Direct access to 2-amino-3-cyano-4H-pyrans/4H-chromenes by [ONO]-pincer supported nickel(ii) phosphine complexes via dehydrogenative annulation of alcohols

Abstract

A highly efficient and convenient multicomponent one-pot synthesis of 4H-pyran/4H-chromene analogues has been demonstrated via acceptor-less dehydrogenative annulation (ADA) of alcohols employing newly designed Ni(II)–ONO pincer complexes as catalysts. A discrete set of Ni(II) complexes (C1–C3) incorporating O^N^O-chelating hydrazone ligands are synthesised and thoroughly characterized using analytical and spectroscopic methods (FT-IR, NMR and HRMS). Single-crystal X-ray diffraction study unambiguously confirms the ONO coordination mode and reveals a square-planar geometry around the nickel center. The catalytic efficiency of the complexes is established by the facile synthesis of 4H-pyran/4H-chromene derivatives through the dehydrogenative coupling of primary alcohols with dimedone or 4-hydroxycoumarin and malononitrile, affording the desired products with a yield of up to 91% using 2 mol% catalyst loading. The current protocol exhibits a broad substrate scope with high atom economy, enabling the construction of diverse 4H-pyran/4H-chromene derivatives (24 examples) and H₂ and H₂O as the only by-products. Control experiments suggest the involvement of key intermediates such as aldehydes and benzylidenemalononitrile. Furthermore, a successful gram-scale synthesis underscores the potential industrial applicability of this sustainable catalytic protocol.