Nonoxidovanadium(IV) Complex-Catalyzed Synthesis of 2-Amino-3-cyano-4H-pyrans/4H-chromenes, Biscoumarins, and Xanthenes under Green Conditions

Reaction of [(VO)-O-IV(acac)(2)] (Hacac = acetylacetone) with a Mannich base, N,N,N ',N '-tetrakis(2-hydroxy-3,5-di-tert-butyl benzyl)-1,2-diaminoethane (H4L, I) in a 1:1 molar ratio in MeOH, leads to the formation of the nonoxidovanadium(IV) complex [(VL)-L-IV] (1). Air stable complex 1 has been characterized using various spectroscopic techniques, DFT calculations, and single-crystal X-ray studies. 1 adopts distorted octahedral geometry where ligand coordinates through all coordination functionalities available. This complex has been used as a catalyst in the one-pot, three-component synthesis of 2-amino-3-cyano-4H-pyrans using 1,3-dicarbonyls (1,3-cyclohexanedione, dimedone, barbituric acid, and 4-hydroxycoumarin), malononitrile, and various substituted aromatic aldehydes in equimolar amounts employing ethanol as a green solvent. The catalytic reaction revealed that the multicomponent synthesis of 4H-pyrans and chromenes is greatly influenced by both types of 1,3-dicarbonyl compound employed and the nature of the substituent on the aromatic ring of the aldehyde. Synthesized catalyst has also been used in the synthesis of pharmacologically relevant oxygen-containing heterocycles, specifically, 1,8-dioxo-octahydro-1H-xanthenes and biscoumarins. The possible mechanism for the synthesized one-pot, multicomponent product has been proposed by isolating intermediate(s) generated during synthesis.