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Sensing chiral amines via supramolecular chemistry and circular dichroism spectrometry
textIn chapter 1 the principles behind circular dichroism spectroscopy and exciton coupled circular dichroism spectroscopy are outlined, and examples are cited that illustrate the utility of these methods in the determination of absolute configuration and ee of chiral amines. This provides background and context for this thesis, which mostly pertains to the sensing of chirality in amines. An exciton coupled circular dichroism method based on the induction of helical chirality in an organometallic host for sensing chiral amines is presented in chapter 2. The method can be used to determine absolute configuration by relating the sign of the first Cotton effect of the host-amine complex to the handedness of the amine. Analysis of the primary circular dichroism optical data is by principal component analysis allows for differentiation of the analytes based on their idendity and handedness. A novel circular dichroism method for detecting chiral amines is discussed in chapter 3. The method uses a highly efficient derivatization method to convert the primary amine into a bidentate imine. Three equivalents of the imine are then assembled together by coordination to Fe(II). The proximity and chiral orientation of the imines leads to exciton coupled circular dichroism, which is of utility in the determination of absolute configuration. Additionally, there is a metal-to-ligand charge transfer band in the visible region that can be used to develop calibration curves, which allow for the determination of the enantiomeric excess of unknown samples with an absolute error of ±5%. Chapter 4 details another imine based circular dichroism method for chiral amines. The method uses a commercially available aldehyde, Fe(II), and circular dichroism spectrometry to sense chirality in amines. It is shown that the circular dichroism signals in the ultraviolet spectrum vary predictably with the handedness of the chiral amine, which has potential applications in the determination of absolute configuration. By developing calibraton curves, signals in the visible spectrum can be used to determine enantiomeric excess with an absolute error of ±6%. Analyzing the primary circular dichroism optical data with linear discriminant analysis allows for differentiation between amines based on their identity and handedness. Finally, chapter 5 illustrates the potential of using the thermodynamic parameters of partitioning between water and octanol as a predictive tool for estimating the contributions of hydrophobicity to host-guest binding events. This is done by showing a relationship between the thermodynamics of partitioning and thermodynamics of hydrophobic binding events for a series of guests and cyclodextrin. A plot of the thermodynamic parameters of binding of a variety of guests to cyclodextrin as a function of the thermodynamic parameters of partitioning between water and octanol shows a linear relationship for a series of alcohols.Chemistr
A Simple Method for the Determination of Enantiomeric Excess and Identity of Chiral Carboxylic Acids
In Situ Assembly of Octahedral Fe(II) Complexes for the Enantiomeric Excess Determination of Chiral Amines Using Circular Dichroism Spectroscopy
A method for discriminating between α-chiral primary
amine
enantiomers is reported. The method utilizes circular dichroism (CD)
spectroscopy and a sensing ensemble composed of 2-formyl-3-hydroxypyridine
(<b>4</b>) and FeÂ(II)Â(TfO)<sub>2</sub>. Aldehyde <b>4</b> reacts rapidly with chiral amines to form chiral imines, which complex
FeÂ(II) to form a series of diastereomeric octahedral complexes that
are CD-active in both the UV and visible regions of the spectrum.
NMR studies showed that for enantiomerically pure imine complexes,
the Δ-<i>fac</i> isomer is preferred. A statistical
analysis of the distribution of stereoisomers accurately modeled the
calibration curves for enantiomeric excess (ee). CD signals appearing
in the UV region were bisignate, and the nulls of the CD signals were
coincident with maxima in the UV spectrum, consistent with exciton
coupling. Time-dependent density functional theory and semiempirical
calculations confirmed that the CD signals in the UV region arise
from coupling of the π–π* transitions in the imine
chromophores and that they can be used to describe the signs and magnitudes
of the curves accurately. The CD signals in the visible region arise
from metal-to-ligand charge-transfer bands, and these signals can
be used to determine the ee values of chiral amines with an average
absolute error of ±5%. Overall, the strategy presented herein
represents a facile in situ assembly process that uses commercially
available simple reagents to create large optical signals indicative
of ee values
In Situ Assembly of Octahedral Fe(II) Complexes for the Enantiomeric Excess Determination of Chiral Amines Using Circular Dichroism Spectroscopy
A method for discriminating between α-chiral primary
amine
enantiomers is reported. The method utilizes circular dichroism (CD)
spectroscopy and a sensing ensemble composed of 2-formyl-3-hydroxypyridine
(<b>4</b>) and FeÂ(II)Â(TfO)<sub>2</sub>. Aldehyde <b>4</b> reacts rapidly with chiral amines to form chiral imines, which complex
FeÂ(II) to form a series of diastereomeric octahedral complexes that
are CD-active in both the UV and visible regions of the spectrum.
NMR studies showed that for enantiomerically pure imine complexes,
the Δ-<i>fac</i> isomer is preferred. A statistical
analysis of the distribution of stereoisomers accurately modeled the
calibration curves for enantiomeric excess (ee). CD signals appearing
in the UV region were bisignate, and the nulls of the CD signals were
coincident with maxima in the UV spectrum, consistent with exciton
coupling. Time-dependent density functional theory and semiempirical
calculations confirmed that the CD signals in the UV region arise
from coupling of the π–π* transitions in the imine
chromophores and that they can be used to describe the signs and magnitudes
of the curves accurately. The CD signals in the visible region arise
from metal-to-ligand charge-transfer bands, and these signals can
be used to determine the ee values of chiral amines with an average
absolute error of ±5%. Overall, the strategy presented herein
represents a facile in situ assembly process that uses commercially
available simple reagents to create large optical signals indicative
of ee values