Polytrifluoromethylation versus Polyfluorination of the Isomers of Kekulé Benzene and Phenol: A Theoretical Study

Abstract

The study of the polyfluorination and polytrifluoromethylation effects on electronic structure and intrinsic acidities has been performed using DFT B3LYP and NBO calculations for the valence isomers of benzene and phenol, i.e., the structures of prismane, benzvalene, Kekulé, and Dewar systems. Also the isodesmic reaction analysis approach to estimate the effects of the substituents on the acidity of the compounds has been used. Although in some systems the additivity of fluorine substituents was more than 100%, the acidifying effects of the fluorine substituents were comparable to that of the CF(3) in only one case. Isodesmic reaction analysis of substituent effects shows that steric effects of poly-CF(3) substitution in the alicyclic cage compounds are significantly smaller. The relative thermodynamic stabilities of the valence isomers of Kekulé benzene were significantly lower than that of the aromatic cycle. The introduction of fluorine substituents often destabilized the compounds even further. Out of the fluorinated hydroxy derivatives of prismane, Dewar benzene, and benzvalene, not all are predicted to be stable enough to be able to undergo a reversible protonation-deprotonation process. In the case of several hydroxy derivatives deprotonation is accompanied by the rupture of a C(α)-C(β) bond and in some cases by the rearrangement to the corresponding phenolate anion. The isomerization of benzene derivatives resulted in more cases where the acidities increased compared with the respective phenol derivatives. The only hydroxy compound with significantly higher acidity than the respective phenol was pentafluorinated 2-OH-Dewar benzene, which was expected to rival the gas-phase acidity of perfluoro-1-adamantanol, currently the most acidic experimentally measured perfluorinated alcohol.