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

Functional Organic Zeolite Analogues 159
redox catalysts and have been incorporated in a variety of porous structures. As
for the guests, there are good examples of hydrocarbon incorporation.
The use of efficient solid catalysts may play an essential role in the construction
of waste-free and workup-free molecular transformations which are
friendly to the environment and resource-saving.
5.3
Organic Zeolite Analogues as Enzyme Mimics in Water
From the environmental point of view, the most ideal solvent is water. This is
also the medium where enzymatic reactions take place. There are a couple of
questions in the use of water for the zeolite analogues. One is as for the maintenance
of structures; can a hydrogen-bonded or metal-coordinated network
survive in an aqueous environment? The other is with respect to the efficiency
of catalysis; even when the structure is maintained, can a guest be bound to the
host via either hydrogen bonding or coordination to the metal in preference to
the water? Both questions arise from the very polar nature of water, which acts
as a strong hydrogen-bond former as well as a potent ligand to a metal ion, especially
a Lewis acid centre.
The strength of the metal-coordination networks against hydrolysis may be
proved by the fact that many of them are prepared by crystallisation from an
aqueous medium. Use of water-tolerable Lewis acids such as lanthanoid metals
may be an interesting extension. The water-tolerance of a hydrogen-bonded
network is also proved. For example, host 35 (Fig. 10), when immersed in water,
not only maintains its hydrogen-bonded network but actually incorporates a
large number (~ 21) of water molecules in each huge cavity and still retains
single-crystallinity. In a similar manner, host 29 (Fig. 9) adsorbs ~ 16 water
molecules. Furthermore, this host preferentially binds highly hydrophilic guest
molecules such as ethanol, ethyl acetate and acetonitrile in water via host-guest
hydrogen bonding. This suggests that cavity-bound polar guests can be activated
via hydrogen bonding in water in a similar manner as in enzymatic catalyses.
In fact, there is a remarkable apparent relevance between microporous
organic solids and enzyme proteins in the maintenance of structures and functions.
Acknowledgements. The support of our research activities by CREST (Core Research for Evolutional
Science and Technology) of Japan Science and Technology Corporation and also by
Grant-in-Aids from the Ministry of Education, Science, and Culture of the Japanese Government
is greatly acknowledged. I am also grateful to my capable co-workers, especially Dr. K.
Endo (CREST, Kyushu University) and Dr. K. Kobayashi (Tsukuba University) for their collaboration.
6
References
1. Breck DW (1974) Zeolite molecular sieves, structure, chemistry, and use. Wiley, New York
2. Herron N (1991) In: Atwood JL, Davies JED, MacNicol DD (eds) Inclusion compounds, vol
5, chap 3. Academic Press, Oxford
3. Suib SL (1993) Chem Rev 93:803