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Catalysts 2017, 7(9), 264; doi:10.3390/catal7090264

Article
Mechanistic Insight into the 2° Alcohol Oxidation Mediated by an Efficient CuI/L-Proline-TEMPO Catalyst—A Density Functional Theory Study
Siyu Li 1, Lin Cheng 1,*Orcid, Qi Wu 2, Qiancheng Zhang 1, Jucai Yang 1 and Juming Liu 1,*
1
College of Chemical Engineering, Inner Mongolia University of Technology, Inner Mongolia Key Laboratory of Theoretical and Computational Chemistry Simulation, Hohhot 010051, China
2
High Performance Computing Center of Jilin University, Changchun 130022, China
*
Correspondence: Tel.: +86-0471-6575722 (L.C.)
Received: 10 August 2017 / Accepted: 31 August 2017 / Published: 5 September 2017

Abstract

:
Density functional theory (DFT) calculations have been performed to investigate the 2° alcohol oxidation to acetophenone catalyzed by the CuI/L-Proline-2,2,6,6- tetramethylpiperidinyloxy (TEMPO) catalyst system. Seven possible pathways (paths A→F) are presented. Our calculations show that two pathways (path A and path B) are the potential mechanisms. Furthermore, by comparing with experimental observation, it is found that path A—in which substrate alcohol provides the proton to OtBu to produce HOtBu followed by the oxidation of substrate directly to product acetophenone by O2—is favored in the absence of TEMPO. Correspondingly, path B is likely to be favored when TEMPO is involved. In path B, the O–O bond cleavage of CuI–OOH to CuII–OH species occurs, followed by acetophenone formation assisted by ligand (L)2ˉ. It is also found that the cooperation of ligand (L)2ˉ and TEMPO plays an important role in assisting the formation of the product acetophenone in path B.
Keywords:
alcohol oxidation; reaction mechanism; density functional theory; aerobic oxidation; energetic span model

1. Introduction

Selective alcohol oxidation to the corresponding carbonyl product is among the most important and common transformations in organic chemical synthesis [1,2]. Recently, extensive investigations for aerobic alcohol oxidation have been focused on the design of environmentally friendly oxidation catalysts [3,4]. During the past few decades, many of the developed catalysts for aerobic alcohol oxidation are noble metal complexes, such as Pd [5,6,7], Au [8,9,10] and Ru [11,12,13,14] complexes, etc. However, by considering the limitations on the synthetic scope of these systems and their rarity, their potential uses in large-scale applications are limited. Recently, a class of copper-based catalysts has emerged as highly effective catalysts for aerobic alcohol oxidation [15,16,17,18,19,20,21,22]. However, these Cu-based catalysts usually show efficient activity for primary alcohol (1° alcohol), but the activity for secondary alcohol oxidation (2° alcohol) is unsatisfactory. Until now, only a few groups have developed Cu-based catalysts for the aerobic oxidation of 2° alcohols [23,24,25,26,27,28,29]. Meanwhile, the ligands for most of the reported Cu-based catalysts are confined to 1,10-phenanthroline (Phen), 2,2′-bipyridine (bipy) and their derivatives. For instance, Markó and co-workers have developed the catalyst (phen)CuCl and co-catalyst dialkylazodicarboxylates which are used to oxidize a wide range of 1° and 2° benzylic, allylic, and aliphatic alcohols [23,24]. Another copper-catalyzed 2° alcohol oxidation catalyst, introduced by Knochel and co-workers [25], utilizes a fluoroalkyl-substituted bipyridyl ligand with CuBr-Me2S as a catalyst in a fluorous biphasic system. Recently, a highly effective catalyst system reported by Stahl et al. made breakthroughs in aerobic alcohol oxidation [26,27,28]. This catalyst system [26], which features a CuI salt in combination with 4,4′-dimethoxy-2,2′-bipyridine (MeObipy), N-methyl-imidazole (NMI) and 9-Azabicyclo[3.3.1]nonane N-Oxyl (ABNO), mediates aerobic oxidation of all classes of alcohols (including 1° and 2° allylic, benzylic, and aliphatic alcohols) with nearly equal efficiency. Although most of these catalysts are efficient for 2° alcohols oxidation, the limited type of ligands for the Cu-based catalysts limit the development on the design of more efficient catalyst systems.
Fortunately, Ding and coworkers reported a CuI/L-Proline-2,2,6,6-tetramethylpiperidinyloxy (TEMPO) catalyst (L-proline: N,O-didentate ligand; commercially available and inexpensive), which has high activity in the aerobic oxidation of 2° alcohols under mild conditions [29]. It should be pointed out that this catalyst system could also catalyze 2° alcohol oxidation without TEMPO, though with less efficiency. Therefore, the CuI/L-Proline-TEMPO catalyst system is expected to be very attractive. Although Ding and coworkers have proposed a plausible mechanism based on the related studies reported by Stahl’s group, a deep understanding and a clear description of the reaction pathways are still missing. Moreover, mechanistic insight of CuI/TEMPO-mediated alcohol oxidation has been the subject of debate. Three of the most prominent mechanistic proposals for the CuI-TEMPO catalyst system are depicted in Scheme 1. (1) Sheldon and co-workers presented a reaction mechanism in which one TEMPO assists the formation of the CuII center, and the second TEMPO coordinated to the CuII center plays the same role as a coordinated tyrosine radical in the galactose oxidase mechanism (Scheme 1, Sheldon) [30]; (2) Stahl et al. reported a mechanism featuring two separate half-reactions: (a) catalyst oxidation by O2; (b) alcohol oxidation mediated by CuII and TEMPO (Scheme 1, Stahl) [31,32]; (3) In contrast to Stahl’s proposals, Brückner et al. proposed a modified reaction mechanism (Scheme 1, Brückner) [33]. They found TEMPO is postulated to stabilize active (bpy)(NMI)CuII-O2ˉ-TEMPO species. Besides the uncertain mechanism, understanding the role of the L-proline ligand in the catalytic cycle could also be beneficial in providing useful information for developing new ligands to assist the alcohol oxidation reaction. Herein, we present DFT calculations for the CuI/L-Proline-TEMPO catalyst system. We hope the investigation of the catalytic reaction mechanism will provide useful insight for the development of new synthetic aerobic oxidation catalysts.

2. Results and Discussion

2.1. Reaction Models

Since Ding and co-workers selected 1-phenethyl alcohol as the model substrate to determine the optimal condition in Ref. [29], 1-phenethyl alcohol was selected to represent 2° alcohol in this work. Under the optimal reaction condition (1.0 mmol alcohol, 5 mol % TEMPO, 1.0 equiv tBuOK, 5 mol % CuI, 5 mol % L-proline), Ding and co-workers demonstrate that L-proline is in its deprotonated form (denoted as (L)2ˉ in the following discussion) under alkaline conditions, and this ligand (L)2ˉ coordinates to CuI ion to form the (L)2ˉ-ligated CuI species. On the basis of the reaction mechanisms proposed by Stahl et al. [31,32], Brückner et al. [33] and Sheldon et al. [30], two CuI complexes bearing ligand (L)2ˉ were considered as the probable catalyst models (Model A: [(L)2ˉ(CuI)(ˉOtBu)]2ˉ(0); Model B: [(L)2ˉCu(TEMPO)]ˉ(229)).

2.2. Possible Reaction Mechanisms

2.2.1. Catalyst Model A

Starting from complex [(L)2ˉ(CuI)(ˉOtBu)]2ˉ(0), six pathways (paths AE) are explored (Scheme 2). Among them, path A, path B and path B′ are proposed in this work, while paths CE are proposed on the basis of the reaction mechanisms presented by Stahl et al. [31,32], Brückner et al. [33] and Ding et al. [29], respectively. As seen in Scheme 2, in process 1014, the product acetophenone is formed by the assistance of oxygen. Subsequently, three pathways (Paths A, B and B′) from 14 are explored. Path A proceeds through the replacement of the product acetophenone by substrate PhCH(OH)CH3. While in path B and B′, TEMPOH and TEMPO are used to replace the product acetophenone.
● Formation of Product Acetophenone in Process 1014
From 0, coordination of substrate PhCH(OH)CH3 to 0 gives 1 ([(L)2ˉ(CuI)(PhCH(OH)CH3)(ˉOtBu)]2ˉ). Next, a proton migration from substrate alcohol to ˉOtBu species generates 2 ([(L)2ˉ(CuI)(PhCH(Oˉ)CH3)(HOtBu)]2ˉ). The energy barrier for this proton transfer process, 111TS1–2, is 17.7 kcal/mol (Figure 1). Subsequently, the weakly coordinated closed-shell singlet HOtBu molecule is replaced by open-shell triplet O2 to generate 3 in a singlet (13, S = 0) and triplet (33, S = 1) states. In 33/13, the spin populations on Cu center, O2 species, PhCH(Oˉ)CH3 and ligand (L)2ˉ are +0.51/+0.53, +1.13/−0.90, +0.10/+0.09, and +0.26/+0.28, respectively. These data indicate that 33/13 possess the [(L)2ˉ(PhCH(Oˉ)CH3)CuII(OOˉ)]2ˉ character. In the following step, the OOˉ species abstracts the H atom from the Cα–H bond of PhCH(Oˉ)CH3 moiety (3→4 in Figure 1). As seen in Figure 1, this H abstraction step involves a spin crossover from the S = 1 ground state to the S = 0 transition state (1TS3–4). Therefore, the minimum energy crossing point (MECP) is calculated by MECP program [34,35,36]. An accurate structure of MECP is located as 3′-MECP (∆E = 12.3 kcal/mol). Through 3′-MECP, 33 crosses to the singlet potential energy surface leading to 14 ([(L)2ˉ)(PhC(O)CH3)CuI(OOH)]2ˉ). After 14, three pathways (paths A, B and B′) are explored.
● Path A
In path A, after 14, the product acetophenone is hereby replaced by PhCH(OH)CH3 to form 15 (Figure 1). Then, the proton migrates from PhCH(OH)CH3 to –OOH moiety to give 16 ([(L)2ˉ(PhCH(O)CH3)CuI(H2O2)]2ˉ). Finally, H2O2 is replaced by O2 to regenerate 3 to finish the catalytic cycle. To assess the efficiency of the catalytic cycle in path A, the energetic span model introduced by Kozuch and Shaik [37,38,39,40,41] was used to identify the Turnover Frequency (TOF)-determining intermediate (TDI) as 16 and the TOF-determining transition state (TDTS) as 1TS3–4, corresponding to an energy span (δE) of 23.6 kcal/mol (calculated by AUTOF program [37,38,39,40,41], based on the theoretically obtained energy profile in Figure 1). Moreover, it is noted that TEMPO was not involved in the catalytic cycle in path A. Thus, path A is likely the favorable pathway for the CuI/L-Proline-TEMPO catalyst system without TEMPO.
● Path B
1. TEMPO or TEMPOH (1-Hydroxy-2,2,6,6-tetramethylpiperidine) coordination?
From 14, another two pathways are designed (path B and path B′ in Scheme 2). The product acetophenone is replaced by TEMPO in path B’, while replaced by TEMPOH in path B. The calculated results show that path B′ should be excluded for the fairly high energetic span (δE = 55.7 kcal/mol, Figure S1 in the Supplementary Materials). The high energetic span for path B′ could trace to the sterically sensitive TEMPO. Stahl and coworkers demonstrate that steric effects of the nitroxyl radical have a key influence on their reactivity toward alcohol oxidation [26,34]. Therefore, direct binding of TEMPO to Cu center in path B′ (232234) is likely to be more susceptible to steric hindrance than path B (17110), where interaction occurs at the second coordination sphere of Cu center. Thus, only path B is discussed below.
As we know, there is no TEMPOH adding to the catalytic system at the beginning of the reaction. In finding where TEMPOH comes from, we speculate that the origin of TEMPOH likely comes from path F at the beginning of the reaction (details in Section 2.2.2). As the reaction goes on, the catalyst is consumed and the concentration of TEMPOH ([TEMPOH]) increases. When [TEMPOH] reaches a certain concentration, path B would be initiated. Then, TEMPOH replaces the product acetophenone to form two isomers 17 and 17′ (Scheme 3). In 17, the proton on TEMPOH forms a hydrogen bond with the proximal oxygen atom of the CuI–OOH species, while in 17′, TEMPOH is hydrogen-bonded to the distal oxygen. It is found that 17 is 7.5 kcal/mol lower than 17′. Subsequently, the formation of either complex 18 or complex 135 from 17 to 17′ (Scheme 3) was computed. The formation of 18 from 17 requires overcoming an energy barrier that is 1.4 kcal/mol higher than forming 135 from 17′. These results indicate that the formation of 18 is energetically more favorable than the formation of 135. Thus, only the favorable pathway from 17 is discussed below. The corresponding energy profile is shown in Figure 2. Detailed information for the pathway from 17′ is collected in Supplementary Materials (Figure S2).
2. Formation of Cu–OH Species
In process 1718, TEMPOH could assist the homolysis or heterolysis of the O–O bond in the CuI–OOH moiety to form either CuII=O and OH or a CuIII–O2ˉ and ˉOH. Thus, the closed singlet and open shell singlet potential energy surfaces in process 1718 are explored. It is found that the open shell singlet 1uTS7–8 converges to the closed singlet 1TS7–8 during the optimization process. This result demonstrates that TEMPOH assists the heterolytic cleavage of the O–O bond of CuI–OOH moiety. To get clearer insight into this O–O bond cleavage, changes in the electronic structure from 17 to 18 are examined. The results show that the electronic configurations of Cu center in 17 is d2yzd2z2d2xzd2xyd2x2-y2, indicating the CuI oxidation state, while that in 18 is d2yzd2z2d2xzd2xyd0x2-y2, implying the CuIII oxidation state. This result further supports the heterolytic cleavage of the O–O bond. Moreover, Wiberg Bond Index (WBI) is calculated by Multiwfn program [42]. It is found that, as the O–O bond is weakened, the O–Cu bond is strengthened in process 171TS7–818 (O–O: 1.300.130.00; O–Cu: 0.871.601.72). This is further verified by the bond length variations for the O–O and O–Cu bonds (O–O: 1.495 Å3.069 Å8.172 Å; O–Cu: 1.878 Å1.712 Å1.685 Å). Thus, in process 1718, the heterolytic cleavage of the O–O bond generates the CuIII–O2ˉ moiety and ˉOH species. Subsequently, ˉOH species dissociate, followed by the H atom transfer from TEMPOH to CuIII–O2ˉ species to form CuII–OH species and TEMPO (18110). After 110, dissociation of TEMPO results in the formation of CuII–OH species (211). This CuII–OH species has been reported as the active species in Stahl’s catalyst system (CuI(OTf)-TEMPO) [32].
3. Alcohol Oxidation
In the following steps (211213), a proton transfers from substrate PhCH(OH)CH3 to ˉOH moiety to form a Cu–alkoxide adduct 213 ([(L2ˉ)CuII(PhCH(Oˉ)CH3)(H2O)]ˉ). From 213, the singlet H2O molecule is replaced by doublet TEMPO to generate 14a in singlet (114a) and triplet (314a) (Figure S3). Then, the H atom transfers from alkoxide either to the oxygen atom of η1-TEMPO via TS14a-15a_O or to the unbound nitrogen atom of η1-TEMPO via TS14a–15a_N (identified by Baerends et al. [43,44]. and Stahl et al. [32]). The calculated energy barriers for both pathways are >28.7 kcal/mol (TS14a-15a_N/TS14a-15a_O relative to 213 in Figure S3). These data indicate that the Cα–H bond activation step might not be assisted by TEMPO. Then, the question is what assists this step. To answer this question, let us to gain insight into the electronic structure of 213. The molecular orbital for 213 illustrates that one unpaired electron is located in Cu dx2-y2 orbital (29.2% Cu character) with a significant contribution of Pz orbital of the N(L)2ˉ atom (34.3% N character) (Scheme 4). This result suggests that the N(L)2ˉ atom could abstract the H atom from alkoxide. The corresponding activation barrier for this H atom transfer is 22.8 kcal/mol (Figure 2). Therefore, instead of TEMPO, ligand (L)2ˉ assists the Cα–H bond activation. However, product acetophenone is not formed in process 213214. In 214, the spin populations are +0.00 on Cu, +0.00 on ligand (L)2ˉ, and +1.00 on PhC(O)CH3 moiety, implying the [(HL2ˉ)CuI(PhC(O)CH3)(H2O)]ˉ character. Thus, another oxidized agent must be needed to give the product acetophenone. In this case, TEMPO is supposed to assist the formation of acetophenone (116117). Finally, OtBu species replace TEMPOH and acetophenone to reproduce 0 to close the catalytic cycle. Obviously, TEMPO and ligand (L)2ˉ act together to assist the formation of product acetophenone. The efficiency of the catalytic cycle in path B was also assessed by the AUTOF program. The results identify 213 as TDI and 2TS13–14 as TDTS, corresponding to an energetic span (δE) of 22.8 kcal/mol. Since TDI and TDTS are the key factors affecting TOF [37], the electron transfer process during 2132TS13–14 is discussed. The spin population on Cu center decreases from +0.41 to +0.06 (2132TS13–14), illustrating that a fraction of β-electron has been migrated to Cu center. This fraction of β-electron comes from the homolytic cleavage of the Cu–NL2– bond. The corresponding α-electron migrates to ligand (L)2ˉ (especially to NL2– atom), which should result in the increase of α-spin populations on ligand (L)2ˉ. However, it is noted that the spin population on ligand (L)2ˉ decreases from +0.57 to +0.47. This change is related to the homolytic cleavage of Cα–H bond of substrate. The Cα–H bond homolytic cleavage could give a fraction of β-spin electron to ligand (L)2ˉ, to make the α-spin population on ligand (L)2ˉ decrease to +0.47. The corresponding α-electron migrates to the substrate to make the α-spin population on the substrate increase from +0.03 to +0.53 in the process 2132TS13–14. Therefore, two bonds (Cu–NL2– and Cα–H bonds) are partially in homolytic cleavage in the process 2132TS13–14. This conclusion is further verified by WBI analysis. The results show that the Cu–NL2– and Cα–H bonds are weakened (55% of Cu–NL2– and 45% of Cα–H bonds are broken at 2TS13–14), while the Cu–Osub bond remains (Table 1). Meanwhile, the bond length variations for the Cu–NL2– and Cα–H bonds in process 2132TS13–14 (Cu–NL2–: 1.871 Å2.189 Å; Cα–H: 1.104 Å1.347 Å) are observed as well.
● Path C
Path C is suggested based on the reaction mechanism disclosed by Stahl and co-workers (Scheme 2). The computed Gibbs free-energy profile is provided in Figure S4. Initially, oxidation of 0 with oxygen gives 3,118. For 318 (118), calculated spin populations on Cu, O2 species and ligand (L)2ˉ are +0.46 (+0.52), +1.22 (−0.85) and +0.25 (+0.27), respectively, implying the CuII–OO•− character. Then, we suppose that another 0 coordinates to 3,118 to form a binuclear CuII species Cu2O2 (19). For 19, three dominant bonding motifs are considered (side-on μ-η22-peroxo, bis-μ-oxo-CuIII and trans end-on μ-η11-superoxo) [45]. However, all the efforts to locate the binuclear CuII species failed. They all dissociate to 0 and 18 during the optimization. Therefore, another possibility is discussed. From 18, OtBu moiety is replaced by PhCH(OH)CH3 to form 20, followed by another 0 coordination to form 21. Nonetheless, the formation of 21 is endothermic 29.0 kcal/mol. Clearly, such a complex 21 is unfavorable to be formed. Moreover, the transition state 3TS22–23 in the following H migration process from TEMPOH to Cu–OO•− moiety is not located in our work. Thus, path C should be ruled out.
● Path D
Path D is suggested based on the reaction mechanism reported by Brückner et al. (Scheme 2). From 18, TEMPO initially coordinates to form 24. This step is endothermic by 19.6 kcal/mol (Figure S5), meaning 24 is unfavorable to be generated. Even if 24 is formed, the activation energy barrier for the following concerted step (3253TS25–26) is still high for a reaction occurring at room temperature (51.1 kcal/mol; 3TS25–26 relative to 10 in Figure S5). Thus, path D should also be excluded.
● Path E
Path E is suggested based on the reaction mechanism reported by Ding and co-workers (Scheme 2). From 318, TEMPOH comes to 18 to form a hydrogen bond to O atom of CuII–OO•− moiety in 327. This initial step is endothermic by 13.7 kcal/mol. Then, TEMPOH gives an H atom to the CuII–OOˉ moiety to form CuII–OOH and TEMPO (28). However, all the efforts to locate the transition state TS27–28 and intermediate 28 failed; they all come back to 27 during the optimization. Therefore, this pathway should be ruled out as well.
As a result, for catalyst model A, path B′, path C, path D and path E are not preferred. Path A and path B are the possible routes (details in Section 2.2.3).

2.2.2. Catalyst Model B

For catalyst model B, path F is explored. Path F is proposed based on Sheldon’s mechanism [30]. The corresponding energy profiles and optimized structures are illustrated in Figure 3. In path F, the starting structure is [(L2ˉ)Cu(TEMPO)]ˉ (229), and two possible binding motifs ([(L2ˉ)CuII(TEMPOˉ)]ˉ or [(L2ˉ)CuI(TEMPO)]ˉ) were taken into account. The calculated singlet occupied molecular orbital for 229 is shown in Scheme 4. Obviously, the unpaired electron is located on Cu dx2-y2 orbital with a significant contribution of the σ orbital of the connecting atoms. This spin distribution explains the α-spin populations on TEMPO (+0.33) and L-Proline (+0.22). In addition, the O–N bond length of TEMPO group is 1.413 Å in 229, consistent with the 1.410 Å in TEMPOˉ anion (1.285 Å in TEMPO radical). Combining the above results, it is obvious that 229 is in the [(L2ˉ)CuII(TEMPOˉ)]ˉcharacter. Subsequently, the addition of PhCH(OH)CH3 to 229 leads to the formation of 230. From 230, the following step is proton migration from PhCH(OH)CH3 to TEMPO anion to form TEMPOH via 2TS30–31 (19.4 kcal/mol relative to 229). Thus, TEMPOH species generating from 229231 in path F would be the origin of the TEMPOH species at the beginning of the reaction. In the following step, replacing TEMPOH by TEMPO leads to 214a, followed by an H atom abstraction step (14a15a). The energy barrier for this H abstraction step is 41.4 kcal/mol, which is fairly high for a reaction occurring at room temperature. Thus, path F should be excluded as well.

2.2.3. Preliminary Mechanistic Assessment

Overall, the three previous prominent mechanistic proposals (paths C, D and F) presented by Stahl et al. [31,32], Brückner et al. [33], Sheldon et al. [30] and Ding’s mechanistic proposal (path E) are not preferred. For path A and path B, the energy spans are 23.6 kcal/mol and 22.8 kcal/mol, respectively, indicating that both pathways are possible routes. Experimentally, Ding and coworkers demonstrate that this CuI/L-Proline-TEMPO catalyst system could catalyze 2° alcohols without TEMPO. Thus, based on the aforementioned calculation results, path A would be the main route when TEMPO is not involved in this catalyst system. Then the question arises: which pathway is the dominant reaction pathway for this catalyst system in the presence of TEMPO? We tentatively assign path B as the dominant pathway for this CuI(L-Proline)/TEMPO-mediated alcohol oxidation reaction. Our preliminary explanations are as follows: (1) One can see that the energy span of path A (23.6 kcal/mol) is 0.8 kcal/mol higher than that of path B (22.8 kcal/mol); (2) The formation of 17 in path B is found to be more favorable than the formation of 15 in path A (Figure 2). Meanwhile, 17 can be transferred to 211 via a high exergonicity of the process (7+TEMPOH→11+TEMPO). This might deplete 17 very fast and thus reduce the probability of the formation of 15; (3) Ding and coworkers found that the conversion of product increases from 10% (TEMPO-free) to >99% (with TEMPO). This experimental observation shows that TEMPO should be involved in the catalytic cycle. Thus, path B is likely to be the dominant pathway for this CuI/L-Proline-TEMPO catalyst system and the cooperation of ligand (L)2− and TEMPO plays an important role in assisting the formation of the product acetophenone.
In a word, although we can tentatively assign path B as the dominant pathway for the CuI/L-Proline-TEMPO catalyst system, more evidence is clearly necessary. For the case without TEMPO, path A is the main route.

3. Computational Details

All the calculations were carried out with the Gaussian 09 series of programs [46]. The B3LYP [47,48,49] functional with a standard 6-31+G(d) basis set was used for geometry optimizations in gas phase, and frequency calculations were carried out at the same level of theory. The intrinsic reaction coordinate (IRC) approach was used to confirm that the transition state connects the two relevant minima [50,51]. The combination of using B3LYP for geometry optimizations and M06 for single point calculations has been successfully applied to investigate various transition metal-catalyzed reactions [52,53,54,55,56,57,58,59,60,61,62,63,64], including organocopper systems [60,61,62,63,64]. Thus, to further refine the energies, single-point energy calculations were performed on the stationary points at M06/6-311+G(d,p) level [65,66] with the SMD solvation model [67] and N,N-Dimethylformamide (DMF) as the solvent using the gas-phase optimized structures. For comparison purposes, the key intermediates and transition states were subject to reoptimization with B3LYP/6-31+G(d)/SMD(DMF) and single-point energy calculations with M06/6-311+G(d,p)/SMD(DMF), and the outcomes were consistent with the M06/6-311+G(d,p)/SMD(DMF)//B3LYP/6-31+G(d) results (Table S1A in the Supplementary Materials). Spin populations are reported by Mulliken population analysis from single-point energy calculations. Geometrical counterpoise correction and the dispersion correction were also calculated by using gCP program and DFT-D3 program of Grimme [68,69,70,71,72]. To ensure our conclusions are not affected by the choice of the computational method used in this study, the key intermediates and transition states were recalculated with different DFT methods. According to benchmarking research [73,74], two commonly used modern functionals—the nonlocal hybrid meta GGA TPSSh [75] and Wb97xd [76] (including empirical dispersion)—were tested. These test calculations show that the relative energetic spans between the competing pathways are not affected by the choice of methods (Table S1B in the Supplementary Materials). In addition, the MECP in this work was located with the code developed by Harvey and co-workers at the B3LYP/6-31G(d) level [34,35,36] and only the electronic energy (ΔE) was evaluated at the M06/6-311+G(d,p)/SMD(DMF)//B3LYP/6-31G(d) level. Some open-shell calculations for the anti-ferromagnetic coupled singlet state resulted in a degree of spin contamination. Therefore, an energy correction was estimated from the Heisenberg spin-Hamiltonian formalism [77,78,79]. A similar energy correction approach was used in previous studies [80,81,82,83]. Detailed cartesian coordinates and corrections to Gibbs free energies for each reported structures have been presented in Tables S2 and S3.

4. Conclusions

We have studied the reaction mechanisms for the oxidation of 1-phenethyl alcohol to the corresponding acetophenone by CuI/L-Proline-TEMPO catalyst system by use of the density functional method. Seven pathways (paths AF) in models A and B were presented. The kinetic assessments based on the Energetic Span Model are used to discriminate the possible pathways. The calculated results show that three previous prominent mechanistic proposals (paths C→F) and Ding’s proposal (path E) are excluded as possible mechanisms for the CuI/L-Proline catalyst system. Instead, two new reaction mechanisms (path A and path B) are presented. The calculated energy spans of path A and path B are 23.6 kcal/mol and 22.8 kcal/mol, respectively. In combination with the experimental result, we suppose that both pathways (path A and path B) are possible routes. Path B is likely to be the dominant catalytic cycle for the CuI/L-Proline-TEMPO catalyst system. In contrast, path A is supposed to be the main route for the case without TEMPO. Besides, by further investigation into alcohol oxidation part in path B, it is found that TEMPO is not directly involved in the Cα–H bond cleavage of PhCH(O)CH3 moiety in the TOF determining Transition State (TS13–14). Instead, ligand (L)2 actually acts as the H atom sink to locate the H atom of the Cα–H bond in the PhCH(O)CH3 moiety in TDTS TS13–14 and TEMPO assists the formation of the product acetophenone.

Supplementary Materials

The following are available online at www.mdpi.com/2073-4344/7/9/264/s1. Table S1A: Comparison of the energy barriers for processes 171TS7–8 and 2132TS13–14 between the optimized structures in the gas phase and the solvent; Table S1B: Comparison of energetic spans (δE) for path A and path B at different levels of theories; Table S2: Cartesian coordinates of all structures considered in this work (B3LYP level); Table S3: Relative corrections to Gibbs free energies for each reported structures; Figure S1: The calculated Gibbs free energy profiles for path B′. The energy values are in kcal/mol; Figure S2: The calculated Gibbs free energy profiles for the process of 7′→44. The energy values are in kcal/mol; Figure S3: The calculated Gibbs free energy profiles for the process of 213115a. The energy values are in kcal/mol; Figure S4: The calculated Gibbs free energy profiles for Path C. The energy values are in kcal/mol; Figure S5: The calculated Gibbs free energy profiles for Path D. The energy values are in kcal/mol.

Acknowledgments

The authors thank the National Natural Science Foundation of China (NSFC) (Grant No. 21343007, 21403117), the Natural Science Foundation of the Inner Mongolia Autonomous Region (Grant No. 2016MS0206) and Program for Innovative Research Team in Universities of Inner Mongolia Autonomous Region (NMGIRT-A1603).

Author Contributions

Lin Cheng and Juming Liu conceived and designed the experiments; Siyu Li performed the experiments; Qi Wu, Qiancheng Zhang and Jucai Yang analyzed the data; Siyu Li and Lin Cheng wrote the paper.

Conflicts of Interest

The authors declare no conflict of interest.

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Scheme 1. Three prominent mechanistic proposals for the CuI/2,2,6,6-tetramethylpiperidinyloxy (TEMPO)-mediated alcohol oxidation reaction.
Scheme 1. Three prominent mechanistic proposals for the CuI/2,2,6,6-tetramethylpiperidinyloxy (TEMPO)-mediated alcohol oxidation reaction.
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Scheme 2. Six possible reaction mechanisms for model A, i.e., Paths A→E.
Scheme 2. Six possible reaction mechanisms for model A, i.e., Paths A→E.
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Figure 1. The calculated Gibbs free energy profiles for process 04 and path A. The solid blue line represents the triplet spin state; the dashed black line represents the singlet state. The energy values are in kcal/mol.
Figure 1. The calculated Gibbs free energy profiles for process 04 and path A. The solid blue line represents the triplet spin state; the dashed black line represents the singlet state. The energy values are in kcal/mol.
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Scheme 3. Two probabilities for 2,2,6,6-tetramethylpiperidinyloxy (TEMPO) coordination.
Scheme 3. Two probabilities for 2,2,6,6-tetramethylpiperidinyloxy (TEMPO) coordination.
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Figure 2. The calculated Gibbs free energy profiles for path B. The dashed line represents the singlet state; the wave line represents the doublet state. The energy values are in kcal/mol.
Figure 2. The calculated Gibbs free energy profiles for path B. The dashed line represents the singlet state; the wave line represents the doublet state. The energy values are in kcal/mol.
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Scheme 4. Molecular orbital diagram for 213 and 229. (Contour values = ±0.05).
Scheme 4. Molecular orbital diagram for 213 and 229. (Contour values = ±0.05).
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Figure 3. The calculated Gibbs free energy profiles and the optimized geometries for path F. The dashed line represents the singlet state; the solid blue line represents the triplet spin state; the wave line represents the doublet state. The energy values are in kcal/mol.
Figure 3. The calculated Gibbs free energy profiles and the optimized geometries for path F. The dashed line represents the singlet state; the solid blue line represents the triplet spin state; the wave line represents the doublet state. The energy values are in kcal/mol.
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Table 1. Distances and Wiberg Bond Index (WBI) for Cu–NL2, Cα–H and Cu–Osub for 213 and 2TS13–14.
Table 1. Distances and Wiberg Bond Index (WBI) for Cu–NL2, Cα–H and Cu–Osub for 213 and 2TS13–14.
StructuresCu–NL2Cα–HCu–OsubCu–NL2 (WBI)Cα–H (WBI)Cu–Osub (WBI)Cu–NL2 Formed %Cα–H Formed %Cu–Osub Formed %
2131.871 Å1.104 Å1.888 Å1.0370.8230.828100100100
2TS13142.189 Å1.347 Å1.892 Å0.4660.4510.78944.954.895.3
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