Letter Cite This: Org. Lett. XXXX, XXX, XXX−XXX
pubs.acs.org/OrgLett
Rhodium(III)-Catalyzed Directed C−H Dienylation of Anilides with Allenes Leads to Highly Conjugated Systems Chiranjit Ghosh, Prajyot Jayadev Nagtilak, and Manmohan Kapur* Department of Chemistry, Indian Institute of Science Education and Research Bhopal, Bhopal Bypass Road, Bhauri, Bhopal 462066, Madhya Pradesh, India
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S Supporting Information *
ABSTRACT: Allenes are unique coupling partners in transition-metal-catalyzed C−H functionalization leading to a variety of products via alkenylation, allenylation, allylation, and annulation reactions. The outcome is governed by both the reactivity of the allene and the formation and stability of the organometallic intermediate. An efficient Rh(III)-catalyzed, weakly coordinating group-directed dienylation of electronically unbiased allenes is developed using an N-acyl amino acid as a ligand. Further elaboration of the dienylated products to construct polycyclic compounds is also described.
I
diene systems are widely employed in the stereoselective synthesis of polycyclic compounds as well as in natural product synthesis.8 Glorius and co-workers reported an Rh-catalyzed C−H dienylation with vinyl amides and allenyl carbinol carbonates to obtain dendralene as well as arenes with diene substituents.9a The presence of the carbinol carbonate moiety was deemed mandatory for β-oxygen elimination from a sevenmembered rhodacycle intermediate to yield the product. Fu and co-workers also demonstrated a dienylation of vinyl carbamates, aryl amides, and ketones with electron-deficient (preactivated for nucleophilic attack) allenes via β-hydride elimination from an allyl-Rh intermediate.9b We report herein the dienylation of anilides with electronically unbiased allenes and its extrapolation via synthesis of polycyclic frameworks and substituted indole derivatives (Scheme 1). Several directing groups, such as amides, imines, ketones, carboxylic acid, etc., have been employed as effective groups to direct the proximal C−H bond functionalization with allenes.6,10 In this regard, to the best of our knowledge, there is no report on the successful C−H dienylation of anilides with electronically unbiased allenes as the coupling partner.11 With our group’s continued interest in exploring anilide-directed C− H functionalization,12 we initiated our studies by probing
n recent years, transition-metal-catalyzed C−H functionalizations, which enabled formation of new C−C as well as C−X bonds, have been extensively explored in an atom- and step-economical manner.1 Unsaturated hydrocarbons such as alkenes, alkynes, and allenes contain π(C−C)-bonds and therefore have strong affinity to C−M bonds thus making them versatile coupling partners in C−H functionalization reactions.2,3 Among these candidates, allenes are unique coupling partners in transition metal catalysis since they have three carbon centers of versatile reactivity based on both electronic and steric factors.4 In recent years, the application of allenes in this field has shown great incremental growth.2−6 Depending on the steric and electronic factors of allenes and the nucleophilic nature of R−M (formed after C−H activation), two different types of carbometalation intermediates are formed in a chemo- and regioselective fashion (Scheme 1). Based on the nature of these two intermediates, a variety of products can arise either via protonolysis, βhydride elimination, or cyclization.5,6 In 2009, Krische and coworkers disclosed the first iridium-catalyzed C−H functionalization with allenes,6a which was later explored by several research groups including those of Ma, Ackermann, Glorius, Mascareñas, Cheng, Cramer, and others.6 Dienes represent a class of highly valuable and important building blocks, which are widely used in a range of reactions such as cycloadditions, polymerizations, metathesis, and addition reactions.7 Electronically and stereochemically biased © XXXX American Chemical Society
Received: March 18, 2019
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DOI: 10.1021/acs.orglett.9b00958 Org. Lett. XXXX, XXX, XXX−XXX
Letter
Organic Letters Scheme 1. C−H Functionalization Reactions with Allenes and Present Work
Scheme 3. Substrate Scope for C−H Dienylation of Substituted Pivaloylanilide and Diaryl Substituted Allenesa−e
Scheme 2. Substrate Scope for C−H Dienylation of Substituted Acetanilide and Substituted Aryl Allenesa−e
a
All yields are isolated yields and reactions were performed using optimized conditions of Tables S1 and S2. bUnless otherwise mentioned, for all reactions: anilide (1 equiv) and allene (1.2 equiv) were used. cFor unsubstituted anilides and p-substituted anilides: allene (1 equiv) and anilide (1.5 equiv) were used. dFor diaryl substituted allenes: MeOH/toluene (1:2) solvent system was used. eBased on recovered starting material, sluggish reaction, reactions are performed with anilide (1 equiv) and allene (1.5 equiv). NR = No reaction.
(SI) for detailed optimization studies). The reaction progressed smoothly in the presence of [Cp*RhCl2]2 as the catalyst along with AgSbF6 as the additive to generate the active cationic species. In the present case, the use of a protic source seemed to improve the efficiency of the transformation, and after considerable optimization, pivaloyl glycine (Piv-Gly-OH) was found to be the best protic additive among various others.13 Among the solvents scanned, methanol came across as a good representative, and in many cases the MeOH−toluene mixture worked well. Oxidants such as Cu(OAc)2, Cu(OAc)2·H2O, Ag2O, Cu(OTf)2, and O2 were found to be less efficient than AgOAc. Other transition-metal-derived catalysts (Pd or Ru) were found to be ineffective for this transformation. With the
a
All yields are isolated yields and reactions were performed using optimized conditions of Table S1. bUnless otherwise mentioned, for all reactions: anilide (1 equiv) and allene (1.2 equiv) were used. cFor unsubstituted anilides and p-substituted anilides: allene (1 equiv) and anilide (1.5 equiv) were used. dFor anilides with electron-withdrawing substituents: bottle-grade tBuOH was used as the solvent. eReaction performed at 1 mmol scale.
various reaction conditions for the dienylation of anilides with electronically unbiased aryl allenes through Rh(III)-catalyzed C−H activation (see Table S1 in the Supporting Information B
DOI: 10.1021/acs.orglett.9b00958 Org. Lett. XXXX, XXX, XXX−XXX
Letter
Organic Letters Scheme 4. Diels−Alder Reaction in Synthesis of Polycyclic Compounds
Scheme 6. Control Experiments and Kinetic Isotope Effect Study
Scheme 5. Synthesis of Indole Derivatives
a
Based on recovered starting material.
optimized conditions in hand, we investigated the effect of diverse substituents on both the acetanilide and the aryl ring of the allenes (Scheme 2). In general, the dienylation of various anilides as well as substituted aryl allenes yielded moderate to good transformations for almost all substrates. Electron-rich as well as electron-neutral anilides were compatible with all allenes possessing variable electronic nature under the optimized conditions to deliver products with satisfying yields. Halogen substituents on both the coupling partners were also well-tolerated. The structure of the dienylated product was confirmed unequivocally by X-ray crystallography of compound 3o (CCDC 1880572). Interestingly, better yields were observed for electron-deficient anilides when bottle-grade t BuOH was used as a solvent instead of MeOH. Pivaloylanilides also furnished dienylation products in good to moderate yields with good functional group tolerance (Scheme 3). The reaction yielded a good outcome for the different substituents on the alkyl chain of the allenes in the toluene−MeOH solvent mixture system. Trifluoroacetanilide and trisubstituted allenes were notable failures in this coupling reaction. The poor-coordinating ability of the trifluoroacetanilide and the steric effects on the trisubstituted allene could be attributed to these failures. Compared to the styryl allenes, a relatively sluggish conversion was observed for nonstyrenebased symmetric aliphatic allenes (3ai−3ap, Scheme 3). The 1,3 diene system has been extensively utilized for enantioselective borylation, hydroamination, hydroacylation,
hydroaminomethylation, and the construction of highly substituted organic frameworks.8i−m After successfully synthesizing several electronically biased diene systems, where one double bond is electronically richer than the other one, we explored the synthetic utility of the dienylated products.9 Treatment of dienophiles with suitable dienes would be useful in constructing six-member carbocycles, as depicted in Scheme 4. In the case of unsymmetrical dienophiles, the selectivity in this reaction was governed by secondary orbital effects. The structure of the Diels−Alder product was confirmed unequivocally by X-ray crystallography of compound 5a (CCDC 1903553). We also envisioned the use of the amide as an internal nucleophile for the construction of heterocycles of synthetic value. Under iodo-amidation conditions,14 we successfully synthesized highly substituted indoles, a very important building block for natural products and pharmaceuticals (Scheme 5). This reaction progresses via formation of the C
DOI: 10.1021/acs.orglett.9b00958 Org. Lett. XXXX, XXX, XXX−XXX
Letter
Organic Letters
oxidation regenerates the active Rh(III) catalyst for the next cycle. Intermediate E was detected in ESI-MS (see the SI for details). In summary, we have developed a rhodium-catalyzed, directed C−H dienylation of anilides using electronically unbiased allenes as the coupling partners. The reaction features excellent regioselectivity and a good substrate scope. Diversified synthetic utility of the C−H dienylation product was reflected in the construction of carbocycles via Diels− Alder reactions as well as the synthesis of highly substituted indole derivatives via an iodo-amidation method. The roles of solvent and the nature of the amino acid ligand were found to be important in the transformation.
Scheme 7. Plausible Catalytic Cycle
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ASSOCIATED CONTENT
S Supporting Information *
The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.orglett.9b00958. General experimental procedures, characterization details, and copies of 1H and 13C NMR spectra of new compounds (PDF) Accession Codes
iodonium ion at the more electron-rich and sterically crowded olefin, followed by elimination of HI. To understand the mechanistic pathway of the C−H dienylation, a number of control experiments were performed. During the optimization of the reaction conditions, we observed the formation of Piv-Gly-OMe in the reaction mixture. To investigate the plausible role of the ligand, the methyl ester of the ligand was utilized under the optimal reaction conditions in parallel reactions in MeOH and toluene as solvents. We observed a decreased yield in MeOH, but a decent increase in yields in toluene (Scheme 6A). This seemed to indicate that both the acid and ester forms of the ligand played an important role in the catalytic cycle. To investigate whether the C−H metalation step was ratelimiting, we carried out kinetic isotope effect studies. A competition reaction of a mixture of [D2]-4-methyl acetanilide and 4-methyl acetanilide with the allene 2 gave a kinetic isotope effect of 1.01 (by NMR) whereas parallel reactions resulted in a kinetic isotope effect value of 0.94 (by GC). These experiments indicate that the C−H activation is not involved in the rate-limiting step (Scheme 6B). Next, to check the reversibility of the C−H bond activation, the reaction was performed in MeOH-d4 with added D2O and a significant amount of deuterium incorporation in both the product and recovered anilide was observed (Scheme 6C). Second, a D−H exchange experiment of the 99% deuterated anilide shows a significant loss of the deuterium label (Scheme 6C). From these two experiments it can be concluded that the C−H activation step is a reversible one. On the basis of the above experiments and previous literature reports,6 we propose a plausible reaction mechanism for dienylation reactions (Scheme 7). The first step of the transformation could be a combination of electrophilic metalation and anilide directed ortho C−H activation to provide the rhodacycle intermediate A. Coordination of the allene to this rhodacycle followed by carborhodation via migratory insertion into the allene results in an eightmembered rhodacycle C, which is in equilibrium with π-allyl rhodacycle intermediate D. This upon β-hydride elimination delivers the targeted product 3, and AgOAc mediated
CCDC 1880572 and 1903553 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
Chiranjit Ghosh: 0000-0001-5941-7343 Manmohan Kapur: 0000-0003-2592-6726 Notes
The authors declare no competing financial interest.
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ACKNOWLEDGMENTS Funding from CSIR-India and SERB-India (EMR/2016/ 004298/OC) is gratefully acknowledged. C.G. and P.J.N. thank UGC-India and CSIR-India, respectively, for research fellowships. We thank the CIF, IISERB, for the analytical data and the Director, IISERB, for funding and the infrastructure facilities.
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REFERENCES
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DOI: 10.1021/acs.orglett.9b00958 Org. Lett. XXXX, XXX, XXX−XXX
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DOI: 10.1021/acs.orglett.9b00958 Org. Lett. XXXX, XXX, XXX−XXX
