Polyaniline, Polyaniline/SiO2 and Poly(4-vinylpyridine): as Highly Efficient and Recyclable Green Heterogeneous Basic Catalysts for the Three-Component Synthesis of Tetrahydrobenzopyran and 3,4-Dihydropyrano[c]Chromene Derivatives

Polyaniline, polyaniline/SiO2, and poly(4-vinylpyridine) are highly effective base catalysts for the synthesis of tetrahydrobenzo[b]pyran and 3,4-dihydropyrano[c]chromene derivatives by the one-pot three-component condensation reaction of aryl aldehydes, malononitrile and α-hydroxy or α-amino activated C–H acids such as 1,3-cyclohexanedione, dimedone, 4-hydroxy-6-methylpyrone, 4-hydroxycoumarin, 1,3-dimethylbarbituric acid, and 1,3-dimethyl-6-amino uracil. The remarkable advantages of this new procedure are high yields, short experimental time, mild reaction condition, low cost and easy preparation of the catalysts, and no need for any workup and purification after completion of the reaction. In addition, these catalysts exhibited excellent recoverability without a negligible decrease of their activities after at least six cycles of reaction.