om0c00236_si_002.xyz (132.44 kB)
Osmium-Promoted Transformation of Alkyl Nitriles to Secondary Aliphatic Amines: Scope and Mechanism
dataset
posted on 2020-05-21, 19:41 authored by Juan C. Babón, Miguel A. Esteruelas, Ana M. López, Enrique OñateThe 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{NCHR}(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-(NHCHCH2OMe)}(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.