Selective reductive amination of aldehydes from nitro compounds catalyzed by molybdenum sulfide clusters†

Amines are the most important building blocks employed in modern medicinal chemistry. Secondary amines are regularly prepared by the alkylation or reductive amination of the parent amines through transition metal, typically precious metals, catalyzed reactions. In the past few years, the catalytic alkylation of amines with alcohols under hydrogenborrowing conditions as well as the catalytic reductive amination of carbonyl compounds using hydrogen as a reductant have been proposed as environmentally friendly procedures to produce N-substituted amines. An actual challenge in catalysis research is to take low value starting materials and convert them into high value products using non-toxic earth abundant materials through an atom efficient procedure. Nitro compounds are an economic feedstock to provide primary amines and their use in reductive amination processes is highly attractive since prior isolation of the amine is not required. Our groups have recently shown that well-defined Mo3S4 cuboidal clusters are efficient catalysts for the chemoselective reduction of nitroarenes to anilines using different reducing agents. Interestingly, the diamino cluster of the formula [Mo3S4Cl3(dmen)3] + (dmen = N,N′-dimethylethylenediamine), as depicted in Fig. 1, performs this transformation under mild conditions and using molecular hydrogen, the most “green” and least expensive reducing agent. On this basis, we became interested in its use as a catalyst for the related reductive amination of aldehydes with nitroarenes and nitroalkanes to generate secondary amines in a straightforward manner. Domino catalytic transformations starting from nitroarenes and carbonyl compounds directed towards the preparation of N-alkylated aryl amines are not so common and they are usually done in the heterogeneous phase using palladium, platinum, rhodium or gold nanocatalysts. The N-alkylation of nitroarenes with aldehydes catalyzed by palladium operates under ambient hydrogen pressure at room temperature when alcohols are used as solvents. However, the formation of byproducts such as benzylalcohol and toluene derivatives limits the selectivity of the process. Several cost-effective carbonsupported catalysts based on nanostructured Fe2O3 and Co-Co3O4 partially encapsulated by nitrogen-enriched graphene layers (Fe2O3/NGr@C and Co-Co3O4/NGr@C, respectively) have also been recently developed by some of us for the tandem reductive amination between nitro compounds and carbonyl compounds using hydrogen as a reductant. In general, these heterogeneous processes employing non-noble metals require more demanding conditions (110–170 °C and 50–70 bar H2) and contrary to the general assumption that anhydrous con-