Molecular Encapsulation: Organic Reactions in Constrained Systems

Book Preface

The inclusion of small guest molecules within suitable host compounds results in constrained systems that imbue novel properties upon the incarcerated organic substrates. For example, the chromophoric nature of dye molecules is considerably altered upon complexation.

And the targeted delivery of active pharmaceutical ingredients can be facilitated in vivo . So, understandably, supramolecular tactics are becoming widely employed and this treatise spotlights them. Indeed, the knowledge gleaned from constructing such assemblies can be applied to develop innovative molecular devices. Insofar as molecular constraint infl uences chemical reactivity, including chemo – , regio – , and stereoselectivity, new synthetic routes become plausible. Often, the impact of encapsulation on product formation is substantial. Therefore, special attention must be paid to the stabilization and chemistry of short – lived, high – energy intermediates, since they are key compounds that govern the outcome of reactions. Common among these transient species is their ability to subsequently undergo an array of reaction mechanisms that include the generation of unusual or strained products. However, the conventional drawback has been that high product yields are sacrifi ced due to a lack of selectivity across low – energy barriers. The use of a constrained system, however, offers the means to solve this problem by steering the reaction trajectory along a desired pathway. In general, we provide a broad overview of many different approaches that aim to manipulate chemical reactions. We anticipate that this endeavor will be thought provoking and ultimately facilitate the exchange of ideas between various scientifi c disciplines.

This volume comprises 17 chapters. It is spearheaded by an account of molecular recognition being used to precisely control photochemical reactions and foster enantiomeric excess. The supramolecular reaction proceeds under mild conditions, wherein a highly reactive excited – state intermediate readily generates products that are diffi cult to obtain thermally. Such inductions of enantioselectivity by chiral auxiliary are especially  appealing. The following three chapters expound upon the cyclodextrin macrocyclic hosts, whose wide availability and ability to form inclusion complexes makes them miniature reaction vessels par excellence . Chapter 2 deals with the use of natural and derivatized Schardinger dextrins to accelerate reactions, such as transesterifi cations and Diels – Alder reactions, and to mimic enzymes, e.g. , ribonucleases and transaminases. Chapter 3 focuses on the development of molecular reactors. The primary role of these containers is to increase the selectivity of a chemical transformation, which also may occur by decreasing the overall rate of the process. In chapter 4 , special cyclodextrin – mediated reactions are discussed. Since guest complexation offers the possibility of performing organic reactions in aqueous media, microsolvent effects are feasible, the protection of water – sensitive intermediates is achieved, and conformational control during the reaction is realized. Indeed, this approach has great appeal for environmental chemistry. Chapter 5 demonstrates the potential of nanoreactor technology, using zeolites as an example. These inorganic inclusion compounds are widely used on an industrial scale during petroleum refi ning, for example, because they catalyze the isomerization and oligomerization of alkenes. The utility of these materials is due to the presence of active sites in well – defi ned cavities that enable customized control of reaction selectivity. And though these natural catalysts are readily available, easily reusable, and therefore cost – effective, the facile preparation of a container of suffi cient size suitable for the regulation of a chemical reaction is an ongoing challenge. To fashion cages as appropriate nano – reactors, a novel approach avoiding tedious multistep synthesis is becoming widespread. It entails the self – assembly of noncovalent capsules. The results obtained using these organized media are highlighted in chapter 6 .