Reverse micellar aggregates: Effect on ketone reduction. 2. Surfactant role

Article abstract of DOI:10.1021/jo049172v

Kinetics of the reduction of 3-chloroacetophenone (CAF) with sodium borohydride (NaBH₄) were followed by UV-vis spectroscopy at 27.0 °C in different reverse micellar media, toluene/BHDC/ water and toluene/AOT/water, and compared with results in an isooctane/AOT/water reverse micellar system. AOT is sodium 1,4-bis-2-ethylhexylsulfosuccinate, and BHDC is benzyl-n-hexadecyl dimethylammonium chloride. The kinetic profiles were investigated as a function of variables such as surfactant and NaBH₄ concentration and the amount of water dispersed in the reverse micelles, W ₀ = [H₂O]/[surfactant]. In all cases, the first-order rate constant, k_obs, increases with the concentration of surfactant as a consequence of incorporating the substrate into the interface of the reverse micelles where the reaction takes place. The reaction is faster at the cationic interface than at the anionic one probably because the negative ion BH ₄^- is part of the cationic interface. The effect of the external solvent on the reaction shows that reduction is favored in the isooctane/ AOT/water reverse micellar system than that with an aromatic solvent. This is probably due to BH₄^- being more in the water pool of the toluene/AOT/water reverse micellar system. The kinetic profile upon water addition depends largely on the type of interface. In the BHDC system, k_obs increases with W₀ in the whole range studied while in AOT the kinetic profile has a maximum at W₀ ～5, probably reflecting the fact that BH₄^- is part of the cationic interface while, in the anionic one, there is a strong interaction between water and the polar headgroup of AOT below W₀ = 5 and, above that, BH ₄^- is repelled from the interface once the water pool has formed. Application of a kinetic model based on the pseudophase formalism, which considers the distribution of the ketone between the continuous medium and the interface and assumes that reaction takes place only at the interface, has enabled us to estimate rate constants at the interface of the reverse micellar systems. At W₀ < 10, it was considered that NaBH₄ is wholly at the interface and, at W₀ ≥ 10, where there are free water molecules, also the partitioning between the interface and the water pool was taken into account. The results were used to evaluate CAF and NaBH ₄ distribution constants between the different pseudophases as well as the second-order reaction rate constant of the reduction reaction in the micellar interface.

Full text of DOI:10.1021/jo049172v

Reverse Micellar Aggregates: Effect on Ketone Reduction. 2.
Surfactant Role
N. Mariano Correa, Daniel H. Zorzan, Loredana D’Anteo,
Ermanno Lasta, Marco Chiarini, and Giorgio Cerichelli*
Dipartimento di Chimica Ingegneria Chimica e Materiali. Facolt a` di Scienze MM FF NN,
Universit a` degli Studi dell’Aquila, L’Aquila 67100, Italy, and INCA Consorzio Interuniversitario
Nazionale “La Chimica per l’Ambiente” UdR Aquila1
cerichel@univaq.it
Received May 17, 2004
4
Kinetics of the reduction of 3-chloroacetophenone (CAF) with sodium borohydride (NaBH ) were
followed by UV-vis spectroscopy at 27.0 °C in different reverse micellar media, toluene/BHDC/
water and toluene/AOT/water, and compared with results in an isooctane/AOT/water reverse
micellar system. AOT is sodium 1,4-bis-2-ethylhexylsulfosuccinate, and BHDC is benzyl-n-hexadecyl
dimethylammonium chloride. The kinetic profiles were investigated as a function of variables such
as surfactant and NaBH
4
concentration and the amount of water dispersed in the reverse micelles,
W
0
) [H O]/[surfactant]. In all cases, the first-order rate constant, kobs, increases with the
2
concentration of surfactant as a consequence of incorporating the substrate into the interface of
the reverse micelles where the reaction takes place. The reaction is faster at the cationic interface
-
than at the anionic one probably because the negative ion BH
4
is part of the cationic interface.
The effect of the external solvent on the reaction shows that reduction is favored in the isooctane/
AOT/water reverse micellar system than that with an aromatic solvent. This is probably due to
-
BH
4
being more in the water pool of the toluene/AOT/water reverse micellar system. The kinetic
profile upon water addition depends largely on the type of interface. In the BHDC system, kobs
increases with W in the whole range studied while in AOT the kinetic profile has a maximum at
∼5, probably reflecting the fact that BH
one, there is a strong interaction between water and the polar headgroup of AOT below W
0
-
W
0
4
is part of the cationic interface while, in the anionic
) 5
4
is repelled from the interface once the water pool has formed. Application of
0
-
and, above that, BH
a kinetic model based on the pseudophase formalism, which considers the distribution of the ketone
between the continuous medium and the interface and assumes that reaction takes place only at
the interface, has enabled us to estimate rate constants at the interface of the reverse micellar
systems. At W
where there are free water molecules, also the partitioning between the interface and the water
pool was taken into account. The results were used to evaluate CAF and NaBH distribution
0
< 10, it was considered that NaBH
4 0
is wholly at the interface and, at W g 10,
4
constants between the different pseudophases as well as the second-order reaction rate constant
of the reduction reaction in the micellar interface.
Introduction
entrapped in a film of surfactant and dispersed in a low-
polarity bulk solvent.1
-19
Several surfactants are able to aggregate in nonaque-
ous solvents to yield reverse micellar systems. In recent
years, these systems have attracted considerable atten-
tion, particularly because of their resemblance to biomem-
branes and their catalytic properties in chemical and
enzymatic reactions. These systems have a molecular
heterogeneity due to the amphiphilic nature of the
surfactant which forms the interface between water and
the nonaqueous solvent. Small solute particles can be
located in three different compartments: (a) the external
organic solvent, (b) the micellar interface formed by a
surfactant monolayer, and (c) the internal water pool. In
this way, these systems contain aqueous microdroplets
Among the anionic surfactants that form reverse
micelles, the best known are the systems derived from
the AOT (sodium 1,4-bis-2-ethylhexylsulfosuccinate) in
(
1) Fendler, J. H. Membrane Mimetic Chemistry; Wiley: New York,
1982.
(
2) Pileni, M. P. Structure and Reactivity of Reverse Micelles;
Elsevier: New York, 1989.
(3) De, T. K.; Maitra, A. Adv. Colloid Interface Sci. 1995, 59, 95.
(
4) Moulik, S. P.; Paul, B. K. Adv. Colloid Interface Sci. 1998, 78,
9
9.
(
5) Silber, J. J.; Biasutti, M. A.; Abuin E.; Lissi, E. Adv. Colloid.
Interface Sci., 1999, 82, 189.
(
6) Garc ´ı a-R ´ı o, L.; Leis, J. R.; Pena, E.; Iglesias, E. J. Phys. Chem.
1
993, 97, 3437.
(7) Correa, N. M.; Durantini E. N.; Silber, J. J. J. Org. Chem. 1999,
4, 5757.
6
(8) Correa, N. M.; Durantini, E. N.; Silber, J. J. J. Org. Chem. 2000,
*
To whom correspondence should be addressed.
65, 6427.
1
0.1021/jo049172v CCC: $27.50 © 2004 American Chemical Society
Published on Web 11/04/2004
J. Org. Chem. 2004, 69, 8231-8238
8231

Correa et al.
different nonpolar media. AOT has a well-known V-
shaped molecular geometry, giving rise to stable reverse
micelles without cosurfactant. AOT has the remarkable
ability to solubilize a large amount of water with values
water reverse micellar system and how the ketone
structure affects the kinetic results. Due to the lack of
kinetic study in different reverse micellar media, we
thought it worthwhile to kinetically investigate how
different cationic and anionic interfaces and external
solvents influence the reduction of 3-chloroacetophenone
(CAF) in reverse micelles.
of W
0
, (W
0
) [H O]/[AOT]), as large as 40-60 depending
2
on the surrounding nonpolar medium, the solute, and the
temperature; however, the droplets size depends only on
2,3,5
0
the water amount, W .
In this paper, we report data on the kinetics of
reduction by NaBH of the CAF in toluene/BHDC/water,
4
The system benzyl-n-hexadecyldimethylammonium chlo-
ride (BHDC) in toluene was chosen as a cationic polar
system in comparison with AOT. It is known that BHDC
forms reverse micelles in nonpolar solvents and water
toluene/AOT/water, and isooctane/AOT/water reverse
micelles. These systems were chosen to compare the
effects of organic solvents and anionic and cationic
interfaces over the kinetic profiles. The results show that
the reaction is faster in the cationic over the anionic
systems and the addition of water can accelerate or
retard the reaction depending on the nature of the
interface. On the other hand, isooctane as an external
solvent seems to allow the borohydride ion to be located
more into the interfacial region, probably shrinking it,
making reaction faster in isooctane/AOT/water than in
toluene/AOT/water reverse micellar system.
0
can be solubilized up to W of about 20 depending on the
external solvent. These reverse micelles seem to have
properties that are characteristics of other reverse mi-
cellar systems. This is brought out by the nature of the
water pool in BHDC, which shows that properties (NMR,
IR, QB absorption spectra) of bulk water are approached
only when more water than necessary for surfactant
solvation is present. Micellar sizes are similar to those
reported for AOT reverse micellar systems.1
1,20
4
Ketone reduction, by NaBH in the presence of sur-
factants in water, shows that micellar aggregates influ-
2
1-23
Experimental Section
ence regioselectivity and stereoselectivity.
Studies
performed in chiral reverse micellar systems24 show that
prochiral ketones are reduced to optically active alcohols.
A systematic study25 of the reaction performed in cetyl-
trimethylammonium bromide (CTAB) and cetyltrimeth-
ylammonium chloride (CTAC) in water shows, for a series
of aromatic ketones, a decrease (20-50-fold) of the
second-order reaction rate constants in the micellar
pseudophase relative to those in water due to specific
micellar interactions. In part 1 of our work (see ref 39),
we have shown results on the reduction in isooctane/AOT/
General Methods. UV-vis spectra were recorded on a
spectrophotometer equipped with a multiple cell holder ther-
mostated at 27.0 ( 0.1 °C. This temperature was chosen due
2
0
to the BHDC Kraft point. Control experiments of CAF
reduction by NaBH (under reverse micelle conditions) showed
4
that CAF disappears and the correspondent alcohol is the only
formed product (by GC analysis). Moreover, in the absence of
NaBH , no reaction occurred and there were no changes in
4
the UV-vis spectra.
Materials. 3-Chloroacetophenone (CAF 99%) and toluene
HPLC grade 99.7%) from Aldrich were used without further
(
purification. Sodium bis(2-ethylhexyl)sulfosuccinate (AOT) and
benzyl-n-hexadecyldimethylammonium chloride (BHDC) from
Sigma (>99%) were dried under reduced pressure and were
(
9) (a) Boscatto, L.; Chiacchiera, S. M.; Correa, N. M.; Durantini,
E. N.; Zingaretti, L.; Silber, J. J. Atual. Fis.-Quim. Org. Brasil 1999,
1, 534. (b) Correa, N. M.; Durantini, E. N.; Silber, J. J. Atual. Fis.-
Quim. Org. Brasil 2000, 12, 96.
10) Falcone, R. D.; Correa, N. M.; Biasutti, M. A.; Silber, J. J.
Langmuir 2002, 18, 2039.
11) Biasutti, M. A.; Correa, N. M.; Silber, J. J. In Molecular
1
2 5 4
kept under vacuum over P O until use. NaBH (98%) from
Acros Organics was used as received. Water was triply
distilled.
(
(
Procedures. A 0.5 M stock solution of surfactant was
prepared by dissolving the necessary amount of surfactant in
toluene. To obtain optically clear solutions, they were shaken
in a sonicating bath. The appropriate amount of stock solution
to obtain a given concentration of surfactant in the micellar
media was transferred into the cell, and the water was added
using a calibrated microsyringe. The amount of water present
in the system is expressed as the molar ratio between water
Interactions in Reverse Micelles Current Topics in Colloid and Interface
Research of Research Trends; Go n˜ i, F. M., Hoffmann, H., Eds.; Research
Trends: Trivandrum, India, 1999; Vol. 3, p 35.
(
12) Monduzzi, M.; Caboi, F.; Moriconi, C. Colloids Surf. A: Physi-
cochem. Eng. Aspects 1997, 129-130, 327.
13) Hamada, K.; Ikeda, T.; Kawai, T.; Kon-No, K. J. Colloid
Interface Sci. 2001, 233, 170.
(
(
14) Ken e´ z, P. H.; Carlstr o¨ m, G.; Fur o´ , I.; Halle, B. J. Phys. Chem.
992, 96, 9524.
15) Garc ´ı a-R ´ı o, L.; Leis, J. R.; Iglesias, E. J. Phys. Chem. 1995, 99,
1
(
and the surfactant (W
0 2
) [H O]/[surfactant]). The stock
1
2318.
solution of NaBH (0.1 M) was prepared by dissolving in a
4
(
(
16) Pileni, M. P. J. Phys. Chem. 1993, 97, 6961.
17) Luisi, P. L.; Straub, B. In Reverse Micelles, 8th ed.; Plenum
water solution of NaOH (0.1 M) the necessary amount of solid,
and the solution was stored in a refrigerator prior to use.
Press: London, 1984.
-
26-30
NaOH (0.1 M) in the water stabilizes BH
ate amount of stock solution of NaBH
reverse micellar system. Ionic concentrations in the reverse
micelles are referred to that of water, i.e., [NaBH ) 0.1 M.
When the surfactant concentration is changed at a given value
of W , total amounts of water and NaBH differ in every case,
4
.
The appropri-
(
18) Ballesteros, A.; Bornscheuer, U.; Capewel, A.; Combes, D.;
was added to the
Condoret, J. S.; Koenig, K.; Kolisis, F. N.; Marty, A.; Mengue, U.;
4
Scheper, T.; Stamatis H.; Xenakism, A. Biocatal. Biotransform. 1995,
1
3, 1.
(
4 w
]
19) Liou, J. Y.; Huang, T. M.; Chang, G. G. J. Chem. Soc., Perkin
Trans. 2, 1999, 2171.
0
4
(
20) McNeil, R.; Thomas, J. K. J. Colloid Interface Sci. 1981, 83,
but the ratio is always the same. The concentration of hydride
species was assumed to be four times that of the analytical
5
7.
(
21) (a) Nikles, J. A.; Sukenik, C. N. Tetrahedron Lett. 1982, 23,
4
4
211. (b) Jaeger, A. D.; Ward, M. D.; Martin, C. A. Tetrahedron 1984,
0, 2691. (c) Jursic, B.; Sunko, D. E. J. Chem. Res. 1988, 202.
(
22) Natrajan, A.; Ferrara, J. D.; Youngs, W. J.; Sukenik, C. N. J.
(26) Davis, R. E.; Swain, C. G. J. Am. Chem. Soc. 1960, 82, 5949.
(27) Davis, R. E.; Bromels, E.; Kibby C. L. J. Am. Chem. Soc. 1962,
84, 885.
(28) Davis, R. E. J. Am. Chem. Soc. 1962, 84, 892.
(29) Gardiner, J. A.; Collat, J. W. J. Am. Chem. Soc. 1965, 87, 1692.
(30) Kreevoy, M. M.; Hutchins, J. E. C. J. Am. Chem. Soc. 1972,
94, 6371.
Am. Chem. Soc. 1987, 109, 7477.
(
23) Zhang, Y.; Fan, W.; Lu, P.; Wang, W. Synth. Commun. 1988,
1
8, 1495.
(
(
24) Zhang, Y.; Sun, P. Tetrahedron: Asymmetry 1996, 7, 3055.
25) Cerichelli, G.; Coreno, M.; Mancini, G. J. Colloid Interface Sci.
1
993, 158, 33.
8232 J. Org. Chem., Vol. 69, No. 24, 2004

Reverse Micellar Aggregates
concentration31,32 due to the presence in the BH -
identical hydrogen atoms.
ion of four
4
Kinetics. Reactions were followed spectrophotometrically
by the disappearance of the absorbance of CAF at 300 nm,
-
3
[
CAF] ) 1.3 × 10 M at 27.0 ( 0.1 °C, using as blank a
0
solution of surfactant of identical concentration and W
start a kinetic run, a stock solution of the ketone was added
20 µL) to a thermostated cell containing NaBH in the reverse
micellar solution. First-order plots were obtained with an
excess of NaBH . Kinetics fitted the first-order integrated
. To
(
4
4
equation in a satisfactory way (r > 0.999) for the whole period
of time (almost 10t1/2). The mean standard deviation of the
kinetic data, using different samples, was less than 5%.
Determination of the Binding Constant of Ketones
(Kket) to AOT Reverse Micellar System. According to the
39
procedure followed in part 1, the absorbances of toluene
solutions of CAF at a fixed wavelength were recorded and
averaged with different AOT concentrations (0-0.4 M) at
W ) 5, 8, and 10, obtained by diluting the stock AOT solution
0
-
3
(
AOT 0.4 M, CAF 1.14 × 10 M) with the toluene CAF
solution. Spectra were run at 27.0 °C, the blank being a
solution of surfactant of identical concentration and W
Values of Kket were calculated with a least-squares fit to
0
.
3
3
eq 1
FIGURE 1. Dependence of kobs on the BHDC concentra-
tion for the reaction between CAF and NaBH in water/
0
BHDC/toluene reverse micellar system at different W .
4
f
b
(
ꢀ + ꢀ [AOT]Kket)[ketone]T
λ
A )
(1)
-3
[
[
CAF] ) 1.3 × 10 M. In all cases, [NaBH
4
]waterpool ) 0.1 M.
(
1 + Kket[AOT])
NaOH]waterpool ) 0.1 M. The dotted line shows the fitting by
λ
eq 8 for W ) 5 and eq 9 for W ) 10 and 20.
where A is the absorbance at different AOT concentrations
0
0
f
b
and ꢀ and ꢀ are the molar extinction coefficients for the
ketones in isooctane and at the interface of the reverse
micelles, respectively. [AOT] is the surfactant concentration
SCHEME 1
T
and [ketone] is the total ketone concentration. It should be
borne in mind that precise spectroscopic estimation of the
constants is ruled out by the impossibility of having all the
b
ketone at the interface, and therefore, the value of ꢀ can only
be estimated. The spectroscopic estimation of Kket was there-
the corresponding alcohol (Scheme 1) as in homogeneous
fore based on the analysis of the spectroscopic data using a
media,3 in direct micelles, and AOT reverse micellar
7,38
25
b
fitting procedure with the ꢀ as an adjustable parameter.
systems: see part 1.39
A large excess of NaBH was always used, and reac-
4
Kk e4 t- 3v6 alues were also determined by using Ketelaar’s
3
eq 2
tions follow first-order kinetics. Effects of the micro-
heterogeneous media were investigated as follows.
Reaction in Toluene/BHDC/Water Reverse Micel-
lar Systems. (1) Effects of BHDC Concentration.
Figure 1 shows the kinetic profile with varying BHDC
1
1
)
+
λtol
λ
f
b
A
- A
(ꢀ - ꢀ )[ketone]
T
1
1
(2)
f
b
[
AOT]
(
ꢀ - ꢀ )[ketone] K
T ket
concentrations at W ) 5, 10, and 20, where kobs increases
0
where Aλtol is the absorbance in toluene and the other terms
almost linearly with increasing [BHDC] over the whole
range. These results probably indicate that saturation
of CAF, in the micellar interface, is not reached in the
range of the [surfactant] used. Moreover, Figure 1 shows
that kobs increases with the water content (W0).
are as defined above. By plotting the left-hand side term of eq
2
vs 1/[AOT], Kket can be calculated from the slopes and
intercepts.
These procedures were not feasible for the CAF solution in
the BHDC reverse micelles due to very low values of the
distribution constants, probably below the limit of sensitivity
of the method. There were no spectral changes with the
variation of [BHDC] between 0 and 0.4 M.
(
2) Effect of NaBH
4
Concentration. Figure 2 shows
the effect of NaBH concentration on the reduction of
4
CAF by NaBH
BHDC and W
with [NaBH
4
in cationic reverse micelles at 0.2 M of
) 5, 10, and 20. kobs’s increase linearly
]. The reaction is faster at high water
0
Results
4
Reaction of CAF with NaBH
4
in toluene/BHDC/water
content as observed with varying [BHDC].
and toluene/AOT/water reverse micellar system yields
(3) Effect of Water Dispersed, W . Figure 3 shows
0
effects of changing W
NaBH
up to W
noticeably. The maximum amount of water (W
0
on kobs with [BHDC] ) 0.3 M and
(
31) Adams, C.; Gold, V.; Reuben, D. M. E. J. Chem. Soc., Perkin
Trans. 2 1977, 1466.
32) Sablayrolles, C.; Contastin, A.; Ducourant, B.; Fruchier, A.;
Chapat, J.-P. Bull. Soc. Chim. Fr. 1989, 467.
33) Sepulveda, L.; Lissi, E.; Quina, F. Adv. Colloid Interface Sci.
986, 25, 1.
34) Ketelaar, J. A. A.; Van de Stolpe, C.; Gersmann, H. R. Recl.
Trav. Chim. 1951, 70, 499.
35) Falcone, R. D.; Correa, N. M.; Biasutti, M. A.; Silber, J. J.
Langmuir 2000, 16, 3070.
36) Correa, N, M.; Durantini, E. N.; Silber, J. J. J. Colloid Interface
Sci. 2001, 240, 573.
[
W
]
4 w
) 0.1 M. The value of kobs increases slowly with
∼ 10, and then the rate constant increases
) that can
(
0
0
0
(
be reached in this system is around 22 and the minimum
1
(
(37) Brown, H.; Ichikawa, K. Tetrahedron 1957, 1, 214.
(38) Boyer, B.; Lamaty, G.; Pastor, J.-P.; Roque, J.-P. Bull. Soc.
Chim. Fr. 1989, 463.
(39) Correa, NM.; Zorzan, H. D., Chiarini, M.; Cerichellli, G. J. Org.
Chem. 2004, 69, 8224.
(
(
J. Org. Chem, Vol. 69, No. 24, 2004 8233

Correa et al.
FIGURE 4. Dependence of kobs on the AOT concentration for
FIGURE 2. Variation of kobs as a function of the NaBH
concentration for the reaction between CAF and NaBH in
water/BHDC/toluene reverse micellar system at different W
4
4
the reaction between CAF and NaBH in water/AOT/toluene
4
-
3
reverse micellar system at different W
M. In all cases, [NaBH
M. The dotted line shows the fitting by eq 8 for W
eq 9 for W ) 8 and 10.
0
. [CAF] ) 1.3 × 10
0
.
4
]waterpool ) 0.1 M. [NaOH]waterpool ) 0.1
-
3
[
CAF] ) 1.3 × 10 M. In all cases, [BHDC] ) 0.2 M.
0
) 5 and
[
NaOH]waterpool ) 0.1 M. The dotted line shows the fitting by
0
0 0
eq 8 for W ) 5 and eq 9 for W ) 10 and 20.
FIGURE 3. Variation of kobs as a function of W
reaction between CAF and NaBH in water/BHDC/toluene
reverse micellar system. [CAF] ) 1.3 × 10 M. In all cases,
0
for the
FIGURE 5. Variation of k as a function of the NaBH4
obs
4
concentration for the reaction between CAF and NaBH in
4
-
3
water/AOT/toluene reverse micellar system at different
-
3
[
BHDC] ) 0.3 M. [NaBH
4
]
waterpool ) 0.1 M. [NaOH]waterpool
)
W . [CAF] ) 1.3 × 10 M. In all cases, [AOT] ) 0.3 M.
0
0.1 M.
[NaOH]
waterpool
) 0.1 M. The dotted line shows the fitting by
eq 8 for W ) 5 and eq 9 for W ) 8 and 10.
0 0
0
W is 3. Reverse micelles with water content below 3 were
not obtained at the temperature used.
concentrations in the whole range. These results are
probably showing that saturation of the ketones in the
micellar interface is not reached in the range of surfac-
tant concentrations used. Moreover, from Figure 4 it can
be inferred that kobs diminishes with increasing water
Reaction in Toluene/AOT/Water Reverse Micellar
System. (1) Effects of AOT Concentration. The
kinetics of the reaction was studied by varying AOT
concentrations between 0 and 0.4 M, keeping the other
conditions fixed. For W
was 0.15 M. Figure 4 shows the kinetic results at W
0
) 10, the lowest [AOT] reached
content (W
Effect of NaBH
of NaBH concentration, the reaction was carried out by
0
).
0
)
4
Concentration. To study the effect
5
, 8, and 10 for CAF: kobs increases on increasing AOT
4
8234 J. Org. Chem., Vol. 69, No. 24, 2004

Reverse Micellar Aggregates
SCHEME 2
SCHEME 3
0
FIGURE 6. Variation of kobs as a function of W for the
4
reaction between CAF and NaBH in water/AOT/toluene
-
3
reverse micellar system. [CAF] ) 1.3 × 10 M. In all cases,
AOT] ) 0.3 M. [NaBH
.1 M.
where KCAF and KBH are distribution constants of CAF
[
0
4
]waterpool ) 0.1 M. [NaOH]waterpool )
and NaBH
4
between the organic phase and the interface
and between the interface and the water pool, respec-
b
b
w
tively. nCAF , nNaBH4 , and nNaBH4 are the occupation
numbers of CAF and NaBH respectively. Super- and
using 0.3 M of AOT at W
shown in Figure 5, where kobs increases linearly with
NaBH ]. The reaction is fastest at low water content as
0
) 5, 8, and 10. The results are
4
subscripts f, b, and w indicate the organic phase, the
micellar pseudophase, and the water pool, respectively.
The occupation number is n ) [solute] /[surf], where surf
b
represents the surfactant. The occupation number is
defined according to the place where the substrate is
located inside the aggregate, in the case of CAF there is
[
4
was observed by varying [AOT].
0
Effect of Water Dispersed, W . The effects of chang-
ing the value of W on kobs, keeping AOT and (NaBH )
0 4 w
concentrations constant, are shown in Figure 6. Values
of kobs increase as W increases up to W
∼ 5 and then
decrease until W
0
0
only the occupation number relative to the CAF at the
0
∼ 8 to finally remain practically
b
interface (nCAF ) [CAF]
b
/[surf]); on the other hand,
0
constant. The maximum W that can be reached in this
4
NaBH is wholly in the aggregate (reverse micelle surface
system is about 10.
and water pool), and therefore, there are two occupation
numbers relative to NaBH , the one relative to NaBH
Discussion
4
4
b
In regard to the reaction environment, NaBH
4
is not
at the surface (nNaBH4 ) [NaBH
4
]
b
/[surf]) and the one
w
soluble in low polarity organic solvents and CAF is poorly
soluble in water, and consequently, the reaction can only
occur at the interface of the aggregate, with the surfac-
tant providing a new region where the reactants can be
relative to NaBH
4
in the water pool (nNaBH4 ) [NaBH
4
]
w
/
[surf]).5
At W
can be proposed
arbitrary and drown only for illustration purposes).
0
< 10, the mechanism summarized in Scheme 2
6,41
(curvature and relative size are
6
-9,15,40,41
in contact.
considered for BH
Therefore, in the kinetic model we
only the equilibrium partitioning
-
6-8
At W
0
g 10 Scheme 3 can be proposed
(curvature
4
between water and the reverse micellar interface and for
the CAF only the partitioning between the organic phase
and the reverse micellar interface.
and relative size are arbitrary and drown only for
illustration purposes).
The rate of the reaction can be expressed by eq 5 at
3
9
As in part 1 and on the same basis, we write the
following equations:
W
0
0
< 10 and by eq 6 at W g 10
d[P]
dt
)
)
k′ [CAF ][NaBH ]
4 T
(5)
(6)
b
b
nCAFb
KCAF
)
(3)
(4)
[
CAF]f
d[P]
dt
k′ [CAF ][NaBH ]
4 b
b
b
b
n
NaBH4
KBH
)
where [P] is the concentration of the obtained product
and k′ is the bimolecular interfacial rate constant. The
w
n
NaBH4
b
concentrations in square brackets refer to the total
volume of reverse micelles.
(
40) Iglesias, E.; Leis, J. R.; Pe n˜ a, M. E. Langmuir 1994, 10, 662.
41) Durantini, E. N.; Borsarelli, C. D. J. Chem. Soc., Perkin Trans.
If [NaBH
4
]
T
. [CAF] the reaction has a first-order
(
2
1996, 719.
kinetics form. Then with eqs 3-6, together with the mass
J. Org. Chem, Vol. 69, No. 24, 2004 8235

Correa et al.
TABLE 1. Kinetic Parameters and Distribution Constants for the Reaction of CAF and NaBH4 in Different Reverse
Micellar Systems at 27 °C
kb/M- min-1
1
10 KCAF/M
2
-1
a
b
c
W0 ) 20a
W0 ) 20
W0 ) 10
water
a
a
b
W0 ) 10
W0 ) 10
W0 ) 10
kw)/M min-1
-1
(
system
W0 ) 5a,b,c
W0 ) 8
b
W0 ) 15
W0 ) 5a,b
W0 ) 8
b
W0 ) 15c
d
e
e
d
e
e
toluene/BHDC
0.28 ( 0.04
0.49 ( 0.04
1.61 ( 0.04
0.12 ( 0.02
0.11 ( 0.02
0.14 ( 0.02
f
g
g
0
.30 ( 0.05
0.52 ( 0.05
1.65 ( 0.05
3.46 ( 0.02
h
i
i
h
i
i
toluene/AOT
0.045 ( 0.002 0.036 ( 0.002 0.032 ( 0.002
90 ( 5
120 ( 5
110 ( 5
j
k
k
l
l
l
0
.043 ( 0.002 0.037 ( 0.002 0.033 ( 0.002
89
110
100
130
120
m
m
m
8
0
isooctane/AOTn 0.21 ( 0.04
0.16 ( 0.04
0.15 ( 0.04
220 ( 10
210 ( 10
200 ( 10
a
W0’s for the toluene/BHDC/water system. b W0’s for the toluene/AOT/water system. W0’s for the isooctane/AOT/water system. d From
c
e
Figure 1 fitted by eq 8 (kb was obtained by correction with eq 10; see text). From Figure 1 fitted by eq 9 (kb was obtained by correction
with eq 10; see text). f From Figure 2 fitted by eq 8 (kb was obtained by correction with eq 10; see text). From Figure 2 fitted by eq 9 (kb
g
h
i
was obtained by correction with eq 10; see text). From Figure 4 fitted by eq 8 (kb was obtained by correction with eq 10; see text). From
j
Figure 4 fitted by eq 9 (kb was obtained by correction with eq 10; see text). From Figure 5 fitted by eq 8 (kb was obtained by correction
with eq 10; see text). From Figure 5 fitted by eq 9 (kb was obtained by correction with eq 10; see text). l From eq 1. The errors here were
not included and are higher than the ones which come from the fitting due to the reason expressed in the Experimental Section. They are
estimated to be around 50%. m From eq 2, note that the errors here were not included and are higher than the ones which come from the
fitting due to the reason expressed in the Experimental Section. They are estimated to be around 50%. KBH ) 100 ( 10 from eq 9 in the
system toluene/BHDC/water. KBH 0.50 ( 0.02 from eq 9 in the system toluene/AOT/water. Parameter values are calculated using 0.995
confidence level in nonlinear regression. n Data taken from ref 39.
k
balance for the ketones and NaBH
expression for the rate (eq 7) and kobs (eq 8 for W
4
, we obtain the final
could not calculate the distribution constant of CAF in
toluene/BHDC/water by UV spectroscopy.
0
< 10
and eq 9 for W
0
g 10)
By fitting the experimental data with eq 8 for W
and eq 9 for the other W ’s (Figure 1), values of KCAF
BH, and k can be obtained, and the results are shown
in Table 1. The value of KBH is shown in footnote m of
Table 1. As can be inferred, k is higher at W ) 20 than
at the other W used, as was previously shown. Even at
0
) 5
0
,
d[P]
dt
K
b
)
kobs[CAF]T
(7)
b
0
with
0
the maximum amount of water, CAF is less reactive in
the micellar interface than in water, probably because
reaction is occurring at the interface and not in the water
pool.
The partition constants for CAF are very small and
0
practically independent of W , indicating the high affinity
of the substrate for the organic pseudophase and that
reaction happens at the interface as pointed out above.
Even with a very small amount of CAF in the interface,
b 0
k at W ) 20 is only 0.5 times lower than in water,
probably reflecting the fact that cationic interfaces are
stabilizing the transition state. For other aromatic ke-
(
4k′ K [NaBH ] [surf])
b CAF 4 T
kobs
)
(8)
(9)
(
1 + KCAF[surf])
(
4k′ KCAFKBH[NaBH ] [surf])
b
4 T
kobs
)
(
1 + KCAF[surf])(1 + KBH)
We note that, if KBH . 1, eq 9 converges to eq 8.
To obtain the micellar second-order rate constants k
for absolute comparison of reactivity in different media,
the molar reaction volume at the interface should be
known. This can be taken from the molar volume of AOT
b
4
tones, reductions by NaBH were studied in normal
and BHDC in the reverse micellar system, which can be
taken as 0.38 and 0.44, respectively.6-8,42
micelles of CTAB and CTAC with a ∼50-fold decrease of
the second-order reaction rate constants in the cationic
In this way, a conventional second-order rate constant
with conventional units M s ) is given by
2
5
-
1
-1
6,25
micellar pseudophase relative to those in water. It
seems, however, that cationic reverse micelles are en-
hancing the reaction rates probably because of the higher
(
k ) k′ vj
(10)
b
b
4
NaBH concentration at the interface of these kinds of
where vj is the surfactant molar volume.
aggregates.
Reaction in the Toluene/BHDC/Water System.
4 0
Analyzing the distribution constant of NaBH at W )
10 and 20 it appears that this large value is reflecting
that, due to electrostatic attraction, the effective concen-
tration of NaBH
micellar system is very high. In the interior of the water
microdroplets OH and Cl were present and competing
with the BH
The increase of kobs with increasing BHDC concentration
0
(Figure 1) at all W ’s that have been studied could be due
to the gradual incorporation of CAF into the micellar
aggregate, but there is no interface saturation, probably
reflecting the low value of the distribution constant.
Moreover, the dependence of kobs on the surfactant
concentration seems to be linear. Equations 8 and 9 show
that when the distribution constant of the ketone is very
low and the product KCAF[surf] is negligible, in compari-
son to unity, kobs would exhibit a linear relationship with
the surfactant concentration. In fact, as noted above, we
4
at the interface of cationic reverse
-
-
-
4
at the surface of the reverse micelle
25
surface. Previous studies carried out in direct cationic
micelles of CTAC have shown that exchange between
-
-
OH and BH
4
can be neglected because addition of a
large amount of hydroxide ion did not affect the reactivity
-
4
of BH . Similarly, the favorable ion exchange constant
-
-25
-
- 43-45
of BH
4
over Cl
and of Cl over OH ,
measured
(42) Lang, J.; Jada, A.; Malliaris, A. J. Phys. Chem. 1988, 92, 1946.
in cationic direct micelle, encourage us to assume that
8236 J. Org. Chem., Vol. 69, No. 24, 2004

Reverse Micellar Aggregates
the majority of borohydride is at the interface so that we
can neglect the influence of OH and Cl on the reaction
kinetic.
with those obtained kinetically. As can be inferred, k
lower at W ) 10 than at 5 as seen in Figure 4. As
previously discussed for the BHDC system, eqs 8 and 9
also show that, if [AOT] and W are fixed, there should
be a linear relationship between kobs and [NaBH ], and
by the introduction of the proper values of KCAF and KBH
values of k can be recalculated for each system at W
b
is
-
-
0
Also if the data of Figure 1 are fitted with eq 8, k
) 10 and 20 are 0.48 ( 0.02 and 1.58 ( 0.02,
respectively. It appears that, in toluene/BHDC/water
b
at
0
W
0
4
,
-
reverse micelles, BH
interface without partitioning into to the water pool.
Equations 8 and 9 also show that, if [AOT] and W are
fixed, there should be a linear relationship between kobs
and [NaBH ] and by the introduction of the values of KCAF
and KBH the values of k could be recalculated for each
system at W ) 5, 10, and 15 by fitting the new set of
experimental data (kobs vs [NaBH ]) with these equations.
The results are compared in Table 1, and good estimates
of k within experimental error are obtained by two
4
is practically anchored at the
b
0
)
5, 8, and 10 by fitting the new set of experimental data
with these equations (Figure 5). The results are compared
0
in Table 1. The k
methods agree reasonably.
0
Values of k at every W studied are always higher in
b
values obtained by two independent
4
b
b
toluene/BHDC than in toluene/AOT, reflecting the fact
that anionic reactants, such as the borohydride, are more
tightly bound to a cationic interface. In this way, the local
effective concentration of the ions is greater in a cationic
interface than in an anionic one making reaction faster.
This fact could be confirmed by looking at values of KBH
in both systems, the KBH(toluene/BHDC)/KBH(toluene/AOT) ratio
being 200. The catalytic effect of the cationic interface
over the anionic one was also found in other types of
0
4
b
independent methods.
The kinetic profile upon the addition of water (Figure
3
) shows that kobs increases modestly with W
0
up to 10
which probably indicates that the strong interactions
-
between BH
4
ion and the cationic polar head of the
1
,41,49,50
reaction where not only ions are involved
but also
BHDC are competing for the nucleophilic attack of the
neutral molecules.8 In other words, a critical point in
micellar second-order kinetics is the value attributed to
vj that is derived from partial molar factor. A reduced
“real reaction volume” that cannot be measured in any
way corresponds to a change in reaction rate.
,9b
ion on CAF when there is only hydration water (W <
0
-10) at a rigid interface.1
1,46
8
0
At W > 10, the interfacial
rigidity in benzene/BHDC/water reverse micelles de-
41,47
creases with the value of W
in kobs with the water amount observed above W
0
,
so the sharp increase
> 10
0
It is interesting to note that the value of KCAF is lower
in the toluene/BHDC than in the toluene/AOT system.
It seems that CAF has less affinity for cationic interfaces
as observed with other aromatic compounds in different
could be due to a higher accessibility of the reactants in
the micellar interface. This effect allows a better contact
between CAF and NaBH
micellar interface. In other words, at a low W
unsolvated BHDC interacts with BH
4
, favoring reaction at the
level,
stabilizing the
initial state of the ion more effectively than when it is
0
reactions.8
,9b,41
Literature H NMR spectroscopic data
1
51
-
4
show a specific interaction between AOT headgroups and
benzene. It appears that the aromatic ring of CAF is
responsible for its stronger interaction with the polar
headgroup of AOT than with BHDC. Accordingly, stron-
25
fully solvated at W
in normal micelles have shown that BH
0
> 10. Moreover, previous studies
-
4
has a higher
-
-
affinity for a cationic surfactant than Cl and Br .
Reaction in the Toluene/AOT/Water System. Com-
parison between the Different Interfaces. To analyze
the effect of different interfaces on the rates of reduction
+
ger interactions between the Na and an aromatic
+
compound than N(Me)
4
and the π system are reported
in the literature.52
The kinetic profile upon addition of water is quite
peculiar (Figure 6). At W > 5, the kinetic profiles show
that the rate diminishes with the water amount until
) 8 and then is practically constant. For W < 5, the
rate increases with W . Our interpretation of those data
is that up to W ) 5 water is solvating the AOT polar
headgroup, making less relevant Na BH
4
of CAF by NaBH , the kinetic profile was investigated
in toluene/AOT/water anionic reverse micellar system.
As shown in Figure 4, kobs increases with increasing
0
AOT concentration at all W ’s studied. This is probably
due to the gradual incorporation of CAF into the reverse
micellar surface. There is no interfacial saturation but
the profile is not as linear as was observed in the toluene/
BHDC/water reverse micelles, probably reflecting a low
value of the distribution constant however higher than
in the cationic micelles. By fitting the experimental data
0
W
0
0
0
0
+
-
4
ion pairing,
so that the rate increases. At W
0
g 5 CAF is also being
solvated and rates decrease.
-
At W
0
∼ 8, the water pool is formed and BH
4
is ejected
> 8,
] at the interface is practically constant as is the
from the anionic interfacial reaction region. At W
0
with eq 8 at W
), values of KCAF, KBH, and k
Table 1. The literature value of the molar volume of AOT,
0
) 5 and eq 9 for W
0
) 8 and 10 (Figure
-
[
BH
4
4
b
were obtained and are in
reaction rate. These results agree with the previous
statement that the reaction is taking place at the
interface of the aggregate and not in the water pool. It
can be seen from Table 1 that the reaction is faster in
pure water than in the reverse micelles media. It must
6
-8,48
vj , is 0.38.
Also in Table 1 can be found values for
the CAF distribution constants obtained independently
by UV spectroscopy that are in a very good agreement
0
be noted that W cannot be superior than 10 while in
toluene/BHDC/water reverse micellar systems values of
around 25 can be reached. This profile is quite different
(
43) Bartet, E.; Gamboa, C.; Sepulveda, L. J. Phys. Chem. 1980, 84,
2
72.
(
(
44) Quina, F. H.; Chaimovich, H. J. Phys. Chem. 1979, 83, 1844.
45) Chaimovich, H.; Bonilha, J. B. S.; Politi, M. J.; Quina, F. H. J.
Phys. Chem. 1979, 83, 1851.
(
46) Correa, N. M.; Biasutti, M. A.; Silber, J. J. J. Colloid Interface
Sci. 1996, 184, 570.
47) Jada, A.; Zana, R.; Lang, R.; Makhloudfi, E.; Hirsh, E.; Candau,
S. J. J. Phys. Chem. 1990, 94, 387.
48) Lang, J.; Jada, A.; Malliaris, A. J. Phys. Chem. 1988, 92, 1946.
(49) Terrier, F. Nuclehophilic Aromatic Displacement, the Influence
of the Nitro Group; VCH: New York, 1991.
(50) Bunton C. A.; Robinson, L. J. Org. Chem. 1969, 34, 780.
(51) Heatley, F. J. Chem. Soc., Faraday Trans. 1 1988, 84, 343.
(52) MA, J. C.; Dougheerty, D. A. Chem Rev. 1997, 97, 1303.
(
(
J. Org. Chem, Vol. 69, No. 24, 2004 8237

Correa et al.
from the one found in the cationic system (Figure 3)
ketone among the continuous medium and the interface
and the assumption that reaction takes place only at the
interface, has enabled us to estimate rate constants in
the interface of the different reverse micellar systems.
-
where BH
4
is part of the double layer of the cationic
interface while in the anionic one it is ejected into the
water pool due to electrostatic repulsion.
Solvent Effects on Anionic Interface. To investi-
gate the effect of external solvent on the CAF reduction
At W
interface and, at W
0
) 5, it was considered that all NaBH
4
is at the
g 10, where there are free water
0
by NaBH
4
, it is useful to compare the results in toluene/
molecules, partitioning between the interface and the
water pool was also considered. Rate constants increase
with the surfactant concentration for every system and
at every water content (W ) as a consequence of incor-
0
porating the substrates into the interface of the reverse
micellar system but the saturation of the interface by the
AOT/water reverse micellar system with those in iso-
octane/AOT/water aggregates.39
The reaction (Table 1) is faster in the isooctane/AOT/
water system and the CAF distribution constant is higher
in these aggregates. In the literature,
11,46,53
several dif-
ferences can be found comparing behaviors of reverse
micelles of AOT/alkanes with those of AOT/aromatic
hydrocarbon. The thermodynamics of micellization show
that clustering of benzene molecules around AOT head-
groups appears to be more ordered than with saturated
substrate is never reached due to the low value of KCAF.
The reaction is faster in the cationic interface than in
-
the anionic one probably because BH4 is part of a
cationic interface. Moreover, the distribution constant of
CAF is around 900 times higher in AOT reverse micelles
than in BHDC reverse micelles, showing that the pres-
alkanes such as isooctane.54 At the same value of W
, the
aggregation number of AOT is five times higher in the
0
ence of the NaBH in the interface is the key factor. The
4
5
5
56
alkanes than in benzene.
high value of KCAF in AOT/toluene reverse micellar
Moreover, while AOT reverse micelles in saturated
hydrocarbons solubilize large amounts of water; those
reverse micelles in benzene solubilize only small amounts
systems is probably due to a specific interaction between
the aromatic ketone and the polar head of the surfactant.
The effect of the external solvent on the reaction shows
5
7
1
of water. However, H NMR spectroscopy shows that
that reduction is favored in isooctane/AOT over toluene/
free water can be detected in AOT-benzene reverse
-
AOT reverse micellar systems, probably due to BH
4
micellar systems at low W
0
values.51 Furthermore, these
being largely present in the water pool of the toluene/
AOT reverse micelles. Also, KCAF is lower in the presence
of toluene as a solvent, due to the aromatic nature of both
molecules.
Effects of water addition differ largely depending on
the type of interface. In toluene/BHDC/water reverse
studies give evidence of specific interactions between
AOT headgroups and benzene. 13C NMR spectroscopy
shows that the extent of water penetration into AOT
reversed micelles is greater in aromatic than in saturated
hydrocarbons. In short, it seems that reversed micelles
of AOT in benzene or toluene are smaller but have
greater water penetration in the nonpolar pseudophase
of the micelle than with AOT in alkanes. It seems that,
58
0
micellar systems, kobs increases with W probably due to
the fact that the layer rigidity diminishes with the
amount of water making possible a larger accessibility
of the reactants in the micellar interface. By solvating
due to the greater water penetration in the toluene/AOT
-
0 4
system and the presence of free water, at lower W , BH
-
+
the BHDC headgroup reducing BH
effects allow better contact between CAF and NaBH
favoring reaction at the micellar interface. On the other
hand, in toluene/AOT/water, kobs is at a maximum at W
4
N pairing, these
is more in the water pool and, for these reasons, the
reaction is slower than in isooctane/AOT reverse micellar
system. Note that the value of KBH in the isooctane/AOT
system is higher (0.7) than that in the toluene/AOT one.
The KCAF value in toluene/AOT is about half that in
isooctane/AOT reverse micellar systems, which is not
surprising if the aromatic natures of the solute and
4
,
0
ca. 5, probably showing that, below this value, water is
interacting strongly with the polar head of the surfactant,
leaving the ketones and the borohydride (located at the
interface) more reactive. Above this threshold water
9b,41
organic solvent are considered;
i.e., CAF has a higher
molecules probably start to interact with CAF, and once
affinity for the organic phase when toluene rather than
an alkane is the external solvent.
-
the water pool is formed, BH
interfacial reaction region.
4
is ejected from the anionic
These results seem to provide a good model for predict-
ing a reduction of the ketones in reverse micellar systems.
Moreover, this study can be extended to control the
stereoselectivity of the reaction since interiors of these
aggregates, which are similar to the polar parts of
enzymes, are the reactive centers of the whole system,
and a chiral environment can easily be created by
dissolving asymmetric solutes in the interior.
Conclusions
The influence of different interfaces and the external
solvent on the mechanism of the reaction between CAF
and NaBH were investigated in water/AOT/toluene and
4
water/BHDC/toluene and also compared with that in
water/AOT/isooctane reverse micellar system by UV
-
spectroscopy. A kinetic model based on the pseudophase
formalism, which considers the distributions of the
Acknowledgment. We gratefully acknowledge the
financial support for this work by the Ministero Affari
Esteri (MAE Direzione Generale per la Promozione e
la Cooperazione Culturale), CNR Progetto Finalizzato
Beni Culturali. Fundacion Antorchas N.M.C. is Scien-
tific Member of CONICET (Consejo Nacional de Inves-
tigaciones Cientificas y Tecnologicas, Argentina).
(
53) Correa, N. M.; Biasutti, M. A., Silber, J. J. J. Colloid Interface
Sci. 1995, 172, 71.
(
54) Mukerherjee, K.; Moulik, S. P.; Murkherjee, D. P. Langmuir
1
993, 3, 494.
(
(
(
55) Verbeeck, A.; De Schryver, F. C. Langmuir 1987, 494.
56) Ueda, M.; Schelly, Z. A. J. Colloid Interface Sci. 1988, 124, 673.
57) Goto A.; Yoshioka, H.; Kishimoto, H.; Fujita, T. Langmuir 1992,
8
9
, 441.
(
58) Yoshino, A.; Okabayashi, H.; Yoshida, T. J. Phys. Chem. 1994,
8, 7036.
JO049172V
8238 J. Org. Chem., Vol. 69, No. 24, 2004