Immobilized Nonenzymatic Catalysts
There are also many uses for special nonenzymatic polymeric catalysts. For instance, polymer-bound crown ethers, cryptates, and channel compounds can be immobilized to act as polymeric phase transfer catalysts. The catalytic activity is based on selective complex formation. An example is a use of polystyrene-attached oxygen heterocycles [18]-crown-6 or a cryptand [222] to catalyze replacements of bromine in n-octyl bromide by an iodine or by a cyanide group [47]:
A 95% yield is achieved. The catalytic activity, as a result of the complexation of the cations, results in an increased nucleophilicity of the anions. Interactions of ions and ion pairs with vinyl polymers of crown ethers were shown to be considerably more efficient than such interactions with unattached crown ethers [48]. Also, studies of diazo-4,7,13,16-tetraoxacyclooctadecane bound to polyacrylamide gel show an enhancement of cationic complexation when compared to ligands that are not bound to polymers [49]. On the other hand, polymer-bound crown ethers do not offer any advantage over unbound ligands in the Koening–Knorr reaction [50]. The catalytic properties and solute-binding capabilities of the pendant crown ethers and glyme ligands apparently depend on the spacing between the ligands. They also depend upon the structure and length of the chains connecting the ligand bound ions and the solvent [51]. In low polarity solvents, the ligands activate anionic reactants through modification of their ion pair structures. Ion exchange resins have been used for a long time now to catalyze some reactions. This is mentioned, for instance, in Chap. 7 in the section on epoxy resins. Basic ion exchange resins can also be used in condensations of furfural with aliphatic aldehydes [52]. Astill different kind of polymeric catalyst is one that has pendant photosensitizers attached. To be effective, the sensitizer portion must absorb light and undergo a transition from a singlet to a longer-lived triplet state (see Sect. 10.4). It must then, without emitting radiation, activate a substrate molecule and return to the ground state. Some dyes function in this manner. An example is Rose Bengal. When it is attached to cross-linked polystyrene [53], it can be used to produce singlet oxygen.
The excited oxygen in turn hydroperoxidizes olefins. The structure of polymer-bound Rose Bengal can be illustrated as follows:
The photosensitized hydroperoxidation reaction of olefins [53] can be shown as follows:
Metathesis catalysts including Grubbs ruthenium catalyst (see Chap. 5) are hard to separate from the reaction products. Chemists therefore sought ways to overcome this problem by immobilizing the catalyst on various supports. Buchmeister recently reviewed various polymer-supported metathesis catalysts [54]. A few are reported here. Early, Grubbs described a phosphine-derivatized polystyrene supported catalyst [55]
Similarly, a Grubbs type ruthenium catalyst on a phosphine-derivatized polystyrene [56] can be illustrated as follows:
This catalyst, however, turned out to be considerably less active in ring opening metathesis polymerization of norborn-2-ene and in metathesis polymerization of cis-2-pentene. Better results were obtained when a ruthenium-based metathesis type catalyst was immobilized via an N-heterocyclic carbene [57]. Additional improvements in performance were obtained when the catalyst was immobilized on a monolithic silica rods:
