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Article

Understanding the Heteroatom Effect on the Ullmann Copper-Catalyzed Cross-Coupling of X-Arylation (X = NH, O, S) Mechanism

by
Diego M. Andrada
1,2,*,
Silvia M. Soria-Castro
3,
Daniel A. Caminos
3,
Juan E. Argüello
3 and
Alicia B. Peñéñory
3
1
Krupp-Chair of General and Inorganic Chemistry, University of Saarland, Campus C4.1, 66123 Saarbruecken, Germany
2
Fachbereich Chemie, University of Marburg, Hans-Meerweinstrasse 4, 35032 Marburg, Germany
3
INFIQC, Departamento de Química Orgánica, Facultad de Ciencias Químicas, University of Córdoba, Ciudad Universitaria, Córdoba X5000HUA, Argentina
*
Author to whom correspondence should be addressed.
Catalysts 2017, 7(12), 388; https://doi.org/10.3390/catal7120388
Submission received: 14 November 2017 / Revised: 7 December 2017 / Accepted: 11 December 2017 / Published: 13 December 2017
(This article belongs to the Special Issue Catalysis Assisted by Calculations: From Organic and Metal Catalysts to Enzymes)
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Abstract

:
Density Functional Theory (DFT) calculations have been carried out in order to unravel the governing reaction mechanism in copper-catalyzed cross-coupling Ullmann type reactions between iodobenzene (1, PhI) and aniline (2-NH, PhNH2), phenol (2-O, PhOH) and thiophenol (2-S, PhSH) with phenanthroline (phen) as the ancillary ligand. Four different pathways for the mechanism were considered namely Oxidative Addition–Reductive Elimination (OA-RE), σ-bond Metathesis (MET), Single Electron Transfer (SET), and Halogen Atom Transfer (HAT). Our results suggest that the OA-RE route, involving CuIII intermediates, is the energetically most favorable pathway for all the systems considered. Interestingly, the rate-determining step is the oxidative addition of the phenyl iodide to the metal center regardless of the nature of the heteroatom. The computed energy barriers in OA increase in the order O < S < NH. Using the Activation Strain Model (ASM) of chemical reactivity, it was found that the strain energy associated with the bending of the copper(I) complex controls the observed reactivity.

Graphical Abstract

1. Introduction

Nowadays, copper-catalyzed cross-coupling Ullmann type reactions are considered among the most useful processes in the synthesis to build C–C, C–N, C–O, C–S, C–P, and C–Se bonds [1,2,3,4,5,6]. Despite its undoubtedly current success, after the seminal work by Fritz Ullmann [7,8,9] and Iram Goldberg [10] in the early 20th century, these reactions did not garner considerable attention from the synthetic community. This was a consequence of the harsh reaction condition requirements such as stoichiometric amounts of copper, strong bases, high temperatures, and long reaction times [11]. These drawbacks were overcome at the beginning of the new century with the introduction of the ancillary chelating ligands (L) approach by the groups of Taillefer [12] and Buchwald [13] representing thus a major breakthrough in copper chemistry. The use of bidentate ligands like phenanthroline [14,15,16,17,18], diamines [19,20,21], amino acids [22,23,24], and 1,3-diketones [25,26] among others [27,28,29,30] has not only met the demands of reliability, mildness, and functional group tolerance but also interesting modern features such as orthogonal reactivity [26]. The applications so far known have been spread over a diverse range of areas from pharmaceutical [31] and natural products [32,33,34] to materials fields [35,36].
The mechanistic details of how the carbon-heteroatom C–X (X = N, O, and S) bonds are formed have remained for a long time somewhat unclear [4]. Since the rate-limiting step is usually at the initial stages of the reaction, the detection of intermediate species has been very rare. Thus, most of the mechanistic proposals which shed light on this area are drawn from kinetic experimental and computational studies [37,38]. While most of the evidence collected so far has shown that the active catalyst is a ligand-copper(I)-nucleophile [(L)CuI(Nu)] species, there is no such agreement on the way this catalyst activates the aryl halide (ArY, Y = Cl, Br and I). The most invoked routes are based on either one-electron redox processes through the radical intermediate CuI/CuII catalytic cycle namely Single Electron Transfer (SET) and Halogen Atom Transfer (HAT) [4,39,40,41,42], or on two-electron redox processes via a CuI/CuIII catalytic cycle like Oxidative Addition–Reductive Elimination (OA-RE) or σ-bond Metathesis (MET) (Scheme 1) [37,38,43,44,45,46,47,48,49,50].
In the last twenty years, a particular effort has been addressed to elucidate the mechanistic details concerning the catalytic copper-catalyzed N-, O-, and S-arylations. Thus, Buchwald et al. started a program to kinetically investigate the N-arylation of amides and alkylamines [20,21,52,53] and O-arylation of aliphatic alcohols [26,54,55]. They clearly showed that the aryl halide activation occurs by the [(L)CuI(Nu)] complexes. In this regard, Hartwig et al. were able to isolate and characterize a number of copper imidate, amidate, amino, phenoxide, and thiophenoxide complexes which are chemically competent to couple a variety of aryl halides [40,42,56]. The studies carried out on the reactivity of these complexes, including kinetic studies and radical clock experiments, argued against pathways involving radical intermediates. Conversely, Buchwald and Houk by experimental and computational methods claimed that the SET and HAT mechanisms are energetically more favorable for Ullmann N- and O-arylation of alkyl amines and alcohols with aryl iodides [51]. Further support to these one-electron mechanisms was given by a number of detailed theoretical studies on the N-, O-, and S-arylation reported by Zhang and co-workers [57,58,59,60,61]. They confirmed that [(L)CuI(Nu)] species is indeed the most reactive towards aryl halide among all the other possible copper species. Additionally, they concluded that N-arylation reactions should occur via an OA-RE mechanism while for the O- and S-arylation reaction HAT is the most likely pathway. In this context, we recently reported the use of Ullmann reactions under conventional and microwave conditions as a simple and economical process to accomplish a variety of thioacetates and sulfur heterocycles [62]. Lately, we have also contributed to the understanding of this S-arylation reaction mechanism by a combination of experimental and computational studies [63]. Remarkably, no evidence of the presence of radical species was found during the reaction process.
In this sense, very little is known about the factors leading to the observed mechanisms and their influence on reactivity. In line with our ongoing interest in the underlying mechanisms on Ullmann-type reactions, we present herein a detailed Density Functional Theory (DFT) computational study on the copper-catalyzed cross-coupling reaction between phenyl iodine and aniline, phenol and thiophenol as the nucleophile in the presence of 1,10-phenanthroline (phen) as a ligand (Scheme 2). In an effort to understand the mechanism of these transformations, we examined the energy profile of different reaction pathways. A deeper understanding of the origins of the reaction barriers was obtained by means of the so-called activation strain model (ASM).

2. Results and Discussion

We first considered the thermodynamics of the formation of the active species [(phen)CuI(NHPh)], [(phen)CuI(OPh)], and [(phen)CuI(SPh)] and their disproportion (Scheme 3). As it was reported elsewhere [57,60,64], there are many copper species that can be formed depending on the reaction conditions. The disproportionation process usually leads to inactive species. Table 1 collates the thermodynamic data for the formation in the gas phase (gas), toluene (tol), and acetonitrile (acn) in the presence of Cs2CO3 and KOtBu as bases. The Gibbs energies computed for the formation of the complexes [(phen)CuI(XPh)] follow the trend according to the pKa of the nucleophiles, i.e., pKa(PhNH2) = 27.0, pKa(PhOH) = 9.95, and pKa(PhSH) = 6.5. In concordance with the experiments outcomes [60,65], the formation of the [(phen)CuI(SPh)] complex is favored in all the conditions here computed, while the formation of [(phen)CuI(NHPh)] and [(phen)CuI(OPh)] requires stronger bases. After their formation, these complexes can further disproportionate to generate the ionic species [CuI(phen)2]+ and [CuI(XPh)2]–. This phenomenon has been experimentally and computationally observed by Fiaschi et al. [66,67], Chen et al. [42], and Zhang et al. [59,60,61]. Our results show that this disproportionation is unfavorable in nonpolar solvents while it becomes favored when the dipolar constant of the solvent increases, in good agreement with previously reported results [59,60,61].
Figure 1 displays the geometry of the [(phen)CuI(XPh)] complexes together with their dissociation energies. In all complexes, the copper is planar three-coordinated. Interestingly, the 1,10-phenanthroline ligand is not symmetrically bound to the metal center. The CuI–N(phen) bond distance differences are bigger for the amido complex (0.322 Å) and smaller for the thiophenoxide complex (0.086 Å). Such a distortion from the trigonal geometry of the d10 metal center as T-shaped has been observed in other complexes [68]. This is a consequence of the strong σ- and π-donor character of the ligands which leads to the electron-rich copper(I) center, leaving the second nitrogen less coordinated. On the other hand, the Cu–X bond distances are predicted to be 1.860 Å (X = N), 1.845 Å (X = O), and 2.185 Å (X = S). The Cu–NHPh bond distance is within the range of distances reported for the CuI-amido complexes (~1.865 Å). As expected, the Cu–O and Cu–S bond distances are shorter than those found for the dimer, i.e., 2.066 Å [66] and 2.340 Å [42] for Cu–O and Cu–S bonds, respectively. Additionally, the Cu–S distance is also slightly longer than those measured by Kaim and co-worked for thiolate copper complexes (ca. 2.145 Å) [68]. The computed Cu–X bond dissociation energies clearly indicate that stability of the complexes decreases in the sense NH > S > O.
We next calculated the energy of the key steps for the most probable pathways for the formation of cross-coupling products PhXPh (3-X, X = NH, O, and S) starting with the complexes [(phen)CuI(NHPh)], [(phen)CuI(OPh)], and [(phen)CuI(SPh)]. We considered only the four most appealed mechanisms displayed in Scheme 1. Figure 2 displays the computed Gibbs energy profiles for the N-, O-, and S-arylation of PhI in the gas phase. Additionally, Table 2 summarizes the effects of the solvent in these energy profiles.
The single electron transfer from the [(phen)CuI(XPh)] complex to PhI forming the radical cation CuII intermediate complex, phenyl radical, and anion I– was examined first [69]. The Gibbs free energy required to transfer the electron from the cooper complexes to the iodobenzene is significantly high, i.e., +107.0, +118.1, and +120.2 kcal/mol for 2-NH, 2-O and 2-S, respectively. The Gibbs energy barriers of this process were estimated using the Marcus-Hush theory [70,71,72,73,74,75,76,77] within the Saveant’s model [78,79,80] for the SET coupled with the cleavage of the Ph–I bond (see the Supplementary Materials pages S2–S4 for details) [51]. The activation barriers obtained are +116.0, +131.9, and +135.1 kcal/mol for 2-NH, 2-O, and 2-S, respectively. A strong effect of the solvent is observed trigged by the stabilization of the iodine anion, as can be observed from entries 7 and 8 of Table 2.
Halogen Atom Transfer HAT has been suggested to govern the mechanism in the reaction of O- and S-arylations [59,60]. In this route, PhI transfers the iodine atom to the [(phen)CuI(XPh)] center leading to the formation of a complex [(phen)CuI(I)(XPh)]• and a phenyl radical. The phenyl radical is attached then to the nucleophile moiety in [(phen)CuI(I)(XPh)]• radical intermediate to form the coupling product (3-X). Our DFT calculations predict that the intermediates in the gas phase lie at 25.2, 27.0, and 31.2 kcal/mol when X is NH, O, and S, respectively. The calculations in toluene and acetonitrile as solvents estimate a stabilization between 1 kcal/mol and 3 kcal/mol for the intermediates. The results in toluene particularly indicate that the radicals formed are more stable that those reported by Houk et al. [51] and Zhang et al. [59,60], i.e., +43.0 kcal/mol for [(phen)CuI(I)(NHMe)]•, +34.0 kcal/mol for [(phen)CuI(I)(OMe)]•, +28.7 kcal/mol for [(phen)CuI(Br)(OPh)]•, +32.2 kcal/mol for [(phen)CuI(I)(SPh)]•. We estimated the barrier energy by the Marcus-Hush equation (see Supplementary Materials page S4), for all the reaction studied the HAT Gibbs activation energy is above 30 kcal/mol.
The two-electron reaction mechanisms MET and OA-RE pathway were considered. On the one hand, the Transition State TSMET consists in concerted C(PhI)–I and Cu–X bond breaking and C(PhI)–X and Cu–I bond formation. The congested four-membered transition states have high energy penalties as is reflected in the activation barriers, i.e., +26.7, +27.6, and +27.9 kcal/mol. The OA-RE route instead involves the same bond formation/cleavage but in two elementary steps. The first step is the oxidative addition and consists of the cleavage of the Ph-I bond via a three-membered transition state (TSOA) to afford a five-coordinated copper(III) Intermediate [(phen)CuIII(I)(Ph)(XPh)] (INTOA/RE). The consecutive second step forms the final product Ph–X–Ph and the catalysts [(phen)CuI(I)] through TSRE. The energies of all structures along the OA-RE pathway exhibited in Figure 2 reveal that this is the most favored reaction mechanism. The first transition state TSOA is predicted to have relative Gibbs free energies in the gas phase of +17.3, +13.2, and +16.8 kcal/mol for the reaction with PhNH2, PhOH, and PhSH, respectively. These transition states are strongly developed for O-arylation given the Cu–C bond length of 2.042 Å, while for N- and S-arylation the bond formation is less developed, i.e., Cu–C bond length 2.177 Å and 2.086 Å, respectively. The resulting intermediates [(phen)CuIII(I)(Ph)(XPh)] are predicted to have Gibbs free energies of +6.5, +9.2, and +8.2 kcal/mol with respect to the reactants for X = NH, O, and S (in that order). The reductive elimination step is faster than the oxidative addition step for all the complexes studied. Thus, the energies needed to proceed from INTOA/RE are 1.9 (X = NH), 9.8 (X = O), and 4.6 (X = S) kcal/mol. The overall solvent effect increases the relative energies of the transition state and intermediate structures with respect to the reactants. This effect can be ascribed to the less polar nature of the intermediate structures in comparison with the reactant species.
The overall results indicate that the copper-catalyzed cross-coupling reaction of N-, O-, and S-arylation undergoes an OA-RE reaction mechanism. These outcomes are in sharp contrast with the observations reported by Buchwald et al. [51] and Zhang et al. [59,60] where the operating mechanisms are the one-electron pathways HAT or SET. The energy barriers in toluene shown in Table 2 can be compared with those reported in the literature. Our values are in general lower than those informed before, to mention, the N-arylation of amide (+28.7 kcal/mol) [57], the O-arylation (+34.2 kcal/mol) [59], and the S-arylation (+39.3 kcal/mol) [60] reactions. Indeed, a direct comparison can be done with the last value shown which corresponds to the reaction between PhI and PhSH. The energy profiles recomputed with different functionals (B3LYP, M06, PBE0, TPSSh, PBE + D3(BJ) and TPSSh + D3(BJ) see Supplementary Materials, Tables S1–S6) led to small differences. It is noteworthy that the functionals without dispersion corrections give comparable energies between the HAT and the OA-RE pathways. However, those methods which include dispersion corrections estimate a significant reduction of the OA-RE energies. A similar effect was pointed out for similar copper-catalyzed reactions by Yu et al. [64] and Huang et al. [81]. In this regard, we recomputed copper-catalyzed Ullmann O-arylation of methanol studied by Houk and Buckwald (see Table S7 in the Supplementary Materials) [51]. The energy barrier of the HAT route remains comparable being +31.9 kcal/mol while the reported value is +34.0 kcal/mol [51]. Remarkable, in the case of the OA-RE route the energy barrier of the oxidative addition shows a significant reduction of about 25 kcal/mol. Our computed value is +18.8 kcal/mol while the reported one is +43.2 kcal/mol [51]. Such a strong effect of the dispersion corrections on transition states in different catalytic systems has been repeatedly observed and revisited [82,83].
Thermodynamics of the whole [(phen)CuI(XPh)] + 1 → 3-X + [(phen)CuII] conversion in the gas phase is favored by −37.3 kcal/mol for aniline, −27.2 kcal/mol for phenol, and −20.5 kcal/mol for thiophenol. When solvents are considered the conversions are more favored by around 1.3 to 3.6 kcal/mol. As the more feasible is the reaction mechanism OA-RE, to determine the rate-determining step in solution, we have to compare the Gibbs free energy differences of the [(phen)CuI(XPh)] + 1 → TSOA and INTOA/RE → TSRE steps. This is due to the fact that intermediates are stable enough in solution to be in thermal equilibrium with the environment. Therefore, in solution, the activation of PhI becomes the rate-determining step for all the cross-coupling calculated. These results are in agreement with the general kinetic observations [40,56]. Interestingly, the predicted activation energy barriers are consistent with the increasing reactivity trend NH (+4.2 kcal/mol) < S (+3.7 kcal/mol) < O (+0.2 kcal/mol).
To gain more quantitative insight into the above reactivity trends and the factors controlling the barrier heights for the oxidative additions, the Activation Strain Model (ASM, see Computational details) was applied. Figure 3 shows the activation strain diagrams (ASD) where the computed potential energy surfaces ΔE(ζ) and its energy contributing terms along the Intrinsic Reaction Coordinate are drawn in this case over the Cu–C bond formation. The potential energy surface is dissected into the strain associated with deformation of the individual reactants (ΔEstrain(ζ)) and the instantaneous interaction between the deformed reactants (ΔEint(ζ)). The ASDs nicely illustrate the effect of the nature of the heteroatom X on the oxidative addition step of the cross-coupling reactions. Thus, for the reaction involving [(phen)CuI(NHPh)], the strain energy increases smoothly from the beginning of the process and only becomes highly destabilizing close to the transition state (ΔEstrain‡ = +30.4 kcal/mol). In addition, the interaction energy between the deformed reactants become strongly stabilizing at longer Cu–C bond distances (ΔEint‡ = −26.2 kcal/mol). As a result, a relatively high activation barrier is computed at a relatively early stage of the process. In the same way, the reaction between 1 and [(phen)CuI(SPh)] has the same shape for the strain energy (ΔEstrain‡ = +39.0 kcal/mol) but the interaction energy is less stabilizing than for the [(phen)CuI(NHPh)] complex. The interaction energy then results in being the dominant effect at a relatively advanced development of the Cu–C bond formation (ΔEint‡ = −35.3 kcal/mol). In the case of [(phen)CuI(OPh)], the strain energy varies more smoothly than its analogues, the stabilizing effect of the interaction term can compensate the strong destabilizing effect of the deformation energy (ΔEstrain‡ = +36.6 kcal/mol vs. ΔEint‡ = −36.4 kcal/mol) only when the Cu–C is almost formed. Clearly, the controlling factor to give origin to the barrier height of the oxidative addition of PhI to [(phen)CuI(XPh)] is the energy needed to deform the reactants from their initial equilibrium geometries to the geometries they adopt in the corresponding transition state.
Figure 4 displays the partitioning of the strain energy into contributions of each reactant. The diagrams indicate a different situation depending on the nature of the heteroatom. On the one hand, when X is NH the major contribution to the total strain energy is the deformation associated with the copper(I) complex (ΔEstrain‡[(phen)CuI(NHPh)] = +17.6 kcal/mol vs. ΔEstrain‡(PhI) = +12.7 kcal/mol). It is well-known, that such a change in the complex angle is required to achieve the maximum orbital overlap between the interacting orbitals of the fragments [84,85]. In this case, the occupied d orbital should interact with the σ*(C–I) vacant orbital. The ΔEstrain(ζ) curve of the oxidative addition N-arylation is dominated by the deformation of the copper active complex at the begging of the process and it varies slightly along the intrinsic reaction coordinate (IRC). In contrast, the deformation of the [(phen)CuI(OPh)] and [(phen)CuI(SPh)] complexes costs about 2.1 kcal/mol less than the amido analogue. Thus, at the transition state the energy needed to deform the PhI is higher than to deform the complexes ΔEstrain‡[(phen)CuI(OPh)] = +16.7 kcal/mol vs. ΔEstrain‡(PhI) = +19.9 kcal/mol and ΔEstrain‡[(phen)CuI(SPh)] = +16.3 kcal/mol vs. ΔEstrain‡(PhI) = +19.7 kcal/mol. In these cases, the curves have a significant contribution by the deformation of the complex, whereas in the proximities of the transition state (Cu–C distance of ca. 2.11 Å) the deformation of PhI becomes prevalent.
Additionally, the interaction energy between the deformed reactants can be further analyzed within the energy decomposition analysis (EDA) framework. As graphically shown in Figure 5 for the oxidative addition in the N-arylation of aniline (Figures S1 and S2 for O-arylation and S-arylation, respectively), the main contribution to the total interaction between PhI and [(phen)CuI(NHPh)] comes from the electrostatic attraction. Indeed, the ΔEelst term contributes around 52% to the total attraction in the transition state while the ΔEorb and ΔEdisp terms do it with about 38% and 10%, respectively. The contribution of the orbital interaction term is very significant, and it is mainly the result of the 3d(Cu)-σ*(Ph–I) interaction. The absolute values for the electrostatic terms (ΔEelst‡) are −73.1, −93.7 and −95.1 kcal/mol for X = NH, O, and S, respectively, while the orbital term (ΔEorb‡) give −53.8, −71.7, and −68.8 kcal/mol in the same order. However, when these terms are compared at the same Cu–C bond distance, no real differences are observed. Although the nature of the heteroatom could affect the electronic feature of the metal center, no such change is reflected in the EDA analysis. The computed interactions between PhI and [(phen)CuI(XPh)] reactants are similar regardless of the nature of X after the copper complex is deformed. Therefore, our data suggest that the reactivity of the Ullmann type cross-coupling reactions are controlled by the energy needed to deform the reacting compounds.
It is worth mentioning that the ΔEdisp term is about ca. –15 kcal/mol stabilization at the transition state. This number counts the overestimation on the transition state energy of the OA-RE mechanism of the former DFT studies.

3. Computational Details

All geometries were optimized without symmetry constraint within the DFT (density functional theory) framework using the B3LYP functional [86,87] in combination with the Ahlrichs def2-SVP basis function [88]. These calculations were performed using the Gaussian 09 optimizer (Gaussian 09, Revision D.01, Gaussian, Inc., Wallingford, CT, USA, 2013) [89] together with TurboMole V6.0 (TURBOMOLE GmbH, Karlsruhe, Germany, 2007) [90]. The stationary points were located with the Berny algorithm [91] using redundant internal coordinates. Analytical Hessians were computed to determine the nature of stationary points (one and zero imaginary frequencies for transition states and minima, respectively) [92] and to calculate unscaled zero-point energies (ZPEs) as well as thermal corrections and entropy effects using the standard statistical-mechanics relationships for an ideal gas [93]. Transition structures (TSs) show only one negative eigenvalue in their diagonalized force constant matrices, and their associated eigenvectors were confirmed to correspond to the motion along the reaction coordinate under consideration using the intrinsic reaction coordinate (IRC) method [94]. The electronic energies were improved by single point calculations at the B3LYP + D3(BJ)/def2-TZVPP [95,96] level of theory. For comparison additional calculations were performed at M06/def2-TZVPP [97], TSPPh/def2-TZVPP [98,99], TSPP + D3(BJ)/def2-TVPP [100], PBE0/def2-TZVPP [101], and PBE + D3(BJ)/def2-TZVPP (see Supplementary Materials) [102,103]. Unless otherwise stated, Gibbs energies were computed at 298.15 K. For these calculations the toluene and acetonitrile solvents were described by nonspecific solvent effects within the self-consistent reaction field (SCRF) approach in Tomasi’s formalism [104,105].
The Wiberg Bond Indices [106] and NPA [107,108] atomic partial charges were calculated at the B3LYP + D3(BJ)/def2-TZVPP//B3LYP/def2-SVP level of theory using GAUSSIAN 09 (Gaussian 09, Revision D.01, Gaussian, Inc., Wallingford, CT, USA, 2013) [89] and GENNBO5.9 programs (Theoretical Chemistry Institute, University of Wisconsin, Madison, WI, USA, 2009) [109].
The origins of the reaction barriers were studied by the so-called Activation-Strain Model of reaction profiles (ASM) [110], also known as distortion/interaction model [111]. This model consists of a systematic extension of the fragment approach from the equilibrium structures to the transition state as well as the nonstationary point along the intrinsic reaction coordinate. Thus, within this approach the potential energy surface ∆E(ζ) is dissected, along the reaction coordinate ζ, into the strain or deformation ∆Estrain(ζ) related to the energy needed to deform the individual fragments, and the interaction energy ∆Eint(ζ) between these deformed moieties Equation (1)
Δ E ( ζ ) = Δ E s t r a i n ( ζ ) + Δ E int ( ζ )
The term ∆Estrain(ζ) is associated with the rigidity of the reactants and the extent to which the group should reorganize such that the reaction step can occur, while the term ∆Eint(ζ) accounts for the strength of the interaction between the fragments depending on their mutual orientation and electronic structure. Overall, the interplay between these two terms leads to the point along the ζ where the transition state arises. In the specific cases studied here the copper–carbon (Cu–C) bond formation was taken as the reaction coordinate.
Additionally, the interaction energy can be separated in the light of the Energy Decomposition Analysis (EDA) method, which was developed by Morokuma [112] and by Ziegler and Rauk [113,114]. The bonding analysis focuses on the instantaneous interaction energy ∆Eint of a bond A–B between two fragments A and B in the particular electronic reference state and in the frozen geometry AB. This energy is divided into four main components (Equation (2)).
Δ E int = Δ E e l s t + Δ E P a u l i + Δ E o r b + Δ E d i s p
The term ∆Eelst corresponds to the classical electrostatic interaction between the unperturbed charge distributions of the prepared atoms (or fragments) and it is usually attractive. The Pauli repulsion ∆EPauli is the energy change associated with the transformation from the superposition of the unperturbed wave functions (Slater determinant of the Kohn-Sham orbitals) of the isolated fragments to the wave function Ψ0 = NÂ[ΨAΨB], which properly obeys the Pauli principle through explicit antisymmetrization (Â operator) and renormalization (N = constant) of the product wave function. It comprises the destabilizing interactions between electrons of the same spin on either fragment. The orbital interaction ∆Eorb accounts for charge transfer and polarization effects [115]. In the case when the Grimme dispersion corrections [95,96] are computed, the term ∆Edisp is added to Equation (2). Further details on the EDA method [116,117] and ASM applications to the analysis of the chemical reactions can be found in the literature [118,119].
Herein, the basis sets for all elements have triple-ζ quality augmented by two sets of polarizations functions and one set of diffuse function. Core electrons were treated by the frozen-core approximation. This level of theory is denoted BP86 + D3(BJ)/TZ2P+ [120]. Scalar relativistic effects were incorporated by applying the zeroth-order regular approximation (ZORA) [121].

4. Conclusions

From the computational study reported in this paper, the following conclusions can be drawn: (i) the copper-catalyzed Ullmann-type N-, O-, and S-arylation cross-coupling reactions take place via oxidative addition—reductive elimination mechanism through penta-coordinated CuIII intermediates. (ii) In good agreement with the experimental findings, the oxidative addition of iodobenzene to active copper complexes [(phen)CuI(XPh)] is the rate-determining step. (iii) The polarity of the solvent increases the relative energy of the intermediate structures in the OA-RE pathway given their less polar nature. (iv) The reactivity is reduced by changing the heteroatom nature following the order X = O > S > NH. (v) This trend in the oxidative addition is controlled by the strain energy associated with the deformation of the reactants from their equilibrium geometries to the geometries they adopt in the transition state. The differences observed are mainly ascribed to the [(phen)CuI(XPh)] geometry change.

Supplementary Materials

Cartesian coordinates (in Å) and total energies (in a.u., noncorrected zero-point vibrational energies included) of all the stationary points discussed in the text are available online at www.mdpi.com/2073-4344/7/12/388/s1. Table S1: Gibbs Free Energies (kcal/mol) for Key Stationary Points in the Mechanisms of Ullmann-Type X-Arylation Reactions (X = NH, O, S) in different solvents at the PCM-[B3LYP/def2-TZVPP//B3LYP/def2-SVP] level of theory, Table S2: Gibbs Free Energies (kcal/mol) for Key Stationary Points in the Mechanisms of Ullmann-Type X-Arylation Reactions (X = NH, O, S) in different solvents at the PCM-[M06/def2-TZVPP//B3LYP/def2-SVP] level of theory, Table S3: Gibbs Free Energies (kcal/mol) for Key Stationary Points in the Mechanisms of Ullmann-Type X-Arylation Reactions (X = NH, O, S) in different solvents at the PCM-[TPSSh/def2-TZVPP//B3LYP/def2-SVP] level of theory, Table S4: Gibbs Free Energies (kcal/mol) for Key Stationary Points in the Mechanisms of Ullmann-Type X-Arylation Reactions (X = NH, O, S) in different solvents at the PCM-[TPSS + D3(BJ)/def2-TZVPP//B3LYP/def2-SVP] level of theory, Table S5: Gibbs Free Energies (kcal/mol) for Key Stationary Points in the Mechanisms of Ullmann-Type X-Arylation Reactions (X = NH, O, S) in different solvents at the PCM-[PBE/def2-TZVPP//B3LYP/def2-SVP] level of theory, Table S6: Gibbs Free Energies (kcal/mol) for Key Stationary Points in the Mechanisms of Ullmann-Type X-Arylation Reactions (X = NH, O, S) in different solvents at the PCM-[PBE + D3(BJ)/def2-TZVPP//B3LYP/def2-SVP] level of theory, Figure S1: EDA of the interaction energy for the oxidative addition reaction of PhI (1) to [(phen)CuI(OPh)] projected onto the forming Cu–C bond. All data have been computed at the BP86 + D3(BJ)/TZ2P+//B3LYP/def2-SVP level, Figure S2: EDA of the interaction energy for the oxidative addition reaction of PhI (1) to [(phen)CuI(SPh)] projected onto the forming Cu–C bond. All data have been computed at the BP86 + D3(BJ)/TZ2P+//B3LYP/def2-SVP level.

Acknowledgments

This work was supported by the Universität des Saarlandes. The work at Córdoba was supported by CONICET, Agencia Nacional de Promoción Científica y Técnica (ANPCyT) and SeCyT-UNC.

Author Contributions

D.M.A. conceived, performed the calculations, analyzed the data, and wrote the paper. S.M.S.-C., D.A.C., J.E.A., and A.B.P. analyzed the data and wrote the paper.

Conflicts of Interest

The authors declare no conflict of interest.

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Catalysts 07 00388 sch001 550
Scheme 1. The four most invoked reaction mechanisms of Ullman-type reactions. Single Electron Transfer (SET), Halogen Atom Transfer (HAT), Oxidative Addition–Reductive Elimination (OA-RE) and σ-bond Metathesis (MET) [51].
Scheme 1. The four most invoked reaction mechanisms of Ullman-type reactions. Single Electron Transfer (SET), Halogen Atom Transfer (HAT), Oxidative Addition–Reductive Elimination (OA-RE) and σ-bond Metathesis (MET) [51].
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Catalysts 07 00388 sch002 550
Scheme 2. Copper-catalyzed cross-coupling Ullmann type reaction studies. Base (Cs2CO3 and KOtBu) and solvent (toluene and acetonitrile).
Scheme 2. Copper-catalyzed cross-coupling Ullmann type reaction studies. Base (Cs2CO3 and KOtBu) and solvent (toluene and acetonitrile).
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Catalysts 07 00388 sch003 550
Scheme 3. Equilibrium between the possible copper species (X = NH, O, and S).
Scheme 3. Equilibrium between the possible copper species (X = NH, O, and S).
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Catalysts 07 00388 g001 550
Figure 1. Optimized structures (B3LYP/def2-SVP) and bond dissociation energies D0(Cu-X) (B3LYP + D3(BJ)/def2-TZVPP//B3LYP/def2-SVP) for the complexes [(phen)CuI(NHPh)], [(phen)CuI(OPh)], and [(phen)CuI(SPh)]. Bond lengths and angles are in [Å] and [°], respectively. The hydrogen atoms attached to carbon atoms are omitted for clarity.
Figure 1. Optimized structures (B3LYP/def2-SVP) and bond dissociation energies D0(Cu-X) (B3LYP + D3(BJ)/def2-TZVPP//B3LYP/def2-SVP) for the complexes [(phen)CuI(NHPh)], [(phen)CuI(OPh)], and [(phen)CuI(SPh)]. Bond lengths and angles are in [Å] and [°], respectively. The hydrogen atoms attached to carbon atoms are omitted for clarity.
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Catalysts 07 00388 g002a 550Catalysts 07 00388 g002b 550
Figure 2. Computed Gibbs energy profile (ΔG [kcal/mol] at 298.15 K) at the B3LYP + D3(BJ)/def2-TZVPP//B3LYP/def2-SVP of the cooper-catalyzed cross-coupling reaction: (a) PhI (1) and aniline (2-NH); (b) PhI (1) and phenol (2-O); (c) PhI (1) and thiophenol (2-S). Oxidative Addition–Reductive Elimination (OA-RE) (—); Methatesis (MET) (—); Halogen Atom Transfer (HAT) (—) and Single Electron Transfer (SET) (—). The hydrogen atoms attached to carbon atoms are omitted for clarity.
Figure 2. Computed Gibbs energy profile (ΔG [kcal/mol] at 298.15 K) at the B3LYP + D3(BJ)/def2-TZVPP//B3LYP/def2-SVP of the cooper-catalyzed cross-coupling reaction: (a) PhI (1) and aniline (2-NH); (b) PhI (1) and phenol (2-O); (c) PhI (1) and thiophenol (2-S). Oxidative Addition–Reductive Elimination (OA-RE) (—); Methatesis (MET) (—); Halogen Atom Transfer (HAT) (—) and Single Electron Transfer (SET) (—). The hydrogen atoms attached to carbon atoms are omitted for clarity.
Catalysts 07 00388 g002aCatalysts 07 00388 g002b
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Figure 3. Activation Strain Model (ASM) diagram of the oxidative addition reaction of PhI (1) to [Cu(phen)XPh] where X = NH (blue curves), X = O (red curves), and X = S (green curves). The position of the corresponding TSs is indicated by a dot. All data were computed at the B3LYP + D3(BJ)/def2-TZVPP//B3LYP/def2-SVP level of theory.
Figure 3. Activation Strain Model (ASM) diagram of the oxidative addition reaction of PhI (1) to [Cu(phen)XPh] where X = NH (blue curves), X = O (red curves), and X = S (green curves). The position of the corresponding TSs is indicated by a dot. All data were computed at the B3LYP + D3(BJ)/def2-TZVPP//B3LYP/def2-SVP level of theory.
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Catalysts 07 00388 g004 550
Figure 4. Contributions to the total strain energy along the reaction coordinate of the oxidative addition reaction of PhI (1) to [(phen)CuI(XPh)] where X = NH (blue curves), X = O (red curves), and X = S (green curves). The position of the corresponding TSs is indicated by a dot. All data were computed at the B3LYP + D3(BJ)/def2-TZVPP//B3LYP/def2-SVP level of theory.
Figure 4. Contributions to the total strain energy along the reaction coordinate of the oxidative addition reaction of PhI (1) to [(phen)CuI(XPh)] where X = NH (blue curves), X = O (red curves), and X = S (green curves). The position of the corresponding TSs is indicated by a dot. All data were computed at the B3LYP + D3(BJ)/def2-TZVPP//B3LYP/def2-SVP level of theory.
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Figure 5. Energy decomposition analysis (EDA) of the interaction energy for the oxidative addition reaction of PhI (1) to [(phen)CuI(NHPh)] projected onto the forming Cu–C bond. All data were computed at the BP86 + D3(BJ)/TZ2P +//B3LYP/def2-SVP level.
Figure 5. Energy decomposition analysis (EDA) of the interaction energy for the oxidative addition reaction of PhI (1) to [(phen)CuI(NHPh)] projected onto the forming Cu–C bond. All data were computed at the BP86 + D3(BJ)/TZ2P +//B3LYP/def2-SVP level.
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Table
Table 1. Gibbs Free Energies (kcal/mol) for active copper [(phen)CuI(XPh)] (X = NH, O, and S) complex formation (Reaction 1) and disproportionation (Reaction 2) in different solvents.1.
Table 1. Gibbs Free Energies (kcal/mol) for active copper [(phen)CuI(XPh)] (X = NH, O, and S) complex formation (Reaction 1) and disproportionation (Reaction 2) in different solvents.1.
ReactionPhNH2PhOHPhSH
ΔGgasΔGtolΔGacnΔGgasΔGtolΔGacnΔGgasΔGtolΔGacn
1 (Cs2CO3)15.28.22.38.50.6−5.8−4.2−12.4−19.4
1 (KOtBu)0.2−3.2−7.7−6.5−10.8−15.8−19.2−23.8−29.4
244.713.7−3.440.410.6−6.046.916.4−0.7
1 All calculations were computed at the PCM-[B3LYP + D3(BJ)/def2-TZVPP//B3LYP/def2-SVP] level of theory.
Table
Table 2. Gibbs Free Energies (kcal/mol) for Key Stationary Points (SP) in the mechanisms of Ullmann-Type X-Arylation Reactions (X = NH, O, S) in different solvents 1.
Table 2. Gibbs Free Energies (kcal/mol) for Key Stationary Points (SP) in the mechanisms of Ullmann-Type X-Arylation Reactions (X = NH, O, S) in different solvents 1.
EntrySPPhNH2PhOHPhSH
ΔGgasΔGtolΔGacnΔGgasΔGtolΔGacnΔGgasΔGtolΔGacn
1TSOA17.318.920.413.215.517.016.819.020.5
2INTOA/RE6.57.58.59.211.413.28.29.310.0
3TSRE8.48.27.919.020.120.712.814.115.1
4TSMet26.728.029.227.630.032.027.929.731.6
5TSHAT30.329.532.631.631.835.134.734.637.7
6INTHAT25.223.722.227.027.026.331.230.830.3
7TSSET116.056.333.9131.967.941.5135.170.043.0
8INTSET107.056.224.3118.167.835.6120.269.737.7
9Prod−37.3−39.6−40.9−27.2−28.2−28.5−20.5−21.8−22.1
1 All calculations were computed at the PCM-[B3LYP + D3(BJ)/def2-TZVPP//B3LYP/def2-SVP] level of theory.

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Andrada, D.M.; Soria-Castro, S.M.; Caminos, D.A.; Argüello, J.E.; Peñéñory, A.B. Understanding the Heteroatom Effect on the Ullmann Copper-Catalyzed Cross-Coupling of X-Arylation (X = NH, O, S) Mechanism. Catalysts 2017, 7, 388. https://doi.org/10.3390/catal7120388

AMA Style

Andrada DM, Soria-Castro SM, Caminos DA, Argüello JE, Peñéñory AB. Understanding the Heteroatom Effect on the Ullmann Copper-Catalyzed Cross-Coupling of X-Arylation (X = NH, O, S) Mechanism. Catalysts. 2017; 7(12):388. https://doi.org/10.3390/catal7120388

Chicago/Turabian Style

Andrada, Diego M., Silvia M. Soria-Castro, Daniel A. Caminos, Juan E. Argüello, and Alicia B. Peñéñory. 2017. "Understanding the Heteroatom Effect on the Ullmann Copper-Catalyzed Cross-Coupling of X-Arylation (X = NH, O, S) Mechanism" Catalysts 7, no. 12: 388. https://doi.org/10.3390/catal7120388

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