Solvent-free, uncatalyzed asymmetric “ene” reactions of N-tert-butylsulfinyl-3,3,3-trifluoroacetaldimines: a general approach to enantiomerically pure α-(trifluoromethyl)tryptamines

Basis set dependence of S[double bond, length as m-dash]O stretching frequencies and its consequences for IR and VCD spectra predictions

Benchmarking functionals and basis sets for the computational prediction of molecular properties is usually done on very small model systems. Larger organic molecules containing heavier second row atoms are not the typical model structures. We herein present the first survey of basis sets and functionals for the prediction of the IR and VCD spectra of chiral tosylates and sulfinates as we noted drastic deviations between computed harmonic frequencies obtained at B3LYP/6-311++G(2d,p) level of theory and those observed in experimental solution phase IR and VCD spectra. We show that the harmonic frequencies of the asymmetric and symmetric S[double bond, length as m-dash]O stretching modes of tosylates are predicted at significantly too low vibrational frequencies if the employed basis set does not provide higher order polarization functions. The results of our benchmarks show that at least the 6-311G(3df,2dp) basis (or equivalent Dunning and Ahlrichs variants) should be used.

Absolute Configurations of Synthetic Molecular Scaffolds from Vibrational CD Spectroscopy

Vibrational circular dichroism (VCD) spectroscopy is one of the most powerful techniques for the determination of absolute configurations (AC), as it does not require any specific UV/vis chromophores, no chemical derivatization, and no growth of suitable crystals. In the past decade, it has become increasingly recognized by chemists from various fields of synthetic chemistry such as total synthesis and drug discovery as well as from developers of asymmetric catalysts. This perspective article gives an overview about the most important experimental aspects of a VCD-based AC determination and explains the theoretical analysis. The comparison of experimental and computational spectra that leads to the final conclusion about the AC of the target molecules is described. In addition, the review summarizes unique VCD studies carried out in the period 2008-2018 that focus on the determination of unknown ACs of new compounds, which were obtained in its enantiopure form either through direct asymmetric synthesis or chiral chromatography.

CF3: an overlooked chromophore in VCD spectra. A review of recent applications in structural determination

The VCD spectra of several chiral compounds containing the CF3 group are reviewed and analyzed. The list of compounds contains pharmaceutically relevant molecules as well as simple model molecules, having the value of case studies. In particular we point out the importance of the sign of the VCD band relative to some stretching normal mode of CF3 in the region 1110-1150 cm-1, as diagnostic of the configuration of stereogenic carbons C* to which the CF3 group is bound: the correspondence (-) ↔ (R) and (+) ↔ (S) holds for 100% of 1-aryl-2,2,2-trifluoroethanols. DFT calculations confirm these conclusions, but for the rule established here they serve just as a check. This rule is tested on two new compounds, namely N-tert-butanesulfinyl-1-(quinoline-4-yl)-2,2,2-trifluoroethylamine, 8, and 4-[2-(2,2,2-trifluoro-1-hydroxyethyl)pyrrolidin-1-yl]-2-(trifluoromethyl)benzonitrile, 10, both containing two stereogenic elements, one of them being an asymmetric carbon C* of unknown configuration binding a CF3 group. Discussion of the general validity of the rule is provided and some further tests are run on compounds in well-established drugs.

Mannich-type addition of 1,3-dicarbonyl compounds to chiral tert-butanesulfinyltrifluoroacetaldimines. Mechanistic aspects and chiroptical studies

Mannich-type addition of 1,3-dicarbonyl compounds to (SS)-N-tert-butanesulfinyltrifluoroacetaldimine has been carried out under dramatically different conditions. The stereochemical outcome was quite different when the reaction was carried out under solvent-free conditions, at high temperature and without any catalyst or additive, compared with the DBU catalyzed reaction in dichloromethane solution at low temperature. Mechanistic aspects of the reaction under both the conditions are discussed. In this respect, vibrational (VCD) and electronic circular dichroism (ECD) and optical rotatory dispersion (ORD) experiments proved to be valuable tools for determining the absolute configurations of the reaction products.

Asymmetric Synthesis of Spirooxindoles via Nucleophilic Epoxidation Promoted by Bifunctional Organocatalysts

Taking into account the postulated reaction mechanism for the organocatalytic epoxidation of electron-poor olefins developed by our laboratory, we have investigated the key factors able to positively influence the H-bond network installed inside the substrate/catalyst/oxidizing agent. With this aim, we have: (i) tested a few catalysts displaying various effects that noticeably differ in terms of steric hindrance and electron demand; (ii) employed α-alkylidene oxindoles decorated with different substituents on the aromatic ring (11a-g), the exocylic double bond (11h-l), and the amide moiety (11m-v). The observed results suggest that the modification of the electron-withdrawing group (EWG) weakly conditions the overall outcomes, and conversely a strong influence is unambiguously ascribable to either the N-protected or N-unprotected lactam framework. Specifically, when the NH free substrates (11m-u) are employed, an inversion of the stereochemical control is observed, while the introduction of a Boc protecting group affords the desired product 12v in excellent enantioselectivity (97:3 er).