Direct synthesis of polyamides via catalytic dehydrogenation of diols and diamines

Rethinking amide bond synthesis

One of the most important reactions in organic chemistry--amide bond formation--is often overlooked as a contemporary challenge because of the widespread occurrence of amides in modern pharmaceuticals and biologically active compounds. But existing methods are reaching their inherent limits, and concerns about their waste and expense are becoming sharper. Novel chemical approaches to amide formation are therefore being developed. Here we review and summarize a new generation of amide-forming reactions that may contribute to solving these problems. We also consider their potential application to current synthetic challenges, including the development of catalytic amide formation, the synthesis of therapeutic peptides and the preparation of modified peptides and proteins.

Heterogeneously catalyzed synthesis of primary amides directly from primary alcohols and aqueous ammonia

In the presence of a manganese oxide based octahedral molecular sieve (OMS-2), a range of primary amides could be synthesized directly from primary alcohols and ammonia. The observed catalysis was heterogeneous, and the recovered catalyst could be reused many times without an appreciable loss of its catalytic performance.

Metal-ligand cooperation by aromatization-dearomatization: a new paradigm in bond activation and "green" catalysis

In view of global concerns regarding the environment and sustainable energy resources, there is a strong need for the discovery of new, green catalytic reactions. For this purpose, fresh approaches to catalytic design are desirable. In recent years, complexes based on "cooperating" ligands have exhibited remarkable catalytic activity. These ligands cooperate with the metal center by undergoing reversible structural changes in the processes of substrate activation and product formation. We have discovered a new mode of metal-ligand cooperation, involving aromatization-dearomatization of ligands. Pincer-type ligands based on pyridine or acridine exhibit such cooperation, leading to unusual bond activation processes and to novel, environmentally benign catalysis. Bond activation takes place with no formal change in the metal oxidation state, and so far the activation of H-H, C-H (sp(2) and sp(3)), O-H, and N-H bonds has been demonstrated. Using this approach, we have demonstrated a unique water splitting process, which involves consecutive thermal liberation of H(2) and light-induced liberation of O(2), using no sacrificial reagents, promoted by a pyridine-based pincer ruthenium complex. An acridine pincer complex displays unique "long-range" metal-ligand cooperation in the activation of H(2) and in reaction with ammonia. In this Account, we begin by providing an overview of the metal-ligand cooperation based on aromatization-dearomatization processes. We then describe a range of novel catalytic reactions that we developed guided by these new modes of metal-ligand cooperation. These reactions include the following: (1) acceptorless dehydrogenation of secondary alcohols to ketones, (2) acceptorless dehydrogenative coupling of alcohols to esters, (3) acylation of secondary alcohols by esters with dihydrogen liberation, (4) direct coupling of alcohols and amines to form amides and polyamides with liberation of dihydrogen, (5) coupling of esters and amines to form amides with H(2) liberation, (6) selective synthesis of imines from alcohols and amines, (6) facile catalytic hydrogenolysis of esters to alcohols, (7) hydrogenolysis of amides to alcohols and amines, (8) hydrogenation of ketones to secondary alcohols under mild hydrogen pressures, (9) direct conversion of alcohols to acetals and dihydrogen, and (10) selective synthesis of primary amines directly from alcohols and ammonia. These reactions are efficient, proceed under neutral conditions, and produce no waste, the only byproduct being molecular hydrogen and/or water, providing a foundation for new, highly atom economical, green synthetic processes.