Porous organic polymer bearing triazine and pyrene moieties as an efficient organocatalyst
Graphical abstract
We report a new secondary amine linked triazine and pyrene containing microporous organic polymer (TrzPyPOP) through the polycondensation reaction and it is used as a very efficient organocatalyst for the synthesis of dihydropyrimidones via multicomponent coupling reaction.
Introduction
Convenient and eco-friendly strategies for the synthesis of dihydropyrimidones (DHPMs) [1] have attracted considerable attention over the years in medicinal chemistry as these compounds worked efficiently for the treatment of cardiovascular diseases [2]. DHPMs show close structural as well as compositional resemblance with nifedipine, often used to regulate high blood pressure. Further, the activity of the DHPMS can be tuned by changing the axial aromatic group, which bisect the boat conformation [3]. Thus, continuous research efforts are directed for the modification of the reactants and reaction conditions [4] for better yield of DHPMs together with its high purity in shorter reaction time. In this context, multicomponent condensation reaction between reactive small organic molecules in the presence of suitable catalyst can offer viable route for the synthesis of DHPM derivatives. In synthetic organic chemistry multicomponent reactions (MCR) [5] are very demanding especially in the discovery of novel drug molecules. In the year 1891, Italian chemist Petro Biginelli [6] has introduced a one-pot multicomponent reaction featuring the condensation between ethyl acetoacetate, benzaldehyde and urea. This reaction has versatile potential for the synthesis of a series of DHPM derivatives. The reaction can easily be carried out by heating the reactants in the presence of solvent with catalytic amount of hydrochloric acid at the refluxing temperature. Being a one-pot process involving multiple reactants, the products obtained from the MCR are economically favorable too. However, under this reaction conditions often moderate yields of the products are obtained together with no scope for recyclability.
Subsequently scientific community has paid huge attention to develop several homogeneous and heterogeneous catalytic systems for synthesizing analogous heteroatom containing chemicals [7] by exploring different synthetic routes. Several heterogeneous catalysts can catalyzes the modified Biginelli reactions with good recyclability, easiness in separation of the catalysts from reaction mixture together with enhanced purity of the product, leaching of active metal sites are the major issues in these cases for prolonged operation. In this context metal-free heterogeneous base catalyst or organocatalysts [8] bearing reactive organic functional groups at the catalyst surface can be utilized to carry out this MCR for the synthesis of value added DHPMs. Biginelli condensation reaction has been conducted over a large variety of homogeneous and heterogeneous catalysts such as Fe(NO3)3.9H2O [9], 12-molybdophosphoric acid [10], Fe(III)/Si-MCM-41 [11], poly(2-acrylamido-2-methylpropane sulphonic acid) [12], polystyrene-supported Al(III) [13], Ce-MCM-41 [14] etc. The magnetic Fe3O4 nanoparticles supported imidazolium-based ionic liquids [15] and many more surface functionalization strategies [[16], [17], [18], [19], [20]] have been undertaken for catalyst modification as well as enhancement of the rate of this reaction. Previously we had reported the Fe3O4 nanoparticles supported functionalized SBA-15 [21] as magnetically recoverable nanocatalyst for carrying out the Biginelli condensation reaction. But optimization of the product yields under mild reaction conditions and purity concern are still remained as major issues for this reaction. Moreover, only few basic organocatalysts are reported so far for this Biginelli condensation reaction.
Porous organic materials have attracted significant interest in solving several issues of energy and environment in the recent times. Covalent organic frameworks (COFs) [[22], [23], [24]], covalent triazine frameworks (CTFs) [25], conjugated microporous polymers (CMPs) [26,27] and related other porous organic polymers (POPs) [[28], [29], [30], [31]] are intensively studied in several frontline applications like catalysis [[32], [33], [34]], selective gas adsorption [35], water purification [36], light harvesting [37] and so on. Among all these porous organic materials, POPs have more flexibility in the synthesis due to their high specific surface area, ease of synthesis in bulk quantities, ease of the introduction of the desired functional group and high chemical stability. As POPs are insoluble in all common organic solvents, they can be used as a heterogeneous catalyst for a series of organic transformation and after reaction the catalyst can be easily isolated, which offer high recyclability of catalyst after several reaction cycles. Further, abundance of aminal linkages in the POP network is helpful for carrying out the base catalyzed reactions [38]. Herein we report the facile metal-free base catalyzed Biginelli reaction over a new triazine and pyrene containing porous organic polymer TrzPyPOP as a heterogeneous organocatalyst synthesized through a very convenient polycondendation reaction between 1,4-bis(4,6-diamino-s-triazin-2-yl)-benzene and pyrene-1-carboxaldehyde as shown in Scheme 1.
Section snippets
Chemicals
Terephthalonitrile, dicyandiamide, 1-pyrene-carboxaldehyde and the aldehydes used as substrates for the catalytic reactions, like 4-methylbenzaldehyde, 4-bromobenzaldehyde, 4-chlorobenzaldehyde, 4-formylbenzonitrile, 4-nitrobenzaldehyde and furan-2-carbaldehyde, were purchased from Sigma-Aldrich. Ethyl acetoacetate, ethyl cyanoacetate were received from Loba Chemie, whereas thiourea and urea were procured from Merck India. Organic solvents such as 2-metoxyethanol, dimethylsulphoxide (DMSO),
Results and discussion
We have synthesized the triazine based four armed amine functionalized ligand (SL-1) by microwave assisted thermal condensation (at 195℃) between terepthalonitrile and dicyandiamide under alkaline medium (Scheme 1). This is followed by the polycondensation reaction between SL-1 with pyrene-1-carboxaldehyde under refluxing conditions to yield TrzPyPOP. The as synthesized material has been characterized by several characterization tools and these are summarized below.
Conclusions
Our experimental observations suggested that a novel hierarchically porous organic polymer TrzPyPOP bearing pyrene and triazine moieties can be synthesized through a convenient polycondensation reaction between four armed amine and pyrene aldehyde. TrzPyPOP showed hierarchical porous polymeric structure with BET surface area 1016 m2 g−1. By virtue of its high specific surface area, hierarchical porosity in nanoscale, and mild surface basicity due to presence of aminal linkages, this porous
CRediT authorship contribution statement
Sabuj Kanti Das: Methodology, Data curation, Formal analysis, Writing - original draft. Avik Chowdhury: Data curation, Formal analysis, Writing - original draft. Debabrata Chakraborty: Data curation, Formal analysis. Utpal Kayal: Data curation, Formal analysis. Asim Bhaumik: Supervision, Funding acquisition, Writing - review & editing.
Declaration of Competing Interest
There are no conflicts to declare.
Acknowledgements
SKD wishes to thank UGC New Delhi and IACS for his senior research fellowship. AC wishes to thank CSIR, New Delhi for a senior research fellowship. DC wishes to thank DST-INSPIRE for a senior research fellowship. AB wishes to thank DST-SERB, New Delhi for a core research grant (Grant number CRG/2018/000230).
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