The ionic liquid effect on the Boulton-Katritzky reaction: a comparison between substrates of different structure

Article abstract of DOI:10.1016/j.tet.2015.05.003

Abstract The mononuclear rearrangement of heterocycles, also called Boulton-Katritzky reaction, was studied in ionic liquid solution using N-(5-phenyl-1,2,4-oxadiazol-3-yl)-N'-(4-nitrophenyl)-formamidine as substrate. The investigation was carried out using piperidine as basic catalyst and several ionic liquids differing in both cation and anion structure. Kinetic data collected were compared with the ones previously reported using (Z)-phenylhydrazone of 3-benzoyl-5-phenyl-1,2,4-oxadiazole to have information about the effect due to the different structure of the alkyl chain borne on the substrate. Furthermore, data were analysed on the grounds of polarity, Kamlet-Taft solvent parameters, but taking also in consideration the structural features of the solvent used. On the whole, the results obtained seem to indicate that the 'ionic liquid effect' can be explained considering the structural features of the constituent ions, their effect on the structure of solvent media and their ability to interact with the transition state of the target reaction.

Full text of DOI:10.1016/j.tet.2015.05.003

Accepted Manuscript
The ionic liquid effect on the Boulton-Katritzky reaction: a comparison between
substrates of different structure
Francesca D’Anna, Daniela Millan, Renato Noto
PII:
S0040-4020(15)00642-0
10.1016/j.tet.2015.05.003
TET 26718
DOI:
Reference:
To appear in: Tetrahedron
Received Date: 30 January 2015
Revised Date: 20 April 2015
Accepted Date: 1 May 2015
Please cite this article as: D’Anna F, Millan D, Noto R, The ionic liquid effect on the Boulton-Katritzky
reaction: a comparison between substrates of different structure, Tetrahedron (2015), doi: 10.1016/
j.tet.2015.05.003.
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The ionic liquid effect on the Boulton-
Katritzky reaction: a comparison between
substrates of different structure
Leave this area blank for abstract info.
Francesca D’Anna,* Daniela Millan and Renato Noto
H
Ph
N
N
N
N
N
Me
R
Me
N
N
O
N
BF
4
PF
6
SbF
6
R'
Me Bu
N
N
H
N
Ph
Pip
IL
O
N
Bu
NTf
2
N(CN)
2
Bu
N
Et
Et
Et
NO
2
N
Me Bu
NO
2

1
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Tetrahedron
journal homepage: www.elsevier.com
The ionic liquid effect on the Boulton-Katritzky reaction: a comparison between
substrates of different structure
a
b,
a
Francesca D’Anna ∗ , Daniela Millan and Renato Noto
a
Dipartimento STEBICEF, Università degli Studi di Palermo, Viale delle Scienze-Parco d’Orleans II, 90128 Palermo (Italy)
Facultad de Química, Pontificia Universidad Católica de Chile, Santiago 6094411, Chile.
b
Dedicated to Prof. Alan Katritzky for his significant contribution in heterocyclic chemistry
ARTICLE INFO
ABSTRACT
Article history:
Received
Received in revised form
Accepted
The mononuclear rearrangement of heterocycles, also called Boulton-Katritzky reaction, was
studied in ionic liquid solution using N-(5-phenyl-1,2,4-oxadiazol-3-yl)-N’-(4-nitrophenyl)-
formamidine as substrate. The investigation was carried out using piperidine as basic catalyst
and several ionic liquids differing in both cation and anion structure. Kinetic data collected were
compared with the ones previously reported using (Z)-phenylhydrazone of 3-benzoyl-5-phenyl-
Available online
1
,2,4-oxadiazole to have information about the effect due to the different structure of the alkyl
chain borne on the substrate. Furthermore, data were analysed on the grounds of polarity,
Kamlet-Taft solvent parameters, but taking also in consideration the structural features of the
solvent used.
On the whole, the results obtained seem to indicate that the “ionic liquid effect” can be
explained considering the structural features of the constituent ions, their effect on the structure
of solvent media and their ability to interact with the transition state of the target reaction.
Keywords:
Ionic Liquids
MRH reaction
Base catalysis
2
009 Elsevier Ltd. All rights reserved
1
. Introduction
Reactivity in ionic liquid (IL) solutions has been one of the
result from entropic effects occurring in these media but which
do not operate in conventional organic solvents (COS).
2
2-27
topics investigated since their first development in the middle of
990s. The so-called “IL effect” has been frequently claimed,
However, notwithstanding different papers and current
thoughts about reactivity in IL solution, this topic is still the
object of intense debate. This arises from the possibility of
varying IL properties by simply changing the cation or anion
structure, but also from dissimilar effects that the same IL might
exert on different reactions. The above considerations confirm
what was proposed at the end of 1990 by Armstrong et al. about
the probability that effects observed could be a function not only
of the IL nature, but also of the substrate or reaction used as
1
including two different observations. The first one has a mere
kinetic nature and takes into account the effect that ILs are able
1-6
to exert on rate of reactions without affecting mechanism. In
the second one, a change in the outcome of reactions is
7-14
detected.
In some cases, the “IL effect” has been rationalized on the
grounds of their solvent properties, as assessed using classical
solvent descriptors such as polarity and Kamlet-Taft solvent
28
probe.
9,15,16
In the framework of our interest in studying ILs properties and
applications, we have performed in IL solution some classical
parameters describing a classical solvent effect.
However, it
is worth mentioning that together with the classical “IL effect” a
supramolecular “IL effect” has also been claimed. This picture
takes in consideration the ionic lattice persisting in these ionic
media in the liquid state and stemming from Coulomb
interactions. This provides a certain structural order degree that,
in the presence of an aromatic cation or anion, is further
29-31
organic reactions.
Among these, we have paid attention to the
mononuclear rearrangement of heterocycles (MRH), also called
32-35
the Boulton-Katritzky reaction.
This reaction is an azole-
azole interconversion widely studied in conventional solvents
36
37
using both basic and acidic catalysts.
17,18
increased by the occurrence of π-π or cation-π interactions.
On the grounds of the above considerations, ILs have been
described as supramolecular fluids and reactivity in IL solution
has been explained bearing in mind their ability to act as confined
reaction media. Their kinetic effect is mainly ascribed to
supramolecular interactions established with ground or transition
19-21
state.
In this light, the supramolecular “IL effect” can also
—
——
∗
Corresponding author. Tel.: +3909123897540; fax: +39091596825; e-mail: francesca.danna@unipa.it

2
Tetrahedron
ACCEPTED MANUSCRIPT
Scheme 1. Representation of reaction studied and ILs used.
Data collected by us, considering the base catalysed
The target reaction was studied in nineteen different ILs
(Scheme 1). Both aliphatic and aromatic ILs were used. As far as
aromatic ILs are concerned, we tried to evaluate the relevance of
different factors such as the nature of the aromatic cation
(imidazolium and pyridinium), cation acidity as accounted for the
presence of a methyl group on C2 of the imidazolium ion
rearrangement of (Z)-phenylhydrazone of 3-benzoyl-5-phenyl-
32,33
34
1
,2,4-oxadiazole in solution of mono-,
di-, and task specific
35
dicationic ILs, demonstrated the positive effect exerted on the
above reaction by aromatic ILs. These ones, being able to
establish π-π and π-dipole interactions, stabilize the cyclic quasi-
aromatic transition state of the reaction and favour its outcome.
Together with solvent and catalyst properties, the above reaction
is heavily affected by the nature of the side chain borne on the
starting oxadiazole ring. Indeed, it determines properties of the
final heterocycle and consequently acts on the nature of the
+
+
([bmim ] and [bm im ]), and the effect deriving from the
2
presence of a different alkyl chain length on the imidazolium ion.
Furthermore, we also used benzylimidazolium-based ILs to
assess the effect due to the extension of π-surface area of the
cation on the outcome of the reaction. The role played by the
36,38
+
transition state.
anion was tested using [bmim ]-based ILs having anions
-
differing in size, shape and coordination ability such as [BF ],
4
In light of the above considerations, the main goal of this
paper is to have a better understanding of the “IL effect” on the
MRH reaction. To pursue this aim, considering the significant
role played by side chain structure of the substrate, we studied
the MRH reaction of the N-(5-phenyl-1,2,4-oxadiazol-3-yl)-N’-
-
-
-
-
[
PF ], [SbF ], [NTf ] and [N(CN) ]. The target reaction was
6 6 2 2
-
also performed in [NTf ]-based ILs having different aliphatic
cations such as [bmpyrr ], [bmpip ] and [bEt N ]. Comparison
2
+
+
+
3
between pyrrolidinium and piperidinium based ILs could allow
us to assess the relevance of geometry and flexibility of a cyclic
cation. On the other hand, the use of ammonium-based IL could
give information about the role played by the increase in
conformational freedom of the cation on going from cyclic to
acyclic structures.
(
4-nitrophenyl)-formamidine (1) into the corresponding 1-aryl-3-
benzoylamino-1,2,4-triazole (2) (Scheme 1). Bearing in mind
data previously reported by us, this study could reveal
information about side chain effect on the MRH reaction, never
investigated so far in IL solutions. As base catalyst, we used
piperidine (Pip) and we performed a kinetic investigation at 298
K under pseudo-first order conditions. In particular, the catalyst
concentration ranged from 0.002 M up to 0.02 M, whereas
substrate concentration stayed constant and was equal to 0.0002
M. The investigation was carried out by means of UV-vis
measurements at 340 nm.
Kinetic data were analysed on the grounds of classical solvent
parameters. Furthermore, the structural order degree of solvents
used was evaluated performing Resonance Light Scattering
(RLS) measurements.

3
2
. Results and discussion
yellow to pale red, probably indicating the occurrence of
secondary processes.
ACCEPTED MANUSCRIPT
Comparison between reactivity in COS and IL solutions. The
In all other IL solutions, unlike ACN solution, we observed
the triazole formation, as accounted for by UV-vis spectra
recorded as a function of time (see Fig. 1 of Supplementary data)
and TLC analysis of the reaction mixtures at the end of the
reaction. Furthermore, in all cases, the plot of kobs as a function of
Pip concentration showed a linear trend. Fit of experimental trend
according to equation (1):
MRH reaction of (1) was previously studied in ACN solution at
13 K, using different basic catalysts such as piperidine (Pip),
triethylamine (TEA) and DABCO. Results reported indicate
that the triazole formation occurred only using tertiary amines.
Indeed, in the presence of Pip, the formation of p-nitroaniline and
3
36
3
-amino-5-phenyl-1,2,4-oxadiazole was observed as
a
consequence of the hydrolysis of the imino bond in the side
chain. Furthermore, at 313 K in ACN solution and using TEA as
catalyst, the plot of experimental rate constant versus TEA
concentration showed a downward curvilinear trend. The above
trend was ascribed to the occurrence of a two step mechanism in
which the triazole formation took place only after the preliminary
formation of a base-substrate ion pair.
kobs = i + kII [Pip]
(1)
allowed calculation of second-order rate constants (kII)
reported in Table 1 (kobs values as a function of Pip concentration
are summarized in Table 1 of Supplementary data). Moreover, in
the above cases we detected negative intercept values that, on the
grounds of previous reports, were ascribed to the occurrence of
In some IL solutions we observed specific behaviour. Indeed,
in [SbF ]-based ILs, the reaction was very slow (more than a
6
week was needed to have a conversion lower than 40 %) and kobs
detected did not show a regular trend as a function of amine
32
an acid-base interaction between Pip and IL cation.
concentration. On the other hand, in [bmpy][NTf ] solution, we
2
observed a change in the colour of the reaction mixture from pale
Table 1. Second-order rate constants for the MRH of (1) in IL solution at 298 K and IL mole fractions (Χ ) in the reaction
IL
mixtures
-
1
-1 a
Entry
1
IL
[bmim][BF
ΧIL
kII (M s )
4
]
0.70
0.177 (0.006)
0.044 (0.003)
2
[bmim][PF
6
]
0.69
[
N(CN)
2
]-based ILs
3
4
5
[bmim][N(CN)
[hmim][N(CN)
[omim][N(CN)
2
]
2
]
2
]
0.68
0.64
0.61
0.240 (0.010)
0.397 (0.020)
0.034 (0.001)
[
NTf
[bmim][NTf
[emim][NTf
2
]-based ILs
6
7
8
9
2
]
0.58
0.60
0.56
0.53
0.57
0.48
0.47
0.48
0.47
0.46
0.071 (0.002)
0.026 (0.002)
0.018 (0.001)
0.026 (0.001)
0.029 (0.001)
0.054 (0.003)
0.070 (0.003)
0.061 (0.002)
0.060 (0.003)
0.0016 (0.0001)
2
]
[hmim[NTf
2
]
[omim][NTf
2
]
1
1
1
1
1
1
0
1
2
3
4
5
2 2
[bm im][NTf ]
[Bzmim][NTf
2
]
[Bzbim][NTf
2
]
[bmpyrr][NTf
2
]
[bmpip][NTf
2
]
[bEt N][NTf ]
3
2
a
Standard deviations are given in parenthesis.
the ionic solvent media were able to induce
outcome of the target process.
a change in the
Linear dependence of kobs on Pip concentration shows that,
unlike from data collected in ACN solution, the rate of reaction
does not tend to a limiting value. This evidence, together with
triazole formation as the only reaction product, allows stating that
Reactivity in IL solution. In IL solution, second order rate
-1 -1 -1 -1
constants range from 0.400 M s down to 0.0016 M s , with
the largest value detected in [hmim][N(CN) ] and the lowest one
2
detected in [bEt N][NTf ] solution. The above range allows a
3
2

4
Tetrahedron
IL MANlogkSC
certain dependence of reaction rate from the
A
n
C
atu
C
re
E
o
P
f
T
th
E
e
D
U
=
R
-6.
I
3
P
7
T
+ 4.85 (2.75)·α +0.29 (0.50)·β + 0.14 (0.23)·π*
II
2
used. To understand the origin of this dependence, we first
considered the polarity of the ILs used. Indeed, data previously
reported for MRH reactions in COS revealed that, with the
catalyst being the same, reaction rates increase with solvent
polarity. In Table 2 of the Supplementary data, polarity
parameters (E (30) and ENR) for ILs used are reported.
Furthermore, in Fig. 2 of Supplementary data plots of logkII
versus E (30) or E
values, analysis of the above plots indicates that IL polarity does
not allow rationalizing of reactivity data. Indeed, for this
parameter, we obtained a very scattered plot with no obvious
trend. This could be the result of very narrow range of ENR values
R = 0.513 n = 12 (4)
but without giving good results. On the whole, the above
results obtained seem to indicate that factors other than solvent
microscopic properties as evaluated by α, β and π* parameters
affect the reaction studied.
T
T
NR
are displayed. As accounted for by ENR
40
According to a previous report, we also tried to correlate
reactivity data with IL mole fraction in the reaction mixtures (ΧIL
values are reported in Table 1). In our case, this parameter range
from 0.46 < ΧIL < 0.70. However, perusal of data reported in
Table 1 seems to indicate a scarce influence of IL amount on the
outcome of the reaction.
(215.3 < ENR < 218.2 kJ/mol) that does not account for large
differences detected in reactivity data.
A different way to analyse data collected is to take into
consideration structural features of the cation and anion and try to
correlate changes in reactivity with the variations they are able to
induce on the crystal lattice of ILs.
A different situation was observed as far as E (30) values
were concerned. Indeed, in this case, the plot of logkII as a
T
function of E (30) values did not show a clear trend (Fig. 2b of
T
Supplementary data), but a careful look at two different areas of
the plot (50.5 < E (30) < 53.0 kcal/mol and 68.8 < E (30) < 70.2
kcal/mol, Fig. 2c-d of Supplementary data) shows an increase in
T
T
Anion Effect. To evaluate the role played by the different
nature of the anion, we took into account [bmim ]-based ILs. As
stated, the anions used differ in size, shape, cross-linking and
coordination ability. Furthermore, as the cation is the same, the
anion also determines the viscosity of the solvent media, with the
+
logk with solvent polarity. Interestingly, the above area identify
II
two IL classes, the first one comprising imidazolium salts, with
the exception of [bm im][NTf ], and the second one including
2
2
[
N(CN) ]- and benzylimidazolium based ILs. In both cases, an
2
highest viscosity detected for [bmim][PF ] and the lowest for
6
increase of logk with solvent polarity was detected. The above
II
[bmim][NTf ] (η = 450 and 52 cP for [bmim][PF ] and
2
6
41
-
trends seem to indicate that, unlike COS, solvent polarity in this
case does not allow an univocal treatment of reactivity data,
unless in the IL group, sub-groups formed by ILs having a
similar structure are considered.
[
bmim][NTf ], respectively). Among the anions used, [BF ]
2 4
-
and [PF ] have high symmetry and a significant cross-linking
6
-
-
ability. On the other hand, both [N(CN) ] and [NTf ] are
2
2
trigonal, but the former has the highest basicity and coordination
ability (see Table 2 of Supplementary data).
In light of the above results, we tried to analyse the “IL effect”
39
using Kamlet-Taft descriptors. These descriptors are reported in
Table 2 of the Supplementary data. Furthermore, in Fig. 3 of the
Analysis of the k values reported in Table 1 shows that the
II
-
-
reactivity decreases along the following order: [N(CN) ] > [BF ]
> [NTf ] > [PF ]. The above trend perfectly recalls that for β
2
4
-
-
Supplementary data, plots of logk as a function of α, β or π* are
II
2 6
displayed. It is worth remembering that the above parameters
account for solvent microscopic properties, such as hydrogen
bond donor ability (α), hydrogen bond acceptor ability (β) and
dipolarity or polarizability (π*). Analysis of the above plots
indicates that these parameters are not able to explain the
reactivity trend observed, if taken alone. In view of this, we tried
to apply a linear solvation energy relationship (LSER):
values (β = 0.942, 0.327, 0.263 and 0.202 for [bmim][N(CN)2],
bmim][BF ], [bmim][NTf ] and [bmim][PF ], respectively),
[
4
2
6
indicating that the anion coordination ability plays a significant
role in determining the reactivity trend observed. This result
closely agrees with the one we collected performing the MRH
reaction in IL solution and using the (Z)-phenylhydrazone of the
33
3
-benzoyl-5-phenyl-1,2,4-oxadiazole as substrate. In that case,
we ascribed the effect observed to the anion ability to stabilize
the transition state and to increase the nucleophilicity of the NH
phenylhydrazonic. It is worth noting that, if this effect is also
operative in this case, it seems less relevant. Indeed, reactivity
ratios significantly decreased on going from (Z)-phenylhydrazone
to arylformamidine (kII,[bmim][BF4]: kII,[bmim][NTf2]: kII,[bmim][PF6] = 20:
2.7: 1 for (Z)-phenylhydrazone and 4: 2.5: 1 for arylformamidine,
respectively), indicating a lower sensitivity of the latter transition
state to the anion nature.
logk = const + a·α +b·β + c·π*
(2)
II
Such an approach has been successfully applied in IL solution
to explain the data collected for different organic reactions.
16,9,15
In our case, multiparametric analysis did not give satisfactory
results. Indeed, taking in consideration all ILs used, we obtained
the eq. 3:
The reactivity trend obtained could also be analysed, taking
into account the structural organization of ILs used. Indeed, data
previously collected for MRH reaction performed in IL solution
indicated that, for aromatic ILs, a further stabilizing effect of the
transition state could derive from solvents ability to establish π-π
logk = -3.97 + 2.51 (2.26)·α +0.46 (0.50)·β + 0.06 (0.22)·π*
II
2
R = 0.368 n = 15
(3)
32
interactions. Obviously, the relevance of the above effect
should increase in parallel with the structural organization degree
of the solvent. On this subject, an evaluation of ILs organization
could be obtained using Resonance Light Scattering (RLS)
measurements that accounts for the presence of aggregates in
detecting a very poor correlation, as indicated by the
correlation coefficient and standard deviations. On the other
hand, dissecting ILs on the grounds of cation nature and taking
into account only imidazolium based ILs, the correlation
coefficient slightly improved as accounted for by eq. 4:
4
2,43
solution and allows having an assessment of their sizes.
It is
worth remembering that RLS intensities (IRLS) are proportional to
the square of aggregates volume. In the case of ILs, as a
consequence of differences in size of the cation or anion, similar

5
IRLS values could be detected for aggregates for
A
me
C
d
C
by
E
a
P
di
T
ff
E
ere
D
nt MAN
T
U
he
S
p
C
res
R
en
I
ce of a longer alkyl chain could induce different
P
T
number of IL units. IRLS values detected at 522 nm for all ILs
used are reported in Table 3 of the Supplementary data. Analysis
effects. The first one due to the increase in van der Waals
interactions that should induce a parallel increase in the order
degree of solvent media favouring the progress of the reaction.
The above favourable effect could be counterbalanced by a
concomitant rise in solvent viscosity and alkyl chain
conformational freedom. These latter effects could become
operative beyond a certain length and disfavour the reaction as a
consequence of decrease in structural order degree and hindrance
of contact between reagents. It is worth remembering that the
different packing of solvent system as a function of alkyl chain
length has been frequently considered able to induce different
+
of IRLS values for [bmim ]-base ILs shows that the size of the
aggregates decreased along the following trend: [bmim][N(CN)2]
>
[bmim][PF ] > [bmim][BF ] > [bmim][NTf ]. With the only
6 4 2
exception of [bmim][PF ], the above trend foresees a structural
6
order degree decreasing in parallel with the reactivity trend. The
discrepancy observed in the case of [bmim][PF ] could be
6
ascribed to the presence of a different number of IL units
constituting aggregates, as a consequence of the biggest anion
size. Alternatively, it could be also due to the high viscosity of
this IL that could hamper contact between reagents and slow
down the rate of reaction.
4
8,49
solvation of reagents through cation or alkyl chain.
On the
grounds of the above considerations, it can be stated that the
maximum reaction rate represents the best balance among above
factors. However, a careful analysis of data displayed in Fig. 1
indicates that the role played by alkyl chain length is also
affected by the IL anion. Indeed, for [NTf ]-based ILs, the
highest reaction rate was detected in the presence of the butyl
Irrespective of any effect exerted by the IL anion, it is
important to explain why, the arylformamidine rearrangement
feels less of a positive effect exerted by a more ordered solvent
system with respect to (Z)-phenylhydrazone. In our opinion, it
could be ascribed to the different aromaticity of triazoles
obtained from (Z)-phenylhydrazone and arylformamidine.
Indeed, on the grounds of DFT calculations, transition states of
MRH reaction have been described as cyclic and quasi-
2
chain; whereas for [N(CN) ]-based ILs it was obtained for the
2
hexylimidazolium derivative.
It is worth mentioning that we previously detected a similar
situation studying the base-catalysed Kemp elimination reaction
in some [NTf ]- and [SbF ]-based ILs. To explain the above
results, the dependence of ILs viscosity on the anion nature and
size of aggregates present in the ILs used should be taken into
account. In Fig. 2, the plot of IRLS is reported as a function of
alkyl chain length for [NTf ]- and [N(CN) ]-based ILs.
44
45-47
aromatic. On the other hand, aromaticity indexes
account
19
for a lower aromaticity of 1,2,4-triazole with respect to 1,2,3-
triazole (in the case of Bird aromaticity index I = 81 and 88 for
2 6
45
1
,2,4- and 1,2,3-triazole, respectively). In turn, the different
aromaticity of products should derive from different
a
aromaticity of the corresponding transition states that in the case
of the arylformamidine could be less stabilized by IL ability of
giving π-π interactions.
2
2
Cation Effect. For the cation effect, we first analysed data
collected using [NTf ]-based ILs. Comparison between second-
2
order rate constants collected in [bmim][NTf2] and
[
bm im][NTf ] (entries 6 and 10 of Table 1) shows that the rate
2 2
of reaction decreases in parallel with cation ability to give
hydrogen bond. We observed a similar trend analysing the
33
reactivity of (Z)-phenylhydrazone. Furthermore, in two cases
we also detected similar reactivity ratios
k , = 2.5 and 2.0 for arylformamidine
33
(
/k ,
II [bmim][NTf2] II [bm2im][NTf2]
and (Z)-phenylhydrazone, respectively).
[
NTf ]-based ILs also allow evaluation of the effect deriving
2
from different alkyl chain length on the imidazolium ion. In Fig.
, the trend of k values is shown as a function of different alkyl
chain length and different anion nature. Rate constant firstly
increases with alkyl chain length until reaching a maximum value
after which it significantly decreases.
Figure 2. Plot of RLS intensity as a function of alkyl chain
1
II
length for [NTf ] and [N(CN) ]-based ILs.
2
2
A careful analysis of data collected shows that, in the case of
[
[
[
NTf ]-based ILs, the size of aggregates decreases going from
emim][NTf2] to [bmim][NTf2], but increases going to
2
omim][NTf ]. The above trend does not parallel the reactivity
2
trend. According to reactivity data, RLS measurements foresee
the presence of more extended aggregates in [hmim][N(CN)2].
It is well known that viscosity increases with alkyl chain
length and comparison between values previously reported for
[
bmim][NTf ] and [bmim][N(CN) ] demonstrates that [N(CN) ]-
2
2
2
based ILs are less viscous than [NTf ]-based ILs (η = 52 and 31.8
2
5
0
cP for [bmim][NTf ] and [bmim][N(CN) ], respectively). On
2
2
the grounds of the above statements, data collected seems to
indicate that in [NTf ]-based ILs, the reactivity observed was a
2
balance between viscosity and structural order degree effect. On
the other hand, in [N(CN) ]-based ILs, where structural
Figure 1. Plot of k values as a function of alkyl chain length.
2
II
organization plays a more significant role with respect to
viscosity, the first factor prevails giving rise to the highest
reaction rate in the highest ordered solvent media.

6
Tetrahedron
ce MAN
A further parameter to analyse is the exte
A
nsi
C
on
C
o
E
f π
P
-s
T
u
E
rfa
D
Q
U
uit
S
e
C
in
R
te
I
restingly, we have verified that the side chain
P
T
area of the cation on going from imidazolium to
benzylimidazolium based ILs. Unlike data collected for (Z)-
phenylhydrazone, an increase in π-surface area of the cation did
not induce significant changes in reactivity. Indeed, reactivity
stayed constant on going from [bmim][NTf ] to [Bzbim][NTf ]
structure present on the 1,2,4-oxadiazole ring plays a significant
role in determining the “IL effect” observed. Indeed, unlike data
we previously collected in the case of (Z)-phenylhydrazone,
arylformamidine rearrangement results less affected by the
structural order degree of IL and its ability to establish π-π
interactions with the target transition state. Probably, this can be
mainly ascribed to differences in aromaticity of 1,2,4- and 1,2,3-
triazole derivatives obtained as reaction products.
2
2
(entries 6 and 12 of Table 1) and slightly decreased on going
from [bmim][NTf ] to [Bzmim][NTf ] (entries 6 and 11 of Table
2
2
1
). This result could be further support to the hypothesis
concerning a lower sensitivity of transition states deriving from
arylformamidine to IL ability to give π-π interactions.
Finally, one of the main questions we wished to solve was the
possibility to apply LSER relationship to this classical organic
reaction performed in IL solution. Unlike some previous
The last topic to take into consideration is the effect exerted
by the aliphatic or aromatic nature of the IL cation. Comparison
of the data collected in [bmim][NTf2], [bmpyrr][NTf2],
16,1
reports, the above approach fails in the attempt to rationalize
the data collected. Among other things, kinetic data show how in
some cases cations having a different structure give rise to quite
similar reactivities. Probably, this indicates that the reactivity of
the arylformamidine in IL solution is a result of the balance
among different factors such as polarity, cation and anion
structural features and repercussions they have on the IL lattice.
All these factors act on the transition state stabilization and
cannot be represented by a single equation.
[
bmpip][NTf ] and [bEt N][NTf ] (entries 6 and 12-15 of Table
2 3 2
1
) shows that reactivity decreases along the following series:
[bmpyrr][NTf2] [bmpip][NTf2] >>
bEt N][NTf ]. It is worth noting that the most significant change
[
[
bmim][NTf2]
>
≈
3
2
+
was detected on going from [bmim ]-based ILs to ammonium IL,
whereas only a modest decrease was obtained on going from
+
[
bmim][NTf ] to aliphatic but cyclic cations such as [bmpyrr ]
2
+
and [bmpip ]. It is worth noting that a similar trend has been
recently detected by Guirado et al. studying the reduction of 9-
0
fluorenone in IL solution. In this case, the quite similar E
values obtained in [bmpyrr][NTf ] and [bmpip][NTf ] were
4
. Experimental Section
51
Materials. Commercial
1,4-dioxane,
[bmim][BF4],
2
2
[
bmim][PF ], 4-nitroaniline, N,N-diethyl-p-nitroaniline, Nile Red
6
ascribed to similar pre-organization and steric hindrance of
cations.
and Reichardt’s dye 30 were purchased and used without further
purification. Substrates 1 and 2 were prepared according to a
53
previously reported procedure. [bmim][NTf ], [bm im][NTf ],
In our case, the similar k values collected in the above ILs
2
2
2
II
[
emim][NTf2],
[hmim][NTf2],
[omim][NTf2],
[bmpy][NTf2],
show that the target reaction does not feel the effect due to a
different cation shape and geometry. This result is different from
the one detected performing Kemp elimination reaction in the
[Bzbim][NTf
], [Bzmim][NTf
], [bmpyrr][NTf ], [bmpip][NTf ],
2 2 2
2
[
[
bEt N][NTF ], [bmim][SbF6], [hmim][SbF6] [omim][SbF6],
bmim][N(CN)2], [hmim][N(CN)2], [omim][N(CN)2], were
3
2
19
same ILs. Furthermore, in the above case as well as studying
52
aryl azides formation in ILs, we also detected an inverse trend
prepared by anion metathesis of the commercially available
corresponding chloride with NaSbF , LiNTf , or NaN(CN)
according to a previously reported procedure.
for kII values collected in [bmim][NTf ] and [bEt N][NTf ]
6
2
2
2
3
2
54
solution. Indeed, we observed a significant increase in the rate of
reaction on going from aromatic to aliphatic IL. In the current
case, the significantly low reactivity detected in [bEt N][NTf ]
All ionic liquids were dried on a vacuum line at 60 °C for at
least 2h before use, then stored in a dryer under argon and over
calcium chloride. Piperidine was freshly distilled before use.
3
2
solution could be ascribed to its tetrahedral structure that,
compared to other “bidimensional” aliphatic cations, could
hinder the base approach or the formation of stabilizing
interactions with the transition state of the target reaction.
Kinetic Measurements and Calculations. UV-vis spectra and
kinetic measurements were carried out by using
a
spectrophotometer equipped with a Peltier temperature controller,
able to keep the temperature constant within 0.1 K. The sample
for a typical kinetic run was prepared by mixing into a quarz
cuvette (optical path 0.2 cm), 500 µL of IL, 50 µL of a solution
of substrate in 1,4-dioxane and then 25 µL of a concentrated
solution of amine in 1,4-dioxane, previously thermostated. The
Finally, comparison with data previously collected for (Z)-
phenylhydrazone in [bmim][NTf ] and [bmpyrr][NTf ] further
2
2
supports the idea that the transition state deriving from the
present substrate is less sensitive to the positive effect deriving
from cation aromaticity as accounted for by different reactivity
ratios (kII,[bmim][NTf2]: kII,[bmpyrr][NTf2] = 44 and 1.2 in the case of (Z)-
phenylhydrazone and arylformamidine, respectively).
–4
substrate concentration was constant and equal to 2.0·10 M; the
-3
piperidine concentration ranged from 0.2–2.0·10 M. The
reactions were studied over at least three half-lives. In all cases,
the correlation coefficients were higher than 0.9998. The
apparent first-order rate constants obtained were reproducible
within ± 5%. All kinetic data were analysed by means of the
KALEIDAGRAPH 4.0 software package.
3
. Conclusions
The main goals of this paper were the study of the “IL effect”
on the MRH reaction and analysis of the influence of the side
chain structure borne on the starting 1,2,4-oxadiazole. As for the
“
IL effect”, data collected show that these solvent media are able
Determination of solvent parameters. The determination of
solvent parameters was carried out by mixing into a quarz cuvette
optical path 0.2 cm), 500 µL of IL and 75 µL of a concentrated
to positively act on the mechanism of the reaction and, differently
from COS, allow the obtainment of 1,2,4-triazole derivative
using Pip as basic catalyst. Probably, this is a consequence of the
organized structure of ILs that, providing confined environments,
hamper that base attack along directions favouring substrate
(
solution of spectroscopic probe in 1,4-dioxane. The concentration
–4
of the probe was equal to 2.0·10 M. The obtained solution was
-
-
thermostated at 298 K.
hydrolysis. Anion symmetry being the same ([PF ] and [SbF ]),
6
6
the above effect seems to be a function of anion size as
demonstrated by the poor reactivity detected in [bmim][SbF6].
RLS
measurements.
Resonance
Light
Scattering
measurements were carried out on a spectrofluorophotometer
employing a synchronous scanning mode in which the emission
and excitation monochromators were preset to identical

7
wavelengths. The RLS spectrum was recorded
nm setting both excitation and emission slit widths at 1.5 nm.
A
fr
C
omCE30
P
0
T
to
E
6
D
00 MAth
N
an
U
ks
S
p
C
ro
R
ject ICM-P10-003-F CILIS, granted by ‘‘Fondo
I
P
T
de Innovación para la Competitividad’’ from Ministerio de
Economía, Fomento y Turismo, Chile.
Acknowledgments
We thank University of Palermo (2012-ATE-0405) and
MIUR (FIRB 2010 RBFR10BF5V) for financial support. D.M
2
6.
Keaveney, S. T.; Schaffarczyk McHale, K. S.; Haines,
R. S.; Harper, J. B. Org. Biomol. Chem. 2014, 12, 7092-
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Frenna, V.; Vivona, N.; Consiglio, G.; Spinelli, D.;
Supplementary Material
ACCEPTED MANUSCRIPT
Mezzina, E. Journal of the Chemical Society, Perkin
Supplementary material for this article: tables of kinetic data,
polarity parameters, Kamlet-Taft solvent parameters, IRLS values
Transactions 2 1993, 1339-1343.
5
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Cammarata, L.; Kazarian, S. G.; Salter, P. A.; Welton,
T. Phys. Chem. Chem. Phys. 2001, 3, 5192-5200.
as a function of ionic liquid, plots of logk as a fuction of polarity
or Kamlet-Taft solvent parameters are available.
II

ACCEPTED MANUSCRIPT
Supplementary Data for
The ionic liquid effect on the Boulton-Katritzky reaction: a comparison between substrates of
different structure
a
b
a
Francesca D’Anna, * Daniela Millan and Renato Noto
Dipartimento STEBICEF, Università degli Studi di Palermo, Viale delle Scienze Ed. 17, 90128 Palermo
Italy)
Facultad de Química, Pontificia Universidad Católica de Chile,Santiago 6094411, Chile.
a
(
b
Figure 1. UV-vis spectra collected as a function of time collected at 298 K, in [bmim][BF ] solution and in the presence of Pip
4
(0.014 M).
Figure 2. Plots of logk as a function of polarity parameters and Kamlet-Taft solvent parameters.
II
Table 1. Observed Rate Constants (kobs) for the MHR reaction of (1) in IL solution, at 298 K, as a function of Pip concentration.
Table 2. Polarity parameters (E (30) and ENR) and Kamlet-Taft solvent parameters corresponding to ILs used.
T
Table 3. IRLS detected at 522 nm as a function of ionic liquid.

ACCEPTED MANUSCRIPT
0
0
0
0
0
,5
,4
,3
,2
,1
0
A
t=0
t=5
t=10
t=20
2
50
300
350
λ (nm)
400
450
Figure 1. UV-vis spectra collected as a function of time collected at 298 K, in [bmim][BF ] solution and in the presence of Pip
4
(0.014 M).
0
(
-0,4
a)
(
b)
-
-
-
0,5
-0,6
-0,8
-
1
-
1
1,5
-1,2
-1,4
-1,6
-1,8
-
2
2,5
-
3
2
15
216
217
218
219
220
45
50
55
60
65
70
75
ENR (kJ/mol)
E (30) (kcal/mol)
T
-
0,6
0,8
-0,4
-0,6
-0,8
-1
(
c)
(d)
-
-
1
-1,2
-1,4
-1,6
-1,8
-1,2
-1,4
-1,6
5
0,5
51
51,5
52
52,5
53
68,8
69
69,2
69,4
69,6
69,8
70
70,2
E (30) (kcal/mol)
E (30) (kcal/mol)
T
T

ACCEPTED MANUSCRIPT
Figure 2. Plots of logk as a function of: (a) ENR values; (b) E (30); (c) E (30) in the range 50.5 < E (30) < 53.0
II
T
T
T
kcal/mol; (d) 68.8 < E (30) < 70.2 kcal/mol; (e) α ; (f) β; (g) π*.
T

ACCEPTED MANUSCRIPT
Table 1. Observed Rate Constants (kobs) for the MHR reaction of (1) in IL solution, at 298 K, as a function of Pip concentration.
3
-1 a
IL
[Pip] (M)
10 k (s )
obs
0
0
.002
.004
0.510
0.506
1.270
1.820
2.480
3.320
0.048
0.146
0.177
0.369
0.612
0.707
[
bmim][BF4]
0.007
0
0
0
0
0
.010
.014
.018
.002
.004
[
bmim][PF6]
0.007
0
0
0
.010
.014
.018
[
N(CN) ]-based ILs
2
0
0
.002
.004
0.353
1.132
1.650
2.800
3.380
4.210
0.622
1.966
2.780
4.320
5.220
7.000
[
[
[
bmim][N(CN)2]
hmim][N(CN)2]
omim][N(CN)2]
0.007
0
0
0
0
0
.010
.014
.018
.002
.004
0.007
0
0
0
.010
.014
.018
0
.004
0.0084
0.0589
0.1840
0.3330
0.4630
0.007
0
0
0
.010
.014
.018
[
NTf ]-based ILs
2
0
0
.002
.004
0.155
0.360
0.462
0.690
0.977
1.320
0.033
0.082
0.140
0.196
0.320
0.463
0.035
0.069
0.116
0.165
0.252
[
bmim][NTf2]
0.007
0
0
0
0
0
.010
.014
.018
.002
.004
[
emim][NTf2]
0.007
0
0
0
0
0
.010
.014
.018
.002
.004
[
[
hmim][NTf2]
omim][NTf2]
0.007
0
0
.010
.014
0
0
.002
.004
0.046
0.096
0.140
0.217
0.365
0.448
0.007
0
0
0
.010
.014
.018

ACCEPTED MANUSCRIPT
0
0
.002
.004
0.069
0.143
0.212
0.289
0.433
0.531
0.052
0.159
0.271
0.463
0.753
0.865
0.067
0.240
0.413
0.670
0.860
1.130
0.123
0.248
0.360
0.588
0.870
1.070
0.020
0.080
0.240
0.460
0.690
0.780
[
bm im][NTf ]
0.007
2
2
0
0
0
0
0
.010
.014
.018
.002
.004
[
Bzmim][NTf2]
0.007
0
0
0
0
0
.010
.014
.018
.002
.004
[
Bzbim][NTf2]
0.007
0
0
0
0
0
.010
.014
.018
.002
.004
[
bmpyrr][NTf2]
0.007
0
0
0
0
0
.010
.014
.018
.002
.004
[
bmpip] [NTf2]
0.007
0
0
0
.010
.014
.018
0
.004
0.009
0.012
0.018
0.025
0.030
[
bEt N][NTf ]
0.007
3
2
0
0
0
.010
.014
.018
a
Rate constants were reproducible within ± 5%.

ACCEPTED MANUSCRIPT
Table 2. Polarity parameters (E (30) and ENR) and Kamlet-Taft solvent parameters corresponding to ILs used.
T
*
Entry
E (30)
ENR
kJ/mol)
217.3
217.7
α
β
π
IL
T
(
kcal/mol)
a
a
a
a
a
a
a
1
2
[bmim][BF4]
[bmim][PF6]
52.7
52.2
0.613
0.614
0.327
0.202
1.078a
1.034
a
a
[
N(CN) ]-based ILs
2
b
b
b
b
b
3
4
5
[bmim][N(CN)2]
[hmim][N(CN)2]
[omim][N(CN)2]
70.2
215.7b
215.3b
215.7
1.800b
0.942b
0.942b
0.943
1.016
1.053
1.029
b
b
69.6 b
1.730
b
b
68.9
1.700
[
NTf ]-based ILs
2
a
a
a
a
a
a
c
c
a
a
a
6
7
8
9
[bmim][NTf2]
[emim][NTf2]
[hmim][NTf2]
[omim][NTf2]
[bm im][NTf ]
51.5
52.5
51.8
50.8
48.1
69.6
69.9
48.4
218.2
217.9
217.9
217.9
217.7
216.2
216.1
0.622
0.684
0.643
0.590
0.384
1.770
1.780
0.263
0.323
0.308
0.365
0.299
0.320
0.290
0.243
0.955a
a
a
a
0.967
a
a
a
a
0.971a
a
a
a
0.940
a
a
a
a
1
1
1
1
1
1
0
1
2
3
4
5
0.987c
2
2
c
c
c
[Bzmim][NTf2]
[Bzbim][NTf2]
[bmpyrr][NTf2]
[bmpip][NTf2]
[bEt N][NTf ]
1.020c
c
c
c
0.980
a
a
a
a
0.409
0.433
0.999
0.971a
a
a
a
a
a
47.8
219.0
217.5
0.338
0.303
0.889a
0.948
a
a
3
2
a
1
b
c
2
From Ref. ; Determined in this work; From Ref.
Table 3. IRLS detected at 522 nm as a function of ionic liquid.
Entry
IL
IRLS
1
2
[bmim][BF4]
[bmim][PF6]
78.5
98.1
3
4
5
[bmim][N(CN)2]
[hmim][N(CN)2]
[omim][N(CN)2]
119
129
87.4
6
7
8
9
[bmim][NTf2]
[emim][NTf2]
[hmim] [NTf2]
[omim][NTf2]
[bm im][NTf ]
63.1
133
214
264
63.1
152
248
76.4
263
102
1
1
1
1
1
1
0
1
2
3
4
5
2
2
[Bzmim][NTf2]
[Bzbim][NTf2]
[bmpyrr][NTf2]
[bmpip][NTf2]
[bEt N][NTf ]
3
2
(
(
1)
2)
D'Anna, F.; La Marca, S.; Lo Meo, P.; Noto, R. Chem. Eur. J. 2009, 15, 7896.
D'Anna, F.; Marullo, S.; Vitale, P.; Noto, R. Eur. J. Org. Chem. 2011, 2011, 5681.