Design strategy parallel reactions

Small-molecule libraries may be created by a variety of methods. These include traditional solution chemistry performed in parallel using multiple reaction vessels so that the individualized chemistry performed in each vessel is, in effect, spatially encoded and hence the structure of the product is implicit by vessel. Alternatively, the chemistry can be performed on a solid support, usually polystyrene beads, in individual vessels, or the supports can be combined into pools where each bead is viewed as a separate reaction vessel. This allows a whole range of design strategies to be employed (vide supra). According to a recent literature survey, two-thirds of reported libraries are prepared on solid support and one-third by solution chemistry. ... 

The introduction of fluorine substituents instead of hydrogen may be a useful strategy for the synthesis of metalation-resistant NHCs this has obvious parallels with the use of fluorine in medicinal chemistry to prevent metabolism of drugs by oxidation reactions at that C-H position. Af-Pentafluorophenylmethylene-substituted NHC complexes of iridium and rhodium showed no evidence of the activation of C-H, C-C or C-F bonds.Grubbs and co-workers also prepared a NHC ligand designed to be resistant to ort/ o-metalation by virtue of the presence of ort/ o-fluorine substituents, and the metathesis catalysts prepared from these ligands displayed increased activity when compared to traditional SIMes-based catalysts. 

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