Deep eutectic ionic liquids based on DABCO-derived quaternary ammonium salts: A promising reaction medium in gaining access to terpyridines

Abstract

Owing to the directional H-bonding, coordination and p-stacking abilities, terpyridines have been widely used as supramolecular tectons in molecular architectures, skeletons in molecular devices and metallopolymers, and are gaining importance in medicinal chemistry. In this paper, we have synthesized, characterized and applied deep eutectic ionic liquids (DEILs) based on 1,4-diazabicyclo[2.2.2]octane; triethylenediamine (DABCO)-derived quaternary ammonium salts for the preparation of terpyridines. These DEILs were synthesized through N-alkylation of DABCO with haloalkanes (1-bromopentane or 1-bromoheptane) followed by mixing and heating with methanol or polyethylene glycol as a hydrogen bond donor. The synthesized DEILs were structurally characterized by IR and NMR. The formation of deep eutectic solvent was confirmed by freezing point depression, it composition was investigated through phase diagram, and its thermal stability was determined through differential scanning calorimetry, derivative thermogravimetry and thermal gravimetric analysis studies. Further, these DEILs were investigated for their effectiveness towards synthesis of 2,2':6',2"-terpyridine, 3,2':6',3"-terpyridineand 4,2':6',4"-terpyridine derivatives through Kröhnke reaction. The results show that these three types of terpyridines can be obtained in reasonable yields (80%-97%) by the one-pot reaction of 2-, 3- or 4-acetylpyridine with a variety of aromatic aldehydes in the presence of DEIL as a reaction medium, sodium hydroxide as a base and ammonium acetate as a cyclizing agent. This methodology is highly efficient and cost-effective for synthesis of symmetrical as well as unsymmetrical terpyridines. Importantly, these DEILs can be reused several times without an obvious loss of activity and are non-toxic, low-volatile, biodegradable and highly thermally stable. Therefore, these DEILs as a non-conventional reaction medium for the synthesis of terpyridines provides appealing opportunities to be investigated in the domain of green synthesis.

Graphical abstract

Keywords

terpyridine / deep eutectic solvent / ionic liquid / Kröhnke reaction / DABCO

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