Communication
Bidentate NHC-Cobalt Catalysts for the Hydrogenation of Hindered
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ABSTRACT: Herein, we report a series of easily accessible
bidentate N-heterocyclic carbene (NHC) cobalt catalysts, which
enable the hydrogenation of hindered alkenes under mild
conditions. The four-coordinated bidentate NHC-Co(II) com-
plexes were characterized by X-ray diffraction, elemental analysis,
ESI-HRMS, and magnetic moment measurements, revealing a
distorted-tetrahedral geometry and a high-spin configuration of the
metal center. The activity of the in situ formed catalytic system,
which was obtained from easily available NHC precursors, CoCl2,
and NaHBEt , was identical with those of well-defined NHC-cobalt catalysts. This highlights the potential utility of this reaction
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system.
lkene hydrogenation is one of the most effective methods
for the construction of saturated C−C bonds. Although
Examples of Co-catalyzed hydrogenation of hindered tri-
and tetrasubstituted substrates are scarce (Scheme 1b). In
2013, Chirik’s group reported a catalytic hydrogenation of
sterically hindered alkenes using bis(arylimidazol-2-ylidene)-
A
transition-metal-catalyzed hydrogenation of alkenes has been
widely used in organic synthesis and industry, the hydro-
genation of hindered tri- and tetrasubstituted alkenes is
challenging because it is difficult for the sterically shielded
reactive sites to access the metal centers of homogeneous
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pyridine cobalt complexes. In 2016, Lu and co-workers
reported a chiral imidazoline iminopyridine cobalt complex for
enantioselective hydrogenation of trisubstituted alkenylboro-
1
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catalysts. In this respect, Crabtree and co-workers reported a
nates. In 2019, Jacobi von Wangelin and co-workers
cationic iridium complex that is an effective catalyst for the
disclosed a bis(imino)acenaphthene-Co(I) complex that is
highly active in the hydrogenation of tri- and tetrasubstituted
2
hydrogenation of hindered alkenes. Recently, a series of chiral
2
3
catalysts has been developed for the asymmetric hydrogenation
substrates. Recently, Chirik’s group reported pyridine-
diimine) (PDI) cobalt complexes for the asymmetric
1
,3
of multisubstituted alkenes. However, the success of alkene
hydrogenation is mainly based on the use of precious-metal
(
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hydrogenation of sterically hindered alkenes. These well-
defined Co(I) complexes with alkyl, hydride, or 1,5-cyclo-
octadiene ligands are often difficult to synthesize and are air
sensitive. In 2018, Chirik and co-workers partially solved this
problem by using a catalytic system generated from bidentate
phosphine ligands, Co(II) salts, and zinc powder. Very
recently, other cobalt catalysts based on phosphine ligands
were reported for the hydrogenation of tri- and tetrasubstituted
2
,4
4,5
6
catalysts: e.g., Ir, Rh, and Ru.
The development of new catalysts that are based on earth-
abundant, inexpensive, and less toxic first-row transition metals
rather than precious metals is important in terms of
sustainability. In recent years, significant progress has been
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7
8
made in iron- and nickel-catalyzed alkene hydrogenation
reactions. Cobalt, which is a congener of rhodium and iridium,
has also attracted increasing interest in developing effective
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alkenes. However, the high cost and oxygen sensitivity of
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catalysts for the hydrogenation of unsaturated substrates, such
phosphine ligands hindered their broad application in large-
scale synthesis. Therefore, the discovery of more easily
available and robust ligands to replace phosphine ligands is
highly desirable for the development of practical catalysts. N-
1
0
10a,f,h,11
10c,12
as carbonyl compounds, imines,
heterocycles,
nitriles,
16N-
1
0f,13
14
15
carboxylic acids, esters, and CO2.
A
series of cobalt catalysts has also been developed for alkene
hydrogenation (Scheme 1). An early example was reported by
Budzelaar’s group in 2005. They disclosed a diiminopyridine−
cobalt complex as an effective catalyst for the hydrogenation of
Received: July 23, 2020
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mono- and gem-disubstituted alkenes. Subsequently, a
1
0a
number of cobalt catalysts, which were reported by Hanson,
11a,c
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19
8a,10h,19,20
Jacobi von Wangelin,
Chirik, Lu, and others,
showed excellent activities in mono- and disubstituted alkene
hydrogenation (Scheme 1a).
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XXXX American Chemical Society
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Organometallics XXXX, XXX, XXX−XXX