Pd(II)-Catalyzed Cyclization–Oxidation of Urea-Tethered

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Letter pubs.acs.org/OrgLett

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

Pd(II)-Catalyzed Cyclization−Oxidation of Urea-Tethered Alkylidenecyclopropanes Leyi Tao and Min Shi* State Key Laboratory of Organometallic Chemistry, Center for Excellence in Molecular Synthesis, University of Chinese Academy of Sciences, Shanghai Institute of Organic Chemistry, Chinese Academy of Sciences, 345 Lingling Road, Shanghai 200032, China S Supporting Information *

ABSTRACT: A Pd(OAc)2-catalyzed intramolecular oxidative cyclization of urea-tethered alkylidenecyclopropanes with urea as a nitrogen source through a Pd(II)/Pd(IV) catalytic cycle has been presented, giving the corresponding cyclobuta[b]indoline derivatives in moderate to good yields with a broad substrate scope. The reaction proceeds through a ring expansion of alkylidenecyclopropane along with the nucleophilic attack of nitrogen atom onto the in situ generated palladium carbenoid species as well as an oxidation process.

I

Scheme 1. Previous Work and This Work

ndoline scaffolds are widely found in natural products and medicinal drugs, which exhibit a broad range of biological activities.1 Moreover, cyclopenta[b]indolines or cyclohexa[b]indolines are a core structure of many fluorescent materials used in fluorescence sensors and in vivo fluorescence imaging of blood−brain disruption.2 In 2007, Darius’s group first disclosed that cyclobuta[b]indoline analogues exhibited a good pharmacological profile since they have a highly selective affinity to the MT1 and MT2 receptors.3 However, as compared with cyclopenta[b] indolines4 or cyclohexa[b]indolines,5 cyclobuta[b]indoline skeleton still rarely appears in synthetic organic chemistry, and its synthetic methods are limited. Currently, the primary synthetic method for the construction of cyclobuta[b]indolines was only realized by [2 + 2]-dearomatization of indoles in the presence of Au(I) or Fe(III) catalyst.6 Thus, it is necessary to develop more useful and general synthetic approaches for the synthesis of functionalized cyclobutane-fused indolines. Alkylidenecyclopropanes (ACPs) are highly strained but readily accessible molecules, which can serve as useful building blocks in organic synthesis. In the past few years, a variety of synthetic methods have been developed for the rapid construction of complex polycyclic frameworks including indoline scaffolds based on the metal-catalyzed cyclization of ACPs.7,8 In 2006, Fürstner9 and Shi10 independently reported that the ring expansion of ACPs into nonsubstituted cyclobutene derivatives could take place upon Pt(II) and Pd(II) catalysis, representing a new ring-opening isomerization process through a carbenoid intermediate (Scheme 1a). On the other hand, the intramolecular oxidative palladiumcatalyzed amination of unactivated olefins using a Pd(II)/ Pd(IV) catalytic cycle has recently attracted much attention from organic chemists because this catalytic cycle can provide valuable methodologies for the preparation of (poly)heterocyclic scaffolds including the indoline motifs.11 The Pd(II)/Pd(IV) catalytic cycle was first reported in 1978,12 and the subsequent mechanistic studies revealed that Pd(IV) © XXXX American Chemical Society

intermediates are able to mainly undergo direct reductive elimination13a,11c,13b−h or an SN2-type nucleophilic attack to afford the desired products.14,11m In 2005, Muñiz’s group reported the first intramolecular palladium-catalyzed oxidative diamination of alkenyl-substituted urea molecules employing urea groups as nitrogen sources, affording cyclic urea products in good yields through a Pd(II)/Pd(IV) catalytic cycle using iodosobenzene diacetate as stoichiometric oxidant (Scheme 1b).11n Inspired by these interesting findings, we envisaged that ureatethered ACPs could also give nitrogen-containing polycyclic compounds through a palladium-catalyzed oxidative amination or diamination in the presence of oxidant. Herein, we report a novel synthetic protocol for the construction of cyclobuta[b]indoline skeleton through a Pd(II)/Pd(IV) catalytic cycle under mild conditions (Scheme 1, this work). This Pdcatalyzed cyclization reaction features an intramolecular Received: April 3, 2018

A

DOI: 10.1021/acs.orglett.8b01047 Org. Lett. XXXX, XXX, XXX−XXX

Letter

Organic Letters

(for more detailed information, see Tables S1−S4 in the Supporting Information). With the optimal conditions in hand, we next investigated the generality of this reaction using various urea-tethered alkylidenecyclopropanes 1a−ad as substrates (Scheme 2). As

nucleophilic attack of nitrogen atom to the in situ generated cyclobutyl palladium carbenoid intermediate along with the formation of Pd(IV) upon oxidation and the subsequent reductive elimination. First, we utilized substrate 1a for the initial examination using Pd(OAc)2 (10 mol %) as the catalyst and PhI(OAc)2 (3.0 equiv) as the oxidant with 2.0 equiv of NMe4Cl/NaOAc as a base at 25 °C in ethyl acetate (EtOAc) under argon atmosphere. We were pleased to find that the desired polycyclic product 2a was given in 71% isolated yield within 24 h (Table 1, entry 1). Encouraged by this result, we attempted to further

Scheme 2. Substrate Scope of 1a,b

Table 1. Optimization of the Reaction Conditionsa

entry

cat (mol %)

base

solvent

yieldb (%)

1 2 3 4 5 6 7c 8d 9 10 11 12 13 14 15 16e 17f 18g

Pd(OAc)2 Pd(TFA)2 PdBr2 Pd(MeCN)2Cl2 Pd(PhCN)2Cl2 PdCl2 Pd(OAc)2 Pd(OAc)2 Pd(OAc)2 Pd(OAc)2 Pd(OAc)2 Pd(OAc)2 Pd(OAc)2 Pd(OAc)2 Pd(OAc)2 Pd(OAc)2 Pd(OAc)2 Pd(OAc)2

NMe4Cl/NaOAc NMe4Cl/NaOAc NMe4Cl/NaOAc NMe4Cl/NaOAc NMe4Cl/NaOAc NMe4Cl/NaOAc NMe4Cl/NaOAc NMe4Cl/NaOAc DIPEA DABCO Et3N K2CO3 NMe4Cl/NaOAc NMe4Cl/NaOAc NMe4Cl/NaOAc NMe4Cl/NaOAc NMe4Cl/NaOAc NMe4Cl/NaOAc

EtOAc EtOAc EtOAc EtOAc EtOAc EtOAc EtOAc EtOAc EtOAc EtOAc EtOAc EtOAc CH2Cl2 CH3CN MTBE EtOAc EtOAc EtOAc

71 64 48 63 50 48 trace trace 55 0 trace 22 63 55 59 29 66 26

a Unless otherwise specified, all reactions were carried out using 1 (0.2 mmol), Pd(OAc)2 (10 mol %), PhI(OAc)2 (0.6 mmol), and NMe4Cl/ NaOAc (0.4 mmol) in EtOAc (4.0 mL). bIsolated yield.

a

Unless otherwise specified, all reactions were carried out using 1a (0.2 mmol), oxidant (0.6 mmol), catalyst (10 mol %), and base (0.4 mmol) in solvent (4.0 mL), 25 °C, 24 h. bIsolated yield. cUsing PhI(TFA)2 to replace PhI(OAc)2. dUsing PhI(OPiv)2 to replace PhI(OAc)2. eReaction at 0 °C. fReaction at 50 °C. gReaction at 80 °C.

compared with 1a, substrate 1b, in which R1 is a phenethyl group, produced the desired product 2b in 67% yield. When R1 group is an aromatic substituent, we found that substrates 1i, 1j, and 1l having an electron-poor aromatic ring gave the desired products in better yields than those bearing electron-rich ones, presumably because the desired products have an electron-rich aromatic ring that is not stable under oxidation conditions. The methyl group could be introduced at the ortho-, para-, or metaposition of the benzene ring, affording the corresponding products 2c, 2d, and 2e in 46%, 48%, and 56% yields, respectively. Substrates 1g and 1h having a dimethyl- and a trimethyl-substituted benzene ring also furnished the desired products 2g and 2h in 33% and 40% yields. Substrate 1m with a trichloro-substituted benzene ring afforded the product 2m in 23% yield, perhaps due to the steric effect. For substrate 1n having a strongly electron-withdrawing nitro group at the benzene ring, the corresponding product 2n was produced in 33% yield because it is labile during the purification on silica gel column chromatography. Furthermore, substrates 1o−t with aliphatic substituents were also tolerated, giving the desired products 2o−t in moderate to good yields ranging from 31− 74%. When R1 is a benzyl group, we also examined the substituent effect at the aromatic ring in the ACP moiety and

optimize the reaction conditions, and the results are summarized in Table 1. First, we screened various palladium catalysts such as Pd(TFA)2, PdBr2, Pd(CH3CN)2Cl2, Pd(PhCN)2Cl2, and PdCl2 in this reaction and identified that Pd(OAc)2 was the best catalyst for this cyclization−oxidation reaction (entries 2−6). The use of PhI(TFA)2 or PhI(OPiv)2 as the oxidant to replace PhI(OAc)2 only gave trace of 2a under identical conditions (entries 7 and 8). Therefore, PhI(OAc)2 was the suitable oxidant in this transformation. Using DIPEA, Et3N, DABCO, or K2CO3 as the base did not improve the yield of 2a (entries 9−12). The other organic solvents such as CH2Cl2, MeCN, and MTBE were also examined, giving 2a in moderate yields ranging from 55%−63% (entries 13−15), indicating that EtOAc was the solvent of choice. Carrying out the reaction at 0 °C or raising the reaction temperature also did not improve the reaction outcome (entries 16−18). Overall, this Pd(OAc)2-catalyzed cyclization−oxidation should be carried out in EtOAc using PhI(OAc)2 (3.0 equiv) as the oxidant and NMe4Cl/NaOAc (2.0 equiv) as a base at 25 °C B

DOI: 10.1021/acs.orglett.8b01047 Org. Lett. XXXX, XXX, XXX−XXX

Letter

Organic Letters found that regardless of whether an electron-donating or electron-withdrawing substituent was introduced, the reactions proceeded smoothly at room temperature, affording the desired products 2u−ac in 36−70% yields. The naphthyl moiety was also compatible, giving the desired product 2ad in 33% yield. The structures of 2a and 2v have been determined by X-ray diffraction. Their ORTEP drawing and X-ray crystal data are presented in the Supporting Information. In this Pd(II)catalyzed cyclization−oxidation process, the electron-rich double bond in ACP can be also oxidized by PhI(OAc)2, giving some byproducts and one of these byproducts has been confirmed by 1H NMR and ESI mass spectra (see the Supporting Information). With the purpose of searching for other nitrogen sources, we utilized substrates 1ae−aj to replace substrate 1a in this reaction, but none of the desired product was formed, suggesting that the use of urea groups as nitrogen sources is essential in this Pd(OAc)2-catalyzed cyclization−oxidation process (Scheme 3).

Scheme 5. Proposed Reaction Mechanism

Pd(IV) species V. Reductive elimination from intermediate V provides the desired product 2 along with the regeneration of Pd(OAc)2 catalyst for the next catalytic cycle. Although the use of a catalytic amount of NMe4Cl/NaOAc is possible in this reaction, giving 2a in 56% yield (see Table S4 in the Supporting Information), using 2.0 equiv of NMe4Cl/NaOAc can furnish 2a in 71% yield. In conclusion, we have disclosed a novel Pd(OAc)2-catalyzed intramolecular oxidative cyclization of urea-tethered ACPs with urea as a nitrogen source through a Pd(II)/Pd(IV) catalytic cycle. This reaction featured a ring expansion of alkylidenecyclopropane along with the nucleophilic attack of nitrogen atom in urea moiety onto the in situ generated carbenoid intermediate as well as an oxidation process, affording the corresponding cyclobuta[b]indoline derivatives in moderate to good yields with a broad substrate scope. Further investigations on expanding the scope and applications of this synthetic method are ongoing in our laboratory.

Scheme 3. Examination of Other Substrates

To further illustrate the synthetic utility of this transformation, the obtained 2a was reduced by LiAlH4 in THF to give the corresponding quite labile product 3a in 23% yield, which could be easily transformed into 4a or 4b in moderate yield upon treatment with allyl bromide or acryloyl chloride (Scheme 4). Moreover, a gram-scale synthesis of 2a was realized by use of 1.11 g (4.0 mmol) of 1a under the standard conditions, affording 2a in 61% isolated yield (0.82 g) (Scheme 4).

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Scheme 4. Further Transformation and Gram-Scale Synthesis of 2a

ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.orglett.8b01047. Experimental procedure and characterization data for all compounds (PDF) Accession Codes

CCDC 1535801 and 1583767 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.

On the basis of the previous work15,9,10 and our own examinations, a plausible reaction mechanism has been outlined in Scheme 5. First, the coordination of Pd(OAc)2 to substrate 1 generates the corresponding cyclopropylmethyl cationic intermediate I, which undergoes a ring-expansion process through cyclopropyl carbon migration to afford the corresponding palladium carbenoid intermediate II. Deprotonation of the NH group in the urea moiety by NMe4OAc gives intermediate III. An intramolecular nucleophilic attack of nitrogen atom onto the carbene center produces the corresponding intermediate IV, which is oxidized by PhI(OAc)2 to furnish a

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. ORCID

Min Shi: 0000-0003-0016-5211 Notes

The authors declare no competing financial interest. C

DOI: 10.1021/acs.orglett.8b01047 Org. Lett. XXXX, XXX, XXX−XXX

Letter

Organic Letters

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ACKNOWLEDGMENTS We are grateful for financial support from the National Basic Research Program of China [(973)-2015CB856603], the Strategic Priority Research Program of the Chinese Academy of Sciences (Grant No. XDB20000000) and sioczz201808, and the National Natural Science Foundation of China (Nos. 20472096, 21372241, 21572052, 20672127, 21421091, 21372250, 21121062, 21302203, 20732008, 21772037, and 21772226).

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DOI: 10.1021/acs.orglett.8b01047 Org. Lett. XXXX, XXX, XXX−XXX