Influence of Substituents on the Rotational Energy Barrier of Axially Chiral Biphenyls, II

Autor: Christian Roussel, Christian Wolf, Detlev H. Hochmuth, Wilfried A. König
Rok vydání: 2006
Předmět:
Zdroj: Liebigs Annalen. 1996:357-363
ISSN: 1099-0690
0947-3440
Popis: The influence of substituents on the rotational energy barrier of a series of 2,2′-bis(trifluoromethyl)- and 2,2′-diisopropylbiphenyl derivatives with various substitution patterns was investigated by dynamic gas chromatography (DGC) with selectively modified cyclodextrins and by high-performance liquid chromatography (HPLC) on microcrystalline cellulose triacetate combined with polarimetry. Investigation of the steric effect of substituents was possible by gradually increasing the bulkiness of substituents in the ortho positions of 2,2′-disubstituted biphenyls and determination of the corresponding energy barriers. The buttressing effect of alkyl groups adjacent to the ortho substituents is discussed. Comparison of the energy barriers of 2,2′-bis(trifluoromethyl)biphenyl (1) with its derivatives possessing alkyl groups in positions 3 and 5, respectively, allows a discrimination between the electronic and steric contributions of the meta substituents and thus the determination of the influence on the energy barrier. The electronic effect on the rate of rotation, i.e. the rate of enantiomerization, of axially chiral biphenyls bearing substituents in the para positions depends on the aromatic system to which they are attached. Electron-donating groups decrease the rotational energy barrier of 2,2′-bis(trifluoromethyl)biphenyl derivatives but increase the barrier of 2,2′-diisopropylbiphenyl derivatives. Electron-accepting groups exhibit the opposite behavior. The unexpected effects in the case of para-substituted 2,2′-diisopropylbiphenyl derivatives are possibly related to C–H/π interactions.
Databáze: OpenAIRE
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