Computational Analysis of the Intramolecular Oxidative Amination of

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

Computational Analysis of the Intramolecular Oxidative Amination of an Alkene Catalyzed by the Extreme π‑Loading N‑Heterocyclic Carbene Pincer Tantalum(V) Bis(imido) Complex Guangchao Liang, T. Keith Hollis,* and Charles Edwin Webster* Department of Chemistry and Center for Computational Sciences, Mississippi State University, Mississippi State, Mississippi 39762-9573, United States S Supporting Information *

ABSTRACT: The extreme π-loading of a (CCC-NHC) pincer Ta(V) bis(imido) complex was previously reported. This complex catalyzes the cyclization of α,ω-aminoalkenes (2,2-diphenylpent-4-en-1-amine) to produce three different products in varying proportions: cyclic imine from oxidative amination (OA), reduction product (RP) from hydrogenated substrate, and hydroamination (HA) (Helgert, T. R.; et al. Organometallics 2016, 35, 3452). Various plausible pathways for the reactions generating cyclic imine product from oxidative amination, the reduction of substrate from hydrogen transfer, the cyclic amine product from hydroamination, and the dehydrogenation of hydroamination product were evaluated using density functional theory computations. RP is the thermodynamic product, while OA and HA are the kinetic products, with HA being lower in energy than OA. Multiple pathways for the generation of OA product were examined. The lowest free energy of activation (ΔG⧧) of the rate-determining-step (RDS) during the oxidative amination was calculated to be 28.8 kcal mol−1. The ΔG⧧ of the RDS for the generation of reduction product is 42.8 kcal mol−1and for the generation of hydroamination product is 41.8 kcal mol−1. The overall turnover-limiting step of the proposed catalytic cycle of the conversions of substrate 3 is the regeneration of the Ta(V) bis(imido) intermediate 7 from Ta(V)-hydride amido intermediate 13 (42.8 kcal mol−1, TS-19-7). An amido hydride Ta intermediate 13 is the computed resting state of the proposed catalytic cycle. High temperature significantly favored the formation of OA-4 and RP-5 and also promoted the dehydrogenation of HA-6. An alternative for the generation of OA-4 with the participation of the NHC as a proton shuttle and through a σN-π-σC isomerization pathway is also discussed. The computational results are consistent with the experimentally observed product ratios and selectivity.

1. INTRODUCTION High activity of transition-metal imido (MNR) complexes in C−H bond activation, olefin polymerization, and olefin metathesis has been reported over the past few decades.1−5 The concept of π-loading, caused by the strong πd−πp orbital interaction between the metal d orbital and the p orbital of the π-donor imido ligand, has been used to explain the high activities in these catalytic reactions.6−8 Computational studies on C−H bond activation demonstrated that methane activation was more exothermic for neutral tris(imido) complexes than related bis(imido) and mono(imido) complexes, and the activation barriers for methane C−H activation of d2 imido complexes were significantly higher than those of d0 analogues.7 The special bonding of transition-metal imido (MNR) complexes was also used as a principle in the design of new alternative Ziegler−Natta catalysts for olefin polymerization.3,9 Group 5 mono(imido) and group 6 bis(imido) complexes as polymerization catalysts have also been reported.10−14 Transition-metal imido complexes (MNR) generally follow the outer-sphere mechanism in transfer hydrogenation, and a metal-hydride amido intermediate (MH-NHR) is generated via two steps of asynchronous proton transfer.15−18 © XXXX American Chemical Society

The [2π + 2π] cycloaddition reaction between the transitionmetal imido (MNR) and unsaturated substrates was also noteworthy.19−22 The catalytic applications of transition metal complexes with N-heterocyclic carbene ligands (NHCs), which are excellent two-electron donors, have been investigated previously.23−27 Compared to the general π-loading TaNR mono(imido) complex, the orbital interaction between the rigidly meridional CCC-NHC pincer ligand (a bis carbene with a coordinated carbon from the phenylene bridge) and the bent Ta(V)(NR)2 bis(imido) fragment maximizes π-loading and affords extreme π-loading in the (CCC-NHC)Ta(V) bis(imido) complex. Extreme π-loading can enhance catalytic activity.17,28,29 The TaN imido bond length and TaN stretching frequency can be used as metrics to show the relative strength of π-loading (Chart 1). Computational results demonstrate that the TaN imido bond lengths of the rigidly meridional CCC-NHC pincer Ta(V) imido complexes are longer than those in previously reported Ta(V) imido complexes (1.848 and 1.853 Å for 2, and Received: February 16, 2018

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DOI: 10.1021/acs.organomet.8b00097 Organometallics XXXX, XXX, XXX−XXX

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Organometallics

products including the cyclic imine (OA-4) from oxidative amination (37−71%), an equivalent of hydrogenated substrate (RP-5, 32−50%), and varying proportions of cyclic amine (HA-6) from hydroamination (0−31%).31 As an extension of our previously proposed pathways for the catalytic reaction of amino-alkene catalyzed by (CCC-NHC) Ta(V) bis(imido) complex,31 the detailed theoretical results from density functional theory (DFT) computations are presented in this contribution. The generation of cyclic imine product (OA-4) from oxidative amination, formation of the reduction product (RP-5), generation of cyclic amine product of hydroamination (HA-6), and dehydrogenation of the hydroamination product were investigated. The possibility of a nonancillary role of the N-heterocyclic carbene in the pincer ligand is also discussed.

Chart 1. Comparisons of the Computed TaN Bond Distances and ν(TaN) of Various Ta(V) Imido Complexes

2. COMPUTATIONAL METHODS Computational studies were performed using Gaussian 09 (Revision D01)33 utilizing three different density functionals, including PBEPBE,34,35 M06,36 and B97-137 (the Becke B9738 exchange and correlation modified by Handy, Tozer, and co-workers), and G3B3 (one of the Gaussian model chemistry methods, which uses complex energy computations).39 Geometry optimizations were carried out in the gas phase with density functional PBEPBE/BS1. Select geometry optimizations were performed with M06/BS2 or B97-1/BS3. In basis set 1 (BS1), the effective core potential (ECP, LANL2DZ) and the associated modified-LANL2DZ40,41 basis set were employed for Ta, basis sets of LANL2DZ (d, p)40,42 and related LANL2DZ ECP were treated with Cl and I atoms, and the 6-31G (d′)43−46 basis sets were used for all other atoms (C, O, N, and H). For all PBE computations, the density fitting approximation,47,48 nondefault self-consistent field convergence (10−6), and pruned fine integration grids (75 radial shells and 302 angular points per shell) were used in gas-phase geometry optimizations. For all M06 computations, pruned ultrafine integration grids (99 radial shells and 590 angular points per shell) were used. For all B97-1 computations, pruned fine integration grids (75 radial shells and 302 angular points per shell) were used. In BS2, the LANL2DZ ECP and associated LANL08 basis set40,49 were employed for Ta and the 6-311+G**50−52 basis sets were used for all other atoms (C, N, and H). In BS3, Ta was treated with cc-pVTZ-pp53 basis sets and the Stuttgart ECP,54 and all other atoms (C, N, and H) were treated with cc-pVTZ.55,56 In BS4, Ta was treated with modified-LAN2TZ(f)40,49,57 basis sets and the LANL2DZ ECP, and all other atoms (C, N, and H) were treated with 6-311++G(2df,2p).50−52 Frequency computations were performed in order to verify the nature of all stationary points. For all located transition states, the displacement of the imaginary frequency of the transition state was visualized, and the motion of the mode was confirmed to connect the corresponding minima. Then, a simplified approach using the mass-weighted displacements from the vibrational mode of the imaginary frequency of the TS was used to manually generate starting geometries for minima optimizations. These starting geometries are slightly displaced from the converged transition state geometry. In each case, optimizations of these displaced geometries lead “downhill” and were confirmed to lead to the two connected minima on either side of the transition state. Free energy corrections were determined at 1 atm and 298.15 K (the values have not been corrected for higher temperatures used in the experiments; however, the general trends hold). Test computations for the exchange of t-butylamine with amine substrate in complex 2 (with two n-butyl substituents on the CCC ligand, instead of two methyl substituents) showed that the inclusion of the n-butyl groups on the CCC ligand only slightly increased the free energy of activation (ΔG⧧) for the exchange. Furthermore, computations with n-butyl groups would unnecessarily complicate the location of minima and transition state because of conformational flexibility. Therefore, for computational convenience for all subsequent computations, the monomer (CCC-NHC)Ta(V) bis(imido) model complex 2 with methyl groups (R′ = CH3) in place of the n-butyl groups was employed as the model for the catalytic

1.830 and 1.820 Å for A and B, respectively, Chart 1). TaN stretching frequencies of the TaN imido group are also consistent with the comparisons of π-loading. A lower TaN stretching frequency for the (CCC-NHC)Ta(V) bis(imido) complex (1238 cm−1 for 2) compared to the Ta(V) bis(imido) complexes is observed (1280 and 1278 cm−1 for A and B, respectively). In general, the Ta(V) imido complex with more π-loading has a longer TaN imido bond length and lower TaN stretching frequency. By employing the concept of extreme π-loading, by design, (CCC-NHC)Ta(V) bis(imido) and mono(imido) complexes should exhibit increased catalytic activity.30,31 Catalytic oxidative amination of an amino-alkene (2,2-diphenylpent-4-en-1-amine; see Scheme 1) with the extremely Scheme 1. Catalytic Reaction of 2,2-Diphenylpent-4enylamine by a (CCC-NHC)Ta(V) Bis(imido) Complex (1; See Chart 1)a

a OA-4 = oxidative amination product, RP-5 = reduction product, and HA-6 = hydroamination product from substrate 3.

π-loaded (CCC-NHC)Ta(V) bis(imido) complex (Chart 2) was recently reported.31 In contrast to the (CCC-NHC)Zr(IV) (amido)32 analogues which produced mostly hydroamination product, (CCC-NHC)Ta(V) bis(imido) complex yielded three Chart 2. Structures of (CCC-NHC)Ta(V) Bis(imido) Complex 1 and the Computational Model 2 (1d is the Complex 1 Dimer, and 1m is the Complex 1 Monomer)

B


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Organometallics Scheme 2. Hypothesized Pathways for the Catalytic Reaction of Substituted Pent-4-enylamine (Substrate 3) with the (CCC-NHC)Ta(V) Bis(imido) Complex

complex 1d (Chart 2). Again, for computational convenience, the phenyl groups of the substrate 3 (2,2-diphenylpent-4-enylamine) were also replaced by methyl groups (R = CH3). Gas-phase Gibbs free energy of activation (ΔG⧧) and Gibbs free energy of reaction (ΔG°) are given in the figures and schemes in kcal mol−1. All energies are relative to the complex 7. Solvation effects were modeled by computing single-point computations on PBEPBE/BS1 gas-phase geometries using the solvation model based on density (SMD)58 with solvent parameters consistent with toluene utilizing the M06 functional with BS2 (M06/BS2) or the PBEPBE functional with BS1 (PBEPBE/ BS1). SMD-M06/BS2//PBEPBE/BS1 yielded slight variations in the solvation energies when compared to the results from SMD-PBEPBE/ BS1//PBEPBE/BS1 (for brevity, further discussion of these results is relegated to the Supporting Information). Gas-phase B97-1/BS4// PBEPBE/BS1 Gibbs free energies are presented in the main text (the thermodynamic corrections are from PBEPBE/BS1 results). SMD-M06 and SMD-PBEPBE single-point energies are given in the Supporting Information. The M06 and B97-1 functionals have been shown by us59 and others60−62 to be accurate methodologies for organometallic systems. The qualitative results from PBEPBE/BS1, PBEPBE/BS4//BS1, M06/BS3, and B97-1/BS4 are similar and do not significantly affect the arguments presented below. Comparisons of these results are presented in the Supporting Information. On the basis of G3B3 results, RP-5 is the thermodynamic product, while OA-4 and HA-6 are the kinetic products, with HA-6 being lower in energy than OA-4 (see the Supporting Information).

3. RESULTS AND DISCUSSION To understand the oxidative amination of an alkenyl amine catalyzed by the (CCC-NHC)Ta(V) bis(imido) complex, possible mechanisms were hypothesized as illustrated in Scheme 2. Discussions on the proposed mechanism are divided in the following sections: (1) conversion of precatalyst to active catalyst via imido ligand exchange (2 → 7, involving complexes 16, 17, and 18; see Figures S1−S5); (2) oxidative amination of substrate 3 catalyzed by complex 10 yielding enamine oxidative amination product (the pyrroline isomer) OA-4i (7 → 8 → 10 → 13; see Figure 1); (3) regeneration of complex 7 from complex 13 by the reduction of substrate 3 to RP-5 (13 → 7, involving species 13b and 19; see Figure 2); (4) isomerization of enamine 4i to OA-4 (4i → 4, involving species 7 and 20; see Figure 3); (5) hydroamination via 8 → 9 → 7 producing cyclic amine HA-6 (see Figure 4); (6) participation of the NHC as a proton shuttle in oxidative amination yielding 4 (10 → 11 → 14 → 15 → 13; see Figure S14); (7) the σN-π-σC isomerization of 10 → 12 generating 4 directly (10 → 12 → 13; see Figure S15); and (8) intermediate 7 catalyzed dehydrogenation of HA-6 generating OA-4 (7 → 13, involving species 22; see Figure 5). Paths 1, 6, and 7 are discussed mostly in the Supporting Information. Paths 2, 3, 4, 5, and 8 are discussed in the main text. C


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Figure 1. Free energy diagram of the proposed pathway for generation of oxidative amination product 4i (the isomer of pyrroline 4). See Scheme 2 for the definition of [Ta]. ΔG°/ΔG⧧ are given in kcal mol−1. All energies are relative to the complex 7. Color code: green, Ta; blue, N; gray, C; white, H. Selected distances of the core structures are given in Å; other atoms are omitted for clarity.

(1) Conversion of Precatalyst to Active Catalyst via Imido Ligand Exchange (see Figures S1−S5). The first part of the proposed mechanism is the imido ligand exchange between the precatalyst 2 and substrate 3 generates the active catalyst 7 (Scheme 2, Figures S1−S5).63 The imido ligand exchange was accomplished by two steps of asynchronous proton transfer, and the computed ΔG° for the net reaction of imido ligand exchange is +2.1 kcal mol−1 (2 → 7) (Scheme 2, Figure S1). The production of the bis(imido) complex 7 is irreversible because of the loss of 1 equiv of t-butylamine. (2) Oxidative Amination of Substrate 3 Catalyzed by Intermediate 10 Directly Yielding Enamine Product 4i (Figure 1). Following the irreversible production of the bis(imido) complex 7, a [2 + 2] cycloaddition32,64−67 of the CC bond and TaN imido bond (TS-7-8) generates a bicyclic cyclometaled imido complex 8 (7 → 8, Figure 1). The trigonal pyramidal geometry of the terminal CH2 unit and the relatively longer Ta−C bond distance (2.41 Å) in intermediate 8 suggest that the Ta−C bond is a relatively weak bond in intermediate 8 (see Cartesian coordinates in the Supporting Information for the full 3D structure). The proton transfer from the bridgehead CH to the Ta metal center (TS-8-10) produces the methylene amide bonded Ta-hydride complex 10. The bond length of Ta−N(substrate) in the transition state TS-8-10 is longer than that of TS-7-8 (2.14 Å vs 1.93 Å, Figure 1).

This longer bond length is caused by a decrease of the double bond character from a TaN imido in complex 7 to a Ta−N amido in complex 10. The subsequent proton transfer from the NH2 group of substrate 3 to the N of the methylene amide in complex 10 (TS-10-13) generates a free 4,4-dimethyl-2methylenepyrrolidine (4i) and the Ta-hydride amido complex 13. A similar process between an activated amine and a coordinated palladium complex generating the palladium-hydride species in the vinylarene hydroamination was proposed previously.68 (3) Regeneration of Complex 7 from Complex 13 by the Reduction of Substrate 3 to RP-5 (Figure 2). Enamine product 4i can be released from 13+4i to produce the Ta-hydride amido complex 13 (see Figure 1). A rotamer of complex 13 can be produced via the rotation of the amido ligand of the deprotonated substrate 3 (see Figure S6) to produce complex 13b. Complex 13b can reduce substrate 3 to the reduction product 5.69−72 The reduction of substrate 3 to generate product 5 catalyzed by an amido Ta-hydride complex 13b is accomplished by an asynchronous process of a hydride transfer step (TS-13b-19) and a proton transfer step (TS-19-7) through bis(imido) Ta-hydride intermediate 19 (Figure 2). A similar reduction process of aromatic olefin catalyzed by a Rh complex has been proposed previously.73,74 Intermediate 19, generated by the hydride transfer step (TS-13b-19), has a relatively weak Ta−H−C agostic interaction. The Ta−H bond distances in intermediate 13b, D


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Figure 2. Free energy diagram of the proposed pathway for generation of reduction product 5. See Scheme 2 for the definition of [Ta]. ΔG°/ΔG⧧ are given in kcal mol−1. All energies are relative to the complex 7. Color code: green, Ta; blue, N; gray, C; white, H. Selected distances of the core structures are given in Å; other atoms are omitted for clarity.

hydrogen acceptors were not able to be reduced in our previous experimental study. (4) Isomerization of Enamine 4i to OA-4 (Figure 3). Four different pathways for the isomerization of enamine 4i to OA product pyrroline 4 were located: (1) intramolecular monomer isomerization, (2) a homodimer isomerization, (3) amine (RNH2) catalyzed isomerization, and (4) metal-complex 7 catalyzed isomerization. The catalyst-free monomer and dimer isomerizations to convert enamine 4i to OA-4 (3,3,5-trimethyl3,4-dihydro-2H-pyrroline) are illustrated in Figures S7 and S8, respectively. However, the relatively high ΔG⧧ of the monomeric process (65.7 kcal mol−1, relative to OA-4) and dimeric process (42.9 kcal mol−1, relative to OA-4) showed monomer and dimer isomerization processes were highly unfavorable. The amine catalyzed isomerization from enamine 4i to OA-4 was also investigated using CH3NH2 as a model catalyst (mimic of substrate 3) (Figure S9). The results from these computations show that the amine catalyzed isomerization from 4i to OA-4 proceeds through two synchronous intermolecular proton transfer processes with a ΔG⧧ of 41.8 kcal mol−1 (relative to OA-4 and a free CH3NH2). The isomerization from 4i to OA-4 catalyzed by the bis(imido) complex 7 is the lowest energetic pathway (25.4 kcal mol−1; see Figure 3). The addition of enamine 4i to complex 7 forms a methylene amide bonded amido complex 20 via TS-4i (7 → 20, Figure 3). The subsequent proton transfer (20 → 7) from the amido ligand to the terminal methylene of the bonded amide (TS-4, Figure 3)

TS-13b-19, and intermediate 19 are 1.94, 2.04, and 2.25 Å, respectively. A slightly shorter Ta−N(substrate) bond length in the transition state of TS-19-7 than that of TS-13b-19 (1.99 Å vs 2.02 Å, Figure 2) was observed. This difference is caused by the increase of the double bond character from Ta−N amido in complex 13b to TaN imido in complex 7. The asynchronous proton transfer formed a fleeting Ta intermediate 19 (13b → 19) with a ΔG⧧ of 41.4 kcal mol−1. The subsequent asynchronous proton transfer (19 → 7) has a ΔG⧧ of 42.8 kcal mol−1, slightly higher than that of the previous proton transfer, which is the rate-determining step during the generation of product 5 from the reduction of 3. Another alternative pathway for the generation of RP-5 exists. Two equivalents of substrate (3) could react with complex 2 and lead to the loss of 2 equiv of t-butylamine, resulting in a species (26) in which both t-butyl imides have been exchanged and replaced with “substrate” imido ligands. Complex 26 could then react with a third equivalent of substrate (3), and after one cycle, 1 equiv of 4i is produced, along with complex 27 (a Ta(V)-hydride amido intermediate, which is an analogue to complex 13). From complex 27, a low-energy transition state (TS-27-28, 29.4 kcal mol−1) can perform an intramolecular reduction of the ethylene unit of the imido substrate. The resulting complex, 28, can then react with another equivalent of substrate (3) to release RP-5 and regenerate complex 26 (see Figure S28 for further discussion). This lowerenergy intramolecular pathway could explain why sacrificial E


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Figure 3. Free energy diagram of the proposed pathway for the isomerization of enamine 4i to oxidative amination product 4. See Scheme 2 for the definition of [Ta]. ΔG°/ΔG⧧ are given in kcal mol−1. All energies are relative to the complex 7. Color code: green, Ta; blue, N; gray, C; white, H. Selected distances of the core structures are given in Å; other atoms are omitted for clarity.

generates free pyrroline 4 and regenerates complex 7. The ΔG⧧ (25.4 kcal mol−1 of TS-4i, Figure 3) for the conversion from 4i to OA-4 catalyzed by bis(imido) complex 7 was computed to be much lower than alternatives (65.7, 42.9, and 41.8 kcal mol−1; see Figures S7−S9). (5) Hydroamination via 8 → 9 → 7 Producing HA-6 (Figure 4). At species 8, there is a partitioning between OA vs HA kinetic products. The first intermolecular proton transfer (TS-8-9) from the NH2 group of a free reactant substrate 3 to the methylene group (8 → 9; see Figure 4) of the bicyclic cyclometalated imido complex 8 forms bis(amido) imido complex 9. Hydroamination product 6 is generated by a second proton transfer via TS-9-7 (9 → 7, Figure 4) from the amido ligand formed from the deprotonated substrate 3 to the N atom in the amide ligand. The first proton transfer that breaks the bicyclic unit is the rate-determining step with a ΔG⧧ of 41.8 kcal mol−1 (TS-8-9, Figure 4 and Figure S10) in the generation of hydroamination product 6. Alternative amido complexes 9b and 9c (rotamers of complex 9) were also studied (Figure S11). The rotation of the cyclic amido in 9c assisted the direct proton transfer from the CH group of the cyclic amide to the Ta metal center, which generates the amido Ta-hydride complex 13 and a molecule of OA-4 with a ΔG⧧ of 45.7 kcal mol−1 for the RDS (TS-8b-9b, Figure S12). Compared to the addition of substrate 3 to complex 8 (TS-8-9), the release of cyclic amino product 6 (TS-9-7) significantly elongates the Ta−N(cyclic amine) bond

length (2.54 Å in TS-9-7 vs 2.15 Å in TS-8-9, Figure 4). The increased double bond character from Ta−N amido in complex 8 to TaN imido in complex 7 shortens the Ta−N(substrate) bond length in the transition state of TS-9-7 compared to that of TS-8-9 (2.00 Å vs 2.31 Å, Figure 4). The selectivity for oxidative amination over hydroamination could be addressed by the different pathways from intermediate 8. Two different intramolecular proton transfer processes from complex 8 were located: (1) generation of the Ta(V)-hydride mono(amido) complex 10 (TS-8-10, 27.5 kcal mol−1; see Figure 1) and (2) generation of the Ta(V) bis(amido) complex 9 (TS-8-9, 41.8 kcal mol−1; see Figure 4). The second proton transfer process is the RDS for the generation of HA-6 (TS-8-9; see the discussion below). A higher ΔG⧧ for the synchronous N−H bond activation of the NH2 group of substrate 3 and Ta−C bond cleavage of the bicyclic cyclometalated imido complex 8 (TS-8-9, 41.8 kcal mol−1, Figure 4) compared to the intramolecular proton transfer generating Ta(V)-hydride mono(amido) complex 10 (TS-8-10, 27.5 kcal mol−1, Figure 2) is consistent with the observation that oxidative amination is the dominant reaction compared to the hydroamination reaction. This result is in contrast to the selectivity in the Rh catalyzed intramolecular hydroamination of alkenes with amines.75,76 In that Rh system, the rate of proton transfer to form amine from the saturated 18-electron aminoalkyl rhodium intermediate was faster than that of F


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Figure 4. Free energy diagram of the proposed pathway with the lowest computed free energy for the generation of HA-6. See Scheme 2 for the definition of [Ta]. ΔG°/ΔG⧧ are given in kcal mol−1. All energies are relative to the complex 7. Color code: green, Ta; blue, N; gray, C; white, H. Selected distances of the core structures are given in Å; other atoms are omitted for clarity.

β-hydrogen elimination, which favored the hydroamination reaction.75 (6) Participation of the NHC as a Proton Shuttle in Oxidative Amination Yielding 4 (see Figure S14). The possibility of the NHC carbon shuttling the hydrogen was also located on the PES. Discussion and the illustrations are included in the SI (Figure S14). (7) The σN-π-σC Isomerization of 10 → 12 Generating 4 Directly (See Figure S15). Beside the above-discussed pathways (Figures 3 and 5), two other pathways for the generation of OA-4 including the participation of the NHC as a proton shuttle (10 → 11 → 14 → 15 → 13) and the involvement of σN-π-σC isomerization (10 → 12 → 13) were also investigated (Figures S14−S15). Relatively high ΔG⧧’s for the RDS for these two pathways were found: the generation of OA-4 via (1) the NHC carbene as a proton shuttle (40.4 kcal mol−1 for TS-14-15 in Figure S14) and (2) isomerization of the CC−N unit (50.0 kcal mol−1 for TS-12-13d in Figure S15). The lowest ΔG⧧ for the RDS during the generation of OA-4 was 28.8 kcal mol−1 (TS-10-13, Figure 1). (8) Intermediate 7 Catalyzed Dehydrogenation of HA-6 Generating OA-4 (Figure 5). Direct dehydrogenation

of HA-6 to generate oxidative amination product OA-4 might be catalyzed by the bis(imido) complex 7 via two proton transfer steps (TS-7-22 and TS-22-13b) (see Figure 5).69,77 The first intermolecular proton transfer (N−H bond activation, 7 → 22) via TS-7-22 (40.1 kcal mol−1) from the NH group of hydroamination product 6 to the N atom of an imido group in the bis(imido) complex 7 produces an imido-amido intermediate 22. The second intermolecular proton transfer (C−H bond activation, 22 → 13b) via TS-22-13b (39.1 kcal mol−1) from the CH of deprotonated HA-6 to the Ta metal center finally generates a molecule of OA-4 and a Ta-hydride amido complex 13b (a rotamer of complex 13). A slightly longer Ta−N(substrate) bond length in the transition state of TS-22-13b (2.02 Å) than that of TS-7-22 (1.98 Å, Figure 5) was observed. This decrease was caused by a decrease of the TaN imido double bond character in complex 7 compared to complex 13b. Similar results on the Ta−N(substrate) bond lengths of TS-13b-19 and TS-19-7 (2.02 Å vs 1.99 Å) were also presented in Figure 2. An alternative pathway for the dehydrogenation of HA-6 to generate OA-4 catalyzed by the bis(imido) complex 7 was investigated (see Figure S13): proton transfer from the CH group of HA-6 (instead of the NH group) to the N atom of one G


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Figure 5. Free energy diagram of the proposed pathway for dehydrogenation of hydroamination product 6 to generate oxidative amination product 4. See Scheme 2 for the definition of [Ta]. ΔG°/ΔG⧧ are given in kcal mol−1. All energies are relative to the complex 7. Color code: green, Ta; blue, N; gray, C; white, H. Selected distances of the core structures are given in Å; other atoms are omitted for clarity.

of the imido groups in complex 7 occurring first (54.9 kcal mol−1 for TS-7-22r), then followed by proton transfer from the NH group (52.7 kcal mol−1 for TS-22r-13b) of deprotonated HA-6. Discussion of the Hypothesized Pathways. Experimental results on the catalytic oxidative amination of substrate 3 (2,2-diphenylpent-4-en-1-amine) by the (CCC-NHC)Ta(V) bis(imido) complex 1d showed that the ratios of the three products (OA-4, RP-5, and HA-6) were temperature-dependent (Table S1).31 These ratios suggested two important results. First, the concentration of substrate 3 affects the yield of reduction product 5, which is consistent with the proposed pathway illustrated in Figure 2. The generation of RP-5 was accomplished by consuming two molecules of substrate 3 (Figure 2). The ΔG⧧ of the RDS during the generation of 5 was 42.8 kcal mol−1 (TS-19-7, Figure 2). However, the generation of OA-4 from oxidative amination was more favorable (28.8 kcal mol−1 for TS-10-13, Figure 1 and Scheme 3). With lower loading of substrate 3, the generation of RP-5 was limited by the competing reaction, the generation of OA-4 (7 → 8 → 10 → 13 vs 13b → 19 → 7 in Figure 2). Second, the yield of HA-6 from hydroamination was affected by temperature. The ΔG⧧ of the RDS in the generation of HA-6 was 41.8 kcal mol−1 (TS-8-9, 8 → 9 → 7 in Figure 4). Once the temperature was increased, the dehydrogenation of HA-6 to generate OA-4 (that was unfavorable at lower temperatures) became a viable pathway. The proposed catalytic

Scheme 3. Gibbs Free Energies of Activation (ΔG⧧ in kcal mol−1) of the Rate-Determining Steps (RDSs) for the Generations of OA-4, RP-5, and HA-6 through Various Pathways

cycle for the conversion of amino-alkene substrate 3 to generate the OA-4, RP-5, and HA-6 by the (CCC-NHC)Ta(V) bis(imido) complex is illustrated in Scheme 4.

4. CONCLUSIONS Theoretical studies on the catalytic oxidative amination of substrate 3 (2,2-diphenylpent-4-en-1-amine) by the (CCCNHC)Ta(V) bis(imido) complex 1d were performed using H


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acceptor design must include a primary aminoalkene that cannot undergo cyclization, but that is a better hydrogen acceptor than the substrate. It would also benefit from accelerated exchange after hydrogenation. Complementarily, high activity could be reached with proper modification on the CCC-NHC platform that will promote the conversion between the Ta-hydride amido complex (13) and Ta bis(imido) complex (7). Proper optimization of temperature and concentration will allow OA to the exclusion of HA. Finally, these optimizations will dramatically improve the prospects for synthetic applicability and asymmetric catalysis through desymmetrization of appropriate substrate.

Scheme 4. Catalytic Conversions of Substrate 3 To Generate OA-4, RP-5, and HA-6a

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ASSOCIATED CONTENT

* Supporting Information S

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.organomet.8b00097. DFT computed thermochemical data for the reactions during the conversion of 3; fluxional processes of 3; process of imido ligand exchange; relative energies of rotomers of complex 13; pathways of monomeric and dimeric conversion of 4i to OA-4; pathways of amine model (CH3NH2) catalyzed conversion of 4i to OA-4; Ta(III) involved high activation energy pathway for the generation of OA-4; pathways of the generation of OA-4 with an NHC severing as a proton shuttle; σN-π-σC isomerization involved generation of OA-4; gas-phase PBEPBE/BS1 Gibbs free energies; results from solvation single-point computations SMD/M06/BS2//PBEPBE/ BS1 and SMD/PBEPBE/BS1//PBEPBE/BS1; results from gas-phase M06/BS2//PBEPBE/BS1, B97-1/ BS3//PBEPBE/BS1, and PBEPBE/BS4//PBEPBE/BS1 single-point computations (PDF) DFT optimized Cartesian coordinates (XYZ)

See Scheme 2 for the definition of [Ta]. ΔG°/ΔG⧧ are given in kcal mol−1. a

DFT computations. The computational results are consistent with the experimentally observed product ratios and selectivity. From this current study, RP-5 is the thermodynamic product, while OA-4 and HA-6 are the kinetic products, with HA-6 being lower in energy than OA-4. The following conclusions for the mechanism of generation of OA-4, RP-5, and HA-6 may be drawn: (1) imido ligand exchange between the t Bu-imido and substrate 3 generates the bis(imido) complex 7, which is proposed to be the active catalyst in the catalytic cycle; (2) complex 10 catalyzes the conversion of substrate 3 to oxidative amination isomer enamine 4i and generates the Ta-hydride amido complex 13; (3) complex 7 catalyzes the facile conversion from enamine 4i to OA-4; (4) complex 7 also can catalyze the dehydrogenation of HA-6; (5) complex 13 can react with substrate 3 to generate product 5, while concomitantly regenerating complex 7; (6) the overall turnover-limiting step of the proposed catalytic cycle is the regeneration of intermediate 7 from 13 (42.8 kcal mol−1, TS-19-7); (7) complex 13 is the resting state of the proposed catalytic cycle; (8) the ΔG⧧ of the RDS in the generation of RP-5 (42.8 kcal mol−1, TS-19-7) was higher than that in the generation of OA-4 (28.8 kcal mol−1, TS-10-13); (9) the synchronous N−H bond activation of substrate 3 and Ta−C bond cleavage of complex 8 (41.8 kcal mol−1, TS-8-9) was found to be the RDS for the generation of HA-6; (10) higher temperatures could promote the dehydrogenation of cyclic amine product 6 to generate oxidative amination product 4; and (11) a pathway involving an intramolecular reduction of the ethylene unit of a bound imido substrate (TS-27-28) could explain why sacrificial hydrogen acceptors were not able to be reduced in our previous experimental study. These results suggest the following items for catalyst design. Given the lower free energy of activation for intramolecular transfer hydrogenation, a sacrificial

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AUTHOR INFORMATION

Corresponding Authors

*E-mail: [email protected] (T.K.H.). *E-mail: [email protected] (C.E.W.). ORCID

Guangchao Liang: 0000-0001-7235-958X T. Keith Hollis: 0000-0002-5470-9811 Charles Edwin Webster: 0000-0002-6917-2957 Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS We thank Mississippi State University High Performance Computing Collaboratory (HPC2) and the Mississippi Center for Supercomputing Research (MCSR) for computing support. This study was supported by the National Science Foundation (OIA-1539035).

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REFERENCES

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Computational Analysis of the Intramolecular Oxidative Amination of

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