Highly efficient proline ester-based nickel catalysts for Michael addition of thiophenols to α,β-enones

Article abstract of DOI:10.1002/jccs.201200386

Two N₃O₂ pentadentate ligands, BMPP and BPPP, were prepared for synthesizing highly efficient nickel catalysts, [Ni(BMPP)(CH ₃CN)](ClO₄)₂ (1) and [Ni(BPPP)(CH ₃CN)](BPh₄)(ClO₄) (2), for thia-Michael addition of thiophenols to α,β-enones. X-ray structures of 1 and 2 revealed that a labile CH3CN molecule was bound to the nickel center of the catalysts. ESI-MS spectroscopy indicated that thiolate replaced the bound CH3CN molecule and coordinated to the nickel center during the catalytic cycle.

Full text of DOI:10.1002/jccs.201200386

JOURNAL OF THE CHINESE
CHEMICAL SOCIETY
Article
Highly Efficient Proline Ester-based Nickel Catalysts for Michael Addition of
Thiophenols to a,b-Enones
Way-Zen Lee,a* Chien-Wei Chiang,a Girish M. Kulkarnia and Ting-Shen Kuob
aDepartment of Chemistry, National Taiwan Normal University, No. 88, Sec. 4, Ting-Chow Rd, Taipei, Taiwan, 11677,
R.O.C.
bInstrumentation Center, Department of Chemistry, National Taiwan Normal University, No. 88, Sec. 4, Ting-Chow Rd,
Taipei, Taiwan, 11677, R.O.C.
(Received: Jul. 14, 2012; Accepted: Oct. 11, 2012; Published Online: Dec. 17, 2012; DOI: 10.1002/jccs.201200386)
Two N3O2 pentadentate ligands, BMPP and BPPP, were prepared for synthesizing highly efficient nickel
catalysts, [Ni(BMPP)(CH3CN)](ClO4)2 (1) and [Ni(BPPP)(CH3CN)](BPh4)(ClO4) (2), for thia-Michael
addition of thiophenols to a,b-enones. X-ray structures of 1 and 2 revealed that a labile CH3CN molecule
was bound to the nickel center of the catalysts. ESI-MS spectroscopy indicated that thiolate replaced the
bound CH3CN molecule and coordinated to the nickel center during the catalytic cycle.
Keywords: Nickel complexes; Proline derivatives; Pyridine derivatives; Conjugate addition;
Thia-Michael addition.
INTRODUCTION
Transition metallic catalysts have emerged as a pow-
metric conjugate addition of thiols to a 3-(2-alkenoyl)-2-
oxazolidinone.9
erful tool in organic syntheses for past decades. Several or-
ganic synthetic methods, such as aldol reactions,1 Mi-
chael,2 aza-Michael3 and thia-Michael additions,4 are
known for C-C, C-N, and C-S bond formation reactions.
These widely used organic syntheses are examples of atom
efficient reactions. Moreover, such reactions are not only
extensively applied in the synthesis of pharmaceutical and
natural products, but also provide different synthetic routes
for the conception of atom economy which describes the ef-
ficient conversion of chemical reactions involving chemo-,
regio-, diastereo- and enantio-selectivity with minimum
waste.5
RESULTS AND DISCUSSION
Synthesis and characterization
Recently, we have reported a low coordinate nickel
catalyst, LNiCl2 (L = bis(1-methylbenzimidazoyl-2-meth-
yl)-10-camphor-sulfonamide), for thia-Michael addition of
thiophenols to a,b-enones with good yields.4b Moreover,
with the inspiration from the active site of urease, we have
also developed a bio-inspired dinuclear nickel catalyst,
[L’Ni2(CH3CN)(THF)](ClO4)3 (L’ = 2,6-bis(bis(2-pyridyl-
methyl)aminomethyl)benzoate), for thia-Michael addition
in excellent yields.4c In order to develop highly efficient
catalysts for thia-Michael addition of thiophenols to a,b-
enones and to grasp the insight of the intermediate within
the catalytic cycle, two nickel complexes, [Ni(BMPP)-
(CH3CN)](ClO4)2 (1) and [Ni(BPPP)(CH3CN)](ClO4)2 (2),
coordinated by a pentadentate ligand, 2,6-bis(((S)-2-meth-
yloxycarbonyl)-1-pyrrolidinyl)methyl)pyridine (BMPP)
or 2,6-bis(((S)-2-(2-propyloxycarbonyl)-1-pyrrolidinyl)-
methyl)pyridine (BPPP), and a solvent molecule were syn-
thesized by reacting [Ni(CH3CN)6](ClO4)2 with BMPP or
BPPP in CH3CN (Scheme I). The proline ester-based
ligand, BMPP and BPPP, were prepared from the nucleo-
philic substitution of 2,6-bis(bromomethyl)pyridine with
corresponding proline esters.11
Many Michael addition reactions catalyzed by metal
complexes were reported in literature. For instance, Gunnoe
et al. have developed a series of monomeric (NHC)Cu(SR)
(R = Ph, Bn) complexes for catalyzing the addition of vari-
ous S-H bonds over olefins to produce “anti-Markovnikov”
products.6 Among those organometallic catalysts, only a
few nickel complexes have shown the capability to cata-
lyze such addition reactions.7 Shibasaki and co-workers
used a dinickel Schiff base complex for direct catalytic
asymmetric aldol reaction of b-keto esters with formalde-
hyde with excellent yields and ee values.8 Also, Kanemasa
et al. developed a tridentate chiral ligand, 4,6-dibenzo-
furandiyl-2,2’-bis(4-phenyloxazoline), designated as
DBFOX/Ph, and its Ni(II) complex can catalyze the asym-
Single crystals of 1 suitable for X-ray diffraction
* Corresponding author. E-mail: wzlee@ntnu.edu.tw
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Lee et al.
Table 1. Matric data for complexes 1 and 2
Scheme I Synthetic procedure
1
2
Ni(1)-N(1)
Ni(1)-N(2)
Ni(1)-N(3)
Ni(1)-N(4)
Ni(1)-O(1)
Ni(1)-O(3)
N(2)-Ni(1)-N(4)
N(3)-Ni(1)-N(1)
O(1)-Ni(1)-O(3)
2.153(5) Å
1.988(5) Å
2.151(6) Å
2.031(6) Å
2.053(4) Å
2.086(4) Å
178.4(3)°
2.167(3) Å
1.987(3) Å
2.136(4) Å
2.038(4) Å
2.073(2) Å
2.048(2) Å
176.36(15)°
160.22(13)°
177.18(12)°
158.8(2)°
177.4(2)°
test Michael donor to react with 2-cyclohexenone (2 mmol)
as a test Michael acceptor. The reaction was carried out
with only 0.5 mol% loading of catalyst 1 or 2 in dry CH3CN
(2 mL) at ambient temperature for 2 hr. While 1 was em-
ployed as the catalyst, almost quantitative yield of the addi-
tion product was obtained. However, a little decrease in the
yield of the reaction was observed when 2 was applied as
the catalyst for the same reaction. In order to distinguish
the catalytic efficiency between catalysts 1 and 2, we have
used less effective substrate, unsubstituted thiophenol, to
react with 2-cyclohexenone. The considerable difference
was seen in the yields of the products (1 for 83%, Table 2,
entry 3, and 2 for 65%). Meanwhile, the same reaction per-
formed without catalyst 1 or 2 only yielded in 42%. This re-
sult suggested that catalyst 2 is a less efficient catalyst for
the thia-Michael addition due to the sterically hindered
isopropylester group of BPPP.
analysis were obtained by slow diffusion method; whereas,
complex 2 had to undergo the replacement of an anion of
ClO4- to BPh4- to form [Ni(BPPP)(CH3CN)](ClO4)(BPh4)
(2’) for crystallization. The molecular structures of 1 and 2’
exhibited in Fig. 1 and 2 revealed that the Ni(II) center of
both 1 and 2’ possessed an slightly distorted octahedral ge-
ometry with very similar bond lengths and bond angles
around the Ni(II) center (Table 1). Notably, complexes 1
and 2’were bound with a CH3CN molecule, which could be
replaced by a substrate with affinity donors such as N, O or
S. The bound substrate will be activated to facilitate the
catalytic reaction.
The same reaction conditions were applied to the sub-
strates of acyclic a,b-enones and different thiophenols. Re-
action of 3-pentenone with 4-methoxythiophenol gave an
excellent yield (96%, Table 2, entry 5) of the addition prod-
Catalytic thia-Michael addition of a,b-enones
To fulfill our aspiration for atom economy of organic
synthesis, 4-methoxythiophenol (1 mmol) was chosen as a
Fig. 2. Thermal ellipsoid representation of 2’ at 50 %
probability level. Hydrogen atoms, solvent
molecules and counter anions of 2’ are omitted
for clarity.
Fig. 1. Thermal ellipsoid representation of 1 at 50 %
probability level. Hydrogen atoms, solvent
molecules and counter anions of 1 are omitted
for clarity.
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Nickel Catalysts for Thio-Michael Addition
and 7-9). The enantiomeric excess of the reaction products
was examined by a chiral HPLC; however, the ee values of
products were not as good as we expected.
Based on our previous work and this study, we hy-
pothesize that the mechanism for the thia-Michael addition
catalyzed by complex 1 involves the in situ formation of
nickel-thiolate intermediate from the reaction of complex 1
and the added thiol with the assistance of the hydrogen in-
teraction between the enone substrate and the thiol proton.
The thiolate then nucleophilically attacks to the b–double
bond of the protonated enone to form a 1,4-addition enol.
After tautomerisation the thia-Michael addition product
was produced and replaced by the next thiol molecule to
complete the catalytic cycle (Scheme II).
uct. However, with 4-methylthiophenol the yield was de-
creased to 67% (Table 2, entry 6). With unsubstituted thio-
phenol the yield for the reaction adduct was lowered to
64% (Table 2, entry 7). Contrastively, with 4-nitrothiophe-
nol comparatively good yield (80%, Table 2, entry 8) of the
reaction adduct was observed. The higher yield for the case
of 4-methoxythiophenol may be attributed to greater nu-
cleophilicity of the thiol. It is noteworthy that the reaction
of the bulky enone, 5-methyl-3-hexenone, with 4-meth-
oxythiophenol gave a moderately good yield of 77% (Table
2, entry 9). With other thiophenols, 4-methylthiophenol,
4-nitrothiophenol and thiophenol the yields of the products
were decreased to 70%, 53% and 36% (Table 2, entries 10,
12 and 11), respectively. The steric hinderance at the b-car-
bon of the enone will be the major factor for the low yields.
The quantitative yield for the reaction of 2-cyclo-
Plausible mechanism for the Ni-catalytic
thia-1,4-addition reaction
Scheme II
+
O
N
O
R1
R2
N
N
Ni
S
O
R
OMe
H+
MeO
hexenone with 4-methoxythiophenl encouraged us to study
the reactivity of 4-, 3- and 2-methoxythiophenls with 2-cy-
clohexenone, 2-cyclopentenone and 2-cycloheptenone.
Excellent yields for the conjugate addition products were
observed in the reaction of 2-cyclohexenone with 3- and
2-methoxythiophenols (Table 3, entries 5 and 6). Com-
pared to 2-cyclohexenone, reactions of 2-cyclopenenone
and 2-cycloheptenone with 4-, 3- and 2-methoxythiophe-
nols gave lower yields of the products (Table 3, entries 1-3
O
2+
R1
+
R2
N
RSH H+, CH3CN
N
O
O
N
N
N
N
Ni
Ni
O
O
R
N
RSH H+, CH3CN
OMe
S
OMe
MeO
MeO
O
2+
N
R1
R2
O
N
N
Ni
O
SR
S
R
OMe
MeO
RSH
R2
H
O
R1
J. Chin. Chem. Soc. 2013, 60, 245-250
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To evaluate the proposed mechanism, 2-cyclohex-
rated enones at ambient temperature. The advantages of-
fered by catalysts 1 and 2 for thia-Michael addition are that
the reactions can be carried out at a mild condition and
completed within 2 hr. High yields for several conjugate
addition products were observed. More importantly, the
nickel catalyst 1, [Ni(BMPP)(CH3CN)](ClO4)2, is very ef-
ficient, and only 0.5 mol% of catalyst loading is needed for
thia-Michael addition of thiophenols to a,b-enones. Mod-
ification for the supporting ligands, BMPP and BPPP, is
undergoing for better enantioselectivity of addition prod-
ucts.
enone and thiophenol were individually added to the dry
CH3CN solution of 1. The ESI-MS spectrum of the reaction
mixture of excess 2-cyclohexenone and 1 showed that no
additional peak was appeared besides that of [Ni(BMPP)-
(ClO4)]+ (1 - CH3CN - ClO4 , m/z = 518.0, Figure S2).
-
Whereas, addition of excess thiophenol to complex 1 af-
forded a peak at m/z = 527.9, which was expected to be
[Ni(BMPP)(SPh)]+ (Fig. 3a and S3). In addition, we have
prepared a separate sample by adding NaOMe to the dry
CH3CN solution of thiophenol for deprotonation, and sub-
sequently transferring the resulting solution to an equal
molar solution of 1. The color of the reaction mixture was
changed from light purple to deep brown. A dominant peak
at m/z = 527.9 corresponding to [Ni(BMPP)(SPh)]+ was
appeared in the ESI-MS spectrum of the deep brown solu-
tion (Fig. S4). Moreover, addition of excess 2-cyclohex-
enone into the solution of 1 with excess thiophenol resulted
the obviously decay of the peak of [Ni(BMPP)(SPh)]+ in
ESI-MS spectrum (Fig. 3b and S5). This result indicated
that the nickel-thiolate species supported by BMPP or
BPPP was generated with the presence of the enone sub-
strate during the catalytic cycle.
EXPERIMENTAL SECTION
Methods and Materials
All manipulations were performed under nitrogen using
standard Schlenk techniques. Acetonitrile was distilled once from
P2O5 and freshly distilled from CaH2 before use. Diethyl ether
was dried in a sodium benzophenone still prior to use. All other
chemicals were of analytical or spectroscopic grade purchased
from commercial sources and used without further purification.
2,6-Bis[[(S)-2-(isopropyloxycarbonyl)-1-pyrrolidinyl]methyl]-
pyridine (BMPP) was synthesized according to published litera-
ture procedure.10 UV-vis spectra were recorded on Agilent 7453
spectrophotometers. ESI-MS spectra were recorded on a Thermo
Finnigan LCQ Advantage spectrometer. Elemental analyses for
C, H, and N were performed on a Perkin-Elmer 2400 analyzer at
the NSC Regional Instrumental Center at National Taiwan Uni-
versity, Taipei, Taiwan.
CONCLUSIONS
In conclusion, two five-coordinate proline ester-
based ligands, BMPP and BPPP, were prepared to synthe-
size nickel catalysts 1 and 2, which were coordinated by a
labile CH3CN molecule. Both complexes are efficient cata-
lysts for thia-Michael addition of thiophnols to a,b-unsatu-
X-ray Structure Determination
Crystals of suitable size for X-ray diffractometer were se-
lected under a microscope and mounted on the tip of a glass fiber,
which was positioned on a copper pin. The X-ray data for com-
plexes 1 and 2 were collected on a Bruker-Nonius or a Bruker
Kappa CCD diffractometer, respectively, employing graph-
ite-monochromated Mo KR radiation at 200 K and the q-2q scan
mode. The space groups for all complexes were determined on the
basis of systematic absences and intensity statistics, and the struc-
tures of 1 and 2 were solved by direct methods using SIR92 or
SIR97 and refined with SHELXL-97. An empirical absorption
correction by multiscans was applied to both structures. All
non-hydrogen atoms were refined with anisotropic displacement
factors. Hydrogen atoms were placed in ideal positions and fixed
with relative isotropic displacement parameters. Selected crystal-
lographic data for 1 and 2 are given in Table 4. Detailed crystallo-
graphic information for complexes 1 and 2 is provided in the Sup-
porting Information (CIF).
Fig. 3. The ESI-MS spectra of (a) complex 1 with ex-
cess thiophenol in CH3CN and (b) the former
solution with the addition of excess 2-cyclo-
hexenone.
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Nickel Catalysts for Thio-Michael Addition
Table 4. Selected X-ray crystallographic data for complexes 1
and 2’
66.16; H, 8.45; N, 10.06. Found: C, 65.96; H, 8.45; N, 10.80.
Synthesis of [(BMPP)Ni(CH3CN)](ClO4)2·H2O (1)
CH3CN (20 mL) was added to a mixture of BMPP (0.3615
g, 1.0 mmol) and [Ni(CH3CN)6](ClO4)2 (0.5039 g, 1.0 mmol) in a
100 mL Schlenk flask to give a light purple solution. The resulting
solution was stirred at room temperature for 2 h, and subsequently
evaporated under vacuum and washed by diethyl ether (3 × 50
mL). The solvent of the purple residue was removed under vac-
uum, and then the residue was disolved in CH3CN (10 mL). The
resulting CH3CN solution was filtered, and the filtrate was
recrystallized by slow diffusion of diethyl ether into the CH3CN
solution at 4 °C. Purple crystals of 1 were obtained over three
days (0.5225 g, 79%). UV-vis (CH3CN; lmax, nm (e, M-1cm-1)):
330 (70), 530 (6), 820 (26). Anal. Calcd for NiC21H32N4Cl2O13: C,
37.20; H, 4.76; N, 8.26. Found: C, 37.36; H, 4.59; N, 8.40.
Synthesis of [(BPPP)Ni(CH3CN)](ClO4)2 (2) and
[(BPPP)Ni(CH3CN)](ClO4)(BPh4) (2’)
1
2’
empirical formula C21H30Cl2N4NiO12
C49H58BClN4NiO8
935.96
fw
660.10
T (K)
200(2)
200(2)
wavelength (Å)
cryst syst
space group
a (Å)
b (Å)
c (Å)
0.71073
Monoclinic
P21
9.2086(2)
13.9316(3)
11.8492(3)
90
111.6070(10)
90
1413.32(6)
2
0.71073
Monoclinic
P21
13.4364(12)
11.2713(10)
15.7702(14)
90
100.450(2)
90
2348.7(4)
2
a (deg)
b (deg)
g (deg)
V (Å3)
Z
Dcalcd (g cm-3)
m (mm-1)
cryst size (mm)
goodness of fit
R1
1.551
0.942
0.38 ´ 0.3 ´ 0.2
1.068
0.0603
1.323
0.527
0.40 ´ 0.34 ´ 0.09
0.993
0.0436
CH3CN (10 mL) was added to a mixture of BPPP (0.0835 g,
0.2 mmol) and [Ni(CH3CN)6](ClO4)2 (0.1008 g, 0.2 mmol) in a 50
mL Schlenk flask to give a light purple solution of 2. The resulting
solution was stirred at room temperature for 1 h, and an CH3CN
solution of NaBPh4 (0.0342 g, 0.1 mmol) was added to the reac-
tion solution for metathesis. The resulting solution was evapo-
rated under vacuum, and the residue was dissolved in CH2Cl2 (15
mL). The resulting solution was filtered, and the purple filtrate
was evaporated under vacuum and washed by diethyl ether (3 ×
20 mL). The solvent of the residue was removed under vacuum,
and CH3CN (4 mL) was then added. The CH3CN solution was fil-
tered, and the filtrate was recrystallized by slow diffusion of di-
ethyl ether into the CH3CN solution at 4 °C. Purple-blue crystals
of 2’ were obtained over three days (0.1216 g, 65%). UV-vis
(CH3CN; lmax, nm (e, M-1cm-1)): 330 (82), 540 (25), 820 (34)
(same for both 2 and 2’). Anal. Calcd for NiC49H58N4ClBO8 (2’):
C, 62.88; H, 6.25; N, 5.99. Found: C, 62.23; H, 5.80; N, 5.93.
General Procedure for the Conjugate Addition Cata-
lyzed by Complexes 1 and 2’
wR2
0.1738
0.1013
Synthesis of 2,6-Bis[[(S)-2-(isopropyloxycarbonyl)-1-
pyrrolidinyl]methyl]pyridine (BPPP)
2,6-Bis(bromomethyl)pyridine (3.72 mmol, 0.9769 g) was
placed in a 250 mL round-bottomed flask equipped with a mag-
netic stirring bar under N2 and dissolved in dry THF (100 mL).
NEt3 (26 mmol, 3.65 mL) was added to the reaction mixture at 0
°C and the mixture was stirred at same temperature for 30 min. A
THF solution of L-proline isopropyl ester (7.8 mmol, 1.5086 g)
was then added to the reaction solution and stirred at room tem-
perature for 24 h. The resulting solution was dried under reduced
pressure, and then extracted with water (100 mL) and CH2Cl2 (3 ×
250 mL). The combined organic phase was washed by brine, and
was dried over anhydrous MgSO4 and then filtered, and the
volatiles were removed under reduced pressure to give a pale yel-
low oil. The remaining crude oil was purified by column chroma-
tography on silica gel (hexane/EtOAc/NEt3 = 4/1/0.05). The col-
orless oil product of BPPP was obtained in 60% yield (0.8983 g).
1H-NMR (400 MHz, CDCl3): d 7.58 (t, J = 7.68 Hz, 1H), 7.35 (d, J
= 7.68 Hz, 2H), 5.01 (m, J = 6.28 Hz, 2H), 4.06-4.03 (d, J = 13.84
Hz, 2H), 3.72-3.68 (d, J = 13.84 Hz, 2H), 3.34-3.30 (dd, J = 8.96
Hz, J = 5.52 Hz, 2H), 3.07-3.05 (m, 2H), 2.50-2.47 (q, J = 8.96
Hz, 2H), 2.11 (m, 2H), 1.92 (m, 4H), 1.88 (m, 2H), 1.23-1.20 (dd,
J = 6.24 Hz, J = 3.08 Hz, 12H) ppm. 13C-NMR (100 MHz,
CDCl3): d 173.64, 158.35, 136.73, 121.28, 67.70, 65.50, 60.08,
53.28, 29.25, 23.27, 21.81 ppm. Anal. Calcd for C23H35N3O4: C,
To a light purple acetonitrile solution (2 mL) of catalyst 1 or
2’ (0.5 mol%), a,b-enone (2.0 mmol) was added under a nitrogen
atmosphere with stirring for 5 min. As a thiol (1.0 mmol) was
added to the reaction mixture, the color of the solution became
brown. The resulting solution was stirred at room temperature for
2 h, and the solvent was evaporated under vacuum. The residue
was then purified by column chromatography on silica gel (hex-
ane/EtOAc = 10/1) to give the conjugate adduct.
ACKNOWLEDGEMENTS
The authors gratefully acknowledge the financial
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support from the National Science Council of Taiwan
(NSC 98-2119-M-003-002-MY3 to W.-Z. L.) and National
Taiwan Normal University (101T3030B3 and 101J1A3103).
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