Palladium/copper complexes co-catalyzed highly enantioselective ring opening reaction of azabenzonorbornadienes with terminal alkynes

Article abstract of DOI:10.1002/adsc.201300748

A chiral palladium/copper (Pd/Cu) cocatalytic system which could enable asymmetric ring opening reaction of azabenzonorbornadienes with terminal alkynes is reported. This system comprises palladium(II) acetate [Pd(OAc) ₂]/(R)-xyl-Binap and copper(I) triflate (CuOTf). A good substrate scope was found, with excellent enantioselective outcomes (up to >99.9% ee). A reaction mechanism involving two metal complexes is also proposed.

Full text of DOI:10.1002/adsc.201300748

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DOI: 10.1002/adsc.201300748
Palladium/Copper Complexes Co-Catalyzed Highly
Enantioselective Ring Opening Reaction of
Azabenzonorbornadienes with Terminal Alkynes
Baomin Fan,a,* Sifeng Li,a,b Hualei Chen,a Zhiwu Lu,a Shanshan Liu,a
Qingjing Yang,a Lu Yu,a Jianbin Xu,a Yongyun Zhou,a and Jun Wangb,*
a
Key Laboratory of Chemistry in Ethnic Medicinal Resources, State Ethnic Affairs Commission & Ministry of Education,
Yunnan University of Nationalities, Kunming 650500, Peopleꢀs Republic of China
Fax : (+86)-871-6591-3103; e-mail: adams.bmf@hotmail.com
Department of Chemistry, South University of Science and Technology of China, Shenzhen, Guangdong 518055, Peopleꢀs
b
Republic of China
Fax : (+86)-755-8624-5672; e-mail: wang.j@sustc.edu.cn
Received: August 17, 2013; Published online: October 9, 2013
Supporting information for this article is available on the WWW under http://dx.doi.org/10.1002/adsc.201300748.
Abstract: A chiral palladium/copper (Pd/Cu) co-
catalytic system which could enable asymmetric
ring opening reaction of azabenzonorbornadienes
with terminal alkynes is reported. This system com-
enantioselective ring opening reaction of oxa- and
azabenzonorbornadienes with terminal alkynes was
reported by Cheng and co-workers in 2002.[8] Never-
theless, successful chiral catalysts developed for the
enantioselective version of this transformation are
still rare. In 2008, Hayashi and co-workers investigat-
ed this challenge by using a rhodium catalyst associat-
ed with the (R)-DTBM-Segphos ligand.[9] This exam-
ple represents the sole effective chiral catalyst report-
ed so far in this area. Although up to 99% ee could
be achieved in this reaction, the chiral rhodium cata-
lyst still encountered a limited substrate scope. Only
sterically bulky acetylenes such as (triisopropylsilyl)-
prises palladium(II) acetate [PdACTHNUTRGENUN(G OAc)2]/(R)-xyl-
Binap and copper(I) triflate (CuOTf). A good sub-
strate scope was found, with excellent enantioselec-
tive outcomes (up to >99.9% ee). A reaction mech-
anism involving two metal complexes is also pro-
posed.
Keywords: asymmetric catalysis; azabenzonorbor-
nadienes; ring opening reaction; terminal alkynes
AHCTUNGTREGaNNNU cetylene resulted in a high enantioselective outcome.
Thus, the development of new and effective chiral cat-
alytic systems for this transformation is still in signifi-
cant demand. The exploration of a general catalyst
Asymmetric ring opening reactions of heterobicyclic which could handle a broad substrate scope remains
alkenes with carbo-nucleophiles have been developed a great challenge.
rapidly in the past decade.[1] This synthetic protocol
Our group has been involved in the asymmetric re-
has emerged as an important and versatile carbon- actions of norbornadiene derivatives in the past years.
carbon bond-forming reaction. Indeed, with appropri- We have established two different chiral iridium cata-
ate chiral transition metal complexes employed, lysts and realized the asymmetric [2+2]cycloaddition
a number of different organometallic reagents, for in- reaction[10] and the asymmetric hydroalkynylation re-
stance, organozinc,[2] lithium,[3] Grignard,[4] alumi- action.[11] Using (S)-NMDPP as chiral ligand, we had
num[5] and boron reagents,[6] have been found as feasi- also developed an efficient iridium catalyst for the
ble nucleophiles in this type of reactions. However, asymmetric ring opening reaction of oxabenzonorbor-
the successful application of neutral carbo-nucleo- nadienes with phenols or naphthols.[12] Recently, we
philes in this type of reactions is still a challenge.
found that the complex of PdCl2 and Xantphos could
Terminal alkynes are neutral, easily available and promote the ring opening reaction of oxa/azabenzo-
have good further transformation ability.[7] If terminal norbornadienes smoothly by employing arylacetylenes
alkynes could be employed as carbo-nucleophiles in as nuleophiles.[13] In continuation of our study, here
the ring opening reactions of heterobicyclic alkenes, it we report a highly effective chiral Pd/Cu co-catalytic
could be seen as a significant improvement. The first system for the first general asymmetric ring opening
example of an achiral nickel catalyst for the non-
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Table 1. The initial establishment of the chiral Pd catalytic system.[a]
Entry
Ligand
Time [h]
Yield [%][b]
ee [%][c]
1[d,e]
2[d,e]
3[d,e]
4
5
6
(R)-Binap
72
72
72
24
5
14
18
28
58
77
90
86
64
89
52
34
88
(R)-tol-Binap
(R)-xyl-Binap
(R)-Binap
(R)-tol-Binap
(R)-xyl-Binap
1
[a]
Reaction conditions: 2a (0.2 mmol), 1a/2a/PdACTHNUTRGNE(UNG OAc)2/ligand/AgOTf (5:1:0.05:0.06:0.10), in DCE (2 mL) at 708C (oil bath
temperature) under argon for the indicated period of time; DCE=dichloroethane.
Yield of isolated product after column chromatography.
[b]
[c]
[d]
[e]
The ee values were determined by HPLC using the chiral stationary phase Chiralcel OD-H.
The reaction was carried out without AgOTf.
1
The reaction was not completed, and the yield was determined by H NMR.
reaction of azabenzonorbornadienes with simple ter- (Table 2). At 708C, commonly used THF, toluene, di-
minal alkynes. oxane, THP and MTBE all provided good yields and
Our initial trials employing Pd/chiral phosphine cat- ees (entries 2–6). Along with a good yield, the highest
alyst in the asymmetric ring opening reaction of aza- ee of 3aa (up to 89%) was given by DME, and only
benzonorbornadienes and terminal alkynes were dis- 10 min were needed to complete the reaction
appointing. The model reaction of phenylacetylene (entry 7). It should be noted that i-PrOH (isopropyl
(1a) and azabenzonorbornadiene 2a resulted in low alcohol) could also be used as solvent here and gave
product yields, although good enantioselectivities a good ee value but a poor yield for the product
were observed (Table 1, entries 1–3). We rationalized (entry 8), which means this Pd/Lewis acid catalytic
À
that the relatively high stability of the C N bond in system holds special activity for terminal alkynes
azabenzonorbornadiene 2a and the low nucleophilici- against alcohols. The temperature screen showed that
ty of phenylacetylene (1a) would be responsible for on increasing the reaction temperature the enantiose-
the failure. In order to tackle this problematic issue, lectivity of the reaction deteriorated (entry 9). Lower-
a Lewis acid was added as co-catalyst in the reaction ing the reaction temperature resulted a rise in the
for activating the phenylacetylene 1a[14] and/or the enantioselectivity (entries 10–13), and 97% ee with
azabenzonorbornadiene 2a.[15] To our delight, superior 92% yield could be obtained at 08C (entry 12). Even
results were obtained. After the addition of AgOTf, at À208C, the reaction could still take place smoothly
both the rate of reaction and yield of desired product and was completed in 3 h, but the result was not obvi-
were increased. The reaction between 2a and 1a pro- ously distinguishable from that at 08C (entry 13).
ceeded smoothly at 708C in DCE when (R)-Binap,
The effects of Lewis acids and palladium precursors
Pd(OAc)2 and AgOTf were employed. The sterically were also investigated (Table 3). Use of AgPF6 and
ACHTUNGTRENNUNG
more congested (R)-xyl-Binap was found to be the AgBF4 resulted in low catalytic activity (entries 2 and
most effective ligand, which offered good-to-excellent 3), and AgSbF6 also proved to be inferior (entry 4).
product yield and enantioselectivity (entry 6).[16]
Beside AgOTf, other trifluoromethanesulfonate salts,
Using (R)-xyl-Binap as chiral ligand, the conditions including Cu(OTf)2, Zn(OTf)2, Fe(OTf)3 and
for this Pd(OAc)2/AgOTf co-catalyzed asymmetric Fe(OTf)2 were tested (entries 5–8). CuOTf was found
ring opening reaction were further optimized to be the best choice (entry 9). Pd(acac)2, PdCl2 and
A
R
ACHTUNGTRENNUNG
A
ACHTUNGTRENNUNG
AHCTUNGTRENNUNG
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Palladium/Copper Complexes Co-Catalyzed Highly Enantioselective Ring Opening Reaction
Table 2. Optimization of the reaction conditions.[a]
Table 3. Screening of Lewis acids and palladium pre-
AHCTUNGTRENNUNG
cursors.[a]
Entry Temp. [oC] Solvent Time
Yield [%][b] ee [%][c]
Entry [Pd]
Lewis acid Time Yield [%][b] ee [%][c]
1
2
3
4
5
6
70
70
70
70
70
70
70
70
90
50
r.t
0
DCE
THF
toluene 10 h
dioxane 3 h
THP
MTBE 4 h
DME
i-PrOH 72 h
DME
DME
DME
DME
DME
1 h
24 h
90
61
86
91
86
94
88
74
78
67
80
85
89
69
87
90
92
97
98
1
Pd
Pd(OAc)2 AgPF6
Pd(OAc)2 AgBF4
Pd(OAc)2 AgSbF6
Pd(OAc)2 Cu
Pd(OAc)2 Zn
Pd(OAc)2 Fe(OTf)3 56 h
Pd(OAc)2 Fe(OTf)2 60 h
Pd(OAc)2 CuOTf
(acac)2 CuOTf
G
1 h
92
20
12
trace
91
90
70
78
97
24
6
97
99
65
/
2[d]
3[d]
4[d]
5
72 h
72 h
72 h
4 h
N
ACHTUNGTRENNUNG
90
68
78
73
97
97
70
61
96
91
96
–
6
7
8
9
7
10 min 91
13
5 min 87
15 min 90
30 min 90
8[d]
9
ACHTUNGTRENNUNG
ACHTUNGTRENNUNG
1 h
10
11
12
13
10[d] Pd
72 h
72 h
72 h
20 h
72 h
11[d] PdCl2
CuOTf
CuOTf
CuOTf
1 h
3 h
92
94
12[d] PdBr2
9
80
24
À20
13
PdI2
[a]
14[d,e] Pd
ACHTUNGTRENNUNG
G
Reaction conditions: 2a (0.2 mmol), 1a/2a/PdACTHNUGTRENUNG(OAc)2/(R)-
15[f]
16
10 min 96
72 h trace
xyl-Binap/AgOTf (5:1:0.05:0.06:0.10), in 2 mL solvent at
the stated oil bath temperature under argon for the indi-
cated period of time. THF=tetrahydrofuran, THP=tet-
rahydrapyran, MTBE=methyl tert-butyl ether, DME=
dimethoxyethane.
Yield of isolated product after column chromatography.
The ee values were determined by HPLC using the chiral
stationary phase Chiralcel OD-H.
–
[a]
Reaction conditions: 2a (0.2 mmol), 1a/2a/[Pd]/(R)-xyl-
Binap/Lewis acid (5:1:0.05:0.06:0.10), in DME (2 mL) at
08C under argon for the indicated period of time.
Yield of isolated product after column chromatography.
The ee values were determined by HPLC using the chiral
stationary phase Chiralcel OD-H.
[b]
[c]
[b]
[c]
[d]
The reaction was not completed, and the yield was deter-
[d]
1
The reaction was not completed, and the yield was deter-
mined by H NMR.
1
mined by H NMR.
5 mol% CuOTf was added.
15 mol% CuOTf was added.
[e]
[f]
PdBr2 were not suitable catalyst precursors, and gen-
erally resulted in low product yields (entries 10–12).
PdI2 gave
(entry 13). The best stoichiometry of CuOTf used was (entries 13 and 14).
found to be two equivalents of Pd(OAc)2. Lowering In addition to the alkyne moiety, an array of aza-
a comparable result with PdACHTUNTGERNUN(G OAc)2 products in high yields and high enantioselectivities
AHCTUNGTRENNUNG
the amount of CuOTf resulted in an obvious drop of benzonorbornadienes was also tested in the ring
the reaction rate, yield and ee (entry 14). Further in- opening reaction with phenylacetylene (Table 5). Sub-
creasing the loading of CuOTf was shown to be un- stituted azabenzonorbornadienes 2 were proven to be
necessary, although the reaction rate could be acceler- applicable substrates. With regard to the Boc (tert-
ated (entry 15). And CuOTf was proven to be an un- but
ACHTUNGTRENoNUNG xycarbonyl group) protected azabenzonorborna-
suitable metallic precursor (entry 16). dienes, there were no deleterious effect of substitu-
With the optimized reaction conditions in hand, tents on the aromatic ring (entries 1–5). A methoxy-
a series of substituted terminal alkynes 1 was exam- carbonyl group was also found to be a suitable pro-
ined (Table 4). Arylacetylenes with electron-donating tecting group. Excellent enantioselectivity with good
or electron-withdrawing groups could all react with 2a yield could be obtained for the reaction of the corre-
smoothly and provided the ring-opening products in sponding substrate (entry 6). When Bz (benzoyl
good yields and excellent enantioselectivities (en- group), Ts (p-toluenesulfonyl group) and Ns (p-nitro-
tries 1–12). Remarkably, a Br group was well-tolerat- benzenesulfonyl group) were used to protect the
ed by this chiral Pd/Cu catalytic system (entry 8). This amino group in azabenzonorbornadienes, an obvious
compatibility would be versatile for further function- decrease of reaction rate was observed. This problem
alization using cross-coupling methods.[17] The hy- could be solved by raising the reaction temperature
droxy group was found to be compatible with these and increasing the loading of CuOTf. When the reac-
conditions (entry 12). Alkyl and triethylsilyl (TES) tions were carried out at 508C, good yields could be
substituted acetylenes could furnish the ring opening achieved with excellent enantioselectivities (>99.9%
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Baomin Fan et al.
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Table 4. Pd-catalyzed asymmetric ring opening reaction of
2a with various terminal alkynes.[a]
Table 5. Pd-catalyzed asymmetric ring opening reaction of
substituted azabenzonorbornadienes with phenylacetylene
1a.[a]
Entry
R
1
Time
[h]
Yield
[%][b]
ee
[%][c]
1
2
3
4
5
6
7
8
Ph
1a
1b
1c
1d
1e
1f
1g
1h
1i
1j
1k
1l
1m
1n
1
97
97
97
98
90
94
81
87
86
88
98
93
97
98
97
94
93
93
91
95
96
97
>99.9
96
99.3
91
95
95
Entry R1, R2
2
Time[h] Yield [%][b] ee [%][c]
4-MeOC6H4
3-MeOC6H4
3,5-diMeOC6H3
4-MeC6H4
4-PhOC6H4
4-CF3OC6H4
4-BrC6H4
4-CF3C6H4
4-FC6H4
16
16
6
4
30
6
1
H, Boc
2a
2b
2c
1
2
5
4
2
97
83
98
89
82
89
61
81
62
97
90
93
94
2
3
OMe, Boc
Me, Boc
4
5
-OCH2O-, Boc 2d
Br, Boc 2e
96
6
4
6
H, C(O)OCH3 2f 24
98
9
7[d,e]
8[d,e]
9[d,e]
H, Bz
H, Ts
H, Ns
2g 36
2h 45
2i 160
>99.9
>99.9
>99.9
10
11
12
13[d]
14
24
127
6
70
2
4-CNC6H4
4-HOCH2C6H4
n-Bu
[a]
Reaction conditions: 2 (0.2 mmol), 1a/2/PdACTHNUGTRENUNG(OAc)2/(R)-
Et3Si
xyl-Binap/CuOTf (5:1:0.05:0.06:0.10), in DME (2 mL) at
08C under argon for the indicated period of time.
Yield of isolated product after column chromatography.
The ee values were determined by HPLC using the chiral
stationary phase Chiralcel OD-H or AD-H.
[a]
Reaction conditions: 2a (0.2 mmol), 1/2a/PdACTHNUGTRENUNG(OAc)2/(R)-
[b]
[c]
xyl-Binap/CuOTf (5:1:0.05:0.06:0.10), in DME (2 mL) at
08C under argon for the indicated period of time.
Yield of isolated product after column chromatography.
The ee values were determined by HPLC using the chiral
stationary phase Chiralcel OD-H or AD-H.
[b]
[c]
[d]
[e]
The reaction was carried out in an oil bath at 508C.
15 mol% CuOTf was added.
[d]
15 mol% CuOTf was added.
Various aryl- or alkylacetylenes could be applied as
suitable nucleophiles in the reaction, and OH, CN,
ee) (entries 7–9). Apart from the azabenzonorborna- Br, F, and TES groups remain intact during the
dienes, we have surveyed the oxabenzonorborna- course of reaction. Further detailed mechanistic inves-
dienes as substrates in this asymmetric ring opening tigations and pharmaceutical synthetic applications
reaction. However, only poor yields of ring opening are in progress.
products were obtained up to now.
A proposed mechanism is shown in Scheme 1. The
phenylacetylene reacted with the copper Lewis acid
Experimental Section
to produce copper acetylide species B. Transmetalla-
tion of B with A, which was generated by a combina-
tion of Pd precursor and chiral ligand, resulted in the
formation of intermediate C. The coordination of the
alkene moiety D to intermediate C gave species E.
The intermediate F was formed by the migratory in-
Typical Procedure for the Synthesis of 3aa
PdACTHUNTRGENNG(U OAc)2 (2.2 mg, 0.01 mmol), (R)-xyl-Binap (8.8 mg,
0.012 mmol) and 1.0 mL DME were added to a Schlenk
tube under an argon atmosphere. The resulting solution was
sertion of a phenylethynyl group to the carbon- stirred at room temperature for 30 min, then 0.01 mmol
2CuOTf·toluene (5.2 mg, CAS: 37234-97-2) was added and
stirring was continued for additional 10 min. Azabenzonor-
bornadienes 2a (48.6 mg, 0.2 mmol) in DME (1.0 mL) was
added, and the mixture was stirred for additional 20 min.
After lowering the temperature of the mixture to 08C, phe-
nylacetylene 1a (111.3 mL, 1.0 mmol) was added with TLC
(thin layer chromatography) monitoring until the complete
consumption of 2a was seen- After vacuum evaporation of
the solvent, the residue was purified by silica gel column
chromatography to provide the desired product 3aa as
carbon double bond. Following beta-nitrogen elimina-
tion, the C N bond is broken and produced the inter-
mediate G. After a simple cation exchange, 3aa was
afforded as the final ring opening product.
In summary, we have successfully developed
a highly effective chiral Pd/Cu co-catalytic system for
asymmetric ring opening reactions of azabenzonor-
bornadienes with terminal alkynes (up to >99.9%
ee). This co-catalytic system exhibits wide substrate
À
compatibility and good functional group tolerance. a yellow solid; yield: 67.2 mg (97%); mp 110–1128C; 97%
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Palladium/Copper Complexes Co-Catalyzed Highly Enantioselective Ring Opening Reaction
Scheme 1. Mechanism for the Pd2+/Cu+-catalyzed asymmetric ring opening reaction.
ee by HPLC (Daicel Chiralcel OD-H, 25 cmꢂ0.46 cm, n-
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=
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1
4.58 min, tminor =6.36 min; H NMR (400 MHz, CDCl3): d=
7.38–7.29 (m, 3H), 7.28–7.18 (m, 5H), 7.14–7.04 (m, 1H),
6.55 (d, J=9.2 Hz, 1H), 6.05–5.95 (m, 1H), 5.32–4.95 (m,
2H), 3.77 (s, 1H), 1.47 (s, 9H); 13C NMR (CDCl3,
100 MHz): d 155.57, 134.64, 132.35, 131.83, 128.48, 128.23,
128.16, 128.04, 127.95, 126.77, 126.20, 125.97, 123.15, 86.36,
83.09, 79.73, 50.80, 32.96, 28.46; HR-MS: m/z=345.1713,
calcd. for C23H23NO2 [M]+: 345.1729.
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Acknowledgements
We thank the National Natural Science Foundation of China
(21362043, 21162040, 20902080), the Government of Yunnan
Province (2012FB170) and the State Key Laboratory of Ele-
mento-organic Chemistry, Nankai University, (201206) for fi-
nancial support.
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