Osmium-Promoted Transformation of Alkyl Nitriles to Secondary Aliphatic Amines: Scope and Mechanism

The transformation of alkyl nitriles to symmetrical and asymmetrical secondary aliphatic amines promoted by the hexahydride complex OsH₆(PⁱPr₃)₂ (1) is described, and the mechanisms of the reactions involved are established. Complex 1 catalyzes the aforementioned transformations of aryl-, pyridyl-, and alkoxy-functionalized alkyl nitriles with linear or branched chains. The formation of the secondary amines involves primary imines, primary amines, and secondary imines as organic intermediates. The reactions take place under mild conditions (toluene, 100 °C, and 4 bar of H₂). Stoichiometric reactions of 1 with pivalonitrile and 2-methoxyacetonitrile have allowed us to isolate the trihydride azavinylidene derivatives OsH₃{NCHR}­(PⁱPr₃)₂ (R = ^tBu (3), CH₂OMe (4)). Their formation involves the insertion of the N–C triple bond of the substrates into an Os–H bond of the unsaturated tetrahydride OsH₄(PⁱPr₃)₂ (A), which is generated by reductive elimination of H₂ from the hexahydride precursor. The reaction of these trihydride azavinylidene species with H₂ is the key step for the reduction of the N–C triple bond of the nitriles. In the absence of H₂, the attack of A to the azavinylidene ligand produces the rupture of its C­(sp²)–C­(sp³) bond. As a consequence of this attack and the presence of primary imines and amines in the reaction media, the binuclear complexes (PⁱPr₃)₂H₄Os­(μ-CN)­OsH₃{κ¹-N-(NHCHCH₂OMe)}­(PⁱPr₃)₂ (5) and (PⁱPr₃)₂H₄Os­(μ-CN)­OsH₃{κ¹-N-(NH₂CH₂CH₂OMe)}­(PⁱPr₃)₂ (6) have been isolated and characterized by X-ray diffraction analysis, for 2-methoxyacetonitrile. DFT calculations reveal noticeable similarities between the hydrogenations of nitriles to primary imines and those of primary imines to primary amines.