Polymer combinatorial libraries catalysts

Kobayashi, S. Combinatorial Library Synthesis Using Polymer-supported Catalysts. In Combinatorial Chemistry, Fenniri, H., Ed., Oxford University Press Oxford, U.K., 2000 pp 421-432. 

The last work pertaining to the discovery of new catalysts is perhaps the most novel approach to be reported thus far. In one of the earliest approaches taken toward catalyst development, Menger et al. (61) attempted to find catalysts for phosphate ester hydrolysis. A series of eight functionalized carboxylic acids were attached to polyallylamine in various combinations. Each of these polymers were then treated with one of three metals, Mg2+, Zn2+, or Fe3+. The different members of each library were identified by the relative percentages of each carboxylic acid attached to the polyamine. For example, one polymer possessed 15% Oct, 15% Imi, 15% Phe, and 5% Fe3+. There is no attempt to identify the location of the various carboxylic acids in a given polymer. This approach is novel since each system consists of an ensemble of different ligands with the carboxylic acids positioned in various locations. Each polymer within a given ratio of carboxylic acids consists of a combinatorial library of potential catalysts. 

The basic concepts of selection experiments with dynamic combinatorial libraries (DCLs) were articulated more than 10 years ago (see Chapter 1). Since then, a number of applications have emerged. This includes the discovery new enzyme inhibitors, receptors, and catalysts, as well as the synthesis of novel materials such as responsive gels and polymers (see Chapters 2-5). A recent addition to the list of applications is the utilization of dynamic combinatorial chemistry (DCC) for analyhcal purposes. This chapter summarizes the main ideas and results in this area. 

Fast screening of materials for their catalytic activity has gained popularity in the area of polymer electrolyte fuel cells. The method involves preparing a combinatorial library of various candidate catalysts of different composition and a quick evaluation of a key electrochemical parameter such as open-circuit potential or high-frequency impedance. This enables a quick screening of an array of materials to identify... 

Several combinatorial approaches to the discovery of transition metal based catalysts for olefin polymerization have been described. In one study Brookhart-type polymer-bound Ni- and Pd-(l,2-diimine) complexes were prepared and used in ethylene polymerization (Scheme 3).60,61 A resin-bound diketone was condensed with 48 commercially available aminoarenes having different steric properties. The library was then split into 48 nickel and 48 palladium complexes by reaction with [NiBr2(dme)] and [PdClMe(COD)], respectively, all 96 pre-catalysts being spatially addressable. 

Although the library of catalysts was actually very small, this combinatorial approach was shown to work surprisingly well. It remains to be seen if truly high throughput can be put into practice, which would require on-line methods for the detection and characterization of the particles bearing the polymers. 

As a final example it should be mentioned that precondensed enones, prepared by standard Knoevenagel condensation of the aldehyde with the CH-acidic carbonyl component, when reacted with thioureas provided 1,3-thiazines 37, which are isomeric to thio-Biginelli compounds of the general formula 14 (see Figure 4.5). A published report describes the combinatorial synthesis of a library of 29 derivatives of thiazines 37 utilizing polymer-supported reagents and catalysts [168]. 

Silyl enol ethers are versatile reagents in organic synthesis [83]. They are used as isol-able enolate equivalents and many useful reactions have been developed using silyl enol ethers [83]. As a new approach to exploit an efficient method for combinatorial synthesis [84], silyl enol ethers were successfully immobilized on to a polymer. Polymer-supported silyl enol ethers (PSSEEs) were prepared according to Sch. 10 [85]. In aldol reactions of PSSEEs with aldehydes, it was again found that Sc(OTf)3 was an efficient catalyst [86]. An example of the preparation of a 1,3-diol library by use of PSSEEs is shown in Sch. 11. In all cases, the reactions proceeded smoothly to afford the corresponding 1,3-diols in good yields. 1,3-Diols are successfully cleaved from the... 

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