Polymer-bound urea catalysts

Figure 6.15 Polymer-bound Schiff base thiourea catalyst 41 bearing 5-pivaloyl-substitution and its nonimmobilized urea analog 42 optimized for the asymmetric Strecker reaction of aromatic and aliphatic aldimines.

Apart from the commonly used NaOCl, urea—H2O2 has been used/ With this reaction, simple alkenes can be epoxi-dized with high enantioselectivity. The mechanism of this reaction has been examined.Radical intermediates have been suggested for this reaction, polymer-bound Mn -salen complex, in conjunction with NaOCl, has been used for asymmetric epoxidation. Chromium-salen complexes and ruthenium-salen complexes have been used for epoxidation. Manganese porphyrin complexes have also been used. Cobalt complexes give similar results. A related epoxidation reaction used an iron complex with molecular oxygen and isopropanal. Nonracemic epoxides can be prepared from racemic epoxides with salen-cobalt(II) catalysts following a modified procedure for kinetic resolution. 

The main side product is a bisureide derived from trapping the N-acyliminium ion intermediate by excess urea. Prolonging the reaction times and increasing the temperature allows the acid catalyst to decompose the side product and regenerate the N-acyliminium ion. The latter can then be irreversibly trapped by the polymer-bound 3-ketoester. 

An approach to imidazolones started from polymer-bound a-diazo-p-ketoester 33, which was transformed to intermediate 35 by treatment with urea 34 in the presence of a rhodium carboxylate catalyst (Scheme 10) [76]. Treatment of the resin-bound insertion product 35 with 10% TFA at room temperature afforded the resin-bound imidazolone 36 within 1 h. The polymer-bound imidazolone could then be cleaved by transesterification to give esters 37 or by a diversity building amidation reaction to provide amides 38. After preparative TLC, the products were obtained in yields of 19-84% (14 examples). 

---