K. Shimizu, K. Ohshima, A. Satsuma
Oct 5, 2009
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Abstract
Aromatic and aliphatic amides are functional groups of great importance in polymers, natural products, and pharmaceuticals. Synthesis of amides is mostly based on activated acid derivatives (acid chlorides and anhydrides) or rearrangement reactions induced by an acid or base. Alternative procedures include the Staudinger ligation, aminocarbonylation of aryl halides, oxidative amidation of aldehydes, and amidation of alcohols with excess amount of hydrogen acceptor. However, all these methods require stoichiometric amounts of various reagents and lead to equimolar amounts of byproducts. A more environmentally friendly protocol, first reported by Milstein et al., is the direct amidation from alcohols and amines catalyzed by Ru complexes accompanied by H2 removal. They proposed that the reaction is based on alcohol dehydrogenation to give an aldehyde intermediate which reacts with the amine to give a hemiaminal that is subsequently dehydrogenated to the amide. In this highly chemoselective reaction, a hydrogen acceptor is not needed and the “dearomatized” aminomethyl phosphinomethyl pyridine as a pincer ligand plays an active role in the hydrogen abstraction and liberation process (cooperative ligand). To date, only two homogeneous catalysts using platinum-group metal (PGM), Ru, with molecularly designed cooperative ligands, have been reported. However, these expensive catalysts do not tolerate secondary amines and are problematic in terms of the catalyst/product separation and necessity of special handling of metal complexes. From the environmental and economic point of view, a reaction with inexpensive (PGM-free) heterogeneous catalyst is desired. Metal clusters have interesting chemical properties, which are unusual for bulk solids. For example, it is well known that gold as an inert d element shows similar or higher catalytic activity for various reactions than PGM-based catalysts when several nanometer-sized gold clusters are supported on a specific support. Silver as a less expensive Group IB metal is very popular in research field of cluster synthesis. However, compared with a research of gold catalysis, surprisingly less attempts have been focused on specific catalysis of silver cluster as well as a structure–activity relationship of silver cluster catalysis. Among a few examples, g-alumina-supported silver cluster (Ag/Al2O3) [9,10] reported by our group acts as heterogeneous catalyst for the oxidant-free dehydrogenation of alcohols and one-pot C C cross-coupling reaction from secondary and primary alcohols. The unprecedented activity of Ag/Al2O3 for reactions initiated by C H activation of alcohols led us to investigate the possibility of using Ag/Al2O3 as new environmentally benign catalyst for the title reaction. Herein, we demonstrate the first example of a heterogeneously catalyzed reaction of alcohols with amines to form amides and H2 using the easily prepared and inexpensive heterogeneous catalyst, Ag/Al2O3. Systematic studies on the structure–activity relationship are also shown to provide a new synthetic strategy of C H activation catalyst using non-PGM material with inorganic cooperative ligands, that is, OH/OH groups on alumina. The catalyst precursors were prepared by impregnating oxides with an aqueous solution of silver nitrate followed by evaporation to dryness at 80 8C and calcination at 600 8C. By controlling the H2 reduction temperature, silver clusters (5 wt %) with similar size were supported on various metal oxides: Ag/MOx-5 (MOx =CeO2, MgO, ZrO2, Al2O3, SiO2). Our previous results of Ag K-edge extended X-ray absorption fine structure (EXAFS) showed that silver species in all samples are in a range 0.84–3.0 nm. For Al2O3-supported catalysts, a series of catalysts with average particle diameter from 0.73 to 30 nm was also prepared by changing the Ag content (1, 3, 5, 10, 50 wt %); the particle diameter increased with Ag content (see Figure S1 in the Supporting Information). [a] Dr. K.-i. Shimizu, K. Ohshima, Prof. Dr. A. Satsuma Department of Molecular Design and Engineering Graduate School of Engineering, Nagoya University Nagoya 464-8603 (Japan) Fax: (+81) 52-789-3193 E-mail : kshimizu@apchem.nagoya-u.ac.jp Supporting information for this article is available on the WWW under http://dx.doi.org/10.1002/chem.200901896.