Capsules softball

Differently from the previous chiral softball, in this case the dissymmetric inner surface of the cavity easily sensed chiral guests, providing different diastereoisomeric complexes in different amounts. Each diastereoisomer is characterized by four different NH resonances because of the complete loss of symmetry of the complex. The system described was extensively investigated pursuing the synthesis of a total of six different glycoluril monomers (21-26) with various spacers in terms of size and electronic properties on the two sides of the bicyclic six-membered unit, and the relative homodimeric capsules were studied with a total of 20 chiral enantiopure guests (Fig. 23 and Table 2) [53]. 

The cavity volume varies from 190 to 390 A3 and the average size of the guest is in the range 160-225 A3 for camphor derivatives and 145-175 A3 for pinane derivatives. Therefore, the proper matching of host and guest was crucial to obtain complexes characterized by the proper fitting and packing 

This effect is added to the steric interactions imparted by the asymmetric shape of the cavities, and to more weak interactions like CH/h. Nevertheless, in a few cases d.e. was above 50% and never more then 60% probably because the average distance between the capsule and the chiral content is not optimized, bearing in mind also that even in the presence of a good fit and high PC, still about half of the cavity s volume is not filled and this space surrounds the guests and is also interposed between them. 

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Fig. 21 Chiral homodimeric softball capsule from an achiral component bearing two different glycoluril residues. The R1 and R2 groups have been removed for clarity...

The structural diversity of softball capsules has also been analyzed by replacing the aryl spacer in the monomer with isomeric ethylene groups (shown in Figure 4-6) to form the congeners 40 guest 40 and 41 guest 41, referred to as wiffle balls [39], Various molecules were encapsulated in 40 guest-40, but no template effects were reported... 

Figure 6.2 a) Cylindrical capsule A. b) Softball capsule B. In the modelled structures some substituents, hydrogen atoms and hydrogen bonds have been omitted for clarity. Reproduced with permission from 12 /] Copyright 2002 Wiley-VCH Verlag GmbH ... 

Figure 7 Homodimeric softball capsule hosting two molecules of benzene as guests.

Figure 8 Chiral memory effect displayed by softball capsule templated with a chiral guest that is subsequently replaced by an achiral one, leading to an enantiomerically enriched softball that slowly racanizes.

Two different dynamic processes take place and can be followed by kinetic measurements (i) the exchange of guests and (ii) the exchange of the capsule monomers. In a first experiment [22]paracydo-phane 14 is added to softball 7-7 loaded with adamantane 8. The cydophane replaces adamantane within several hours. Then, in a second experiment the heterodimerization of 6-6 and 7-7 is studied. 

Fig. 26 A Chiral memory effect applied to the construction of a chiral softball homo-dimeric capsule made of achiral units. B Racemic softball binds a chiral guest leading preferentially to one diastereoisomer. Fast exchange, compared to capsule s racemization rate, with a better achiral guest allows preparation of enantioenriched capsule...

Figure 5.12 shows the Rebek softballs. Two C-shaped monomers can form a capsule self-assembled through a seam of hydrogen bonds. Inside the cavity, the... 

Fig. 5.12. Rebek s softballs — dimeric self-assembling capsules. The cavity volume can be tuned by the length of the spacer introduced between the center piece and glycoluril binding sites of the monomers. The methylated monomer (bottom) is not able to form dimeric capsules. Several hydrogen bonding donors and acceptors (arrows) that are responsible for dimerization, are implemented in each building block. A computer-generated model of an example of a dimeric capsule is shown in the box. Right Typical guest molecules which can be encapsulated in the cavity.

Figure 4-6 Rebek s dimeric softball and wiffle-ball capsules.

Fig. 1 Capsule formation and early examples of Rebek s self-assembling eapsules self-replicating dimer 1-1, the tennis ball 2. and the softball 3.

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