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

Functional Organic Zeolite Analogues 155
In the absence of any catalyst, the reaction is ~ 90% endo-selective and is very
slow with a half-life of t = 500 h at 25 °C. The reaction is catalyzed by the solid
state of the host (29) almost equally when the latter is used as pulverized powders
(with a turnover rate constant of 0.33 h –1 ) or as nonpulverized pieces of an
approximate size of 1 ¥ 1 ¥ 1 mm (0.20 h –1 ). The catalyzed reaction gives rise to
the endo product (68 endo) with a 96% selectivity. In reference to the criteria
shown above, the present heterogeneous catalysis is approximately an order of
magnitude more efficient than the corresponding homogeneous catalysis exhibited
by soluble resorcinol and shows a remarkably small size effect for the
solid catalyst.
When immersed in a mixture of reactants, the host readily forms a 1:2:2
adduct 29 ◊ 2(acrolein) ◊ 2(diene) in seconds. The isolated adduct undergoes a
gradual intracavity Diels-Alder reaction with an approximate half-life of 2 h.
The resulting product adduct 29 ◊ 2(68 endo), when dipped in the reaction mixture,
undergoes a facile product/reactant exchange to regenerate the reactant
adduct 29 ◊ 2(acrolein) ◊ 2(diene) in minutes. These results, coupled with the
negligibly small size effect for the catalyst, suggest that the principal catalytic
sites are the internal cavities and not the surface of the solid catalyst.
The plausible mechanism (Fig. 17) involves (1) simultaneous binding of two
reactants in a cavity, (2) facilitated (~250-fold as compared with the spontaneous
process) but still rate-determining intracavity reaction which is preorganized
and hence stereoselective, and (3) product/reactant exchange resulting in
turnover of the catalyst. In light of the crystal structure for an analogous adduct
29 ◊ 2(ethyl acrylate)◊(diene), there is no doubt that the proximity of dienophile
and diene is responsible for the acceleration of the present bimolecular process
in the cavity.However,it should not be overlooked that the dienophile hydrogenbonded
to the host (O-H◊◊◊O-H◊◊◊O=C-C=C) must be activated electronically,
although separation of an overall effect into steric and electronic factors is not
easy.
The effect of hydrogen bonding becomes clearer when one examines a unimolecular
process which should be free from proximity effects. Host 29 also
catalyzes the intramolecular ene reaction of an unsaturated terpenoid aldehyde
cytronellal (69) to isopulegol (70) (Eq. 3) [79].
69 70
In the initially formed 1:2 adduct 29 ◊ 2(69), substrate 69 is hydrogen-bonded to
the host with its alkenyl group in an extended conformation. Nevertheless, the
present cyclization reaction in the cavity is ~ 100-fold faster than the spontaneous
process. The acceleration should be primarily ascribed to the hostsubstrate
hydrogen bonding.
The Diels-Alder and related ene reactions provide a mechanistic prototype of
the zeolitic catalyses played by microporous organic solids. From a practical
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