Amides are versatile building blocks in synthetic organic chemistry, presenting a wide range of pharmacological applications, and are used as raw materials in industry for the large-scale production of engineering plastics, detergents and lubricants. The development of green procedures for the synthesis of this relevant class of compounds from various starting materials, which replace antiquated methods using carboxylic acid derivatives and amines, is therefore of prime interest in modern chemistry. In this review article, a survey of metal-catalyzed synthetic approaches of amides conducted in an environmentally friendly aqueous medium is given.

The hydration of nitriles is an atom economical route to generate primary amides of great academic and industrial significance. From an academic perspective, considerable progress has been made toward the development of transition metal catalysts able to promote this hydration process under mild conditions. In this context, with regard to activity, selectivity, functional group compatibility and modes of reactivity, the most versatile nitrile hydration catalysts discovered to date are based on ruthenium complexes. Herein, a comprehensive account of the different homogeneous ruthenium catalysts described in the literature is presented. Heterogeneous ruthenium-based systems are also discussed.

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Three different series of novel mononuclear arene-ruthenium(II) complexes containing aminophosphine ligands, namely, [RuCl 2 {κ 1 (P)-2-Ph 2 PC 6 H 4 CH 2 NHR}(η 6 -arene)], [RuCl 2 {κ 1 (P)-3-Ph 2 -PC 6 H 4 CH 2 NHR}(η 6 -arene)], and [RuCl 2 {κ 1 (P)-4-Ph 2 PC 6 H 4 CH 2 NHR}(η 6 -arene)] (arene = C 6 H 6 , p-cymene, 1,3,5-C 6 H 3 Me 3 , C 6 Me 6 ; R = i Pr, t Bu; all combinations), have been synthesized and fully characterized. These readily accessible species are efficient catalysts for the selective hydration of organonitriles into amides under challenging reaction conditions, i.e., pure aqueous medium in the absence of any cocatalyst, being much more active than their corresponding nonfunctionalized triphenylphosphine counterparts [RuCl 2 (PPh 3 )(η 6 -arene)]. The results obtained in this study indicate that the (amino-phosphine)ruthenium(II) complexes operate through a "bifunctional catalysis" mechanism in which the ruthenium center acts as a Lewis acid, activating the nitrile molecule, and the P-donor ligand acts as a Brønsted base, the pendant amino group generating the real nucleophile of the hydration process, i.e., the OHgroup.

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Several rhodium(I) complexes [RhCl(COD) (PR 3 )

The rearrangement of aldoximes to primary amides has been studied using the readily available arene-ruthenium(II) complex [RuCl2(η6-C6Me6){P(NMe2)3}] (5 mol %) as catalyst. Reactions proceeded cleanly in pure water at 100 °C without the assistance of any cocatalyst, affording the desired amides in high yields (70–90%) after short reaction times (1–7 h). The process was operative with both aromatic, heteroaromatic, α,β-unsaturated, and aliphatic aldoximes and tolerated several functional groups. Reaction profiles and experiments using 18O-labeled water indicate that two different mechanisms are implicated in these transformations. In both of them, nitrile intermediates are initially formed by dehydration of the aldoximes. These intermediates are then hydrated to the corresponding amides by the action of a second molecule of aldoxime or water. A kinetic analysis of the rearrangement of benzaldoxime to benzamide is also discussed.

The ruthenium(II) arene dimer [{RuCl(μ-Cl)(η 6 -p-cymene)} 2 ] readily reacted with 4 equiv of guanidines ( i PrHN) 2 CNR (R = i Pr (1a), 4-C 6 H 4 t Bu (1b), 4-C 6 H 4 Br (1c), 2,4,6-C 6 H 2 Me 3 (1d), 2,6-C 6 H 3 i Pr 2 (1e)) in toluene at room temperature to generate the mononuclear complexes [RuCl{κ 2 N,N′-C(NR)(N i Pr)NH i Pr}(η 6 -p-cymene)] (2a−e) and the easily separable guanidinium chloride salts [( i PrHN) 2 C(NHR)][Cl] (3a−e). Compounds 2a−e and 3a−e were fully characterized by elemental analysis and IR and NMR spectroscopy. The structures of [RuCl{κ 2 N,N′-C-(N i Pr) 2 NH i Pr}(η 6 -p-cymene)] (2a) and [RuCl{κ 2 N,N′-C(N-4-C 6 H 4 t Bu)(N i Pr)NH i Pr}(η 6 -p-cymene)] (2b) were also determined by X-ray diffraction analysis. Regardless of the steric requirements of the aromatic substituents, a nonsymmetric coordination of the guanidinate anions in 2b−e was observed, in complete accord with theoretical calculations (DFT) on the corresponding [RuCl{κ 2 N,N′-C(NR)(N i Pr)-NH i Pr}(η 6 -p-cymene)] and [RuCl{κ 2 N,N′-C(N i Pr) 2 NHR}(η 6 -p-cymene)] models. Remarkably, complexes 2a−e were active catalysts for the redox isomerization of allylic alcohols in the absence of base, which represents the first catalytic application known for ruthenium guanidinate species.