Cu(OTf)2-catalyzed selective opening of aryl and vinyl epoxides with carbonyl compounds to give 1,3-dioxolanes

Article abstract of DOI:10.1055/s-2005-864795

Copper(II) triflate catalyzes the ring-opening of aryl- and vinyl-substituted epoxides with various carbonyl compounds to furnish 1,3-dioxolanes under mild conditions. Alkyl- and alkoxycarbonyl-substituted epoxides remain unchanged under reaction conditio

Full text of DOI:10.1055/s-2005-864795

8
54
LETTER
Cu(OTf) -Catalyzed Selective Opening of Aryl and Vinyl Epoxides with
2
Carbonyl Compounds to Give 1,3-Dioxolanes
1
C
P
u(OTf) -Cata
a
lyze
d
Selec
v
tive
O
penin
e
g
of
A
ryl an
l
d
V
inyl
EpoK
2
Shire BioChem Inc., 275 Armand-Frappier Blvd., Laval, Québec, H7V 4A7, Canada
E-mail: pavel.krasik@torcan.com
Received 23 June 2004
A series of experiments was carried out at various temper-
atures using conventional catalysts to determine, if the
conditions for selective opening of aryl-substituted ep-
Abstract: Copper(II) triflate catalyzes the ring-opening of aryl- and
vinyl-substituted epoxides with various carbonyl compounds to
furnish 1,3-dioxolanes under mild conditions. Alkyl- and alkoxy-
carbonyl-substituted epoxides remain unchanged under reaction oxides can be established. We found that BF ·OEt (2
3 2
conditions. This allows selective opening of aryl-substituted mol%) converts both styrene oxide and 1,2-epoxybutane
epoxides in the presence of alkyl-substituted ones.
to the corresponding 1,3-dioxolanes (temperature range
Key words: epoxide-opening, dioxolanes, copper triflate, chemo- –20 to –78 °C). SnCl
is less reactive towards dioxolane
4
selectivity
formation then BF ·OEt , however, both styrene oxide
3
2
and 1,2-epoxybutane form 1,3-dioxolanes with compara-
ble rates at –78 °C and at higher temperatures. TiCl and
4
AlCl demonstrated comparable reactivity towards alkyl-
and aryl-substituted epoxides at various temperatures.
Epoxides are important and useful intermediates for the
synthesis of organic structures due to their ease of prepa-
ration and their propensity for strain-induced ring-open-
ing reactions. Transformation of epoxides to 1,2-diol
moieties have been applied to synthesis of complex natu-
3
Herein, we report the mild and selective opening of aryl-
and vinyl-substituted epoxides catalyzed by Cu(OTf)2.
Alkyl-substituted epoxides are not reactive under the de-
scribed conditions.
2
ral compounds. 1,3-Dioxolanes are useful blocks in syn-
thetic chemistry as synthetic equivalents of 1,2-diols.3
Lewis acid-catalyzed epoxide-opening with carbonyl
compounds has attracted considerable attention as a con-
venient way of synthesizing 1,3-dioxolanes. A number of
Lewis acids and transition metal complexes including
,4
Copper(II) triflate acts as a catalyst to transform epoxides
into the corresponding 1,3-dioxolanes. While this reaction
is very fast in the case of aryl- and vinyl-substituted ep-
oxides [styrene oxide-opening with acetone gives 100%
conversion to 2,2-dimethyl-4-phenyl-1,3-dioxolane (1)
after 1 h at –20 °C] alkyl-substituted epoxides do not react
under these conditions. A competition experiment
5
6
7
8
9
BF ·OEt , titanium salts, AlCl , iron(III) salts, BiCl ,
3
2
0
3
3
1
11
8b,12
13
CuSO4, RuCl , tin(IV) salts,
and MeReO3, have been used to catalyze this transfor-
mation. The most active catalysts (BF ·OEt , TiCl and
SnCl4 ) catalyze the formation of 1,3-dioxolane at
iridium complexes
3
4
1
(
equimolar mixture of styrene and butene oxides was used
5
6b
3
2
4
as a substrate) demonstrated the complete conversion of
styrene oxide to the corresponding dioxolane. Butene ox-
8
b
8
2+ 13
–
78 °C. Iron(III) salts,
[CpIr(NCMe) ] ,
and
3
ide remained unchanged under the reaction conditions (1
1
4
MeReO3 catalyze epoxide-opening with carbonyl com-
17
h at –20 °C). Moreover, the Cu(OTf) -catalyzed reac-
9
3
10
2
pounds at room temperature while BiCl , CuSO , and
RuCl3 convert epoxides to 1,3-dioxolanes at reflux.
4
tion of alkyl-substituted epoxides with carbonyl com-
pounds is quite sluggish at room temperature (Table 1).
1
1
15
Despite the substantial difference in activity of these cat-
alysts reaction rates for epoxides having different substi-
Table 1 Copper(II) Triflate-Catalyzed Opening of the Substituted
1
6
tution patterns are very similar. None of the catalysts
described in the literature demonstrates any preference for
opening of epoxides having substituents with specific
steric or electronic requirements.
Epoxides with Acetonea
As a part of our research program aimed at developing
synthetic methods for substituted dioxolanes, we attempt-
ed to identify catalysts and conditions that can be applied
to differentiate epoxides with aryl and alkyl substitution in
the dioxolene formation reaction.
Entry
R
Temp (°C)
Conversion (%)
1
2
3
Phenyl
CH OC H
5
–20
25
100
2
2
6
Et
CH Br
25
Nr
Nr
4
25
2
a
SYNLETT 2005, No. 5, pp 0854–0856
2
1.
0
3.
2
0
0
5
The reactions were carried out using 2 mol% of Cu(OTf) , reaction
2
Advanced online publication: 09.03.2005
time: 1 h.
DOI: 10.1055/s-2005-864795; Art ID: S06004ST
©
Georg Thieme Verlag Stuttgart · New York

LETTER
Cu(OTf) -Catalyzed Selective Opening of Aryl and Vinyl Epoxides
855
2
Scheme 1 Cu(OTf) -catalyzed styrene oxide-opening with acetone
2
Copper(II) triflate-catalyzed styrene oxide reaction with epoxide addition). Side reactions can be minimized using
acetone produced a number of by-products resulting from a large (10 equiv) excess of acetone to furnish dioxolane
the epoxide rearrangement and the subsequent reactions 1 in 95% isolated yield18 (Table 2). Phenyl-acetaldehyde
namely phenyl-acetaldehyde
Scheme 1).
2
and dioxolane
3
2 and dioxolane 3 were not detected in the reaction mix-
ture under these conditions.
(
When one equivalent of acetone was used for the reaction, The same reaction conditions were applied to various car-
0–20% of both by-products were formed depending on bonyl compounds and epoxides (Table 2) to afford good
the exact reaction conditions (temperature, the rate of the to high yields of 1,3-dioxolanes.19
1
The reaction of an enantiopure styrene oxide was tested.
Table 2 Cu(OTf) -Catalyzed Epoxide-Opening with Carbonyl
Compounds
2
a
(R)-Styrene oxide reacts with acetone in the presence of
Cu(OTf)220 with inversion of configuration to give enan-
Entry Epoxide
1
Carbonyl Productb
compound
Yield (%) tiomerically enriched (S)-2,2-dimethyl-4-phenyl-1,3-di-
oxolane (4:1 S:R ratio).
95
92
84
94
63
In summary, an efficient method to selectively convert
aryl- and vinyl-substituted epoxides into 1,3-dioxolanes
in the presence of alkyl- and alkoxycarbonyl-substituted
epoxides is reported.
2
Acknowledgment
3
The authors thank Ms. Josee Dugas for GC experiments, Mr. Danny
Lafrance and Dr. Gregory Bydlinski for valuable suggestions.
4c
References
(1) New address: P. Krasik, Torcan Chemical Ltd. P.O. Box
5c,d
3
3
08, 110 Industrial Parkway North, Aurora, Ontario, L4G
H4, Canada; e-mail: pavel.krasik@torcan.com.
(
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8c
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(
3) Greene, T. W.; Wuts, P. Protective Groups in Organic
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(
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(
8, 466. (b) Nagata, T.; Takai, T.; Yamada, T.; Imagawa, K.;
a
Mukaiyama, T. Bull. Chem. Soc. Jpn. 1994, 67, 2614.
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Reactions were carried out at –20 °C, 10 equiv of the carbonyl
(
(
compound, 2 mol% of Cu (OTf) unless noted otherwise.
2
b
All the products had physical data identical to that reported in the
literature.
c
The product have been isolated as a cis/trans mixture.
The reaction was carried out at r.t., slow (0.08 mL/h) addition of
d
675.
styrene oxide.
e
The starting material was a 1:1 mixture of cis- and trans-isomers.
Synlett 2005, No. 5, 854–856 © Thieme Stuttgart · New York

8
56
P. Krasik et al.
LETTER
(
9) Mohammadpoor-Baltork, I.; Khosropour, A. R.; Aliyan, H.
layer was separated. The reaction mixture was analyzed by
H NMR. Complete conversion to 1,3-dioxolane was
1
Synth. Commun. 2001, 31, 3411.
(
(
(
10) Hanzlik, R. P.; Leinwetter, M. J. Org. Chem. 1978, 43, 438.
11) Iranpoor, N.; Kazemi, F. Synth. Commun. 1998, 28, 3189.
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Chem. 1999, 582 (2), 358.
detected for styrene oxide. 1-Butene oxide remains
unchanged.
(18) Typical procedure for epoxide-opening with carbonyl
compounds is as follows: Cu(OTf) (2 mol%) was dissolved
2
(
in CH Cl (1 mL) and dry acetone (1.5 mL, 10 equiv). The
2
2
reaction mixture was cooled to –20 °C, styrene oxide (2
mmol) was added dropwise. The reaction mixture was
stirred for 1 h at –20 °C, poured onto sat. aq NaHCO3
solution (5 mL). CH Cl (10 mL) was added and the organic
(
(
(
14) Zhu, Z.; Espenson, J. H. Organometallics 1997, 16, 3658.
15) Typical catalyst loadings are 2–20 mol%.
16) Data presented for K CoW O -catalyzed epoxide-opening
5
12
4
2
2
with carbonyl compound suggest that aryl-substituted
epoxides react with higher rates than alkyl-substituted ones.
However, no specific comparisons of reactivity are made.
See: Habibi, M. H.; Tangestaninejad, S.; Mirkhani, V.;
Yadollahi, B. Catal. Lett. 2001, 75, 205.
layer was separated. The product was purified by flash
chromatography (10% EtOAc in hexane) to give 2,2-
dimethyl-4-phenyl-1,3-dioxolane in 95% yield. H NMR
1
(400 MHz, CDCl ): d = 1.52 (s, 3 H), 1.58 (s, 3 H), 3.72 (dd,
3
J = 8.2, 8.1 Hz, 1 H), 4.34 (dd, J = 8.1, 6.3 Hz, 1 H), 5.10
1
3
(
17) The competition experiment was carried out as follows:
Cu(OTf) (2 mol%) was dissolved in CH Cl (1 mL) and dry
(dd, J = 7.9, 6.3 Hz, 1 H), 7.20–7.60 (m, 5 H). C NMR (400
MHz, CDCl ): d = 139.48, 128.99, 128.50, 126.68, 110.15,
2
2
2
3
acetone (1.5 mL, 10 equiv). The reaction mixture was cooled
to –20 °C, a mixture of styrene oxide (1 mmol) and 1-butene
oxide (1 mmol) was added dropwise. The reaction mixture
was stirred for 1 h at –20 °C, poured onto sat. aq NaHCO3
solution (5 mL). CH Cl (10 mL) was added and the organic
78.40, 72.14, 27.06, 26.45.
(19) For all compounds the physical data corresponded to that
published earlier.
(20) Conditions: 5 mol% of Cu(OTf) , –80 °C, 178 h.
2
2
2
Synlett 2005, No. 5, 854–856 © Thieme Stuttgart · New York