Mechanism and Regioselectivity of the Iron-Catalyzed Hydroboration

Article Cite This: Organometallics XXXX, XXX, XXX−XXX

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Mechanism and Regioselectivity of the Iron-Catalyzed Hydroboration of N‑Heteroarenes: A Computational Study Jia-Yi Chen and Rong-Zhen Liao* Key Laboratory of Material Chemistry for Energy Conversion and Storage, Ministry of Education, Hubei Key Laboratory of Bioinorganic Chemistry and Materia Medica, Hubei Key Laboratory of Materials Chemistry and Service Failure, School of Chemistry and Chemical Engineering, Huazhong University of Science and Technology, Wuhan 430074, China

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S Supporting Information *

ABSTRACT: Iron complex Cp*(Ph2PC6H4S)Fe was recently reported to regioselectively catalyze the hydroboration of N-heteroarenes with pinacolborane (HBpin). Density functional calculations were performed to elucidate the reaction mechanism and to rationalize the regioselectivity. Three different reaction pathways were considered, and the hydrogen atom transfer pathway was found to be the most favorable one. The reaction started with a hydrogen atom transfer from pyridine−HBpin complex to the metal center of the catalyst. This leads to the generation of a pyridine−Bpin radical and an iron(III)-hydride intermediate. Then, the hydrogen atom can be transferred to either ortho- or para-position of the borylated N-heteroarenes, affording the corresponding products. The origin of the regioselectivity was found to be interaction-controlled from distortion/interaction analysis. The suggested mechanism can rationalize very well the regioselectivity of all other N-heteroarene substrate.

1. INTRODUCTION Catalytic regioselective reduction of N-heteroarenes through hydroboration1 or hydrosilylation1d,2 has attracted great attention because the reduced dihydropyridyl unit is prevalent in biologically active compounds and pharmaceuticals, and especially the dihydropyridine family finds wide applications in organic synthesis and transformations.3 The significance of this chemistry is also reflected by proceeding selective dearomatization at mild reaction conditions.2g,4 Considerable progress has been made in transition-metal catalyzed 1,2- and 1,4selective hydrosilylation or hydroboration of N-heteroarenes.1a−f,2a,c,d,5 However, systems established using base-metal catalyst are rare. The studies of fundamental reactions between the metal catalyst and the N-heteroaromatic substrate or reducing agent such as hydroboranes and hydrosilanes is essential to understand the initial activation step of the catalysis, providing mechanistic insight into the regioselectivity.6 In principle, the activity of the catalysts is very dependent on nature of the metal and its coordination sphere. For example, two entirely different types of initial activations were recently found for the nickel-catalyzed hydroboration of N-heteroarenes. Findlater et al. reported 1,4-regioselctive hydroboration by using a convenient Ni(acac)2/PCyp3 system that is initiated by binding the substrate to the metal center,5d while Wang’s and our groups reported cooperative Ni−O reactivity enables Cp*Ni(1,2-Ph2PC6H4O) to activate the B−H bond of HBpin, generating an active nickel(II) hydride with an oxygenstabilized boron moiety for the 1,2-hydroboration.7 By © XXXX American Chemical Society

contrast, iron-thiolate complex Cp*Fe(1,2-Ph2PC6H4S) is capable of capturing the B−H bond of 9-BBN and BH3 forming a stable iron-borne adduct, but it cannot cleave the B−H bond. Interestingly, this iron-thiolate compound efficiently proceeds 1,2-hydroboration of N-heteroarenes by HBpin at ambient temperature (Scheme 1).1e Stoichiometric reactions demonstrated that the catalysis is initiated by binding of the substrate to the metal center through N atom coordination since Cp*Fe(1,2-Ph2PC6H4S) is stable toward HBpin. Cooperative metal−ligand reactivity for abundant metal catalysis is particular of interest, which potentially expands the scope of using base metals for important transformations.8 In the context of partial reduction of pyridines and its derivatives, the established system are Ru−S and Ni−O cooperative catalyst.2f,7 In the case of iron-thiolate based catalysis, although the thiolate site was proposed to facilitate the B−H bond cleavage and the borenium ion delivery, the related stoichiometric reactions have not been established.1e In this paper, we present a detailed mechanistic investigation by density functional calculations. First, three different plausible pathways for the hydroboration of pyridine will be discussed to establish the most favorable mechanism. Then, the results for the regioselectivity in the hydroboration of other substrates1e will be presented. Importantly, the calculations can reproduce Received: May 2, 2019

A

DOI: 10.1021/acs.organomet.9b00292 Organometallics XXXX, XXX, XXX−XXX

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Organometallics

described by 6-311+G(2df,2p). The B3LYP*-D3 functional has been shown to perform better in describing relative energies of different spin states for transition metal complexes.13 For comparison, single-point calculations were also performed at the SMD-M06L-D3/SDD-6-311+G(2df,2p) level.14 For the broken-symmetry(BS)open-shell singlet state, a spin projection technique was uesd to get more accurate energies (eq 1, where Δunr,ST is the open-shell BS singlet− triplet gap from unrestricted calculations and ΔST is a correction which accounts for ⟨S2⟩T and ⟨S2⟩BS).15 The triplet was assumed to be a pure spin state and the open-shell BS singlet is only contaminated by the triplet component, which allows for an estimation of the open-shell BS singlet−triplet gap for pure states.16

Scheme 1. Structure of Catalyst 1 and Its Catalyzed Reactions1e

ΔST =

2 Δunr,ST ⟨S ⟩T − ⟨S2⟩BS 2

(1)

For all species, the concentration correction of 1.9 kcal/ molat room temperature was added, which accounts for the change of the standard state from 1 atm (24.5 L/mol, 298.15 K) for an ideal gas to 1 M (1 mol/L) in solution.17 To validate the accuracy of the density functional used, open-shell domain-based local pair natural orbital coupled cluster (DLPNO−CCSD(T)) single-point energy calculations in the gas phase were performed for selected key stationary points using the def2-SVP and def2-TZVP basis sets. Twopoint complete basis set (CBS) limit extrapolation were then carried out to alleviate the basis set incompleteness error using ORCA 4.1 (for details, see the Supporting Information).18,19

and explain the regionselectivities of seven different substrates very well.

2. COMPUTATIONAL DETAILS The quantum chemical calculations were carried out using density functional theory (DFT) with the B3LYP-D39 functional (with Grimme’s D3 dispersion), as implemented in the Gaussian16 program package. 10 For geometry optimizations, the SDD11 pseudopotential was used for Fe, the 6-311+G* basis set was used for P and S, and the 6-31G** basis set was used for the H, B, C, N, and O elements. The frequencies were computed analytically at the same level of theory as the geometry optimizations to identify the nature of all stationary points being either minimum (no imaginary frequency) or transition state (only one imaginary frequency) and also to obtain the Gibbs free energy corrections at 298.15 K. On the basis of these optimized geometries, the final and solvation energies were obtained as single-point corrections by applying the SMD12 continuum solvation model (in benzene solution) employing the B3LYP*-D313 functional (15% Hartree−Fock exchange and D3 dispersion from B3LYP-D3) and a larger basis set, where all elements, except Fe, were

3. RESULTS AND DISCUSSION 3.1. Uncatalyzed Hydroboration of Pyridine. Our investigation started with the uncatalyzed reaction between pyridine and HBpin. The coordination of the pyridine nitrogen atom to the boron atom of HBpin20 leads to the formation of a stable complex, Int1, which lies at +5.4 kcal/mol relative to the two separated substrates. Geometric constrain dictates the hydride transfer takes place to the ortho-carbon of pyridine via a four-membered ring transition state (TSuncat, Figure 1), associated with a very high barrier of 52.7 kcal/mol (Figure 2). A similarly high barrier has been observed in the addition of HBpin to alkenes.21 The addition of a second HBpin molecule as a hydride transfer bridge has also been considered, however, the barrier is still very high, being +36.9 kcal/mol. At the

Figure 1. Structures of transition states for the uncatalyzed hydroboration of pyridine. Distances are given in angstroms. Relative energies are given in kcal/mol relative to two separated substrates, and the imaginary frequencies are also indicated. B


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Figure 2. Gibbs free energy diagram (kcal/mol) for the uncatalyzed hydroboration of pyridine in benzene at the SMD B3LYP*-D3//B3LYP-D3 level. The DLPNO−CCSD(T) results for all stationary points were shown in red in parentheses.

DLPNO−CCSD(T) basis set limit level,18f,19 the barrier was calculated to be 42.0 kcal/mol. These results suggest that a catalyst must be used to lower the barrier for the hydroboration reaction. 3.2. Fe-Catalyzed Hydroboration of Pyridine. We then investigated the iron-catalyzed hydroboration of pyridine, the simplest and representative N-heteroarene. It should be mentioned that the dinitrogenbridged diiron compound [Cp*(Ph2PC6H4S)Fe]2(μ-N2) (labeled 1′, Figure S2) formed by the binding of N2 to two iron complexes has been identified by X-ray structure analysis.1e The ground state of 1’ is a singlet and the quintet state is +2.9 kcal/mol higher in energy. The calculated N−N bond distance is 1.13 Å, which is the same as that in the crystal structure (Table S1).1e The N−N distance is very close to that of free N2 molecule, which suggest that the N2 molecule is weakly coordinated between the two iron centers.22 In fact, the mononuclear catalyst 1 (Figure 3) could be regenerated by dissolving crystals of 1′ in benzene solvent,23 with the release of a N2 molecule. The reaction mechanism (labeled as mechanism A) proposed by Wang and co-workers was investigated,1e in which the pyridine first coordinates to the catalyst.1b Our previous calculations24 showed that 16-electron complex 1 (Figure 3) prefers to be a triplet, while the singlet and the

quintet are 10.1 and 3.9 kcal/mol higher, respectively. The coordination of pyridine to 1 was found to be slightly exergonic by 0.6 kcal/mol (Figure 4). This coordination leads to the formation of an 18-electron complex, A-Int1, which is a quintet at the SMD B3LYP*-D3//B3LYP-D3 level in benzene. A-Int1 was shown to be paramagnetic in C6D6 solvent with magnetic susceptibility of 2.83 μB,1e suggesting the formation of a triplet species in C6D6 solution. Geometry optimization of A-Int1 in the gas phase using eight different density functionals including four pure functionals M06-L-D3,14 MN15L,25 TPSSTPSS-D3,26 and PBE-D3,27 as well as four hydrid functionals M06-D3,28 MN15,29 TPSSh-D3,26 and PBE0D3,27,30 showed that the singlet structure agrees well with the crystal structure, with much lower RMSD for the critical metal−ligand coordination distances. The typical RMSD (see Tables S2−S10) is in the range of 0.02−0.03 Å for the singlet and 0.11−0.15 Å for the triplet, while it is in the range of 0.16−0.21 Å for the quintet. These results strongly suggested that the spin state of the complex in the crystal structure is a singlet. However, spin crossover31 from singlet to triplet takes place during its solvation in C6D6 solution. In this case, B3LYP-D3 gave somewhat larger energy gap between the singlet and triplet, while M06-L-D3 gave more reasonable results. For the reaction mechanism and regioselectivity studied below, we found that M06-L-D3 and M06-L-D3// B3LYP-D3 gave very similar barriers (see Tables S11−S13) and the same conclusion. Therefore, all energies reported herein are on the basis of the B3LYP-D3 geometries. Starting from A-Int1 (Figure S3), the HBpin substrate can undergo an electrophilic addition to the thiolate of the catalyst, which is coupled with the hydride transfer from the boron atom to the pyridine substrate. Two different transition states have been optimized, with the hydride transferred to either the ortho-carbon (A-TS1C2) or the para-carbon (A-TS1C4). Unexpectedly, A-TS1C2 (Figure 5) has a barrier of 37.5 kcal/ mol relative to A-Int1, and the barrier for A-TS1C4 is even higher, at 51.7 kcal/mol. Downhill from A-TS1C2, a transit intermediate, A-Int2C2 (Figure S3), has been located, from which the 1,2-hydroboration product is formed with a very small barrier. For the 1,4-addition, the 1,4-hydroboration product is formed directly from A-TS1C4, and the whole reaction is exergonic by 5.8 kcal/mol. The barriers for both pathways are even higher than the uncatalyzed pathway as

Figure 3. Optimized structure of 1. Distances are given in angstroms. Spin density on Fe is indicated in red italics for the triplet state. For clarity, unimportant hydrogen atoms are not shown. C


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Figure 4. Gibbs free energy diagram (kcal/mol) for Mechanism A calculated at the SMD B3LYP*-D3//B3LYP-D3 level in benzene. S, T, and Q indicate closed-shell singlet, triplet, and quintet, respectively. The DLPNO−CCSD(T) results for catalyst 1 are shown in red in parentheses.

Figure 5. Transition state structures for mechanism A. For clarity, unimportant hydrogen atoms are not shown. Distances are given in angstroms. Spin densities on Fe and the Cp* ring are shown in red italics for the quintet state. Relative energies are given in kcal/mol relative to 1 plus two separated substrates. The imaginary frequencies are also indicated.

pyridine. In B-TS2, the scissile B−H bond is 2.27 Å. The barrier for B-TS3 is 1.4 kcal/mol higher than B-TS2. In the resulting intermediate B-Int3 (Figure S4), the critical B−N distance is 1.59 Å, and the distance between the hydride and the ortho-carbon C2 is 2.24 Å. In contrast, it is 3.50 Å between the hydride and the para-carbon C4. From B-Int3, geometric constrain dictates that the hydride can only be transferred to ortho-carbon of pyridine. B-TS4C2 has been optimized with an imaginary frequency of 431.7 i cm−1, which mainly relates to the critical hydride transfer from Fe to C1. The barrier was calculated to be only 3.5 kcal/mol relative to B-Int3, while BTS4C4 (Figure S4) has a much higher barrier of 16.1 kcal/mol. In B-TS4C2, the scissile Fe−H bond is 1.59 Å, and the key H1−C1 and S−B bonds are 1.64 and 2.08 Å, respectively. In this mechanism, the catalyst activates HBpin by the generation of a reactive FeII−H,1b,2f,8b while the boron center functions as a Lewis acid to activate the pyridine substrate,7 which becomes more electrophilic to accept a hydride from Fe. Importantly, the 1,2-addition product is exclusively obtained via this mechanism. In order to obtain the 1,4-dearomatized products, other pathway has to be operative.

discussed above, suggesting that other pathways must operate to account for the observed high reactivity and selectivity. Two alternative mechanistic scenarios can be envisioned, which have different order of reactions involving the catalyst and the two substrates. First, the catalyst reacts with HBpin, followed by reaction with pyridine (labeled as mechanism B).32 Second, the catalyst reacts directly with the pyridine− HBpin complex Int1 (labeled as mechanism C).7 In mechanism B, the thiolate performs a nucleophilic attack on the boron moiety of HBpin, in concomitant with the hydride coordination to Fe. The optimized transition state BTS1 is shown in Figure 6, the barrier was calculated to be 18.2 kcal/mol in the singlet state, and the resulting intermediate, BInt1 (Figure S4), lies at +12.9 kcal/mol relative to 1 plus HBpin. In B-Int1, the FeII−H1 (distance of 1.56 Å) interacts with the boron moiety with a B−H1 distance of 1.31 Å. Coordination of the pyridine nitrogen atom to the boron moiety turns to proceed in a stepwise manner, namely, cleavage of the B−H bond (B-TS2) followed by the formation of the B−N bond (B-TS3). B-TS2 (Figure 7) has a barrier of 15.1 kcal/mol in the singlet state relative to B-Int1 plus D


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Figure 6. Structures of transition states for mechanism B. For clarity, unimportant hydrogen atoms are not shown. Relative energies are given in kcal/mol relative to 1 plus two separated substrates. Distances are given in angstroms. The imaginary frequencies are also indicated.

Figure 7. Gibbs free energy diagram (kcal/mol) for mechanism B calculated at the SMD B3LYP*-D3//B3LYP-D3 level in benzene. S, T, and Q indicate closed-shell singlet, triplet, and quintet, respectively. The DLPNO−CCSD(T) results for the ground state of all stationary points are shown in red in parentheses.

A third plausible pathway (mechanism C) is that the pyridine−HBpin complex could react with the catalyst through a hydrogen atom transfer from the boron center to the iron center.7 The optimized transition state C-TS1 is shown in Figure 8, and the barrier was calculated to be 28.0 kcal/mol in triplet state (Figure 9). The broken-symmetry open-shell singlet state and the quintet state barriers are 5.0 and 3.2 kcal/ mol higher, respectively. In C-TS1, the distances of the scissile B−H bond and the nascent Fe−H bond are 1.82 and 1.60 Å, respectively. This leads to the formation of a FeIII−H pyridine−Bpin radical intermediate C-Int1, which lies at +14.2 kcal/mol. C-Int1 is a triplet, and the spin densities on Fe and the pyridine−Bpin moiety are 1.07 and 0.98,

respectively. In the next step, the hydrogen atom can be delivered from the iron center to either the ortho- (C-TS2C2) or the para-position (C-TS2C4) of the pyridine moiety. CTS2C2 has a barrier of only 6.1 kcal/mol relative to C-Int1 in the triplet state, while C-TS2C4 is 2.3 kcal/mol higher in energy than C-TS2C2. The higher barrier for C-TS2C4 suggested that this complex preferentially catalyzes 1,2addition of HBpin to pyridine. In C-TS2C2, the breaking Fe−H bond distance is 1.61 Å, while the C1−H1 bond is shortened to 1.82 Å. In addition, the spin densities on Fe, H1 and the pyridine−Bpin moiety are 1.52, −0.15, and 0.71, respectively. E


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Figure 8. Transition state structures for mechanism C. For clarity, unimportant hydrogen atoms are not shown. Distances are given in angstroms. Spin densities on selected atoms and groups are shown in red italics for the spin state with the lowest energy. Relative energies are given in kcal/ mol relative to 1 plus two separated substrates. The imaginary frequencies are also indicated.

Figure 9. Gibbs free energy diagram (kcal/mol) for mechanism C calculated at the SMD B3LYP*-D3//B3LYP-D3 level in benzene. S, BS, T, and Q indicate closed-shell singlet, the broken-symmetry open-shell singlet, triplet, and quintet, respectively. The DLPNO−CCSD(T) results for the triplet state are shown in red in parentheses.

As shown in Figures 7 and 9, B-TS3 is the rate-limiting for pathway B with a total barrier of 30.0 kcal/mol, while C-TS1 is the rate-limiting step for pathway C with a total barrier of 28.6 kcal/mol. In addition, B-Int3 and C-Int1 could interconvert by association/dissociation of the B−S bond.7 As a consequence, both B-TS3 and C-TS1 may contribute to the rate as the calculated barriers are quite close, with an energy difference of 1.4 kcal/mol. Importantly, B-TS4 and C-TS2 are the selectivity-determining step. For the formation of the 1,2product, the reaction can proceed via both C-TS2C2 (barrier of 20.9 kcal/mol) and B-TS4C2 (21.9 kcal/mol). For the formation of the 1,4-product, the reaction takes place via C-

TS2C4, but its barrier is 2.3 kcal/mol higher than C-TS2C2. By using transition state theory, the ratio of 1,2-product/1,4product can be estimated to be 57:1. These results are in good agreement with the previous experimental results, in which only 1,2-product was observed. The quite high total barrier of 28.6 kcal/mol also explains why the yield is only 39% in 24 h at room temperature, with a catalyst load of 1 mol %.1e It should be pointed out that the barrier for the interconversion of 1,2product to 1,4-product was calculated to be 26.1 kcal/mol (backward barrier from the kinetic product to the TS for the thermodynamic product), which is lower than the rate-limiting barrier of 28.6 kcal/mol. To reproduce the regioselectivity, the F


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phase were carried out at the DLPNO−CCSD(T)/CBS18f,19 level for mechanisms B and C. The Gibbs free energy correction and the solvation effects were taken from the B3LYP data. These results were shown in red in parentheses in Figures 4, 7, and 9. It can be seen that the DLPNO−CCSD(T) method gave a somewhat lower barrier for all stationary points except transition state B-TS1 and intermediate B-Int1. Importantly, it also suggested that in mechanism C the first step is rate-determining and that the 1,2-addition is preferred with a barrier difference of 2.5 kcal/mol (2.3 kcal/mol for B3LYP*-D3). Furthermore, the single-point energies calculated using M06-L-D314 are consistent with the B3LYP*-D3 results (see Figure S5) and gave the same regioselectivity. For example, the corresponding barrier difference is 2.2 kcal/mol (21.9 kcal/mol for C-TS2C2 and 24.1 kcal/mol for C-TS2C4). 3.3. Rationalization of Regioselectivity for Other Substrates. The previous experimental studies showed that functional groups such as Me, F, and benzofused Nheterocycles were well-tolerated under the reaction conditions. To further verify the suggested mechanism (pathways B and C) and to test the accuracy of the method used, five additional substrates (Scheme 2) were chosen to rationalize the regioselectivity.1e The calculated energies for key intermediates and selectivity-determining transition states are showed in Table 1 (for details, see Figures S6−S25, Table S14−S16). For 2-methylpyridine, the reaction was performed using 2.5 mol % of 1, and the yield was 62% in 48 h. The product ratio (3b′′ vs 3b′) was estimated to be 66:34. The calculations revealed that the barriers were 25.9 and 26.1 kcal/mol, respectively, for the generation of 1,6-product (3b′′) and 1,4product (3b′), respectively. In addition, the formation of 1,2product (3b) has a much higher barrier (29.9 kcal/mol). The barrier difference of 0.2 kcal/mol can be converted to a ratio of 58:42, which can be considered to be in excellent agreement with the experimental results. It should be noted that the calculated interconversion barrier (27.3 kcal/mol) is slightly lower than the total barrier (28.1 kcal/mol), while it should be somewhat higher for the kinetically controlled reaction. The energy difference is within the error of the method, and it should be considered to be acceptable. For 3-methylpyridine, the experiment gave only 3c and 3c′′ with a ratio of 93:7. Interestingly, the calculations revealed that the coordination of 3-methylpyridine to the iron center to form A-Int13MP was found to be exergonic by 2.4 kcal/mol. As a consequence, the overall barriers for C-TS4C2, C-TS2C4, and C-TS2C6 were calculated to be 22.7 kcal/mol, 26.2 and 23.0 kcal/mol, respectively. The same barrier for C-TS2C2 and CTS2C6 suggested that 1,2-addition and 1,6-addition would compete with each other at the beginning. However, the 1,2product (3c) is 2.0 kcal/mol lower in energy compared with the 1,6-product (3c′′), and the 1,6-product (3c′′) can be converted to 1,2-product (3c) with a barrier of 23.2 (i.e., 20.3 + 2.9) kcal/mol. The barrier for the interconversion is the lowest among all the substrates under investigation. With the extension of the reaction time, 1,2-product and 1,6-product may reach thermodynamic equilibrium. The hydroboration of 3-methylpyridine is thus thermodynamic control, and an energy difference of 2.0 kcal/mol between 1,2-Prod and 1,6Prod gave a product ratio of 97:3. It should be pointed out that the formation of the 1,4-product (3c′) is thermodynamically most favorable; however, the conversion of 1,2-product (3c′′) to 1,4-product (3c′) has a much higher barrier of 28.7 (i.e., 4.9 + 23.8) kcal/mol.

rate-limiting barrier should be at least slightly lower than that for the interconversion. This barrier difference is within the error bar of the B3LYP*-D3 functional, typically about 3 kcal/ mol for relative energies of significantly different structures. However, for the investigation of regioselectivity and stereoselectivity, the error bar is much smaller due to very similar transition state structures and error cancellation. It should be pointed out that the calculated barrier of 28.6 kcal/mol seems to be somewhat overestimated as the reaction were performed at room temperature, for which the barrier should be less than 25 kcal/mol.33 It should be noted that all three possible pathways have been considered in our recent study on the Cp*(Ph2C6H4O)Ni catalyzed hydroboration of pyridine.7 For that catalyst, pathway B is exclusively preferred with a barrier of 21.8 kcal/mol, and only 1,2-addition takes place. The hydrogen atom transfer pathway is strongly disfavored as the formation of the radical pair, a NiIII-hydride species and a PyBpin radical, is endergonic by 23.1 kcal/mol. The different metal−ligand combination is thus crucial to tune the reaction pathway, thus provide different regioselectivities. To understand the origin of the regioselectivity of this reaction, distortion/interaction analysis on the selectivitydetermining step (C-TS2) was performed by using the activation-strain model.34 In the analysis, the full model is divided into two parts, the FeIII−H complex and the pyridine− Bpin radical. Single−point calculations were performed for each part in C-Int1, C-TS2C2, and C-TS2C4. The total activation energy (without Gibbs free energy collection) for the selectivity-determining step is associated with two components, a distortion energy (ΔE⧧dist) and an interaction energy (ΔE⧧int). As showed in Figure 10, the total distortion

Figure 10. Distortion/interaction analysis for C-TS2 calculated at the SMD B3LYP*-D3//B3LYP-D3 level in benzene.

energy is of the same magnitude for the attack on C2 and C4; however, the interaction energy for C2 (−18.2 kcal/mol) is much larger than that for C4 (−14.8 kcal/mol). The regioselectivity for this step is thus mainly interactioncontrolled.35 To test the accuracy of the density functional used, singlepoint calculations for the most favorable spin state in the gas G


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Organometallics Scheme 2. Regioselective Hydroboration of Pyridines and Benzofused N-Heterocyclesa

a For consistency, the atom labeling scheme for isoquinoline (2f) is similar to that for quinoline (2e). The blue checkmark symbol means this kind of product was detected in experiment, while the red “×” symbol means that was not detected in experiment.1e

Table 1. Calculated Relative Energies for Key Intermediates and Transition State for Different Substrates at the SMD B3LYP*D3//B3LYP-D3 Level in Benzene mechanism C substrates b

pyridine (2a) 2-methylpyridine (2b) 3-methylpyridine (2c) 3-fluoropyridine (2d) quinline (2e) isoquinoline (2f)

products

A-Int1

TS1

Int1

TS2C2

TS2C4

TS2C6

total barrier

1,2-Prod

1,4-Prod

1,6-Prod

IC barriera

−0.6 1.4 −2.4 −0.7 1.3 0

28.0 29.5 25.5 25.4 23.6 21.7

14.2 18.7 14.2 13.9 10.6 9.2

20.3 29.9 20.3 19.8 16.8 27.5

22.6 26.1 23.8 23.2 17.1 33

20.3 25.9 20.6 23.9 36 14.2

28.6 28.1 27.9 26.1 22.3 21.7

−3.5 −2.1 −4.9 −5.3 −9.4 7.5

−5.8 −2.1 −6.8 −6.9 −10.4 15.7

−3.5 −1.2 −2.9 −1.6 19 −14.3

26.1 27.3 23.2c 28.5 26.5 41.8

a

Interconversion (IC) barrier from the kinetically more favored product to thermodynamically more favored product (backward barrier from the kinetic product to the TS for the thermodynamic product). bThe 1,2-product and the 1,6-product for substrate 3a are the same species. cBarrier for the conversion of 1,6-Prod to 1,2-Prod.

with an energy difference of only 0.3 kcal/mol (product ratio of 62:38). Interconversion of 1,2-product (3e) to 1,4-product (3e′) has a barrier of 26.5 (i.e., 9.4 + 17.1) kcal/mol, which is considerably higher than the total barrier (22.3 kcal/mol). As a result, interconversion may not take place for this substrate. The calculated product ration of 62:38 should be considered as in reasonable agreement with experimental ratio of 38:62. For isoquinoline, only 3f′′ was obtained. The calculations showed that the coordination of isoquinoline to the catalyst is isogonic (Table 1). In addition, the formation of 1,6-product (3f′′) is both kinetically and thermodynamically much more favorable. The total barrier for isoquinoline is 21.7 kcal/mol, which is the lowest among all the substrates. Indeed, the

For 3-fluoropyridine, only 3d was obtained under the experimental condition. Indeed, the calculations favor the 1,2addition pathway, with a barrier of only 20.5 kcal/mol for CTS2C2. The barrier is 3.4 kcal/mol lower than that for CTS2C4. The 1,4-product (3d′) is slightly more stable, but the conversion of 1,2-product (3d) to 1,4-product (3d′) has a barrier of 28.5 kcal/mol (5.3 + 23.2). As a consequence, the product distributuon should be kinetically controlled for 3fluoropyridine. A mixed products 3e and 3e′ (ratio of 38:62) were obtained for the hydroboration of quinoline. The formations of 1,2product (3e) and 1,4-product (3e′) have very similar barriers, 16.8 kcal/mol for C-TS2C2 and 17.1 kcal/mol for C- TS2C4, H


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Organometallics experiments showed that isoquinoline gave the highest reactivity, with full conversion at room temperature in 1 h.1e By assuming a turnover number of 99 in 1 h (99% yield with 1% mol catalyst loading),1e the rate constant can be estimated to be approximately 100 h−1, which corresponds to a barrier of 19.5 kcal/mol using the classical transition state theory. The calculated barrier of 21.7 kcal/mol at the B3LYP*-D3 level is thus in good agreement with the experimental results, with an error bar of about 2 kcal/mol. Importantly, the formations of both 1,2-product (3f) and 1,4-product (3f′) are endergonic. The deuterium isotope effect has been measured for isoquinoline using DBpin, which gave a value of kH/kD = 2.33.1e We have calculated the deuterium isotope effects for both mechanisms B and C by using isoquinoline as the substrate. These were done by recalculating the Gibbs free collections from the harmonic frequencies using the mass of deuterium instead of proton. The computed Gibbs free energy difference was then converted to isotope effect using classical transition state theory. The KIE values for B-TS3IQ and CTS1IQ were calculated to be 1.62 and 3.07, respectively. The experimental KIE of 2.22 is thus in between these two calculated values. These results also suggest that both pathways may contribute to the apparent rate constant.

Scheme 3. Suggested Catalytic Cycle for the Hydroboration of N-Heteroarenes on the Basis of the Present Calculations

4. CONCLUSION In summary, we have performed density functional calculations to elucidate the detailed reaction mechanism and regioselectivity of an iron-catalyzed hydroboration of N-heteroarenes. The uncatalyzed reaction was found to have a very high barrier, which confirms that an efficient catalyst is needed to mediate this reaction. The originally proposed mechanism (pathway A) was found to have extremely high barriers, ruling out its possibility. In this pathway, the ligand sulfide acts as a nucleophile to attack the boron center, which is coupled with a hydride transfer from the boron center to the substrate. Instead, two other plausible mechanisms were proposed on the basis of the present calculations. In pathway B, the catalyst reacts with HBpin first through a nucleophilic attack of the ligand sulfide on the born center of HBpin, followed by hydride transfer from HBpin to the iron center. This is followed by the coordination of the substrate nitrogen to the boron center. Finally, the hydride is transferred from the iron center to the substrate carbon. Due to geometric constrains, the hydride is preferentially delivered to C2. The third mechanism is a hydrogen atom transfer pathway C, as depicted in Scheme 3. In the first step, the catalyst reacts with the HBpin−pyridine complex directly through hydrogen atom transfer. This leads to the formation of a Py-Bpin radical and a FeIII−H complex. The second step involves a hydrogen atom transfer from the iron center to either the ortho- or paraposition of the borenium-activated pyridine. The first step is the rate-limiting step with a barrier of 28.6 kcal/mol, while the second step is the selectivity-determining step. For the pyridine substrate, the regioselectivity trends are correctly reproduced by the present calculations, and the 1,2-addition is kinetically more favorable. The two suggested pathways B and C were then used to provide rationalization of the observed regioselectivity for other N-heteroarenes substrates. For all cases, pathway C has somewhat lower barrier. The suggested mechanism, namely, pathway C, can reproduce very well the selectivity trend. It should be pointed out that the hydroborations of 3-

methylpyridine and quinolone are suggested to be thermodynamically controlled.

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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.9b00292. Figures and energy diagram (PDF) Coordinates (XYZ)

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. ORCID

Rong-Zhen Liao: 0000-0002-8989-6928 Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was supported by the National Natural Science Foundation of China (21873031) and the Fundamental Research Funds for the Central Universities (2017KFKJXX014). We acknowledge helpful discussion with Prof. Wenguang Wang and insightful comments from anonymous reviewers and the editor.

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REFERENCES

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Mechanism and Regioselectivity of the Iron-Catalyzed Hydroboration

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