Boronic acids : structural and mechanistic studies and application as macromolecular sensing systems
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Boronic acids, particularly those that carry an ortho-aminomethyl group, have been extensively utilized in the field of molecular recognition in recent years thanks to their ability to reversibly bind to a wide variety of polyol substrates. They have been shown to form cyclic boronate esters rapidly upon reaction with 1,2- and 1,3-diols, catechols, carbohydrates and hydroxycarboxylic acids, making them attractive as potential sensing units. While they have found broad application in this forum, the mechanism by which they work is still up for debate. This work begins in Chapter 1 with a review of the fundamentals of ¹¹B NMR spectroscopy and its application on the analysis of boronic acid-containing systems. The focus of Chapter 2 turns toward systems with an o-iminomethylphenylboronic acid moiety. This species can be formed easily through a three-component assembly, though physical understanding of this complex lags behind. With the fundamentals of ¹¹B NMR spectroscopy detailed previously, the results obtained when utilizing this technique to study both the structure and mode of interaction in these species will be presented. In Chapter 3 we give a comprehensive review of the data and conclusions that have been published by different groups about one of the most successful fluorescent sugar sensors of this kind, first introduced by Seiji Shinkai in 1994. Additionally, it delineates the experimental results obtained by our group when attempting to answer some of the remaining questions. In Chapter 4 we report the use of the aforementioned multi-component assembly as an enantioselective sensor for α-chiral primary amines. Using circular dichroism, the ee% of these analytes could be accurately determined with this system. Additionally, enantio- and chemodiscrimination was possible by employing chemometric tools like PCA and LDA. Finally, Chapter 5 is a compilation of efforts to expand the use of these sensing systems into synthetic organic chemistry research labs. In collaboration with Xumu Zhang at Rutgers University, we have implemented a previously developed system to analyze the product of an asymmetric hydrogenation of an imine to create a chiral amine. A proof of concept study on a novel automated circular dichroism plate reader prototype aimed to increasing sample throughput was completed at New York University with Professor Bart Kahr.