The first example of asymmetric hydrogenation of imines with Co 2(CO)8/(R)-BINAP as catalytic precursor

Article abstract of DOI:10.1016/j.molcata.2012.08.019

The first example of asymmetric hydrogenation of imines using Co ₂(CO)₈/(R)-BINAP/H₂/CO system was developed. The reaction conditions were screened with a wide range of N-aryl benzophenone ketimines, obtaining products with excellent yields and good enantiomeric excess. Moreover, a pathway is suggested based on the isolation and characterization of several catalyst intermediates.

Full text of DOI:10.1016/j.molcata.2012.08.019

Journal of Molecular Catalysis A: Chemical 366 (2013) 17–21
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Journal of Molecular Catalysis A: Chemical
journal homepage: www.elsevier.com/locate/molcata
The first example of asymmetric hydrogenation of imines with
Co2(CO)8/(R)-BINAP as catalytic precursor
Manuel Amézquita-Valencia∗, Armando Cabrera∗
Instituto de Química, Universidad Nacional Autónoma de México, Ciudad Universitaria, Circuito Exterior, Coyoacán 04510, México D.F., Mexico
a r t i c l e i n f o
a b s t r a c t
Article history:
Received 28 May 2012
Received in revised form 20 August 2012
Accepted 21 August 2012
Available online 30 August 2012
The first example of asymmetric hydrogenation of imines using Co2(CO)8/(R)-BINAP/H2/CO system was
developed. The reaction conditions were screened with a wide range of N-aryl benzophenone ketimines,
obtaining products with excellent yields and good enantiomeric excess. Moreover, a pathway is suggested
based on the isolation and characterization of several catalyst intermediates.
© 2012 Elsevier B.V. All rights reserved.
Keywords:
Phosphine ligand
Dicobalt octacarbonyl
Asymmetric hydrogenation
Imines
Cobalt catalysis
1. Introduction
Overall, the last reaction was applied in the synthesis of asymmetric
2-cyclopentanones using Co2(CO)8 modified with chiral bidentade
Asymmetric catalysis is one of the most important tools for
the synthesis of enantiopure chemicals [1], due to its high atom
economy when compared to methods based on the resolution of
racemates. In this context, the enantioselective reduction of prochi-
ral ketimines is among the most reliable and efficient approaches
to obtain the corresponding optically active amines [2–4], which
are themselves found in various natural or medicinal compounds
[5–8]. Likewise, several methodologies have been developed for
the enantioselective reduction of imines [9–28], the asymmetric
hydrogenation of functionalized imines such as acyclic aromatic
N-aryl imines [29], ␣-fluorinated iminoesters [30,31] and acyclic
imines [32], with a variety of complexes of Ir [32,33], Ru [34], Rh
[19] and organocatalysts [35,36] have been employed in asym-
metric hydrogenations and/or transfer hydrogenations of imines.
Noteworthy, Co2(CO)8/modified phosphine complexes have never
been used as catalysts in asymmetric hydrogenation of imines,
even though, phosphine dicobalt octacarbonyl derivatives play an
important role as catalytic promotors in other organic transfor-
mations, for example: hydroformylation of alkenes (also known as
oxo process discovered in 1938 by Otto Roelen) [37,38], amidocar-
bonylation reaction [39,40], synthesis of quinolines [41], synthesis
of ␤-lactams [42] and the Pauson–Khand reaction (PKR) [43–45].
phosphines as catalytic precursor. Furthermore, in 2003, Gibson’s
group studied the PKR mechanism of Co2(CO)8/BINAP system using
the synthesis of bicyclic cyclopentenones; based on their results
(they reported yields above 80% and ee up to 70%), postulated the
existence of (BINAP)(CO)Co-ꢀ-(CO)2-Co(CO)3 as catalyst precursor
[46] and the isolation of a cobalt hydride (BINAP)(CO)2CoH [47].
Therefore, we became interested in using of this hydride as a pre-
cursor in the reduction of prochiral imines, leading us to the first
example of the catalytic asymmetric hydrogenation of imines with
Co2(CO)8/(R)-BINAP.
2. Experimental
2.1. General
All reactions and manipulations were carried out under nitro-
gen atmosphere by using Schlenk-type techniques. 1H NMR, 13C
NMR and 31P NMR spectra were obtained on a JEOL GX300 Bruker-
Avance 300, Varian Unity 300 (300, 75 and 121 MHz respectively)
spectrometers in CDCl3 as solvent at 25 ◦C. IR spectra were recorded
on a Nicolet FTIR Magna 750 spectrophotometer. Optical rota-
tions were measured on a Perkin–Elmer 343 spectropolarimeter.
Mass spectra were obtained using a JEOL JMS-SX102A instrument
with m-nitrobenzyl alcohol as the matrix (FAB+ mode). Elemen-
tal analyses for some compounds were obtained on an Elemental
Analyzer CE-440. HPLC analyses were performed on a Hewlett
Packard 1100 system with UV-DAD. Separations were achieved on a
∗
Corresponding authors. Fax: +52 5 6162217/6162207.
E-mail addresses: majo00@yahoo.com (M. Amézquita-Valencia),
arcaor1@unam.mx (A. Cabrera).
1381-1169/$ – see front matter © 2012 Elsevier B.V. All rights reserved.
http://dx.doi.org/10.1016/j.molcata.2012.08.019

18
M. Amézquita-Valencia, A. Cabrera / Journal of Molecular Catalysis A: Chemical 366 (2013) 17–21
Table 2
Daicel Chiracel OD-H (25 mm × 4.6 mm) column. X-ray deter-
mination was collected on a Bruker SMART APEX CCD area
diffractometer by the ␻-scan method.
Optimization of the asymmetric hydrogenation of imine 3a.a
2.2. Hydrogenation of imines
Imines were synthesized according to a literature procedure
[48]. Typical procedure: in a Schlenk tube under nitrogen atmo-
sphere, a solution of imine (100 mg), Co2(CO)8 (1 mol%) and
(R)-BINAP (2 mol%) in THF (10 mL) was transferred to a 45 mL stain-
less Steel Parr vessel which was pressurized at 450 psi with a H2/CO
mixture in a 1/3 ratio. The reactor was placed in a preheated oil bath
at 120◦ with magnetic stirring for 15 h. The reaction mixture was
purified by column chromatography using hexane and ethyl acetate
as eluents.
.
Entry
Co2(CO)8 (mol%)
Ligand
Yield (%)b
ee (%)c
1
2
3
4
5
6
7
8
9
10
11d
12e
13f
14g
5
2
5
3
2
1
1
1
1
1
1
1
1
1
L1
L1
L2
L2
L2
L2
L3
L4
L5
L6
L3
L3
L3
L3
31
30
79
80
79
81
86
83
40
34
89
95
82
95
–
–
–
–
–
–
93
53
5
3. Results and discussion
In our preliminary studies, we determined the influence of
solvent, temperature, and carbon monoxide pressure, in the hydro-
genation of imine 3a (Eq. (1)) with Co2(CO)8/rac-BINAP (Table 1).
The reduction of 3a was not observed when working only with
hydrogen (H2) (entry 1), however, in the presence of CO/H2 mix-
tures (3:1) (entries 2–4 and 7) good yields were obtained, behavior
that suggests the stabilization of the catalytic species by CO partial
pressure [40].
1
92
93
90
63
a
Reaction conditions: imine 3a (100 mg), 1 mol% Co2(CO)8/ligand (1/2) in dry
THF (10 mL), 24 h, 450 psi with a 1/3 ratio (H2/CO), ligand: PPh3 L1, rac-BINAP L2,
(R)-BINAP L3, (R)-Tol-BINAP L4, (S-S)-Me-DUPHOS L5, (S,S)-DIOP L6.
b
Isolated yield.
Determined by HPLC.
20 h.
15 h.
c
d
e
f
12 h.
g
Co2(CO)8/ligand (ratio: 1/1), 15 h.
Our initial evaluation began with hydrogenation of imine 3a as
the model substrate, finding an optimal H2/CO ratio (1:3) in THF.
Afterwards, we tested the influence of different monodentate and
bidentate phosphine ligands in the conversion yields; the results
are summarized in Table 2.
(1)
At room temperature, the hydrogenation product was not
observed (entry 5) but, a temperature increase (120 ◦C) favored
conversion and reaction yields (entries 4 and 6). The use of a
coordinating solvent (THF) led to the best enantiomeric excess
(95%ee) (entry 11), that contrasts with lower values obtained
when carrying out the reaction in other solvents such as:
toluene (67%ee), benzene (75%ee) and methylene chloride (52%ee)
(entries 8–10).
The use of monophosphine ligand L1 resulted in poor yields
(entries 1 and 2), unlike the biphosphine ligands L2–L6, we found
that low catalyst loading (1%) did not affect the conversion of 3a
(entries 3–6). However, low yields were obtained when using Me-
DUPHOS L5 or DIOP L6 ligands (entries 9 and 10), compared to
those observed for L4 (entry 8) and (R)-BINAP L3, which achieved
the best result (entry 7). After 24 h we found in this reaction a lower
concentration of the by-product 5a (less than 6%), which was iso-
lated and fully characterized (Eq. (2)), same results were obtained
using the fluorinated imine 2b as substrate in catalytic amounts
of Co2(CO)8/L3 (1 mol%). Moreover, when we used previously pre-
pared amine 4a in the same reaction conditions, we found again the
product 5a (25%). Finally when this last reaction was carried out in
the absence of Co2(CO)8/L3 we did not find by-product 5a. This by-
product was not formed when the reaction time was reduced to
15 h (entries 11–13).
Table 1
Effect of the H2/CO ratio, solvent and temperature in hydrogenation of imine 3a.a
Entry
Temperature (◦C)
H2/CO ratio
Yields (%)b
ee%g
1
2
3
4
5
120
120
120
120
25
100
120
120
120
120
120
1/0
1/1
1/2
1/3
1/3
1/3
1/4
1/3
1/3
1/3
1/3
0
0
0
87
0
20
73
91
75
80
86
0
0
0
0
0
0
0
67
75
52
95
6
7
8c
9d
10e
11f
a
Reaction time 24 h and 450 psi pressure, 100 mg of imine 3a, 5 mol%
Co2(CO)8/rac-BINAP (1/2) and dry tetrahydrofuran (THF, 10 mL) as solvent.
b
Isolated yield.
(2)
c
The reaction was carried out in dry toluene (10 mL) as solvent and (R)-BINAP.
The reaction was carried out in dry benzene (10 mL) as solvent and (R)-BINAP.
The reaction was carried out in dry methylene chloride (10 mL) as solvent and
d
Noteworthy, the use of an equimolar mixture of
CO2(CO)8/ligand resulted in good reaction yield but low enantio-
selectivity (entry 14), this lack of selectivity could be related to the
existence of two hydride species, HCo(CO)4 and HCo(CO)2–BINAP
e
(R)-BINAP.
f
The reaction was carried out in dry THF as solvent and (R)-BINAP.
g
Determined by HPLC.

M. Amézquita-Valencia, A. Cabrera / Journal of Molecular Catalysis A: Chemical 366 (2013) 17–21
19
Table 3
Asymmetric hydrogenation of imines from substituted anilines.a
.
Entry
R1
R2
Product
Yield (%)b
ee (%)c
1
2
3
4
5
6
7
8
4-Me
4-Me
4-MeO
4-MeO
2-MeO
3-Cl
4-Cl
4-F
4-Cl
4-F
4-Cl
4-Cl
4-F
2a
2b
2c
2d
2e
2f
98
96
99
97
98
98
94
95
81
78
83
70
7
4
4
91
3-Cl
3-Cl
2g
2h
4-Me
a
Reaction conditions: 100 mg of imine, 1 mol% Co2(CO)8/(R)-BINAP (1/2), dry THF
(10 mL), 15 h, 450 psi with a 1/3 ratio (H2/CO).
b
Isolated yield.
Determined by HPLC.
c
Fig. 1. Crystallographic structure of compound 2b [49].
(Eq. (3)), which are competing in the hydrogenation
step.
may be ascribed to both steric and electronic effects (entries 7–9,
41–72%ee). Moderated 36–58%ee were achieved with R1 = m-F or
para and meta CF3 groups in conjunction with p-F and p-Cl on the
benzophenone moiety (entries 10–13, 36–58%ee). Noteworthy, the
methyl group on the aromatic ring in R1 or R2 leads again to an
increase in enantioselectivity (entries 4–6 and 9).
Regarding the pathway of this reaction, we have developed
several stoichiometric experiments and we found the species con-
figuring the equilibrium shown in Scheme 1.
(3)
The reaction of Co2(CO)8 A with BINAP at room temperature in a
stoichiometric 1:2 ratio results in the formation of B (Scheme 1) as a
brown powder compound in good yield (91% respect to Co2(CO)8).
Red crystals of B (this compound crystallizes with a THF molecule),
suitable for X-ray studies, were grown from slowly diffusion of
Under the optimized reaction conditions [Co2(CO)8/(R)-BINAP,
THF, 15 h, H2/CO (1:3) 450 psi], a wide variety of imines were tested
to examine the reaction scope, starting with the synthesis of imines
derived from anilines with different substituents. Thus, the pres-
ence of different arylic groups attached to the nitrogen atom in the
imine can be tolerated with excellent yields (entries 1–8, 94–98%),
as shown in Table 3.
Table 4
Asymmetric hydrogenation of imines from substituted benzylamines.a
These substrates indicate a substituent effect on enantio-
selectivity, thus, the presence of p-Cl or p-F substituents on
the benzophenone moiety and p-Me or p-MeO groups on the
amine moiety, resulted in good enantioselectivities (entries 1–4,
70–83%ee), but a lower value was obtained when o-MeO was sub-
stituted on the aromatic ring in R1 (entry 5, 7%ee). The reduction
of enantioselectivity may be attributed to the steric hindrance of
the ortho substituent in the substrate. Furthermore, the inclusion
of a m-Cl substituent in the R1 resulted in low enantioselectivity
(entries 6 and 7, 4%ee), however, the introduction of a p-methyl
group on the aromatic ring in R2 led to an increase in enantioselec-
tivity (entry 8, 91%ee). In this context, we were able to determine
the structure of amine 2b by X-ray diffraction studies (Fig. 1).
This methodology was also applied to the asymmetric hydro-
genation of imines from benzylamines derivatives. The first
attempts resulted in excellent yields and good enantiomeric
excesses; the results are shown in Table 4.
As mentioned before, the conversion of all substrates resulted
in excellent yields (93–98%), although, the influence of the sub-
stituents affected the enantiomeric excess. Thus, the presence
of p-Cl, p-F or p-Me groups on the benzophenone moiety gave
excellent enantioselectivities (entries 1–3, 73–95%ee), meanwhile,
when R1 is a methyl group, slightly higher enantioselectivities were
obtained (entries 4–6, 96–99%ee). The variation of the substituent
attached to the benzylamine moiety for p-Cl, p-CF3 or m-CF3 groups
resulted in a negative effect on the enantioselectivities, which
.
Entry
R1
R2
Product
Yield (%)b
ee (%)c
1
2
H
H
H
4-Cl
4-F
4-Me
4-Cl
4-F
4-Me
4-Me
4-Me
4-Me
4-Cl
4-F
4a
4b
4c
4d
4e
4f
4g
4h
4i
95
96
98
94
96
96
97
94
93
97
93
93
95
95
94
73
99
99
96
41
44
72
37
36
45
58
3
4
5
6
7
8
9
10
11
12
13
4-Me
4-Me
4-Me
4-Cl
4-CF3
3-CF3
3-F
4-CF3
3-CF3
3-CF3
4j
4k
4l
4-Cl
4-F
4m
a
Reaction conditions: 100 mg of imine, 1 mol% Co2(CO)8/(R)-BINAP (1/2), dry THF
(10 mL), 15 h, 450 psi in a 1/3 ratio (H2/CO).
b
Isolated yield.
Determined by HPLC.
c

20
M. Amézquita-Valencia, A. Cabrera / Journal of Molecular Catalysis A: Chemical 366 (2013) 17–21
Scheme 1. Equilibriums found with the system Co2(CO)8/L-L/H2/CO.
pentane into a solution of B in THF (Fig. 2). IR spectrum of B shows
five absorptions for the CO stretching frequencies at 2042, 1967,
1916, 1868, 1787 cm−1, this data is in accord with that reported
by Gibson’s group [46]. In the FAB+ spectra of B the appropriate
ions peaks are found, additionally, the ESI mass spectrum shows a
characteristic fragmentation pattern of successive CO elimination.
The 31P NMR spectrum reveals a broad signal at 41.4 ppm, which is
assigned to specie B [46].
In order to obtain compound C, a temperature was increased
until reflux conditions favored C formation. The IR spectrum shows
two absorption bands at 2011 and 2030 cm−1 for terminal CO lig-
ands. In the FAB+ and ESI mass spectra, the appropriate molecular
ions peaks are found, while, the recorded 31P NMR spectrum reveals
a single resonance at ı 61.08 ppm. Compound C is thermally sensi-
tive at atmosphere pressure and slowly decomposes in solution at
25 ◦C. The hydride complex D was prepared from C in hydrogena-
tion conditions without substrate and it was possible to detect the
hydride moiety of D by the appearance of a band at 2063 cm−1 in
the IR spectra [50,51]. In addition, the ESI mass spectra showed
a 710 m/z [M−CO] peak corresponding to [HCo(L-L)CO] moiety E
(Scheme 2), even though, the molecular ion was not observed.
The proposed mechanism sequence of this reaction is outlined
in Scheme 2, the pathway is presumed in the absence of a more
thorough study. As the reaction is conducted under CO atmosphere,
carbon monoxide dissociation–association is likely to be reversible
in many of these species (Scheme 1). The formation of a cobalt
hydride D is the first reaction step, followed by the loss of a CO
ligand from D giving species E (16e electrons), this species forms
Scheme 2. Proposed mechanism sequence.
the cobalt intermediate F by coordination of the imine moiety.
Afterwards, an imine insertion into the Co H bond would give an
amino-cobalt species G [52] with concomitant binding of the CO
ligand. The regeneration of the active species D could occur by dif-
ferent processes (hydrogenolysis, sigma-bond-metathesis reaction
or even oxidative addition–reductive elimination) in the presence
of H2.
4. Conclusion
In summary, we reported for the first time the enantiose-
lective reduction of imines using dicobalt octacarbonyl system
modified with (R)-BINAP as ligand, where the methyl group sub-
stituent is a key feature for good enantioselectivity. In addition,
a mechanism is suggested based on the isolation and charac-
terization of several cobalt species. Therefore, this methodology
provides an attractive alternative to imines reduction under mild
conditions.
Acknowledgments
We thank DGAPA-UNAM for financial support (PAPIIT
IN203209) and CONACyT (Fellowship 229448). Also, we gratefully
acknowledge A. Toscano, S. Hernández-Ortega, J. Pérez, L. Velasco,
Rocio Patin˜o Maya, Ma. Angeles-Pen˜a, L.C. Marquez and E. Garcia
for technical support.
Fig. 2. Molecular structure of compound B. Hydrogen atoms are omitted for clarity
[49].

M. Amézquita-Valencia, A. Cabrera / Journal of Molecular Catalysis A: Chemical 366 (2013) 17–21
21
Appendix A. Supplementary data
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Supplementary data associated with this article can be
found, in the online version, at http://dx.doi.org/10.1016/j.
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