Synthesis of new chiral ferrocenyl P,N-ligands with a benzoxazole ring and their application in Ag-catalyzed asymmetric [3+2] cycloaddition

Article abstract of DOI:10.1016/j.tetlet.2013.05.003

New chiral ferrocenyl P,N-ligands with a benzoxazole ring as the N-donor group have been synthesized from 2-aminophenol through a three-step transformation and successfully employed in the Ag-catalyzed asymmetric [3+2] cycloaddition of azomethine ylides with electron-deficient alkenes. High diastereoselectivities, excellent enantioselectivities, and good yields have been achieved for a variety of substrates, demonstrating the potential of these new P,N-ligands in asymmetric catalysis.

Full text of DOI:10.1016/j.tetlet.2013.05.003

Accepted Manuscript
Synthesis of new chiral ferrocenyl P,N-ligands with a benzoxazole ring and their
application in Ag-catalyzed asymmetric [3+2] cycloaddition
Sheng Yan, Cheng Zhang, Ya-Hui Wang, Zhong Cao, Zhuo Zheng, Xiang-Ping
Hu
PII:
S0040-4039(13)00751-X
DOI:
http://dx.doi.org/10.1016/j.tetlet.2013.05.003
TETL 42903
Reference:
To appear in:
Tetrahedron Letters
Please cite this article as: Yan, S., Zhang, C., Wang, Y-H., Cao, Z., Zheng, Z., Hu, X-P., Synthesis of new chiral
ferrocenyl P,N-ligands with a benzoxazole ring and their application in Ag-catalyzed asymmetric [3+2]
cycloaddition, Tetrahedron Letters (2013), doi: http://dx.doi.org/10.1016/j.tetlet.2013.05.003
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Synthesis of new chiral ferrocenyl P,N-
ligands with a benzoxazole ring and their
application in Ag-catalyzed asymmetric
[3+2] cycloaddition
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Sheng Yan, Cheng Zhang, Ya-Hui Wang, Zhong Cao,* Zhuo Zheng and Xiang-Ping Hu*
O
N
Ph
N
N
O
R
O
R
N
O
AgOAc (3 mol%)
PPh2
1b (3.3 mol%)
Fe
N
Ph
+
Et3N (10 mol%)
CO2Et
N
H
PhMe, -20 o
C
CO2Et
O
(Rc,Sp)-1b
endo-adducts
up to 99% ee

1
Tetrahedron Letters
journal homepage: www.elsevier.com
Synthesis of new chiral ferrocenyl P,N-ligands with a benzoxazole ring and their
application in Ag-catalyzed asymmetric [3+2] cycloaddition
Sheng Yana,b, Cheng Zhangb, Ya-Hui Wangb, Zhong Caoa,*, Zhuo Zhengb and Xiang-Ping Hub, ∗
a School of Chemistry and Biological Engineering, Changsha University of Science and Technology, Changsha 410004, China
b Dalian Institute of Chemical Physics, Chinese Academy of Sciences, Dalian 116023, China
ARTICLE INFO
ABSTRACT
Article history:
Received
Received in revised form
Accepted
New chiral ferrocenyl P,N-ligands with a benzoxazole ring as the N-donor group have been
synthesized from 2-aminophenol through a three-step transformation and successfully employed
in the Ag-catalyzed asymmetric [3+2] cycloaddition of azomethine yelides with electron-
deficient alkenes. High diastereoselectivities, excellent enantioselectivities and good yields have
been achieved for a variety of substrates, demonstrating the potential of these new P,N-ligands
in asymmetric catalysis.
Available online
Keywords:
P,N-ligand
2009 Elsevier Ltd. All rights reserved.
benzoxazole
asymmetric catalysis
silver
cycloaddition
Asymmetric catalysis is one of the most powerful methods for
the preparation of enantiomerically pure compounds. In this
context, a number of metal complexes with hybrid chiral P,N-
ligands bearing an oxazoline as the N-donor group have been
recognized as powerful catalysts in a variety of asymmetric
catalytic reactions such as hydrogenation, allylic alkylation, and
so on.1 In sharp contrast, few chiral P,N-ligands with an
oxazole/benzoxazole ring as the N-donor moiety have been found
to be efficient in catalytic asymmetric reactions presumably due
to the lack of α-central chirality close to the N-donor atom as that
in oxazoline ligands (Figure 1).2 However, the increased rigidity
caused by the presence of a benzoxazole ring in the ligand
backbone may be advantageous in some cases to create an
efficient chiral environment around the central metal. The most
important task for the development of an efficient chiral
phosphine-benzoxazole ligand should be to search for an
appropriate chiral skeleton.
In our recent work, we have developed a series of chiral P,N-
ligands based on chiral 1-ferrocenylethylamine skeleton. These
ligands displayed excellent enantio-induction in the Pd-catalyzed
asymmetric allylic alkylation,3 Ru-catalyzed asymmetric
cyclopropanation,4 Cu-catalyzed asymmetric propargylic
amination,5 Cu-catalyzed asymmetric [3+3] cycloaddition,6 and
so on.7 The success of these ligands in asymmetric catalysis
demonstrated the potential of 1-ferrocenylethylamine as the
chiral ligand backbone. As an effort toward further exploration of
the application of this chiral skeleton in asymmetric catalysis,8
herein we report the synthesis of a new type of chiral P,N-ligands
with a benzoxazole ring as the N-donor moiety from 1-
ferrocenylethylamine derivatives, which were found to be highly
effective in the Ag(I)-catalyzed asymmetric [3+2] cycloaddition
of azomethine ylides with some electron-deficient alkenes.
NMe2
NH2
OH
O
S
a
PPh2
N
H
*
Fe
3
2
O
*
O
Ph
Ph
Ph
Ph
P
P
b
(Rc,Sp)-PPFA 5a
N
N
c
R
O
N
R
O
N
Figure 1. Structure of P-oxazoline and P-benzoxazole ligands
Cl
NHR
PPh2
N
4
———
PPh2
Fe
d
Fe
∗ Corresponding author. Tel.: +86-411-84379276; fax: +86-411-84684746; e-mail: xiangping@dicp.ac.cn
(Rc,Sp)-PPFNH2 5b: R = H
(Rc,Sp)-PPFNHMe 5c: R = Me
(Rc,Sp)-1a: R = H
(Rc,Sp)-1b: R = Me

2
Tetrahedron Letters
Scheme 1. Synthesis of chiral ferrocenyl P,N-ligands (Rc,Sa)-1a-b.
Reagents and conditions: (a) KOH, EtOH/H2O (5/1), refluxing, 3 h; (b)
POCl3/PCl5, CH2Cl2, rt, 12 h; (c) Ac2O, 110 oC, NH3 or MeNH2,
MeOH, 80 oC; (d) i-Pr2NEt, DMF, rt-50 oC, 12 h.
Table 1. Catalytic asymmetric [3+2] cycloaddition reaction of N-(4-chlorobenzylidene)glycine ethyl ester 6a with N-
phenylmaleimide 7a
Ph
N
Ph
N
COOEt
N
O
O
O
O
O
[Ag]/L (3 mol%)
+
N
Ph
+
base (10 mol%)
Solvent
p-Cl-Ph
CO2Et
p-Cl-Ph
CO2Et
N
H
N
H
4-ClC6H4
O
6a
endo-8a
7
exo-8a
entry
1
ligand
1a
Ag salt
base
solvent
toluene
toluene
toluene
toluene
toluene
CH2Cl2
THF
temperature
Yield (%)b
Endo/exo
Ee (%)d
50
AgOAc
AgOAc
AgOAc
AgOTf
AgBF4
Et3N
Et3N
Et3N
Et3N
Et3N
Et3N
Et3N
Et3N
Et3N
-
rt
85
93
90
97
82
93
93
78
92
94
92
87
91
94
93
89
94
>95/5
95/5
2
1b
1c
rt
95
3
rt
>95/5
90/10
92/8
87
4
1b
1b
1b
1b
1b
1b
1b
1b
1b
1b
1b
1b
1b
1b
rt
27
5
rt
64
6
AgOAc
AgOAc
AgOAc
AgOAc
AgOAc
AgOAc
AgOAc
AgOAc
AgOAc
AgOAc
AgOAc
AgOAc
rt
90/10
93/7
97
7
rt
74
8
CH3CN
Et2O
rt
95/5
97
9
rt
93/7
95
10
11
12
13
14
15
16e
17f
toluene
toluene
toluene
toluene
toluene
toluene
toluene
toluene
rt
95/5
93
i-Pr2NEt
DBU
DABCO
Et3N
Et3N
Et3N
Et3N
rt
>95/5
>95/5
95/5
95
rt
31
rt
94
40
0
95/5
94
95/5
96
-10
-20
>95/5
>95/5
97
98
a Reactions were performed in 3 mL of solvent with 0.5 mmol of substrate 6a, 10 mol% of base additives, and 3 mol% of catalyst prepared in situ from Ag salts
and 1.1 equiv of ligand at indicated temperature for 2 h
b Isolated yields.
c endo/exo-Ratio was determined by 1H NMR.
d Enantiomeric excesses were determined by chiral HPLC.
e Reaction time, 6 h.
f Reaction time, 12 h.
into the desired phosphine-benzoxazole ligands 1a-b11 in high
O
yields by the reaction with chiral phosphine-amine intermediates,
O
(Rc,Sp)-PPFNHR,12 under the basic condition.
Cl
OH
NMe2
PPh2
O
N
N
a
4
PPh2
PPh2
For a comparison, (Rc,Sp)-1c with a oxygen linker was also
Fe
b
Fe
Fe
prepared in a similar procedure as shown in Scheme 2.
With these newly developed ferrocenyl phosphine-
benzoxazole ligands in hand, we then examined their efficiency
in the Ag-catalyzed asymmetric [3+2] cycloaddition of
azomethine ylides with N-phenylmaleimide.13 Initially, the
cycloaddition of N-(4-chlorobenzylidene)glycine ethyl ester (6a)
with N-phenylmaleimide 7 was selected as a model reaction. The
reaction was performed in Et2O at rt in the present of a catalytic
amount of Et3N (10 mol%) and the catalyst (3 mol%) in situ
prepared from silver salts and 1.1 equiv of ferrocenyl phosphine-
benzoxazole ligands 1a-c,14 and the results are summarized in
Table 1. In comparison with ligand 1a bearing a N-H proton,
ligand 1b with a N-methyl substituent showed significantly
improved enantioselectivity in this model reaction, giving the
endo-cycloadduct 8a in good diastereoselectivity (95/5) with
(Rc,Sp)-5a
(Rc,Sp)-5d
(Rc,Sp)-1c
Scheme 2. Synthesis of chiral ferrocenyl P,N-ligand (Rc,Sa)-1c.
Reagents and conditions: (a) Ac2O, 110 C, 3 h, and then LiAlH4,
THF, reflux for 12 h; (b) n-BuLi, Et2O, and then reflux for 12 h.
o
Chiral phosphine-benzoxazole ligands 1a-b were synthesized
from 2-aminophenol according to the procedure shown in
Scheme 1. Initially, 2(3H)-benzoxazolethione (3) was prepared
as a white needle in 82% yield by the reaction of 2-aminophenol
with CS2 in refluxing aqueous EtOH for 3 hours.9 The treatment
of 2(3H)-benzoxazolethione with POCl3 and PCl5 in CH2Cl2 at
room temperature generated 2-chlorobenzoxazole 4 as a colorless
oil in 60% yield.10 Finally, 2-chlorobenzoxazole 4 was converted

3
enantioselectivity of up to 95% ee (entry 1 vers entry 2). With
(Rc,Sp)-1c as ligand, the reaction gave the cycloadducts in slightly
decreased enantioselectivity (87% ee) but with the same
configuration as that obtained with (Rc,Sp)-1a (entry 3). This
result suggested that in this class of P,N-ligands, the N-atom in
the benzoxazole ring rather than the one directly attached to the
stereogenic carbon center is the actual N-donor atom. The
reaction proved to be highly sensitive to the catalyst precursors.
Thus, the use of AgOTf and AgBF4 resulted in dramatically
decreased enantioselectivities (entries 4 and 5). The nature of the
diastereoselectivities than those with an electron-donating
group (entries 8-9 vers entries 5-7). An azomethine ylide derived
from 1-naphthylaldehyde also proved to be a suitable substrate in
this transformation, producing endo-cycloadduct in 98% ee
(entry 10). 2-Thienyl substrate 6k was also efficient, giving
endo-cycloadduct in 88% ee (entry 11). More importantly, the
presence catalytic system showed good reactivity for alkyl
substituted substrate 6l, giving endo-cycloadduct in 91% ee
(entry 12).
To further extend the scope of this catalytic system, the scope
and generality of this catalytic system with regard to iminoesters
and dipolarophiles were also investigated (Figure 1). The present
catalytic system is remarkably tolerant and remains high in
reactivity for azomethine ylides derived from amino esters other
than glycinate. Thus, under the optimized reaction conditions, the
azomethine ylide derived from phenylalanine successfully
reacted with N-phenylmaleimide 7, leading to perfect endo-
selectivities (>95/5) and excellent enantioselectivities (96% ee).
A variety of other dipolarophiles was also subjected to this
reaction. With N-methylmaleimide, high endo-selectivity
(endo/exo > 95/5) and good enantioselectivity (94% ee) was
achieved. However, for dimethyl maleate and tert-butyl arcylate,
the present catalytic system proved to be less efficient, displaying
very low enantioselectivity.
solvent
has
some
influence
on
the
conversion,
diastereoselectivity and enantioselectivity (entries 6-9). Further
work on the screening of various bases indicated that Et3N was
the best choice (entries 10-13). The reaction temperature had
some impact on the reactivity and enantioselectivity, and the
reaction performed at lower temperature tended to give better
enantioselectivity (entries 14-17). When the reaction temperature
was carried out at -20 οC, up to 98% ee was achieved (entry 17).
Table 2. Catalytic asymmetric [3+2] cycloaddition of imino
esters 6a-l with N-phenylmaleimide 7a
O
N
Ph
7
Ph
N
Ph
N
O
R
N
O
R
O
O
R
O
AgOAc (3 mol%)
1b (3.3 mol%)
Ph
+
N
N
O
O
O
O
Et3N (10 mol%)
CO2Et
CO2Et
N
H
N
H
PhMe, -20 o
C
CO2Et
6a-l
Ph
CO2Me
endo-8a-l
exo-8a-l
p-Cl-Ph
CO2Et
p-Cl-Ph
N
H
N
H
Entry Substrate (R)
Yield (%)b
endo/exo c
95/5
ee (%)d
98
8n
8m
93% yield (endo/exo >95/5)
94% ee (endo)
80% yield (endo/exo >95/5)
96% ee (endo)
1
6a: 4-ClC6H4
6b: 3-ClC6H4
6c: 2-ClC6H4
6d: Ph
94
94
92
90
89
87
94
85
80
85
75
80
2
>95/5
>95/5
>95/5
94/6
95
3
98
MeO2C
p-Cl-Ph
CO2Me
CO2Et
t-BuO2C
4
97
5
6e: 4-FC6H4
6f: 4-CF3C6H4
6g: 4-BrC6H4
6h: 4-MeC6H4
6i: 4-MeOC6H4
6j: 1-naphthyl
6k: 2-thienyl
6l: cyclohexyl
98
p-Cl-Ph
CO2Et
N
H
N
H
6
94/6
97
8p
8o
7
>95/5
92/8
95
84% yield (endo/exo >95/5)
95% yield (endo/exo = 95/5)
25% ee (endo)
19% ee (endo)
8
>99
98
Figure 1. Cycloadducts of the reaction between imino esters with
other dipolarophiles.
9
90/10
95/5
10
11
12
98
In conclusion, we have developed a new family of chiral
ferrocenyl P,N-ligands with a benzoxazole fragment through an
efficient and simple synthesis route. These ligands were
evaluated in the Ag(I)-catalyzed asymmetric [3+2] cycloaddition
of azomethine ylides with various electronic-deficient alkenes, in
which high endo-selectivities and good to excellent
enantioselectivities (up to >99% ee) were achieved. Further
applications of these new ligands in other type of asymmetric
reactions are in progress.
>95/5
93/7
88
91
a Reactions were performed in 3 mL of toluene with 0.5 mmol of substrate
6a-j, 10 mol% of Et3N, and 3 mol% of catalyst prepared in situ from AgOAc
and 1.1 equiv of 1b at -20 oC for 12 h
b Isolated yields.
c endo/exo-Ratio was determined by 1H NMR.
d
Enantiomeric excesses were determined by chiral HPLC.
Acknowledgments
Under the optimized reaction conditions, the scope of the
present catalytic system in the 1,3-dipolar cycloaddition was
carried out. As shown in Table 2, various imino esters 6a-i
prepared from various substituted benzaldehydes reacted with N-
phenylmaleimide 7 to give the corresponding endo-adducts 8a-i
in high yields and good to perfect enantioselectivities (95->99%
ee) (entries 1-9). The results disclosed that the electronic property
of the substituent on the phenyl ring had little effect in the
enantioselectivities, but had some influence on the
diastereoselectivities (entries 1, 4-9). The substrates bearing an
electron-withdrawing substituent tended to give higher
Support for this research from the Dalian Institute of Chemical
Physics (CAS). The authors also thank Prof. Hongchao Guo for
providing several chiral materials (synthesized in the National
Key Technologies R&D Program of China, 2012BAK25B03,
CAU) for ligand synthesis.

4
Tetrahedron
MHz, CDCl3): δ 1.59 (d, J = 7.2 Hz, 3H), 2.33 (s, 3H), 3.76 (s,
1H), 4.13 (s, 5H), 4.32 (s, 1H), 4.50 (s, 1H), 5.81-5.83 (m, 1H),
6.81-6.86 (m, 4H), 6.89-6.93 (m, 1H), 7.00-7.09 (m, 3H), 7.28-
7.30 (m, 4H), 7.44-7.48 (m, 2H); 31P NMR (162 MHz, CDCl3): δ -
25.7; 13C NMR (100 MHz, CDCl3): δ 18.3, 28.6, 51.4 (d, J = 8.9
Hz), 68.7, 69.9 (d, J = 3.5 Hz), 70.0, 72.1 (d, J = 4.5 Hz), 77.8 (d,
J = 10.2 Hz), 91.9, 92.1, 108.3, 115.7, 119.5, 123.3, 127.6 (d, J =
7.2 Hz), 128.0 (d, J = 17.4 Hz), 128.1, 128.7, 132.5, 132.7, 134.4,
134.6, 136.8 (d, J = 9.9 Hz), 137.8 (d, J = 9.5 Hz), 143.5, 148.6,
161.0. HRMS (m/z) calcd. for C32H29FeN2OP: 544.1367, found:
544.1367.
References and notes
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Catalysts; Zhou, Q.-L., Ed.; Wiey-VCH: Weinheim, 2011; pp
221-256; (b) Tannert, R.; Pfaltz, A. In Phosphorus (III) Ligands in
Homogeneous Catalysis: Design and Synthesis; Kamer, P. C. J.;
van Leeuwen, W. N. M., Eds.; Wiley, 2012; pp 235-260.
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Dieguez, M.; Andersson, P. G. Chem. Eur. J. 2010, 16, 620-638;
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Selected physical and spectral data for (Rc,Sp)-1c: yellow solid; mp:
o
1
196-198 C; [α]D25: -346.6 (c 0.53, CH2Cl2); H NMR (400 MHz,
CDCl3): δ 1.85 (d, J = 7.2 Hz, 3H), 3.81 (s, 1H), 4.17 (s, 5H), 4.43
(s, 1H), 4.83 (s, 1H), 5.91-5.93 (m, 1H), 6.68-6.93 (m, 9H), 7.29-
7.48 (m, 5H); 31P NMR (162 MHz, CDCl3): δ -26.4; 13C NMR
(100 MHz, CDCl3): δ 16.9, 48.6 (d, J = 8.0 Hz), 69.0, 70.1 (d, J =
3.5 Hz), 70.7, 72.7 (d, J = 4.5 Hz), 77.8 (d, J = 10.2 Hz), 89.8,
90.1, 109.5, 109.7, 121.3, 122.7, 127.4 (d, J = 6.9 Hz), 127.8,
128.0 (d, J = 7.6 Hz), 129.1, 131.9 (d, J = 20 Hz), 134.7 (d, J = 20
Hz), 136.4 (d, J = 9.1 Hz), 136.8 (d, J = 9.2 Hz), 142.3, 152.7.
HRMS (m/z) calcd. for C31H26FeNO2P: 531.1051, found:
531.1047.
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cycloadditon of azomethine ylides. Under nitrogen atomosphere, a
solution of AgOAc (0.015 mmol) and ligand 1b (0.0165 mmol) in
3 mL of toluene was stirred at room temperature for 1 h. The
solution was cooled to -20 oC, and then added imino esters (0.5
mmol), Et3N (7.5 μL, 0.05 mmol) and N-phenylmaleimide 7 (0.6
mmol) successively. After 12 h, the mixture was passed through a
short column of silica gel and the diastereometric ratio (endo/exo)
was determined by the NMR spectroscopic analysis of the crude
product. The residue was then purified by column chromatography
on silica gel, and was submitted to ee analysis by HPLC with a
chiral column.
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11. Selected physical and spectral data for (Rc,Sp)-1a: yellow solid;
mp: 120-124 oC; [α]D25: -304.6 (c 0.50, CHCl3); 1H NMR (400
MHz, CDCl3): δ 1.53 (d, J = 6.4 Hz, 3H), 3.79 (s, 1H), 4.02 (s,
5H), 4.30 (s, 1H), 4.50 (s, 1H), 5.11-5.15 (m, 1H), 5.71 (br, 1H),
6.94-6.98 (m, 1H), 7.06-7.13 (m, 7H), 7.32-7.35 (m, 4H), 7.48-
7.52 (m, 2H); 31P NMR (162 MHz, CDCl3): δ -24.6; 13C NMR
(100 MHz, CDCl3): δ 22.4, 48.9 (d, J = 4.1 Hz), 69.2, 69.9, 70.2 (d,
J = 4.4 Hz), 72.1 (d, J = 4.2 Hz), 95.1, 108.5, 116.2, 120.4, 123.5,
128.0, 128.1, 128.2, 129.1, 132.6 (d, J = 18.8 Hz), 134.9 (d, J =
20.6 Hz), 136.6 (d, J = 7.6 Hz), 138.7 (d, J = 8.3 Hz), 143.1, 148.3,
160.4; HRMS (m/z) calcd. for C31H27FeN2OP: 530.1210, found:
530.1209.
Selected physical and spectral data for (Rc,Sp)-1b: yellow needle;
o
mp: 154-156 C; [α]D25: -364.1 (c 0.50, CHCl3); 1H NMR (400