Microwave assisted N-propargylation of imidazole using alkaline promoted carbons

Applied Catalysis A: General 240 (2003) 287–293

Microwave assisted N-propargylation of imidazole using alkaline promoted carbons V. Calvino-Casilda a , A.J. López-Peinado a , J.L.G. Fierro b , R.M. Mart´ın-Aranda a,∗ a

Departamento de Qu´ımica Inorgánica y Qu´ımica Técnica, UNED, Senda del Rey, 9, E-28040-Madrid, Spain b Instituto de Catálisis y Petroleoqu´ımica, CSIC, Campus Cantoblanco, E-28049-Madrid, Spain Received 14 May 2002; received in revised form 12 July 2002; accepted 6 August 2002

Abstract Two basic activated carbons (Na+ and Cs+ -Norit) have been used as catalysts in the synthesis of N-propargyl imidazole. The effect of the microwave irradiation has been studied. Under the experimental conditions used higher yields and selectivities of N-propargyl imidazole than those obtained using other basic media were reached. The carbons were characterized by thermal analysis, nitrogen adsorption and X-ray photoelectron spectroscopy. Most of the basic sites in the promoted carbons have 13.3 ≤ pKa ≤ 16.5. The order or basicity is Na+ -Norit < Cs+ -Norit. The basicity enhancement is directly connected with the size of the alkaline cation. The yield of the N-propargyl imidazole presents a maximum for 0.04 g of Cs+ -Norit at 300 W in only 3 min of microwave irradiation. © 2002 Elsevier Science B.V. All rights reserved. Keywords: Basicity; Microwave irradiation; Imidazole propargylation; Activated carbons

1. Introduction Recently, microwave ovens have been introduced to chemical laboratories as an alternative heating system to conventional heating methods [1–3]. The main feature of this activating system is the rate enhancement of reactions, due to temperature and pressure effects [4,5]. However, in some catalytic organic reactions activated by microwaves, apart from rate enhancement, selectivity is also modified [6,7]. The anionic alkylation of N-heterocycles is the primary route to obtain pharmaceutically important intermediate products [8]. N-propargyl heterocycles of type A (Scheme 1) are of interest because of their pharma∗ Corresponding author. Tel.: +34-1-3987351; fax: +34-1-3986697. E-mail address: [email protected] (R.M. Mart´ın-Aranda).

cological properties. Their synthesis in basic media affords usually low yields of the desired product, affording a mixture of N-allyl and N-propargyl derivatives. The practical use of base catalysts has largely been restricted to the production of speciality chemicals. Many solid base catalysts often tend to deactivate more rapidly than their acidic counterparts. However, there are numerous classes of reactions that can be catalyzed by heterogeneous bases [9]. Recently, activated carbons have been used to catalyze efficiently organic synthesis, because of their extended surface area, microporous structure, and high degree of surface reactivity [10,11]. Alkaline carbons may also be appropriate solids to selectively catalyze base reactions. In the present study, we investigate the effect of the microwave irradiation on the activity and selectivity of two alkaline promoted carbons for the reaction between imidazole and propargyl bromide.

0926-860X/02/$ – see front matter © 2002 Elsevier Science B.V. All rights reserved. PII: S 0 9 2 6 - 8 6 0 X ( 0 2 ) 0 0 4 5 6 - 8

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V. Calvino-Casilda et al. / Applied Catalysis A: General 240 (2003) 287–293 Table 1 Characterization of the catalysts Catalyst

Scheme 1. N-propargyl imidazole.

Carbon is known in microwave terminology as a very “lossy” material, which means that it is a very efficient absorber of microwave energy and converts that energy to heat. In this study we demonstrate how this can be used to reduce the reaction temperature and how the selectivity is modified.

2. Experimental 2.1. Catalysts preparation An activated carbon, RX-1 EXTRA Norit, has been employed as pristine carbon. Two alkaline carbons have been prepared by ionic exchange of Na+ and Cs+ using 0.43 and 0.075 M solutions of sodium and cesium carbonates, respectively, with 2 wt.% metal content for about 60 h at 353 K. In both cases, the liquid-to-solid ratio was 10 by weight. The samples were filtered and washed to give a carbonate-free material. After drying for 16 h at 383 K, the carbons were crushed and sieved to 0.250 mm particle size. 2.2. Catalysts characterization The Norit RX-1 EXTRA carbon is a well-characterized steam activated peat-char, which has a total open pore volume of 1.054 cm3 g−1 [12] and a wide range of pores (V1 = 0.455 cm3 g−1 , V2 = 0.108 cm3 g−1 and V3 = 0.491 cm3 g−1 ; V1 = pore volume of φ < 7.5 nm, V2 pore volume of 7.5 < φ < 50 nm and V3 , pore volume of φ > 50 nm) [13]. The apparent specific areas of the carbon samples were calculated by the BET method from N2 adsorption data at 77 K, measured in a Micromeritics ASAP 2010 equipment (Table 1). Photoelectron spectra (XPS) were acquired with a VG ESCALAB 200R spectrometer equipped with a hemispherical electron analyzer and Mg K␣ (hν = 1253.6 eV, 1 eV = 1.6302 × 10−19 J) X-ray source.

SBET pH (m2 g)

Norit RX-1 1882 Na+ -Norit RX-1 1541 Cs+ -Norit RX-1 1686

Ash (%)

8.0 3.1 8.3 4.8 10.3 5.5

M2 O (%)

Metal (at-g/100 g cat.)

– 1.7 2.4

– 0.055 0.017

The powder samples were pressed into aluminum holders and mounted on a sample rod placed in the pretreatment chamber of the spectrometer. After outgassing at room temperature for 1 h, they were moved within the analysis chamber. The residual pressure in the ion-pumped analysis chamber was maintained below 5 × 10−9 mbar during data acquisition. The intensities of C 1s, O 1s, and Na 1s or Cs 3d5/2 peaks were estimated by calculating the integral of each peak after smoothing and subtraction of the “S”-shaped background and fitting the experimental curve to a combination of Gaussian and Lorentzian lines of variable proportion. The binding energies (BEs) were referenced to the major C 1s component at 284.9 eV, this reference giving BE values with an accuracy of ±0.1 eV. The ash contents of the studied catalysts were obtained by thermogravimetric analysis (TG/DTA Seiko System 320). The calculated metal contents (Table 1) indicate that the exchange capacity of Norit RX-1 EXTRA carbon for sodium is around three times the exchange capacity for cesium. This is not surprising taking into account that the surface of the active carbon has a determined density of surface groups with negative charge, which interact with the M+ ions by electrostatic forces during the adsorption–impregnation process. For reasons of steric hindrance, and principally at the micropores of the substrate, it is obvious that the Na+ ions (ionic radius = 0.095 nm) can easily access the centers located in micropores, whereas the Cs+ ions (ionic radius = 0.169 nm) have to overcome a diffusional barrier. The pHs of the samples studied were measured following the method described by Rivera and Ferro [14] using an Omega pH-meter, model PHB-62. The basicity of the promoted carbons was evaluated [15] following the method described by Corma et al. [16] by Knoevenagel condensation between benzaldehyde and malonic esters.

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2.3. Microwave activated reaction Activity and selectivity of the catalysts were determined using a multimode microwave oven. A microwave transparent all Teflon (PTFE) autoclave was used. The microwave equipment employed in this work is a Sanyo EM 881 microwave oven operated at the fixed-frequency of 2450 MHz. Imidazole (2 mmol) and different amounts of the catalyst were blended in the Teflon vessel and propargyl bromide (2.5 mmol) was added. The mixture was irradiated in the microwave at different powers (300 and 750 W). After cooling, the reaction products were extracted with acetone (20 ml) and filtered. The reaction was followed using a Konik KNK-3000-HRGC gas chromatograph (GC) equipped with a 60 m long. BP1 capillary column. The mass spectra of the products were obtained on a Hewlett-Packard HP5971 A spectrometer. The reactivity is expressed in terms of the amount of A obtained in wt.%. 2.4. Conventional heating reaction The conventional heating experiments were carried out in a batch reactor. The same ratio of reactants than that for microwave activation is used. Imidazole (2 mmol) and the corresponding catalyst were mixed in a Pyrex flask. Then, propargyl bromide (2.5 mmol) was added in absence of any solvent. The mixture was

289

heated to the reaction temperature (303 and 333 K). The samples were taken periodically, following the evolution of the reaction by GC between 15 and 60 min. 3. Results and discussion 3.1. Catalysts characterization Table 1 summarizes the apparent specific area, the pH and the metal contents of the catalysts. The pristine carbon, RX-1 EXTRA Norit, exhibits a basic pH (8.0), which increases only slightly in the sample exchanged with sodium, but considerably in the Cs-Norit sample. In spite of the exchange capacity for the cesium being around three times less than the capacity of the sodium, the highest basicity of the first cation could be responsible for the increase observed in the pH of the sample (10.3). In order to get a more precise idea about the chemical state and the relative dispersion of the alkaline metals at the surface of the carbon, a surface analysis of the different samples by X-ray photoelectron spectroscopy was carried out. The BEs of C 1s and O 1s core levels, and the characteristic inner levels of the alkaline elements are given in Table 2, together with the M/C atomic ratios, determined from the peak intensities and the tabulated sensibility atomic

Table 2 Binding energies (eV) of core electrons and atomic ratios calculated by XPS Catalyst

C 1s

Norit RX-1

284.9 286.4 288.9 290.6 292.5

(57) (23) (7) (6) (7)

284.9 286.2 287.5 289.4

(57) (15) (15) (13)

530.9 (44) 532.7 (56)

1072.5a

0.0050

284.9 286.5 288.1 289.7

(59) (20) (10) (11)

530.9 (47) 532.6 (53)

723.9b

0.0027

Na+ -Norit RX-1

Cs+ -Norit RX-1

O 1s

Values in parenthesis indicate the percentage of each peak. a Na 1s. b Cs 3d 5/2 .

531.2 532.5 533.9 535.2

M (29) (41) (19) (11)

M/C atomic ratio

–

–

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factors [17]. Peak synthesis procedures revealed several components in the C 1s core level. A major peak at 284.9 eV and another three (four in the case of the Norit pristine) centered at high BEs can be discerned. The peak located at 284.9 eV can be assigned to the C–C bonds of graphitic-like structure of the carbon. The H-containing species (–CH–) should be included in the same peak, due to the small chemical shift between these species and the C–C ones. A second peak with a 15–23% ratio of the total area is observed around 286.4 eV, which can be associated with C–O bonds in alcohols. The third component close to 288 eV, in general less intense than the former, is attributed to ketonic species (C=O) and the last one, above 289 eV, to more oxidized (–COO–) or carbonates species. In the Norit RX-1 sample, besides this peak, which is observed at 290.6 eV, another component around 292.5 eV is detected. This last peak, due to a shake-up satellite (␲ → ␲∗ ) produced in the photoionization process, is not observed in the alkaline-containing samples. Similarly, the O 1s line profile is quite complex, especially in the Norit sample. The first component centered at 531 eV, is due to C–O and/or COO species of the carbonaceous support. The second one, at 532 eV, is attributed to the hydroxides (and carbonates) of the corresponding alkaline metals. In the Norit carbon sample, another two components above 533 eV are observed, which could be assigned to molecular water, probably with different interaction degree with the surface or different localization, for example, at the external surface and micropores. The BEs of the inner electrons of the alkaline elements fit well with hydroxide species, although carbonates species cannot be discarded, due to the proximity of the BEs of these last species. Nevertheless, it does not mean that Na+ and Cs+ are not ion exchanged on surface negative groups. As it is known, the surface of carbon materials contains negative groups that easily interact with alkaline cations when the corresponding ion exchange treatment is carried out. This ion exchange generates the active sites. By contrast, the exposure to ambient condition can generate a partial carbonation of the surface by reaction with CO2 , as detected by XPS. This is a frequent process in basic solid materials. Hence, the alkaline carbon is considered as the true catalyst.

With respect to the M/C atomic ratios, they tend to diminish when the size of the alkaline cation increases as a consequence of the lower exchange in the case of cesium, as determined by thermogravimetric analysis. However, the XPS results show a slight cesium enrichment at the surface, because the M/C ratio is around 2.4 times less for the cesium than for the sodium, meanwhile the content in metal (at-g M/100 g carbon) determined by thermogravimetric analysis is three times higher for the Na-Norit than for the Cs-Norit carbon. Using the Knoevenagel condensation as a probe reaction of basicity for inorganic solids, it has been found that most of the active sites in the alkaline promoted carbons have 13.3 ≤ pKa ≤ 16.5. Considering that the NH group of imidazole presents a pKa = 14.5 [18] these carbons are appropriate catalysts to ionize imidazole and to perform the imidazole propargylation. 3.2. Catalytic experiments under microwave irradiation In general, the propargylation of N-heterocycles in basic media, affords several products (Scheme 2) with low yield and selectivity for the desired N-propargyl derivative A [19]. Allenes B and ynamines C, which are stabilized by resonance, are formed together with the compound A [20]. Under microwave irradiation, the N-propargyl derivative A is selectively obtained when imidazole is alkylated with propargyl bromide. This was also the case when using Mg oxide as the catalyst [7]. The mass spectrum of the reaction product [MS m/s: 106(M+ ), 79 (100), 52, 39] confirms that the only product obtained is the compound A. The blank experiments give small amount of product, so the reaction can be negligible. In this study, the influence of some reaction parameters, such as basicity and amount of the catalyst, irradiation time and irradiation power, on the activity and selectivity of the reaction have been investigated. 3.2.1. Effect of the alkaline promoter and irradiation power The propargylation of imidazole was performed on Na+ - and Cs+ -promoted carbons. Several microwave irradiation experiments at different times for a fixed

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Scheme 2. Alkylation of imidazole with propargyl bromide.

power and amount of catalyst were carried out. Table 3 summarizes the conversion values obtained with both alkaline carbons, and indicates the temperatures attained inside the microwave oven during all the runs. The temperatures were measured inside the reaction vessels located in the microwave oven after each experiment. It can be observed that, at all the reaction times, the order of activity is Cs+ -Norit > Na+ -Norit. This trend agrees with the characterization of the samples, which indicates that the basicity of the Norit carbon increases with the size of the alkaline promoter. Thus, conversions around 83% are obtained in only 3 min of reaction at 300 W when Cs+ -Norit is used as catalyst. The activity of Cs+ -Norit is 1.2 times higher than the activity of Na+ -Norit. This ratio is maintained at other studied power. Selectivities are always >99%. For comparison, conventional heating in a batch system was carried out at 303 and 333 K, which are the same temperatures as attained under microwave heating. To check the external and internal diffusion Table 3 Microwave irradiation of imidazole (2 mmol) with propargyl bromide (2.5 mmol) using 0.02 g of Na+ - and Cs+ -Norit promoted carbons Conversion (%) 80 W

Na+ -Norit Cs+ -Norit Temperature (K) a

Time (min).

150 W

a series of experiments using imidazole and propargyl bromide, and Na- and Cs-carbons as catalysts, were carried out changing the stirring rates (1000 and 3500 rpm) and the particle size (≤0.074, ≤0.14, and ≤0.25 mm in diameter). The results obtained indicate that, in this range of conditions, the reaction is controlled by neither external nor internal diffusion. Thus, we can compare the activity and selectivity of the carbons under these reaction conditions. The conversions obtained using the same catalysts under conventional heating are shown in Table 4. A comparative study of the conversion levels achieved under microwave irradiation with those obtained under conventionally thermal activation, reveals that the microwave activation performs much better than thermal activation since higher yields are reached in less time. After 60 min of reaction at 333 K, conversion around 33% is obtained under thermal activation. Nevertheless, under microwave irradiation in only 3 min at 300 W, conversions of 83% are obtained on Table 4 Thermal activation of imidazole (2 mmol) with propargyl bromide (2.5 mmol) using 0.02 g of Na+ - and Cs+ -Norit promoted carbons Temperature (K)

3a

1a

3a

1a

3a

33.2 41.5 302

49.1 55.2 306

47.4 56.3 312

65.0 72.3 321

52.4 71.6 314

70.1 83.4 333

Conversion (%) Na+ -Norit

Cs+ -Norit

303

5 15 30 60

0.0 2.9 6.5 15.6

0.0 4.6 9.3 21.0

333

5 15 30 60

0.0 4.3 9.2 28.1

0.0 8.0 13.6 33.2

300 W

1a

Time (min)

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Fig. 1. Effect of the amount of catalyst in the N-propargylation of imidazole under microwave activation using as catalysts: (a) Na+ -Norit; (b) Cs+ -Norit. Reaction time: 3 min.

Cs-Norit sample. It may be observed hot particles of the carbon in the reaction mixture after the microwave experiments, which contribute to a faster reaction. 3.2.2. Influence of the catalyst amount The influence of the catalyst amount has also been studied under microwave activated reactions. 0.02 g and 0.04 g of catalysts (Na+ -Norit and Cs+ -Norit) have been employed at 80, 150, and 300 W of irradiation power. Fig. 1 displays the conversion values obtained with both catalysts. The product A is selectively formed, being the conversion values comprised between 50 and 99%. It is observed that for a given amount of catalyst, the yield of A increases with the irradiation time keeping constant the selectivity.

of Cs+ -Norit carbon is used. Conversion values of 98% with 100% selectivity are obtained in only 3 min of reaction. These results show the great usefulness of the microwave irradiation of a basic carbon catalyst in the target reaction. This method can be extended to the preparation of other N-propargyl heterocycles, which are key precursors for many fine chemicals.

Acknowledgements Financial Support of this work by Spanish CICYT (project MAT2001–0319) is gratefully acknowledged. Norit pristine carbon has been kindly supplied by Norit Company. References

4. Conclusions Alkaline carbons were found to be very efficient catalysts for the N-propargylation of imidazole. The basicity of the carbon is enhanced by the presence of the alkaline metal on the surface. The microwave heating can greatly improve the catalyst activity and selectivity of N-propargylation of imidazole over alkaline promoted carbons. Under these experimental conditions, the formation of environmental hazardous residues is avoided. It can be concluded that the activity increases with the irradiation power, giving the highest values of N-propargyl imidazole when 0.04 g

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