Molecular reactors

Exercise. Argue that the number of neutrons in a molecular reactor is a Markov process. 

The molecular size pore system of zeolites in which the catalytic reactions occur. Therefore, zeolite catalysts can be considered as a succession of nano or molecular reactors (their channels, cages or channel intersections). The consequence is that the rate, selectivity and stability of all zeolite catalysed reactions are affected by the shape and size of their nanoreactors and of their apertures. This effect has two main origins spatial constraints on the diffusion of reactant/ product molecules or on the formation of intermediates or transition states (shape selective catalysis14,51), reactant confinement with a positive effect on the rate of the reactions, especially of the bimolecular ones.16 x ... 

It has been concluded that, in most cases, catalytic reactions over zeolites occur within their intracrystalline cages and channels. Zeolite catalysts can therefore be considered as a succession of nano or molecular reactors. The consequence is that the activity, selectivity, but also the stability of all the reactions carried out over zeolite catalysts, depend (slightly or significantly) on the shape and size of cages, channels and of their apertures, hence that shape selectivity is a general characteristic of zeolite catalyzed reactions. 

Easton CJ, Lincoln SF, Barr L, Onagi FI (2004) Molecular reactors and machines applications, potential, and limitations. Chem Eur J 10 3120-3128... 

Easton et al. have discussed the applications, potential, and limitations of molecular machines and reactors [15]. They define molecular reactors as miniature vessels for the assembly of reactants at the molecular level, in order to change the nature of chemical transformations. Molecular machines consist of interrelated parts with separate functions, and perform some kind of work, at the molecular level. However, practical examples are not based on individual functions of singlemolecule devices. 

C. J. Easton, Cyclodextrins as molecular reactors, next chapter in this book. 

This behavionr is in many ways like that displayed by enzymes and other authors have therefore used the term artificial enzymes instead of molecular reactors. However, the need for a different term has become more important since recent rapid advances in molecular biology have provided tools for protein engineering to produce entirely new catalysts that might be more obviously classed as artificial enzymes, while molecular reactors encompass a much broader range of species. Another important distinction is... 

The problem with this system is that the molecular reactor offers very little in the way of a real practical advantage The ratio of formation of the dyes has been changed in favour of indirubin but only by reducing the overall yield of the two dyes by a factor of almost thirty and the yield of indirubin by Ihirteen times. This is almost certainly the result of complexation of indoxyl and isatin by the cyclodextrin, increasing their effective steric bulk and reducing the frequency of their productive coUisions. At the same time the rates of hydrolytic decomposition of indoxyl and isatin are largely unaffected, so these processes become more dominant. Similar effects are likely to be common with molecular reactors unless complexation increases the desired reactivity in some other way. 

The examples described above are just a selection of those reported, chosen to illustrate what has already been achieved using cyclodextrin-based molecular reactors. The concepts have been clearly d onstrated and it is reasonable to expect that practical applica-... 

C. J. Easton, J. B. Harper, S. F. Lincoln, lV,lV-Bis(6 -deoxy-ot-cyclodextrin-6 -yl)urea as a molecular template in the formation of indigoid dyes. New J. Chem., 1998, 22, 1163—1165 J. B. Harper, C. J. Eastorr, S. F. Lincoln, A cyclodextrin-based molecular reactor to template the formation of indigoid dyes. Tetrahedron Lett., 2003, 44, 5815-5818. 

L. Barr, S. F. Lincoln, C. J. Easton, A cyclodextrin molecular reactor for the regioselective synthesis of 1,5-disubstituted-l,2,3-triazoles, Supramol. Chem., 2005, 17, 547-555. 

The inner cavity provided by the symmetrical and cylindrical capsule 7 7 was exploited as reaction flask in multiple studies. For example, it was shown that the 1,3-dipolar cycloaddition reaction between phenylazide 13 and phenylacetylene 14 (Huisgen reaction) was 20,000-fold accelerated when the 12a 12a container was used as molecular vessel compared to the reaction in bulk solution. Likewise, the regioselectivity of the reaction was modified affording one single region-isomer, exclusively the 1,4-adduct [34, 35]. These observations were explained by the increase in local concentration and the close and correct disposition of molecules 13 and 14 experienced in the capsule s cavity. Many other studies related to the use of the cylindrical 12a 12a capsule as molecular reactor have been reported [36-38] but for the sake of brevity, they will not be discussed in this chapter. 

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