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Anthraquinone substituted by 2-methyl-1-naphthyl groups in positions 1,8 yields syn (meso) and anti (racemic) isomers (red and yellow colored, respectively) that interconvert with a barrier of 35.4 kcal mol(-1) in solution. Their structures were identified by NOE experiments in solution and X-ray diffraction in the solids. The racemic anti form (C(2) point group) entails two atropisomers that were separated by enantioselective HPLC: the absolute configuration was assigned by TD-DFT simulation of the ECD spectrum. Two atropisomers were also separated and assigned in the case of anthraquinone bearing a single 2-methyl-1-naphthyl substituent in position 1.

By making use of low-temperature dynamic NMR spectroscopy, the rotation barriers about the sp3-sp2 bond have been determined in a number of hindered benzyl alcohols symmetrically substituted in the ortho positions, the substituents being F, Cl, Br, and Me. The free energies of activation covered the range 4.6-10.1 kcal mol-1. Ab initio computations matched satisfactorily the trend of these values and predicted the conformation adopted by these compounds. In one case, this result could be also confirmed by the X-ray diffraction structure. In the case of the corresponding methyl ethers two barriers could be measured, corresponding to the passage across two distinguishable transition states: the higher barriers covered the range 5.0-8.1 kcal mol-1 and the lower ones the range 4.7-6.2 kcal mol-1.

4-Aryl-3-bromo-N-benzylmaleimides and 3,4-biaryl-N-benzylmaleimides have been synthesized by a modified Suzuki cross-coupling reaction from 3,4-dibromo-N-benzylmaleimide. The conformational studies by dynamic NMR and DFT calculations showed that the interconversion barrier between the two available skewed conformations is under steric control. When the aryl group was a 2-methylnaphthyl, thermally stable atropisomers were isolated by enantioselective HPLC and their absolute configurations were assigned by TD-DFT simulations of the ECD spectra.

Simple model systems based on the 2,11-dithia-[3,3]-metaparacyclo-phane skeleton were designed and synthesized to study the effect of substituents on the intramolecular aromatic-aromatic interactions between benzene rings. X-Rays structures established that in their more stable conformation these metaparacyclophanes feature partially overlapped aromatic rings (interplanar distance of about 3.5 Å) with the planes of the aromatic systems arranged in a slightly tilted disposition (interplanar angles ranging from 5 to 19°). Calculations showed that these derivatives underwent topomerization by flipping of the meta-substituted ring over the para-substituted one, a process in which the two rings adopted a continuum of edge-to-face dispositions, including also an orthogonal one, which were less stable than the starting face-to-face arrangement. The energy barriers to the isomerization process were experimentally determined by variable temperature NMR spectroscopy, using an internal temperature standard to safely assess even minor differences in energy (estimated experimental error ± 0.1 kJ mol-1). The variation of the barriers as a function of the different substitution on the interacting ring was small and apparently not related to the effect of the substituents on the polarity of the -systems. An explanation based on the charge penetration effect seemed more suitable to rationalize the observed trends in the barriers.

The free energies of activation for the aryl-aryl rotation of 17 biphenyl derivatives, bearing a heavy heteroatom (S, Se, Te, P, Si, Sn) as ortho substituent, have been measured by variable temperature NMR. These numbers, so called B values, represent a meaningful measure of the steric hindrance exerted by the selected substituents. DFT computations match quite satisfactorily the experimental barriers and the ground state geometries as well (determined, in two cases, by X-ray diffraction). The present values extend the available list of B values and thus provide an enlarged basis for the compilation of the space requirements of standard substituents, based solely on experimental determinations.

We describe herein the investigation of the stereodynamic processes occurring in a series of 1-aryl-2,2-dimethylindanols, by dynamic NMR. When the aryl moiety is a mesityl or a 2-methyl-1-naphthyl, the rotational barrier exceeds the 25 kcal/mol, so that stable atropisomers are observed. In two cases, all the chiral-atropisomeric species have been separated by enantioselective HPLC, and the comparison between theoretical and experimental electronic circular dichroism spectra allowed the absolute configuration assignment of all the isolated species to be obtained. Chirality, 2011. © 2011 Wiley-Liss, Inc.

Anti and syn conformers, due to restricted sp(2)-sp(2) bond rotation, were detected in hindered 1,8-diarylbiphenylenes, the aryl moieties being phenyl groups bearing 0micron-alkyl substituents such as Me, Et, i-Pr, and t-Bu. By means of low-temperature NOE experiments, the corresponding structures were assigned and were found to be in agreement with the results of single-crystal X-ray diffraction. The interconversion barriers of these conformers were determined by line-shape simulation of the variable-temperature NMR spectra and the experimental values were reproduced satisfactorily by DFT calculations. In the case of the bulkiest aryl substituent investigated (i.e., 2-methylnaphthalene), the syn and anti atropisomers were stable enough as to be separated at ambient temperature. The two enantiomers (M,M and P,P) of the isomer anti were also isolated by enantioselective HPLC, and the theoretical interpretation of the corresponding CD spectrum allowed the absolute configuration to be assigned.

Syn and anti conformers in similar proportions were observed at ambient temperature for the title compounds. The conformational assignment of the two anthrones was obtained by the observation of different multiplicity of the methylene NMR signals, whereas that of the anthraquinone derivative was determined by NOE experiments. The anti-to-syn interconversion barriers were obtained by line-shape simulation of the temperature-dependent NMR spectra, and by saturation transfer experiments. In one case the X-ray diffraction indicated that the syn is the only structure observed in the crystals.

The rotation barriers of two 1,8-ditolylanthracene derivatives have been measured by accurate line-shape simulation of their variable-temperature NMR spectra. Both values were found to be more than twice as large as those previously reported in the literature.

NMR spectra of biphenyl derivatives bearing a single CR2OH substituent in the ortho position indicate that they exist as sp (more stable) and ap (less stable) conformers, due to the restricted rotation about the Ar-CR2OH bond. When R = Et (compound 2) the corresponding rotation barrier was determined (7.5 kcal mol-1) by line shape simulation of the low-temperature NMR spectra. Introduction of the prochiral i-Pr group in the position 3' of a biphenyl with the CMe2OH substituent in the position 2 (4) allowed the determination of the enantiomerization barrier (due to the Ar-Ar bond rotation) for the stereolabile axially chiral atropisomers (13.95 kcal mol(-1)). DFT computations of these barriers were all in agreement with the experiments. Biphenyls bearing two CR2OH groups in the 2,2' positions were found to exist as configurationally stable atropisomers: when R = Me (7) they were separated by enantioselective HPLC and the absolute configuration assigned on the basis of the corresponding CD spectra. In solution, compounds 6 (R = H) and 7 (R = Me) were found to originate a dimer, due to H-bond interactions between two enantiomers. In the case of 7, the free energy of activation (9.5 kcal mol-1) for the exchange of the monomer with the dimer could be measured, for the first time, by dynamic NMR. The conformational preferences, predicted by computations for the biphenyls with two CR2OH substituents in the 2,2' positions, were confirmed by X-ray diffraction in the case of R = H (6), R = Me (7), and R = i-Pr (9).

Dynamic NMR spectroscopy allowed, for the first time, the determination of the pi-barriers responsible for the enantiomerisation processes in derivatives bearing two aryl substituents bonded to a planar framework: this could be achieved in the case of 1,8-di-m-tolylbiphenylene (1) and 1,8-di-m-xylylbiphenylene (2). In derivative 1 the three possible conformers predicted by DFT computations (anti-in, syn and anti-out) were detected and, in addition, the steric barrier responsible for their interconversion could be measured. The barriers predicted by DFT calculations were found in satisfactory agreement with the experimental data.