Iron-Catalyzed Dehydrogenative [4 + 2] Cycloaddition of Tertiary

Letter pubs.acs.org/acscatalysis

Iron-Catalyzed Dehydrogenative [4 + 2] Cycloaddition of Tertiary Anilines and Enamides for the Synthesis of Tetrahydroquinolines with Amido-Substituted Quaternary Carbon Centers Mi-Na Zhao,† Le Yu,† Rong-Rong Hui, Zhi-Hui Ren, Yao-Yu Wang, and Zheng-Hui Guan* Key Laboratory of Synthetic and Natural Functional Molecule Chemistry of Ministry of Education, Department of Chemistry & Materials Science, Northwest University, Xi’an 710127, P. R. China S Supporting Information *

ABSTRACT: An iron-catalyzed dehydrogenative [4 + 2] cycloaddition reaction of tertiary anilines and enamides for the synthesis of tetrahydroquinolines with amido-substituted quaternary carbon centers has been developed. The reaction proceeds through iron-catalyzed dehydrogenation of tertiary anilines followed by nucleophilic addition/intramolecular cyclization with enamides to afford tetrahydroquinolines with amido-substituted quaternary carbon centers in good yields. The reaction tolerates a wide range of functional groups and proceeds under mild conditions. The mechanism for the dehydrogenative [4 + 2] cycloaddition has been confirmed by DFT calculations. KEYWORDS: iron-catalyzed, dehydrogenative [4 + 2] cycloaddition, enamide, tertiary anilines, tetrahydroquinoline dienes, [4 + 2] cycloaddition of aromatic π-components is considerable challenging because of the high stability of aromatic rings.10 In this paper, we describe the development of an iron-catalyzed dehydrogenative [4 + 2] cycloaddition of tertiary anilines and enamides for the synthesis of tetrahydroquinolines with amido-substituted quaternary carbon centers. Enamides which are versatile building blocks in organic synthesis have been widely applied in nucleophilic addition reactions to synthesize pharmaceutical compounds.11,12 Recently, intramolecular nucleophilic addition of enamides to electrophiles, such as aldehydes and nitriliums, has been developed for the efficient construction of azaheterocycles.13,14 In light of the electrophilic nature of N-aryl iminium ion, we hypothesized that a nucleophilic dienophile, such as enamides, would be matched for the dehydrogenative [4 + 2] cycloaddition reaction. Additionally, [4 + 2] cycloaddition of N-aryl iminium ion and 1,1-disubstituted enamides may produce sixmembered rings bearing amido-substituted quaternary carbon centers. We began our study by utilizing N,N-dimethylaniline 1a and enamide 2a as model substrates to test the dehydrogenative [4 + 2] cycloaddition reaction (Table 1). We were pleased to find that tetrahydroquinoline with amido-substituted quaternary carbon center 3aa was indeed observed in 8% yield in the presence of TBHP in DCE at 60 °C (Table 1, entry 1). To

[4 + 2] Cycloaddition reaction is one of the most efficient and atom-economic ways to construct six-membered rings.1 Over the past decades, [4 + 2] cycloaddition strategies have been widely applied in the synthesis of six-membered natural products, biologically active compounds, and pharmaceuticals (Scheme 1a).2 However, the direct construction of sixmembered compounds bearing quaternary carbon centers by [4 + 2] cycloaddition reaction is still a challenging task.3,4 Furthermore, the inherent character of the [4 + 2] cycloaddition is the use of conjugated dienes or heterodienes as the starting materials, which is sometimes not conveniently accessible. Recently, dehydrogenative [4 + 2] cycloaddition reactions have emerged as a promising protocol for the construction of six-membered complex molecular scaffolds (Scheme 1b). The pioneering Ni/AlMe3-cocatalyzed dehydrogenative [4 + 2] cycloaddition of formamides with alkynes through double C−H activation has been developed by the Nakao and Hiyama group.5 Pd-catalyzed dehydrogenative [4 + 2] cycloaddition of simple terminal olefins with maleimides has been developed by White and co-workers.6,7 In addition, a Lewis acid and NHC cocatalyzed oxidative [4 + 2] cycloaddition of enals and trifluoromethyl ketones for the enantioselective construction of unsaturated lactones has been developed by Chi and co-workers.8 Very recently, a DDQmediated dehydrogenative [4 + 2] cycloaddition of 2-methyl-3alkylindoles with dienophiles has been developed to synthesize tetrahyrocarbazoles, carbazoles, and heteroacenes by the Zhang group.9 Inspired by these achivements, we envisioned that the dehydrogenation of tertiary anilines would generate a N-aryl iminium ion which may act as a [4π]-diene for cycloaddition reaction (Scheme 1). However, in comparison with conjugated © XXXX American Chemical Society

Received: March 23, 2016 Revised: April 26, 2016

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ACS Catalysis Table 1. Optimization of the Reaction Conditionsa

Scheme 1. Different Strategies for [4 + 2] Cycloaddition Reaction

entry

catalyst

oxidant

solvent

t (°C)

yield (%)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15

--CuCl CuBr CuI FeCl2 FeCl3 Fe(OTf)3 FeCl3 FeCl3 FeCl3 FeCl3 FeCl3 FeCl3 FeCl3 FeCl3

TBHP TBHP TBHP TBHP TBHP TBHP TBHP TBHP TBHP TBHP DDQ BQ DTBP TBHP TBHP

DCE DCE DCE DCE DCE DCE DCE toluene dioxane CH3CN CH3CN CH3CN CH3CN CH3CN CH3CN

60 60 60 60 60 60 60 60 60 60 60 60 60 40 80

8 16 25 5 34 46 19 32 5 65 0 0 27 54 42

a Reaction condition: 1a (0.3 mmol), 2a (0.2 mmol), catalyst (20 mol %), oxidant (0.4 mmol), solvent (3 mL); isolated yields. TBHP = tertbutyl hydrogenperoxide (5−6 M in nonane), DDQ = 2,3-dichloro-5,6dicyano-1,4-benzoquinone, BQ = p-benzoquinone, DTBP = di-tertbutyl peroxide.

enamide proceeded smoothly to afford the desired tetrahydroquinoline 3am in 66% yield. These results indicated that the electronic nature of the enamides has little effect on the [4 + 2] cycloaddition reaction. 2-Naphthyl enamide 2n participated in the reaction smoothly to give the desired tetrahydroquinoline 3an in 56% yield. Furthermore, heterocyclic enamides, such as thienyl enamide 2o and pyridyl enamide 2p, proceeded smoothly as well to give the corresponding tetrahydroquinolines with amido-substituted quaternary carbon centers 3ao− 3ap in 70% and 65% yields, respectively. It should be noted that the corresponding ketone was observed as the main byproduct in the above reactions. The N-propionyl enamide 2q showed similar reactivity with N-acetyl enamide 2a to give the corresponding tetrahydroquinoline 3aq in 66% yield. The cyclic enamide 2r was also successfully performed the dehydrogenative [4 + 2] cycloaddition reaction to give the tetracyclic product 3ar in 52% yield. Furthermore, aliphatic enamides 2s and 2t were tolerated in the reaction to afford the tetrahydroquinolines 3as−3at in moderate yields. However, no reaction occurred when βsubstituted enamide, such as N-styrylacetamide, was used as the substrate. Next, the scope of tertiary anilines was investigated, and the results are summarized in Table 3. N,N-Dimethylanlines with methyl, methoxy, or halogen groups such as fluoro, chloro, and bromo on the aromatic ring reacted smoothly to give the corresponding tetrahydroquinolines with amido-substituted quaternary carbon centers 3ba−3ga in good yields. The reaction of N,N-dimethyl-3-methylaniline 1c gave two regioisomers 3ca and 3ca′ in total 63% yield with a 3:1 ratio, and the reaction mainly occurred at the less sterically hindered position of N,N-dimethyl-3-methylaniline 1c. N,N-Dimethyl-1naphthylamine 1h also underwent the reaction to give the corresponding tetrahydroquinoline 3ha in 43% yield. However,

improve the reaction efficiency, Cu and Fe salts were chosen as the catalysts because they were regarded as efficient catalysts for dehydrogenation of tertiary amines.15−17 Expectedly, a 25% yield of product 3aa was obtained when CuBr was used as catalyst, while CuCl and CuI were inferior to CuBr (Table 1, entries 2−4). Gratifyingly, FeCl3 was found to be the effective catalyst by screening of different iron catalysts, to give the desired tetrahydroquinoline 3aa in 46% yield (Table 1, entries 5−7). For further improving the reaction efficiency, different solvents such as toluene, 1,4-dioxane, CH3CN, and different oxidants such as DDQ, BQ, DTBP, were optimized (Table 1, entries 8−13). It was found that the optimal solvent and oxidant for the reaction is CH3CN and TBHP, respectively (Table 1, entry 10). Finally, the reaction temperature was also varied, and 60 °C gave the best result (Table 1, entries 14−15). With the optimized reaction conditions established, a series of enamides were investigated for extending the substrate scope (Table 2). This Fe-catalyzed dehydrogenative [4 + 2] cycloaddition reaction displayed good functional group tolerance. Enamides with electron-neutral or electron-donating groups on aryl rings, such as alkyl, phenyl, methoxyl, and amino, all gave the corresponding tetrahydroquinolines with amido-substituted quaternary carbon centers 3ab−3ah in good yields. Enamides with an electron-withdrawing group such as fluoro, chloro, bromo, and iodo were also tolerated and afforded the corresponding tetrahydroquinolines 3ai−3al in moderate to good yields. Moreover, trifluoromethyl substituted 3474

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ACS Catalysis Table 2. Fe-Catalyzed Dehydrogenative [4 + 2] Cycloaddition of Enamides and N,N-Dimethyl Anilinea

Table 3. Fe-Catalyzed Dehydrogenative [4 + 2] Cycloaddition of Enamide 2a and Tertiary Anilinesa

a

Reaction condition: 1 (0.3 mmol), 2a (0.2 mmol), FeCl3 (20 mol %), TBHP (5−6 M in nonane) (0.4 mmol), CH3CN (3 mL); Isolated yields. bThe regioisomeric ratio was determined by NMR analysis.

To demonstrate the utility of this reaction, we have carried out the dehydrogenative [4 + 2] cycloaddition of N,N-dimethyl aniline 1a and enamide 2u (Scheme 2). The tetrahydroquinoScheme 2. Synthesis and Application of Tetrahydroquinolines

a

Reaction conditions: 1a (0.3 mmol), 2 (0.2 mmol), FeCl3 (20 mol %), TBHP (5−6 M in nonane) (0.4 mmol), CH3CN (3 mL); isolated yields. bDCE was used as solvent.

only trace of the desired tetrahydroquinoline was observed when ortho-bromo-N,N-dimethylanline 1i or 4-cyano-N,Ndimethylaniline was used as the substrate. It was assumed that these sterically hindered and electron-poor tertiary anilines are less reactive in the dehydrogenation step. Furthermore, it was found that the reaction prefer to occur on N-methyl substituted tertiary anilines. When N-ethyl-Nmethylaniline 1j, N-benzyl-N-methylaniline 1k, and ethyl 2(methyl(phenyl)amino)acetate 1l were employed as the substrates, the tetrahydroquinolines 3ja−3la were obtained as the only products in low yields, along with recovered the most of starting materials (Table 3). No reaction occurred when N,N-diethylaniline was employed as the substrate. In addition, aliphatic amines, such as pyrrolidine and piperidine, were inactive under the reaction conditions.

line 3au, which is a valuable synthetic intermediate, was obtained in 49% yield under the standard conditions. The reaction is particularly useful, as the resulting product tetrahydroquinoline 3au could be applied to synthesis ethyl 4-(dimethylamino)-1-methyl-1,2,3,4-tetrahydroquinoline-4-carboxylate 4 and (4-(dimethylamino)-1-methyl-1,2,3,4-tetrahydroquinolin-4-yl) methanol 5 that has been applied in the treatment of gastrointestinal diseases.18,19 3475

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ACS Catalysis

tertiary aniline 1a was performed under the standard conditions (Scheme 5). The dehydrogenative coupling product 3av′ was

On the basis of the above results, tentative mechanisms for this dehydrogenative [4 + 2] cycloaddition reaction are proposed in Scheme 3. First, the dehydrogenation of N,N-

Scheme 5. Cross Dehydrogenative Coupling of Tertiary Aniline 1a and Enamide 2v

Scheme 3. Tentative Mechanism for Dehydrogenative [4 + 2] Cycloaddition Reaction

dimethylaniline 1a in the presence of [Fe]/TBHP generated the iminium ion 1a′.17 Next, two pathways are possible for the following steps. In one case (path a), nucleophilic addition of enamide 2a to iminium ion 1a′ afforded the intermediate A, which undergoes a subsequent intramolecular cyclization and deprotonation to give the final tetrahydroquinoline 3. Alternatively (path b), the intermediate B was directly formed through a concerted transition state TS-3. Then, deprotonation of the intermediate B generated the tetrahydroquinoline 3. In order to verify the availability of the above presented mechanisms, DFT calculations was performed with B3LYP functional and the 6-31G(d) basis set. Although we have set different initial guess geometry in the transition-state optimization for the first step addition along the two pathways, we only obtained the same optimized structure for the stepwise pathway (path a). The six-membered ring transition state TS-3 corresponding to the concerted pathway (path b) cannot be located. The free energy profile for the pathway a and the structure of transition states are given in Scheme 4. The

obtained in 50% yield instead of the tetrahydroquinoline 3av. The product 3av′ was formed via tautomerization of the intermediate A′ due to the steric effect of ortho-methyl substituent on enamide 2v. This result indicates that the path a is reasonable for this dehydrogenative [4 + 2] cycloaddition reaction, which is consistent with the result of DFT calculations. In conclusion, an iron-catalyzed dehydrogenative [4 + 2] cycloaddition of tertiary anilines and enamides for the synthesis of tetrahydroquinolines with amido-substituted quaternary carbon centers has been developed in this paper. The reaction tolerated a wide range of functional groups and proceeded under mild conditions. DFT calculations and experimental investigation suggested that the reaction proceeded through a dehydrogenation/nucleophilic addition/intramolecular cyclization cascade pathway. Further studies on the scope of the reaction is in progress in our laboratory.

Scheme 4. DFT-Computed Energy Surface

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acscatal.6b00849. Detailed experimental procedures, characterization data and copies of NMR spectra for all products, and Cartesian coordinates of all computed structures (PDF)

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ASSOCIATED CONTENT

S Supporting Information *

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Author Contributions †

These authors contributed equally (M.-N.Z. and L.Y.).

Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was supported by generous grants from the National Natural Science Foundation of China (21272183 and 21472147) and Fund of the Rising Stars of Shanxi Province (2012KJXX-26).

calculated activation free energy of nucleophilic addition of enamide 2a and iminium ion 1a′ to form intermediate A via transition state TS-1 is only 10.3 kcal/mol, and subsequently, the reaction overcomes the transition state TS-2 barrier of only 8.6 kcal/mol to afford the intermediate B. It is obvious that path a is favorable in the thermodynamic control reaction of [4 + 2] cycloaddition of 1a′ and 2a. To further confirm the path a, dehydrogenative [4 + 2] cycloaddition of ortho-methyl substituted enamide 2v and

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