Surfactant-based reactions, sodium dodecyl sulfate

Scarso and Strukul observed that anionic micelles based on sodium dodecyl-sulfate (SDS) efficiently solubilized Pt bis-cationic complexes and ketones in the apolar core of the micelles. This favored the contact between substrate and catalyst and, more importantly, led to a higher steric control of the asymmetric reaction thanks to the more ordered nano-environment present in the micelles compared to bulk organic solvents. Each substrate required the dedicated optimization of the catalyst, the surfactant and the experimental conditions since the distribution of the substrates and catalysts is greatly affected by the kind of surfactant and aggregate considered (Scheme 23.42). Overall, in all the cases tested, an increase in enantioselectivity was observed for the asymmetric BV oxidation of meso-4-substituted cyclohexanones with bis-diphenylphoshinobinaphthyl (BINAP) as the hgand and SDS as the surfactant. A different scenario was present in the kinetic resolution of chiral cyclobutanones or with meso cyclobutanones where an increase of enantioselectivity was observed with the neutral polyoxyethanyl-a-tocopheryl sebacate (PTS) surfactant (Scheme 23.42). 

It has been shown that the addition of a small amount of the anionic surfactant sodium dodecyl sulfate (SDS) to a microemulsion based on nonionic surfactant increased the rate of decyl sulfonate formation from decyl bromide and sodium sulfite (Scheme 1 of Fig. 2) [59,60]. Addition of minor amounts of the cationic surfactant tetradecyltrimethylammonium gave either a rate increase or a rate decrease depending on the surfactant counterion. A poorly polarizable counterion, such as acetate, accelerated the reaction. A large, polarizable counterion, such as bromide, on the other hand, gave a slight decrease in reaction rate. The reaction profiles for the different systems are shown in Fig. 12. More recent studies indicate that when chloride is used as surfactant counterion the reaction may at least partly proceed in two steps, first chloride substitutes bromide to give decyl chloride, which reacts with the sulfite ion to give the final product [61]. 

Consequently, new investigations dealing with reactions in micellar solutions composed of functional surfactants, or mixtures of inert and functional surfactants, continue to appear in the literature. An interesting study of acid-catalyzed hydrolysis of 2-(p-tetradecyloxyphenyl)-l,3-dioxolane (p-TPD) in aqueous sodium dodecyl sulfate (SDS) solutions has been reported [13]. In this case,/ -TPD behaves as a non-ionic functional surfactant and apparently forms non-ideal mixed micelles with the anionic surfactant (SDS). Based on the observed kinetic data, the authors propose that, at elevated temperatures, the thermodynamic non-ideality results in the manifestation of two populations of micelles, one rich in SDS and the other rich in/>-TPD. 

When the surfactant was sodium dodecyl sulfate, some butanol had to be present to achieve the microemulsion. A typical run used 2 mmol aryl iodide, 0.5 g surfactant, 1 mL butanol, 10 mL water, and 2 mmol base. The butanol was unnecessary with nonionic surfactants, such as the one derived from 1-dodecanol with 23 eq of ethylene oxide. The advantages of such a system include the following (a) No organic solvent is needed. The substrate is the oil phase, (b) The microemulsions form without the need for vigorous agitation, (c) No excess base is needed, in contrast with some reactions in which phase-transfer catalysis is used, (d) The surfactants can be recovered and recycled. They are inexpensive and biodegradable. 

Asgher et al. (2011b) have also reported another FI-CL method to determine retinol and a-tocopherol in pharmaceuticals and human blood serum based on the enhancement effect of the lucigenin CL reaction in alkaline medium. Surfactants including polyoxyethylene lauryl ether (Brij-35), Triton X-100, cetyltrimethyl ammonium bromide (CTAB) and sodium dodecyl sulfate (SDS) enhance lucigenin CL intensity. With Brij-35, the enhancement was 67% for retinol and 58% for a-tocopherol. With CTAB, the enhancement was 16% for retinol whereas for a-tocopherol, the CL intensity was quenched by up to 95%. Retinol could be determined specifically in the presence of a-tocopherol using CTAB. 

In the dipeptide 43 (Figure 24.15) the additional H-bond could be made by COOH group whereas as the hydrophobic moiety is the tryptophan sidereaction conditions showed that the presence of a base and a surfactant was essential for success of the reaction. Usually, N-methylmorpholine (NMM)/sodium dodecyl sulfate (SDS) or l,4-diazabicyclo[2.2.2]octane (DABCO)/ poly(ethylene glycol) 400 (PEG400) were employed as additive, and the reactions were carried out in water at 0°C. In the reaction system, the base might form the carboxylate with catalyst, which could increase the solubility in water the surfactant could increase the interfacial areas of water and organic reagent (62). 

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