CâH Activation versus Ring Opening and Inner- versus Outer-Sphere

Article Cite This: J. Org. Chem. XXXX, XXX, XXX−XXX

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C−H Activation versus Ring Opening and Inner- versus OuterSphere Concerted Metalation−Deprotonation in Rh(III)-Catalyzed Oxidative Coupling of Oxime Ether and Cyclopropanol: A Density Functional Theory Study Ran Meng, Siwei Bi,* Yuan-Ye Jiang, and Yuxia Liu

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School of Chemistry and Chemical Engineering, Qufu Normal University, Qufu 273165, People’s Republic of China S Supporting Information *

ABSTRACT: The Rh(III)-catalyzed oxidative coupling of oxime ether (S1) and cyclopropanol (S2) with Cu(II) as the oxidant features the combination of C−H activation and strained ring opening. The sequential order of C−H activation versus ring opening was investigated with the aid of density functional theory calculations. Prior ring opening due to the release of ring strain is found to be favored over the prior C−H activation. For the prior ring-opening mechanisms, the outer-sphere concerted metalation− deprotonation (CMD) mechanism in C−H bond activation is energetically favored. The outer-sphere CMD mechanism proposed in this work favors solvent effects and affords the N→Rh binding that allows a directing role of the Schiff base group. In conclusion, the reaction was suggested to undergo prior ring opening followed by C−H activation via the outer-sphere CMD mechanism. similar mechanism involving β-H elimination as shown in Scheme 2.6,7 Theoretical and computational studies on TM-catalyzed oxidative coupling have been extensively reported.8 However, to our knowledge, the oxidative couplings specifically involving the combination of C−H activation and three-membered ring opening (Scheme 1b,c) have not been probed theoretically and computationally. Experiments demonstrated that the intermolecular coupling reactions with Cu(II) as the external oxidant (eqs 5 and 6) have high efficiency, high selectivity, and a broad substrate scope and proceed under mild conditions. Stimulated by advantages and novelty of this reaction, we are interested in exploring reaction mechanisms with eq 5 as the target reaction because illumination of mechanistic issues is vital for deeply understanding the reaction nature and the origin of relevant issues such as selectivity and so on. We also anticipate this study would provide important references for designing relevant reactions.

1. INTRODUCTION Transition metal (TM)-catalyzed C−C coupling reactions extremely contribute to organic molecular diversity and complexity, which can be classified into electroneutral, oxidative, and reductive couplings. TM-catalyzed electroneutral cross-coupling reactions have been extensively and deeply investigated both experimentally and theoretically, which were recognized by the Nobel Prize in Chemistry 2010 “for palladium-catalyzed cross-couplings in organic synthesis”.1 TM-catalyzed oxidative coupling reactions have currently been a hot topic in organic and pharmaceutical syntheses to which dramatically increasing attention has been given.2 As compared to electroneutral coupling, the oxidative coupling has such advantages as having no need for substrate prefunctionalization and being relatively easy to deal with experimentally.2 From the perspective of oxidants used, the oxidative coupling can be divided into two categories, one with internal oxidants and the other with external oxidants. For the TM-catalyzed oxidative coupling reactions that involve combination of C−H activation and three-membered ring opening, research mainly focused on the redox-neutral reactions. As exemplified in Scheme 1a, eq 1 features the arene substrate as the oxidant,3 and eq 2 highlights the threemembered ring substrate as the oxidant.4 Only one intramolecular oxidative reaction with an external oxidant was reported (Scheme 1b).5 More specifically, for eq 4 containing a combination of C−H activation and strained ring opening, only two reactions were reported recently by Li and coworkers (eqs 56 and 67). Both reactions were proposed to undergo a © XXXX American Chemical Society

2. COMPUTATIONAL METHODS All of the calculations were performed with the Gaussian 09 software package.9 The geometry optimizations were conducted using the M0610 functional with the Lanl2dz11 basis set for rhodium and 631G(d,p)12 basis set for the other atoms. In addition, polarization functions were added for Rh (ζf = 1.350).13 To verify all stationary Received: July 10, 2019

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DOI: 10.1021/acs.joc.9b01868 J. Org. Chem. XXXX, XXX, XXX−XXX

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The Journal of Organic Chemistry

Solvent effects were considered by performing single-point selfconsistent reaction field (SCRF) calculations using the SMD15 model. The solvent used in calculations was methanol, the same as that used in the experiment. The single-point energies based on the gas-phase optimized structures were computed with the M06 functional. Also, the polarization functions were added for Rh (ζf = 1.350). Dispersion corrections at the same level of theory were added by using the D316 version of Grimme’s dispersion with Becke−Johnson damping to give a better description of long-range weak interactions. The M06−D3 method has been employed extensively in the study of TM-catalyzed organic reactions.17 The higher level SDD18 basis set was used for rhodium and the 6-311++G(d,p) basis set for the other atoms. An ultrafine grid19 was assigned to the DFT calculations to avoid possible integration grid errors. The Gibbs free energy change in solution was first obtained by adding the Gibbs free energy correction in the gas phase to the electronic energy calculated in the solution, and then we made further corrections as described below. Heat of 1.9 kcal/mol was added to all species20 to account for the standard state change from 1 atm to 1 mol/L at 298.15 K. Meanwhile, we employed the THERMO program21 developed by Fang’s group to calculate the solution translational entropy to give corrections for the calculated solutionphase Gibbs free energies involving two-to-one (or one-to-two) transformations. It is worth noting that the method, optimization in the gas phase followed by single-point calculation, was extensively utilized and is still in use currently since it well simulated the mechanistic issues of TM-catalyzed organic reactions.22

Scheme 1. Combination of C−H Activation with ThreeMembered Ring Opening

3. RESULTS AND DISCUSSION The reaction mechanism for eqs 5 and 6 has been proposed by Li and coworkers (Scheme 2).6,7 The arene substrate undergoes C−H activation induced by Cp*Rh(OAc)2 to afford I and then proton transfer with cyclopropanol is followed to generate II. Ring opening of II delivers III from which two possible pathways were proposed. Direct C−C reductive elimination from III gives VI and the product. Alternatively, β-H elimination/olefin insertion/C−H reductive elimination from III can also generate VI and the product. Oxidation of VI by Cu(II) regenerates Cp*Rh(OAc)2. In this work, eq 5 in Scheme 1d was taken as the target reaction from which we attempted to explore the reaction mechanisms. We first examine the mechanisms put forward experimentally and then shed light on the insights from alternative mechanisms proposed in this study. 3.1. Mechanism with Prior C−H Activation Followed by Ring Opening. 3.1.1. C−H Activation with Cp*Rh(OAc)2 as an Active Catalyst: Path a. We first examine the mechanism shown in Scheme 2, which features the prior C− H bond activation and then the three-membered ring opening. Figure 1 shows the Gibbs free energy profile calculated for reaction 5, referred to as path a. Previous reports demonstrated that the active catalyst Cp*Rh(OAc)2 (K) could be derived from [Cp*RhCl2]2 and CsOAc.23 Step (K + S1 → 2a) enables the C−H bond activation via an inner-sphere concerted metalation−deprotonation (CMD) mechanism with an activation barrier of 20.7 kcal/mol. Note that no binding of N→Rh in TS1a occurs with N···Rh being 3.5 Å. Addition of cyclopropanol (S2) allows proton transfer giving 4a that then triggers the three-membered ring opening via TS4a, generating 5a. Coordination isomerism allows the formation of the more stable complex (6a), indicating that N→Rh is remarkably stronger than O→Rh. Finally, C−C reductive elimination (6a→7a) takes place via TS6a with a barrier of 19.1 kcal/mol. The more stable product-coordinated 8a is formed with π(CO)→Rh in place of the agostic C−H···Rh interaction.

Scheme 2. Proposed Mechanism for Reaction Eqs 5 and 6 by Li and Coworkers

points as either the minima (zero imaginary frequencies) or first-order saddle points (one imaginary frequency), vibration frequencies were obtained at the same level of theory as that for structural optimizations. Calculations of intrinsic reaction coordinates (IRC)14 have also been conducted to confirm the connection between transition structures and minima. B


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Figure 1. Calculated Gibbs energy profile for reaction eq 5 with Cp*Rh(OAc)2 as the active catalyst shown in Scheme 2 (path a). The bond distances are given in angstrom (Å).

Figure 2. Calculated Gibbs energy profile for reaction eq 5 with Cp*Rh(OAc)+ as the active catalyst (path b). The bond distances are given in angstrom (Å).

conditions (room temperature). (2) Calculated results suggest that the ring opening process is rate-determining, which is contrary to the experimental observations that the C−H bond activation is rate-determining. (3) The nonbonding of N and Rh in TS1a cannot render the Schiff base group playing a directing function in the C−H activation step. Li and coworkers also proposed an alternative reaction pathway that undergoes β-H elimination from 6a.6 Since 6a is not available due to the high activation barrier associated with the ring opening, the mechanism containing β-H elimination from 6a can be excluded although the reversible β-H

Product P is released from 8a, and the ensuing Cp*Rh(I) species is oxidized by the external oxidant Cu(OAc)2 to regenerate Cp*Rh(OAc)2, completing the catalytic cycle. Path a has three features: (i) the reaction undergoes prior C−H activation and then ring opening, (ii) the prior C−H activation proceeds via an inner-sphere CMD mechanism, and (iii) neutral Cp*Rh(OAc)2 serves as the active catalyst. This pathway was concluded to be infeasible, which can be accounted for by three reasons. (1) The calculated overall barrier (28.8 kcal/mol from the starting materials to TS4a) is too high and thus inconsistent with the experimental C


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Figure 3. Calculated Gibbs energy profile featuring ring opening followed by C−H activation with Cp*Rh(OAc)2 as the active catalyst (path c). The bond distances are given in angstrom (Å).

whether the neutral Cp*Rh(OAc)2 or the cationic Cp*Rh(OAc)+ acts as the active catalyst, both pathways are unreasonable because the ring opening rather than the C−H activation is calculated to be rate-determining, which is inconsistent with the experimental observations. Alternative mechanisms still need to be explored. Therefore, we turn our attention to the mechanisms with prior ring opening followed by C−H activation. 3.2. Mechanisms with Prior Ring Opening Followed by C−H Activation. 3.2.1. C−H Activation via the InnerSphere CMD Mechanism: Path c. In this section, the mechanism with prior ring opening followed by C−H activation via the inner-sphere CMD mechanism was examined. As shown in Figure 3, coordination of cyclopropanol S2 to K gives 1c, and proton transfer allowed the formation of 2c. Ring opening occurs via TS2c giving 3c. A coordination isomerism takes place from 3c to 4c with the carbonyl coordination replaced by the acetate oxygen. 4c then engages the arene C−H activation via TS4c through the innersphere CMD mechanism generating 5c. Adduct 4c′ formed by 4c and S1 was not located due to high steric repulsion. Note that the nitrogen atom cannot coordinate to Rh in TS4c because 5c is formed as an 18e species without the nitrogen coordination. Coordination isomerism leads to N→Rh bonding giving 6a, again indicating that the N→Rh bonding is much stronger than the keto O→Rh bonding. The following steps from 6a are the same as those shown in Figure 1. The rate-determining step is found to be the arene C−H activation (4c via TS4c→5c), which is consistent with the experimental observations. However, the high activation barrier of 31.2 kcal/mol does not agree with experimental conditions (room temperature). The disadvantage of this mechanism is the nonbinding of nitrogen with Rh, implying that the nitrogen atom cannot play a directing function. In other words, the meta- and para-C−H activations might also occur. For this consideration, the meta- and para-position C− H activations were calculated. The relative energies of the

elimination was confirmed by the H/D exchange experiment. The detailed Gibbs free energy profile calculated for the pathway from 6a via β-H elimination/olefin insertion/C−H reductive elimination to 8a is given in the Supporting Information (Figure S1). These calculated results imply that a more reasonable reaction mechanism is expected to rationalize the reversible β-H elimination. 3.1.2. C-H Activation with Cp*Rh(OAc)+ as the Active Catalyst: Path b. Considering that the above mechanisms with neutral Cp*Rh(OAc)2 as the active catalyst are not viable, cationic Cp*Rh(OAc)+ as the active catalyst is examined herein (referred to as the ionic mechanism). Both the neutral24 and ionic25 mechanisms were extensively reported. As shown in Figure 2, dissociation of an acetate from the neutral catalyst K followed by addition of S1 gives the cationic intermediate 2b. Note that the acetate dissociation is thermodynamically accessible in methanol but is not in nonpolar solvents such as heptane, toluene, and so on. The thermodynamic data in different solvents, associated with the acetate dissociation from Cp*Rh(OAc)2, have been calculated using the SMD model (Table S1). 2b triggers the C−H bond activation via the innersphere CMD mechanism, giving the rhodacyclic intermediate 3b. Note that, differing from TS1a in the neutral mechanism, the N→Rh coordination is present in TS2b, enabling the Schiff base group to play a directing role. Substitution of the resulting acetic acid with S2 affords 5b, which then undergoes a barrierless proton transfer with acetate giving the neutral species 4a. 4a is lower in electronic energy but higher in Gibbs free energy than 5b as a result of entropy decrease. The whole process from K + S1 + S2 → 4a is calculated to be kinetically available. However, this mechanism is still not viable because the subsequent steps from 4a shown in Figure 1 are kinetically unreachable under the experimental conditions. Therefore, the ionic mechanism with Cp*Rh(OAc)+ as the active catalyst can be ruled out. Paths a and b highlight the prior arene C−H bond activation and then the three-membered ring opening. In summary, D


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The Journal of Organic Chemistry transition states associated with the ortho-, meta-, and paraposition C−H activations by the CMD mechanism are given in Scheme 3. The meta- and para-position C−H activations are

mentioned in Figure 3, the C−H activation was proposed through the outer-sphere CMD mechanism. It has been confirmed that a combination of 3d with an acetate cannot form 4c′, and the path shown in Figure 3 is impracticable. Thus, we proposed that 3d would undergo an outer-sphere CMD mechanism for the C−H activation. As shown in TS4d, the outer-sphere acetate anion acts as a Lewis base to abstract the arene C−H hydrogen atom, giving the cyclorhodated intermediate 5d. The formation of the H-bonds of acetic acid with the carbonyl moiety gives the more stable intermediate 6a. The following steps from 6a to 8a are the same as those shown in Figure 1. Examining the whole energy profile, one can see that this reaction mechanism becomes more reasonable compared to those studied above. (1) The overall barrier for the prior ring opening process (20.8 kcal/mol, 1c→TS2c) is lower than that for the later C−H activation process (24.6 kcal/mol, 3c→ TS4d). Thus the C−H activation process is suggested to be rate-determining, which is consistent with the experimental determinations. (2) The rate-determining barrier 24.6 kcal/ mol calculated for the C−H activation process is in agreement with experimental conditions (room temperature). (3) Strong N→Rh binding in TS4d enables the Schiff base group to display a directing function, leading to the ortho-C−H activation made accessible. Clearly, the meta- and para-C−H activation cannot be available due to the short arm of the Schiff base group. Therefore, we suggested that the reaction undergoes such a mechanism involving prior three-membered ring opening and then the arene C−H bond activation via the outer-sphere CMD process. The reversible β-H elimination demonstrated by H/D exchange experiment can be rationalized from the calculation results. As shown in Scheme 4, 3d can easily undergo β-H elimination to afford 6d with a barrier of 4.7 kcal/mol. It should be noted that the β-H elimination is infeasible from III to IV since the mechanism proposed in Scheme 2 is

Scheme 3. Calculated Transition States Related to Ortho-, Meta-, and Para-Position C−H Activation by the CMD Mechanisma

a

Gibbs free energies and electronic energies in parentheses are given in kcal/mol.

unexpectedly found to be favored over the ortho-position C− H activation. Clearly, these results are inconsistent with the experimental fact that only the ortho-position C−H bond was selectively activated. Therefore, this proposed mechanism is not suitable for the reaction. 3.3. C−H Activation via the Outer-Sphere CMD Mechanism: Path d. In this section, we focus our attention on the prior ring-opening pathway with the later C−H activation from 3c via the outer-sphere CMD mechanism. The detailed calculated Gibbs free energy profile is shown in Figure 4. The steps from K + S2 to 3c that contain the prior threemembered ring opening process are same as those shown in Figure 3. Instead of undergoing the C−H activation via the inner-sphere CMD mechanism from 3c as shown in Figure 3, 3c dissociates an acetate and then accepts a cyclopropanol molecule, giving the cationic species 2d. Cleavage of the Rhcarbonyl bond affords a 16e species, 3d, which is ready for the subsequent C−H activation via the CMD process. As

Figure 4. Calculated Gibbs free energy profile with the C−H activation via the outer-sphere CMD process. The bond distances are given in angstrom (Å). E


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The Journal of Organic Chemistry Scheme 4. H/D Exchange through β-H Elimination

Figure 5. Calculated Gibbs free energy profile for the formation of 1d with Cp*Rh(OAc)+ as the key intermediate. The bond distances are given in angstrom (Å).

Scheme 5. Geometries of Three Rate-Determining Transition States for Paths a−d with Key Dataa

a

The bond distances are given in Å, and the relative energies are in kcal/mol.

inoperative (Figure S1). 6d undergoes H/D exchange to afford 6d-D. Considering that the β-H elimination transition state F


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Figure 6. Summary on the Gibbs free energy profiles of paths a−d together with their mechanistic features inside boxes. The relative Gibbs free energies are given in kcal/mol.

TS3d is lower in energy than the rate-determining TS4d, the β-H elimination is predicted to be reversible in accordance with the experimental observations. We also examined the possibility for formation of 1d with Cp*Rh(OAc)+ as the key intermediate (Figure 5). The catalyst K dissociates an acetate anion and then coordinates with S2 giving the cationic species 1e. The proton transfer is followed to give 3e that then undergoes ring opening to generate 4e. Dissociation of the resulting acetic acid molecule from 4e gives 1d. Considering that TS3e is just slightly less stable than TS2c by 0.8 kcal/mol, the two paths for forming 1d in Figures 4 and 5 may be competitive.

H activation. As seen from Figure 1, the C−H activation process from the starting point to 4a is thermodynamically unfavorable by 2.8 kcal/mol from which the ring-opening step via TS4a follows. TS4c is a C−H activation transition state with the prior ring-opening process. As seen from Figure 3, the ring-opening process from the starting point to 4c is thermodynamically favored by 8.7 kcal/mol from which the C−H activation step via TS4c follows. Thus, 4c is 11.5 kcal/ mol lower in energy than 4a. As a result, TS4c becomes lower in energy than TS4a even though the barrier from 4c to TS4c (31.2 kcal/mol) is higher than the one from 4a to TS4a (26.0 kcal/mol). In summary, release of ring strain from the prior ring-opening process is the major reason leading to TS4c being more stable than TS4a. Second, relative stability of TS4c and TS4d was analyzed. Both transition states feature the prior ring-opening process and differ from the CMD modes for the later C−H activation. TS4c and TS4d show similar stability in the gas phase with ΔΔE = 0.3 but differ in solution with ΔΔE = 5.9 and ΔΔG = 6.6 kcal/mol. The distinct stability of both transition states in solution is clearly caused by the solvent effect. TS4d involves partial charge separation as indicated by the dipole moments of 4d (10.11 D), TS4d (6.76 D), and 5d (2.58 D) where 4d involves a complete charge separation and 5d is a neutral species. Therefore, TS4d has a larger solvent effect compared to TS4c leading to the former being more stable. Corresponding data with structures of the three transition states optimized in solution were also computed. In the gas phase, ΔΔE = −0.7, while in the solution, ΔΔE = 5.0 and ΔΔG = 5.5 kcal/mol (Scheme S1). These calculation results also support the above arguments. To give a clearer comparison for the four pathways, we collect the four Gibbs free energy profiles into one diagram (Figure 6). The pathways with prior C−H activation followed by ring-opening are put in the right-side of the figure (paths a and b), and those with prior ring-opening followed by C−H

4. DISCUSSION ON THE PROPOSED REACTION MECHANISMS As described above, two types of reaction mechanisms were investigated, one features prior C−H activation, and the other highlights prior ring opening. For the mechanism with prior C−H activation, using Cp*Rh(OAc)2 and Cp*Rh(OAc)+ as the active catalysts was proposed (paths a and b). For the mechanism with a prior ring-opening process, the inner- and outer-sphere CMD process was considered for the later C−H activation (paths c and d). Paths a and b go through the same rate-determining transition state TS4a, while paths c and d undergo the rate-determining transition states TS4c and TS4d, respectively. Geometries of the three transition states are given in Scheme 5. The data given in the figure are relative Gibbs free energies in the solvent, electronic energies in the solvent (in parentheses), and electronic energies in the gas phase (in square brackets). TS4d was set to be the zero reference point. Here, we explain why path d is more reasonable than paths a−c by analyzing the relative stability of the three rate-determining transition states TS4a, TS4c, and TS4d. The three transition states have the same molecular composition. First, the relative stability of TS4a and TS4c is analyzed. TS4a is a ring-opening transition state with the prior arene C− G


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The Journal of Organic Chemistry Scheme 6. Favorable Catalytic Cycle Derived from DFT Calculations (Path d)

activation are put in the left-side of the figure (paths c and d). The features of the four pathways are presented inside boxes. Note that the subsequent steps from 6a are not shown in the figure since such steps are not rate-determining. Trivial intermediates are also omitted in the figure for simplicity. It is seen that the intermediates in paths c and d are lower in energy than those in paths a and b, indicating that the prior ring-opening process is thermodynamically favored over the prior C−H activation process (starting point to 3c vs starting point to 4a). One can understand on a whole why path d is most favored over the other pathways from the features summarized in each box. In order to more clearly see the favorable reaction mechanism (path d), the catalytic cycle derived from DFT calculations was depicted in Scheme 6. Proton transfer from K + S2 delivers 2c, which then undergoes ring opening generating 3c. Thereafter, substrate oxime ether (S1) replaces the acetate yielding cationic species 2d with acetate as the counterion. 2d undergoes the outer-sphere CMD mechanism via transition state TS4d to activate the arene C−H bond, giving 6a. At the end, C−C reductive elimination from 6a obtained product-coordinated 7a. Liberation of the product affords the low-valence complex Cp*Rh that is oxidized by Cu(II) regenerating the active catalyst K, completing the catalytic cycle.

calculations. Four possible pathways were considered in this study as summarized below.

1. Paths a and b are inaccessible since the thermodynamically unfavorable prior C−H activation makes the later ring opening process rate-determining, which is inconsistent with the experimental observations where the C−H activation process is rate-determining. 2. Path c is also less favored since the Schiff base group cannot play a directing role when the inner-sphere CMD mechanism is proposed for C−H activation, and accordingly the activation barrier calculated for C−H activation is too high. 3. Path d is reasonable because (a) prior ring opening makes the intermediate before C−H activation lower in energy and (b) the charge separation caused by the outer-sphere CMD mechanism for C−H activation gives a larger solvent effect, leading to the C−H activation transition state being lower in energy. 4. This study provides important references for understanding these reactions in considering the sequence of C−H activation versus ring opening and favorability of inner- versus outer-sphere CMD mechanism in activation of the C−H bond.

5. CONCLUSIONS The Rh(III)-catalyzed oxidative coupling of oxime ether (S1) and cyclopropanol (S2) with Cu(II) as the oxidant, which features the combination of C−H activation and strained ring opening, was studied with the aid of density functional theory H


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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.joc.9b01868. β-H elimination energy barrier, key transition state energies in methanol, dissociation of Cp*Rh(OAc)2, Gibbs free energies, and Cartesian coordinates (PDF)

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. ORCID

Siwei Bi: 0000-0003-3969-7012 Yuan-Ye Jiang: 0000-0002-4763-9173 Yuxia Liu: 0000-0003-1139-8563 Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was jointly supported by the National Natural Science Foundation of China (nos. 21873055 and 21702119) and Natural Science Foundation of Shandong Province, China (no. ZR2019MB016).

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