A study on the substituent effects of norbornadiene derivatives in iridium-catalyzed asymmetric [2 + 2] cycloaddition reactions

Article abstract of DOI:10.1039/c3ob27382b

Employing a series of norbornadiene derivatives as substrates, the effects of various substituents on the Ir-catalyzed asymmetric [2 + 2] cycloaddition reactions with arylacetylenes were studied. It was found that the atom forming the short bridge chain had a great effect on the enantioselectivity of the reaction. Heteroatoms, such as oxygen and nitrogen, always resulted in excellent enantioselectivity. However, carbon atoms could decrease the enantioselective control ability of the catalyst over the reaction. The groups on the unreacted carbon-carbon double bond were found to have but a little effect on the reaction. Based on the results of the experiments, a mechanism was also hypothesized for the reaction.

Full text of DOI:10.1039/c3ob27382b

Organic &
Biomolecular Chemistry
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A study on the substituent effects of norbornadiene
derivatives in iridium-catalyzed asymmetric [2 + 2]
cycloaddition reactions†
Cite this: Org. Biomol. Chem., 2013, 11,
2294
Jun Hu,‡ Qingjing Yang,‡ Lu Yu, Jianbin Xu, Shanshan Liu, Chao Huang, Lin Wang,
Yongyun Zhou and Baomin Fan*
Employing a series of norbornadiene derivatives as substrates, the effects of various substituents on the
Ir-catalyzed asymmetric [2 + 2] cycloaddition reactions with arylacetylenes were studied. It was found
that the atom forming the short bridge chain had a great effect on the enantioselectivity of the reaction.
Heteroatoms, such as oxygen and nitrogen, always resulted in excellent enantioselectivity. However,
carbon atoms could decrease the enantioselective control ability of the catalyst over the reaction. The
groups on the unreacted carbon–carbon double bond were found to have but a little effect on the reac-
tion. Based on the results of the experiments, a mechanism was also hypothesized for the reaction.
Received 8th December 2012,
Accepted 29th January 2013
DOI: 10.1039/c3ob27382b
www.rsc.org/obc
oxabenzonorbornadienes with terminal alkynes, providing
chiral cyclobutene products with up to 99% ee.11 Interestingly,
Introduction
Promoted by extensive studies, transition metal catalyzed reac- by simply changing the diphosphine ligand from planar chiral
tions of norbornadiene derivatives and terminal alkynes have to axial chiral (e.g. SYNPHOS), no [2 + 2] cycloaddition pro-
shown high reactivity and amazing diversity in recent years. ducts were observed; instead hydroalkynylation products were
Catalyzed by different transition metals or under different cata- generated by direct addition of the C–H bond of terminal
lytic conditions, norbornadiene derivatives and terminal alkynes to the CvC bond of norbornadiene deratives.12 We
alkynes can undergo hydroalkynylation reaction,1 ring-opening had noticed that the bridge atoms in norbornadiene deriva-
reaction,2 homo-Diels–Alder reaction,3 cross coupling reac- tives had a strong effect on the enantioselectivity of hydroalky-
tion,4 [2 + 1],5 [2 + 2],6 [2 + 2 + 2]7 and other8 cycloaddition nylation reaction, since high to excellent enantioselectivities
reactions, leading to various cyclic products. Among all these could be obtained for carbo-norbornadienes (up to 97% ee),12a
reactions, the [2 + 2] cycloaddition reaction of norbornadiene and only moderate to good enantioselectivities were obtained
derivatives with terminal alkynes is even more outstanding, for oxa-norbornadienes (up to 85% ee).12b Enlightened by
since fused and strained cyclobutenes are generated as pro- these results, obvious substituent effects of norbornadiene
ducts, and four or more stereocenters can be formed at the derivatives on the Ir-catalyzed asymmetric [2 + 2] cycloaddition
same time.9 However, maybe due to the high reactivity of the reaction with terminal alkynes were expected. In this paper, we
sp1 C–H bond of terminal alkynes towards norbornadiene sub- report the results of applying a series of norbornadiene deriva-
strates, only limited progress had been achieved for the cata- tives with various substituents as substrates in the Ir-catalyzed
lytic [2
+
2] cycloaddition reaction of norbornadiene asymmetric [2 + 2] cycloaddition reaction with arylacetylenes.
Additionally, based on the experimental results, a reaction
derivatives with terminal alkynes.6,10
Recently, we found that iridium complexes employing mechanism was also hypothesized.
planar chiral ligands, such as Xylyl-phanephos, can promote
the asymmetric [2
+
2] cycloaddition reaction of
Results and discussion
Asymmetric [2 + 2] cycloaddition reaction of norbornadiene
derivatives with arylacetylenes
Key Laboratory of Chemistry in Ethnic Medicinal Resources, State Ethnic Affairs
Commission and Ministry of Education, Yunnan University of Nationalities,
Kunming, 650500, China. E-mail: adams.bmf@hotmail.com;
Fax: (+86)-871-5910017
Under the reaction conditions, which had been optimized in
our previous work, a series of substituted oxabenzonorborna-
dienes were used in the asymmetric [2 + 2] cycloaddition reac-
tion with phenylacetylene (Table 1). It seems that the
†Electronic supplementary information (ESI) available: Copies of 1H and 13C
NMR spectra of new compounds, and HPLC chromatograms. See DOI:
10.1039/c3ob27382b
‡These two authors contributed equally to this paper.
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substituents on the phenyl ring have little effect on the
enantioselectivity of the reaction, since all of the substrates
1b–1g could react with phenylacetylene smoothly to generate
the corresponding cyclobutene products with excellent ee values
(entries 2 to 7). Though the MeO group and Br group on the
phenyl ring gave comparable yields and similar ee values for
the reaction products, they did affect the reaction rate greatly
(entry 2 and entry 3). 48 hours were needed to complete the
reaction for substrate 1b and only 12 hours for 1c, which
means that electron-donating groups could decrease the reac-
tion rate, however electron-withdrawing groups increased it.
The steric hindrance effect of the substituents on the phenyl
ring was not very obvious, since substrates bearing different
bulky substituents on the phenyl ring got similar reaction
results (entry 4 to entry 7). However, the steric hindrance on
the bridge head carbons of the substrate was fatal for the reac-
tion. For example, the methyl groups in substrates 1h–1j
blocked the reactions completely, and no transformation took
place at all (entry 8 to entry 10).
Besides oxabenzonorbornadienes, three oxanorbornadienes
1k–1m without a benzo group were also tested in this iridium-
catalyzed asymmetric [2 + 2] cycloaddition reaction (Table 2).
Instead of phenylacetylene, 3,5-dimethoxyphenylacetylene was
used here in order to make the purification of products easier.
From the results, it could be seen that the benzo group in the
substrates was not necessary for the reaction to take place. Pro-
moted by the Ir-catalyst, all of these three substrates could
Table 1 The iridium-catalyzed asymmetric [2 + 2] cycloaddition reaction of
oxabenzonorbornadienes with phenylacetylenea
Entry
1
Oxanorbornadienes
Time (h)
20
Yieldb (%)
79
eec (%)
99
2
48
76
94
3
4
5
12
24
36
60
52
85
96
93
95
6
20
50
94
Table 2 The expansion of oxanorbornadienes as substrates for the iridium-
catalyzed asymmetric [2
dimethoxyphenylacetylenea,b
+
2] cycloaddition reaction with 3,5-
7
8
24
30
52
93
N. R.
/
Entry
1
Oxanorbornadienes
Time (h)
24
Yieldc (%)
35
eed (%)
89
9
30
30
N. R.
N. R.
/
/
2
3
24
20
30
22
92
94
10
a Reaction conditions:
Cl]2 : xylyl-phanephos = 1 : 2 : 0.025 : 0.065, in THF (2 mL) at 90 °C
1
(0.3 mmol), 1 : phenylacetylene : [Ir(COD)- under Ar for the indicated period of time. b Under the same reaction
1 (0.3 mmol), 1 : arylacetylene : [Ir(COD)-
a Reaction conditions:
Cl]2 : xylyl-phanephos = 1 : 2 : 0.025 : 0.065, in THF (2 mL) at 90 °C conditions, 92%, 94% and 96% ee values could be obtained for the
under Ar for the indicated period of time. b Isolated yield by column [2 + 2] cycloaddition products of 1k, 1l and 1m with phenylacetylene
chromatography. c ee % were determined by chiral HPLC using a respectively. c Isolated yield by column chromatography. d ee % were
Chiralcel OD-H or AS-H column.
determined by chiral HPLC using a Chiralcel AD-H or OJ-H column.
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react readily with 3,5-dimethoxyphenylacetylene to generate these azabenzonorbornadienes were proven suitable substrates
the desired [2 + 2] cycloaddition products with high enantio- for this reaction. To the three N-Boc protected substrates 1n,
selectivities. Though the products’ ee values were distributed 1o and 1p, their cyclobutene products generated with phenyl-
in a narrow range, it could still be concluded that bulky substi- acetylene had high optical purities and moderate to good yields
tuents on the unreacted carbon–carbon double bond were (entry 1 to entry 3). The complex of [Ir(COD)Cl]2 and Xylyl-pha-
favourable for high enantioselective control of the reaction, nephos also showed excellent enantioselective control ability
since the products’ ee values increased slightly from 89% to in the [2 + 2] cycloaddition reaction of sulfonyl protected aza-
94%, with the change of the ester groups from –COOMe to benzonorbornadienes 1q, 1r and 1s (entry 4 to entry 6).
–COOEt and then to –COOBut.
Though the corresponding cyclobutene product had 90% ee
Since the corresponding cyclobutene products have great value, the drawback of the p-nitrobenzenesulfonyl group (Ns)
potential to be useful intermediates for the synthesis of poly- was evident. The lowest yield was obtained for the reaction of
cyclic alkaloids, the use of azabenzonorbornadienes as sub- azabenzonorbornadiene 1s, as well as the lowest enantioselec-
strates in this reaction has particular importance for organic tivity (entry 6). It seems that the reduction of the electron
chemistry. Some azabenzonorbornadienes (1n to 1s) were then cloud density on the nitrogen atom is disfavourable for the
tested in this Ir-catalyzed system (Table 3). Fortunately, all of reaction, which implies that the nitrogen atoms on the bridge
chains of these substrates maybe participate in the coordi-
nation to Ir metal during the catalytic process.
Benzonorbornadienes, which have no heteroatoms on the
bridge chain, were then employed in this catalytic cyclo-
addition reaction (Table 4). Promoted by the chiral Ir-catalyst,
Table 3 The application of azabenzonorbornadienes as substrates for the
iridium-catalyzed asymmetric [2
phenylacetylenea
+
2] cycloaddition reaction with
several benzonorbornadienes with different substituents on
the benzo-ring (1t to 1x) reacted with phenylacetylene
smoothly to generate the desired cyclobutene products in high
yield. However, lower enantioselective control ability of the
chiral iridium catalyst for these carbon-bridged norbornadiene
substrates was shown, since only 69% to 80% ee values were
obtained (entry 1 to entry 5). Benzonorbornadienes 1y and 1z,
which have bulky substituents on the carbon-bridge, were
inert in this catalytic system, since no reactions took place
after heating and stirring for a long time (entries 6 and 7).
Entry
1
Benzonorbornadiene
Time (h)
48
Yieldb (%)
45
eec (%)
94
2
3
16
16
63
44
92
93
Mechanism for Ir-catalyzed [2 + 2] cycloaddition
To hypothesize the [2 + 2] cycloaddition reaction mechanism,
the following two reactions should be mentioned. (a) During
our expanding of the scope of alkynes, two representative
internal alkynes 3a and 3b had been explored in the Ir-catalytic
system to react with oxabenzonorbornadiene 1a. Surprisingly,
after two days’ heating and stirring, only 1-naphthol 4 (about
30% yield) and some unreacted oxabenzonorbornadiene
1a were separated out, and none of the targeted [2 + 2] cyclo-
addition products could be detected. Independent
experiments had proved that the complex of [Ir(COD)Cl]2 and
(R)-Xylyl-phanephos can catalyze the isomerization of oxaben-
zonorbornadiene 1a to 1-naphthol. So, it can be concluded
that, different from the Rh-catalyzed [2 + 2] cycloaddition reac-
tion of norbornadiene derivatives,10p this Ir-catalyzed one is
effective only for terminal but not internal alkynes. (b) Under
the optimized reaction conditions, when spiro chiral dipho-
sphine ligand (R)-xyl-SDP13 was used instead of (R)-xylyl-phane-
phos, oxabenzonorbornadiene 1a could also react with
phenylacetylene. However, not only [2 + 2] cycloaddition
4
12
64
94
5
6
20
24
57
31
95
90
a Reaction conditions:
1 (0.3 mmol), 1 : phenylacetylene : [Ir(COD)-
Cl]2 : xylyl-phanephos = 1 : 2 : 0.025 : 0.065, in THF (2 mL) at 90 °C
under Ar for the indicated period of time. b Isolated yield by column product 2a, but also the hydroalkynylation product 5, were
chromatography. c ee % were determined by chiral HPLC using a
generated. The former was isolated in 12% yield with 28% ee,
Chiralcel OD-H or AD-H column. Boc = t-butoxycarbonyl group, Ts = p-
and the latter was isolated in 20% yield with 44% ee
(Schemes 1–3).
toluenesulfonyl group, Bs
nitrobenzenesulfonyl group.
= benzenesulfonyl group, Ns = p-
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Table 4 The using of benzonorbornadienes as substrates for the iridium-cata-
lyzed asymmetric [2 + 2] cycloaddition reaction with phenylacetylenea
Scheme 2 The use of (R)-xyl-SDP as chiral ligand in the Ir-catalyzed [2 + 2]
cycloaddition reaction.
Entry
1
Benzonorbornadiene
Time (h)
60
Yieldb (%)
85
eec (%)
76
2
3
24
60
86
89
69
73
4
24
26
80
5
6
24
60
26
71
/
N. R.
Scheme 3 Hypothesized mechanism for the Ir-catalyzed asymmetric [2 + 2]
cycloaddition reaction.
7
60
N. R.
/
reductive elimination of the iridium(III) part will give the
hydroalkynylation product E, further addition of the Ir–H
bond to the triple bond in C is going on in this cycle, and the
metallocyclopentenyl intermediate D is constructed. As the
last step, the reductive elimination of iridium from D produces
the [2 + 2] cycloaddition product F and the intermediate A to
complete the cycle.
a Reaction conditions:
1 (0.3 mmol), 1 : phenylacetylene : [Ir(COD)-
Cl]2 : xylyl-phanephos = 1 : 2 : 0.025 : 0.065, in THF (2 mL) at 90 °C
under Ar for the indicated period of time. b Isolated yield by column
chromatography. c ee % were determined by chiral HPLC using a
Chiralcel OD-H column.
Conclusions
A series of norbornadiene derivatives had been employed as
substrates in Ir-catalyzed asymmetric [2 + 2] cycloaddition reac-
tion with arylacetylenes to investigate the substituent effects
on the reaction. It was found that the kind of atoms on the
bridge chain was decisive to the enantioselective level of the
reaction. Heteroatoms with a weak coordinating ability, such
Scheme 1 Exploration of internal alkynes in the Ir-catalyzed [2 + 2] cyclo-
addition reaction.
Based on the results of all experiments, a mechanism was as oxygen and nitrogen, always resulted in excellent enantio-
hypothesized for this [2 + 2] cycloaddition reaction. The cata- selectivity for the [2 + 2] cycloaddition reaction, and the pro-
lytic cycle is started by the oxidative addition of terminal alkyne ducts generally had more than 90% ee values. However,
to the chiral Ir(I) complex to generate the intermediate A. The carbon atoms on the bridge chain decreased the enantioselec-
coordination of the alkene moiety to the chiral iridium(III) tivity of the reaction greatly, and the corresponding products’
complex A forms B. After the migratory insertion of the phenyl- ee values mainly ranged from 69% to 80%. The groups on the
ethynyl group to the carbon–carbon double bond in an unreacted carbon–carbon double bond, including various sub-
exo manner, the intermediate C is produced. Although the stituted benzo-rings and ester groups, had but a little effect on
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the reaction. Bulky steric hindrance on the bridge-head AS-H, i-propanol–hexane = 10/90, flow rate 1.0 mL min−1, λ =
carbons or bridge chains could block the reaction completely. 254 nm): tminor = 16.8 min, tmajor = 22.7 min.
Based on all the experiment results, an interesting reaction
Cyclobutene 2c. White solid, 60% yield, mp 149–150 °C,
mechanism had also been hypothesized. Further efforts to 96% ee; 1H NMR (400 MHz, CDCl3): δ 7.61 (s, 1H), 7.55
understand and improve this Ir-catalyzed asymmetric [2 + 2] (s, 1H), 7.47–7.45 (m, 2H), 7.39–7.35 (m, 2H), 7.32–7.26
cycloaddition reaction are still ongoing.
(m, 1H), 6.44 (s, 1H), 5.10 (s, 1H), 4.98 (s, 1H), 3.09 (d, J =
3.6 Hz, 1H), 2.76 (d, J = 3.6 Hz, 1H); 13C NMR (100 MHz,
CDCl3): δ 146.56, 146.27, 145.76, 132.76, 128.59, 128.44,
126.45, 125.23, 125.13, 124.95, 122.32, 76.19, 75.45, 46.48,
43.94; HRMS (ESI) calcd for C18H12OBr2 [M]+, 401.9255, found
401.9260; HPLC (Chiralcel OD-H, i-propanol–hexane = 10/90,
Experimental section
General remarks
flow rate 1.0 mL min−1, λ = 254 nm): tmajor = 13.0 min, tminor
=
The reactions were carried out under argon atmosphere by
using Drybox (Mikrouna, Supper 1220/750). Norbornadiene
derivatives were synthesized according to the published pro-
cedures.14 Arylacetylenes were purchased from reagent compa-
nies and distilled before use, and other commercial reagents
were used as received without further purification. Anhydrous
THF was distilled from sodium benzophenone ketyl prior to
use. 1H NMR and 13C NMR spectra were recorded on a Bruker-
Avance 400 spectrometer. Chemical shifts (δ) were reported in
ppm with tetramethylsilane as an internal standard, and
J values were given in Hz. The ee values were determined by an
Agilent 1260 Series HPLC using Daicel AD-H, OD-H, OJ-H or
AS-H chiral columns with a mixture of i-propyl alcohol and
n-hexane as the eluent. Melting points were measured on
X-4 melting point apparatus and uncorrected. High resolution
mass spectra (HRMS) were performed on a VG Autospec-3000
spectrometer. Column chromatography was performed with
silica gel (200–300 mesh) with petroleum ether and ethyl
acetate as eluents.
14.8 min.
Cyclobutene 2d. White solid, 52% yield, mp 149–151 °C,
93% ee; 1H NMR (400 MHz, CDCl3): δ 7.51–7.49 (m, 2H),
7.40–7.37 (m, 2H), 7.33–7.31 (m, 1H), 6.89 (s, 1H), 6.84 (s, 1H),
6.50 (s, 1H), 6.00–5.97 (m, 2H), 5.08 (s, 1H), 4.95 (s, 1H), 3.07
(d, J = 3.6 Hz, 1H), 2.74 (d, J = 3.6 Hz, 1H); 13C NMR (100 MHz,
CDCl3): δ 147.13, 146.06, 138.90, 138.29, 133.13, 128.49,
128.16, 127.15, 124.94, 102.13, 102.06, 101.21, 75.88, 47.13,
44.58; HRMS (ESI) calcd for C19H14O3Na [M + Na]+, 313.0840,
found 313.0849; HPLC (Chiralcel OD-H, i-propanol–hexane =
10/90, flow rate 1.0 mL min−1, λ = 254 nm): tminor = 23.6 min,
tmajor = 25.8 min.
Cyclobutene 2e. White solid, 85% yield, mp 113–115 °C,
95% ee; 1H NMR (400 MHz, CDCl3): δ 7.51–7.49 (m, 2H),
7.40–7.37 (m, 2H), 7.33–7.31 (m, 1H), 6.87 (s, 1H), 6.82 (s, 1H),
6.46 (s, 1H), 5.06 (s, 1H), 4.93 (s, 1H), 4.23 (s, 4H), 3.06 (d, J =
3.6 Hz, 1H), 2.74 (d, J = 3.6 Hz, 1H); 13C NMR (100 MHz,
CDCl3): δ 147.01, 141.72, 138.15, 137.57, 133.15, 128.48,
128.14, 127.11, 124.95, 109.69, 109.59, 76.44, 75.70, 64.32,
47.40, 44.88; HRMS (ESI) calcd for C20H16O3 [M]+, 304.1099,
found 304.1100; HPLC (Chiralcel OD-H, i-propanol–hexane =
General procedure for the Ir-catalyzed [2 + 2] cycloaddition
reactions
Under the protection of argon in a drybox, the mixture of 10/90, flow rate 1.0 mL min−1, λ = 254 nm): tmajor = 24.8 min,
[Ir(COD)Cl]2 (5.1 mg, 0.0075 mmol), (R)-xylyl-phanephos tminor = 31.8 min.
(13.4 mg, 0.0195 mmol) and THF (2 mL) was stirred at room
Cyclobutene 2f. Yellow solid, 50% yield, mp 201–202 °C,
temperature for 30 minutes in a reaction tube. After the 94% ee; 1H NMR (400 MHz, CDCl3): δ 8.78–8.75 (m, 2H),
addition of a norbornadiene derivative (0.3 mmol), the stirring 8.13–8.00 (m, 1H), 7.96–7.94 (m, 1H), 7.71–7.66 (m, 4H),
was continued for another 20 minutes. After the addition of 7.58–7.56 (m, 2H), 7.44–7.40 (m, 2H), 7.34–7.33 (m, 1H), 6.64
arylacetylene (0.6 mmol), the reaction tube was sealed with a (s, 1H), 5.78 (s, 1H), 5.65 (s, 1H), 3.16 (d, J = 3.2 Hz, 1H), 2.83
rubber septum, and then moved out of the drybox to an oil (d, J = 3.2 Hz, 1H); 13C NMR (100 MHz, CDCl3): δ 147.42,
bath. The mixture was stirred at 90 °C (bath temperature) 140.55, 139.86, 133.12, 130.11, 130.07, 128.60, 128.29, 127.52,
until the reaction was complete. After vacuum evaporation 126.94, 126.27, 126.21, 124.97, 124.10, 124.04, 123.71, 123.66,
of the solvent, the residue was purified by column 76.32, 75.48, 47.34, 44.89; HRMS (ESI) calcd for C26H18O [M]+,
chromatography through silica gel to yield the aimed product. 346.1358, found 346.1354; HPLC (Chiralcel AS-H, i-propanol–
The ee value of the product was determined by chiral HPLC hexane = 5/95, flow rate 1.0 mL min−1, λ = 254 nm): tminor
analysis. 24.4 min, tmajor = 28.7 min.
Cyclobutene 2b. White solid, 76% yield, mp 173–175 °C, Cyclobutene 2g. White solid, 52% yield, mp 168–170 °C,
=
94% ee; 1H NMR (400 MHz, CDCl3): δ 7.51–7.47 (m, 2H), 93% ee; 1H NMR (400 MHz, CDCl3): δ 7.49–7.47 (m, 2H),
7.37–7.34 (m, 2H), 7.29–7.25 (m, 1H), 6.98 (s, 1H), 6.92 (s, 1H), 7.39–7.35 (m, 2H), 7.30–7.24 (m, 1H), 6.89 (s, 2H), 6.49 (s, 1H),
6.48 (s, 1H), 5.10 (s, 1H), 4.97 (s, 1H), 3.90 (s, 3H), 3.89 (s, 3H), 5.20 (s, 1H), 5.07 (s, 1H), 3.05 (d, J = 3.6 Hz, 1H), 2.72 (d, J =
3.05 (d, J = 3.6 Hz, 1H), 2.72 (d, J = 3.6 Hz, 1H); 13C NMR 3.6 Hz, 1H), 2.37 (s, 3H), 2.31 (s, 3H); 13C NMR (100 MHz,
(100 MHz, CDCl3): δ 147.79, 147.76, 147.15, 137.58, 136.99, CDCl3):
δ
146.81, 143.13, 142.60, 133.20, 128.51,
133.22, 128.47, 128.12, 127.20, 124.94, 104.82, 104.74, 128.17, 128.08, 128.06, 126.98, 126.85, 126.82, 124.90, 75.46,
76.82, 76.07, 56.39, 47.41, 44.84; HRMS (ESI) calcd for 74.70, 46.12, 43.65, 18.13, 18.08; HRMS (ESI) calcd for C20H18O
C20H18O3 [M]+, 306.1256, found 306.1259; HPLC (Chiralcel [M]+, 274.1358, found 274.1357; HPLC (Chiralcel OD-H,
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i-propanol–hexane = 5/95, flow rate 1.0 mL min−1, λ = 254 nm):
tminor = 7.6 min, tmajor = 10.1 min.
Cyclobutene 2p. White solid, 44% yield, mp 197–198 °C,
93% ee; 1H NMR (400 MHz, CDCl3): δ 7.60–7.56 (m, 2H),
Cyclobutene 2k. Yellow Oil, 35% yield, 89% ee; 1H NMR 7.46–7.36 (m, 5H), 6.42 (s, 1H), 5.07 (s, 1H), 5.00 (s, 1H), 2.93
(400 MHz, CDCl3): δ 6.57 (d, J = 2.3 Hz, 2H), 6.47 (s, 1H), 6.41 (d, J = 3.2 Hz, 1H), 2.74 (d, J = 3.2 Hz, 1H), 1.01 (s, 9H); 13C
(t, J = 2.16 Hz, 1H), 5.00 (s, 1H), 4.90 (s, 1H), 3.86 (s, 3H), 3.83 NMR (100 MHz, CDCl3): δ 154.25, 147.17, 145.14, 144.80,
(s, 3H), 3.81 (s, 6H), 3.19 (d, J = 3.4 Hz, 1H), 2.88 (d, J = 3.8 Hz, 133.17, 128.72, 128.51, 128.46, 125.56, 125.39, 125.12, 122.09,
1H). 13C NMR (100 MHz, CDCl3): δ 163.10, 163.06, 161.00, 79.87, 59.32, 58.86, 46.96, 45.07, 28.28, 27.70; HRMS (ESI)
146.99, 143.82, 143.30, 134.38, 127.36, 103.19, 100.49, 78.57, calcd for C23H21NO2Br2 [M]+, 500.9939, found 500.9952; HPLC
77.88, 55.41, 52.44, 52.36, 44.56, 41.91. HRMS (ESI) calcd for (Chiralcel AD-H, i-propanol–hexane = 10/90, flow rate 1.0 mL
C20H20O7 [M]+, 372.1209, found 372.1205; HPLC (Chiralcel min−1, λ = 254 nm): tmajor = 7.6 min, tminor = 9.2 min.
AD-H, i-propanol–hexane = 30/70, flow rate 1.0 mL min−1, λ =
254 nm): tminor = 8.4 min, tmajor = 18.3 min.
Cyclobutene 2q. White solid, 64% yield, mp 186–188 °C,
94% ee; 1H NMR (400 MHz, CDCl3): δ 7.46–7.44 (m, 2H),
Cyclobutene 2l. White solid, 30% yield, mp 89–90 °C, 92% 7.38–7.34 (m, 2H), 7.32–7.28 (m, 3H), 7.07–7.05 (m, 1H),
1
ee; H NMR (400 MHz, CDCl3): δ 6.57 (d, J = 2.1 Hz, 2H), 6.47 7.00–6.92 (m, 3H), 6.86 (s, 1H), 6.84 (s, 1H), 6.47 (s, 1H), 4.92
(s, 1H), 6.41 (t, J = 2.28 Hz, 1H), 5.00 (s, 1H), 4.89 (s, 1H), (s, 1H), 4.80 (s, 1H), 2.99 (d, J = 3.2 Hz, 1H), 2.67 (d, J = 3.2 Hz,
4.35–4.26 (m, 4H), 3.81 (s, 6H), 3.19 (d, J = 3.56 Hz, 1H), 2.88 1H), 2.22 (s, 3H); 13C NMR (100 MHz, CDCl3): δ 146.66, 143.33,
(d, J = 3.50 Hz, 1H), 1.35 (s, 3H), 1.34 (s, 3H). 13C NMR 142.59, 142.50, 136.05, 132.97, 128.79, 128.53, 128.31, 127.62,
(100 MHz, CDCl3): δ 162.86, 162.81, 161.00, 147.01, 143.60, 126.68, 126.49, 125.06, 120.68, 120.59, 62.27, 61.72, 47.58,
143.19, 134.43, 127.40, 103.12, 100.61, 78.59, 77.91, 61.44, 45.00, 21.33; HRMS (ESI) calcd for C25H21NO2S [M]+, 399.1293,
55.38, 44.68, 42.03, 14.12. HRMS (ESI) calcd for C22H24O7 [M]+, found 399.1299; HPLC (Chiralcel AD-H, i-propanol–hexane =
400.1522, found 400.1528; HPLC (Chiralcel OJ-H, i-propanol– 10/90, flow rate 1 mL min−1, λ = 254 nm): tminor = 14.5 min,
hexane = 10/90, flow rate 1.0 mL min−1, λ = 254 nm): tmajor
25.3 min, tminor = 30.0 min.
=
tmajor = 27.5 min.
Cyclobutene 2r. White solid, 57% yield, mp 166–168 °C,
Cyclobutene 2m. Yellow oil, 22% yield, 94% ee; 1H NMR 95% ee; 1H NMR (400 MHz, CDCl3): δ 7.47–7.35 (m, 8H),
(400 MHz, CDCl3): δ 6.57 (d, J = 2.2 Hz, 2H), 6.46 (s, 1H), 7.10–7.04 (m, 3H), 6.99–6.89 (m, 3H), 6.48 (s, 1H), 4.95 (s, 1H),
6.40 (t, J = 2.26 Hz, 1H), 4.92 (s, 1H), 4.80 (s, 1H), 3.80 (s, 6H), 4.83 (s, 1H), 3.00 (d, J = 4.1 Hz, 1H), 2.69 (d, J = 3.6 Hz, 1H);
3.19 (d, J = 3.48 Hz, 1H), 2.86 (d, J = 3.50 Hz, 1H), 1.55 (s, 9H), 13C NMR (100 MHz, CDCl3): δ 143.15, 142.43, 139.01, 132.95,
1.53 (s, 9H). 13C NMR (100 MHz, CDCl3):
δ 161.99, 132.91, 131.81, 128.93, 128.56, 128.38, 128.21, 127.29, 126.63,
161.90, 160.97, 147.09, 144.08, 143.12, 134.54, 127.52, 103.01, 126.30, 125.43, 125.07, 123.26, 121.32, 120.64, 120.56, 62.35,
100.64, 82.42, 82.32, 78.69, 55.36, 44.85, 42.20, 28.17, 28.14. 61.83, 47.60, 45.00; HRMS (ESI) calcd for C24H19NO2S [M]+,
HRMS (ESI) calcd for C26H32O7 [M]+, 456.2148, found 385.1137, found 385.1139; HPLC (Chiralcel AD-H, i-propanol–
456.2156; HPLC (Chiralcel OD-H, i-propanol–hexane = 10/90, hexane = 20/80, flow rate 1.0 mL min−1, λ = 254 nm): tminor
=
flow rate 1.0 mL min−1, λ = 254 nm): tminor = 5.6 min, tmajor
=
7.2 min, tmajor = 14.4 min.
11.6 min.
Cyclobutene 2s. White solid, 31% yield, mp 210–211 °C,
Cyclobutene 2n. White solid, 45% yield, mp 116–117 °C, 90% ee; 1H NMR (400 MHz, CDCl3): δ 7.92–7.89 (m, 2H),
1
94% ee; H NMR (400 MHz, CDCl3): δ 7.50–7.36 (m, 5H), 6.98 7.62–7.58 (m, 2H), 7.45–7.42 (m, 2H), 7.39–7.32 (m, 3H), 7.09
(s, 1H), 6.94 (s, 1H), 6.47 (s, 1H), 5.07 (s, 1H), 4.97 (s, 1H), 3.90 (m, 1H), 7.02 (m, 1H), 6.98–6.93 (m, 2H), 6.47 (s, 1H), 4.98
(s, 3H), 3.89 (s, 3H), 2.92 (d, J = 3.6 Hz, 1H), 2.72 (d, J = 3.2 Hz, (s, 1H), 4.88 (s, 1H), 3.04 (d, J = 3.6 Hz, 1H), 2.74 (d, J = 3.6 Hz,
1H), 1.01 (s, 9H); 13C NMR (100 MHz, CDCl3): δ 154.37, 147.68, 1H); 13C NMR (100 MHz, CDCl3): δ 149.29, 146.75, 145.01,
147.41, 147.35, 136.56, 136.04, 133.60, 129.58, 128.38, 128.23, 143.03, 142.32, 132.60, 128.68, 128.63, 126.88, 126.49, 124.99,
125.15, 124.96, 105.18, 105.09, 104.96, 79.32, 59.86, 59.40, 123.36, 120.85, 120.75, 62.78, 62.06, 47.44, 44.90; HRMS (ESI)
56.32, 47.87, 46.02, 28.34, 27.75; HRMS (ESI) calcd for calcd for C24H18N2O4S [M]+, 430.0987, found 430.0990; HPLC
C25H27NO4 [M]+, 405.1940, found 405.1938; HPLC (Chiralcel (Chiralcel AD-H, i-propanol–hexane = 10/90, flow rate 1.0 mL
OD-H, i-propanol–hexane = 5/95, flow rate 1.0 mL min−1, λ = min−1, λ = 254 nm): tminor = 18.0 min, tmajor = 30.3 min.
254 nm): tmajor = 14.3 min, tminor = 18.7 min.
Cyclobutene 2t. White solid, 85% yield, mp 103–104 °C,
Cyclobutene 2o. White solid, 63% yield, mp 202–204 °C, 76% ee; 1H NMR (400 MHz, CDCl3): δ 7.48–7.46 (m, 2H),
1
92% ee; H NMR (400 MHz, CDCl3): δ 7.49–7.35 (m, 5H), 7.11 7.38–7.34 (m, 2H), 7.29–1.26 (m, 1H), 6.92 (s, 1H), 6.86 (s, 1H),
(s, 1H), 7.09 (s, 1H), 6.45 (s, 1H), 5.05 (s, 1H), 4.96 (s, 1H), 2.92 6.46 (s, 1H), 3.89 (s, 3H), 3.87 (s, 3H), 3.13 (s, 1H), 2.98 (s, 1H),
(d, J = 3.2 Hz, 1H), 2.72 (d, J = 3.2 Hz, 1H), 2.26 (s, 3H), 2.25 2.88 (d, J = 3.2 Hz, 1H), 2.55 (d, J = 3.2 Hz, 1H), 1.94 (d, J = 9.6
(s, 3H), 1.00 (s, 9H); 13C NMR (100 MHz, CDCl3): δ 154.49, Hz, 1H), 1.67 (d, J = 9.6 Hz, 1H); 13C NMR (100 MHz, CDCl3):
147.57, 141.88, 141.48, 134.36, 134.24, 133.68, 129.48, 128.35, δ 147.74, 146.76, 140.62, 139.83, 133.88, 128.61, 128.51,
128.16, 125.14, 124.97, 121.57, 121.33, 79.15, 59.62, 59.09, 127.98, 125.06, 106.75, 106.61, 56.41, 47.79, 45.23, 41.86,
47.80, 45.96, 28.35, 27.77, 19.97; HRMS (ESI) calcd for 41.46, 40.73; HRMS (ESI) calcd for C21H20O2 [M]+, 304.1463,
C25H27NO2 [M]+, 373.2042, found 373.2043; HPLC (Chiralcel found 304.1467; HPLC (Chiralcel OD-H, i-propanol–hexane =
AD-H, i-propanol–hexane = 10/90, flow rate 1.0 mL min−1, λ = 5/95, flow rate 1.0 mL min−1, λ = 254 nm): tmajor = 13.7 min,
254 nm): tmajor = 6.1 min, tminor = 8.6 min.
tminor = 15.3 min.
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Cyclobutene 2u. White solid, 86% yield, mp 144–146 °C,
Organic & Biomolecular Chemistry
Notes and references
69% ee; 1H NMR (400 MHz, CDCl3): δ 7.52–7.47 (m, 2H),
7.46–7.43 (m, 2H), 7.33–7.29 (m, 1H), 6.85 (s, 1H), 6.80 (s, 1H),
6.48 (s, 1H), 5.94–5.91 (dd, J = 9.8, 1.6 Hz, 2H), 3.12 (s, 1H),
2.98 (s, 1H), 2.88 (d, J = 3.2 Hz, 1H), 2.56 (d, J = 3.6 Hz, 1H),
1.94 (d, J = 9.2 Hz, 1H), 1.71 (d, J = 9.2 Hz, 1H); 13C NMR
(100 MHz, CDCl3): δ 147.61, 144.70, 141.70, 140.89, 133.70,
128.48, 128.32, 127.87, 124.92, 103.88, 103.74, 100.52, 47.41,
44.85, 41.68, 41.32, 40.54; HRMS (ESI) calcd for C20H16O2 [M]+,
288.1150, found 288.1153; HPLC (Chiralcel OD-H, i-propanol–
1 (a) A. Tenaglia, L. Giordano and G. Buono, Org. Lett., 2006,
8, 4315; (b) K. Kohno, K. Nakagawa, T. Yahagi, J. C. Choi,
H. Yasuda and T. Sakakura, J. Am. Chem. Soc., 2009, 131,
2784; (c) D. Gatineau, L. Giordano and G. Buono, J. Am.
Chem. Soc., 2011, 133, 10728.
2 (a) D. K. Rayabarapu, C. F. Chiou and C. H. Cheng, Org.
Lett., 2002, 4, 1679; (b) T. Nishimura, E. Tsurumaki,
T. Kawamoto, X. X. Guo and T. Hayashi, Org. Lett., 2008, 10,
4057.
3 (a) M. Lautens, W. Tam, J. C. Lautens, L. G. Edwards,
C. M. Crudden and A. C. Smith, J. Am. Chem. Soc., 1995,
117, 6863; (b) O. Pardigon, A. Tenaglia and G. Buono, J.
Org. Chem., 1995, 60, 1868; (c) A. Tenaglia and S. Gaillard,
Org. Lett., 2007, 9, 3607; (d) G. Hilt and F.-X. D. Mesnil,
Tetrahedron Lett., 2000, 41, 6757.
4 A. Tenaglia and S. Marc, J. Org. Chem., 2008, 73, 1397.
5 (a) J. Bigeault, L. Giordano and G. Buono, Angew. Chem.,
Int. Ed., 2005, 44, 4753; (b) J. Bigeault, L. Giordano, I.
D. Riggi, Y. Gimbert and G. Buono, Org. Lett., 2007, 9,
3567; (c) J. Bigeault, I. D. Riggi, Y. Gimbert, L. Giordano
and G. Buono, Synlett, 2008, 1071; (d) A. Tenaglia and
S. Marc, J. Org. Chem., 2006, 71, 3569.
hexane = 3/97, flow rate 0.5 mL min−1, λ = 254 nm): tmajor
12.3 min, tminor = 14.2 min.
=
Cyclobutene 2v. White solid, 89% yield, mp 105–107 °C,
73% ee; 1H NMR (400 MHz, CDCl3): δ 7.48–7.45 (m, 2H),
7.38–7.33 (m, 2H), 7.28–7.26 (m, 1H), 6.80 (s, 1H), 6.75 (s, 1H),
6.43 (s, 1H), 4.22 (s, 4H), 3.09 (s, 1H), 2.94 (s, 1H), 2.88 (d, J =
3.2 Hz, 1H), 2.55 (d, J = 3.2 Hz, 1H), 1.91 (d, J = 9.6 Hz, 1H),
1.65 (d, J = 9.6 Hz, 1H); 13C NMR (100 MHz, CDCl3): δ 147.47,
141.34, 140.69, 140.57, 133.72, 128.47, 128.23, 127.84, 124.93,
111.09, 110.92, 64.36, 47.55, 45.00, 41.30, 40.78, 40.19;
HRMS (ESI) calcd for C21H18O2 [M]+, 302.1307, found
302.1300; HPLC (Chiralcel OD-H, i-propanol–hexane = 5/95,
flow rate 1.0 mL min−1, λ = 254 nm): tmajor = 5.8 min, tminor
=
10.9 min.
Cyclobutene 2w. White solid, 26% yield, mp 162–163 °C,
80% ee; 1H NMR (400 MHz, CDCl3): δ 7.49–7.47 (m, 2H),
7.38–7.34 (m, 2H), 7.29–7.27 (m, 1H), 7.08 (s, 1H), 7.02 (s, 1H),
6.45 (s, 1H), 3.13 (s, 1H), 2.98 (s, 1H), 2.89 (d, J = 3.2 Hz, 1H),
2.56 (d, J = 3.2 Hz, 1H), 2.25 (s, 3H), 2.24 (s, 3H), 1.90 (d, J =
9.6 Hz, 1H), 1.65 (d, J = 9.6 Hz, 1H); 13C NMR (100 MHz,
CDCl3): δ 147.43, 146.05, 145.29, 133.78, 133.15, 128.48,
128.28, 127.83, 124.93, 122.94, 122.78, 47.50, 44.96, 41.36,
40.98, 40.25, 19.91; HRMS (ESI) calcd for C21H20 [M]+,
272.1565, found 272.1563; HPLC (Chiralcel OD-H, i-propanol–
6 (a) T. Mitsudo, H. Naruse, T. Kondo, Y. Ozaki and
Y. Watanabe, Angew. Chem., Int. Ed., 1994, 33, 580;
(b) D. J. Huang, D. K. Rayabarapu, L. P. Li, T. Sambaiah and
C. H. Cheng, Chem.–Eur. J., 2000, 6, 3706; (c) Y. Kuninobu,
P. Yu and K. Takai, Chem. Lett., 2007, 36, 1162; (d) A. Allen,
K. Villeneuve, N. Cockburn, E. Fatila, N. Riddell and
W. Tam, Eur. J. Org. Chem., 2008, 4178; (e) K. C. Chao,
D. K. Rayabarapu, C. C. Wang and C. H. Cheng, J. Org.
Chem., 2001, 66, 8804.
7 (a) D. J. Huang, T. Sambaiah and C. H. Cheng, New
J. Chem., 1998, 22, 1147; (b) T. Sambaiah, D. J. Huang
and C. H. Cheng, J. Chem. Soc., Perkin Trans. 1, 2000,
195.
8 D. K. Rayabarapu, T. Sambaiah and C. H. Cheng, Angew.
Chem., Int. Ed., 2001, 40, 1286.
9 For the catalytic [2 + 2] cycloaddition reactions applying
other alkenes, see: A. Nishimura, M. Ohashi and S. Ogoshi,
J. Am. Chem. Soc., 2012, 134, 15692 and references cited
therein.
hexane = 3/97, flow rate 0.5 mL min−1, λ = 254 nm): tminor
8.0 min, tmajor = 8.9 min.
=
Cyclobutene 2x. White solid, 26% yield, mp 140–142 °C,
1
71% ee; H NMR (400 MHz, CDCl3): δ 7.52 (s, 1H), 7.47–7.44
(m, 3H), 7.39–7.35 (m, 2H), 7.32–7.29 (m, 1H), 6.43 (s, 1H),
3.16 (s, 1H), 3.03 (s, 1H), 2.89 (d, J = 3.2 Hz, 1H), 2.57 (d, J =
3.2 Hz, 1H), 1.92 (d, J = 10.0 Hz, 1H), 1.70 (d, J = 10.0 Hz,
1H); 13C NMR (100 MHz, CDCl3):
δ
149.41, 148.74,
147.01, 133.26, 128.57, 128.16, 127.53, 126.78, 126.61,
124.94, 120.66, 46.48, 43.93, 41.48, 40.89, 40.38; HRMS 10 For the catalytic [2 + 2] cycloaddition reaction of norbor-
(ESI) calcd for C19H14Br2 [M]+, 399.9462, found 399.9457;
nadiene derivatives with internal alkynes, see:
(a) B. T. Mitsudo, K. Kokuryo and Y. Takegami, J. Chem.
Soc., Chem. Commun., 1976, 722; (b) R. W. Jordan,
P. L. Marquand and W. Tam, Eur. J. Org. Chem., 2008, 80;
(c) R. W. Jordan, P. R. Khoury, J. D. Goddard and W. Tam,
J. Org. Chem., 2004, 69, 8467; (d) R. W. Jordan,
K. Villeneuve and W. Tam, J. Org. Chem., 2006, 71, 5830;
(e) R. W. Jordan and W. Tam, Org. Lett., 2001, 3, 2367; (f) R.
W. Jordan and W. Tam, Tetrahedron Lett., 2002, 43, 6051;
(g) K. Villeneuve and W. Tam, Angew. Chem., Int. Ed., 2004,
43, 610; (h) K. Villeneuve, N. Riddell and W. Tam, Tetra-
hedron, 2006, 62, 3823; (i) A. Tenaglia and L. Giordano,
HPLC (Chiralcel OD-H, i-propanol–hexane
= 3/97, flow
rate 0.5 mL min−1, λ = 254 nm): tmajor = 9.5 min, tminor
=
10.4 min.
Acknowledgements
We thank the National Natural Science Foundation of
China (21202142, 21162040, 20902080) and the Department of
Education of Yunnan Province (2010CI08) for financial
support.
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Paper
Synlett, 2003, 2333; ( j) J. Treutwein and G. Hilt, Angew. 14 (a) W. M. Best, P. A. Collins, R. K. Mcculloch and D. Wege,
Chem., Int. Ed., 2008, 47, 6811; (k) N. Riddell and W. Tam,
J. Org. Chem., 2006, 71, 1934; (l) R. R. Burton and W. Tam,
J. Org. Chem., 2007, 72, 7333; (m) N. Cockburn, E. Karimi
and W. Tam, J. Org. Chem., 2009, 74, 5762; (n) R. R. Burton
and W. Tam, Tetrahedron Lett., 2006, 47, 7185;
(o) K. Villeneuve and W. Tam, Organometallics, 2006, 25,
843; (p) T. Shibata, K. Takami and A. Kawachi, Org. Lett.,
2006, 8, 1343.
Aust. J. Chem., 1982, 35, 843; (b) G. Werner and
H. Butenschön, Eur. J. Org. Chem., 2012, 3132;
(c) Y. D. Xing and N. Z. Huang, J. Org. Chem., 1982, 47, 140;
(d) Y. H. Cho, V. Zunic, H. Senboku, M. Olsen and
M. Lautens, J. Am. Chem. Soc., 2006, 128, 6837;
(e) K. Y. Jung and M. Koreeda, J. Org. Chem., 1989, 54, 5667;
(f) H. L. Goering, A. C. Backus, C. S. Chang and
D. Masilamani, J. Org. Chem., 1975, 40, 1533;
(g) M. J. Panigot, J. F. Waters, U. Varde, J. K. Sutter and
C. N. Sukenik, Macromolecules, 1992, 25, 530;
(h) M. Lautens, K. Fagnou and V. Zunic, Org. Lett., 2002, 4,
3465; (i) D.M. Hodgson, M. W. P. Bebbington and P. Willis,
Org. Lett., 2002, 4, 4353; ( j) J. W. Coe, M. C. Wirtz,
C. G. Bashore and J. Candler, Org. Lett., 2004, 6, 1589;
(k) R. R. Burton and W. Tam, Org. Lett., 2007, 9, 3287;
(l) H. Hart, C. Y. Lai, G. C. Nwokogu and S. Shamouilian,
Tetrahedron, 1987, 43, 5203.
11 B. M. Fan, X. J. Li, F. Z. Peng, H. B. Zhang, A. S. C. Chan
and Z. H. Shao, Org. Lett., 2010, 12, 304.
12 (a) B. M. Fan, Q. J. Yang, J. Hu, C. L. Fan, S. F. Li, L. Yu,
C. Huang, W. W. Tsang and F. Y. Kwong, Angew. Chem., Int.
Ed., 2012, 51, 7821; (b) J. Hu, Q. J. Yang, J. B. Xu, B. M. Fan,
J. Wang, C. Y. Lin, Z. X. Bian and A. S. C. Chan, Org.
Biomol. Chem., 2013, 11, 814.
13 J. H. Xie, L. X. Wang, Y. Fu, S. F. Zhu, B. M. Fan, H. F. Duan
and Q. L. Zhou, J. Am. Chem. Soc., 2003, 125, 4404.
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