Fourier transform near-infrared vibrational circular dichroism used for on-line monitoring the epimerization of 2,2-dimethyl-1,3-dioxolane-4-methanol: A pseudo racemization reaction.

Near-infrared (near-IR) Fourier transform vibrational circular dichroism (FT-VCD) spectroscopy has been used to monitor the epimerization of (S)-(+)-2,2-dimethyl-1,3-dioxolane-4-methanol (S-DDM). The near-IR-VCD spectra display clear isolated VCD bands at the range of 4700-5050 cm(-1) resulting from the OH stretch-bend combination bands of S-DDM, which were found to decrease in intensity with increasing reaction time. The near-IR-VCD spectra of 10 reference samples obtained were subjected to partial least-squares (PLS) regression analysis, and the results were used to build predictive models for enantiomeric excess (EE) determination. Multivariate regression was carried out on three different sets of spectra, corresponding to the epimerization of S-DDM in three different solvents: methylcyclohexane, carbon tetrachloride and tetrahydrofuran. The effects of solvent in DDM epimerization are discussed in terms of the relative stabilization of the reaction intermediate of the DDM epimerization reaction. The results of these near-IR-VCD studies for the determination of EE highlights the potential of VCD for in situ real-time process monitoring of the reaction kinetics of chiral molecules in solution.

Enantiomeric excess determination by fourier transform near-infrared vibrational circular dichroism spectroscopy: simulation of real-time process monitoring.

The first use of near-infrared (NIR) Fourier transform vibrational circular dichroism (FT-VCD) to follow changes in the enantiomeric excess (EE) of chiral sample molecules in time using a flow-cell sampling apparatus is reported. Simultaneous changes in the fractional composition and the EE of a mixture of two different chiral molecules were monitored as a function of time. This simulates the progress of the chemical reaction from a chiral reactant to a chiral product where the mole fractions and EE values of both species may change with time. For the molecules studied, alpha-pinene, camphor, and borneol, the accuracy of following EE changes for one species alone is approximately 2%, while for simultaneously following EE changes in two species it is approximately 3% for 30 min sampling periods at 16 cm(-1) spectral resolution. These findings demonstrate the potential for VCD to be used in the NIR region for real-time monitoring of the composition and %EE of chemical reactions involving the synthesis of chiral molecules.

Determination of enantiomeric excess in samples of chiral molecules using fourier transform vibrational circular dichroism spectroscopy: simulation of real-time reaction monitoring.

The first use of Fourier transform vibrational circular dichroism (FT-VCD) to follow changes in the percent enantiomeric excess (% EE) of chiral molecules in time using a flow cell sampling apparatus is reported. FT-VCD, as opposed to dispersive scanning VCD, eliminates the need to scan the VCD spectrum in time to monitor the % EE at more than one spectral location. The first use of partial least-squares chemometric analysis to determine % EE values from kinetic sets of VCD spectral data is also reported. These two advances have been used to monitor simultaneously changes in the fractional composition and the % EE of a mixture of two different chiral molecules. This simulates the progress of the chemical reaction from a chiral reactant to a chiral product where the % EE of both molecules can change with time. For the molecules studied, alpha-pinene, camphor, and borneol, the accuracy of following % EE changes for one species alone is approximately 1%, while for simultaneously following % EE changes in two species is approximately 2% for 10-20-min sampling periods at 4 cm(-)(1) spectral resolution. This accuracy can be increased for the same collection times or maintained for shorter periods of collection by lowering the spectral resolution. These findings demonstrate the potential for VCD to be used for real-time monitoring of the composition and % EE of chemical reactions involving the synthesis chiral molecules.

Extension of fourier transform vibrational circular dichroism into the near-infrared region: continuous spectral coverage from 800 to 10 000 cm(-1).

We report the first vibrational circular dichroism (VCD) spectra with continuous coverage from 800 cm(-1) in the mid-infrared (MIR) region to 10 000 cm(-1) in the near-infrared (NIR) region. This coverage is illustrated with MIR and NIR absorbance and VCD spectra of 2,2-dimethyl-dioxolane-4-methanol (DDM), alpha-pinene, and camphor that serve as calibration samples over this entire region. Commercially available, dual-source Fourier transform (FT) MIR and NIR VCD spectrometers were equipped with appropriate light sources, optics, and detectors, and were modified for dual-polarization-modulation (DPM) operation. The combination of liquid-nitrogen- and thermoelectric-cooled HgCdTe (MCT) detectors, as well as InGaAs and Germanium (Ge) detectors operating at room temperature, permitted collection of the desired absorbance and VCD spectra across the range of vibrational fundamental, combination band, and overtone frequencies. The spectra of DDM and alpha-pinene were measured as neat liquids and recorded for both enantiomers in the various spectral regions. Spectra for camphor were all measured in CCl(4) solution at a concentration of 0.6 M, except for the carbonyl-stretching region, where a more dilute concentration was used. The typical anisotropy ratios (g) of the three molecules were estimated with respect to their strongest VCD bands in each spectral region. It was found that for all three molecules in the spectral regions above 2000 cm(-1), anisotropy ratios are approximately the same order (10(-5)) of magnitude. However, in the MIR region, the typical anisotropy ratios are significantly different for the three molecules. This study demonstrates that with modern FT-VCD spectrometers modified for DPM operation, VCD spectra can be measured continuously across a wide spectral range from the MIR to nearly the visible region with an unsurpassed combination of signal-to-noise ratio and spectral resolution.

Quantitative analysis of incomplete HPLC resolution of enantiomers. Fit of polarimetric detection for D- and L-phenylalanine to a gaussian function.

This report compares laser-based polarimetric and UV data for quantitating incompletely resolved enantiomers by HPLC. Using L- and D-phenylalanine as a working model, response data is shown across the entire detection region while emphasizing the regions at or near 100% L, 100% D and 50:50 L:D at resolutions between 0.4 and 1.4. In general, the poorer the resolution, the greater the improvement in detectability with the polarimeter when compared to UV detection. This is due to the inherent bipolar nature of the polarimetric signal, which creates a well-defined crossing point for integration of an enantiomeric pair. In our previous work, we described the improvement in measurement precision when applying a bipolar gaussian peak model to the raw chromatographic peak data. This study will measure the model's effect on improving accuracy. This study should be applicable to other chiroptical detection strategies that produce a bipolar signal for enantiomers, such as circular dichroism and circularly polarized luminescence.

Determination of the absolute configuration of a key tricyclic component of a novel vasopressin receptor antagonist by use of vibrational circular dichroism.

We present the results of a study using vibrational circular dichroism (VCD) for (+)-1, which furnished an unambiguous determination of its absolute configuration as S. The most abundant conformation of (+)-1 in CDCl(3) solution was also established.

Reductive Amination of Aldehydes and Ketones with Sodium Triacetoxyborohydride. Studies on Direct and Indirect Reductive Amination Procedures(1).

Sodium triacetoxyborohydride is presented as a general reducing agent for the reductive amination of aldehydes and ketones. Procedures for using this mild and selective reagent have been developed for a wide variety of substrates. The scope of the reaction includes aliphatic acyclic and cyclic ketones, aliphatic and aromatic aldehydes, and primary and secondary amines including a variety of weakly basic and nonbasic amines. Limitations include reactions with aromatic and unsaturated ketones and some sterically hindered ketones and amines. 1,2-Dichloroethane (DCE) is the preferred reaction solvent, but reactions can also be carried out in tetrahydrofuran (THF) and occasionally in acetonitrile. Acetic acid may be used as catalyst with ketone reactions, but it is generally not needed with aldehydes. The procedure is carried out effectively in the presence of acid sensitive functional groups such as acetals and ketals; it can also be carried out in the presence of reducible functional groups such as C-C multiple bonds and cyano and nitro groups. Reactions are generally faster in DCE than in THF, and in both solvents, reactions are faster in the presence of AcOH. In comparison with other reductive amination procedures such as NaBH(3)CN/MeOH, borane-pyridine, and catalytic hydrogenation, NaBH(OAc)(3) gave consistently higher yields and fewer side products. In the reductive amination of some aldehydes with primary amines where dialkylation is a problem we adopted a stepwise procedure involving imine formation in MeOH followed by reduction with NaBH(4).