H–Y zeolite is a versatile heterogeneous catalyst for the synthesis of β-nitroamines

Applied Catalysis A: General 388 (2010) 211–215

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H–Y zeolite is a versatile heterogeneous catalyst for the synthesis of ␤-nitroamines Kenichi Komura ∗ , Yuki Taninaka, Yoshifumi Ohtaki, Yoshihiro Sugi Department of Materials Science and Technology, Faculty of Engineering, Gifu University, Yanagido 1-1, 501-1193 Gifu, Japan

a r t i c l e

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Article history: Received 15 April 2010 Received in revised form 20 August 2010 Accepted 25 August 2010 Available online 27 September 2010 Keywords: H–Y zeolite Aza-Henry reaction Heterogeneous catalysis ␤-Nitroamine

a b s t r a c t Heterogeneous aza-Henry reactions of aldimes with nitromethane were examined with zeolites as catalysts. Among the zeolite used, H–Y zeolite showed satisfactory catalytic activity for various aldimines, resulting in corresponding ␤-nitroamines in good yields. H–Y zeolite also showed the excellent reusability until the 3rd usage without loss of its activity; further, the calcination of deactivated H–Y zeolite allowed it to rejuvenate its catalytic activity. Solid-state 13 C MAS NMR revealed the predominant activation of an aldimine into the H–Y zeolite. © 2010 Elsevier B.V. All rights reserved.

1. Introduction

2. Experimental

Aza-Henry reaction, a nucleophilic addition of nitroalkanes to imines, is an important reaction in organic synthesis [1]. The reaction is because it directly offers the valuable chemical intermediates, ␤-nitroamines; these can be feasibly transformed to vicinal diamines by the reduction [2–6], or to ␣-amino acids by Nef reactions [7,8]. Typically, the reaction has been carried out by homogeneous catalysts such as organic weak bases or Lewis acids [9–13]. However, the usage of heavy metal salts gives some drawbacks, such as a tedious work-up, separation of catalyst, and waste production. To overcome these issues, researchers have paid much attention to the development of heterogeneous catalysts for environmental benign organic syntheses. Especially, zeolites, a microporous aluminosilicate with ordered pores, have been used for catalysts in fine chemical syntheses [14], because they have acid sites in their frameworks and are reusable by some reactivation such as calcination under air flow. Recently, we have found that Y zeolite works as an efficient catalyst for the hydroamination of methyl acrylate to aniline [15–17]. This encouraged us to develop some other fine chemical syntheses over zeolite as a versatile heterogeneous catalyst. Here, we wish to report the heterogeneous aza-Henry reaction of aldimines with nitromethane over zeolite catalysts. H–Y zeolite found to be a suitable catalyst for this reaction with good yields.

All of reagents are commercially available and were used without any purification. Na–Y and H–Y zeolite (SiO2 /Al2 O3 (SAR) = 5.6, HSZ-320NAA and HSZ-320 HOA) and H-MOR (SAR = 25, HSZ650HOA) were purchased from Tosoh Corporation, Tokyo, Japan. H-BEA (SAR = 25, CP 814B-25, Zeolyst International, Valley Forge, PA, USA). Prior to use, all zeolite samples were calcined by the following calcination program: room temperature to 550 ◦ C (heating rate 1 ◦ C/min), 550 ◦ C for 7 h under air flow. Sc(OTf)3 and the ionexchanged resin Nafion® (NR50, 0.8 mmol H+ /g) were purchased from Tokyo Kasei and Aldrich, respectively. The acidity of zeolites was determined by ammonia temperature-programmed desorption (NH3 -TPD) using TPD66 (Bel Japan). The specific surface area and the pore volume were determined by the BET method using nitrogen adsorption analysis (BELSORP 28SA; Bel Japan) at 77 K. Estimation of yields and chemical structure of product were performed by NMR measurements in CDCl3 , and solid-state 13 C magic angle scanning (MAS) NMR was also carried out with an ECA-500 (JEOL).

∗ Corresponding author. E-mail address: [email protected] (K. Komura). 0926-860X/$ – see front matter © 2010 Elsevier B.V. All rights reserved. doi:10.1016/j.apcata.2010.08.054

2.1. Aza-Henry reaction To a test tube containing a magnetic stir bar and H–Y zeolite (50 mg) was added a solution of aldimine (1 mmol) in nitromethane (1.5 ml). The resulting mixture was then heated at 60 ◦ C for 9 h. After cooling to ambient temperature, the mixture was filtrated to remove the catalyst and the residue was washed with chloroform (2 ml × 3). The obtained filtrate was concentrated by evapora-

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Scheme 1. Table 1 Aza-Henry reaction of 2a and nitromethane.a Run

Catalystb

Surface area (m2 /g)c

Acid amount (mmol/g)d

Conversion of imine (%)e

1 2 3 4 5 6 7 8 9 10

None Sc(OTf)3 f Nafion® g H–Y (5.6) H–Y (80) H–Y (320) H-BEA (25) H-MOR (30) H-ZSM-5 (40) Na–Y (5.5)

– – – 700 660 725 613 220 170 720

– – 0.80 0.54 0.01 n.d.h 0.60 0.99 1.15 –

<1 96 13 99 48 33 77 7 8 26

a b c d e f g h

Yield (%)e 2a

2b

<1 75 13 82 46 31 72 7 6 26

0 21 0 17 2 2 5 0 2 0

Reactions were performed by using imine (0.5 mmol) and zeolite (50 mg) in nitromethene (1.5 ml, 28 mmol) at 50 ◦ C for 6 h. Number in parentheses is SiO2 /Al2 O3 ratio. Measured by N2 adsorption using BET method. Measured by NH3 -TPD. Values are estimated from h-temperature peak. Estimated by 1 H NMR. 10 mol% of catalyst was used. Amount of installed H+ was 10 mol%. Not detectable.

tion and dried in vacuo. Resulting crude mixture was resolved in CDCl3 and measured by 1 H NMR to determine the yield: comparing with methyne proton (CH) of product, aryl protons (Ph–H) of substrate and ␤-nitrostyrene. In the reaction screenings given in Tables 1 and 2, the reaction was performed at 90 ◦ C for 6 h under the same amounts of substrates and catalyst as above. 2.2. Reuse and recycle experiment The reaction was performed using the same supporting conditions as mentioned above. After the reaction was completed, the resulting mixture was filtrated and washed with chloroform. Collected H–Y zeolite catalyst was dried in vacuo and put into a fresh test tube containing a magnetic stirring bar again. To this was added the solution of imine 1a in nitromethane to demonstrate the reaction. After the 4th time running, collected

Table 2 Effects of solvent in aza-Henry reaction over H–Y zeolite.a Run

Solvent

Conversion of imine (%)

1 2 3 4 5 6 7 8 9 10

MeOH EtOH 2-Propanol CH3 NO2 Toluene Chloroform THF EtOAc DMF CH3 NO2 /H2 Ob

75 48 36 99 40 43 0 0 0 >99

Yield (%) 2a

2b

65 42 29 82 32 29 – – – 84

10 6 7 17 8 14 – – – 16

a Reactions were performed by using imine (0.5 mmol), zeolite (50 mg) and nitromethane (2.5 mmol) in solvent (1.5 ml) at 50 ◦ C for 6 h. b 0.5 mmol of H2 O was added.

H–Y zeolite catalyst was calcined at 550 ◦ C under air flow and used. 2.3. Solid-state 13 C MAS NMR measurement of imine occluded HY zeolite Samples of imine and CH3 NO2 occluded HY zeolite were prepared as follows: the prescribed amounts of imine or CH3 NO2 were resolved in dry toluene, and to the resulting clear solution was added HY zeolite. After stirring for 2 h at room temperature, toluene was removed by reduced pressure. The obtained zeolite powder was further exposed under vacuum for 6 h at room temperature, and then each sample was put into a solid-state NMR sample folder (4 mm zirconia rotor) immediately. 13 C MAS NMR measurement was done at 5 kHz spinning. 3. Results and discussion We examined the zeolite catalyzed aza-Henry reaction using an N-p-anisyl-p-tolualdimine (1a) in nitromethane as a model reaction, as shown in Scheme 1. Aldimine 1a was easily prepared from p-tolualdehyde and p-anisidine, and the reaction 1a with nitromethane gives a 2-(p-anisyl)amino-2-p-tolyl-1-nitroethane (2a) as a main product; however, the ␤-nitrostyrene derivative (2b) was observed as a minor product. Table 1 summarizes the results of the aza-Henry reaction. The reaction was performed at 50 ◦ C for 6 h in the presence of catalyst. The reaction did not occur in the absence of catalyst (run 1). By comparing with conventional homogeneous catalysts, we carried out the reaction by using 10 mol% of Sc(OTf)3 as a catalyst [18]. The high conversion of 1a was observed and the yield of 2a was 75% accompanying with 21% of 2b. On the other hand, the ion-exchanged resin as a versatile heterogeneous acid catalyst, Nafion catalyzed the reaction of 1a with nitromethane to afford 2a with low yield (run

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Table 3 Aza-Henry reaction of aldimines in nitromethane over H–Y zeolite.a Run

R

Product

Yield of ␤-nitroamineb

1

81 (10)

2

90 (9)

3

75 (25)

4

79 (17)

5

44 (51)

6

47 (17)

7

44 (15)

a b

All reactions were performed by using imine (0.5 mmol) and nitromethane (1.5 ml, 27.9 mmol) in the presence of H–Y (SiO2 /Al2 O3 = 5.6, 50 mg) at 60 ◦ C for 9 h. Estimated by 1 H NMR, and the number of parenthesis indicates the yield of the corresponding ␤-nitrostryrene derivative.

3). However, H–Y (SiO2 /Al2 O3 (SAR) = 5.6) and H-Beta (SAR = 25) zeolites with 3-dimensional 12-membered ring (MR) showed high catalytic activity: H–Y zeolite gave the product 2a in 82% yield with quantitative conversion and H-Beta zeolite was in 72% yield, respectively (runs 4 and 7). It can be found that H–Y zeolite showed the catalytic activity comparable with the homogeneous catalyst, Sc(OTf)3 . The higher yield of H–Y zeolite than of H-Beta is due to the different reaction space into the zeolite: H–Y has larger reaction space (super cage) than H-Beta. However, H-MOR (1-dimensional 12 MR: SAR = 30) and H-ZSM-5 (3-dimensional 10-MR: SAR = 40) zeolites exhibited much lower activities: 7% and 6% yields of 2a, respectively (runs 8 and 9). It is notable that, although H–Y zeolite has the lowest acidity among the used zeolites, the catalytic activity

is highest to give 2a in 82% yield. This suggests that pore size and structure of zeolite affect the catalysis, and that H–Y zeolite is the most appropriate catalyst for the aza-Henry reaction. However, we found that the yield of 2a was decreased with increasing in SAR of H–Y zeolite (runs 5 and 6), suggesting that acidity of the zeolite is also essential for catalysis. Na–Y zeolite (SAR = 5.5) gave 2a in 26% yield (run 6), although it is effective catalyst for the hydroamination [15]. These results show that both pore structure and acidity of zeolite are essential, and that H–Y zeolite with low SAR is a suitable catalyst to achieve the aza-Henry reaction. The influences of solvents are summarized in Table 2. Among used solvents, nitromethane was found to be the best to afford the 2a in 82% yield with excellent conversion of 1a. A polar solvent

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Fig. 1. Effect of reaction temperature in the aza-Henry reaction over H–Y zeolite. Fig. 2. Reusability of H–Y zeolite in the aza-Henry reaction of 1a with nitromethane.

such as MeOH gave 2a in 65% yield; however, EtOH, 2-propanol, toluene, chloroform, and THF gave much lower yields, and no reaction occurred in ethyl acetate and DMF. Interestingly, the addition of water did not remarkably affect the catalysis, showing the same product distribution of 2a and 2b in nitromethane (run 10). Consequently, such results indicate that nitromethane is the most appropriate solvent in the aza-Henry reaction over H–Y zeolite. We then examined the influence of the reaction temperature shown in Fig. 1. The gradual increasing in yield of 2a was observed at 30 ◦ C, and the reaction was reached in 55% yield after 12 h. While the reaction was carried out at 60 ◦ C, the yield of 2a was increased with the reaction period, and the good yield (81%) was obtained for 9 h. However, the reaction was finished after 3 h with a steep yield gradient at 90 ◦ C, affording 2a in 83% yield. Although the reaction was rapidly finished at 90 ◦ C, the yield of the desired product 2a was almost same as that at 60 ◦ C. These results indicate that the reaction of 1a over H–Y zeolite that take place at 60 ◦ C for 9 h should provide suitable results. Table 3 summarizes the aza-Henry reaction over H–Y zeolite using various aldimines in nitromethane. The reaction of N-(panisyl)benzaldimines derived from benzaldehyde and p-anisidine gave the corresponding ␤-nitroamine in 81% yield (run 1). N-(pAnisyl)phenylaldimines with p-substituted methyl, methoxy and chloro groups also gave good yields (runs 2–4) irrespective of their electron-donating or withdrawing natures. However, the reaction of N-(p-anisyl)-p-nitrobenzaldehyde imine gave the mixture of ␤-nitroamine and the ␤-nitrostyrene derivative with almost the same ratio. This is because that a nitro substituent at para position is the strongest electron-withdrawing nature, which leading to the elimination to form ␤-nitrostyrene derivative (entry 5). H–Y zeolite also catalyzed the reaction of N-(p-anisyl) heterocyclic aldimines, derived from 2-furfural and 2-thiophenaldehyde, and showed moderate yields in 47% and 44%, respectively. Recycle experiments in the reaction of 1a in nitromethane were examined over H–Y zeolite; results are shown in Fig. 2. The yield of 2a remained satisfactory until the 3rd use: 80% (1st), 83% (2nd), 83% (3rd); however, it decreased slightly at 4th use (74%). H–Y zeolite after 4th use was rejuvenated by the calcination at 550 ◦ C for 4 h under air flow, resulting in 85% yield of 2a. This suggests that H–Y zeolite is a promising solid catalyst for aza-Henry reaction with excellent reusability. The aza-Henry reaction is one of the typical nucleophilic addition reactions: nitroalkanes work as a nucleophile to attack the

electron deficient carbon atom of an imine. Therefore, one can consider two types of reaction pathways catalyzed by zeolite: one is the nitromethane activated path and the other is the imine activated path. Recently, we [17] and Onaka et al. [19,20,21] independently reported the availability of solid-state 13 C MAS NMR technique of the occluded organic molecule into the zeolite to discuss the catalysis associating with the activation manner. In order to clarify which substrates are activated by H–Y zeolite, we examined solid-state 13 C MAS NMR measurements of the aldimine or the nitromethane occluded H–Y zeolite (abbreviated as imine@H–Y and CH3 NO2 @H–Y, respectively). The spectra of a liquid nitromethane and a solid aldimine were compared; no significant distinct peak was observed in the spectrum of CH3 NO2 @H–Y (Figure not shown), while in Fig. 3, quite different peak patterns in 13 C NMR charts were obviously detected in the imine@H–Y. Due to the drastic peak change, we cannot fully assign their patterns; however, it is evident that the H–Y zeolite

Fig. 3. 13 C MAS NMR spectra of imine 1a (down) and imine 1a occlude into H–Y zeolite (imine@H–Y) (up), respectively.

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In conclusion, we have developed the heterogeneous aza-Henry reaction over zeolite catalyst. Corresponding ␤-nitroamines can be obtained in good yields over H–Y zeolite catalyst, and H–Y zeolite showed the excellent reusability and recyclability. Thus, H–Y zeolite is a versatile heterogeneous catalyst. Further research is now ongoing, and it will be report in near future. References

Fig. 4. Plausible reaction mechanism of the aza-Henry reaction over zeolite catalyst.

would affect the interaction with imine more effectively than with nitromethane. Based upon the above results and the considerations of the catalytic mechanism by a typical Brønsted and/or Lewis acid in organic synthesis, we speculate a plausible reaction mechanism shown in Fig. 4. Because the acid sites in the framework of H–Y zeolite should be responsible for catalysis, acid sites of H–Y zeolite would activate the imine by the interaction with lone pairs of nitrogen atoms to enhance the electron deficiency of carbon atoms, then a reactive tautomer of nitromethane attacks to the imine, resulting in yield of a product, accompanying with regenerating the acid site again. However, it is necessary to carry out more experiments to prove our speculated reaction mechanism.

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