The Study On Lewis Acid Promoted Dioxygen Activation By Chromium Complex For Olefin Oxidation

Catalytic oxidation is one of the representative reactions in chemical,pharmaceutical and fine chemical fields.In current industrial oxidation processes,dioxygen based catalytic oxidations are generally achieved through thermo-assisted dioxygen activation,which leads to the happening of versatile radical chain processes with low product selectivity,thus causes serious pollution with high oxidant costs.In nature,redox metal ions mediated dioxygen activation and oxidations generally happen smoothly with high selectivity under room temperature,which can be attributed to that,the stereo-chemical structure and different modulators including hydrogen bond and transition metal ions can synergistically catalyze dioxygen activation and next oxidations by redox enzymes.Thus,based on the understanding of the enzymatic mechanisms,modelling enzymatic dioxygen activation to achieve industrially catalytic oxidations has become the dream of scientists.In the present studies,inspired by the role of hydrogen bond(Br(?)nsted acid,H~+)in enzymatic dioxygen activation,we investigated Lewis acid(Br(?)nsted acid vs Lewis acid)modulated high valent chromium complex for electron transfer and oxygenation reactions,Lewis acid promoted low valent chromium complex activating dioxygen with further catalytic oxidations.It was found that,the presence of Lewis acid can obviously accelerated Cr(V)=O species mediated electron transfer reaction and olefinic C=C bond cleavage reactions.However,due to the instability of Cr(V)=O species under the reaction conditions,its stoichiometric oxygenation efficiency was pretty poor.Next,we further investigated Lewis acid promoted dioxygen activation by low valent chromium complex for olefin oxidation by C=C bond cleavage.It was found that the presence of Lewis acid can sharply improve Cr(Ⅲ)complex mediated dioxygen activation in olefin C=C bond cleavage reaction,and a stronger Lewis acid provided a better promotional effect.In oxidations of styrene with different substituents,it was found that the presence of an electron-rich substituent benefits olefin oxidation happening,indicating that the C=C bond cleavage is an electrophilic oxidation process.Inspired by the dioxygenation mechanism of indoleamine 2,3-dioxygenase in literature,we proposed that,in the present studies,the presence of Lewis acid can stabilize the superoxo radical,which was produced by low valent chromium complex mediated dioxygen activation,and enhance its electrophilic capability in C=C bond cleavage,thus leads to sharply improved efficiency in olefin oxidations.This work confirmed that,the presence of Lewis acid does promote redox metal ions mediated dioxygen activation,and further improve the catalytic oxidation in olefinic C=C bond cleavage,which may have provided a new clue in catalyst design for redox metal ions mediated dioxygen activation and catalytic oxidations under ambient conditions.