Cobalt-Catalyzed Chemo- and Enantioselective Hydrogenation of Conjugated Enynes

Article abstract of DOI:10.1002/anie.202106566

Asymmetric hydrogenation is one of the most powerful methods for the preparation of single enantiomer compounds. However, the chemo- and enantioselective hydrogenation of the relatively inert unsaturated group in substrates possessing multiple unsaturated bonds remains a challenge. We herein report a protocol for the highly chemo- and enantioselective hydrogenation of conjugated enynes while keeping the alkynyl bond intact. Mechanism studies indicate that the accompanying Zn²⁺ generated from zinc reduction of the Co^II complex plays a critical role to initiate a plausible Co^I/Co^III catalytic cycle. This approach allows for the highly efficient generation of chiral propargylamines (up to 99.9 % ee and 2000 S/C) and further useful chemical transformations.

Full text of DOI:10.1002/anie.202106566

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Accepted Article
Title: Cobalt-Catalyzed Chemo- and Enantioselective Hydrogenation of
Conjugated Enynes
Authors: Yanhua Hu, Zhenfeng Zhang, Yangang Liu, and Wanbin
Zhang
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Cobalt-Catalyzed Chemo- and Enantioselective Hydrogenation of
Conjugated Enynes
Yanhua Hu,[a] Zhenfeng Zhang,[b] Yangang Liu,[a,b] and Wanbin Zhang*[a,b]
Abstract: Asymmetric hydrogenation is one of the most powerful
methods for the preparation of single enantiomer compounds.
However, the chemo- and enantioselective hydrogenation of the
relatively inert unsaturated group in substrates possessing multiple
unsaturated bonds remains a challenge. We herein report a protocol
for the highly chemo- and enantioselective hydrogenation of
conjugated enynes while keeping the alkynyl bond intact. Mechanism
studies indicate that the accompanying Zn2+ generated from zinc
reduction of the CoII complex plays a critical role to initiate a plausible
CoI/CoIII catalytic cycle. This approach allows for the highly efficient
generation of chiral propargylamines (up to 99.9% ee and 2000 S/C)
and further useful chemical transformations.
general 4-aryl/alkyl-substituted conjugated enynes even under
very low hydrogen pressure of 1 atm (Scheme 1).[5] Therefore, a
more universal and efficient catalytic system is highly desired for
the chemo- and enantioselective hydrogenation of such
substrates.
The difference in the ability to selectively activate alkenyl and
alkynyl groups should be the critical factor to achieve effective
chemo- and stereocontrol in the asymmetric hydrogenation of
conjugated enynes. Ideally, only the alkenyl group is activated
and not the alkynyl group. However, the activation of alkynyl
group can be easily realized using Rh metal due to its large atomic
radius and strong electron transport capacity, eventually leading
to the failure of selective control in the previous attempts.[5] In view
of this, we envisaged that the cognate Co metal with a smaller
atomic radius and lower electron transport capacity could be used
to selectively activate the alkenyl group via the formation of a
chelating ring with the amido group but having relatively low ability
to activate the alkynyl group, thus realizing the selective
hydrogenation of enynes with the alkynyl group left intact.
Transition metal-catalyzed asymmetric hydrogenation is one of
the most practical and efficient methods for the preparation of
enantiopure compounds.[1] The Nobel Prize in chemistry was
awarded for outstanding achievements in this field at the
beginning of this century.[2] However, to date, most studies have
been limited to the enantioselective reduction of substrates
bearing a single unsaturated bond.[3] Less attention has been
directed towards the simultaneous chemo- and enantioselective
hydrogenation of the relatively inert unsaturated group in
substrates possessing multiple unsaturated bonds, despite there
being a huge demand in practical applications.[4] It is understood
that the reactivity of the C≡C bond is usually much higher than
other unsaturated C=C, C=N, and C=O bonds in the catalytic
reduction reaction. Therefore, how to preserve the versatile and
functional C≡C bond is a significant challenge in asymmetric
hydrogenation. Due to the similar coordination activation mode,
the chemo- and enantioselective hydrogenation of the C=C bond
with C≡C bond left intact is extremely difficult. Although the
presence of a directing acyl group and bulky 4-silyl substituent is
effective for improving the chemoselectivity, the classic
diphosphine-Rh catalytic systems are unsuitable for the more
Scheme 1. Enantioselective Hydrogenation of Conjugated Enynes Catalyzed
by Rhodium and Cobalt.
Following the pioneering work on the Co-catalyzed asymmetric
hydrogenation of alkenes in 1973, sluggish progress has been
made in this area.[6] It was not until the last decade that Chirik’s
group successively developed effective bis(imino)pyridine-CoI
and bisphosphine-CoII-Zn catalytic systems for the asymmetric
hydrogenation of unfunctionalized and functionalized olefins.[6c-h]
Similarly, the Lu’s and Zhang’s groups have developed effective
asymmetric hydrogenation procedures for terminal alkenes and
multi-substituted acrylic acids, respectively.[6i-m] Our group also
developed the first efficient Co-catalyzed asymmetric
hydrogenation of substituted imines enabled by a directing NHBz
group and nonbonding interaction between the ligand and
[a]
Y. Hu, Prof. Dr. Y. Liu and Prof. Dr. W. Zhang
Shanghai Key Laboratory for Molecular Engineering of Chiral Drugs
Frontier Science Center for Transformative Molecules
School of Chemistry and Chemical Engineering
Shanghai Jiao Tong University
800 Dongchuan Road, Shanghai 200240, China
E-mail: wanbin@sjtu.edu.cn
Dr. Z. Zhang, Prof. Dr. Y. Liu and Prof. Dr. W. Zhang
School of Pharmacy
[b]
Shanghai Jiao Tong University
800 Dongchuan Road, Shanghai 200240, China
Supporting information for this article is given via a link at the end of
the document.((Please delete this text if not appropriate))
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10.1002/anie.202106566
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substrate.[7] These seminal works have attracted widespread
attention from academia and industry because of the
demonstrated high potential of Earth-abundant metal catalysts.[8]
However, these Co catalysts are only alternatives but not
supplements to the cognate Rh and Ir catalysts in asymmetric
hydrogenation. Herein, the first efficient Co-catalyzed chemo- and
enantioselective hydrogenation of conjugated enynes has been
realized, a reaction which is unsuccessful using Rh catalysts
(Scheme 1). The methodology shows the benefits of using Earth-
abundant catalysts in overcoming limitations in substrate scope
commonly encountered with rare precious metals.
The conjugated enyne 1a, bearing 2-acetamido and 4-phenyl
groups, was chosen as the model substrate to evaluate the
performance of commercially available chiral diphosphine ligands
in the Co-catalyzed asymmetric hydrogenation (Table 1). The
tentative reactions were carried out under 40 atm hydrogenation
o
pressure at 50 C in MeOH over 24 hours. Generally, C2-linked
diphosphine ligands showed higher efficiency than C3- and C4-
Table 1. Condition optimization.
Entry[a]
Ligand
Solvent
MeOH
MeOH
MeOH
MeOH
MeOH
MeOH
CH3CN
CH3CN
CH3CN
2a/2a’ [%][b]
55/24
10/0
ee [%][c]
10
1
2
(R,R)-Me-DuPhos
(R,R)-BenzP*
(R,R)-QuinoxP*
(S,S)-BDPP
94
3
92/8
96
Scheme 2. Substrate Scope. Conditions: 1 (0.2 mmol), CoCl2 (1 mol %), (R,R)-
QuinoxP* (1.1 mol %), Zn dust (10 mol %), H2 (40 atm), CH3CN (1 mL), 50 oC,
21 h, unless otherwise noted. Yields of isolated products are given. The ee
values were determined by HPLC using chiral columns. [a] 10 h.
4
trace
--
5
(R,Sp)-JosiPhos
(R)-BINAP
trace
--
6
trace
--
7
(R,R)-QuinoxP*
(R,R)-QuinoxP*
(R,R)-QuinoxP*
95/5
99.4
99.4
99.4
linked ligands (entries 1-3 vs 4-6). For example, (R,R)-Me-
DuPhos was active but gave poor chemoselectivity and low
enantioselectivity (entry 1). The P-stereogenic diphosphine (R,R)-
BenzP* bearing a phenyl backbone has dramatically reduced
activity and satisfactory stereocontrol, affording the desired
product 2a with 10% yield and 94% ee (entry 2). To our delight,
high chemoselectivity and excellent enantioselectivity were
obtained when (R,R)-QuinoxP*[9] bearing a quinoxalino backbone
was employed (entry 3). Only trace amounts of product could be
detected using other chiral diphosphine ligands (entries 4–6). To
8[d]
9[e]
41/0
>99/1
[a] Reaction conditions: 0.2 mmol substrate 1a, 1 mol % CoCl2, 1.1 mol %
Ligand, 10 mol % Zn dust, 40 atm H2, 1 mL solvent, 50 oC, 24 h, unless
otherwise noted. [b] The conversions were calculated from 1H NMR spectra.
[c] The ee values were determined by HPLC using chiral columns. [d] H2 (20
atm). [e] 21 h.
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10.1002/anie.202106566
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improve both the chemoselectivity and stereoselectivity for the
formation of 2a, we carefully investigated other parameters which
may influence this reaction (see Table S1 in the Supporting
Information). The use of CoCl2 as the cobalt source, Zn dust as
the reducing agent, and CH3CN as the reaction solvent were
chosen as the optimal reaction conditions (entry 7). Reducing the
hydrogenation pressure from 40 to 20 atm provided an increase
in the chemoselectivity, but led to worse conversion (entry 8).
Finally, shortening the reaction time from 24 h to 21 h gave
satisfactory chemoselectivty with excellent enantioselectivity
(entry 9).
With the optimal reaction conditions established for the
hydrogenation, we began investigating the substrate scope of the
conjugated enynes (Scheme 2). Substrates with both electron-
donating and electron-withdrawing groups at different positions
on the phenyl ring were hydrogenated with high yields and
excellent enantioselectivities (2a-p), and several products were
obtained with almost perfect enantioselectivities of 99.9% ee.
Disubstituted, 2-naphthyl, and heteroaryl substrates were also
amenable to the reaction conditions, affording the corresponding
products (2q-2y) with satisfactory chemo- and enantioselectivities.
It was necessary to shorten the reaction time to generate products
2l and 2v in high yields. The absolute configuration of 2a was
determined to be R by X-ray crystallographic analysis and the
other products were then assigned by analogy to 2a (see the
Supporting Information for details).[10]
Scheme 3. Mechanism Studies.
It was reported that both CoI and Co0 active species could be
obtained by single-electron reduction of CoII complexes using the
Zn-protocol.[11] To identify the active species involved in this
catalytic cycle, CoI complexes were emplyed for further
investigations. Firstly, we found that catalyst generated in situ
from commercially available CoCl(PPh3)3 and (R,R)-QuinoxP* did
not produce any product at all after 24 hours in the absence of
additives or with the addition of NaBArF or zinc reductant
(Scheme 3a). To our surprise, the addition of 2 mol % of ZnCl2 to
this system dramatically promoted the hydrogenation reaction,
affording the desired product in 69% conversion and 99.3% ee
(Scheme 3a). Further increasing the dosage of ZnCl2 to 10 mol %
gave complete conversion to a mixture of 2a and 2a’ with 77/23
ratio (Scheme 3a). The enantioselectivity using CoCl(PPh3)3 is
the same as that using the CoCl2-Zn system and the phenomenon
is similar to our previous work concerning ZnCl2-promoted Rh-
catalyzed hydrogenation.[12] This suggests that the present
catalytic system probably involves a cationic CoI species
generated from the reduction of CoII to CoI followed by the
abstraction of the coordinated halide on CoI by the ZnCl2 activator.
The deuterium-labeling experiments using D2 gas furnished
exclusively 1,2-d2 incorporated hydrogenation product 2a-d,
indicating that the hydrogen gas is hydrogen source (Scheme 3b-
c). Based on the above experimental results and literature
reports,[6f,11,12] a plausible CoI/CoIII redox mechanism has been
proposed (see Scheme S2 for details).
Scheme 4. Application Studies.
Using the (R,R)-QuinoxP*-CoCl2-Zn catalytic system, a gram-
scale hydrogenation of 1a at a substrate/catalyst (S/C) loading of
2000 was successful with the addition of ZnCl2, giving the desired
product 2a with complete conversion, 98% yield and 98.9% ee
(Scheme 4 up). This performance represents the highest catalytic
efficiency reported for the Co-catalyzed asymmetric
hydrogenation of C=C bond to date. The diversified reduction of
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In conclusion, we have developed the first efficient Co-catalyzed
chemo- and enantioselective hydrogenation of conjugated
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and with excellent enantioselectivities (up to 99.9% ee and 2000
TON) and further utilized for several valuable transformations. A
plausible CoI/CoIII catalytic cycle has been proposed and the
activating effect of the accompanying Zn2+ generated from the
zinc reduction of the CoII complex has been confirmed.
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Acknowledgements
This work was supported by National Key R&D Program of China
(No. 2018YFE0126800), National Natural Science Foundation of
China (Nos. 21620102003, 21831005, 91856106, 21991112,
22071150), and Shanghai Municipal Education Commission (No.
201701070002E00030). We also thank the Instrumental Analysis
Center of Shanghai Jiao Tong University.
Keywords: asymmetric hydrogenation • chemoselectivity •
cobalt • enynes • propargylamines
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Yanhua Hu, Zhenfeng Zhang, Yangang
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Cobalt-Catalyzed Chemo- and
Enantioselective Hydrogenation of
Conjugated Enynes
The Co-catalyzed chemo- and enantioselective hydrogenation of conjugated
enynes have been realized for the synthesis of chiral propargylamines with high
efficiency (up to 99.9% ee and 2000 TON). Mechanism studies indicate that the
accompanying Zn2+ generated from zinc reduction of the CoII complex exhibits an
activating effect for the initiation of plausible CoI/CoIII catalytic cycle.
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