Bicyclic Olefin Cocatalyzed C–H Bond Amidation by

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Cp*Rh(III)/Bicyclic Olefin Cocatalyzed C−H Bond Amidation by Intramolecular Amide Transfer Xiaoming Wang, Tobias Gensch, Andreas Lerchen, Constantin G. Daniliuc, and Frank Glorius* Westfälische Wilhelms-Universität Münster, Organisch-Chemisches Institut, Corrensstraße 40, 48149 Münster, Germany S Supporting Information *

ABSTRACT: A bicyclic olefin was discovered as a cocatalyst in a Cp*Rh(III)-catalyzed C−H bond amidation proceeding by an intramolecular amide transfer in N-phenoxyacetamide derivatives. Combining experimental and theoretical studies, we propose that the olefin promotes a Rh(III) intermediate to undergo oxidative addition into the O−N bond to form a Rh(V) nitrenoid species and subsequently direct the nitrenoid to add to the ortho position. The amide directing group plays a dual role as a cleavable coordinating moiety as well as an essential coupling partner for the C−H amidation. This methodology was successfully applied to the late-stage diversification of natural products and a marketed drug under mild conditions.

I. INTRODUCTION Transition-metal-catalyzed C−H activation has become one of the most efficient and straightforward synthetic strategies for the direct functionalization of inert C−H bonds.1 Unsaturated molecules, including alkenes or alkynes, have been widely applied as coupling partners in this area, as the carbon−metal bond resulting from C−H activation can add to the unsaturated systems, representing an important means to construct new frameworks. Unique among these transformations is the Catellani reaction,2 where norbornene acts as a transient mediator, directing a palladium-catalyzed C−H activation (Scheme 1a). This reaction, discovered by Catellani and further developed by the groups of Catellani, Lautens, and others has become a powerful method for the aromatic multifunctionalization. The release of norbornene back into the catalytic cycle after ortho C−H functionalization is a unique feature that has long intrigued the community but has not been replicated in other catalytic systems. Thus, discovering novel catalytic systems employing olefins as transient mediators in C−H activation and utilizing new roles of these mediators other than directing the C−H activation itself2,3 remain formidable challenges with great opportunities to achieve new transformations. Recently, Rh(III)-catalyzed C−H activation assisted by an oxidizing directing group has evolved to be a mild and redoxeconomic strategy for the construction of various structurally diverse organic molecules (Scheme 1b).4,5 It is recognized that in this strategy the same moiety acts both as the directing group, facilitating C−H activation, and as an internal oxidizing agent by the cleavage of an oxidizing bond (X−Y). Following the reaction of the initially formed rhodacycle with a coupling partner, the X−Y bond can oxidize rhodium either before or after a reductive step. The unit “Y” which is generated from this © 2017 American Chemical Society

Scheme 1. Olefins as Cocatalysts in Transition-MetalCatalyzed C−H Activation

cleavage can be a leaving group or further react with the coupling partner to form a new bond.6 In all known cases, the coupling partners are involved in the products. Inspired by the Catellani reaction, a dual catalytic system including Cp*Rh(III) and a transient mediator seems feasible if the coupling partner could be released after the cleavage of an oxidizing directing Received: March 19, 2017 Published: April 18, 2017 6506

DOI: 10.1021/jacs.7b02725 J. Am. Chem. Soc. 2017, 139, 6506−6512

Article

Journal of the American Chemical Society group (X−Y) and the cleaved unit “Y” could be transferred to the arene (Scheme 1b). Here, we describe a Cp*Rh(III)/olefin cocatalyzed ortho C− H amidation of N-phenoxyacetamide derivatives 1 by an intramolecular amide transfer (Scheme 1c). 7-Oxa(tetrafluorobenzo)norbornadiene 2e is used as a cocatalyst, and it is proposed to promote a Rh(III) intermediate to undergo oxidative addition into the O−N bond to form a Rh(V) nitrenoid species and subsequently direct the nitrenoid to add to the ortho position. Moreover, the directing group (amide unit) acts not only as a cleavable coordinating group7 but also as an essential coupling partner for the C−H amidation. This method was successfully applied to the latestage diversification of natural products and a marketed drug.

room temperature. According to the proposed mechanism of the rearrangement,8 we assumed that an electron-poor olefin would inhibit the rearrangement, possibly promoting the amide-transfer pathway. Indeed, using tetrafluoro-substituted olefin 2b, the yield of product 3a was improved to 65%. The reaction with 2c gave the product 3a in 54% yield, along with 41% of the remaining olefin 2c. When 2d was used, the reaction gave 3a (49%) and carboamination product 5d (45%). All of these results illustrated that the reaction outcome depends on the structure of the olefin. Prompted by the different results from the olefins 2a, 2b, and 2c, 7-oxa(tetrafluorobenzo)norbornadiene 2e was synthesized and tested in the reaction. To our delight, 3a was obtained in high yield (93%) and 55% of the olefin 2e remained after the reaction. Control experiments indicated that the lost 2e (45%) decomposed to undefined compounds under the reaction conditions (for details, see the Supporting Information). Furthermore, we investigated the feasibility of developing a Rh(III)/olefin-cocatalyzed amide transfer reaction. Using 2b, 2c, and 2e as cocatalysts (20 mol %), the desired product 3a was obtained in 37%, 25%, and 94% yield, respectively. The product 3a was obtained in lower yield (52%) when the amount of 2e was reduced to 10 mol %. The reaction failed to produce 3a in the absence of either the Rh catalyst or 2e, indicating that the present reaction is cocatalyzed by both Rh and the olefin. 2. Scope and Late-Stage Diversification. The scope of this reaction was investigated under the optimized reaction conditions (Scheme 2). N-Phenoxyamides with phenylacetylor cyclohexanecarbonyl substituents on nitrogen afforded the corresponding products 3b and 3c in good yields. However, no product was obtained with a N-Ts group. N-Phenoxyaceta-

II. RESULTS AND DISCUSSION 1. Discovery of Olefins as Cocatalysts. In our previous work,8 we used arenes bearing oxyacetamide (O-NHAc) as an oxidizing directing group6a in a coupling with 7-azabenzonorbornadiene (2a) delivering bridged polycyclic molecules with a Wagner−Meerwein-type rearrangement as a key step. During this work, a trace amount of 2-acetamidophenol was detected in some cases. We found that this side product cannot be formed without the addition of the olefin, suggesting that the assistance of the olefin is essential for this amide-transfer process.9 Inspired by this serendipitous finding, we developed a novel catalytic system for C−H amidation10 employing Cp*Rh(III) and olefins as the catalysts. However, one of the biggest challenges is the selectivity in this complex system where numerous side reactions such as rearrangement, naphthylation, olefin hydroarylation, or olefin carboamination can be expected according to previously known reactivity.11−13 We initiated our studies with the investigation of the influence of various bicyclic olefins 2 in the Rh(III)-catalyzed intramolecular reaction of N-phenoxyacetamide (1a) (Table 1). In the presence of [Cp*Rh(CH3CN)3](SbF6)2 (5.0 mol %) and NaOAc (50 mol %), with olefin 2a, the reaction afforded the rearranged product 4 (72%) along with the amide-transfer product 3a in 20% yield, while 20% of 2a remained after 12 h at

Scheme 2. Substrate Scopea

Table 1. Investigation of Bicyclic Olefins in the Cp*RhCatalyzed Amide Transfera

a

a

Reaction conditions: 1a (0.1 mmol), 2 (0.1 mmol), [Cp*Rh(CH3CN)3](SbF6)2 (5.0 mol %), and NaOAc (50 mol %) in CH2Cl2 (0.25 mL) at room temperature for 12 h. 1H NMR yields are shown. b Results in parentheses were obtained using 20 mol % of 2.

Reaction conditions: 1 (0.2 mmol), 2e (20 mol %), [Cp*Rh(CH3CN)3](SbF6)2 (5.0 mol %), and NaOAc (50 mol %) in CH2Cl2 (0.5 mL) at room temperature for 12 h. bNMR yield with 100 mol % 2e. 6507

DOI: 10.1021/jacs.7b02725 J. Am. Chem. Soc. 2017, 139, 6506−6512

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Journal of the American Chemical Society

3. Experimental Mechanistic Studies. Mechanistic studies using a combined experimental and computational approach were conducted to understand the reaction, specifically: (1) Which roles does the olefin play? (2) What is the origin of the selectivity of the amide transfer? (3) How is the olefin released from the intermediate? In a crossover experiment using 1a-CD3 and 1e as the substrates under the standard conditions (Scheme 4a), only the

mides 1 bearing methyl, phenyl, methoxy, and halide substituents at the para positions reacted smoothly to the products 3e−i in 63−71% yield. The substrates bearing p-ester and p-CF3 groups only afforded the desired products 3j and 3k in 25% and 9% yield, respectively. Furthermore, these two substrates (1j and 1k) were applied in reactions with 100 mol % 2e under standard conditions. The crude 1H NMR of the reaction mixtures showed the formation of the desired products (3j in 47% and 3k in 28%) along with the corresponding olefin carboamidation products (30% and 26%, respectively), suggesting that the electron deficiency of these substrates influences the selectivities of the reaction pathways, causing consumption of the catalytic amounts of olefin. For unsymmetrical substrates such as meta-substituted and meta, para-disubstituted substrates, the reactions showed excellent selectivity for the C−H bond with less steric hindrance, delivering products 3l and 3m in 70% and 84% yield, respectively. Additionally, an ortho substitutent was tolerated, giving the corresponding product 3n in 84% yield. To our delight, the catalytic system also tolerated functional groups such as alkyl alcohol, cyano, and ester groups, which would be decomposed under the strongly acidic conditions in traditional nitration-reduction protocols for the synthesis of 2-aminophenols.14 In addition, this reaction also showed tolerance for large steric hindrance, showcased by the thymol-derived substrate 1r containing an iPr group at the 2-position and a methyl group at the 5-position that still underwent the C−H amidation smoothly at the 6-position. Phenols are ubiquitous structural motifs found in a wide range of natural products and pharmaceuticals.15 Site-selective amidation of inert C−H bonds in these bioactive skeletons represents a unique strategy for accessing new pools of functionalized analogues.16 To show the applicability of the reaction developed herein, 1s, 1t, and 1u derived from estrone, tyrosine, and clofoctol, respectively, were prepared and subjected to the optimized conditions (Scheme 3). To our delight, the catalytic amide transfer took place smoothly to afford 3s, 3t, and 3u in 51%, 60%, and 45% yield, respectively. Importantly, exclusive site-selectivity was observed in the presence of multiple reactive C−H bonds and functional groups like ketone and NHBoc, which can be challenging in late-stage diversification via C−H activation.

Scheme 4. Experimental Mechanistic Studiesa

a1

H NMR yields are given. bHydrogens have been omitted for clarity. For additional details, see the Supporting Information.

products 3a-CD3 and 3e were detected, indicating that the ortho amidation proceeds via an intramolecular pathway. Addition of TEMPO or BHT as radical scavengers had a negligible effect on the reaction, suggesting that the reaction is most likely not based on a radical pathway. Furthermore, a stable cyclometalated Rh(III) complex 6 was obtained upon treatment of substrate 1a with [Cp*RhCl2]2.17 The reaction of complex 6 with 20 mol % or 1 equiv of 2e at room temperature afforded a new Rh complex 7, corresponding to the deprotonated product coordinated to rhodium as confirmed by XRD (Scheme 4b), in 48% and 59% NMR yield, respectively. These results show that the amide transfer can proceed from metalacycle 6 and 2e without any other additives. Using the complex 7 as a catalyst (Scheme 4c) under the standard conditions only afforded 3a in 9% NMR yield due to low conversion of 1a, indicating that the regeneration of active catalyst from 7 requires an additional species. The addition of HOAc (20 mol %) to this catalytic reaction afforded the product 3a in >99% NMR yield, very similar to the result (96%) using Cp*Rh(OAc)2·H2O as the catalyst. Thus, complex 7 is most likely involved in the catalytic cycle, and HOAc,

Scheme 3. Diversification of Natural Products and a Marketed Drug under Mild Conditionsa

a

Reaction conditions: 1 (0.2 mmol), 2e (20 mol %), [Cp*Rh(CH3CN)3](SbF6)2 (5.0 mol %), and NaOAc (50 mol %) in CH2Cl2 (0.5 mL) at room temperature for 12 h. 6508

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Journal of the American Chemical Society Scheme 5. Free Energy Profile of the Ortho-Amidation Process and Key Transition-State-Optimized Structuresa

a

Selected nuclear distances in Å. Hydrogens on Cp* have been omitted for clarity.

Rh(V) nitrenoid species (C). Recent computational studies also identified similar Cp*Rh(V) nitrenoid complexes as key intermediates in Cp*Rh(III) catalysis both with oxidizing directing groups21a−e and with amidation reagents.20 After O− N cleavage, nitrenoid addition to the ortho carbon has a low barrier of 7.2 kcal mol−1 (TSCD), generating a spirocyclic, dearomatized intermediate D. An alternative concerted process generating this intermediate directly from an isomer of B without the formation of a Rh(V) nitrenoid was located with a very high barrier of 41.6 kcal mol−1 (TSB″D), which is clearly disfavored in comparison to the stepwise process and in line with the results from Scheme 4d. An elimination of the olefin (ΔG⧧ = 9.2 kcal mol−1, TSDE) from D restores aromaticity and generates the product−Rh complex E (corresponding to the isolated complex 7 with the olefin) from which the final product is released by protonation. No pathway was found for an olefin release from C prior to nitrenoid addition. The aromatic nitrenoid addition is a novel mechanistic step in this catalytic system and crucial for the success of this reaction. Frontier molecular orbital analysis revealed the electronic nature of this reaction (Figure 1).21e The highest

generated during the formation of the rhodacycle, is essential for the release of the active catalyst from 7. To further learn about the amidation process, including the distinction between a stepwise pathway and a concerted pathway, as well as the nature of the key intermediates, we investigated the amidation reactions of (2-hydroxyphenyl)boronic acid (8) with the amidation reagent 918 using the Cp*Rh(OAc)2·H2O catalyst (Scheme 4d). Arylboronic acids have been used in Cp*Rh(III) catalysis as precursors for aryl− Rh species formed by transmetalation.19 In the Cp*Rh(III)catalyzed C−H amidation using 9, a key Rh(V) nitrenoid intermediate was proposed by Chang, producing the C−N coupled product by migratory insertion of the nitrenoid into a Rh−C bond.18,20 When 8 and 9 were reacted with 5 mol % of Cp*Rh(OAc)2·H2O, only protodeboration could be detected, indicating that the proposed phenyl−Rh intermediate does not react with the amidation reagent directly. When olefin 2e (1.0 equiv) was present, the amidation product 3a was detected in 67% NMR yield. Besides this product, the olefin carboamidation product 5e was also formed, again suggesting the phenyl amidation needs the assistance of 2e. The reaction between 8 and 2e in the absence of amidation reagent 9 gave the naphthylation product 10, formed from β-oxygen elimination and dehydration, in 36% yield. These control reactions illustrated that a stepwise pathway (O−N bond cleavage before the formation of the new C−N bond) is most likely involved and a Rh(V) nitrenoid intermediate21 may be the key intermediate in the Cp*Rh/2e cocatalyzed amide-transfer reaction of N-phenoxyacetamides. 4. Computational Studies. To understand the reaction mechanism, a detailed computational analysis22 with DFT was undertaken using the Gaussian 09 package at the M06/6311+G(d,p)+SDD(Rh)/SMD(DC M)//M06/6-31 G(d)+Lanl2DZ(Rh) level;23,24 for full details and additional considerations, see the Supporting Information. In the most favorable pathway (Scheme 5), the rhodacycle generated from C−H activation adds to the exo face of the olefin with a low barrier of 9.2 kcal mol−1 (TSAB). Consecutively, oxidative addition of Rh into the O−N bond occurs from a higher energy conformer of the seven-membered metalacycle (B′) with a barrier of 15.0 kcal mol−1 (TSB′C) forming what is formally a

Figure 1. Kohn−Sham orbital plots (isodensity value: 0.04 au) of frontier orbitals for C and TSCD.

occupied molecular orbital (HOMO) in C corresponds to a π orbital of the phenyl moiety, while the lowest unoccupied molecular orbital (LUMO) represents the π* orbital of the Rh nitrenoid. In the HOMO of transition state TSCD, both the phenyl π- and the nitrenoid π*-orbitals are present with a bonding interaction between N and the ortho C, a signature for electrophilic attack by the nitrenoid to the nucleophilic arene 6509

DOI: 10.1021/jacs.7b02725 J. Am. Chem. Soc. 2017, 139, 6506−6512

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Journal of the American Chemical Society

Rh(III) to add into the O−N bond, forming a nitrenoid species that is further directed toward electrophilic addition to the arene by the olefin. In order to regain aromaticity, the olefin is released back to the catalytic cycle. The present methodology also shows potential applications in the late-stage diversification of natural products and marketed drugs. The reaction pathway found in this study reveals a rare example of an olefin as a cocatalyst instead of a coupling partner in transition-metal catalysis. This is a novel catalytic system with an olefin as transient mediator in C−H activation with the unprecedented role of the mediator to direct O−N, rather than C−H cleavage. Thus, this work might stimulate future work for the development of new olefin-cocatalyzed reactions.

moiety. Presumably, the concomitant reduction of Rh is a driving force of this process. According to previous reports and observed side products, a number of different processes can potentially take place in this system.6,12 We found that β-O-elimination from B and reductive C−O elimination from C both have feasible but uncompetitive barriers, in line with the absence of the corresponding naphthylated or tetrahydrodibenzofuran products in the experiments. Migratory insertion of the nitrenoid into the Rh−C bond in C results in carboamination products, as observed previously.6 The corresponding barrier is 5.3 kcal mol−1 higher in energy than for the ortho amidation. Conversely, with the carbocyclic olefin 2d, both amidation barriers are much closer (ΔΔG⧧ = 1.6 kcal mol−1), matching the similar yields of the ortho amidation and carboamination products shown in Table 1. Presumably, steric and electrostatic repulsion between the bicyclic moiety and the nitrenoid acetyl group govern the selectivity for ortho amidation. In agreement with these findings is the observed decrease in selectivity for the electrophilic addition with electron-poor substrates in favor of the carboamination (3j and 3k, Scheme 2). 5. Proposed Mechanism. Based on the experimental and computational results, we propose the mechanism shown in Scheme 6. After C−H activation generating rhodacycle 6, olefin

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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/jacs.7b02725. Experimental details of data acquisition and additional discussion; additional mechanistic computations and Cartesian coordinates of optimized structures (PDF) X-ray data for compounds 3t, 6, and 7 (CIF) Computed structures (XYZ)

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Scheme 6. Proposed Mechanism

AUTHOR INFORMATION

Corresponding Author

*[email protected] ORCID

Frank Glorius: 0000-0002-0648-956X Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS We acknowledge financial support from the Alexander von Humboldt Foundation (X.W.) and the Deutsche Forschungsgemeinschaft (Leibniz Award). We are grateful to Frederik Sandfort for experimental assistance. We also thank Dr. Manuel van Gemmeren, Dr. Christian Mück-Lichtenfeld, and Suhelen Vásquez-Céspedes (all WWU Münster) for helpful discussions and the ZIV Münster/PALMA for computational resources.

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insertion forms the seven-membered rhodacycle B. The bicyclic nature of 2e facilitates the insertion and stabilizes B by inhibiting β-H elimination. Consecutively, the Rh(V) nitrenoid C is formed by oxidative addition of Rh into the O−N bond. This intermediate was also proposed to be formed in the reaction of (2-hydroxyphenyl)boronic acid 8 with the amidation reagent 9 and 2e (Scheme 4d).20,21 At this stage, the electron-withdrawing substituents in 2e disfavor the rearrangement pathway.8 An electrophilic ortho nitrenoid addition promoted by repulsion from the bicyclic olefin generates a spirocyclic, dearomatized intermediate D while at the same time reducing Rh. In order to regain aromaticity, the olefin is released in a subsequent elimination, furnishing the Rh−product complex 7. Protonation by HOAc then releases the product.

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III. SUMMARY AND CONCLUSIONS In summary, we have developed a Cp*Rh(III)/bicyclic olefincocatalyzed intramolecular C−H amidation reaction of Nphenoxyacetamide derivatives under mild and neutral conditions. Through experimental and computational studies, we elucidated that reaction with the olefin cocatalyst enables 6510

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