An acyl group makes a difference in the reactivity patterns of cytochrome P450 catalyzed N-demethylation of substituted N,N-dimethylbenzamides-high spin selective reactions

This paper addresses the experimentally observed mechanistic differences between the cytochrome P450-eatalyzed N-demethylation of substituted N,N-dimethylanilines (DMA) and of N,N-dimethylbenzamides (DMBA). The two reactions of these substrates are initiated by C-H activation of the methyl groups on the nitrogen. Thus, the DMA reactions exhibit small deuterium kinetic isotope effects (KIEs), and these KIEs and the corresponding reaction rates exhibit a linear response to the electronic nature of the para substituent. By contrast, the DMBA reactions exhibit large KIEs; the KIEs and reaction rates do not at all respond to the nature of the para substituent. Accordingly, the present paper uses density functional theoretical calculations to address these reactivity patterns in para-substituted DMBA and compare these results to those obtained for the DMA reactions previously (Wang, Y.; Kumar, D.; Yang, C. L.; Han, K. L.; Shaik, S. J. Phys. Chem. B 2007, 111, 7700). The theoretical calculations reproduce the experimental trends of narrow variations in rates and KIEs. It is shown that the above mechanistic differences between the two reaction series of DMA and DMBA are caused by the ability of the para substituent to maintain a conjugation path between the C-H reaction center and the aryl moiety. Furthermore, the computational results show a new feature of reactivity, namely, that the N-demethylation of DMBA proceeds by a spin-selective reaction via the high spin state of the active species of the enzyme. This conclusion is reinforced by the match of the calculated and experimental KIE values.