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Letters in Organic Chemistry, 2016, 13, 143-147 143

Ultrasound-Promoted Synthesis of -amino Carbonyl Compounds via a Mannich Reaction Catalyzed by Ionic Liquids

Hua Qian*, Wang Kai and Juejie Zhen

School of Chemical Engineering, Nanjing University of Science and Technology, Nanjing, 210094,

China

Received April 16, 2015: Revised September 19, 2015: Accepted December 04, 2015

Abstract:

Background: Mannich reaction is one of the most important carbon-carbon bond formation reactions

in organic synthesis. Traditional methods require a large amount of ungreen catalysts or much longer

reaction time, which limits the scope of their application. So, a recyclable catalyst with high activity

and selectivity is highly appreciated, and the highly shortened reaction time is also very appealing.

Methods: An efficient and facile process to prepare a series of -amino carbonyl compounds was found via Mannich reac-

tions catalyzed by caprolactam-based Brønsted acidic ionic liquids under ultrasonic irradiation.

Results: [Capl][BF4] was the most effective catalyst in the Mannich reaction, and good yields was gained within 2 6 h.

The activity and stability of the catalyst was maintained very well even after five times, and ultrasound can effectively

shorten the reaction time and enhance the yield at ambient condition.

Conclusion: A convenient procedure for Mannich reaction using acidic ILs catalyst under ultrasonic irradiation has been

designed with many superiorities, such as simple manipulation, less reaction time and high yields. The Mannich reaction

takes place in no presence of organic solvents such as toluene or ethanol, etc. The new synthetic method reported in this

paper would make appreciable contribution to the subject of environmental friendly chemistry and it is available for in-

dustrial applications.

Keywords: Ionic liquid, mannich reaction, ultrasound sonochemistry.

INTRODUCTION

Mannich reaction is one of the most important carbon-carbon bond formation reactions in organic synthesis [1-3]. The formed -amino carbonyl compounds are critical inter-mediates in the preparation of natural products, pharmaceuti-cals and versatile synthetic intermediates [4-7]. In early days, traditional methods of Mannich reactions were achieved by “adding ketones into Schiff bases” and “amine exchange reaction” [8]. However, this procedure is plagued by a num-ber of serious disadvantages and has limited its application. Therefore, during the past few decades, the area of finding new ways to preapre -amino carbonyl compounds or their derivatives has come into focus. The preferable route is to use one-pot strategy which allows a wide range of structural variations by adjusting the contents of the three components. In this procedure, a series of catalyst systems were devel-oped including proline [9, 10], ionic liquids (IL)

[11, 12],

rare earth perfluorooctane [13, 14], heteropolyacids [15], and other organic catalysts [16-18], etc. However, these methods require a large amount of ungreen catalysts or much longer reaction time (8 24 h), which limits the scope of their

*Address correspondence to this author at the School of Chemical Engineer-

ing, Nanjing University of Science and Technology, Nanjing, 210094, China; Tel./Fax: +86-025-84315601; E-mail: qianhua@njust.edu.cn

application. So, a recyclable catalyst with high activity and selectivity is highly appreciated, and the highly shortened reaction time is also very appealing.

As known to us all, ultrasound has been applied in a large

variety of chemical conversion for its ability to enhance rate and improve yields. In addition, it can also facilitate organic

conversions in environmental conditions in some cases

which otherwise require extreme temperature and pressure [19, 20]. So, ultrasound is employed with the aim to greatly

shorten the long reaction time. For acid catalyzed Mannich

reactions, acidic ILs are deemed as one of the most promis-ing alternatives of conventional acid catalysts. Meanwhile,

the use of non-volatile ILs should provide a clue to force less

volatile substrates to undergo the cavitational activation. These liquids are less volatile, which should change appre-

ciably the features of cavitation in this bulk [21, 22]. Consid-

ering these, we designed to combine both these advantages to induce new focuses.

Different from common reaction conditions, we find that,

under ultrasound activation condition together with the use of an appropriate IL, it is feasible to efficiently accomplish

the Mannich reaction in one-pot operation, which is also a

promising substitute for conventional homogeneous/ heterogeneous acidic catalysts.

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144 Letters in Organic Chemistry, 2016, Vol. 13, No. 2 Qian et al.

RESULTS AND DISCUSSION

Acidities and Ion Conductivity of Caprolactam-based ILs

The measurement of the Brønsted acidic scale of ILs was conducted on an ultraviolet-visible light spectrophotometer, with 4-nitroaniline used as an indicator, according to the literature previously reported [23]. The Hammett function, H0, is calculated using eq 1:

H0 = pK(I)aq + log([I]/[IH+]) (1)

where pK(I)aq is the pKa value of 4-nitroaniline (pK(I)aq =

0.99), and [I] and [IH+] are, respectively, the molar concen-

trations of the unprotonated and protonated forms of 4-

nitroaniline. This value can be regarded as the relative acid-

ity of the ILs.

Fig. (1). Absorption spectra of 4-nitroaniline in various ILs in di-

chloromethane.

The results of acidities of four ILs are shown in Fig. (1). The maximal absorbance of the unprotonated form of 4-nitroaniline was observed at 350 nm in CH2Cl2 (Fig. 1. spec-tra a). We could determine the [I]/[IH

+] ratio by measuring

the absorbance when each IL was added (spectra b-e), and then the Hammett function (H0) was calculated (Table 1).

Ultimately, we obtained the acidity order of four ILs:

[Capl][BF4] [Capl][NO3] [Capl][TFA] [Capl][TSA]

Meanwhile, [Capl][BF4] possessesd the highest ion con-ductivity, due to its lower viscosity and smaller cation size. The acidity and ion conductivity play an important role in increasing the catalytic activities.

Ultrasonic Irradiation and Various ILs in the Mannich

Reaction

Three-component Mannich reaction was firstly investi-gated by using the simple substrates such as benzaldehyde, acetophenone and aniline (Table 2).

As shown in Table 2, the Mannich reaction hardly pro-ceeded without catalyst. When [Capl][BF4] was used as the catalyst, we finally obtained the expected -amino carbonyl product with a high yield of 87% (Table 1, entry 2). How-ever, the long reaction time was disappointing.

It is obvious to demonstrate the effect of the use of ultra-sound from Table 1, entries 2-3. Under the ultrasonic irradia-tion, reaction time was greatly shortened and the yield was enhanced for quick mass transfer. Meanwhile, other ILs with different anions were also investigated with the yield of 84%-91%.

Compared with the acidity order of four ILs, the order of their catalytic activity embodied by the corresponding yield

Table 1. Hammett function (H0) values and ion conductivity of ILs at 60°C.

Ionic Liquids [Capl][BF4] [Capl][TFA] [Capl][TSA] [Capl][NO3]

H0 -0.27 2.76 2.91 1.83

Ion Conductivity

(μS/cm) 1130 570 309 236

Table 2. Mannich reaction catalyzed by different ILsa.

Entry IL

Ultrasound Time (h) Isolated Yield (%)

1 none none 24 2

2 [Capl] [BF4] none 12 87

3 [Capl] [BF4] ))))b 2 94

4 [Capl][TFA] )))) 2 87

5 [Capl][TSA] )))) 2 84

6 [Capl][NO3] )))) 2 91

a Reaction condition: benzaldehyde (4 mmol), acetophenone (4 mmol), aniline (4 mmol) and IL (0.08 mmol) were stirred at room temperature. b “))))” means ultrasonic irradiation.

250 300 350 400 450

0.0

0.2

0.4

0.6

0.8

1.0

1.2

1.4

e

dc

a

b

Abso

rban

ce

Wavelength/nm

a: blankb: [Capl][TSA]c: [Capl][TFA]d: [Capl][NO3]e: [Capl][BF4]

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Ultrasound-Promoted Synthesis of -amino Carbonyl Compounds Letters in Organic Chemistry, 2016, Vol. 13, No. 2 145

is not quite similar, which can be explained by the synergis-tic combination of ILs and ultrasonic irradiation.

It has been observed that ion liquids with ultrasound can promote the activity in chemical reactions as they combine both the advantages in enhancing efficiency of chemical con-versions [21, 22]. Meanwhile, the ultrasound utilization ratio can be further increased by those ILs which have high ion conductivity [24], so as to accelerate the reaction. So, the acidity and ion conductivity of ILs are both crucial in the reaction.

Mannich Reactions with Various Substituted Reactants

Using [Capl][BF4]

Under the optimized condition, we demonstrated the re-actions of various substituted reactants using [Capl][BF4] as catalyst. The results are given in Table 3.

As shown in Table 3, under the ultrasound irradiation, all reactants with both electron-withdrawing and electron-donating substituents reacted smoothly at room temperature, and the reactions were completed within 2-6 h to provide good yields (61-97%). Compared with literature reported before, this procedure presents many superiorities, such as moderate condition, less reaction time, and higher yields.

Recycling Tests of the Catalyst

The recovery and reuse performance are the most impor-tant aspects for catalysts in a green process. Thus, the recy-clability of [Capl][BF4] was investigated by using benzalde-hyde, acetophenone and aniline as model substrates. [Capl][BF4] was recovered by removing water under vacuum (70°C/110 mmHg) for 1 h, and reused for the next cycle. The

yields remained almost the same even after the catalyst had been recycled five times (Fig. 2).

EXPERIMENTS

Chemicals and Equipments

All chemicals reported in this manuscript were obtained from commercial sources of reagent grade.

Electrospray ionization mass spectroscopy (ESI-MS) was recorded on Finnigan TSQ Quantumm ultra AM LC/MS instrument.

1H NMR was carried out with Bruker DRX 300

instrument, using TMS as an internal standard. The viscosity was measured by a capillary viscometer in a water bath at 60°C. The ion conductivity was measured using a DDS-11A digital conductivity-meter. Relative acidity was conducted on a Model UV-240 spectrophotometer. Sonication was per-formed in a Hiton KGD-250B ultrasonic cleaner with a rated input power of 120 W.

Synthesis and Characterization of Caprolactam-based

ILs

The synthesis of caprolactam-based ILs ([Capl][X]) was carried out by the reaction of caprolactam and corresponding acid. The preparation of [Capl][BF4] was regarded as an ex-ample and others were as similar.

In a general synthesis method, 0.1 mol caprolactam was mixed with 30 mL deionized water in a 100 mL round-botton flask under an ice bath atmosphere, 0.1 mol fluoboric acid was added in a dropwise manner within 30 min. The reaction temperature was controlled under 50

°C for 4 h and

was vigorously stirred. The deionized water was removed under reduced pressure. The obtained solid mass was dried

Table 3. Mannich reaction catalyzed by [Capl][BF4] under ultrasonic irradiationa.

Entryb

R1 R2 R3 Time(h)c

Isolated Yield (%) m.p. (lit)/°C

1 H H H 2 94 168-171(169-171) [25]

2 H H 4-CH3 2 96 167-171(170-171) [25]

3 H H 4-OCH3 2 97 176-179(177-179) [25]

4 H H 4-Cl 4 94 169-171(170-171) [25]

5 H H 4-NO2 4 70 183-185(184-186) [25]

6 4-Cl H H 4 75 117-120(119-120) [25]

7 4-CH3 H H 2 91 136-138(139-140) [26]

8 4-OCH3 H H 2 94 147-149(148-150) [27]

9 4-NO2 H H 6 61 142-145(145-146) [28]

10 H 4-CH3 H 6 67 129-131(131-132) [29]

11 H 4-Cl H 6 64 116-118(117-118) [29]

12 H 4-NO2 H 6 83 108-111(108-109) [29]

13 H 4-OCH3 H 6 72 123-125(123-125) [30]

a Reaction condition: aldehyde (4 mmol), ketone (4 mmol), amine (4 mmol) and IL(0.08 mmol) were stirred under ultrasonic irradiation at room temperature. b The structure of products were characterized by 1H NMR and verified by reference [27], [30] and [31]. C The reaction was monitored by TLC.

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146 Letters in Organic Chemistry, 2016, Vol. 13, No. 2 Qian et al.

under vacuum at 80°C for 2 h. [Capl][BF4] was obtained with

a total yield of 89%. The other acids used to synthesize ILs were separately trifluoroacetic acid (TFA), p-toluenesulfonic acid (TSA) and concentrated nitric acid.

The prepared [Capl][X] was obtained with a high yield of 83%-89% and they were all moisture stable. All of them can be easily miscible with various polar solvents, such as methanol, acetonitrile, water and acetone. Most of them are liquids except for [Capl][NO3] [32, 33].

[Capl][BF4] 1H NMR (300 MHz, d6-DMSO): 1.45-

1.66 (m, 6H), 2.38 (q, 2H), 3.08 3.13 (t, 2H), 8.26 (s, 1H), 11.98 (s, 1H).

13C NMR (75.5 MHz, d6-DMSO): 22.86,

29.15, 30.08, 35.41, 42.18, 179.25. MS (ESI): m/z 114.1 (cation). Elemental analysis, Calcd: C, 35.85, H, 6.03, N, 6.97; Found: C, 35.80, H, 6.08, N, 6.91.

[Capl][TFA] 1H NMR (300 MHz, CDCl3): 1.65-1.82

(m, 6H), 2.51 (q, 2H), 3.27 (q, 2H), 8.06 (s, 1H), 15.89 (s, 1H).

13C NMR (75.5 MHz, CDCl3): 22.74, 28.86, 30.25,

35.76, 42.83, 115.23, 160.99, 181.02. MS (ESI): m/z 114.1 (cation). Elemental analysis, Calcd: C, 39.07, H, 5.63, N, 6.51; Found: C, 39.01, H, 5.69, N, 6.58.

[Capl][TSA] 1H NMR (300 MHz, CDCl3): 1.42-1.62(m,

6H), 2.32-2.35 (t, 2H), 3.08-3.10(t, 2H), 4.73(s, 2H), 7.39-7.46(m, 4H), 7.67-7.68(d, 3H).

13C NMR (75.5 MHz, CD-

Cl3): 21.91, 28.75, 30.08, 35.70, 43.28, 128.04, 131.96, 134.35, 144.83, 179.28, 29.57. MS (ESI): m/z 114.1 (cati-on). Elemental analysis, Calcd: C, 54.71, H, 6.72, N, 4.91; Found: C, 54.64, H, 6.80, N, 4.99.

[Capl][NO3] 1H NMR (300 MHz, CDCl3): 1.64-1.84

(m, 6H), 2.56 2.63 (t, 2H), 3.33 3.43 (t, 2H), 8.41 (s, 1H).

13C NMR (75.5 MHz, CDCl3): 22.61, 29.04, 30.16, 35.69,

42.91, 179.27. MS (ESI): m/z 114.2 (cation). Elemental

analysis, Calcd: C, 40.90, H, 6.88, N, 15.90; Found: C, 40.83, H, 6.82, N, 15.88.

Reaction Procedure

A mixture of aromatic aldehyde (4 mmol), aromatic ke-tone (4 mmol), aromatic amine (4 mmol) and IL (0.1 mmol) was added into a 25-mL flask, and stirred in a thermostatted ultrasonic cleaning bath at room temperature under ultra-sonic irradiation and monitored by TLC until the reaction was fully completed. The reactants and product mixture were washed with water and recrystallized using anhydrous etha-nol. [Capl][X] was recovered by removing water from solu-tion under vacuum (70°C /110 mmHg) for 1 h and after sev-eral attempt to the catalyst remained highly reactivie.

CONCLUSION

In summary, a convenient procedure for Mannich reac-tion using acidic ILs catalyst under ultrasonic irradiation has been designed with many superiorities, such as simple ma-nipulation, less reaction time and high yields. The Mannich reaction takes place in the abesence of organic solvents such as toluene or ethanol, etc. Compared with other catalysts, caprolactam-based ILs are readily available, in favor of envi-ronment, efficiency and reused ability for more than five

Fig. (2). Recycling and reuse of [Capl][BF4]3.

HO

R1

O

R2

NH2

R3 R2NH

O

R1

R3IL

r.t.

))))

Scheme 1. Mannich reaction catalyzed by ILs under ultrasonic irradiation.

1 2 3 4 580

85

90

95

100

1 2 3 4 580

90

100

Yiel

d of

nitr

atio

n/%

Recycle times

Yield of nitration/%

Reco

very

ratio

of c

atal

yst/%

Recovery ratio of catalyst/%

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Ultrasound-Promoted Synthesis of -amino Carbonyl Compounds Letters in Organic Chemistry, 2016, Vol. 13, No. 2 147

times and maintenance of quite high activity. Meanwhile, the method which combines the advantages of ILs and ultra-sound can remarkably enhance the reaction rate and are pre-sented as a potential alternative method for different substi-tuted -amino carbonyl compounds and other fine chemicals. Therefore, the new synthetic method reported in this paper would make appreciable contribution to the subject of environmental friendly chemistry and it is available for industrial applications.

CONFLICT OF INTEREST

The authors confirm that this article content has no con-flict of interest.

ACKNOWLEDGEMENTS

The authors gratefully acknowledge the financial support of the National Science Foundation of China (21406116) and research fund of Nanjing University of Science and Tech-nology.

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