Rapid Optical Determination of Enantiomeric Excess, Diastereomeric Excess, and Total Concentration Using Dynamic-Covalent Assemblies: A Demonstration Using 2-Aminocyclohexanol and Chemometrics

Abstract

Optical analysis of reaction parameters such as enantiomeric excess (ee), diastereomeric excess (de), and yield are becoming increasingly useful as assays for differing functional groups become available. These assays typically exploit reversible covalent or noncovalent assemblies that impart optical signals, commonly circular dichroism (CD), that are indicative of the stereochemistry and ee at a stereocenter proximal to the functional group of interest. Very few assays have been reported that determine ee and de when two stereocenters are present, and none have targeted two different functional groups that are vicinal and lack chromophores entirely. Using a CD assay that targets chiral secondary alcohols, a separate CD assay for chiral primary amines, a UV–vis assay for de, and a fluorescence assay for concentration, we demonstrate a work-flow for speciation of the enantiomers and diastereomers of 2-aminocyclohexanol as a test-bed analyte. Because of the fact the functional groups are vicinal, we found that the ee determination at the two stereocenters is influenced by the adjacent center, and this led us to implement a chemometric patterning approach, resulting in a 4% absolute error in full speciation of the four stereoisomers. The procedure presented herein would allow for the total speciation of around 96 reactions in 27 min using a high-throughput experimentation routine. While 2-aminocyclohexanol is used to demonstrate the methods, the general workflow should be amenable to analysis of other stereoisomers when two stereocenters are present.

Recent advances in analytical techniques for high throughput experimentation

“…Optical analysis, on the other hand, can be rapidly performed on plate formats and is thus more amenable to HTA. Numerous probe designs and optical assays that yield ee values via CD, fluorescence, or UV absorbance have been developed and reported 116,117 . A recent publication by Pilicer et al 118 .…”

Dynamic Amide Formation by Reversible Nitrone Exchange of Potassium Acyltrifluoroborates (KATs) and Hydroxylamines in Aqueous Conditions

“…The hydroxylamine used in 3 was benzyl hydroxylamine, and its isotopically labeled version d7-benzyl hydroxylamine was used in 10, making the nitrones 9 / 3 a pair of isotopomers, and 11 / 10 another pair of isotopomers. As the KATs and hydroxylamines exchange a statistical mixture will be reached in which [9]: [3] and [11]: [10] approach unity. We assume the ionization yield in LCMS for isotopomers to be the same, so that ion count ratios of isotopomers from LC-ESI-MS analysis may serve as a good estimate of the actual ratio between isotopomer concentrations, which is an indicator of the exchange process.…”

Optical monitoring of polymerizations in droplets with high temporal dynamic range

“…19,20,[24][25][26][27][28][29] At the other extreme, the inherent scalability of optical methods makes them attractive readouts for massively parallel, high-throughput combinatorial testing of reaction conditions. [30][31][32][33][34][35] Pairing this approach with machine-learning methods also enables accelerated discovery and screening of new functional materials. [36][37][38][39] Ideally, such combinatorial searches will occur in chemical environments that strongly resemble the conditions that would be used in an industrial setting.…”