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

Reaction of [V^IVO(acac)₂] (Hacac = acetylacetone) with a Mannich base, <i>N</i>,<i>N</i>,<i>N</i>',<i>N</i>'-tetrakis(2-hydroxy-3,5-di-<i>tert-</i>butyl benzyl)-1,2-diaminoethane (H₄L, <b>I</b>) in a 1:1 molar ratio in MeOH, leads to the formation of the nonoxidovanadium(IV) complex [V^IVL] (<b>1</b>). Air stable complex <b>1</b> has been characterized using various spectroscopic techniques, DFT calculations, and single-crystal X-ray studies. <b>1</b> 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-4<i>H</i>-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 4<i>H</i>-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-1<i>H</i>-xanthenes and biscoumarins. The possible mechanism for the synthesized one-pot, multicomponent product has been proposed by isolating intermediate(s) generated during synthesis.