Kinetic study of the addition of trihalides to unsaturated compounds in ionic liquids. Evidence of a remarkable solvent effect in the reaction of ICl2-

Article abstract of DOI:10.1021/jo049318q

The kinetic constants and activation parameters for the reactions of Br₃^- and ICl₂^- with some alkenes and alkynes have been determined in the ionic liquids [bmim][PF₆], [emim][Tf₂N], [bmim][Tf₂N], [hmim][TF₂N], [bm₂im] [Tf₂N], and [bpy][TF₂N] (where emim = 1-ethyl-3-methyl-imidazolium, bmim = 1-butyl-3-methylimidazolium, hmim = 1-hexyl-3-methylimidazolium, bm₂im = 1-butyl-2,3-dimethylimidazolium, bpy = butylpyridinium, PF₆ = hexafluorophosphate, and Tf ₂N = bis(trifluoromethylsulfonyl)imide) and in 1,2-dichloroethane. The rates of both reactions increase on going from 1,2-dichloroethane to ILs. Evidence suggests that, while the hydrogen bonding ability of the imidazolium cation is probably the main factor able to increase the rate of the addition of ICl₂^- to double and triple bonds, this property has no effect on the electrophilic addition of Br₃^- to alkenes and alkynes. Furthermore, in the case of the ICl₂^- reaction, the hydrogen bonding ability of ILs can be exploited to suppress the unwanted nucleophilic substitution reaction on the products by the Cl ^- anion.

Full text of DOI:10.1021/jo049318q

Kin etic Stu d y of th e Ad d ition of Tr ih a lid es to Un sa tu r a ted
Com p ou n d s in Ion ic Liqu id s. Evid en ce of a Rem a r k a ble Solven t
-
Effect in th e Rea ction of ICl2
Cinzia Chiappe* and Daniela Pieraccini
Dipartimento di Chimica Bioorganica e Biofarmacia, via Bonanno 33, 56126 Pisa, Italy
cinziac@farm.unipi.it
Received April 23, 2004
-
-
The kinetic constants and activation parameters for the reactions of Br3 and ICl2 with some
alkenes and alkynes have been determined in the ionic liquids [bmim][PF6], [emim][Tf2N],
[bmim][Tf2N], [hmim][TF2N], [bm2im][Tf2N], and [bpy][TF2N] (where emim ) 1-ethyl-3-methyl-
imidazolium, bmim ) 1-butyl-3-methylimidazolium, hmim ) 1-hexyl-3-methylimidazolium, bm2im
) 1-butyl-2,3-dimethylimidazolium, bpy ) butylpyridinium, PF6 ) hexafluorophosphate, and Tf2N
) bis(trifluoromethylsulfonyl)imide) and in 1,2-dichloroethane. The rates of both reactions increase
on going from 1,2-dichloroethane to ILs. Evidence suggests that, while the hydrogen bonding ability
of the imidazolium cation is probably the main factor able to increase the rate of the addition of
ICl2- to double and triple bonds, this property has no effect on the electrophilic addition of Br3- to
alkenes and alkynes. Furthermore, in the case of the ICl2- reaction, the hydrogen bonding ability
of ILs can be exploited to suppress the unwanted nucleophilic substitution reaction on the products
by the Cl- anion.
-
In tr od u ction
about the use of ICl2 as an electrophile have been
reported6 also in molecular solvents. Recently, we have
shown2c that trihalide ILs can be generated by addition
of a proper halogen (Br2 or ICl) to an imidazolium halide
and that they can be used as reagent-solvents for the
stereospecific halogenation of double and triple bonds in
ILs. The addition of both trihalides to alkenes and
alkynes proceeds with complete anti stereoselectivity,
while the regioselectivity, observable in the case of
Recently, room-temperature ionic liquids (ILs) have
gained a great deal of attention due to their excellent
electrochemical properties, as well as their promise as
new green reaction media in organic synthesis.1 Because
of the potential importance of ILs as solvents for various
industrial processes, we have begun a systematic study
of their effects on rate constants of some representative
elementary reactions occurring through ionic intermedi-
ates, including electrophilic additions to double and triple
bonds,2 aliphatic nucleophilic substitutions,3 aromatic
electrophilic substitutions.4
-
the ICl2 addition, depends on the alkene or alkyne
structure.2 Aryl-substituted double or triple bonds give
exclusively the Markovnikov adducts, while alkyl-
substituted alkenes give mixtures of Markovnikov or
anti-Markovnikov adducts, the percentage depending on
the relative steric hindrance of the substituents.2c
In the present study, we examine the effect of some
widely employed ionic liquids on the kinetic behavior of
-
-
the addition of Br3 and ICl2 to alkenes and alkynes.
Herein, we report the kinetic constants and the activa-
tion parameters for the reactions of Br3- and ICl2- with
some alkenes and alkynes (compounds 1-3) in six
different ILs and in a typical molecular solvent for these
reactions, 1,2-dichloroethane (DCE).
-
The electrophilic properties of Br3 have been studied
extensively in chlorinated solvents,5 where this species
is generally produced by reaction of Br2 with a tetra-
alkylammonium bromide. At variance, only few data
(1) (a) Holbrey, J . D.; Seddon, K. R. Clean Products Processes 1999,
1, 223-236. (b) Earle, M. J .; Seddon, K. R. Pure Appl. Chem. 2000,
72, 1391-1398. (c) Welton, T. Chem. Rev. 1999, 99, 2071-2083. (d)
Wasserscheid, P.; Keim, M. Angew. Chem., Int. Ed. 2000, 39, 3772-
3789. (e) Sheldon, R. Chem. Commun. 2001, 2399-2407. (e) Olivier-
Bourbigou, H.; Magna, L. J . Mol. Catal. A 2002, 182, 419-437. (f)
Dupont, J .; de Souza, R. F.; Suarez, P. A. Z. Chem. Rev. 2002, 102,
3667-3692. (g) Wilkes, J . S. J . Mol. Catal. A 2004, 214, 11.
(2) (a) Chiappe, C.; Capraro, D.; Conte, V.; Pieraccini, D. Org. Lett.
2001, 3, 1061-1063. (b) Chiappe, C.; Conte, V.; Pieraccini, D. Eur. J .
Org. Chem. 2002, 2831-2837. (c) Bortolini, O.; Bottai, M.; Chiappe,
C.; Conte, V.; Pieraccini, D. Green Chem. 2002, 4, 621-627.
(3) Chiappe, C.; Pieraccini, D.; Saullo, P. J . Org. Chem. 2003, 68,
6710-6715.
(4) Chiappe, C.; Pieraccini, D. ARKIVOC 2002, 249-255.
(5) Bellucci, G.; Chiappe, C.; Lo Moro, G. J . Org. Chem. 1997, 62,
3176-3182 and references therein.
(6) Kajigaeshi, S.; Morikavi, M.; Fujisaki, S.; Okamoto, T. Bull.
Chem. Soc. J pn. 1990, 63, 3033. Sket, B.; Zupet, P.; Zupan, M.
Tetrahedron 1990, 46, 2503.
10.1021/jo049318q CCC: $27.50 © 2004 American Chemical Society
Published on Web 08/13/2004
J . Org. Chem. 2004, 69, 6059-6064
6059

Chiappe and Pieraccini
-
TABLE 1. Secon d -Or d er Ra te Con sta n ts for th e Rea ction of Br 3 w ith Alk en es a n d Alk yn es in Ion ic Liqu id s a n d DCE
a t 25 °Ca
1a
1b
1c
3a
3b
-1
-1
-1
-1
-1
solvent
kBr (M s-1
)
kBr (M s-1
)
kBr (M s-1
)
kBr (M s-1
)
kBr (M s-1
)
-
-
-
-
-
3
3
3
3
3
[bmim][PF6]
[emim][TF2N]
[bmim][TF2N]
[hmim][TF2N]
[bm2im][TF2N]
DCE
3.4 × 10-2
1.2 × 10-1
1.5 × 10-1
8.4 × 10-2
4.2 × 10-2
ndb
6.7 × 10-2
1.2 × 10-1
7.6 × 10-2
4.1 a 10-2
ndb
2.7 × 10-4
5.4 × 10-4
1.9 × 10-4
1.3 × 10-4
1.2 × 10-4
1.8 × 10-5
9.3 × 10-4
9.5 × 10-4
6.4 × 10-4
4.3 × 10-4
2.7 × 10-4
5.6 × 10-5
1.8 × 10-3
3.2 × 10-3
2.8 × 10-4
1.5 × 10-4
1.3 × 10-4
2.6 × 10-4
ndb
a
b
-
Standard deviations in the values of kBr were always less than 5% and more usually between 2 and 3% of the quoted values. Too
3
fast to be measured using a UV-vis spectrophotometer. nd ) not determined.
-
TABLE 2. Ap p a r en t Activa tion P a r a m eter s for th e Rea ction of Br 3 w ith 1c a n d 3a a
-1
kBr (298 K) (M s-1
)
Ea,obsd (kJ mol-1
)
∆Hq (kJ mol-1
)
∆Sq (J K-1 mol-1
)
-
3
1c
3a
DCE
1.8 × 10-5
1.9 × 10-4
6.2 × 10-4
5.6 × 10-5
9.3 × 10-4
9.5 × 10-4
6.4 × 10-4
2.7 × 10-4
69.6 (4)
68.2 (4)
56.1 (4)
67.8 (4)
57.8 (3)
55.7 (4)
60.5 (4)
61.7 (4)
67.1 (4)
65.7 (4)
53.6 (4)
65.3 (4)
55.3 (3)
53.2 (4)
58.0 (4)
59.2 (4)
-111 (2)
-95 (2)
[bmim][Tf2N]
[bpy][Tf2N]
DCE
[bmim][PF6]
[emim][TF2N]
[bmim][TF2N]
[bm2im][TF2N]
-127 (4)
-104 (2)
-118 (3)
-124 (2)
-113 (2)
-116 (2)
a
Standard deviations are given in parentheses.
SCHEME 1
The different kinetic behavior characterizing the two
addition reactions is discussed considering a different
charge development in the two transition states.
The rate constants for bromination of substrates 1c, 3a ,
and 3b with tetrabutylammonium tribromide ([Bu4N]-
[Br3]) in DCE are also reported in Table 1.
All the reactions were followed for at least two half-
lives. It is immediately evident that the reactions in ILs
are slightly accelerated (5-30 times) in comparison to
the molecular solvent (DCE) and that the values of
kinetic constants depend on the IL structure. In Table 2
are reported the activation parameters related to the
reaction of alkene 1c and alkyne 3a .
Kinetic and product distribution data2 suggest a very
similar mechanism for the reaction in ILs and in the
molecular solvent.
Resu lts a n d Discu ssion
Br om in a tion Ra tes. The kinetic constants for bro-
mination of several alkenes and alkynes, 1-pentene (1a ),
styrene (1b), ethyl trans-cinnamate (1c), 1-phenylacet-
-
ylene (3a ), and phenylpropyne (3b), using [bmim][Br3
]
as a halogenating agent, were determined in six ionic
liquids, [bmim][PF6], [emim][Tf2N], [bmim][Tf2N], [hmim]-
[TF2N], [bm2im][Tf2N], and [bpy][TF2N] (where emim )
1-ethyl-3-methylimidazolium, bmim ) 1-butyl-3-methyl-
imidazolium, hmim ) 1-hexyl-3-methylimidazolium,
bm2im ) 1-butyl-2,3-dimethylimidazolium, bpy ) butyl-
pyridinium, PF6 ) hexafluorophosphate, and Tf2N ) bis-
(trifluoromethylsulfonyl)imide). In all examined ILs, the
reaction gave the corresponding dibromo adducts arising
from an anti stereospecific addition process.
The reaction rates were measured spectrophotometri-
cally, under pseudo-first-order conditions (in the presence
of a large excess of alkyne or alkene), by monitoring the
disappearance of Br3-. All reactions obeyed the second-
order rate law of eq 1
In chlorinated solvents (DCE, chloroform, dichloro-
-
methane) it has been shown5,7 that the addition of Br3
to alkenes and alkynes takes place through a concerted
mechanism, reported in Scheme 1. The process involves
as a first step an equilibrium between two complexes of
-
bromine, that bearing to Br3 and that giving the 1:1
alkyne-Br2 π-complex (Kπ-complex). The equilibrium con-
stant KC is given by the ratio of the formation constants
-
of the two species; KC ) Kπ-complex/KBr . The product- and
3
the rate-determining nucleophilic attack of the bromide
ion on the initially formed alkene- or alkyne-Br2
π-complex gives directly the anti addition product. Prac-
tically no intermediate is formed in this reaction; the
bond-making and bond-breaking steps are indeed con-
-d[Br3-]/dt ) kBr -[Br -][S]
(1)
3
3
where [S] is the substrate (alkene or alkyne) concentra-
(7) Bianchini, R.; Chiappe, C.; Lo Moro G.; Lenoir, D.; Lemmen, P.;
Golberg, N. Chem. Eur. J . 1999, 5, 1570-1580.
-
tion. The kBr values at 25 °C are reported in Table 1.
3
6060 J . Org. Chem., Vol. 69, No. 18, 2004

Halogenation in Ionic Liquids
certed, although not necessarily simultaneous. Almost
pentacoordinated carbon atoms characterize the charge-
diffuse rate-determining transition state.
ing.11 In a reaction of the type of Br3- addition, in which
both bond making and bond breaking occur in the
transition state, the effect of the viscosity depends on the
position of the transition state. At variance with the
situation characterizing the addition in all the other ILs,
in [bmim][Br] the reaction occurs in the presence of a
large excess of Br-. This anion is probably the true
nucleophile of the addition process, and its presence in a
large excess may affect the position of the transition
state; bond making may prevail over bond breaking.
Viscosity can in this case enhance the reaction rate. At
variance, in all the other ILs a later transition state is
more probable and the effect of the viscosity decreases,
becoming the opposite if the bond breaking is more
advanced with respect to the bond making.
This mechanism is completely different from that
characterizing the addition of Br2 to alkenes and alkynes
in chlorinated solvents, which occurs through the for-
mation of an ionic intermediate, a bromiranium (or
bromirenium), or â-bromocarbenium tribromide ion pair.5,7
Although Br2 is a much better electrophile than Br3-, due
to the high formation constant of the tribromide ion in
chlorinated solvents,5,7 the addition of free Br2 is not able
to compete with the addition of Br3-. Also in ILs the
-
charge-diffuse Br3 anion is much more stable than the
Br- species;8 practically no free bromine is present in
-
solution,2c and Br3 is the sole electrophile both in DCE
and ILs.2c In DCE, an aprotic solvent of medium polarity,
the electrophile (Br3-) and the nucleophilic species (the
The values of the kinetic constants and activation
parameters, which are really apparent activation param-
-
formed Br- anion or the same Br3 anion) are surely
-
eters being kBr the product of two constants (KC and k2),
3
present in solution as ion pairs with the tetrabutyl-
ammonium cation. Since the activation parameters in ILs
are very similar to the parameters characterizing the
reaction of [Bu4N][Br3] in DCE, much like in the ionic
media the anions are coordinated by one or more cations
of the ionic liquid, forming species very similar to ion
pairs. The presence of ion pairs in ILs has been proposed9
also by Welton and co-worker on the basis of the kinetic
behavior of nucleophilic substitution reactions in some
ILs and dichloromethane.
Although we propose the same mechanism in the two
media (ionic and molecular), it is evident from the data
reported in Table 1 that the reactions in ILs are generally
faster that in DCE, showing that ILs are able to affect
the stability of reactants and/or transition state, probably
the latter being more important.
cannot give direct information about the properties of ILs.
Anyway, the comparison with the values related to the
reaction in molecular solvents may be useful for increas-
ing the knowledge of these new reaction media.
In chlorinated solvents, the reaction rate increases on
going from more polar DCE to less polar chlorinated
solvents, in agreement with the formation of a transition
state having more charge delocalization than the reac-
tants. Furthermore, reaction rate is positively affected
by the ability of the solvent (for example, chloroform) to
give hydrogen bonding.5 In particular, it was suggested5
that the electrophilic solvatation by hydrogen bonding
to the leaving bromide ion is the main factor affecting
reactivity when the reaction is carried out in chloroform.
Using solvatochromic dyes and partition methods, Welton
et al. have shown that the examined ILs have a different
hydrogen bonding acidity; [bmim][Tf2N] should be among
the employed ILs, having the higher hydrogen bonding
ability.12 Actually, the data reported in Table 2 show that
the activation parameters are very similar in all inves-
tigated ILs. Therefore, they seem to be not significantly
affected by the different ability of the investigated ILs
to give hydrogen bonding, although generally the activa-
tion enthalpies are lower in ILs than in DCE. The
apparent lower ability of the ionic liquids, with respect
chloroform, to affect reactivity through hydrogen bonding
can be a consequence of the fact that this effect is due to
the IL cation. Cation exerts the hydrogen bonding donor
ability both on the leaving and on the entering Br- anion.
If bond making and bond breaking occur simultaneously
in the rate-determining step, the hydrogen bonding
ability of the IL has practically no effect. At variance, in
chloroform the nucleophilic Br- is present in solution as
a tight ion pair with the counteranion, the tetraalkyl-
ammonium cation, and therefore the formation of hydro-
gen bonding between the entering Br- and the solvent
is not important. The solvent can instead hydrogen bond
with the liberated leaving group (the Br- anion), favoring
the Br-Br bond breaking. Therefore, although we hy-
pothesize the same mechanism in molecular and ionic
In a previous study on bromination of alkynes (includ-
ing 3a and 3b) in [bmim][Br], a very viscous IL, we have
proposed2b the hypothesis that the viscosity of the reac-
-
tion medium might affect the rate of Br3 addition.
Particularly, viscosity seemed2b to increase the kinetic
-
constant when, as in the case of alkyne 3a (kBr ) 1.25
3
× 10-3 M-1s-1), the substrate structure favored an earlier
transition state, in which bond making is favored with
respect to bond breaking. Since the viscosity of the ionic
liquids used in this work has been recently determined,10
we first checked if, also under the new conditions, the
viscosity played a main role in the bromination of 3a . At
25 °C, the order of increasing viscosity is [emim][Tf2N]
< [bmim][Tf2N] = [bpy][Tf2N] < [hmim][Tf2N] < [bm2im]-
[Tf2N] < [bmim][PF6] , [bmim][Br]. The data reported
in Table 1 for alkyne 3a clearly show that the reaction
rates do not depend on the viscosity of the ionic liquid
alone. A trend may be found only considering a more
homogeneous class of ILs, [emim][Tf2N] < [bmim][Tf2N]
< [hmim][Tf2N], although in this case the reaction rates
increase as the solvent viscosities decrease. Actually, the
viscosity can have opposite effects on the reaction rate;
indeed it favors bond making but disfavors bond break-
(8) Grodkowski, J .; Neta, P. J . Phys. Chem. A 2002, 106, 11130-
11134.
(9) Lancaster, N. L.; Salter, P. A.; Welton, T.; Young, G. B. J . Org.
Chem. 2002, 67, 8855-8861.
(10) (a) Dzyuba, S.; Bartsch, R. A. ChemPhysChem 2002, 3, 161-
166. (b) Carda-Broch, S.; Berthold, A.; Armstrong, D. W. Anal. Bioanal.
Chem. 2003, 375, 191-199. (c) Okoturo, O. O.; VanderNoot, T. J . J .
Electroanal. Chem. 2004, 568, 167-181.
(11) Swiss, K. A.; Firestone, R. A. J . Org. Chem. 1999, 103, 5369-
5372.
(12) (a) Crowhurst, L.; Mawdsley, P. R.; Perez-Arlandis, J . M.;
Salter, P. A.; Welton, T. Phys. Chem. Chem. Phys. 2003, 5, 2790-2794.
(b) Anderson, J . L.; Ding, J .; Welton, T.; Armstrong, D. W. J . Am. Chem.
Soc. 2002, 124, 14247-14254.
J . Org. Chem, Vol. 69, No. 18, 2004 6061

Chiappe and Pieraccini
-
TABLE 3. Secon d -Or d er Ra te Con sta n ts for th e Rea ction of ICl2 w ith Alk en es a n d Alk yn es in Ion ic Liqu id s a n d DCE
a t 25 °Ca
1a
1b
1d
1e
2
3a
-1
-1
-1
-1
-1
-1
solvent
kICl (M s-1
)
kICl (M s-1
)
kICl (M s-1
)
kICl (M s-1
)
kICl (M s-1
)
kICl (M s-1
)
-
-
-
-
-
-
2
2
2
2
2
2
[bmim][PF6]
[emim][TF2N]
[bmim][TF2N]
[hmim][TF2N]
[bpy][TF2N]
DCE
7.1 × 10-4
4.1 × 10-3
5.3 × 10-4
1.0 × 10-3
1.8 × 10-3
1.1 × 10-4
5.8 × 10-4
1.6 × 10-3
7.8 × 10-4
2.8 × 10-4
9.6 × 10-4
ndc
1.8 × 10-3
1.0 × 10-2
4.7 × 10-3
3.6 × 10-3
7.6 × 10-3
3.6 × 10-4
7.0 × 10-3
1.1 × 10-2
7.2 × 10-3
5.2 × 10-3
1.1 × 10-2
5.9 × 10-4
2.6 × 10-3
3.4 × 10-5
7.7 × 10-5
3.1 × 10-5
2.9 × 10-5
5.9 × 10-5
ndc
ndb
7.3 × 10-3
4.9 × 10-3
1.0 × 10-2
7.5 × 10-4
a
b
-
Standard deviations in the values of kICl were always less than 5% and more usually between 2 and 3% of the quoted values. Too
2
fast to be measured using a UV-vis spectrophotometer. c Rapid darkening of the solution prevented the evaluation of the kinetic constant.
solvents, the hydrogen bonding between Br- and the
SCHEME 2
solvent can play a different role in the two media.
Iod in a tion Ra tes. The kinetic behavior of the addition
-
of ICl2 to several alkenes (1-pentene, 1a ; styrene, 1b;
trans-2-pentene, 1d ; cis-2-pentene, 1e; cyclopentene, 2)
and to phenylacetylene, 3a , has been examined both in
ILs and DCE, using in this latter case tetrabutylammo-
-
nium ICl2 ([Bu4N][ICl2]) as an iodochlorinating agent.
The kinetics of iodochlorination were measured spectro-
-
photometrically by monitoring the disappearance of ICl2
-
at 336 nm (absorption maximum of ICl2 in ILs). They
were carried out under pseudo-order conditions, working
in the presence of at least a 10-fold excess of the
substrate. All reactions obeyed the second-order rate law
of eq 2
-d[ICl2-]/dt ) kICl -[ICl -][S]
(2)
-
While the rates of the electrophilic addition of ICl2 to
double and triple bonds increase on going from DCE to
ILs, the substitution reactions are generally slower in
these latter media. In particular, the decomposition of
the ICl2- species may be completely repressed performing
the reaction in the ILs having the higher hydrogen
bonding donor ability. These solvents not only increase
the rate of the addition process but also reduce the
nucleophilicity of Cl- anion, reducing the rate of the
substitution process, in agreement with the recently
reported data about nucleophilic substitution reactions
in ILs.9 In these ILs, it is therefore generally possible to
isolate the corresponding iodochloro derivatives in high
yield, without observation of reactant decomposition and
formation of the corresponding dichloro adducts.
2
2
where [S] is the substrate (alkene or alkyne) concentra-
-
tion. The kICl values at 25 °C are reported in Table 3.
2
The rate constants for iodochlorination of substrates 1a ,
1d , 1e, and 2 with [Bu4N][ICl2] in DCE are also reported
in Table 3. All the reactions carried out in ILs were
followed for at least two half-lives. At variance, only the
initial rate constants (<10% of conversion) could be
measured in the case of the reactions performed in DCE,
due to the progressive darkening of the solutions. In the
case of 1b and 3a , the darkening of the solutions was so
fast that also the initial rate constants could not be
evaluated. This phenomenon has been attributed to the
formation of I3-. The formation of this species is probably
a consequence of the reaction of Cl- with the addition
product(s), the iodochloro adduct(s). This substitution
reaction gives the corresponding dichloro derivative(s)
and an increase in the amount of I- (see Scheme 2). The
formation of small amounts of dichloro adducts has been
The complete anti stereoselectivity,2c which character-
izes this reaction as well as the addition of Br3-, associ-
ated with the similar kinetic behavior suggests for this
reaction a mechanism analogous to that proposed for the
addition of Br3-. However, the increase in the reaction
rate observed for the addition of ICl2- on going from DCE
to ILs is generally higher than the increase character-
-
evidenced frequently in the reactions of ICl2 addition
to alkenes, in particular when the alkene structure favors
the substitution process (arylalkenes).
-
izing the addition of Br3 in the same solvents. This is
The I- anion reacts with ICl2- to gives I3-, through the
multiequilibria pathway reported in Scheme 2. The
in agreement with a different charge distribution in the
two transition states or a different stabilization of the
two ground states.
Also for the addition of ICl2- the reaction rates do not
correlate with IL viscosity alone. Furthermore, at vari-
-
formation of I3 has been confirmed by the appearance
of the corresponding absorption band in the UV spec-
trum. This anion has in ILs a peculiar absorption band
centered at 365 nm. Therefore, on the basis of these
newer results, we attribute the very low reaction rates,
previously observed2c using [Bu4N][ICl2-] as a reagent in
chlorinated solvents, to the partial transformation of the
-
ance with the Br3 addition, the activation parameters
(reported in Table 4 for the reaction of olefin 1d ) show a
higher dependence on the IL structure, although the
activation energy values, ∆G298q, are very similar in all
examined ILs.
The constancy of ∆Gq may be explained considering the
possibility of having in these solvents an isokinetic
-
-
ICl2 species in I3
.
On the contrary, the substitution reaction is not able
to compete significantly with the addition process in ILs.
6062 J . Org. Chem., Vol. 69, No. 18, 2004

Halogenation in Ionic Liquids
-
TABLE 4. Ap p a r en t Activa tion P a r a m eter s for th e Rea ction of ICl2 w ith 1d a
KICl (298 K) (M s-1
)
Ea,obsd (kJ mol-1
)
∆Hq (kJ mol-1
)
∆Sq (J K-1 mol-1
)
∆G298q (kJ mol-1)
-1
-
2
DCE
3.6 × 10-4
1.0 × 10-2
4.7 × 10-3
1.8 × 10-3
7.6 × 10-3
1.8 × 10-3
30.6 (3)
33.3 (5)
17.8 (1)
44.8 (3)
28.4 (3)
78.0 (6)
28.1 (3)
30.8 (5)
15.3 (1)
42.3 (3)
25.9 (3)
75.5 (6)
-215 (2)
-178 (1)
-238 (1)
-159 (1)
-198 (3)
-26 (1)
92
84
84
89.5
86
[emim][TF2N]
[bmim][TF2N]
[bm2im][TF2N]
[bpy][Tf2N]
[bmim][PF6]
83
a
Standard deviations are given in parentheses.
F IGURE 1. Enthalpy-entropy compensation plot for the
reaction of 1d with ICl2 in ILs (•) and DCE (9).
-
-
F IGURE 2. Arrhenius plots for the reaction of 1d with ICl2
in ILs.
relationship (IKR) and/or a compensation effect. Similari-
ties and differences between these two concepts have
been recently discussed.13
to indicate that IKR does not exist, or if existing it is not
inclusive of all the solvents. The existence of an isokinetic
relationship is usually taken to mean that the reaction
parameter that is varied, in this case the solvent, is
operating on only one type of interaction in the system,
while the absence can point to a complex interplay of
effects.
According to the definition, the linear correlation found
between enthalpies and entropies of activation (Figure
-
1) for the reaction of 1d with ICl2 in the examined ILs
and DCE clearly illustrates the presence of a compensa-
tion effect.
A correlation of this type has been recently found also
for a diffusion-controlled process carried out in several
imidazolium ILs and some molecular solvents.14 However,
such linear correlations between ∆Hq and ∆Sq are often
coincidental or are the fruit of an inappropriate or
incomplete description of the kinetic model, and there-
fore caution is required in their interpretation. A true
test of the existence of an IKR is generally considered13
the behavior shown by the Arrhenius plots. An IKR
would result in a convergence of the Arrhenius lines to a
single point, referred as the isokinetic temperature (IKT),
where the difference among the rate constants is at a
minimum.
Examination the Arrhenius plots for the reaction of 1d
in the investigated ILs (Figure 2) shows a convergence
of the lines with a similar intersection point for [emim]-
[Tf2N], [bmim][Tf2N], and [bpy][Tf2N], but deviations can
be evidenced for [bmim][PF6] and [bm2im][Tf2N], which
present another intersection point.
In our case the presence of two intersection points
seems to indicate that not only the hydrogen bonding
ability but also at least another factor are able to affect
the rate of ICl2- addition. Considering the physicochem-
ical properties of the examined ILs, we may hypothesize,
as explained below, that the hydrogen bonding donor
ability of the IL is the main or sole factor affecting
reactivity in [emim][Tf2N], [bmim][Tf2N], and [bpy][Tf2N],
while in the other two solvents there is the additional
contribution of the viscosity. The viscosities of [bmim]-
[PF6] and [bm2im][Tf2N] are indeed significantly different
from those of the other ILs.
With more detailed examination of the activation
parameters, it emerges that in [bmim][Tf2N], at variance
with the behavior in the other ionic solvents, the activa-
tion enthalpy is strongly reduced, while the ∆Sq value is
significantly increased. Considering that this is the IL
having the relatively higher hydrogen bonding donor
ability, the activation parameters seem to suggest that
this solvent property is probably the main factor affecting
the reaction rate. In agreement with this hypothesis, the
activation enthalpy increases (and the activation entropy
decreases) when the reaction is performed in an ionic
liquid having a cation less suitable to give hydrogen
bonding such as [bm2im] or [bpy]. In the imidazolium
salts, the presence of a methyl group at C-2 significantly
reduces the hydrogen bond acidity of the imidazolium
cation. Analogously, the pyridinium cation is expected
to form weaker, if any, hydrogen bonds. Furthermore, it
Since the two points are not very far from each other,
a program for linear regressions with a common point of
intersection has been applied to establish if the deviations
may be due to experimental errors.15 Calculation seems
(13) Liu, L.; Gou, Q.-X Chem. Rev. 2001, 101, 673-695.
(14) McLean, A. J ., Muldoon, M. J .; Gordon, C. M. Dunkin, I. R.
Chem. Commun. 2002, 1880-1881.
(15) Ouvrard, C.; Berthelot, M.; Lamer, T.; Exner, O. J . Chem. Inf.
Comput. Sci. 2001, 41, 1141-1144. IKR Program: The Isokinetic
Relationship Program. http://www.sciences.univ-nantes.fr/spectro/
publicat.htm (accessed March, 2004).
J . Org. Chem, Vol. 69, No. 18, 2004 6063

Chiappe and Pieraccini
SCHEME 3
of a conventional solvent with a single ionic liquid not
necessarily implying that the new conditions are the best
ones. The ability of the ionic liquids to affect the reaction
rate depends, however, on the reaction; the addition of
-
ICl2 is more affected by the ionic liquid structure than
the analogous reaction of Br3-, although both are faster
in ionic liquids than in molecular solvents.
Finally, important information about the features of
ionic liquids and on the correlation between structure-
solvent properties can be obtained from kinetic study of
well-known reactions. These results show the potential
effects of the cations of the ILs as hydrogen bonding
donors in electrophilic addition reactions in ionic liquids
and corroborate previous data17 showing that the capa-
bility of imidazolium ILs for hydrogen bonding can be
exploited to suppress unwanted substitution reactions.
is more frequently claimed16 that a strong anion-cation
association such as that characterizing [bmim][PF6] is
able to reduce the cation hydrogen bonding ability. It is
worth noting that the reaction in this latter IL is
characterized by the higher value of the activation
enthalpy but by the lower activation entropy. This
behavior may be attributed to the greater cross-linking
that distinguishes the ILs bearing the [PF6]- anion
compared to [Tf2N]-. A similar behavior has been ob-
served14 also previously for the triplet energy transfer
from triplet benzophenone to naphthalene.
Exp er im en ta l Section
P r ep a r a tion of [bm im ][Br 3] a n d [bm im ][ICl2]. An ap-
propriate amount of [bmim][Cl] or [bmim][Br] was weighed
in a calibration flask, and the IL was refrigerated at 0 °C. An
equimolar amount of ICl or Br2 was added at the same
temperature, and the trihalide ionic liquid obtained was stored
in the dark. Attention was paid to avoid contamination of the
solution from atmospheric humidity.
Kin etic Measu r em en ts. Solutions of [bmim][Br3] or [bmim]-
[ICl2] in ILs were prepared by weighing the halogenating
species into known volumes of the desired ionic liquid.
Analogously, solutions of [Bu4N][Br3] in DCE were prepared
by weighing the reagent into known volumes of the solvent.
Solutions of [Bu4N][ICl2] in DCE were prepared adding an
equimolar amount of ICl to solutions of [Bu4N][Cl] in DCE.
Concentrations were also checked by UV measurements.
All these solutions were prepared shortly before use, pro-
tected from daylight, and adjusted to twice the desired initial
concentrations in the kinetic runs. Aliquots of these pre-
thermostated solutions were mixed with equal volumes of
prethermostated solutions of the unsatured compounds of
suitable concentrations.
The reactions of compounds 1a , 1b, 1d , 1e, 2, and 3d with
[bmim][ICl2] in ionic liquids or with [Bu4N][ICl2] in DCE were
carried out at halogen concentrations ranging from 1 x 10-2
to 1 x 10-1 M in the presence of at least a 10-fold excess of the
unsatured compound, following the disappearance of the
dichloroiodate at 336 nm. For the reactions of compounds 1a -c
and 3a ,b with [bmim][Br3] in ionic liquids or with [Bu4N][Br3]
in DCE, a halogen concentration ranging from 1 x 10-3 to 1 x
10-2 M was used and the disappearance of the halogen was
followed at 336 nm. The experiments were repeated at least
in triplicate and accepted on condition of a 5% maximum
difference of the respective absorption/time curves. The second-
Finally, it must be remarked that the activation
parameters for the reaction in [emim][Tf2N] are quite
different from those related to the same reaction in
[bmim][Tf2N], despite the structural similarity between
these two ILs, suggesting a different ability of the two
cations to give hydrogen bonding. This may be attributed
to a different anion-cation interaction.
-
In conclusion, the solvent effect in the addition of ICl2
to double bonds can be explained by the degree of
stabilization of the leaving chloride ion via hydrogen
bonding to the cation of the ionic liquid. The different
sensibility showed by the two investigated reactions
(addition of ICl2- and Br3-) toward the hydrogen bonding
ability of the IL may be related to a different charge
distribution in the respective transition states, which is
probably a consequence of the different hydrogen bonding
acceptor ability of the two departing halides. The inter-
action IL cation‚‚‚chloride should be stronger than
cation‚‚‚bromide.
We retain therefore that in the case of ICl2- addition,
the bond breaking precedes bond making, the transition
state has greater iodiranium character (in this case, we
cannot exclude the formation of a iodiradium intermedi-
ate), and the attack by chloride is not in itself the rate-
determining step, although it might be part of a concerted
process. In this situation, the ability of the IL to form
hydrogen bonding with the leaving Cl- plays a main role
(Scheme 3).
-
-
order rate costants, kBr and kICl , obtained by fitting the
absorption/time data to the approp2riate integrated rate equa-
tion, are reported in Tables 1 and 3. The apparent activation
parameters, reported in Tables 2 and 4, were obtained from
Arrhenius plots.
3
Con clu sion
-
In conclusion, this work shows that ICl2 may be a
useful electrophile for the functionalization of double or
triple bonds in particular when it is used in ionic liquids,
where the higher stability of the product(s) toward
substitution reaction reduces or avoids the formation of
dichlorides and the decomposition of the reactant.
Furthermore, it supports the more recent data showing
that not all ionic liquids are the same, the replacement
Ack n ow led gm en t. We thank MIUR and University
of Pisa for funding this project.
Su p p or tin g In for m a tion Ava ila ble: General experimen-
tal methods. This material is available free of charge via the
Internet at http://pubs.acs.org.
J O049318Q
(16) Aggarwal, A.; Lancaster, N. L.; Sethi, A. R.; Welton T. Green
Chem. 2002, 4, 517-520. Chiappe, C.; Imperato, G.; Napolitano, E.;
Pieraccini, D. Green Chem. 2004, 6, 33-36.
(17) Ross, J .; Xiao, J . Chem. Eur. J . 2003, 9, 4900-4906.
6064 J . Org. Chem., Vol. 69, No. 18, 2004