Conformational Studies by Dynamic NMR. 93.¹ Stereomutation, Enantioseparation, and Absolute Configuration of the Atropisomers of Diarylbicyclononanes

Abstract

The structure of 9-(4-methoxyphenyl)-9-(3-methylphenyl) bicyclo[3.3.1]nonane (1) has been determined by single-crystal X-ray diffraction, and the rotation barriers about the two aryl-C9 bonds have been measured by variable temperature NMR spectroscopy in solution. In the case of 9-(4-methoxyphenyl)-9-(1-naphthyl)bicyclo[3.3.1]nonane (3), the barrier involving the naphthyl-C9 rotation was found to be so high as to allow the physical separation of the two atropisomers by enantioselective HPLC at ambient temperature: the absolute configuration could be established on the basis of the corresponding CD spectra. It was also observed that the Ar-C9 rotation barriers of monoaryl-substituted nonanes are much lower than those of the corresponding diaryl-substituted nonanes.

Benzoxazine Atropisomers: Intrinsic Atropisomerization Mechanism and Conversion to High Performance Thermosets

“…In order to elucidate the difference in isomerism, NMR spectra were also acquired using a Varian Oxford AS300 (300 MHz), a Bruker Ascend III 500 MHz, and a Varian Inova 600 MHz NMR sepctrometers. Cross-polarization magic-angle spinning (CP-MAS) solid-state 13 C NMR spectra were collected using a Bruker Avance III 400 MHz instrument, using adamantance as reference. Powder samples were packed into a 4 mm zirconia rotor with a spinning rate of 10 000 Hz, and the number of transients was 6000 scans.…”

“…Moreover, atropisomerism has rarely been reported in other types of chemical structures beyond biaryls and related compounds. 13,14 Benzoxazine has been widely investigated and reported due to its outstanding properties for applications. 15−21 Benzoxazine resin is known to have the highest molecular-design flexibility among all the polymeric materials and can be tailored to desired properties.…”

Recent Advances in Stereodynamics and Conformational Analysis by Dynamic NMR and Theoretical Calculations

“…The low-temperature NMR spectra allowed the determination of the rotation barrier of the 4-methoxyphenyl group (10.2 kcal mol -1 ) and also that of the 3-methylphenyl substituent (10.8 kcal mol -1 ), the latter corresponding to an enantiomerisation process between two stereolabile atropisomers (enantiomers). [94] Scheme 5.…”

B Values as a Sensitive Measure of Steric Effects

“…[29] In all cases investigated, the DG°value was found to be invariant in the given temperature range, thus implying a negligible activation entropy DS°. [30] Quantum-chemical calculations: A complete conformational search was preliminarily carried out by means of the Molecular Mechanics force field (MMFF), [31] using the Monte-Carlo method implemented in the package TITAN 1.0.5. [32] The most stable conformers thus identified were subsequently energy-minimized by density functional theory (DFT) computations.…”