Cobalt-catalyzed hydrogenation of β-enamino esters using an internal mixture of bidentate and monodentate ligands

Article abstract of DOI:10.1016/j.jorganchem.2014.06.028

Different β-amino esters have been obtained in good yields by means of octacarbonyldicobalt-catalyzed hydrogenation of β-enamine esters in the presence of mixture of a bidentate phosphine chiral (R-BINAP) and a monodentate phosphine achiral (PPh₃

Full text of DOI:10.1016/j.jorganchem.2014.06.028

Journal of Organometallic Chemistry 768 (2014) 145e150
Contents lists available at ScienceDirect
Journal of Organometallic Chemistry
journal homepage: www.elsevier.com/locate/jorganchem
Cobalt-catalyzed hydrogenation of
b-enamino esters using an internal
mixture of bidentate and monodentate ligands
*
*
ꢀ
Manuel Amezquita-Valencia , Armando Cabrera
ꢀ
ꢀ
ꢀ
Instituto de Química, Universidad Nacional Autonoma de Mexico, Ciudad Universitaria, Circuito Exterior, Coyoacan, 04510 Mexico, D.F., Mexico
a r t i c l e i n f o
a b s t r a c t
Article history:
Different
hydrogenation of
b
-amino esters have been obtained in good yields by means of octacarbonyldicobalt-catalyzed
-enamine esters in the presence of mixture of a bidentate phosphine chiral (R-BINAP)
Received 23 April 2014
Received in revised form
11 June 2014
Accepted 26 June 2014
Available online 9 July 2014
b
and a monodentate phosphine achiral (PPh3). Likewise, a non-symmetric Co/BINAP/PPh3 complex was
isolated. This new compound was used in the hydrogenation reaction under different conditions and the
results suggest that this heterocombination could be responsible for improving enantiomeric excess in
the reduction products.
© 2014 Elsevier B.V. All rights reserved.
Keywords:
Phosphine ligands
Asymmetric hydrogenation
b
-enamino esters
Cobalt catalysis
Mixture of ligands
Introduction
aniline in the presence of aromatic solvents by azeotropic removal
of water. However, this method has been displaced by the use of
Co2(CO)8-catalyzed reactions have played key roles for the
synthesis of useful organic compounds in both academia and in-
dustry. In general, the principal application of dicobalt octacarbonyl
is in carbonylation reactions, however; this complex has been
effective in different reactions such as 1,3-oxazinan-4-ones syn-
thesis [1], cyclodimerization of 1,4-dilithio-1,3-butadienes [2],
hydrosilylation of amides [3]. This ability in the activation of small
molecules reveals the potential application of Co2(CO)8 in different
catalytic reactions. On the other hand, enamines represent an
important class of compounds due to their occurrence in natural
products [4]. Likewise, these compounds have received significant
attention in organic synthesis due to their versatility as in-
termediates in the synthesis of heterocycles with a variety of bio-
logical activities, such as anticonvulsant [5], anti-inflammatory [6]
and anticancer compounds [7]. Therefore, several protocols are
available for the synthesis of enamines. The most common method
Lewis acids [8,9], because they are easy to handle, inexpensive and
relatively stable to air and moisture. On the other hand, chiral
amino esters are important compounds that can be used as build-
ing blocks for the synthesis of
well as other classes of compounds with biological and pharma-
cological activity [12,13]; because of this, the interest in -amino
esters has continued to grow. In this context, -enamino esters are
convenient precursors in the synthesis of chiral -amino esters by
b-peptides and b-lactams [10,11] as
b
b
b
asymmetric hydrogenation. In this case, a number of transition
metal catalytic systems have been developed wherein focus of the
studies has been on Rh, Ir and Ru [14e16]. Zhang and co-workers
first reported the rhodium-catalyzed asymmetric hydrogenation
of N-aryl
b-enamino esters with good to excellent enantiose-
lectivities [17]. They also extended this strategy to the asymmetric
hydrogenation of unprotected enamino esters catalyzed using
iridium complexes [18]; the first report of this reaction was made
by the Merck and Solvias groups [19]. Likewise, few iridium com-
plexes have been used in the asymmetric hydrogenation of N-aryl
is a reaction between b-keto esters or b-dicarbonyl compounds and
b-enamines. For example, Renaud et al. found that [IrCl(COD)]
complex was active in this reaction with good yields. However,
when they worked with chiral ligands (phosphoramidite ligands)
in the same reaction no chiral induction was found [20]. More
recently, Vries et al. working with [Ir(COD)2]BArF complexes in the
* Corresponding authors. Fax: þ52 5 6162217, þ52 5 6162207.
ꢀ
E-mail addresses: majo00@yahoo.com (M. Amezquita-Valencia), arcaor1@unam.
mx (A. Cabrera).
http://dx.doi.org/10.1016/j.jorganchem.2014.06.028
0022-328X/© 2014 Elsevier B.V. All rights reserved.

ꢀ
M. Amezquita-Valencia, A. Cabrera / Journal of Organometallic Chemistry 768 (2014) 145e150
146
presence of a mixture of chiral and achiral monodentate ligands
obtained good yields and good enantioselectivities [21]. In the case
of ruthenium, there are a few reports of the hydrogenation of N-aryl
Crystallography
Crystallographic and experimental details of crystal structure
determination are listed in Table 1. Single crystal for X-ray
diffraction was obtained by slow diffusion of pentane into a THF
solution of II. Single crystal was mounted on a glass fiber at
room temperature. The crystal was then placed on a Bruker
b
-enamino esters; the first example was reported by Moroi et al. at
Takasato [22]. In 2010, Hebbache et al. reported the hydrogenation
of N-benzyl -enamino esters using different ruthenium complexes
b
under acidic conditions [20]. Despite these examples, there have
been few reports on the hydrogenation of this type of compound,
especially using cobalt as a catalytic precursor. It is noteworthy that
we recently used a cobalt-modified complex for the hydrogenation
SMART APEX CCD diffractometer, equipped with Mo K
a radia-
tion. Systematic absence and intensity statistic were used in
space group determinations. The structure was determined us-
ing direct methods [25]. Anisotropic structure refinement was
achieved using full-matrix, least-squares techniques on all non-
hydrogen atoms. All hydrogen atoms were placed in idealized
positions, based on hybridization, with isotropic thermal
parameter fixed at 1.2 times the value of the attached atom.
Structure solution and refinement was performed using
SHELXTL v 6.10 [26].
of imines and
Here, we aim to extend this methodology to the hydrogenation of
N-aryl -enamino esters using Co2(CO)8 and a mix of ligands con-
a-enamino ketones, obtaining good results [23,24].
b
sisting of chiral bidentate phosphine and achiral monodentate
phosphine.
Experimental section
General procedure for synthesis of
b-enamino esters
General
A mixture of the -dicarbonyl compound (2 mmol), the amine
b
All reactions and manipulations were carried out under ni-
trogen atmosphere using Schlenk-type techniques. Column
chromatography was performed on silica (70e230 and
230e400 mesh). The 1H, 13C NMR spectra were recorded on a
Bruker-Advance 300 spectrometer in CDCl3 as solvent at 25 ꢀC.
Mass spectra were obtained using a JEOL JMS-SX102A instru-
ment. Elemental analyses for compounds were obtained on an
Elementary Analyzer CE-440. IR spectra were recorded on a
Nicolet FTIR magna 750 spectrophotometer. Optical rotations
were measured on a PerkineElmer 343 spectropolarimeter. HPLC
analyses were performed on a Hewlett Packard 1100 system with
UV-DAD. Separations were achieved on a Diacel Chiracel OD-H
(2 mmol) and CoCl2 (1 mol%) was stirred at room temperature for
12 h. The crude reaction mixture was passed through a pad of
Na2SO4. Subsequently, the solvent was evaporated under reduced
pressure to provide the crude product. Further purification was
carried out by silica gel flash chromatography with hexane-ethyl
acetate (8:2) to afford pure
yields.
b-enamino esters with excellent
General procedure for hydrogenation reaction of b-enamino esters
The hydrogenation was performed on a 4712 model Parr
reactor. In a Schlenk tube under nitrogen atmosphere, corre-
and OD columns. X-ray determination was collected on
a
sponding b-enamino ester (100 mg, 0.037 mmol) was added to a
Bruker SMART APEX CCD area diffractometer by the -scan
u
solution of Co2(CO)8 (1 mol%), (R)-BINAP (2 mol%) and PPh3 (2 mol
%) in THF (10 mL), the reaction mixture was stirred for 10 min.
Then, the solution was transferred to a 45 mL steel Parr vessel
previously purged with vacuum-nitrogen. The reaction vessel 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 120ꢀ with magnetic
stirring for 36 h. At the end of the reaction time, the reactor was
cooled and the gases were liberated. The solution was concen-
trated under reduced pressure and resulting product was purified
by chromatography column using hexane and ethyl acetate as
eluents.
method. All chemicals and solvents were used as received unless
otherwise stated. Tetrahydrofuran (Na, benzophenone), hexane
(Na, benzophenone), and methylene chloride (P2O5) were
distilled under nitrogen prior to use.
Table 1
Crystal data and structure refinement for II.
II$THF
Empirical formula
Formula weight (g molꢁ1
Temperature
Crystal color
Crystal system
Space group
Crystal size (mm)
a (Å)
C68H49Co2O5P3
1160.84
298(2) K
Brown
Triclinic
P-1
0.451 ꢂ 0.251 ꢂ 0.056
11.1513(4)
13.7071(5)
20.2515(8)
105.384(2)
90.599(3)
103.826(2)
2889.06(19)
2
)
Mercury drop experiment
Following the above described procedure for hydrogenation
reaction; four drops of elemental Hg were added. After 36 h, the
solution was filtered and purified by column chromatography on
silica gel.
b (Å)
c (Å)
a
b
g
(ꢀ)
(ꢀ)
(ꢀ)
Results and discussion
V (Å3)
Initial hydrogenation experiments were performed at 450 psi
of H2/CO pressure and 120 ꢀC, using 1 mol% of catalyst with Co/
L* ¼ 1/2 prepared in situ. Enamine 1a was chosen as model
substrate; the results are summarized in Table 2. First, different
Z
Dcal (mg/cm3)
1.334
0.780
2.084e25.38
24,647
10,524
m
(mmꢁ1
)
2
q
(ꢀ)
Reflections collected
Independent reflections
Rint
times and temperatures were probed to obtain b-amino ester 2a.
We observed that the [Co2(CO)8/rac-BINAP] catalytic system
exhibited higher reactivity at 120 ꢀC with 3 h of time reaction
(entries 1, 2, 3 and 4). In contrast, if the reaction temperature was
less than 120 ꢀC the catalytic activity decreased (entries 5 and 6).
Furthermore, the catalytic efficiency of the cobalt complexes in
0.0486
R1 [I > 2
s(I)]
0.0619
wR2 [all data]
0.1952
Goodness-of-fit
1.011
0.810 and ꢁ0.490
Max./min. Dr (e Åꢁ3
)

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M. Amezquita-Valencia, A. Cabrera / Journal of Organometallic Chemistry 768 (2014) 145e150
147
Table 2
Hydrogenation of 1a
Table 3
b
-enamino esters under various conditions.a
Asymmetric hydrogenation of b
-enamines using Co2(CO)8 with mixed ligands.a
Entry Temperature (ꢀC) Time (h) Ligand
Yield (%)b ee (%)c
1
2
3
4
5
6
7
8
120
120
120
120
100
25
120
120
120
120
120
120
24
15
7
3
7
24
7
7
7
7
rac-BINAP
rac-BINAP
rac-BINAP
rac-BINAP
rac-BINAP
rac-BINAP
(R)-BINAP
(R)-Tol-BINAP
(R)-H8-BINAP
(R,R)-DIOP
90
89
90
89
0
rac
rac
rac
rac
e
Entry
Co/L*/L
Product
Yield (%)b
ee (%)c
1
2
3
4
5
6
7
8
1/1/1
1/1/2
1/1/3
1/2/1
1/3/1
1/2/2
1/3/3
1/1/1
1/2/1
1/2/2
1/2/2
1/2/2
1/2/2
1/2/2
1/2/2
2a
2a
2a
2a
2a
2a
2a
2b
2b
2b
2a
2a
2a
2a
2a
75
51
43
75
81
37
41
70
71
22
85
91
91
92
89
0
0
12
2
1
20
4
0
e
91
92
95
71
rac
rac
rac
rac
rac
rac
9
0
8
10
11
12
9
7
7
(R,R)-Me-DuPHOS 81
(R)-PROPHOS 84
10
11d
12e
13f
14g
15
37
20
21
22
21
20
a
Reaction conditions: 0.37 mmol of 1a enamine, 1 mol% Co2(CO)8 (0.0037 mmol),
(0.0075 mmol) ligand, 10 mL THF, H2/CO (1:3, 450 psi), 120 ꢀC.
b
Isolated yield.
c
Determined by HPLC analysis with Daicel chiralcel OD-H and OD columns.
a
Reaction conditions: 0.37 mmol of 1a or 1b enamine, 1 mol% of Co2(CO)8
(0.0037 mmol), ligands: L* ¼ (R)-BINAP (0.0075 mmol) and L ¼ PPh3 (0.0075 mmol),
10 mL THF, H2/CO (1:3, 450 psi), 120 ꢀC, 7 h.
the asymmetric hydrogenation reaction of
b-enamines was
b
Isolated yield.
evaluated; for this purpose the racemic ligand was changed to
(R)-BINAP. In this case, product 2a was obtained as a racemic
mixture (entry 7). To improve the enantiomeric excess, different
chiral phosphorus ligands such as (R)-Tol-BINAP, (R)-H8-BINAP,
(R,R)-DIOP, (R,R)-Me-DuPHOS and (R)-PROPHOS were probed;
unfortunately, all cases gave racemic mixtures (entries 8e12).
Similar results were found by Enthaler et al. for the hydrogena-
c
Determined by HPLC analysis with Daicel chiralcel OD-H and OD columns.
24 h.
30 h.
36 h.
d
e
f
g
48 h.
between Co2(CO)8, (R)-BINAP and PPh3 could be responsible for
the increase in enantioselectivity. On the other hand, the yield
of the reaction increased to up to 91% when the reaction was
carried out for 36 h (entries 11e14). In order to test the ho-
mogeneity of the catalytic system a mercury drop experiment
was done (See Supplementary material). The result showed no
inhibition of the process and no significant differences in yield
was observed, this result is consistent with a homogeneous
process (entry 15).
The effect of the different monodentate ligands in the asym-
metric hydrogenation reaction was also investigated; the results
are shown in Table 4. These experiments were performed using a
Co/L*/L ratio of 1:2:2 at 450 psi and 120 ꢀC in tetrahydrofuran for
36 h.
tion of
b-dehydroamino acid catalyzed by iridium complexes
with (R,R)-DIOP, (S)-BINAP and (S,S)-Me-DuPHOS [27].
In order to improve the enantiomeric excesses, we decided
to use a mixture of chiral and achiral ligands. This idea has
been developed by Reetz [28,29] and Feringa [30,31] for more
than a decade. They also demonstrated that it is possible to
increase enantioselectivity and reactivity by using mixtures of
chiral monodentate ligands [32]. In this context, they have
postulated that the corresponding heterocombination gener-
ated from a mixture of ligands coordinated to a metal center is
more selective than the corresponding homocombination [32].
Therefore, for our initial experiments we decided to work with
a mixture of (R)-BINAP and PPh3. The monodentate ligand was
selected based on excellent results reported in the literature on
asymmetric hydrogenation reactions of carbonecarbon double
bonds [21,31,33]. Initially, the Co2(CO)8/(R)-BINAP ratio was
kept constant at 1:1 and only the PPh3 ratio was varied. The
results are presented in Table 3. When we increased the
quantity of monodentate ligand, a decrease in the reaction
yields was found and no chiral induction was observed in
products 2aeb (entries 1, 2 and 8). The reaction with ratio Co/
L*/L (1:1:3) gave a moderate yield and slightly increased the
enantioselectivity (entry 3). Then, we investigated the effect of
increasing the BINAP ratio. No chiral induction was observed
when the bidentate ligand was increased to ratios of 1:2:1 or
1:3:1. A similar result was obtained to that with 1:3:3 (entries
4, 5, 9 and 7). Interestingly, the Co/L*/L (1:2:2) combination
increased the enantiomeric excess from 12% to 22% and 8% to
37% for products 2a and 2b, respectively (entries 6 and 10).
These results may suggest that the 1:2:2 ratio is essential for
asymmetric induction, showing that the heterocombination
When the reaction was carried out in the presence of the L1
ligand the expected product 2a was isolated with a yield of 72%;
however, this reaction yield was lower compared to when the
Table 4
The effect of different achiral monodentate ligands on the hydrogenation reaction of
1a.a
Entry
Achiral ligand
Yield (%)b
ee (%)c
1
2
3
4
5
P(p-CH3C6H4)3 L1
P(o-CH3C6H4)3 L2
72
46
24
81
83
0
0
0
0
1
P(C6H11
) L3
3
P(OC6H4)3 L4
Sb(C6H4)3 L5
a
Reaction conditions: 0.37 mmol of 1a enamine,
1
mol% of Co2(CO)8
(0.0037 mmol), ligands: (R)-BINAP (0.0075 mmol) and
L
¼
achiral ligand
(0.0075 mmol), 10 mL THF, H2/CO (1:3, 450 psi), 120 ꢀC, 36 h.
b
Isolated yield.
c
Determined by HPLC analysis with Daicel chiralcel OD-H column.

ꢀ
148
M. Amezquita-Valencia, A. Cabrera / Journal of Organometallic Chemistry 768 (2014) 145e150
Table 5
Table 6
Hydrogenation of enamine 1a with various chiral bidentate phosphines.a
Asymmetric hydrogenation of b
-enamines using Co/(R)-BINAP/PPh3.a
Entry
Chiral ligand
Yield (%)b
ee (%)c
1
2
3
4
5
(R)-Tol-BINAP
(R)-H8-BINAP
(R,R)-DIOP
(R,R)-Me-DuPHOS
(R)-PROPHOS
76
73
70
72
74
1
2
2
3
1
a
Reaction conditions: 0.37 mmol of 1a enamine,
1
mol% of Co2(CO)8
(0.0037 mmol), ligands: Chiral ligand (L*) (0.0075 mmol) and PPh3 (L)
(0.0075 mmol), 10 mL THF, H2/CO (1:3, 450 psi), 120 ꢀC, 36 h.
Entry
R1
R2
Product
Yield (%)b
ee (%)c
b
Isolated yield.
1
2
3
4
5
6
7
8
H
H
H
H
H
H
H
H
H
H
H
H
H
2a
2b
2c
2d
2e
2f
2g
2h
2i
2j
2k
2l
2m
2n
2o
2p
2q
2r
93
85
91
90
89
93
92
89
91
88
90
89
82
86
89
84
93
87
80
88
85
82
86
22
37
7
7
5
c
Determined by HPLC analysis with Daicel chiralcel OD-H column.
p-MeO
p-Me
p-Cl
PPh3 ligand was used (entry 1, Table 4 vs. entry 13, Table 3).
Changing the substituent position on the ligand resulted in a
poor yield (entry 2). This behavior could be associated with a
steric effect in the heterocombination generated with L2 and (R)-
BINAP (entry 3), making it impossible to form a mixed ligand
species; a similar result was obtained with ligand L3 (entry 4). In
contrast, good yields were obtained with triphenylphosphite L4
and triphenylstibine L5 as ligands. Unfortunately, all previous
heterocombinations did not improve the enantioselectivity of the
reaction. In addition, we explored the use of five different chiral
bidentate phosphine ligands (L*) in combination with triphe-
nylphosphine (L) in the hydrogenation of 1a. The reaction was
carried out with a Co/L*/L ratio of 1:2:2 in tetrahydrofuran under
450 psi (H2/CO) pressure and 120 ꢀC for 36 h. The results are
summarized in Table 5. Regrettably, low enantioselectivity was
obtained using the different bidentate ligands. Even so, the cat-
alytic systems showed a decrease in catalytic activity (entries
1e5).
p-F
m-Me
m-Cl
o-Me
o-MeO
o-Et
o-Br
o-OH
H
19
5
19
38
6
32
26
18
20
7
43
21
4
9
10
11
12
13
14
15
16
17
18
19
20
21
22
23
p-MeO
p-MeO
p-MeO
p-MeO
p-MeO
p-MeO
p-MeO
p-MeO
p-MeO
p-Me
p-Me
p-MeO
p-Me
p-Cl
p-F
m-Me
m-Cl
o-Me
o-Et
H
p-Cl
2s
2t
2u
2v
2w
6
25
15
24
32
a
Reaction conditions: 0.37 mmol of enamine 1aew,
1
mol% of Co2(CO)8
(0.0037 mmol), (R)-BINAP (0.0075 mmol) and PPh3 (0.0075 mmol), ratio ¼ 1/2/2,
In summary, the best results were obtained using Co2(CO)8/(R)-
BINAP/PPh3 (1:2:2) under 450 psi of pressure with 1 mol% of
catalyst loading in tetrahydrofuran for 36 h. Using this optimized
catalytic system, we explored the scope of the hydrogenation re-
10 mL THF, H2/CO (1:3, 450 psi), 120 ꢀC, 36 h.
b
Isolated yield.
c
Determined by HPLC analysis with Daicel chiralcel OD-H and OD columns.
action using various N-aryl
b-enamino esters; the results are
summarized in Table 6. In general, good yields were obtained (up to
93%). The results revealed that the substitution on the aromatic ring
of the substrate has no effect on the reaction yield. Similarly, the
hydrogenation reaction tolerated both electron-donating and
electron-withdrawing groups on the substrate, whereas enantio-
meric excesses were affected for the substituent position. Thus, in
the absence of a substituent on R2, good yields were obtained with
moderate ee% values (entries 1, 13 and 22). However, the existence
of a p-MeO group on R2 presented a slight increase in enantiose-
lectivity (entries 2 and 14), while the reaction proceeded with poor
enantioselectivity in the presence of p-Me (entries 3 and 15).
When the reaction was realized with substrates containing p-Cl
or p-F groups, the desired products resulted but with lower ee%
(entries 4e5). In contrast, p-MeO and p-Me substituents attached to
the benzoylacetate moiety in combination with p-Cl or p-F on the
amine moiety gave the best enantioselectivity (entries 16 and 17
and 23). This increase in enantiomeric excess may be attributed to
the electronic effects associated with the substituent in position R1.
meta Substituents on the amine moiety were also tested; however,
low values of ee% were obtained (entries 6 and 7, 18 and 19).
Notably, substituents on the ortho position of the amine moiety
provided the product with an increased ee% value. The small
improvement in the ee could be associated with both electronic
and steric effects of the substituent at the ortho position, although a
lower ee% was obtained when the substituent was changed from a
methyl to an ethyl group (entries 8e12, and 20 and 21).
of triphenylphosphine (0.1 mmol) to a solution of Co2(CO)8
(0.1 mmol) in THF resulted in the PPh3Co(CO)7 (I) complex (Eq. (1)).
Then, 0.1 mmol of (R)-BINAP was added to a solution of complex I,
resulting in the expected product (II). Compound II is a brown
powder that is thermally stable at atmospheric pressure but de-
composes in solution at 25 ꢀC. Crystals of II were grown by layering
a saturated tetrahydrofuran solution of II with pentene. The 31P
NMR spectrum of (II) crystals showed three signals at
d 29.2, 28.7,
28.4 ppm corresponding to BINAP and PPh3. A few minutes later
two new signals can be observed (ꢁ5.1 ppm PPh3 and ꢁ14.2 ppm
BINAP free). This behavior is associated with decoordination of the
ligands present in molecule II.
(1)
The IR spectrum of II showed four absorptions representing CO
stretching frequencies at 2046, 2028, 1993 and 1934 cmꢁ1, which
could be assigned to COterminal groups. The molecular structure is
shown in Fig. 1. Compound II crystallizes in the triclinic space group
P-1 and displays a dinuclear arrangement in which the phosphine
ligands are observed in anti-position with respect to the CoeCo
bond. This structure reveals two different phosphine ligands and
five terminal carbonyls forming a distorted trigonal bipyramidal
geometry around the Co atoms. The length of the Co(2)e
P(3) ¼ 2.188(17) bond is comparable with different complexes re-
ported in the literature [24]. In addition, the Co(1)eCo(2)e
P(3) ¼ 161.796(6)ꢀ angle indicates a slight deviation from linearity.
Finally, in order to probe the participation of the hetero-
combination from cobalt in the hydrogenation reaction, we decided
to synthesize a non-symmetrical cobalt complex. First, the addition

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M. Amezquita-Valencia, A. Cabrera / Journal of Organometallic Chemistry 768 (2014) 145e150
149
probability of hydride V may be responsible for this increase in
enantioselectivity. These results support the proposal that enan-
tiomeric excess is modulated by the heterocombination between
cobalt/BINAP/PPh3.
Conclusion
In summary, we have demonstrated that Co2(CO)8/(R)-BINAP/
PPh3 is a good catalytic system for the hydrogenation of
b-
enamine esters. The desired amino esters were obtained in good
yields but with low enantioselectivities. Finally, a mixture of two
different ligands in combination with a new cobalt complex
showed that such a heterocombination may improve the enan-
tiomeric excess.
Acknowledgments
Fig. 1. ORTEP diagram showing the molecular structure of [((R)-BINAP)
Co(CO)2eCo(CO)3(PPh3)] (II) [35]. Thermal ellipsoids are shown at the 40% probability
level, and hydrogen atoms are omitted for clarity. Selected bond distances (Å): Co(1)e
P(1), 2.2726(12); Co(1)eP(2), 2.1920(13); Co(2)eP(3), 2.1880(17); Co(1)eCo(2),
2.7170(9); Co(1)eC(21), 1.775(6); Co(1)eC(22), 1.7554(6); Co(1)eC(23), 1.780(7);
Co(1)eC(24), 1.752(8); Co(1)eC(25), 1.760(8). Selected bond angles (ꢀ): P(1)eCo(1)e
P(2), 95.55(5); P(1)eCo(1)eCo(2), 84.79(16); P(2)eCo(1)eCo(2), 156.75(5); P(3)e
Co(2)eCo(1), 161.79(6); C(22)eCo(1)eC(21), 127.5(2); C(24)eCo(2)eC(23), 122.3(3);
C(24)eCo(2)eC(25), 122.9(3); C(25)eCo(2)eC(23), 113.5(3). Molecule II has a residual
disorder of 18% in the two phenyl groups at the PPh3.
We are grateful for the financial support from DGAPA-PAPIIT
(IN203209) and CONACyT (Fellowship 229448). Also, we gratefully
ꢀ
ꢀ
ꢀ
acknowledge A. Toscano, S. Hernandez-Ortega, J. Perez-Flores, L.
~
~
Velasco, Rocio Patino Maya, Ma. Angeles-Pena, Elizabeth Huerta
Salazar, L.C. Marquez and E. García for technical support.
Appendix A. Supplementary material
CCDC 982740 contains the supplementary crystallographic data
for this paper. These data can be obtained free of charge from The
Cambridge Crystallographic Data Centre via www.ccdc.cam.ac.uk/
data_request/cif.
Similarly, there is also a deviation from linearity for angle Co(2)e
Co(1)eP(2) ¼ 156.75(5); this could be attributed to the bulkiness of
the BINAP. The CoeCo bond length was found to be 2.717(9) Å and
is similar to other cobalt complexes [23,24,34].
As mentioned before, the asymmetric induction could be asso-
ciated with heterocombination between Co2(CO)8, PPh3 and BINAP.
In order to examine this hypothesis, we decided to work with
complex II in the asymmetric hydrogenation of substrate 1b. It is
worth mentioning that complex II in the presence of an extra equiv.
of PPh3 and BINAP may generate different species in equilibrium
and it is possible to propose different cobalt hydrides as shown in
Eq. (2), since the coordination ability of the ligands may generate
both homocombination and heterocombination. The structures of
hydrides (III), (IV) and (VI) are suggested based on previously re-
ports in the literature [36e38], we were unable to detect them
experimentally but they must exist in the reaction media and only
specie (V) can be responsible for the ee observed. We observed that
hydride (III) cannot induce ee (vide supra) on the reaction product.
Appendix B. Supplementary data
Supplementary data related to this article can be found at http://
dx.doi.org/10.1016/j.jorganchem.2014.06.028.
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(2)
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