Reactions in Nanoreactors

The encapsulation method uses preformed metal complexes and can thus avoid the formation of undesired species in the solid matrix. This strategy could be basically applied for the encapsulation of various kinds of molecular catalysts in the nanoreactor. 

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Although the research field is still in its infancy, several examples of reactions, wherein self-assembled nanoreactors are applied and are shown to dramatic enhance or alter reactivity, have appeared, thereby demonstrating the power of the supramolecular concept. Detailed studies are required to fully understand the mechanisms behind the effects observed when carrying out reactions in nanoreactors. The results obtained so far sketch a bright prospective, as reactions have been observed that are unique to those carried out in capsules. In this review we have focussed on reactions that take place inside the capsules. However, molecular capsules have also been used to control reactions that take place outside the capsule for example by controlling the release of reagents, making the nanoreactor applications virtually unlimited. ... 

Fig. 3 Concept of nanoengineering of oxide catalytic surface in terms of nanoreactor array, some of the possibilities offered by this concept (in particular in terms of realizing multifunctional catalysts for cascade reactions in nanoconfined liquids) and a SEM image of an array of Xi02 nanotubes produced by anodic oxidation of Ti foils. Source Centi et alN...

Finally, reactions in the surface nanoreactors were found to be important for biological objects such as DNA and pheromones some hypotheses were advanced that the analogues of the surface nanoreactors might have played a significant role in biological evolution. 

The process of miniemulsion allows in principle the use of all kinds of monomers for the formation of particles, which are not miscible with the continuous phase. In case of prevailing droplet nucleation or start of the polymer reaction in the droplet phase, each miniemulsion droplet can indeed be treated as a small nanoreactor. This enables a whole variety of polymerization reactions that lead to nanoparticles (much broader than in emulsion polymerization) as well as to the synthesis of nanoparticle hybrids, which were not accessible before. 

Later, more sophisticated supramolecular complexes capable of improved molecular recognition started to be studied. New supramolecular approaches to constmct synthetic biohybrid catalysts were developed [190]. An example is the giant amphi-philes, formed by a (hydrophilic) enzyme headgroup and a synthetic apolar tail. These biohybrid amphiphilic compounds self-assemble in water to yield enzyme fibers and enzyme reaction vessels (nanoreactors [202]). 

Figure 6.1 a) Simplified reaction profiles of a reaction in the bulk solution (dashed line) and of a reaction within a nanoreactor (blue line), b) Simplified reaction profiles of a reaction leading to product D which is destabilized by the nanoreactor (solid line) compared to the bulk solution (dashed line), and of a reaction leading to product E which is stabilized by the nanoreactor (blue line) compared to the bulk solution (dashed line). [6] Reproduced by permission of The Royal Society of Chemistry (RSC)... 

The octahedral coordination cage D described by Fujita and co-workers has been applied as a nanoreactor for bimolecular Diels-Alder reactions in water. Suspending 9-(hydroxymethyl)anthracene 7 and A-cyclohexylmaleimide 8a in an aqueous solution of... 

B. Gall, M. Bortenschlager, O. Nuyken, R. Weberskirch, Cascade reactions in polymeric nanoreactors mono (Rh)- and bimetallic (Rh/lr) miceUar catalysis in the hydroaminomethyla-tion of 1-octene, Macromol. Chem. Phys., 2008, 209, 1152. 

Thus, the stated above results have shown that for curing reaction proceeding in fractal space the reaction rate constant reduction is typical. The formation of a large number of microgels with smaller molecular weight in comparison with reaction in Euclidean space at the same conversion degree is also typical for such reaction. The dimensional border between nanoreactor and nanoparticle for the considered curing reaction has been obtained. 

This may be quite important in processes in which the ionic strength is determinant such as sol-gel transitions or chemical reactions in microemulsions [137-141]. Double-tailed surfactants such as dioctyl sulfosuccinate or diallQ lmethylammonium salts are likely to produce either vesicles (with excess water) or inverse W/O microemulsions with a polar core [142,143] that is used as a nanoreactor for a score of processes such as esterification or hydrolysis [144] in which enzymes are immobilized in an organogel [145]. Organogels can be made so that their structure depends on the composition of the microemulsion [146-148]. 

The basic idea employed by Lipshutz involves the use an amphiphilic or surfactant-type molecule as a platform to solubilize the components of an organic reaction in water. Essentially, the amphiphile forms micelles in an aqueous environment, and these can act as micellar nanoreactors, in which the metal-catalyzed organic transformation can proceed. This approach is attractive, as it employs commercially available catalysts without the need for time-consuming or costly catalyst modifications. The amphiphilic surfactant molecule initially employed for these studies was PTS (131), which contains an unsym-metrical, sebacic acid-derived diester functionalized at one end with a lipophilic a-tocopherol unit and a hydrophilic PEG unit at the other (Figure 5.28) [115]. 

Figure 16 Application of amphiphilic homopolymers as nanoreactors for reactions in water. (Reproduced from Ref. 41. American Chemical Society, 2005.)...

Rostamnia S, Xin H. Pd(OAc)2 SBA-15/PrEn nanoreactor a highly active, reusable and selective phosphine-free catalyst for Suzuki-Miyaura cross-coupling reaction in aqueous media. Appl Organometal Chem 2013 27 348-52. 

One of the most common types of reactions to be template using CB[ ] nanoreactors is photochemical cycloadditions a few examples were discussed in Section 5.4 [120-124]. A number of other examples will be discussed here [126-137]. Svoboda and Kbning reviewed template photochemical reactions back in 2006, and had a small section on such reactions in molecular flasks [126]. Although most examples at that time involved cyclodextrins as the molecular flask (a nice descriptive name for a nanoreactor), there were a few examples at that time involving CB[7] and CB[8]. Those examples and some more recent ones will be discussed here [127-137]. 

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