Acceptorless oxidant-free dehydrogenation of amines catalyzed by Ru–hydride complexes of amide-acid/ester ligands

Abstract

Traditional dehydrogenation of amines involves the transfer of hydrogen molecule(s) from a substrate to an acceptor. In acceptorless dehydrogenation, hydrogen gas is liberated without an oxidant, providing an efficient synthetic method. Acceptorless dehydrogenation of primary amines to nitriles without using an oxidant or hydrogen acceptor is significant yet challenging. Herein, we present efficient Ru-based catalysts capable of carrying out such a transformation with hydrogen gas as the only by-product. A new class of air and moisture-stable ruthenium–hydride complexes (1–4) of amide-acid/ester-based ligands have been synthesized and characterized. Crystal structures of two representative complexes, 2 and 3, illustrate the bidentate N–O coordination mode of the ligands. At the same time, additional binding sites are occupied by one hydride, one CO, and two PPh₃ co-ligands. The catalytic behavior of these complexes is explored towards the oxidant-free, acceptorless, and selective dehydrogenation of primary and secondary amines affording nitriles and imines, respectively. Among four Ru(II) complexes, complex 2 showed the best catalytic activity for the dehydrogenation of amines. A wide variety of both primary and secondary amines were utilized to explore the substrate scope. The catalytic system tolerated both electron-withdrawing and electron-releasing substituents on amine substrates. Various control experiments and mechanistic studies were carried out to support the dehydrogenation of amines by using complex 2 as a representative catalyst.