Copper(I)-catalyzed [2+2] photocycloaddition of nonconjugated alkenes in room-temperature ionic liquids

Article abstract of DOI:10.1055/s-2007-965971

Intra- and intermolecular copper(I)-catalyzed [2+2] photocycloaddition reactions of nonconjugated alkenes can be achieved efficiently in room-temperature ionic liquids such as [tmba][NTf₂]. The ionic liquid can be recovered and recycled. No cyc

Full text of DOI:10.1055/s-2007-965971

SPECIAL TOPIC
1247
Copper(I)-Catalyzed [2+2] Photocycloaddition of Nonconjugated Alkenes in
Room-Temperature Ionic Liquids
[2+2]
P
hotocyclo
h
a
ddition in
I
a
onic
L
iquid
n
s
chal K. Malik,a,b Michel Vaultier,*b Subrata Ghosh*a
a
Department of Organic Chemistry, Indian Association for the Cultivation of Science, Jadavpur, Kolkata, 700 032, India
Fax +91(33)24732805; E-mail: ocsg@iacs.res.in
b
Université de Rennes 1, Institute de Chimie, UMR CNRS 6510, Avenue de General Leclerc, 35042 Rennes, France
E-mail: michel.vaultier@univ-rennes1.fr
Received 27 January 2007
Generally, copper(I) trifluoromethanesulfonate catalyzed
photochemical reactions are carried out in benzene or in
an ether, such as diethyl ether or dioxane, as solvent in
which the sensitive catalyst is stable and the diene–cop-
Abstract: Intra- and intermolecular copper(I)-catalyzed [2+2] pho-
tocycloaddition reactions of nonconjugated alkenes can be achieved
efficiently in room-temperature ionic liquids such as [tmba][NTf2].
The ionic liquid can be recovered and recycled. No cycloadduct
could be isolated when the same reaction is carried out under iden- per(I) complex is soluble. As a solvent, benzene is not en-
tical conditions in [bmim][NTf2], which undergoes extensive de-
composition.
vironmentally benign. Ethers generally undergo
polymerization when irradiated in the presence of cop-
Key words: catalysis, copper, cycloadditions, ionic liquids, photo-
chemistry
per(I) trifluoromethanesulfonate catalyst producing sig-
nificant amount of undesired material from which
isolation of pure products is very difficult. To obviate
these problems when using these solvents, we considered
The [2+2] photocycloaddition of alkenes to form cyclo- the possibility of using a room-temperature ionic liquid
butanes is probably the most well-studied and useful reac- (RTIL)7 as solvent. RTILs7,8 are gaining importance as al-
tion in organic synthesis amongst various photochemical ternatives to conventional organic solvents for various or-
processes. Unlike cycloaddition1 of an alkene to an enone ganic transformations. Apart from being a solvent, on
(lmax >300 nm), which can be achieved by irradiation di- many occasions RTILs have been reported to have a sig-
rectly or in the presence of sensitizer, cycloaddition be- nificant effect on the reaction rate8 and yield. However,
tween two nonconjugated alkenes (lmax 190–200 nm) very little attention has been paid9 to performing photo-
requires catalysis by transition-metal salts.2 Of the various chemical reactions in RTILs, mainly due to the lack of in-
metal salts investigated copper(I) salts especially cop- formation on the stability of RTILs on irradiation.
per(I) trifluoromethanesulfonate, available as bis(cop- Photochemistry in ionic liquids9 has so far been carried
per(I) trifluoromethanesulfonate)–benzene complex,3 has out exclusively with pyridinium and imidazolium salts,
been found to be the most efficient catalyst for cycloaddi- which have little absorption above 300 nm. However,
tion reactions. The copper(I)-catalyzed photocyclo- there is no report on photochemistry in RTILs based on
addition2 of alkenes 1 proceeds through ground-state ammonium salts. Pulse radiolysis10 has demonstrated that
formation of a 2:1 alkene–copper(I) complex 2, which imidazolium-based RTILs produce electrons that reacts
shows UV absorption at 230–240 nm. Thus, this complex with the cationic part of the ionic liquid to form the corre-
can be photoexcited when irradiated leading to cycloaddi- sponding radical, which can initiate reactions of photoac-
tion to afford cyclobutanes 3 (Scheme 1). The most re- tive substrates, while ammonium salt based RTILs
markable feature of copper(I)-catalyzed photocyclo- produce solvated electrons that do not react with the par-
addition reactions is their regio- and stereoselectivity es- ent ions. Thus photocycloaddition in RTILs based on im-
pecially in the case of intramolecular cycloadditions. In- idazolium and ammonium salts is worthy of investigation.
tramolecular copper(I)-catalyzed [2+2] photocyclo- Herein we report copper(I)-catalyzed [2+2] photocy-
addition of 1,6-dienes having allylic substituents proceeds cloaddition reactions of nonconjugated alkenes in RTILs.
stereoselectively4 and it is this aspect that has made it syn-
Initially we chose intramolecular photocycloaddition of
thetically very useful as demonstrated by us5 and others.6
1,6-diene 4a (Table 1). Irradiation of the diene 4a11 in di-
ethyl ether solution in the presence of copper(I) trifluo-
romethanesulfonate with a light source of 254 nm is
Cu
+
R
R
R
R
R
R
known to produce the cyclobutane derivative 5a in 76%
yield. However, when a solution of this diene 4a in 1-bu-
tyl-3-methylimidazolium bis(trifluoromethylsulfonyl)im-
ide ([bmim][NTf2]) in the presence of copper(I) tri-
fluoromethanesulfonate was irradiated with light of wave-
length 254 nm, extensive decomposition of the ionic liq-
uid occurred; no product could be isolated from this
reaction. When the diene 4a was irradiated in trimeth-
1
2
3
Scheme 1
SYNTHESIS 2007, No. 8, pp 1247–1250
Advanced online publication: 28.02.2007
DOI: 10.1055/s-2007-965971; Art ID: C00107SS
© Georg Thieme Verlag Stuttgart · New York
x
x
.
x
x
.2
0
0
7

1248
C. K. Malik et al.
SPECIAL TOPIC
MeOOCH2C
yl(butyl)ammonium bis(trifluoromethylsulfonyl)imide
([tmba][NTf2]) under the above conditions, the cyclobu-
tane derivative 5a was isolated after purification in 75%
yield, comparable with the yield obtained from reaction of
the diene 4a in diethyl ether. The ionic liquid, after isola-
tion of the product and purification, can be reused. To ex-
plore the generality, a number of 1,6-dienes 4b–f were
irradiated in [tmba][NTf2] under identical condition and
the results are summarized in Table 1. It may be noted that
photocycloaddition in [tmba][NTf2] works well not only
to produce azabicyclo[3.2.0]heptanes 5a–d, but also to
produce the oxa analogue 5e and bicyclo[3.2.0]heptane
5f.
CH2COOMe
H
H
H
H
CuOTf
[tmba][NTf2]
hν
CH2COOMe
8
9
Scheme 2
chemical reactions. The ionic liquid can be conveniently
recovered and recycled with equal efficiency. However,
[bmim][NTf2] which has slight absorption at 300 nm can
not be used for copper(I)-catalyzed photoreactions.
For column chromatography, silica gel (60–120 mesh) was used. 1H
and 13C NMR spectra were recorded on a Bruker DPX 300 spec-
trometer in CDCl3 (300 MHz for 1H, 75 MHz for 13C) unless other-
wise stated. HRMS were recorded on Varian Mat 311 and Micro
Mass QTOF spectrometers. The dienes 4a,11 4b,11 4d,14 and 4f15
were prepared following literature procedures.
Table 1 Copper(I) Trifluoromethanesulfonate Catalyzed [2+2]
Photocycloaddition in [tmba][NTf2]
H
CuOTf, [tmba][NTf2]
X
X
hν, 24 h
R2
R1
R2
R1
Ethyl N-Allyl-N-but-2-enylcarbamate (4c)
5
4
To a magnetically stirred suspension of NaH (0.48 g, 20 mmol) pre-
washed with hexane in DME (25 mL) was added ethyl N-allylcar-
bamate (2.6 g, 20 mmol) dropwise via a syringe at r.t.; stirring was
continued for an additional 3 h. Then 1-bromobut-2-ene (3.24 g, 24
mmol) was added and the resulting mixture was stirred at r.t. for 20
h and refluxed for 2 h. After cooling, H2O (40 mL) was added and
the mixture was acidified with 10% HCl (20 mL). The mixture was
extracted with Et2O. The organic extract was washed with brine,
dried (Na2SO4), and concentrated under vacuum. Distillation of the
residual oil afforded the diene 4c;11 yield: 2.8 g (77%).
Diene
4a
X
R1
Me
H
R2
H
Product Yielda (%)
NCO2Et
NCO2Et
NCO2Et
NCO2Bn
O
5a
5b
5c
5d
5e
5f
75 (76)
65 (74)
57
4b
H
4c
H
Me
H
4d
H
75
4e
H
Ph
H
79b (75)
60
1H NMR: d = 1.17 (t, J = 7.0 Hz, 3 H), 1.59 (d, J = 6.4 Hz 3 H), 3.72
(d, J = 9.9 Hz, 4 H), 4.06 (q, J = 7.0 Hz, 2 H), 5.01–5.13 (m, 2 H),
5.30–5.35 (m, 1 H), 5.45–5.52 (m, 1 H), 5.61–5.74 (m, 1 H).
4f
C(CO2Me)2
H
13 C NMR: d = 14.5 (Me), 17.5 (Me), 48.0 (CH2), 61.1 (CH2), 116.4
(CH2), 126.3 (CH), 128.3 (CH), 133.8 (CH), 156.1 (CO).
a Yields in parenthesis are for reactions in Et2O.
b Yield based on recovered starting material.
Allyl Cinnamyl Ether (4e)
To a magnetically stirred ice cold suspension of NaH (1.16 g, 48.5
mmol), prewashed with hexane, in anhyd THF (30 mL) was added
dropwise cinnamyl alcohol (5 g, 37.3 mmol). The mixture was
stirred at r.t. for 2 h and then allyl bromide (5.41 g, 44.7 mmol) was
added and stirring was continued for a further 1 h. The mixture was
quenched by addition of H2O (2 mL) and extracted with Et2O. The
organic extract was washed with brine and dried (Na2SO4). The sol-
vent was removed under reduced pressure and the residual oil was
chromatographed to afford the ether 4e as a yellow liquid; yield: 5.8
g (90%).
1H NMR: d = 4.09 (d, J = 5.6 Hz, 2 H), 4.20 (dd, J = 0.79, 5.7 Hz,
2 H), 5.27 (d, J = 10.3 Hz, 1 H), 5.38 (d, J = 17.3 Hz, 1 H), 6.01 (m,
1 H), 6.31–6.40 (m, 1 H), 6.67 (d, J = 15.9 Hz, 1 H), 7.26–7.46 (m,
5 H).
H
H
H
H
H
H
H
H
H
H
H
H
6
7
Figure 1
Intermolecular [2+2] photocycloaddition in [tmba][NTf2]
was found to proceed in an even better yield than in con-
ventional organic solvents. Thus, dicyclopentadiene pro-
duced the dimer 6 in 71% yield against 48% reported12 in
benzene. Similarly, norbornene gave the dimer 713 in 90%
yield (Figure 1). Finally, irradiation of the cyclopentene
13C NMR: d = 70.9 (CH2), 71.3 (CH2), 117.2 (CH2), 126.2 (CH),
derivative 8 produced, under identical condition, a mix- 127.8 (CH), 128.7 (CH), 132.5 (CH), 134.9 (CH), 136.9 (C).
HRMS (ESI): m/z [M + Na]+ calcd for C12H14ONa: 197.0942;
found: 197.0916.
ture of the adduct 9 and its diastereomer (8:1) in 56% yield
(Scheme 2). The head–tail structure and cis-anti-cis ste-
reochemistry of the major adduct is based on analogy to
photodimerization of substituted cyclopentenone deriva-
tives.1a
Ethyl 1-Methyl-3-azabicyclo[3.2.0]heptane-3-carboxylate (5a);
Typical Procedure for Photochemical Reactions in [tmba][NTf2]
The substrate 4a (120 mg, 0.66 mmol) and [tmba][NTf2] (3 mL)
were placed in a quartz tube (200 mm/10 mm). While purging argon
through this soln, CuOTf·benzene (26 mg, 0.05 mmol) was added
and the argon purge was continued till the entire mixture became al-
In conclusion, we have demonstrated that an ammonium
salt based RTIL such as [tmba][NTf2] is quite stable to
UV irradiation and can be an efficient solvent for photo-
Synthesis 2007, No. 8, 1247–1250 © Thieme Stuttgart · New York

SPECIAL TOPIC
[2+2] Photocycloaddition in Ionic Liquids
1249
most homogeneous (ca. 15 min.) The open end of the tube was then
sealed with a rubber septum. The tube was then placed in a Rayonet-
type reactor fitted with 20 W, 254 nm lamps (12) and irradiated for
48 h. The mixture was then transferred to a separatory funnel and
extracted with Et2O (3 × 10 mL). The combined Et2O extracts were
washed with cold 25% aq NH3 soln (2 × 5 mL) and dried (Na2SO4).
Evaporation of the ether in vacuo followed by column chromatog-
raphy (silica gel) of the residual mass afforded the cyclobutane 5a;
yield: 90 mg (75%).
1H NMR (300 MHz, CDCl3): d = 1.17 (s, 3 H), 1.21 (t, J = 7.1 Hz,
3 H), 1.50–1.55 (m, 1 H), 1.70–1.79 (m, 1 H), 1.83–1.88 (m, 1 H),
2.02–2.12 (m, 1 H), 2.33 (m, 1 H), 2.88 (d, J = 10.6 Hz, 1 H), 3.31
(m, 1 H), 3.53 (s, 2 H), 4.08 (q, J = 14.3 Hz, 2 H).
13C NMR (75 MHz, CDCl3): d = 13.8 (Me), 20.3 (CH2), 23.1 (CH),
29.8 (CH2), 41.7 (C), 42.7 (C), 52.0 (CH2), 58.0 (CH2), 59.9 (CH2),
154.6 (CO).
HRMS (EI): m/z [M]+ calcd for C10H17NO2: 183.1259; found:
183.1257.
6-Phenyl-3-oxabicyclo[3.2.0]heptane (5e)
Isolated in 79% yield (based on recovered diene after 6 h irradia-
tion).
1H NMR: d = 2.11–2.16 (m, 1 H), 2.23–2.27 (m, 1 H), 2.91–2.96
(m, 2 H), 3.18–3.20 (m, 1 H), 3.44–3.54 (m, 1 H), 3.54–3.59 (m, 1
H), 3.91–3.96 (m, 2 H), 7.12–7.20 (m, 5 H).
13C NMR: d = 31.9 (CH2), 35.4 (CH), 42.0 (CH), 47.3 (CH), 74.1
(CH2), 74.6 (CH2), 125.9 (CH), 126.4 (CH), 128.4 (CH), 146.2 (C).
HRMS (EI): m/z [M]+ calcd for C12H14O: 174.1044; found:
174.1047.
Dimethyl Bicyclo[3.2.0]heptane-3,3-dicarboxylate (5f)
The diene 4f (120 mg, 0.57 mmol) gave after irradiation 5f; yield:
72 mg (60%).
1H NMR: d = 0.52 (m, 2 H), 2.09 (m, 2 H), 2.28 (m, 4 H), 2.76 (m,
2 H), 3.63 (s, 3 H), 3.7 (s, 3 H).
13C NMR: d = 23.3 (CH2), 37.5 (CH), 40.7 (CH2), 51.5 (OMe), 51.7
(OMe), 62.8 (C), 171.8 (CO), 172.0 (CO).
HRMS (EI): m/z [M – OCH3]+ calcd for C10H13O3: 181.0864. found:
Recovery of [tmba][NTf2]
The ionic liquid left after extraction of the product was dissolved in
CH2Cl2 (20 mL) and washed with 25% aq NH3 soln. The CH2Cl2
soln was heated under reflux with activated charcoal (0.5 g) for 30
min, cooled to r.t. and then filtered through a filter paper. CH2Cl2
was distilled out from the filtrate and the residue in the flask was
dried in vacuo (0.01 mbar) at 60 °C. The ionic liquid recovered in
this way was reused.
181.0866.
Dimethyl Tricyclo[5.3.0.02,6]decane-3,8-diacetate (9)
Isolated in 56% yield (based on recovered starting material).
1H NMR: d = 1.49 (m, 6 H), 1.71 (m, 2 H), 1.93 (t, J = 5.4 Hz, 2 H),
2.08 (m, 2 H), 2.25 (m, 4 H), 2.45 (m, 1 H), 2.81 (m, 1 H), 3.57 (s,
3 H), 3.59 (s, 3 H).
13C NMR: d = 25.6 (CH2), 30.7 (CH2), 32.3 (CH2), 33.02 (CH2),
33.61 (CH2), 36.8 (CH2), 37.9 (CH), 38.3 (CH), 39.5 (CH), 40.6
(CH), 44.7 (CH), 51.2 (CH3), 51.3 (CH3), 173.6 (CO), 173.7 (CO).
Ethyl 3-Azabicyclo[3.2.0]heptane-3-carboxylate (5b)
The diene 4b (120 mg, 0.71 mmol) on irradiation gave 5b; yield: 78
mg (65%).
HRMS (EI): m/z [M]+ calcd for C16H24O4: 280.1674; found:
280.1699.
1H NMR (200 MHz, CDCl3): d = 1.26 (t, J = 7.1 Hz, 3 H), 1.67–
1.72 (m, 2 H), 2.14–2.22 (m, 2 H), 2.88–2.91 (m, 2 H), 3.21–3.29
(m, 2 H), 3.55 (m, 2 H), 4.14 (q, J = 14.2 Hz, 2 H).
6-Phenyl-3-oxabicyclo[3.2.0]heptane (5e) by Irradiation of
Diene 4e in Diethyl Ether
13C NMR (50 MHz, CDCl3): d = 15.2 (Me), 24.9 (CH2), 37.5 (CH),
38.4 (CH), 53.4 (CH2), 61.3 (CH2), 156.3 (CO).
A magnetically stirred soln of diene 4e (1 g, 5.75 mmol) in anhyd
Et2O was irradiated internally with a Hanovia 450 W medium pres-
sure mercury vapor lamp through a water-cooled quartz immersion
well for 6 h. The mixture was washed with ice-cold 35% aq NH3
soln and brine, and dried (Na2SO4). Removal of solvent followed by
column chromatography of the residual mass afforded the adduct 5e
(750 mg, 75%) as a colorless oil. Spectral data obtained for this
sample were identical to those prepared by irradiation in ionic liquid
as described above.
HRMS (EI): m/z [M]+ calcd for C9H15NO2: 169.1102; found:
169.1105.
Ethyl 6-Methyl-3-azabicyclo[3.2.0]heptane-3-carboxylate (5c)
The diene 4c (105 mg, 0.57 mmol) on irradiation gave 5c; yield: 60
mg (57%).
1H NMR (200 MHz, CDCl3): d = 1.11 (d, J = 6.8 Hz, 3 H), 1.2 (t,
J = 6.6 Hz, 3 H), 1.59–2.09 (m, 2 H), 2.21–2.80 (m, 3 H), 3.2 (m, 2
H), 3.47–3.73 (m, 2 H), 4.08 (d, J = 14.2 Hz, 2 H).
13C NMR (50 MHz, CDCl3): d = 15.2 (Me), 16.5 (Me), 22.0 (CH),
27.7 (CH), 32.9 (CH2), 33.5 (CH), 52.6 (CH2), 53.1 (CH2), 61.3
(CH2), 156.1 (CO).
HRMS (EI): m/z [M]+ calcd for C10H17NO2: 183.1259; found:
183.1254.
Acknowledgment
S.G. thanks Department of Science and Technology, Government
of India for Ramanna Fellowship. C.K.M. thanks CSIR, New Delhi
for a Senior Research Fellowship and French Embassy, India for a
Sandwich Theses scholarship, 2004.
Benzyl 3-Azabicyclo[3.2.0]heptane-3-carboxylate (5d)
The diene 4d (120 mg, 0.52 mmol) on irradiation gave 5d; yield: 90
mg (75%).
1H NMR: d = 1.72 (m, 2 H), 2.09 (m, 2 H), 2.94–2.80 (m, 3 H), 3.24
(m, 1 H), 3.53 (m, 1 H), 5.06–5.09 (s, 2 H), 7.18–7.32 (m, 5 H),
3.95–4.01 (m, 1 H).
13C NMR: d = 23.5 (2 × CH2), 36.0, 37.0 (CH), 52.0, 52.3 (CH2),
65.7 (CH2), 126.7, 126.8, 126.9, 127.41, 127.43 (5 CH of Ph), 136.1
(C), 154.5 (CO).
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Synthesis 2007, No. 8, 1247–1250 © Thieme Stuttgart · New York

1250
C. K. Malik et al.
SPECIAL TOPIC
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Synthesis 2007, No. 8, 1247–1250 © Thieme Stuttgart · New York