Molecular clipping

Sijbesma, R. P., Nolte, R. J. M. Molecular Clips and Cages Derived from Glycoluril. 175, 57-100 (1995). 

Scheme 4.6 Self-sorting behavior of C-shaped molecular clips 13 and ( )-14.

Wu AX, Chakraborty A, Eettinger JC, Elowers RA, Isaacs L. Molecular clips that undergo heterochiral aggregation and self-sorting. Angew Chem Int Ed 2002 41 4028-4031. 

The structural diversity associated with bicyclic 5-5 fused heterocyclic systems containing two heteroatoms in each ring has been noted previously in both CHEC(1984) <1984CHEC(6)1027> and CHEC-II(1996) <1996CHEC-11(7)115>. In recent years this diversity has also been reflected in the number of applications that have been found for these compounds which include organic metals, molecular clips and receptors and molecular magnets. These fields will be examined in due course through this chapter. 

More recently, Wu et a/. <2006JOC4502> have described an extensive crystallographic study of 11 different molecular clips based on the glycoluril skeleton 11. 

Since this work, a number of other molecular clips have been synthesised. Extensive studies in the synthesis of these compounds have been carried out by Pan and Wu who have described a new class of clips which bind aromatic guests and are soluble in nonpolar organic solvents <2006SL49>. The synthesis of this new class of molecular clip is also based on a novel synthetic pathway utilising aromatic amines outlined in Scheme 17. 

Molecular Clips and Cages Derived from Glycoluril... 

The term Molecular Clip has been coined for molecules of type 2. That these molecules do indeed possess the geometric features of a clip is apparent from the X-ray structure of the tetramethoxy derivative 3a (Fig. 2) [lla,b]. The o-xylylene moieties of this molecule define a tapering cavity, the walls of which are at an angle of 39.5 with the centers of the benzene rings 6.67 A apart. The carbonyl groups of the glycoluril moiety, which are hydrogen-bond acceptor sites, are separated by 5.52 A. It was also possible to obtain a crystal structure of the chiral dibromo-derivative 4 of clip 3 (Fig. 3). This compound was separated into its enantiomers by HPLC on a chiral stationary phase [12]. 

In order to increase the n-n interaction with the aromatic guests, two molecular clips with naphthalene walls (compounds 8 and 9) were synthesized. These molecules had quite unexpected properties. Surprisingly, the clip with 1,4-dimethoxynaphthalene cavity walls (8) did not bind guest molecules [15]. The reason for this behavior became clear when the X-ray structure was solved [16] (Fig. 6). All four methoxy groups of 8 were found to point into the cavity, completely blocking the access of a potential guest to the carbonyl groups. A similar structure may also be present in solution. 

With the objective of developing a selective epoxidation catalyst, molecular clip 3a was functionalized with Ni(II)- or Mn(III)-salophen groups (see 31a and 31b) [33]. Salen and salophen complexes of Mn(III) are known to epoxidize alkenes in the presence of an oxygen donor [34]. The X-ray structure of the... 

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