Synthesis of 1,2-Dihydroquinolines by Co(III)-Catalyzed [3 + 3

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Synthesis of 1,2-Dihydroquinolines by Co(III)-Catalyzed [3 + 3] Annulation of Anilides with Benzylallenes Ramajayam Kuppusamy, Rajagopal Santhoshkumar, Ramadoss Boobalan, Hsin-Ru Wu, and Chien-Hong Cheng ACS Catal., Just Accepted Manuscript • DOI: 10.1021/acscatal.7b04087 • Publication Date (Web): 24 Jan 2018 Downloaded from http://pubs.acs.org on January 24, 2018

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ACS Catalysis

Synthesis of 1,2-Dihydroquinolines by Co(III)-Catalyzed [3 + 3] Annulation of Anilides with Benzylallenes Ramajayam Kuppusamy,† Rajagopal Santhoshkumar,† Ramadoss Boobalan,† Hsin-Ru Wu‡ and ChienHong Cheng*,† † ‡

Department of Chemistry, National Tsing Hua University, Hsinchu 30013, Taiwan. Instrumentation Center, MOST, National Tsing Hua University, Hsinchu 30013, Taiwan.

Supporting Information Placeholder ABSTRACT: A cobalt-catalyzed C–H/N–H annulation of anilides with allenes to synthesize 1,2-dihydroquinolines is described. The reaction proceeds via a C–H activation, allene insertion, followed by β-hydride elimination and an intramolecular 1,4-addition to a butadiene-group-containing intermediate. Allenes act as a three-carbon source in the present C–H activation strategy. KEYWORDS: cobalt, C–H activation, annulation, allenes, anilides, 1,4-addition

The metal-catalyzed inert C–H functionalization has emerged as an efficient methodology in organic synthesis owing to their atom- and step-economical manner.1,2 In particular, the development of cost-effective reactions using more abundant firstrow transition metal catalysts has received great attention in recent years.3,4 In this context, less expensive cobalt catalysts have played an essential role in C−H activation for selective C−C bond formation.4 Of these reactions, the oxidative annulation of arenes with π-components such as alkynes, alkenes and allenes has become a versatile and straightforward strategy for the synthesis of diverse heterocycles.5,6

Scheme 1. Annulation of Arenes with Allenes Allenes are interesting unsaturated coupling partner due to the presence of three reactive carbon centres7 resulting in rich regio-, chemo-, and diastereoselectivity.8 In the C–H functionalization with allenes, the selectivity is generally the focus of the study. Until now, allenes are known to act as one or two carbon sources in transition-metal-catalyzed C–H annulation (Scheme 1).6,9 In this context, we recently demonstrated a cobalt-catalyzed oxidative C–H/O–H annulation of 2vinylphenols with allenes for the synthesis of 2H-chromenes in which allene was a one carbon source.6a Similarly, bidentate directing group-assisted reactions provided [4 + 2] annulation products in which allenes acted as a two carbon source via either metal-π-allyl or metal-alkenyl intermediates.6b-g Encouraged by these results and our continuous research on cobalt-

catalyzed C–H activation reactions,5d-e,6a,6g,10 we are highly curious to know the selectivity of allenes with different arenes using more abundant cobalt catalyst. Herein, we wish to report the synthesis 1,2-dihydroquinolines(1,2-DHQ) by cobalt(III)catalyzed oxidative [3 + 3] annulation of anilides with allenes. 1,2-DHQ is a privileged heterocyclic platform for the synthesis of biologically active molecules such as antioxidant, antibacterial, natural products, and antijuvenile hormone insecticides.11 Initially, buta-2,3-dien-1-ylbenzene (2a) was treated with N-(p-tolyl)acetamide (1a)12 in the presence of [Cp*Co(CO)I2] (0.020 mmol), AgSbF6 (0.040 mmol), Cu(OAc)2·H2O (0.040 mmol) and Ag2O (0.20 mmol) in 1,2-dichloroethane at 80 °C for 15 h. The reaction gave 1-(4,6-dimethyl-2-phenylquinolin1(2H)-yl)ethan-1-one (3aa) in 60% yield with a trace of 4,6dimethyl-2-phenylquinoline (4aa) (see Supporting Information). The effect of solvent on the product yield was tested using different solvents. Among the screened solvents, nitromethane was most effective, giving product 3aa in 90% yield (Table 1, entry 1). The presence of acetate appears important in enhancing the product yield. Various metal acetates were tested, and Cu(OAc)2·H2O afforded the highest yield (Table 1, entries 1‒7). Similarly, AgSbF6 is also a crucial additive for the success of the reaction whereas other additives such as NaSbF6, AgPF6, AgOTf and AgBF4 were less effective (Table 1, entries 8‒12). Next, we tried different oxidants for the present catalytic reaction. Among the oxidants, Ag2CO3 gave the desired product 3aa in 95% yield and other oxidants provided 22‒88% yields (Table 1, entries 13‒18). These results indicate that acetate ion boosts the removal of proton in the C–H activation step, but a high concentration would inhibit the coordination of substrates to cobalt and reduces the product yield. The HOAc produced during the reaction will likely be neutralized with the poor soluble Ag2CO3 to form AgOAc which acts as the oxidant and regenerate acetate ion. As a result, the OAcconcentration is kept at low concentration level in the catalytic solution.

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Under the optimized reaction conditions, the scope of anilides 1 was examined using 2a as the allene substrate in the present cobalt-catalyzed [3 + 3] annulation reaction (Table 2). Acetanilide and para-substituted (OMe and Ph) acetanilides 1b-d proceeded well with 2a to give products 3ba-da in excellent yields. Similarly, halogen functionalities on the aryl ring of acetanilides 1e-f were amenable to the reaction conditions, providing products 3ea–fa in 75‒78% yields. Gratifyingly, acetanilides 1g-h with a para-substituted electron withdrawing group proceeded very well to afford the annulation products 3ga–ha in good to excellent yields. The structure of 3ga was further confirmed by X-ray crystallographic analysis. Nitro substituted acetanilide 3i also participated well in the reaction, but the reaction gave the desired product 3ia and aromatized product 4ia in 65% and 13% yields, respectively. meta-Substituted acetanilide 1k provided single regioisomeric product 3ka in 86% yield in which the C–H activation occurred at the less-hindered position of 1k. Similarly, the reaction of N-(naphthalen-1yl)acetamide (1l) and N-(naphthalen-2-yl)acetamide (1m) with 2a gave the annulation product 3la-ma in 82% and 77% yields, respectively. Sterically-hindered 2-substituted acetanilides 1n-o reacted well with 2a under the optimized reaction conditions to give 3na-oa in good yields. Finally, 2,5dimethoxy acetanilide 1p also participated in the reaction to give product 3pa in 73% yield. Table 1. Optimization Studiesa,b

Entry

Additive 1

Additive 2

Oxidant

3aa Yield (%)

1 2 3

Cu(OAc)2·H2O Cu(OPiv)2 Mn(OAc)3·2H2O

AgSbF6 AgSbF6 AgSbF6

Ag2O Ag2O Ag2O

90 20 54

4 5 6 7 8 9 10 11 12 13 14c 15c 16 17c 18c

NaOAc Fe(OAc)2 AcOH -Cu(OAc)2·H2O Cu(OAc)2·H2O Cu(OAc)2·H2O Cu(OAc)2·H2O Cu(OAc)2·H2O Cu(OAc)2·H2O Cu(OAc)2·H2O Cu(OAc)2·H2O Cu(OAc)2·H2O Cu(OAc)2·H2O Cu(OAc)2·H2O

AgSbF6 AgSbF6 AgSbF6 AgSbF6 NaSbF6 AgPF6 AgOTf AgBF4 -AgSbF6 AgSbF6 AgSbF6 AgSbF6 AgSbF6 AgSbF6

Ag2O Ag2O Ag2O Ag2O Ag2O Ag2O Ag2O Ag2O Ag2O Ag2CO3 AgOTf Cu(OAc)2·H2O O2 AgBF4 AgOAc

0 14 48 48 16 82 74 66 0 95 18 62 24(14)d 22(26)d 82

a

Unless otherwise mentioned, all reactions were carried out using 1a (0.20 mmol), 2a (0.30 mmol), [Cp*Co(CO)I2] (0.020 mmol), additive 1 (0.040 mmol), additive 2 (0.040 mmol), oxidant (0.20 mmol) and MeNO2 (2.0 mL) at 80 °C for 15 h under N2. bYields were determined by the 1H NMR integration method. cOxidant (0.40 mmol). dYield of 4aa is in the parenthesis.

This present catalytic reaction was successfully extended to various substituents on the nitrogen of anilines 1q-t with 2a

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(Table 2). Interestingly, 1,1-dimethyl-3-(p-tolyl)urea (1q) proceeded well in the reaction, delivering [3+3] annulation products 3qa and 4aa (= 4qa) in 26% and 60% yields, respectively. Other anilines 1r-t with a different acyl (RCO) or ester group tolerated well in the present reaction conditions to afford the expected products 3ra-ta in good yields. Next, we explored the reaction of different allenes 2b-i with 1a under the optimized reaction conditions (Table 3). Thus, the reactions of ortho, meta, and para substituted benzylallenes 2b-h with 1a afforded products 3ab-ah in good to excellent yields. Similarly, 1-naphthylmethyl allene 2i also afforded product 3ai in 92% yield. It is noteworthy that alkyl or internal allenes are not suitable for the present reaction. A possible reason is that internal allenes are more difficult for coordination and insertion, while long chain alkyl allenes readily undergo carboncarbon double bond migration leading to unidentified products. Table 2. Cobalt-Catalyzed Annulation of Anilides 1 with Allene 2a.a,b

a

1 (0.33 mmol), 2a (0.50 mmol), [Cp*Co(CO)I2] (0.030 mmol), AgSbF6 (0.070 mmol), Cu(OAc)2·H2O (0.070 mmol), Ag2CO3 (0.33 mmol) and CH3NO2 (2.0 mL) at 80 °C for 15 h under N2. b Isolated yields. cRatio of 3:4.

To understand the mechanism of the present cobaltcatalyzed annulation reaction, we conducted a series of deuterium-labelling experiments (Scheme 2). Thus, treatment of [D5]-1b under the standard reaction conditions gave 50% D/H exchange in the ortho position of [D5]-1b. In a similar manner, the reaction [D5]-1b with 2a gave product [D5]-3ba in 82%

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ACS Catalysis yield with 13% D/H exchange (Scheme 2a). These results reveal that the C–H activation step is reversible. Next, the competition reaction of a mixture of [D5]-1b and 1b with 2a gave an intermolecular kinetic isotopic effect (KIE) of 3.4 whereas the parallel reaction provided a KIE of 1.8 (Scheme 2b). These results indicate that the C–H activation step may be involved in the rate limiting step. In addition, we prepared diene 5aa from substrates 1a and 2a using a ruthenium complex as catalyst (Scheme 2c). Treatment of 5aa under the standard reaction conditions in the presence or absence of cobalt catalyst proves that intramolecular 1,4-addition takes place in presence of cobalt catalyst without silver or copper additive (Scheme 2d). These results indicated that 5aa is plausibly a catalytic intermediate and support that the reaction proceeds via intramolecular 1,4-addition of N-H to the diene group in 5aa. Table 3. Cobalt-Catalyzed Annulation of Anilide 1a with Allenes 2.a,b

ceeds via a β-hydride elimination and a 1,4-addition of N–H with diene. 1,2-DHQ could be readily transformed into useful 1,2,3,4-tetrahydroquinolines and quinolines.

Scheme 2. Mechanistic Studies

a

As in footnote a, Table 2. bIsolated yields.

Furthermore, 1,2-DHQ was easily converted to the corresponding quinoline and, 1,2,3,4-tetrahydroquinoline which are present in various bioactive molecules and natural products.13 Thus, the reaction of product 3aa with potassium hydroxide gave quinoline 4aa in 87% yield. Likewise, 3aa was hydrogenated in the presence of Pd/C under hydrogen atmosphere to provide tetrahydroquinoline 6aa in excellent yield (Scheme 3). Based on the results of mechanistic studies and literature precedence,5i,9c,14 a possible mechanism for the cobalt(III)catalyzed [3 + 3] oxidative annulation of anilides with allenes is proposed in Scheme 4. The reaction starts by the generation of active cobalt(III) complex I from [Cp*Co(CO)I2], Cu(OAc)2·H2O and AgSbF6. Ortho C–H metalation of anilide 1a with I followed by allene insertion, β-hydride elimination and reductive elimination gives 5aa and Co(I). Then, Co(I) oxidation by silver ion, deprotonative coordination of N–H to Co(III), followed by 1,4-addition of N-Co to the diene group in IV provides product 3aa. In summary, we have successfully demonstrated a new cobalt-catalyzed C–H/N–H annulation of anilides with allenes to synthesize 1,2-DHQ. In the reaction, allene formally acts as a three-carbon source in C–H activation approach to give [3 + 3] annulation product in first time. The reaction possibly pro-

Scheme 3. Synthesis of Quinoline and Tetrahydroquinoline

Scheme 4. Plausible Mechanism

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ASSOCIATED CONTENT AUTHOR INFORMATION Corresponding Author *E-mail: [email protected]. Home page: http://mx.nthu.edu.tw/~chcheng/. Notes

The authors declare no competing financial interest. Supporting Information

The Supporting Information is available free of charge on the ACS Publications website at DOI: General experimental procedures, characterization details, and copies of 1H and 13C NMR spectra of new compounds (PDF) X-ray data for 3ga (CIF)

ACKNOWLEDGMENT We thank the the Ministry of Science and Technology of the Republic of China (MOST 105-2633-M-007-003) for support of this research.

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