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

Functional Organic Zeolite Analogues 149
there is a rather strict upper limit for the size of guests able to be accommodated.
As noted above, two guest (ketone or ester) molecules (R 1COR 2; R 1,
R 2= alkyl, aryl or alkoxy) are bound in each cavity of the 2D network 30 with
their carbonyl groups hydrogen-bonded to the host in the sheet and their alkyl,
aryl or alkoxy groups extending into the neighbouring sheets. In reference to the
top view 31 (Fig. 9) of the sheets, the affinity of a guest would then be governed
by the intersheet distance. This in fact results in a remarkable chain-length
selectivity. The optimal chain-length in R 1 and R 2 is 4. Thus, 4,4-ketones such as
5-nonanone, benzophenone and propyl or isobutyl benzoate constitute a class of
highest affinity guests. The shorter one (£ 3) with a reduced affinity is still able
to be accommodated, but the longer one (£ 5) is hardly incorporated; the
(4,4) vs (5,5) selectivity being 5-nonanone/6-undecanone @ 100/0. Ethyl benzoate
(a 3,4-ketone) is one of the suitable guests, while its alkyl/aryl exchanged
isomer, phenyl propionate (a 2,5-ketone) shows almost no affinity [52].
A similar cooperation of hydrogen bonding and hydrophobic/van-der-Waals
cavity packing may explain the preferential binding of alcohol from an aqueous
alcohol solution [70].
What is remarkable about zeolite pores is that various pore sizes are available.
Pore size control is an important target of designed organic zeolite analogues.
There are a number of promising approaches. One is concerned with thickness
control. The huge cavity of height ~10 Å provided by the diresorcinol 2D net 30
(Fig. 9) is more than enough to incorporate two molecules of a simple guest, e.g.
alkyl benzoate. The branching in the alkyl moiety allows better packing of the
cavity and hence affinity-enhancing, i.e. isopropyl>ethyl and isobutyl>propyl
[52]. Monoresorcinol host 32 (Fig. 9), on the other hand, forms a 1:1 adduct in
which the guest must be inserted in a much thinner (~3.5 Å) cavity (34), which
fits for an aromatic ring having a p-electron thickness of ~3.5 Å as well as a linear
alkyl group. This results in a remarkable thickness or linear/branched selectivity
for the ketone guests. Competition, for example, between diethyl ketone
and diisopropyl ketone or between ethyl phenyl ketone and isopropyl phenyl
ketone results in an exclusive incorporation of the former [53]. The branched
isopropyl group is too thick to be incorporated in the cavity without breaking
the hydrogen-bonded polyresorcinol chains which are essential for maintaining
the cavity. Attempted crystallization of host 32 from isopropyl benzoate
affords a quite different 1:2 (host to guest) adduct having no network
structure.
The size and shape of the pores may be systematically changed by modifying
the hosts. This approach, however, often fails since even an apparently slight
structural change could dramatically alter the crystal structure. A family of
chiral bicyclic diols including 47, 50, and 52 (Fig. 13) are remarkable in this
respect, since the key spiral spine motif 48 is well conserved in the family, while
the size of the internal canal depends on the stereochemistry at the OH-bearing
carbon, i.e. 50 vs 52, and the presence or absence of an alkyl bridge i.e. 47 vs 52
[61]. The canal cross-sectional areas (e.g. 49, 51, and 53) can be changed in a
range from 0 to 35 Å 2 and a great variety of guest molecules with different sizes,
including those as wide as ferrocene and as long as squalene, can be included in
tuned intrahelix canals.