Reverse micellar aggregates: Effect on ketone reduction. 1. Substrate role

Article abstract of DOI:10.1021/jo049173n

The reduction of three aromatic ketones, acetophenone (AF), 4-methoxyacetophenone (MAF), and 3-chloroacetophenone (CAF), by NaBH₄ was followed by UV-vis spectroscopy in reverse micellar systems of water/AOT/isooctane at 25.0 °C (AOT is sodium 1,4-bis-2- ethylhexylsulfosuccinate). The first-order rate constants, k_obs, increase with the concentration of surfactant due to the substrate incorporation at the reverse micelle interface, where the reaction occurs. For all the ketones the reactivity is lower at the micellar interface than in water, probably reflecting the low affinity of the anionic interface for BH ₄^-. Kinetic profiles upon water addition show maxima in k_obs at W₀ ≈ 5 probably reflecting a strong interaction between water and the ionic headgroup of AOT; at W₀ < 5 by increasing W₀ BH₄^- is repelled from the anionic interface once the water pool forms. The order of reactivity was CAF ? AF > MAF. Application of a kinetic model based on the pseudophase formalism, which considers distribution of the ketones between the continuous medium and the interface, and assumes that reaction take place only at the interface, gives values of the rate constants at the interface of the reverse micellar system. At W₀ = 5, we conclude that NaBH₄ is wholly at the interface, and at W₀ = 10 and 15, where there are free water molecules, the partitioning between the interface and the water pool has to be considered. The results were used to estimate the ketone and borohydride distribution constants between the different pseudophases as well as the second-order reaction rate constant at the micellar interface.

Full text of DOI:10.1021/jo049173n

Reverse Micellar Aggregates: Effect on Ketone Reduction. 1.
Substrate Role
N. Mariano Correa, Daniel H. Zorzan, Marco Chiarini, and Giorgio Cerichelli*
Dipartimento di Chimica, Ingegneria Chimica e Materiali. Facolta` di Scienze MM FF NN Universita`
degli Studi dell’Aquila, 67100 L’Aquila, Italy, and INCA Consorzio Interuniversitario Nazionale
“La Chimica per l’Ambiente” UdR Aquila1
cerichel@univaq.it
Received May 17, 2004
The reduction of three aromatic ketones, acetophenone (AF), 4-methoxyacetophenone (MAF), and
3-chloroacetophenone (CAF), by NaBH4 was followed by UV-vis spectroscopy in reverse micellar
systems of water/AOT/isooctane at 25.0 °C (AOT is sodium 1,4-bis-2-ethylhexylsulfosuccinate). The
first-order rate constants, kobs, increase with the concentration of surfactant due to the substrate
incorporation at the reverse micelle interface, where the reaction occurs. For all the ketones the
reactivity is lower at the micellar interface than in water, probably reflecting the low affinity of
the anionic interface for BH4-. Kinetic profiles upon water addition show maxima in kobs at W0 ≈
5 probably reflecting a strong interaction between water and the ionic headgroup of AOT; at W0 <
-
5 by increasing W0 BH4 is repelled from the anionic interface once the water pool forms. The
order of reactivity was CAF . AF > MAF. Application of a kinetic model based on the pseudophase
formalism, which considers distribution of the ketones between the continuous medium and the
interface, and assumes that reaction take place only at the interface, gives values of the rate
constants at the interface of the reverse micellar system. At W0 ) 5, we conclude that NaBH4 is
wholly at the interface, and at W0 ) 10 and 15, where there are free water molecules, the partitioning
between the interface and the water pool has to be considered. The results were used to estimate
the ketone and borohydride distribution constants between the different pseudophases as well as
the second-order reaction rate constant at the micellar interface.
Introduction
In principle, reverse micellar systems can affect reac-
tion rates by two main processes;14 they can increase or
diminish the energy of the transition state of the reaction,
thus affecting the rate constants. This is what is called
the medium effect, and it takes into account all the
properties of the interface such as micropolarity, micro-
viscosity, ionic strength, charge density, etc. The other
effect is that they can change the reactant location which
is called the solubilization effect. In the case of bimo-
lecular reactions, solubilization of the reactants in the
interfacial region can significantly accelerate reactions
due to a “concentration” effect, while reactions of segre-
gated reactants may be retarded. When both reactants
Chemical reactivity in reverse micellar systems is an
interesting object of study because these media are
macroscopically homogeneous and isotropic but are het-
erogeneous on the molecular scale. These systems contain
aqueous microdroplets entrapped in a film of surfactants
and dispersed in a low-polarity bulk solvent. In this way,
and depending on the water content, three pseudophases
are present in reverse micellar systems, water, interface,
and organic ones that allow compartmentalization of
solubilized species at the microscopic level.1-9 The physi-
cal characteristics of water microdroplets differ from
those of bulk water, mainly in regard to mobility,
polarity, and properties affected by its high ionic strength
and by the electronic influence of the charged surfactant
headgroups.5,6,10-13
(8) Correa, N. M.; Durantini, E. N.; Silber, J. J. J. Org. Chem. 2000,
65, 6427.
(9) (a) Boscatto, L.; Chiacchiera, S. M.; Correa, N. M.; Durantini,
E. N.; Zingaretti, L.; Silber, J. J. In Atual. Fis.-Org. Brasil 1999, 11,
534. (b) Correa, N. M.; Durantini, E. N.; Silber, J. J. Atual. Fis.-Org.
Brasil 2000, 12, 96.
* To whom correspondence should be addressed.
(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.
(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
Interactions in Reverse Micelles Current Topics in Colloid and Interface
Research of Research Trends; Gon˜i, F. M., Hoffmann, H., Eds.; Research
Trends: Trivandrum, India, 1999; Vol. 3, p 35.
(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,
99.
(12) Monduzzi, M.; Caboi, F.; Moriconi, C. Colloids Surf. A: Physi-
cochem. Eng. Aspects 1997, 129-130, 327.
(5) Silber, J. J.; Biasutti, M. A.; Abuin, E.; Lissi, E. Adv. Colloid
Interface Sci. 1999, 82, 189.
(13) Hamada, K.; Ikeda, T.; Kawai, T.; Kon-No, K. J. Colloid
Interface Sci. 2001, 233, 170.
(6) Garc´ıa-R´ıo, L.; Leis, J. R.; Pen˜a, E.; Iglesias, E. J. Phys. Chem.
1993, 97, 3437.
(14) Eicke, H. F.; Parffit, G. D. Interfacial phenomena in apolar
media; Surfactant Science Series; Marcel Dekker, Inc.: New York,
1987; Vol. 21.
(7) Correa, N. M.; Durantini, E. N.; Silber, J. J. J. Org. Chem. 1999,
64, 5757.
10.1021/jo049173n CCC: $27.50 © 2004 American Chemical Society
Published on Web 11/04/2004
8224
J. Org. Chem. 2004, 69, 8224-8230

Reverse Micellar Aggregates
Procedures. A 0.5 M stock solution of surfactant was
prepared by dissolving the necessary amount of surfactant in
isooctane. 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, was transferred
into the cell, and the water was added with a calibrated
microsyringe. The amount of water present in the system is
expressed as the molar ratio between water and the surfactant
(W0 ) [H2O]/[surfactant]). NaBH4 was dissolved in water in a
concentration of 0.1 M before being added to the microemulsion
and stored in a refrigerator prior to use. NaOH ) 0.1 M was
are in the water droplet or at the interface, they are
concentrated as in a nanoreactor, and since its size is
easily varied, it is relatively easy to assess the properties
of the micellar system.15,16 Also, reverse micellar systems
are of interest as a reaction medium because they are
powerful models of biological systems.17-19
In water, micelles influence regioselectivity and stereo-
selectivity20-22 of the reduction of ketones with borohy-
dride; i.e., in chiral reverse micellar systems23 prochiral
ketones are reduced to optically active alcohols. A sys-
tematic study24 of the reaction in cetyltrimethylammo-
nium bromide (CTAB) and cetyltrimethylammonium
chloride (CTAC) in water shows that there are, for a
series of aromatic ketones, decreases (20-50 fold) in
second-order reaction rate constants in the micellar
pseudophase, relative to those in water. Due to the lack
of kinetic studies in reverse micellar systems we have
investigated this reaction kinetically in isooctane/AOT/
water reverse micellar system.
present in the water solution in order to stabilize the ion
- 25-29
BH4
.
NaBH4 is not soluble in isooctane, so ionic concen-
trations in the reverse micelles are referred to that of water,
i.e., [NaBH4]W ) 0.1 M. When the surfactant concentration is
changed at a determined value of W0, total amounts of water
and NaBH4 differ in every case, but the ratio is always the
same.
The concentration of hydride species was assumed to be four
times that of the analytical concentration30,31 due to the
-
presence in the BH4 ion of four identical hydrogen atoms.
Kinetics. Reactions were followed spectrophotometrically
by monitoring the decrease of the absorbance of the ketones
at the following wavelengths and ketones analytical concen-
trations: MAF, 263, 270, and 277 nm; [MAF] ) 5 × 10-5 M;
CAF, 278, 289, and 297 nm; [CAF] ) 3 × 10-4 M; AF, 271,
278, and 286 nm; [AF] ) 3 × 10-4 M at 25.0 ( 0.1 °C. As blank
was used a solution of surfactant of identical concentration
and W0. To start the reaction, the appropriate amount of
ketone stock solution in isooctane was added (20-30 µL) to a
thermostated cell containing NaBH4 in the reverse micellar
system. When [NaBH4] is changed, the method is similar to
that described above, but with a 0.15 M stock solution of
NaBH4. Dilution of this stock solution to the desired [NaBH4]
is made before adding it to the reverse micellar system.
In every case, first-order plots were obtained with the
addition of excess NaBH4. The kinetic data fit the first-order
integrated equation in a satisfactory way (r > 0.999 for almost
10t1/2). When the reaction was too slow, the rate constant was
calculated from the initial rates. The 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. The absorbances
of isooctane solutions of each ketone at a fixed wavelength
(shown above) were recorded and averaged in the presence of
different AOT concentrations (0-0.4 M) at W0 ) 5, 10, and
15, obtained by diluting the stock AOT solution (AOT 0.4 M,
CAF or AF 3 × 10-4 M, MAF 5 × 10-5 M) with the isooctane-
ketone solution. The spectra were run at 25.0 °C, the blank
being a solution of surfactant of identical concentration and
W0. Values of Kket were calculated with a least-squares fit to
eq 132
We report below data on the kinetics of reduction by
NaBH4 of three aromatic ketones, acetophenone (AF),
4-methoxyacetophenone (MAF), and 3-chloroacetophe-
none (CAF), in isooctane/AOT/water reverse micelles to
investigate how the reverse micellar system and the
ketones structure influence the reaction rate. The results
show that the reaction is faster at low water content and
decreases with the amount of dissolved water depending
on the nature of the ketones.
Experimental Section
General Methods. UV-vis spectra were recorded on a
spectrophotometer equipped with a multiple cell holder ther-
mostated at 25.0 ( 0.1 °C. Control experiments of ketones
reduction by NaBH4 (under reverse micelle conditions) showed
that the ketone disappears and the correspondent alcohol is
the only formed product (by GC analysis). Moreover, in the
absence of NaBH4, no reaction occurred and there were no
changes in the UV-vis spectra.
Materials. Acetophenone, (AF; 99%), 4-methoxyacetophe-
none (MAF; 98%), 3-chloroacetophenone (CAF; 99%), and 2,2,4-
trimethylpentane (isooctane HPLC grade; 99.7%) from Aldrich
were used without further purification. Sodium bis(2-ethyl-
hexyl)sulfosuccinate (AOT) from Sigma (>99%) was dried
under reduced pressure and was kept under vacuum over P2O5
until it was used. NaBH4 (98%) from Acros Organics was used
as received. Water was distilled three times.
(ꢀf + ꢀb[AOT]Kket)[ketone]T
Aλ )
(1)
(15) Garc´ıa-R´ıo, L.; Leis, J. R.; Iglesias, E. J. Phys. Chem. 1995, 99,
12318.
(1 + Kket[AOT])
(16) Pileni, M. P. J. Phys. Chem. 1993, 97, 6961.
(17) Luisi, P. L.; Straub, B. In Reverse Micelles, 8th ed.; Plenum
Press: London, 1984.
(18) Ballesteros, A.; Bornscheuer, U.; Capewel, A.; Combes, D.;
Condoret, J. S.; Koenig, K.; Kolisis, F. N.; Marty, A.; Mengue, U.;
Scheper, T.; Stamatis, H.; Xenakism, A. Biocatal. Biotransform. 1995,
13, 1.
(19) Liou, J. Y.; Huang, T. M.; Chang, G. G. J. Chem. Soc, Perkin
Trans. 2 1999, 2171.
where Aλ is the absorbance at different AOT concentration, ꢀf
and ꢀb are the molar extinction coefficients for the ketones in
isooctane and at the interface of the reverse micelles respec-
(25) Davis, R. E.; Swain, C. G. J. Am. Chem. Soc. 1960, 82, 5949.
(26) Davis, R. E.; Bromels, E.; Kibby, C. L. J. Am. Chem. Soc. 1962,
84, 885.
(20) (a) Nikles, J. A.; Sukenik, C. N. Tetrahedron Lett. 1982, 23,
4211. (b) Jaeger, A. D.; Ward, M. D.; Martin, C. A. Tetrahedron 1984,
40, 2691. (c) Jursic, B.; Sunko, D. E. J. Chem. Res. 1988, 202.
(21) Natrajan, A.; Ferrara, J. D.; Youngs, W. J.; Sukenik, C. N. J.
Am. Chem. Soc. 1987, 109, 7477.
(27) Davis, R. E. J. Am. Chem. Soc. 1962, 84, 892.
(28) Gardiner, J. A.; Collat, J. W. J. Am. Chem. Soc. 1965, 87, 1692.
(29) Kreevoy, M. M.; Hutchins, J. E. C. J. Am. Chem. Soc. 1972,
94, 6371.
(30) Adams, C.; Gold, V.; Reuben, D. M. E. J. Chem. Soc., Perkin
Trans. 2 1977, 1466.
(22) Zhang, Y.; Fan, W.; Lu, P.; Wang, W. Synth. Commun. 1988,
18, 1495
(31) Sablayrolles, C.; Contastin, A.; Ducourant, B.; Fruchier, A.;
Chapat, J.-P. Bull. Soc. Chim. Fr. 1989, 467.
(32) Sepulveda, L.; Lissi, E.; Quina, F. Adv. Colloid Interface Sci.
1986, 25, 1.
(23) Zhang, Y.; Sun, P. Tetrahedron: Asymmetry 1996, 7, 3055.
(24) Cerichelli, G.; Coreno, M.; Mancini, G. J. Colloid Interface Sci.
1993, 158, 33.
J. Org. Chem, Vol. 69, No. 24, 2004 8225

Correa et al.
TABLE 1. Kinetic Parameters and Distribution Constants for the Reaction of Ketones with NaBH4 in Water/AOT/
Isooctane Reverse Micellar Systems at 25.0 °C
kb/M-1 min-1 a
water (kw)/M-1 min-1
Kket/M-1
W0 ) 10
ketones
MAF
W0 ) 5
W0 ) 10
W0 ) 15
W0 ) 5
W0 ) 15
0.020b ( 0.005
0.018d ( 0.005
(13.5)
0.017c ( 0.005
0.015e ( 0.005
(15.8)
0.0083c ( 0.0005
0.0088e ( 0.0005
(32.5)
1.0b ( 0.1
1.1c ( 0.1
1.5c ( 0.1
0.27 ( 0.02
3.46 ( 0.02
1.14 ( 0.02
1.1f
1.0f
1.4f
0.9g
1.1g
1.6g
CAF
AF
0.21b ( 0.04
0.28d ( 0.05
(16.4)
0.16c ( 0.04
0.17e ( 0.05
(21.2)
0.15c ( 0.04
0.14e ( 0.05
(22.4)
2.2b ( 0.1
2.5f
2.1c ( 0.1
2.0f
2.0c ( 0.1
1.9f
2.3g
1.9g
2.1g
0.057b ( 0.005
0.055d ( 0.005
(18.2)
0.022c ( 0.005
0.020e ( 0.005
(47.3)
0.019c ( 0.005
0.018e ( 0.005
(54.7)
0.9b ( 0.1
0.8f
0.7c ( 0.1
0.8f
0.8c ( 0.1
0.7f
0.9g
0.9g
0.7g
a In parentheses (kw/kb). b From Figure 1A-C fitted by eq 8 (kb was obtained by correction with eq 10; see text). c From Figure 1A-C
fitted by eq 9 (kb was obtained by correction with eq 10; see text). d From Figure 2A-C fitted by eq 8 (kb was obtained by correction with
eq 10; see text). e From Figure 2A-C fitted by eq 9 (kb was obtained by correction with eq 10; see text). f From eq 1. The errors here are
not included and are higher than the one which comes from the fitting due to the reason expressed in the Experimental Section. They are
estimated to be around 50%. g From eq 2. The errors here are not included and are higher than that which comes from the fitting due to
the reasons expressed in the Experimental Section. They are estimated to be around 50%. KBH ) 0.7 ( 0.1 from eq 9 for W0 ) 10 and 15.
Parameter values calculated using 0.995 confidence level in nonlinear regression.
tively, [AOT] is the surfactant concentration, and [ketone]T is
the total ketone concentration referred to total solution volume.
It should be borne in mind that precise spectroscopic estima-
tion of the constants is ruled out by the impossibility of having
all the ketone at the interface, and therefore, the value of ꢀb
can only be estimated. The spectroscopic estimation of Kket was
therefore based on an analysis of the spectroscopic data using
a fitting procedure with the ꢀb as an adjustable parameter.
Kket was also determined by using Ketelaar’s equation33-35
(2)
Reaction in Water. Kinetic results were obtained in
water at [OH-] ) 0.1 M by varying [NaBH4]W between
0.01 and 0.15 M for AF, MAF, and CAF and are shown
in Table 1, where the first-order rate constants, kobs follow
the sequence CAF > AF > MAF.
Reaction in the Water/AOT/Isooctane Reverse
Micellar System. (1) Effects of AOT Concentration.
Kinetics were studied at constant W0, [NaBH4]W, and
[NaOH]W and varying AOT concentrations between 0 and
0.4 M. Figure 1A-C show the kinetic results at W0 ) 5,
10, and 15 for CAF, AF, and MAF, respectively, where
kobs increases on increasing AOT concentrations over the
whole range. These results show that the saturation of
the ketones at the micellar interface is not reached in
the surfactant concentrations range used, although for
CAF the kinetic profiles show a more pronounced down-
ward curvature. Moreover, data in Figure 1 A-C show
that, for all the ketones, kobs diminishes with increasing
water content (W0).
1
1
)
+
A
λisoo - Aλ (ꢀf - ꢀb)[ketone]T
1
1
(2)
(ꢀf - ꢀb)[ketone]TKket
[AOT]
where Aλisoo is the absorbance in isooctane and the other terms
are defined above. Plotting the left-hand side term of eq 2 vs
1/[AOT] gives the value of Kket from the slopes and intercepts.
(2) Effect of NaBH4 Concentration. To study the
effect of NaBH4 concentration, the reaction was carried
out with 0.3 M of AOT at W0 ) 5, 10, and 15. Figure
2A-C shows the results where kobs increase linearly with
[NaBH4]W. Reactions are faster at low water content as
was observed by varying [AOT].
Results
The reactions of the ketones with NaBH4 in water and
in isooctane/AOT/water reverse micellar system yield the
corresponding alcohols (see below) as previously reported
in homogeneous media36,37 and in normal micelles.24
(3) Effect of W0. The effects of changing W0 on kobs
keeping AOT and (NaBH4)w concentrations constant are
shown in Figure 3A-C for CAF, AF, and MAF, respec-
tively. The value of kobs increases with W0 up to ∼5 and
then decreases until W0 ∼10 and then remains practically
constant.
Discussion
To elucidate how microheterogeneous media affect the
reduction reaction several variables were investigated as
follows.
In water, CAF is the most and MAF the least reactive
ketone. These results are, as expected, because, the
m-chlorine atom in CAF exerts a substrate inductive
effect; consequently, it is the most reactive ketone. The
reactions were carried out at [NaOH] ) 0.1 M. Compar-
ing values of the rate constants with the one obtained in
more diluted aqueous NaOH ([NaOH] ) 0.1 M versus
[NaOH] ) 0.0163 M),24 a small acceleration of the
reaction rate is observed probably due to the ionic
strength of the media.
(33) Ketelaar, J. A. A.; Van De Stolpe, C.; Gersmann, H. R. Recl.
Trav. Chim. 1951, 70, 499.
(34) Falcone, R. D.; Correa, N. M.; Biasutti, M. A.; Silber, J. J.
Langmuir 2000, 16, 3070.
(35) Correa, N, M.; Durantini, E. N.; Silber, J. J. J. Colloid Interface
Sci. 2001, 240, 573.
(36) Brown, H.; Ichikawa, K. Tetrahedron 1957, 1, 214.
(37) Boyer, B.; Lamaty, G.; Pastor, J.-P.; Roque, J.-P. Bull. Soc.
Chim. Fr. 1989, 463.
8226 J. Org. Chem., Vol. 69, No. 24, 2004

Reverse Micellar Aggregates
SCHEME 1
In regard to the reaction environment the NaBH4 is
insoluble in isooctane, the ketones are poorly soluble in
water,24 and the anion BH4- has the same charge of the
AOT interface reverse micelles. Considering all these
aspects, reaction can only occur at the interface of the
aggregates, with the surfactant providing a new region
where the reactants can be in contact as previously found
with other kinds of reactants and reactions in reverse
micelles.6-9,15,38,39 For these reasons, in the kinetic model
we considered for BH4- only the equilibrium partitioning
between the water and the reverse micellar interface and
for the ketones only the partitioning between the organic
phase and the reverse micellar interface.
SCHEME 2
Moreover, up to W0 < 8-10, all the water is bound to
Na+ and at the ionic headgroup of the surfactant, and
only above this value free water exists.1,3-5,11 A quantita-
tive model may be written based on the pseudophase
treatment. It was considered that at W0 < 10, only two
pseudophases were present, the interface and the bulk
organic solvent and all the BH4- resides at the interface
(under this conditions no water pool is present), while
the ketones are distributed between two environments.
At W0 g 10, the pseudophase model was extended to
include three pseudophases (water pool, isooctane, and
AOT), each of which is considered as being uniformly
distributed in the total volume of the reverse micellar
sistem.6 In this way, we consider the distribution of the
ketones between the organic pseudophase and the inter-
face, and the distribution of NaBH4 between the interface
and the water pool where it will be repelled from an
anionic interface.
In this model, the distribution of the reactants among
the pseudophases can be described by the partition
constant, with solute concentrations in the micelles
expressed as a function of the occupation number n )
[solute]b/[AOT]. In this sense, one avoids having to
explicitly define the molar reaction volume of the inter-
face,5,6,10 and the concentration in square brackets refer
to the analytical bulk concentration in moles per liter of
total solution volume5
occupation number is defined according to the place
where the substrate is located inside the aggregate, and
in the case of the ketone there is only the occupation
b
number relative to the ketone at the interface (nket
)
[ketone]b/[AOT]); on the other hand, NaBH4 is wholly in
the aggregate (reverse micelle surface and water pool,
depending on the water content W0); therefore, there are
two occupation numbers relative to NaBH4, the one
b
relative to NaBH4 at the surface (nNaBH ) [NaBH4]b/
4
[AOT]) and the one relative to NaBH4 in the water pool
w
(nNaBH ) [NaBH4]w/[AOT]).5
4
At W0 < 10 the mechanism summarized in Scheme 1
can be proposed6,39 (curvature and relative size are
arbitrary and drawn only for illustration purposes), and
at W g 10 the mechanism shown in Scheme 26-8 can be
proposed (curvature and relative size are arbitrary and
drawn only for illustration purposes).
b
nket
Kket
)
(3)
(4)
The rate of the reaction can be expressed by eq 5 at
W0 < 10 and by eq 6 at W0 g 10
[ketone]f
b
n
NaBH4
KBH
)
w
d[P]
dt
n
NaBH4
) k′b[ketoneb][NaBH4]T
) k′b[ketoneb][NaBH4]b
(5)
(6)
where Kket and KBH are the distribution constants of the
ketones and NaBH4 between the organic phase and the
interface and between the interface and the water pool,
d[P]
dt
respectively. nbket, nbNaBH , and nw
are the occupation
NaBH4
numbers of the ketones4and the NaBH4 defined above,
respectively. Sub- and superscripts f, b, and w indicate
the organic phase, the micellar pseudophase, and the
water pool, respectively. These equation applies at a fixed
value of W0 and when [substrates]T , [AOT]. The
where [P] is the concentration of the obtained product
and k′b is the bimolecular interfacial rate constant. The
concentrations in square brackets refer to the total
volume of reverse micelles.
If [NaBH4]T . [ketone]T, the reaction has a first-order
kinetic form. Then with eqs 3-6, and the mass balance
for the ketones and NaBH4, we obtain the final expres-
sion for the rate (eq 7) and kobs (eq 8 for W0 < 10 and eq
(38) Iglesias, E.; Leis, J. R.; Pen˜a, M. E. Langmuir 1994, 10, 662.
(39) Durantini, E. N.; Borsarelli, C. D. J. Chem. Soc., Perkin Trans.
2 1996, 719.
J. Org. Chem, Vol. 69, No. 24, 2004 8227

Correa et al.
FIGURE 1. Dependence of kobs on the AOT concentration for
the reaction between ketones and NaBH4 in water/AOT/
isooctane reverse micellar system at different W0: (A) [CAF]
) 3 × 10-4 M; (B) [AF] ) 3 × 10-4 M; (C) [MAF] ) 5 × 10-5
M. In all cases, [NaBH4]waterpool ) 0.1 M. The dotted line shows
the fitting by eq 8 (A) and eq 9 (B and C).
FIGURE 2. Variation of kobs on the NaBH4 concentration for
the reaction between ketones and NaBH4 in water/AOT/
isooctane reverse micellar system at different W0: (A) [CAF]
) 3 × 10-4 M; (B) [AF] ) 3 × 10-4 M; (C) [MAF] ) 5 × 10-5
M. In all cases, [AOT] ) 0.3 M. The dotted line shows the
fitting by eq 8 (A) and eq 9 (B and C).
8228 J. Org. Chem., Vol. 69, No. 24, 2004

Reverse Micellar Aggregates
9 for W0 g 10). Note that eq 9 reduces to eq 8 when KBH
is . 1, i.e., for W0 < 10. with
d[P]
dt
) kobs[ketone]T
(7)
(8)
(4k′bKket[NaBH4]T[AOT])
(1 + Kket[AOT])
kobs
)
(4k′bKketKBH[NaBH4]T[AOT])
kobs
)
(9)
(1 + Kket[AOT])(1 + KBH
)
To obtain the micellar second-order rate constants kb
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
in the reverse micellar system, which can be taken as
0.38.6-8,40
In this way, a conventional second-order rate constant
(with conventional units M-1s-1) is given by6,24
kb ) k′bvj
where vj is the surfactant molar volume.
The increase of kobs with increasing AOT concentrations
(Figure 1A-C) at all W0’s could be due to the gradual
transfer of the ketones from organic pseudophase into
the reverse micellar surface; however, there is no inter-
facial saturation, reflecting the low values of the distri-
bution constants. By fitting the experimental data with
eq 8 (Figure 1A) and eq 9 (Figure 1B,C), the Kket and kb
values are obtained and the results are in Table 1. The
KBH value is shown in the footnote of Table 1 and was
only determined by fitting the experimental results with
eq 9; i.e., it was not experimentally measured as the
ketones distribution constants. Values of Kket obtained
by UV spectroscopy are in very good agreement with the
kinetic values (Table 1). For all the ketones studied, kb
is lower at W0 15 than at 5 as can be seen from Figure 1
A-C.
CAF is the most and MAF the least reactive in the
micellar interface, as in water. This behavior reflects the
electron-withdrawing and -donating properties of the
aromatic substituents.41 The ketones are less reactive in
the micellar interface than in water, showing that this
region at W0 ) 5 is less favorable for the reaction,
probably reflecting the low BH4- affinity for the anionic
interface. It must be noted that, at W0 ) 5, where all the
NaBH4 is located at the interface, reactivity of all the
ketones, relative to those in water (kb/kw), are practically
the same independent from the ketones’ structure, but
at higher water content, when BH4- is repelled from the
interface, the rate constants are much lower in the
aggregate than in water. It must be pointed that the
properties of the interface vary much more at low water
content than at a higher one.5
FIGURE 3. Variation of kobs with respect to W0 for the
reaction between ketones and NaBH4 in water/AOT/isooctane
reverse micellar system: (A) [CAF] ) 3 × 10-4 M; (B) [AF] )
3 × 10-4 M; (C) [MAF] ) 5 × 10-5 M. In all cases, [AOT] ) 0.3
M.
The partition constants for the ketones are quite small
accordingly with the large affinity of the reactants for
the organic pseudophase. These constants are practically
independent of the water content, showing that the
binding is to the interface and not to the water pool. The
(40) Lang, J.; Jada, A.; Malliaris, A. J. Phys. Chem. 1988, 92, 1946.
(41) Stewart, R.; Teo, K. C. Can. J. Chem. 1980, 58, 2491.
J. Org. Chem, Vol. 69, No. 24, 2004 8229

Correa et al.
m-Cl derivative has the largest distribution constant even
for other kinds of reaction carried out in AOT reverse
micellar systems, such as aromatic nucleophilic substitu-
tion of dinitrobenzene halides by aliphatic amines.9 It
seems that chlorine substituents, in their structure,
increase the affinity for the AOT interface in reverse
micelles.
Analyzing the distribution constant of NaBH at W0 )
10 and 15, the low value obtained suggests that the
incorporation of NaBH4 into the interface of AOT reverse
micellar system is not favored due to electrostatic repul-
sion. Therefore, the ion largely resides in the water pool
of the aggregate.
Equations 8 and 9 also show that, if [AOT] and W0 are
fixed, there should be a linear relationship between kobs
and [NaBH4], and by the introduction of the values of
Kket and KBH the values of kb could be recalculated for
each system at W0 ) 5, 10, and 15 by fitting the new set
of experimental data with these equations (Figure 2). The
results are reported in Table 1. As shown, good estimates
of kb, within experimental error, are obtained by two
independent methods.
Conclusions
The mechanism of reaction between different aromatic
ketones and NaBH4 was investigated in a water/AOT/
isooctane reverse micellar system by UV spectroscopy.
Application of a kinetic model based on the pseudophase
formalism, which considers distribution of the ketones
between the continuous medium and the interface and
assumes that reaction takes place only at the interface,
has enabled us to obtain values of the rate constants in
the interface of the AOT reverse micellar system. At W0
) 5, it was considered that all of the NaBH4 is at the
interface. At W0 ) 10 and 15, where there are free water
molecules, partitioning between the interface and the
water pool was also considered. The observed rate
constants of reaction (kobs) increase with increasing
surfactant concentration for every ketone and at every
water content (W0) as a consequence of incorporating the
substrates into the interface of the reverse micellar
system, but saturation of the interface is never reached
due to the low value of Kket. For all the ketones studied
the reactivity is lower at the micellar interface than in
In all the systems studied, the kinetic profile upon
addition of water is quite peculiar and difficult to explain
(Figure 3). At W0 > 5, the kinetic profiles show that the
reaction rate decreases with the water content until W0
) 10, and then it remains practically constant. For W0 <
5, the reaction rate increases with W0. Our interpretation
of those data is that up to W0 ) 5, water is solvating the
water, probably reflecting the low affinity for the anionic
interface of BH4
-
.
The kinetic profiles upon water addition show maxima
in rates at W0 ca. 5: a probable explanation of this
experimental evidence is that at low W0 water is strongly
interacting with the polar headgroup of the surfactant,
leaving the ketones and the NaBH4 (located at the
interface) more reactive. Above this threshold, water
molecules probably start to interact with the reactants,
and once the water pool is formed, BH4- is repelled from
the anionic interface where the reaction takes place.
These results provide a good model to predict micellar
catalysis, in reverse micellar systems, for ketone reduc-
tion. Moreover, this study can be extended to control the
stereoselectivity because the core of these aggregates,
which are similar to the polar parts of enzymes, are the
reactive centers of the whole system and can easily create
a chiral environment for dissolving asymmetric solutes
in their interiors.
AOT polar headgroup, making less relevant Na+ BH4
-
-
ion pairing and leaving the ketones and BH4 more
reactive. At W0 > 5, the reactants are being solvated and
rates decrease. At W0 ∼ 10, the water pool is formed and
-
BH4 is repelled from the anionic interfacial reaction
region. At W0 > 10, [BH4-] at the interface is practically
constant as is the reaction rate. These results suggest
that the reaction is taking place at the interface of the
aggregate and not in the water pool because, as can be
seen from Table 1, for all the ketones the reaction is
faster in water.
Among the three ketones studied, acetophenone has
shown the lowest value of Kket. This is the one which is
located more in the oil-side of the interface, far away from
the NaBH4. Maybe this is the explanation for the bigger
difference found between the reaction rate in water and
in the reverse micellar system for this ketone (see Table
1).
The highest value of W0 ) 15 is much lower than that
of ca. 70 that can be reached in isooctane/AOT/water
reverse micellar systems,2,4,5 showing that BH4- and HO-
are disturbing the reverse micellar system, probably due
to the salt concentrations inside the aggregates.3,4,11
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 a Sci-
entific Member of CONICET (Consejo Nacional de
Investigaciones Cientificas y Tecnologicas).
JO049173N
8230 J. Org. Chem., Vol. 69, No. 24, 2004