Pyrazines, encapsulated

Multiple guests have further been used as kinetic templates for creating multicapsules with one guest per chamber (Fig. 13).92 The dynamic characteristics and the overall potential of such intriguing carceplexes have not been fully examined though there was some indication that (a) encapsulation of three molecules of methyl acetate inside 23 was cooperative and (b) encapsulated pyrazines would in 24 modulate each other s magnetic environment.92,94 It is also worth mentioning that from the synthetic standpoint the preparation of capsule 25 was accomplished in an excellent 30% yield.94... 

Pyrazine, 1,4-dioxane, dimethyl sulfide, DMSO and so on were found to be good templating molecules, whereas JV-methyl-2-pyrrolidene (NMP), dimethylformamide (DMF), DMA and so on were poor templating molecules. The general observations which were made are as follows (1) for the reported carcerand a maximum of seven nonhydrogen atoms is allowed (2) the guest should be neither too polar nor too nonpolar (3) higher symmetry results in easier carcaplex formation (4) secondary amines are not encapsulated and (5) cyclic molecules act as better templates than acyclic molecules. 

Rebek and co-workers have studied the self-assembly of molecules containing substituted ureas.65 Dimerization of two calixarenes (where Bn is benzyl and Ar is 4-fluo-rophenyl ) (7.8) by hydrogen bonding produces a host that encapsulates benzene, fluorobenzene, p-difluorobenzene, and pyrazine, but not toluene. 

Picolinic acid also accelerates the H2O2 oxidations but less efficiently than pyrazine-2-carboxylic acid. It has been demonstrated recendy that the vanadium complex with picolinic acid, VO(PA)2 , encapsulated into the NaY zeolite retains solution-like activity in the liquid-phase oxidation of hydrocarbons [16a], It is noteworthy that pyrazine-2-carboxylic acid accelerates the hydrocarbon oxidation catalyzed by CH3Re03 [25 b]. Employing a (+)-camphor derived pyrazine-2-carboxylic acid as a potential co-catalyst in the CHsReOj-catalyzed oxidation of methyl phenyl sulfide with urea-H202 adduct, the corresponding sulfoxide was obtained with an e.e. of 15% [16b]. 

Similar effects have been observed based on double crown-ether encapsulation of the CT ion and a symmetrically substituted derivative, and similar conclusions have been reached concerning the applicability of a three-site delocalization mechanism. In both the solvent and crown-encapsulation studies, however, the intervalence energy shifts are only about half as large as expected based on a simple t/7r(Ru)—7r (pyrazine)—t/7r(Ru) delocalization scheme. The additional involvement of pyrazine tt orbitals could account for the smaller than expected environmental effects. 

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