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

Functional Organic Zeolite Analogues 157
undergoes a facilitated and highly stereoselective (98%) intracavity Diels-Alder
reaction [65]. The product, however, resists being exchanged by the reactants,
resulting in a product inhibition of the catalysis. While the essential difference
between the acrolein and ethyl acrylate systems is not clear, the diminished
exchangeability of the bulkier acrylate product is not unexpected. The hydrogen-bonded
network in host 29 is flexible enough to adjust to the guest structures.
This may be a good aspect as far as guest binding is concerned. However,
such an induced-fit “stabilization” of the adduct may make the desorption of
included guest more difficult (Fig. 16). An efficient turnover of a catalyst is
based on a good balance of sorption and desorption. It is likely that for this purpose
we need more rigid and less guest-sensitive networks.
4.3
Immobilization of Soluble Metal Complexes
Hydrogen-bond donor atoms (O and N) are potential ligands for metal ions.
This suggests that the hydrogen-bonded network is convertible to a metalcoordinated
one via H + /M n+ exchange. Treatment of apohost 29 with
TiCl 2(OCH(CH 3) 2) 2 affords an insoluble amorphous solid formulated as
29 4– ◊ 2[TiCl(OCH(CH 3) 2] (Ti-host) with concomitant liberation of one molecule
of HCl and isopropanol (29+2[TiCl 2(OCH(CH 3) 2) 2] Æ 29 4– ◊ 2[TiCl(OCH
(CH 3) 2]+HCl+(CH 3) 2CHOH) (29 4– is the deprotonated tetraanion of 29). This
formulation suggests that the hydrogen-bonded network (O-H◊◊◊O-H) in apohost
29 has been replaced by a metal-organic network (O – –Ti 4+ –O – where
Ti 4+ =Ti 4+ Cl(OCH(CH 3) 2). When dipped in liquid ethyl acetate, alkyl benzoate,
alkyl acrylate or acrolein as a guest, Ti-host binds 4 moles (i.e. 2 moles on each
4-coordinate Ti 4+ centre) of the guest. Gas adsorption studies indicate that
the guest binding to Ti-host is much more reversible than to apohost 29
(Fig. 16). Hydrocarbon guests such as benzene,p-xylene and 1,3-cyclohexadiene
(4–6 moles) are also accomodatable.
Ti-host remarkably catalyzes the highly endo-selective (> 99%) acrolein-1,3cyclohexadiene
Diels-Alder reaction (Eq. 2) [80]. The half-lives of this reaction
in the absence and presence (3 mol%) of a catalyst are t = 500 h (no catalyst), 50
h (apohost 29 as an insoluble catalyst), 1 h (TiCl 2(OCH(CH 3) 2) 2 as a soluble catalyst)
and ~ 5 min (Ti-host as an insoluble catalyst). Thus, as a Lewis acid Ti-host
shows a much higher activity than its soluble counterpart. As a solid catalyst, it
allows easy product-catalyst separation, recovery of the catalyst without deactivation
and hence its repeated use. In addition, Ti-host is also capable of catalyzing
the Diels-Alder reaction between ethyl acrylate and 1,3-cyclohexadiene,
which the apohost 29 fails to catalyze because of the lack of desorption of the
product from the cavities.
A soluble Ti 4+ Lewis acid can thus be immobilized with a known hydrogenbonded
supporting network as a microporous multi-ligand. This simple
strategy would in principle be applicable to various organic networks and
metal ions.