The traditional homogeneous access to aromatic amine derivatives is a nucleophilic aromatic substitution of the corresponding aryl halides. The halogen atom is usually relatively inert to amination reaction unless it is activated by the presence of electron withdrawing groups. Consequently, there has been particular emphasis over the past decade on the synthesis of metal complexes that are active catalysts for the preparation of aromatic amines. This tutorial review focuses on the use of metal-based complexes for the direct amination of aryl halides with ammonia.

The formation of carbon-carbon bonds by transition metal-catalysed direct functionalisation of C–H bonds has recently emerged as a greener alternative to traditional cross-coupling reactions. Despite tremendous improvement of the catalytic efficiency allowing reactions under milder conditions or lower catalyst loadings, this type of transformation still suffers from a major limitation with regards to green chemistry concerning the reaction media since the preferred solvents for these transformations are the undesirable and toxic, high boiling point, polar, aprotic, N-methylpyrrolidinone (NMP), dimethylformamide (DMF) or dimethylacetamide (DMAc). Recently, efforts have been made to perform these reactions in greener or more environmentally acceptable media. This review summarises the contributions made in this direction during the past few years.

Ru(OAc)₂(p-cymene) catalyses the directed dehydrogenative alkenylation of N-aryl pyrazoles by styrene and alkyl acrylates in the presence of a catalytic or stoichiometric amount of Cu(OAc)₂·H₂O in air; the acetic acid solvent plays a key role. With arene electron-donating groups, ruthenium-catalysed ortho-dialkenylation with alkyl acrylates can be obtained. A new method to generate the oxidative homocoupling of N-phenylpyrazole is provided with the Ru(OAc)₂(p-cymene) catalyst.

The ruthenium catalysed direct functionalisation of arene C–H bonds by aryl halides is reported. Reactions were performed in diethyl carbonate (DEC) instead of N-methylpyrrolidone (NMP), the solvent of choice used in most ruthenium catalysed C–H bond transformations. The use of diethyl carbonate facilitates the workup procedure thus reducing the amount of waste water. The slight loss of activity due to the use of diethyl carbonate is counterbalanced by the improvement of the catalyst efficiency achieved by a judicious choice of additives. Several arenes containing an N-heterocycle as a directing group have been diarylated.