Encapsulation of Reactive Intermediates

Jean-Luc Mieusset and Udo H. Brinker Institut fUr Organische Chemie, Universitat Wien, A-1090 Wien, Austria 

The inclusion of molecules is a very common technique in the pharmaceutical sciences to protect sensitive compounds and increase their usability and their storage time. For example, drugs were enclosed in host molecules like cyclodextrins. This topic has been previously reviewed.  

The same approach can be applied to organic chemistry where very labile species can be encapsulated. In this way, species that usually would rapidly decompose or polymerize even at low temperatures can be stored at room temperature, especially when the guest has no chance to escape from its host. To achieve this goal, hemicarcerands have been mostly used, leading to the preparation of cages containing highly reactive species such as 1,3-cyclobutadiene, cycloheptatetraene, or benzyne. This subject is presented in detail in a specially dedicated chapter (Reactions inside Carcerands, Ralf Warmuth). 

Similarly, very useful results can be obtained if the labile species is not incarcerated but can easily be released from its cage. Obviously, this property is significant in order to have a chance to use the guest or its products. In this section, we will first present what can be done to increase the thermodynamic and kinetic stability of a labile species with the aim to increase its relative concentration during the reaction or to make it more persistent. Second, we will show a few examples to demonstrate how to stabilize some 

Molecular Encapsulation Organic Reactions in Constrained Systems Edited by Udo H. Brinker and Jean-Luc Mieusset 2010 John Wiley Sons, Ltd 

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The encapsulation of reactive organometallic complexes is not restricted to the anionic [Ga4(L13)6]12- cages. Thus, Fujita and coworkers were able to generate the coordinatively unsaturated complex [Cp Mn(CO)2] (Cp = C5H4Me) within a self-assembled [M Le] coordination cage 28 (Fig. 20) (132). Photoirradiation of solid 27 gave complex 28, the crystal structure of which confirms the presence of the unsaturated pyramidal [CpMn(CO)2] fragment. The direct observation of such intermediates is... 

The rising need for new separation processes for the biotechnology industry and the increasing attention towards development of new industrial eruyme processes demonstrate a potential for the use of liquid membranes (LMs). This technique is particularly appropriate for multiple enzyme / cofactor systems since any number of enzymes as well as other molecules can be coencapsulated. This paper focuses on the application of LMs for enzyme encapsulation. The formulation and properties of LMs are first introduced for those unfamiliar with the technique. Special attention is paid to carrier-facilitated transport of amino acids in LMs, since this is a central feature involved in the operation of many LM encapsulated enzyme bioreactor systems. Current work in this laboratory with a tyrosinase/ ascorbate system for isolation of reactive intermediate oxidation products related to L-DOPA is discussed. A brief review of previous LM enzyme systems and reactor configurations is included for reference. 

The hydrophobic cavity of M4L5 cage and its strong affinity for cationic guests can be exploited for encapsulation and stabilization of reactive intermediates [18]. 

According to previous reports, under Brpnsted acid catalysis, three classes of products can be expected (Fig. 9.16). Four stereoisomeric products 13 and products 12a-d can be generated as major and minor products, respectively. Additionally, product 14 can be formed from condensation of 13 with 11. Using a coordination cage as a catalyst, 11 can be encapsulated within the hydrophobic interior of the cage, as confirmed by HNMR technique. It was established that, in contrast to cyclization in acidic aqueous solution, the hydrophobic interior of the cage prevents the capture of reactive intermediates by water [29], Cyclization followed by elimination resulted in diastereomeric products (Fig. 9.17) [10]. 

The process for preparing linear poly-p-xylylenes by pyrolytic polymerization of di-p-xylylenes has been extended to include the formation of p-xylylene copolymers. Pyrolysis of mono-substituted di-p-xylylenes or of mixtures of substituted di-p-xylylenes results in formation of two or more p-xylylene species. Copolymerization is effected by deposition polymerization on surfaces at a temperature below the threshold condensation temperature of at least two of the reactive intermediates. Random copolymers are produced. Molecular weight of polymers produced by this process can be controlled by deposition temperature and by addition of mercaptans. Unique capabilities of vapor deposition polymerization include the encapsulation of particulate materials, the ability to replicate very fine structural details, and the ability of the monomers to penetrate crevices and deposit polymer in otherwise difficultly accessible structural configurations. 

N NMR spectroscopy has been used extensively in the study of dinitrogen complexes, and a large body of data now exists on these compounds (143). It has also been used to study reactive intermediates Attempts, so far unsuccessful, have been made to identify the unknown ion [HN2]+ in the diazotization of NH3 with H N02 (144). The cyclic anion [CNy] and the isomers of [HCN7] formed in its protonation have been examined (145). Cluster compounds containing encapsulated nitrogen atoms (146) and mononuclear nitro complexes have also been studied (147) by both N and N NMR spectroscopy. The latter show a range of shifts from 66 to 174 ppm. 

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