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Mechanism of Rh(III)-catalyzed cyclopropanation using N-enoxyphthalimides and alkenes: Insights from DFT calculations
Tetrahedron 72 (2016) 8456e8462
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Mechanism of Rh(III)-catalyzed cyclopropanation using Nenoxyphthalimides and alkenes: Insights from DFT calculations Jiaojiao Deng, Xiuling Wen, Zhiping Qiu, Juan Li* Department of Chemistry, Jinan University, Huangpu Road West 601, Guangzhou, Guangdong, 510632, PR China
a r t i c l e i n f o
a b s t r a c t
Article history: Received 31 August 2016 Received in revised form 2 November 2016 Accepted 4 November 2016 Available online 11 November 2016
The Rh(III)-catalyzed cyclopropanation reaction using N-enoxyphthalimides and alkenes developed by Rovis group [J. Am. Chem. Soc. 2014, 136, 1129211295] provided an efficient method for the constructions of trans 1,2-disubstituted cyclopropanes in which an elegant control of the diastereoselectivities was achieved. In the current report we aimed at uncovering the mechanism and diastereoselectivity of the reactions using density functional theory (DFT) calculations. By comparing the energies of all possible pathways, we found that a novel mechanism involving a four-membered Rh(V) species is the energetically most favorable one. In this pathway, the four-membered Rh(V) intermediate is formed by sequential CeH activation, alkene insertion and NeO bond cleavage steps, and the final cyclopropane product is formed via an reductive elimination process. The NeO bond cleavage was found to be the diastereoselectivity-determining, which was reproduced well the experimentally observed selectivity. By analyzing using the distortion/interaction model, it was found that the distortion energy plays a main role in determining the diastereoselectivity. © 2016 Elsevier Ltd. All rights reserved.
Keywords: Density functional calculations Diastereoselectivity Cyclopropanation Rhodium CeH activation
1. Introduction Rh(III)-mediated Ce;H activation has recently emerged as a powerful and atom-economical approach for the construction of heterocyclic compounds.1 Notably, this approach is generally reliant on the presence of a suitable directing group in the substrate molecule to achieve high levels of regioselectivity and reactivity.2 However, these reactions usually require the addition of an external oxidant. Several green strategies have recently been developed for Rh-mediated CeH activation reactions involving the use of a direct oxidizing group as an internal oxidant, thereby avoiding the need for an external oxidant.3 Functional groups containing an NeO bond can be used as an internal oxidant, with the cleavage of the NeO bond usually resulting in the release of the O-linked portion.4 In contrast, studies pertaining to the release of the N-linked portion of an NeO bond are scarce.5 The cyclopropyl functional group is a core chemical structure that can be found in a broad range of biologically active agents and functional materials.6 Based on their important biological properties and unique structural characteristics, considerable research
* Corresponding author. E-mail address: [email protected] (J. Li). http://dx.doi.org/10.1016/j.tet.2016.11.017 0040-4020/© 2016 Elsevier Ltd. All rights reserved.
efforts have been directed towards the development of new methods for the synthesis of cyclopropane rings. Most of these efforts have focused on the transition metal-catalyzed cyclopropanation of alkenes with diazo reagents to prepare functionalized cyclopropanes.7 However, diazo compounds can be thermally unstable and difficult to prepare, and the development of new catalytic cyclopropanation reactions involving alternative reagents is therefore highly desired. In this regard, Rovis's group reported the first Rh(III)-catalyzed cyclopropanation of alkene substrates with N-enoxyphthalimides, which proceeded via a CeH activation pathway in high yield and diastereoselectivity with the concomitant release of phthalimide (Scheme 1).8 Considering the significance of the coupling reaction depicted in Scheme 1 and the current poor understanding of its mechanism, it is important to elucidate the detailed reaction mechanism, including the origins of the observed diastereoselectivity. The proposed mechanism by Rovis et al. for Rh(III)-catalyzed CeH activation involves Rh(III) or Rh(I) intermediates as shown in Scheme 2. C formed from the CeH activation of B will rearrange to D via a alkene insertion, which in turn undergoes cyclopropanation, providing E. Then, two distinct pathways can be possible. For path a, the b-H elimination will afford the Rh(I) species F, which would undergo a tautomeric rearrangement to form G. The final step in this process would be a NeO bond cleavage to allow for the
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Scheme 1. Rh(III)-catalyzed cyclopropanation reaction of N-enoxyphthalimides and alkenes.
Scheme 2. Experimentally postulated mechanism for Rh(III)-catalyzed cyclopropanation reaction.
regeneration of the catalyst and the concomitant release of the disubstituted trans-cyclopropane product 3 and phthalimide 4. For the alternative path b, a NeO bond cleavage in I occurs to afford the product 3 with the regeneration of the catalyst A. Despite of the above proposals, other possible mechanisms, such as NeO bond cleavage preceding cyclopropanation step to give the Rh(V) nitrene, cannot be ruled out. A Rh(III)/Rh(V) catalytic cycle has been proposed in several other reports.9 Additionally, it is important to examine which step is diastereoselectivity-determining and what factors control the diastereoselectivity of the reaction. Another important issue that needs to be addressed is the coordination site of the Rh center in some key intermediates, which will impact the energy barrier. Here we report the first computational study on the mechanism of the Rh(III)-catalyzed cyclopropanation of phenyl-N-enoxyphthalimide. Elucidation of the mechanisms will help us to understand the complicated experimental findings. 2. Computational details The molecular geometries of the complexes were optimized using DFT calculations at the M06 level.10 Frequency calculations were also performed at the same level of theory to identify all of the stationary points as minima (zero imaginary frequencies) or transition states (one imaginary frequency), as well as the free energies at 298.15 K. An IRC11 analysis was performed to confirm that all of the stationary points were smoothly connected to each other. The Rh atoms in this analysis were described using the LANL2DZ basis set, including a double-valence basis set with the Hay and Wadt
effective core potential.12 Polarization functions were added for Rh (zf ¼ 1.350).13 The 6-31G* 14 basis set was used for the other atoms. Single-point energy calculations were conducted using the polarizable continuum model (PCM)15 to evaluate the solvent effects for all of the gas phase optimized species with trifluoroethanol as the solvent. In the PCM calculations, the SDD16 and 6-311þþG** basis sets were used for Rh and all of the other atoms, respectively. We also performed single-point energy calculations using the DFT-D3 empirical dispersion correction by Grimme.17 All of the calculations were performed using the Gaussian 09 packages.18
3. Results and discussion The first step in the cyclopropanation reaction would involve the coordination of 1 to [CpiprRhOAc]þ (ipr ¼ isopropylcyclopentadienyl). The results of our DFT calculations revealed that this step would be exergonic by 5.3 kcal/mol (Scheme 3). The next step in this process would be a concerted metalation-deprotonation (CMD) reaction to give B1.19 The free energy of the CMD transition state TSB-B1 was determined to be 10.8 kcal/mol. The subsequent removal of a single molecule of neutral acetic acid and the coordination of an acetate ligand for Rh center would afford intermediate C. The CeH activation is a irreversible process, which is consistent with the deuterium labeling experiments.8 The coordination of the alkene substrate to Rh center would then lead to the formation of intermediate C1. The subsequent migratory insertion of the alkene moiety in intermediate C1 into its RheC bond would occur via transition state TSC1-D to give D. As shown in Fig. 1, the distances of RheC1 and RheC2 bond in D are
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Scheme 3. Mechanism for the transformation from A to D. The solvent- and dispersion-corrected relative free energies in the PCM model have been given in kcal/mol.
Fig. 1. Optimized structures of TSC1-D and D along with key bond lengths (in angstroms) and atomic numbering schemes. Hydrogen atoms have been omitted for clarity.
2.298 Å and 2.197 Å, respectively, which implies that the effective coordination of the p bond of the double bond to the Rh center. Intermediate D would then be converted to E via a cyclopropanation step, which would occur with an energy barrier of 11.3 kcal/mol (Fig. 2). Starting from intermediate E, there are two possible pathways involves Rh(I) or Rh(III) intermediates for the formation of the final product as proposed in Scheme 2. For path a (Fig. 2), intermediate E would undergo a facile isomerization step to give the agostic intermediate E1, followed by a concerted CMD/ reductive elimination to afford the Rh(I) species E2 via transition state TSE1-E2 with an energy barrier of 8.0 kcal/mol relative to E1 (i.e., 25.7 kcal/mol relative to D). Intermediate E2 would then release a single molecule of AcOH to give F, which would undergo an isomerization to give the isomer G. The subsequent cleavage of the OeN bond in G would lead to the oxidation of its Rh(I) center via transition state TSG-H to give the Rh(III) center in H. The coordination of a single molecule of AcOH to the Rh center would then lead to the protonation of the N atom of the phthalimide group via
transition state TSH1-H2 with the concomitant release of phthalimide 4. Finally, the protonation of the cyclopropane ring in H2 by the second AcOH molecule would lead to the formation of the disubstituted trans-cyclopropane product 3 with the regeneration of CpiprRh(OAc)2. Based on the results shown in Scheme 3 and Fig. 2, we concluded that the transition state for the concerted CMD/reductive elimination step would be the rate-determining step for path a and that the overall energy barrier for this pathway would be 25.7 kcal/mol. The concerted CMD/reductive elimination could precede the cyclopropanation step. The relative energy of the transition state for the cyclopropanation step TSD′-F (17.7 kcal/mol in Scheme S1) was found to be higher than that of TSE1-E2 (4.5 kcal/mol) in path a, which indicated that this pathway could be ruled out. After the formation of H1, it is possible that the protonation of the cyclopropane ring could occur prior to that of the N atom of the phthalimide moiety. However, the transition state TSH1-J1′ for the cyclopropanation step (Scheme S2) was 17.2 kcal/ mol higher in energy than H1, making this pathway unlikely. As shown in path b, the oxidation state of the Rh(III) center could remain unchanged throughout the entire pathway (Fig. 3). Firstly, the isomerization of E gives rise to I. The cleavage of the OeN bond in I would lead to the formation of the Rh(III) complex J via TSI-J, which would be the rate-determining transition state. Intermediate J would then dissociate to give J1 together with the final product 3. Finally, J1 would be readily protonated by acetic acid to yield 4 with the regeneration of CpiprRh(OAc)2. The overall free energy barrier for path b was calculated to be 26.4 kcal/mol, which corresponds to the energy of TSI-J relative to D. The energy barriers for paths a and b were calculated to be 25.7 and 26.4 kcal/mol, which appeared to be too high to allow for the coupling reaction to occur under the experimental conditions (21 C).8 Furthermore, the differences in the barriers for the
Fig. 2. Energy profiles calculated for the processes from intermediate D based on path a. The solvent- and dispersion-corrected relative free energies in the PCM model have been given in kcal/mol. 1 þ A is taken as the zero energy reference point.
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Fig. 3. Energy profiles calculated for the processes from intermediate E based on path b. The solvent- and dispersion-corrected relative free energies in the PCM model have been given in kcal/mol. 1 þ A is taken as the zero energy reference point.
formation of the trans and cis isomers in paths a and b were inconsistent with the observed diastereoselectivity. For paths a and b, the diastereoselectivity of the cyclopropanation reaction would be determined by the OeN bond cleavage step. Transition states TSG-H and TSG-cisH for the formation of the trans and cis isomers (H and cisH) in path a were determined to have almost identical free energy (6.1 vs 6.0 kcal/mol). This result suggests that the cyclopropanation reaction would result in a 1:1 mixture of trans and cis isomers. The 7.9 kcal/mol energy difference between transition states TSI-J and TSI-cisJ for the formation of the trans and cis isomers (J and cisJ) in path b implies that the corresponding ratio of J: cisJ is far more than 12:1 in the experimental observation.8 Based on these results, we proceeded to investigate an alternative mechanism for the cyclopropanation reaction to account for these experimental observations.8 Further calculations revealed that the cyclopropanation could proceed via a Rh(V)-mediated pathway, with the cleavage of the
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OeN bond preceding the cyclopropanation step (path c in Fig. 4). The first step in path c would be the isomerization of D to give D1, which would undergo an OeN bond cleavage step via TSD1-K to give intermediate K with an energy barrier of 19.2 kcal/mol relative to D. As shown in Fig. 5, the interatomic distances of RheN in K (2.060 Å) is shorter than that in D1 (2.385 Å), which further confirms the conversion of the RheN dative-bond to a RheN s-bond. The Rh(V) intermediate K is a seven-coordinated species with four-membered rhodacycle (see Fig. 1). The stable seven-coordinated Rh(V) complexes have been reported.20 K would then undergo an easily reductive elimination via transition state TSK-J1′ to generate intermediate J1′ with the concomitant release of the disubstituted transcyclopropane product 3. Finally, the protonation of the nitrogen atom in J2 with a single molecule of AcOH would result in the formation 4 and regenerate CpiprRh(OAc)2. Compared with paths a (25.7 kcal/mol) and b (26.4 kcal/mol), path c had the lowest overall energy barrier (19.2 kcal/mol). The results of these calculations therefore demonstrate that the relative free energies of the transition states for the cleavage of the OeN bond in path c are responsible for determining the diastereoselectivity of this cyclopropanation reaction. The energy difference of 1.8 kcal/mol between transition states TSD1-K and TSD1-cisK implied that the corresponding ratio of trans and cis isomers would be approximately 21:1, which is consistent with the experimentally observed ratio of 12:1.8 As reported by Rovis et al.,8 the Rh(III)catalyzed cyclopropanation of alkene substrates with N-enoxyphthalimides take place in low yield when other cyclopentadienyl ligands are used (Table 1). For other cyclopentadienyl ligands, we calculated the rate-determining step according to the favorable path c for the Cpipr ligand. The differences in the free energy barriers of the cleavage of the OeN bond step among the all cyclopentadienyl ligands correlate well with their yield in the experiment.8 These extra calculations further confirm that the path c is reasonable. To develop a better understanding of the observed chemoselectivity, we calculated the arene CeH activation. However, the arene CeH activation step had an energy barrier of 29.8 kcal/mol, making it much less favorable than the vinyl CeH activation step,
Fig. 4. Energy profiles calculated for the process from intermediate D based on path c. The solvent- and dispersion-corrected relative free energies in the PCM model have been given in kcal/mol. 1 þ A is taken as the zero energy reference point.
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Fig. 5. Optimized structures of D1, TSD1-K and K along with key bond lengths (in angstroms) and atomic numbering schemes. Hydrogen atoms have been omitted for clarity.
Table 1 Distortion/interaction analysis of key TS for different cyclopentadienyl ligands. Relative free energies and electronic energies are given in kcal/mol. Entry 1 2 3 4 5 6 7
Cpx *
Cp CpCF3 Cp1 Cp2 Cpt CpE Cpipr
DEact
DEdist-cat
DEdist-org
DEdist
DEint
DGact
Yield(%)
22.9 22.9 23.5 21.7 27.5 23.6 21.0
13.1 27.2 14.1 13.9 29.3 26.0 14.1
41.1 47.4 40.2 38.2 45.4 46.8 39.3
54.2 74.6 54.3 52.1 74.7 72.8 53.4
31.3 51.7 30.8 30.4 47.2 49.2 32.4
21.8 21.7 25.1 22.8 26.8 23.6 19.2
63 56 27 40 22 40 79
which had an energy barrier of 16.1 kcal/mol. The result of this calculation was also consistent with the experimentally observed chemoselectivity.8 As shown above, the results of these calculations have shown that the Rh(V) pathway would be kinetically favored over the Rh(I) and Rh(III) pathways for the Rh(III)-catalyzed cyclopropanation of ethyl acrylate with N-enoxyphthalimide. The Rh(I) and Rh(III) pathways would result in the advanced formation of unstable cyclopropane, which would lead to a pronounced increase in the overall energy demand of the subsequent processes. We used the distortion/interaction model21 to gain insight into the diastereoselectivity (Table 1). The distortion/interaction model is also called the activation-strain model,22 which has been used to elucidate the reactivities and selectivities in different reactions, especially organometallic reactions.20,21 In bimolecular reaction, the activation barrier (DEz) is dissected into distortion energy (DEdist) and interaction energy (DEint). DEdist is the energy required to distort the reactants from their equilibrium geometries to the geometries they adopt in the transitional states. DEint is the energy of interaction between the distorted fragments. Although the OeN bond cleavage step is not a bimolecular reaction, distortion/interaction model can be still applied to this unimolecular processes by using complex D as a reference, as suggested by Fern andez and Bickelhaupt.23 The distortion/interaction model has also been successfully employed on some unimolecular system recently.24 For the OeN bond cleavage step considered herein, each structure was decomposed into two fragments (the catalyst [CpiprRh(OAc)]þ and the organic moiety
(org)).24a In the two transition structures for the OeN bond cleavage as shown in Fig. 6, there is a much smaller distortion energy for TSD1-K (DEdist-cat ¼ 14.1 kcal/mol and DEdist-org ¼ 39.3 kcal/mol) compared to TSD1-cisK (DEdist-cat ¼ 22.9 kcal/mol and DEdistorg ¼ 49.1 kcal/mol, albeit with a larger interaction energy). The results indicate that the distortion energy is the major factor controlling the diastereoselectivity. The distortion/interaction model was also applied to explain the highest reactivity of Cpipr ligand among the several cyclopentadienyl ligands. The calculated distortion and interaction energies of the reactants are listed in Table 1. The low yields for CpCF3,
Fig. 6. Distortion/interaction model for the diastereoselectivity.
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Cpt and CpE ligands are clearly controlled by the much larger distortion energies of both the catalyst and organic moiety compared to the Cpipr ligand, although their interaction energies are significantly more negative. The distortion and interaction energies for Cp* and Cp1 ligands are both higher than that for Cpipr ligand. Compared to Cp2 ligand, Cpipr ligand has slightly higher DEdist, but more negative interaction energy DEint. Therefore, the Cpipr ligand tends to avoid the sterically crowded region, which is the main reason for its highest reactivity among several ligands. 4. Conclusions In conclusion, Rovis et al. recently reported the Rh(III)-catalyzed cyclopropanation of alkene substrates with N-enoxyphthalimides to give the corresponding trans-1,2-disubstituted cyclopropanes. In this study, we have successfully used DFT calculations to elucidate the mechanism and diastereoselectivity of this cyclopropanation reaction. The free energy profiles for several possible reaction pathways have been calculated and compared. The results of these calculations have shown that the generally accepted mechanisms for this reaction, including those involving the formation of Rh(I) and Rh(III) intermediates for the formation of the cyclopropane, required high activation energies and were inconsistent with the observed diastereoselectivity. Instead, we have identified a mechanism involving a four-membered ring containing a Rh(V) species as the lowest energy pathway, which would be initiated by a CeH activation reaction. The subsequent migratory insertion of the alkene substrate would result in the formation of an s-alkylrhodium(III) complex, which would undergo an OeN bond cleavage step to give a four-membered ring containing Rh(V). This catalytic cycle would then close with a CeC reductive elimination reaction, leading to the formation of the final cyclopropane product. This Rh(V) mechanism predicted a final trans/cis product ratio of 21:1, which was in reasonable agreement with the experimentally observed ratio of 12:1. Analysis of the key transition state by the distortion/interaction model indicated the distortion energy plays a key role in determining the diastereoselectivity. Acknowledgements This work was supported by the National Natural Science Foundation of China (Grant No. 21573095), the Fundamental Research Funds for the Central Universities (Grant No. 21615405) and the high-performance computing platform of Jinan University. Appendix A. Supplementary data Supplementary data related to this article can be found at http:// dx.doi.org/10.1016/j.tet.2016.11.017. References 1. Reviews on Rh-catalyzed Ce;H activation (a) Satoh T, Miura M. Chem Eur J. 2010;16:11212; €ge T, Liu F, Glorius F. Chem Soc Rev. 2011;40:4740; (b) Wencel-Delord J, Dro (c) Song G, Wang F, Li X. Chem Soc Rev. 2012;41:3651; (d) Colby DA, Bergman RG, Ellman JA. Chem Rev. 2010;110:624; € der N, Glorius F. Adv Synth Catal. 2014;356:1443; (e) Kuhl N, Schro (f) Wendlandt AE, Suess AM, Stahl SS. Angew Chem Int Ed. 2011;50:11062; (g) Cho SH, Kim JY, Kwak J, Chang S. Chem Soc Rev. 2011;40:5068; (h) Arockiam PB, Bruneau C, Dixneuf PH. Chem Rev. 2012;112:5879. 2. (a) Chatani N. Topics in Organometallic Chemistry: Directed Metallation. vol. 24. Berlin, Germany: Springer-Verlag; 2007; (b) Murai S, Kakiuchi F, Sekine S, et al. Nature. 1993;366:529; (c) Ros A, Fernandez R, Lassaletta JM. Chem Soc Rev. 2014;43:3229; (d) Zhang F, Spring DR. Chem Soc Rev. 2014;43:6906. 3. (a) Mo J, Wang L, Liu Y, Cui X. Synthesis. 2015;47:439; (b) Huang H, Ji X, Wu W, Jiang H. Chem Soc Rev. 2015;44:1155; (c) Patureau FW, Glorius F. Angew Chem Int Ed. 2011;50:1977.
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Mechanism of Rh(III)-catalyzed cyclopropanation using Nenoxyphthalimides and alkenes: Insights from DFT calculations Jiaojiao Deng, Xiuling Wen, Zhiping Qiu, Juan Li* Department of Chemistry, Jinan University, Huangpu Road West 601, Guangzhou, Guangdong, 510632, PR China
a r t i c l e i n f o
a b s t r a c t
Article history: Received 31 August 2016 Received in revised form 2 November 2016 Accepted 4 November 2016 Available online 11 November 2016
The Rh(III)-catalyzed cyclopropanation reaction using N-enoxyphthalimides and alkenes developed by Rovis group [J. Am. Chem. Soc. 2014, 136, 1129211295] provided an efficient method for the constructions of trans 1,2-disubstituted cyclopropanes in which an elegant control of the diastereoselectivities was achieved. In the current report we aimed at uncovering the mechanism and diastereoselectivity of the reactions using density functional theory (DFT) calculations. By comparing the energies of all possible pathways, we found that a novel mechanism involving a four-membered Rh(V) species is the energetically most favorable one. In this pathway, the four-membered Rh(V) intermediate is formed by sequential CeH activation, alkene insertion and NeO bond cleavage steps, and the final cyclopropane product is formed via an reductive elimination process. The NeO bond cleavage was found to be the diastereoselectivity-determining, which was reproduced well the experimentally observed selectivity. By analyzing using the distortion/interaction model, it was found that the distortion energy plays a main role in determining the diastereoselectivity. © 2016 Elsevier Ltd. All rights reserved.
Keywords: Density functional calculations Diastereoselectivity Cyclopropanation Rhodium CeH activation
1. Introduction Rh(III)-mediated Ce;H activation has recently emerged as a powerful and atom-economical approach for the construction of heterocyclic compounds.1 Notably, this approach is generally reliant on the presence of a suitable directing group in the substrate molecule to achieve high levels of regioselectivity and reactivity.2 However, these reactions usually require the addition of an external oxidant. Several green strategies have recently been developed for Rh-mediated CeH activation reactions involving the use of a direct oxidizing group as an internal oxidant, thereby avoiding the need for an external oxidant.3 Functional groups containing an NeO bond can be used as an internal oxidant, with the cleavage of the NeO bond usually resulting in the release of the O-linked portion.4 In contrast, studies pertaining to the release of the N-linked portion of an NeO bond are scarce.5 The cyclopropyl functional group is a core chemical structure that can be found in a broad range of biologically active agents and functional materials.6 Based on their important biological properties and unique structural characteristics, considerable research
* Corresponding author. E-mail address: [email protected] (J. Li). http://dx.doi.org/10.1016/j.tet.2016.11.017 0040-4020/© 2016 Elsevier Ltd. All rights reserved.
efforts have been directed towards the development of new methods for the synthesis of cyclopropane rings. Most of these efforts have focused on the transition metal-catalyzed cyclopropanation of alkenes with diazo reagents to prepare functionalized cyclopropanes.7 However, diazo compounds can be thermally unstable and difficult to prepare, and the development of new catalytic cyclopropanation reactions involving alternative reagents is therefore highly desired. In this regard, Rovis's group reported the first Rh(III)-catalyzed cyclopropanation of alkene substrates with N-enoxyphthalimides, which proceeded via a CeH activation pathway in high yield and diastereoselectivity with the concomitant release of phthalimide (Scheme 1).8 Considering the significance of the coupling reaction depicted in Scheme 1 and the current poor understanding of its mechanism, it is important to elucidate the detailed reaction mechanism, including the origins of the observed diastereoselectivity. The proposed mechanism by Rovis et al. for Rh(III)-catalyzed CeH activation involves Rh(III) or Rh(I) intermediates as shown in Scheme 2. C formed from the CeH activation of B will rearrange to D via a alkene insertion, which in turn undergoes cyclopropanation, providing E. Then, two distinct pathways can be possible. For path a, the b-H elimination will afford the Rh(I) species F, which would undergo a tautomeric rearrangement to form G. The final step in this process would be a NeO bond cleavage to allow for the
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Scheme 1. Rh(III)-catalyzed cyclopropanation reaction of N-enoxyphthalimides and alkenes.
Scheme 2. Experimentally postulated mechanism for Rh(III)-catalyzed cyclopropanation reaction.
regeneration of the catalyst and the concomitant release of the disubstituted trans-cyclopropane product 3 and phthalimide 4. For the alternative path b, a NeO bond cleavage in I occurs to afford the product 3 with the regeneration of the catalyst A. Despite of the above proposals, other possible mechanisms, such as NeO bond cleavage preceding cyclopropanation step to give the Rh(V) nitrene, cannot be ruled out. A Rh(III)/Rh(V) catalytic cycle has been proposed in several other reports.9 Additionally, it is important to examine which step is diastereoselectivity-determining and what factors control the diastereoselectivity of the reaction. Another important issue that needs to be addressed is the coordination site of the Rh center in some key intermediates, which will impact the energy barrier. Here we report the first computational study on the mechanism of the Rh(III)-catalyzed cyclopropanation of phenyl-N-enoxyphthalimide. Elucidation of the mechanisms will help us to understand the complicated experimental findings. 2. Computational details The molecular geometries of the complexes were optimized using DFT calculations at the M06 level.10 Frequency calculations were also performed at the same level of theory to identify all of the stationary points as minima (zero imaginary frequencies) or transition states (one imaginary frequency), as well as the free energies at 298.15 K. An IRC11 analysis was performed to confirm that all of the stationary points were smoothly connected to each other. The Rh atoms in this analysis were described using the LANL2DZ basis set, including a double-valence basis set with the Hay and Wadt
effective core potential.12 Polarization functions were added for Rh (zf ¼ 1.350).13 The 6-31G* 14 basis set was used for the other atoms. Single-point energy calculations were conducted using the polarizable continuum model (PCM)15 to evaluate the solvent effects for all of the gas phase optimized species with trifluoroethanol as the solvent. In the PCM calculations, the SDD16 and 6-311þþG** basis sets were used for Rh and all of the other atoms, respectively. We also performed single-point energy calculations using the DFT-D3 empirical dispersion correction by Grimme.17 All of the calculations were performed using the Gaussian 09 packages.18
3. Results and discussion The first step in the cyclopropanation reaction would involve the coordination of 1 to [CpiprRhOAc]þ (ipr ¼ isopropylcyclopentadienyl). The results of our DFT calculations revealed that this step would be exergonic by 5.3 kcal/mol (Scheme 3). The next step in this process would be a concerted metalation-deprotonation (CMD) reaction to give B1.19 The free energy of the CMD transition state TSB-B1 was determined to be 10.8 kcal/mol. The subsequent removal of a single molecule of neutral acetic acid and the coordination of an acetate ligand for Rh center would afford intermediate C. The CeH activation is a irreversible process, which is consistent with the deuterium labeling experiments.8 The coordination of the alkene substrate to Rh center would then lead to the formation of intermediate C1. The subsequent migratory insertion of the alkene moiety in intermediate C1 into its RheC bond would occur via transition state TSC1-D to give D. As shown in Fig. 1, the distances of RheC1 and RheC2 bond in D are
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Scheme 3. Mechanism for the transformation from A to D. The solvent- and dispersion-corrected relative free energies in the PCM model have been given in kcal/mol.
Fig. 1. Optimized structures of TSC1-D and D along with key bond lengths (in angstroms) and atomic numbering schemes. Hydrogen atoms have been omitted for clarity.
2.298 Å and 2.197 Å, respectively, which implies that the effective coordination of the p bond of the double bond to the Rh center. Intermediate D would then be converted to E via a cyclopropanation step, which would occur with an energy barrier of 11.3 kcal/mol (Fig. 2). Starting from intermediate E, there are two possible pathways involves Rh(I) or Rh(III) intermediates for the formation of the final product as proposed in Scheme 2. For path a (Fig. 2), intermediate E would undergo a facile isomerization step to give the agostic intermediate E1, followed by a concerted CMD/ reductive elimination to afford the Rh(I) species E2 via transition state TSE1-E2 with an energy barrier of 8.0 kcal/mol relative to E1 (i.e., 25.7 kcal/mol relative to D). Intermediate E2 would then release a single molecule of AcOH to give F, which would undergo an isomerization to give the isomer G. The subsequent cleavage of the OeN bond in G would lead to the oxidation of its Rh(I) center via transition state TSG-H to give the Rh(III) center in H. The coordination of a single molecule of AcOH to the Rh center would then lead to the protonation of the N atom of the phthalimide group via
transition state TSH1-H2 with the concomitant release of phthalimide 4. Finally, the protonation of the cyclopropane ring in H2 by the second AcOH molecule would lead to the formation of the disubstituted trans-cyclopropane product 3 with the regeneration of CpiprRh(OAc)2. Based on the results shown in Scheme 3 and Fig. 2, we concluded that the transition state for the concerted CMD/reductive elimination step would be the rate-determining step for path a and that the overall energy barrier for this pathway would be 25.7 kcal/mol. The concerted CMD/reductive elimination could precede the cyclopropanation step. The relative energy of the transition state for the cyclopropanation step TSD′-F (17.7 kcal/mol in Scheme S1) was found to be higher than that of TSE1-E2 (4.5 kcal/mol) in path a, which indicated that this pathway could be ruled out. After the formation of H1, it is possible that the protonation of the cyclopropane ring could occur prior to that of the N atom of the phthalimide moiety. However, the transition state TSH1-J1′ for the cyclopropanation step (Scheme S2) was 17.2 kcal/ mol higher in energy than H1, making this pathway unlikely. As shown in path b, the oxidation state of the Rh(III) center could remain unchanged throughout the entire pathway (Fig. 3). Firstly, the isomerization of E gives rise to I. The cleavage of the OeN bond in I would lead to the formation of the Rh(III) complex J via TSI-J, which would be the rate-determining transition state. Intermediate J would then dissociate to give J1 together with the final product 3. Finally, J1 would be readily protonated by acetic acid to yield 4 with the regeneration of CpiprRh(OAc)2. The overall free energy barrier for path b was calculated to be 26.4 kcal/mol, which corresponds to the energy of TSI-J relative to D. The energy barriers for paths a and b were calculated to be 25.7 and 26.4 kcal/mol, which appeared to be too high to allow for the coupling reaction to occur under the experimental conditions (21 C).8 Furthermore, the differences in the barriers for the
Fig. 2. Energy profiles calculated for the processes from intermediate D based on path a. The solvent- and dispersion-corrected relative free energies in the PCM model have been given in kcal/mol. 1 þ A is taken as the zero energy reference point.
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Fig. 3. Energy profiles calculated for the processes from intermediate E based on path b. The solvent- and dispersion-corrected relative free energies in the PCM model have been given in kcal/mol. 1 þ A is taken as the zero energy reference point.
formation of the trans and cis isomers in paths a and b were inconsistent with the observed diastereoselectivity. For paths a and b, the diastereoselectivity of the cyclopropanation reaction would be determined by the OeN bond cleavage step. Transition states TSG-H and TSG-cisH for the formation of the trans and cis isomers (H and cisH) in path a were determined to have almost identical free energy (6.1 vs 6.0 kcal/mol). This result suggests that the cyclopropanation reaction would result in a 1:1 mixture of trans and cis isomers. The 7.9 kcal/mol energy difference between transition states TSI-J and TSI-cisJ for the formation of the trans and cis isomers (J and cisJ) in path b implies that the corresponding ratio of J: cisJ is far more than 12:1 in the experimental observation.8 Based on these results, we proceeded to investigate an alternative mechanism for the cyclopropanation reaction to account for these experimental observations.8 Further calculations revealed that the cyclopropanation could proceed via a Rh(V)-mediated pathway, with the cleavage of the
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OeN bond preceding the cyclopropanation step (path c in Fig. 4). The first step in path c would be the isomerization of D to give D1, which would undergo an OeN bond cleavage step via TSD1-K to give intermediate K with an energy barrier of 19.2 kcal/mol relative to D. As shown in Fig. 5, the interatomic distances of RheN in K (2.060 Å) is shorter than that in D1 (2.385 Å), which further confirms the conversion of the RheN dative-bond to a RheN s-bond. The Rh(V) intermediate K is a seven-coordinated species with four-membered rhodacycle (see Fig. 1). The stable seven-coordinated Rh(V) complexes have been reported.20 K would then undergo an easily reductive elimination via transition state TSK-J1′ to generate intermediate J1′ with the concomitant release of the disubstituted transcyclopropane product 3. Finally, the protonation of the nitrogen atom in J2 with a single molecule of AcOH would result in the formation 4 and regenerate CpiprRh(OAc)2. Compared with paths a (25.7 kcal/mol) and b (26.4 kcal/mol), path c had the lowest overall energy barrier (19.2 kcal/mol). The results of these calculations therefore demonstrate that the relative free energies of the transition states for the cleavage of the OeN bond in path c are responsible for determining the diastereoselectivity of this cyclopropanation reaction. The energy difference of 1.8 kcal/mol between transition states TSD1-K and TSD1-cisK implied that the corresponding ratio of trans and cis isomers would be approximately 21:1, which is consistent with the experimentally observed ratio of 12:1.8 As reported by Rovis et al.,8 the Rh(III)catalyzed cyclopropanation of alkene substrates with N-enoxyphthalimides take place in low yield when other cyclopentadienyl ligands are used (Table 1). For other cyclopentadienyl ligands, we calculated the rate-determining step according to the favorable path c for the Cpipr ligand. The differences in the free energy barriers of the cleavage of the OeN bond step among the all cyclopentadienyl ligands correlate well with their yield in the experiment.8 These extra calculations further confirm that the path c is reasonable. To develop a better understanding of the observed chemoselectivity, we calculated the arene CeH activation. However, the arene CeH activation step had an energy barrier of 29.8 kcal/mol, making it much less favorable than the vinyl CeH activation step,
Fig. 4. Energy profiles calculated for the process from intermediate D based on path c. The solvent- and dispersion-corrected relative free energies in the PCM model have been given in kcal/mol. 1 þ A is taken as the zero energy reference point.
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Fig. 5. Optimized structures of D1, TSD1-K and K along with key bond lengths (in angstroms) and atomic numbering schemes. Hydrogen atoms have been omitted for clarity.
Table 1 Distortion/interaction analysis of key TS for different cyclopentadienyl ligands. Relative free energies and electronic energies are given in kcal/mol. Entry 1 2 3 4 5 6 7
Cpx *
Cp CpCF3 Cp1 Cp2 Cpt CpE Cpipr
DEact
DEdist-cat
DEdist-org
DEdist
DEint
DGact
Yield(%)
22.9 22.9 23.5 21.7 27.5 23.6 21.0
13.1 27.2 14.1 13.9 29.3 26.0 14.1
41.1 47.4 40.2 38.2 45.4 46.8 39.3
54.2 74.6 54.3 52.1 74.7 72.8 53.4
31.3 51.7 30.8 30.4 47.2 49.2 32.4
21.8 21.7 25.1 22.8 26.8 23.6 19.2
63 56 27 40 22 40 79
which had an energy barrier of 16.1 kcal/mol. The result of this calculation was also consistent with the experimentally observed chemoselectivity.8 As shown above, the results of these calculations have shown that the Rh(V) pathway would be kinetically favored over the Rh(I) and Rh(III) pathways for the Rh(III)-catalyzed cyclopropanation of ethyl acrylate with N-enoxyphthalimide. The Rh(I) and Rh(III) pathways would result in the advanced formation of unstable cyclopropane, which would lead to a pronounced increase in the overall energy demand of the subsequent processes. We used the distortion/interaction model21 to gain insight into the diastereoselectivity (Table 1). The distortion/interaction model is also called the activation-strain model,22 which has been used to elucidate the reactivities and selectivities in different reactions, especially organometallic reactions.20,21 In bimolecular reaction, the activation barrier (DEz) is dissected into distortion energy (DEdist) and interaction energy (DEint). DEdist is the energy required to distort the reactants from their equilibrium geometries to the geometries they adopt in the transitional states. DEint is the energy of interaction between the distorted fragments. Although the OeN bond cleavage step is not a bimolecular reaction, distortion/interaction model can be still applied to this unimolecular processes by using complex D as a reference, as suggested by Fern andez and Bickelhaupt.23 The distortion/interaction model has also been successfully employed on some unimolecular system recently.24 For the OeN bond cleavage step considered herein, each structure was decomposed into two fragments (the catalyst [CpiprRh(OAc)]þ and the organic moiety
(org)).24a In the two transition structures for the OeN bond cleavage as shown in Fig. 6, there is a much smaller distortion energy for TSD1-K (DEdist-cat ¼ 14.1 kcal/mol and DEdist-org ¼ 39.3 kcal/mol) compared to TSD1-cisK (DEdist-cat ¼ 22.9 kcal/mol and DEdistorg ¼ 49.1 kcal/mol, albeit with a larger interaction energy). The results indicate that the distortion energy is the major factor controlling the diastereoselectivity. The distortion/interaction model was also applied to explain the highest reactivity of Cpipr ligand among the several cyclopentadienyl ligands. The calculated distortion and interaction energies of the reactants are listed in Table 1. The low yields for CpCF3,
Fig. 6. Distortion/interaction model for the diastereoselectivity.
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Cpt and CpE ligands are clearly controlled by the much larger distortion energies of both the catalyst and organic moiety compared to the Cpipr ligand, although their interaction energies are significantly more negative. The distortion and interaction energies for Cp* and Cp1 ligands are both higher than that for Cpipr ligand. Compared to Cp2 ligand, Cpipr ligand has slightly higher DEdist, but more negative interaction energy DEint. Therefore, the Cpipr ligand tends to avoid the sterically crowded region, which is the main reason for its highest reactivity among several ligands. 4. Conclusions In conclusion, Rovis et al. recently reported the Rh(III)-catalyzed cyclopropanation of alkene substrates with N-enoxyphthalimides to give the corresponding trans-1,2-disubstituted cyclopropanes. In this study, we have successfully used DFT calculations to elucidate the mechanism and diastereoselectivity of this cyclopropanation reaction. The free energy profiles for several possible reaction pathways have been calculated and compared. The results of these calculations have shown that the generally accepted mechanisms for this reaction, including those involving the formation of Rh(I) and Rh(III) intermediates for the formation of the cyclopropane, required high activation energies and were inconsistent with the observed diastereoselectivity. Instead, we have identified a mechanism involving a four-membered ring containing a Rh(V) species as the lowest energy pathway, which would be initiated by a CeH activation reaction. The subsequent migratory insertion of the alkene substrate would result in the formation of an s-alkylrhodium(III) complex, which would undergo an OeN bond cleavage step to give a four-membered ring containing Rh(V). This catalytic cycle would then close with a CeC reductive elimination reaction, leading to the formation of the final cyclopropane product. This Rh(V) mechanism predicted a final trans/cis product ratio of 21:1, which was in reasonable agreement with the experimentally observed ratio of 12:1. Analysis of the key transition state by the distortion/interaction model indicated the distortion energy plays a key role in determining the diastereoselectivity. Acknowledgements This work was supported by the National Natural Science Foundation of China (Grant No. 21573095), the Fundamental Research Funds for the Central Universities (Grant No. 21615405) and the high-performance computing platform of Jinan University. Appendix A. Supplementary data Supplementary data related to this article can be found at http:// dx.doi.org/10.1016/j.tet.2016.11.017. References 1. Reviews on Rh-catalyzed Ce;H activation (a) Satoh T, Miura M. Chem Eur J. 2010;16:11212; €ge T, Liu F, Glorius F. Chem Soc Rev. 2011;40:4740; (b) Wencel-Delord J, Dro (c) Song G, Wang F, Li X. Chem Soc Rev. 2012;41:3651; (d) Colby DA, Bergman RG, Ellman JA. Chem Rev. 2010;110:624; € der N, Glorius F. Adv Synth Catal. 2014;356:1443; (e) Kuhl N, Schro (f) Wendlandt AE, Suess AM, Stahl SS. Angew Chem Int Ed. 2011;50:11062; (g) Cho SH, Kim JY, Kwak J, Chang S. Chem Soc Rev. 2011;40:5068; (h) Arockiam PB, Bruneau C, Dixneuf PH. Chem Rev. 2012;112:5879. 2. (a) Chatani N. Topics in Organometallic Chemistry: Directed Metallation. vol. 24. Berlin, Germany: Springer-Verlag; 2007; (b) Murai S, Kakiuchi F, Sekine S, et al. Nature. 1993;366:529; (c) Ros A, Fernandez R, Lassaletta JM. Chem Soc Rev. 2014;43:3229; (d) Zhang F, Spring DR. Chem Soc Rev. 2014;43:6906. 3. (a) Mo J, Wang L, Liu Y, Cui X. Synthesis. 2015;47:439; (b) Huang H, Ji X, Wu W, Jiang H. Chem Soc Rev. 2015;44:1155; (c) Patureau FW, Glorius F. Angew Chem Int Ed. 2011;50:1977.
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