Tris(pyrazolyl)borate Cobalt-Catalyzed Hydrogenation of C=O, C=C, and C=N Bonds: An Assistant Role of a Lewis Base

Article abstract of DOI:10.1021/acs.orglett.9b00679

The combination of tris(pyrazolyl)borate cobalt complexes and Lewis base is developed as an efficient catalyst precursor in the homogeneous hydrogenation. A broad substrate scope including carbonyls, alkenes, enamines, and imines is reduced with 60 atm of H₂ at 60 °C. Mechanistic studies support the hydrogenation operates through a frustrated Lewis pair (FLP)-like reduction process. These results highlight the development of novel non-noble metal catalytic processes, when combined with the diverse small molecule activation chemistry associated with FLPs.

Full text of DOI:10.1021/acs.orglett.9b00679

Letter
pubs.acs.org/OrgLett
Cite This: Org. Lett. XXXX, XXX, XXX−XXX
Tris(pyrazolyl)borate Cobalt-Catalyzed Hydrogenation of CO,
CC, and CN Bonds: An Assistant Role of a Lewis Base
Yang Lin,† De-Ping Zhu,‡,§ Yi-Ran Du,‡,§ Rui Zhang,*,† Suo-Jiang Zhang,‡,§ and Bao-Hua Xu*,‡,§
†State Key Laboratory of Chemical Engineering, East China University of Science and Technology, Shanghai 200237, P. R. China
‡Beijing Key Laboratory of Ionic Liquids Clean Process, Key Laboratory of Green Process and Engineering, State Key Laboratory of
Multiphase Complex Systems, Institution of Process Engineering, Chinese Academy of Sciences, Beijing 100190, P. R. China
§College of Chemistry and Chemical Engineering, University of Chinese Academy of Sciences, Beijing, 100049, P. R. China
S
* Supporting Information
ABSTRACT: The combination of tris(pyrazolyl)borate cobalt
complexes and Lewis base is developed as an efficient catalyst
precursor in the homogeneous hydrogenation. A broad substrate
scope including carbonyls, alkenes, enamines, and imines is
reduced with 60 atm of H2 at 60 °C. Mechanistic studies support
the hydrogenation operates through a frustrated Lewis pair
(FLP)-like reduction process. These results highlight the
development of novel non-noble metal catalytic processes, when combined with the diverse small molecule activation
chemistry associated with FLPs.
into pincer-cobalt-based systems for CO bonds hydro-
he catalytic hydrogenation of unsaturated organic
genation. A heteroatom-free bis(anthracene)-cobaltate catalyst
T
substrates with H2 plays a crucial role in the industrial
synthesis of fine chemicals.1,2 Despite considerable develop-
developed by Wolf and von Wangelin also carries out this
transformation, citing a low valent Co(I)−H intermediate for
the observed reactivity.9 Besides, Elsevier and de Bruin
reported a combination of Co(BF4)2 and tridentate phosphine
(IV) for a related reductive transformation of carboxylic acids
to alkanols.8e The reduction mainly proceeds with two steps
via an aldehyde intermediate, and the operative mechanism
involves a rate-limiting step of hydride migration from
Co(II)−H to the aldehyde carbon. Experimental investigations
into the approach of non-noble cobalt-catalyzed CO bonds
hydrogenation and studies on the reactivity difference from the
other unsaturated bonds, such as CC bonds, with the same
catalytic system would further advance the potential of such
strategies.
Tris(pyrazolyl)borates (Tp) represent one of the current
well-established ligand systems.10 However, reports on Tp-
based catalysts toward hydrogenation were sporadic despite
significant achievements made in the C(sp3)−H oxidative
functionalization.11 To the best of our knowledge, only one
example of (Tp)rhodium thiolate complex was described by
Seino and Mizobe in the chemoselective hydrogenation of C
N over the CO bond.12 Herein, a scorpionate cobalt-
catalyzed and Lewis base assisted reduction of CO bonds is
demonstrated. Besides, CC and CN bonds are well
tolerated with such a system. Mechanistic studies suggest a
frustrated Lewis pair (FLP)-like13 combination of a scorpio-
nate Co(II) complex with 1,4-diazabicyclo[2.2.2]octane
(DABCO), which bears sufficient cumulative Lewis acid/
ments, transition-metal-mediated catalysis still confronts a key
challenge in terms of the replacement of expensive noble
metals by earth-abundant, inexpensive base metals.3−9
Following this perspective, a few novel cobalt catalysts for
homogeneous hydrogenation have been disclosed in recent
years with the implementation of rational ligand design,
wherein pincer-based complexes and those solely featuring
phosphine ligands have been extensively studied.4−8 The
hydrogenation of CO bonds, albeit less progressive as
compared to that of CC and CN bonds,5,6a,9 has been
accessed with three well-defined cobalt complexes (Scheme 1,
I−III). For instance, recent work by Hanson6a and Kempe6b
demonstrates the utility of incorporating bifunctional motifs
Scheme 1. Homogenous Cobalt Complexes for CO Bond
Hydrogenation
Received: February 22, 2019
© XXXX American Chemical Society
A
DOI: 10.1021/acs.orglett.9b00679
Org. Lett. XXXX, XXX, XXX−XXX

Organic Letters
Letter
Lewis base (LA/LB) strength and a suitable steric profile to
activate H2 and provides the requisite Co(II)−H species for
hydrogenation thereafter.
Table 1. Catalytic Hydrogenation of Benzaldehyde (2a)
a
with TpR,R (R = CF3, Ph) Complexes 1a−d
Half-sandwich scorpionate complexes (TpR,RM(Cl) (1), M
= Co, Zn, Ag; R = CF3, Ph) were easily approached by
metathesis reaction between the corresponding potassium salt
(TpR,RK) and proper transitional metal salts.14 In addition to
the already published complexes 1b and 1c,15 a new cobalt
complex stabilized by a TpCF3,CF3 ligand (1a) was synthesized,
characterized, and applied. Besides, the cobalt adduct of
scorpionate substituted by a less electron-deficient phenyl
group (1d) was also utilized to probe the potential electronic
effect imposed by a Tp ligand. The molecular structures of 1a,
1b, and 1d16 were determined by X-ray crystal diffraction
analysis (Figures S1−S3). In the solid state, either the cobalt or
zinc ion is surrounded by one chlorido ligand and three
coordinating N atoms of the Tp ligand in a distorted
tetrahedral fashion, which is reasonable for a d7(cobalt)/
d10(zinc) high-spin configuration. Besides, the average Co−N
bond length of 1a is slightly longer by ca. 0.018 Å than that in
1d. By contrast, the length of the Co−Cl bond is relatively
short in 1a as compared to that in 1d (1a: 2.159(6) Å vs 1d:
2.196(3) Å). We also found the difference in the Co−Cl (1a)
and Zn−Cl (1b) bond length is shorter by ca. 0.04 Å than the
radius difference between the Co(II) and Zn(II) ion,17
suggesting a stronger σ-bonding between Co(II) and Cl ions.
Hydrogenation of benzaldehyde (2a) was investigated to
identify an efficient Tp-based catalyst (Tables 1 and S1). To
our delight, benzyl alcohol (3a) was obtained with a 94% yield
in the presence of 1a (2 mol %), AgBF4 (5 mol %), and
DABCO (20 mol %) with 60 atm of H2 at 60 °C, 3 h (Table 1,
entry 1). The catalytic active species derived from a
combination of 1a, AgBF4, and DABCO was proposed to be
[TpCF3,CF3Co(DABCO)]BF4, which was attained by chloride
b
entry
1 (mol %)
1a (2.0)
1a (2.0)
1a (2.0)
1a (2.0)
1a (2.0)
1a (2.0)
−
1a (2.0)
1a (1.0)
1a (2.0)
1a (2.0)
Tp(CF3)2K (2.0)
CoCl2 (2.0)
Co(BF4)2 (2.0)
HTp(CF3)2 (2.0)
1c (2.0)
base
time (h)
yield (%)
1
2
3
4
5
6
7
DABCO
NEt3
2,4,6-collidine
P4-tert-butyl
IDipp
3
3
3
3
3
10
3
3
3
3
10
3
3
3
3
3
3
3
94
0
0
12
0
0
0
0
51
75
35
0
0
0
0
0
0
24
−
DABCO
DABCO
DABCO
DABCO
c
8
d
9
e
10
f
11
12
13
DABCO
DABCO
DABCO
DABCO
DABCO
DABCO
DABCO
DABCO
c
14
c
15
c
16
17
18
1d (2.0)
1b (2.0)
a
Reaction conditions: 2a (0.2 mmol), 1 (0−2 mol %), AgBF4 (5 mol
b
%), base (20 mol %), THF (2 mL), H2 (60 atm), 60 °C. Yields
determined by GC analysis using biphenyl as an internal standard.
14a
c
d
e
f
AgBF4 (0). AgBF4 (2.5 mol %). AgBF4 (2 mol %). DABCO (8
removal of 1a with AgBF4 and a subsequent reaction with
mol %). DABCO: 1,4-diazabicyclo[2.2.2]octane. P4-tert-butyl: tert-
butyl tris-[tris(dimethylamino)phosphoranylidene]phosphomidic tri-
amide. IDipp: 1,3-bis(2,6-diisopropylphenyl)imidazol-2-ylidene.
DABCO. The reactivity of Lewis bases other than DABCO
dropped remarkably (entries 2−5). Besides, the components
1a, AgBF4, and DABCO were each found to be indispensable
(entries 6−8), and their amounts cannot be further reduced to
satisfy the high reactivity (entries 9−11). Both synthetic
substrates leading to 1a, potassium complex (TpCF3,CF3K), and
cobalt precursor (CoCl2) were investigated independently, but
with no reactivity being detected (entries 12 and 13). The
combination of DABCO with either Co(BF4)2 (entry 14) or
HTp(CF3)2 (entry 15) was not reactive. The poor performance
of silver complex 1c and DABCO (TpCF3,CF3Ag(DABCO))
(entry 16) disfavors an active ingredient of 1c potentially
formed by a metathesis reaction between 1a and AgBF4 during
the catalysis of 1a (entry 1). Other scorpionate complexes by
substitution with electron-withdrawing groups at the same
position was also complicated: weak groups, such as fluoro
(2e) and methyl ester (2h), were well tolerated versus strong
trifluoromethyl (2i) or nitrile (2j) groups which performed
poorly. Besides, halogen-substituted benzyl alcohols (3f, g)
were not obtained at all. One of the most likely reasons could
be that the LA/LB interaction between Co and DABCO was
competitively interrupted with abundant substrates bearing a
proper Lewis acidic or basic moiety, which thereby brought the
H2 cleavage to a standstill. As further evidence of this
assumption, not only perfluorophenyl (2m), consisting of a
more electron-deficient formyl C−H, but also thiophene-2-
carbaldehyde (2o) bearing a strong sulfur donor (vs 2n)
provided a poor yield. The amount of DABCO was
consequently increased to preserve the crucial Co(II)−
DABCO interaction for these unsuccessful substrates (2b, d,
f, g, i). As expected, a moderate to excellent yield of the
corresponding alkanol was obtained (3f, g, i), except for 3b
and 3d. We also found significant steric effects when the ortho-
position was substituted. For instance, the yield of ortho-
methoxyl benzyl alkanol (3p: 89%) was slightly lower as
compared to that of the para-substituted one (3c: 94%) and
no product was detected when both ortho-positions were
blocked (3q). Not only aromatic aldehydes but also aliphatic
either incorporating a less electron-deficient ligand (TpPh,Ph
,
1d, entry 17) or replacing the transition metal with zinc (1c,
entry 18) were found to be far less reactive. Priliminary efforts
in searching for a proper Lewis base to couple with 1b for
catalytic hydrogenation of 2a were found to be unsuccessful
(Table S1). It is worthy to note that a zinc complex of C5Me5
and N-heterocyclic carbenes studied by Stephan exhibited an
FLP-like H2 cleavage, which has been successfully used in the
catalytic CN bond hyrogenation.4j
We next examined the substrate scope and limitations of this
newly developed protocol (Scheme 2). para-Methoxyl
benzaldehyde (2c) proceeded smoothly, but that substituted
with electron-donating groups of either stronger phenol (2b)
or weaker methyl (2d) did not react. The situation of
B
DOI: 10.1021/acs.orglett.9b00679
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Organic Letters
Letter
Scheme 2. Hydrogenation of Aldehydes Catalyzed by
Scheme 3. TpCF3,CF3Co-Catalyzed Hydrogenation of Alkene,
Enamine, and Imine
a
a
TpCF3,CF3Co Complex 1a
a
General conditions: substrate (0.2 mmol), 1a (2 mol %), AgBF4 (5
mol %), DABCO (20 mol %), H2 (60 atm), 60 °C, THF (2 mL).
Yields determined by GC analysis through using areas of peak
b
normalization method. Yields determined by NMR spectroscopy
using dibromomethane as an internal standard.
a
General conditions: substrate (0.2 mmol), 1a (2 mol %), AgBF4 (5
mol %), DABCO (20 mol %), H2 (60 atm), 60 °C, THF (2 mL).
methylstyrene (4e), but no alkanol product was obtained with
para-methylbenzaldehyde (Scheme 2, 2d) under identical
conditions. For conjugated enone 4f, the CC bond was
again selectively reduced. Not only aromatic alkenes (4a−4f)
but also aliphatic alkenes (4g, 4h) were reduced with high
yields. Next, enamines as a kind of amino-substituted alkenes
were also examined. The resulting saturated amines (5i−5k)
were detected in excellent yields, with high chemoselectivity
toward the CC bond. Finally, we were interested in the
catalytic reduction of imines. Again, a notable functional group
tolerance was observed. The corresponding amines (7a−7e)
were detected in good to excellent yields, except for 7d and 7e,
which bears bromide and sterically hindered substituents,
respectively. We also investigated the catalytic hydrogenation
of 2a and 4f on a scale of 1 mmol, which offers 3a and 5f with
90% and 81% yield, respectively, after reaction at 60 °C under
60 atm of H2 for 10 h (Scheme S1).
Yields determined by GC analysis through using areas of peak
b
c
normalization method. DABCO (120 mol %). Isolated yield.
d
Yields determined by NMR spectroscopy using dibromomethane as
an internal standard. Undecanal was detected with 9% yield. 3-
e
f
Phenylpropanal and (E)-3-phenylprop-2-en-1-ol were detected with
g
75% and 17% yield, respectively. 100 °C, (E)-3-phenylprop-2-en-1-ol
was detected with 34% yield.
ones (2r−2v) were successfully transformed, with acceptable
yields of the corresponding alkanols being obtained under
identical conditions. The poor performance of 2u and 2v was
attributed to the sterically hindered aldehyde therein. The
conformation of the cyclohexyl ring in 2v is altered by
introducing a vinyl group, which renders the formyl group
accessible, and a moderate yield of alkanol product (3w) was
thereby reached. In the case of 10-undecenal (2x), saturated
aldehyde was produced with one double bond being reduced.
Besides, the preferential reduction of CC over CO bond
was detected in the hydrogenation of 2y under standard
conditions, forming the saturated aldehyde with 75% yield,
which is in contrast to that of Kempe’s catalyst.6b A full
conversion with both groups being reduced to 3y was detected
upon reaction at 100 °C for 25 h.
To understand the initial reaction of catalysis, the formation
of transient [TpCF3,CF3Co(DABCO)]BF4 (A, Scheme 4) and
LA/LB interaction between Co(II) and DABCO was probed
by in situ NMR experiments (Figure S4). Evidently, the
reaction of AgBF4 with 1a (1:1) at 60 °C for 3 h led to a pale
white solid of AgCl and a transparent colorless solution, of
1
which H NMR features an upfield shift of pyrazol protons to
Taking note of the good performance of 1a for the CO
reduction, we focused on unsaturated substrates bearing CC
and CN bonds (Scheme 3). Hydrogenation of CC bonds
as compared to CO bonds with this system exhibits a broad
substituent tolerance. Herein, the para-halide (4b, c) and para-
trifluoromethylstyrene (4d) proceeded excellently. In addition,
a high yield of alkane product was obtained in the case of para-
7.21 ppm. Subsequent reaction with 1 equiv of DABCO
resulted in a lavender solution, and a slight downfield shift of
the pyrazol protons to 7.25 ppm was detected. It is consistent
with the presence of a donor−acceptor interaction upon
1
coordination. However, the same H NMR spectrum shows
only a single resonance for the DABCO protons, suggesting
rapid exchange between an adduct and FLP. We, thus,
C
DOI: 10.1021/acs.orglett.9b00679
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Organic Letters
Letter
150 mol % [H-DABCO]BF4, whereas proton integration at the
β-position remained intact. In a similar fashion, H NMR
Scheme 4. Proposed Mechanism for the Catalytic
Hydrogenation of CX (X = O, C, N) Bonds
1
analysis of the product resulted from a competitive reaction
between H2 and [D-DABCO]BF4 (150 mol %) showed a
partial deuteration at the α-position (1.73 H) as well as
nondeuterated β-C−H (2.00 H). These data support an FLP-
like reduction pathway (I), wherein the reduction of 4f is
initiated by hydride addition to the Michael (β) position
followed by protonation of the resulting cobalt-enolate
intermediate, yet the hydrogenolysis pathway (II) cannot be
ruled out. Herein, a large amount of [H/D-DABCO]BF4 was
requisite to detect the H/D scrambling probably because the
protonation does not locate as a rate-limiting step. Finally, the
involvement of [H-DABCO]BF4 during the FLP-like hydro-
genation process with the newly developed protocol was
further confirmed by cross experiments toward the reactivity of
in situ formed tris(pyrazolyl)borate Co(II)−H species from
the reaction of 1a with NaBEt3H (Table S2).19 Hydrogenation
of 2a and 4a did not occur with the use of a combination of 1a
and NaBEt3H but proceeded in the presence of additional [H-
DABCO]BF4 under identical conditions.
supposed exposure of a solution of complex A in THF to about
60 atm of H2 affords an FLP-like activation of H2 in which the
Co(II) and DABCO reacted as an LA/LB combination to
generate the transient hydridocobalt complex B and
ammonium salt [H-DABCO]BF4 (Scheme 4).
To probe the apparent differences between the hydro-
genation of the CO (2) and CC (4) bond, partial
poisoning studies for hydrogenation of 2a and 4a were
performed using TEMPO (2,2,6,6-tetramethyl-piperidine-1-
oxyl) as a radical inhibitor and pyridine as a strongly binding
poison (Table 3). Neither of these two reagents could replace
Two possible pathways for the subsequent reduction were
proposed as denoted by I and II in Scheme 4. One possibility
involves an FLP-like reduction sequence characterized by
initial H-transfer to the CX (X = O, N, C) bond of organic
substrates followed by exchange of the resulted complex C for
acidic [H-DABCO]BF4 (path I).18 In this regard, substrate
activation by protonation with [H-DABCO]BF4 may prefer-
entially occur over H-transfer in the case of specific substrates
(omitted from Scheme 4 for clarity).13a Alternatively, the
cobalt complex C was thought to undergo hydrogenolysis
toward Co(II)−X σ-bond affording the product and
regenerating the catalyst hydridocobalt complex B (path
II).8e In this case, DABCO only functions as an initiator to
form the hydridocobalt catalyst. To probe these possibilities,
labeling experiments of competitive hydrogenolysis with H2
versus protonation by [H-DABCO]BF4 were conducted using
enone 4f as the substrate (Table 2 and Figure S5). The
participation of [H-DABCO]BF4 as a proton donor (H+-
donor) in the reduction of 4f with D2 was demonstrated by an
apparent H/D scrambling at the α-position in the presence of
a
Table 3. Partial Poisoning Studies
b
b
entry
DABCO (x)
additive (y)
Y3a (%)
Y5a (%)
1
2
3
4
5
6
(20)
−
−
(20)
(20)
(20)
−
100
0
0
trace
71
0
99
0
0
12
99
83
pyridine (20)
TEMPO (20)
pyridine (20)
TEMPO (20)
TEMPO (100)
a
Reaction conditions: 2a or 4a (0.2 mmol), 1a (2 mol %), AgBF4 (5
mol %), DABCO (x mol %), additive (y mol %), H2 (60 atm), 60 °C,
b
THF (2 mL), 10 h. Determined by GC analysis using biphenyl as an
internal standard.
a
Table 2. Deuterium-Labeling Experiments
DABCO in the catalytic hydrogenation (entries 2 and 3 vs
entry 1). With pyridine, both reactions were completely
inhibited by 1 equiv per DABCO (entry 4), but for TEMPO,
they proceeded smoothly with only a slightly decreased
efficiency being detected in the reduction of 2a (entry 5).
This suggests pyridine poisons the cobalt center by acting as a
stronger and less sterically hindred base than DABCO, whereas
the basicity of TEMPO is insufficient to disrupt the FLP
interaction, thereby rendering the H2 cleavage still possible. To
our suprise, 1 equiv per substrate of TEMPO was found to be
sufficient to suppress the hydrogenation of 2a most likely
through functioning as a hydride abstractor toward Co−H
species,20 but the reduction of 4a again remained intact under
identical conditions (entry 6). Thus, we could propose hydride
tranfer from Co−H to a carbonyl carbon is relatively hindered
as compared to a vinyl moiety, which renders the hydro-
c
1H integration
b
H2/D2
H+-donor (x)
yields (%)
α
β
H2
D2
D2
D2
H2
−
−
>99
>99
>99
>99
>99
2.00
1.09
1.22
1.61
1.73
2.00
1.36
1.33
1.29
2.00
[H-DABCO]BF4 (50)
[H-DABCO]BF4 (150)
[D-DABCO]BF4 (150)
a
Reaction conditions: 4f (0.2 mmol), 1a (2 mol %), AgBF4 (5 mol
%), DABCO (20 mol %), H+-donor (x mol %), H2/D2 (60 atm), 60
b
°C, THF (2 mL), 24 h. Determined by GC analysis using areas of
c
1
peak normalization method. Determined by H NMR analysis.
D
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Organic Letters
Letter
(2) For selected reviews of noble metal-catalyzed hydrogenation:
(a) Noyori, R.; Ohkuma, T. Angew. Chem., Int. Ed. 2001, 40, 40−73.
(b) Werkmeister, S.; Junge, K.; Beller, M. Org. Process Res. Dev. 2014,
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genation of the former more sensitive to radical inhibitors. In
this regard, the hydrogenation of the CO bond is more
susceptible to substrates bearing a X−H (X = O, C) group of a
highly acidic nature. As we found in a comparison between
acetophenone (8a) and therein methyl fluorinated 2,2,2-
trifluoroacetophenone (8b) (Scheme S2), hydrogenation did
not occur with 8a but proceeded excellently in the case of 8b
under identical conditions.
Ligand-assisted strategies by preincorporating proper basic
or acidic moieties into the ligand have been largely used in the
transition metal catalysts design for homogeneous hydro-
genation reactions.2,4 The coupled metal and ligand therein
function as an internal LA−LB pair with a defined distance to
activate H2 during the catalysis. From this investigation and a
related zinc complex of C5Me5 studied by Stephan,4j there is
now increasing evidence that such an FLP-like H2 cleavage can
also be conducted with a combination of non-noble metal
complexes and external bases. The powerful catalytic chemistry
of transition metal complexes, when combined with the diverse
small molecule activation chemistry associated with FLPs,
should allow the development of new catalytic processes that
are beyond the capabilities of current non-noble metal
catalysis.
(4) For selected reviews of non-noble metal-catalyzed hydro-
̈
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V.; Beller, M. Chem. - Eur. J. 2019, 25, 122−143. For recent articles of
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Chem. Soc. 2013, 135, 11736−11739. (i) Liu, T.-B.; DuBois, D. L.;
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ASSOCIATED CONTENT
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S
The Supporting Information is available free of charge on the
ACS Publications website at DOI: 10.1021/acs.or-
glett.9b00679.
Preparation and characterization of TpR,RM(Cl) com-
plexes; additional experimental procedures; spectro-
scopic, GC-MS, and crystallographic data (PDF)
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CCDC 1895289−1895291 contain the supplementary crys-
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free of charge via www.ccdc.cam.ac.uk/data_request/cif, or by
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AUTHOR INFORMATION
Corresponding Authors
■
*E-mail: r.zhang@ecust.edu.cn.
*E-mail: bhxu@ipe.ac.cn.
ORCID
Rui Zhang: 0000-0003-1370-8289
Bao-Hua Xu: 0000-0002-7222-4383
Notes
The authors declare no competing financial interest.
ACKNOWLEDGMENTS
■
We are grateful to the National Natural Science Foundation of
China (Petrochemical Joint Fund U1662133, No. 21374029)
and K. C. Wong Education Foundation (No. GJTD-2018-04)
for financial support.
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