3-Arylcoumarin Scaffolds From 3-Chlorocoumarin and Arylboronic Acids via Site-Selective CCl Bond Activation With Palladium Complexes of N∩O Chelating Hydrazones

Abstract

Using palladium hydrazone complex as a homogeneous catalyst for SMC, we synthesized a library of 15 variant 3-arylcoumarins. In this context, we report a set of four coordinated palladium(II) hydrazone complexes 1 and 2 having the general formula [Pd(L)(Cl)(PPh₃)] by reacting the precursor complex [PdCl₂(PPh₃)₂] with the hydrazone ligands HL₁ or HL₂. Single crystal XRD and conventional spectroscopic techniques were employed to characterize both complexes. From the catalytic application point of view, the new complexes 1 and 2 were tested as homogeneous catalysts towards the conversion of 3-chlorocoumarins into 3-arylcoumarins by activating the challenging CCl functionality. A minimal catalyst loading of complex 2 (0.01 mol%), afforded a series of 3-arylcoumarins with a maximum of 92% yield and reusability up to five cycles. The present methodology is simple and easy to carry out at room temperature utilizing a green solvent (ethanol) in an open flask with a spectrum of aryl boronic acids.