Abstract: The reaction mechanism of Cα–H hydroxylation of 4-chloro-N-cyclopropyl-N-isopropylaniline catalyzed by cytochrome P450 was investigated using the density functional theory. The Becke’s three-parameter hybrid exchange functional and the Lee-Yang-Parr correlation functional (B3LYP) were used in the structure optimization and single-point energy calculations. There are two Cα–H hydroxylation reaction pathways for 4-chloro-N-cyclopropyl-N-isopropylaniline catalyzed by P450. One is Cα–H hydroxylation at the cyclopropyl group, and the other is Cα–H hydroxylation at the isopropyl group. Our calculations demonstrate that the Cα–H activation at both the cyclopropyl group and the isopropyl group is a hydrogen atom transfer process, and the reaction is concerted. Since the Cα–H activation energy on the high-spin quartet state is much higher than that on the low-spin (LS) doublet state, the Cα–H hydroxylation proceeds in a spin-selective manner, mostly on the LS state. Comparison of the energy barriers for the two reaction pathways predicts a preponderance of Cα–H hydroxylation at the cyclopropyl group over that on the isopropyl group by roughly a ratio of 1.8:1, which means that the N-decyclopropylation branch is 64% and the N-deisopropylation branch is 36% during the dealkylation of 4-chloro-N-cyclopropyl-N-isopropylaniline. This is in agreement with former experimental results.

Key words: density functional theory, transition state, hydrogen atom transfer, energy barrier, reaction rate, reaction mechanism