Nickel-catalyzed asymmetric transfer hydrogenation of conjugated olefins

Article abstract of DOI:10.1039/c5cc01632k

Asymmetric transfer hydrogenation of electron-deficient olefins is realized with nickel catalysts supported by strongly σ-donating bisphosphines. Deuterium labeling experiments point to a reaction sequence of formate decarboxylation, asymmetric hydride in

Full text of DOI:10.1039/c5cc01632k

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ARTICLE TYPE
Nickel-Catalyzed Asymmetric Transfer Hydrogenation of Conjugated Olefins
Siyu Guo, Peng Yang and Jianrong (Steve) Zhou*
Received (in XXX, XXX) Xth XXXXXXXXX 20XX, Accepted Xth XXXXXXXXX 20XX
DOI: 10.1039/b000000x
5
Asymmetric transfer hydrogenation of electron-deficient
Previously, Pfaltz et al. invented cobalt-catalyzed asymmetric
35 reduction using borohydrides.6 Recently, Chirik group reported
cobalt-catalyzed hydrogenation of styrenes and enamides, but the
cobalt catalysts were sensitive to air and moisture and the types
of olefins that gave high ee were quite limited.7 In recent years,
achiral nickel complexes were found to have non-stereoselective
40 hydrogenation activity towards unsaturated bonds.8 Hamada et al.
reported nickel-catalyzed hydrogenation of ketone groups of α-
amino-β-ketoesters under dynamic kinetic resolution conditions
(around 80% ee).9 Recently our group disclosed highly
stereoselective nickel catalysts for transfer hydrogenation of
45 enamides and hydrazones having directing groups.10 Formic acid
was used as a safe and easy-to-handle hydrogen source. It has a
high volume/density of H2 and is a promising hydrogen storage
material.11 In recent years, highly efficient metal catalysts were
developed for decomposition of formic acid to release H2.12 In
50 comparison, high-pressure hydrogen gas and liquid are
commonly acknowledged as safety hazard during storage,
transport and use.
olefins is realized with nickel catalysts supported by strongly
σ-donating bisphosphines. Deuterium labeling experiments
points to a reaction sequence of formate decarboxylation,
asymmetric hydride insertion and non-stereoselective
10 protonation of resulting nickel enolates.
Asymmetric hydrogenation is the state-of-the-art in
homogeneous metal catalysis and it is practiced on large scales in
manufacturing of chiral pharmaceuticals and agrochemicals.1
Today, chiral catalysts of noble metals Rh,2 Ru3 and Ir4 dominate
15 the field of asymmetric hydrogenation. Expensive noble metals
themselves contributed to a fraction of total cost of hydrogenation
processes, in addition to costly chiral bisphosphines.
Furthermore, the mining and purification of these rare metals
from ores are energy demanding and costly. These metals are
20 produced in only dozens of tons a year worldwide and are very
expensive, often thousands-fold more so than abundant metals
such as copper and nickel. They are highly toxic to human and
ecosystems. The heavy metal residues in pharmaceutical active
ingredients must be reduced to ppm levels according to FDA
Table 1 Solvent effect for a model hydrogenation of ethyl (E)-β-
25 regulations. Waste treatment after catalytic hydrogenation also 55 methylcinnamate (GC yields and conversion on 0.1 mmol scale)
incurs additional costs. In comparison, base metals like iron,
nickel and copper are much cheaper, less toxic or even nontoxic,
and are being produced in millions of tons a year.5
NiBr2(DME) 4 mol%
(R)-Me-Duphos 4.8 mol%
Me
Me
CO2Et
CO2Et
Ph
Ph
HCO2H / Et3N (5 : 2)
80 oC, 12 h
NiBr2 (DME) 4 mol%
Bisphosphine 4.8 mol%
Me
Me
model reaction
CO2Et
HCO2H + Et3N
(5 : 2)
CO2Et
Ph
Ph
R
i-PrOH, 80 oC, 12 h
Entry
Conditions
Conv (%) Yield (%) Ee (%)
1
2
3
4
5
6
7
8
9
MeOH
EtOH
45
98
95
99
88
97
53
72
83
45
95
92
92
87
96
39
55
81
90
90
91
91
91
93
92
93
94
R
H
t-Bu
H
H
P
P
P
n-BuOH
i-PrOH
DMF
P
P
H
P
P
H
R
R
t-Bu
t-Bu
P
t-Bu
t-Bu
H
t-Bu
(2S)-BPE
(1S)-TangPhos
(1S)-DuanPhos
R = Ph 22%, 82% ee
R = i-Pr 0%
(S)-Binapine
48%, 49% ee
0%
Diglyme
THF
0%
Me
Me
Me
N
N
P
t-Bu
t-Bu
Toluene
PhCF3
R
P
P
R
PCy2
Me
Me
Fe
P
PCy2
Fe
P
P
Me
R
R
Me
(R,S)-CyPP-Cy
(2R)-DuPhos
(R)-QuinoxP*
(2R)-Me-BPF
R = Me 92%, 91% ee
R = Et or i-Pr 0%
0%
5%, 29% ee
0%
Herein, we report a nickel/DuPhos catalyst for asymmetric
60 transfer hydrogenation of conjugated olefins using formic acid.13
DuPhos, which was invented by Mark Burk previously, was
uniquely active and gave 91% ee value in the model reaction (Fig
30
Fig 1 Performance of chiral bisphosphines in a model reaction of
β-methylcinnamate
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1). The performance of other bisphosphines was unsatisfactory.
(S)-Binapine was completely inactive. Other bisphosphine
showed little activity, including Ph-BPE, Me-DPF, DuanPhos,
QuinoxP*, and Josiphos ligands.14 TangPhos was moderately
gave 94% ee in the presence of a TangPhos catalyst, while the
DuPhos catalyst gave only 65% ee (Fig 2b). The reaction can be
scaled up to gram scale with 2 mol% of nickel (Fig 2c). In a case
of p-chloro-β-methylcinnamate, its aryl-Cl bond was reduced to
5
active.15 Less donating (biaryl)bisphosphines including BINAP, 30 C-H in the isolated product. It was probably caused by oxidative
Segphos and DIPAMP were completely inactive. PHOX (Pfaltz
ligand) and Feringa’s phosphoramidite did not form active
catalysts. Iron, cobalt and copper salts were tested with Me-
DuPhos and did not form active catalysts.
Isopropanol was used as a solvent in isolation experiments.
The nickel catalyst worked well in several other alcohols and
diglyme (Table 1). No hydrogenation activity was detected with
10 atm of H2.
addition of the C-Cl bond to a nickel(0) species. A β,β-
dialkylacrylate was also attempted which afforded a moderate
54% ee. Simple styrene-type derivatives did not react.
98%D
NiBr2(DME) 4 mol%
(R)-Me-DuPhos 4.8 mol%
10
Me
Me
O
D
(a)
CO2Et
CO2Et
Et3N
(5 : 2)
Ph
Ph
D
OD
PhCF3, 80 oC, 24 h
100% conv
D
43%D
D
59%D
+
[(L-L)]Ni-X]
DCO2D
NiBr2(DME) 4 mol%
(R)-Me-DuPhos 4.8 mol%
deuteration
Me
Me
(a)
CO2Et
CO2Et
Me
Ph
Ph
- CO2
+
D
Ph
(L-L)
HCO2H / Et3N (5 : 2)
+
CO2Et
[(L-L)]Ni-OC(O)D]
[(L-L)]Ni-D]
i-PrOH, 80 o
C
91% ee, 92%
olefin
insertion
+
Ni
Other examples
Me
decarboxylation
or O-enolate
Me
Me
CO2Et
CO2Et
CO2Et
31%D
Me
O
Me
MeO
F3C
cat. Ni / Me-DuPhos
(b)
(c)
D
Ph
Me
Et3N
(5 : 2)
CO2Et
CO2Et
H
O
OD
Ph
PhCF3, 80 oC, 24 h
100% conv
90% ee, 92%
92% ee, 94%
95% ee, 93%
D
37%D
D
55%D
Me
Me
Me
Me
CO2Et
CO2Et
CO2Et
O
_
+
(L-L)Ni0
[(L-L)]Ni-D]
H
OD
OMe
H
O
35
94% ee, 88%
91% ee, 92%
80% ee, 82%
Fig 3 Deuterium labeling experiments and reaction mechanism
Me
Me
Me
Et
CO2Et
When we attempted the model reaction using [D2]formic acid
CO2Et
CO2Et
S
CO2Et
Bn
Ph
(Fig 3a), the β position was fully deuterated as expected. Both at
N
40 α and αʹ positions were partially deuterated, too. The deuterium
content at α and αʹ positions added up to around 100%. Most
likely, the main pathway consists of formate decarboxylation on
nickel, hydride insertion of the olefin and subsequent non-
stereoselective protonation of resulting enolates (Fig 3a).16 This
45 pathway is distinct from syn-addition of H2 in classical dihydride
and monohydride pathways using noble metal catalysts.17
Surprisingly, when we used [D1]formic acid (Fig 3b), besides
deuteration at α positions, a significant amount of deuterium
ended up in β position (about 30%D). In control experiments, no
50 extra deuteration occurred when the methylcinnamate and its
product were treated with [D1]formic acid. Direct hydride transfer
from a formate to a metal-bound methylcinnamate via a six-
membered transition state is inconsistent with >100% deuterium
incorporation.
54% ee, 87%
82% ee, 92%
92% ee, 88%
90% ee, 92%
O
Me
Me
CO2Et
CO2Et
CO2Me
CO2t-Bu
Ph
Ph
94% ee, 92%
88% ee, 93%
94% ee, 98%
93% ee, 98%
Me
Me
CN
CONEt2
Ph
Ph
91% ee, 85%
82% ee, 87%
NiBr2(DME) 4 mol%
(S)-TangPhos 4.8 mol%
CO2Me
CO2Me
CO2Me
(b)
CO2Me
Ph
Ph
Ph
HCO2H / Et3N (5 : 2)
Toluene, 80 o
C
94% ee, 97%
55
We propose a minor reaction pathway that involves an
equilibrium of (DuPhos)Ni(H)+ and (DuPhos)Ni0 via reversible
deprotonation. This allows a deuteron of [D1]formic acid to
become a nickel deuteride and eventually add to the β position of
β-methylcinnamate (Fig 3c).18 The presence of a nickel(0) species
NiBr2(DME) 2 mol%
(R)-Me-DuPhos 2.4 mol%
Me
Me
(c)
CO2Et
CO2Et
Ph
HCO2H / Et3N (5 : 2)
diglyme, 90 oC, 3 d
90% ee, 92%
5 mmol
0.88 gram
15
Fig 2 Examples of transfer hydrogenation (isolated yields from
60 was supported by hydrodechlorination of an aryl C-Cl bond in p-
chloro-β-methylcinnamate. As another piece of evidence for
nickel(0), when Ni(PPh3)4 was used as a nickel precursor, the
model reaction in Table 1 became much slower, but it still
afforded 14% conversion after 24 hours at 100oC.
0.5 mmol of olefins)
(E)-Cinnamates having small β groups cis to the ester groups
20 reacted to afford good ee (Fig 2). The (Z)-geometric isomer
afforded only <20% ee, however. Electron-donating or
withdrawing groups on aryl groups were well tolerated, as well as
thiophene and pyridine rings. Cyclic olefins were also
hydrogenated efficiently to give tetralines. Furthermore, the ester
25 groups can be replaced with amides and nitriles. α-Phenylmaleate
65
In summary, we herein report a nickel-catalyzed transfer
hydrogenation of conjugated olefins. The nickel catalyst must be
supported by a strongly σ-donating bisphosphine, probably for
efficient decarboxylation of a formate anion. No directing groups
are needed on olefins, unlike enamides and ketone hydrazones
2 | Journal Name, [year], [vol], 00–00
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DOI: 10.1039/C5CC01632K
that we reported previously.10 Deuterium labelling experiments
point to a major pathway involving formate decarboxylation,
hydride insertion into olefins and protonation of resulting nickel
enolates.
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We thank Singapore GSK-EDB green and sustainable
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A nickel catalyst is used for asymmetric hydrogenation of
electron-deficient olefins using formic acid as a hydrogen source
5
R1
R1
nickel catalyst
CO2Et
CO2Et
R2
R2
formic acid
>90% ee
Me
Me
ligands:
Me-DuPhos
P
P
Me
Me
4 | Journal Name, [year], [vol], 00–00
This journal is © The Royal Society of Chemistry [year]