Intramolecular general base catalyzed ester hydrolysis. The hydrolysis of 2-aminobenzoate esters

Article abstract of DOI:10.1021/jo0103017

Rate constants have been obtained for the hydrolysis of the trifluoroethyl, phenyl, and p-nitrophenyl esters of 2-aminobenzoic acid at 50°C in H₂O. The pseudo-first-order rate constants, k_obsd, are pH independent from pH 8 to pH 4 (the pK_a of the amine group conjugate acid). The 2-aminobenzoate esters hydrolyze with similar rate constants in the pH-independent reactions, and these water reactions are ～2-fold slower in D₂O than in H₂O. The most likely mechanism involves intramolecular general base catalysis by the neighboring amine group. The rate enhancements in the pH-independent reaction in comparison with the pH-independent hydrolysis of the corresponding para substituted esters or the benzoate esters are 50 - 100-fold. In comparison with the hydroxide ion catalyzed reaction, the enhancement in k_obsd at pH 4 with the phenyl ester is 10⁵-fold. Intramolecular general base catalyzed reactions are assessed in respect to their relative advantages and disadvantages in enzyme catalysis. A general base catalyzed reaction can be more rapid at low pH than a nucleophilic reaction that has a marked dependence on pH and the leaving group.

Full text of DOI:10.1021/jo0103017

J . Org. Chem. 2002, 67, 3179-3183
3179
In tr a m olecu la r Gen er a l Ba se Ca ta lyzed Ester Hyd r olysis. Th e
Hyd r olysis of 2-Am in oben zoa te Ester s
Thomas H. Fife,* Randhir Singh, and Ramesh Bembi
Department of Biochemistry, University of Southern California, Los Angeles, California 90089-9560
tfife@hsc.usc.edu
Received March 19, 2001
Rate constants have been obtained for the hydrolysis of the trifluoroethyl, phenyl, and p-nitrophenyl
esters of 2-aminobenzoic acid at 50 °C in H
independent from pH 8 to pH 4 (the pK of the amine group conjugate acid). The 2-aminobenzoate
esters hydrolyze with similar rate constants in the pH-independent reactions, and these water
O than in H O. The most likely mechanism involves intramolecular
2
O. The pseudo-first-order rate constants, kobsd, are pH
a
reactions are ∼2-fold slower in D
2
2
general base catalysis by the neighboring amine group. The rate enhancements in the pH-
independent reaction in comparison with the pH-independent hydrolysis of the corresponding para
substituted esters or the benzoate esters are 50-100-fold. In comparison with the hydroxide ion
5
catalyzed reaction, the enhancement in kobsd at pH 4 with the phenyl ester is 10 -fold. Intramolecular
general base catalyzed reactions are assessed in respect to their relative advantages and
disadvantages in enzyme catalysis. A general base catalyzed reaction can be more rapid at low pH
than a nucleophilic reaction that has a marked dependence on pH and the leaving group.
Classical general base catalysis, in which a base
partially abstracts a proton from a water molecule in the
transition state, has been suggested for a number of
enzymatic reactions.1 For example, the deacylation of
acyl R-chymotrypsins has been considered to occur by a
general base mechanism involving histidine-57 (I).1,5
general base mechanism in enzymatic reactions are
therefore not apparent.
Chemical intramolecular reactions resemble the intra-
-6
1
complex reactions of enzymes. A general base mecha-
nism has been found in ester and amide hydrolysis
reactions in cases where a nucleophilic reaction is
precluded, or difficult.8
-15
A direct comparison of intra-
molecular general base catalyzed reactions and intra-
molecular nucleophilic reactions of structurally similar
esters would allow a rigorous evaluation of relative
mechanistic advantages and disadvantages, which could
in turn provide a chemical basis for the proposed mech-
anisms in enzymatic reactions.
2
-Substituted benzoate esters are ideal compounds for
the kinetic investigation of intramolecular reactions
because of both the rigid steric relationship between the
substituent group and the ester carbonyl and the sys-
tematic structural variations that are possible. A basic
group in the 2-position of benzoate esters with which
nucleophilic attack is sterically difficult might preferen-
tially function in hydrolysis reactions as an intramolecu-
lar general base. We have therefore in this work deter-
mined rate constants for hydrolysis of the esters II-IV
with which the neighboring amine group cannot easily
act as a nucleophile because of the four-membered ring
transition state that would be required; a tetrahedral
intermediate resulting from nucleophilic attack by the
General base catalyzed hydrolysis reactions involve
restriction of at least one water molecule in the transition
state and therefore should be less favorable than nucleo-
philic reactions that proceed by direct attack of a
functional group at the reaction center. It is generally
thought that nucleophilic reactions will occur when
sterically possible. Current concepts can more easily
explain the large rate constants in enzymatic reactions
7
in terms of nucleophilic processes. The reasons for a
(1) (a) Bruice, T. C.; Benkovic, S. J . Bioorganic Mechanisms;
1
6
Benjamin: New York, 1966. (b) Bruice, P. Y. Organic Chemistry;
Prentice Hall: Englewood Cliffs, NJ , 1995; Chapter 24.
neighboring amine would be highly strained. It is
important to determine the steric limitations of both
(2) Lipscomb, W. N. Acc. Chem. Res. 1970, 3, 81.
(
3) (a) Hangauer, D. G.; Monzingo, A. F.; Matthews, B. W. Biochem-
istry 1984, 23, 5730. (b) Christianson, D. W.; Lipscomb, W. N. J . Am.
Chem. Soc. 1986, 108, 4998. (c) Christianson, D. W.; Lipscomb, W. N.
Acc. Chem. Res. 1989, 22, 62.
(8) Fife, T. H.; Benjamin, B. M. Chem. Commun. 1974, 525.
(9) J encks, W. P.; Carriuolo, J . J . Biol. Chem. 1959, 234, 1272.
(10) Felton, S. M.; Bruice, T. C. J . Am. Chem. Soc. 1969, 91, 6721.
(11) Bruice, P. Y.; Bruice, T. C. J . Am. Chem. Soc. 1974, 96, 5523.
(12) Bender, M. L.; Kezdy, F. J .; Zerner, B. J . Am. Chem. Soc. 1963,
85, 3017.
(13) Fife, T. H.; Hutchins, J . E. C. J . Am. Chem. Soc. 1972, 94, 2837.
(14) Kirby, A. J .; Lloyd, G. T. J . Chem. Soc., Perkin Trans. 2 1976,
1748, 1753.
(4) (a) Cotton, F. A.; Hazen, E. E.; Legg, M. J . Proc. Natl. Acad. Sci.
U.S.A. 1979, 76, 2551. (b) Oefner, C.; Suck, D. J . Mol. Biol. 1986, 192,
05. (c) Suck, D.; Oefner, C. Nature 1986, 321, 620.
6
2
(5) Bender, M. L.; Kezdy, F. J . J . Am. Chem. Soc. 1964, 86, 3704.
(6) Fife, T. H.; Milstien, J . B. Biochemistry 1967, 6, 2901.
(7) Lightstone, F. C.; Bruice, T. C. J . Am. Chem. Soc. 1996, 118,
595.
(15) Fersht, A. R.; Kirby, A. J . J . Am. Chem. Soc. 1967, 89, 4857.
1
0.1021/jo0103017 CCC: $22.00 © 2002 American Chemical Society
Published on Web 04/17/2002

3
180 J . Org. Chem., Vol. 67, No. 10, 2002
Fife et al.
general base and nucleophilic reactions. The intra-
molecular nucleophilic reactions of 2-aminomethylbenzo-
ate esters were reported in a preceding paper.17
There have been few previous examples of intramo-
lecular general base catalysis by a primary amine group
8
18
of the type in II-IV. Capon and Ghosh reported that
the amine group of III did not participate in the hydroly-
sis reaction, but they only determined rate constants
at high and low pH (>8.5 and <2). We have found
pronounced participation by the neighboring group in the
hydrolysis reactions at pH < 8.5.
F igu r e 1. Plot of log kobsd vs pH for the hydrolysis of phenyl
2-aminobenzoate in H O at 50 °C (µ ) 0.1 M with KCl).
2
Exp er im en ta l Section
Ma ter ia ls. The trifluoroethyl, phenyl, and p-nitrophenyl
esters of anthranilic acid have been reported previously and
1
9,20
were prepared by the literature procedures.
tallization from chloroform, the hydrochloride of phenyl 2-amino-
2
12ClNO :
After recrys-
benzoate had an mp ) 149 °C. Anal. Calcd for C13
C, 62.52; H, 4.81; N, 5.61. Found: C, 62.26; H, 4.44; N, 5.20.
H
Phenyl 4-aminobenzoate was prepared by reduction of
phenyl 4-nitrobenzoate in absolute ethanol with a PtO
2
2
0
catalyst by the method of Berti et al. The ester had an mp )
2
0
1
67 °C (lit. 169-172 °C).
Infrared and NMR spectra were consistent in all cases with
the expected ester structure. Buffers were prepared from
reagent-grade material. Amine buffer components were freshly
distilled or recrystallized prior to use.
Kin etic Meth od s. The hydrolysis of 2- and 4-aminoben-
zoate esters in water (µ ) 0.1 M with KCl) was followed by
monitoring the changes in absorbance at appropriate wave-
lengths between 245 and 400 nm with a Beckman DU-7000
spectrophotometer. The spectra at the conclusion of the
reactions quantitatively matched those of 2- or 4-aminobenzoic
acid and the alcohol or phenol. Repetitive scans during the
course of the reactions gave no evidence for the formation of
an accumulating intermediate. To initiate the reactions, 30
µL of an acetonitrile solution of the ester was added to the
buffer in the cuvette. Buffers included chloroacetate, formate,
acetate, cacodylate, imidazole, Tris, morpholine, trimethyl-
amine, and carbonate. Significant buffer catalysis was not
observed under conditions of the experiments (0.02 M buffer).
The reactions were followed to completion, and infinity points
were stable. The values of kobsd, the pseudo-first-order rate
constants, were computer calculated using a rigorous least-
squares procedure. The reaction solution pH values were
measured with either a Radiometer model 22 pH meter with
a GK 2303 C combination electrode or a Beckman model 3500
pH meter with a combination electrode standardized against
F igu r e 2. Plot of log kobsd vs pH for the hydrolysis of phenyl
4
-aminobenzoate in H
2
O at 80 °C (µ ) 0.1 M with KCl).
Resu lts
Figure 1 shows the plot of log kobsd vs pH for hydrolysis
of phenyl 2-aminobenzoate (III) in H O at 50 °C with
2
-
µ ) 0.1 M with KCl. The plot indicates OH catalysis at
pH > 9, and a reaction at pH < 9 that is dependent on
the base species of the 2-amino group. The plot bends
downward near pH 4 to give a slope of 1.0. The equation
0
for kobsd is given in eq 1; k has the
Ka
kobsd ) (k + k K /a )
(1)
0
OH
w
H
[
]
K + aH
a
value 3.2 × 10 s-1, kOH is 1.8 M
-
4
-1 -1
s
, and pKapp is 3.8.
-
4
-1
H O
D O
2
In D
2
O, k
0
is 1.8 × 10
s
(k
0
2
/k
0
) 1.8). The
w
Mallinckrodt standard buffer solutions. The value of K at 50
-
14
hydrolysis of the corresponding phenyl 4-aminobenzoate
°
C was taken to be 5.5 × 10
.
Computer modeling of the esters was carried out with a
was too slow to measure conveniently at 50 °C. The
Silicon Graphics Indigo 2 workstation. Geometry was opti-
mized with AMl semiempirical calculations. Energies were
determined with Hartree-Fock 6-31G* ab initio and density
functional pBP.DN* calculations.
reactions were, therefore, followed at 80 °C in H O. As
2
seen in Figure 2, there is no apparent pK
a
, and the
pH-independent reaction from pH 3 to 8.5 has kobsd
)
-
4
-1
1
× 10
Trifluoroethyl 2-aminobenzoate (IV) gives a plot of
log kobsd for hydrolysis in H
s .
(16) The instability of a tetrahedral intermediate was indicated by
2
O vs pH (µ ) 0.1 M) that
density functional and ab initio calculations. See Discussion.
(17) Fife, T. H.; Chauffe, L. J . Org. Chem. 2000, 65, 3579.
(18) Capon, B.; Ghosh, B. Ch. J . Chem. Soc. B 1966, 472.
(19) Staiger, R. P.; Miller, E. B. J . Org. Chem. 1959, 24, 1214.
(20) Berti, G.; Carboni, S.; DeFazio, R.; DiMarco, A.; Ghione, M.
is similar at 50 °C to that for III in Figure 1. Again,
the reaction is pH independent from pH 4 to 8.5. The
value of k
profile is 4.7 × 10
0
from the pH-independent region of the
-
4
-1
Farmaco Ed. Sci. 1957, 12, 15; Chem. Abstr. 1959, 53, 19954e.
s . Trifluoroethyl 4-aminobenzoate

Hydrolysis of 2-Aminobenzoate Esters
J . Org. Chem., Vol. 67, No. 10, 2002 3181
Ta ble 1. Ra te Con sta n ts for Hyd r olysis of Ester s II a n d
Sch em e 1
IV a t 50 °C a n d µ ) 0.1 M w ith KCl
4
-1
4
-1
ester kobsd × 10 s
pH
ester
kobsd × 10 s
pH
II
4.04
2.2
2.5
3.0
3.3
3.7
4.0
4.25
4.4
4.7
4.9
5.3
5.5
5.75
3.45
4.0
4.3
4.6
4.9
IV (contd)
5.41
4.28
5.28
5.03
4.40
4.30
4.34
4.70
5.39
4.25
5.55
5.40
6.03
11.07
13.41
24.96
36.7
5.4
5.7
6.1
6.5
6.8
7.0
7.45
7.65
7.9
8.0
8.1
8.6
9.0
9.75
5
8
.61
.13
2
2
2
2
2
2
2
2
4
5
4.5
6.1
4.6
7.4
7.1
8.6
4.2
9.9
3.1
8.5
4.52
IV
5
3
4
3
.21
.32
.91
.98
10.1
10.5
11.0
neutral pH region, but at 50 °C, the rate constants are
relatively small (III is more reactive than the 4-substi-
tuted ester by an estimated factor of at least 25-50-fold
at 50 °C). At 80 °C, the 4-substituted ester hydrolyzes
was prepared but was not of sufficient solubility in
water for accurate kinetic measurements. Trifluoroethyl
-
6
-1
21
benzoate has k
p-Nitrophenyl 2-aminobenzoate hydrolyzes in H
0 °C (µ ) 0.1 M with KCl) in a pH-independent reaction
at pH > 4; pKapp ) 3.5. The value of the rate constant
0
) 5 × 10
s
at 50 °C.
3
.2-fold more slowly in the pH-independent reaction than
III at 50 °C. The pH-independent reaction of phenyl
-aminobenzoate continues through the pH 4 region; as
2
O at
5
4
seen in Figure 2, there is no downward bend in the pH-
rate constant profile.
When the leaving group is made poorer with the
-
3
-1
for the pH-independent reaction is 2 × 10
s . Again,
the plot of log kobsd vs pH (not shown) bends downward
at pKapp to give a slope of 1.0. The observed rate constants
for the reaction of II and IV are given in Table 1.
a
trifluoroethyl ester IV, with which the pK of the leaving
group is 12.4,24 participation by the neighboring amine
group also occurs, and the rate constants for pH-
independent hydrolysis are closely similar to those of
Discu ssion
a
III. Thus, the leaving group pK has little effect in
The most striking feature of the plot of log kobsd vs pH
for the hydrolysis of phenyl 2-aminobenzoate (III) in
Figure 1 is the large pH-independent region extending
from pH 9 to 4. The reaction represented by the plateau
in the profile is undoubtedly a water reaction. Substituted
phenyl benzoate esters undergo a pH-independent hy-
drolysis reaction that is nearly independent of substitu-
tion in the acyl group.22 However, the rate constants are
considerably less than those for hydrolysis of III; the
reported rate constant for pH-independent hydrolysis
these intramolecular reactions. The k for the para-nitro
0
derivative is also similar to that for IV, even though
the pK of the leaving groups differ by 5.4 pK units. A
a
a
â ) 0 was found previously in the pH-independent water
lg
reactions of picolinate esters.2
5,26
In those reactions, the
rate constants did not vary appreciably as the leaving
group was varied from dinitrophenol (pKa ) 4.4) to
trifluoroethanol (pK ) 12.4). A â ∼ 0 indicates an early
a
lg
transition state with little or no C-O bond breaking.
The pH-independent hydrolysis of III is characterized
-
5
-1 22
of phenyl benzoate in water at 90 °C is 3 × 10
s
.
H2O
D2O
by a D
is consistent with proton transfer in the transition state.
A slower reaction in D O than H O has characterized
other reactions thought to involve intramolecular general
2
O solvent isotope effect (k
0
/k
0
) near 2, which
Near pH 4, the plot of Figure 1 bends downward to give
27
a slope of 1.0 (pKapp ) 3.8). The downward bend corre-
2
2
sponds with the expected pK
a
of the 2-amino group.23
Therefore, protonation of the amino group retards the
rate of the hydrolysis reaction. Rate retardation would
not be expected on the basis of electronic effects; the
increased electron withdrawal of the protonated amine
should enhance the ease of attack of a water molecule at
the carbonyl carbon. Thus, the neutral amine base is
important in the water reaction of III.
That the amine base of III is directly participating in
the pH-independent hydrolysis reaction is clear from the
fast rate of hydrolysis in comparison with the corre-
sponding 4-amino-substituted ester. A pH-independent
water reaction of the latter ester also occurs in the
10-15
base catalysis,
whereas a nucleophilic reaction should
have a solvent isotope effect near unity. Thus, the amine
base participation is most reasonably considered to occur
via Scheme 1 and the general base mechanism (V) in
which a proton is partially removed from a water mol-
ecule as it attacks the carbonyl group. There could be a
hydrogen bonding interaction between the amine base
and the attacking water molecule. That the intramolecu-
lar reactions of II-IV are not nucleophilic via sterically
unfavorable four-membered ring transition states is also
indicated by the very small leaving group dependence
(
24) Ballinger, P.; Long, F. A. J . Am. Chem. Soc. 1960, 82, 795.
(25) âlg is the slope of a plot of the log of the rate constant vs
the pK of the leaving group.
(26) Fife, T. H.; Przystas, T. J . J . Am. Chem. Soc. 1985, 107,
1041.
(27) Protons attached to the attacking water molecule and those
hydrogen bonded to the developing negative charge on the carbonyl
group could undergo changes that might account for the isotope effect
of 1.8.
(
21) Fife, T. H.; DeMark, B. R. J . Am. Chem. Soc. 1979, 101,
7
379.
a
(
22) Mollin, J .; Baisa, A. Collect. Czech. Chem. Commun. 1978,
4
3, 304.
(23) pK
a
values of anthranilic acid are 2.05 and 4.95 at 25 °C:
Kortum, G.; Vogel, W.; Andrussow, K. Dissociation Constants of
Organic Acids in Aqueous Solution; Butterworths: London, 1961; Vol.
4
76, p 380.

3
182 J . Org. Chem., Vol. 67, No. 10, 2002
Fife et al.
-
in the reaction, which is not typical of the nucleophilic
reactions of weakly basic amines.28 The effect of the
leaving group is large in the nucleophilic reactions of the
and OH catalyzed hydrolysis reactions are compared at
the same pH, then the enhancement in kobsd due to
general base catalysis will increase by a factor of 10 for
each decrease in pH of one unit at pH values above the
1
7,29
corresponding 2-aminomethylbenzoate esters (âlg ∼ 1.0)
and those of o-aminophenylacetate esters.30
a
pK of the general base. The enhancement of the observed
rate constant can therefore become quite large (10 -fold
5
in the case of III at pH 4). This is because of the decrease
-
in concentration of the fully ionized nucleophile (OH )
with decreasing pH.
Gen er a l Ba se Ca ta lysis. The reactions of II-IV
provide an excellent opportunity to assess the relative
advantages and disadvantages of general base mecha-
nisms in enzymatic reactions. General base catalysis
gives relatively small rate enhancements, but a precise
The amine nitrogen of II-IV is held rigidly quite close
to the carbonyl group carbon; AMl semiempirical calcula-
tions indicate with III and IV that the nonbonded
distances are 2.99 and 3.08 Å, respectively. Unlike other
8
steric alignment is not required. A nucleophilic mecha-
nism, on the other hand, can give large rate constants
at pH values near neutrality and is subject to further
apparent intramolecular general base catalyzed reac-
17,30
general acid-base catalysis.
At pH 7, phenyl 2-amino-
tions,1
0-15
there are no steric impediments to nucleophilic
methylbenzoate releases phenol 20 000-fold faster than
III. However, the steric fit is highly important in the
nucleophilic reactions. A steric situation comparable to
the formation of a five- or six-membered ring transition
state is very likely required in enzymatic nucleophilic
attack except the necessity of a four-membered ring
transition state. The energy of the hypothetical tetra-
hedral intermediate (VI) from the density functional or
ab initio calculations, is much higher than that of the
isomeric reactant (∆ ) +50.4 kcal/mol).31 The C-N-C
bond angle is 88.82° in VI.
33
reactions. Consequently, if a variety of structurally
dissimilar substrates are acted upon by a hydrolytic
enzyme, then a mechanism without rigid steric con-
straints would be advantageous. It is now clear that a
further point in favor of the general base mechanism is
the lack of dependence of the rate constants on the
leaving group (âlg = 0). As a consequence, a large group
of different substrates can be utilized without loss of
catalytic efficiency.
The difference in pH dependence for nucleophilic and
general base catalyzed reactions can lead to a significant
advantage of the general base mechanism at pH values
near or below neutrality. When the steric fit of an
intramolecular nucleophile to the carbonyl is very good,
the nucleophilic attack will be facile. The tetrahedral
intermediate should then be present at high concentra-
tion. Breakdown of the intermediate to products can be
rate determining and pH dependent; the cyclization of
Thus, the calculations indicate that nucleophilic attack
in II-IV would be energetically unfavorable and there-
fore unlikely; the transition state for nucleophilic attack
would necessarily lie much closer to the tetrahedral
intermediate than to reactants. In this case, the calcula-
tions support and help explain the conclusions derived
from the experimental data.
The rate enhancement provided by mechanism V can
be calculated by comparison with the rate constant for
the water reaction of the 4-substituted derivative or that
of the unsubstituted benzoate ester. In those reactions,
a water molecule very likely acts as a general base,
partially abstracting a proton from another water mol-
ecule as it attacks at the carbonyl. In bimolecular general
base catalyzed hydrolysis reactions, the Bronsted â
values usually range from 0.2 to 0.5.32 A small â implies
only moderate proton transfer from the water molecule
in the transition state. Thus, the moderate calculated
rate enhancement for intramolecular general base ca-
talysis in the hydrolysis of III (<100-fold) reflects the
facility of the comparison reaction and a small â coef-
ficient. A small â is consistent with a small rate enhance-
ment due to the basic group. Note, however, that if the
calculated kobsd values for the intramolecular general base
2
-aminomethylbenzoate esters proceeds with apparent
-
17,29
OH catalysis.
involving attack of an incipient hydroxide ion will be pH
independent at pH values greater than the pK of the
In contrast, a general base reaction
a
base catalyst. Consequently, at low pH values, a general
base mechanism can in fact give the largest rate. With
the present series of compounds, IV reacts faster than
trifluoroethyl 2-aminomethylbenzoate at pH < 5. The
mechanism for the latter ester cannot change to general
base catalysis at pH < 5 because of amine protonation
and the favorable nucleophilic attack that traps the
functional group free base in a tetrahedral intermediate
(eq 2).
(
28) J encks, W. P.; Gilchrist, M. J . Am. Chem. Soc. 1968, 90,
2
2
622. Blackburn, G. M.; J encks, W. P. J . Am. Chem. Soc. 1968, 90,
638.
(
29) Fife, T. H.; DeMark, B. R. J . Am. Chem. Soc. 1976, 98,
6
978.
(
(
30) Fife, T. H.; Duddy, N. W. J . Am. Chem. Soc. 1983, 105, 74.
31) The ∆ values of the internal energies were similar employing
(33) In the deacylation of the p-nitrophenoxycarbonyl serine-195 acyl
R-chymotrypsin, the leaving group is p-nitrophenol. Histidine-57 acts
as a nucleophile, but the reaction is quite hindered, which is probably
the result of poor steric fit: Hutchins, J . E. C.; Fife, T. H. J . Am. Chem.
Soc. 1972, 94, 8848.
density functional pBP.DN* or 6-31G* ab initio calculations.
32) (a) J encks, W. P.; Carriuolo, J . J . Am. Chem. Soc. 1961, 83,
743. (b) Fife, T. H.; McMahon, D. M. J . Org. Chem. 1970, 35, 3699.
(
1

Hydrolysis of 2-Aminobenzoate Esters
J . Org. Chem., Vol. 67, No. 10, 2002 3183
As the leaving group becomes poorer, the rate advan-
tage of a general base mechanism near pH 7 would
become larger,34 in view of the large effect of the leaving
group in the analogous intramolecular nucleophilic reac-
catalyst is advantageous when â is small, since a high
concentration of the base form is then present at mod-
erate pH. Carboxyl groups have been implicated as
2
-4
general base catalysts with several enzymes.
tions.17 Reduction of the pK
of the nucleophile would
further increase the relative kinetic advantage of the
general base reaction. A low pK for a general base
a
Ack n ow led gm en t. This work was supported by
research grants from The National Science Foundation
and The National Institutes of Health.
a
(34) This assumes that a general base catalyzed reaction can occur
when the leaving group is poor. A pH-independent hydrolysis reaction
of ethyl picolinate cannot be detected (ref 26).
J O0103017