Novel catalytic properties of pyridyl-bonded Pd-catalysts in cross-couplings of aryl halides with phenyl boronic acid under aerobic conditions

Abstract

A new series of stable, easily accessible, efficient and recyclable phosphine free Pd-catalysts have been developed for their catalytic activity using Suzuki–Miyaura cross-coupling as model reaction system. The 2-benzoylpyridine-N(4)-substitutedthiosemicarbazones form compounds of composition, [Pd{(2-py-N)(Ph)CN–NC(–S)–N⁴HR}Cl] (R =H, Me, Et, Ph), which have been labeled as Cat 1(H), Cat 2(Me), Cat 3(Et) and Cat 4(Ph) – based on R-substitution at –N⁴HR. The catalytic activity of these complexes with central core, {Pd(NNS-L)Cl}, has been explored by using simple aryl bromides and aryl iodides as substrates for S-M coupling reactions. Thus the C–C cross-couplings of a series of aryl halides (R–X), Ph–X, p-NC–C₆H₄–X, p-MeO–C₆H₄–X and p-Me–C₆H₄–X (X = Br, I), with phenyl boronic acid have been successfully carried out in the presence of a Pd-catalyst using K₂CO₃ base in aqueous ethanol medium at 50 °C under aerobic conditions. Each catalyst activated C–Br and C–I bonds of aryl halides and formed di-aryl products, Ph–Ph, p-NC–C₆H₄–Ph, p-MeO–C₆H₄–Ph and p-H₃C–C₆H₄–Ph, from both type of halides with high isolated yields and TON. The reactivity of C–X bonds of aryl halides (Y–C₆H₄–X; X = Br, I) towards oxidation addition varied in the order: Y = CN > H > MeO ∼ Me (Y is trans to X). These are initial, preliminary studies that may enable future work establishing these complexes as promising catalysts for related transformations. The catalytic system has interesting key features such as aqueous ethanol media, recyclability, and phosphine-free nature.