On chirality transfer in electron donor-acceptor complexes. A prediction for the sulfinimine···BF3 system

Chirality transfer observed in Raman optical activity spectra

Chirality transfer (also called induced chirality) is a phenomenon present in chiroptical spectra that manifests itself as a new band or bands of an achiral molecule interacting with a chiral one. In the Raman optical activity (ROA) spectroscopy, the bands of achiral solvents have been recently observed, but the latest papers have shown that they corresponded to the new ECD-Raman (eCP-Raman) effect. Here, we show an unambiguous example of chirality transfer observed in the ROA spectra. The spectra registered for the (1:1) mixtures of achiral benzonitrile with the enantiomers of 2,2,2-trifluoro-1-phenylethanol, 1-phenylethanol, and 1-phenylethylamine exhibited the v(CN) vibration band at about 2230 cm^-1. The ROA measurements were repeated several times to ensure the reliability of the phenomenon. Calculations revealed the CN···HO or CN···HNH hydrogen bond formation accompanied by the π···π or CH···π interactions. The interaction strength was shown to be an important factor for the pronouncement of the ROA chirality transfer effect.

Solvation of 2-(hydroxymethyl)-2,5,7,8-tetramethyl-chroman-6-ol revealed by circular dichroism: a case of chromane helicity rule breaking

The primary goal of this work is to clarify why 2-(hydroxymethyl)-2,5,7,8-tetramethyl-chroman-6-ol {(S)-TMChM} deviates from the chromane helicity rule under solvent change. The rule, applicable to determining the absolute configuration of molecules containing the chromane chromophore, binds the sign of the 1Lb Cotton effect (CE) with the helicity of the dihydropyran ring. In case of TMChM, however, this CE exhibits extreme solvent dependence: it is negative in non-coordinating solvents and positive in coordinating ones, irrespective of the helicity of the heterocyclic ring. TD-DFT calculations using PCM and hybrid solvation models were performed to explain origin of this phenomenon. It turned out that the 1Lb CE sign directly depends on the position of the phenolic OH group at carbon atom C6 (OHC6). In the absence of interactions with solvents (as in CCl4 or nC6H14) or when a solvent plays proton donor role (as in CHCl3), the OHC6 lies in the phenyl plane and the 1Lb CE sign follows the P/M helicity rule. In contrast, in proton acceptor solvents, like DMSO, CH3OH or CH3CN, the OHC6 group is deflected from the phenyl plane, and the 1Lb CE sign of individual (S)-TMChM conformers depends on the sector in which the OHC6 is located. Thus, in solution, the 1Lb CE sign is an average over different orientations of the OHC6 group and can be positive (as in DMSO and CH3OH) or negative (as in CH3CN) which means that it does not follow the chromane helicity rule. The impact of OHC6 on the 1Lb CE sign and thus the conclusions for the stereochemistry of chromans are demonstrated here for the first time. Additionally, a comparison of experimental and simulated ECD spectra, supported by VCD data, allowed to determine the geometry of intermolecular clusters formed in different solvents.

On the non-additivity of the substituent effect in ortho-, meta- and para-homo-disubstituted benzenes

The non-additivity of the substituent effect in para-, meta-, and ortho- homo-disubstituted benzenes on π-valence orbitals is smaller than that on σ-ones. The former increases while the latter decreases with π-electron-donating character of the substituent which demonstrates the role of hyperconjugation in the substituent effect.

Vibrational energy distribution analysis (VEDA): scopes and limitations

The principle of operations of the VEDA program written by the author for Potential Energy Distribution (PED) analysis of theoretical vibrational spectra is described. Nowadays, the PED analysis is indispensible tool in serious analysis of the vibrational spectra. To perform the PED analysis it is necessary to define 3N-6 linearly independent local mode coordinates. Already for 20-atomic molecules it is a difficult task. The VEDA program reads the input data automatically from the Gaussian program output files. Then, VEDA automatically proposes an introductory set of local mode coordinates. Next, the more adequate coordinates are proposed by the program and optimized to obtain maximal elements of each column (internal coordinate) of the PED matrix (the EPM parameter). The possibility for an automatic optimization of PED contributions is a unique feature of the VEDA program absent in any other programs performing PED analysis.

Absolute configurations of fungal and plant metabolites by chiroptical methods. ORD, ECD, and VCD studies on phyllostin, scytolide, and oxysporone

The absolute configuration (AC) of the bioactive metabolites phyllostin (1) and scytolide (2), two hexahydro-1,4-benzodioxines produced by Phyllosticta cirsii, and oxysporone (3), a dihydrofuropyranone recently isolated from a strain of Diplodia africana, has been assigned by computational analysis of their optical rotatory dispersion (ORD), electronic circular dichroism (ECD), and vibrational circular dichroism (VCD) spectra. Computational prediction of ORD, ECD, and VCD allowed us to assign (3S,4aR,8S,8aR) AC to naturally occurring (-)-1, while (4aR,8S,8aR) AC was assigned to (-)-2 employing only ECD and VCD, because in this case ORD analysis turned out to be unsuitable for AC assignment. Theoretical prediction of both ORD and ECD spectra of 3 led to assignment of (4S,5R,6R) AC to (+)-3. In this case a satisfactory agreement between experimental and calculated VCD spectra was obtained only after taking into account solvent effects. This study shows that in the case of flexible and complex natural products only a concerted application of more than a single chiroptical technique permits unambiguous assignment of absolute configuration.

Intermolecular alkyl radical additions to enantiopure N-tert-butanesulfinyl aldimines

The sulfinyl group in (R)-N-tert-butanesulfinyl aldimines provides efficient control of the stereoselectivity in the intermolecular reactions with alkyl radicals. The methodology is applicable to aryl, heteroaryl, benzyl, and alkynyl imines, even those containing CN, CO2Me, COR, and OH groups. The best results are attained with hindered radicals (tertiary and secondary ones) without C═N bond reduction. This reaction complements the well-established organometallic additions to N-sulfinyl aldimines to obtain enantiomerically pure functionalized α-branched primary amines.