Ruthenium-catalysed chemoselective alkylation of nitroarenes with alkanols

The alkylation of nitroarenes with alkanols catalysed by a phosphinesulfonate chelated ruthenium complex [Ru-1] was developed. It exhibits very different reactivity and chemoselectivity depending on acidic-basic conditions. Under either neutral or acidic conditions, nitroarenes were converted to secondary and tertiary anilines, respectively, with secondary and primary alkanols. By contrast, the same catalyst promotes the N-monoalkylation of nitroarenes with primary alkanols under alkaline conditions, while both N-monoalkylation with secondary alkanols and N-dialkylation with primary alkanols are inhibited. Mechanistic studies demonstrated that the novel reactivity under alkaline-free conditions is attributed to the sulfonate ligand-assisted O-H activation of alkanols, thus enabling the reduction of nitroarenes and nucleophilic C-N coupling in a concerted approach. Furthermore, it follows the traditional two-step approach, taking the corresponding imines as the intermediates, under alkaline conditions. Based on such features, diverse secondary and tertiary anilines were obtained directly from nitroarenes using the same catalyst [Ru-1].