Influence of Substituents in Fluorobenzene Derivatives on the Cytochrome P450-Catalyzed Hydroxylation at the Adjacent Ortho Aromatic Carbon Center

Abstract

In a previous study, the in vivo cytochrome P450-catalyzed regioselectivity of aromatic ring hydroxylation for a series of (poly)fluorobenzenes could be quantitatively predicted by the calculated frontier orbital density distribution in the aromatic ring [Rietjens et al. (1993) Biochemistry 32, 4801-4812]. However, the relative small fluorine, its size almost comparable to a hydrogen, is not expected to influence the regioselectivity of aromatic hydroxylation due to steric hindrance. The aim of the present study was to investigate the influence of substituents larger than a hydrogen or fluorine on the possibilities for hydroxylation at the adjacent carbon center. First, the in vivo regioselectivity of aromatic ring hydroxylation of a series of C4-substituted fluorobenzenes was investigated. The results obtained demonstrate that a chlorine and cyano C4 substituent do not hamper hydroxylation at the positions ortho to the C4 carbon atom. For 4-chloro- and 4-cyanofluorobenzene, the observed regioselectivity of aromatic hydroxylation correlated with the regioselectivity predicted on the basis of the frontier orbital density distribution. In contrast, a bromine and iodine substituent affected the hydroxylation at the adjacent ortho carbon centers, reducing it to respectively 40 and 6% of the calculated intrinsic reactivity of the carbon centers. Additional experiments were performed to investigate whether the regioselectivity of the aromatic hydroxylation of the C4-substituted fluorobenzene model compounds was influenced by changes in the cytochrome P450 enzyme pattern. Results obtained demonstrate that for these relatively small substrates the regioselectivity of their hydroxylation was not significantly influenced by several cytochrome P450 inducers. This suggests that the active sites of the cytochromes P450 catalyzing the aromatic hydroxylation do not impose a stereoselective orientation of the aromatic rings with respect to the iron-oxo porphyrin reaction center. Thus, the working hypothesis for additional experiments was that the deviations for the regioselectivity of aromatic hydroxylation observed for 4-bromo- and 4-iodofluorobenzene may be ascribed to steric hindrance by the bromine and iodine substituents hampering the attack of the cytochrome P450 iron-oxo species on the adjacent carbon centers in the benzene derivative. This working hypothesis was further tested by investigating whether useful steric correction factors could be derived from the results obtained with the series C4-substituted fluorobenzenes. These correction factors should make it possible to correct calculated relative reactivities of carbon sites for steric hindrance by substituents positioned ortho with respect to the carbon to be hydroxylated. This will make it possible to better explain and predict the regioselectivities for other chlorine-, bromine-, iodine-, and cyano-containing fluorobenzenes. The in vivo regioselectivity of aromatic ring hydroxylation of a series of five chlorine-, bromine-, iodine-, or cyano-containing fluorobenze...

Biotransformation of fluorophenyl pyridine carboxylic acids by the model fungus Cunninghamella elegans

“…Some researchers have correlated the regiospecificity of cytochrome P450-catalysed hydroxylation of aromatic compounds with calculated frontier orbital density distribution (Koerts et al, 1997, Rietjens et al, 1993. Here, calculated frontier orbital density distributions at high-level computations (see supporting information for details) confirm the regiospecificity of CYP-catalysed hydroxylation for compounds 1-3 and 5 where the highest HOMO densities are found at the 4' carbon sites in these starting molecules.…”

Fluorination of aromatic compounds with N-fluorobenzenesulfonimide under solvent-free conditions

“…The mass spectra were obtained on an Hewlett-Packard G1800A GC-MS system consisting of an HP 5971 mass-selective detector and an HP 5890 Series II gas chromatograph. The structure of the obtained compounds was confirmed by the 1 H and 19 F NMR and mass spectra which were consistent with published data for 2-fluoro-1,3,5-trimethylbenzene, 2,4-difluoro-1,3,5-trimethylbenzene [39], 3-fluoro-1,2,4,5-tetramethylbenzene [39], 1-fluoro-2,3,4,5,6-pentamethylbenzene [40], 1-fluoro-2,4-dimethylbenzene, 2-fluoro-1,3-dimethylbenzene* [41], 2-and 4-fluorophenols [41][42][43], 2,4-difluorophenol [42], 2-fluoro-4-methylphenol [41], 2-fluoro-1-methoxy-4-methylbenzene, 4-chloro-2-fluoro-1-methoxybenzene [19], 2-fluorobenzene-1,3-diol, 4-fluorobenzene-1,3-diol, 4,6-difluorobenzene-1,3-diol [29,44], 1-fluoronaphthalen-2-ol [31], 2-and 4-fluoronaphthalen-1-ols [23], and 2-and 4-fluoro-1-methoxybenzenes [19,41].…”

Influence of structural and functional modifications of selected genotoxic carcinogens on metabolism and mutagenicity – a review

“…The bioactivation or elimination of halobenzene derivatives such as fluorobenzene is influenced by the regioselectivity and extent of CYP450 catalyzed monooxygenation, which in turn is caused by the size of the substituent [208]. It is demonstrated that chlorine and cyano substitution at C-4 position of fluorobenzene did not hamper hydroxylation at the positions ortho to the C-4 carbon atom whereas bromo and iodo substitution reduced it.…”