Evaluation of hemilabile Pd(II) NNS and NNSe non-symmetric pincers in Suzuki–Miyaura cross-coupling: Unexpected selective nitrile hydration of 4-bromonitrobenzene in mild conditions

Abstract

In this work, four non-symmetric NNS and NNSe Pd(II) pincer complexes (1–4) supported by tridentate iminophosphorane ligands bearing both the indole heterocycle and a varying chalcogenoether pending arm [XR = SPh (1), SePh (2), SMe (3), SeMe (4)], were tested in the Suzuki–Miyaura cross coupling reaction between phenyl boronic acid and aryl bromides. The modest catalytic activity of complex 3 under conventional heating (110 °C), was boosted under microwave irradiation, leading to excellent yields of coupling under remarkably mild conditions: 5 min of reaction time at 70 °C under aerobic conditions. The complexes supported by NNS ligands proved to be more robust and superior catalysts than those supported by NNSe ligands, which tended to decompose under catalytic conditions. Among the four complexes, pincer 3 also demonstrated the best catalytic performance and was tested in the coupling of a variety of p-substituted bromobenzenes. Notably, the catalytic performance of complex 3 was unaffected by the mercury drop test, and with most substrates in the series, the percentage of conversion reflected the aryl bromides’ Hammett parameter. However, little to no coupling occurred with p-bromobenzonitrile and the reaction with p-nitrobenzene resulted in only limited conversion yields. In fact, when activated using microwave irradiation, complex 3 unexpectedly catalyzed the nitrile hydration of p-bromobenzonitrile, yielding p-bromobenzamide under remarkably mild Suzuki–Miyaura coupling conditions.