Letter Cite This: ACS Catal. 2018, 8, 1308−1312
pubs.acs.org/acscatalysis
Co(III)-Catalyzed Coupling-Cyclization of Aryl C−H Bonds with α‑Diazoketones Involving Wolff Rearrangement Xinwei Hu,†,¶ Xun Chen,†,§,¶ Youxiang Shao,‡ Haisheng Xie,† Yuanfu Deng,† Zhuofeng Ke,*,‡ Huanfeng Jiang,† and Wei Zeng*,† †
Key Laboratory of Functional Molecular Engineering of Guangdong Province, School of Chemistry and Chemical Engineering, South China University of Technology, Guangzhou 510641, China ‡ School of Materials Science and Engineering, PCFM Lab, Sun Yat-sen University, Guangzhou 510275, China § School of Pharmacy, Hainan Medical University, Haikou 571199, China S Supporting Information *
ABSTRACT: An unusual cobalt(III)-catalyzed cross-coupling/cyclization of aryl C−H bonds of N-nitrosoanilines with α-diazoβ-ketoesters has been achieved. This protocol features a unique combination of Csp2-H activation/Wolff rearrangement process, allowing for the rapid assembly of quaternary 2-oxindoles. The empirical evidence and density functional theory (DFT) calculations reveal the trapping process of transient acceptor ketene intermediates by cobalt metallocycles. KEYWORDS: cobalt catalysis, coupling-cyclization, α-diazoketones, Wolff rearrangement, quaternary 2-oxindoles
K
Scheme 1. Transition-Metal-Catalyzed Selective Transformations of Diazo Compounds
etenes are extremely important transient synthons in organic chemistry.1 Over the past decades, transitionmetal-catalyzed cycloaddition and cross-coupling of ketenes with different reaction partners have been evidenced to enable assembly of complex carbonyl compounds,2 in which most types of ketenes belong to donor ketenes such as α-alkyl ketenes and α-alkylaryl ketenes (generated in situ from acyl chlorides).2j,k In comparison, the chemical transformations involving acceptor ketenes such as α-acyl ketenes have not been well established even under the photolysis and high-temperature conditions, possibly because acceptor ketenes are frequently unavailable,3 albeit various acceptor ketenes could provide a potential platform to construct structurally diverse carbonyl compounds. Therefore, developing new and efficient catalytic systems to trap transient acceptor ketenes under mild conditions is highly desirable. Direct carbenoid functionalization of inert C−H bonds belongs to one of the most challenging topics in organic synthesis.4 Recently, chelation-assisted cross-coupling of C−H bonds with diazo compounds has been extensively investigated for site-selectively installing a new C−C bond onto a coupling partner.5 Most particularly, cobalt-catalyzed C−H functionalization has aroused great attention due to its low cost and environmental benignity.6,7 In this context, Glorius,8 Ackermann,9 and Wang10 successively reported the Co(III)-catalyzed cross-coupling of aryl C−H bonds with diacceptor diazo compounds through carbene migratory insertion under chelation-assistance of pyridines, ketamines, amidines, and pyrazoles (Scheme 1a). However, transition-metal-catalyzed © XXXX American Chemical Society
Wolff rearrangement of diazo compounds still remains rather elusive under C−H activation system. Up to now, only noble metal salts could allow for the Wolff rearrangement of αdiazoketones, resulting in the formation of donor ketenes (Scheme 1b),11 but earth-abundant first-row transition-metalcatalyzed Wolff rearrangement has not been reported yet. As the continuation of our interest in cobalt-catalyzed C−H functionalization,12 we herein explored a unprecedented cyclic Received: October 26, 2017 Revised: January 11, 2018 Published: January 16, 2018 1308
DOI: 10.1021/acscatal.7b03668 ACS Catal. 2018, 8, 1308−1312
Letter
ACS Catalysis coupling of N-nitrosoanilines13 (A) with diacceptor diazo compounds, in which cobalt complex-promoted Wolff rearrangement delivered acceptor α-acyl ketenes. This protocol represents a unique platform to combine C−H activation/ Wolff rearrangement for assembling quaternary 2-oxindoles14 (C) (Scheme 1c), which are the cornerstone of many natural products and medicinal molecules.15 Our initial attempt to construct quaternary 3,3-disubstituted 2-oxindole (3a) involved cyclized coupling of N-nitrosaniline (1a) with α-diazo-β-ketoester (2a) in the presence of NaOAc (20 mol %) and various cobalt catalysts (at 120 °C for 24 h under Ar; Table 1, entries 1−6). To our delight, we quickly
optimize the reaction conditions with different solvent systems including toluene and DMF. Unfortunately, these solvents did not deliver the desired product 3a at all (entries 19 and 20) [see Supporting Information (SI) for more details]. To explore the scope of the Co(III)-catalyzed 3,3disubstituted-2-oxindole synthesis, an array of different functional groups were introduced into the benzene ring of Nmethyl-N-nitrosoaniline 1. As summarized in Table 2, substitution at the 2- or 4-position of the phenyl ring did not obviously affect the yield of the 2-oxindoles 3. Electrondonating substitutents such as methyl and methoxyl group were effective and provided 74−80% yields of products (3a−3d). Meanwhile, reactions also proceeded smoothly with various
Table 1. Optimization of Reaction Conditionsa
Table 2. Substrate Scopea,b
entry
catalysts
additives
solvent
yield (%)b
1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20
CoCl2 Co(acac)2 Co(OAc)3 Co(acac)3 Cp*CoLnc Cp*Co(CO)I2 Cp*Co(CO)I2 Cp*Co(CO)I2 Cp*Co(CO)I2 Cp*Co(CO)I2 Cp*Co(CO)I2 Cp*Co(CO)I2 Cp*Co(CO)I2 Cp*Co(CO)I2 Cp*Co(CO)I2 Cp*Co(CO)I2 Cp*Co(CO)I2 Cp*Co(CO)I2 Cp*Co(CO)I2 Cp*Co(CO)I2
KOAc KOAc KOAc KOAc KOAc KOAc NaOAc NaOPiv PivOH FeCl3 Cu(OAc)2 B(C6F5)3 Sc(OTf)3 Zn(OAc)2 Zn(OAc)2 Zn(OAc)2 Zn(OAc)2 Zn(OAc)2 Zn(OAc)2 Zn(OAc)2
DCE DCE DCE DCE DCE DCE DCE DCE DCE DCE DCE DCE DCE DCE DCE DCE DCE TCEh toluene DMF
0 13 0 0 18 26d 19d 25d 24d 11d
