New ruthenium(ii) catalysts enable the synthesis of 2-amino-4H-chromenes using primary alcohols via acceptorless dehydrogenative coupling

Abstract

A series of biologically important 2-amino-4H-chromenes functionalized with different substituents have been synthesized through a one-pot multicomponent reaction catalysed by p-cymene Ru(II) organometallic complexes encompassing N–O chelated carbazole based hydrazone ligands. A panel of p-cymene Ru(II) complexes were synthesized and characterized using various spectral (FT-IR, UV-vis, NMR and HR-MS) and analytical methods. The molecular structure of one of the complexes was corroborated with the help of a single-crystal X-ray diffraction study. 2-Amino-4H-chromene derivatives have been readily assessed under mild conditions via ruthenium(II) catalyst mediated acceptorless dehydrogenative coupling of substituted benzyl alcohols, resorcinol, and malononitrile. The present catalytic protocol furnishes a variety of 2-amino-4H-chromenes in high yields of up to 95% from a wide range of readily available primary alcohols without the use of any oxidant/additives utilizing low catalyst loading. A plausible mechanism for the catalytic synthesis of 2-amino-4H-chromene compounds has been described via the formation of an aldehyde and benzylidenemalononitrile intermediates with the discharge of water and hydrogen as by-products. Interestingly, a medicinally important tacrine analogue was successfully constructed with good yields from the synthesized 2-amino-4H-chromenes.