Tertiary Aromatic Amides Imides and Ureas

The oligomers described in this section resemble most of those described in Section 1.3 in that they consist of a string of p-conjugated units separated by single bonds. They thus possess a reduced number of accessible conformers. However, in these cases none of the schemes shown in Fig. 1.2 apply: unambiguous conformational preferences do not exist at the single bonds separating the p-(systems). Rotation about these bonds is nevertheless restricted by various factors, such as intramolecular p-p stacking between aromatic units and steric hindrance associated with bends in the backbone architectures. The bends arise from the preferred cis conformation of tertiary amides of aromatic acids and aliphatic-aromatic secondary amines (Fig. 1.9), in which the two aryl groups project to the same side of the amide bond [115]. Even though no clear conformational preferences are defined at the aryl-NCO and aryl-CON linkages, rotation about these bonds is hindered by steric effects and weak attractive electrostactic interactions between aryl groups. When the molecules are soluble in protic solvents, interactions between the aryl groups are reinforced by solvophobic effects (see Chapter 3, Section 3.4.1).

Fig. 1.9 Cis-trans equilibrium of aliphatic aromatic tertiary amides. See Itai et al. [115].

Fig. 1.9 Cis-trans equilibrium of aliphatic aromatic tertiary amides. See Itai et al. [115].

This pattern has been efficiently exploited in oligomers derived from aliphatic-aromatic tertiary ureas and guanidines [81, 116], imides [117, 118], and amides [119-122]. In urea and imide functional groups, two cis conformations are combined, resulting in a strong kink between each aryl group that allows almost perfect face to face stacking of adjacent aryl rings. In the solid state, these oligomers adopt folded ladder-like conformations consisting of a pseudo-helical arrangement of imide or urea moieties around a central column of stacked aryl groups (Figs. 1.10a, b and d) [81, 117]. Tertiary amides give rise to less pronounced kinks and thus less pronounced interactions between adjacent aryl groups in the sequence. However, solid state structures show that these oligomers adopt compact helical

Fig. 1.10 Formulae and crystal structures at the same scale of tertiary ureas, amide and imide oligomers. (a)-(b) front view and side view; (c) top view and side view; (d) two possible conformations. References of examples (a)-(d) are from [81], [81], [120] and [117], respectively.

conformations where aryl-aryl contacts are observed between nonadjacent units (Fig. 1.10c) [120].

Evidence that solution conformations resemble those observed in the solid state come from NMR studies that show NOE and upfield shifts of the proton signals involved in p-p stacking. However, the absence of strong directional interactions in the folded conformation leads one to suspect that the solution structures are not as well-defined as the solid state structures. At each aryl-amide, aryl-imide or aryl-urea linkage, several conformers are compatible with p-p stacking of the aryl moieties. This is well illustrated by the two crystal structures shown in Fig. 1.10d. They correspond to two distinct folded conformations of the same backbone where the upper and lower naphthyl groups have been flipped upon rota-

Fig. 1.11 Schematic representation of possible conformers of the oligomers shown in Fig. 1.10d [117].

tion of the naphthyl-imide linkage by 180°. Even though these two structures belong to two molecules which possess methyl and benzyl residues, respectively there is no reason to exclude an equilibrium between the two conformers for both species, as illustrated in Fig. 1.11. To the extent that the structures shown in Fig. 1.10 may be called helices, such rotations about aryl-imide, aryl-urea and aryl-amide linkages amount to locally inverting the helix handedness. It is remarkable that such inversions may occur many times within an oligomer without disrupting the overall arrangement of the stacked aryl groups. Thus, the solution conformations of aromatic tertiary amide, urea and imide oligomers presumably consist of an ensemble of closely related conformers.

The equilibria involved between these conformers may be shifted upon introducing a chiral bias at each aliphatic substituent of the backbone nitrogen atoms: a chiral side group gives a local preference in favor of the left-handed or the right-handed helical turn. Thus, in oligoimides [118], and polyamides [120, 122] possessing a chiral aliphatic residue at every unit, strong induced circular dichroism bands are observed, suggesting a long range helical order in the backbone. From the studies above however, it might be hypothesized that the effect of chiral residues is local and that ''majority rules'' and ''sergeant and soldiers'' principles apply to a limited extent in these systems (see Chapters 11 and 12).

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