Mononuclear rearrangement of heterocycles in ionic liquids catalyzed by copper(II) salts

Article abstract of DOI:10.1016/j.tet.2008.09.055

The reactivity of E- and Z-phenylhydrazones of 3-benzoyl-5-phenyl-1,2,4-oxadiazole in the presence of CuCl₂ and Cu(ClO₄)₂·6H₂O has been studied in four imidazolium ionic liquids [bmim][X] (X=BF₄^-, PF₆^-, SbF₆^- and CF₃SO₃^-). The reaction may follow different mechanistic patterns, depending on the nature of the ionic liquid anion, accounting for both qualitative and kinetic data. In the presence of CuCl₂, two processes take place at the same time, i.e., the E?Z isomerization and the rearrangement of Z-isomer into the relevant 4-benzoylamino-2,5-diphenyl-1,2,3-triazole. In contrast, in the presence of Cu(ClO₄)₂·6H₂O, the rearrangement occurs only in solution of [bmim][BF₄] and [bmim][CF₃SO₃]. Collected data show that the effect of the ionic liquid on the isomerization and the rearrangement is different. In particular, a higher cross-linking degree seems to favour the triazole, but disfavour the E?Z isomerization.

Full text of DOI:10.1016/j.tet.2008.09.055

Tetrahedron 64 (2008) 11209–11217
Contents lists available at ScienceDirect
Tetrahedron
journal homepage: www.elsevier.com/locate/tet
Mononuclear rearrangement of heterocycles in ionic liquids catalyzed
by copper(II) salts
,
a
a
a
,
b
Francesca D’Anna a , Vincenzo Frenna , Salvatore Marullo , Renato Noto , Domenico Spinelli
*
*
a
`
Dipartimento di Chimica Organica ‘E. Paterno`’, Universita degli Studi di Palermo, Viale delle Scienze-Parco d’Orleans II, 90128 Palermo, Italy
Dipartimento di Chimica Organica ‘A. Mangini’, Universita degli Studi di Bologna, Via S. Giacomo 11, 40126 Bologna, Italy
b
`
a r t i c l e i n f o
a b s t r a c t
Article history:
Received 20 June 2008
Received in revised form 1 September 2008
Accepted 18 September 2008
Available online 26 September 2008
The reactivity of E- and Z-phenylhydrazones of 3-benzoyl-5-phenyl-1,2,4-oxadiazole in the presence of
CuCl2 and Cu(ClO4)2$6H2O has been studied in four imidazolium ionic liquids [bmim][X] (X¼BFꢀ4 , PF6ꢀ, SbF6ꢀ
and CF3SOꢀ3 ). The reaction may follow different mechanistic patterns, depending on the nature of the ionic
liquid anion, accounting for both qualitative and kinetic data. In the presence of CuCl2, two processes take
place at the same time, i.e., the E$Z isomerization and the rearrangement of Z-isomer into the relevant
4-benzoylamino-2,5-diphenyl-1,2,3-triazole. In contrast, in the presence of Cu(ClO4)2$6H2O, the re-
arrangement occurs only in solution of [bmim][BF4] and [bmim][CF3SO3]. Collected data show that the
effect of the ionic liquid on the isomerization and the rearrangement is different. In particular, a higher
cross-linking degree seems to favour the triazole, but disfavour the E$Z isomerization.
Keywords:
Ionic liquids
Mechanism reaction
MRH reaction
Ó 2008 Elsevier Ltd. All rights reserved.
1. Introduction
sometimes called ‘designer solvents’. Therefore ILs are particularly
suitable for transition metal catalysis, providing significant ad-
Ionic liquids have been attracting growing attention during the
last decade.1 Owing to their unusual and intriguing characteristics,
new developments have been disclosed in a wide range of research
fields, from organic synthesis2 to electrochemistry,3 materials sci-
ence4 and so forth. Most of the focus has been put on the so-called
room temperature ionic liquids (RTILs), i.e., liquid salts at room
temperature. They are generally composed by organic cations and
polyatomic fluorinated anions. Ionic liquids (ILs) generally exhibit
low melting points (<100 ꢁC), low vapour pressure and
flammability.
vantages in terms of improved catalyst stability and activity.8 For
example their ability to dissolve organometallic species allows
catalyst immobilization making the use of specifically tailored li-
gands unnecessary.
On the other hand, since ILs are poorly miscible with polar or-
ganic solvents, catalyst recycling is also facilitated. Moreover a quite
recent review has given a detailed picture on the use of chiral
imidazolium ILs and on their applications in asymmetric reactions.9
A way to gather information about the complex behaviour of
these solvents is the study of a reaction whose mechanism in
conventional solvents is well defined. In particular, using aromatic
ILs, a suitable probe reaction should proceed through a highly or-
However, one of the most striking feature is their high structural
order degree. In particular, in addition to Coulomb and hydrogen
bond interactions, for ILs of aromatic cations, also CH-
p, quadru-
dered transition state, having a different extension of p-conjuga-
pole- , and stacking play a significant role in determining the
p
p
–
p
tion with respect to the reactants. Thus, the mononuclear
rearrangement of heterocycles (MRH),10 which allows azole-to-
azole interconversion of suitable heterocyclic compounds, has been
recently proven to be a good candidate.
This reaction has been extensively investigated in conventional
solvents10d as well as in more complex systems such as micelles11
and cyclodextrins.12 Investigations carried out recently have shown
that the amino-catalyzed rearrangement of the Z-phenylhydrazone
of 3-benzoyl-5-phenyl-1,2,4-oxadiazole (1-Z) into the relevant
4-benzoylamino-2,5-diphenyl-1,2,3-triazole (1-T) is favoured in IL
solution (Scheme 1). The higher reactivity has not been ascribed to
a possible higher polarity of these solvent media, but to their
organized structure and organizing ability.13
characteristics of ILs.5 Dupont showed that imidazolium-based ILs
can be considered as supramolecular polymers consisting of
a highly extended network of hydrogen bonds linking anions and
cations together.6 As a result, structural order persists to a signifi-
cant extent in the liquid and even in the gas phase.
It has been shown that slight modifications in the ILs’ cation or
anion nature are able to induce radical changes in both rate and
selectivity of a particular reaction.7 This is the reason why RTILs are
* Corresponding authors. Tel.: þ39 091596919; fax: þ39 091596825.
E-mail addresses: fdanna@unipa.it (F. D’Anna), rnoto@unipa.it (R. Noto).
0040-4020/$ – see front matter Ó 2008 Elsevier Ltd. All rights reserved.
doi:10.1016/j.tet.2008.09.055

11210
F. D’Anna et al. / Tetrahedron 64 (2008) 11209–11217
0.5
H
Ph
Ph
Nβ
N
Ph
Ph
NHPh
0.45
0.4
Isomerization
N
N
N
O
N
MRH
N
N
N
NNα
H
Ph
Ph
O
O
Ph
Ph
1-T
1-E
1-Z
Scheme 1. General description of substrates reactivity.
0.35
0.3
The reactivity of both E- and Z-phenylhydrazones of the
3-benzoyl-5-phenyl-1,2,4-oxadiazole is also affected by the pres-
ence of Lewis acids such as copper(II) salts. Data in methanol col-
lected recently have shown that the outcome of the reaction
depends on the nature of the copper salt counteranion.14 Indeed, in
the presence of salts of poorly coordinating anions, such as sulfate
or perchlorate, only the 1-E$1-Z isomerization has been detected.
On the other hand, salts having strongly coordinating anions (able
to behave as bases), such as halides or acetate, could promote the
rearrangement of 1-Z into the relevant 1-T, too.
Both reactions (isomerization and rearrangement) involve the
coordination of the metal cation by the imino nitrogen atom (Nb) in
the side chain. In the presence of strongly coordinating anions, this
allows both the rotation around the C–N double bond and the
increase in acidity of the hydrogen of the Na–H bond, which favours
the following rearrangement.
0.25
0.2
0.15
0.1
0.05
0
270
320
370
(nm)
420
Figure 1. UV–vis spectra of 1-Z (0.0002 M) in [bmim][CF3SO3] recorded at 313 K as
a function of time (time range¼105 min).
they showed a decrease of 1-Z (lmax at 365 nm) and an increase of
1-T (lmax at 295 nm) concentrations (Fig. 1).
Thus, bearing in mind the particular sensitivity of these reactions
to both the copper salts and the nature of the IL, we supposed that
they might be good candidates to test the properties of ILs as solvents
for transition metal promoted reactions on heterocyclic compounds.
Hence, we undertook a study on the reactivity of 1-E and 1-Z, in the
presence of CuCl2 and Cu(ClO4)2$6H2O in four imidazolium ionic
liquids, namely [bmim][BF4], [bmim][CF3SO3], [bmim][PF6] and
[bmim][SbF6] (bmim¼1-butyl-3-methylimidazolium).
The presence of a sharp isosbestic point agreed with the rear-
rangement of 1-Z into the relevant 1-T in which the tri-
fluoromethanesulfonate anion, an extremely weak base in
conventional solvents and having a low hydrogen bond basicity,16
acts as a general base and promotes the reaction.
Subsequently, we verified, from a qualitative point of view, that
the presence of copper salts did not induce any change in the
outcome of the reaction. To this end, we carried out the reactions in
the presence of equimolar amounts of CuCl2 and Cu(ClO4)2$6H2O at
313 K. All reactions were monitored for 4 h (see Section 4). The
obtained results are reported in Table 5 of Supplementary data.
The data collected show that 1-E$1-Z always occurs irre-
spective on the IL or Cu(II) salt used. In contrast, 1-Z/1-T rear-
rangement does not take place when solutions of Cu(ClO4)2$6H2O
in [bmim][SbF6] or [bmim][PF6] are used. Furthermore, the com-
parison with data previously collected in methanol14 shows that
the latter reaction proceeds faster in IL. Indeed, higher yields were
obtained in the half of the time (1-T yield in MeOH: 18.0–27.0% in
8 h). Finally, no significant amount of by-products was observed.
These ionic liquids differ significantly for the coordinating
ability and symmetry of the anion and are able to induce a different
packing and structural order degree. Furthermore these ILs show
different viscosity and
b
values.15 In this connection, some litera-
ture reports have underlined that changes in these parameters can
lead to different reaction outcomes.7b
We chose copper(II) salts differing in the anion nature. In par-
ticular, they bear highly (Clꢀ) and loosely (ClOꢀ4 ) coordinating an-
ions, respectively, able to ‘live in’ and ‘live out’ of the cation
coordination sphere, respectively.
Other copper salts, such as CuBr2, CuSO4$5H2O and
Cu(OAc)2$H2O, were not tested because of their low solubility in
methanol, the co-solvent used in this work (see Section 4). The
reactivity of the title substrates was investigated by means of
qualitative tests and kinetic measurements. All of the qualitative
tests were carried out at 313 K by treating each substrate with an
equimolar amount of copper(II) salt. Since the two substrates (1-E
and 1-Z) and the rearrangement product (1-T) are active in differ-
ent regions of the spectrum, the kinetic investigation was carried
out at 313 K by means of UV–vis measurements at various cop-
per(II) salt concentrations, ranging from 1.5ꢂ10ꢀ3 M to 1.2ꢂ10ꢀ2 M.
2.2. Kinetic study
In order to understand the above results we studied the re-
activity in ILs as a function of copper salt concentration. As pre-
viously pointed out, hydrazones 1-E and 1-Z and the relevant
triazole 1-T are active in different regions of the UV–vis spectrum.
The kinetic investigation was carried out at 313 K by means of UV–
vis measurements, recording the absorbance values at nine wave-
lengths ranging from 335 nm to 375 nm as a function of the time.
The overall reactivity of hydrazones 1 is summarized in Scheme 2
where kE,Z and kZ,E are the rate constants related to the reversible
isomerization reaction and kZ,T is the rate constant for the rear-
rangement, respectively. Details concerning the reaction mixture
analysis and the determination of the rate constants are reported in
Supplementary data.
2. Results and discussion
2.1. Qualitative approach
Firstly, we checked behaviour of the substrates, in IL solution, in
the absence of copper salts. According to previous literature re-
sults,13 1-Z and 1-E stay unchanged at 313 K in solution of
[bmim][BF4], [bmim][PF6] and [bmim][SbF6]. A remarkably differ-
ent result was obtained in solution of [bmim][CF3SO3], where 1-Z
rearranged into the relevant 1-T. This was confirmed by the UV–vis
spectra of 1-Z recorded as a function of time in [bmim][CF3SO3]:
kE,Z
kZ,E
Scheme 2. 1-E$1-Z isomerization and 1-Z/1-T rearrangement.
kZ,T
1-E
1-Z
1-T

F. D’Anna et al. / Tetrahedron 64 (2008) 11209–11217
11211
active complex, formed in a fast step, has a 1:1 stoichiometry
(substrate: copper salt) (Scheme 3). If the 1-E isomer is the
starting material, the above equilibria can be considered.
According to this mechanistic hypothesis, experimental data
have been fitted by means of Eq. 1:
(a)
0.012
0.011
0.01
À
À
Á
kE;Z ðKEÞCu½CuðIIÞꢃ
1 þCðuKEÞ ½CuðIIÞꢃ
0.009
0.008
0.007
0.006
0.005
0004
kobs
¼
¼
Cu
Á
kZ;E ðKZÞCu½CuðIIÞꢃ
1 þCðuKZÞ ½CuðIIÞꢃ
kobs
(1)
Cu
where (KE)Cu and (KZ)Cu are, respectively, the binding equilibrium
constants for the formation of the adducts between 1-E or 1-Z and
Cu(II), while (kE,Z Cu and (kZ,E)Cu represent the isomerization rate
constants. In Table 1 the relevant results of kinetic data fit are
reported.
)
0.001
0.0015
0.002
[CuCl2] (M)
0.0025
0.003
0.0035
Our experimental data, as shown in Table 1, allowed to de-
termine, in the presence of all copper salts, rate constant values
relevant to the 1-E/1-Z isomerization. On the other hand, rate
constant values concerning the 1-Z/1-E isomerization were
determined only in the presence of Cu(ClO4)2$6H2O. In the pres-
ence of CuCl2 the 1-Z isomer fastly rearranges into the corre-
sponding 1-T (see later), preventing the formation of a significant
concentration of 1-E.
According to data previously collected in methanol solution, for
both isomers two parallel pathways can be depicted as shown in
Scheme 4.
A deeper analysis of the data reported in Table 1 shows that in
the same IL 1-E isomerizes faster in the presence of CuCl2 than in
the presence of Cu(ClO4)2$6H2O and that the rates of 1-E/1-Z
isomerization are also affected by the nature of the IL used. Indeed,
(b)
0.0035
0.003
0.0025
0.002
0.0015
0.001
0.0005
0
for the same copper salt, slightly higher (kE,Z Cu values were
)
detected in [bmim][SbF6] than in [bmim][PF6]. This trend could be
a result of the hydrogen bond acceptor ability of the IL anion. In-
deed, the decrease in isomerization rate constants parallels the
increase in
and [bmim][PF6], respectively).15
Probably, an IL such as [bmim][PF6] with a higher
b
values of the ILs (
b
¼0.146 and 0.207 for [bmim][SbF6]
0.002 0.003 0.004 0.005 0.006 0.007 0.008 0.009
[CuCl2] (M)
b
value and
a larger cross-linking degree shows a more organized structure,
where the rotation around the imino double bond and the relevant
isomerization are more difficult. A similar result was previously
reported by Asano et al. on studying the thermal Z to E isomeri-
zation of azobenzenes in IL.17 It is noteworthy that the isomeriza-
tion can also proceed through a mechanism with inversion at
nitrogen.18 Nevertheless, in our opinion this mechanistic pattern is
not operative because of the binding with the copper cation.
The comparison between reactivity data in methanol14 and in
ILs allows us to draw an interesting conclusion: indeed, the relative
rate constants of the 1-E$1-Z isomerization increase along the
series: MeOH<[bmim][PF6]<[bmim][SbF6].19
In general, the reaction proceeds faster in ILs than in methanol,
but the differences detected are not as high as one would expect on
the grounds of the very different characteristics of the involved
solvents. In order to understand these results, opposite effects
compensating each other should be considered. Indeed, ILs favour
the reaction as a consequence of a weaker reagent solvation,
allowing the transition state to be reached more easily. However,
with respect to methanol, they disfavour the isomerization on the
basis of the strong solvent–solvent interactions that, according to
previous literature report,20 agree for slow response of the solvent
to change in the solute.
Figure 2. Plots of observed kinetic constants concerning 1-E/1-Z isomerization in
the presence of CuCl2 in: (a) [bmim][PF6] and (b) [bmim][BF4] at 313 K.
2.2.1. 1-E$1-Z Isomerization
The plots of the observed rate constants (kE,Z and kZ,E) versus
Cu(II) concentration showed different trends as a function of the IL
used. In general, either parabolic (for [bmim][BF4] and
[bmim][CF3SO3] solutions) or hyperbolic trends (for [bmim][PF6]
and [bmim][SbF6] solutions) were obtained, suggesting that the
‘actual’ course of the reaction is different.
As a consequence of this, experimental data will be discussed as
a function of the used IL. In Figure 2, kE,Z as a function of CuCl2
concentration in [bmim][BF4] and in [bmim][PF6] solution are
shown.
Data at variable concentration of copper salt, in different ionic
liquids, are reported in Supplementary data (Tables 6–11).
2.2.1.1. Kinetic measurements in [bmim][PF6] and [bmim][SbF6]. The
hyperbolic trend obtained for kE,Z or kZ,E in [bmim][PF6] and in
[bmim][SbF6] accounts for a mechanism in which the kinetically
(K )
E Cu
(kE,Z)Cu
1-E + Cu(II)
1-E/Cu(II)
1-Z/Cu(II)
1-Z + Cu(II)
Since in the presence of Cu(ClO4)2$6H2O the isomerization is the
only reaction observed, the KE,Z constant value, corresponding
to the 1-E$1-Z equilibrium can be calculated. A comparison be-
tween data in methanol14 and in ILs shows that the KE,Z values
(kZ,E Cu
)
(K )
Z Cu
Scheme 3. 1-E/Cu(II) complexation equilibrium and 1-E$1-Z copper-assisted
isomerization.

11212
F. D’Anna et al. / Tetrahedron 64 (2008) 11209–11217
Table 1
Binding equilibrium constants and rate constants concerning the 1-E$1-Z isomerization in the presence of CuCl2 and Cu(ClO4)2$6H2O in [bmim][PF6] and
[bmim][SbF6] at 313 K
Salt
IL
(KE)Cu (Mꢀ1
)
103 (kE,Z
)
Cu (sꢀ1
)
Ra
(KZ)Cu (Mꢀ1
)
103 (kZ,E
)
Cu (sꢀ1
)
Ra
CuCl2
CuCl2
Cu(ClO4)2$6H2O
Cu(ClO4)2$6H2O
[bmim][PF6]
[bmim][SbF6]
[bmim][PF6]
[bmim][SbF6]
280ꢅ30
150ꢅ20
990ꢅ50
900ꢅ100
24ꢅ2
0.997
0.997
0.997
0.997
31ꢅ2
6.3ꢅ0.1
9.7ꢅ0.3
280ꢅ30
1.03ꢅ0.06
0.94ꢅ0.03
0.994
0.993
920ꢅ110
a
Correlation coefficient.
Ph
Ph
Ph
NHPh
NHPh
NHPh
(KE)Cu
N
N
N
N
N
N
M +
N
N
N
M
M
Ph
Ph
Ph
O
O
O
1-E
1-E/Cu
1-E/Cu''
Kinetically
inactive
(kZ,E Cu (kE,Z)Cu
)
NHPh
N
Ph
M
N
Ph
δ−
M
N
δ+
Ph
O
N
N
1-E/Cu''
N
NHPh
Ph
O
TS
Kinetically
inactive
(kZ,E Cu
)
(kE,Z Cu
)
Ph
Ph
PhHN
N
M
M
N
Ph
N
N
N
N
N H
Ph
(KZ)Cu
N
+ M
N
N
H
N
Ph
O
Ph
O
Ph
Ph
O
1-Z/Cu'
1-Z/Cu
1-Z
Kinetically
inactive
M indicates Cu2+ in the instance Cu(ClO4)2 6H2O
M indicates CuX2 in the instance of CuCl2
Scheme 4. General scheme for the 1-E$1-Z isomerization in the presence of copper salts.
increase from methanol to [bmim][SbF6] along the series: MeOH
(4.1)<[bmim][PF6] (6.1)<[bmim][SbF6] (10.0).
agrees with Eq. 2, where ku and (kII
first-order and the third-order rate constants concerning the
)
or (kII
)
represent the
E,Z Cu
Z,E Cu
Generally speaking ILs stabilize the Z isomer with respect to the
E isomer to a greater extent than methanol. The highest thermo-
dynamic stability of 1-Z arises from the intramolecular hydrogen
bonds between the hydrazonic hydrogen atom of the side chain and
nitrogen atoms of the heterocyclic ring. This effect should be en-
hanced by the weakest solvating ILs. Accordingly, KE/Z reaches the
maximum value in [bmim][SbF6], which is characterized by the
spontaneous and the trimolecular pathway, respectively.
kobs [ kuDðkIIE;ZÞCu½CuðIIÞꢃ
(2)
2
In Table 2 the results of the kinetic data fit are reported. In three out
of five instances the calculated ku values are meaningless (negative
values), while in the other two their contribution to the overall
isomerization process appears negligible (kII/kuꢄ105 Mꢀ2).
Also in this case a two-step mechanism should be hypothesized.
Obviously, considering this hypothesis we are aware of the fact that
rate constant values reported in Table 2 are conditioned by binding
equilibrium constants, and that our experimental data do not allow
to determine them.
lowest Kamlet–Taft
accept hydrogen bonds from the solute molecule.
b
value,15 and therefore by the lowest ability to
2.2.1.2. Kinetic measurements in [bmim][BF4] and [bmim][CF3SO3]. In
[bmim][BF4] and in [bmim][CF3SO3] the fit of experimental data
Table 2
Rate constants concerning the 1-E$1-Z isomerization in the presence of CuCl2 and Cu(ClO4)2$6H2O in [bmim][BF4] and [bmim][CF3SO3] solution at 313 K
Salt
IL
103 ku (sꢀ1
ꢀ0.2ꢅ0.2
)
(kII
)
(sꢀ1 Mꢀ2
)
(kII
)
(sꢀ1 Mꢀ2
Z,E Cu
)
Ra
E,Z Cu
CuCl2
CuCl2
CuCl2
Cu(ClO4)2$6H2O
Cu(ClO4)2$6H2O
[bmim][BF4]
[bmim][BF4]
[bmim][CF3SO3]
[bmim][BF4]
[bmim][CF3SO3]
49ꢅ4
0.981
0.987
0.994
0.984
0.986
ꢀ0.04ꢅ0.02
0.02ꢅ0.02
ꢀ0.34ꢅ0.06
0.06ꢅ0.01
6.5ꢅ0.4
14ꢅ1
10ꢅ0.4
5.5ꢅ0.4
a
Correlation coefficient.

F. D’Anna et al. / Tetrahedron 64 (2008) 11209–11217
11213
As can be seen from data reported in Table 2, in the presence of
CuCl2, in [bmim][CF3SO3] the 1-Z/1-E isomerization was not
detected. Probably in this case 1-Z isomer quickly rearranges into
the relevant 1-T, preventing the formation of a significant con-
centration of 1-E.
The main course of the reaction seems to be determined by
a species involving two CuX2 units. The structure of kinetically
active species can be depicted in the instance of CuCl2. Indeed,
considering the high coordinating ability of chloride anion and
taking in mind the literature reports about the formation of binu-
clear complexes with CuHal2,21 it should be a halo-bridge copper
complex such as I.
(a)
0.7
0.6
05
0.4
0.3
0.2
0.1
0
Ph
Cl
Cl
Cl
Cl
Cu
Cu
Ph
N
N
N
N
Ph
O
H
I
In order to get information concerning the structure of the
kinetically active species in the presence of Cu(ClO4)2$6H2O, we
recorded the UV–vis spectrum of this copper salt both in
methanol and [bmim][BF4] (Fig. 3a and b). The spectrum
recorded in [bmim][BF4] reveals the presence of a weak ab-
sorption band centred at 600 nm. This latter band is absent in
the spectrum recorded in methanol and does not belong to the
ionic liquid.22
This absorption band should be the result of some weak in-
teractions in the ILs that should allow a tighter association between
the perchlorate anion and the copper cation.23 Alternatively, it
should be due to the coordination of the tetrafluoroborate anion,
which in the reaction mixture is in overwhelming excess with re-
spect to the copper cation.
In order to choose between these hypotheses, we recorded the
UV–vis spectrum of Cu(BF4)2$xH2O in [bmim][BF4] (Fig. 3c). As can
be seen, it shows the same absorption band present in the spectrum
of Cu(ClO4)2$6H2O in [bmim][BF4], confirming the in situ formation
of this copper salt. Hence, the kinetically active species should have
a dinuclear copper complex structure.24
A further analysis of the data reported in Table 2 shows that the
isomerization is affected both by the IL and the copper salt used. In
particular, for the same copper salt, the reaction proceeds faster in
[bmim][BF4] than in [bmim][CF3SO3] (4.9 and 1.8 times for CuCl2
and Cu(ClO4)2$6H2O, respectively). On the other hand, for the same
IL ([bmim][BF4]), Cu(ClO4)2$6H2O seems to be more efficient (2.1
times) with respect to CuCl2 in catalyzing the 1-Z/1-E
isomerization.
-0.1
24
20
400
480
560
(nm)
640
720
800
880
(b)
1.2
1
0.025
0.02
0.015
0.01
0.8
0.6
0.4
0.2
0
0.005
450 500 550 600 650 700 750
240
320
400
480
560
(nm)
640
720
800
880
(c)
0.25
0.2
0.15
0.1
0.05
0
2.2.2. 1-Z/1-T Rearrangement
Kinetic data for 1-Z/1-T rearrangement were collected in
CuCl2/[bmim][X] (Xꢀ¼SbF6ꢀ, PF6ꢀ and BFꢀ4 ). The rearrangement in-
duced by Cu(ClO4)2$6H2O was studied in [bmim][BF4]. The
[bmim][CF3SO3] was not used since the rearrangement proceeded
spontaneously in this IL. Hyperbolic (for [bmim][PF6] and
[bmim][SbF6]) and parabolic (for [bmim][BF4]) trends, as a function
of copper salt concentration, were detected. In Figure 4, kZ,T values
as a function of CuCl2 concentration collected in [bmim][PF6] and in
[bmim][BF4] solution are reported.
It is noteworthy that, as a consequence of Cu(BF4)2 formation
(see above), in [bmim][BF4] the rearrangement took place also in
the presence of Cu(ClO4)2$6H2O. Because different behaviours were
detected as a function of IL used, for the sake of clarity, experi-
mental data will be discussed separately (data collected as a func-
tion of copper salt concentration, in different ILs solutions, are
reported in Tables 6–11 of Supplementary data).
240
320
400
480
560
(nm)
640
720
800
Figure 3. (a) UV–vis spectrum of Cu(ClO4)2$6H2O 1.98ꢂ10ꢀ3 M in methanol. (b) UV–
vis spectrum of Cu(ClO4)2$6H2O 1.98ꢂ10ꢀ3 M in [bmim][BF4]. (c) UV–vis spectrum of
Cu(BF4)2$xH2O 2.49ꢂ10ꢀ3 M in [bmim][BF4].

11214
F. D’Anna et al. / Tetrahedron 64 (2008) 11209–11217
Table 3
(a)
0.02
0.015
0.01
0.005
0
Binding equilibrium constants and rate constants concerning the 1-Z/1-T re-
arrangement in the presence of CuCl2 in [bmim][PF6] and [bmim][SbF6] solution at
313 K
Salt
IL
(KZ)Cu (Mꢀ1
)
103 (kZ,T
)
Cu
(sꢀ1
)
Ra
CuCl2
CuCl2
[bmim][PF6]
[bmim][SbF6]
70ꢅ12
59ꢅ7
24ꢅ1
0.995
0.998
180ꢅ17
a
Correlation coefficient.
According to this mechanistic hypothesis, experimental data
have been fitted by means of Eq. 3
À
Á
kZ;T ðKZÞCu½CuðIIÞꢃ
1 þCðuKZÞ ½CuðIIÞꢃ
kobs
¼
(3)
Cu
where (kZ,T)Cu is the rate constant for the rearrangement and (KZ)Cu
0.001 0.002 0.003 0.004 0.005 0.006 0.007 0.008
[CuCl2] (M)
is the binding equilibrium constant for the formation of the adduct
between 1-Z and CuCl2. In Table 3 the relevant results of kinetic
data fit are reported.
(b)
0.00012
0.0001
8 10-5
6 10-5
4 10-5
2 10-5
0
Also in this case according to data previously collected in
methanol, the overall course of the reaction can be depicted as in
Scheme 6.
Analysis of the data reported in Table 3 evidences that the 1-Z/
Cu adduct, different from the 1-E/Cu one, is more stable in
[bmim][SbF6] than in [bmim][PF6]. However, according to data
previously discussed about the 1-E$1-Z isomerization, the rate of
the rearrangement increases when the 1-Z/Cu adduct stability
decreases. The triazole formation is also affected by the nature of
the IL anion. In particular, the reaction is faster in [bmim][PF6] than
in [bmim][SbF6].
Furthermore, with respect to the isomerization, in this case the
IL anion influence seems to be a little more (twice) significant. The
collected data show that the structural order degree of the ILs exert
an opposite effect on the isomerization and the rearrangement
reactions. In particular, the latter one is favoured by ILs having
a higher organization degree. This result agrees with data pre-
viously reported by us about the effect of the organized structure
and the organizing ability of imidazolium ILs, which is able to fa-
vour the rearrangement stabilizing the transition state by means of
0.002 0.003 0.004 0.005 0.006 0.007 0.008 0.009
[CuCl2] (M)
Figure 4. Plots of observed kinetic constants concerning 1-Z/1-T rearrangement in
the presence of CuCl2 in (a) [bmim][PF6] and (b) [bmim][BF4] at 313 K.
electrostatic and p–p
interactions as well.13
An interesting point to be analyzed is the nature of the base that,
in this case, favours the hydrazonic proton transfer and conse-
quently the triazole formation. According to data previously
reported in methanol,14 the coordination of the imino nitrogen by
Cu(II) induces an electronic shift and causes an acidity increase.
Under such a condition, the occurrence of a bifunctional catalysis
can be supposed, as well as proposed for other nucleophilic
2.2.2.1. Kinetic measurements in [bmim][PF6] and in [bmim][SbF6]. The
kinetic trends of kZ,T versus [CuCl2] concentration account for
a two-step mechanism (Scheme 5).
(KZ)Cu
(kZ,T Cu
)
1-Z + Cu(II)
T/Cu
1-Z/Cu
Scheme 5.
Ph
Ph
Ph
δ-
δ+
CuCl2
CuCl2
N
N H
Ph
(KZ)Cu
B
N
N
N
N
N
N H
δ+
δ−
+
CuCl2
N
N H B
N
N
Ph
Ph
O
O
Ph
Ph
O
1-Z
Ph
TS
1-Z/Cu
(kZ,T Cu
)
H
H
N
Ph
N
Ph
Ph
Ph
CuCl2
+
N
N
N
N
CuCl2
O
O
N
N
Ph
T/Cu
Ph
T
Scheme 6. Details on the 1-Z/CuCl2 complexation equilibrium and the relevant 1-Z/1-T rearrangement.

F. D’Anna et al. / Tetrahedron 64 (2008) 11209–11217
11215
substitution.25 The Cu(II) acts as a Lewis acid and the relevant anion
present in the metal coordination sphere acts as a Lewis base.
However, also the IL anion could play an important role. Indeed,
the increase in hydrazonic proton acidity could be so significant to
allow proton transfer also by means of a very weak base such as the
IL anion. This hypothesis is supported by the literature reports
about acid dissociation in IL solution of methylene active
compounds26 or of 2,4-dinitrophenol27 also in the absence of an
added base.
It is noteworthy that in [bmim][PF6] and in [bmim][SbF6], as
well as in methanol, the 1-Z isomer shows a dichotomic behaviour.
Indeed, the 1-Z/1-T rearrangement is promoted by CuCl2 and not
by Cu(ClO4)2$6H2O. This behaviour in methanol was ascribed to the
different ability of copper salt anion to ‘live in’ or ‘live out’ of the
metal coordination sphere.14
In order to verify whether a similar behaviour could take place
also in ILs, we carried out kinetic tests to evaluate the effect of
added chloride ion. Thus, variable amounts of [bmim][Cl] have been
added to a fixed concentration of Cu(ClO4)2$6H2O in [bmim][SbF6]
to examine how the substrate reactivity was affected. Collected
data show that in the binary mixture [bmim][SbF6]/[bmim][Cl] and
in the presence of Cu(ClO4)2$6H2O also the 1-Z/1-T rearrange-
ment occurs (Fig. 5a). (Data collected at different [bmim][Cl] con-
centrations are reported in Table 14 of Supplementary data.)
Probably, the addition of [bmim][Cl] causes the in situ formation
of CuCl2. Nevertheless, as long as chloride concentration increases,
the rate of the rearrangement reaction decreases. A similar effect
was detected for the 1-E/1-Z isomerization (Fig. 5b).
In conventional organic solvents, UV–vis spectra show that the
addition of chloride ion to a CuCl2 solution favours the formation of
chlorocuprates complexes such as CuClꢀ3 and CuCl4.2,28 These, in
general, should have a lower ability to interact with substrates, as
a consequence of the saturation of the metal coordination sphere.
This hypothesis seems to be effective also in IL. Indeed, the UV–vis
spectrum of Cu(ClO4)2$6H2O recorded in the binary mixture
[bmim][SbF6]/[bmim][Cl] perfectly accounts for the presence of
tetrachlorocuprate anion (Fig. 5c).
(a)
0.004
0.0035
0.003
0.0025
0.002
0.0015
0.001
0.0005
0.005
0.01
[bmimCl] (M)
0.015
0.02
(b)
0.003
0.0025
0.002
0.0015
0.001
0.0005
0
2.2.2.2. Kinetic measurements in [bmim][BF4]. As previously said, in
[bmim][BF4], the 1-Z/1-T rearrangement took place both in the
presence of CuCl2 and Cu(ClO4)2$6H2O. In both cases a parabolic
trend of kZ,T versus Cu(II) concentration was detected. Experimental
data were fitted according to Eq. (4).
kobs [ kuDðkII ;TÞ ½CuðIIÞꢃ
(4)
2
Z
Cu
where ku and (kII
Z,T)Cu represent the first-order and the third-order
rate constants concerning the spontaneous and the trimolecular
pathway, respectively. The relevant results of kinetic data fit are
reported in Table 4 (entries 1 and 2). In every case the calculated
ku values are meaningless (negative values).
Once more, kII values underline that the only real pathway in-
volves a substrate–copper complex having a 1:2 stoichiometry. The
kinetically active complex should have a dinuclear copper structure
(see above). In general, in [bmim][BF4] the 1-Z/1-T rearrangement
proceeds faster in the presence of Cu(ClO4)2$6H2O than in the
presence of CuCl2 and the occurrence of the reaction in the pres-
ence of the former salt seems to be the major mechanistic change
with respect to what detected in methanol.14
0.005
0.01
[bmimCl] (M)
0.015
0.02
(c)
0.5
0.45
0.4
0.35
0.3
0.25
0.2
Probably, as it occurs for isomerization, the kinetically active
catalyst is the Cu(BF4)2 salt formed in situ. This hypothesis was
confirmed by kinetic measurements carried out in [bmim][BF4]
in the presence of increasing amounts of Cu(BF4)2$xH2O (data
0.15
0.1
Table 4
0.05
0
Rate constants concerning the 1-Z/1-T rearrangement in the presence of
Cu(ClO4)2$6H2O, CuCl2 and Cu(BF4)2$xH2O in [bmim][BF4] at 313 K
250
350
450
550
(nm)
650
750
850
Entry Salt
105 ku (sꢀ1
)
kII (sꢀ1 Mꢀ2
)
kII (sꢀ1 Mꢀ2
)
Ra
Z,T
Z,E
1
2
3
4
CuCl2
ꢀ0.1ꢅ0.4
1.4ꢅ0.1
10ꢅ1
0.988
0.984
0.993
0.988
Figure 5. Plots of observed kinetic constants concerning: (a) the 1-E/1-Z isomeri-
zation and (b) the 1-Z/1-T rearrangement at 313 K in the presence of fixed concen-
tration of Cu(ClO4)2$6H2O (0.001 M) and variable amounts of [bmim][Cl] in
[bmim][SbF6] solution. (c) UV–vis spectrum of Cu(ClO4)2$6H2O (0.001 M) and
[bmim][Cl] (0.0186 M) in [bmim][SbF6].
Cu(ClO4)2$6H2O ꢀ2.8ꢅ0.5
Cu(BF4)2$xH2O
Cu(BF4)2$xH2O
ꢀ8ꢅ2
ꢀ8ꢅ1
5.9ꢅ0.3
3.8ꢅ0.2
a
Correlation coefficient.

11216
F. D’Anna et al. / Tetrahedron 64 (2008) 11209–11217
collected at different Cu(BF4)2$xH2O concentrations are reported in
Table 15 of Supplementary data). Experimental results show that
this copper salt is able to promote the isomerization and the rear-
rangement as well. Also in this case parabolic trends for observed
rate constant values versus Cu(BF4)2$xH2O concentrations were
obtained. Fit of experimental data gave results reported in Table 4
(entries 3 and 4).
4.3. Kinetic measurements
UV–vis spectra for kinetic measurements were carried out by
using a spectrophotometer equipped with a Peltier temperature
controller, in order to keep the temperature constant within 0.1 K.
The sample for a typical kinetic run was prepared by mixing into
a quartz cuvette (light path 0.2 cm) 500
mL of ionic liquid, 50
mL of
Analysis of data reported in the table shows that this copper salt
in [bmim][BF4] is less efficient than Cu(ClO4)2$6H2O in catalyzing
the rearrangement, but anyway more efficient than CuCl2.
a substrate solution in methanol and then 25
m
L of a concentrated
solution of copper salt in methanol, previously thermostated. A
mixture with the same composition of the one undergoing mea-
surements, containing all components but the substrate, was used
as a blank. The initial concentration of substrate was fixed to
1.0ꢂ10ꢀ4 M. The copper salts concentration ranged from
1.5ꢂ10ꢀ3 M to 1.2ꢂ10ꢀ2 M. All of kinetic data were analyzed by
means of Kaleidagraph 3.0 software.
3. Conclusions
The collected data show that the reactivity of both E- and Z-
phenylhydrazones of 3-benzoyl-5-phenyl-1,2,4-oxadiazole in ILs is
affected by the presence of CuCl2 or Cu(ClO4)2$6H2O. In particular,
different mechanistic patterns arise, depending on the nature of the
copper salt anion, with the isomerization promoted by both copper
salts and the rearrangement preferably promoted by CuCl2.
Acknowledgements
We thank MIUR (PRIN 20066034372) and the Universities of
Palermo and Bologna for financial support.
Comparison with data previously collected in MeOH14 shows
that the studied reactions proceed faster in IL than in MeOH.
However, these reactions are differently affected by the nature of
the anion of ILs. In particular, according to data previously repor-
ted,13 a high structural order degree favours the rearrangement,
Supplementary data
Qualitative, kinetic data and calculation method are provided.
Supplementary data associated with this article can be found in the
online version, at doi:10.1016/j.tet.2008.09.055.
stabilizing the transition state by means of electrostatic and
p–p
interactions. On the other hand, the same factor disfavours the
isomerization on the grounds of a slower response of the solvent to
structural changes in the solute, arising from the rotation around
the imino double bond.
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4. Experimental section
4.1. Materials
Commercial methanol was distilled before use. Commercial
CuCl2 and Cu(ClO4)2$6H2O were dried in a muffle over phosphorous
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19. (Cu(ClO4)2$6H2O:(kE,Z
)
Cu;MeOH:ðkE;Z Þ
ꢃ:ðkE;Z
Þ
¼1:2.0:3.1.
Cu;½bmimꢃ½SbF6ꢃ
6
(kZ,E
ðkE;Z
)
Þ
Cu;MeOH:ðkZ;EÞ
:ðkZ;EÞCu;½bmimꢃ½PF ¼1:1.3:1.2. CuCl2:(kE,Z
)
:
Cu;MeOH
:ðCkuE;;½Zbmimꢃ½PF
Þ
¼1:11.4:14.7).
6ꢃ
Cu;½bmimꢃ½SbF6ꢃ
Cu;½bmimꢃ½PF6
ꢃ
Cu;½bmimꢃ½SbF6ꢃ
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