Article Cite This: Organometallics XXXX, XXX, XXX−XXX
Revealing the Structure and Reactivity of the Active Species in the FeCl2−TBHP System: Case Study on Alkene Oxidation Zhiliang Huang,† Xiaotian Qi,‡ Jyh-Fu Lee,§ and Aiwen Lei*,†,| †
The Institute for Advanced Studies (IAS), College of Chemistry and Molecular Sciences, Wuhan University, Wuhan 430072, People’s Republic of China | State Key Laboratory of Organometallic Chemistry, Shanghai Institute of Organic Chemistry, Chinese Academy of Sciences, 345 Lingling Road, Shanghai 200032, People’ s Republic of China ‡ School of Chemistry and Chemical Engineering, Chongqing University, Chongqing 400030, People’s Republic of China § National Synchrotron Radiation Research Center, Hsinchu 30076, Taiwan S Supporting Information *
ABSTRACT: A mechanism involving a quintet ferryl [FeIVO] species has been disclosed for the oxidation of alkene under the FeCl2−TBHP system. Both theoretical and experimental results suggested that the quintet ferryl species [FeIVO] was produced by the reaction of FeCl2 with TBHP. A Mulliken atomic spin density distribution on [FeIVO] showed that the O site has strong radical character and could easily react with alkene to form a carbon radical intermediate. This radical could be further oxidized by TBHP to lead to the CC bond cleavage of alkene.
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INTRODUCTION
However, due to the lack of further evidence, debates on the key intermediates still remain. Recently, we revealed that 1,1-diphenylethylene (1a) could be oxidized to afford benzophenone (2a) in 84% yield at room temperature in the presence of FeCl2 and TBHP (eq 1). In the
The iron−tert-butyl peroxide (TBHP) system is a well-developed combination for oxidative reactions.1a−i In the past few decades, it has been widely used toward the construction of C−O and C−X (X = C, N, etc.) bonds via C−H hydroxylation,2a,b crossdehydrogenation coupling,3 oxidative coupling reactions,4a−c etc. However, the mechanism of the reaction of the iron salt with TBHP has not yet been settled satisfactorily.1a,5a,b Undoubtedly, it is of great importance to elucidate the nature of the iron− TBHP system, since this knowledge will not only be helpful for the new iron-catalyzed reaction design but also provide some information for understanding the oxidation mechanism of ironcontaining oxygenases.6 The mechanistic investigation of the reaction of iron with TBHP has by now a history of decades,7 and intense discussions always focus on the characterization of the key intermediates.8a−d To date there are mainly two views: (1) a high-valent iron species is the key intermediate, as supported by the isolation and characterization of high-valent non-heme or heme iron oxo or peroxo in the presence of TBHP,9a−f and (2) the corresponding oxy radical, which is produced by O−H or O−O bond homolysis, is the key intermediate for oxidation.8b,c,10 Generally, the well-known high-valent iron intermediates are considered too active and unstable to live in the extra ligand-free system.11a−e Additionally, up to now, they have not yet been detected in the FeCl2−TBHP system even at ultralow temperature. Hence, a great number of people believe the high-valent iron species might not exist in the FeCl2−TBHP system without extra ligand.5a,8b,c © XXXX American Chemical Society
meantime, only a trace amount of 2a could be obtained when FeCl2 is absent, indicating that FeCl2 plays a vital role in this transformation. To the best of our knowledge, accumulated evidence shows that the ferryl species [FeIVO] can be formed in the FeCl2−H2O2 solution (Fenton’s reagent).12a−d In this regard, we suspected that a similar [FeIVO] species might also exist in the FeCl2−TBHP system and work as the key intermediate for the oxidation of the CC bond of 1a. Therefore, density functional theory (DFT) calculations and experimental characterization have been utilized to study the mechanism of the oxidation of 1a by FeCl2 and TBHP. Delightfully, our investigation supported the formation of a [FeIVO] species in the mixture of FeCl2 and TBHP, and it could react with 1a immediately, exergonic by 37.4 kcal/mol. Herein, we share our latest discoveries to bring some fresh ideas to the the iron− TBHP oxidation system. Received: March 29, 2018
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DOI: 10.1021/acs.organomet.8b00184 Organometallics XXXX, XXX, XXX−XXX
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calculated by M06 is provided for a discussion of the energy. The Mulliken atomic spin density of certain atoms was also calculated using the same method. The optimized structures were displayed using CYLview. Additionally, the MECP location program developed by Harvey and co-workers was used in this study to gain the structures of MECPs at the B3LYP/6-31G(d) level of theory. The solvation single-point energies of the MECPs, which were calculated at the M06/6-311+G(d,p) level in acetonitrile, have also been summarized. It is also noteworthy that all of the iron complexes considered in this study are in the high spin state. The corresponding low spin state structures were determined to be energetically unfavorable.
COMPUTATIONAL METHODS
All of the DFT calculations were carried out with the GAUSSIAN 09 series of programs. The DFT method B3LYP13a,b with a standard 6-311G(d) basis set was used for geometry optimizations. Harmonic vibrational frequency calculations were performed for all of the stationary points to confirm if the points were local minima or a transition structure and to derive the thermochemical corrections for the enthalpies and free energies. In this study, the stability of the wave function has been tested for singlet, triplet, quintet, and sextet state intermediates. All of the test results confirmed that the wave function is stable under the perturbations considered. The solvation effects were considered by single-point calculations on the gas-phase stationary points with an SMD continuum solvation model.14 The M0615 functional with the 6-311+G(d,p) basis set was employed to calculate the solvation single-point energies in an acetonitrile solvent to provide more accurate energy information. The Gibbs free energy of each stationary point as
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RESULTS AND DISCUSSION Mechanistic Investigation of the Oxidation of Olefin by the FeCl2−TBHP System. Initially, the oxidation of CC bonds of olefins by the FeCl2−TBHP system was tested. As shown in Scheme 1, various olefins could be oxidized successfully. For example, 1,1-diarylethylenes containing electrondonating or electron-withdrawing groups are suitable for this oxidation reaction (2b−d). Halo-substituted 1,1-diarylethylenes are also tolerated in this transformation to afford the corresponding ketones in good yields (2e−h). 2-(1-Phenylvinyl)naphthalene could be oxidized by FeCl2 and TBHP efficiently as well (2i). (1-Cyclohexylvinyl)benzene and 1-chloro-4-vinylbenzene will also be transferred to the desired aldehydes by this oxidation reaction (2j,k). Hence, these results concluded that the oxidation of CC bonds of olefins, which has attracted a great deal of attention,16 is a good model for the mechanistic investigation. In the experiment, the addition of FeCl2 facilitated the oxidation of olefin, since only a trace amount of 1a could be oxidized to 2a at room temperature by TBHP without FeCl2. To determine the role of FeCl2 in this transformation, DFT calculations were performed. The optimal structure of FeCl2 in acetonitrile (CH3CN) solution was studied first, and computational results showed that the quintet Fe(II) complex A1 (Figure 1) coordinated by two Cl and two CH3CN groups is favored over other structures, including the low spin state Fe(II) complexes (see the Supporting Information for details). The structure of A1 was characterized by XAS spectroscopy, and experimental results are shown in Figure S1 in the Supporting Information. The Mulliken atomic spin density of Fe(II) in A1 is determined to be 3.73
Scheme 1. Oxidation of CC Bonds of Olefins by the FeCl2− TBHP Systema
a
The reaction was carried out with FeCl2 (0.25 mmol, 28.0 mg), alkene (0.25 mmol), and TBHP (1.5 mmol, 5 M in decane) in CH3CN (2 mL) under N2 at room temperature for 8 h. Isolated yield.
Figure 1. Free energy profile for the generation of [FeIVO] species A6 from quintet Fe(II) species A1 and TBHP. B
DOI: 10.1021/acs.organomet.8b00184 Organometallics XXXX, XXX, XXX−XXX
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could be formed through transition state A5-ts (Figure 2); this process is endergonic by 12.9 kcal/mol, and the activation free energy is 25.0 kcal/mol. Meanwhile, another minimum energy crossing point, 5-MECP, was obtained for this radical substitution process. Optimized structures suggests that the O2−H bond lengths in 5-MECP and A5-ts are 1.88 and 1.14 Å, respectively, which confirms that 5-MECP occurs earlier before A5-ts. From an energetic point of view, the relative energies of 4-MECP and 5-MECP are found to be much lower than those of adjacent transition states, indicating that spin crossover during the generation of A6 is a facile process. Other pathways for the reaction of A1 and TBHP were safely ruled out (see the Supporting Information for more details). Therefore, the theoretical calculation results suggested that the [FeIVO] species could be formed in the ligand-free iron-TBHP system. Furthermore, an Mulliken atomic spin density analysis shows that the spin densities located on Fe(IV) and O in A6 are 3.18 and 0.45, respectively (Figure 2). These data reveal that the oxygen atom would be very active, as it has strong radical character. In order to stabilize the [FeIVO] species for spectroscopic characterization, an electron-rich ligand TMC (1,4,8,11tetramethyl-1,4,8,11-tetraazacyclotetradecene) was employed in this system.11e As shown in Figure 3, the XANES (X-ray absorption near-edge structure) spectrum of Fe(OTf)2(TMC) displays a small peak at 7112.3 eV, which is assigned to a 1s → 3d Fe(II) transition.17 After the addition of TBHP, Fe(II) is oxidized, as evidenced by a right shift of edge energy. The pre-edge energy and edge energy of the oxidized iron complex are at 7113.8 and 7125.2 eV, respectively, consistent with values previously reported for oxoiron(IV) species.18 In addition, XANES spectra exhibiting a large pre-edge area is also a feature of oxoiron(IV) species, which is caused by its low-symmetry distorted structure.17 Hence, the [FeIVO] species could be formed and spectroscopically characterized in the reaction of ferrous salt with TBHP by introducing TMC as the ligand, which coincides well with our theoretical calculations. Since the quintet [FeIVO] species A6 is extremely active, it could react with 1a immediately to afford the carbon radical species A7. As shown in Figure 4, this process is highly exergonic by 37.4 kcal/mol. Subsequently, the radical substitution between A7 and TBHP occurs through transition state A8-ts, forming
Figure 2. Optimized structures of several key intermediates and transition states. The values in parentheses are Mulliken atomic spin densities of certain atoms, and the other values are bond lengths, given in Å.
(Figure 2), which confirms that the spin state of A1 is a quintet. Thus, A1 is regarded as the reaction initiator and set as the relative zero free energy point in following calculations. In addition, the spin states of other structures were also studied. Unless otherwise noted, the high spin state is more favored in this reaction system (see the Supporting Information for details). The subsequent reaction of A1 with TBHP is shown in Figure 1. In the beginning, a ligand exchange between A1 and TBHP occurs to afford A2 with a free energy increase of 5.9 kcal/mol. Then, homolytic cleavage of the O−O bond leads to the sextet Fe(III) intermediate A4 and tert-butoxyl radical via transition state A3-ts with an activation free energy of 12.8 kcal/mol. Moreover, the minimum energy crossing point (MECP) 4-MECP was also located between A3-ts and A4. Structural analysis shows that the O−O bond lengths in A3-ts and 4-MECP are 1.75 and 2.99 Å (Figure 2), respectively, which clearly revealed the activation process of the O−O bond in TBHP. After hydrogen abstraction by the tert-butoxyl radical, the quintet [FeIVO] species A6
Figure 3. XANES spectra of Fe(OTf)2(TMC) and the mixture of Fe(OTf)2(TMC) and TBHP at −40 °C in CH3CN. C
DOI: 10.1021/acs.organomet.8b00184 Organometallics XXXX, XXX, XXX−XXX
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Figure 4. Free energy profile for the generation of product 2a from the quintet [FeIVO] species A6 and 1a.
Figure 5. Structural information on several key intermediates and transition states. The values in parentheses are Mulliken atomic spin densities of certain atoms, and the other values are bond lengths, given in Å.
tert-butoxyl radical forms intermediate A10 with a free energy increase of 7.1 kcal/mol. Then, A10 undergoes a hydrogen abstraction process to generate the key quintet Fe(IV) species A12 via transition state A11-ts with an activation energy of 18.7 kcal/mol. Another minimum energy crossing point, 10-MECP, was also obtained after A10. The structural information shown in Figure 5
the sextet Fe(III) intermediate A9 with a free energy decrease of 33.6 kcal/mol. The activation free energy of this step is 25.0 kcal/mol. As the spin state of A9 is a sextet and A7 is a quintet, 9-MECP was located between A8-ts and A9. The relative energy is found to be −74.7 kcal/mol, which is only 2.2 kcal/mol higher than that of A9. Hydrogen bonding between A9 and a D
DOI: 10.1021/acs.organomet.8b00184 Organometallics XXXX, XXX, XXX−XXX
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the National Synchrotron Radiation Research Center. The Program of Introducing Talents of Discipline to Universities of China (111 Program) is also appreciated.
suggests that 10-MECP occurs before A11-ts. Moreover, the relative energy of 10-MECP is determined to be 14.4 kcal/mol lower than that of A11-ts. Finally, the oxidation product 2a and intermediate A15 were generated through C−C bond cleavage transition state A13-ts from A12; the activation barrier is 13.8 kcal/mol, and the overall decrease of free energy is 53.2 kcal/mol. Herein, these calculation results concluded that the [FeIVO] species is active and could efficiently promote the oxidation of 1a. Additionally, benzophenone (2a) and benzaldehyde (2l) are detected as the oxidation products when ethene-1,1,2triyltribenzene is employed as the substrate (eq 2). This result
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further proves the possibility of the above oxidation mechanism, which involves a C−C bond cleavage process to lead to the formation of two CO bonds.
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CONCLUSION In summary, a mechanism involving a quintet ferryl [FeIVO] species has been proposed and disclosed for the oxidation of alkenes with a ligand-free FeCl2−TBHP system. DFT calculation results suggested that the [FeIVO] species A6 is produced by the reaction of FeCl2 with TBHP via a single electron transfer (SET) process and a hydrogen transfer (HT) process; the overall activation barrier is 25.0 kcal/mol. The Mulliken atomic spin density distribution on [FeIVO] showed that the O site has strong radical character and could easily react with alkene to form the carbon radical intermediate A7. The radical A7 could be further oxidized by TBHP to lead to the CC bond cleavage of alkene that is exergonic by 84.3 kcal/mol in all.
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ASSOCIATED CONTENT
S Supporting Information *
The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.organomet.8b00184. Experimental details and spectral data for all compounds (PDF) Cartesian coordinates for the calculated structures (XYZ)
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REFERENCES
AUTHOR INFORMATION
Corresponding Author
*E-mail for A.L.:
[email protected]. ORCID
Aiwen Lei: 0000-0001-8417-3061 Notes
The authors declare no competing financial interest.
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ACKNOWLEDGMENTS This paper is dedicated to Professor Xiyan Lu on the occasion of his 90th birthday. This work was supported by the National Natural Science Foundation of China (21390402, 21520102003, 21702150), the 973 Program (2012CB725302), the CAS Interdisciplinary Innovation Team and the Hubei Province Natural Science Foundation of China (2017CFA010), and the China Postdoctoral Science Foundation (BX201600114, 2016M602340). XAFS data were collected at beamline 17C1 of E
DOI: 10.1021/acs.organomet.8b00184 Organometallics XXXX, XXX, XXX−XXX
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DOI: 10.1021/acs.organomet.8b00184 Organometallics XXXX, XXX, XXX−XXX
