Iridium-catalyzed asymmetric ring-opening reactions of oxabenzonorbornadienes with amine nucleophiles

Article abstract of DOI:10.1021/om100384q

We have explored a new iridium-catalyzed ring-opening reaction of oxabenzonorbornadienes with a variety of primary aromatic amine or N-substituted piperazine nucleophiles, affording the corresponding products in excellent yields (up to 99%) with moderate enantioselectivity (25-81% ee). The trans configuration of product 2d was confirmed by X-ray crystallography.

Full text of DOI:10.1021/om100384q

Organometallics 2010, 29, 3477–3480 3477
DOI: 10.1021/om100384q
Iridium-Catalyzed Asymmetric Ring-Opening Reactions
of Oxabenzonorbornadienes with Amine Nucleophiles
Dingqiao Yang,*,† Yuhua Long,*,† Junfang Zhang,† Heping Zeng,†
Sanyong Wang,‡ and Chunrong Li‡
†School of Chemistry and Environment, South China Normal University, Guangzhou 510006, People’s
Republic of China, and ‡Guangdong Food Industry Institute, Guangzhou 510308, People’s Republic of China
Received April 30, 2010
Summary: We have explored a new iridium-catalyzed ring-
opening reaction of oxabenzonorbornadienes with a variety of
primary aromatic amine or N-substituted piperazine nucleo-
philes, affording the corresponding products in excellent yields
(up to 99%) with moderate enantioselectivity (25-81% ee).
The trans configuration of product 2d was confirmed by X-ray
crystallography.
reactions have attracted increasing attention because those
reactions are characterized with excellent yields and high
enantioselectivity,2 as demonstrated by the research works
reported by Lautens,3 Hou,4 Cheng,5 et al. Many transition-
metal complexes, including copper,6 palladium,7 iron,8 ruthe-
nium,9 rhodium,10 and nickel11 may be used for the asymmetric
ring-opening reactions of oxabicyclic alkenes. In recent de-
cades, the transition metal iridium12 has been widely used as
a catalyst for the synthesis of enantiomerically pure compounds.
In addition, various nucleophiles have been explored for this
type of reaction. For example, Lautens and co-workers
reported the rhodium-catalyzed asymmetric ring opening
of oxabenzonorbornadiene with a wide range of nucleophiles
such as phenols,3d organoboronic acids,3e dialkylzincs,3f,g carb-
oxylates,3h sulfur nucleophiles,3i amines,3j etc.
Ring-opening reactions of oxabicyclic alkenes represent a
useful method in modern organic synthesis.1 Research topics
related to transition-metal-catalyzed asymmetric ring-opening
*To whom correspondence should be addressed. E-mail: yangdq@
scnu.edu.cn (D.Y.); yuhualong68@hotmail.com (Y.L.). Tel: þ86 20
39310068. Fax: þ86 20 85210087.
Recently, we have reported13 a promising catalytic system
for enantioselective asymmetric ring-opening reactions with
the transition metal iridium. To further explore this novel
catalytic system, in this paper, we report our new findings on
ring-opening reactions of oxabicyclic alkenes. The novel
catalytic system demonstrates a potential useful method
for the synthesis of trans-1,2-dihydronaphthalenol deriva-
tives. The products of these reactions are interesting in
themselves as potential therapeutic agents and are valuable
building blocks for complex polycyclic skeletal motifs.
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Results and Discussion. We began our studies by optimi-
zing the iridium catalyst system (Table 1). Aniline was used
as nucleophile to react with oxabenzonorbornadiene (1a) in
the presence of iridium (1.5 mol %) and bisphosphine ligand
(3 mol %). We first chose the inexpensive ligand 1,10-bis-
(diphenylphosphino)ferrocene (DPPF), so as to validate the
catalytic activity of the iridium complex (Table1 1, entry 1).
Then we chose three different chiral ligands which have been
widely used in asymmetric ring-opening reactions. We found
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B. X.; Sun, J.; Hou, X. L. Org. Lett. 2004, 6, 2833–2835.
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r
2010 American Chemical Society
Published on Web 07/15/2010
pubs.acs.org/Organometallics

3478 Organometallics, Vol. 29, No. 16, 2010
Yang et al.
Table 1. Condition Screening for Iridium-Catalyzed Asymmetric Ring Opening of Oxabenzonorbornadiene 1a with Anilinea
entry
ligand
DPPF
(R,S)-PPF-PtBu2
(S)-BINAP
solvent
additiveb
tempc (°C)
time (h)
yieldd (%)
eee (%)
1
2
3
4
5
6
7
8
THF
THF
THF
THF
DME
CH3CN
toluene
dioxane
THP
CH2Cl2
THF
THF
80
80
80
80
90
90
110
100
100
50
50
48
2
23
36
91
93
91
24
51
87
81
33
n.r.
25
52
71
94
0
13
35
61
53
31
17
7
(S)-p-Tol-BINAP
(S)-p-Tol-BINAP
(S)-p-Tol-BINAP
(S)-p-Tol-BINAP
(S)-p-Tol-BINAP
(S)-p-Tol-BINAP
(S)-p-Tol-BINAP
(S)-p-Tol-BINAP
(S)-p-Tol-BINAP
(S)-p-Tol-BINAP
(S)-p-Tol-BINAP
(S)-p-Tol-BINAP
2
12
12
51
12
5
9
46
63
10
11
12
13
14
15
8
NH4F
NH4Cl
NH4Br
NH4I
80
80
80
80
55
50
50
24
2
50
59
62
79
THF
THF
THF
Bu4NI
80
a The reaction was carried out with 1a (0.20 mmol) and 3.0 equiv of aniline (0.9 mmol) in solvent (2.0 mL) in the presence of [Ir(COD)Cl]2 (1.5 mol %)
and ligand (3.0 mol %). b 1.0 equiv of additive. c Oil bath temperature. d Isolated yield after silica gel column chromatography. e Determined by HPLC
with a Chiralcel AD-H column or OD-H column.
that the ligands (S)-p-Tol-BINAP and (S)-BINAP gave 2a
in reasonable yields (91 and 93%, respectively), but only the
ligand (S)-p-Tol-BINAP offered product 2a with an enantio-
selectivity (61%) higher than that of other ligands (Table 1,
entries 2-4) in THF. Next, we investigated the impact of
solvents on the reaction. It was found that the reactions were
slower in DME, THP, dioxane, toluene, CH2Cl2, and MeCN
than in THF (Table 1, entries 5-10). On the basis of those
observations, we chose THF as a solvent. To further opti-
mize the reaction conditions, we had investigated five ammo-
nium halides as additives. Control experiments indicated
that, in the presence of Bu4NI, reactions underwent smoothly
and the yield was improved to 94% and the enantioselectivity
was up to 79% ee; however, no reaction occurred when NH4F
was used (Table 1, entries 11-15). The use of halide additives
may affect the enantioselectivity of the asymmetric ring-open-
ing reaction with amines. Removing the chloride ligand from
the coordination sphere of the iridium and replacing it with
iodide prior to the addition of reagents and additives further
improves the enantioselectivity. Those findings suggested that
the additive might play an important role in transition-metal
catalysis. Furthermore, we observed the reactivity of halides in
the order F<Cl<Br<I. Experimental evidence supports the
hypothesis that the halide additives are acting at the iridium
metal (Table 1, entries 11-15).
On the basis of these findings, we finalized our optimal con-
ditions as the following: 1.5 mol % of catalyst [Ir(cod)Cl]2,
3 mol % of chiral ligand (S)-p-Tol-BINAP, 3 equiv of pri-
mary amine with 1 equiv of additive in THF at 80 °C and
stirring for 2 h.
Having identified conditions which were favorable for the
iridium-catalyzed ring opening of 1a with aniline, we were
interested in understanding how the substituted groups in
aniline would impact the ring-opening reaction of 1a. Thus,
1a reacted with substituted anliline nucleophiles to give the
corresponding ring-opening products 2a-n (Table 2).
To evaluate the scope of the reaction, anilines with electron-
withdrawing substituent groups were used as nucleophilic
reagents; the reaction results are summarized in Table 2.
Table 2. Scope of Ring-Opening Reactions of
Oxabenzonorbornadiene 1a with Substituted Anilinea
entry
R
product time (h) yieldb (%) eec (%)
1
2
3
4
5
6
7
8
H
2a
2b
2c
2d
2e
2f
2g
2h
2i
2j
2k
2l
2m
2n
2
3
3
3
21
2
2
4.5
4.5
3
3.5
3
3.5
3
94
95
80
96
58
96
98
98
97
96
96
94
96
97
78
35
56
57
45
25
47
54
33
50
50
61
55
54
4-Br
2-Br
3-Br
2,4-dibromo
4-Cl
4-CH3
2-CH3
3-CH3
4-CH(CH3)2
4-CH2CH2CH3
4-OCH3
9
10
11
12
13
14
2,4-dimethoxy
N-naphthyl
a The reaction was carried out with 1a (0.30 mmol), 3.0 equiv of
substituted aniline (0.9 mmol) and 1.0 equiv of additive in THF (2.0 mL)
at 80 °C (oil bath temperature) in the presence of [Ir(COD)Cl]2 (1.5 mol %)
and (S)-p-Tol-BINAP (3.0 mol %). b Isolated yield after silica gel column
chromatography. c Determined by HPLC with a Chiralcel AD-H column
or OD-H column.
When R=2,4-dibromo, the yield was decreased to 58% and
a longer reaction time was required; The enantioselectivity
was the lowest when 4-chloroaniline was used as a nucleo-
phile reagent (only 25% ee) (Table 2, entry 6). However,
treating oxabenzonorbornadiene (1a) with 3-bromoaniline,
we obtained good yield (96%) and acceptable enantioselectivity
(57% ee) (Table 2, entry 4). In the ring-opening reactions of
oxabenzonorbornadiene (1a) with electron-donating groups,
we found that, in almost all cases, we obtained good yields
(94-98%) with moderate enantioselectivity (33-61% ee)
(Table 2, entries 7-14).

Communication
Organometallics, Vol. 29, No. 16, 2010 3479
Table 3. Scope of Asymmetric Ring-Opening Reactions of
Dimethoxy-Substituted Oxabenzonorbornadienes 1b,c with
Substituted Anilinesa
Table 4. Scope of Asymmetric Ring-Opening Reactions of
Substituted Oxabenzonorbornadienes 1a,b with N-Substituted
Piperazinesa
entry substrate
R
product time (h) yieldb (%) eec (%)
time yieldb
product (h)
entry substrate
R
(%) eec(%)
1
2
3
4
5
6
7
8
1b
1b
1b
1b
1b
1b
1b
1b
1b
1b
1b
1c
1c
1c
H
3a
3b
3c
3d
3e
3f
20
20
5.5
5.5
9
3
87
93
52
54
21
97
98
73
97
95
38
97
99
97
64
51
70
66
81
26
64
68
32
77
66
52
52
50
4-Br
2-Br
3-Br
1
2
3
4
1a
1a
1a
1a
4-methoxyphenyl
2-fluorophenyl
2-chlorophenyl
2,5-
difluorophenyl
4-methoxyphenyl
2-fluorophenyl
2o
2p
2q
2r
10
10
8
85
84
89
97
54
55
50
43
2,4-dibromo
4-CH3
2-CH3
10
3g
3h
3i
3
5.5
3
5
6
1b
1b
3l
3m
24
24
62
79
59
38
3-CH3
9
4-OCH3
2,4-dimethoxy
N-naphthyl
H
4-Br
4-CH3
a The reaction was carried out with substrate (0.30 mmol), 3.0 equiv of
N-substituted piperazines (0.9 mmol), and 1.0 equiv of additive in THF
(2.0 mL) at 80 °C (oil bath temperature) in the presence of [Ir(COD)Cl]2
(1.5 mol %) and (S)-p-Tol-BINAP (3.0 mol %). b Isolated yield after silica
gel column chromatography. c Determined by HPLC with a Chiralcel AD-H
column.
10
11
12
13
14
3j
6
20
3
3
3
3k
4a
4b
4c
a The reaction was carried out with substrate (0.30 mmol), 3.0 equiv of
substituted aniline (0.9 mmol), and 1.0 equiv of additive in THF (2.0 mL)
at 80 °C (oil bath temperature) in the presence of [Ir(COD)Cl]2 (1.5 mol
%) and (S)-p-Tol-BINAP (3.0 mol %). b Isolated yield after silica
gel column chromatography. c Determined by HPLC with a Chiralcel
AD-H column or OD-H column.
Scheme 1
To further explore the reaction scope, we used several
substituted oxabenzonorbornadienes 1b,c to perform the
reactions (Table 3). We found that substrate 1c underwent
addition reactions with various primary aromatic amines
proceeded at 80 °C in good yields (97-99%) but with lower
enantioselectivities (50-52% ee) (Table 3, entries 12-14)
than for the dimethoxyoxabenzonorbornadiene 1b (Table 3,
entries 1-11). However, under the same reaction conditions,
the dimethoxy-substituted oxabenzonorbornadiene 1b with
primary aromatic amines afforded the corresponding ring-
opening addition products with lower yields and higher
enantioselectivities (Table 3, entries 1-11) than for other
substrates. As shown in Table 3, when R = 2-Br, 4-CH3,
4-OCH3 on aniline, the yields were high and the enantios-
electivities were lower (Table 3, entries 2, 6, and 9).
The structures of the ring-opening products were deter-
mined and characterized by 1H and 13C NMR, IR, HRMS,
and elemental analysis. We found that these products possess
a trans configuration. The absolute configuration of the ring-
opening product 2d was confirmed by single-crystal X-ray
analysis. The crystal was obtained by solvent evaporation
from its solution in dichloromethane, acetone, ethanol, and
petroleum ether. Its configuration was assigned as 1R,2R, and
the hydroxyl group and amine group are in a trans pattern.
Having investigated the ring-opening reactions of oxaben-
zonorbornadiene with primary aromatic amines, we wish
to explore the same ring-opening reaction with secondary
amines. Thus, protected piperazine nucleophiles were attempted.
The results are shown in Table 4. The results demonstrated
that the ring-opening reaction of oxabenzonorbornadiene
with protected piperazines offered reasonable yields and
moderate entioselectivies in the presence of iridium catalyst
(Table 4, entries 1-6). The absolute configuration of 2r was
analyzed by X-ray crystallography, and the 1-hydroxyl group
and 2-piperzinyl group of 2r were found to be in trans posi-
tions. Its configuration was also assigned as 1S,2S.
On the basis of our observations and findings, we propose
the reaction mechanism detailed in Scheme 1. The chiral dimeric
iridium complex A is first formed. The oxygen atom and the
double bond of oxabenzonorbornadiene 1a are then reversibly
coordinated to the iridium center of the catalyst to give the inter-
mediate B. Oxidative insertion of B into the C-O bond forms
C. Then, attack of the amine nucleophile along with configu-
rational inversion is proposed to occur in an SN2 displacement
of the iridium catalyst. The trans-1,2-dihydronaphthalenol
product 2 is subsequently released, and A is regenerated.
Conclusion. In conclusion, we have explored a new iridium-
catalyzed ring-opening reaction of oxabenzonorbornadienes

3480 Organometallics, Vol. 29, No. 16, 2010
Yang et al.
with a variety of primary aromatic amine or N-substituted
piperazine nucleophiles to afford the corresponding pro-
ducts with moderate enantioselectivity (25%∼81% ee) in
excellent yields (up to 99%). New chiral catalysts allowed the
isolation of ring-opened products in higher enantiomeric
excess using lower catalyst loadings. Studies on further ex-
pansion of the scope and synthetic utility of this Ir-catalyzed
method are also being pursued in our laboratory.
254 nm). Retention times were 31.2 min (major) and 40.8 min
(minor). [R]20 = þ159.4° (c = 1.00, CHCl3). IR (KBr,
D
cm-1): 3233 (s), 3057 (w), 3014 (m), 1601 (s), 1498 (s), 1450
(w), 1425 (w), 1312 (m), 1185 (m), 1046 (m), 783 (s), 695 (s).
1H NMR (400 MHz, CDCl3): δ 7.47 (d, J=6.4 Hz, 1H), 7.29
(t, J = 5.6 Hz, 2H), 7.21 (t, J = 7.6 Hz, 2H), 7.15 (d, J =
6.4 Hz, 1H), 6.78 (t, J=6.8 Hz, 1H), 6.71 (d, J=7.6 Hz, 2H),
6.56 (d, J=9.6 Hz, 1H), 6.01 (d, J=7.2 Hz, 1H), 4.84 (d, J=
7.6 Hz, 1H), 4.31 (d, J = 4.4 Hz, 1H), 3.37 (s, 1H), 2.60 (s,
1H); 13C NMR (100 MHz, CDCl3): δ 146.6, 135.6, 131.9,
129.5, 128.5, 128.5, 128.2, 128.0, 127.0, 126.7, 118.4, 114.1,
71.5, 55.6. MS (ESI): calcd m/z for C16H15NO (Mþ) 237.12,
found 238.02 (M þ H)þ. Anal. Calcd for C16H15NO: C,
80.98; H, 6.37; N, 5.90. Found: C, 81.21; H, 6.64; N, 6.12.
The ARO reactions of oxabenzonorbornadiene (1a) and
the dimethoxy-substituted oxabenzonorbornadienes 1b,c
with primary aromatic amines or N-substituted piperazines
are the same as the above representative procedure. See the
Supporting Information for details of the syntheses of the
new compounds 2b-r, 3a-m, and 4a-c.
Experimental Section. Representative Procedure for the Asym-
metric Ring-Opening Reactions of Oxabenzonorbornadiene
1a with Primary Amine Nucleophiles. A 5.0 mL round-
bottom flask fitted with a reflux condenser was flame-dried
under a stream of nitrogen and cooled to room tempera-
ture. [Ir(COD)Cl]2 (3.0 mg, 1.5 mol %) and (S)-p-Tol-BINAP
(6.1 mg, 3 mol %) were simultaneously added and followed
by addition of anhydrous tetrahydrofuran (2.0 mL). After
the mixture was stirred for 10 min, 1.0 equiv. Bu4NI (110.7 mg)
was added. After stirring for another 10 min, oxabenzonor-
bornadiene 1a (43.2 mg, 0.3 mmol) was added and the
mixture was heated to reflux, followed by adding primary
aromatic amine (3 equiv. to 1a). The temperature was con-
tinuously increased to 80 °C until the reaction was completed
as judged by thin layer chromatography. The solvent was
removed in vacuo and the crude mixture was purified by
column chromatography (Silica Gel: 200-300 mesh) to give
the target product.
Acknowledgment. We are grateful to the National
Natural Science Foundation of China (Nos. 20772036
and 20802021) and the Natural Science Foundation of
Guangdong Province (Nos. 8251063101000002 and 7005804)
for financial support.
(1R,2R)-2-Phenylamino-1,2-dihydro-naphthalen-1-ol (2a).
Following the representative procedure, 2a was obtained as
a white solid (66.8 mg, 94%). Rf = 0.14 on silica gel (1/10
ethyl acetate/petroleum ether, v/v). Mp: 93-95 °C. The ee
was determined to be 78% using HPLC analysis on a Chiralcel
AD-H column (90/10 hexane/2-propanol, 0.5 mL/min, λ =
Supporting Information Available: Text, figures, and tables
giving experimental procedures and full characterization data
for all new compounds, including optical rotations, IR, 1H and
13C NMR, MS, and elemental analyses, and X-ray structure data
for compounds 2d,r. This material is available free of charge via
the Internet at http://pubs.acs.org.