A theoretical study of reactivity and regioselectivity in the hydroxylation of adamantane by ferrate(VI).
The conversion of adamantane to adamantanols mediated by ferrate (FeO(4)(2)(-)), monoprotonated ferrate (HFeO(4)(-)), and diprotonated ferrate (H(2)FeO(4)) 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 degrees ) and 2-adamantanol by the abstraction of a secondary hydrogen atom (2 degrees ). 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 degrees )/(2 degrees ) for the competing H-atom abstraction reactions is calculated to be 9.30 at 75 degrees C, which is fully consistent with an experimental value of 10.1.
Institute for Fundamental Research of Organic Chemistry, Kyushu University, Fukuoka 812-8581, Japan.
Yoshihito
Y
Shiota
Naoki
N
Kihara
Takashi
T
Kamachi
Kazunari
K
Yoshizawa