Palladium/zinc co-catalyzed syn-stereoselectively asymmetric ring-opening reaction of oxabenzonorbornadienes with phenols

Article abstract of DOI:10.1002/chem.201500816

A new palladium/zinc co-catalyst system associated with chiral (R)-Difluorphos for asymmetric ring-opening reaction of oxabenzonorbornadienes with phenols is reported. This catalyst system allows the formation of cis-2-aryloxy-1,2-dihydronaphthalen-1-ol products in good yields (up to 95-% yield) with excellent enantioselectivities (up to 99-% ee). The cis-configuration of the product has been confirmed by X-ray crystal structure analysis. To the best of our knowledge, it represents the first example in ring-opening reactions of bicycloalkenes with heteronucleophiles in a syn-stereoselective manner. Open the ring: A new palladium/zinc co-catalyst associated with chiral (R)-Difluorphos for asymmetric ring-opening reaction of oxabenzonorbornadienes with phenolic or naphtholic nucleophiles was developed, which afforded the corresponding cis-2-aryloxy-1,2-dihydronaphthalen-1-ol products in good yields (up to 95-% yield) with excellent enantioselectivities (up to 99-% ee).

Full text of DOI:10.1002/chem.201500816

DOI: 10.1002/chem.201500816
Communication
&
Asymmetric Catalysis |Hot Paper|
Palladium/Zinc Co-Catalyzed syn-Stereoselectively Asymmetric
Ring-Opening Reaction of Oxabenzonorbornadienes with Phenols
[
b]
[a]
[a, d]
[a]
[a]
[c]
Sifeng Li, Jianbin Xu, Baomin Fan,*
Zhiwu Lu, Chaoyuan Zeng, Zhaoxiang Bian,
[a]
[b]
Yongyun Zhou, and Jun Wang*
[
6b,12]
ners by choosing different metal catalysts, such as Rh,
Abstract: A new palladium/zinc co-catalyst system associ-
ated with chiral (R)-Difluorphos for asymmetric ring-open-
ing reaction of oxabenzonorbornadienes with phenols is
reported. This catalyst system allows the formation of cis-
[2e,3a,b]
[2a,b,f,6c,13]
[14]
[15]
Cu,
Pd,
Pt and Ni. Yet, for the ring-opening
addition of oxabenzonorbornadienes with oxygen- or nitro-
gen-based nucleophiles, only trans-configuration products
[7,8,10,11,16]
were generally observed with Rh, Ir or Ru catalysts.
To
2
-aryloxy-1,2-dihydronaphthalen-1-ol products in good
yields (up to 95% yield) with excellent enantioselectivities
up to 99% ee). The cis-configuration of the product has
the best of our knowledge, there has been no example in ring-
opening reaction of oxabenzonorbornadienes with heteroatom
nucleophiles to afford cis-2-aryloxy-1,2-dihydronaphthalen-1-ol.
In fact, they are important scaffolds for total synthesis of many
(
been confirmed by X-ray crystal structure analysis. To the
best of our knowledge, it represents the first example in
ring-opening reactions of bicycloalkenes with heteronu-
cleophiles in a syn-stereoselective manner.
[17]
bioactive compounds.
Recent literatures showed that cis-2-aryloxy-1,2-dihydro-
naphthalen-1-ols could be accessed by cross-coupling of cis-
1
,2-dihydronaphthalen-1,2-diol with ArBF K in the presence of
3
[18]
copper(II) acetate. The parent cis-1,2-dihydronaphthalen-1,2-
Transition metal-catalyzed asymmetric ring-opening (ARO) re-
actions of oxabenzonorbornadienes have attracted continuous
interest and extensive study due to their advantages of
straightforward access to chiral hydronaphthalenes, in which
they are widely exist as the subunit in natural products and
diol could be afforded by bacterial oxidation of aromatic mole-
[19]
cules
or by multi-step synthesis from ortho-vinylbenzalde-
[20]
hyde.
Indeed, a general and stereocontrolled method for
easy access of cis-dihydronaphthalen-1,2-diols with various
substituents is still in demand. In this regard, the ARO pathway
leading to cis-2-aryloxy-1,2-dihydronaphthalen-1-ol products in
one step would be a desirable approach.
[
1]
bioactive molecules. Considerable progresses have been
made for this type of reactions since Lautens’s initial work.
With the aid of various chiral transition-metal cata-
[
2–6]
lysts, a variety of carbo-
and heteroatom nucleo-
[
7–11]
philes
reacted with oxabenzonorbornadienes
successfully leading to substituted hydroxyl-dihydro-
naphthalenes. In the ring-opening addition of car-
bonucleophiles to oxabenzonorbornadienes, both cis
and trans configured products can be obtained in
a highly diastereoselective and enantioselective man-
Scheme 1. Asymmetric ring-opening reaction of oxabenzonorbornadienes with O- or N-
based nucleophiles.
In continuing our research interest of ring-opening reaction
+
[
a] J. Xu, Prof. B. Fan, Z. Lu, C. Zeng, Y. Zhou
[21,22]
using carbonucleophiles,
herein we report a transition
YMU-HKBU Joint Laboratory of Traditional Natural Medicine
Yunnan Minzu University, Kunming 650500 (P. R. China)
E-mail: adams.bmf@hotmail.com
metal/Lewis acid co-catalyst system for asymmetric ring-open-
ing reaction of oxabenzonorbornadienes with phenolic or
naphtholic nucleophiles. Remarkably, this method gave exclu-
sive syn-selective outcome the first time, in which it is comple-
mentary to previous methods for trans-configuration product
formation (Scheme 1).
+
[
b] S. Li, Prof. J. Wang
Department of Chemistry
South University of Science and Technology of China
Shenzhen, Guangdong, 518055 (P. R. China)
E-mail: wang.j@sustc.edu.cn
[
c] Prof. Z. Bian
School of Chinese Medicine
Hong Kong Baptist University
We embarked this investigation using oxabenzonorborna-
diene 1a and phenol 2a as benchmark substrates with
Pd(OAc) catalyst and Lewis acid Zn(OTf) as co-catalyst in com-
2
2
[d] Prof. B. Fan
bination with a range of chiral diphosphine ligands (Table 1).
Gratifyingly, commercially available ligand (R)-Binap gave cis-2-
phyloxy-1,2-dihydronaphthalen-1-ol 3aa in 66% yield with
State Key Laboratory of Elemento-organic Chemistry
Nankai University, Tinajim, 300071 (P. R. China)
+
[
] These authors contributed equally to this work.
7
6% ee in DCE at room temperature for 12 h (Table 1, entry 1).
Supporting information for this article is available on the WWW under
http://dx.doi.org/10.1002/chem.201500816.
The best product enantioselectivity was obtained by employ-
Chem. Eur. J. 2015, 21, 1 – 6
1
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These are not the final page numbers! ÞÞ

Communication
[23]
main product (Table 2, entries 12, 14–16). Com-
pared with Zn(OTf) , AgSbF resulted in a higher
Table 1. Ligand screening for Pd/Zn-catalyzed ARO reaction of oxabenzonorbornadiene
[
a]
2
6
1
a with phenol 2a.
product yield but
a
lower enantioselectivity
(
Table 2, entry 13). CuOTf gave almost the same
performance as Zn(OTf) (Table 2, entry 17). The re-
2
action still proceeded well even the catalyst load-
ing was decreased to 1.5 mol% Pd(OAc) (Table 2,
2
entry 18).
The substrate scope were compiled in Table 3.
No significant effect on phenolic nucleophile as
the corresponding ring-opening products 3aa–3aj
could be obtained in good yields (78–95%) with
good enantioselectivities (88–97%). Sterically hin-
dered 1-naphthol 2k resulted in moderate yield
(
50%) with good ee (91%). 2-Naphthol and 7-me-
thoxyl-2-naphthol also reacted with 1a smoothly,
and afforded the desired product in good yield
with high ee (entries 12–13). The cis-configuration
of the product was confirmed by X-ray crystal
[24]
[
b]
[c]
Entry
Ligand
t [h]
Yield [%]
ee [%]
structure analysis (3ad, Figure 1).
To further extend the substrate scope of this
transformation, substituted oxabenzonorborna-
dienes 1b–f with various substituents were exam-
ined (Table 4). The addition of phenol 2a to oxa-
benzonorbornadienes 1b–f proceeded smoothly to
afford the corresponding ring-opening products in
good yields and excellent enantioselectivities. Par-
ticularly noteworthy is that the steric hindrance
caused by the substituents at the oxabenzonorbor-
nadienes 1b and 1c did not have deleterious
effect on both yields and ees of the products
1
2
3
4
5
6
7
8
9
(R)-Binap
(R)-Tol-Binap
(R)-xylyl-Binap
12
20
16
10
32
20
3
96
20
6
66
66
71
55
51
63
66
trace
70
80
64
91
76
50
8
(R)-H
8
-Binap
28
66
72
94
ND
77
81
À61
35
(R)-Synphos
(R)-Segphos
(R)-Difluorphos
(R)- Phanephos
(S)-MeO-Biphep
(S)-Cl-MeO-Biphep
(R)-SDP
10
1
1
52
60
12
(R)-(S)-Josiphos
[
(
(
a] Reaction conditions: Pd(OAc)
3.6 mol%) in DCE (2 mL) was stirred at room temperature for 30 min under Ar atm. 1a
0.3 mmol) and 2a (1.5 mmol) were added, and the reaction mixture was stirred at
2
(3.0 mol%), Zn(OTf)
2
(6.0 mol%), and ligand
(
Table 4, entries 1 and 2). The bromo group re-
mained intact under these reaction conditions
Table 4, entry 5), that allows further potential func-
tionalization using traditional cross-coupling meth-
room temperature for indicated period of time. [b] Yield of isolated product. [c] Deter-
mined by HPLC analysis. DCE=1,2-dichloroethane. ND=not determined.
(
ing (R)-Difluorphos as the ligand (Table 1, entry 7). (R)-(S)-Josi-
phos gave high yield but inferior enantioselectivity (entry 12).
Thus, we chose (R)-Difluorphos as the ligand of choice for fur-
ther optimization.
The reaction conditions for this Pd(OAc) /Zn(OTf) co-cata-
2
2
lyzed asymmetric ring-opening reaction were further surveyed
Table 2). Solvents such as CH Cl , THF, toluene, DME, dioxane,
(
2
2
THP and MTBE resulted in moderate-to-good yields with high
ee values (Table 2, entries 2–8). The highest ee of 3aa (97%)
along with a good yield (84%) was afforded in toluene
(entry 4). The effects of the temperature were investigated. In-
creasing the reaction temperature to 508C, decreased the yield
of 3aa to 46% without affecting the ee (entry 9). Lowering the
reaction temperature to 08C resulted in a similar yield and
enantioselectivity, but 32 h was required for full conversion
Figure 1. ORTEP drawing of 3ad.
(
entry 10). The addition of Zn(OTf) is crucial for the reaction to
2
proceed. When the reaction was carried out in the absence of
Zn(OTf) , only trace amount of product was detected (Table 2,
2
entry 11). Formation of product 3aa was not observed when
the reaction was co-catalyzed by Lewis acid AgOTf, AgPF6,
AgBF , or Cu(OTf) , while 1-naphthol only was obtained as the
ods. Remarkably, all the ring-opening products were exclusive-
ly in cis-configuration with 70–84% yield and 97–99% ee
values.
4
2
&
&
Chem. Eur. J. 2015, 21, 1 – 6
www.chemeurj.org
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ÝÝ These are not the final page numbers!

Communication
[
a]
Table 2. Optimization of reaction conditions.
Table 3. Pd/Zn-catalyzed ARO reaction of 1a with various phenols and
naphthols.
[
a]
[
b]
[c]
Entry
Solvent
DCE
T [8C]
Lewis acid
t [h]
Yield [%]
ee [%]
[
b]
[c]
Entry
1
Aryl phenols
t [h]
Yield [%]
84
ee [%]
1
2
3
4
5
6
7
8
9
RT
RT
RT
RT
RT
RT
RT
RT
50
0
RT
RT
RT
RT
RT
RT
RT
RT
Zn(OTf)
Zn(OTf)
Zn(OTf)
Zn(OTf)
Zn(OTf)
Zn(OTf)
Zn(OTf)
Zn(OTf)
Zn(OTf)
Zn(OTf)
–
2
2
2
2
2
2
2
2
2
2
3
6
8
8
3
96
5
96
1
32
96
96
20
96
96
96
10
8
66
42
76
84
73
63
64
60
94
94
95
97
96
96
96
96
96
97
ND
ND
90
ND
ND
ND
97
97
CH
2
Cl
2
2a
2b
2c
2d
8
97
THF
toluene
DME
dioxane
THP
MTBE
toluene
toluene
toluene
toluene
toluene
toluene
toluene
toluene
toluene
toluene
2
3
4
8
8
8
79
92
80
96
94
93
46
81
10
11
trace
trace
90
trace
trace
trace
83
[
d]
12
13
14
15
16
17
18
AgOTf
5
2e
12
78
89
AgSbF
6
[
[
[
d]
d]
d]
AgPF
AgBF
6
4
6
7
8
9
2 f
2g
2h
2i
8
8
8
8
95
96
91
95
93
88
96
95
Cu(OTf)
CuOTf
2
[
e]
Zn(OTf)
2
84
[
(
a] Reaction conditions: Pd(OAc)
R)-Difluorphos (3.6 mol%) in solvent (2 mL) was stirred at room tempera-
2
(3.0 mol%), Lewis acid (6.0 mol%), and
ture for 30 min under Ar atm. 1a (0.3 mmol) and 2a (1.5 mmol) were
added, and the reaction mixture was stirred at room temperature for indi-
cated period of time. [b] Yield of isolated product. [c] Determined by HPLC
analysis. [d] 1-Naphthol was obtained as main product. [e] 1.5 mol%
2
Pd(OAc) and 1.8 mol% (R)-Difluorphos were used instead. DCE=dichloro-
ethane, THF=tetrahydrofuran, DME=dimethoxyethane, THP=tetrahydra-
pyran, MTBE=methyl tert-butyl ether. ND=not determined.
10
2j
24
24
87
50
96
91
11
2k
On the basis of results, a proposed mechanism is shown in
Scheme 2. Chiral palladium A generated by Pd(OAc) and (R)-
2
1
2
2l
12
48
96
90
89
95
Difluorphos coordinates with oxabenzonorbornadiene 1a and
phenol to form intermediate B. Pd inserts to the OÀH bond of
phenol to give the intermediate C. Then b-elimination of
13
2m
[
a] Reaction conditions: Pd(OAc)
2 2
(1.5 mol%), Zn(OTf) (6.0 mol%), and
(
R)-Difluorphos (1.8 mol%) in toluene (2 mL) was stirred at room tempera-
ture for 30 min under Ar atm. 1a (0.3 mmol) and 2a–m (1.5 mmol) were
added, and the reaction mixture was stirred at room temperature for indi-
cated period of time. [b] Yield of isolated product. [c] Determined by
HPLC analysis.
oxygen opens the furyl ring and gives the ring-opened species
D. Subsequent hydrolysis liberates ring-opened product 3aa.
The Pd complex A is regenerated. It should be noted that in
this proposed mechanism, Zn(OTf) is important for the reactiv-
2
ity and stereoselectivity.
In conclusion, we have successfully developed a new Pd/Zn
catalyst system associated with chiral (R)-Difluorphos for asym-
metric ring-opening addition of oxabenzonorbornadienes with
phenolic or naphtholic nucleophiles. This protocol has the
characteristic of mild reaction conditons (e.g., room tempera-
ture) and broad substrate scope. The cis-2-aryloxy-1,2-dihydro-
naphthalen-1-ol products were generally obtained in good
Scheme 2. Proposed mechanism for the Pd/Zn co-catalyzed asymmetric
ring-opening reaction.
Chem. Eur. J. 2015, 21, 1 – 6
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&
These are not the final page numbers! ÞÞ

Communication
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Oxabenzonorbornadien
t [h]
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ee [%]
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2
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Zn(OTf)
2
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3
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[
6
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[
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(
first example in ring-opening reaction of oxabenzonorborna-
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7
[
25]
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Keywords: asymmetric catalysis · oxabenzonorbornadienes ·
phenols · ring-opening reaction · syn-stereoselectivity
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Communication
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Received: December 22, 2014
Revised: February 27, 2015
Published online on && &&, 0000
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ꢀ 2015 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
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Communication
COMMUNICATION
&
Asymmetric Catalysis
S. Li, J. Xu, B. Fan,* Z. Lu, C. Zeng, Z. Bian,
Y. Zhou, J. Wang*
&& – &&
Open the ring: A new palladium/zinc
co-catalyst associated with chiral (R)-Di-
fluorphos for asymmetric ring-opening
reaction of oxabenzonorbornadienes
with phenolic or naphtholic nucleo-
philes was developed, which afforded
corresponding cis-2-aryloxy-1,2-dihydro-
naphthalen-1-ol products in good yields
(up to 95% yield) with excellent enan-
tioselectivities (up to 99% ee).
Palladium/Zinc Co-Catalyzed syn-
Stereoselectively Asymmetric Ring-
Opening Reaction of
Oxabenzonorbornadienes with
Phenols
An Asymmetric Ring-Opening Reaction
For the ring-opening addition of oxabenzonorborna-
dienes with oxygen- or nitrogen-based nucleophiles,
only trans-configuration products were observed gener-
ally with Rh, Ir or Ru catalysts. In the Communication
by B. Fan, J. Wang and co-workers on page && ff.,
a new palladium/zinc co-catalyst associated with chiral
(
R)-Difluorphos for asymmetric ring-opening reaction of
oxabenzonorbornadienes with phenol was developed,
which afforded corresponding cis-2-aryloxy-1,2-dihydro-
naphthalen-1-ol products in good yields (up to 95%
yield) with excellent enantioselectivities (up to 99% ee).
&
&
Chem. Eur. J. 2015, 21, 1 – 6
www.chemeurj.org
6
ꢀ 2015 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
ÝÝ These are not the final page numbers!