Geometric aspects influencing N-NTB transition - implication of intramolecular torsion

Herein we report a comprehensive study on novel carbonyl- and ethenyl-linked symmetric dimers that combine synthesis, mesomorphic properties and molecular modelling. The study has been focused on the impact of geometry imposed by the linkage group on the incidence of the twist-bend nematic (N_TB) phase. Comparison of the mesomorphic properties of these two series complemented with computational studies of conformational space around the linkage group points molecular curvature and intramolecular torsion plays important role in the appearance of the N_TB phase and can be regarded as the basic structural requirements for design of new twist-bend nematogen materials.     

Wasseruntersuchung

Ohne Zusammenfassung     

Sulfur Ylide Promoted Synthesis of N‐Protected Aziridines: A Combined Experimental and Computational Approach

A range of N‐protected aziridines [N‐Tosyl (N‐Ts), N‐2‐trimethylsilylethanesulfonamide (N‐SES), N‐tert‐butoxycarbonylamido (N‐Boc), and N‐o‐nitrobenzenesulfonamide (oNs)] were prepared in moderate to good yield and with high enantiomeric excess of both isomers starting from N‐protected imines, using a sulfonium salt derived from Eliel’s oxathiane. The diastereoselectivities of the reactions are influenced by the imine N‐protecting group, the imine substituent, and the sulfide structure. An unusual cis selectivity was observed in the formation of N‐tosyl‐2‐phenyl‐3‐tert‐butylaziridine and N‐o‐trimethylsilylethanesulfonamide‐2‐phenyl‐3‐tert‐butylaziridine, which was explained by using computational models. The analysis suggests that betaine formation in the case of N‐tosyl‐tert‐butylaldimine aziridination using oxathiane benzyl sulfonium ylide 1 ′ is reversible and that the selectivity is determined at the rotation step, which is unusual for semistabilized ylide aziridination. We have shown herein that the steric bulk of an imine substituent, in combination with a sterically demanding sulfonium ylide, can also affect the reversibility of the reaction. This is the first example of this sort involving aziridinations using semistabilized ylides.     

Lanthanide chelates of fluorinated β-diketones. Part II magnetic moments and electronic spectra of lanthanide chelates of four fluorinated β-diketones and the crystal structure of [Er(m · MeC6H4CO=CHCOF3)3(H2O)2] · H2O

Summary Magnetic and spectral data are reported for the complexes Ln(RCO=CHCOCF₃)₃ ·nH ₂O (Ln=lanthanide; R=p BrC₆H₄,m-MeC₆H₄,o-MeC₆H₄, Bu-t; n=2 or 3). An x-ray crystal structure determination of Er(m-MeC₆H₄CO=CH-COCF₃)₃ · 3 H₂O showed that the complex is eight-coordinate with three bidentate -diketonato and two aquo ligands; the third water molecule is hydrogen-bonded to one of the coordinated water molecules. The configuration about the erbium atom approximates to a square antiprism. The magnetic moments of the samarium and europium chelates are temperature-dependent, whereas those of the other paramagnetic lanthanides obey the Curie-Weiss law. The visible spectra of the chelates of Pr, Nd, Sin, Eu, Tb, Dy, Ho, Er, and Tm display line-like absorption bands typical of lanthanide ions. Hypersensitive transitions were observed for Nd, Sm, Eu, Dy, Ho, and Er.Part I,Transition Met. Chem., 8, 298 (1983).     

Evidence for a non-RRKM photochemical decay. Triplet state behavior of alicyclic ketones

By means of triplet-triplet energy transfer to biacetyl, the population of the vibrationally relaxed triplet state T⁰₁ of the alicyclic ketones cycloheptanone, cyclohexanone, and cyclopentanone was examined as a function of the excitation wavelength. This population, as measured in terms of the ketone triplet yield φ_T(λ), was compared with the excitation energy dependence of the photochemical quantum yield. This comparison demonstrated that the main photochemical path of these ketones originates from higher vibrational levels of T₁. Thus, φ^T(λ) reflects the branching ratio between a non-RRKM photochemical decay and the vibrational relaxation process to T₁. Moreover, φ_T(λ) was found to decrease significantly in the order cycloheptanone > cyclohexanone > cyclopentanone. This observation reflects, at least in part, the influence of the internal degrees of freedom on the vibrational relaxation rate.     

Inversion of enantioselectivity in quinine-mediated desymmetrization of glutaric meso-anhydrides

An unexpected inversion of enantioselectivity, dependent on the degree of quinine loading, was observed during the desymmetrization of glutaric meso-anhydrides. Decrease in catalyst loading from 1.6 equiv to 0.1 equiv caused a clear inversion of stereochemistry—from about 40% ee of (R)-configuration to about 40% ee of the opposite enantiomer. The effect of various carboxylic acid additives was also studied.