Tetraloop tetra-urea

Figure 5.8 Formula survey of tetra-urea calix[4]arenes used as precursors for bis-, tris-, and tetraloop tetra-ureas (Y = alkyl).

Bulky residues can be attached to the urea groups of a tetra-urea under conditions where it forms a heterodimer (a pseudoro-taxane) with a bis-, tris-, or tetraloop tetra-urea. Often this strategy is called stoppering (see Scheme 5.7), because stoppers are attached to the axles. 

Figure 5.19 Molecular shape of rotaxane 23 (n = 6), a hetero-dimeric assembly of a tetraloop tetra-urea 9 (stick representation) and tetra-tosylurea 4 (space filling), based on MD simulations. Ether groups (OY = OC5Hnn,) areomitted in the formula.

As shown in Section 5.5, bisloop or tetraloop tetra-ureas (8, 9) do not form homodimers, while monoloop compounds (e.g., 15) form selectively only one homodimer. These selectivities may be summarized as follows ... 

The inability of bis-, tris-, or tetraloop compounds to form homodimers and the general tendency of tetra-urea calix[4]arenes to dimerize can be further exploited for the synthesis of mechanically interlocked molecules. A 1 1 mixture of tetra-ureas 5 or 6 with bis- or tetraloop compounds 8 or 9 (in practice the non-reactive loop component is added in a small excess) contains exclusively heterodimers (e.g., 5-8, 5-9, or 6-9), since this is the only possibility, to have all the urea functions involved in the favorable hydrogen-bonded belt.5 Again this is easily evidenced by the complete absence of peaks for the homodimer 5-5, or 6-6 in the H NMR spectra (for an example see Figure 5.11). 

The preorganization of functional groups attached to the urea residues could also be used for the synthesis of multiple rotaxanes. Schematically, this is shown for tetra[2] rotaxanes in Scheme 5.20. Bis[2]rotaxanes will be available analogously, using a bisloop compound 8 instead of the tetraloop compound 9. 

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