Suzuki-Miyaura cross-coupling reaction catalyzed by Pd(II) complexes of N,O-donor Schiff base ligands

Abstract

The catalytic activities of Pd(II) complexes synthesized from bidentate N,O-donor ligands were evaluated for the Suzuki-Miyaura CC coupling reaction between haloarenes (iodo-/bromo-/chloro-) and electron-donating/electron-withdrawing groups containing phenylboronic acids. The N,O-donor ligands were synthesized from salicylaldehyde, 3,5-di-tert-butylsalicylaldehyde, 3-tert-butylsalicylaldehyde, 5-tert-butylsalicylaldehyde with aniline and derivatives of aniline. The single crystal of complexes 5c, 5f, 5h, 5j, and 5t were diffracted by single crystal XRD, and crystal structure of these Pd(II) complexes represents trans geometry. Different Pd(II) complexes of the ligands with electronic and steric effects were evaluated in the Suzuki-Miyaura reaction were explored. Moreover, the reaction parameters such as substrate to base (K2CO3) ratio, temperature, solvent, catalyst loading, steric and electronic effect of phenylboronic acid and aryl halides for the catalytic coupling were investigated. The Pd(II) catalyst 5a was highly active across a broad range of electron donating/electron withdrawing aryl iodides and aryl bromides. The catalyst 5a (0.05 mol%) in the presence of K2CO3 base at 120 °C in DMF shows an excellent activity towards a wide range of phenyl boronic acids containing electron donating/electron withdrawing groups and the corresponding products were obtained in 45–99% yields after 2–4 h. Moreover, the Pd(II) complex 5a was reused in three continuous cycles by adding the reactants (iodobenzene and phenylboronic acid) after their complete conversion to the product, and the catalyst took a longer time for conversion of the substrate in each continuous recycle. The turnover number of the Pd(II) complex 5a was found to be 1960 which was comparatively better than other Pd(II) salen complexes 5b-u.