Nucleophilic and acid catalyst behavior of a protic ionic liquid in a molecular reaction media. Part 1

Article abstract of DOI:10.1002/poc.1501

This work presents a new approach using ionic liquids, namely ethylammonium nitrate. The aim was to analyze how the addition of small amounts of a protic ionic liquid to a pure molecular solvent, modifies the microscopic characteristics of a reaction medium. In order to achieve this, a kinetic study of nucleophilic aromatic substitution reactions between 1-fluoro-2, 4-dinitrobenzene and 1-butylamine or piperidine was developed in this type of binary mixtures. We have detected nucleophiles competition originated by the presence of the ionic solvent at very low concentrations, observing the ethylamine derivative as the main substitution product. Moreover, in the light of previous results we have confirmed that the protic ionic liquid can act as both Broensted acid and/or nucleophile. In this connection, we have selected the nucleophilic addition of amines to carbonyl compounds as reaction model. The protic ionic liquid in the presence of aromatic aldehydes substituted by electron-donating groups makes possible the formation of the corresponding imines with good yields. The results demonstrated that the influence of the protic ionic liquid is very important in the course of both reaction systems. Copyright

Full text of DOI:10.1002/poc.1501

Research Article
Received: 19 September 2008,
Revised: 12 November 2008,
Accepted: 12 November 2008,
Published online in Wiley InterScience: 30 December 2008
(www.interscience.wiley.com) DOI 10.1002/poc.1501
Nucleophilic and acid catalyst behavior
of a protic ionic liquid in a molecular
reaction media. Part 1
Claudia G. Adama,y, Graciela G. Fortunatoa and Pedro M. Mancinia
*
This work presents a new approach using ionic liquids, namely ethylammonium nitrate. The aim was to analyze how
the addition of small amounts of a protic ionic liquid to a pure molecular solvent, modifies the microscopic
characteristics of a reaction medium. In order to achieve this, a kinetic study of nucleophilic aromatic substitution
reactions between 1-fluoro-2,4-dinitrobenzene and 1-butylamine or piperidine was developed in this type of binary
mixtures. We have detected nucleophiles competition originated by the presence of the ionic solvent at very low
concentrations, observing the ethylamine derivative as the main substitution product. Moreover, in the light of
¨
previous results we have confirmed that the protic ionic liquid can act as both Bronsted acid and/or nucleophile. In this
connection, we have selected the nucleophilic addition of amines to carbonyl compounds as reaction model. The
protic ionic liquid in the presence of aromatic aldehydes substituted by electron-donating groups makes possible the
formation of the corresponding imines with good yields. The results demonstrated that the influence of the protic
ionic liquid is very important in the course of both reaction systems. Copyright ß 2008 John Wiley & Sons, Ltd.
Keywords: protic ionic liquid; aromatic nucleophilic substitution; imine synthesis; ethylammonium nitrate; acid catalysts
strongly protic solvent mixtures of molecular solvent/ionic liquid
type, as a new solvent.
INTRODUCTION
In the last few years designing solvents has become an important
task for chemists, the challenge consisting in developing a
‘‘desired solvent’’ with particular properties for each application.
In connection with this, ionic liquids (ILs) turn up as a new type of
solvents suitable for developing physicochemical processes.[1–11]
IL technology has been successfully applied in several classic
organic chemistry processes.[12,13] A great number of investi-
gations are centered in characterizing ionic systems by
measuring viscosity, boiling point, melting point, and additionally
their microscopic properties. Other works were focused on
modulating their physical and chemical properties through
modifications in their anions or cations.[1–11] Recently, protic ionic
liquids (PILs) have been extensively used in organic syn-
thesis,[14–16] as catalysts,[17] and as self-assembly media.[18,19]
Drummond and Greaves[20] published an extensive review on
PILs, analyzing the goodness of these ILs as protic solvents in
several reactions. Nevertheless, there are relatively few works
based on kinetic studies. Even though some of these works
analyze different reactive systems varying the nature of the IL, the
data kinetic treatments result insufficient.[21–25]
The main purpose of this work is to study how a reaction
medium is modified, at microscopic-molecular level, by adding
small amounts of a PIL to a molecular solvent, in order to improve
a reaction process. Hence, a specific aim is to evaluate if the
presence of small amounts of a PIL could affect the course of a
chemical reaction. In connection with this, our attention is drawn
to analyze the possible participation of the ionic liquid both as
acid catalyst and/or potential nucleophilie.
With these purposes in mind, we have selected two organic
reactions as models: (i) the aromatic nucleophilic substitution
reaction (SNAr) between 1-fluoro-2,4-dinitrobenzene (FDNB) and
1-butylamine (BU) and piperidine (PIP), respectively, in order not
only to highlight the possible nucleophiles competition but also
to attempt the determination of the reaction rate constant for
ethylamine (EA) as nucleophile; (ii) the addition of amines to
substituted aromatic carbonyl compounds, in order to evaluate
the behavior of EAN as dual acid catalyst and nucleophile.
In a recent work, we analyzed the microscopic features of
binary solvent systems formed by a molecular solvent and an IL
cosolvent (ethylammonium nitrate, EAN).[26] This study was
focused on the identification of solvent mixtures with relevant
solvating properties. We proposed three individual solvent
mixtures, which were selected from the explored systems on
the basis of their hydrogen-bonding ability, as ‘‘new liquids’’
suitable to provide a medium with particular microscopic solvent
features. These results showed that it is possible to prepare
´
Correspondence to: P. M. Mancini, Departamento de Qu´ımica, Area Qu´ımica
*
´
Organica, Facultad de Ingenier´ıa Qu´ımica, Universidad Nacional del Litoral,
Santiago del Estero 2829 (3000) Santa Fe Repu´blica, Argentina.
E-mail: pmancini@fiq.unl.edu.ar
a
C. G. Adam, G. G. Fortunato, P. M. Mancini
´
´
Departamento de Qu´ımica, Area Qu´ımica Organica, Facultad de Ingenier´ıa
Qu´ımica, Universidad Nacional del Litoral, Santiago del Estero, Santa Fe
Repu´blica, Argentina
y
Researcher from CONICET, cadam@fiq.unl.edu.ar.
J. Phys. Org. Chem. 2009, 22 460–465
Copyright ß 2008 John Wiley & Sons, Ltd.

NEW APPROACH USING IONIC LIQUIDS
RESULTS AND DISCUSSION
The selected PIL was EAN. This IL was chosen taking into account
that it is a protic solvent and it has been extensively used. The
molecular solvent selected was acetonitrile (AcN). The selection
was done on the basis of its microscopic properties. This solvent is
dipolar relatively strong hydrogen bond acceptor (HBA), with
weak hydrogen bond donor (HBD) properties (ETN ¼ 0.46,
*
p ¼ 0.75, a ¼ 0.19, and b ¼ 0.40). It is important to remark that
the binary solvent system [AcN þ EAN], which molecular-
microscopic properties have been determined recently, exhibits
synergism on the acidity property.[26]
Solvent effect on chemical processes
Aromatic nucleophilic substitution reaction (SNAr). Kinetic
determinations
A kinetic study of the SNAr reactions between FDNB with BU and
PIP was developed in order to analyze the influence of EAN in the
reaction system. The reactions were carried out at 258C, in AcN,
and in the presence of low EAN concentrations (6.25 ꢀ 10ꢁ4 M up
to 0.3 M). They were performed under pseudo-first order
conditions, and at constant amine concentration (0.002 M).
The mechanism of SNAr reactions has been widely studied.
Since the first pioneering studies,[27] different contributions have
been made towards the understanding of solvent effects on SNAr
reactions[28,29] showing how extensive and complex the
interactions of the substrate and/or the intermediates with the
solvent molecules can be. We have analyzed the solvent effects
on the SNAr reactions carried out in molecular solvent mixtures,
and we interpreted them by means of multiparametric corre-
lation analysis between empirical solvent parameters and the kA
second-order rate constants.[30–34] Further, the n-butylammonium
nitrate salt effects on rate constants for the reaction of
2,4-dinitrochlorobenzene with n-butylamine were previously
studied in chloroform and ethanol. The results indicated that the
addition of this salt has almost no effect on the reaction rate.[35]
The spectra of reaction with the studied SNAr reactions for PIP
and BU are presented in Figs. 1 and 2, respectively. For the
reaction with PIP, a decrease in the absorbance values of the
Figure 1. Spectra of SNAr reaction between FDNB (5 ꢀ 10ꢁ5 M) and PIP
(0.002 M) at variable concentrations of EAN (corresponding to Table 1)
Scheme 1 presents the equilibrium between EAN and PIP. The
values corresponding to the equilibrium constants for PIP and EA
in AcN were obtained from the literature.[36,37] This equilibrium
modifies the reaction system and the quantitative yield of the
product will depend on the equilibrium constant values
proposed in Scheme 1.
In order to confirm the non-participation of the nitrate ion in
this equilibrium, we selected the ethylammonium acetate (EAAc)
as ionic solvent and applied the same methodology previously
cited. In this experiment, only the N-(2,4-dinitrophenyl) ethyl-
amine was observed as product.
substitution product corresponding to this amine (lmax
¼
382 nm), can be observed at low EAN concentrations (from 0
to 0.025 M). In contrast, the formation of a new species with lmax
at 350 nm was detected at EAN concentrations ꢂ0.05 M. The
above result can be understood by taking into account that EAN
¨
can act as a Bronsted acid and it can establish an acid–base
equilibrium with amines, making possible to predict nucleophiles
competition.
For the reaction with BU, the above differences in the
absorbance values with the increase of the EAN concentrations,
cannot be observed because the wavelengths of absorption for
both products are similar (Fig. 2). As it can be observed, the
spectra for the products from both amines at high EAN
concentrations are coincident. This result is the expected one.
In the light of these results, we carried out the corresponding
reactions with both amines at preparative scale. The
N-(2,4-dinitrophenyl) ethylamine substitution product was
1
observed in both cases, which was confirmed by H NMR, 13C
Figure 2. Spectra of SNAr reaction between FDNB (5 ꢀ 10ꢁ5 M) and BU
NMR, and mass spectroscopy.
(0.002 M) at variable concentrations of EAN (corresponding to Table 1)
J. Phys. Org. Chem. 2009, 22 460–465
Copyright ß 2008 John Wiley & Sons, Ltd.
www.interscience.wiley.com/journal/poc

C. G. ADAM, G. G. FORTUNATO AND P. M. MANCINI
Scheme 1.
In this direction, we have calculated the amine effective molar
concentration through Equation (1). Where x is the advance
grade, CEAN the EAN molar concentration, which is variable, and
CAmine is the constant amine (PIP or BU) molar concentration.
X2
Keq
¼
(1)
ðCEAN ꢁ xÞðCAmine ꢁ xÞ
Table 1 shows the results obtained from Equation (1), varying
the EAN concentration and at 0.002 M of BU or PIP. It can be
deduced that, for concentrations of EAN ꢂ0.05 M, the effective
concentrations of PIPeff or BUeff are similar to the substrate
concentration. At this point, the EA concentration is close to
0.002 M, being this amine the only effective nucleophile.
Moreover, under the present conditions, these reactions are
not base catalyzed. We support this assumption based on the
abundant evidence of strong hydrogen-bonding interactions
between amines and protic solvents. Several contributions have
revealed that the strong solvation of the amine molecule by
protic solvent is responsible for the observed decrease in rate.[30]
Scheme 2 shows the reaction mechanism in all solvents when
primary amines are the nucleophiles. In this type of SNAr reactions
base catalysis is not operative so the formation of the zwitterionic
intermediate is rate limiting and the kA second-order rate
constants is simplified to kA ¼ k1.
Pseudo-first order rate constants, koTbs, for the SNAr reaction with
BU
EA as nucleophile in the presence of PIP (koPbIPs) or BU (k ) are
obs
listed in Table 2. As it can be observed, at low concentrations of
EAN, two processes are taking place simultaneously as con-
sequence of the competition of both nucleophiles, EA with PIP or
BU, respectively.
At this point and considering the protic characteristic of EAN,
the pseudo-first order constant derives in Equation (2).
koTbs ¼ kAEA½EAꢃ þ kAAmine½Amineꢃ
(2)
At EA concentration of 0.002 M, the k and k (sꢁ1) values
PIP
obs
BU
obs
are very similar, corresponding wholly to the reaction with EA. For
this condition, the kAEA value is 2.62 (l molꢁ1 sꢁ1). It is interesting to
note that, for the reaction with BU in AcN pure solvent and at the
same concentration of amine, the kABU value is approximately 7.54
(l molꢁ1 sꢁ1).[32]
The three-dimensional space, plotted from the data corre-
PIP
BU
obs
sponding to Tables 1 and 2, presents the k and k values as a
obs
function of the concentrations of EA, BU, and PIP, respectively
(Fig. 3). As it can be observed, the variation of the koTbs values
corresponding to both amines (PIP and BU) in pure AcN (plane
YZ) is due to the fact that the SNAr reaction with PIP is base
catalyzed while the corresponding to BU is not base catalyzed.[32]
Additionally, the plot shows a decrease of the koTbs values until
both reactive processes converge to a single value, the koTbs value
corresponding to the reaction with the EA.
www.interscience.wiley.com/journal/poc
Copyright ß 2008 John Wiley & Sons, Ltd.
J. Phys. Org. Chem. 2009, 22 460–465

NEW APPROACH USING IONIC LIQUIDS
compound and amine, in consequence, it is generally necessary
to remove water and/or employ an excess amount of the
reactants for satisfactory conversion to product. At weakly acidic
pH, the addition of amines to carbonyl compounds can follow
either a concerted or a stepwise mechanism. The predominant
mechanism depends on the basicity of the amine and the mutual
equilibrium affinity for adduct formation between the carbonyl
compound and the amine. Recently, a series of solid-state
reactions between gaseous amines and aldehydes to render
imines have been reported.[41]
In this direction, we analyzed the behavior of 4-(dimethyl)-
aminobenzaldehyde, 4-hidroxy benzaldehyde, benzaldehyde,
and 4-nitrobenzaldehyde in the presence of EAN. The aromatic
aldehydes were selected in view of the electronic effects of the
p-substituents: electron-donating and electron-withdrawing
groups. Benzaldehyde was chosen as reference. The reactions
were carried out at preparative level and the formation of the
expected products was confirmed. The experimental results
showed that the electronic nature of the substituent groups in
the aromatic ring played a key role in the quantitative conversion
to imine product. The experimental yields followed the
order: 4-(dimethyl)aminobenzaldehyde > 4-hidroxy benzaldehy-
de > benzaldehyde ffi 4-nitrobenzaldehyde (97%, 64, 14, and
10%, respectively). For benzaldehyde and 4-nitrobenzaldehyde,
the corresponding imines were obtained with poor yields. It is
interesting to note that the above reactivity trend for the selected
aldehydes followed an opposite direction to the one expected if
reactions would be carried out under usual conditions. As a result,
in the presence of EAN, the imine product formation is promoted
by electron-donating substituents in the aromatic ring. In these
cases, these substituent groups favor the acid–base equilibrium
between EAN and aldehydes.
Scheme 2.
Table 2. Pseudo-first order rate constants, koTbs (sꢁ1) for the
SNAr reaction between FDNB (5 ꢀ 10ꢁ5 M) and ethylamine in
the presence of PIP (koPbIPs) and BU (k ), measured at 258C
BU
obs
[EA]/103 M
koTbs(s1)
0.55
0.946
1.69
1.95
1.97
2
PIP
k
BU
0.1349
0.0127
0.0848
0.0128
0.0286
0.0099
0.0112
0.0081
0.01
0.0087
0.0051
0.0054
obs
k
obs
Addition reactions of carbonyl compound with a nitrogen
nucleophile: Imine formation
While the employ of pure ILs as reaction media have been
investigated in a great variety of chemical processes, reactions in
which ILs take part as reagents were less investigated. Taking into
¨
account that EAN is a Bronsted acid and so, it can act as an acid
catalyst generating at the same time a nucleophilic specie, it was
interesting to explore the dual behavior of EAN in nucleophilic
addition reactions to carbonyl compounds. An important number
of chemical and biochemical processes are initiated by the
addition of a nitrogen nucleophile to a carbonyl group. These
processes have been the subject of extensive study.[38–40] Due to
their diverse reactivity, imines are common substrates in a wide
range of chemical transformations. However, the equilibrium in
these reactions usually lies in favor of the free carbonyl
In order to get a deeper insight of the reaction feature, we
analyzed the influence of small amounts of EAN (0.05 M ꢄ
[EAN] ꢄ 0.3 M) to a solution of 4-(dimethyl)aminobenzaldehyde
in pure AcN, following the methodology previously applied for
SNAr reactions. Figure 4 shows the following plots: (a) the
Figure 4. Spectra of 4-(dimethyl)aminobenzaldehyde (---) at 10ꢁ4 M in
pure AcN; product of the reaction (__---__) and influence of EAN concen-
tration (—)
Figure 3. Pseudo-first order rate constants, koTbs for the SNAr reaction in
the presence of PIP (koPbIPs) and BU (k ), at 258C
BU
obs
J. Phys. Org. Chem. 2009, 22 460–465
Copyright ß 2008 John Wiley & Sons, Ltd.
www.interscience.wiley.com/journal/poc

C. G. ADAM, G. G. FORTUNATO AND P. M. MANCINI
Scheme 3.
spectrum corresponding to the aldehyde in pure AcN
General methods
([aldehyde] ¼ 10ꢁ4 M) (dot line); (b) the spectra obtained as
EAN is added to the aldehyde solution, at different concen-
trations of the IL (solid line), and (c) the spectrum corresponding
to the pure imine product in AcN. (dash-dot line). From the
analysis of the plots, it can be concluded that, as EAN
concentration in the media is increased, a new species appears
at lmax near to 400 nm, corresponding to the formation of the
imine product.
All the products were obtained stirring the reaction mixture over
night at room temperature. Yields were determined by weight,
purity by 1H and 13CNMR and mass spectroscopy. All NMR spectra
were taken at a 500 MHZ Bruker Unity instrument in CDCl3. These
values were verified with those previously reported.[41,42]
The spectrophotomectric measurements and kinetic exper-
iments were preformed on PerkinElmer Lambda 40 UV/Vis
spectrophotometer, equipped with a thermostatic cell holder.
The kinetics of the reactions was run under pseudo-first order
conditions with the substrate (FDNB) as a minor component at
258C.
In the light of these results, a simplified reaction scheme is
presented in Scheme 3.
N-(2,4-dinitrophenyl) ethylamine was obtained by the reac-
tion of FDNB, PIP, and EAN (1:10:20, respectively). The reaction
was followed by TLC (Hexane/EtOAc, 10:1). The reaction mixture
was cleaned up by chromatography silica gel column.
4-[(Ethylimino) methyl]-N,N-dimethylbenzenamine was
obtained as yellow solids by the reaction of 4-(dimethyl)-
aminobenzaldehyde and with EAN (1:7), measured pH value for
this mixture was about 6. The solid obtained was washed
repeatedly with diethyl ether (2 ꢀ 5 ml) to give a 97% yield.
4-[(ethylimino) methyl] phenol was obtained by the reaction
of 4-(hydroxy)benzaldehyde and with EAN (1:8) measured pH
value for this mixture was about 6. The imine was extracted from
the reaction mixture with dichlomethane, and purified by
chromatography silica gel column. This product was obtained
with a 64% yield.
CONCLUSIONS
Based on the overall results presented in this work, we can
conclude the following:
ꢅ For the SNAr reactions studied, it was possible to confirm that
EAN can take part in an acid–base equilibrium with amines,
even at very low concentrations ([EAN] ꢂ 0.05 M), generating a
nucleophile species which can participate in the reaction. The
substitution product, N-(2,4-dinitrophenyl) ethylamine was
obtained and therefore the kA second-order rate constant
for ethylamine could be determinated.
ꢅ For the nucleophilic addition reactions of amines to substi-
tuted aromatic aldehydes, where acid catalysis is required, the
use of EAN seems to be convenient. The EAN can take part in an
acid–base equilibrium with aromatic aldehydes substituted by
electron-donating group. The imine products from the
selected aldehyde could be obtained, confirming the dual
Acknowledgements
¨
behavior of EAN as Bronsted acid and potential nucleophile.
The participation of these salts as potential nucleophiles in this
type of process allows developing new synthesis routes.
Financial support form Universidad Nacional del Litoral (U.N.L),
C.A.I R D 2006 (projects: 33-183 and 33-182) and Agencia Nacio-
´
´
nal de Promocion Cient´ıfica y Tecnica-U.N.L (Project PIC-TO:
The model reactions presented in this work constitute suitable
examples of how the microscopic feature of a reactive system can
be modified by adding aliquots of a protic ionic liquid to a
molecular solvent. The acidity of the resultant binary mixtures
plays a key role in acid-catalyzed reactions with the possibility of
replacing traditional acid reagents. Moreover, the whole reactive
system can be modulated with the aim of not only to promote
acid-catalyzed reactions but also to generate nucleophilic species
in situ. In connection with this, the design of ionic liquids can be
formulated according to the particular requirements of a reactive
system.
36189) is gratefully acknowledged.
REFERENCES
[1] J. D. Holbrey, K. R. Seddon, Clean Products and Processes 1999, 1,
223–236.
[2] T. Welton, Chem. Rev. 1999, 99, 2071–2083.
[3] P. Wassercheid, W. Keim, Angew. Chem. Int. Ed. Engl. 2000, 39,
3772–3789.
[4] H. Olivier-Bourbigou, L. Magna, J. Mol. Catal. A: Chem. 2002, 182–183,
419–437.
[5] H. Z. Zhao, Phys. Chem. Liq. 2003, 41, 545–557.
[6] C. F. Poole, J. Chromatgr. A 2004, 1037, 49–82.
[7] C. T. Martins, B. M. Sato, O. A. El Seoud, J. Phys. Chem. B 2008, 112,
8330–8339.
[8] P. Wasserchied, T. Welton, Ionic Liquids in Synthesis, Wiley-VCH,
Weinheim, 2007.
[9] R. Rogers, K. R. Seddon, Ionic liquids as Green Solvents: Progress and
Prospects, ACS, Washington DC, 2003.
[10] C. Reichardt, Pure Appl. Chem 2004, 76, 1903–1919.
[11] C. Reichardt, Green Chem. 2005, 7, 339–351.
EXPERIMENTAL
Materials
All the amines, reagents and the solvents were prepared and/or
purified as reported previously.[30–34] The EAN was prepared from
a concentrated aqueous solution of ethylamine (70%) and nitric
acid (70%).[12,13]
www.interscience.wiley.com/journal/poc
Copyright ß 2008 John Wiley & Sons, Ltd.
J. Phys. Org. Chem. 2009, 22 460–465

NEW APPROACH USING IONIC LIQUIDS
[12] A. C. Cole, J. L. Jensen, I. Natai, K. L. T. Tran, K. J. Weaver, D. C. Forber, J.
H. Davisar, J. Am. Chem. Soc. 2002, 124, 5962.
[13] D. W. Morrison, D. C. Forber, Jr J. H. Dravis, Tetrahedron Lett. 2001, 42,
6053–6055.
[14] R. V. Hangrarge, D. V. Jarikote, M. S. Shingare, Green Chem. 2002, 4,
266–268.
[27] J. F. Bunnett, R. J. Morath, J. Am. Chem. Soc. 1955, 77, 5051–5165.
[28] C. Reichardt, Solvent and Solvent Effects in Organic Chemistry,
Wiley-VCH, Weinheim, 2003.
[29] C. Reichardt, Chem. Rev. 1994, 94, 2319–2358.
[30] R. D. Martinez, P. M. Mancini, L. R. Vottero, N. S. Nudelman, J. Chem.
Soc. Perkin Trans. 2 1986, 1133–1138.
[15] K. K. Laali, V. J. Gettwert, J. Org. Chem. 2001, 66, 35–40.
[16] Y. Hu, J. Chen, Z. G. Le, Q. G. Zheng, Synth. Commun. 2005, 35, 739–
744.
[31] P. M. Mancini, A. J. Terenzani, C. Adam, L. R. Vottero, J. Phys. Org. Chem.
1997, 10, 849–860.
´
[32] P. M. Mancini, A. J. Terenzani, C. Adam, A. Perez, L. R. Vottero, J. Phys.
[17] T. Jiang, H. X. Gao, B. X. Han, G. Y. Zhao, Y. H. Chang, W. Z. Wu, L. Gao, G.
Y. Yang, Tetrahedrom Lett. 2004, 45, 2699–2701.
[18] T. L. Greaves, A. Weerawardena, C. Fong, C. J. Drummond, Langmuir
2007, 23, 402–404.
[19] T. L. Greaves, A. Weerawardena, C. Fong, C. J. Drummond, J. Phys.
Chem. B 2007, 111, 4082–4088.
[20] T. L. Greaves, C. Drummond, J. Chem. Rev. 2008, 108, 206–2237.
[21] N. L. Lancanster, T. Welton, G. B. Young, J. Chem. Soc., Perkin Trans. 2
2001, 2267–2270.
[22] C. Adam, L. Gracia-R´ıo, A. Godoy, J. R. Leis, Green Chem. 2006, 8,
596–598.
Org. Chem. 1999, 12, 207–220.
[33] P. M. Mancini, A. J. Terenzani, C. Adam, L. R. Vottero, J. Phys. Org. Chem.
1999, 12, 713–724.
[34] P. M. Mancini, A. J. Terenzani, C. Adam, L. R. Vottero, J. Phys. Org. Chem.
1999, 12, 430–440.
[35] S. D. Ross, M. Finkelstein, J. Am. Chem. Soc. 1957, 79, 6547–6554.
[36] E. P. Serjeant, Potentiometry and Potentiometric Titration, Krieger
Publishing Company, Malabar, Florida 32950 1984.
[37] J. F. Coetzee, G. R. Padmamabhan, J. Am. Chem. Soc. 1965, 87, 5005.
[38] W. P. Jencks, Catalysis in Chemistry and Enzymology, McGraw-Hill, New
York, 1969.
[23] C. Chiappe, D. Pieraccini, J. Org. Chem. 2004, 69, 6059–6064.
[24] I. Newington, J. M. Perez-Arlandis, T. Welton, Org. Lett. 2007, 9,
5247–5250.
[39] W. P. Jencks, Chem. Rev. 1972, 72, 705–718.
[40] I. M. C. Brigghente, L. R. Vottero, A. J. Terenzani, R. A. Yunes, J. Phys.
Org. Chem. 1991, 4, 107–112.
[25] L. Crowhurst, R. Falcone, L. Llewellyn Lancaster, V. Llopis-Mestre, T.
Welton, J. Org. Chem. 2006, 71, 8847–8853.
[26] P. M. Mancini, G. G. Fortunato, L. R. Vottero, Phys. Chem. Liq. 2004, 42,
625–632.
[41] G. Kaupp, J. Schmeyers, J. Boy, Thetrahedron 2000, 56, 6899–6911.
[42] B. Gierczyk, B. Leska, B. Nowak-Wydra, G. Schroeder, G. Wojcie-
chowski, F. Bartl, B. Brzezinski, A. Mickiewicz, J. Mol. Struct. 2000,
524, 217–225.
J. Phys. Org. Chem. 2009, 22 460–465
Copyright ß 2008 John Wiley & Sons, Ltd.
www.interscience.wiley.com/journal/poc