Abstract
The synthesis of a series of known β-aminocyclohexanones has been accomplished using pentaerythrityl tetramethyl imidazolium phosphotungstate (C(MIM-PTA)4) as a new tetradentate acidic catalyst. It was prepared via condensation of pentaerythrityl tetrabromide with methyl imidazole. Then, bulky anion H2PW12O401− was substituted with Br− in the structure. This tetradentate catalyst provides designable cations and anions. Anions have two types of acids, acidic protons, and metals with Lewis acidity. In order to test the efficient catalytic behavior of the tetradentate catalyst, a controlled reaction was performed using benzaldehyde, aniline and cyclohexanone. Imine from the condensation of benzaldehyde and aniline was observed in the absence of ionic catalyst instead of desired products. Thus, this reaction would be attractive because of the time, energy, and raw material saving considerations because of the absence of isolation of intermediates and stereospecificity. The catalyst shows high catalytic activity such that after four recycles the product was obtained with high yield and purity. This reaction was performed at room temperature. Although high temperature could improve the reaction rate, it contributes to side reactions and oxidation of aldehyde and amine. The catalyst was characterized by elemental analysis, FT-IR spectroscopy, 1H NMR, 13C NMR, and TGA.
Highlights
The catalyst shows high catalytic activity such that after four recycles the product was obtained with high yield and purity
We describe the successful synthesis and use of a tetradentate polyoxometalate (Fig. 1) for the condensation of aldehyde, ketone and amine in the Mannich-type coupling
One of the general route to make heterogeneous polyoxometalate catalysts is by loading them in porous materials. This heterogeneous catalyst showed a high activity in many catalytic reactions, but they suffered aggregation and leaching of the catalyst due to the weak interaction with supports
Summary
The Mannich reaction is one of the most useful strategies for C–C bond formation in organic synthesis.[1,2] It is a practical and efficient method for the construction of both allylic and benzylic amines, important targets that are found in many biologically active molecules and which are useful synthetic intermediates employed in the total syntheses of alkaloids and other complex molecules.[3,4,5,6,7] This reaction is considered to be a very effective strategy for the synthesis of useful compounds as building blocks in pharmaceuticals and natural products such as antimicrobial,[8] cytotoxic,[9] anticancer[10] and analgesic activity.[11,12]. The general catalysts for standard Mannich reactions mainly involve various acids Most of these strategies suffer from problems such as using corrosive reagents, expensive and large amounts of catalyst, long reaction times, harmful reaction media ( uorinated solvent), and low yields. In this regard, aromatic amines with ortho-substitution in most cases gave trace or even no products as a result of a large steric hindrance effect. We describe the successful synthesis and use of a tetradentate polyoxometalate (Fig. 1) for the condensation of aldehyde, ketone and amine in the Mannich-type coupling This designable catalyst has some important properties such as a heterogenic Brønsted and Lewis catalyst, the presence of oxygen as a base, and recyclability
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