Elsevier

Molecular Catalysis

Volume 484, March 2020, 110744
Molecular Catalysis

Knoevenagel-Doebner condensation promoted by chitosan as a reusable solid base catalyst

https://doi.org/10.1016/j.mcat.2019.110744Get rights and content

Highlights

  • Chitosan is used as solid base for Knoevenagel-Doebner condensation reaction

  • Catalyst enjoys high stability under the reaction conditions

  • Direct synthesis of unsaturated carboxylic acids is reported

  • Catalyst is used for five cycles with no loss in its activity

Abstract

The development of green and sustainable processes using naturally occurring biopolymers is becoming one of the suitable remedies to replace the conventional catalytic systems that generate large amount of byproducts with high risk factors. In this context, although Knoevenagel-Doebner condensation reaction has been reported with many organocatalysts including proline, no attempts were made to develop heterogeneous catalysts with environmental concerns. Considering these factors in mind, the title reaction is studied with chitosan as a heterogeneous solid base catalyst for the synthesis of α,β-unsaturated carboxylic acids through the condensation followed by decarboxylation reactions. Chitosan offers many advantages including high stability as evidenced by leaching, reusability tests, wide substrate scope and providing higher yields of the desired products with high purity. Powder X-ray diffraction (XRD), Fourier transform infrared (FT-IR), scanning electron microscope (SEM) and elemental analysis revealed that there are no major changes in the structural integrity and morphology of chitosan before and after catalysis under the optimized reaction conditions.

Introduction

Organocatalysts have been employed as preferential catalysts in synthetic organic transformations [[1], [2], [3], [4]] due to its environmental benignity and absence of metals. Therefore, organocatalysts mediated transformations afford products with very high purity without contamination of metal traces and hence, considerable interests are devoted to develop these types of catalysts in the synthesis of medicinally important and biologically active molecules [[5], [6], [7], [8]]. Chitosan is one of the naturally occurring biodegradable polymers [[9], [10], [11]] that can be easily prepared by the deacylation of chitin [12,13]. The presence of free amino groups in chitosan skeleton (Fig. 1) can behave as a basic site that can promote base catalyzed reactions and can be used as an alternative solid base catalyst instead of conventional basic catalysts. The use of chitosan as a solid base catalyst offers many advantages than other solid base catalysts including environmental benignity, biodegradability, availability of high density of basic sites, absence of metals and high stability in organic as well as aqueous medium. Due to these facinating properties, chitosan has been widely utilized as heterogeneous catalysts in wide variety of base-catalyzed organic reactions such as aldol condensation [[14], [15], [16]], Knoevenagel condensation [14,15,[17], [18], [19], [20], [21], [22]], Henry reaction [15], Michael addition [15,23] and multicomponent reaction [24,25]. On the other hand, due to its facile recovery from the reaction mixture and its ability to reuse in consecutive cycles, the use of chitosan offers additional merits compared to homogeneous basic catalysts [17,18].

Knoevenagel-Doebner condensation is one of the viable strategies to prepare directly α,β-unsaturated carboxylic acids from the reaction of aldehydes and malonic acids as starting entities in the presence of basic catalysts by the liberation of harmless by-products such as water and carbon dioxide [[26], [27], [28]]. Some of the notable applications of α,β-unsaturated carboxylic acids are i) found in lignin related structures [29], ii) used as synthons in organic chemistry [30,31], used as anti-oxidants [32,33], anti-UV ingredients [34,35] and building blocks for the synthesis of bioactive molecules [36,37]. These types of compounds were prepared with conventional organic bases like pyridine, piperidine and 4-dimethylaminopyridine as homogeneous catalysts [[38], [39], [40]]. Though these bases exhibited better performances by providing higher yields, the utility of these soluble catalysts is limited due to their toxic nature. Also, bifunctional polymeric catalyst [41] and microwave-assisted methods [42] were also developed to access α,β-unsaturated carboxylic acids. Nevertheless, these methods showed some limitations such as tedious catalyst preparation procedures and microwave reactor size. Recently, Allais and coworkers have reported the synthesis of p-hydroxycinnamic acids in moderate to good yields involving proline as organocatalyst [43]. In one side, all these previously discussed organocatalysts have showed superior activity in terms of yield, but, on other side, recovery and reusability of these catalysts are very tedious due to their complicated separation processes from the reaction mixture. Scheme 1 provides various catalytic methods developed for the synthesis of α,β-unsaturated carboxylic acids through Knoevenagel-Doebner reaction and compares them with the present work using chitosan as solid catalyst.

In recent years, pristine or modified chitosan solids have been widely employed as heterogeneous solid catalysts for various organic transformations [[44], [45], [46], [47], [48], [49], [50]]. Hence, considering the drawbacks with other methods and to overcome some of these limitations, herein, the present work reports the synthesis of α,β-unsaturated carboxylic acid derivatives through Knoevenagel-Doebner condensation starting from benzaldehyde (1) and malonic acid (2) using chitosan as a reusable heterogeneous solid base catalyst under milder conditions as shown in Scheme 2. Further, chitosan promotes the condensation of 1 with 2 to give the respective condensation product through Knoevenagel condensation promoted by the basic sites which further undergoes decarboxylation by eliminating carbon dioxide to afford α,β-unsaturated carboxylic acid as the final product.

Section snippets

Materials

Chitosan (Sigma reference number 448,869), malonic acid and aldehyde derivatives were procured from Sigma Aldrich and Alfa Aesar and used as received. N, N’-Dimethylformamide (DMF) and other solvents were received from Merck and Chemical Drug House commercial suppliers and they were used without any additional purification.

Instrumentation methods

Powder XRD patterns were measured in the refraction mode in a Bruker D2 Phaser X-ray diffractometer (30 kV, 10 mA) using the Cu Kα (λ =1.5406 Å) radiation. FT-IR analysis was

Results and discussion

The degree of N-deacetylation in the commercial chitosan (i.e., the average number of D-glucosamine units per 100 monomers expressed in percentage) was >75% and the basicity value due to amino groups was reported to be around 4.5 mmol/g. [19] Furthermore, the BET surface area of chitosan was ranging between 2−5 m2/g which is very low compared to other solid basic catalysts. [48]

The structural aspects of commercially available chitosan were characterized by powder XRD, FT-IR, elemental analysis

Conclusions

In summary, the present work has shown a simple and facile strategy for the preparation of α,β-unsaturated carboxylic acids through the condensation of 1 and 2 using chitosan as a heterogeneous solid base catalyst. Although wide ranges of homogeneous organocatalysts have been reported for this reaction, the recovery of these catalysts strongly impose serious limitations. In contrast, the use of chitosan as a heterogeneous solid base catalyst offer many advantages like biodegradability,

CRediT authorship contribution statement

Nagaraj Anbu: Visualization, Investigation, Data curation, Writing - original draft. Surendran Hariharan: Supervision, Writing - review & editing. Amarajothi Dhakshinamoorthy: Conceptualization, Methodology, Software, Validation.

Acknowledgements

AD thanks the University Grants Commission, New Delhi, for the award of an Assistant Professorship under its Faculty Recharge Programme. AD also thanks the Science Engineering Research Board (SERB), Department of Science and Technology, India, for the financial support through Extra Mural Research Funding (EMR/2016/006500).

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