Solvation of N,C-Protected Valine: Interactions with DMSO and a Chiral Solvating Agent

Small protected amino acids find applications in many fields of chemistry. Their solvation, however, is often only studied in aqueous solution. In this contribution, the solvent-dependent conformational preferences of N,C-protected Boc-Val-NH-C₃H₇ are investigated by a combined VCD spectroscopic and theoretical approach. Replica exchange molecular dynamics simulations at the PM6 level and DFT calculations are both found to predict the conformational preferences in chloroform very well, which is confirmed by comparison of experimental and calculated IR and VCD spectra. In the case of DMSO, we find that solute–solvent interactions have to be taken into account explicitly in the simulation of the spectra. Furthermore, we show that the experimental spectra are better resembled by considering the conformational distribution obtained from the REMD simulations than from DFT. Finally, we also show that the weak interaction of Boc-Val-NH-C₃H₇ with a chiral solvating agent also induces a small VCD spectral signature

Explicit Solvation of Carboxylic Acids for Vibrational Circular Dichroism Studies: Limiting the Computational Efforts without Losing Accuracy

On the basis of a comprehensive analysis of experimental and theoretical IR and vibrational circular dichroism (VCD) spectra, we make suggestions for solvation schemes for carboxylic acids. More specifically, we have studied two chiral carboxylic acids in solvents of different polarity and hydrogen bonding capabilities and verify previously proposed solute–solvent structures for their general applicability. Explicit solvation with acetonitrile-d3 is shown to be most important for carboxylic acid groups directly attached to a stereocenter, while dimethylsulfoxide-d6 should always be considered explicitly in spectra analysis. In order to circumvent the need to consider dimerization with calculations on the full homodimer in nonpolar solvents such as chloroform-d1, we quantitatively evaluate the quality of truncated models. Methanol-d4 is concluded to be the most challenging solvent for VCD studies of carboxylic acids, as the acid strength affects the hydrogen bonding strength to methanol and thus significantly determines the degree and effect of solvation. With the help of matrix-isolation IR spectroscopy, we also characterize the monomeric species of α-phenylpropionic acid

Strong Solvent-Dependent Preference of Δ and Λ Stereoisomers of a Tris(diamine)nickel(II) Complex Revealed by Vibrational Circular Dichroism Spectroscopy

In the present study, we use vibrational circular dichroism (VCD) spectroscopy to investigate the metal-centered Δ and Λ chirality of a tris­(diamine)­nickel­(II) complex. Chiral diphenylethylenediamine is chosen as the ligand, which puts the Δ and Λ isomers of the complex in a diastereomeric relationship. X-ray crystallography indicates an equal preference of both stereoisomers in the solid state. This equal preference is also supported by the related density functional theory calculations. A comparison between the experimental and calculated VCD spectra also proves the existence of both isomers in an acetonitrile solution. However, a significant shift of the equilibrium toward the Λ diastereomer is found for the complex in dimethyl sulfoxide. This solvent-induced preference for a particular absolute configuration is hypothesized to arise from a stronger and more effective solvation of the Λ isomer. The observation that the solvent can significantly influence and shift an equilibrium between two diastereomeric forms is expected to have important implications on structural analysis and on how reaction mechanisms are rationalized

Simultaneous Resonance Raman Optical Activity Involving Two Electronic States

In the present work, the first observation of strong resonance Raman optical activity (RROA) involving more that one resonant electronic state is reported. The chiral transition metal complex bis-(trifluoroacetylcamphorato) copper­(II), abbreviated Cu­(tfc)₂, exhibits both resonance Raman (RR) and RROA spectra with laser excitation at 532 nm. Vibrational assignments for this complex were carried out by comparing the non-RR spectra of Cu­(tfc)₂ excited at 1024 nm to density functional theory (DFT) calculations. The theory of the single-electronic-state (SES) RROA is extended to the next simplest level of theory involving two resonant electronic states (TES) without interstate vibronic coupling as an aide to the interpretation of the observed TES-RROA spectra. Based on measured UV–vis electronic absorbance spectra and corresponding TD-DFT calculations, the most likely two states associated with the RROA spectra are identified

Absolute Configuration of the Polyketide Natural Product (−)-Enterocin

The absolute configuration of the polyketide natural product (−)-enterocin was established by two independent approaches. In the first approach, synthetic enterocin with a defined configuration was compared to the natural product. While identical in all scalar properties, the compound displayed an opposite specific rotation and a different chiral HPLC retention time when compared with (−)-enterocin. In a second approach, the vibrational circular dichroism (VCD) of the natural product was measured and shown to be opposite to the calculated VCD of its enantiomer

Absolute Configuration of the Polyketide Natural Product (−)-Enterocin

The absolute configuration of the polyketide natural product (−)-enterocin was established by two independent approaches. In the first approach, synthetic enterocin with a defined configuration was compared to the natural product. While identical in all scalar properties, the compound displayed an opposite specific rotation and a different chiral HPLC retention time when compared with (−)-enterocin. In a second approach, the vibrational circular dichroism (VCD) of the natural product was measured and shown to be opposite to the calculated VCD of its enantiomer

Controlled Flexible Coordination in Tripodal Iron(II) Phosphane Complexes: Effects on Reactivity

The possibility to alter properties of metal complexes without significant steric changes is a useful tool to tailor the reactivity of the complexes. Herein we present the synthesis of iron complexes with the tripodal phosphane ligands Triphos and Triphos^Si and report on their different coordination properties. Whereas reaction of Triphos^Si and FeX₂ (X = Cl, Br) exclusively afforded (Triphos^Si)­FeX₂ with a κ²-coordinated ligand, the homologous C-derived Fe complexes show rapid conversion in solution to afford [(Triphos)­Fe­(CH₃CN)₃]­[Fe₂Cl₆] or [(Triphos)­Fe­(CH₃CN)₃]­[FeBr₄], respectively. The structural conversion was found to be temperature- and solvent-dependent and was accompanied by a linear change of the overall magnetization. The different ligand influence was shown to have a significant effect on the ability of (Triphos^Si)­FeCl₂ and (Triphos)­FeCl₂ to perform the Sonogashira cross-coupling reaction of 4-iodotoluene and phenyl acetylene as well as the hydrosilylation of acetophenone. The results presented herein show the different coordination properties of two structurally homologous tripodal ligands and demonstrate the importance of geometrically controlled ligand field splitting on the stability and reactivity of metal complexes. The C/Si exchange therefore provides a simple and straightforward tool to manipulate properties and reactivity of metal complexes

Anharmonicity Effects in the Vibrational CD Spectra of Propylene Oxide

This study reports the first vibrational circular dichroism (VCD) spectra of propylene oxide isolated in an argon matrix. The narrow bandwidth achieved, in addition to the rich VCD features, allows a thorough analysis of the spectra and detailed comparison with the calculated harmonic and anharmonic IR and VCD spectra. Several bands in the fingerprint region are identified as nonfundamental modes based on the harmonic calculation, and three of them feature significant VCD intensity. Anharmonic VCD intensity calculations were carried out using a newly developed methodology based on the second-order level of vibrational perturbation theory. All new bands observed can be undoubtedly assigned to combination or overtone modes except the band at 1486.4 cm^–1, which is tentatively identified to contain contributions from an overtone and a combination mode. The study also reveals that the current theoretical approach for anharmonic contributions to VCD spectra needs to be further improved when there is an accidental resonance involved

Controlled Flexible Coordination in Tripodal Iron(II) Phosphane Complexes: Effects on Reactivity

The possibility to alter properties of metal complexes without significant steric changes is a useful tool to tailor the reactivity of the complexes. Herein we present the synthesis of iron complexes with the tripodal phosphane ligands Triphos and Triphos^Si and report on their different coordination properties. Whereas reaction of Triphos^Si and FeX₂ (X = Cl, Br) exclusively afforded (Triphos^Si)­FeX₂ with a κ²-coordinated ligand, the homologous C-derived Fe complexes show rapid conversion in solution to afford [(Triphos)­Fe­(CH₃CN)₃]­[Fe₂Cl₆] or [(Triphos)­Fe­(CH₃CN)₃]­[FeBr₄], respectively. The structural conversion was found to be temperature- and solvent-dependent and was accompanied by a linear change of the overall magnetization. The different ligand influence was shown to have a significant effect on the ability of (Triphos^Si)­FeCl₂ and (Triphos)­FeCl₂ to perform the Sonogashira cross-coupling reaction of 4-iodotoluene and phenyl acetylene as well as the hydrosilylation of acetophenone. The results presented herein show the different coordination properties of two structurally homologous tripodal ligands and demonstrate the importance of geometrically controlled ligand field splitting on the stability and reactivity of metal complexes. The C/Si exchange therefore provides a simple and straightforward tool to manipulate properties and reactivity of metal complexes