- Foldamers as reactive sieves: Reactivity as a probe of conformational flexibility
-
A series of m-phenyleneethynylene (mPE) oligomers modified with a dimethylaminopyridine (DMAP) unit were treated with methyl sulfonates of varying sizes and shapes, and the relative reactivities were measured by UV spectrophotometry. Using a small-molecule DMAP analogue as a reference, each of the methyl sulfonates was shown to react at nearly identical rate. In great contrast, oligomers that are long enough to fold, and hence capable of binding the methyl sulfonate, experience rate enhancements of 18-1600-fold relative to that of the small-molecule analogue, depending on the type of alkyl chain attached to the guest. Three different oligomer lengths were studied, with the longest oligomers exhibiting the fastest rate and greatest substrate specificity. Even large, bulky guests show slightly enhanced methylation rates compared to that with the reference DMAP, which suggests a dynamic nature to the oligomer's binding cavity. Several mechanistic models to describe this behavior are discussed.
- Smaldone, Ronald A.,Moore, Jeffrey S.
-
p. 5444 - 5450
(2008/02/04)
-
- Mechanisms of hydrolysis and related nucleophilic displacement reactions of alkanesulfonyl chlorides: pH dependence and the mechanism of hydration of sulfenes
-
pH-rate profiles, primary kinetic isotope effects, deuterium substitution patterns, and pH-product ratios in the presence of added nucleophiles provide evidence for the following overlapping set of mechanisms for the hydrolysis of methanesulfonyl chloride (1) (in 0.1 M KCl at 25 °C): (a) pH ≤ 1-6.7, reaction with water by direct nucleophilic attack on the sulfonyl chloride; (b) pH ≥ 6.7-11.8, rate-determining attack by hydroxide anion to form sulfene (2), which is then trapped by water in a fast step; and (c) pH ≥ 11.8, sulfene formation and sulfene trapping by hydroxide anion; careful inspection showed no sign of sulfene formation in the reaction with water or of direct displacement by hydroxide anion. This pattern, with appropriate variations in the values of pHi (the pH at which two competing mechanisms have the same rate), is apparently general for simple alkanesulfonyl chlorides having at least one hydrogen on the carbon bearing the sulfonyl group. Azide and acetate anions react with 1 below pHi for 1 (6.7) by direct nucleophilic substitution at the sulfur, but above pHi by trapping of the sulfene. 2-Chlorophenoxide anion reacts with 1 below pH 6.7 by both (a) direct displacement to form the ester and (b) elimination to form the sulfene. Above pH 6.7, sulfene is formed from the sulfonyl chloride by reaction with either 2-chlorophenoxide or hydroxide ion; this is followed by trapping of the sulfene with 2-chlorophenoxide, water, or hydroxide. The possibility of the 2-chlorophenoxide anion acting as a general base promoting the reaction of water with either 1 and 2 was examined, but no sign of either process was detected.
- King,Lam,Skonieczny
-
p. 1743 - 1749
(2007/10/02)
-
- MECHANISTIC VARIATION IN ALKANESULFONYL CHLORIDE HYDROLYSIS AND RELATED REACTIONS
-
Kinetic and product ratio studies are consistent with the following mechanisms for the hydrolysis of methanesulfonyl chloride: (a) in acidic medium (pH 1-6) via a direct substitution on sulfur (SN2-S), (b) in mildly basic medium (pH 8-10) by way of sulfene (CH2=SO2) formation followed by trapping with water, and (c) in strongly basic solution (pH >10) via sulfene with trapping by the hydroxide ion.The reactions of primary and secondary alkanesulfonyl chlorides are qualitatively similar.
- King, J. F.,Lam, J. Y. L.,Skonieczny, S.
-
p. 177 - 180
(2007/10/02)
-
