Solvent Mediating a Switch in the Mechanism for Rhodium(III

Apr 17, 2017 - Herein we carried out a detailed theoretical mechanistic exploration for the reactions to elucidate the switch between carboamination a...
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Solvent Mediating a Switch in the Mechanism for Rhodium(III)Catalyzed Carboamination/Cyclopropanation Reactions between N‑Enoxyphthalimides and Alkenes Yang-Yang Xing, Jian-Biao Liu,* Xie-Huang Sheng, Chuan-Zhi Sun, Fang Huang, and De-Zhan Chen* College of Chemistry, Chemical Engineering and Materials Science, Collaborative Innovation Center of Functionalized Probes for Chemical Imaging in Universities of Shandong, Institute of Molecular and Nano Science, Shandong Normal University, Jinan 250014, P. R. China S Supporting Information *

ABSTRACT: Recently, a new synthetic methodology of rhodiumcatalyzed carboamination/cyclopropanation from the same starting materials at different reaction conditions has been reported. It provides an efficient strategy for the stereospecific formation of both carbon- and nitrogen-based functionalities across an alkene. Herein we carried out a detailed theoretical mechanistic exploration for the reactions to elucidate the switch between carboamination and cyclopropanation as well as the origin of the chemoselectivity. Instead of the experimentally proposed RhIII−RhI−RhIII catalytic mechanism, our results reveal that the RhIII−RhV−RhIII mechanism is much more favorable in the two reactions. The chemoselectivity is attributed to a combination of electronic and steric effects in the reductive elimination step. The interactions between alkene and the rhodacycle during the alkene migration insertion control the stereoselectivity in the carboamination reactions. The present results disclose a dual role of the methanol solvent in controlling the chemoselectivity. Scheme 1).20,21 Recently, Piou and Rovis reported a rhodium(III)-catalyzed cyclopropanation reaction using N-enoxyphthalimides and alkenes (eq 3 in Scheme 1).22 Interestingly, further optimization of the reaction conditions resulted in a high selectivity for syn-carboamination products from the same reactants (eq 4 in Scheme 1).23 Undoubtedly, the reaction methodology (eq 4 in Scheme 1) provides a new way of synthesizing functionalized alkenes because it allows the stereoselective incorporation of both carbon- and nitrogenbased functionalities across an alkene. The origin of the chemoselectivity (Scheme 1, eq 3 vs eq 4) is proposed to be produced by the solvent effect. When using trifluoroethanol (TFE) as the solvent, the proposed mechanism involves the generation of an alkene insertion intermediate in which the coordinatively unsaturated metal ligates the enol alkene moiety. The intermediate subsequently undergoes migratory insertion and oxidative addition to give a cyclopropane product. Conversely, in the more nucleophilic methanol (MeOH), the solvent is assumed to cleave the C− N bond in the phthalimide moiety, forming the amido ester. Thus, a bidentate DG is assumed to be generated in situ, leading to reductive elimination rather than cyclopropanation. In addition, previous theoretical studies have clearly revealed different types of internal oxidants that would influence the reaction mechanism.24 The detailed density functional theory (DFT) mechanistic calculations by Xia on the reaction shown

1. INTRODUCTION Transition-metal-catalyzed C−H functionalization has proven to be a powerful approach for organic synthesis; however, control of the site selectivity remains an ongoing challenge.1−3 The most widely applied synthetic strategy to govern a functionalized C−H bond is the use of a directing group (DG).4 Coordination of a DG to a transition metal may significantly influence the inherent steric/electronic preferences and ultimately affect the functionalized position. To date, strongly σ-donating or π-accepting nitrogen-, sulfur-, or phosphorus-containing DGs, as well as weaker coordinating moieties (e.g., carboxylates, ketones, esters, or amides), are widely employed as DGs.5 The development of DGs that serve as internal oxidants has revolutionized the oxidative C−H functionalization, which was independently introduced by several research groups.6−10 Compared with analogous reactions using external oxidants, the internal oxidant strategy has the following advantages: increased reactivity under milder reaction conditions, greater selectivity, and larger functional group tolerance.11 In practice, the moiety in the substrate serving as both a DG and an oxidant typically contains a cleavable N−N or N−O bond.12−17 In the pioneering work by Fagnou and Glorius, efficient rhodium(III)catalyzed C−H activation leading to dihydroisoquinolinones from benzamides and alkenes was achieved by employing hydroxamate DGs (eq 1 in Scheme 1).18,19 In another key example, Liu and Lu reported the rhodium(III)-catalyzed redox-neutral coupling of N-phenoxyacetamides and alkenes by using the phenoxyamide as the internal oxidant (eq 2 in © 2017 American Chemical Society

Received: February 22, 2017 Published: April 17, 2017 5392

DOI: 10.1021/acs.inorgchem.7b00450 Inorg. Chem. 2017, 56, 5392−5401

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Scheme 2. Modified Mechanism of Rhodium(III)-Catalyzed Carboamination and Cyclopropanation Reactions Using NEnoxyphthalimides and Alkenes

in eq 1 (Scheme 1) reveal that HN−OPiv is a strong internal oxidant and promotes the RhIII/RhV catalytic cycle.25 Similar

results were reported by recent theoretical work of Wu and coworkers on the reactions of N-phenoxyacetamides with 5393

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Inorganic Chemistry alkynes.26 Therefore, it is interesting to understand the role of these kinds of novel oxidizing DGs in the reactions studied herein. Considering that the detailed mechanisms of the rhodiumcatalyzed cyclopropanation and carboamination reactions remain unclear, in particular the underlying causes for the chemoselectivity and stereoselectivity, we herein report a mechanistic study of the reactions by means of DFT calculations. The computed geometric details allow us to check the proposed coordination patterns in the key intermediates. More importantly, in terms of mechanistic understanding, our study attempts to gain insight into the role of the solvent in control of the selectivity.

2. COMPUTATIONAL METHODS The real catalysts and substrates were employed in all of the computations. Geometry optimizations and frequency calculations were performed at the B3LYP/BS1 level27,28 in MeOH using the SMD solvation model.29 BS1 designates a mixed basis set of ECP28MDF_AVDZ for the rhodium atom30,31 and 6-31G(d,p) for other atoms. The results of frequency calculations were examined to confirm that each structure is a local minimum (no imaginary frequency) or a transition state (only one imaginary frequency). Key transition-state structures were further confirmed by intrinsic reaction coordinate (IRC) calculations to connect the corresponding reactants and products. Thermodynamic corrections at 298.15 K and 1 atm for all structures in MeOH were obtained by harmonic frequency calculations at the B3LYP/BS1 level. To obtain solvation-corrected free energies, we performed single-point energy calculations at the B3LYP-D3(BJ) 32 /ECP28MDF_AVDZ/6-311++G(d,p) level in MeOH with SMD on the B3LYP/BS1-opimized geometries. The present method is found to perform well in reproducing experimental observations (see Table S1 for details). All calculations were performed using the Gaussian 09 package.33 Natural bond order (NBO) calculations were performed by the GenNBO 5.0 program34 using the wave function obtained from the B3LYP/BS1 level in MeOH with the SMD model on the selected systems. The 3D structures were prepared using CYLview.35

Figure 1. Free-energy profiles for the ring-opening step. Energies are relative to 1a + nMeOH (n = 1−3) and are mass-balanced.36

kcal/mol), indicating the important role of the surrounding solvent molecules. Further inclusion of more MeOH molecules has little influence on the barrier. ii. Formation of a Five-Membered Rhodacycle. The reactions shown in eq 4 in Scheme 1 were carried out in the presence of a catalytic amount of [Cp*tBuRh(CH3CN)3](SbF 6) 2 and excess cesium 1-adamantylcarboxylate (1AdCO2Cs) additive. The 1-adamantylcarboxylate-ligated species Cp*tBuRh(O2CAd)2 (Cat) would be generated in situ and chosen as the active catalyst. As shown in Figure 2, coordination of the amido ester 3 to the catalyst gives complex 4, which is slightly endergonic. Starting from 4, consecutive N− H and C−H activation occurs to generate the five-membered rhodacycle 7. Both of the two steps are proposed to involve the concerted metalation deprotonation (CMD) process,2,3 which has been investigated widely in various transition-metalcatalyzed oxidative coupling reactions.37,38 The N−H deprotonation proceeds via TS3 with a barrier of 29.1 kcal/mol, followed by the release of 1-AdCO2H into the reaction medium.39 Interestingly, the subsequent C−H activation is very facile, only crossing a barrier of 8.6 kcal/mol (TS5 relative to 6). For analogous rhodium(III)-catalyzed reactions involving N−H and aromatic C−H activation reported previously,24−26 the DFT results show that the barriers of aromatic C−H activation are higher than the values of the preceding N−H deprotonation, which differ dramatically from our results shown in Figure 2. Considering that the bonding differences between the alkenyl and aryl C(sp2)−H are small, it is of interest to analyze the underlying factors responsible for the different kinetic barriers. An examination of the optimized structures reveals that the geometry of intermediate 6 is close to its corresponding transition state TS5 (Figure 3). The distortion required to achieve a transition structure is therefore lower, which finally results in a low barrier according to the distortioninteraction model.40−43 The results could be further illustrated by the IRC calculation results shown in Figure 4. Different from deprotonation of the aromatic substrates,44 Rh−C bond formation in our system is continuous; however, its change is remarkably small.

3. RESULTS AND DISCUSSION On the basis of Rovis’ reports and our calculations, the reaction mechanism was modified and extended to incorporate both the carboamination and cyclopropanation reactions, as shown in Scheme 2. In each case, from the alkene insertion intermediate, two possible catalytic cycles (RhIII/RhI and RhIII/RhV) are considered and compared. Significantly, as shown below, there is an overwhelming preference for RhIII/RhV over RhIII/RhI, and the pathway involving a rhodium hydride complex in the originally proposed mechanism of cyclopropanation reaction should be reconsidered based on our calculated results. 3.1. Mechanism of Carboamination and Stereoselectivity. i. Ring-Opening Reaction. First, in the presence of MeOH, it is hypothesized that the solvent would open the phthalimide to form the phthalimide-derived amido ester. The free-energy profiles for the ring-opening process are shown in Figure 1. Opening of the phthalimide moiety by only one MeOH molecule is kinetically impossible, with a high energy barrier of 41.6 kcal/mol. We thus consider the MeOH-assisted ring-opening reaction and compute two other possible pathways. As shown in Figure 1, the additional MeOH molecules mediate the concerted hydrogen transfer to the nitrogen atom and attack the carbon atom, which significantly decreases the energy barriers of the ring-opening process. Finally, the pathway involving three MeOH molecules is kinetically feasible, with an attainable activation energy (27.9 5394

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Figure 2. Free-energy profiles for N−H and C−H activation. Energies are relative to Cat + 3 and are mass-balanced.

prior to opening of the phthalimide group, and the detailed results are given in Figure S1. However, this route is unfavorable with significantly higher transition states in the ring-opening step. iii. Stereoselectivity and RhIII−RhV−RhIII versus RhIII−RhI− RhIII Cycle. The resulting 16-electron rhodacycle 7 subsequently undergoes alkene coordination by two different patterns from each of the two faces of the Rh−C bond, resulting in four distinct π complexes. Accordingly, subsequent migratory insertions proceed via four different transition states, resulting in the corresponding insertion intermediates. Therefore, a total of four possible products would be generated, and their relationships are shown in Scheme 3. For each of the insertion intermediates, both the RhIII−RhI−RhIII and RhIII−RhV−RhIII catalytic mechanisms are considered. We discuss the most

Figure 3. Optimized structures of TS5 and its intermediate precursor 6, with select bond distances given in angstroms. Hydrogen atoms, except for the transferring proton, are omitted for clarity.

Scheme 3. Four Different Migratory Insertion Patterns Resulting in the Corresponding Final Products

Figure 4. Calculated bond distances and electronic energy (ΔE) along the IRC of C−H activation.

Another parallel pathway in which C−H activation occurs prior to N−H deprotonation is also considered (see the red path in Figure 2). In the comparable path, the barrier for C−H activation is 27.7 kcal/mol, and the following N−H deprotonation occurs also with a negligible barrier. A comparison of the two free-energy profiles shows that the red path is slightly more favorable. Wu and co-workers26 have thoroughly studied the possibility of additional MeOH coordination in the intermediates and transition states of a closely related redox-coupling reaction of N-phenoxyacetamides with alkynes, and their results indicate that additional MeOH coordination is unfavorable in free energy. Therefore, we did not take the explicit solvents into consideration in C−H and N−H activation and in the following key steps. In addition, as shown in Scheme 2, it is possible that C−H activation occurs 5395

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Figure 5. Free-energy profiles for the most favorable alkene insertion and for the oxidation addition and reductive elimination. Energies are relative to Cat + 3 and are mass-balanced.

distance relatively longer. For TS6-s2 and TS6-s3, because of the more steric hindrance during the coordination of alkene, the amido ester moiety distorts greatly to accommodate the incoming alkene, and significant structural changes are found. The influence of distortion and interaction on the activation energies can be quantitatively studied by the distortion− interaction analysis (see Figure 7), and the results are listed in

favorable pathway here (Figure 5) and provide the other less favorable ones in Figures S2−S4.45 Alkene insertion has been found to be crucial for the final product stereoselectivity;46 hence, we have considered four possibilities of insertion mentioned above. As shown in Figure 5, the most favorable alkene insertion proceeds with an energy barrier of 21.8 kcal/mol (TS6 relative to 7). We next examine the structures of the four distinct transition states (Figure 6) to understand their differences in energy. In TS6, the hydrogenbonding and C−H/π interactions exist in the alkene and substrate, making them closer. In TS6-s1, however, the methyl group points toward the hinder phenyl group, and the shorter H···H distance (2.40 Å) further manifests the steric repulsion. The resulting steric hindrance makes the Rh−Calkene bond

Figure 7. Distortion−interaction analysis for the alkene insertion transition states.

Table 1. For the four transition states, the distortion energies (ΔEdist⧧) of complex 7 in TS6-s2 and TS6-s3 are much more positive than those of TS6 and TS6-s1, which is in good agreement with the observed structural deviation shown in Figure 6. In spite of the more negative interaction energies (ΔEint⧧) of TS6-s2 and TS6-s3, the final activation energies (ΔEact⧧) are relatively higher, making the insertion of alkene from the right face of 7 (see Scheme 3) unfavorable. Compared with TS6-s1, the interaction energy ΔEint⧧ between two deformed reactants (2a and 7) in TS6 is much more negative, which is consistent with the interaction differences in the two

Figure 6. Optimized structures of alkene insertion transition states, with Rh−Calkene and C−Calkene bond distances given in angstroms and their relative free energies in kilocalories per mole. Irrelevant hydrogen atoms are omitted for clarity. 5396

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moiety adopts a chair conformation (Figure 9), in which a strong steric effect is found within the 6-membered ring. Thus,

Table 1. Results of Distortion−Interaction Analysis for the Alkene Insertion Transition Statesa TS

ΔEdist⧧(2a)

ΔEdist⧧(7)

ΔEint⧧

ΔEact⧧

TS6 TS6-s1 TS6-s2 TS6-s3

24.0 18.5 23.0 18.9

32.1 30.0 42.9 41.1

−46.6 −36.2 −49.9 −41.7

9.5 12.3 16.0 18.3

a

The results are obtained at the B3LYP-D3/ECP28MDF_AVDZ/6311++G(d,p) level. The detailed values along the reaction coordinate are given in Figure S5.

transition states. Besides the stronger electrostatic interactions existing in TS6, the orbital interactions during the alkene insertion (mainly involving σRh−C and π*alkene as well as σ*Rh−C and πalkene) are also expected to be stronger, in view of the bigger orbital overlap at the relatively shorter Rh−Calkene and C−Calkene bond distances (Figure 6). Finally, the much stronger interaction between the deformed 2a and 7 in TS6 makes it the most stabilized transition state, which indicates that the (1S,2S)-3a product should be formed preferentially. It was originally proposed that the alkene insertion generated the coordinatively saturated intermediate 9′, in which the rhodium atom coordinated to the in situ formed ester group (Figure 8). However, we located another more stable

Figure 9. Optimized structures of 10, 11, TS7, and TS9, with select bond distances given in angstroms. Hydrogen atoms are omitted for clarity.

the formation of the C−N bond is not preferential. For the RhIII/RhV cycle, the Rh−N bond varies as 2.19 → 1.86 → 2.11 Å for 9 → 10 → 12, which is in accordance with the ongoing elementary steps. In summary, our calculated results indicate that the N−O bond is a strong internal oxidant and favors the RhIII/RhV catalytic cycle. The stable intermediate 12 undergoes two protonation steps to generate (1S,2S)-5a, which is converted by a ring-closing reaction to the more stable carboamination product (1S,2S)-3a (see Figure 10). For 12, the nitrogen atom is protonated by 1AdCO2H via the CMD-type transition state TS11 to form 14, and the facile protonation just crosses an energy barrier of 9.8 kcal/mol. The subsequent protonation at the carbon atom Figure 8. Optimized structure of 9 and the experimentally proposed 9′, with select bond distances given in angstroms and their relative free energies. Irrelevant hydrogen atoms are omitted for clarity.

intermediate 9, with coordination of the enol alkene fragment instead of the ester group. The resulting insertion intermediate 9 undergoes sequential oxidative addition and reductive elimination to generate 12, which is strongly exergonic by up to 54.7 kcal/mol. There have been many examples of metalmediated reactions involving the key rhodium(IV) intermediates,25,47−52 and the RhIII−RhV−RhIII mechanism is found to be more favorable than the RhIII−RhI−RhIII mechanism in a number of reactions.24,26,53,54 The same conclusion is found in the reaction studied herein. The results in Figure 5 compare the RhIII−RhV−RhIII (via TS7 and TS9) and RhIII−RhI−RhIII (via TS8 and TS10) mechanisms. Apparently, the former mechanism is both thermodynamically and kinetically feasible, while the latter one has a forbidden high barrier (up to 53.7 kcal/mol for TS8 relative to 9). Similar results are found in the other three paths listed in Scheme 3 (see Figures S2−S4). In each case, the experimentally proposed RhIII/RhI cycle crosses a much higher kinetic barrier. Now we explain the difference between the two pathways by examining the structures of the key intermediates. For 11, the forming oxazine-like heterocycle

Figure 10. Free-energy profiles for protonation. Energies are relative to Cat + 3 and are mass-balanced. 5397

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Figure 11. Free-energy profiles for the whole catalytic cycle of cyclopropanation. Energies are relative to Cat + 1a and are mass-balanced.

requires a relatively higher energy of 14.7 kcal/mol (TS13 relative to 14). Alternatively, protonation could occur first for the carbon atom (see the blue path), while this route requires a much higher energy (25.9 kcal/mol for TS12 relative to 12) and thus is kinetically unfavorable. The same trend is found in the other three paths (see Figures S6−S8). In addition, the energy barriers for the second protonation step in paths s1−s3 (see the red paths in Figures S6−S8) are 19.1, 23.4, and 23.3 kcal/mol, respectively, which are all higher than the barrier of TS13. In summary, starting from intermediate 7, the route affording product (1S,2S)-3a prevails in the following key elementary steps. 3.2. Mechanism of Cyclopropanation. The free-energy profiles for the whole catalytic cycle of cyclopropanation are displayed in Figure 11, along with the structures of stationary points. The C−H activation spans an energy barrier of 31.0 kcal/mol and generates a highly unstable intermediate 19. This step is endergonic by 18.1 kcal/mol (19 relative to 18), and the following coordination of alkene 2a further raises the energy of the system. The subsequent alkene insertion transition state TS15 is 0.8 kcal/mol higher than the C−H activation transition state TS14, which can be mainly attributed to the higher energy of 19, in view of the alkene insertion only requiring a barrier of 13.7 kcal/mol (TS15 relative to 19).55 Analogous to the results in Figure 5, the alkene insertion into 19 is an exergonic process with an energy release of 14.8 kcal/mol. From 21, sequential oxidative addition and reductive elimination occur to generate the final cyclopropanation product 4a, which is exergonic by up to 57.6 kcal/mol. A remarkable difference with the results in Figure 5 is the much higher barrier of the oxidative addition step, which is up to 27.8 kcal/mol (TS17 relative to 21). From 23, we also considered the C−N reductive elimination (via

TS18′; see Figure S10) that would result in 3a, whereas TS18′ is 14.0 kcal/mol higher than TS18. We have also considered another path that begins with reductive elimination, followed by oxidative addition. Again, the RhIII/RhI catalytic cycle (the blue path) is highly unfavorable, with significantly higher transition state TS19. In addition, the originally proposed mechanism involving β-hydrogen elimination is impossible, considering the much higher transition state TS19′ and the rhodium hydride intermediate 24′. The experimentally observed deuterium extrusion in the deuterium-labeling experiments might be due to the enolization of intermediate 23. In summary, the most favorable pathway of the cyclopropanation reaction is featured with three successive transition states (TS14, TS15, and TS17) that are comparable in energy. Experimentally, the cyclopropane 4a is favored over the carboamination product 3a in the TFE solvent. As listed in Table 2, the higher transition states along the cyclopropanation Table 2. Relative Free Energies (in kcal/mol) of Key Transition States along the Cyclopropanation in TFE and MeOH Solvents MeOH (ε = 32.6) TFE (ε = 26.7)

TS14

TS15

TS17

29.9 26.8

30.7 28.9

30.0 25.7

are found to be stabilized in a less polar solvent. In other words, our results reveal a dual role of the MeOH solvent on the chemoselectivity; that is, cyclopropanation is inhibited in the more polar MeOH, and the reaction of MeOH with substrate 1a initiates a more favorable carboamination pathway. 5398

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Figure 12. Free-energy profiles for reductive elimination starting from 10. Energies are relative to Cat + 3 and are mass-balanced.

3.3. Origins for Chemoselectivity. The low barrier of C− N reductive elimination in the carboamination reaction (see TS9 in Figure 5) intrigues us to explore the possibility of C−C reductive elimination from intermediate 10, which would directly furnish the cyclopropane 4a. Interestingly, the barrier of C−C bond formation is also considerably low, only 2.1 kcal/ mol higher than the C−N bond formation (Figure 12).56 The energy difference between TS9 and TS21 gives a calculated ratio of 3a:4a = 34.6:1, which is consistent with the experimental result (3a:4a = 8.4:1). Our calculated results indicate that the reductive elimination is the chemoselectivity-controlling step in both the cyclopropanation and carboamination reactions. The steric and electronic effects on reductive elimination from palladium(II) complexes have been widely investigated.57 Experimental and theoretical studies show that reductive elimination would be faster from the complexes containing the more electrondonating ligands than those containing more electron-poor ligands. To understand the chemoselectivity of the reactions, we computed the natural population analysis (NPA) charges in the precursors 10 and 23. As listed in Table 3, in intermediate 10, the NPA charges suggest the bond formation between C1 and the more negatively charged N would be more favorable, which is in agreement with the energy difference between TS9 and TS21. Referring to 23, although N is even much more

negative, there is a strong steric effect caused by the big Nphth group during C−N reductive elimination, which leads to a higher energy barrier.

4. CONCLUSIONS We have performed a DFT mechanistic study on the rhodium(III)-catalyzed cyclopropanation/carboamination reactions between N-enoxyphthalimides and alkenes in the MeOH solvent. In contrast with the experimentally proposed mechanism of cyclopropanation, the catalytic cycle involves C−H activation, alkene insertion, oxidation addition, reductive elimination, and protodemetalation. Compared with carboamination, the cyclopropanation reaction is kinetically unfavorable, with three relatively high transition states. In MeOH, the solvent cleaves the C−N bond of the phthalimide moiety, so a new DG is formed, initiating the carboamination route. The carboamination reaction proceeds via seven sequential steps: ring opening, C−H activation, N−H activation, alkene insertion, oxidation addition, reductive elimination, and protonation. The initiating ring opening mediated by the solvent is a crucial step, and the alkene migration insertion controls the stereoselectivity. The present results show that the interactions between the alkene and the 16-electron rhodacycle drives the selectivity. The resulting alkene insertion intermediate prefers the RhIII−RhV−RhIII mechanism rather than the experimentally proposed RhIII−RhI−RhIII mechanism, and the former is both thermodynamically and kinetically more favorable. The cyclopropane product can also be generated in the catalytic cycle of carboamination, which is kinetically more favorable than the cyclopropanation pathway. In each reaction, the chemoselectivity is kinetically controlled by the reductive elimination step, in which the C−C and C−N bond formations compete with each other. Both electronic and steric effects play crucial roles. In the carboamination pathway, the electronic effect dominates, so the bond formation between the carbon and the more negative nitrogen is preferential. However, in the cyclopropanation pathway, the steric effect introduced by the big Nphth group makes the C−N bond formation difficult. Our detailed computational studies provide a deep understanding of the switch mechanism mediated by the polar MeOH solvent for rhodium(III)-catalyzed carboamina-

Table 3. NPA Charges of 10 and 23

C1 C2 N

10

23

−0.41 −0.38 −0.44

−0.31 −0.33 −0.63 5399

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Inorganic Chemistry

(12) Wu, S.-Z.; Zeng, R.; Fu, C.-L.; Yu, Y.-H.; Zhang, X.; Ma, S.-M. Rhodium-Catalyzed C−H Functionalization-Based Approach to EightMembered Lactams. Chem. Sci. 2015, 6, 2275−2285. (13) Hyster, T. K.; Ruhl, K. E.; Rovis, T. A Coupling of Benzamides and Donor/Acceptor Diazo Compounds to Form γ-Lactams via Rh(III)-Catalyzed C−H Activation. J. Am. Chem. Soc. 2013, 135, 5364−5367. (14) Huckins, J. R.; Bercot, E. A.; Thiel, O. R.; Hwang, T.; Bio, M. M. Rh(III)-Catalyzed C−H Activation and Double Directing Group Strategy for the Regioselective Synthesis of Naphthyridinones. J. Am. Chem. Soc. 2013, 135, 14492−14495. (15) Liu, B.-Q.; Song, C.; Sun, C.; Zhou, S.-G.; Zhu, J. Rhodium(III)Catalyzed Indole Synthesis Using N−N Bond as an Internal Oxidant. J. Am. Chem. Soc. 2013, 135, 16625−16631. (16) Zhang, Z.-X.; Jiang, H.; Huang, Y. Ruthenium-Catalyzed RedoxNeutral C−H Activation via N−N Cleavage: Synthesis of NSubstituted Indoles. Org. Lett. 2014, 16, 5976−5979. (17) Zhao, D.-B.; Shi, Z.-Z.; Glorius, F. Indole Synthesis by Rhodium(III)-Catalyzed Hydrazine-Directed C−H Activation: Redox-Neutral and Traceless by N−N Bond Cleavage. Angew. Chem., Int. Ed. 2013, 52, 12426−12429. (18) Rakshit, S.; Grohmann, C.; Besset, T.; Glorius, F. Rh(III)Catalyzed Directed C−H Olefination Using an Oxidizing Directing Group: Mild, Efficient, and Versatile. J. Am. Chem. Soc. 2011, 133, 2350−2353. (19) Guimond, N.; Gorelsky, S. I.; Fagnou, K. Rhodium(III)Catalyzed Heterocycle Synthesis Using an Internal Oxidant: Improved Reactivity and Mechanistic Studies. J. Am. Chem. Soc. 2011, 133, 6449−6457. (20) Shen, Y.-Y.; Liu, G.-X.; Zhou, Z.; Lu, X.-Y. Rhodium(III)Catalyzed C−H Olefination for the Synthesis of ortho-Alkenyl Phenols Using an Oxidizing Directing Group. Org. Lett. 2013, 15, 3366−3369. (21) Liu, G.-X.; Shen, Y.-Y.; Zhou, Z.; Lu, X.-Y. Rhodium(III)Catalyzed Redox-Neutral Coupling of N-Phenoxyacetamides and Alkynes with Tunable Selectivity. Angew. Chem., Int. Ed. 2013, 52, 6033−6037. (22) Piou, T.; Rovis, T. Rh(III)-Catalyzed Cyclopropanation Initiated by C−H Activation: Ligand Development Enables a Diastereoselective [2 + 1] Annulation of N-Enoxyphthalimides and Alkenes. J. Am. Chem. Soc. 2014, 136, 11292−11295. (23) Piou, T.; Rovis, T. Rhodium-Catalysed syn-Carboamination of Alkenes via a Transient Directing Group. Nature 2015, 527, 86−90. (24) Chen, W.-J.; Lin, Z.-Y. Rhodium(III)-Catalyzed HydrazineDirected C−H Activation for Indole Synthesis: Mechanism and Role of Internal Oxidant Probed by DFT Studies. Organometallics 2015, 34, 309−318. (25) Xu, L.; Zhu, Q.; Huang, G.-P.; Cheng, B.; Xia, Y.-Z. Computational Elucidation of the Internal Oxidant-Controlled Reaction Pathways in Rh(III)-Catalyzed Aromatic C−H Functionalization. J. Org. Chem. 2012, 77, 3017−3024. (26) Yang, Y.-F.; Houk, K. N.; Wu, Y.-D. Computational Exploration of RhIII/RhV and RhIII/RhI Catalysis in Rhodium(III)-Catalyzed C−H Activation Reactions of N-Phenoxyacetamides with Alkynes. J. Am. Chem. Soc. 2016, 138, 6861−6868. (27) Becke, A. D. Density-Functional Thermochemistry. III. The Role of Exact Exchange. J. Chem. Phys. 1993, 98, 5648−5652. (28) Lee, C.; Yang, W.; Parr, R. G. Development of the Colle-Salvetti Correlation-Energy Formula into a Functional of the Electron Density. Phys. Rev. B: Condens. Matter Mater. Phys. 1988, 37, 785−789. (29) Marenich, A. V.; Cramer, C. J.; Truhlar, D. G. Universal Solvation Model Based on Solute Electron Density and on a Continuum Model of the Solvent Defined by the Bulk Dielectric Constant and Atomic Surface Tensions. J. Phys. Chem. B 2009, 113, 6378−6396. (30) Figgen, D.; Peterson, K. A.; Dolg, M.; Stoll, H. EnergyConsistent Pseudopotentials and Correlation Consistent Basis Sets for the 5d Elements Hf−Pt. J. Chem. Phys. 2009, 130, 164108. (31) Peterson, K. A.; Figgen, D.; Dolg, M.; Stoll, H. EnergyConsistent Relativistic Pseudopotentials and Correlation Consistent

tion/cyclopropanation reactions between N-enoxyphthalimides and alkenes.

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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.inorgchem.7b00450. Additional computational results, energies, and Cartesian coordinates of all optimized structures (PDF)

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AUTHOR INFORMATION

Corresponding Authors

*E-mail: [email protected] (J.-B.L.). *E-mail: [email protected] (D.-Z.C.). ORCID

Jian-Biao Liu: 0000-0002-2550-3355 Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was financially supported by the National Natural Science Foundation of China (Grant 21375082).

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REFERENCES

(1) Huang, Z.-X.; Lim, H.-N.; Mo, F.-Y.; Young, M. C.; Dong, G.-B. Transition Metal-Catalyzed Ketone-Directed or Mediated C−H Functionalization. Chem. Soc. Rev. 2015, 44, 7764−7786. (2) Balcells, D.; Clot, E.; Eisenstein, O. C−H Bond Activation in Transition Metal Species from a Computational Perspective. Chem. Rev. 2010, 110, 749−823. (3) Sperger, T.; Sanhueza, I. A.; Kalvet, I.; Schoenebeck, F. Computational Studies of Synthetically Relevant Homogeneous Organometallic Catalysis Involving Ni, Pd, Ir, and Rh: an Overview of Commonly Employed DFT Methods and Mechanistic Insights. Chem. Rev. 2015, 115, 9532−9586. (4) Chen, Z.-K.; Wang, B.-J.; Zhang, J.-T.; Yu, W.-L.; Liu, Z.-X.; Zhang, Y.-H. Transition Metal-Catalyzed C−H Bond Functionalizations by the Use of Diverse Directing Groups. Org. Chem. Front. 2015, 2, 1107−1295. (5) Gensch, T.; Hopkinson, M. N.; Glorius, F.; Wencel-Delord, J. Mild Metal-Catalyzed C−H Activation: Examples and Concepts. Chem. Soc. Rev. 2016, 45, 2900−2936. (6) Wu, J.-L.; Cui, X.-L.; Chen, L.-M.; Jiang, G.-J.; Wu, Y.-J. Palladium-Catalyzed Alkenylation of Quinoline-N-oxides via C−H Activation under External-Oxidant-Free Conditions. J. Am. Chem. Soc. 2009, 131, 13888−13889. (7) Tan, Y.-C.; Hartwig, J. F. Palladium-Catalyzed Amination of Aromatic C−H Bonds with Oxime Esters. J. Am. Chem. Soc. 2010, 132, 3676−3677. (8) Ng, K.; Chan, A. S. C.; Yu, W. Pd-Catalyzed Intermolecular orthoC−H Amidation of Anilides by N-Nosyloxycarbamate. J. Am. Chem. Soc. 2010, 132, 12862−12864. (9) Guimond, N.; Gouliaras, C.; Fagnou, K. Rhodium(III)-Catalyzed Isoquinolone Synthesis: the N−O Bond as a Handle for C−N Bond Formation and Catalyst Turnover. J. Am. Chem. Soc. 2010, 132, 6908− 6909. (10) Huang, H.-W.; Ji, X.-C.; Wu, W.-Q.; Jiang, H.-F. Transition Metal-Catalyzed C−H Functionalization of N-oxyenamine Internal Oxidants. Chem. Soc. Rev. 2015, 44, 1155−1171. (11) Patureau, F. W.; Glorius, F. Oxidizing Directing Groups Enable Efficient and Innovative C−H Activation Reactions. Angew. Chem., Int. Ed. 2011, 50, 1977−1979. 5400

DOI: 10.1021/acs.inorgchem.7b00450 Inorg. Chem. 2017, 56, 5392−5401

Article

Inorganic Chemistry Basis Sets for the 4d Elements Y−Pd. J. Chem. Phys. 2007, 126, 124101. (32) Grimme, S.; Antony, J.; Ehrlich, S.; Krieg, H. A Consistent and Accurate ab initio Parametrization of Density Functional Dispersion Correction (DFT-D) for the 94 Elements H−Pu. J. Chem. Phys. 2010, 132, 154104. (33) Frisch, M. J.; Trucks, G. W.; Schlegel, H. B.; Scuseria, G. E.; Robb, M. A.; Cheeseman, J. R.; Scalmani, G.; Barone, V.; Mennucci, B.; Petersson, G. A.; Nakatsuji, H.; Caricato, M.; Li, X.; Hratchian, H. P.; Izmaylov, A. F.; Bloino, J.; Zheng, G.; Sonnenberg, J. L.; Hada, M.; Ehara, M.; Toyota, K.; Fukuda, R.; Hasegawa, J.; Ishida, M.; Nakajima, T.; Honda, Y.; Kitao, O.; Nakai, H.; Vreven, T.; Montgomery, J. A., Jr.; Peralta, J. E.; Ogliaro, F.; Bearpark, M.; Heyd, J. J.; Brothers, E.; Kudin, K. N.; Staroverov, V. N.; Kobayashi, R.; Normand, J.; Raghavachari, K.; Rendell, A.; Burant, J. C.; Iyengar, S. S.; Tomasi, J.; Cossi, M.; Rega, N.; Millam, J. M.; Klene, M.; Knox, J. E.; Cross, J. B.; Bakken, V.; Adamo, C.; Jaramillo, J.; Gomperts, R.; Stratmann, R. E.; Yazyev, O.; Austin, A. J.; Cammi, R.; Pomelli, C.; Ochterski, J. W.; Martin, R. L.; Morokuma, K.; Zakrzewski, V. G.; Voth, G. A.; Salvador, P.; Dannenberg, J. J.; Dapprich, S.; Daniels, A. D.; Farkas; Foresman, J. B.; Ortiz, J. V.; Cioslowski, J.; Fox, D. J. Gaussian 09, revision D.01; Gaussian, Inc.: Wallingford, CT, 2009. (34) Glendening, E. D.; Badenhoop, J. K.; Reed, A. E.; Carpenter, J. E.; Bohmann, J. A.; Morales, C. M.; Weinhold, F. NBO 5.0 ; Theoretical Chemistry Institute, University of Wisconsin, Madison, WI, 2001. (35) Legault, C. Y. CYLview, 1.0b; Université de Sherbrooke: Sherbrooke, Canada, 2009; http://www.cylview.org. (36) For the ring-opening step, it is found that activation free energies without entropy correction are unreasonably high (the raw values of TS1 and TS2 are up to 39.5 kcal/mol), given that the reaction proceeds at room temperature. To correct the entropy contribution arising from translational motion, the free energies were corrected by using the Whitesides method. See: Mammen, M.; Shakhnovich, E. I.; Deutch, J. M.; Whitesides, G. M. Estimating the Entropic Cost of Self-Assembly of Multiparticle Hydrogen-Bonded Aggregates Based on the Cyanuric Acid·Melamine Lattice. J. Org. Chem. 1998, 63, 3821−3830. For a discussion of the difficulties of obtaining accurate free energies in solution, see: Liu, J.-B.; Sheng, X.H.; Sun, C.-Z.; Huang, F.; Chen, D.-Z. A Computational Mechanistic Study of Amidation of Quinoline N-Oxide: The Relative Stability of Amido Insertion Intermediates Determines the Regioselectivity. ACS Catal. 2016, 6, 2452−2461. (37) Song, G.-Y.; Wang, F.; Li, X.-W. C−C, C−O and C−N Bond Formation via Rhodium(III)-Catalyzed Oxidative C−H Activation. Chem. Soc. Rev. 2012, 41, 3651−3678. (38) Duan, P.-P.; Yang, Y.-F.; Ben, R.; Yan, Y.-Y.; Dai, L.; Hong, M.; Wu, Y.; Wang, D.-Q.; Zhang, X.-H.; Zhao, J. Palladium-Catalyzed Benzo[d]isoxazole Synthesis by C−H Activation/[4+ 1] Annulation. Chem. Sci. 2014, 5, 1574−1578. (39) We failed to locate the transition state TS3 in solution, and the value was thus based on the structure optimized in the gas phase. We believe that the difference is small and would not alter the final results. (40) Ess, D. H.; Houk, K. N. Distortion/Interaction Energy Control of 1,3-Dipolar Cycloaddition Reactivity. J. Am. Chem. Soc. 2007, 129, 10646−10647. (41) Gorelsky, S. I.; Ghosh, S.; Solomon, E. I. Mechanism of N2O Reduction by the μ4-S Tetranuclear CuZ Cluster of Nitrous Oxide Reductase. J. Am. Chem. Soc. 2006, 128, 278−290. (42) Gorelsky, S. I. Origins of Regioselectivity of the PalladiumCatalyzed (aromatic)C−H Bond Metalation-Deprotonation. Coord. Chem. Rev. 2013, 257, 153−164. (43) van Zeist, W. J.; Bickelhaupt, F. M. The Activation Strain Model of Chemical Reactivity. Org. Biomol. Chem. 2010, 8, 3118−3127. (44) Neufeldt, S. R.; Jiménez-Osés, G.; Huckins, J. R.; Thiel, O. R.; Houk, K. N. Pyridine N-Oxide vs Pyridine Substrates for Rh(III)Catalyzed Oxidative C−H Bond Functionalization. J. Am. Chem. Soc. 2015, 137, 9843−9854.

(45) It is worth noting that the rhodacyclic intermediate 7 is expected to be prochiral at the metal center. Rotation of the rhodacycle around the axis perpendicular to the Cp*tBu ring would generate the corresponding diastereomer. The structural difference between the insertion transition states of the diastereomers mainly lies in the spatial position of the tert-butyl group. In this paper, we only considered the most stable conformers. (46) Semakul, N.; Jackson, K. E.; Paton, R. S.; Rovis, T. Heptamethylindenyl (Ind*) Enables Diastereoselective Benzamidation of Cyclopropenes via Rh(III)-Catalyzed C−H Activation. Chem. Sci. 2017, 8, 1015−1020. (47) Brayshaw, S. K.; Sceats, E. L.; Green, J. C.; Weller, A. S. C−C σ Complexes of Rhodium. Proc. Natl. Acad. Sci. U. S. A. 2007, 104, 6921−6926. (48) Li, J.; Qiu, Z.-P. DFT Studies on the Mechanism of the Rhodium(III)-Catalyzed C−H Activation of N-Phenoxyacetamide. J. Org. Chem. 2015, 80, 10686−10693. (49) McBee, J. L.; Escalada, J.; Tilley, T. D. High Oxidation State Rhodium and Iridium Bis(silyl)dihydride Complexes Supported by a Chelating Pyridyl-Pyrrolide Ligand. J. Am. Chem. Soc. 2009, 131, 12703−12713. (50) Schröder, N.; Wencel-Delord, J.; Glorius, F. High-Yielding, Versatile, and Practical [Rh(III)Cp*]-Catalyzed Ortho Bromination and Iodination of Arenes. J. Am. Chem. Soc. 2012, 134, 8298−8301. (51) Park, S. H.; Kwak, J.; Shin, K.; Ryu, J.; Park, Y.; Chang, S. Mechanistic Studies of the Rhodium-Catalyzed Direct C−H Amination Reaction Using Azides as the Nitrogen Source. J. Am. Chem. Soc. 2014, 136, 2492−2502. (52) Vyboishchikov, S. F.; Nikonov, G. I. Rhodium Silyl Hydrides in Oxidation State + 5: Classical or Nonclassical? Organometallics 2007, 26, 4160−4169. (53) Xing, Y.-Y.; Liu, J.-B.; Tian, Y.-Y.; Sun, C.-Z.; Huang, F.; Chen, D.-Z. Computational Mechanistic Study of Redox-Neutral Rh(III)Catalyzed C−H Activation Reactions of Arylnitrones with Alkynes: Role of Noncovalent Interactions in Controlling Selectivity. J. Phys. Chem. A 2016, 120, 9151−9158. (54) Zhou, T.; Guo, W.; Xia, Y.-Z. RhV-Nitrenoid as a Key Intermediate in RhIII-Catalyzed Heterocyclization by C−H Activation: A Computational Perspective on the Cycloaddition of Benzamide and Diazo Compounds. Chem. - Eur. J. 2015, 21, 9209−9218. (55) The results of the other three possible migratory insertion patterns are given in Figure S9. (56) The detailed results of the red path are given in Figure S11. (57) Hartwig, J. F. Electronic Effects on Reductive Elimination to Form Carbon−Carbon and Carbon−Heteroatom Bonds from Palladium(II) Complexes. Inorg. Chem. 2007, 46, 1936−1947.

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