New C2-symmetric chiral tetraaza ligands and their application for [Ru(p -cymene)Cl2]2-catalyzed asymmetric transfer hydrogenation

Article abstract of DOI:10.1080/15533174.2011.613084

Two new chiral ligands (S, S, S, S)-N, N'-bis(2-p-toluene- sulfonylaminocyclohexyl) ethylenediamine (2a) and (S, S, S, S)-N, N'-bis(2-p-toluenesulfonylaminocyclohexyl) trimethylenediamine (2b) were prepared and confirmed by means of elemental analysis, IR

Full text of DOI:10.1080/15533174.2011.613084

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New C2-Symmetric Chiral Tetraaza Ligands and
Their Application for [Ru(p-cymene)Cl2]2-Catalyzed
Asymmetric Transfer Hydrogenation
Yiling Hong a , Huajie Tan a , Jin Qiu a & Liang Shen a
a College of Material Chemistry and Chemical Engineering, Hangzhou Normal University ,
Hangzhou , P. R. China
Accepted author version posted online: 14 Mar 2012.Published online: 01 May 2012.
To cite this article: Yiling Hong , Huajie Tan , Jin Qiu & Liang Shen (2012) New C2-Symmetric Chiral Tetraaza Ligands and
Their Application for [Ru(p-cymene)Cl2]2-Catalyzed Asymmetric Transfer Hydrogenation, Synthesis and Reactivity in Inorganic,
Metal-Organic, and Nano-Metal Chemistry, 42:4, 502-506, DOI: 10.1080/15533174.2011.613084
To link to this article: http://dx.doi.org/10.1080/15533174.2011.613084
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Synthesis and Reactivity in Inorganic, Metal-Organic, and Nano-Metal Chemistry, 42:502–506, 2012
Copyright ꢀ Taylor & Francis Group, LLC
C
ISSN: 1553-3174 print / 1553-3182 online
DOI: 10.1080/15533174.2011.613084
New C2-Symmetric Chiral Tetraaza Ligands and Their
Application for [Ru(p-cymene)Cl2]2-Catalyzed Asymmetric
Transfer Hydrogenation
Yiling Hong, Huajie Tan, Jin Qiu, and Liang Shen
College of Material Chemistry and Chemical Engineering, Hangzhou Normal University, Hangzhou,
P. R. China
symmetric 1, 2-cyclohexanediamine have been proven to be
Two new chiral ligands (S, S, S, S)-N, Nꢁ-bis(2-p-toluene-
sulfonylaminocyclohexyl) ethylenediamine (2a) and (S, S, S, S)-N,
Nꢁ-bis(2-p-toluenesulfonylaminocyclohexyl) trimethylenediamine
(2b) were prepared and confirmed by means of elemental analysis,
popular chiral building blocks for the synthesis of chiral aux-
iliaries and chiral reagents, chiral sensors, and chiral deriva-
tives or solvating agents.[10–12] In this work, we report on the
preparation of two C2-symmetric chiral tetraaza ligands, (S, S,
S, S)-N, Nꢀ-bis(2-p-toluenesulfonylaminocyclohexyl) ethylene-
diamine (2a) and (S, S, S, S)-N, Nꢀ-bis(2-p-toluenesulfonyl-
aminocyclohexyl) trimethylenediamine (2b), which are used in
the Ru-catalyzed asymmetric transfer hydrogenation of ace-
tophene, giving the corresponding optically active secondary
alcohols in good conversion (87–98%) and low enantiometric
excess (4–27%).
1
IR, H NMR, and MS. The catalytic effect for asymmetric trans-
fer hydrogenation of acetophenone has been investigated using
[Ru(p-cymene)Cl2]2 and above C2-symmetric chiral tetraaza lig-
ands in 2-propanol/KOH. The chiral 1-phenylethanol was obtained
with good conversion (87–98%) and low enantiometric excess
(4–27%).
Keywords acetophene, asymmetric transfer hydrogenation, chiral
tetraaza ligands, [Ru(p-cymene)Cl2]2
EXPERIMENTAL
INTRODUCTION
Optically active secondary alcohols are important intermedi- Materials and Methods
ates for the construction of many commercially attractive phar-
maceuticals, agrochemicals, fragrances, flavors and specialty
materials.[1–2] Asymmetric transfer hydrogenation (ATH) of ke-
tones is now realized to be one of the most powerful, practi-
cal, and versatile tools to access chiral secondary alcohols and
amines in organic synthesis because of its excellent selectivity,
operational simplicity, and wide substrate scope.[3–5]
Chiral ligands play a key role in ATH to fulfill the high cat-
alytic activity and enantioselectivity. Recently, chiral tetraaza
ligands have received much attention not only for their simple
synthesis from easily available inexpensive precursors, but also
for their more stabile in air than phosphine ligands.[6–8] More-
over, they may provide multicoordination sites on the metal
center and can be easily modified to tune the steric and elec-
tronic properties of the resulting metal complex.[9] The C2-
All the catalysis reactions were carried out under an atmo-
sphere of dried nitrogen in oven-dried glassware with mag-
netic stirring. Acetophenone and 2-propanol were purified by
standard methods. [Ru(p-cymene)Cl2]2 was purchased from
Alfa Aesar (Ward Hill, Massachusetts, UK) and used as re-
ceived without further purification. Thin-layer chromatogra-
phy was performed on 0.25 mm silica gel plates. Visu-
alization was accomplished with UV light. Melting points
were measured with a X-5 micromelting point apparatus and
are uncorrected. Optical rotations were determined with a
PerkinElmer (Waltham, MA, USA) 341 polarimeter with a
20
sodium lamp and are reported as follows: [α]589 = 100α/
lc(c = g/100mL, solvent, l = 1 dm). Infrared (IR) spectra were
measured on a Bruker (Madison, WI, USA) Tensor 27 spec-
1
trometer. The H NMR spectra were recorded on a 400 MHz
Bruker spectrometer and reported in ppm using tetramethylsi-
lane as internal standard. Mass spectra were measured with an
Agilent (Santa Clara, CA, USA) 5979 spectrometer. Analytical
gas chromatography was performed on a Shimadzu (Nakagyo-
ku, Kyoto, Japan) GC-2014. Analytical high-performance liquid
chromatography was performed on an Elite (Dalian, Liaoning,
China) P230 using a Daicel (Kita-ku, Osaka, Japan) Chiralcel
OZ-H column (4.6 mm × 0.25 m).
Received 7 July 2011; accepted 7 August 2011.
The authors express their gratitude to the Zhejiang Provincial Nat-
ural Science Foundation of China for financial Support through Project
No. Y4090056.
Address correspondence to Liang Shen, College of Material
Chemistry and Chemical Engineering, Hangzhou Normal University,
Hangzhou 310016, P. R. China. E-mail: shenchem@hotmail.com
502

ASYMMETRIC TRANSFER HYDROGENATION OF ACETOPHENONE
503
2862.04 ( CH ), 1599.24, 1453.88 (C C of benzene),
1327.07 ( SO2 ), 1161.36, 1090.26 (C N of secondary
amine), 817.43. 1H NMR(CDCl3, 400 MHz), δ (ppm): 7.815,
7.794, 7.778, 7.757[d, 4H, J = 8.4 Hz, J = 8.4 Hz, CH of
phenyl], 7.314, 7.294[d, 4H, J = 8.0 Hz, CH of phenyl],
3.008–2.986[m, 2H], 2.796–2.787[m, 2H], 2.631, 2.621, 2.606,
2.596, 2.581, 2.570[dt, 2H, J = 4.0 Hz, CH of cylcohexane],
2.422[s, 6H, CH3], 2.352, 2.342, 2.325, 2.316, 2.300, 2.290[dt,
2H J = 4.0 Hz, CH of cyclohexane], 1.917–1.777[m, 4H,
Procedure for Preparation of the Chiral Ligands
Preparation of (1S, 2S)-(+)-N-p-toluenesulfonyl-1,
2-cyclohexanediamine(TsCYDN)
TsCYDN was prepared according to literatures.[13,14] Pale
yellow solid, mp: 107.2–108.0◦C; [α]589 = +64.9◦(c 0.4,
20
CH2Cl2); Selected IR, υ(cm–1): 3348.74, 3286.53 (N H),
2942.12 ( CH3), 2921.42 ( CH2 ), 2856.24 ( CH ),
1594.04, 1493.59, 1446.91 (C C of benzene), 1325.17
( SO2 ), 1161.92 (C N of primary amine), 1115.11,
1093.50 (C N of secondary amine), 812.44. 1H NMR
(CDCl3, 400 MHz), δ (ppm): 7.772, 7.751[d, 2H, J = 8.4 Hz,
CH of phenyl], 7.286, 7.305[d, 2H, J = 8.4 Hz, CH of
phenyl], 2.646–2.585[dt, 1H, CH ], 2.419[s, 3H, CH3],
2.352–2.292[dt, 1H, CH ], 1.196–1.792 [m, 2H, CH2 ],
1.641–1.591[m, 2H, CH2 ], 1.132–1.061[m, 4H, CH2 ].
MS(EI): Anal. Calcd. for C13H21SO2N2 [M+H]+: 269.388;
Found: 269.105.
CH2- of cyclohexane], 1.695–1.591[m, 4H,
CH2- of
cyclohexane], 1.534, 1.506, 1.454, 1.426[dd, 2H, J = 11.2 Hz]
1.196–1.025[m, 8H, CH2- of cyclohexane]. MS(EI): Anal.
Calcd. for C29H45S2O4N4 [M+H]+: 577.830; Found: 577.769.
Typical Procedure for Asymmetric Transfer
Hydrogenation
A
mixture of metal precursor [Ru(p-cymene)Cl2]2
(0.02mmol) and the chiral ligand 2a or 2b(0.042mmol) in freshly
distilled 2-propanol was stirred at 80◦C for 2–3 h under nitrogen
atmosphere. A solution of potassium hydroxide in 2-propanol
was added and the reaction mixture was stirred for another
2–3 h. Then the acetophenone (4 mmol) was added, and the
reduction was conducted at given temperature for the time indi-
cated (monitored by TCL).[15] After completion of the reaction,
the reaction was quenched with saturated aqueous solution of
NaCl, neutralized with solution of HCl and extracted with Et2O
(3 × 10 mL). The organic phase were combined, dried over
anhydrous Na2SO4, filtered, concentrated under reduced pres-
sure, and purified by flash chromatography on silica gel.[16,17]
The enantiometric excess (ee) was determined by HPLC and the
chemical conversation was determined by GC.[18,19]
Preparation of (S, S, S, S)-N, Nꢀ-bis(2-p-
toluenesulfonylaminocyclohexyl)ethylenediamine (2a)
(1S, 2S)-TsCYDN (0.2680 g, 1.0 mmol) and 1, 2-
dibromoethane (0.0940 g, 0.5 mmol) were placed in a glass
sealed vessel and heated at 110◦C for 28 h. The crude product
was dissolved in dichloromethane (30 mL) and washed with
a 20% NaOH solution (20 mL). The compound was extracted
with dichloromethane (3 × 20 mL), dried over Na2SO4, filtered,
and concentrated under reduced pressure. The crude product,
which consisted of the tetraaza ligand and unreacted TsCYDN,
was purified by preparative thin layer chromatogram. Filemot
solid, mp: 70.0–71.8◦C; [α]589 = +45.3◦(c 0.4, CH2Cl2);
20
Selected IR, υ(cm–1): 3266.85 (N H), 2932.68 ( CH2 ),
2858.48 ( CH ), 1598.70, 1451.07 (C C of benzene),
1326.17 ( SO2 ), 1161.77, 1092.34 (C N of secondary
1
amine), 816.26. H NMR(CDCl3, 400 MHz), δ (ppm): 7.823,
RESULTS AND DISCUSSION
7.803 [d, 4H, J = 8.0 Hz, CH of phenyl], 7.302, 7.282
[d, 4H, J = 8.0 Hz, CH of phenyl], 3.072–3.055 [m, 4H],
2.649–2.541 [m, 4H, CH of cyclohexane], 2.419 [s, 6H,
CH3], 2.082 [m, 2H, CH2- of cyclohexane], 1.689–1.589
[m, 6H, CH2- of cyclohexane], 1.209–1.173 [m, 8H, CH2-
of cyclohexane]. MS(EI): Anal. Calcd. for C28H43S2O4N4
[M+H]+: 563.804; Found: 563.589.
Synthesis of Chiral Ligands
The general approach to synthesize C2-symmetric chiral
tetraaza ligands is outlined in Scheme 1. In the first step, an ex-
cess of the 1,2-diaminocyclohexane must be used and the reac-
tion was carried out below 0◦C. Otherwise it was easy to produce
byproducts. Ligands 2a and 2b were prepared in free-solvent and
homogeneous systems. The reaction is strongly influenced by
the temperature. When TsCYDN and 1, 2-dibromoethane, or 1,
3-dibromopropane stay at homogeneous, the reactions proceed
smoothly at 110◦C.[20] If not, it is difficult to separate tetraaza
ligands from the crude product. We adopt preparative thin layer
chromatogram to purify the tetraaza ligands.
Preparation of (S, S, S, S)-N, Nꢀ-bis(2-p-
toluenesulfonylaminocyclohexyl)trimethylenediamine (2b)
(1S, 2S)-TsCYDN (0.2680 g, 1.0 mmol) and 1, 3-
dibromopropane (0.1010 g, 0.5 mmol) were placed in a glass
sealed vessel and heated at 110◦C for 28 h. The crude product
was dissolved in dichloromethane (30 mL) and washed with
a 20% NaOH solution (20 mL). The compound was extracted
with dichloromethane (3 × 20 mL), dried over Na2SO4, filtered,
Asymmetric Transfer Hydrogenation of Acetophenone
The catalyst was generated in situ by refluxing ligands 2a
and concentrated under reduced pressure. The crude product, and 2b with [RuCl2(p-cymene)]2 (2.2:1) in 2-propanol and used
which consisted of the tetraaza ligand and unreacted TsCYDN, in the asymmetric transfer hydrogenation of acetophenone with
was purified by preparative thin-layer chromatogram. Filemot 2-propanol as the hydrogen source and KOH as a promoter at
solid, mp: 111.0–112.8◦C; [α]589 = +47.6◦(c 0.4, CH2Cl2); 80◦C under nitrogen atmosphere.[21] A comparison of Ru(II)-
20
Selected IR, υ (cm–1): 3405.48 (N H), 2935.13 ( CH2 ), ligand 2a/2b catalysts for the asymmetric transfer hydrogenation

504
Y. HONG ET AL.
NH2
NH2
O
H3C
S
Cl
O
NEt3/CH2Cl2
NH2
NH
HN
HN
Br
NH
HN
HN
Br
Br
Br
110¡ æ
NH
Ts
110¡ æ
NH
Ts
NH
Ts
Ts
Ts
2a
1 TsCYDN
2b
SCH. 1. Synthesis of the C2-asymmetric chiral tetraaza ligands.
of acetophenone by 2-propanol in the presence of KOH is sum- the absolute configuration of the 1-phenylethanol was highly
marized in Table 1.
dependent on that of chiral ligands.
Based on the results, (R) enantiomer of the 1-phenylethanol
The chiral ligands (2a and 2b) exhibited good catalytic activ-
was obtained when the chiral ligand was (S) enantiomer un- ity similar to chiral diamine TsCYDN ligand, but lower enan-
der the experiment conditions. As noted by Gao et al.,[7,22–24] tioselectivity than TsCYDN.[1,25] This was due to the presence of
TABLE 1
A comparison of Ru(II)-ligand 2a/2b catalysts for the asymmetric transfer hydrogenation of acetophenone by 2-propanol in the
presence of KOH
OH
O
[Ru(p-cymene)Cl2]2/2a-2b
CH3
CH3
iPrOH, KOH
NH
HN
NH
HN
HN
HN
Ts
NH
Ts
NH
Ts
Ts
2a
2b
Sr. no.
Ligand
S/M/L/base
Temperature (◦C)
Conv. (%)
ee (%)
T (h)
Configuration
1
2
3
4
5
6
7
8
9
2a
2a
2a
2a
2b
2b
2b
2b
2b
2b
100:1:1.1:5
80
80
80
80
80
80
80
80
80
80
87.37
98.31
95.45
93.68
90.42
97.39
94.14
93.93
88.86
86.51
6.77
7.9
4.2
racemic
26.8
13.12
7.4
4.24
3.6
18
18
18
18
18
18
18
18
18
18
R
R
R
–
R
R
R
R
R
R
100:1:1.1:10
100:1:1.1:25
100:1:1.1:40
100:1:1.1:5
100:1:1.1:10
100:1:1.1:25
100:1:1.1:40
100:1:1.1:100
100:1:1.1:150
10
3.72
Reaction conditions: catalyst, 2.0 × 10–5 mol; Ligand, 4.4 × 10–5 mol; Acetophenone, 4.0 × 10–3 mol; IPA, 25 mL. Conversion was determined
by GC; R configuration was confirmed by HPLC. S = mubstrate, M = metal, L = ligand, base = KOH.

ASYMMETRIC TRANSFER HYDROGENATION OF ACETOPHENONE
505
Cl
NH
HN
HN
Ru
NH
Ts
Ts
Cl
1
-HCl
KOH
OH
CH
OH
CH
HN
NH
H C
CH
3
Ph
CH
3
3
Ru
N
NH
Ts
CH
Ts
H
H C
3
3
Ph
Cl
O
C
CH
C
2
3
H
O
H
HN
NH
N
NH
Ru
Ru
HN
NH
Ts
HN
NH
Ts
Ts
Ts
Cl
Cl
5
3
H
HN
HN
NH
Ru
O
O
C
NH
Ts
C
Ts
CH
H C
Ph
CH
Cl
3
3
3
4
SCH. 2. Mechanistic proposal of ATH of acetophenone catalyzed by chiral tetraaza Ru complexes.
the C-C linker in ligands which could increase the flexibility of
Mechanism
Refer to Noyori’s catalysis mechanism,[27], Scheme 2 is pre-
sumed to illustrate the pathway of ATH of acetophenone with
2-propanol catalyzed by 18-electron chiral tetraaza Ru chloride
1 and KOH as base. The strong base is required for the irre-
versible elimination of HCl from 1 forming the 16e Ru amide
complex 2. Dehydrogenation of 2-propanol by 2 and 3 gives
the 18-electron Ru hydride 4, which hydrogenates acetophe-
none forming chiral 1-phenylethanol with recovery of 2. These
ketone/alcohol redox processes occur via a Ru-H-C-O-H-N six-
membered transition structure as shown in 5. The Ru center and
nitrogen ligand simultaneously participate in both forward and
reverse steps of the hydrogenation transfer.
the backbone and consequently decrease the enantioselectivity
of the catalyst.[8]
Table 1 shows that the results using chiral ligand 2b gave
higher enantioselectivities than ligand 2a. We reason that mov-
ing away the active center by a linker from 1, 2-ethylene to 1,
3-trimethylene will enhance the enantiometric excess.
It is well known that the catalyst-base ratio is essential to
activate the catalysts for hydrogen transfer.[26] The enantiomeric
purity were significantly affected by the amount of base. A
lower amount of base gives a higher ee of alcohol with good
conversation. Furthermore, with the amount of base increased,
the ee value was lower, and the conversation decreased slowly.

506
Y. HONG ET AL.
12. Canivet, J.; Suss-Fink, G. Water-soluble arene ruthenium catalysts con-
CONCLUSION
taining sulfonated diamine ligands for asymmetric transfer hydrogenation
of α-aryl ketones and imines in aqueous solution. Green. Chem. 2007, 9,
391–397.
In conclusion, the catalytic effect for ATH of acetophe-
none using [Ru(p-cymene)Cl2]2 and novel C2-symmetric chiral
tetraaza ligands (2a and 2b) in 2-propanol/KOH has been inves-
tigated. Only low enantioselectivity was observed even though
the good catalytic activity of chiral ligands was exhibited. This
can be attributed to (a) the presence of the C-C linker in ligand
moiety substantially increase the flexibility of the backbone and
consequently decrease the enantioselectivity, (b) moving away
the active center by a linker will enhance the reactivity, and (c)
the amount of base remarkably affects the enantioselectivity.
These results provide a useful insight for the design of more
efficient C2-symmetric chiral tetraaza ligands.
13. Ng, K.; Somanathan, R.; Walsh, P. J. Synthesis of homochiral pentadentate
sulfonamide-based ligands. Tetrahedron: Asymmetry 2001, 12, 1719–1722.
14. Cortez, N. A.; Aguirre, G.; Hake, M. P.; Somanathan, R. Water-soluble
chiral monosulfonamide cyclohexane-1,2-diamine-RhCp∗ complex and its
application in the asymmetric transfer hydrogenation (ATH) of ketones.
Tetrahedron Lett. 2007, 48, 4335–4338.
15. He, W.; Liu, P.; Zhang, B. L.; Sun, X. L.; Zhang, S. Y. Efficient iridium and
rhodium-catalyzed asymmetric transfer hydrogenation using 9-amino(9-
deoxy)cinchona alkaloids as chiral ligands. Appl. Organomet. Chem. 2006,
20, 328–334.
16. Murata, K.; Ikariya, T.; Noyori, R. New chiral rhodium and iridium com-
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of aromatic ketones. J. Org. Chem. 1999, 64, 2186–2187.
17. Martins, J. E. D.; Clarkson, G. J.; Wills, M. Ru (II) Complexes of N-
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Ketones and Imines. Organic Lett. 2009, 11(4), 847–850.
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