Rotaxanes 3 , unsymmetrical

The successful synthesis of both symmetrical and unsymmetrical [3]rotaxanes revealed the strength of the threading approach - even though the neutral template responsible for the formation of the rotaxanes offers only weak interactions, two macrocycles can be mechanically bound to an elongated amide thread. 

Figure 27. Syntheses of symmetrical 67 and unsymmetrical 66 amide-linked [3]rotaxanes.

Figure 32. Porphyrin blocking groups offer the possibility of further supramolecular deri-vatization. Amide-linked rotaxane with porphyrin blocking groups 85, its Zn complex 86, and an unsymmetrically capped rotaxane 87.

The bridging of sulfonamide groups with diiodo compounds can also be applied to the intermolecular coupling of rotaxanes. When rotaxane 80e was treated with 95 under standard conditions, 76% of the wheel-bis[2]rotaxane 102 and 8% of the iodine-substituted rotaxane 101 were isolated (Figure 40) [46]. 101 is an excellent substrate for the synthesis of higher and unsymmetric rotaxane assemblies. 

Figure 49. The first cycloenantiomeric [2]rotaxane - the different atom sequence of wheel with respect to the unsymmetric axle creates this rare case of chirality.

In 1996 we reported on the first covalent dimerization of the [2]rotaxane with sulfonamide function in its wheel component to give a [3]rotaxane 32 (bis[2]rotaxane) [49], Subsequently, we expanded the concept of covalent bridging and produced more elaborate rotaxane assemblies such as an unsymmetrical bis[2]rotaxane 33, and 34,... 

Undoubtedly, the most important stereochemical feature is chirality. The [2]-rotaxane depicted schematically in Figure 2.12a is made with a macrocycle and a dumbbell that are not themselves chiral but oriented by appropriate substitution patterns on these components. Vogtle and co-workers described cycloenantiomeric rotaxanes that are based on the amide templating functionality (Figure 2.13).36 Racemic [2]-rotaxane 34 was synthesized in 20% yield by reaction of unsymmetrical acid dichloride 32 with trityl aniline 33 in the presence of macrocycle 31. The object and its mirror image are created by different sequences of the amide and sulfonamide linkers in both the dumbbell and the macrocyclic component. If the order of the atoms within the dumbbell is examined, in one enantiomer the sulfonamide nitrogen atom is... 

A number of other simple rotaxanes based on cyclodextrins have been reported. Rotaxanes incorporating more than one threaded cyclodextrin moiety are also known. One such system involves a linear component consisting of a 16-methylene chain unsymmetrically terminated with carbaxole and viologen... 

Figure 7 shows another /i-CD-based pseudo[3]rotaxane host-gnest system R5 complexing with an unsymmetric guest compound comprising ferrocene and azobenzene... 

In particular, when the TFPB salt of n-hexylbenzylammonium was added to a CDCI3 solution of calix[6]arene host lb, the NMR spectrum clearly evidenced the formation of both pseudo[2]rotaxane diastereoisomers endo-alkyl)- and endo-benzyl)-8ai clh (Fig. 30.7) in a 4 1 ratio [17]. Interestingly, the unsymmetrical threading experiment performed with n-alkylbenzylammonium cations bearing shorter alkyl chains, such as pentyl and butyl Sc", led to a higher endo-aUcyl/endo-benzyl ratio of 10 1 and 30 1, respectively [17]. From these results we have deduced the following stereochemical ndo-alkyl rule [27] (Fig. 30.7) ... 

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