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Z. Fu and J. Chen
activation barriers (with differences ≤2 kcal/mol) on both spin states and thus is ratedetermining in the hydroxylation process. However, the second hydroxyl rebound
step on the doublet state is barrierless, in contrast to a slight rebounding barrier
needed for the quartet state reaction.
The rate-determining HAT step in C–H hydroxylation is generally characterized
by large kinetic isotope effects (KIEs). KIEs refer to the change in reaction rates
when one atom (typically the H atom) of the reactant is replaced by one of its isotopes (e.g., deuterium) [29]. According to the study by Li et al. [30], KIEs can
serve as a spin-state reactivity probe because the TSR possesses similar KIEs values,
whereas significantly discrepant KIEs for two spin states indicate a spin-selective
reactivity (SSR) scenario in H-abstractions by Compound I. In addition, Shaik et al.
[15] found a linear correlation between the activation barriers of alkane hydroxylation by Compound I with the corresponding bond dissociation energies (BDEs)
of C–H bonds. BDEs are defined as the energy difference between the optimized
substrate molecule (Sub-H) and the optimized substrate radical (Sub·) and H atom,
i.e., BDE(Sub-H) = E(Sub-H) − E(Sub·) − E(H). However, this linear correlation
becomes insignificant for C–H bonds with lower BDEs, e.g., Cα-H bonds adjacent
to strong electron-withdrawing groups.
17.3.1.2 Mechanisms for Heteroatom (N, S, P) Oxidation
Heteroatom (N, S, P) oxidation pertains to another type of important reaction catalyzed by P450 enzymes. N-containing amines can be activated by P450 enzymes into
products that may induce genotoxicity and carcinogenicity. Metabolism of secondary
and tertiary amines by P450 enzymes would primarily proceed via N-dealkylation
for which two possible mechanisms (Fig. 17.6) are reported [29]. Analogous to the
HAT scheme for C–H bond hydroxylation, one mechanism suggests an initial Cα–H
abstraction by Compound I with the yield of an alkane radical and PorFe
IV –OH intermediate, which is followed by hydroxyl rebound to form alcohol amine products.
The other mechanism hypothesizes that a single electron transfer (SET) from the
amine N atom to Compound I results in a N cation radical, which is then deprotonated to the alcohol amine. Bifurcate HAT and SET mechanisms have long been a
controversial issue; only recently, quantum chemical calculations have ascertained
that HAT is generally lower in activation barriers and thus more favorable than SET.
N-dealkylation of secondary and tertiary amines would yield primary amines
as products. Further oxidation of primary aromatic amines (ArNH 2 ) can proceed
via the following four mechanisms as shown in Fig. 17.7, three of which adopt
Compound I as the oxidant, i.e., HAT followed by hydroxyl rebound, oxygen addition
rearrangement (OAR), and SET followed by proton transfer (PT). The other pathway,
however, involves the iron-superoxide species (FeOO
2− ) in the catalytic cycle as the
active oxidant. Computational studies [31] have revealed that the HAT and HOrebound pathways require the lowest energy barrier and hence are the principal
pathways for ArNH 2 oxidation by P450 enzymes. Nevertheless, significant energy
barrier increases were observed for the rate-limiting HAT with the enhancement
Z. Fu and J. Chen
activation barriers (with differences ≤2 kcal/mol) on both spin states and thus is ratedetermining in the hydroxylation process. However, the second hydroxyl rebound
step on the doublet state is barrierless, in contrast to a slight rebounding barrier
needed for the quartet state reaction.
The rate-determining HAT step in C–H hydroxylation is generally characterized
by large kinetic isotope effects (KIEs). KIEs refer to the change in reaction rates
when one atom (typically the H atom) of the reactant is replaced by one of its isotopes (e.g., deuterium) [29]. According to the study by Li et al. [30], KIEs can
serve as a spin-state reactivity probe because the TSR possesses similar KIEs values,
whereas significantly discrepant KIEs for two spin states indicate a spin-selective
reactivity (SSR) scenario in H-abstractions by Compound I. In addition, Shaik et al.
[15] found a linear correlation between the activation barriers of alkane hydroxylation by Compound I with the corresponding bond dissociation energies (BDEs)
of C–H bonds. BDEs are defined as the energy difference between the optimized
substrate molecule (Sub-H) and the optimized substrate radical (Sub·) and H atom,
i.e., BDE(Sub-H) = E(Sub-H) − E(Sub·) − E(H). However, this linear correlation
becomes insignificant for C–H bonds with lower BDEs, e.g., Cα-H bonds adjacent
to strong electron-withdrawing groups.
17.3.1.2 Mechanisms for Heteroatom (N, S, P) Oxidation
Heteroatom (N, S, P) oxidation pertains to another type of important reaction catalyzed by P450 enzymes. N-containing amines can be activated by P450 enzymes into
products that may induce genotoxicity and carcinogenicity. Metabolism of secondary
and tertiary amines by P450 enzymes would primarily proceed via N-dealkylation
for which two possible mechanisms (Fig. 17.6) are reported [29]. Analogous to the
HAT scheme for C–H bond hydroxylation, one mechanism suggests an initial Cα–H
abstraction by Compound I with the yield of an alkane radical and PorFe
IV –OH intermediate, which is followed by hydroxyl rebound to form alcohol amine products.
The other mechanism hypothesizes that a single electron transfer (SET) from the
amine N atom to Compound I results in a N cation radical, which is then deprotonated to the alcohol amine. Bifurcate HAT and SET mechanisms have long been a
controversial issue; only recently, quantum chemical calculations have ascertained
that HAT is generally lower in activation barriers and thus more favorable than SET.
N-dealkylation of secondary and tertiary amines would yield primary amines
as products. Further oxidation of primary aromatic amines (ArNH 2 ) can proceed
via the following four mechanisms as shown in Fig. 17.7, three of which adopt
Compound I as the oxidant, i.e., HAT followed by hydroxyl rebound, oxygen addition
rearrangement (OAR), and SET followed by proton transfer (PT). The other pathway,
however, involves the iron-superoxide species (FeOO
2− ) in the catalytic cycle as the
active oxidant. Computational studies [31] have revealed that the HAT and HOrebound pathways require the lowest energy barrier and hence are the principal
pathways for ArNH 2 oxidation by P450 enzymes. Nevertheless, significant energy
barrier increases were observed for the rate-limiting HAT with the enhancement