Iron-catalyzed highly enantioselective reduction of aromatic ketones with chiral P2N4-type macrocycles

Article abstract of DOI:10.1002/adsc.201100733

Novel P₂N₄-donors containing chiral 22-membered macrocyclic ligands have been synthesized and the structures have been determined by an X-ray diffraction study. The catalytic systems in situ generated from triiron dodecarbonyl, Fe₃(CO)₁₂, and the chiral macrocyclic ligand exhibited high activity (TOF up to 1940 h ^-1) and excellent enantioselectivity with up to 99% ee in the asymmetric transfer hydrogenation of various aromatic ketones. Copyright

Full text of DOI:10.1002/adsc.201100733

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DOI: 10.1002/adsc.201100733
Iron-Catalyzed Highly Enantioselective Reduction of Aromatic
Ketones with Chiral P2N4-Type Macrocycles
Shenluan Yu,a Weiyi Shen,a Yanyun Li,a,* Zhenrong Dong,a Yaqing Xu,a Qi Li,a
Juanni Zhang,a and Jingxing Gaoa,*
a
State Key Laboratory of Physical Chemistry of Solid Surfaces and National Engineering Laboratory for Green Chemical
Productions of Alcohols, Ethers and Esters, College of Chemistry and Chemical Engineering, Xiamen University, Xiamen
361005, Peopleꢀs Republic of China
Fax : (+86)-592-218-3047; phone: (+86)-592-218-0116; e-mail: yanyunli@xmu.edu.cn or jxgao@xmu.edu.cn
Received: September 20, 2011; Revised: November 19, 2011; Published online: February 28, 2012
Supporting information for this article is available on the WWW under
http://dx.doi.org/10.1002/adsc.201100733.
Abstract: Novel P2N4-donors containing chiral 22-
membered macrocyclic ligands have been synthe-
sized and the structures have been determined by
an X-ray diffraction study. The catalytic systems in
situ generated from triiron dodecarbonyl,
Fe3(CO)12, and the chiral macrocyclic ligand exhib-
ited high activity (TOF up to 1940 hÀ1) and excel-
lent enantioselectivity with up to 99% ee in the
asymmetric transfer hydrogenation of various aro-
matic ketones.
Keywords: asymmetric catalysis; iron; ketones;
macrocycle ligands; transfer hydrogenation
Optically active alcohols are very important inter-
mediates, which are widely used in the pharmaceuti-
cals, agrochemicals, fragrances, and flavors industries.
Scheme 1. The structures of chiral ligands L1–L4.
The asymmetric reduction of prochiral ketones is one attracted more and more attention.[5] In 2004, we first-
of the most efficient and practical methods to attain ly reported the asymmetric transfer hydrogenation
chiral alcohols.[1] In the course of enantioselective cat- (ATH) of aromatic ketones with the catalysts in situ
alytic reactions, chiral ligands play a key role in asym- generated from iron complexes and chiral PNNP li-
metric induction. In 1996, we developed a series of gands.[3c] From then on, Morris et al. have applied the
chiral open-chain PNNP-type ligands (L1 and L2, chiral PNNP-Fe complexes to the asymmetric hydro-
Scheme 1),[2] which have been widely applied by other genation of ketones with excellent activity.[3i,j] Bellerꢀs
researchers and us in the asymmetric reduction of ke- group also reported the enantioselective reduction of
tones and imines, oxidative kinetic resolution of race- imines catalyzed by Fe-based catalysts containing
mic alcohols, asymmetric epoxidation of alkenes, PNNP ligands.[3k] In continuation of our study on
asymmetric cyclopropanation of alkenes and so on.[3] chiral aminophosphine ligands and related catalysis,
These results showed that the chiral open-chain herein we report for the first time the synthesis of
PNNP-type ligands are versatile. Generally, the most novel chiral 22-membered macrocycle ligands and
efficient catalysts for the asymmetric reduction of ke- their application in the highly enantioselective, iron-
tones are based on precious metals such as ruthenium, catalyzed reduction of aromatic ketones.
rhodium and iridium.[4] However, due to the low cost,
easy availability and safety, chiral iron catalysts have 22-membered macrocycles L3 and L4 (Scheme 1) is
The synthesis of the P2N4-donor containing chiral
818
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Adv. Synth. Catal. 2012, 354, 818 – 822

Iron-Catalyzed Highly Enantioselective Reduction of Aromatic Ketones
Scheme 2. Synthesis of chiral macrocycles L3 and L4.
outlined in Scheme 2. In the presence of anhydrous
Na2SO4, L3 was synthesized by cyclocondensation of
bis(o-formylphenyl)phenylphosphane and (R,R)-1,2-
diaminocyclohexane in chloroform at 508C (yield:
75%). The IR spectrum of L3 exhibited a strong C=N
stretch at 1648 cmÀ1. The 31P NMR spectrum of the
imine in CDCl3 solution showed two singlets (d=
À15.3 and À22.2 ppm). These may be due to the two
isomers in the solution. Crystals of L3 suitable for X-
Figure 2. X-ray structure of chiral macrocycle L4.
ray diffraction were grown from CHCl3/hexane
(Figure 1).[6]
Having synthesized the macrocycle ligands L3 and
L4, we investigated the iron-catalyzed enantioselec-
tive reduction of ketones with these chiral 22-mem-
bered macrocycle ligands.
Initially, we chose readily available propiophenone
as a model substrate and investigated the ATH with
the catalysts in situ generated from a variety of iron
complexes and P, N-containing ligands L1–L4
(Table 1). Of these ligands, macrocycle L4 exhibited
excellent enantioselectivity when combined with
Table 1. Screening of the ATH on propiophenone with vari-
ous iron sources.[a]
Entry Iron source
Ligand Conversion[b] ee[b]
[%]
[%]
1
2
3
Fe3(CO)12
Fe3(CO)12
Fe3(CO)12
Fe3(CO)12
Fe3(CO)12
L1
L2
L3
L4
L4
L4
L4
54
10
22
93
98
<1
7
30
44
29
98
98
–
74
–
–
Figure 1. X-ray structure of chiral macrocycle L3.
4
5[c]
6
AHCTUNGTRENNUNG
Reduction of diimino L3 with excess NaBH4 was
carried out in ethanol, affording the corresponding di-
amino L4 in 80% yield. The 31P NMR spectrum of L4
showed a singlet at d=À27.1 ppm in accordance with
two equivalent phosphino groups. The C2-symmetry
of this macrocycle was also confirmed by the X-ray
structure analysis (Figure 2).[6]
Recently, chiral macrocycles containing O, N, S or
combinations thereof have been successfully applied
in the field of enantioselective catalysis, such as asym-
metric epoxidation, aldol reaction, cyclopropanation
and hydrogenation.[7] However, chiral macrocycles
with P, N donors are very rare[8] and have seldom
been employed in asymmetric reactions to date.[9]
7
8
9
(PPh3)2 L4
<1
2
G
L4
10
11
12
13
14
15
N
G
80
63
2
<1
<1
2
98
98
–
–
–
G
L4
L4
L4
L4
L4
G
–
[a]
[b]
[c]
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Shenluan Yu et al.
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Fe3(CO)12, while the ligand L3, which has an analo- ents at the ortho position, due to the steric hindrance
gous structure, displayed inferior activity and low of the substituents (Table 2, entries 2, 5 and 10). For
enantioselectivity. These results could be attributed to the reduction of more hindered aromatic ketones
the promoting effect of NH functions in the ligand (Table 2, entries 12–17), excellent enantioselectivities
L4.[2] The best enantioselectivity (98% ee) was (98–99% ee) were observed with an obvious decrease
obtained with the trinuclear iron carbonyl in activity. 2-Acetonaphthone was also efficiently re-
clusters
[PPN][HFe3(CO)11]. In contrast, the mono- and di-nu- 1 h (Table 2, entry 18). The ATH of heteroaromatic
clear iron carbonyl clusters, iron(II) or iron(III) salts ketones also gave satisfactory results (Table 2, en-
Fe3(CO)12,
[Et3NH]
G
or duced with 93% conversion and 96% ee at 558C for
A
ACHTUNGTRENNUNG
AHCTUNGTRENNUNG
showed poor activity in the ATH of propiophenone. tries 19–21). Thus, 3-acetylpyridine, 2-propionylthio-
It is interesting to note that the reaction is promoted phene and 2-butyrylthiophene could be reduced
by addition of some ammonium salt such as NH4Cl smoothly with good to excellent enantioselectivities
(Table 1, entry 5), although the role of ammonium in (90–96% ee). Unfortunately, the ATH of alkyl ke-
the catalytic cycle is unclear.
tones only gave good to moderate activities and low
With the optimized conditions in hand, we next ex- enantioselectivities (such as for 3-methyl-2-butanone:
amined the asymmetric reduction of a wide range of 658C, 12 h, 58% conversion, 45% ee; for 4-methyl-2-
aromatic ketones using the catalytic system of pentanone: 608C, 4 h, 83% conversion, 35% ee).
Fe3(CO)12 in combination with macrocycle L4
The remarkable efficiency of the iron catalyst was
(Table 2). For acetophenone and its derivatives, the further demonstrated at a low catalyst loading (0.1–
catalyst system showed outstanding enantioselectivi- 0.02 mol%, S/C=1000–5000) (Table 3), giving almost
ties under mild reaction conditions (91–99% ee; identical ee values of products with a TOF of up to
Table 2, entries 1–11). The electronic properties of 1940 hÀ1 (Table 3, entry 8). Good conversions and sat-
substituents on the aromatic ring had insignificant isfactory enantiomeric excesses were still obtained for
impact on the enantioselectivity, although the me- the reduction of acetophenone and 4’-chloroacetophe-
thoxy substituent gave the products in slightly lower none at an S/C ratio of 5000:1 (Table 3, entries 3 and
enantioselectivities (Table 2, entries 8 and 9). Howev- 9). The iron-based catalytic system is the first success-
er, the activities are significantly affected by substitu- ful example of chiral macrocycles used for the asym-
Table 2. Scope of the iron-catalyzed ATH of ketones.[a]
Entry
R1
R2
Temperature [8C]
Time [h]
Conversion[b] [%]
ee[b] [%]
1
2
3
4
5
6
7
8
Ph
Me
Me
Me
Me
Me
Me
Me
Me
Me
Me
Me
Et
n-Pr
i-Pr
n-Bu
Cy
Me
Me
Me
Et
55
75
55
75
75
55
65
55
75
65
65
75
65
75
65
65
65
55
50
75
65
1
2
1
0.5
2
0.5
0.5
1
0.5
16
3
97
64
92
94
92
98
99
92
86
23
99
98
96
90
96
81
95
93
99
87
85
98
99
96
96
97
95
97
91
93
95
95
98
99
99
99
99
99
96
90
96
96
o-Me-C6H4
m-Me-C6H4
p-Me-C6H4
o-Cl-C6H4
m-Cl-C6H4
p-Cl-C6H4
m-MeO-C6H4
p-MeO-C6H4
o-NO2-C6H4
p-CN-C6H4
Ph
9
10
11
12
13
14
15
16
17
18
19
20
21
1
1
Ph
Ph
Ph
Ph
12
1
12
12
1
2
2
2
Ph2CH
2-naphthyl
3-pyridyl
2-thienyl
2-thienyl
n-Pr
[a]
Conditions: ketone (1 mmol), L4 (0.5 mol%), Fe3(CO)12 (0.5 mol%), NH4Cl (6 mol%), KOH (12 mol%), i-PrOH
(10 mL).
[b]
Determined by GC and HPLC analysis.
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Adv. Synth. Catal. 2012, 354, 818 – 822

Iron-Catalyzed Highly Enantioselective Reduction of Aromatic Ketones
Table 3. Iron-catalyzed ATH of ketones at higher S/C.[a]
Entry
R1
R2
S/C
Time [h]
Conversion[b] [%]
ee[b] [%]
TOF [hÀ1]
1
2
3
4
5
6
7
8
Ph
Ph
Ph
Me
Me
Me
Me
Me
Me
Me
Me
Me
Me
Et
1000
2000
5000
1000
1000
1000
2000
2000
5000
1000
1000
1000
1000
1
2
5
1
1
1
3
1
12
1
16
5
91
89
66
89
88
90
95
97
80
97
94
49
61
97
97
97
94
94
95
95
95
93
90
98
99
99
910
890
660
890
880
900
633
1940
333
970
59
m-Me-C6H4
p-Me-C6H4
m-Cl-C6H4
m-Cl-C6H4
p-Cl-C6H4
p-Cl-C6H4
m-MeO-C6H4
Ph
9[c]
10
11
12
13
Ph
Ph
n-Pr
n-Bu
98
122
5
[a]
Conditions: L4 (0.005 mmol), Fe3(CO)12 (0.005 mmol), NH4Cl (0.06 mmol), KOH (0.12 mmol), i-PrOH (25 mL), 758C.
Determined by GC and HPLC analysis.
The reaction was carried out at 808C.
[b]
[c]
metric reduction of a wide spectrum of aromatic ke- Acknowledgements
tones with excellent enantioselectivities. In compari-
We would like to thank the National Natural Science Founda-
son to chiral open-chain PNNP ligands, the macrocy-
cle displays higher chiral efficiency. The cyclic struc-
ture may restrict the configurational flexibility of the
ligand and increase the rigidity, thus giving rise to
high enantioselectivity.
tion of China (No. 20423002; 20923004; 21173176), Program
for Changjiang Scholars and Innovative Research Team in
University (No. IRT1036) and State Key Laboratory of Phys-
ical Chemistry of Solid Surfaces for financial support.
In conclusion, we have developed a novel chiral 22-
member macrocycle ligand which displays excellent
chiral efficiency for the asymmetric reduction of vari-
ous ketones. In view of the high activity, remarkable
enantioselectivity and cheap price of iron catalysts,
the iron-based catalytic system containing a chiral
macrocycle is of significant industrial interest. Further
investigations on the reaction mechanism and the ex-
tension of the catalytic system are currently ongoing
in our laboratory.
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Adv. Synth. Catal. 2012, 354, 818 – 822