Fluorous Phase-Based Enantioselective Fluorescent Screening for Asymmetric Catalytic Reaction and Enantioselective Fluorescent Recognition of Amino Acids

By using 1, 1'-Bi-2-naphthol (BINOL) as the chiral core, a number of fluorinated organic sensors have been developed for the fluorescent recognition of chiral molecules. This thesis has provided an application for rapid enantiomeric excess determination and high throughput catalyst screening by using a type of fluorophilic fluorescent sensor in the fluorous phase.
The excellent chiral recognition ability of the BINOL-perfluoro-diketone sensors has been expanded to a variety of chiral amino alcohols containing a secondary amine group in the fluorous phase [ef = (IS-I0)/(IR-I0) value up to 100]. They also gave large nonlinear response toward the enantiomeric composition of the amino alcohols in perfluoro-hexanes (FC-72). The enantiomerically pure probes, as well as the racemic probe, were all applied to facilitate the screening of the catalysts for the asymmetric reaction of a meso-epoxide with an alkyl amine. The fluorescence-based ee measurements have been confirmed by HPLC-chiral column analysis which has validated the fluorous phase-based enantioselective fluorescent sensing method. Even with manual screening protocols, we have already discovered a condition with significantly improved enantioselectivity for the polysaccharide-catalyzed asymmetric reaction of 1,2-epoxycyclohexane with an alkyl amine. The entire procedure is automatable and also suitable for high throughput screening by using the probe.
When a novel fluorescent probe made of BINOL and coumarin was excited at λ1 = 365 nm in a neutral buffer solution, it showed fluorescence enhancement at 465 nm in the presence of an amino acid with a very small difference between the two enantiomers. When excited at λ2 = 467 nm, it showed highly enantioselective fluorescence enhancement at 534 nm. This allowed the determination of both concentration and enantiomeric composition of amino acids. A detailed investigation was conducted on the reaction mechanism of the BINOL-coumarin-based fluorescent probe with amino acids. On the basis of the studies including fluorescence spectroscopy, 1HNMR, UV-vis, mass spectroscopy, single-crystal X-ray analysis, and molecular modeling, it was found that the distinctively different fluorescent responses of the probe toward the amino acid at the two excitation wavelengths were due to two different reaction pathways that generate different intermediates and products.
The same fluorescent probe was unexpectedly found to be highly selective at 461 nm for just one specific thiol molecule, cysteine under neutral aqueous conditions with a low applicable concentration (10 µM) and agreeable incubation temperature (37 ℃) in the absence of any other additives. With interference, cysteine was selectively detectable from 1.6 µM to hundreds of µM which covered both normal and abnormal cellular cysteine concentrations of humans.