Tritylium Cation as Low Loading Lewis Acidic ... - ACS Publications

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Tritylium Cation as Low Loading Lewis Acidic Organocatalyst in Povarov Reactions Yu Huang, Chuanhong Qiu, Zhenjiang Li, Weiyang Feng, Haifeng Gan, Jingjing Liu, and Kai Guo* State Key Laboratory of Materials-Oriented Chemical Engineering, College of Biotechnology and Pharmaceutical Engineering, Nanjing Tech University, 30 Puzhu Rd S., Nanjing 211816, China S Supporting Information *

ABSTRACT: Triphenylmethylium cation catalyzed Povarov reactions with excellent yields in 1 h with a remarkably low loading (0.5 mol %) are disclosed. A Lewis acidic carbenium catalysis mechanism was proposed and validated. A three-component Povarov reaction in a batch was transferred into a two-stage microflow process, which resulted in a 60-fold reduction in reaction time from 2 h to 2 min with 89% separated yield.

KEYWORDS: Carbenium, Povarov reaction, Organocatalysis, Low loading, Tetrahydroquinolines

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INTRODUCTION The formal [4 + 2] cycloaddition of an electron-rich dienophile with a 2-azadiene, importantly an N-aryl imine or iminium ion, is one of the most efficient strategies leading to 1,2,3,4tetrahydroquinolines. This named Povarov reaction1 provides diversity of entries into tetrahydroquinolines that have shown a broad range of biological activities.2−6 2-Azadienes are rather unreactive in Povarov and to be activated by coordinating with acidic catalyst or promoter to enhance their electron-deficient character (Scheme 1).7 For electrophilic activations, Lewis acid has been the most commonly used one since the original works of Povarov.1 Kobayashi and co-workers led a great breakthrough in 1995, which showed that a small amount of lanthanide triflates are excellent catalysts for imino Diels−Alder reactions.8 A series of metal-based Lewis acids have been developed to accelerate the reactions.9−13 In the past decade, Brønsted acids came to the center stage since the pioneering works of Akiyama group on the first enantioselective organocatalytic Povarov reaction by chiral phosphoric acid.14−22 Besides metal Lewis acid and phosphoric acid catalysts, alternative catalysts or promoters expanded the spectrum. Representative ones are solid acid,23,24 selectfluor,25,26 radical cations,27,28 and α-chymotrypsin.29 Despite their significance, most of these catalysts suffered from disadvantages such as expensiveness, high loading, long reaction time, and lacking of tunablility. Therefore, developing green and efficient catalysts for this reaction remains a significant challenge. Lewis acidic organocatalyst, a novel and metal-free catalyst that possesses great potential due to its broad variety of possible activation modes, has attracted interest.30,31 One of our focuses was organic ionic salt, which has a low-lying LUMO © XXXX American Chemical Society

that can accept an electron pair as typical Lewis acid. So far, several types of oniums including imidazolium,32,33 pyridinium,34 and phosphonium35 have been documented and shown good catalytic activity for Povarov, although somewhat high catalyst loading and long reaction time are recorded. We envisioned onium of carbon at the center,36 i.e., carbenium ion, a highly active electron deficient species, was a powerful candidate Lewis acidic promoter to Povarov reaction. Carbeniums are a family of versatile species in chemistry. Besides serving as key intermediates in several fundamental transformations, they are highly powerful Lewis acid (co)catalysts for polymerizations.30 The substitution around the trivalent carbon center could provide unique opportunities for tuning their stability and reactivity. However, there is a limited number of examples on carbenium-based Lewis acid catalysis, presumably due to the undefined nature of the carbenium in some reactions. Mukaiyama and co-workers started the pioneering works 30 years ago by using triphenylmethylium (tritylium) salts as catalysts for Mukaiyama aldol reactions37 and Sakurai allylations.38 However, the mechanisms of the reactions have been a subject of controversy because tritylium may initiate formation of silylium, or side product Brønsted acidic species could also promote these reactions. Very recently, Franzén39,40 provided strong support for the carbocationic nature in the catalysis by excluding the involvement of competing silyl-based Lewis acid or derived Brønsted acid in the catalysis. Received: October 28, 2015 Revised: November 24, 2015

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DOI: 10.1021/acssuschemeng.5b01379 ACS Sustainable Chem. Eng. XXXX, XXX, XXX−XXX

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ACS Sustainable Chemistry & Engineering Scheme 1. Lewis and Brønsted Acidic Catalysts in Povarov Reactions

(Table 1, entry 7). Thus, under the optimal reaction conditions, furo-[3,2-c]quinoline was obtained in 90% isolated yield. With the optimum conditions in hand, the scope of the reaction was evaluated on a set of imine/dienophile combinations. A range of imines with different substituents, derived from aromatic aldehydes and amines, readily underwent cycloaddition with 2,3-dihydrofuran (DHF) 2 to produce the corresponding products in good to excellent yields (Table 2). From these results, we found that the reaction was amenable to both electron-withdrawing and electron-donating substituents on either side of the imine CN bond. Even ortho-substituents on the N-aryl ring proved efficient. Besides that, the scope of the TrBF4-catalyzed Povarov reaction was explored with regard to different dienophiles. 3,4-2H-dihydropyran also readily underwent the title reaction to provide polycyclic products in good yields, whereas cyclopentadiene and indene were essentially unreactive under the same reaction conditions, wherein only traces of the corresponding products were observed. This was reasonable since the poor nucleophilic alkenes display poor reactivity.7 The mechanism of Povarov reaction between N-aryl imines and electron-rich olefins has been controversial for decades.41−44 However, a two-step pathway is generally accepted rather than a concerted one. On the basis of the above experimental facts and prior work by others, a possible mechanism of this transformation was proposed (Scheme 2). Initially, N-aryl imine (1) was activated by Lewis acidic complexation with tritylium ion into intermediate I; next, complex I was attacked nucleophilically by electron-rich olefins (2,3-dihydrofuran, 2) to form the corresponding intermediate II; then an intramolecular cyclization of II produced the intermediate III; finally, tautomerization of intermediate III released the final product, and the tritylium was regenerated To confirm the Lewis acidic complexation of tritylium with imine 1, 13C NMR experiments were performed with varying the catalyst loading with respect to the imine 1a.42 The 13C NMR spectrum of imine 1a was run on its own in CDCl3 on a Bruker AV-400 spectrometer and was compared with samples that had been treated with 50, 100, and 150 mol % of TrBF4. As the ratio of TrBF4 to 1a was increased, the most noticeable effect was deshielding on the carbon of the imine, whose chemical shifts changed from δ160.5 to δ162.9 (Figure 1). This evidence suggests a strong binding interaction exists between tritylium and the imine nitrogen atom.

In this paper, we report tritylium tetrafluoroborate (TrBF4) as an organocatalyst in Povarov reactions. A Lewis acidic catalysis mechanism by carbenium was proposed and validated. The carbenium-catalyzed three-component Povarov reaction in batch mode and microflow mode was also investigated

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RESULT AND DISCUSSION In the initial experiment, benzylideneaniline (1a) was chosen as the model substrate to react with 2,3-dihydrofuran (DHF) (2) in the presence of a catalytic amount of TrBF4 (5 mol %) in dichloromethane. After the reaction was stirred at room temperature for 1 h, it was finished and gave the corresponding furo-[3,2-c]quinoline in 70% yield as a mixture of cis- and transisomers, which could be easily separated and purified by column chromatography on silica gel (Table 1). Table 1. Reaction Results of 1a with DHF (2) under Different Reaction Conditions

entry

solvent

1 2 3 4 5 6 7

DCM toluene ether CH3CN THF THF THF

a

cat. TrBF4 TrBF4 TrBF4 TrBF4 TrBF4 TrBF4 TrBF4

(5 mol %) (5 mol %) (5 mol %) (5 mol %) (5 mol %) (0.5 mol %) (0.05 mol %)

time (h)

product ratio (cis/trans)a

overall yield (%)b

0.5 0.5 0.5 0.5 0.5 1 12

63:37 75:25 67:33 45:55 48:52 49:51 47:53

70 64 56 63 92 90 60

The ratio is based on isolation by chromatography. bIsolated yield.

To establish an optimal reaction conditions, various trial reactions were carried out and the results are summarized in Table 1. It has been observed that THF is conducive for the present reaction as compared to ether, acetonitrile, and toluene. To our delight, lowering the TrBF4 loading from 5 to 0.5 mol % resulted in a similar yield of the furo-[3,2-c]quinoline, although twice of the reaction time was needed (Table 1, entry 6). However, when the catalyst loading decreased to 0.05 mol %, the reaction was finished in 12 h and only gave 60% of the product B


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ACS Sustainable Chemistry & Engineering Table 2. Scope of the Reactiona

a All reaction were conducted at room temperature using 0.5 mol % TrBF4 in THF. bIsolated yield. cThe ratio is based on isolation by chromatography (cis: trans).

Besides that, a Brønsted acid formed by the reaction of the carbenium with trace of water, or species from decomposition of the carbenium could be the actual catalysts for the Povarov. To exclude possible Brønsted acid catalysis, we performed the reaction in the presence of sterically hindered base 2,6-ditertbutylpyridine (DBPy) as a proton scavenger.45 When the reaction between benzylideneaniline 1a and 2,3-dihydrofuran 2 was conducted in the presence of TrBF4 (0.5 mol %) and DBPy (1 mol %), the yields of furo-[3,2-c]quinoline were essentially not changed (88% yield) (Scheme 3). These results excluded the possibility of Brønsted acid catalysis. We interpret these findings as proof of an active carbenium being responsible for the catalysis in Lewis acidic nature. One-pot, three-component synthesis of the target products from aldehyde, anilines, and 2,3-dihydrofuran in the presence of TrBF4 (0.5 mol %) was investigated. The reaction was carried out in THF by stirring for 2 h at room temperature, the expected products were obtained, respectively (Scheme 4a). The yields were always somewhat lower, however, which may be attributed to the decomposition or deactivation of the carbenium by water formed during imine formation.

Scheme 2. Proposed Mechanism for the CarbeniumCatalyzed Povarov Reaction

C


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ACS Sustainable Chemistry & Engineering

with a water absorption device (Scheme 4b).46,47 As shown in Scheme 4b, a benzaldehyde solution and aniline solution were pumped into the microreactor I, then the resulting imine solution and 2,3-dihydrofuran solution containing tritylium tetrafluoroborate were pumped into the same microreactor II. To our delight, 89% yield was obtained at a residence time of 2 min at a total flow rate of 5 mL/min (Figure S57 in the Supporting Information). Importantly, the yields were comparable to those of the preformed imine process. This showed the first efficient and scalable three-component Povarov in flow.

To circumvent this obstacle, we adopted a two-stage, threecomponent Povarov reaction through a continuous flow layout

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CONCLUSION We have described a novel protocol for the synthesis of tetrahydroquinolines via Povarov reactions using triphenylmethylium cation as a mild and efficient organocatalyst with remarkably low loading (0.5 mol %). A Lewis acidic catalysis mechanism by carbenium was proposed and validated. A series of pyrano[3,2-c] and furo [3,2-c]-tetrahydroquinolines were obtained in good to excellent yields. Povarov by preformed imines, and in one-pot, three-component reactions afforded the aimed products by tritylium catalyst. Further exploration by two-stage, three-component Povarov in microflow apparatus made the preparation a practical one with high yield (89%) in short time (2 min). It is anticipated that the chiral version of the catalysis will contribute to asymmetric synthesis of some useful tetrahydroquinolines.

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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/acssuschemeng.5b01379.

Figure 1. 13C NMR chemical shift values of the imine carbon atom with increasing ratios of TrBF4 to imine.

Scheme 3. Reaction of Benzylideneaniline with DHF in the Presence of TrBF4 and DBPy

Scheme 4. Attempts of Three-Component Povarov Reaction in the Batch Mode and Microflow Mode

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ACS Sustainable Chemistry & Engineering

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Materials and apparatus, general experimental procedure and respective scanned spectra (1H and 13C NMR) of all the synthesized compounds (PDF).

AUTHOR INFORMATION

Corresponding Author

*K. Guo. Tel.: +86 25 5813 9926. Fax: +86 25 5813 9935. Email: [email protected]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS The authors thank The National Natural Science Foundation of China (21444001), the National High Technology Research and Development Program of China (2011AA02A202), and the Priority Academic Program Development of Jiangsu Higher Education Institutions for financial supports.

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REFERENCES

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