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• Developing sustainable and efficient catalysts for the oxidation of hydrocarbons under mild conditions has attracted considerable interests in synthetic chemistry and the chemical industry [1-5]. In this regard, metalloenzymes utilize earth-abundant metal center (e.g., iron or manganese) to catalyze a wide range of oxidation reactions by activating dioxygen under mild conditions, thereby providing an unique platform for the development of efficient oxidation catalysts [6, 7]. Over the past decades, a plethora of biomimetic catalysts capable of chemo- and enantioselectively oxidizing C=C and C-H bonds have been established [8, 9]. In particular, iron and manganese complexes with linear tetradentate nitrogen (N4) ligands inspired by the nonheme enzymes have been becoming the catalyst choice of chemo- and enantioselective oxidations, owing to their facile synthesis and diversification [10-12]. In these catalytic systems, aqueous hydrogen peroxide (H₂O₂) usually serves as the terminal oxidant, thus making these systems green and sustainable property. It should be noted that carboxylic acid, such as acetic acid, is a pivotal additive for efficient oxidation catalysis with these iron and manganese complexes.

  In the case of nonheme iron(Ⅱ) complexes, the effect of acetic acid (AcOH) was originally observed by Jacobsen and coworkers in the study of practical epoxidation olefin with MEP-Fe^Ⅱ(MeCN)₂(SbF₆)₂/H₂O₂ [MEP = N,N'-dimethyl-N,N'-bis(2-pyridylmethyl)-ethylene-1, 2-diamine]. Upon addition of catalytic amount of AcOH, Fe^Ⅱ(MEP) complex displayed excellent reactivity in the olefin epoxidation [13]. Later, Que and co-workers proposed a carboxylic acid assisted O-O bond cleavage mechanism, in which a highly reactive Fe^Ⅴ-oxo species was generated via a heterolytic O-O bond cleavage of an Fe^Ⅲ-OOH precursor in the presence of acetic acid [14]. The carboxylic acid assisted mechanism in the catalytic epoxidation reactions promoted by nonheme iron catalyst/H₂O₂/carboxylic acid has been supported experimentally and theoretically [15, 16]. In particular, on the basis of density functional theory (DFT) calculations for Fe^Ⅱ(S,S-PDP)/H₂O₂/AcOH catalytic system {PDP = 2-[[2-(1-(pyridin-2-ylmethyl)-pyrrolidin-2-yl)pyrrolidin-1-yl]methyl]pyridine}, the DFT investigations support the involvement of (S,S-PDP)Fe^Ⅳ(O)(AcO^•) as oxidizing species via the O-O bond homolysis from (S,S-PDP)Fe^Ⅲ(κ2-peracetate). This [oxoiron(Ⅳ)-AcO] cation radical has similar electronic structure as that of compound I of P450 [15]. Actually, the multiple-oxidant nature has been observed in nonheme iron systems, largely depending on the binding ligand around iron center. Recently, an [Fe^Ⅴ(O)(OAc)(PyNMe₃)]²⁺ species has been spectroscopically trapped by Costas and co-workers, which is capable of hydroxylating unactivated alkyl C-H bonds [17]. Interestingly, Nam and co-worker have demonstrated that nonheme high-spin iron(Ⅲ)-acylperoxo complex with 13-TMC ligand (13-TMC = 1,4,7,10-tetramethyl-1,4,7,10-tetraazacyclotri-decane) is strong oxidant capable of oxygenating hydrocarbons prior to their conversion into iron-oxo species via O-O bond cleavage [18]. Based on these achievements, DFT calculations have definitely helped to predict the possible active species and to address mechanistic issues, thereby elucidating the multiple-oxidant nature in the catalytic cycle of oxidation reactions in combination with the experimental studies.

  In parallel to iron systems, nonheme manganese complexes bearing chiral N4 ligands have been shown to be effective catalysts in the asymmetric oxidations [10-12]. In these manganese catalytic systems, carboxylic acids (e.g., AcOH) still play a key role in the activation of H₂O₂ together with the manganese complexes. In the past decade, Costas [19, 20], Bryliakov [16, 21] and our group [22-26] have developed a variety of nonheme manganese complexes for efficient oxidation catalysis. Most of this class of effective iron and manganese catalysts thus far possesses same feature that is supported by chiral N4 ligands, thus providing two labile cis-binding sites for H₂O₂ activation. In these reactions, it has been proposed with indirect experimental evidence that the active intermediate is Mn^Ⅴ-oxo species [16, 27]. Compared to well documented mechanism of nonheme iron catalysts, however, the mechanistic aspects concerning the manganese catalysts are underdeveloped. In this context, what kinds of manganese species will involve in the manganese/H₂O₂ catalytic systems, Mn^Ⅴ(O), Mn^Ⅳ(O) cation radical or other possible species [28]? An in-depth understanding of the mechanism is a fundamental task for rational ligand design toward tuning of enantio-control and reactivity. Previously, a variety of manganese complexes with aryl-substituted MCP (MCP = N,N'-dimethyl-N,N'-bis-(2-pyridinylmethylcyclo-hexane-1, 2-diamine) N4 ligands by our group have been showed excellent enantiocontrol in the asymmetric epoxidation of olefins, in which sulfuric acid could significantly enhance the activity for olefin epoxidation [22, 29, 30]. Continuing on this line of research, we report herein the systematic theoretical calculations on the epoxidation mechanism catalyzed by the manganese modeling catalyst, Mn^Ⅱ(R,R-PMCP)(OTf)₂ with H₂O₂ in the presence of sulfuric acid. Based on the DFT results, [Mn^Ⅴ(O)(R,R-PMCP)(SO₄)]⁺ with a triplet ground spin state serves as the active species for the olefin epoxidation. Interestingly, a Mn^Ⅲ-persulfate is likely involved in this catalytic system, however, DFT results suggest that the energy barrier of its C=C double bond epoxidation (18.7 kcal/mol) is higher than that of its transformation to a [Mn^Ⅴ(O)(R,R-PMCP)(SO₄)]⁺ species (9.5 kcal/mol). In addition, the direct use of oxone (potassium peroxymonosulfate, 2KHSO₅·KHSO₄·K₂SO₄) as oxidant also provides a comparable enantioselectivity as that of H₂O₂/sulfuric acid system.

  High-valent metal-oxo species are highly reactive intermediates involved in the oxidation of organic substrates by heme and nonheme enzymes and their model compounds. In the biomimetic manganese catalyst/H₂O₂/acid catalytic system, Mn^Ⅴ-oxo, Mn^Ⅳ-oxo and Mn^Ⅲ-peroxo may be involved as the possible active species as those of nonheme iron catalytic systems. As previously proposed by Bryliakov/Talsi and co-workers, still, the manganese catalytic system started from an Mn^Ⅲ-hydroperoxide species, followed protonation of the hydroperoxide ligand by the coordinated acid in Mn^Ⅲ-hydroperoxo intermediates facilitates the O-O bond cleavage, generating Mn^Ⅴ-oxo species as reactive epoxidizing intermediates. As shown in Scheme 1, possibly, [Mn^Ⅴ(O)(R,R-PMCP)(SO4)]⁺ 3, Mn^Ⅲ-persulfate 4 can be generated from Mn^Ⅱ(R,R-PMCP)(OTf)₂ 1 with H₂O₂ as the oxidant in the presence of sulfuric acid. Previously, we have briefly studied the formation of [Mn^Ⅴ(O)(R,R-PMCP)(SO₄)]⁺ 3 from its precursor, Mn^Ⅲ-OOH species (2) by DFT calculations [29]. The results showed that the transformation occurs reasonably via an O-O bond heterolysis by the aid of coordinated sulphuric acid. For this process, as shown in Fig. 1, the calculated ground state for complex 2 is the quintet spin state. In addition, low-spin singlet 2 is involved, however, the singlet state is more than 33.0 kcal/mol higher in energy than quintet one. In the first step, cleavage of the O-OH bond of 2 leads to the formation of Mn-oxo 3, in which a spin change from the higher quintet spin to triplet spin state occurs (two-state reactivity) [31, 32]. In this case, the transition state (TS) TS23 in triplet and quintet states are found to be 13.0 and 22.4 kcal/mol, respectively. Possibly, cyclic Mn^Ⅲ-persulfate is involved as previously reported (S,S-PDP)Fe^Ⅲ(κ2-peracetate) species. Actually, DFT calculations support the formation the Mn^Ⅲ-persulfate through the O-O bond coupling reaction with a barrier of 11.2 kcal/mol. The potential energies of triplet Mn^Ⅴ-oxo (³3) and quintet Mn^Ⅲ persalt (⁵4) are equal, indicating that both species can possibly exist in the reaction system and act as epoxidizing reagent.

  Scheme 1

  

  Scheme 1.  Possible intermediates for the Mn^Ⅱ(R,R-PMCP)(OTf)₂ with H₂O₂ in the presence of sulfuric acid.

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  Figure 1

  

  Figure 1.  Potential energy profiles for the conversion of Mn^Ⅲ-OOH to manganic persulfate via Mn^Ⅴ-oxo at B3LYP/Def2-TZVPP//TZVP(Mn, OOH, HSO₄, N atoms in the ligand)-6-31G^* (C and H atoms in the ligand) level including zero-point energy and solvation corrections.

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  Fig. 2 shows the key geometric parameters and spin-state energies of Mn^Ⅴ(O) 3, Mn^Ⅲ-persulfate 4 without the resulting H₂O. The bond length of Mn–O in ³3 is 1.699 Å, similar to the Mn-O bond length (i.e., 1.71 Å) of an S = 1 Mn^ⅤO complex, [(S-PMB)Mn^Ⅴ(O)(OAc)]⁺ (S-PMB = (S)-N-methyl-1-(1-methyl-1H-benzo-[d]imidazol-2-yl)-N-((1-((1-methyl-1H-benzo[d]-imidazol-2-yl)methyl)pyrroli-din-2-yl)-methyl)methanamine) [33]. Especially, the ³3 has the shortest distance of O1-O3, providing a justification of the lowest transition state in the formation of Mn^Ⅲ-persulfate 4 than those of singlet- and quintet states (2.326, 3.078 and 3.443 Å).

  Figure 2

  

  Figure 2.  Key geometric parameters and spin-state energies of Mn^Ⅴ(O) 3, Mn^Ⅲ persulfate 4.

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  Then, asymmetric epoxidation by Mn-oxo 3 was calculated, styrene was chosen as a model substrate. As shown in Fig. 3, styrene can bind to the Mn-oxo 3 and two diastereomeric complexes form with different stabilities. In the case of the triplet 3, the energy difference in both diastereomeric complexes is 2.0 kcal/mol. In the first step, a radical intermediate (IM3) is generated after the reaction of Mn-oxo 3 with styrene. The profile shows that this step has the lowest energy barrier of 2.6 kcal/mol on the triplet surface. The calculated results suggest this step is the enantioselectivity-determining step. The relative free energy of TS1(R) is 0.5 kcal/mol higher than that of TS1(S), which suggests the major epoxide product is S-configuration. This result is also agreement with the experimental observation. Inspection of the geometric details of preferred TS1 demonstrates that the phenyl group on the PMCP ligand plays a key role in tuning the enantiocontrol of the reaction. Bulkier group will lead to higher enantioselectivity, for example, (R,R)-DBP-MCP-Mn(OTf)₂ having 3,5-di-tert-butyl-phenyl groups on the ligand provide a better ee value (Table 1, 60% ee vs. 50% ee) [29]. When (R,R)-MCP-Mn(OTf)₂ complex, without phenyl group, was used as catalyst in the epoxidation reaction, it provided a poor yield. Consistently, the formation of Mn^Ⅴ-oxo from (R,R)-MCP-Mn(OTf)₂ has an increased energy barrier of 15.1 kcal/mol (Fig. S1 in Supporting information), compared to that of (R,R)-PMCP-Mn(OTf)₂ (13.0 kcal/mol). Finally, the IM3 undergoes an intramolecular ring closure to produce the epoxide. This step also including a spin state change from triplet to quintet. Besides, to gain more insight into the possible Mn^Ⅲ-persulfate 4, we have computationally examined the epoxidation styrene by 4 (Fig. 4). The styrene epoxidation by 3 is exothermic in each step, while the epoxidation by 4 is endothermic in the first step. Besides, this pathway has an energy barrier of 18.7 kcal/mol, even much higher than that of transformation of 4 to Mn-oxo 3 (9.5 kcal/mol). Taken together, these results imply that Mn^Ⅲ-persulfate 4 is not an epoxidizing species. Instead, it may serve as a reservoir for the formation of reactive Mn^Ⅴ(O) species during the oxidation reactions. In fact, the direct use of Oxone as an oxidant also provided a comparable enantioselectivity as that of H₂O₂ in the presence of sulfuric acid, when (R,R)-DBP-MCP-Mn(OTf)₂ was used as the catalyst (Table 1, entries 5 and 6) [34, 35]. The lower yield attributes to its bad solvability in organic solvent, despite water is used as a solvent.

  Figure 3

  

  Figure 3.  Energy profiles for the styrene epoxidation by Mn^Ⅴ-oxo at B3LYP/Def2-TZVPP//TZVP(Mn)-6-31G^* (other atoms) level including zero-point energy and solvation corrections.

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  Table 1

  

  Table 1.  Asymmetric epoxidation of olefins.^a

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  Figure 4

  

  Figure 4.  Energy profiles for the styrene epoxidation by Mn^Ⅲ-persulfate at B3LYP/Def2-TZVPP//TZVP(Mn)-6-31G^* (other atoms) level including zero-point energy and solvation corrections.

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  To further understand the enantioselectivity, the independent gradient model (IGM) [36] of S-³TS1 and R-³TS1 was established to elucidate the possible interactions. As shown in Fig. 5, the S isomer has an additional T-shaped π-π (T π-π) between phenyl on the ligand and the styrene, while the R isomer has only parallel-displaced π-π (PD π-π) interaction between the pyridine of the ligand and styrene [37]. Besides, the deformation energy of S-³TS1 isomer is about 1.6 kcal/mol lower than that of R isomer. Taken together, the additional T-shaped π-π interaction and less deformation make the S-³TS1 more favored.

  Figure 5

  

  Figure 5.  IGM for two isomers of triplet TS1.

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  In summary, DFT calculations were performed to investigate the mechanism of the asymmetric epoxidation of olefin by (R,R)-PMCP-Mn(OTf)₂/H₂O₂/H₂SO₄ catalyst system. The computational results suggest that [Mn^Ⅴ(O)(R,R-PMCP)(SO₄)]⁺ is an epoxidizing reagent, and it promotes the epoxidation reaction with a low barrier. A cyclic Mn^Ⅲ-persulfate species is also involved in this catalyst system, however, it cannot participate in the epoxidation directly due to the higher energy barrier.

  Declaration of competing interest

  The authors declare that they have no conflicts of interest to this work.

  Acknowledgment

  We acknowledge financial support of this work from the National Natural Science Foundation of China (Nos. 21773273, 21802153).

  Supplementary materials

  Supplementary material associated with this article can be found, in the online version, at doi:10.1016/j.cclet.2021.08.092.

  
  
• 1. [1]

     M. Guo, T. Corona, K. Ray, W. Nam, ACS Cent. Sci. 5 (2019) 13-28. doi: 10.1021/acscentsci.8b00698

  2. [2]

     C.M. Che, V.K.Y. Lo, C.Y. Zhou, J.S. Huang, Chem. Soc. Rev. 40 (2011) 1950-1975. doi: 10.1039/c0cs00142b

  3. [3]

     J.F. Hartwig, M.A. Larsen, ACS Cent. Sci. 2 (2016) 281-292. doi: 10.1021/acscentsci.6b00032

  4. [4]

     A.C. Lindhorst, S. Haslinger, F.E. Kühn, Chem. Commun. 51 (2015) 17193-17212. doi: 10.1039/C5CC07146A

  5. [5]

     Q.S. Sun, W. Sun, Chin. J. Org. Chem. 40 (2020) 3686-3696. doi: 10.6023/cjoc202006008

  6. [6]

     L.J. Que, W.B. Tolman, Nature 455 (2008) 333-340. doi: 10.1038/nature07371

  7. [7]

     K. Ray, F.F. Pfaff, B. Wang, W. Nam, J. Am. Chem. Soc. 136 (2014) 13942-13958. doi: 10.1021/ja507807v

  8. [8]

     W.N. Oloo, L.J. Que, Acc. Chem. Res. 48 (2015) 2612-2621. doi: 10.1021/acs.accounts.5b00053

  9. [9]

     O. Cussó, X. Ribas, M. Costas, Chem. Commun. 51 (2015) 14285-14298. doi: 10.1039/C5CC05576H

  10. [10]

      K.P. Bryliakov, Chem. Rev. 117 (2017) 11406-11459. doi: 10.1021/acs.chemrev.7b00167

  11. [11]

      W. Sun, Q. Sun, Acc. Chem. Res. 52 (2019) 2370-2381. doi: 10.1021/acs.accounts.9b00285

  12. [12]

      L. Vicens, G. Olivo, M. Costas, ACS Catal. 10 (2020) 8611-8631. doi: 10.1021/acscatal.0c02073

  13. [13]

      M.C. White, A.G. Doyle, E.N. Jacobsen, J. Am. Chem. Soc. 123 (2001) 7194-7195. doi: 10.1021/ja015884g

  14. [14]

      R. Mas-Ballesté, L.J. Que, J. Am. Chem. Soc. 129 (2007) 15964-15972. doi: 10.1021/ja075115i

  15. [15]

      Y. Wang, D. Janardanan, D. Usharani, S. Sason, et al., ACS Catal. 3 (2013) 1334-1341. doi: 10.1021/cs400134g

  16. [16]

      O.Y. Lyakin, R.V. Ottenbacher, K.P. Bryliakov, E.P. Talsi, ACS Catal. 2 (2012) 1196-1202. doi: 10.1021/cs300205n

  17. [17]

      J. Serrano-Plana, W.N. Oloo, L. Acosta-Rueda, et al., J. Am. Chem. Soc. 137 (2015) 15833-15842. doi: 10.1021/jacs.5b09904

  18. [18]

      B. Wang, Y.M. Lee, M. Clémancey, M. Costas, et al., J. Am. Chem. Soc. 138 (2016) 2426-2436. doi: 10.1021/jacs.5b13500

  19. [19]

      I. Garcia-Bosch, A. Company, X. Fontrodona, X. Ribas, M. Costas, Org. Lett. 10 (2008) 2095-2098. doi: 10.1021/ol800329m

  20. [20]

      O. Cussó, I. Garcia-Bosch, D. Font, M. Costas, et al., Org. Lett. 15 (2013) 6158-6161. doi: 10.1021/ol403018x

  21. [21]

      E.P. Talsi, K.P. Bryliakov, Coord. Chem. Rev. 256 (2012) 1418-1434. doi: 10.1016/j.ccr.2012.04.005

  22. [22]

      M. Wu, B. Wang, S.F. Wang, C.G. Xia, W. Sun, Org. Lett. 11 (2009) 3622-3625. doi: 10.1021/ol901400m

  23. [23]

      D.Y. Shen, B. Qiu, D.Q. Xu, W. Sun, et al., Org. Lett. 18 (2016) 372-375. doi: 10.1021/acs.orglett.5b03309

  24. [24]

      W.F. Wang, Q.S. Sun, C.G. Xia, W. Sun, Chin. J. Catal. 39 (2018) 1463-1469. doi: 10.1016/S1872-2067(18)63116-X

  25. [25]

      J.Y. Du, C.X. Miao, C.G. Xia, W. Sun, Chin. Chem. Lett. 29 (2018) 1869-1871. doi: 10.1016/j.cclet.2018.04.036

  26. [26]

      B. Qiu, C.G. Xia, W. Sun, Chin. Chem. Lett. 30 (2019) 698-701. doi: 10.1016/j.cclet.2018.10.023

  27. [27]

      J.Y. Du, C.X. Miao, C.G. Xia, W. Sun, et al., ACS Catal. 8 (2018) 4528-4538. doi: 10.1021/acscatal.8b00874

  28. [28]

      R.H. Zhao, X.Y. Chen, Z.X. Wang, Org. Lett. 23 (2021) 1535-1540. doi: 10.1021/acs.orglett.0c04102

  29. [29]

      C.X. Miao, B. Wang, Y. Wang, W. Sun, et al., J. Am. Chem. Soc. 138 (2016) 936-943. doi: 10.1021/jacs.5b11579

  30. [30]

      J. Lin, C.X. Miao, F. Wang, W. Sun, et al., ACS Catal. 10 (2020) 11857-11863. doi: 10.1021/acscatal.0c02634

  31. [31]

      D. Schröder, S. Shaik, H. Schwarz, Acc. Chem. Res. 33 (2000) 139-145. doi: 10.1021/ar990028j

  32. [32]

      D. Usharani, D. Janardanan, C. Li, S. Shaik, Acc. Chem. Res. 46 (2013) 471-482. doi: 10.1021/ar300204y

  33. [33]

      X.X. Li, M. Guo, B. Qiu, W. Nam, et al., Inorg. Chem. 58 (2019) 14842-14852. doi: 10.1021/acs.inorgchem.9b02543

  34. [34]

      T.W.S. Chow, Y. Liu, C.M. Che, Chem. Commun. 47 (2011) 11204-11206. doi: 10.1039/c1cc11999k

  35. [35]

      T.W.S. Chow, E.L.M. Wong, Z. Guo, C.M. Che, et al., J. Am. Chem. Soc. 132 (2010) 13229-13239. doi: 10.1021/ja100967g

  36. [36]

      L. Corentin, R. Gaëtan, K. Hassan, H. Eric, et al., Phys. Chem. Chem. Phys. 19 (2017) 17928-17936. doi: 10.1039/C7CP02110K

  37. [37]

      M.O. Sinnokrot, C.D. Sherrill, J. Phys. Chem. A 110 (2006) 10656-10668. doi: 10.1021/jp0610416

• 

  Scheme 1  Possible intermediates for the Mn^Ⅱ(R,R-PMCP)(OTf)₂ with H₂O₂ in the presence of sulfuric acid.

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  Figure 1  Potential energy profiles for the conversion of Mn^Ⅲ-OOH to manganic persulfate via Mn^Ⅴ-oxo at B3LYP/Def2-TZVPP//TZVP(Mn, OOH, HSO₄, N atoms in the ligand)-6-31G^* (C and H atoms in the ligand) level including zero-point energy and solvation corrections.

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  Figure 2  Key geometric parameters and spin-state energies of Mn^Ⅴ(O) 3, Mn^Ⅲ persulfate 4.

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  Figure 3  Energy profiles for the styrene epoxidation by Mn^Ⅴ-oxo at B3LYP/Def2-TZVPP//TZVP(Mn)-6-31G^* (other atoms) level including zero-point energy and solvation corrections.

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  Figure 4  Energy profiles for the styrene epoxidation by Mn^Ⅲ-persulfate at B3LYP/Def2-TZVPP//TZVP(Mn)-6-31G^* (other atoms) level including zero-point energy and solvation corrections.

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  Figure 5  IGM for two isomers of triplet TS1.

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  Table 1.  Asymmetric epoxidation of olefins.^a

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