Effect of cyclodextrin on the intramolecular catalysis of amide hydrolysis

Article abstract of DOI:10.1021/jo001011d

The hydrolysis reaction of phthalamic acids (HOOCArCONHR, R = p-NO₂Ph 1a, Ph 1b, adamantyl 1c) and N-phenyl maleamic acid 2b was studied in the presence of hydroxypropyl-β-cyclodextrin (HPCD) in acid solution. The reactions of 1a and 1b were studied also in the presence of β-cyclodextrin (β-CD). All the compounds formed inclusion complexes with HPCD, and the association constant was determined from the change in absorption of the substrate when the host is added in the case of 1a (90 M^-1) and 2b (49 M^-1). For 1c (4 × 10⁴ M^-1) a competition method was used, and for 1b the association equilibrium constant was obtained from the kinetic data (37 M^-1) because it is too reactive for the spectrophotometric method. Both cyclodextrins strongly inhibited the reactions, and analysis of the kinetic data for HPCD indicated that the reactions of complexed 1a, 1b, and 2b are at least 10-30 times slower than in the bulk solution whereas 1c reacts only 4.6 times slower when it is complexed. The inhibition is attributed to changes in the geometry of the substrate due to interaction of the carboxylic group and/or the amide with the OH at the rim of the cyclodextrin. The differences in the relative effect observed for 1c are attributed to the formation of a tighter complex with this substrate.

Full text of DOI:10.1021/jo001011d

1548
J . Org. Chem. 2001, 66, 1548-1552
Effect of Cyclod extr in on th e In tr a m olecu la r Ca ta lysis of Am id e
Hyd r olysis
Alejandro M. Granados and Rita H. de Rossi*
Instituto de Investigaciones en Fı´sico Quı´mica de Co´rdoba (INFIQC), Facultad de Ciencias Qu´ımicas,
Departamento de Quı´mica Orga´nica, Universidad Nacional de Co´rdoba, Ciudad Universitaria,
5000 Co´rdoba, Argentina
ritah@dqo.fcq.unc.edu.ar
Received J uly 6, 2000
The hydrolysis reaction of phthalamic acids (HOOCArCONHR, R ) p-NO2Ph 1a , Ph 1b, adamantyl
1c) and N-phenyl maleamic acid 2b was studied in the presence of hydroxypropyl-â-cyclodextrin
(HPCD) in acid solution. The reactions of 1a and 1b were studied also in the presence of
â-cyclodextrin (â-CD). All the compounds formed inclusion complexes with HPCD, and the
association constant was determined from the change in absorption of the substrate when the host
is added in the case of 1a (90 M-1) and 2b (49 M-1). For 1c ( 4 × 104 M-1) a competition method
was used, and for 1b the association equilibrium constant was obtained from the kinetic data (37
M-1) because it is too reactive for the spectrophotometric method. Both cyclodextrins strongly
inhibited the reactions, and analysis of the kinetic data for HPCD indicated that the reactions of
complexed 1a , 1b, and 2b are at least 10-30 times slower than in the bulk solution whereas 1c
reacts only 4.6 times slower when it is complexed. The inhibition is attributed to changes in the
geometry of the substrate due to interaction of the carboxylic group and/or the amide with the OH
at the rim of the cyclodextrin. The differences in the relative effect observed for 1c are attributed
to the formation of a tighter complex with this substrate.
Intramolecular catalysis of amide hydrolysis is a reac-
tives form very strong complexes with â-cyclodextrines10
so we expected a stronger effect. Quite to the contrary,
the reaction of this substrate is inhibited as all the others,
but the effect is less significant than for the other
substrates.
tion that has received attention as a model for enzyme
catalysis.1,2,3 The neighboring carboxylic group adds to
the carbonyl carbon of the amide forming a tetrahedral
intermediate which then gives the anhydride. Since the
kinetic barrier to the hydrolysis of anhydrides is consid-
erably lower than for the hydrolysis of an amide, the
intramolecular addition of a carboxyl group to the amide
produces a catalytic route to hydrolysis. It has been
shown that factors that favor the formation of the cyclic
intermediate have enormous influence on the reaction
rates.4,5 Cyclodextrins, which are cyclic oligomers of R-D-
glucose have a well-defined cavity6 and have been
frequently used as microreactors which can catalyze or
inhibit organic reactions by including the substrate in
their cavity.7,8 Since intramolecular reactions are very
much dependent on the relative position of the reacting
groups and the time that they are at the proper distance
for reaction,9 we considered it interesting to study the
effect of cyclodextrin on the hydrolysis of compounds 1
and 2, and the results are reported here. Substrate 1c
was used because it is known that adamantane deriva-
Resu lts
The reactions of 1a and 1b were studied in the range
of pH 1 to 4 (Table S1).11 The rate of the reactions
decreases slightly between pH 1 and 3 and more between
pH 3 and 4. Increasing hydrogen ion concentration
increases the reaction rate by raising the proportion of
the un-ionized substrate (pKa of 1b ) 3.62)12 which is
the active species. The spectrum of the reaction products
was compared with a solution containing the correspond-
ing aniline and phthalic acid in the expected concentra-
tion for a quantitative reaction. The spectrum as well as
its first and second derivatives match that of a mock
solution. These results indicate that the reaction takes
place as indicated in eq 1 and that not a significant
(1) Kirby, A. J .; Lancaster, P. W. J . Chem. Soc., Perkin 2 1972, 1206.
(2) Suh, J . Chun, K. H. J . Am. Chem. Soc. 1986, 108, 305.
(3) Menger, F. M.; Ladika, M. J . Am. Chem. Soc. 1988, 110, 6794.
(4) Higuchi, T., Eberson, L., Herd, A. K. J . Am. Chem. Soc. 1966,
88, 3805,
(5) Higuchi, T., MiIki, T.; Shah, A. C.; Herd, A. K. J . Am. Chem.
Soc. 1963, 85, 3655.
amount of the imide is formed as was observed under
other reaction conditions. Similar results were obtained
with 1c and 2b which kinetic of hydrolysis was studied
at pH 2.
(6) (a) M. L. Bender, M. Komiyama. Cyclodextrin Chemistry; Springer-
Verlag: New York, 1977. (b) I. Tabushi, Y. Kuroda. In Advances in
Catalysis; Eley, D. D., Pines, H., Weisz, P. B., Eds.; Academic Press:
London, 1983; Vol. 32, pp 417-462.
(10) (a) Shortreed, M. E.; Wylie, R. S.; Macartney, D. H. Inorg Chem
1993, 32, 1824. (b) Cromwell, W. C.; Bystrom, K.; Eftink, M. R. J . Phys.
Chem. 1994, 89, 326.
(7) Takashi, K. Chem. Rev. 1998, 98, 2013.
(8) Viola, L.; de Rossi, R. H. Can. J . Chem. 1999, 77, 860.
(9) Menger, F. M. Acc. Chem. Res. 1985, 18, 128.
(11) Supporting Information.
(12) Morawetz, H.; Shafer, J . A. J . Am. Chem. Soc. 1962, 84, 3783.
10.1021/jo001011d CCC: $20.00 © 2001 American Chemical Society
Published on Web 02/10/2001

Effect of Cyclodextrin on Amide Hydrolysis
J . Org. Chem., Vol. 66, No. 5, 2001 1549
Ta ble 1. Ca lcu la ted Ra tes a n d Equ ilibr iu m Con sta n ts
for th e Rea ction s of 1 a n d 2 in th e P r esen ce of HP CDa
substrate
ko, s-1
Kass., M-1
kCD, s-1
ko/kCD
>27
1a b
5.7 × 10-5 (90 ( 20)c
<2 × 10-6f
<9 × 10-5f
(84 ( 8)d
1b
1c
1.0 × 10-3 (37 ( 3)d
>11
1.2 × 10-4 (4.1 ( 0.6) × 104,e (2.6 ( 0.6) × 10-5 4.6
(1.6 ( 0.1) × 103,d
2b
4.0 × 10-4 (49 ( 3)c
(71 ( 2)d
<3 × 10-5f
>15
Effect of Cyclod extr in . The addition of â-cyclodextrin
(â-CD) or hydroxypropyl-â-cyclodextrin (HPCD) to solu-
tions of 1a and 2b showed a batochromic shift of the
maximum absorption, but no isosbestic point was ob-
served. Most of the study was carried out using HPCD
instead of â-CD because of the more convenient solubility
of the former, which allows determinations at higher host
concentrations. Using the values of the absorption mea-
sured at the wavelength of maximum change, and
assuming 1:1 stoichiometry for the complexes, the equi-
librium constant for the association of 1a and 2b with
HPCD was determined by adjusting the experimental
data to eq 213 (Table 1).
a
Data at 40 °C, in water-ACN 4%, pH 2, ionic strenght 0.5 M
unless otherwise noted. The error in ko is less than 3% in all cases.
The organic cosolvent is 4% dioxane. c Spectrophotometric value,
b
solvent ACN-water 4%v/v except for 1a (dioxane 4% v/v), tem-
perature 25 °C. Kinetic value. e Determined by competition
d
method in 2% ACN and pH 10.47. f Calculated assuming that
k
CDKass.[0.03]/ko ) 0.1.
b(ꢀc - ꢀs)[So]Kass.[HPCD]o
A ) Ao +
(2)
1 + Kass.[HPCD]o
In Figure 1 is shown the data for 1a . Absorbance
values measured at different wavelengths gave the same
equilibrium constant, and this is a good indication that
the complex has 1:1 stoichiometry. In the case of 1b the
equilibrium constant could not be determined because
of the high reactivity of this compound.
On the other hand, the spectrum of 1c does not change
in the presence of â-CD or HPCD in the range of
concentrations useful for the determinations (e0.03
M).14,15 In this case we used the competition method for
the determination of the association constant. The com-
petitor used was phenolphthalein (Pht).16 This compound
has a strong absorption band at 550 nm at pH higher
than 10. This band vanishes when the complex [Pht‚
HPCD] is formed. Adding increasing amounts of 1c to
solutions with constant concentration of Pht and HPCD,
the band at 550 nm increases as Pht is displaced from
the cavity for 1c. Treating these data as indicated in the
literature,13 the value of Kass. was obtained (Table 1).
F igu r e 1. Absorbance of 1a at 370 nm (9) and at 353 nm (b)
vs HPCD concentration. The lines are calculated using eq 2.
[1a ]o ) 5 × 10-5 M, solvent: 4% dioxane in water at pH ) 2,
ionic strenght 0.5 M and 25 °C.
under the same reaction conditions, but this compound
is formed in 20% when the solvent contains 80% diox-
ane.17
The hydrolysis rate decreases in all cases in the
presence of HPCD in a nonlinear fashion (Figure 2 is
representative). The rate of hydrolysis of 1a and 1b also
decreases in the presence of â-CD (Table 2S).
On the other hand, Shafer et al.18 reported that 1b in
0.1 M HCl and at 100 °C gave no more than 0.2% of
phthalimide. These results show that pH and cosolvent
concentration are important factors which determine the
competition between imide and anydride formation. As
stated in the results, we did not find evidence for the
formation of imides, which is in agreement with the
literature data mentioned above.17,18
Discu ssion
It was reported in the literature that compound 1a
produces the imide 3 (Ar ) p-NO2-Ph) when heated at
65.8 °C and at pH 4.42 in dioxane 20%-water 80%, while
compound 1b does not produce the corresponding imide
The mechanism of the reaction as described in the
literature17 is shown in Scheme 1.
(13) Connors, K. A. Binding Constants: The Measurement of Mo-
lecular Complex Stability; Wiley-Interscience: New York, 1987.
(14) Loftsson, T.; Brewster, M. E.; Derendorf, H.; Bodor N. Pharm.
Ztg. Wiss. 1991, 5.
(15) Considering the solubility of HPCD in water, higher concentra-
tions could be used, but then the properties of the solvent changes
and interpretation of the results become too difficult.
(16) (a) Buvaribarcza, A.; Kajtar, J .; Barcza, L. J Inclusion Phenom.
Mol. Recog. 1996, 24, 211. (b) Gray, E.; MacLean, S. A.; Reinsborough,
V. C. Aust. J . Chem. 1995, 48, 551.
Under our reaction conditions, the formation of prod-
ucts is the rate-determining step, and the proton-
catalyzed pathway (k3) becomes significant below pH 1.17
(17) Hawkins, D. M. J . Chem. Soc., Perkin 2 1976, 642.
(18) Brown, J .; Su, S. C. K.; Shafer, J . A. J . Am. Chem. Soc. 1966,
88, 4468.

1550 J . Org. Chem., Vol. 66, No. 5, 2001
Granados and de Rossi
Sch em e 2
found with p-nitroaniline;19 (b) the association of 1c has
a much higher value than that corresponding to the other
substrates, consistent with the fact that adamantane
forms a very strong complex with cyclodextrin;10 (c)
Naptalam (N-1-naphthylphthalanilic acid) does not form
a complex with â-cyclodextrin, and its rate of hydrolysis
is not affected by the presence of cyclodextrins.20 Besides
the Kass. of substrates, 1 and 2 parallel those of the
corresponding amines with â-CD, namely 40 M-1 21
,
260
M-1 22
,
and 1.1 × 105 M-1 23
, for aniline, p-nitroaniline, and
adamantylamine, respectively.
Considering the formation of the inclusion complex of
the substrate with â-cyclodextrin or HPCD, the reaction
can be represented as shown in Scheme 2 where ko
represents the observed rate constant for the reaction in
the bulk solution, kCD is the observed rate constant for
the reaction of the complexed substrate and CD repre-
sents â-CD or HPCD.
F igu r e 2. Dependence of the observed rate constant for the
hydrolysis of 1a (2) (right ordinate), 1b (9), and 2b (b) (left
ordinate) with HPCD concentration. Temperature 40 °C, pH
2 and ionic strenght 0.5 M. The lines are calculated using eq
3 with kCD ) 0.
Under the conditions used in our study (pH ) 2), the
substrate is mainly in its neutral form since the pKa
should be in the order of that of 1b12 or higher. Therefore,
S in Scheme 2 represents the neutral substrate.
Based on this scheme the observed rate constant is
given by eq 3
ko + kCDKass.[CD]
kobs
)
(3)
1 + Kass.[CD]
Since for 1a and 2b the Kass. was determined spectro-
photometrically, we first attempted to calculate kCD by
nonlinear-least-squares analysis of the data24 using eq 3
with only one adjustable parameter, but the value of kCD
obtained has very large error or is negative so it has no
physical meaning. If kCDKass.[CD] < ko, eq 3 can be
rearranged to eq 4
F igu r e 3. Schematic representation of the inclusion com-
plexes formed between compounds 1 or 2 and cyclodextrin. R
represents aryl ring (1a , 1b, and 2b) or adamantane (1c). The
broken line for the aryl ring is to indicate that it is not present
in the case of 2b.
Sch em e 1
K
ass.[CD]
1
1
)
+
(4)
kobs ko
ko
A plot according to eq 4 is linear (Figure 4) and from
the ratio of slope and intercept the value of Kass. can be
calculated and it is in reasonable good agreement with
the spectrophotometric value. It should be noted that the
kinetic value of Kass. is obtained at 40 °C whereas the
spectrophotometric value was determined at 25 °C to
avoid hydrolysis during the measurement. The data for
1a with â-CD (Table 2S) plotted in the same way give a
good straight line and Kass. ) 46 M-1 is obtained. For 1c
a similar plot is not linear, indicating that in this case
The change in the spectrum with the addition of
cyclodextrin indicates that there is some kind of interac-
tion between them. Addition of an equivalent amount of
glucose to 1a does not produce changes in the spectrum
nor in the rate of reaction. We suggest that a complex is
formed with the substrate included in the cavity of
cyclodextrin. The compounds have two binding sites, and
the inclusion might be through any of them. We think
that the complex has the structure shown in Figure 3
based on the following observations: (a) the changes in
the spectrum of 1a with cyclodextrin are similar to those
(19) Buva´ri, A. Barcza, L. J . Chem. Soc., Perkin 2, 1988, 543.
(20) Granados, A.; Nassetta, M. de Rossi, R. H. J . Agric. Food Chem.
1995, 43, 2493.
(21) Veglia, A. V.; de Rossi, R. H. J . Org. Chem. 1988, 53, 5281.
(22) Barra, M.; de Rossi R. H.; de Vargas, E. B. J . Org. Chem. 1987,
52, 5004.
(23) Odashima, K.; Hashimoto, H.; Umezawa, Y. Mikrochim. Acta
1994, 113, 223.
(24) Sigmaplot, Handel Scientific, Version 3.02.

Effect of Cyclodextrin on Amide Hydrolysis
J . Org. Chem., Vol. 66, No. 5, 2001 1551
catalyzed hydrolysis rate by inclusion in the cyclodextrin
cavity could be explained by a microsolvent effect;
however, in this case we would expect about the same
inhibition for all the substrates, and this is not the case.
The mechanism of the reaction involves several reac-
tion steps (Scheme 1). It is known that in maleamic acid
changing the substituents on the double bond has a
significant effect on the rate, and this was attributed to
modification of the equilibrium constant for the formation
of the tetrahedral intermediate.30 The largest acceleration
for intramolecular catalysis is observed when the groups
are held in close proximity in comformationally rigid
molecules as in the maleate system.1,3
To interpret the effect of cyclodextrin on the observed
rate, we have to consider that under the conditions used
in our study, the rate-determining step for the reactions
in water solution is the amine expulsion from the
tetrahedral intermediate.17 We think that it is not likely
that this rate is so much decreased for complexation by
cyclodextrin as to explain the inhibition observed. We
believe that the main effect must be due to a decrease in
the equilibrium constants for the intramolecular proton
transfer to form 4 and/or for the formation of the
tetrahedral intermediate 5 (Scheme 1). The formation of
the zwitterionic intermediate 4 is very important for the
catalysis because it is known that the carboxylate groups
do not catalyze the reaction,1,17 and it could be that the
hydrophobic cavity of cyclodextrin significantly decreases
this equilibrium constant due to changes in the dipolar
moment of the molecule. It is known that the contribution
of dipolar interactions to the formation of inclusion
complexes is minor,31 then we do not expect a great
difference in KZ for free and bound substrates. We suggest
that the inhibition is mainly due to a decrease in the
equilibrium constant for the formation of the tetrahedral
intermediate 5 due to a decrease in the rate of intra-
molecular ring closure. We also suggest that the inclusion
complex of the substrate with the cyclodextrin perturbs
the distribution of conformers in the ground-state, forcing
it to adopt an unfavorable orientation for the intramo-
lecular catalysis. The effectiveness of the inclusion to
modify the conformers distribution should depend on the
geometry of the fit. Compound 1c, having the adaman-
tane with a great affinity for the cavity and very tight
fit, leaves the rest of the molecule free to adopt the
adequate conformation for the reaction and exposed to
the solution. In the case of the other substrates the
complexes are looser so the structure of the complex may
adopt a geometry that maximizes the hydrogen bond
interaction with OH at the rim of the cavity at the
expenses of the conformation more appropriate for the
reaction. It is interesting to note that these reactions can
be considered as a simple model for noncompetitive
inhibition of enzymatic catalysis.
F igu r e 4. Plot of the reciprocal of the observed rate constant
for 1a (2) (right ordinate), 1b (b), and 2b (9) (left ordinate)
vs hydroxypropyl-â-cyclodextrin.
the value of kCDKass. is not negligible compared to the
value of ko. This may be due to the high value of Kass, to
a ratio kCD/ko higher than for the other substrates, or to
both effects. Besides, the data for this substrate could
not be adjusted to eq 3 using the value of Kass. determined
by the competition method. Nonlinear adjustment of the
data yield a value of kCD ) (2.6 ( 0.6) × 10-5 s-1 and
Kass. ) (1.4 ( 0.1) × 103 M-1 which is about 25 times
smaller than the spectrophotometric value. The difference
in the kinetic and spectrophotometric values can be due
to the difference in reaction conditions for the two
methods of determinations. The kinetic data were ob-
tained with acetonitrile as cosolvent in 4% volume
whereas the spectrophotometric data were obtained with
2% ACN.25 It was not possible to determine the equilib-
rium constant in 4% ACN because under these conditions
the spectrum of phenolphthalein changes only very little
with the addition of the amide. Besides, in the spectro-
photometric determination a buffer was used, and it is
known that buffers can significantly influence the as-
sociation equilibrium constants.26 Remarkable effects of
buffers were reported in the association of a â-CD
derivative with p-nitrophenyl phosphate.27 The values of
association equilibrium constant are 6 × 104 M-1, 2.1 ×
105 M-1, and 4.1 × 102 M-1 with 0.01 M Tris, imidazole,
and phosphate, respectively. Considering the good cor-
relation of the data through eq 4, we conclude that ko/
kCD must be 10-30 or higher for substrates 1a , 1b, and
2b, indicating a very strong inhibition. On the other
hand, for 1c this ratio is only 4.6.
The addition of dioxane to water produces a decrease
in the observed rate of hydrolysis of phthalanilic acids,17
and also the rate of hydrolysis of carboxylic esters
decrease and then increase when organic solvents are
added. The complex behavior was attributed to several
factors such as changes in the dielectric constants of the
reaction medium28 and the change from an ionic to polar
transition state.29 The inhibition of the intramolecularly
Exp er im en ta l Section
Aqueous solutions were made up from water purified in a
Millipore apparatus. Acetonitrile Merck HPLC grade was used
as received, and dioxane was purified as in previous work.32
â-Cyclodextrin and HPCD (average degree of substitution 5.9,
(25) We have determined the equillibrium constant for the associa-
tion of phenyl phthalate with HPCD in 2 and 4% ACN and the
corresponding values were 600 and 300 M-1
.
(26) Yi, Z. P.; Huang, Z. Z.; Yu, J . S.; Chen, H. L. Chem Lett. 1998,
1161-2.
(27) Ghosh M.; Zhang R.; Lawler, R. G.; Seto, C. T. J . Org. Chem.
2000, 65, 735.
(30) Aldersley, M. F.; Kirby, A. J .; Lancaster, P. W.; McDonals, R.
S.; Smith, C. R. J . Chem. Soc., Perkin 2 1974, 1487.
(31) Park J . H.; Nah T. H. J . Chem. Soc., Perkin Trans 2 1994,
1359-62.
(28) Laidler, K. J .; Landskrooener, C. A. Trans. Faraday Soc. 1956,
52, 200.
(29) Hyne, J . B. J . Am. Chem. Soc. 1960, 82, 5129.
(32) de Rossi, R. H.; de Vargas, E. B. J . Am. Soc. 1981, 103, 1533.

1552 J . Org. Chem., Vol. 66, No. 5, 2001
Granados and de Rossi
MW 1476) (Roquette) were a gift from Ferromet S.A. Argentina
and were used without further purification.
The pH measurements were done at controlled temperature
and calibrated with buffers prepared in our laboratory accord-
ing to the literature.33
320, and 330 nm for 2b, and the values obtained give very
good fit to eq 2.
The same method was used for phenolphthalein (2.6 × 10-6
M) at pH 10.47 (buffer CO32-/CO3H- 0.1M) in water containing
2% ACN and ionic strength 0.5 M and HPCD within 1.49 ×
105 M and 1.02 × 10-3 M. The absorbance was measured at
550 nm. The value obtained was (1.50 ( 0.04) × 104 M-1 in
good agreement with literature values,16 namely (2.0 ( 0.4) ×
104 M-1 for â-CD and (1.2 ( 0.1) × 104 M-1 for HPCD. For the
determination of the equilibrium constant for 1c a solution
containing phenolphthalein 6 × 10-6 M and HPCD 6 × 10-5
M at pH 10.47 was added to a solution of 1c in ACN so that
the final concentration of the organic solvent was 2% v/v, and
that of 1c varies within 9.22 × 10-6 M and 1.54 × 10-4 M.
Compounds 1a and 1b were prepared by adding the corre-
sponding aniline (0.01mol) to a chloroform solution of phthalic
anhydride17 (yield 62 and 75%, respectively). The product was
recrystallized from chloroform. For compound 1c the solvent
was boiling pyridine, and the product was isolated after
evaporation of the solvent and recrystallized from chloroform
(yield 48%).34 Compound 2b was prepared by adding a solution
of maleic anhydride in acetonitrile to a solution of aniline in
the same solvent (yield 87%). The compounds were character-
ized by GC-MS, and the purity was checked by comparison of
the hydrolyzed solution with a mock solution containing
phthalic acid and the amine.
Kin etic Deter m in a tion s. They were done as reported35 at
40 °C, ionic strength 0.5M (NaCl), and 4% of organic solvent
that is dioxane for 1a and acetonitrile for 1b, 1c, and 2b. HCl
or acetic/acetate buffer were used to regulate the pH. The
observation wavelengths were 380 nm for 1a , 275 for 1b, and
2b and 225 nm for 1c.
Ack n ow led gm en t. This research was supported in
part by the Consejo Nacional de Investigaciones Cien-
tificas y Te´cnicas (CONICET), the Consejo de Investi-
gaciones Cient´ıficas y Tecnolo´gicas de la Provincia de
Co´rdoba, (CONICOR), Agencia Nacional de Ciencia y
Tecnolog´ıa (FONCYT), and the Universidad Nacional
de Co´rdoba, Argentina.
Equ ilibr iu m Deter m in a tion s. For 1a and 2b the method
used was as described previously.36 The absorbance of the
solution was measured at 370 and 353 nm for 1a and 312,
Su p p or tin g In for m a tion Ava ila ble: Table S1 and S2,
containing the observed rate constant for substrates 1 and 2
as function of pH and cyclodextrin concentration. This material
is available free of charge via the Internet at http://pubs.acs.org.
(33) Handbook of Chemistry and Physics, 72nd ed.; Lide, D. R., Ed.;
CRC Press: Boca Raton, FL, 1991-1992; pp 8-30 to 8-35.
(34) Herr, M. E.; J ohnson, R. A.; Murray, H. C.; Lester, M. R.;
Fonken, G. S. J . Org. Chem. 1968, 33, 3201.
J O001011D
(36) Sanrame, C.; de Rossi, R. H.; Arguello, G. J . Phys. Chem. 1996,
100, 8151.
(35) Granados, A.; de Rossi, R. H. J . Am. Chem. Soc. 1995, 117, 3690.