Mechanistic Insights of Allylic Oxidation of Aliphatic Compound by Tetraamido Iron(V) Species: A C-H vs. O-H Bond Activation
The main challenges in synthetic organic chemistry are chemoselective oxidation of C-H bond by natural complexes under mild conditions. Heme and non-heme metal complexes with several oxidants such as dioxygen, hydrogen peroxide, m-chloroperbenzoic acid etc are used for catalytic oxidation of aliphatic C-H bond. Metal catalyzed allylic oxidation of aliphatic compounds is attractive intermediates and these are very useful in pharmaceutical industries. Here we have reported electronic structures and also for the first time mechanistic detail of selective allylic oxidation of the cyclohex-2-enol by an oxidant non-heme iron oxo species using hybrid density functional theory incorporating dispersion effects. Our DFT results show that a significant spin density on the ferryl oxygen is playing an important role to activate O-H and C-H bond of the cyclohex-2-enol. The reaction can be feasible via O-H (pathway a) and C-H (pathway b) bond activation. The initial hydrogen abstraction is the rate-determining step in both the pathways. A significant spin density on carbon and oxygen atom shows that O-H/C-H bond activation occurs through the homolytic cleavage. DFT investigation shows that the O-H bond activation of the studied mechanism can show two-state reactivity whereas C-H bond activation can proceed via single state reactivity. By mapping energy profiles, we have found that the C-H bond activation is relatively preferable over the O-H and oxygen attack. Further, the significant exchange of metal electrons and structural parameters during transition states can control reactivity and can help to design catalyst with better efficiency/selectivity for catalytic reactions.