A theoretical study of reactivity and regioselectivity in the hydroxylation of adamantane by ferrate(VI)

The conversion of adamantane to adamantanols mediated by ferrate (FeO₄^2-), monoprotonated ferrate (HFeO₄^-), and diprotonated ferrate (H₂FeO₄) is discussed with the hybrid B3LYP density functional theory (DFT) method. Diprotonated ferrate is the best mediator for the activation of the C-H bonds of adamantane via two reaction pathways, in which 1-adamantanol is formed by the abstraction of a tertiary hydrogen atom (3°) and 2-adamantanol by the abstraction of a secondary hydrogen atom (2°). Each reaction pathway is initiated by a C-H bond cleavage via an H-atom abstraction that leads to a radical intermediate, followed by a C-O bond formation via an oxygen rebound step to lead to an adamantanol complex. The activation energies for the C-H cleavage step are 6.9 kcal/mol in the 1-adamantanol pathway and 8.4 kcal/mol in the 2-adamantanol pathway, respectively, at the B3LYP/6-311++G** level of theory, whereas those of the second reaction step corresponding to the rebound step are relatively small. Thus, the rate-determining step in the two pathways is the C-H bond dissociation step, which is relevant to the regioselectivity for adamantane hydroxylation. The relative rate constant (3°)/(2°) for the competing H-atom abstraction reactions is calculated to be 9.30 at 75 °C, which is fully consistent with an experimental value of 10.1.