Notes
Catalytic synthesis of benzimidazole derivatives over modified forms of zirconia
T E Mohan Kumara, b, S Z Mohamed Shamshuddina, b, *, Venkatesha, b & S Reena Sarithaa, b
aChemistry Research Laboratory, HMS Institute of Technology, NH4, Kyathsandra, Tumakuru 572 104, Karnataka, India
bResearch and Development Center, Bharathiar University, Coimbatore 572 104, India
Email: mohamed.shamshuddin@gmail.com Received 2 May 2016; revised and accepted 3 November 2016
The solid acid catalysts, ZrO2, Mo(VI)/ZrO2 and Pt-SO42-/ZrO2 have been prepared and characterized for their surface area by BET, total surface acidity by NH3-TPD/ n-butyl amine back titration and crystallinity by powder XRD methods. These solid acid catalysts have been used in the synthesis of various benzimidazole derivatives. The results have been interpreted based on the variation of acidic properties and powder XRD phases of zirconia on incorporation of Mo(VI) or Pt-SO42–
ions. Mo(VI)/ZrO2 is found to be an efficient solid acid catalyst for the synthesis of benzimidazoles with up to ~98% yield. These solid acids were found to be reusable up to at least 5 reaction cycles. Plausible mechanism for the formation of benzimidazoles over protonic acid sites of the solid acid catalysts is proposed.
Keywords: Catalysts, Solid acids catalysts, Zirconia, Modified zirconia, Benzimidazoles
Many organic transformations are assisted by heterogeneous solid acid catalysts such as metal oxides, mixed metal oxides, doped metal oxides, zeolites, resins, clays, etc.1,2. These solid acids can replace liquid acid catalysts like H2SO4, HCl, AlCl3, BF3, H3PO4, SnCl4, ZnCl2, etc.3-6. The main advantages of solid acid catalysts are that they are eco-friendly and green because being non-toxic, non corrosive, easy to recover and reusable for several times. Also, they reduce or even eliminate the production of hazardous materials7-8. Metal oxide based solid acids like zirconia (ZrO2) as well as modified forms of zirconia are widely employed as catalysts in a number of acid catalyzed reactions such as transesterification, esterification, alkylation, acetylation, hydrocarbon isomerization, synthesis of heterocyclic compounds, etc.7-14. The physico-chemical and catalytic properties of zirconia can be modified by incorporating anions such as sulphate, borate, phosphate or cations such Mo(VI), W(V), Cr(III) or Pt-sulphate ions9.
Benzimidazoles are an interesting class of heterocyclic compounds which exhibit a wide range of biological and medicinal properties. They have applications in therapeutic treatment as antihistamines, antiviral, antifungal, antihypertensive, antiulcer, antitumor, etc.15-20. Several methods have been developed in the last decade for the synthesis of substituted benzimidazoles using various catalysts21-28. Many of these methods have one or more drawbacks such as laborious, low yields, tedious separation processes, several side reactions, expensive reagents and catalysts, complex work-up and purification.
In the present article the synthesis and characterization of solid acid catalysts, viz., ZrO2, Mo(VI)/ZrO2 and Pt-SO4
2-/ZrO2 is reported. One of the main objectives of this study was to investigate the catalytic activity of these solid acid catalysts in the synthesis of industrially important benzimidazoles.
The effect of nature of solid acid catalyst, weight of solid acid catalyst, solvents, reaction temperature and reaction time was studied to optimize the reaction conditions. Reactivation and reusability of these solid acid catalysts was also investigated.
Experimental
Hydrated zirconia (Zr(OH)4) was prepared by precipitation method. Typically, 100 g of zirconyl nitrate was dissolved in 250 mL of deionised water and the resulting solution was heated at 80 °C for 10 min. To this solution, aqueous ammonia (1:1) was added drop wise with constant stirring. The obtained precipitatewas filtered, washed with deionized water, dried in hot air oven at 120 °C for 12 h and finely powdered.
Mo(VI)/ZrO2 consisting of 5% Mo(VI) was prepared by taking known amounts of zirconyl hydroxide (4.5 g) and ammonium molybdate (0.16 g) in a china dish and making into a paste with 10 mL of deionised water29, 30. The resulting paste was dried at 120 °C for 12 h in hot air oven and the obtained solid was finely powdered.
Pt-sulphated zirconia was prepared in two steps30: To the previously prepared 10 g of Zr(OH)4, 6 mL of 3 M H2SO4 was added and made into a paste. The resulting paste was dried at 120 °C for 12 h in a hot air oven and the obtained solid was finely powdered.
To the resulting powder, 1% H2PtCl6 solution was added and made into a fine paste, which was again dried in an air oven for 12 h at 120 oC.
All the finely powdered solid acid catalytic materials were calcinated at 550 °C in a muffle furnace for 5 h. The calcinated solid acids are abbreviated as ZrO2 (Z), Mo(VI)/ZrO2 (MZ) and Pt-SO4
2-/ZrO2 (Pt-SZ). Pure zirconia was also calcined at 300 °C in addition to 550 °C.
The solid acid catalysts were characterized for their physico-chemical properties. The specific surface area was obtained by BET method (Quanta Chrome Autosorb Analyzer) while the, surface acidity was measured by NH3-TPD method (Mayura TPD unit) as well as n-butyl amine back-titration method using bromothymol blue indicator. Crystallinity of the materials was studied by powder X-ray diffraction (Philips X’Pert X-ray diffractometer) using CuKα.
A mixture consisting of a substituted aromatic aldehyde, a substituted aromatic diamine in a suitable solvent (ethanol) and a solid acid catalyst (Z or MZ or Pt-SZ) was taken in a reaction flask and the resulting mixture was heated at a particular temperature and the progress of the reaction was monitored by thin layer chromatography (Scheme 1). After the stipulated of reaction time, the reaction mixture was cooled, filtered and the residue was washed with ethanol to recover the solid acid catalyst. The filtrate was evaporated to get the crude reaction product. The desired benzimidazole product was purified by silica gel column chromatography using a suitable mobile phase (ethyl acetate+hexane). The reaction products were characterized by melting point, 1H NMR,
13C NMR spectroscopy (Bruker, 400 MHz) and LC-MS (Varian) techniques.
Results and discussion
The BET surface areas of Z, MZ and Pt-SZ are given in Table 1. The surface area of solid acids was
found to follow the order: Z < MZ < Pt-SZ. The surface area of zirconia was found to increase when incorporated with either Mo(VI) or Pt-SO4
2- ions.
Compared to Z and MZ, Pt-SZ showed the highest surface area which may be due to cracking of ZrO2
into fine particles when treated with SO4
2- ions1. However, an increase in the surface area of ZrO2 upon incorporation of Mo(VI) ions has been attributed to the formation of Mo-O-Zr linkages in MZ31.
When the surface areas of pure zirconia calcined at 300 °C and 550 °C were compared, the zirconia sample calcined at higher temperature was found to have higher surface area than the sample calcined at lower temperature.
The total surface acidity (TSA) values and acid site distribution in Z, MZ and Pt-SZ, calcined at 550 °C, determined by NH3-TPD/ n-butyl amine back titration technique are presented in Table 1. The TSA of the solid acids was found to follow the order: Z< MZ <
Pt-SZ.
Pure ZrO2 (calcined at 300 °C as well as 550 °C) was found to be least acidic when compared to its modified forms i.e., MZ or Pt-SZ. The TSA values indicate that the incorporation of either Mo(VI) or Pt- SO4
2- ions has a significant effect on the acidic EtOH
(solvent) NH2
NH2
+
H O
N H N
+
N N
R
R
R R
R R R
(aromatic
diamine) (aromatic aldehyde)
Zirconia based solid acid catalyst
(2-substituted benzimidazole)
(1,2-substituted benzimidazole)
where R = alkyl, methoxy, F, Cl and CF3 groups at different position of aromatic system.
Reaction of substituted aromatic aldehyde with substituted aromatic 1,2-diamine in presence of ZrO2 based solid acid catalyst.
Scheme 1
Table 1 Physico-chemical properties of the solid acids, ZrO2, MZ and Pt-SZ
Acid site distribution (mmol/g)a Catalyst BET surface
area (m2/g)
W M S VS TSAb
ZrO2 42.9 0.06 0.30 - - 0.36
(0.33)
MZ 148.1 - 0.26 0.76 - 1.02
(1.03)
Pt-SZ 161.8 - - 0.87 0.44 1.31
(1.29)
aW: Weak; M: Medium: S: Strong: VS: Very strong
bTSA values obtained by n-butylamine back titration method.
properties of pure zirconia. A considerable increase in the surface acidity of ZrO2 was observed when it was modified with either Mo(VI) or Pt-SO4
2- ions, which may be attributed to the formation of electron deficient states upon modification. Increase in the surface acidity upon incorporation of Mo(VI) ions could be attributed to the formation of Mo-O-Zr species31.
When acid site distributions of pure ZrO2, MZ and Pt-SZ were compared, it was observed that pure zirconia consists of ‘weak and moderate’ acid sites, MZ consists ‘moderate and strong’ acid sites whereas Pt-SZ consists of ‘strong and very strong’ acid sites32. Further, the TSA values obtained by n-butyl amine back-titration method were found to be in good agreement with that obtained by NH3-TPD method.
PXRD patterns of the solid acids, calcined at 550 °C and used for the present study are shown in Fig. 1. In the case of pure ZrO2, peaks due to both monoclinic phase (2θ (deg.) = 24.5, 28.2, 33.1, 47.3, 56.2) and tetragonal phase (2θ (deg.) = 30.2, 35.3, 49.8, 60.0) are observed. However, upon incorporation of Mo(VI) or Pt-SO4
2- ions the monoclinic phase of ZrO2 disappeared to vanish and only catalytically active phase ,i.e., tetragonal phase of ZrO2 could be observed32. This shows the effect of incorporation of Mo(VI) or Pt-SO4
2-
ions, which has a strong influence on the phase modification of zirconia (monoclinic to the metastable tetragonal).
Further, when the surface area, surface acidity of zirconia and its modified forms were compared with their PXRD patterns, a triangular correlation was found to exist between these properties. It looks as if;
the tetragonal crystalline phase of zirconia could be responsible for higher surface area as well as higher surface acidity of zirconia based solid acid catalysts.
When the PXRD pattern of pure zirconia calcined at 300 °C (Supplementary data, Fig. S1) was compared with the PXRD patterns of either MZ or Pt-SZ (Fig. 1), it was observed that all the three samples comprised the tetragonal phase of zirconia. It has been reported that the tetragonal phase of zirconia formed after the incorporation of either anions or cations into zirconia is catalytically more active32.
The synthesis of benzimidazoles was carried out with different substituted aromatic aldehydes and substituted aromatic diamines in presence of ethanol (solvent) over solid acid catalysts such as Z or MZ or Pt-SZ. However, for optimization studies 3,5-dimethyl- benzaldehyde and 4,5-dimethyl-benzene-1,2-diimine were selected as model substrates. Analysis of the reaction products revealed that 2-substituted and 1,2-substituted benzimidazoles such as 2-(3,5-dimethyl- phenyl)-5,6-dimethyl-1H-benzo[d]imidazole and 1-(3,5- dimethylbenzyl)-2-(3,5-dimethylphenyl)-5,6-dimethyl- 1H-benzo[d]imidazole respectively were formed as the reaction products. Optimization studies were carried out by varying the solid acid catalyst, amount of the solid acid catalyst, solvent, reaction temperature and reaction time. For these studies the molar ratio of the reactants was fixed at 3,5-dimethyl-benzaldehyde: 4,5-dimethyl- benzene-1,2-daimine = 1:1.
For further studies, the reaction products such as 2-(3,5-dimethylphenyl)-5,6-dimethyl-1H-benzo[d]- imidazole and 1-(3,5-dimethylbenzyl)-2-(3,5- dimethylphenyl)-5,6-dimethyl-1H-benzo[d]imidazole have been abbreviated as (product A) and (product B) respectively.
In order to compare the catalytic activity of the solid acid catalysts, calcined at 550 °C, used for the present work (i.e., ZrO2, Mo(VI)/ZrO2, Pt-SO4
2-/ZrO2) and to screen out the facile catalyst for the synthesis of benzimidazoles, the reactions were carried out between 3,5-dimethyl-benzaldehyde (0.65 mL), 4,5-dimethyl-benzene-1,2-daimine (0.6 g) (molar ratio
= 1:1) and ethanol (20 mL) as solvent over these solid acid catalysts. The reactions were carried out by using 0.06 g of the catalyst at 80 oC for 3 h. A correlation between the surface acidity and the catalytic activity
Fig. 1 Powder XRD patterns of (a) ZrO2, (b) MZ and (c) Pt-SZ.
[M indicates monoclinic and T indicates tetragonal phase].
of these solid acids was observed. Pure zirconia being least acidic was found to be least active in synthesis of benzimidazoles with the yields of product A (24%) and product B (11%). Over MZ solid acid catalyst, the yield of product A was found to be 53% and for product B it was 22%. However, when the reactions were carried out with Pt-SZ, other addition products were also formed besides product A (61%) and product B (26%). Hence Pt-SZ was found to be less selective towards the formation of the desired benzimidazole products. Formation of addition products over Pt-SZ can be attributed to the presence of ‘very strong’ acid sites which can decompose either reactants or reaction intermediates or the product molecules during the course of the reaction.
Further, when the reactions were carried out over pure zirconia (calcined at 300 °C) having tetragonal phase , it was found to be less active in synthesis of benzimidazoles (~16% of product A) and (~9% of product B) because of low acidity when compared to either pure zirconia calcined at 550 oC or MZ or Pt-SZ which also have tetragonal phase which is reported to be catalytically active32. Therefore, from this study it could be inferred that Mo(VI)/ZrO2 solid acid is a better catalyst as compared to either ZrO2 or Pt-SO4
2-/ZrO2. This study also indicates that the formation of benzimidazoles require either moderate or strong acid sites which are present to a maximum extent in the MZ catalyst. For further optimization studies, MZ was chosen as the solid acid catalyst.
The reactions were carried out by varying the weight of the solid acid catalyst (MZ) from 0.01 to 0.1 g at 80 °C for 3 h and the results are presented in Fig. 2. It was observed that the yield of benzimidazole (both products A and B) increased when the amount of the catalyst was increased from 0.01 g to 0.06 g and became stable when the weight was increased beyond 0.06 g. Highest yield of benzimidazole was obtained when 0.06 g of MZ was used as the solid acid catalyst. Hence for further studies 0.06 g was used as the optimum weight of MZ solid acid catalyst.
The reactions between 3,5-dimethyl-benzaldehyde and 4,5-dimethyl-benzene-1,2-daimine were carried out in presence of different solvents such as water, ethanol, methanol and dimethyl sulfoxide. The yield of benzimidazoles was lowest over water and highest over ethanol (Table 2). For the present work, ethanol was chosen as the solvent as it is one of the cheapest, easily available and low boiling liquid.
The effect of reaction temperature on the yield of benzimidazoles was studied by varying the temperature from 60 oC to 100 oC. An increase in the yield of benzimidazoles (both products A and B) was observed when the reaction temperature was increased from 60 °C up till 80 oC. Further increase in the reaction temperature beyond 80 oC resulted in the formation of addition products other than product A and product B. Formation of addition products may be attributed to the decomposition of either reactants or reaction intermediates or the products at elevated temperatures.
Therefore, from the present study reaction temperature of 80 oC was found to be a suitable for the synthesis of benzimidazoles.
The reaction time was varied between 1 and 5 h the yield of benzimidazoles (both products A and B) increased with an increase in the reaction time till 3 h and stabilized thereafter beyond 3 h. A reasonably good yield of benzimidazoles (particularly product A and product B) could be obtained for a reaction time of 3 h under the optimized reaction conditions.
Various benzimidazole derivatives (Supplementary data, Table S1) were synthesized under optimized reaction conditions, i.e., as molar ratio of aldehyde:
Fig. 2 Effect of weight of the solid acid catalyst (MZ) on the yield of benzimidazoles. [React. cond.: React. temp. = 80 °C;
react. time = 3 h; molar ratio of 3,5-dimethyl-benzaldehyde:
4,5-dimethyl-benzene-1,2-daimine = 1:1; ethanol = 20 mL].
Table 2 Effect of solvents on the yield (%) of benzimidazoles.
[React. cond.: React. temp. = 80 °C; amt of MZ = 0.06 g; molar ratio of aldehyde:diamine = 1:1].
Solvent Yield of A (%) Yield of B (%)
Water 31.2 11.2
Ethanol 53.1 22.2
Methanol 46.2 12.6
DMSO 47.4 13.2
diamine = 1:1, weight of the catalyst (Mo(VI)/ZrO2) = 0.06 g, solvent = ethanol and reaction temperature = 80 oC. The reactions were carried by using different aromatic aldehydes and diammines by following the procedure as discussed under experimental section.
However, the reaction time was found to vary with the nature of aldehyde or diamine used to synthesize various benzimidazoles. The reaction products were analysed by melting point, 1H NMR, 13C NMR spectroscopy, LC-MS techniques and the analysis data are given as Supplementary Data (Appendix S1).
It is noteworthy that two tautomers were formed in the compounds bearing entries (3, 6, 8-10, 12) which was confirmed by 1H NMR (showed 0.5 protons) and multiple peaks in aromatic region of 13C NMR.
However, only one tautomer was observed in the benzimidazole compounds bearing entries (1, 2, 4, 5, 7 and 11).
After the first reaction cycle, the solid acid catalysts were filtered from the reaction mixture, washed with ethanol, dried at 120 °C for 1 h and calcined at 550 °C for 30 min. The thus- reactivated solid acid catalyst was reused in the next reaction cycle for the synthesis of benzimidazoles under similar reaction conditions. The reactivation and reusability of the used solid acid catalyst was repeated for five times by following the same procedure.
Results show that pure ZrO2 as well as Mo(VI)/ZrO2
could be reused for at least five reaction cycles without any significant loss in their catalytic activity.
However, when Pt-SO4
2-/ZrO2 was reused for five reaction cycles, a gradually decrease in its activity could be observed. This study indicates that pure Z and MZ have better reusability than Pt-SZ (Supplementary data, Tables S2).
Though pure zirconia has good reusability, it is less catalytically active in benzimidazole synthesis as compared to MZ or Pt-SZ. Therefore, Mo(VI)/ZrO2
may be inferred as an efficient and reusable solid acid catalyst for the synthesis of benzimidazole derivatives.
The two plausible mechanisms for the formation of 2-substituted benzimidazoles and 1,2-substituted benzimidazoles using protonic (Brønsted) acid sites of a solid acid catalyst are shown in Scheme 2. One of the mechanisms involve the formation of an imine (a), which with the help of protonic acid site of the catalyst eventually undergoes ring closure by the intramolecular attack of second amino group on C-N double bond to give hydrobenzimidazole (c) that subsequently undergoes aromatization by aerial oxidation under the reaction conditions to afford the desired 2-substituted benzimidazole (d).
The other mechanism involves the formation of a diimine (e) followed by a ring closure reaction utilizing protonic acid site of the catalyst leading to a reaction intermediate (f), which gives the corresponding product (h) by sequential tandem transformation with regeneration of the protonic acid site33.
In summary, this article describes a convenient and efficient method for the synthesis of benzimidazoles
NH2 NH2 N N
H O
OH O-
OH= Protonic acid site N+
N H
H
N C+ N H
O- OH
N N
H O 2
N NH2
OH O-
N+ N H
H H
N H N H
O OH
-
N H N
[O]
R R
R
R
R
R
R
R
R R
R
R
R R
R R
R R
R
R
R ..
..
..
..
..
..
H O
(a) (b) (c)
(d) (e)
(f)
(g)
(h)
Mechanism for the synthesis of benzimidazoles over acid catalysts using protonic acid sites.
Scheme 2
with substituted aromatic aldehydes and substituted aromatic diamines over modified forms of zirconia as solid acid catalytic materials. A correlation between the surface acidity and the catalytic activity of the solid acids was observed. Mo(VI)/ZrO2 consisting of moderate or strong acid sites as well as bearing the catalytically active tetragonal phase of zirconia, has proven to be an efficient and reusable solid acid catalytic material for the synthesis of various benzimidazoles. Pt-SO4
2-/ZrO2 consisting of ‘very strong’ acid sites was found to lose its activity during the course of time when reused. Pure zirconia, calcined at low temperature (300 °C), even though had tetragonal phase, produced very low yield of benzimidazoles because of low acidity when compared to Mo(VI)/ZrO2.
Supplementary data
Supplementary data associated with this article, viz, Tables S1-S2, Fig. S1 and Appendix S1, are available in the electronic form at http://www.niscair.res.in/
jinfo/ijca/IJCA_55A(12)1465-1470_SupplData.pdf.
Acknowledgement
Authors are thankful to Vision Group on Science and Technology, Government of Karnataka (GRD- 375/2014-15) for the financial support. The authors are grateful to the authorities of Bangalore Institute of Technology, Bangalore for BET surface area analysis, Poornaprajna Institute of Scientific Research, Bangalore for PXRD analysis. The authors are also thankful to the Sophisticated Analytical Instrument Facility, Indian Institute of Science Bangalore for NMR and LCMS analysis of benzimidazoles compounds.
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