Steric effects which determine the conformational preferences and stereodynamic processes of aryl fluorenyl ketones

Abstract

The stereodynamic processes and conformational preferences of two classes of aryl fluorenyl ketones have been investigated by means of dynamic NMR spectroscopy, DFT calculations and X-ray diffraction. When the aryl substituent has two hydrogens in the ortho positions, its rotation is independent of that of the fluorene ring. In contrast, if the two ortho hydrogens are replaced by the bulkier methyl groups (e.g. mesityl fluorenyl ketones), the motion of the aryl ring interacts with the fluorene, and the two rings rotate in a correlated manner.

Recent trends in conformational analysis

“…However, it should be noted that as in the case of chemical reactions, there are some cases where the DFT calculations of molecular conformations could not match the experimental evidence 135. A recent example is that of aryl–fluorenyl ketones,136 where DFT calculations failed to find the geometry of the best conformation observed in solution by low‐temperature NOE experiments, despite the use of different models (B3LYP, PBE1PBE, M05‐2X), and the inclusion of the solvent in the calculations.…”

Synthesis and Structural Characterization of Spiro(fluorene-9,4′-imidazolidine)-2′,5′-dione and (9H-Fluorene-9-yl)urea

“…The angle between their mean planes is 86.92(4)°. Analogous features are observed in the crystal structures of related spiro-fluorenyl derivatives [24,25]. In the crystal the molecules form one-dimensional hydrogenbonded chains [graph set R 2 2 (8)] through N-HÁÁÁO interactions ( Fig.…”

Stereodynamics and absolute configuration of stereolabile atropisomers in 2,2‐dimethyl‐1‐aryl‐1‐indanols

“…On the other hand, the DFT approach should be considered suitable to tackle the conformation analysis of the present set of molecules because of its fairly good accuracy coupled with a reasonable computational cost. The barriers to rotation observed in the 2,2-dimethylindanols (1-7) are largely influenced by the bulkiness of the aryl substituent 25,26 , { as shown in Table 1. When the ortho-hydrogens of the aryl group are replaced by methyls (compound 3), the steric hindrance, and consequently, the barrier increases so much that the mesityl rotation is already locked at 258C, as deduced by the 1 H spectrum where the ortho-methyls and the meta-hydrogens show pairs of sharp singlets separated by 750 and 160 Hz, respec- (N being the number of the basis functions, that is, of the orbitals), so this approach would imply computational times close to a week when dealing with larger molecules like compound 5, 6, and 7.…”

“…The barriers to rotation observed in the 2,2-dimethylindanols (1-7) are largely influenced by the bulkiness of the aryl substituent 25,26 , { as shown in Table 1.…”

“…S2 and S3) in good agreement with the values measured in similar compounds. 25,26 Actually, the whole rotational process for 2 consists of two simultaneous rotational processes: indanyl-CH 2 and CH 2 -Ph, which allow to better accommodate the phenyl bulkiness and thus facilitate the whole motion by reducing the steric interactions.…”