Aerobic Oxidative Alkenylation of Weak O-Coordinating

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Cite This: Org. Lett. XXXX, XXX, XXX−XXX

Aerobic Oxidative Alkenylation of Weak O‑Coordinating Arylacetamides with Alkenes via a Rh(III)-Catalyzed C−H Activation Subramanian Jambu,† Ramakrishnan Sivasakthikumaran,† and Masilamani Jeganmohan* Department of Chemistry, Indian Institute of Technology Madras, Chennai 600036, Tamil Nadu, India

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S Supporting Information *

ABSTRACT: A versatile and site-selective rhodium(III)catalyzed aerobic oxidative alkenylation of arylacetamides including primary, secondary, and tertiary amides having a weak O-coordinating acetamide directing group with alkenes is described. In the reaction, air was utilized as a sole oxidant. The reaction was compatible with activated alkenes and maleimides.

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using a weak oxygen-containing directing group on the aromatic molecules. It is important to note that the weak coordinating nature of the acetamide directing group generally leads to the unfavorable metallacycle formation, and also the tautamerizable nature of acetamide makes a weak directing group for the C−H functionalization reaction.9 Owing to these difficulties, the C−H functionalization using acetamide is less explored in the literature. Herein, we report the alkenylation of arylacetamides with alkenes including maleimides in the presence of a rhodium(III) catalyst under air. In the present reaction, ortho alkenylated arylacetamides and 3-arylated maleimides were observed in good yields. Treatment of 2-methyl phenylaceamide 1a with methyl acrylate 2a (2.0 equiv) in the presence of [RhCp*(CH3CN)3][SbF6]2 (3 mol %) and Adm-1-COOH (1-adamantanecarboxylic acid) (30 mol %) under air in ClCH2CH2Cl at 100 °C for 12 h afforded ortho-alkenylated arylacetamide 3aa in 92% isolated yield (Scheme 1). It is interesting to note that the alkenylation reaction proceeds very well under air, and a Cu or Ag oxidant is not needed. The alkenylation reaction was further examined with various solvents such as DME, toluene, THF, CH3CN, TFE, DMF, MeOH, and AcOH instead of DCE. Among them, THF and toluene were partially effective, yielding product 3aa in 77% and 68% yields, respectively. Other solvents were less effective or ineffective for the reaction. The reaction did not proceed in the absence of a rhodium catalyst. Further, the alkenylation reaction was examined with other organic acids such as acetic acid, pivalic acid, and mesitylenic acid instead of 1-adamantanecarboxylic acid. These additives were also partially effective, yielding product 3aa in 76%, 78%, and 73% yields, respectively. The optimization study clearly reveals that [RhCp*(CH3CN)3][SbF6]2 (3 mol

rylacetamides are found to be a key motif in a variety of bioactive molecules, drugs, hormones, proteins, and glycoproteins.1 Due to the potential application, the development of new methods for the functionalization of arylacetamides is highly important as well as desirable. The transitionmetal-catalyzed chemical bond formation via C−H bond activation has gained much attention in organic synthesis for the past two decades.2 For instance, metal-catalyzed ortho alkenylation of substituted aromatics with alkenes via C−H bond activation is a useful method to synthesize vinyl arenes in a highly regio- and stereoselective manner.3 Particularly, in the chelation-assisted deprotonation pathway, alkenylation was achieved at the C−H bond of substituted aromatics with alkenes very effectively.3 Several research groups have devoted substantial effort toward alkenylation of substituted aromatics with alkenes4 in the presence of palladium,5 rhodium,6 and ruthenium7 catalysts. Generally, in the oxidative alkenylation via the deprotonation pathway, an oxidation step that includes a metal with lower oxidation state changing to a higher oxidation state [Pd(0) to Pd(II), Rh(I) to Rh(III), and Ru(0) to Ru(II)] is required to regenerate the active catalyst. Typically, a stoichiometric amount of inorganic or organic oxidant is required to regenerate the active catalyst. In fact, the use of molecular oxygen as the oxidant instead of an inorganic or organic oxidant would be more advantageous as well as the potential application in the C−H functionalization reaction. Recently, the metal-catalyzed C−H activation involving O2 as the sole oxidant has been reported in the literature.8 Mostly, for this type of transformation, a strong chelating group is needed on the aromatic molecules. Meanwhile, a few Rhcatalyzed aerobic C−H activations have also been reported.8e−h It is noteworthy to mention that the aerobic C−H alkenylation by a Rh catalyst requires strongly coordinating nitrogen directing groups.7f To the best of our knowledge, only a few reports are available on a Rh(III)-catalyzed aerobic C−H alkenylation by © XXXX American Chemical Society

Received: January 1, 2019

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DOI: 10.1021/acs.orglett.8b04140 Org. Lett. XXXX, XXX, XXX−XXX

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Organic Letters Scheme 1. Scope of Substituted Arylacetamides 1a−wa

products 3sa and 3ta in 84% and 86% yields, respectively. The secondary N-methyl aryl acetamides 1u and 1v underwent alkenylation with 2a to give products 3ua and 3va in 79% and 70% yields, respectively. Interestingly, primary aryl acetamide 1w was also effectively involved in the alkenylation reaction, furnishing ortho alkenylated arylacetamide 3wa in 80% yield. The scope of the alkenylation reaction was further examined with substituted alkenes 2b−f (Scheme 2). Ethyl acrylate (2b), Scheme 2. Scope of Activated Alkenes

a

All reactions were carried out using substituted amides 1a−w (75 mg), methyl acrylate (2a, 2.0 equiv), [RhCp*(CH3CN)3][SbF6]2 (3 mol %), Adm-1-COOH (30 mol %) under air in ClCH2CH2Cl at 100 °C for 12 h. bMethyl acrylate (2a) (1.0 equiv) was used.

%) and Adm-1-COOH (30 mol %) under air in ClCH2CH2Cl at 100 °C for 12 h are the best conditions for the reaction. The scope of the alkenylation reaction was examined with various substituted arylacetamides 1a−w under the optimized reaction conditions (Scheme 1). The alkenylation reaction was compatible with functional groups such as OMe-, Me-, F-, Cl-, and Br-substituted arylacetamides. The reaction of electrondonating 2-OMe-substituted arylacetamide 1b with 2a provided ortho alkenylated acetamide 3ba in 85% yield. A halogen group such as a F, Cl, and Br substituent at the ortho position of arylacetamides 1c−e delivered ortho alkenylated aromatic acetamides 3ca−ea in 71%, 88%, and 74% yields, respectively. Unsymmetrical meta substituted aromatic acetamides 1f−h also efficiently participated in the reaction, giving alkene derivatives 3fa, 3ga, and 3ha in 95%, 92%, and 89% yields with excellent ortho selectivity. In the reaction, alkenylation takes place at the C6 position of aryl acetamides. The structure of compound 3ha was confirmed by a single crystal X-ray diffraction analysis (CCDC 1874622). The reaction of para-substituted arylacetamides 1i−l with 2a (1.0 equiv) yielded alkenylated products 3ia, 3ja, 3ka, and 3la in 87%, 81%, 67%, and 73% yields, respectively. In the case of para-substituted arylacetamides 1i−l, a very minor amount of ortho alkylated arylacetamides was also observed. The less reactive 2,4-dichloro and 3,5-dichloro arylacetamides 1m and 1n underwent alkenylation smoothly to give alkenylation products 3ma and 3na in 90% and 86% yield, respectively. Gratifyingly, α-methyl-phenylacetamide 1o reacted with 2a affording alkenylated product 3oa in 55% yield. Treatment of tertiary N-cyclohexyl arylacetamides 1p−r with 2a gave the corresponding alkene derivatives 3pa, 3qa, and 3ra in 88%, 81%, and 80% yields, respectively. The other tertiary N,Ndimethyl and N,N-diethyl aryl acetamides 1s and 1t were also smoothly converted into the corresponding alkenylated

n-butyl acrylate (2c), phenyl acrylate (2d), and cyclohexyl acrylate (2e) reacted well with 1a, giving alkene derivatives 3ab−ae in 61%, 57%, 67%, and 68% yields, respectively. Tertiary and secondary arylacetamides 1a and 1u reacted with phenyl vinyl sulphone (2f) furnishing an inseparable mixture of alkenylated (major) and alkylated (minor) products 3af+3af′ and 3uf+3uf′ in 68% (4:1) and 66% (4:1) yields, respectively. Interestingly, meta as well as para substituted aryl acetamides 1v and 1x yielded a 7:1 mixture of alkenylated and alkylated products 3vf+3vf′ in 69% and 3xf+3xf′ in 62% (7:1) combined yields, respectively. When para-methyl- and para-bromo-arylacetamides 1y and 1j reacted with 2a (2 equiv), symmetrical bis alkenylated products 4ya and 4ja were observed in 87% and 86% yields, respectively (Scheme 3). We have successfully demonstrated a stepwise and one-pot divinylation with two different alkenes Scheme 3. Bis Vinylation of Arylacetamides

B

DOI: 10.1021/acs.orglett.8b04140 Org. Lett. XXXX, XXX, XXX−XXX

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Organic Letters

reaction was examined with various N-substituted maleimides; it proceeded smoothly providing the expected alkenylation products 8p−t in moderate yields. Treatment of ortho alkenylated arylactamide 3aa with LiOH (5.0 equiv) in THF/MeOH/H2O (4:1:1) solvent at 80 °C for 12 h gave α,β-unsaturated acid 6 in 86% yield (eq 1). The reaction is highly chemoselective, and only the ester group of alkene was hydrolyzed with the amide group remaining intact.

such as alkenylation of 1i with 2a for 12 h followed by addition of 2b into the same pot in a highly regioselective manner. In the reaction, unsymmetrical 1,3-divinylbenzene derivative 5 was observed in 51% yield in one pot. Recently, maleimides are used as an alkene source for several organic transformations via C−H activation. In most of the cases, maleimides reacted with substituted aromatics providing a Michael-type ortho alkylated aromatics.10 But, the reports on Heck-type alkenylation involving maleimides are scarce.11 It is due to the lack of the β-hydrogen in the syn-periplanar position for the β-hydride elimination. To evade this problem, mostly, 3-arylated maleimides are prepared via the metalcatalyzed cross-coupling of 3-halo maleimides with organoboronic acids or aryl halides with maleimides.12 It is also important to mention that the 3-arylated maleimides are pivotal structural motifs present in various natural products and pharmaceuticals with diverse biological activities.13 When 4-methoxy phenylaceamide 1i was treated with N-ethylmaleimide (7a) (2.0 equiv) in the presence of [RhCp*(CH3CN)3][SbF6]2 (5 mol %) and Cu(OAc)2·H2O (1.0 equiv) under air in 1,2-dimethoxyethane (DME) at 80 °C for 12 h, 3-arylated maleimide 8a was observed in 72% isolated yield (Scheme 4) (for the detailed optimization study, see the Supporting Information). The base Cu(OAc)2·H2O was used to deprotonate the acidic β-hydrogen for the protodemetalation.

To gain more insight into the reaction mechanism, the following deuterium labeling reactions were performed (Scheme 5). The reaction of 1i with CD3COOD (2.0 equiv) Scheme 5. Mechanistic Investigation

Scheme 4. Scope of Arylacetamides with Maleimides

in the presence of [Rh(Cp*)(CH3CN)3)][SbF6]2 (3 mol %) in DCE at 100 °C for 6 h provided D-1i in 95% yield with 49% deuterium incorporation at both ortho carbons as well as 60% deuterium incorporation at the acidic benzylic-carbon. Further, the reaction of 1j with 2a under similar reaction conditions yielded product D-3ja in 65% yield with 38% deuterium incorporation at the ortho carbon and 46% deuterium incorporation at benzylic-carbon. These results clearly reveal that the ortho-C−H bond activation is a reversible process. Next, we performed a radical trapping experiment in the presence of radical scavenger 2,2,6,6-tetramethylpiperidine Noxide (TEMPO). But, there was no significant drop in yield of product, which clearly indicates that the reaction is not proceeding via a single-electron pathway. A plausible reaction mechanism is proposed to account for the present alkenylation reaction in Scheme 6.14 The weak precoordination of the lone pair of oxygen atom of acetamide with the active rhodium species A followed by ortho-metalation via deprotonation pathway provides a six-membered metallacycle intermediate B. Then, the alkene π-bond of methyl acrylate replaces acetic acid and subsequent insertion into the Rh−C alkene bond of intermediate C providing intermediate D. The intermediate D undergoes β-hydride elimination via decoordination of the CONR2 group of intermediate E giving the alkenylated product 3aa and a rhodium(III) hydride complex F which further undergoes reductive elimination to afford Rh(I) complex G. The rhodium(I) complex G

The scope of the alkenylation reaction was examined with various substituted arylacetamides 1 (Scheme 4). The reaction was compatible with para, meta, and ortho substituted arylacetamides. In the reaction, the expected 3-arylated maleimides were observed in good to moderate yields. It is important to note that, in the meta substituted aryl acetamides, the C−H activation selectively takes place at the less hindered C6 position. The structure of compound 8f was confirmed by single crystal X-ray diffraction analysis (CCDC 1887164). The reaction of N-substituted secondary arylacetamides 1 with 7a provided expected products 8l−o in moderate yields. The C

DOI: 10.1021/acs.orglett.8b04140 Org. Lett. XXXX, XXX, XXX−XXX

Organic Letters

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Scheme 6. Proposed Mechanism

Letter

ASSOCIATED CONTENT

* Supporting Information S

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.orglett.8b04140. General experimental procedures and characterization details; 1H NMR and 13C NMR spectra of all compounds; single-crystal X-ray diffraction data for compounds 3ha and 8f (PDF) Accession Codes

CCDC 1874622 and 1887164 contain the supplementary crystallographic data for this paper. These data can be obtained free of charge via www.ccdc.cam.ac.uk/data_request/cif, or by emailing [email protected], or by contacting The Cambridge Crystallographic Data Centre, 12 Union Road, Cambridge CB2 1EZ, UK; fax: +44 1223 336033.

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. ORCID

Masilamani Jeganmohan: 0000-0002-7835-3928 Author Contributions †

S.J. and R.S. contributed equally to this work.

Notes

undergoes aerobic oxidation under air in the presence of AcOH giving the active rhodium(III) catalyst A and water as the sole byproduct. In the reaction, an organic acid provides an acetate source to deprotonate the C−H bond of the aromatic moiety. The alkenylation of aryl acetamides with maleimides proceeds via coordination of the lone pair of the oxygen atom of arylacetamide 1a with a rhodium(III) species H followed by ortho-metalation providing a six-membered rhodocycle intermediate I (Scheme 6). Coordinative syn insertion of maleimide 2 into the Rh−carbon bond of intermediate J gives eight-membered rhodocycle intermediate K. Next, the decoordination of CONR2 forms intermediate L. The syn coplanarity arrangement of a metal with Cβ−H is required for β-hydride elimination. In intermediate L, the rotation of a single bond to obtain syn coplanarity is restricted due to the cyclic system. It is likely that the base acetate ion deprotonates the acidic β-hydrogen of intermediate L to form Heck-type product 8 and intermediate M. Reoxidation of the catalyst by Cu(OAc)2 and O2 regenerates the active catalyst H. In the reaction, Cu(OAc)2 was used to deprotonate the acidic β-hydrogen of intermediate L. In summary, we have developed a rhodium(III)-catalyzed aerobic oxidative ortho-alkenylation of weak O-coordinating arylacetamides (primary, secondary, tertiary) and activated alkenes in good to excellent yields. In the reaction, ortho alkenylated aryl acetamides were observed in a highly regioand diastereoselective manner. Molecular oxygen has been used as a sole oxidant to regenerate the active catalyst. By applying this strategy, symmetrical and unsymmetrical 1,3divinylbenzenes were prepared in good yields. Meanwhile, arylacetamides reacted with maleimides providing 3-arylated maleimides in good yields.

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS We thank the DST-SERB (EMR/2014/000978), India for the support of this research. S.J. thanks IITM for an HTRA Fellowship. R.S. thanks DST-SERB for the postdoctoral fellowship.

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DOI: 10.1021/acs.orglett.8b04140 Org. Lett. XXXX, XXX, XXX−XXX