Ruthenium-catalysed chemoselective alkylation of nitroarenes with alkanols

Abstract

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].