Ionic Modified TBD Supported on Magnetic Nanoparticles: A Highly

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Ionic modified TBD supported on magnetic nanoparticles: a highly efficient and recoverable catalyst for organic transformations Anguo Ying, Hailiang Hou, Shuo Liu, Gang Chen, Jianguo Yang, and Songlin Xu ACS Sustainable Chem. Eng., Just Accepted Manuscript • DOI: 10.1021/ acssuschemeng.5b01757 • Publication Date (Web): 07 Jan 2016 Downloaded from http://pubs.acs.org on January 11, 2016

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ACS Sustainable Chemistry & Engineering

Ionic Modified TBD Supported on Magnetic Nanoparticles: A Highly Efficient and Recoverable Catalyst for Organic Transformations Anguo Ying, a,* Hailiang Hou, b Shuo Liu,c Gang Chen,a Jianguo Yang,a and Songlin Xub,*

(aSchool of Pharmaceutical and Chemical Engineering, Taizhou University, Taizhou 318000, China) (bSchool of Chemical Engineering and Technology, Tianjin University, Tianjin 300072, China) (cCollege of Chemistry, Nankai University, Tianjin 300071, China) Corresponding Authors * Dr. Anguo Ying and Prof. Songlin Xu. Tel./fax: +86 576 88660359. E-mail address: [email protected] or [email protected] (A. Ying), [email protected] (S. Xu). NH2

O R2

+

O

O

R2

R1

N

H N

Fe O Fe3O34 4

R1

R5

12 examples, yield: 63-84 %

R3

R5 R3

R4

N Cl

R1

O

SiO2 OO O Si Stablization of transition state

NH +

H N

N N

R4


N Active catalytic site

R6 X

NH +

R7

N R7

N

R6 X

N

N 12 examples, yield: 69-95 %

ABSTRACT: 1,5,7-Triazabicyclo[4.4.0]dec-5-ene (TBD) with ionic modification supported on magnetic nanoparticles was prepared and characterized by transmission electron microscopy (TEM), Fourier transform infrared (FT-IR), X-ray diffraction (XRD), thermogravimetric analysis (TG), vibrating sample magnetometer (VSM) and elemental analysis. The supported ionic specie

1

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was certified as a highly active and magnetically recoverable catalyst for the synthesis of various substituted ureas and structurally diverse N-heterocyclic compounds. The catalyst can be readily recovered by external magnetie and be reused for 6 times without significant loss in catalytic activity. Ionic tagged catalyst performs much better than its ionic free counterpart in organic transformations. KEYWORDS: Magnetic nanoparticles, Ionic moiety, TBD, Immobilization, Recyclability

INTRODUCTION From the principles of “Green Chemistry”, supported catalysts are extremely important for the design and development of environmentally benign organic transformations.

1-4

Various kinds of

carriers have been explored, including polymers, zeolites, resins, nano-sized materials, etc.

5,6

Among these supports published, the nanoparticles are most frequently utilized as solids to immobilize organic molecular, metal complex, enzyme because of their high surface area and good dispersability in reaction solvents, which render the formed catalysts equal or even higher catalytic activities with their homogeneous counterparts.

7-11

However, these catalysts often with

the diameter of less than 100 nm suffer from the tedious recycling work-up when conventional separation method, such as filtration and centrifugation are used. Therefore, magnetic nanoparticles (MNPs) as heterogeneous support have been emerged as alternative to traditional materials mentioned above. The MNPs-supported catalysts can not only be considered as quasi-homogeneous catalytic compounds but also be easily separated and recovered from the reaction mixture by external magnetic force, which fully cater to the demand of “Green Chemistry”. Some successful applications of MNPs-supported catalysts in organic reactions, for

2


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example, coupling reactions,

12,13

ring-closing metathesis,

14,15

oxidation reaction,

16

Pechmann

reaction 17 and asymmetric reactions 18 have been extensively reported. Due to the unique properties of low vapor pressure, wide liquid range, high thermal stability, excellent dissolution ability,

19-22

ionic liquids (ILs) have been introduced as linker into the

structures of heterogeneous catalysts. To some surprise, the ionic moiety in the catalyst can more or less stablize the transition state of reaction through electrosteric activitaion, which is benificial to

reaction

rate

as

well

as

reaction

chem-

and

stereo-

selectivity.

23-26

1,5,7-Triazabicyclo[4.4.0]dec-5-ene (TBD), a basic guanidine, is widely used as promoter or catalyst in organic synthesis.

27-30

Combining the advantages of magnetic nanoparticles and ionic

liquids and continuation of effort in the development of environmentally benign protocols, 31-33 we design a novel ionic modified TBD grafted on the MNPs (Scheme 1) and its application in the synthesis of N,N-substituted ureas as well as aza-Michael addtion.

MNPs

TBD

ILs

Scheme 1. A Concept to Design Novel Catalyst

EXPERIMENTAL SECTION General Remarks All reagents and solvents in this work are commercially available and were used without purification. Transmission electron microscopy (TEM) was performed with a instrument (JEM-2100) operating at 40-100 kV. Fourier transform infrared spectroscopy (FT-IR) were 3



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recorded on a spectrometer (Nicolet-5700) using KBr pellets. Powder X-ray diffraction data were obtained from a Bruker AXS D8 Advance instrument using Cu Kα radiation. The magnetic measurements were carried out in a vibrating sample magnetometer (VSM) at room temperature. Thermogravimetric analysis was performed under nitrogen using a Shimadzu TGA-50 spectrometer. Elemental analysis was carried out on a Carlo Erba 1160 analyzer. The reaction monitoring was accomplished by thin layer chromatography (TLC) on gel F254 plates. All 1H and 13

C NMR were recorded on a Bruker Avance DPX 400 spectrometer at 400 MHz and 100 MHz

respectively. Chemical shifts were given as δ value with reference to tetramethylsilane (TMS) as the internal standard. Synthesis of Silica Coated Magnetic Nanoparticles (SiO2@Fe3O4, MNPs) Magnetic (Fe3O4) nanoparticles were prepared by the reported coprecipitation method.

34,35

FeCl3·6H2O (11.0 g) and FeCl2·4H2O (4.0 g) were dissolved in deionized water (250 mL) under nitrogen gas with vigorous mechanically stirring at 85 °C for 1 h. The pH value was then adjusted to 9 using the concentrated aqueous ammonia (25 wt %). When the color of the bulk solution turned to black, the magnetic precipitates were separated and washed several times with deionized water until the pH value of the eluent decreased to 7. The obtained Fe3O4 nanoparticles were coated by a layer of silica using sol-gel method.

36

The naked Fe3O4 nanoparticles were highly

dispersed in ethanol (200 mL) by ultrasonic irradiation. The concentrated NH3·H2O (20 mL) and TEOS (10 mL) were successively added into the solution. Then the reaction was stirred for 24 h at room temperature. The resulting MNPs was collected by an external permanent magnet and washed three times with ethanol. Finally, the solids were further dried in a vacuum oven at 60 °C for 10 h to give the black MNPs (SiO2@Fe3O4). 4


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Preparation of Ionic Modified TBD Grafted on Magnetic Nanoparticles (Im-TBD@MNPs) and Ionic-free Counterpart TBD@MNPs Synthesis of intermediate 1 37 The solution of imidazole (1.02 g, 15 mmol) in dried THF (5 mL) was added dropwise to the well stirred THF (5 mL) solution of LiH (0.16 g, 20 mmol) under nitrogen atmosphere at temperature of ≤5 °C (ice bath). Then 1-bromo-3-chloropropane (1.57 g, 10 mmol) was slowly added into the mixture within the temperature of 25 °C. When the reaction was completed, it was quenched by water and removed THF under vacuum. The resultant residue was extracted by dichloromethane (5×5 mL) and the combined organic phase was evaporated under vacuum to afford the intermediate 1. Synthesis of ionic tagged catalyst Im-TBD@MNPs 2 g of MNPs (SiO2@Fe3O4) were dispersed in dried toluene (40 mL) with the assistance of sonication for 1 h, followed by dropwise addition of the solution of 3-chloropropyltriethoxysilane (0.72 mL) and toluene (20 mL) under nitrogen protection with significant stirring at reflux for 48 h. The resultant solids absorbed by external magnetie were washed with ethanol (10×4 mL) and dried under vacuum at room temperature for 6 h to give intermediate 2. Modified MNPs 2 (2 g) were dispersed in dried toluene (20 mL) by sonication for 1 h. An toluene (10 mL) solution of intermediate 1 (0.86 g, 6 mmol) was subsequently added and the reaction was refluxed for 48 h under nitrogen. When the reaction mixture was cooled to room temperature, the supported ionic species 3 was collected by magnetic force, followed by ethanol rinse and dried under vacuum. 1 g of Supported ionic liquid 3 was highly dispersed in ethanol (20 mL) in the presence of 5



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sonication for 1 h. TBD (1.25 g, 5 mmol) in ethanol (5 mL) was dropped into the above solution. At the end of the addition, the reaction was heated to reflux and proceeded for 48 h under nitrogen atmosphere. After magnetic separation, brown crude solids were further rinsed with ethanol (10 mL) for 6 times and dried under vacuum at 50 °C for 12 h to yield the catalyst Im-TBD@MNPs. Synthesis of ionic-free catalyst TBD@MNPs Suspension of modified MNPs 2 (1 g) in ethanol (20 mL) was formed with the aid of sonication for 1 h, followed by slowly addition of solution of TBD (1.25 g, 5 mmol) and ethanol (5 mL). The reaction mixture was refluxed for 48 h under nitrogen protection. The resultant solids were rinsed by ethanol (6×10 mL) and dried under vacuum at 50 °C for 12 h to give the ionic-free counterpart TBD@MNPs. General Procedure for Synthesis of N,N′-substituted Urea in Presence of Catalyst Im-TBD@MNPs An oven dried-flask (25 mL) was charged with aromatic amine (6 mmol), DMC or DEC (3 mmol), and Im-TBD@MNPs (0.1 g). The mixture was reacted at 80 °C. When the reaction was completed (detected by TLC plate), the magnetic catalyst was separated from the reaction mixture by external magnetie and the remained residue was diluted with water (10 mL). Some crystallized solids were the crude product, which was further purified by recrystallization using ethanol to afford the desired condensation product. All products were characterized by NMR analysis. The recovered catalyst was washed with ethanol (10 mL×3), dried at 50 °C under vacuum for 5 h, and was subjected to the next cycle. General Procedure for Im-TBD@MNPs Catalyzed Aza-Michael Addition In a typical reaction, cyclic amine (2.0 mmol), Michael acceptor (2.2 mmol), catalyst 6


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Im-TBD@MNPs (40 mg) in 3 mL ethanol were stirred at room temperature. As the TLC indicated the completion of the reaction, the reaction mixture was diluted by 5 mL ethanol. When an external magnet held the superparamagnetic nanoparticles in the flask, the remained liquid was obtained just by simply decantation and then was evaporated under reduced pressure. The residue was further purified by column chromatography to give the finally products. All products were characterized by NMR analysis. The magnetic recovered catalyst was washed with ethanol and dried at 50 °C under vacuum for 5 h, and was used at the next run.

RESULTS AND DISCUSSION The primary step of this objective is to synthesize and functionalize the magnetic nanoparticles (Scheme 2). Firstly, the Fe3O4 nanoparticles with the outer layer of silica, which not only prevent the aggregation of the nano-sized particles but also provides numerous for surface Si-OH groups for further modifications, 38,39 were produced by chemical co-precipitation method. 34,35 Secondly, 3-(3-chloropropyl)-imidazole 1 was prepared from the reaction between 1-chloro-3-bromopropane and imidazole, followed by reaction with the MNPs supported 3-chorosilylpropane 2 to afford the corresponding imidazolium ionic moiety tethered with chloropropane 3. The key intermediate 3 was reacted with the TBD in dry ethanol under reflux in the presence of nitrogen atmosphere to give the desired catalyst Im-TBD@MNPs. For comparison, ionic free counterpart TBD@MNPs was also produced by simple reaction of TBD and MNPs supported 3-chorosilylpropane 2 (Scheme 3).

7



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EtO Si EtO OEt

SiO2

Fe3O4

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Cl SiO2

Fe3O4

SiO2@Fe3O4 (MNPs)

O O Si O

Cl

Cl

N

2 2 Cl + HN

Br

HLi, dried THF

N

25℃, N2

Fe3O4

O

+

O Si O

N

Cl

-

N Cl

Toluene, N2, reflux

1

H N

N SiO2

N

N

SiO2

(TBD)

Fe3O4

O O Si O

N Cl-

N N

N N

Dried EtOH N2, reflux

Im-TBD@MNPs

3

Scheme 2. Synthesis of Im-TBD@MNPs N

N

SiO2

2, Dried EtOH

N N H

Fe3O4

N2, reflux

O O Si O

N

N

TBD@MNPs

TBD

Scheme 3. Preparation of TBD@MNPs The transmission electron microscopy (TEM) images of the ionic tagged catalyst Im-TBD@MNPs, ionic free counterpart TBD@MNPs and their support MNPs are presented in Figure 1. The average size of the dark nano-Fe3O4 core in three samples is about 12-18 nm with a mostly spherical shape while their protection layer-grey silica shell is about 4-6 nm. As shown in Fig. 1, grafting TBD onto the silica shell did not lead to the significant change in the structure and morphology of MNPs, indicating that the core Fe3O4 (Figure 1a and Figure 1b) remained intact during the immobilization and functionalization process on the surface of MNPs (Figure 1c). The loadings of ionic tagged TBD and ionic free TBD were determined by elemental analysis of nitrogen as 0.275 mmol·g-1 and 0.082 mmol·g-1 respectively.

8


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(a)

(b)

(c)

Figure 1. TEM patterns of Im-TBD@MNPs (a), TBD@MNPs (b) and MNPs (c) By comparing Fourier transform infrared (FT-IR) spectrums of MNPs, TBD@MNPs and Im-TBD@MNPs, successfully functionalization on the surface of the MNPs can be proved in the Figure 2. All samples show the same adsorption bands at 579 cm-1, 1088 cm-1, 3400 cm-1, which correspond to the characteristic vibrations of Fe-O bond, Si-O bond and Si-OH bond respectively. The bands at 2874 cm-1and 2965 cm-1 at sample of Im-TBD@MNPs and TBD@MNPs indicate the vibration of alkyl chain C-H.

40

The efficient immobilization of TBD and imidazolium ionic

moiety grafted on the surface of the MNPs (IL-TBD@MNPs, Figure 2a) was verified by the bands at 1541 cm-1, 1465 cm-1 (C=N and C=C stretching vibration of TBD ring and imidazole ring ), as 9



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well as the band at 3147 cm-1 (C-H stretching vibration of imidazole ring).

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41

Moreover, the

absence of the C-H adsorption of imidazole ring shows the ionic free characteristics of TBD@MNPs, and the relatively lower adsorption signal at 1541 cm-1, 1465 cm-1 may be due to the lower loading of TBD in the TBD@MNPs (Fig. 2b) than that in the Im-TBD@MNPs (Figure 2a).

Figure 2 FT-IR spectrums of Im-TBD@MNPs (a), TBD@MNPs (b) and MNPs (c) X-ray diffraction (XRD) patterns of Im-TBD@MNPs, TBD@MNPs and MNPs are shown in Figure 3. All three samples demonstrate 6 characteristics diffraction peaks at 2θ=30.07°, 35.52°, 43.12°, 53.36°, 57.19° and 62.79°, corresponding (2 2 0), (3 1 1), (4 0 0), (4 2 2), (5 1 1) and (4 4 0) respectively, which match well with the standard nano-Fe3O4 particles (JCPDS file No. 19-0629). The broad peak at 2θ between 20° and 30° indicate the existence of amorphous silica layer formed around the magnetic core.

42,43

To our gratification, the functionalization process on

the surface of MNPs (Figure 3c) to produce supported catalysts Im-TBD@MNPs (Figure 3a), 10


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TBD@MNPs (Figure 3b) have no obvious effect on the formation of well crystallized solids with sharp and strong peaks.

Figure 3. XRD Patterns of Im-TBD@MNPs (a), TBD@MNPs (b), and MNPs (c) The stability of Im-TBD@MNPs and TBD@MNPs was examined by the thermogravimetric (TG) analysis. As shown in Figure 4, the weight loss below 150 °C is due to the evaporation of physically adsorbed water as well as hydrate water molecules. 44 The weight loss about 12.5 % at 252 °C−328 °C of Im-TBD@MNPs was assigned decomposition of TBD and ionic imidazolium moiety. As for the TBD@MNPs, the range between 243 °C−362 °C leads to significant decrease in the weight loss of 5.8 %. From the DTG curves of the two supported catalysts, the decomposition rate of TBD species is much faster than other stages. It is worthy of note that the total TBD content of Im-TBD@MNPs and TBD@MNPs is highly constant with the catalyst loading determined by elementary analysis.

11



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(a)

(b)

Figure 4. TG-DTG analysis for Im-TBD@MNPs (a) and TBD@MNPs (b) Magnetic properties of Im-TBD@MNPs TBD@MNPs and MNPs were investigated using a vibrating sample magnetometer (VSM) with a field of -20000 θe to 20000 θe at room temperature. As shown in Figure 5, the M (H) hysteresis loop for the three samples was completely reversible, which indicated their superparamagnetic characteristics. The catalysts Im-TBD@MNPs and TBD@MNPs demonstrated saturation magnetization values of 43.3 emu•g-1 and 42.7 emu•g-1, respectively, while the unimmobilized MNPs had the value of 58.65 g-1. The reason may be attributed to the grafting of TBD over MNPs. However, the lower value of Figure 5a and Figure 12


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5b is still high enough to ensure the readily recovery of the catalysts from reaction mixture using external magnetic force.

Figure 5. Hysteresis loops of Im-TBD@MNPs (a), TBD@MNPs (b), and MNPs (c) The activity of the two supported catalysts was probed by the condensation reaction of aniline and dimethyl carbonate (DMC) to form N, N′-substituted urea. As demonstrated in Table 1, Im-TBD@MNPs, TBD@MNPs and TBD could efficiently catalyze the reaction forward and afford the product yields of 70 %, 59 %, 67 % respectively under solvent-free condition at 80 °C (Table 1, entries 1,2,4). However, almost no product was detected when the reaction occurred in presence of naked MNPs or in absence of any catalyst (Table 1, entries 3 and 5). Notably, the ionic tagged catalyst Im-TBD@MNPs performs better than ionic-free counterpart TBD@MNPs and TBD. Then we select Im-TBD@MNPs for further optimization of reaction conditions. Moreover, the efficiency of the immobilized catalyst was found to be affected by varying the amount of the catalyst exploited in the reaction. It could be found that the increase in the catalyst loading from 13



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100 mg to 150 mg and 200 mg, no substantial improvement in the product yield was observed while the yield significantly decreased when the amount of catalyst was dropped to 100 mg and 50 mg (Table 1, entries 6,7 vs 8,9). Although the reaction proceeded smoothly in toluene and water, the solvent-free condition was chosen for further investigations from viewpoint of economic and environmental green chemistry (Table 1, entries 10 and 11). Table 1. Evaluation of the Effects of the Catalyst and Solvent on the N, N′-Substituted Urea Reaction of Phenylamine and DMC a NH2 Im-TBD@MNPs

O +

a

O

O

H N

H N O

80

Entry

Catalyst (mg)

Solvent

t (h)

Yield (%)b

1

Im-TBD@MNPs (100)

Solvent Free

8

70

2

TBD @MNPs (335)

Solvent Free

9

59

3

MNPs (200)

Solvent Free

10

Trace

4

TBD (3.8)

Solvent Free

8

67

5

No catalyst

Solvent Free

12

Trace

6

Im-TBD@MNPs (200)

Solvent Free

8

72

7

Im-TBD@MNPs (150)

Solvent Free

8

69

8

Im-TBD@MNPs (50)

Solvent Free

8

41

9

Im-TBD@MNPs (10)

Solvent Free

8

19

10

Im-TBD@MNPs (100)

Water

8

58

11

Im-TBD@MNPs (100)

Toluene

8

70

The reaction was performed with aniline (6 mmol), DMC (3 mmol), 80 °C;b Isolated yield based

on DMC. With the optimal reaction conditions in hand, we then examined the scope of the present nano-sized Im-TBD@MNPs as catalyst for the synthesis of structural-versatile ureas (Table 2). Gratifyingly, various aromatic amines with electron-donating groups at the ring, such as 4-MeO, 2-Me, 3-Me, 4-Me, 4-OEt, reacted smoothly with DMC in presence of Im-TBD@MNPs under solvent-free conditions at 80 °C and generated the corresponding products in good yields of 14


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63 %−78 % (Table 2, entries 1-6). To our disappointment, the electron-deficient groups at the aromatic ring inactivated the reaction and no reaction product was detected (Table 2, entries 7,8,15). It is worth noting that diethyl carbonate (DEC) could also effective electrophile to afford higher yields of desired products than those of DMC, which may attribute the better leaving ability of OEt in comparison with OMe (Table 2, entries 9-14). Table 2. Synthesis of N,N′-Substituted Ureas in Presence of Im-TBD@MNPs a. NH2

O +

R2

O

O

H N

Im-TBD@MNPs R1

80

R1

a

R2

H N R1

O

Entry

R1

R2

t (h)

Yield (%)b

1

H

Me

8

70

2

4-OMe

Me

7

78

3

2-Me

Me

8

63

4

3-Me

Me

8

72

5

4-Me

Me

8

80

6

4-OEt

Me

8

78

7

4-Cl

Me

8

NR c

8

4-NO2

Me

10

NR c

9

H

Et

14

74

10

4-OMe

Et

12

84

11

2-Me

Et

14

66

12

3-Me

Et

14

77

13

4-Me

Et

14

83

14

4-OEt

Et

14

84

15

4-Cl

Et

14

NR c

Reaction conditions: aromatic amine (6 mmol), DMC or DEC (3 mmol), Im-TBD@MNPs (100

mg),solvent-free, 80 °C; b Isolated yield based on DMC or DEC; c No reaction. Aza-Michael addition is one of most atom-economic and highly efficient reactions for synthesis 15



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of N-heterocyclic compounds, the potential therapeutic alternatives with pharmaceutically active properties.

45

Initially, the reaction between imidazole and methyl acrylate was chosen as model

reaction for the optimization of conditions (Table S1). The catalyst Im-TBD@MNPs afforded higher yield of 19 % than TBD@MNPs (Table S1,entries 1,2). As expected, the reaction did not proceed in presence of blank carrier MNPs or without any catalyst (Table S1, entries 3 and 4). The above results determined Im-TBD@MNPs as the desired catalyst for further examinations. When various amounts of catalyst and reaction solvents were exploited, 40 mg and ethanol gave the best results in terms of reaction yield (Table S1, entries 1 and 5-10). Next, the reaction scope was investigated. As shown in Table 3, a variety of structurally diverse imidazoles can react smoothly with α, β-unsaturated carbonyl compounds to generate the corresponding products in 70 %−93 % within 5 hours. The reactivity decreased with the increasing steric hindrance of Michael acceptor (Table 3, entries 1 vs 5 and 6). Moreover, it is possible to carry out the reaction of benzimidazole to afford good yield in 3 hours (Table 3, entry 8). Table 3. Michael Addition of Various Imidazole Derivatives with α, β-Unsaturated Carbonyl Compounds Catalyzed by Im-TBD@MNPs [a] R5 N

NH +

R3

Im-TBD@MNPs R5

R4

R3

EtOH rt

N N

R4

Entry

R3

R4

R5

t (h)

Yield (%)b

1

H

COOMe

H

2

93

2

NO2

COOMe

H

2

90

3

i-Pr

COOMe

H

5

72

4

CH3

COOEt

H

3

84

5

H

COOBu

H

3

84

6

H

COOBu

CH3

3

80

7

NO2

CN

-

1

95

16


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1H-benzo

8 a

COOMe

H

3

70

Reaction conditions: imidazole derivatives (2.0 mmol), α, β-unsaturated carbonyl compounds

(2.2 mmol), Im-TBD@MNPs (40 mg), ethanol (3 mL), room temperature. b

Isolated yield based on imidazole derivatives The reaction was extended to a series of other cyclic amines and Michael acceptors explore the

generality of the catalytic system. As shown in Table 4, morpholine reacted smoothly with methyl acrylate,

ethyl

acrylate,

butyl

acrylate,

acrylamide,

N-(hydroxymethyl)acrylamide,

N,N-dimethylacrylamide to afford the corresponding products in 80 %-95 %. Unlike the our previous report,

46

the long chain of the acrylate esters has no detrimental effect on the reaction

rate (Table 4, entries 1-3). To our pleasure, 1-ethylpiperazine and 1-hydroxyethylpiperazine are also

efficient

Michael

donors

to

react

with

1-morpholinoprop-2-en-1-one

and

N-2-(methyl-4-oxopentan-2-yl)acrylamide and need longer time to finish this reactions, which may be attributed to larger steric hindrance near the carbonyl group (Table 4, entries 7-9). In the case of piperidine as nucleophile, the reaction can tolerate the α, β-unsaturated amides with large polar or bulky substituent on N position (Table 4, entries 10-12). Table 4. Michael Addition of Heterocyclic Amines to α, β-Unsaturated Carbonyl Compounds Catalyzed by Im-TBD@MNPs a Im-TBD@MNPs R6 X

NH +

R7

EtOH rt

R7 R6 X

N

Entry

X

R6

R7

t (h)

Yield (%)b

1

O

-

COOMe

1

95

2

O

-

COOEt

1

93

3

O

-

COOBu

1

90

4

O

-

CONH2

2

93

5

O

-

CONHCH2OH

1.5

84

6

O

-

CON(CH3)2

1.5

80

17



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Page 18 of 29

O 7

N

CH2CH3

8

N

CH2CH2OH

9

N

CH2CH3

10

C

CH3

11

C

CH3

12

C

CH3

N

O

N

O

5.0

84

4.5

81

5

69

2

88

4

81

2

95

O

O

O N H O N

O

O

a

N H

CONHCH2OH

Reaction conditions: heterocyclic amine (2.0 mmol), α,β-unsaturated carbonyl compounds (2.2

mmol), Im-TBD@MNPs (40 mg), ethanol (3 mL), room temperature. b

Isolated yield based on secondary amine From the viewpoint of economical and ecological demands for sustainable chemistry, the

possibility of the magnetic recycling of catalyst was investigated. The reaction of imidazole and methyl acrylate in ethanol was examined in the presence of Im-TBD@MNPs. After the completion of the reaction, the catalyst was separated from the reaction mixture by external magnetic force (Figure 7a and b), followed by decantation of reaction solution, which was purified by column chromatography to afford the adduct. The remained catalyst was washed with ethanol, dried under vacuum, and was directly subjected to the fresh substrates for next cycle. As shown in Figure 6A, the catalyst could be reused for 6 times without obvious loss of catalytic activity, which can also verified by TEM analysis that the structure and morphology of the sixth used catalyst almost remain the same as the fresh one (Figure 7c). Additionally, the catalyst Im-TBD@MNPs can be readily recovered and reused in the reaction of aniline and DMC and generated the desired product in the yield of 60 % even after it was recycled for 6 times (Figure 18


Page 19 of 29

6B).

93

91

100 80 Yield (%)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


70

68

90

87

67

88

65

60

63

85

60

40

B

20

A

0 1

2

3

4

5

6

Cycle

Figure 6. The reusability results of Im-TBD@MNPs catalyzed aza-Michael addition (A) and N, N′-substituted urea reaction (B).

(a)

(b)

(c)

Figure 7. Photographs of the dispersion of catalyst Im-TBD@MNPs in reaction procedure (a), separation of the catalyst with external magnetie (b), and TEM image of reused catalyst (c) A plausible mechanism for the synthesis of N, N′-substituted ureas as well as aza-Michael addition of cyclic amines catalyzed by Im-TBD@MNPs was demonstrated in Figure 8. The two reactions was promoted by the catalyst from two side. On one hand, the lonely electrons of N atom at the guanidine ring, as a mainly active catalytic site of catalyst Im-TBD@MNPs could take away hydrogen atom of amines and then enhance the nucleophilicity of Michael donor. Consequently, the activated N nucleophile react smoothly with DMC or α,β-unsaturated carbonyl 19



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Page 20 of 29

compounds to form the final product. On the other side, the imidazolium ionic moiety is hydrophilic to easily access of substrate to catalytic site. Moreover, the “ionic environment” may play a role on the stabilization of the transition state to accelerate the reaction process,

23

which

was, to some extent, proved by the fact that the ionic tagged catalyst Im-TBD@MNPs perform much better than ionic free counterpart TBD@MNPs (Table 1, entries 1 vs 2 and Table S1, entries 1 vs 2). Both the catalytic role of guanidine and unique property of ionic moiety efficiently facilitate the reaction forward.

Fe O Fe3O34 4

Fe O Fe3O34 4

Fe O Fe3O34 4

SiO2

SiO2

SiO2

OO O Si Stablization of transition state

OO O Si

OO O Si

Rx Ry

N NH Cl

N

N

N

N

Cl

Cl

Active catalytic site N

N N

H

Rx N

N NH

Ry N

N

N

N

N Rx

N

Ry

Activated nucleophile

Figure 8. Plausible mechanism regarding Im-TBD@MNPs for the N, N′-substituted ureas reaction and aza-Michael addition

CONCLUSION In conclusion, a novel TBD with ionic tag grafted on the magnetic nanoparticles (Im-TBD@MNPs) was prepared for the first time. The catalyst is not only suitable for the synthesis of substituted ureas from the reaction of aromatic amines and DMC or DEC, but also for the aza-Michael addition of cyclic amines to various α,β-unsaturated carbonyl compounds with complex structures. The nano-sized catalyst can easily collected by a magnet and no significant 20


Page 21 of 29

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


loss in catalytic activity was detected when it was reused for at least six times. The better performance of ionic tagged Im-TBD@MNPs, compared with ionic free TBD@MNPs exhibits a unique role of ionic moiety in catalytic reaction, which encourage us to develop more immobilized catalysts with ionic tag for organic reactions.

ASSOCIATED CONTENT Supporting Information The supporting information is available free of charge on the ACS publications website at DOI: Optimization reaction conditions of aza-Michael addition of imidazole to methyl acrylate (Table S1). NMR data and spectrums of all products (PDF).

AUTHOR INFORMATION Corresponding Authors * Dr. Anguo Ying and Prof. Songlin Xu. Tel./fax: +86 576 88660359. E-mail address: [email protected] or [email protected] (A. Ying), [email protected] (S. Xu). Notes The authors declare no competing financial interest.

ACKNOWLEDGEMENTS We are grateful for the financial supports for this research by the Zhejiang Provincial Natural Science Foundation of China (No.LY15B060002), and National Natural Science Foundation of China (Grant 21576176, 21106090, 21176170 and 21272169).

21



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Ionic modified TBD supported on magnetic nanoparticles: a highly efficient and recoverable catalyst for organic transformations Anguo Ying, Hailiang Hou, Shuo Liu, Gang Chen, Jianguo Yang, and Songlin Xu NH2

O R2

+

O

O

R2

R1

N

H N

Fe O Fe3O34 4

R1

NH +

R5

12 examples, yield: 63-84 % R5 R3

R4

N Cl

R1

O

SiO2 OO O Si Stablization of transition state

R3

H N

N N

R4


N Active catalytic site

R6 X

NH +

R7

R7

N N

R6 X

N

N 12 examples, yield: 69-95 %

Synopsis We disclosed the preparation of a novel ionic modified TBD grafted on the magnetic nanoparticles, which was used as catalyst for the synthesis of substituted ureas and aza-Michael addition.


Ionic Modified TBD Supported on Magnetic Nanoparticles: A Highly

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