd(0) occurring via reductive elimination, through the transition state TS2-[Pd]-Cl. However, it leads to the coordination intermediate I3-[Pd]-Cl, where the organic molecule holding the new C−O bond remains bonded to palladium. This step is predicted to be the rate-determining step in the activation mechanism for the studied precatalyst [Pd]-Cl-1−4 series. The formation of a hydride by a H-transfer from the tBu moiety to the metal or directly to any of the former allylic carbons was discarded as a reaction manifold.42 Interestingly, the computed barrier TS2[Pd]-Cl for this step is 15.0 kcal/mol above I2-[Pd]-Cl, suggesting significant differences for all [Pd]-Cl-1−4 precatalysts (see Figure 2). The short Pd−C2 bond distance (2.198 Å) for the TS2-[Pd]-Cl-1 reveals why the formation of the C−O bond by the OtBu moiety is more difficult (≈2.800 Å for TS2[Pd]-Cl-2−4). Furthermore, to confirm that the relative low stability of TS2-[Pd]-Cl-1 could be attributed to a disfavored

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RESULTS AND DISCUSSION Precatalyst Series [Pd]-Cl-1−4 Geometries. The analysis of the computed selected Pd−C distances of the DFToptimized geometries of precatalyst series [Pd]-Cl-1−4 suggests that the resulting steric hindrance upon the allyl scaffold modification has a non-negligible effect on the ligand bond to the Pd metal center. In agreement with experiments, Pd−C3 distances (see Scheme 2) remain relatively constant, whereas Pd−C1 distances elongate after the substitution on C1 of the allyl moiety, with the exception of [Pd]-Cl-4, where the Pd−C1 distance is slightly shorter than that of [Pd]-Cl-3 (2.234 vs 2.292 Å, respectively). These geometrical differences have significant effects on the Pd−C1 and Pd−C3 bond length asymmetry by increasing the difference between Pd−C1 and Pd−C3 bond distances in the following trend: [Pd]-Cl-1 < [Pd]-Cl-2 < [Pd]-Cl-3, whereas [Pd]-Cl-4 displays a different behavior because of the conjugation due to the phenyl ring. Additionally, the Mayer bond order (MBO) values40,41 (0.570, 0.503, 0.442, and 0.495, respectively) confirm the expected decrease of the Pd−C1 bond strength in accordance with the computed [Pd]−C1 bond distances. Finally, NPA charges (−0.486, −0.262, −0.052, and −0.310 e−, respectively) show that the negative charge on C1 decreases from [Pd]-Cl-1 to [Pd]-Cl-3, facilitating the nucleophilic attack of the tertbutoxide (OtBu) in the second step of the reaction that generates the active catalyst. Precatalyst Activation Mechanism for the [Pd]-Cl-1−4 Series. Figure 1 depicts the most stable species involved in the activation mechanism for the [Pd]-Cl-1−4 series and their relative Gibbs energy. The one-step reaction mechanism (A in Scheme 2) was discarded because a concerted transition state

Figure 1. Computed stationary points of the activation mechanism for the studied precatalyst [Pd]-Cl-1−4 series. Relative Gibbs energies are given in kcal/mol. C


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Organometallics

Table 1. Computed Stationary Points of the Activation Mechanism for the Studied Precatalyst [M]-Cl-1−4 Series (M = Pd, Ni) (Relative Gibbs Energies Are Given in kcal/ mol) [Pd]-Cl

[Ni]-Cl

1 2 3 4 1 2 3 4

[M]

I1

TS1

I2

TS2

I3

IPr-M0

0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0

0.6 3.2 4.3 −1.2 −1.5 0.9 4.0 0.7

12.6 10.6 8.2 8.0 10.0 9.9 7.7 7.4

7.9 7.7 8.0 1.4 6.8 6.2 5.4 3.0

30.6 23.0 25.0 20.7 30.9 33.3 31.3 35.0

−7.2 −7.3 −6.0 −12.1 9.7 9.9 13.2 6.8

1.3 2.4 9.7 2.7 29.2 29.3 36.7 31.2

energy barrier computed for [Ni]-Cl-3 and the highest barrier for [Ni]-Cl-1. This barrier trend for the [Ni]-Cl-1−4 series is similar to that of Pd-Cl-1−4 series. However, the barriers for the rate-determining reductive elimination step TS2-[Ni]-Cl, which generates the active catalyst species Ni(0)-Cl, are predicted to be higher in energy (24.1−32.0 kcal/mol) relative to the TS2-[Pd]-Cl barriers (15.3−22.7 kcal/mol) in Figure 1 and Table 1. Furthermore, for the [Ni]-Cl-1−4 series, the overall barrier from the most stable intermediate I1-[Ni]-Cl or initial catalyst is predicted to be much higher in energy (31.3− 35.0 kcal/mol) than the overall barrier for the [Pd]-Cl-1−4 series (21.9−30.6 kcal/mol). Finally, the activation mechanism for [Ni]-Cl-1−4 series is endergonic and in particular, the catalytically active IPr-Ni(0) species for [Ni]-Cl-3 is predicted to be quite endergonic (5.4 kcal/mol above TS2-[Ni]-Cl-3), suggesting that the activation mechanism is not favorable for [Ni]-Cl-3. However, the coordination intermediate is much lower in energy but still significantly higher than the initial catalyst [Ni]-Cl by at least 6.8 kcal/mol, for I3-[Ni]-Cl-3. Moreover, to further stabilize the Ni(0) species, i.e. IPr-[Ni], the coordination of a molecule of isopropyl alcohol was found to be 7.7 kcal/mol more stable, however still 11.8 kcal/mol less stable than I3-[Ni]-Cl-1. For palladium, the alcohol binding is less competitive than for I3-[Ni]-Cl-3, by 7.9 kcal/mol, and indeed only 0.6 kcal/mol more stable than the Pd(0) structure, which confirms the high stability of such a structure for this metal. Taken together, these results suggest that the precatalyst activation mechanism for the [Ni]-Cl-1−4 series is energetically more demanding and kinetically more challenging than the activation mechanism for the [Pd]-Cl-1−4 series. A possible explanation can be given based on the fact that the stabilization energy for Ni reduction (M(II) → M(0)),3 which is the negative of the sum of the first and second ionization potential of M, is −595 kcal/mol, whereas for Pd, this value is −640.4 kcal/mol, suggesting that Pd can undergo the reduction process much more efficiently than Ni and thus explains why the predicted barrier for the reductive elimination is lower for the former. Precatalyst Activation Mechanism for [Pd]-Br-1−4 and [Ni]-Br-1−4 Series. In this section, we investigate the effect of the halogen atom variation, by substituting Cl for Br at the Pd metal center, on the precatalyst activation followed by Suzuki−Miyaura reaction mechanism using 4-bromotoluene substrate. To this end, we considered the precatalyst series [Pd]-Br-1−4 derived from [Pd(IPr)(R-allyl)Br]. A point worth mentioning here about the activation mechanism is that after the substitution of Br by OtBu ligand, the energy profile for the [Pd]-Br-1−4 series coincides with the

Figure 2. DFT-optimized geometries for the TS2-[Pd]-Cl-1−4 series along with the primary bond distances given in Å.

C−O bond formation due to the absence of substituents on the allyl, that is, with small sterical hindrance, the analyses of the charges on oxygen for both the I1-[Pd]-Cl1−4 and I2-[Pd]Cl1−4 series of intermediates indicate that electronics do not play a key role here (−0.752, −0.764, −0.768, and −0.756 e−, respectively, for the I2-[Pd]-Cl1−4 series), as sterics do. However, the overall barrier calculated from the most stable intermediate, that is, [Pd]-Cl-1−3 and I1-[Pd]-Cl-4 (ratedetermining intermediate), to the next highest transition state TS2-[Pd]-Cl (rate-determining transition state) for the [Pd]Cl-1−4 series follows the order [Pd]-Cl-1 (30.6 kcal/mol) > [Pd]-Cl-2 (25.0 kcal/mol) > [Pd]-Cl-4 (23.0 kcal/mol) > [Pd]-Cl-3 (21.9 kcal/mol).43 This is in excellent agreement with experimental observations and indeed provides a quantitative explanation for the low yield (12%) displayed by [Pd]-Cl-1 and of the higher yields (≈90%) observed for [Pd]Cl-2−4. The transition states evaluated point out that, for the next intermediate, we do not obtain directly the Pd(0) species IPr-[Pd] as this last process is unfavorable by 8.5, 9.7, 15.7, and 14.8 kcal/mol for [Pd]-Cl-1−4, respectively. Precatalyst Activation Mechanism for [Ni]-Cl-1−4 Series. To understand the effect of metal substitution on the studied Suzuki−Miyaura reaction, we have investigated the activation mechanism for nickel precatalyst [Ni]-Cl-1−4 series,44,45 derived from [Ni(IPr)(R-allyl)Cl]. We have also examined the reaction mechanism for IPr-Ni(0) species. The corresponding energy profile is given in Table 1, together with the corresponding values calculated for the Pd-based series, for the sake of comparison. Focusing on the activation mechanism, I1-[Ni]-Cl intermediates for the [Ni]-Cl-1−4 series are either isoenergetic or slightly more stable (by 1−2 kcal/mol) compared to the corresponding I1-[Pd]-Cl intermediates in Figure 1. The thermodynamic stability of I1-[Ni]-Cl intermediates for the [Ni]-Cl-1−4 series is similar to that of intermediates for the [Pd]-Cl-1−4 series. Next, the first barrier TS1-[Ni]-Cl ranges from 3.7 to 11.5 kcal/mol, with the lowest D


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Organometallics

agreement with the past results by Maseras et al.22 On a general note, the reaction mechanism involves a catalytic cycle consisting of three main steps: oxidative addition,46 transmetalation, and reductive elimination. The reaction begins with the coordination of 4-chlorotoluene to Pd metal in IPr-Pd(0), giving intermediate [Pd]-Cl-I1, which lies 7.3 kcal/mol below the starting reactants. The next step corresponds to the oxidative addition of C−Cl bond of 4-chlorotoluene to Pd metal in [Pd]-Cl-I1, which leads to the formation of the more stable intermediate [Pd]-Cl-I2, lying 13.5 kcal/mol below [Pd]-Cl-I1. This step proceeds via a three-center transition state, [Pd]-Cl-TS1, with an energy barrier of 12.0 kcal/mol above [Pd]-Cl-I1. Then, the coordination of organoborate species via the O atom of the −OH group to [Pd]-Cl-I2 leads to unstable adduct [Pd]-Cl-I3, which lies 3.9 kcal/mol above [Pd]-Cl-I2.47 This step is followed by the release of Cl− anion and the subsequent rearrangement of organoborate species at the metal center leading to intermediate [Pd]-I4. From an energy point of view, [Pd]-I4 is 4.4 kcal/mol less stable than Pd]-Cl-I2 and requires overcoming a barrier ([Pd]-Cl-TS2) of 13.4 kcal/mol. The addition of phenylboronic acid and release of Cl− is the rate-determining step in the catalytic cycle of Figure 3. It is worth noting that Maseras et al. found that the oxidative addition was the rate-determining step in their study of the Suzuki−Miyaura cross-coupling reaction mechanism.24 From [Pd]-I4, the phenyl migration from boron to [Pd] metal occurs via a concerted bond cleavage of the C(Ph)−B bond and the formation of the [Pd]−C(Ph) bond, giving intermediate [Pd]-I5, from which the subsequent release of B(OH)3 leads to the formation of a more stable intermediate [Pd]-I6. This step proceeds through transition state [Pd]-TS3 and requires overcoming a barrier of 6.0 kcal/mol above [Pd]I4. Overall, the transmetalation process for IPr-Pd(0) species is quite feasible, being exergonic by 13.6 kcal/mol with respect to the separated boronic acid and [Pd]-Cl-I2 reactants. The final step corresponds to the formation of the C(Ph)−C(Ph) bond by reductive elimination from [Pd]-I6, through transition state [Pd]-TS4. This is a rather low energy step, with a barrier of only 2.6 kcal/mol, that would release the desired biphenyl product, regenerating the catalyst IPr-Pd(0) with an energy gain of 28.4 kcal/mol. To summarize, our theoretical findings suggest that the studied Suzuki−Miyaura reaction mechanism for catalyst species IPr-Pd(0) is thermodynamically favorable and kinetically affordable with the highest barrier of 13.4 kcal/ mol, which is quite reasonable in view of the fact that the reaction is carried out experimentally at room temperature in 60 min. Cross-Coupling Reaction Mechanism for [Ni] Species [Ni]-Cl-1−4 Series. Focusing on the precatalytic activation in Table 1, it is evident that the overall reaction energy profile for IPr-Ni(0)-catalyzed Suzuki−Miyaura reaction is less exergonic when compared with the energy profile for IPr-Pd(0). Further, bearing the relatively low stability of the catalytic active species IPr-Ni(0) will not be favored (see Tables 1 and 2), and thus it will be omitted, going from I3-[Pd]-Cl to [Ni]-Cl-I0, where the 4-chlorotoluene is bonded yet to the metal, together with the leaving alkene from the preactivation. Based on the different nature of the complexes in the [Ni]-Cl-1−4 series, the stabilization energy of [Ni]-Cl-I0 is 18.3, 15.0, 8.5, and 15.8 kcal/mol for the [Ni]-Cl-1−4 series, respectively, whereas the corresponding species for Pd (i.e., [Pd]-Cl-I0) is unstable for the [Pd]-Cl-1−4 series by 3.5, −1.6, 5.6, and 5.9 kcal/mol,

energy profile for [Pd]-Cl-1−4 series in Figure 1. Thus, we limit our discussion to the first step of the activation mechanism and the corresponding energetics. In the case of [Pd]-Br-1−4 series, I1-[Pd]-Br intermediates (relative energies are +0.7, +0.8, −0.8, and −2.5 kcal/mol for [Pd]-Br-1 to [Pd]-Br-4, respectively) are slightly more stable (by 1−2 kcal/ mol) when compared with the respective I1-[Pd]-Cl intermediates in Figure 1, except for the stabilization by 5.1 kcal/mol for I1-[Pd]-Br3, because of the higher steric hindrance of Br than the entering OtBu moiety. These results indicate that the substitution of halogen by a KOtBu might be faster for [Pd]-Br system than for [Pd]-Cl as bromide is a better leaving group than chloride. Finally, for the sake of comparison, we studied the precatalyst activation for the [Ni]Br-1−4 series for which no significant changes were observed (relative energies are +2.0, +1.5, + 1.0, and 2.1 kcal/mol for [Ni]-Br-1 to [Ni]-Br-4, respectively). Suzuki−Miyaura Cross-Coupling Reaction Mechanism for IPr-Pd(0) Species. Having completed the description of the mechanism of the activation for the [Pd]-Cl-1−4 series, we investigate next the entire catalytic cycle for IPr-Pd(0)mediated Suzuki−Miyaura reaction shown in Scheme 1a. Figure 3 presents the most stable intermediates and their relative Gibbs energies for the studied reaction mechanism, in

Figure 3. Computed stationary points of the Suzuki−Miyaura crosscoupling reaction pathway for active catalyst IPr-Pd(0) species with 4chlorotoluene and phenylboronic acid as substrates (relative Gibbs energies are in kcal/mol). E


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Organometallics

addition with 4-bromotoluene can be explained by taking into account the C−Br bond strength (66.0 kcal/mol) that is 14.8 kcal/mol lower in energy than the C−Cl bond strength (80.8 kcal/mol);49 (iii) the transmetalation step ([Pd]-I2 → [Pd]I4) is predicted to be the rate-determining step, and it requires overcoming a barrier ([Pd]-Br-TS2) of 14.1 kcal/mol, roughly the same barrier as [Pd]-Cl-TS2. Taken together, these observations suggest that the studied Suzuki−Miyaura reaction for IPr-Pd(0) species might be slightly faster with 4bromotoluene than with 4-chlorotoluene, owing to the lower predicted barrier for the oxidative addition. Finally, for the sake of comparison, we studied the Suzuki− Miyaura reaction mechanism for IPr-Ni(0) with 4-bromotoluene and phenylboronic acid (see Table 2). Overall, the corresponding energy profiles for [Ni]-Br-1−4 series are similar to that of [Ni]-Cl-1−4, both from thermodynamic and kinetic perspectives.

Table 2. Computed Stationary Points of the Suzuki−Miyaura Cross-Coupling Reaction Pathway for Active Catalyst IPrM(0) (M = Pd, Ni) Species with 4-Chlorotoluene (X = Cl) and 4-Bromotoluene (X = Br), and Phenylboronic Acid as Substrates (Gibbs Energies Are in kcal/mol) substrate

4-chlorotoluene

4-bromotoluene

catalyst

Pd

Ni

Pd

Ni

[M] X-I0 X-I1 X-TS1 X-I2 X-I3 X-TS2 I4 TS3 I5 I6 TS4 [M]

0.0 3.5 −7.3 4.7 −20.8 −16.9 −7.4 −16.4 −10.4 −23.3 −34.4 −31.8 −62.8

0.0 −18.3 −15.4 −9.6 −46.5 −39.8 −35.5 −38.7 −36.4 −51.1 −57.1 −53.6 −62.8

0.0 2.0 −6.8 0.0 −23.7 −17.1 −9.6 −20.8 −14.8 −27.7 −38.8 −36.2 −67.2

0.0 −18.6 −15.8 −12.3 −45.2 −39.4 −14.6 −43.2 −40.8 −55.6 −61.5 −58.0 −67.2

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CONCLUSIONS In summary, using DFT calculations, we have studied the precatalyst activation mechanism for the series, [Pd]-Cl-1−4, derived from [Pd(IPr)(R-allyl)Cl], followed by the complete description of the Suzuki−Miyaura reaction for the catalytically active IPr-Pd(0) species using 4-chlorotoluene and phenylboronic acid as substrates in isopropyl alcohol as solvent. Our theoretical results suggest that the upper barrier trend, that is, the rate-determining step of the catalytic process described here, corresponding to the activation mechanism for the studied [Pd]-Cl-1−4 series, is in good agreement with the experiments, with [Pd]-Cl-1 performing catalytically worse with an upper barrier of ≈30.0 kcal/mol and [Pd]-Cl-2−4 being equally efficient with a high barrier of ≈20.0−25.0 kcal/ mol. We extended our investigations to [Pd]-Br-1−4 series derived from [Pd(IPr)(R-allyl)Br] species. The main results are the following: (i) in the activation mechanism, the halogen substitution by OtBu ligand is more favorable for the [Pd]-Br1−4 series than for the [Pd]-Cl-1−4 series because bromide is a better leaving group than chloride; (ii) IPr-Pd(0)-mediated Suzuki−Miyaura reaction with 4-bromotoluene is favorable, and when compared to the reaction with 4-chlorotoluene, it is found to be similar, kinetically, as the barrier of the ratedetermining transmetalation step with both substrates is nearly isoenergetic, whereas with 4-bromotoluene, it is 5.2 kcal/mol lower in energy relative to the barrier of the oxidative addition step with 4-chlorotoluene. Finally, for the studied Suzuki− Miyaura reaction, our results indicate that Ni-based complexes ([Ni(IPr)(R-allyl)X]; X = Cl, Br) are poorer catalysts when compared to Pd-based congeners due to the significantly high energy upper barrier (10.0−15.0 kcal/mol higher than that of the Pd catalysts) for the activation of the precatalyst series [Ni]-X-2−4 and because of the less exergonic reaction energy profile for IPr-Ni(0)-mediated catalytic cycle.

respectively. Then, the primary results of the cross-coupling reaction for the [Ni]-Cl-1−4 series can be summarized as follows: (i) in the case of IPr-Ni(0), the substrate 4chlorotoluene bound intermediate [Ni]-Cl-I1 is thermodynamically more stable than the corresponding intermediates [Pd]-Cl-I1 of Figure 3 (15.4 vs 7.3 kcal/mol, respectively). This extra stability is due to the η6-coordination of the phenyl ring of 4-chlorotoluene to the Ni center in the [Ni]-Cl-I1 complex, whereas [Pd]-Cl-I1 intermediate displays an η2coordination; (ii) the oxidative addition step barrier [Ni]-ClTS1 is predicted to be 6.2 kcal/mol lower in energy in comparison with the [Pd]-Cl-TS1 barrier of Figure 3 (5.8 vs 12.0 kcal/mol, respectively); (iii) the rate-determining step ([Ni]-Cl-I2 → [Ni]-I4) of the cross-coupling reaction pathway, which corresponds to addition of phenylboronic acid during the transmetalation process, has a barrier of 10.0 kcal/mol. This barrier for [Ni]-Cl-TS2 is predicted to be 3.4 kcal/mol lower in energy than that of the rate-determining step through [Pd]-Cl-TS2 of Figure 3. Taken together, these results suggest that the nature of the metal plays an important role and that Ni-based complexes are catalytically less efficient than their Pd-based congeners for the studied Suzuki−Miyaura reaction due to the calculated significantly higher barrier for the activation mechanism of the [Ni]-Cl-1−4 series. Consequently, this prediction nearly rules out any possible use of these Ni-based species in crosscoupling reactions at room temperature; however, under harsher conditions, nickel could be active, as well.48 Precatalyst Activation and Suzuki−Miyaura Reaction Mechanism for [Pd]-Br-1−4 and [Ni]-Br-1−4 Series. Next, the reaction energy profile for IPr-Pd(0)-mediated catalytic cycle with the 4-bromotoluene substrate is calculated, and results are included in Table 2. Analogously to the activation mechanism, for the catalytic cycle, the steps after the Br− ion dissociation (i.e., intermediate Pd−I4 and onward) are identical to those of Figure 3. The main results can be summarized as follows: (i) with respect to separated reactants, intermediate Pd-Br-I1 is 0.5 kcal/mol less stable than Pd-Cl-I1 of Figure 1; (ii) the barrier for the oxidative addition step ([Pd]-Br-TS1) is predicted to be 5.2 kcal/mol lower in energy relative to the barrier for [Pd]-Cl-TS1 of Figure 3. The easier oxidative

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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/acs.organomet.7b00114. Cartesian coordinates and absolute energies of all computed species (XYZ) F


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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. ORCID

Paola Belanzoni: 0000-0002-1286-9294 Steven P. Nolan: 0000-0001-9024-2035 Heiko Jacobsen: 0000-0003-0721-8726 Luigi Cavallo: 0000-0002-1398-338X Miquel Solà: 0000-0002-1917-7450 Albert Poater: 0000-0002-8997-2599 Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS A.P. thanks the Spanish MINECO for a project CTQ201459832-JIN. M.S. thanks EU for a FEDER fund (UNGI08-4E003), the Generalitat de Catalunya for project 2014SGR931 and ICREA Academia 2014 prize, and MINECO of Spain through project CTQ2014-54306-P. G.M.M. thanks LLP/ Erasmus Student Placement Programme (A.A. 2012/2013) for funding. S.P.N. acknowledges the ERC for support of this work. This publication is based upon work supported by the King Abdullah University of Science and Technology (KAUST) Office of Sponsored Research (OSR) under Award No. OSR2015-CCF-1974-03. We thank the referees for helpful comments and suggestions.

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