SolVolysis Reactions of p-Nitrophenyl Alkanoates
J. Am. Chem. Soc., Vol. 118, No. 50, 1996 12537
Scheme 1
reactivity by a combination of catalyst-substrate proximity5 and
micellar microenvironment.35a
In the present study, we report an initial example of specific
salting-in effects leading to striking substrate selectivity for the
hydrolysis reaction of p-nitrophenyl alkanoates 2 catalyzed by
1 in 0.05 M aqueous Tris buffer solution. These results provide
new insight into catalytic efficiency and substrate selectivity in
our enzyme-mimic model systems. We propose that the direct
interactions between buffer component and salting-in agent, tris-
(hydroxymethyl)methylammonium ion, and catalyst 1 permit
use of 1 in aqueous solution and contribute to optimum reactivity
with the hexanoate ester 2. A detailed kinetic characterization
of the salting-out effects of sodium chloride on the solvolysis
of 2 catalyzed by 1 in 1:1 (v/v) methanol-water solution is
also reported.
Experimental Section
Materials and Reagents. Synthesis of the poly(siloxane-bis-
(trimethylene)) supported 4-(diallylamino)pyridine (1) has been de-
scribed previously.30 p-Nitrophenyl alkanoates 2 (n ) 2-16) and 1,4-
dioxane were purchased from Sigma Chemical Co. Tris(hydroxy-
methyl)aminomethane, sodium borate, hydrochloric acid, sodium
chloride, methanol, and buffer solution (0.05 M H2PO4-/HPO42-, pH
8.0) were used as received from Aldrich and Fisher.
of hydrophobically associating catalyst-substrate systems that
attempt to identify the optimum substrate structure. And no
previous study has investigated the influence of electrolytes on
the specificity of a “catalyst” that carries the 4-(dialkylamino)-
pyridine function, a true turnover catalyst.14-17,19-21
Kinetic Measurements. The cuvette was filled with 2.5 mL of a
fresh solution containing catalyst in 0.05 M aqueous buffer solutions
at pH 8.0, and the solution was equilibrated for 10 min at 30 ( 1 °C
in the thermostated cell compartment of a Hewlett-Packard Model 8450
spectrophotometer. A fresh stock solution (usually 5 µL) of p-
nitrophenyl alkanoates 2 (n ) 2-16, 2.5 × 10-2 M) in dioxane was
added by microsyringe. The fresh catalyst solutions containing sodium
chloride for kinetic experiments were prepared in 1:1 (v/v) methanol-
aqueous buffer (0.05 M H2PO4-/HPO42-, pH 8.0) solution, where the
4-(dialkylamino)pyridine group of 1 is partially protonated. The pKa
of 1 increases from 6.9 to 7.2 as NaCl concentration is increased to
0.50 M in 1:1 (v/v) methanol-water solutions as a result of an increase
in shielding of protonated pyridinium cation by chloride counterion.
The reaction mixture was quickly mixed by shaking, and the absorbance
at 400 nm was recorded as a function of time. The reactions were
performed for 4-5 half-lives, and the pseudo-first-order rate constants
(kobsd) were obtained as slopes of plots of ln[A∞/(A∞ - At)] vs time,
where A∞ and At are the absorbances at infinite time and time t,
respectively. The first-order rate constants (kobsd) represent the average
of two or three runs, and the experimental error is less than 5%.
4-(Dimethylamino)pyridine and its derivatives are widely
recognized as highly reactive supernucleophilic acyl-transfer
catalysts in nonaqueous media.22,23 Polymers and surfactants
bearing the 4-(dialkylamino)pyridine group have recently
become the subject of extensive research.24-34 Most studies
have focused on the evaluation of these macromolecules as
nucleophilic catalysts for solvolysis of p-nitrophenyl alkanoates
2 in aqueous and mixed aqueous-organic solvent.25,26,29 The
results from these studies have made essential contributions
toward understanding the origins of efficiency and selectivity
in biological and chemical catalysis. Macromolecule 1 contain-
ing the 4-(dialkylamino)pyridine functionality and a bis-
(trimethylene)disiloxane backbone exhibits enzyme-like sub-
strate selectivity for the solvolysis of 2.29 This synthetic polymer
shows highest levels of activity toward p-nitrophenyl tetradec-
anoate when it is used as a nucleophilic catalyst for the
solvolysis of a series of 2 in 1:1 (v/v) methanol-water solution29
(Scheme 1). Such catalytic reactions are appreciably accelerated
by the presence of the anionic surfactant SDS which binds both
substrate and catalyst within micellar aggregates and influences
Results and Discussion
We have examined the effects of the buffers on the solubility
of 1 in aqueous solution at pH 8.0 and 30 °C. In 0.05 M
phosphate and borate aqueous buffer solutions, the solubility
of 1 was found to be similar to its solubility in pure water, and
aqueous solutions of 1 show appreciable turbidity when the
concentration of 1 is increased beyond 2.5 × 10-5 unit mol
L-1. In contrast, we found that in 0.05 M aqueous Tris buffer
solution the solubility of 1 shows a more than 10-fold increase
over that in pure water. An aqueous solution of 1 at 2.5 ×
10-4 unit mol L-1 and 0.05 M Tris buffer remains clear even
after standing for prolonged periods. These results clearly
suggest that tris(hydroxymethyl)methylammonium ion is a
salting-in ion in water in accord with previous observations that
the anions and cations of low charge density tend to salt-in
organic solutes while the ions of high charge density tend to
salt-out such solutes.3,7,17 Therefore, tris(hydroxymethyl)-
methylammonium chloride increases the solubility of the
hydrocarbons and other hydrophobes in water.
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The Salting-In Effects of Tris(hydroxymethyl)methylam-
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