Catalysts incompatible

We investigated the chemoenzymatic synthesis of block copolymers combining eROP and ATRP using a bifunctional initiator. A detailed analysis of the reaction conditions revealed that a high block copolymer yield can be realized under optimized reaction conditions. Side reactions, such as the formation of PCL homopolymer, in the enzymatic polymerization of CL could be minimized to < 5 % by an optimized enzyme (hying procedure. Moreover, the structure of the bifunctional initiator was foimd to play a major role in the initiation behavior and hence, the yield of PCL macroinitiator. Block copolymers were obtained in a consecutive ATRP. Detailed analysis of the obtained polymer confirmed the presence of predominantly block copolymer structures. Optimization of the one-pot procedure proved more difficult. While the eROP was compatible with the ATRP catalyst, incompatibility with MMA as an ATRP monomer led to side-reactions. A successfiil one-pot synthesis could only be achieved by sequential addition of the ATRP components or partly with inert monomers such as /-butyl methacrylate. One-pot block copolymer synthesis was successful, however, in supercritical carbon dioxide. Side reactions such as those observed in organic solvents were not apparent. 

Disadvantages of the synthesis of DNA copolymers without a synthesizer are less convenience and more time consumption but this type of method is especially helpful in the case of solvent or catalyst incompatibilities. The conditions of post-DNA synthesis change depending on factors such as chemo-stability, catalyst and... 

As one would expect, in those cases in which the ionic liquid acts as a co-catalyst, the nature of the ionic liquid becomes very important for the reactivity of the transition metal complex. The opportunity to optimize the ionic medium used, by variation of the halide salt, the Lewis acid, and the ratio of the two components forming the ionic liquid, opens up enormous potential for optimization. However, the choice of these parameters may be restricted by some possible incompatibilities with the feedstock used. Undesired side reactions caused by the Lewis acidity of the ionic liquid or by strong interaction between the Lewis acidic ionic liquid and, for example, some oxygen functionalities in the substrate have to be considered. 

Obviously, there are many good reasons to study ionic liquids as alternative solvents in transition metal-catalyzed reactions. Besides the engineering advantage of their nonvolatile natures, the investigation of new biphasic reactions with an ionic catalyst phase is of special interest. The possibility of adjusting solubility properties by different cation/anion combinations permits systematic optimization of the biphasic reaction (with regard, for example, to product selectivity). Attractive options to improve selectivity in multiphase reactions derive from the preferential solubility of only one reactant in the catalyst solvent or from the in situ extraction of reaction intermediates from the catalyst layer. Moreover, the application of an ionic liquid catalyst layer permits a biphasic reaction mode in many cases where this would not be possible with water or polar organic solvents (due to incompatibility with the catalyst or problems with substrate solubility, for example). 

Application of sulfuric acid as the catalyst is considered more practical for esterification because of its higher boiling point, its incompatibility with benzene, and the stability of nitroacetic acid in the reaction mixture that allows the omission of the final neutralization step. 

Catalyst nanoencapsulation is an excellent fit to the concepts of green chemistry [2] in the area of process intensification - enabling incompatible catalysts to function in the same reactor, thereby achieving what otherwise simply cannot be done. 

In this stoichiometric forerunner, the use of a polymeric support demonstrated the concept of using an immobilization method to prevent reagents from reacting with each other in an undesired manner, permitting a reaction to occur that is not normally possible. By analogy, there are possibilities where various immobilization methods (in this case, we are interested in nanoencapsulation methods) are used to enable two incompatible catalysts to work concomitantly in an otherwise impossible reaction. 

Cascade Reactions with Incompatible Catalysts and Nanoencapsulation... 

In 2006, Poe et al. reported a cascade reaction employing two incompatible catalysts, one of which was microencapsulated [19]. In this case, an organic amine was encapsulated and used in conjunction with a nickel-based Lewis acid catalyst (Scheme 5.4). 

Scheme 5.4 Encapsulation of the amine by cross-linking allows the use of two incompatible catalysts [19],...

Scheme 5.8 DKR of a secondary alcohol using an acidic zeolite racemization catalyst in conjunction with CALB. The zeolite was encapsulated using an Lb L method in order to overcome the incompatibility of the two catalysts.

Scheme 5.10 The use oftwo otherwise incompatible catalysts enabled by their immobilization in a sol-gel matrix (SC) to perform a two-step cascade [25],...

They observed the complete deactivation of the rhodium catalyst whether immobilized or not in the presence of free amines. When no amine was present, styrene formation was not observed. After 17 h of a reaction in which both catalysts were immobilized, the yield of the product, ethylbenzene, was 52%, again demonstrating the principle of enabling two otherwise incompatible catalysts to work concomitantly in order to achieve process intensification. 

To the best of the authors knowledge, these examples comprise most of the literature that fits the criteria of cascade reactions with incompatible catalysts achieved with nanoencapsulation. However, there are many more examples where cascade reactions have been achieved with incompatible catalysts, but without the use... 

Other Cascade Reactions with Incompatible Catalysts - Polydimethylsiloxane (PDMS) Thimbles for Generic Site Isolation... 

Bowden and co-workers demonstrated the utility of such a system in multiple cascade reactions, including the use of an otherwise incompatible combination of a Grubbs catalyst and an osmium dihydroxylation catalyst (Figure 5.2) [34],... 

Moreover, in this example, the solvent systems used are also incompatible. The Grubbs catalyst is used in a relatively dry, nonpolar solvent to dissolve the substrates, whereas the AD-mix is placed in various alcohol-water mixtures. 

The same approach has also been used in a reaction cascade involving 4-dimethy-laminopyridine (DMAP) and an acid catalyst [35], These two catalysts are mutually incompatible as the add quenches the DMAP, but site isolation using a PDMS thimble enables the cascade to proceed successfully (Figure 5.3). 

Although the systems described here have not been used for nanoencapsulated cascade reactions, or of course, for mutually incompatible catalysts, they offer an attractive possibility for the extension of this field, especially given the availability of a wide range of protein-based nanometer-sized cages, such as chaperonins, DNA binding proteins, and the extensive class of viruses [107]. 

The use of multiple otherwise incompatible catalysts allows multistep reactions to proceed in one reaction vessel, providing many potential benefits. In this chapter, literature examples of nanoencapsulation for the purpose of process intensification have been discussed comprehensively. Current efforts in the literature are mostly concentrated in the areas of LbL template-based nanoencapsulation and sol-gel immobilization. Other cascade reactions (without the use of nanoencapsulation) that allow the use of incompatible catalysts were also examined and showcased as potential targets for nanoencapsulation approaches. Finally, different methods for nanoencapsulation were investigated, thereby suggesting potential ways forward for cascade reactions that use incompatible catalysts, solvent systems, or simply incompatible reaction conditions. 

Nonetheless, a wide variety of potential methods are available to achieve the goal of nanoencapsulation for the purpose of facilitating the use of two or more incompatible catalysts in cascade reactions. The many multistep reactions that are of importance in the fine chemicals industry are prime targets for the application of the principles of nanoencapsulation and, therefore, of green chemistry. 

In Fischer-Tropsch synthesis the readsorption and incorporation of 1-alkenes, alcohols, and aldehydes and their subsequent chain growth play an important role on product distribution. Therefore, it is very useful to study these reactions in the presence of co-fed 13C- or 14 C-labeled compounds in an effort to obtain data helpful to elucidate the reaction mechanism. It has been shown that co-feeding of CF12N2, which dissociates toward CF12 and N2 on the catalyst surface, has led to the sound interpretation that the bimodal carbon number distribution is caused by superposition of two incompatible mechanisms. The distribution characterized by the lower growth probability is assigned to the CH2 insertion mechanism. 

Steam reforming is traditionally carried out in large fired furnaces containing many catalyst-containing tubes. There are several requirements in reforming that might normally be considered mutually incompatible ... 

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