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

Abstract

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.