1,1‘-Binaphthyl Based Bis- and Tris-Phosphoric Acids: Syntheses and Application as Fluorescent Chemosensors

The strategy for the synthesis of chiral bis-binaphthyl phosphoric acids involving the use of 1,1’-binaphthyl-monoidide and various linkers (Y or Z) allowed the straightforward synthesis of bis- and even tris-phosphoric acids. In the end, this synthetic approach led to the formation of five bis-1,1’-binaphthyl phosphoric acids (R,R)-1a to (R,R)-1e and one tris-1,1’-binaphthyl phosphoric acid (R,R,R)-1f. The investigation of the sensing ability of the bis-phosphoric acids (R,R)-1a/b/c/d as fluorescence sensors demonstrated the selective sensing ability of (R,R)-1d towards ferric ions (Fe3+) which led to a fluorescence quenching. Based on the chiral nature of the sensors, we were also able to monitor the interaction of the bis-phosphoric acids with the analytes (metal ions) by CD spectroscopy. Both spectroscopic (fluorescence and CD) methods are providing a dual-mode detection for the sensing application. In a second project, we applied the bis-/tris-phosphoric acids (1a-f) in combination with Ni2+ or Eu3+ as fluorescence sensors for chiral bioanalytes (L-/D-amino acids). An array-based fluorescence sensing was developed to generate analyte-specific fluorescence patterns and statistical data analyses (PCA and LDA) were implemented to simplify the data evaluation and to estimate the discrimination ability of the fluorescence array. The array showcased the capability to identify the type of amino acid with high accuracy, 99% and 98% for L- and D-amino acids respectively. The ability of the array to additionally discriminate the configuration of amino acids in a set of 37 different L- and D-amino acids -was found to be satisfying, albeit giving only 93% accuracy. For this approach, it might be helpful to include the enantio-counterparts of the phosphoric acids as additional sensors. In addition to the synthesis of bis-/tris-binaphthyl phosphoric acid diesters, we were also able to furnish the related phosphoric acid monoesters. Partial racemization of the binaphthyl-backbone was observed during the cyclization process leading to a formation of diastereomers of the furane-annelated naphthols, which was confirmed by spectroscopic and X-ray crystal structure analyses. Additional experimental and theoretical studies using a model substrate proved that both racemization and cyclization processes are acid-catalyzed. Since the barrier of the cyclization reaction is lower than the racemization, the cyclization proceeded faster than the racemization. This work successfully concluded an in-depth investigation of the cyclization and racemization mechanism as well as the isolation and characterization of all six isomeric phosphoric acid monoesters. These phosphoric acids are the highly promising compounds for further use in chemosensing and catalysis. The modified synthetic route involving the use of mono-iodo binaphthyl bis-carboxylic acids was successfully applied to form the mono-iodo allylamine (R)-86. This serves as an essential building block which can be connected by various linkers for the generation of bis-binaphthyl-amines. Within the scope of this work, we succeed the formation one bis-binaphthyl-amine with 1,4-ethinyl benzene linker in nine steps. Only the last deprotection step to yield the bis-binaphthyl-amine is still pending. In future, the combination of bis-binaphthyl-phosphoric acids and bis-binaphthyl-amines will provide new possibilities to build complimentary supramolecular double helices.