Facile synthesis of NC(sp3)O pincer palladium complexes and their use as efficient catalysts for Suzuki-Miyaura reaction of aryl bromides in aqueous medium

Abstract

Two NC(sp3)O pincer palladium(II) complexes 3a-3b were readily prepared in high yields in only two steps. Of the first step, catalytic hydrophosphination of 2-alkenoylpyridines and subsequent in situ phosphine oxidation produced the NC(sp3)O pincer preligands 2a-2b. The second step is palladation of the preligands 2a-2b where PdCl2 was used as the Pd source to afford the desired Pd pincers 3a-3b via C(sp3)-H bond activation. Single crystal X-ray diffraction analysis of complex 3a unambiguously confirmed the NCO tridentate coordination mode of the complexes. The two complexes 3a-3b were applied to catalyze the Suzuki-Miyaura reaction. Complex 3b was found to be more efficient and exhibited very high activity in the Suzuki reaction of structurally diverse aryl bromides with arylboronic acids in aqueous ethanol under air. At a reaction temperature of 70 °C, a TON of up to 1.9 × 105 and a TOF of up to 9800 h−1 were achieved. At lower temperatures 3b was still very active, giving a TON of up to 9.5 × 103 and a TOF of up to 3900 h−1 at room temperature.