We
report herein selective catalytic hydrogenation of nitriles
to primary amines with the use of the non-pincer Mn(I) compound fac-[(CO)3Mn{iPr2P(CH2)2PiPr2}(OTf)] (2) as a catalytic precursor
(3 mol %) in the presence of KO
t
Bu (10
mol %) and 2-BuOH as solvent. Benchmark benzonitrile and electron-rich
aromatic and aliphatic nitriles were hydrogenated under rather mild
conditions (7 bar, 90 °C, 15 min) to produce the corresponding
amines in excellent to very good isolated yields (83–97%, six
examples). Increasing the H2 pressure and time (35 bar,
30 min) allowed for the production of (di)amines in excellent yields
(94–98%, three examples) from electron-deficient aromatic nitriles
and terephthalonitrile. Notably, adiponitrile was reduced to hexamethylenediamine
in 53% isolated yield. Finally, mechanistic insights were performed
and suggested unsaturated Mn-hydride species performing the elementary
steps during catalytic turnover.
We report herein the first example of a homogeneous manganese catalyzed transfer hydrogenation of nitriles using 2‐BuOH as the hydrogen source. Compound fac‐[(CO)3Mn{iPr2P(CH2)2PiPr2}Br] (Mn‐1, 3 mol %) exhibited catalytic activity in the presence of KOtBu (10 mol %) for the transfer hydrogenation of benzonitrile to yield a mixture of benzylamine (BA) and N‐sec‐butylidenebenzylamine (SBA). Subsequent acidic hydrolysis yielded isolated benzylamine hydrochloride in 96 %. The title system featured reversible formation of N‐benzylidenebenzylamine (BBA) prior to formation of SBA. A series of amine hydrochlorides was prepared following this methodology (39–92 % isolated yields, 4 examples). Best substrates for this transformation are electron‐rich aromatic nitriles, nonetheless electron‐deficient aromatic as well as aliphatic nitriles were also hydrogenated. Mechanistic studies suggested coordinatively unsaturated Mn‐hydride species performing catalytic turnover.
Mn-,
Fe-, Co-, and Ni-based catalytic precursors have been reported
for the homogeneous 3d transition-metal-catalyzed hydrogenation of
nitriles. We present herein a critical assessment of such reports,
emphasizing experimental setups, selectivity patterns, and mechanistic
aspects fashioning this growing field. Moreover, its successes, drawbacks,
and challenges are highlighted to outline what is next in the design
of catalytic systems for future years.
Amidines
and 2-substituted benzoxazoles were synthesized from N-heterocyclic nitriles under mild conditions (50 °C, 48 h, two
steps) in an atom-economical process that involves addition of methanol,
the solvent, to a nitrile moiety to yield a methyl imidate and the
subsequent extrusion of solvent in the presence of amines to afford
the title compounds. Methyl imidate formation was achieved by developing
a new catalytic pathway using [(dippe)Ni(H)]2 (dippe =
1,2-bis(diisopropylphosphino)ethane), [Ni(cod)2]/dppe,
or [Ni(cod)2]/P(OPh)3 (cod = 1,5-cyclooctadiene,
dppe = 1,2-bis(diphenylphosphino)ethane, P(OPh)3 = triphenyl
phosphite) as the catalyst precursor. Regarding the ligands, for a
given substrate, namely 4-cyanopyridine, the best performance for
the Ni(0)-catalyzed system was found for the σ-donor bidentate
dippe, whereas the monodentate π acceptor P(OPh)3 was less efficient. In relation to the substrates, for a given Ni–dippe
system, steric hindrance and, more importantly, substrate electron-withdrawing
character control imidate formation and thus the yield of amidines
and benzoxazoles.
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