16.5 Application of Synthetic Polymers in Catalysis
Recently, polymer supported catalysts have received much attention in organic synthesis. The polymer support provides potential advantages such as easy separation and recovery of the expensive catalyst from the reaction mixture by simple filtration and allows it for subsequent reuse. The reduced toxicity and odor of polymer supported species are another advantage over conventional solution-phase reactions. Typically, polymer supported catalyst show higher activity as compared with the monomer catalyst; this is due to the increase in the number of reactive centers in the polymer catalysts. Generally Lewis acids can be immobilized to polymer support by covalent bonding or ionic bonding or coordination of polymeric ligands with metal species.
Hodge and coworkers have synthesized polymer supported oxazaborolidines with α,α-diaryl-β-aminoalcohol by Suzuki coupling of α,α-diphenyl-L-prolinol derivatives with cross-linked polystyrene containing phenylboronic acid residue.1,2 This supported catalyst shows high level of enantioselectivity up to 97% ee for the reduction of ketone to alcohol (Scheme 1).
Scheme 1
Itsuno et al. has reported cross-linked polymer supported chiral oxazaborolidinone catalyst for Mukiayama-aldol type reaction between aldehyde and silyl enol ether that provided β-hydroxyketone in >90% ee (Scheme 2). Further, this catalyst was used for asymmetric Mannich reaction and asymmetric allylation reaction.3
Scheme 2
Shibasaki and coworkers have developed a bifunctional chiral catalyst consisting Lewis acid (aluminium metal) and the Lewis base (phosphine oxide), which was successfully applied to asymmetric Reisser-type reaction of trimethysilylcyanide with quinolone and 2-furoyl chloride (Scheme 3). This catalyst afforded high activity and nearly similar enantioselectivity as compared with the monomer.4,5
Scheme 3
Kobayashi et al. has developed hydrophobic polymer supported scandium catalyst for allylation reaction of ketone with tetra-allyltin in water (Scheme 4).6 The same catalyst 5 showed high activity in Mukaiyama-aldol reaction also and the reactions proceeded much faster in water than in other organic solvents.7
Scheme 4
Nanostructured polymer supported Sc(III) catalyst was prepared by cross-linking inverted hexagona (HII) monomer containing sodium sulfonate head group with divinylbenzene, followed by exchanging sodium with scandium(III) nitrate (Scheme 5).8 This catalyst was tested for Mukiyama aldol and Mannich type reactions.
Scheme 5