Ionic interactions are increasingly appreciated as a key, asymmetry-inducing factor in enantioselective catalytic transformations, including those involving Brønsted acid or base catalysis, phase-transfer catalysis, and related processes. However, a detailed understanding of these interactions is often lacking. Herein, we show how an enantiopure anion enforces a chiral conformation onto a catalytically relevant achiral cation. Specifically, we use vibrational circular dichroism (VCD) spectroscopy to monitor the transmission of stereochemical information from a chiral phosphate anion to a flexible manganese(III)-salen cation. We show that VCD can be used to study solvent effects and that the obtained chiroptical data directly and quantitatively correlate with the experimentally observed enantioselectivity in an asymmetric olefin epoxidation reaction.

Propylene oxide (PO) is the ideal candidate to benchmark both new experimental and theoretical approaches. Herein it is used to demonstrate the feasibility of measurements of VCD spectra in liquid rare gases (CryoVCD) and to test recent theoretical developments in the prediction of anharmonic IR and VCD spectra.

Four isomeric azulenylcarbenes were synthesized in argon matrices by photolysis of the corresponding diazo precursors, and the photochemistry of these carbenes was studied. The carbenes and their rearranged products were characterized by IR, UV/Vis, and EPR spectroscopy, and the experimental data were compared to results from DFT calculations. While 2-, 5- and 6-azulenylcarbene show triplet ground states, 1-azulenylcarbene exhibits a singlet ground state, in accord with theoretical predictions. The rearrangements of the azulenylcarbenes give access to a number of unusual C11 H8 isomers, such as other carbenes and strained allenes.

The chiral amine α-phenylethyl amine (PEA) was isolated in cryogenic matrices and investigated using vibrational circular dichroism (VCD) and infrared spectroscopy. The potential energy surface (PES) of PEA features five different conformers connected by relatively low conformational transition states. Based on the IR spectra, it could be confirmed that all conformational energy barriers are passed at a deposition temperature of 20 K, and that only the global minimum conformation of PEA is populated in both argon and nitrogen matrices. However, differences in the calculated and experimental VCD spectra indicate deviations from the minimum structure by perturbation of the phenyl ring as well as of the amine orientation. The degree of the perturbation is found to also depend on the choice of the host gas, which shows the subtle influence of the environment on the conformational distortion of PEA.

The observation of the active species is the goal of most spectroscopic investigations on enantioselective organocatalysts in solution. Although NMR spectroscopy is widely applied, it has low sensitivity for conformational changes or the chiral nature of the interactions. In the present work, we exemplify the use of vibrational circular dichroism (VCD) spectroscopy for the characterization of a chiral thiourea model compound in nonpolar and polar solvents, as well as for a detailed analysis of its interaction with a model reactant. We discuss solvent-induced conformational changes of the thiourea, and provide evidence for an unexpected binding topology between the thiourea and an acetate anion. The results clearly showcase the possibilities offered by using VCD spectroscopy in the characterization of chiral organocatalysts.