Density Functional Theory Study of the Mechanisms of Iron

Structures and Quantum Chemical Parameters. Mohammad Taghi Taghizadeh , Morteza Vatanparast , Saeed Nasirianfar. Chemistry Journal of Moldova 2015...
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Density Functional Theory Study of the Mechanisms of IronCatalyzed Aminohydroxylation Reactions Qinghua Ren,* Shuhui Guan, Xiaoyan Shen, and Jianhui Fang Department of Chemistry, Shanghai University, 99 Shangda Road, Shanghai 200444, People’s Republic of China S Supporting Information *

ABSTRACT: Experimental studies recently show that iron salts are effective catalysts for oxaziridine-mediated oxyamination reactions, in place of the powerful osmium-catalyzed Sharpless aminohydroxylation method. The present study reports a theoretical analysis of the mechanism of the ironcatalyzed aminohydroxylation reaction between vinylbenzene and N-sulfonyloxaziridine substrate using density functional theory (DFT) calculations. Our calculations show that the Fe(II)-catalyzed process is favored over the Fe(III)-catalyzed process for the overall catalytic cycle. The rate-limiting step in the whole catalytic cycle is the process of forming the final aminohydroxylation product from a six-membered-ring intermediate, but the solvation effect in MeCN of this step leads to a lowering of the energy barrier computed using the C-PCM method. The regioselectivity has also been investigated.

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INTRODUCTION Transition-metal catalysts, such as those with palladium,1 nickel,2 ruthenium,3 rhodium,4 and gold,5 are of utmost importance for the formation of carbon−carbon and carbon− heteroatom bonds in modern organic synthesis. However, they are limited by their high price or considerable toxicity, especially in the context of manufacturing on larger scales. Therefore, the first-row transition-metal iron has recently been given more attention, because many simple iron salts have been proven to be highly efficient, cheap, nontoxic, and environmentally friendly precatalysts.6 Iron is the most abundant transition metal in nature and has been applied in human history since ancient times. Iron-catalyzed cross-coupling reactions have already matured into important reactions.7 A theoretical understanding of the reaction mechanisms of transition-metal-catalyzed reactions may shed light on how to design more powerful catalysts and how to control the catalysis processes. Many detailed theoretical studies of the prominent palladium-catalyzed8 and nickel-catalyzed9 reactions have been published. However, theories about the mechanisms of the iron-catalyzed reactions have to some extent now fallen behind, although many successful iron-catalyzed reaction experiments are now being performed in an increasingly routine manner.10 Kochi,11 Furstner,12,6d Bogdanovic,13 Norrby,14 etc. have made contributions to the mechanisms of iron-catalyzed crosscoupling reactions of Grignard reagents. However, the mechanisms of many other iron-catalyzed reactions remain somewhat obscure. It is well-known that the osmium-catalyzed Sharpless aminohydroxylation15 is a powerful method for the rapid transformation of alkenes into 1,2-amino alcohols. Because of © 2014 American Chemical Society

the high price and considerable toxicity of osmium salts, palladium16 and copper17 catalysts have been developed for olefin oxyamination. Furthermore, Yoon18 recently reported that iron salts are also effective catalysts for oxaziridinemediated oxyamination. However, they did not give any theoretical study on the mechanisms of this novel ironcatalyzed reaction. Yoon’s discovery of this iron-catalyzed aminohydroxylation reaction is a useful synthetic advance for oxaziridine activation and has spurred other iron-catalyzed aminohydroxylation experiments.19 It is valuable to perform more fully computational studies exploring the mechanisms of this kind of reaction. Here, we select one typical reaction in the experimental work of Yoon18 as a calculated model (see Scheme 1) in order to gain insight into the mechanisms of iron-catalyzed aminohydroxylation reactions. In the above reaction, one reactant, here denoted as R1, is vinylbenzene. Another reactant is N-sulfonyloxaziridine subScheme 1. Illustration of the Iron-Catalyzed Aminohydroxylation Reaction

Received: November 24, 2013 Published: March 12, 2014 1423

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Figure 1. Outline of the Fe(II)-catalyzed aminohydroxylation reaction mechanism for the product D from R1 and R2. Frequency analysis was performed after the full optimization without any symmetry restriction to ensure that the local minima had 0 imaginary frequencies and the transition state had exactly 1. To calculate the solvation energies, the C-PCM polarizable conductor calculation model24 with the UFF radii on the gas-phase optimized geometries was used. Both the electronic and nonelectronic free energies in solution were added to the gas-phase Gibbs free energies to obtain the solution Gibbs free energies in MeCN. All of the solutionphase free energies reported in this article correspond to the reference state of 1 mol/L and 298 K.

strate, which is denoted as R2. The aminohydroxylation reaction product between R1 and R2 is denoted as D (see Scheme 1). Our thorough theoretical investigation has been applied to this kind of iron-catalyzed aminohydroxylation reaction using density functional theory (DFT) calculations with the B3LYP method.21 The experimental work of Yoon18 used Fe(Br)3 and Fe(acac)3 as the precatalysts. Fe(acac)3 is cheap, is nonhygroscopic, and is as effective as FeBr3, However, Fe(acac)3 is not itself the catalytically active species.18 In other aminohydroxylation reactions of Xu’s experimental work,19 Fe(Br)3 and Fe(Br)2 are used as the catalysts. Therefore, our model calculations will start from the catalysts Fe(Br)3 and Fe(Br)2 when considering Fe(III)-catalyzed or Fe(II)-catalyzed crosscoupling reactions, respectively. Our calculations try to answer the following questions. How does each of the catalytic steps take place? Which step is the rate-determining step in the whole catalytic cycle? Which valence of iron, Fe(III) or Fe(II), is the favored catalyst for these kinds of reactions? Answers to these questions will improve the understanding of these kinds of iron-catalyzed aminohydroxylation reactions.

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RESULTS AND DISCUSSION Fe(II) Catalyst. Track 1 of Catalytic Cycle: [Fe(Br)2] Catalyst First Attacks R1. The path where the catalyst [Fe(Br)2] (CA) first attacks styrene (R1) is called Track 1. Its catalytic cycle is outlined in Figure 1, which includes the two complexes IN1a and IN3a, the three intermediates IN2a, IN4a, and IN5a, and the four transition states TS1a, TS2a, TS3a, and TS4a; detailed optimized structures of these intermediates and transition states are shown in Figures 2 and 3, respectively. Detailed fully optimized structures of the reactants R1 and R2, the catalyst CA, the complexes IN1a and IN3a, the intermediate IN2a, and the transition state TS1a are shown in Figure 2. The fully optimized structures of the intermediates IN4a and IN5a, the transition states TS2a, TS3a, and TS4a, and the final product D are shown in Figure 3. The values of the important bonding distances are also given in Figures 2 and 3 (distances are given in Å). The mechanism of Track 1 includes three basic steps. (I) Oxidation of [Fe(Br)2] gives the three-membered-ring

COMPUTATIONAL METHODS

All quantum mechanical calculations were performed using the GAUSSIAN 03 program. 20 The gas-phase geometries of all compounds were fully optimized by density functional theory (DFT) with the B3LYP hybrid functional.21 The 6-31G(d,p) basis set22 was used for C, H, O, N, Cl, Br, and S, and the SDD quasirelativistic pseudopotential and associated basis set was used for iron.23 1424

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Figure 2. Fully optimized structures of the reactants R1 and R2, the catalyst CA, the intermediates IN1a, IN2a, and IN3a, and the transition state TS1a for Track 1. Distances are given in Å. Color scheme: C, cyan; H, white; O, red; N, blue; S, yellow; Br, orange; Cl, ocher; Fe, pink.

Figure 3. Fully optimized structures of the intermediates IN4a and IN5a, the transition states TS2a, TS3a, and TS4a, and the product D for Track 1. Distances are given in Å. Color scheme: C, cyan; H, white; O, red; N, blue; S, yellow; Br, orange; Cl, ocher; Fe, pink.

structure intermediate IN2a. The approach of styrene (R1) toward the catalyst (CA), [Fe(Br)2], leads to the formation of the complex IN1a. From IN1a, after passing through the transition state TS1a, the intermediate of oxidative addition IN2a is obtained (see Figure 2). In the optimized geometry of IN1a, the iron atom is above the center of the phenyl and is far from the vinyl; the distance between Fe and 1C of the vinyl is 3.28 Å and the distance between Fe and 2C of vinyl is 4.27 Å. The Fe center is then rotated to attack the double bond between 1C and 2C of vinyl in the transition state TS1a. After passing through TS1a, the Fe−1C and Fe−2C bonds are formed in IN2a, i.e. both are 2.12 Å (see Figure 2), accompanying the process where the double bond of vinyl (the distance between 1C and 2C is 1.34 Å) becomes a single bond (the distance between 1C and 2C is 1.38 Å). The threemembered-ring structure 1C−Fe−2C is formed in IN2a. (II) Reactant R2 is added to IN2a to give the six-memberedring structure intermediate IN5a. After the intermediate IN2a is formed, the approach of R2 toward this intermediate leads to the formation of complex IN3a. From IN3a, passing through the second transition state TS2a, the intermediate IN4a is formed. In the fully optimized structure of intermediate IN4a, it can be seen that the four-membered-ring structure Fe−N−C− O is formed (see Figure 3). The Fe−N and Fe−O bond distances are 1.78 and 1.77 Å, respectively, in comparison to 2.84 and 3.46 Å for those in IN3a, respectively. The Fe−1C and Fe−2C separations are extended from 2.16 and 2.17 Å for IN3a (see Figure 2) to 3.47 Å and 4.10 Å for IN4a (see Figure 3), respectively. Then the 1C atom and 2C atom of styrene attack

the Fe center again, to pass through the transition state TS3a, so that the six-membered-ring structure 1C−2C−Fe−N−3C−O intermediate IN5a is formed, accompaned by the breaking of the Fe−O bond (see Figure 3). (III) Finally, reductive elimination of IN5a returns the catalyst CA, [Fe(Br)2]. The N atom and 2C atom in IN5a connect together in this process and pass through the transition state TS4a to form the final product D, the aminohydroxylation compound, while the catalyst CA is regenerated. In the fully optimized structure of TS4a, it can be seen that the N−2C distance is 2.12 Å. After the N−2C bond is formed, the distance becomes 1.48 Å in D (see Figure 3). The other bonds N−3C, 3 C−O, O−1C, and 1C−2C are nearly unchanged. The energy profile for the mechanism of Track 1 when the catalyst [Fe(Br)2] (CA) first attacks styrene (R1) is shown in Figure 4. We can see that the rate-limiting step of the whole catalytic cycle is the formation of the final product D from the six-membered-ring structure intermediate IN5a when passing through the transition state TS4a by 22.73 kcal/mol. This process needs to break the stable six-membered-ring structure to form a five-membered-ring structure, which needs more energy than the other steps. The second highest energy barrier is that for passoing through the transition state TS2a with a value of 20.29 kcal/mol. In this process, the complex IN3a including a three-membered-ring structure is transformed into the intermediate IN4a including a four-membered-ring structure. The third highest energy barrier is that for passing through the transition state TS1a with a value of 19.64 kcal/ mol. In this process, the three-membered-ring structure 1425

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Figure 4. Energy profiles for the mechanism of Track 1 when the catalyst [Fe(Br)2] (CA) first attacks styrene (R1) and the mechanism of Track 2 when the catalyst CA first attacks R2. Energies are given in kcal/mol. The dashed line (in red) is for the process giving the regioisomeric outcome Dd.

Figure 5. Fully optimized structures of the intermediates IN1b and IN2b and the transition state TS1b for Track 2. Distances are given in Å. Color scheme: C, cyan; H, white; O, red; N, blue; S, yellow; Br, orange; Cl, ocher; Fe, pink.

intermediate of oxidative addition IN2a is obtained. The lowest energy barrier is for TS3a, with a value of 17.02 kcal/mol. Track 2 of Catalytic Cycle: [Fe(Br)2] Catalyst First Attacks R2. The path in which the catalyst [Fe(Br)2] (CA) first attacks oxaziridine (R2) is called Track 2. Its catalytic cycle is also outlined in Figure 1. The approach of R2 toward the catalyst CA leads to the formation of the complex IN1b. From IN1b, passing through one transition state TS1b, the four-memberedring structure intermediate IN2b is formed. The fully optimized structures of IN1b, TS1b, and IN2b are shown in Figure 5. In the optimized geometry of IN1b, the distance between Fe and N is 1.93 Å and the distance between Fe and O is 2.92 Å. The Fe center then attacks the N−O bond in the transition state TS1b. After passing through the transition state TS1b, the Fe− N and Fe−O bonds are formed in IN2b. The former length is 1.77 Å, and the latter length is 1.76 Å (see Figure 5). At the same time the N−O bond is broken. The intermediate IN2b has a four-membered-ring structure. Then the approach of another reactant R1 toward to the intermediate IN2b leads to the formation of the intermediate IN4a. The remaining steps have the same mechanism as that for Track 1. The energy profile for the mechanism from CA + R2 to the product D of Track 2 when the catalyst [Fe(Br)2] (CA) first attacks R2 is also shown in Figure 4. It can be seen that the energy barrier to pass through the transition state TS1b is 13.85 kcal/mol, which is much smaller than the energy barrier to pass through TS1a (19.64 kcal/mol) and TS2a (20.29 kcal/mol) in Track 1. The number of steps in Track 2 is also smaller than that of Track 1. Therefore, the path of Track 2, where the catalyst CA first attacks the oxaziridine R2, instead of styrene

R1, should be favored for the mechanisms of Fe(II)-catalyzed processes. The N−O bond in R2 is a single bond, which is more easily broken when the iron center attacks, in comparison with the CC double bond of styrene (R1). At the same time, the fourmembered-ring structure intermediate IN2b forming when the catalyst [Fe(Br)2] (CA) first attacks R2 should be more favored, in comparison with the three-membered-ring structure intermediate IN2a forming when the catalyst [Fe(Br)2] (CA) first attacks R1. Strassner25 reported a mechanism of osimiumcatalyzed aminohydroxylation of olefins. The [3 + 2] mechanism to form a five-membered-ring structure intermediate is more favored than the [2 + 2] mechanism to form a fourmembered-ring structure intermediate. This conclusion is similar to ours: i.e., the formation of a stable five-memberedring structure intermediate is favored. These reasons could explain why Track 2 is more favored than Track 1 for the Fe(II)-catalyzed mechanism. For the whole catalytic cycle of Track 2, the rate-limiting step is also the process passing through TS4a (22.73 kcal/mol), where the six-membered-ring structure is broken to form a fivemembered-ring structure while regenerating the original catalyst [Fe(Br)2]. The second energy barrier is now that for passing through the transition state TS3a (19.10 kcal/mol from IN2b), where the stable six-membered-ring structure is formed. The energy barrier for passing through TS1b (13.85 kcal/mol) is the smallest. 1426

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al.,26 the “energetic span” between the lowest and highest energy species along the reaction path from IN2b passing through IN5a to obtain D is 22.73 kcal/mol, which is the experimentally observed path, but it is 29.48 kcal/mol for the path of passing through IN5d to obtain the regioisomeric aminohydroxylation compound Dd. The conclusion is the same. Furthermore, the energy of Dd is 1.25 kcal/mol higher than that of product D (see Figure 4); therefore, Dd is less stable than D. These calculated results all agree with the regioselectivity of the experimental products of Yoon.18 Fe(III) Catalyst. Track 3 of Catalytic Cycle: [Fe(Br)3] Catalyst First Attacks R2. Following the calculated results of Fe(II)-catalyzed mechanisms, we first select a path similar to that of the favored Track 2, where Fe(Br)3 (CA3) first attacks the reactant R2. The approach of R2 toward the catalyst [Fe(Br)3] leads to the formation of the complex IN1c. From IN1c, passing through one transition state TS1c, the fourmembered-ring structure intermediate of oxidative addition is IN2c. The fully optimized structures of the catalyst CA3, the intermediates IN1c and IN2c, and the transition state TS1c are shown in Figure 7.

Next, we need to think about how to obtain the regioisomeric outcome for the mechanism following Track 2. In the step of the approach of the reactant R1 toward the intermediate IN2b leading to the formation of the intermediate IN4a, if the positions of atom 1C and atom 2C of styrene are exchanged, another complex IN4d will be obtained (see its fully optimized structure in Figure 6).

Figure 6. Fully optimized structures of the intermediates IN4d and IN5d, the transition state TS3d and TS4d, and the product Dd. Distances are given in Å. Color scheme: C, cyan; H, white; O, red; N, blue; S, yellow; Br, orange; Cl, ocher; Fe, pink.

Then the 2C atom of styrene attacks the O atom and the 1C atom of styrene attacks the Fe center. After passing through the transition state TS3d, the six-membered-ring structure 2 C−1C−Fe−N−3C−O intermediate IN5d is formed. Finally, reductive elimination of IN5d returns the catalyst CA, [Fe(Br)2], after passing through the transition state TS4d. The N atom and 1C atom in IN5d connect together to form the final product Dd, the regioisomeric aminohydroxylation compound (see Figure 6). The energy profile for the path of the regioisomeric outcome is also shown in Figure 4 (shown by the red dashed line). The energy barrier from IN2b to IN5d when passing through the transition state TS3d is 25.04 kcal/mol, but the energy barrier from IN2b to IN5a when passing through the transition state TS3a is only 19.10 kcal/mol. It is clear that the path to obtain IN5d is more difficult than that to obtain IN5a. Next, the energy barrier passing through the transition state TS4d from IN5d to form the final regioisomeric aminohydroxylation product Dd is 20.52 kcal/mol, which is closer to the energy barrier passing through TS4a (22.73 kcal/mol). In total, the energy barriers of TS3a and TS4a to form D are all smaller than that of TS3d to form Dd. This means that the regioisomeric aminohydroxylation product Dd is not favored. The conclusion is consistent with the experimental results.18 According to the energetic span model proposed by Shaik et

Figure 7. Fully optimized structures of the catalyst CA3, the intermediates IN1c, IN2c, IN3c, and IN4c, and the transition states TS1c and TS2c for Track 3. Distances are given in Å. Color scheme: C, cyan; H, white; O, red; N, blue; S, yellow; Br, orange; Cl, ocher; Fe, pink.

The approach of another reactant R1, vinylbenzene, toward the intermediate IN2c leads to the formation of the complex IN3c. Then the 1C atom and 2C atom of styrene attack the Fe center to form the six-membered-ring structure 1C−2C−Fe− N−3C−O intermediate IN4c (see Figure 7). The transition state connecting IN3c and IN4c could not be found after many 1427

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whether R1 or R2 first attacks the Fe catalyst. This conclusion is in agreement with the experimental results of Xu.19 Solvation Effect. The calculated Gibbs free energies in the gas phase, ΔGg, and the Gibbs free energies in MeCN solution when carrying out the C-PCM polarizable conductor calculations, ΔGsol, for Tracks 1−3 are shown in Figure 9.

attempts. Finally, the N atom and 2C atom in IN4c connect together and pass through the transition state TS2c to form the final product D, the five-membered-ring structure aminohydroxylation compound, while the catalyst CA, [Fe(Br)3], is regenerated. The fully optimized structure of TS2c is also shown in Figure 7. The energy profile for the mechanism of Track 3 when the catalyst [Fe(Br)3] (CA3) attacks the reactant R2 in the beginning is shown in Figure 8. We can see that the energy

Figure 8. Energy profile for the mechanism of Track 3 when the catalyst [Fe(Br)3] (CA3) first attacks R2. Energies are given in kcal/ mol.

barrier to pass through the transition state TS1c needs 28.82 kcal/mol, which is much higher than the energy barriers to pass through TS1b (13.85 kcal/mol) or to pass through TS1a (19.64 kcal/mol) and TS2a (20.29 kcal/mol). It can be concluded that the Fe(III)-catalyzed process is too difficult in comparison to the Fe(II)-catalyzed mechanisms. This conclusion is consistent with the experimental results of Xu.19 His work showed that Fe(II)/ligand,MeCN complexes are able to catalyze the aminohydroxylation but Fe(III)/ligand,MeCN complexes are inactive. In order to explore the reason that the barrier to the Fe(III)catalyzed transition state TS1c (28.82 kcal/mol) is so much higher than that to the Fe(II)-catalyzed transition TS1b (13.85 kcal/mol), we did more calculations using [FeBr2+] in place of FeBr3 for the Fe(III) catalyst, so that the steric effect could be avoided. The energy barrier of TS1b_[FeBr2+] is 27.21 kcal/ mol, which is still much higher than the energy barriers to pass through TS1b (13.85 kcal/mol) or to pass through TS1a (19.64 kcal/mol). Thus, Track 3 for the Fe(III)-catalyzed path is difficult. We also tried to obtain the mechanism when Fe(Br)3 first attacks the reactant styrene (R1), which favors a path like that of Track 1. Unfortunately, we did not obtain the first transition state TS1a_Fe(III) with either FeBr3 or [FeBr2+] as the catalyst after many attempts (more than 1 year). However, the second transition state TS2a_[FeBr2+] was found when using [FeBr2+] as the catalyst. Its energy barrier is 29.95 kcal/mol, which is also much higher than the energy barriers to pass through TS1b (13.85 kcal/mol) or to pass through TS1a (19.64 kcal/mol) and TS2a (20.29 kcal/mol) when using Fe(II) catalyst. The optimized structures of the transition states TS1b_[FeBr2+] andTS2a_[FeBr2+] are shown in the Supporting Information. In all, the Fe(III)-catalyzed mechanism is not favored over the Fe(II)-catalyzed mechanism for both paths, regardless of

Figure 9. Gibbs free energies ΔGg (solid line) and Gibbs free energies in MeCN solution ΔGsol (dashed line) for the overall catalytic cycle for Tracks 1−3. Energies are given in kcal/mol.

The solid line is ΔGg, and the dashed line is ΔGsol. It can be seen that the ΔGsol values of the processes to pass through the transition states TS1a, TS2a, TS3a, and TS4a in Track 1 are 20.32, 21.46, 8.8, and 14.67 kcal/mol, respectively, and their ΔGg values are 17.91, 19.24, 21.96, and 22.04 kcal/mol, respectively. The solvation effect is not obvious for the processes passing through TS1a and TS2a but is exothermic for the processes passing through TS3a by −13.16 kcal/mol and TS4a by −7.37 kcal/mol. The solvation effect clearly leads to a lowering of the energy barriers of TS3a and TS4a. Similarly, the solvation effect is not obvious for the process passing through the transition state TS1b in Track 2, where the ΔGsol value is 16.25 kcal/mol in comparison with the ΔGg value (13.79 kcal/mol). The ΔGsol value passing through TS1b in Track 2 is still much smaller than that for passing through TS1a (20.32 kcal/mol) and TS2a (21.46 kcal/mol) in Track 1. This means that the path of Track 2 is favored. The solvation effect is also not obvious for the process passing through the transition state TS1c in Track 3, where the ΔGsol value is 28.69 kcal/mol, in comparison with the ΔGg value (27.14 kcal/mol). 1428

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It can be seen that the ΔGsol value passing through TS1c is much larger than that of passing through TS1a or TS1b. Therefore, this also means that the path of Track 3 using the Fe(III) catalyst is more difficult, in comparison with the mechanism using the Fe(II) catalyst of Tracks 1 and 2. The effect of the solvent (MeCN) was taken into account for the structures through single-point calculations at each optimized geometry using the polarized continuum model (C-PCM) at 298.15 K.24 Here, in order to save computing time, Br ligands are used around the iron center, instead of the large ligands of the experimental work. However, the computation of free energy values for the dissociation of ligands that produces charged species is difficult, mainly due to the large entropy effects in dissociation processes in the gas phase and the overestimation of the solvation energy of the resulting charged species.27 Furthermore, the calculations for dissociation processes are quite sensitive to the computational method used and, therefore, the available continuum methods cannot treat rigorously the solvation of charged species;27a thus, we did not consider this further.

computed molecule Cartesian coordinates in .xyz format for convenient visualization. This material is available free of charge via the Internet at http://pubs.acs.org.

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*E-mail for Q.R.: [email protected]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was supported by Innovation funding of Shanghai University (A.10-0101-11-009) and Shanghai Higher Education Connotation Construction “085” project “Materials Genome Engineering” funding (B.58-B111-12-101 and B.58-B111-12103). Q.R. is grateful for the help of Professor Gabriel G. Balint-Kurti and Professor Jeremy N. Harvey of the University of Bristol.

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CONCLUSIONS Transition-metal catalysts are of utmost importance for the formation of carbon−carbon and carbon−heteroatom bonds in modern organic synthesis, but most of them are limited by their high price or considerable toxicity. Iron catalysts, which have the advantages of being cheap, nontoxic, and environmentally friendly, have been proven to be highly efficient for many important reactions. Yoon has recently reported that iron salts are effective catalysts for oxaziridine-mediated oxyamination, in place of the osmium-catalyzed Sharpless aminohydroxylation method. Their discovery is a useful synthetic advance for oxaziridine activation. Our studies report a thorough theoretical analysis of the mechanism of this kind of iron-catalyzed aminohydroxylation reaction between vinylbenzene and Nsulfonyloxaziridine substrate using density functional theory (DFT) calculations. The calculated results show that the path of Track 2, where the iron catalyst first attacks the oxaziridine substrate, is better than the path of Track 1, where the iron catalyst first attacks styrene. The rate-limiting step in the whole catalytic cycle is to form the final aminohydroxylation product from a sixmembered-ring structure intermediate, where the electronic energy barrier is 22.73 kcal/mol. This process needs to break the stable six-membered-ring structure to form a fivemembered-ring structure. The Fe(III)-catalyzed process is difficult to realize, in comparison with the Fe(II)-catalyzed mechanism. Furthermore, the calculated results also show that the regioisomeric aminohydroxylation product Dd is not favored over the product D. The solvation effect is not obvious for the processes passing through the transition states TS1a, TS2a, and TS1b but clearly leads to a lowering of the energy barriers for the steps passing through the transition states TS3a and TS4a, where the Gibbs free energies in MeCN solution, ΔGsol, are 13.16 and 7.37 kcal/ mol lower than the Gibbs free energies in the gas phase, ΔGg, respectively, using the C-PCM method.

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REFERENCES

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ASSOCIATED CONTENT

S Supporting Information *

A table giving calculated energies for all optimized structures, a figure giving optimized structures of the transition states TS1b_[FeBr2+] and TS2a_[FeBr2+], and a text file of all 1429

dx.doi.org/10.1021/om401141r | Organometallics 2014, 33, 1423−1430

Organometallics

Article

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