Bidentate NHC-Cobalt Catalysts for the Hydrogenation of Hindered Alkenes

Communication

Bidentate NHC-Cobalt Catalysts for the Hydrogenation of Hindered

Alkenes

∥

Zeyuan Wei, Yujie Wang, Yibiao Li,* Ra ﬀ aella Ferraccioli, and Qiang Liu*

Cite This: https://dx.doi.org/10.1021/acs.organomet.0c00498

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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

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)-

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

pyridine cobalt complexes. In 2016, Lu and co-workers

reported a chiral imidazoline iminopyridine cobalt complex for

enantioselective hydrogenation of trisubstituted alkenylboro-

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

hydrogenation of hindered alkenes. Recently, a series of chiral

catalysts has been developed for the asymmetric hydrogenation

substrates. Recently, Chirik’s group reported pyridine-

diimine) (PDI) cobalt complexes for the asymmetric

of multisubstituted alkenes. However, the success of alkene

hydrogenation is mainly based on the use of precious-metal

(

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

4,5

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

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

alkenes. However, the high cost and oxygen sensitivity of

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-

10a,f,h,11

10c,12

as carbonyl compounds, imines,

heterocycles,

nitriles,

₁₆N-

0f,13

carboxylic acids, esters, and CO₂.

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

mono- and gem-disubstituted alkenes. Subsequently, a

number of cobalt catalysts, which were reported by Hanson,

11a,c

8a,10h,19,20

Jacobi von Wangelin,

Chirik, Lu, and others,

showed excellent activities in mono- and disubstituted alkene

hydrogenation (Scheme 1a).

XXXX American Chemical Society

Organometallics XXXX, XXX, XXX−XXX

Organometallics

Communication

Scheme 1. Homogeneous Cobalt Catalysts for

Hydrogenation of Alkenes

Table 1. Catalytic Hydrogenation of 1a with NHC Ligands

and CoCl₂

Reaction conditions: 1a (1 mmol), L_N_H_C(0.5 mol %), CoCl (0.5

mol %), NaHBEt (1.5 mol %), THF (0.5 mL), 25 °C, H (10 bar),

6 h. Conversion and yields were determined by GC using biphenyl

as an internal standard.

hydrogen-transfer process is not necessary in this reaction. In

comparison, the pincer ligands L5−L8 only aﬀorded the

product 2a in 26−70% yields under the same reaction

conditions (entries 5−8). These results corroborate our

assumption that bidentate ligands with lower steric hindrance

beneﬁt substrate bonding to the metal center and are therefore

superior to tridentate ligands in the hydrogenation of hindered

alkenes. Further evaluation of additive and solvent e ﬀ ects with

L1 as the ligand was conducted (see Table S1 in the

Supporting Information). The use of KOMe instead of

heterocyclic carbenes (NHCs) are promising alternatives

because of their strong σ-donating abilities and high

tunabilities. NHC precursors, such as imidazole salts, are

usually air stable, nontoxic, and easily accessible from simple

starting materials. In 2018, Deng and co-workers reported

NHC-Co(II) catalysts for alkene hydrogenation with diphe-

NaHBEt as the additive for precatalyst activation resulted in

much lower yields (Table S1 , entries 1 and 6). There was no

reaction when K CO , KOAc, K PO , and KOH were used as

the bases (Table S1 , entries 2 −5). THF was identiﬁed as the

optimum solvent (Table S1, entries 1 and 7−10). The

nylsilane as hydrogen sources. In our previous studies, we

successfully developed a series of tridentate pincer NHC-

Co(II) catalysts with metal−ligand cooperation functionality,

minimum amount of NaHBEt needed to ensure a high

reaction eﬃciency was 3 mol % (Table S1, entries 11 and 12).

The reaction proceeded smoothly even under 2 bar pressure of

H with the assistance of a higher amount of NaHBEt (Table

10g,15d

which enabled the hydrogenation of ketones and esters.

We envisioned that bidentate NHC-Co catalysts with less

steric hindrance and more empty coordination sites could be

used for the hydrogenation of multisubstituted alkenes. Herein,

we report highly eﬃcient robust bidentate Co(II) catalysts for

the hydrogenation of a series of mono-, di-, tri-, and

tetrasubstituted alkenes. The activities of the catalytic systems,

which were formed in situ from easily available NHC

precursors, CoCl , and NaHBEt , are identical with those of

S1 , entry 13).

The catalyst structure was further investigated by synthesiz-

ing two NHC-cobalt complexes from the NHC ligand

precursors L1, L4, and CoCl (Scheme 2a). Ag O was used

as a base to generate a NHC-Ag intermediate. Subsequent

transmetalation with CoCl aﬀorded the NHC-Co complexes,

which were puriﬁed via recrystallization from a mixed

dichloromethane−hexane solvent. These four-coordinated

NHC-Co complexes (Co-1 and Co-4) were fully characterized

by X-ray diﬀraction, elemental analysis, ESI-HRMS, and

magnetic moment measurements. The single-crystal structures

are shown in Scheme 2b. Both Co-1 and Co-4 adopt a

distorted-tetrahedral geometry, indicating a high-spin conﬁg-

uration of the cobalt center. This was veriﬁed by magnetic

susceptibility measurements (4.3 μ for Co-1 and 3.7 μ for

well-deﬁned cobalt catalysts (Scheme 1c).

Optimization and Alkene Hydrogenation. Initially,

three diﬀerent types of NHC ligand precursors L1−L8 were

tested in the Co-catalyzed hydrogenation of 3,3-dimethyla-

crylic acid ethyl ester (1a) (Table 1). The bidentate ligands L1

and L2 with a N−H moiety gave 98% and 95% yields,

respectively, of 2a (entries 1 and 2). Changing the N−H

moiety to N−Me gave ligands L3 and L4, which showed

equally good activities (entries 3 and 4). This indicates that a

N−H moiety to ensure metal−ligand cooperation in the

Co-4). The well-deﬁned complex Co-1 was conﬁrmed to be an

eﬀective catalyst for the hydrogenation of 1a (Scheme 2c).

Organometallics XXXX, XXX, XXX−XXX

Organometallics

Communication

Scheme 2. Synthesis and Reactivity of NHC-Cobalt

Complexes

Scheme 3. Substrate Scope for Tri- and Tetrasubstituted

Alkenes

This indicates that such four-coordinated NHC-Co complexes

are probably reactive catalyst precursors generated from in situ

catalytic systems.

With the optimized reaction conditions in hand, we explored

the substrate scope of the cobalt-catalyzed alkene hydro-

genation. The substrates were expanded to various tri- and

tetrasubstituted alkenes (Scheme 3). The trisubstituted cyclic

enamine 1b was transformed into the target product in 95%

yield by using a 5 mol % catalyst loading and 15 mol % of

NaHBEt under 50 bar of H at 100 °C. Indene derivatives

Reaction conditions unless speciﬁed otherwise: alkene (1 mmol), L1

1 mol %), CoCl (1 mol %), THF (0.5 mL), NaHBEt (3 mol %), 25

2 3

with 3-phenyl and 3-p-methoxyphenyl substituents (1c,d) were

hydrogenated eﬀectively to aﬀord the corresponding indane

products in 92% and 85% isolated yields, respectively, at higher

catalyst loadings and reaction temperatures. A similarly high

yield was achieved with the 2-methyl-substituted indene 1e as

the substrate. Notably, the CC double bonds of multi-

substituted α,β-unsaturated carbonyl compounds (1f−k) were

chemoselectively hydrogenated in good to excellent yields and

various functional groups, e.g. heterocycles, halogens, and aryl

rings, were well tolerated. This chemoselective hydrogenation

of α,β-unsaturated carbonyl compounds using bidentate cobalt

catalysts diﬀered from previously reported results for the use of

pincer PNP cobalt catalysts, in which the carbonyl groups of

α,β-unsaturated carbonyl compounds were selectively reduc-

), NaHBEt (6 mol %), 60 °C, H (50 bar). L1 (5 mol %), CoCl

3 2 2

(5 mol %), NaHBEt (15 mol %), 100 °C, H (50 bar). L1 (20 mol

), CoCl (20 mol %), NaHBEt (60 mol %), 120 °C, H (50 bar).

2 3 2

Various mono- and disubstituted alkenes were also

eﬀectively hydrogenated under mild reaction conditions

(Scheme 4). Speciﬁcally, functional monosubstituted alkenes

bearing heterocyclic, bromide, and amide substituents (1q−s)

were hydrogenated to the corresponding alkane products in

excellent yields. Other nonactivated terminal alkenes (1t,u)

were also successfully reduced under similar reaction

conditions. The hydrogenation of cyclic 1,2-disubstituted

alkenes 1v−x and acyclic 1,2-disubstitued alkene 1y was easily

achieved at room temperature. 1,1-Disubstituted alkenes 1z,aa

were also suitable substrates for this reaction. Notably,

complete hydrogenation of conjugated dienes and isolated

dienes also proceeded smoothly (1ab−ad). These results are in

contrast to those reported by Fout, in which only the

monosubstituted terminal alkene moieties in conjugated dienes

and isolated dienes were hydrogenated at room temper-

ed.

The results indicate that these coordinatively unsatu-

rated bidentate cobalt complexes react preferentially with

alkenes over ketones, albeit with a N−H substructure in the

auxiliary ligands. The activated tetrasubstituted alkene 1l was

successfully hydrogenated to give the desired product in 97%

yield, with the carbonyl group untouched, under relatively

harsh reaction conditions. The more challenging tetrasub-

stituted alkenes 1m,n were also reduced to alkanes in high

yields. However, nonactivated tetrasubstituted alkenes, such as

pentamethylcyclopenadiene (1o) and 2,3-dimethyl-2-norbor-

nene (1p), were not hydrogenated under these reaction

conditions.

20a

ature.

The high functional group tolerance and broad

substrate scope of this transformation show the versatility and

practicality of the in situ NHC-Co catalytic systems.

Organometallics XXXX, XXX, XXX−XXX

Organometallics

The Supporting Information is available free of charge at

Communication

Scheme 4. Substrate Scope for Mono- and Disubstituted

Alkenes

ASSOCIATED CONTENT

sı Supporting Information

https://pubs.acs.org/doi/10.1021/acs.organomet.0c00498.

■

General considerations, preparation of NHC ligands and

cobalt complexes, procedures of catalytic reactions,

characterization data, and H NMR and C NMR

spectra and X-ray crystallographic data for Co-1 (CCDC

2018438) and Co-4 (CCDC 2018441) (PDF )

Accession Codes

CCDC 2018438 and 2018441 contain the supplementary

crystallographic data for this paper. These data can be obtained

free of charge via www.ccdc.cam.ac.uk/data_request/cif, or by

emailing data_request@ccdc.cam.ac.uk, or by contacting The

Cambridge Crystallographic Data Centre, 12 Union Road,

Cambridge CB2 1EZ, UK; fax: +44 1223 336033.

AUTHOR INFORMATION

Corresponding Authors

■

Yibiao Li − School of Biotechnology and Health, Wuyi

University, Jiangmen, Guangdong 529090, People’s Republic of

China; orcid.org/0000-0001-8249-5630;

Email: leeyib268@126.com

Qiang Liu − Center of Basic Molecular Science (CBMS),

Department of Chemistry, Tsinghua University, Beijing 100084,

People’s Republic of China; orcid.org/0000-0002-8402-

029X; Email: qiang_liu@mail.tsinghua.edu.cn

Reaction conditions unless speciﬁed otherwise: alkene (1 mmol), L1

(

1 mol %), CoCl (1 mol %), THF (0.5 mL), NaHBEt (3 mol %), 25

C, H (10 bar), 16 h. Yields were determined by GC using biphenyl

Authors

as an internal standard. Isolated yields. L1 (2 mol %), CoCl (2 mol

Zeyuan Wei − Center of Basic Molecular Science (CBMS),

Department of Chemistry, Tsinghua University, Beijing 100084,

People’s Republic of China

Yujie Wang − Center of Basic Molecular Science (CBMS),

Department of Chemistry, Tsinghua University, Beijing 100084,

People’s Republic of China

), NaHBEt (6 mol %), 60 °C, H (50 bar).

Gram-scale hydrogenation of 1a was performed to evaluate

the eﬃciency of this cobalt catalytic system (Scheme 5). A

turnover number of 760 was achieved with a catalyst loading of

.1 mol % under 50 bar of H at 60 °C for 16 h.

Raﬀaella Ferraccioli − CNR, Istituto di Scienze e Tecnologie

Molecolari (ISTM), 20133 Milan, Italy

Scheme 5. Turnover Number Experiments

Complete contact information is available at:

https://pubs.acs.org/10.1021/acs.organomet.0c00498

Author Contributions

The manuscript was written through contributions of all

authors. All authors have given approval to the ﬁnal version of

the manuscript.

Author Contributions

∥

Z.W. and Y.W. contributed equally to this work.

Notes

In summary, we synthesized and characterized a series of

bidentate NHC-Co complexes, which were successfully used in

the catalytic hydrogenation of hindered alkenes. Various

multisubstituted alkenes were hydrogenated with high

eﬃciency, by taking advantage of the empty coordination

sites and lower steric hindrance of bidentate cobalt catalysts.

Chemoselective hydrogenation of CC double bonds was

achieved in the presence of carbonyl groups and heterocycles,

as well as carbon−halide bonds. Notably, the activities of the

catalytic systems, which were formed in situ from readily

available NHC precursors, CoCl , and NaHBEt , were

The authors declare no competing ﬁnancial interest.

ACKNOWLEDGMENTS

■

We are grateful for the ﬁnancial supports from National

Program for the National Natural Science Foundation of China

(91845107, 21822106), the Beijing Municipal Commission of

Science and Technology (Z181100005118001), and the

Foundation of the Department of Education of Guangdong

Province (2018KZDXM070).

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Organometallics XXXX, XXX, XXX−XXX

Organometallics

Communication

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