Phosphine-free cobalt pincer complex catalyzed: Z -selective semi-hydrogenation of unbiased alkynes

Article abstract of DOI:10.1039/c7cy01994g

Herein, we report a novel, molecularly defined NNN-type cobalt pincer complex catalyzed transfer semi-hydrogenation of unbiased alkynes to Z-selective alkenes. This unified process is highly stereo- and chemo-selective and exhibits a broad scope as well as wide functional group tolerance. Ammonia-borane (AB), a bench-stable substrate with high gravimetric hydrogen capacity, was used as a safe and practical transfer hydrogenating source.

Full text of DOI:10.1039/c7cy01994g

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PAPER
Qingzhu Zhang et al.
Catalytic mechanism of C–F bond cleavage: insights from QM/MM
analysis of fluoroacetate dehalogenase
rsc.li/catalysis

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DOI: 10.1039/C7CY01994G
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Phosphine-free cobalt pincer complex catalyzed Z-selective semi-
†
hydrogenation of unbiased alkynes
a
b
a
a
Received 00th January 20xx,
Accepted 00th January 20xx
Vinod G. Landge, Jayaraman Pitchaimani, Siba P. Midya, Murugan Subaramanian, Vedichi
b,
a,
Madhu, * and Ekambaram Balaraman *
DOI: 10.1039/x0xx00000x
www.rsc.org/
Here, we report a new, molecularly defined NNN-type cobalt Despite the tremendous success of phosphine ligands in
pincer complex catalyzed transfer semi-hydrogenation of unbiased homogeneous catalysis, they have encountered common
alkynes to Z-selective alkenes. This unified process is highly drawbacks. This includes their preparation often with far from
stereo-, and chemo-selective and exhibits a broad scope as well as trivial methods, multi-step syntheses, handling under an inert
wide functional group tolerance. Ammonia-borane (AB), a bench- atmosphere, etc., As a consequence, the phosphine ligands are
stable substrate with high gravimetric hydrogen capacity was used expensive and potentially challenging to make on a large
8
as a safe and practical transfer hydrogenating source.
scale.
Very recently, transfer semihydrogenation of alkynes to Z- and
E-alkenes catalyzed by PNP and NNP-Co(II) pincer complexes,
respectively derived from an electron-rich phosphine ligand
Catalytic hydrogenation of unsaturated compounds is a
fundamental synthetic transformation, finding application in
the agrochemical, pharmaceutical, and commodity chemical
9
was reported by the research group of Liu and Luo.
1
-2
industries. The hydrogenation of alkyne, in general, bears
the dual challenge of product- (E/Z-isomerisation and
overreduction) and stereo-selectivity. Thus, a stereoselective
semihydrogenation of alkynes under mild conditions is a grand
Subsequently, Fout and co-workers developed an elegant
Mes
cobalt(II)
complex,
(
CCC)Co-(H
2
)(PPh
3
)
for
the
semihydrogenation of internal alkynes with E-selectivity using
1
0
molecular H
Zhang and Dong reported cobalt-catalyzed (in situ generated
from Co(II) salt, NaBH , and ethylenediamine) Z-selective
semihydrogenation of internal alkynes under 3 bar of H
2
as the hydrogen source. Based on this report,
2
a
challenge. The majority of the efficient homogeneous
catalytic (transfer)hydrogenation reactions depend on the
4
2
precious and less abundant 4d and 5d transition-metals.
2
1
1
Indeed, one of the major goals in transition-metal catalysis is
the replacement of expensive precious metals by cheap, low-
toxic, and abundant base metals for similar or better
pressure. In all the above approaches, only internal alkyne
was employed for hydrogenation reactions. However,
hydrogenation of terminal alkynes and olefins has not been
investigated. Here, we have now prepared air-stable, easy to
synthesize, NNN-cobalt(II) pincer complexes I-III (Figure 1a).
Remarkably, these phosphine-free Co(II)-complexes efficiently
catalyze the transfer semihydrogenation of unbiased internal
alkynes to Z-alkenes using ammonia-borane as a transfer
hydrogenating source. These reactions operate under mild,
3
-4
reactivity.
In recent times, notable progress has been made employing
well-defined soluble cobalt complexes in various
4
-7
de)hydrogenation reactions. In particular, cobalt catalysts
(
have been reported in the hydrogenation reactions of
4
,5a-e,5o
5a,5f
5a
imines, nitriles,
5b,5e,5m,5t
5n-o
5k-l,5u
olefins,
ketones,
5
esters,
l
5s
1
2
carboxylic acids,
CO
2
,
and N-heterocycles.
neutral, additive and/or base-free conditions, as well as
without the need of any special high-pressure equipment.
Our interest in catalytic hydrogenation began with the
discovery of new, bench-stable, phosphine-free cobalt pincer
complexes. The reaction of the tridentate ligand, Py-NNN with
Importantly, it should be noted that the molecularly-defined
cobalt catalysts mainly used in the (de)hydrogenation
reactions have possessed (electron-rich)phosphine ligands.
CoBr
2
in MeOH at room temperature results in the
corresponding Py-NNN cobalt(II)-pincer complexes in excellent
1
3
yields. Complexes I-III were characterized by HRMS, IR, and
UV analysis. NMR analysis of showed peaks at δ = 84 (s), 44.4
br.s), 24.0 (s), and -29.9 (br.s) and strongly indicates the
paramagnetic nature. The structure of was confirmed by a
single-crystal X-ray diffraction study (Figure 1b).
I
(
I
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Journal Name
provides an efficient, and stereoselective access to vinylsilane
under mild conditions. Notably, unactivated alkynes such as 4-
octyne
amine
(
1t), and (R)-N-benzyl-N-(1-phenylethyl)but-2-yn-1-
(
1u) also efficiently proceeded and gave the
corresponding alkenes in good yields with excellent Z-
selectivity.
Table 1. Scope of internal alkynes.
a
cat. I (2ꢀ4 mol%)
N:BH
N
Co
Figure 1. (a) PyNNN-Cobalt(II)-complexes used for the present study. (b) X-ray crystal-
structure analysis of I with 50% probability of thermal ellipsoids. Selected bond length
R
H
3
3
N
N
R
R
+
o
MeOH, 50ꢀ80
10ꢀ14 h
C
R
R
Br
Br
o
o
R
O
O
[
A ] and angle [ ]: Co(1)-N(1) 2.529(3), Co(1)-N(2) 2.026(3), Co(1)-N(3) 2.351(3), N(1)-
1
2
3
cat. I
Zꢀisomer
Eꢀisomer
Co(1)-N(2) 73.11(12), N(3)-Co(1)-N(2) 77.11(12) N(1)-Co(1)-N(3) 150.03(11), Br(1)-
Co(1)-Br(2) 113.89(3). CCDC no: CCDC 1557308.
Symmetrical alkynes
F
F
MeO
OMe
In a typical experiment, the reaction of diphenylacetylene (1a),
4-16
1
a, 94%b (Z/E:100/0)
1b, 53%, 48%
b
(Z/E:100/0)
1c, 77%, 70%
b
(Z/E:100/0)
1
ammonia-borane (AB),
and a catalytic amount of Py-PNN
Unsymmetrical alkynes
cobalt complex (I) afforded only the Z-isomer 2a (94% yield)
Cl
with the complete conversion of 1a (see ESI for optimization).
Analogous Co(II)-complexes II and III showed similar activity
and stereoselectivity. Gratifyingly, the cobalt complexes have
been readily activated in the presence of AB without any
additional sensitive additives, and the reaction operates under
mild, neutral conditions. It was well-known that borane
reagent forms [Co]−H species, an acꢀve catalyst in the
Me
1
d, 99%, 95%b (Z/E:100/0)
1e, 69% (Z/E:100/0)
CF
1f, 94% (Z/E:100/0)
3
Br
CN
CF
3
1i, 35% (Z/E:100/0)c
1g, 73% (Z/E:100/0)
1h, 36% (Z/E:100/0)
F
1
0,17
hydrogenation reactions.
With the optimized conditions in
CN
NO
2
2
CO Me
hand, we examined the transfer hydrogenation of various
internal alkynes (Table 1). The symmetrical alkynes contain
fluorine (1b) and methoxy (1c) groups and gave the partial-
hydrogenated products in 53% and 77% yields, respectively
with complete Z-selectivity. Remarkably, a sterically hindered
unsymmetrical alkyne (1e) was hydrogenated and gave only
the Z-alkene 2e in 69% yield. Interestingly, halogenated (-Cl
and -Br) diaryl alkynes are highly compatible with our present
catalytic system. Various functional groups such as
trifluoromethyl, cyano, nitro, and ester were highly tolerated
and offered the corresponding alkenes with the complete Z-
selectivity (products 1h in 36%, 1i in 35%, 1j in 83%, 1k in 33%,
and 1l in 90% yields). Furthermore, substrates featuring
1k, 33% (Z/E:100/0)d
1l, 99%, 90%b (Z/E:100/0)
1
j, 83% (Z/E:100/0)
Me
N
m, 72% (Z/E:79/21)e,f
S
1
1n, 77% (Z/E:100/0)
Acylic Polysubstititued alkynes and aliphatic alkynes
Et
OTBS
OH
1o, 99% (Z/E:93/7)
1p, 99%, 91%b (Z/E:100/0)
1q, 52% (Z/E:100/0)
Me
Me
CO
2
Me
TMS
1
r, 84% (Z/E:51/49)f,g
1s, 24% (Z/E:88/12)
1t, 99% (Z/E:100/0)
Ph
Me
N
Me
Ph
u, 99%, 88%b (Z/E:100/0)
pyridyl, and thiophenyl (1m-1n), which are difficult to
1
hydrogenate due to their coordinating ability to the metal
center also exhibited good yields with excellent Z-selectivity
under regular reaction conditions. Acyclic aliphatic alkynes 1o
and 1p smoothly underwent hydrogenation reaction and
yielded the corresponding alkenes 2o in 99% (Z/E : 93:7), and
a
Reaction conditions: 1 (0.5 mmol), ammonia-borane (AB) (0.6 mmol), and 4.0 mol % I
b
in 1 mL of MeOH at 80 °C for 14 h and GC yields of alkenes with Z/E ratios are shown.
c
d
e
Isolated yield. 50 °C for 12 h. 0.8 mmol of AB was used. AB (0.5 mmol) and 2 mol% I
f
was used. Complete hydrogenated product (28% in case of 1m and 16% in case of 1r)
g
was observed. reaction time is 30 min.
2p in 91% (complete Z-selectivity) yields, respectively.
Significantly, alkynes featuring a hydroxyl group 1q were also
formed and gave the Z-alkene in 52% yield. The hydrogenation
of 1r led to the desired 2r in 84% with 1:1 of Z/E selectivity.
The semihydrogenation of alkyne 1s, which would deliver a Z-
vinylsilane, is highly important since vinylsilanes are
Importantly, we were able to extend our system further to
include terminal alkynes (Table 2). Thus, the hydrogenation of
diverse terminal alkynes was tested under optimized
conditions. Substituted terminal alkynes such as methoxy, N,N-
dimethylamine, fluoro, and chloro were smoothly underwent
transfer hydrogenation and afforded the corresponding
alkenes in excellent yields. Aliphatic terminal alkyne 4f
underwent hydrogenation and gave quantitative yield of
semihydrogenated alkene. In most cases, no alkane (or a trace
amount) formation was detected. Interestingly, the
competitive hydrogenation experiment of alkyne and alkene
1
8
indispensable compounds in organic synthesis. Thus, upon
employing trimethylsilyl group (1p and 1s), the reaction
proceeded with complete Z-selectivity, and the silyl group
remained intact under these conditions, whereas acidic
1
9
conditions employed in a Pd-catalyzed alkyne reduction
resulted in cleavage of the TMS group. Thus, our approach
2
| J. Name., 2012, 00, 1-3
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Journal Name
under optimized conditions showed that alkyne reacts much
COMMUNICATION
1
faster than alkene. This result may suggest that the alkyne
3
favours the insertion reaction with the Co-H complex (see ESI).
Table 2. Scope of terminal alkynes.
a
Scheme 2. Selective removal of alkyne impurities from alkenes.
2
mol% I
H3N:BH3
MeOH, 50 oC, 8 h
R
R
To gain insight into the reaction mechanism, control
experiments, and deuterium-labeling studies were performed
4
1
3
MeO
Me2N
F
under standard reaction conditions (Scheme 3). Initially, the
dehydrogenation product of AB was investigated. Under the
standard reaction conditions, AB was completely converted to
4
a, 94%
4b, 79%
4c, 78%
Cl
1
1
B(OMe) , which is the only detected boron compound by
3
B
Me
TBSO
4f, 99%
NMR analysis, along with the generation of semihydrogenated
product 2a and H2 gas (by gas chromatography analysis).
Control experiments showed the dehydrogenation of AB (in
4
d, 96%
4e, 97%
a
Reaction conditions: 4 (0.5 mmol), AB (0.6 mmol), and 2 mol % I in 1 mL of MeOH at
0 °C for 8 h and yields of alkenes are calculated based on GC analysis.
21
5
absence of 1a) delivered B(OMe) and dihydrogen with the
3
1
1
13
complete conversion AB (by B NMR). However, in the
absence of the Co(II) precatalyst, the dehydrogenation of AB is
very low. Deuterium-labeling experiments were carried out to
verify the hydrogen source of the hydrogenated product.
Notably, in absence of AB, no formation of hydrogenated
product was observed. This proves that methanol was not the
Based on this study, we were able to further extend our
system for the step-temperature of switching chemoselectivity
in the hydrogenation reactions (Scheme 1).
reductant. However, when CD OD was used as the solvent, the
3
monodeuterated 2a-[D] was isolated. These results, therefore,
demonstrated that methanol mediates the protonation of the
alkenyl cobalt intermediate. It is noteworthy that no
isomerization of cis-stilbene 2a to trans-stilbene 3a was
observed under standard conditions (a trace amount), thereby
highlighting the importance of present Py-NNN-Co(II) catalytic
Scheme 1. Step-temperature chemoselectivity switch.
To our delight, styrenes bearing a range of electron-donating
as well as electron-withdrawing substituents were
hydrogenated to the corresponding alkanes in excellent yields
o
up to 99% yield) at the higher reaction temperature (80 C)
9
system with the NNP-Co(II) system. The competitive
(
experiment of electronically diverse alkynes (-F vs -OMe)
showed that electron-rich alkynes react faster than electron-
deficient alkynes. This result may suggest that the electron-
rich alkyne favors the alkyne insertion reaction with the
intermediate Co-H complex.
and longer reaction time (Table 3). However, disubstituted
alkenes fail to give the hydrogenated product under regular
conditions.
a
Table 3. Cobalt-catalyzed hydrogenation of terminal alkenes.
a
Reaction conditions: 4 (0.5 mmol), AB (0.6 mmol), and 4 mol % I in 1 mL of MeOH at
8
0 °C for 14 h and yields of alkanes are calculated based on GC analysis.
Encouraged by high chemoselectivity for this cobalt-catalyzed
semihydrogenation of alkynes to alkenes, we believe the
removal of alkyne impurities from alkenes is promising for the
2
0
chemical industry. In this context, a mixture of alkyne 1d and
an excess of cis-alkene 2a 1d 2a: 1/100) was exposed under
(
/
optimized reaction conditions. Notably, alkyne was selectively
hydrogenated to 2d with the complete conversion of 1d
(Scheme 2).
Scheme 3. Mechanistic studies
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In summary, we have demonstrated the general, efficient
stereo-, and chemo-selective semi-hydrogenation of alkynes to
produce Z-alkenes. These reactions proceed under phosphine-
free, additive and/or base-free conditions using a new, well-
defined NNN-type cobalt pincer catalyst and ammonia-borane
as a practical hydrogen source. This unified process exhibits a
broad scope (aliphatic, aromatic and chiral alkynes) as well as
wide functional group tolerance, such as halides, alcohol,
acetal, amine, ether, nitrile, nitro, ester and heterocyclic
motifs. Finaly, the present convenient method was applied for
the selective removal of alkyne impurities from alkenes.
CabreroꢀAntonino, A. Spannenberg, K. Junge, R. Jackstell and M.
Beller, Angew. Chem., Int. Ed., 2017, 56, 3216ꢀ3220; (t) P. Daw, S.
Chakraborty, G. Leitus, Y. DiskinꢀPosner, Y. BenꢀDavid and D. Milstein,
ACS Catal., 2017,
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, 2500ꢀ2504; (u) J. Yuwen, S. Chakraborty, W. W.
, 3735ꢀ3740.
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7
6
7
Coꢀcatalyzed transfer hydrogenation: (a) G. Zhang and S. K. Hanson,
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(
Mastalir, G. Tomsu, E. Pittenauer, G. Allmaier and K. Kirchner, Org.
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This research is supported by the DST-Fast Track Scheme
Am. Chem. Soc., 2016, 138, 10786ꢀ10789.
(SB/FT/CS-065/2013). VGL, and SPM thanks to CSIR for
8
9
(a) D. Spasyuk, S. Smith and D. G. Gusev, Angew. Chem., Int. Ed.,
fellowship. VM acknowledges UGC (No. 30-11/2015 (SA-II)).
2
013, 52, 2538ꢀ2542; (b) E. C. Hansen, D. J. Pedro, A. C. Wotal, N. J.
Gower, J. D. Nelson, S. Caron and D. J. Weix, Nat. Chem. 2016,
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8
,
1
Notes and references
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1
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0
1
K. Tokmic and A. R. Fout, J. Am. Chem. Soc., 2016, 138, 13700ꢀ13705.
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12
Notably, most known cobaltꢀcatalyzed hydrogenation reactions require a
reactive base, an acid, and/or a strong reductant as additive, either for
the activation or preparation of the active cobalt catalyst (ref. 5ꢀ6).
4
0
, 1291ꢀ1299.
2
(a) C. Oger, L. Balas, T. Durand and J.ꢀM. Galano, Chem. Rev., 2013,
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13, 1313ꢀ1350; (b) D. Wang and D. Astruc, Chem. Rev., 2015, 115
,
†
1
3
4
See the ESI.
6621ꢀ6686; (c) K. Radkowski, B. Sundararaju, and A. Fürstner, Angew.
1
Ammoniaꢀborane (NH
3
:BH
3
), an easy to prepare and manipulate solid,
) storage material,
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2
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Heinekey and K. I. Goldberg, J. Am. Chem. Soc., 2006, 128, 12048ꢀ
1
4134.
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1
5
4
5
P. J. Chirik, Acc. Chem. Res., 2015, 48, 1687ꢀ1695.
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(g) M. R. Friedfeld, M. Shevlin, J. M. Hoyt, S. W. Krska, M. T. Tudge
16
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DOI: 10.1039/C7CY01994G
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Catalysis Science & Technology
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DOI: 10.1039/C7CY01994G
TOC
Catalytic partial hydrogenation of alkynes to configurationally defined alkenes is a key
transformation in the bulk and fine chemical production. Indeed, it remains a crucial
application of industrial synthesis, in part due to the control of selectivity that is viable only
through judicious catalyst design. Herein, we report a new, molecularly defined NNN-type
cobalt pincer complex catalyzed semi-hydrogenation of unbiased alkynes to Z-selective
alkenes. The reaction proceeds readily at a lower temperature (50‒80 °C), phosphine ligand-
free, and base-free conditions, with no additive being required.