Pd-catalyzed asymmetric Wacker-type cyclization of o-trisubstituted allylphenols by use of tetraoxazoline ligands

Article abstract of DOI:10.1016/j.tet.2008.07.090

Pd(II)-catalyzed intramolecular Wacker-type cyclizations of o-trisubstituted allylphenols were studied. The chelation-induced axially chiral catalytic system, Pd(CF₃COO)₂-8 (1:1 molar ratio), showed excellent catalytic activities and

Full text of DOI:10.1016/j.tet.2008.07.090

Tetrahedron 64 (2008) 9413–9416
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Pd-catalyzed asymmetric Wacker-type cyclization of o-trisubstituted
allylphenols by use of tetraoxazoline ligands
*
Feijun Wang, Guoqiang Yang, Yong Jian Zhang, Wanbin Zhang
Department of Chemistry, School of Chemistry and Chemical Technology, Shanghai Jiao Tong University, 800 Dongchuan Road, Shanghai 200240, PR China
a r t i c l e i n f o
a b s t r a c t
Article history:
Received 5 June 2008
Received in revised form 17 July 2008
Accepted 22 July 2008
Available online 26 July 2008
Pd(II)-catalyzed intramolecular Wacker-type cyclizations of o-trisubstituted allylphenols were studied.
The chelation-induced axially chiral catalytic system, Pd(CF3COO)2-8 (1:1 molar ratio), showed excellent
catalytic activities and enantioselectivities in the Wacker-type cyclizations of o-trisubstituted allylphe-
nols with up to 94% ee.
Ó 2008 Elsevier Ltd. All rights reserved.
1. Introduction
(see Fig. 1), but only 9% ee for o-trisubstituted allylphenols 2a was
obtained. Therefore, they developed boxax 5b,6 which introduced
Pd(II)-catalyzed intramolecular Wacker-type cyclizations have
emerged as a versatile strategy in the construction of a range of
heterocycles,1 such as number of biologically active compounds2
several functional groups at the C3 and C30 positions of binaphthyls
(see Fig. 1). Using boxax 5b as a ligand, up to 97% ee was obtained
for catalytic cyclization of 2a, but only 9% ee was obtained for
o-tetrasubstituted allylphenol 1. Stoltz and co-workers also repor-
ted this reaction with Pd(II)/sparteine as a catalyst system under
aerobic conditions, however, the enantioselectivities for cycliza-
tions of o-trisubstituted allylphenols are not sufficiently high.7 Very
recently, we reported a series of novel biphenyl oxazoline ligands 6,
7, and 8 (see Figs.1 and 2) and their applications in the Wacker-type
cyclization of o-tetrasubstituted allylphenols.8,9 The best result was
gained with ligands 8, which have four identical chiral oxazoline
groups at four ortho positions in biphenyl. Although ligands 8 have
no any axial chirality due to the molecular symmetry, only one
diastereomer as (S)-axial configuration was obtained upon the
complexing process whether the monometallic (S,aS)-9 or bi-
metallic palladium complexes (S,aS)-10 was formed.9 The catalytic
having
a 2-isopropenyl-2,3-dihydrobenzofuran skeleton that
contains an asymmetric carbon at the 2-position. However, asym-
metric oxidative cyclizations catalyzed with chiral Pd(II)-com-
plexes have received relatively little attention for stereoselective
syntheses of these skeletons.3 Previous attempts at asymmetric
version of this oxidative cyclizations were reported by Hosokawa
and Murahashi,4 nevertheless, whose Wacker-type cyclization of
o-allylphenols by use of a chiral p-allylpalladium complex afforded
dihydrobenzofurans with only 29% ee. Recently, Hayashi and co-
workers reported an important breakthrough in the Pd(II)-cata-
lyzed enantioselective Wacker-type cyclization of o-allylphenols
with chiral bisoxazoline ligands based on binaphthyl backbone
(boxax) (Scheme 1).5 Their researches, however, showed that the
enantioselectivity of this oxidative cyclization largely depended on
the substituent circumstances of allyl group at a substrate. In the
cyclization of o-tetrasubstituted allylphenol 1, 96% ee was obtained
in the presence of p-benzoquinone in methanol by using boxax 5a
Y
O
N
O
N
R
R
R
N
N
R
R'
R'
O
O
R
Y
(S)-boxax 5a
(S)-boxax 5b
Pd(II), Ligand
benzoquinone
methanol
O
O
OH
1 R = Me
2a R = H
R
R'
N
O
3 R = Me
4a R = H
R
H
H
O
R
R
N
N
(CH2)n
Scheme 1. Pd(II)-catalyzed Wacker-type cyclization.
N
O
R
O
O
R'
6
7
* Corresponding author. Tel./fax: þ86 21 5474 3265.
E-mail address: wanbin@sjtu.edu.cn (W. Zhang).
Figure 1. Bisoxazoline ligands with a biaryl backbone.
0040-4020/$ – see front matter Ó 2008 Elsevier Ltd. All rights reserved.
doi:10.1016/j.tet.2008.07.090

9414
F. Wang et al. / Tetrahedron 64 (2008) 9413–9416
reaction solvents (entries 1 and 8), but the enantioselectivities
R
R
O
O
R
R
R
markedly decreased by using THF, toluene, and TFE (2,2,2-tri-
fluoroethanol) (entries 5–7). Furthermore, the presence of water
did not affect the catalytic efficiency obviously (entry 9). It is also
found that the enantioselectivity was largely depended on the
reaction temperature. The enantioselectivity was significantly im-
proved by decreasing the reaction temperature (entries 10 and 11).
The reaction was also carried out at 20 ꢀC for 3 days in acetone, the
highest enantioselectivity 94% ee was acquired (entry 12). Under the
same reaction conditions with entry 1, our ligands 6 with different
axial configuration and the length of the bridge across the 5,50-
position of biphenyls were also used to Pd-catalyzed Wacker-type
cyclizations of o-trisubstituted allylphenol 2a (entries 13–15). How-
ever, only 27% ee was obtained in the cyclization of allylphenol 2a.
With optimized reaction conditions in hand, the generality of
this catalytic system, Pd(CF3COO)2-8 (1:1 molar ratio), was studied
through the Wacker-type cyclization of a series of o-trisubstituted
allylphenols 2b–2e and o-trisubstituted allyl naphthols 2f–2g,
providing corresponding chiral 2,3-dihydrobenzofurans 4b–4e,
dihydronaphtho[1,2-b]furan 4f, and dihydronaphtho[2,1-b]furan
4g. For the substrate 2b with methyl group at 5-position of
o-allylphenol, excellent enantioselectivity was obtained, but only
60% of isolated yield was afforded (Table 2, entry 1). The electronic
property of the group at 4-position of o-allylphenols slightly
effected the enantioselectivity of cyclizations of 2c–2e (entries 2–
4). When 4-position of o-allylphenols was substituted by phenyl
(2c) or fluoro (2d) group, excellent enantioselectivities were
obtained, but slightly lower enantioselectivity was obtained for
cyclization of 2e, which is substituted by methoxy group. The
cyclizations of naphthol derivatives, 2f and 2g, gave the corre-
sponding cyclized products 4f and 4g with good isolated yields and
moderate enantioselectivities (entries 5 and 6).
O
O
O
O
N
R
N
R
R
N
N
N
N
Pd
Pd
Pd
R
N
N
R
N
N
N
N
O
O
O
O
R
R
(S,aS)-9
(S,aS)-10
8
top view
Figure 2. Tetraoxazoline ligands and their palladium complexes.
system, Pd(CF3COO)2-8 in a 1:1 molar ratio, showed excellent cat-
alytic activities and enantioselectivities in the Wacker-type
cyclizations of o-tetrasubstituted allylphenols with up to 99% ee.
Thus, to investigate the generality of this catalytic system, we
report herein asymmetric Wacker-type cyclizations of o-tri-
substituted allylphenols by using of Pd-8 complex as a catalyst.10
2. Results and discussion
Previous work showed that phenyl-substituted tetraoxazoline
(Ph-8) was the best ligand in Pd-catalyzed Wacker-type cycliza-
tions of o-tetrasubstituted allylphenols, so Ph-8 was used in the
same reaction but the allyl group at the phenols is trisubstituted.
Typically, 2-(2-methyl-2-butenyl)phenol (2a) was used as a model
substrate for the oxidative cyclization. The reactions were catalyzed
by 10 mol % of the Pd(II) complexes generated in situ by mixing
Pd(CF3COO)2 with oxazoline-based biphenyl ligands in the pres-
ence of p-benzoquinone as reoxidant in methanol at 60 ꢀC. As
shown in Table 1, it was noteworthy that the chelation-induced
axially chiral Pd-catalyst, Pd(CF3COO)2-8 (1:1 molar ratio), is more
effective than bipalladium catalyst for asymmetric cyclization of 2a
(entries 1 and 2). In order to optimize reaction conditions, the in-
fluence of palladium source, solvent, and reaction temperature was
also studied. The catalytic efficiencies were largely depended on
palladium source, solvent, and reaction temperature. The Pd(II)
complexes formed from Pd(OAc)2 and Pd(CF3COO)2 exhibited
similar catalytic efficiencies (entries 1 and 3). However, markedly
decreased enantioselectivity has been observed with the Pd(II)-
complexes bearing chloride (entry 4). Higher catalytic activities and
enantioselectivities were observed by using acetone or methanol as
3. Conclusion
In summary, Pd(II)-catalyzed intramolecular Wacker-type cy-
clizations of o-trisubstituted allylphenols using tetraoxazoline
ligands were studied. High catalytic activity and excellent
enantioselectivity for cyclization of o-trisubstituted allylphenols
Table 1
Pd(II)-catalyzed intramolecular Wacker-type cyclizations of o-trisubstituted allylphenol 2aa
H
Pd(II), Ligand
benzoquinone
O
OH
H
methanol
4a
2a
Entry
Ligand
Pd source
Pd(II)/L (mol %/mol %)
Solvent
Temperature (ꢀC)
Time (h)
Yieldb
%
eec
%
1
2
3
4
5
6
7
8
8 (R¼Ph)
Pd(CF3COO)2
Pd(CF3COO)2
Pd(AcO)2
10/10
10/5
Methanol
Methanol
Methanol
Methanol
THF
Toluene
TFE
Acetone
5% H2O in methanol
Methanol
Methanol
Acetone
Methanol
60
60
60
60
60
60
60
Reflux
60
40
20
20
60
60
60
24
24
24
24
24
24
24
24
24
72
72
72
24
24
24
94
92
95
35
27
39
78
93
91
90
79
87
94
12
91
82
23
79
4
8 (R¼Ph)
8 (R¼Ph)
10/10
10/10
10/10
10/10
10/10
10/10
10/10
10/10
10/10
10/10
10/20
10/20
10/20
8 (R¼Ph)
PdCl2
8 (R¼Ph)
Pd(CF3COO)2
Pd(CF3COO)2
Pd(CF3COO)2
Pd(CF3COO)2
Pd(CF3COO)2
Pd(CF3COO)2
Pd(CF3COO)2
Pd(CF3COO)2
Pd(CF3COO)2
Pd(CF3COO)2
Pd(CF3COO)2
68
49
16
83
77
86
93
94
27
2
8 (R¼Ph)
8 (R¼Ph)
8 (R¼Ph)
9
8 (R¼Ph)
10
11
12
13
14
15
8 (R¼Ph)
8 (R¼Ph)
8 (R¼Ph)
(S,aR,S)-6 (n¼8, R¼i-Pr)
(S,aS,S)-6 (n¼8, R¼i-Pr)
(S,aR,S)-6 (n¼10, R¼i-Pr)
Methanol
Methanol
23
a
The reactions were catalyzed by Pd(II)-8 complex generated in situ by mixing palladium source with tetraoxazoline 8 in the presence of 4 equiv of p-benzoquinone.
Isolated yield by column chromatography.
The ee was determined by HPLC on a Daicel Chiralcel OD-H column (hexane/2-propanol¼99.0:1.0, flow¼0.5 mL/min). The S absolute configuration was assigned by
b
c
comparison of optical rotations with the known reported data.6

F. Wang et al. / Tetrahedron 64 (2008) 9413–9416
9415
Table 2
forming (S)-2-ethenyl-2-methyl-2,3-dihydrobenzofuran (4a). To
a solution of palladium bis(trifluoroacetate) (14.0 mg, 0.042 mmol)
and 8 (31.1 mg, 0.042 mmol) in methanol (1.0 mL) were added
p-benzoquinone (181.6 mg, 1.68 mmol) and 2-(2-methyl-2-bute-
nyl)phenol (2a) (68.2 mg, 0.42 mmol) in methanol (0.5 mL) at
room temperature. The reaction mixture was stirred at 60 ꢀC for
24 h, concentrated in vacuo, and then chromatographed on silica
gel (eluent: ethyl acetate and petroleum ether) to give compound
4a.
Pd(II)-catalyzed intramolecular Wacker-type cyclizations of o-trisubstituted allyl-
phenols 2b–2e and o-trisubstituted allyl naphthols 2f–2ga
Entry
Substrate
Productb
Yieldc
%
eed
%
1
60
92
O
OH
2b
4b
Ph
F
Ph
F
2
3
4
87
89
90
90
90
83
O
OH
4.2.1. (S)-2-Ethenyl-2-methyl-2,3-dihydrobenzofuran (4a)
Yield 94% (63.0 mg), 82% ee, colorless oil; 1H NMR (400 MHz,
4c
2c
CDCl3):
d 7.11–7.16 (m, 2H, ArH), 6.76–6.88 (m, 2H, ArH), 6.05
(dd, J¼17.6, 11.2 Hz, 1H, CH), 5.31 (dd, J¼17.2, 0.8 Hz, 1H, CH), 5.10
(dd, J¼10.4, 0.8 Hz, 1H, CH), 3.18 (d, J¼14.2 Hz, 1H, CH2), 3.06 (d,
J¼15.6 Hz, 1H, CH2), 1.56 (s, 3H, CH3); 13C NMR (100 MHz, CDCl3):
O
OH
2d
4d
H3CO
H3CO
d
158.8, 141.8, 128.2, 126.6, 125.2, 120.4, 113.0, 109.6, 87.7, 42.2, 26.3.
The ee was determined by HPLC on a Daicel Chiralcel OD-H column
(hexane/2-propanol¼99.0:1.0, flow¼0.5 mL/min).
O
OH
4e
2e
4.2.2. (S)-2-Ethenyl-2,6-dimethyl-2,3-dihydrobenzofuran (4b)
Yield 60%, 92% ee, colorless oil; 1H NMR (400 MHz, CDCl3):
d 7.02
O
O
OH
2f
5
88
92
54
71
(dd, J¼7.6, 15.2 Hz, 1H, ArH), 6.66 (d, J¼7.2 Hz, 1H, ArH), 6.63 (d,
J¼8.0 Hz, 1H, ArH), 6.00–6.09 (m, 1H, CH), 5.27–5.33 (m, 1H, CH),
5.06–5.10 (m, 1H, CH), 3.09 (d, J¼15.6 Hz, 1H, CH2), 2.97 (d,
J¼15.6 Hz, 1H, CH2), 2.20 (s, 3H, CH3), 1.56 (s, 3H, CH3); 13C NMR
4f
(100 MHz, CDCl3):
d 158.6, 142.0, 128.1, 124.7, 121.4, 121.1, 112.7,
6
110.4, 87.8, 41.9, 26.5, 21.7; HRMS (Micromass LCT) calcd for
12H14O: 174.1045, found: 174.1046. The ee was determined by
HPLC on Daicel Chiralcel OD-H column (hexane/2-prop-
OH
2g
C
4g
a
a
All reactions were catalyzed by 10 mol % of the Pd(II)-8 complex generated in
situ by mixing Pd(CF3COO)2 with tetraoxazoline 8 (Pd/ligand 1:1) in the presence of
4 equiv of p-benzoquinone in acetone at 20 ꢀC for 72 h.
anol¼99.0:1.0, flow¼0.5 mL/min).
b
4.2.3. (S)-2-Ethenyl-2-methyl-5-phenyl-2,3-dihydro-
benzofuran (4c)
Absolute configurations for these products were assigned by analogy through
comparison of HPLC elution order with compounds 4a.
c
Yield 87%, 90% ee, colorless oil; 1H NMR (400 MHz, CDCl3):
Isolated yield by column chromatography.
The ee was determined by HPLC on a Daicel Chiralcel OD-H column.
d
d
7.50–7.57 (m, 2H, ArH), 7.27–7.41 (m, 5H, ArH), 6.85 (d, J¼8.8 Hz,
1H, ArH), 6.08 (dd, J¼17.6, 10.8 Hz, 1H, CH), 5.34 (dd, J¼17.2, 0.8 Hz,
1H, CH), 5.11 (dd, J¼10.8, 0.8 Hz, 1H, CH), 3.23 (d, J¼15.6 Hz, 1H,
CH2), 3.10 (d, J¼15.6 Hz, 1H, CH2), 1.58 (s, 3H, CH3); 13C NMR
were observed by using Pd(CF3COO)2-8 (1:1 molar ratio) with up to
94% ee. It is demonstrated that our tetraoxazoline ligand 8 is of
more general utility for Wacker-type cyclization than others that
have been reported previously. The applications of tetraoxazoline
ligands in other asymmetric reactions are underway.
(100 MHz, CDCl3):
d 158.6, 141.7, 141.5, 134.0, 128.8, 127.3, 127.2,
126.9, 126.6, 124.1, 113.1, 109.8, 82.2, 42.2, 26.3; HRMS (Micromass
LCT) calcd for C17H16O: 236.1201, found: 236.1204. The ee was
determined by HPLC on a Daicel Chiralcel OD-H column (hexane/2-
propanol¼99.0:1.0, flow¼0.5 mL/min).
4. Experimental
4.1. General comments
4.2.4. (S)-2-Ethenyl-2-methyl-5-fluoro-2,3-dihydro-
benzofuran (4d)
Yield 89%, 90% ee; 1H NMR (400 MHz, CDCl3):
d 6.77–6.85 (m,
All air- and moisture-sensitive manipulations were carried out
with standard Schlenk techniques under nitrogen. Methanol and
2,2,2-trifluoroethanol (TFE) were distilled from CaH2. The com-
mercially available reagents were used without further purification.
The substrates of Wacker-type cyclization, 2a–2g, were prepared
according to the literature procedures.5a TLC was run on 2 cmꢁ5 cm
silica plate. Column chromatography was run on silica gel (100–200
mesh). 1H NMR (400 MHz) spectra and 13C NMR (100 MHz) spectra
were recorded on a Varian MERCURY plus-400 spectrometer. The ee
values were determined by HPLC using a Daicel Chiralcel OD-H.
HRMS were performed on a Micromass LCTTM at the Analysis and
Research Center of East China University of Science and Technology.
2H, ArH), 6.68 (dd, J¼4.4, 8.8 Hz, 1H, ArH), 6.02 (dd, J¼17.2, 10.8 Hz,
1H, CH), 5.30 (dd, J¼17.2, 0.8 Hz,1H, CH), 5.10 (dd, J¼10.8,1.2 Hz,1H,
CH), 3.15 (d, J¼16.0 Hz,1H, CH2), 3.04 (d, J¼15.6 Hz,1H, CH2),1.54 (s,
3H, CH3); 13C NMR (100 MHz, CDCl3):
d 141.5, 114.3, 114.1, 113.1,
112.4, 112.1, 109.7, 109.6, 88.4, 42.3, 26.2; HRMS (Micromass LCT)
calcd for C11H11FO: 178.0794, found: 178.0795. The ee was de-
termined by HPLC on a Daicel Chiralcel OD-H column (hexane/2-
propanol¼99.5:0.5, flow¼0.5 mL/min).
4.2.5. (S)-2-Ethenyl-2-methyl-5-methoxy-2,3-dihydro-
benzofuran (4e)
Yield 90%, 83% ee, colorless oil; 1H NMR (400 MHz, CDCl3):
4.2. General procedure for the Pd(II)-catalyzed asymmetric
Wacker-type cyclization of o-trisubstituted allylphenols and
naphthols
d
6.64–6.73 (m, 3H, ArH), 6.03 (dd, J¼17.2,10.4 Hz,1H, CH), 5.30 (dd,
J¼17.2, 0.8 Hz, 1H, CH), 5.08 (dd, J¼8.4, 1.2 Hz, 1H, CH), 3.74 (s, 3H,
OCH3), 3.15 (d, J¼16.0 Hz, 1H, CH2), 3.03 (d, J¼15.2 Hz, 1H, CH2), 1.54
(s, 3H, CH3); 13C NMR (100 MHz, CDCl3):
d 154.1, 153.1, 141.8, 127.6,
A typical procedure is given for the reaction of 2-(2-methyl-2-
butenyl)phenol (2a) with a palladium(II) complex of ligand 8
113.0, 112.9, 111.5, 109.5, 87.8, 56.1, 42.6, 26.2; HRMS (Micromass
LCT) calcd for C12H14O2: 190.0994, found: 190.0994. The ee was

9416
F. Wang et al. / Tetrahedron 64 (2008) 9413–9416
determined by HPLC on a Daicel Chiralcel OD-H column (hexane/2-
Acknowledgements
propanol¼99.0:1.0, flow¼0.5 mL/min).
This work was supported by the National Natural Science
Foundation of China, the Ministry of Education of China, and
Nippon Chemical Industrial Co., Ltd.
4.2.6. (S)-2-Ethenyl-2-methyl-2,3-dihydronaphtho[1,2-b]furan (4f)
Yield 88%, 54% ee, colorless oil; 1H NMR (400 MHz, CDCl3):
d 7.80
(d, J¼7.6 Hz, 1H, ArH), 7.69 (d, J¼9.2 Hz, 1H, ArH), 7.54 (d, J¼8.4 Hz,
1H, ArH), 7.43–7.47 (m, 1H, ArH), 7.25–7.31 (m, 1H, ArH), 7.12 (d,
J¼8.4 Hz, 1H, ArH), 6.14 (dd, J¼17.6, 10.8 Hz, 1H, CH), 5.37 (dd,
J¼17.2, 1.2 Hz, 1H, CH), 5.12 (dd, J¼10.8, 1.2 Hz, 1H, CH), 3.46 (d,
References and notes
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J¼5.6 Hz,1H, CH2), 3.33 (d, J¼15.6 Hz,1H, CH2), 1.64 (s, 3H, CH3); 13
C
NMR (100 MHz, CDCl3):
d 156.4, 141.9, 131.1, 129.3, 129.2, 128.9,
126.7, 122.9, 122.7, 117.9, 112.9, 112.4, 88.6, 41.2, 26.6; HRMS
(Micromass LCT) calcd for C15H14O: 210.1045, found: 210.1045. The
ee was determined by HPLC on a Daicel Chiralcel OD-H column
(hexane/2-propanol¼99.0:1.0, flow¼0.5 mL/min).
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4.2.7. (S)-2-Ethenyl-2-methyl-2,3-dihydronaphtho[2,1-b]furan (4g)
Yield 92%, 71% ee, colorless oil; 1H NMR (400 MHz, CDCl3):
d 7.67
(d, J¼8.8 Hz, 1H, ArH), 7.69 (d, J¼9.2 Hz, 1H, ArH), 7.52 (d, J¼8.0 Hz,
1H, ArH), 7.44 (dd, J¼6.8, 8.0 Hz, 1H, ArH), 7.28 (dd, J¼7.2, 8.4 Hz,
1H, ArH), 7.12 (d, J¼8.8 Hz, 1H, ArH), 6.12 (dd, J¼17.2, 10.4 Hz, 1H,
CH), 5.36 (d, J¼17.2 Hz, 1H, CH), 5.11 (d, J¼10.8 Hz, 1H, CH), 3.43 (d,
J¼15.2 Hz, 1H, CH2), 3.30 (d, J¼15.6 Hz, 1H, CH2), 1.63 (s, 3H, CH3);
13C NMR (100 MHz, CDCl3):
d 156.3, 141.9, 131.1, 129.3, 129.2, 128.9,
8. (a) Wang, F.; Zhang, Y. J.; Wei, H.; Zhang, J.; Zhang, W. Tetrahedron Lett. 2007, 48,
4083; (b) Wang, F.; Zhang, Y. J.; Yang, G.; Zhang, W. Tetrahedron Lett. 2007, 48, 4179.
9. Zhang, Y. J.; Wang, F.; Zhang, W. J. Org. Chem. 2007, 72, 9208.
10. The preliminary results for cyclization of 2a have been reported in our previous
paper, see Ref. 9.
126.7, 122.8, 122.7, 117.9, 112.9, 112.4, 88.6, 41.1, 26.5; HRMS
(Micromass LCT) calcd for C15H14O: 210.1045, found: 210.1045. The
ee was determined by HPLC on a Daicel Chiralcel OD-H column
(hexane/2-propanol¼99.0:1.0, flow¼0.5 mL/min).