First graphene oxide promoted metal-free nitrene insertion into olefins in water: Towards facile synthesis of activated aziridines

Article abstract of DOI:10.1039/c7ra09351a

A facile metal-free graphene oxide (GO)-catalyzed synthesis of tosylaziridines using PhINTs as the nitrene source is reported. The reaction involves nitrene insertion into a variety of styrene/nitrostyrene derivatives in the presence of iodine at room tem

Full text of DOI:10.1039/c7ra09351a

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First graphene oxide promoted metal-free nitrene
insertion into olefins in water: towards facile
synthesis of activated aziridines†
Cite this: RSC Adv., 2017, 7, 48723
b
Prashant Shukla,a Suhasini Mahata,b Anjumala Sahu,b Manorama Singh,
b
a
*
Vijai K. Rai
and Ankita Rai
A facile metal-free graphene oxide (GO)-catalyzed synthesis of tosylaziridines using PhI]NTs as the nitrene
source is reported. The reaction involves nitrene insertion into a variety of styrene/nitrostyrene derivatives in
the presence of iodine at room temperature and in water. The envisaged process is highly green,
operationally simple, it employs metal-free catalysis and it affords excellent yields (85–92%) and high
diastereoselectivity (95–98%) of the Z-isomer of the product. The catalyst used could be recycled for
further use in other reactions.
Received 23rd August 2017
Accepted 29th September 2017
DOI: 10.1039/c7ra09351a
rsc.li/rsc-advances
Aziridines have been synthetic targets as well as scaffolds since for bioactive molecules,26 we decided to undertake the aziridi-
Gabriel’s discovery in 1888. They represent an important nation of olens using PhI]NTs as the nitrene source.27
structural motif in drugs and natural products (Fig. 1) and are Recently, we have reported the synthesis of functionalized gra-
highly useful as feedstock in organic synthesis to provide phene oxide,28 which could easily be dispersed in water.
functionalized carbon chains due to the inherent strain in small Keeping the use of water as a reaction medium in mind, we
ring heterocycles.1 In terms of synthetic transformations, the anticipated using the reported Fe-decorated GO as a catalyst for
utility of aziridines arises from their selective ring-opening the present investigation. To our surprise, we were successful in
reactions using various nucleophiles, which oen form the isolating the target aziridine but the yield was not satisfactory.
basis of more complex target syntheses.2,3 Owing to their Then we turned our attention to employing a metal-free cata-
chemical and biological interest, numerous methods are re- lytic process and tried graphene oxide only as a catalyst in the
ported for their synthesis,4–14 including the aziridination of presence of PhI]NTs and I2 and the reaction was successful in
olens,4–6 the addition of carbene and ylide to imines,7–9 and terms of yield of pure product. Herein, we disclose a metal-free
different cyclization protocols.10–14 However, the direct transfer aziridination of styrenes as well as nitrostyrenes using an
of nitrogen atoms to alkenes has been the most practical and equimolar amount of PhI]NTs in the presence of I2 using GO
useful method for the synthesis of aziridines.15 Although as the catalyst in an aqueous medium and at ambient reaction
transition-metal catalyzed aziridination of alkenes has been conditions (Scheme 1). To the best of our knowledge, the azir-
well reported in the literature, metal-free aziridination using idination of olens through graphene oxide-catalyzed nitrene
different nitrogen sources is still an attractive strategy from insertion reactions under metal-free conditions using water as
practical, economical and, of course, environmental points of a solvent has not been reported so far.
view.16–20 The nitrene transfer reaction,21,22 and the use of imi-
In our preliminary experimentation, we rst examined
noiodinanes23 as nitrene precursors and thus the efficient equimolar mixtures of styrene 1a and PhI]NTs 2 with 10 mol%
conditions for catalytic aziridination of olens using iodine(III) of I2 in water at room temperature (Table 1, entry 1), but we did
oxidants are reported.24 Despite previous work, the develop- not isolate product 3a without the use of a catalyst, even upon
ment of more environmentally benign nitrene transfer reac-
tions for the construction of diverse aziridines is still in great
demand. Thus, inspired by these reports and in continuation of
our interest in developing efficient and green catalytic routes25
aSchool of Physical Sciences, Jawaharlal Nehru University, New Delhi, 110027, India.
E-mail: ankitarai@mail.jnu.ac.in; Tel: +91-11-26738808
bDepartment of Chemistry, School of Physical Sciences, Guru Ghasidas
Vishwavidyalaya, Bilaspur (C.G.)-495009, India
† Electronic supplementary information (ESI) available: Characterization details
are given. See DOI: 10.1039/c7ra09351a
Fig. 1 Drugs and natural products containing aziridine.
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1,4-dioxane, THF, MeOH and water (Table 1, entries 7, 10–14).
When the reaction was carried out in aprotic solvents such as
MeCN, DCM, 1,4-dioxane and THF, it afforded the aziridine 3a
in trace amounts (Table 1). However, when using polar protic
solvents, the yield was improved (Table 1, entry 14) and there-
fore, water was found to be the best choice of solvent in the
present optimization process.
Scheme 1 Synthesis of the GO-catalyzed aziridination of olefins.
The role of I2 was also investigated and a reaction of 1a and 2
and GO (15 mg) was performed in the absence of iodine, but no
aziridine 3a was isolated (Table 1, entry 17) which is evidence
that iodine plays a crucial role in the formation of aziridine
rings. When the reaction was performed in the dark it afforded
3a in low yield, showing that the reaction involved a radical
which was promoted by ambient room light in the laboratory
(Table 1, entry 18).
Table 1 Screening of catalysts and solvents for the aziridination of
styrene 1aa
Aer optimization of the reaction conditions, we next
investigated the substrate scope of the present aziridination of
styrene. The substituents on the benzene ring of styrene 1 did
not show any specic effect over the product outcome as well as
the conversion rate. Substrates with electron-donating and
electron withdrawing substituents at the para-position were
tolerated to afford the corresponding aziridine 3 in consistently
good yields (Table 2). Substituents at the ortho or meta positions
on the phenyl ring caused almost no change in reactivity (Table
2, entries 7 and 8). However, when using an aliphatic alkene,
a moderate yield of the corresponding aziridine was obtained.
The formation of product 3 was conrmed by spectral analysis.
In the 1H NMR spectra of 3a (Fig. S1; ESI†), a double doublet at
d 3.78 (J ¼ 7.0, 4.5 Hz) for CH indicates the presence of two
vicinal CH2 protons, further conrmed by two doublets at d 2.99
(J ¼ 7.0 Hz) and d 2.43 (J ¼ 4.5 Hz). Thus, the present optimized
synthesis of stirring an equimolar mixture of styrene 1 and
Entry Catalyst (mg) I2 (mol%) Solvent
Timeb (h) Yieldc,d (%)
1
2
3
4
5
6
7
8
—
—
—
—
10
10
10
10
10
10
10
10
10
10
10
10
10
10
10
10
5
Water
MeOH
MeCN
THF
1,4-Dioxane 12
H2O
H2O
H2O
H2O
Benzene
DCM
DMF
THF
MeOH
H2O
12
12
12
12
—
—
—
—
—
43
92
—
—
—
23
—
29
53
73
92
—
92
—
Fe–GO (15)
GO (15)
rGO (15)
Graphite
GO (15)
GO (15)
GO (15)
GO (15)
GO (15)
GO (10)
GO (20)
GO (15)
GO (15)e
12
10
12
12
10
12
10
10
10
10
10
10
10
9
10
11
12
13
14
15
16
17
18
H2O
H2O
10
H2O
a
Reaction conditions: a mixture of 1a (1 equi.), 2 (1 equi.), I2 (15 mol%)
and GO (15 mg) in 5 mL of water was stirred at room temperature. b The
c
stirring time at room temperature. Yield of isolated and puried
Table 2 Substrate scope of styrene 1 in the present aziridination
product. d Characterized by spectral (IR, H NMR, 13C NMR and EIMS)
1
processa
data. e The reaction was carried out in the dark.
changing the solvent system (MeOH, MeCN, THF and 1,4-
dioxane) (Table 1, entries 2–5). Next, the screening of the cata-
lytic and solvent systems was undertaken, as given in Table 1.
Initially, we used our recently reported catalyst,28 Fe–GO (15 mg)
using water as a solvent, and we successfully isolated the cor-
responding aziridine product 3a in moderate yield (Table 1,
entry 6). Encouraged by the result, we tried several graphene-
based materials as catalysts in the present synthetic protocol
(Table 1, entries 6–9). However, GO was found to be the most
effective among GO, rGO, graphite and Fe–GO, while rGO and
graphite were ineffective for the conversion (Table 1, entries 8
and 9). The optimum catalyst loading for graphene oxide was
found to be 15 mg. When the amount of catalyst decreased from
15 mg to 10 mg, the yield of product 3a reduced (Table 1, entry
15) but the use of 20 mg of the catalyst showed the same yield
(Table 1, entry 16). Solvent screening for the synthesis of 3a was
also undertaken using various solvents such as MeCN, DCM,
Entry
1 (R) styrene
3 (R) product
Timeb (h)
Yieldc,d (%)
1
2
3
4
5
6
7
8
9
10
H, 1a
H, 3a
10
10
12
10
11
10
10
10
11
10
92
91
87
90
91
85
90
89
91
89
4-Me, 1b
4-OMe, 1c
4-Cl, 1d
4-CN, 1e
4-CF3, 1f
2-Me, 1g
3-Me, 1h
4-NO2, 1i
4-Br, 1j
4-Me, 3b
4-OMe, 3c
4-Cl, 3d
4-CN, 3e
4-CF3, 3f
2-Me, 3g
3-Me, 3h
4-NO2, 3i
4-Br, 3j
a
Reaction conditions: a mixture of 1 (0.75 mmol), 2 (0.75 mmol), I2
(0.075 mmol) and GO (15 mg) dispersed in 5 mL of water was stirred
b
c
at room temperature. The stirring time at room temperature. Yield
of isolated and puried product. Characterized by spectral (IR, 1H
d
NMR, 13C NMR and EIMS) data.
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electron releasing and electron withdrawing groups at the para
position were in good agreement with the yield of product 5
(Table 3). Also, the ortho- and meta-substituted phenyl ring
afforded approximately the same yield of pure product 5.
Increasing the mol% of iodine did not increase the yield.
However, decreasing the amount of iodine resulted in
substantial suppression of product yield (Table 3, entries 8 and
9). A similar effect was observed when varying the amount of the
GO catalyst (Table 3, entries 10 and 11). The 1H NMR spectra of
5a shows two doublets at d 2.28 and 4.55 for both CHs in azir-
idine (5a) with the same coupling constant (J ¼ 9.5 Hz) con-
rming the formation of nitroaziridine 5 (Fig. S2, ESI†). Next,
the Z-stereochemistry of aziridines 5 was assigned on the basis
of higher J-values (9.5–9.8 Hz) of 2-H and 3-H than those of (E)-
aziridines (6.5–6.6 Hz). This conforms with the earlier obser-
vations for aziridines reported in the literature.32–35 Further-
more, strong NOEs between 2-H and 3-H in aziridines 5 also
conrm that these protons are on the same face of the molecule
and thus, prove their Z-stereochemistry (Fig. 2). The plausible
mechanism for the formation of aziridines 3 and 5 is given in
Scheme 3. The radical pathway is favoured in the light of the
homolytic weakness of the N–I bond37 and low conversion rate
in the dark using the same reaction conditions (Table 1).
Initially, PhINTs 2 reacts with I2 in the presence of GO, gener-
ating N,N-diiodotosylamide 6, which is the cornerstone of the
present nitrene insertion process. Presumably, homolytic
cleavage of the N–I bond37 of 6 generates the amidyl radical 7,
which is followed by the intramolecular cyclization of 8, assisted
by the iodine free radical to furnish the target molecules 3 and 5
with the regeneration of I2, which is to be reused in the catalytic
cycle (Scheme 3).
Scheme
aziridines.
2 Various synthetic routes for the synthesis of nitro-
PhI]NTs 2, iodine (10 mol%) and GO (15 mg) in water for 10–
12 h was successful and afforded tosylaziridine 3 in 85–92%
yields (Table 2).
Although 2-nitroaziridines appear to be a scaffold for further
transformation, very few reports are available on the synthesis
of C-nitroaziridines12,30–32 using nitroalkene or its derivatives
with a nitrogen source, namely ethyl[(4-nitrobenzenesulfonyl)
oxy]carbamate (NsONHCO2Et)/azide, amine or imine (eqn (1)–
(3) of Scheme 2). Keeping these literature reports in mind, we
extended our envisaged aziridination process to the synthesis of
2-nitro-1-tosylaziridine 5 employing the optimized reaction
conditions in hand. Thus, a mixture of nitrostyrene 4, PhINTs 2,
iodine (10 mol%) and GO (15 mg) was stirred at room temper-
ature in water and the corresponding 2-nitro-1-tosylaziridine
was synthesized in 5–7 h in good to excellent yield (87–91%)
(Table 3). However, nitrostyrenes bearing various types of
Graphene oxide (GO) is a single hexagonal lattice sheet of sp2
and sp3 hybridized C atoms. The oxidation of graphite results in
Table 3 Synthesis of 2-nitro-1-tosylaziridine 5a
Entry
4 (Ar) nitrostyrene
5 (Ar) product
Timeb (h)
Yieldc,d (%)
Z/Ee
1
2
3
4
5
6
7
Ph, 4a
Ph, 5a
5
6
6
7
6
6
5
5
5
5
5
89
87
88
91
88
90
91
89
68
89
61
96 : 04
95 : 05
97 : 03
98 : 02
96 : 04
97 : 03
97 : 03
96 : 04
96 : 04
96 : 04
96 : 04
4-OMeC6H4, 4b
4-ClC6H4, 4c
4-MeC6H4, 4d
2-ClC6H4, 4e
4-NO2C6H4, 4f
3-MeC6H4, 4g
Ph, 4a
Ph, 4a
Ph, 4a
Ph, 4a
4-OMeC6H4, 5b
4-ClC6H4, 5c
4-MeC6H4, 5d
2-ClC6H4, 5e
4-NO2C6H4, 5f
3-MeC6H4, 5h
Ph, 5a
Ph, 5a
Ph, 5a
Ph, 5a
8f
9g
10h
11i
a
Reaction conditions: a mixture of 4 (3.5 mmol), 2 (0.750 mmol), I2 (10 mol%) and GO (15 mg) dispersed in 5 mL of water was stirred at room
temperature. The stirring time at room temperature. Yield of isolated and puried product. Characterized by spectral (IR, 1H NMR, 13C
b
c
d
NMR and EIMS) data. Determined by 1H NMR spectroscopy. Reaction was performed in 15 mol% of I2. Reaction was performed in 5 mol%
e
f
g
of I2. h Reaction was performed with 20 mg of GO. Reaction was performed with 10 mg GO.
i
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Fig. 2 NOE experiment on nitro-aziridines 5.
Fig. 4 Reusability experiment of the GO catalyst.
Scheme 3 Plausible mechanism for the formation of tosylaziridines 3
and 5.
Fig. 5 IR (A) and XRD (B) of GO before and after use in a reusability
experiment.
a number of defects or holes on its surface. The presence of
numerous –COOH groups at the edges of akes as well as at the
edges of defect sites may be utilized for its functionalization,
which makes GO an efficient catalyst (Fig. 3).36 The reusability of
GO as a heterogeneous acid catalyst was also examined and it
temperature (Fig. 4). Comparison of the FT-IR spectra of GO
before and aer use conrmed that the catalyst remained the
same aer the reaction (Fig. 5).
was recovered from the rst batch of reactions by centrifugation Conclusion
followed by washing and drying, then it was reused for six
subsequent batch reactions. Appreciable conversions were
achieved in all recycling experiments carried out at room
In summary, we have disclosed an unprecedented and efficient
route for the synthesis of tosylaziridines using PhI]NTs via
nitrene insertion into olens in the presence of graphene oxide
as a catalyst in water. The envisaged green method has several
advantages over existing procedures for the synthesis of acti-
vated aziridines, viz., higher yields of products, simple opera-
tion, and ambient reaction conditions and would be a better
practical alternative to cater to the needs of academia as well as
industry.
Experimental
Preparation of graphene oxide
GO was synthesized from the oxidation of graphite powder
under oxidising conditions using the modied Hummers
method29 followed by exfoliation in an aqueous solution.
Graphite akes (1 g) were slowly added to ice-cold concentrated
sulfuric acid (23 mL) then continued stirring was performed,
putting the whole mixture in an ice bath along with sodium
nitrate (0.5 g) and then vigorously stirring using a magnetic
Fig. 3 Various functional groups of the graphene oxide used as
a catalyst.
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stirrer. Aer that, potassium permanganate (3 g) was added to graphite has a strong and sharp diffraction peak at 2q ¼ 23.62ꢀ,
the reaction mixture very slowly, keeping the temperature within which corresponds to the (002) plane of the hexagonal graphite
0–5 ꢀC. The mixture was allowed to stir at room temperature for structure with an interlayer spacing of 3.76 A (Fig. S3, ESI†).
ꢀ
30 min forming a thick paste. It was diluted with distilled water Aer chemical oxidation and exfoliation into graphene oxide,
(46 mL) under stirring conditions. The temperature of the solu- the 23.62ꢀ peak disappeared and a wide diffraction peak centred
tion was raised to about 98 ꢀC and the mixture was allowed to stir at 10.13ꢀ appeared, corresponding to the (002) plane and the
ꢀ
for 4 h and was then nally cooled down. Then 100 mL of interlayer spacing of the material was 8.72 A. An increased
deionized water was added followed by slow addition of 3 mL interlayer distance between consecutive carbon basal planes is
H2O2 (30%). The color of the solution changed from dark brown attributed to the intercalation of oxygen functional groups
to yellowish brown. The overall solution was exfoliated under (–COOH, –OH etc.) and water molecules into the carbon layer
stirring followed by centrifugation at 3000 rpm to collect the solid structure. XRD shows various defects and functional groups
mass at the bottom. The remaining solid material was then carrying sp3 hybridized carbon atoms, which are introduced
washed with 50 mL of warm deionised water and 30 mL of 5% during the oxidation process.
ꢀ
HCl. Finally, the brown mass was collected and dried at 50 C
under vacuum to obtain solid graphene oxide. The formation of FT-IR spectrum of graphene oxide (GO)
GO and the presence of various chemical functionalities on the
Fig. S4, ESI† shows the FTIR spectrum of GO, which shows
GO nanosheets have been well characterized and analyzed by
a broad peak at 3403.74 cmꢁ1 in the high-frequency area
TEM, XRD, FTIR and UV-Vis spectroscopy.
attributed to the stretching mode of the O–H bond, revealing
the presence of hydroxyl groups in graphene oxide. The band
observed at 1725.16 cmꢁ1 was assigned to the carboxyl group.
TEM analysis of graphene oxide
The sharp peak found at 1608.79 cmꢁ1 is a resonance peak that
HR-TEM measurements were performed to determine the
can be assigned to the stretching and bending vibrations of the
–OH groups from water molecules adsorbed on graphene oxide.
The peak at 1381.42 cmꢁ1 arises from the C–OH group. The
peak at 1241.65 cmꢁ1 denotes C–O (epoxy group) stretching and
the C–O (alkoxy) stretching vibration bands appear at
1061.08 cmꢁ1. FTIR spectra provide the presence of ample
numbers of oxygen functionalities such as hydroxyl, epoxide,
carboxylate and carbonyl on graphene oxide.
morphology of graphene oxide. Bends and wrinkles were
observed on the graphene oxide nanosheets at several places
(Fig. 6). Graphene oxide nanosheets appear to have a single
layer morphology. Well-dened diffraction spots in a selected
area electron diffraction (SAED) pattern illustrate that the
monolayer graphene oxide formed. However, in the HR-TEM
image, the presence of topological features along with an
overlapping area of graphene oxide nanosheets reveal that they
are highly dispersed in water.
UV-Vis spectrum of graphene oxide
XRD analysis of graphene oxide
The UV-Vis spectrum of graphene oxide is also shown in Fig. S5,
ESI.† According to the absorbance spectra, the main spectrum
of graphene oxide has a strong absorption peak at 234.32 nm,
attributed to the p / p* transition of the C–C conjugated
aromatic domains and a weak absorption (shoulder) at
306.64 nm due to the n / p* transition of the C]O bond.
XRD analysis was used to characterize the crystalline nature and
phase purity of the as-synthesized graphene oxide. Pure
General method for the synthesis of aziridines 3 and 5
Styrene (0.75 mmol) was added to a mixture of iodine
(10 mol%), PhINTs (0.750 mmol) and GO (15 mg) in distilled
water (5 mL). The reaction mixture was stirred at room
temperature for 10 to 12 h. The reaction progress was recorded
periodically and aer completion of the reaction (using TLC),
the reaction mixture was washed sequentially with diethyl ether
(10 mL ꢂ 3), brine solution (10 mL ꢂ 3) and distilled water (10
mL) and it was nally dried over potassium carbonate. Evapo-
ration of the solvent on a rotavapor le the crude solid product
which was puried by column chromatography on silica gel
(ethyl acetate/n-hexane ¼ 1/9) to provide the pure products 3
1
and 5. The products were conrmed by H NMR and 13C NMR
spectroscopy and all the characterization data match those re-
ported in the literature well.15h,32 However, the spectroscopic
data of representative compounds is given. 3a: IR (KBr) nmax
¼
3051, 2838, 1603, 1582, 1455 cmꢁ1. 1H NMR (400 MHz, CDCl3):
Fig. 6 High resolution TEM image and diffraction pattern (inset) of the
graphene oxide nanosheets.
d ¼ 2.39 (d, 1H, J ¼ 4.5 Hz, 3-Ha), 2.99 (d, 1H, J ¼ 7.0 Hz, 3-Hb),
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2.43 (s, 3H, TsCH3), 3.78 (dd, 1H, J ¼ 4.5, 7.0 Hz, 2-H), 7.87 (d,
J ¼ 8.0 Hz, 2Harom), 7.21–7.34 (m, 7Harom) ppm. 13C NMR (100
MHz, CDCl3): d ¼ 23.7, 34.7, 43.8, 125.1, 126.3, 127.5, 128.3,
129.6, 134.1, 136.5, 142.8 ppm. EIMS: m/z ¼ 273 [M+].
5 A. Y. Rulev, A. R. Romanov, E. V. Kondrashov, I. A. Ushakov,
A. V. Vashchenko, V. M. Muzalevskiy and V. G. Nenajdenko,
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2014, 79, 5558; (b) S. Nicholas, N. S. Dolan, R. J. Scamp,
T. Yang, J. F. Berry and J. M. Schomaker, J. Am. Chem. Soc.,
2016, 138, 14658.
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Perkin Trans. 1, 1999, 2293; (b) R. Pellicciari, L. Amori,
N. Kuznetsova, S. Zlotsky and A. Gioiello, Tetrahedron Lett.,
2007, 48, 4911.
C
15H15NO2S: calcd C 65.91, H 5.53, N 5.12; found: C 65.73, H
5.16, N 5.41. 5a: IR (KBr) nmax ¼ 3049, 2835, 1607, 1579, 1509,
1
1459, 1324 cmꢁ1. H NMR (400 MHz, CDCl3): d ¼ 2.12 (s, 3H,
TsCH3), 2.28 (d, 1H, J ¼ 9.5 Hz, 3-H), 4.54 (d, 1H, J ¼ 9.5 Hz, 2-
H), 7.20–7.41 (m, 7Harom), 7.89 (d, J ¼ 8.0 Hz, 2Harom) ppm. 13
C
NMR (100 MHz, CDCl3): d ¼ 23.9, 42.9, 78.1, 125.9, 126.6, 127.5,
128.4, 129.3, 131.5, 137.9, 143.9 ppm. EIMS: m/z ¼ 318 [M+].
C
15H14N2O4S: calcd C 56.59, H. 4.43, N 8.80; found: C 56.88. H
4.26, N 8.47.
9 (a) H. Luo, K. Chen, H. Jiang and S. Zhu, Org. Lett., 2016, 18,
5208; (b) B.-N. Lai, J.-F. Qiu, H.-X. Zhang, J. Nie and J.-A. Ma,
Org. Lett., 2016, 18, 520.
Conflicts of interest
There is no conicts of interest.
10 J. E. G. Kemp, in Comprehensive Organic Synthesis, ed. B. M.
Trost and I. Fleming, Pergamon, Oxford, 1991, vol. 7, p. 467.
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Chemistry, ed. G. W. Gribble and T. L. Gilchrist, Pergamon
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Acknowledgements
We sincerely thank SERB, DST, New Delhi, for the INSPIRE
´
Faculty Research Award (IFA12-CH-61), DST PURSE for 12 (a) N. De Kimpe, R. Verhee, L. De Buyck and N. Schamp,
publication fees support and AIRF, Jawaharlal Nehru Univer-
sity for providing microanalysis and spectra.
Synth. Commun., 1975, 5, 269; (b) N. De Kimpe, L. Moens,
´
R. Verhe, L. De Buyck and N. Schamp, J. Chem. Soc., Chem.
´
Commun., 1982, 19; (c) N. De Kimpe, P. Sulmon, R. Verhe,
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Notes and references
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