Catalysis by Pincer Complexes: Synthesis of Esters, Amides, and Peptides

Abstract

Esters, amides, amines, acetals, and peptides are important industrial chemicals. Production of these chemicals is based onmultistep processes using stoichiometric amounts of acid or base promoters and coupling reagents, and involves reactive intermediates derived from alcohols or acids (Scheme 1.1), which leads to very large amounts waste [1]. Controlling overoxidation of substrates in the synthetic methods of these products is a challenge, and hence stepwise syntheses are performed (see below) [2]. Thus, green processes that can circumvent stoichiometric reagents and the generation of reactive intermediates, such as acid chlorides, and directly deliver stable and useful industrial products, such as esters and amides, in an atom-economical, selective manner without producing waste are needed. Pincer ligands, that is, tridentate ligands that enforce meridional geometry upon complexation to transition metals, result in pincer complexes which possess a unique balance of stability versus reactivity [3]. Transition-metal complexes of bulky, electron-rich ‘‘pincer’’ ligands have found important applications in synthesis, bond activation, and catalysis [4, 5]. Among these, pincer complexes of iPr-PNP (2,6-bis-(di-iso-propylphosphinomethyl)pyridine), tBu-PNP (2,6-bis-(di-tert-butylphosphinomethyl)pyridine), and PNN ((2-(di-tert-butylphosphinomethyl)-6-diethylaminomethyl)pyridine), PNN-BPy (6-di-tert-butylphosphinomethyl-2,2′-bipyridine) ligands exhibit diverse reactivity [6–8]. These bulky, electron-rich pincer ligands can stabilize coordinatively unsaturated complexes and participate in unusual bond activation and catalytic processes. In most processes, homogeneously catalyzed by metal complexes, the ligands, while imparting critical properties on the metal center, do not participate directly in bond-making and -breaking processes with the substrates. In recent years, complexes in which the ligands actively cooperate with the metal center in bondactivation processes have been developed [9]. We have devised novel catalytic systems based on pincer complexes in which the pincer ligands cooperate with the metal center in a synergistic manner and their interplay facilitates the chemical processes. The pincer complexes are based on new pyridineand acridine-type