Page 156 - Academic Press Encyclopedia of Physical Science and Technology 3rd InOrganic Chemistry

P1: GLQ/GLT P2: GPJ Final Pages
Encyclopedia of Physical Science and Technology EN007G-334 June 30, 2001 15:23

Inclusion (Clathrate) Compounds 727

FIGURE 27 Two representations of the structure of perhydro-
triphenylene.

reactions can provide insight. Reactions out of the realm
of true biochemistry have two features in common: They
are catalyzed very effectively, so that they occur rapidly
even under mild conditions, and they are very selective. A
true enzyme mimic must exhibit both of these features.
The most studied class of enzyme mimics is the cy-
clodextrins. The rigid hydrophobic cavity is rimmed with
hydroxyl groups that may be further functionalized.
As an example of a well-studied biomimetic process,
consider the chlorination of anisole:
OCH 3 OCH 3 OCH 3
Cl
HOCl

Cl

In aqueous solution both the ortho and the para isomers
are produced (in ratio of about 40:60, respectively). Ad- FIGURE 29 (a) Propellerlike structure of tri-o-thymotide and (b)

related structure of N, N -dimethyltrianthranilide.
dition of α-cyclodextrin changes the ratio of 4:96. The
ortho position is clearly shielded from attack, as is shown
in Fig. 30. The cyclodextrin also has the effect of sub-
stantially speeding up the chlorination of anisole. This
catalysis is due to the existence of a new pathway for the
reaction, which is also shown in Fig. 30.
The construction of synthetic iron(II) dioxygen carriers
as a model for myoglobin has long been sought. Advances
based on transition metal complexes of cyclidenes have
now provided the result. Cyclidenes, given in Fig. 31, are
composed of two fused rings, one which is useful for metal
coordination and the other for molecular design purposes.
A model which serves to illustrate the biological discrim-
ination of O 2 over CO is displayed in Fig. 32.
The synthesis of molecules with shapes and functional
group orientation (Fig. 33) appropriate for catalytic func-
tions has been achieved by the use of the Kemp triacid as
a structural unit to effect U-turns. This strategy has led to
hosts capable of the recognition of numerous substrates
such as is shown for chiral alcohols in Fig. 34.
Many other classes of enzyme mimics are under in-
FIGURE 28 (a) Saucerlike structure of cyclotriveratrylene and vestigation, but the general principles are covered in the
(b) related octopus molecule. examples above. One of the first benefits expected from