Rate‐limiting step and micellar catalysis of the non‐classical nitro group nucleophilic substitution by thiols in 4‐nitro‐N‐n‐butyl‐1,8‐naphthalimide

Abstract

Nitronaphthalimides are a particularly interesting group of nitroaromatics used for staining cells in hypoxia and in photodynamic therapy. The kinetics of the aromatic nucleophilic substitution of the nitro‐group of 4‐nitro‐N‐n‐butyl‐1,8‐naphthalimide (4‐NBN) by n‐heptanethiol (RSH), thiophenol (ArSH) and 2‐hydroxy‐1‐ethanethiol (HESH) in aqueous solutions containing micelles and in water–methanol mixtures were analyzed. The substitution of the nitro group by aromatic or aliphatic thiols led to fluorescent substitution products. Hexadecyltrimethylammonium chloride micelles (CTAC) increased the rate of thiolysis of 4‐NBN by 1 × 105 (RSH) and 4 × 105 (ArSH) fold. For the reaction of ArSH with 4‐NBN the calculated second‐order rate constant in the micellar phase, k2m, is 1.4 times higher than that in the aqueous phase, k2w. For the reaction of ArSH with 4‐NBN, k2m/k2w = 0.05. Both k2w and k2m decreased with increasing pKa of the thiol, suggesting that thiolate attack is not rate limiting in either the aqueous phase or micellar pseudophase. The high micellar rate acceleration is therefore largely due to concentration of the reagents in the micellar phase and effects on the thiol acid–base equilibrium. Copyright © 2003 John Wiley & Sons, Ltd.

Device Performance and Lifetime of Polymer:Fullerene Solar Cells with UV‐Ozone‐Irradiated Hole‐Collecting Buffer Layers

“…These compare to values of 1200 M −1 reported for 1‐heptanethiol association with DODAC vesicles21 and 150 M −1 for benzyl thiol association with CTAB micelles 22. [Cl − ] b and [Cl − ] f , the concentrations of bound and free chloride ions in the Tris‐HCl buffered DOTAP vesicle solutions were calculated using Eqns (6) and (7), where [RS − ] b and [OH − ] b are the concentrations of vesicle‐bound thiolate23 and hydroxide24 ions. Since K ap is expected to vary as a function of [DOTAP], we used values for K ap calculated by Eqn (8) 19.…”

“…The kinetic data obtained for the hydrolysis of 1 in NaOH solutions in CTAB and DODAC solutions were fitted to the following equation: where K OH/X is the ion‐exchange constant between the hydroxide and the halide at the micellar or vesicle surface, k 2m is the second‐order rate constant in the surfactant phase, k 2w is the second‐order rate constant in the aqueous phase, and [D n ] and V are the concentration and effective volume of the micellized surfactant. The concentrations of the bromide ions (CTAB) or chloride ions (DODAC) in the aqueous and surfactant pseudo‐phases, [X − ] w and [X − ] m , respectively, were calculated using the ion‐exchange equations described in detail elsewhere 23–25. The value of α (0.20), the degree of counter‐ion dissociation at both the micelle and vesicle surface was assumed to be independent of the surfactant concentration, and cmc values of 5.0 × 10 −4 and 6 × 10 −6 M were used for CTAB and DODAC.…”

Reactions in Micellar Systems

“…[114] Iglesias [115] studied the behavior of diketones and b-keto acids in micelles in the presence of acids and bases. It was found that hydrolysis and nitrosylation compete depending on the type of surfactant used (Scheme 9).…”

The effect of the hydrophobic environment on the retro-aldol reaction: comparison to a computationally-designed enzyme

“…values because it is known that rates of micelle-accelerated reactions can drop after such value. 12, 18 Further, we used butylamine (BuAm) as the catalytic amine because its p K a value (10.6) 19 is similar to that of lysine in solution. Whereas the presence of 5% DMSO (needed to solubilize the substrate) and of 50 mM sodium phosphate can alter the c.m.c values reported in Table 1, we did see micelle formation in the experimental conditions used, as judged by surface tension measurements (data not shown).…”