Fluorescence Resonance Energy Transfer (FRET) as a High-Throughput Assay for Coupling Reactions. Arylation of Amines as a Case Study

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Abstract
A solution-phase assay based on fluorescence resonance energy transfer (FRET) was developed for high-throughput screening of palladium catalyzed aminations of aryl halides. Dansylpiperazine was used as the fluorescent component and a chloro- or bromoarene tagged with an azodye as the quenching partner. Fluorescence intensities of reaction aliquots correlated linearly with reaction yield after dilution to appropriate concentrations. A library of 119 phosphine and heterocyclic carbene ligands was evaluated in duplicate reactions of two combinations. In general, the FRET assay displayed excellent reproducibility, with less than 5% of the duplicate experiments showing significant variability in yields. Among reactions producing greater than 50% yield, the average percent uncertainty was just 5%. For a small subset of sterically hindered ligands, differences in yields between 10 and 20% were observed between the substrates bearing dyes for the FRET assay and substrates that are unfunctionalized. However, the remaining catalyst combinations gave yields similar to those expected from literature precedent. In addition to an evaluation of the accuracy of the FRET assay, this work includes the use of the FRET assay to investigate relative activities of various catalysts for the amination of aryl bromides and chlorides and to find conditions for aminations in more polar solvents. Reactions with K3PO4 base in aqueous mixtures of polar and nonpolar organic solvents were shown to be appropriate for the amination chemistry.