198 Topics in Current Chemistry Editorial Board: A. de Meijere KN ...

112 R.E. Meléndez · A.D. Hamilton
structure of 15 to have expanded their cavities from 11 Å (taking into account
van der Waals radius) to 12.7 Å. In 16 the hexamer cavities are distorted and
therefore the diameter is reduced to 10.4 Å (see Fig. 14) [49].
3.2
Amides
A functional group that can parallel the hydrogen bonding directionality of the
carboxylic acid synthon is the amide group . Primary and secondary amides
[17], pyridones [50], and 2-aminopyrimidines [51] form eight-membered ring
hydrogen bonded dimers via the bidentate interaction of the N-H and C=O
groups. However, amides can also form linear chains through hydrogen bonding
to adjacent molecules. For example, in the solid state benzamide (17) forms
dimers and catemers (Fig. 15) [52].
Lauher et al. have successfully predicted the formation of a bidentate chain
pattern formed by ureylenedicarboxylic acids. The analysis of six different
structures has led to the conclusion that this interaction persists even in the presence
of other strong hydrogen bonding groups (carboxylic acid) [53]. Compound
18 shows the formation of catenated bidentate urea interactions. However,
in ureylenedicarboxylic acids of type 19, both urea-urea and carboxylic
acid dimer motifs are present creating a stable sheet arrangement. Although the
bidentate interaction is polar, the adjacent strands in 19 take up an antiparallel
orientation, therefore cancelling polarity in the sheets (Fig. 16).
Wuest et al. have synthesized dipyridones to take advantage of the amide
hydrogen bonding dimer [50, 54]. In an elegant study they separated the pyridone
functional groups by several different spacers. When the dipyridones are
asymmetric, such as 20, they form discrete hydrogen bonded dimers. However,
the symmetric isomer 21 cannot satisfy its hydrogen bonding potential in the
Fig. 15. Formation of dimers and catemers through hydrogen bonding in 17