# Advances in Organic and Organic-Inorganic Hybrid Polymeric Supports for Catalytic Applications
## 3 Organocatalyst Asymmetric Reactions
Effect of achiral linker R and counter anion of iminium cation on the enantioselectivity were investigated in the Diels-Alder reaction of cinnamaldehyde and cyclopentadiene. The chiral imidazolidinone polymer **81c-TFA**, having \(p\)-xylyl as linker and CF\({}_{3}\)CO\({}_{2}\)\({}^{-}\) as counter anion, exhibited the highest enantioselectivities (_exo_ ee 93%, _endo_ ee 97%). After the completion of the reaction, the polymeric catalyst **81c-TFA** was recovered by simple decantation. The acid retreatment of the recovered polymeric catalyst allowed to repeatedly use the catalyst without the loss of catalytic activity. The first example of monolithic reactor containing a chiral organocatalyst was reported [54]. Particularly, the monolithic organocatalyst **84** was provided by radical copolymerisation of  
Figure 29: Synthesis of polyetheres **81a–e(HX)** and use of **81c-TFA** as catalyst in asymmetric Diels-Alder reaction. divinylbenzene and the properly modified enantiomerically pure imidazolidinone **83** inside a stainless steel column in the presence of dodecanol and toluene as porogens and AIBN as initiator (Scheme 30). **Scheme 30. Synthesis of polymer-supported organocatalyst 84 and its application for catalysis in flow. **The enantiomerically pure imidazolidinone **83** was synthesized by the following synthetic strategy. The imidazolidinone **75**, synthesized starting from (_S_)-tyrosine methyl ester, was modified with \(\  
and \(p\)-nitrobenzaldehyde in DMSO:H\({}_{2}\)O (87:13) as solvent and at room temperature. Catalysts **93a-c** showed similar behaviour in terms of conversion, diastereo- and enantioselectivity. Copolymer **93d**, in turn, led to much lower conversion and decrease of stereoselectivity. These results indicated influence on catalytic performance of the functionalization level of the catalytic polymers, since **93d** was much less functionalized than **93a-c**. **Scheme 31. Preparation of polymeric chiral ionic liquids, PCILs-86-89. **  
**Scheme 32. Synthesis of polynorborene-supported proline 93a-d and 94d and their application in aldol reaction. **The catalyst **93b** was also employed in aldol reactions of a wide range of aromatic aldehydes affording the anti-aldol products in high yields and diastereo- and enantioselectivities. Catalyst **94b** provided a series of the aldol adducts with excellent yields and stereoselectivities in a 50:50 mixture of DMF and water as solvent, in shorter reaction times compared to those of catalyst **90b** and without the need for adding acid. The recyclability of polymers **93b** and **94b** was studied in aldol reaction between cyclohexanone and \(p\)-nitrobenzaldehyde. Copolymer **93b** was recycled seven times affording the aldol product with constant stereoselectivity and with only slight decrease of catalytic activity in every cycle. Catalyst **94b**, on the contrary, was recycled and reused for at least seven runs without any appreciable loss in yield or in stereoselectivity, showing the very positive effect of the \(p\)-phenylene carboxylate spacer on the chemical stability of the polynorborhene-supported organocatalyst. Polystyrene-immobilized triazolylproline organocatalyst **97** was prepared and employed in asymmetric cross- and self-aldol reactions of aldehydes (Scheme 33) [59]. The heterogeneous catalyst **97** was obtained by radical copolymerization of the proline derivative **96** with styrene and DVB and successive deprotection of both the Boc group and the _tert_-butyl ester with TFA. The monomer **96**, bearing a pendant vinyl group, was prepared by click reaction between (25,_AR_)-N-Boc-4-azido-L-proline _tert_-butyl ester and bifunctional linker **95** obtained starting from \(p\)-chloromethylstyrene and 4-ethynylbenzyl alcohol. The asymmetric cross-aldol reaction between a series of aromatic aldehydes and donor aliphatic aldehydes catalyzed by 10 mol % of catalyst **97** in water led to aldol adducts in very good yields and ee values.