Polymer-Supported Organocatalyst

Ξ August 3rd, 2008 | → 2 Comments | ∇ Supported Catalyst |

Andrei V. Malkov, Marek Figlus, and Pavel Kocovský, 

J. Org. Chem. 2008, 73, 3985–3995


 

The development of free enviroment methodology is an important goal in organic synthetic chemistry. Here I report a very pretty JOC paper about the synthesis and use of polymer supported organocatalyst for the asymmetric reduction of ketimines with trichlorosilane. 


There are several methods for the reduction of ketimines, and the development of metal-free organocatalyst is a novel synthetic plylosophy with the ambition to replace the traditional transition metal catalysis. 

Several groups have developed, with very good result in terms of yield and ee, a series of organocatalyst derived from α-amino acids to promote asymmetric reduction of prochiral imines. Malkov et al. have initially developed new amino acid derived formamide-type catalysts show below (A); in order to make easier the separation of the catalyst and the product, they have introduced a fluorous tag to the catalyst which simplified the separation to an ordinary filtration through a pad of fluorous silica gel that retained the catayst (B)

 

 

As the next step toward developing a green methodology for the reduction of imines with Cl3SiH, they resolved to anchor the catalyst to a polymer by employing the ether link. They developed several catalysts (C - F) anchored to different kind of resin (Merrifield, Wang TentaGel etc): generally yield and ee are always very good. 

 

 

 

 

The general synthesis of the catalyst is reported below:

 

 

 

Immobilization of the catalyst to the solid support was effected by alkylation of the phenolic hydroxyl with polymeric benzyl chloride using the modified Williamson method or by Mitsunobu Conditions (wiyh polymeric benzyl alcohol).