Micellar catalytic system

The direct synthesis of poly(3-sulfopropyl methacrylate)-fr-PMMA, PSP-MA-fr-PMMA (Scheme 27) without the use of protecting chemistry, by sequential monomer addition and ATRP techniques was achieved [77]. A water/DMF 40/60 mixture was used to ensure the homogeneous polymerization of both monomers. CuCl/bipy was the catalytic system used, leading to quantitative conversion and narrow molecular weight distribution. In another approach the PSPMA macroinitiator was isolated by stopping the polymerization at a conversion of 83%. Then using a 40/60 water/DMF mixture MMA was polymerized to give the desired block copolymer. In this case no residual SPMA monomer was present before the polymerization of MMA. The micellar properties of these amphiphilic copolymers were examined. 

The catalytic principle of micelles as depicted in Fig. 6.2, is based on the ability to solubilize hydrophobic compounds in the miceUar interior so the micelles can act as reaction vessels on a nanometer scale, as so-called nanoreactors [14, 15]. The catalytic complex is also solubihzed in the hydrophobic part of the micellar core or even bound to it Thus, the substrate (S) and the catalyst (C) are enclosed in an appropriate environment In contrast to biphasic catalysis no transport of the organic starting material to the active catalyst species is necessary and therefore no transport limitation of the reaction wiU be observed. As a consequence, the conversion of very hydrophobic substrates in pure water is feasible and aU the advantages mentioned above, which are associated with the use of water as medium, are given. Often there is an even higher reaction rate observed in miceUar catalysis than in conventional monophasic catalytic systems because of the smaller reaction volume of the miceUar reactor and the higher reactant concentration, respectively. This enhanced reactivity of encapsulated substrates is generally described as micellar catalysis [16, 17]. Due to the similarity to enzyme catalysis, micelle and enzyme catalysis have sometimes been correlated in literature [18]. 

Many micellar catalytic applications using low molecular weight amphiphiles have already been discussed in reviews and books and will not be the subject of this chapter [1]. We will rather focus on the use of different polymeric amphiphiles, that form micelles or micellar analogous structures and will summarize recent advances and new trends of using such systems for the catalytic synthesis of low molecular weight compounds and polymers, particularly in aqueous solution. The polymeric amphiphiles discussed herein are block copolymers, star-like polymers with a hyperbranched core, and polysoaps (Fig. 6.3). 

Figure 6.11 Examples of biohybrid catalytic systems, (a) Covalent coupling of a polystyrene tail to the enzyme CALB lipase the resulting biohybrid forms micellar fibers, (b) Cofactor reconstitution A polystyrene tail is connected to the horseradish peroxidase enzyme via the cofactor ferri-protoporphyrin IX. the resulting...

In contrast with the catalytic system based on RuCl(rf-C9H7)(PPh3)2 in micellar solutions [32], the reaction of secondary propargylic alcohols in 2-propanol/ H20 at 100 °C in the presence of 5 mol % of RuCl(Cp)(PMe3)2 leads to conjugated enals with E stereoselectivity (Eq. 12) [88]. 

Cation Requirements. While some of the phospholipases C found in bacteria appear to prefer Ca2+, there are many many reports supporting Zn2+ as the divalent cation of choice. There is some support for the fact that this enzyme is probably a metallo (Zn2+) protein which also requires Ca2+ for catalytic activity, but there is more evidence for the enzyme s ability to influence the surface charge on the micellar substrate system. 

Johnson-Matthey Co. has reported that oleic acid methyl ester or linoleic acid methyl ester can be hydroformylated in micellar media using a water-soluble rhodium complex of monocarboxylated triphenylphosphine 45 as catalyst. As a further example, polyunsaturated linolenic acid methyl ester can be hydroformylated to the triformyl derivative with a selectivity of 55% with a Rh/TPPTS catalytic system in the presence of CTAB (Scheme 1.23). ... 

The earliest examples demonstrating the promise of LLC materials for accelerating chemical reactions involved the use of LLC phases of commercially available ionic or non-charged surfactants in water as nanoscale reaction media. In these systems, the surfactants and resulting LLC phases were not functionalized with any catalytic or reactive groups. All the reactants and catalytic entities were from external sources and solubilized in the LLC domains during reaction. Consequently, the rate acceleration effects observed in these systems can be attributed to the same types of confinement, solubilization, and electronic interactions found in micellar catalysis systems [98-100]. 

Based on the knowledge of micellar catalytic effects, an exclusive photochemical reaction mechanism in micellar solution possibly may be expected or designed to fulfill each particular environmental problem. Thomas J.K.. (1980) proposed four possible photochemical reaction mechanisms that might exist in micellar system (see Figure 2). 

Theoretically, polysoaps may be of interest for all the potential applications mentioned for micellar polymers in the introduction. In practice, the attempted uses of polysoaps cover a much narrower range. Besides a number of diverse suggestions, proposed uses are basically classical colloidal applications, medical or pharmaceutical applications, and catalytic systems. 

The outstanding features of metal clusters prepared in block copolymer micelles [81] are their high catalytic activity combined with high stability. Such micellar catalyst systems can be recovered after reaction by precipitation or ultrafiltration. In many cases high selectivity and stability have been observed. Cyclohexadiene, for instance, is selectively hydrogenated by Pd colloids just to cyclo-octene [69]. High activity and stability of such catalyst particles have been reported for the Heck-reaction with unusually high turnover numbers of... 

This chapter will only deal with catalytic systems covalently attached to the support. Dendrimer [96-101], hyperbranched polymer [102, 103], or other polymer [100] encapsulated catalysts, micellar catalysis [104] and non-cova-lently bound catalysts (via ionic [105,106], fluorous, etc. intercations) are not being treated. Also catalysis with colloidal polymers [ 107,108] and biocatalysts, such as enzymes and RNA, will not be reviewed here. 

Ganguly NC, Mondal P and Barik SK. Iodine in aqueous micellar environment a mild effective ecofriendly catalytic system for expedient s)mthesis of bis(indol-yl)methanes and 3-substituted indolyl ketones. Green Chem. Lett. Rev. 2012 5(1) 73-81. 

The system is designed to hydrolyze tris(4-nitrophenyl)phosphate (TNP), an insecticide. Both hydrophobic and hydrophilic Zn +-cyclen complexes were synthesized and tested for activity. The lipophilic Zn +-cyclen complex forms a co-micellar phase with triton, which creates a hydrophobic environment near the catalyst reactive center. The hydrophobic center allows for the efficient attack at the reaction center, thus leading to substantially enhanced catalytic reactivity. The hydrophilic catalyst, on the other hand, prevents an efficient attack at the reaction center and hence shows no reactivity. The nearly 100-fold difference in rate of the co-micellar catalyst system as compared with the aqueous system stems mainly from the solvability of TNP in the micellar phase and, to a partial extent, to the higher reactivity of Zn + due to the exclusion of H2O. 

The first micellar activation of a transition-metal catalyzed reaction was introduced by Menger et al., who proved the rate and yield enhancements of the oxidation of piperonal into piperonylic acid by KMn04 at 55 °C along with the in situ addition of 0.01 M of cetyltrimethylammo-nium bromide (CTAB). " Transition metal complexes were also extended toward dual catalytic systems in aqueous catalytic Pauson-Khand-type reaction. It was shown that... 

Asymmetric epoxidation of terminal alkenes with hydrogen peroxide was optimized with electron-poor chiral Pt(II) complexes bearing a pentafluorophenyl residue, as described in Section 23.3.1.6. The same catal3rtic system was made more sustainable by the employment of water as the solvent under micellar conditions. Surfactant optimization revealed the preferential use of neutral species like Triton-XIOO to solubihze both the catalyst and substrates. In several cases an increase of the asymmetric induction was observed (Scheme 23.43). The use of an aqueous phase and the strong affinity of the catalyst for the micelle allowed the recycling of the catalytic system by means of phase separation and extraction of the reaction products using an apolar solvent (hexane). The aqueous phase containing the catalyst was reused for up to three cycles with no loss of activity or selectivity. 

Oxidations catalysed by Co/122 (Table 6), Mn/126 (Table 6 R=Me, CH2CH20H) and PdCl3(pyridine) systems or ruthenium complexes and CoBr2 in micellar systems. We note also that the classical Wacker process employing PdCl2/CuCl2 catalytic systems constitutes another application of oxidation reactions in aqueous media. 

Several methods can be used to achieve recyclable catalytic systems, such as the following ones (i) utilization of heterogeneous sofid catalytic materials (ii) formation of dispersed nano-, sol-gel, and micellar systems (iii) phase division, where a homogeneous catalyst and substrate are usually well soluble in one solvent, while the product is soluble in another solvent,... 

Pt(ll) complexes modified with chelating chiral diphosphines are thus far the most efficient catalyst for the asymmetric BV oxidation of five- and six-membered ring cyclic ketones, albeit with low turnover. Recently, Scarso and co-workers optimized the catalytic system bearing chiral diphosphines for the asymmetric BV oxidation of six- and four-membered ring cyclic chiral and mc50-ketones observing enhancements of both activity and enantioselectivity when working in water under micellar conditions. ... 

Our micellar models show unusually high catalytic activities as compared with other related model systems. Foregoing results and discussions may be summarized by referring to a generalized mechanism of catalysis shown in Scheme 5. 

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