Synthesis and structure of binuclear arene Ru(ii) N^O chelating complexes: synthesis of pyrimidinones via acceptorless dehydrogenative annulation using alcohols†

Abstract

We report binuclear arene Ru(II) benzhydrazine complex catalysed eco-friendly, selective and sustainable synthesis of bioactive pyrimidinones via acceptorless dehydrogenative annulation of alcohols. Facile synthesis of three new binuclear ruthenium(II) complexes of general formula [(η⁶-p-cymene)₂Ru₂(L)Cl₂] (BC1–BC3) (where L = biphenyl benzhydrazine derivatives) has been accomplished by the reaction of [(η⁶-p-cymene)₂Ru₂Cl₂(μ-Cl)₂] with biphenyl hydrazine ligands (BL1–BL3). The formation of the newly synthesized ruthenium complexes has been authenticated by analytical and spectral (FT-IR, UV-vis, NMR and HR-MS) techniques. The three-dimensional molecular architecture of one of the representative complexes (BC3) has been ascertained by single crystal XRD study, which revealed the presence of a pseudo octahedral geometry around ruthenium. Furthermore, the catalytic activity of all the complexes has been examined towards the construction of substituted pyrimidinones from the coupling of readily available alcohols, ethyl cyanoacetate and amidine hydrochlorides using acceptorless dehydrogenative annulation (ADA) methodology. A library of 6-oxo-1,6-dihydropyrimidine-5-carbonitrile derivatives (20 examples) has been synthesised using 1 mol% of ruthenium catalyst (BC2) loading with a maximum yield of up to 92% and H₂, H₂O and ethanol as the only by-products. The utility of the existing catalytic protocol has been extended to a large-scale synthesis of one of the derivatives 6-oxo-1,6-dihydropyrimidine-5-carbonitrile in 73% isolated yield. Furthermore, anticancer drug “bropirimine” has been successfully achieved by utilizing the current catalytic protocol.