Visible-light Photocatalytic Selective Aerobic Oxidation Of Alcohols On TiO₂ And Its Mechanistic Implication

The selective oxidation of alcohols to aldehydes or ketones is very formidable to accomplish on the ground that the oxidant is dioxygen and the conditions are carried out at ambient conditions. To this end, UV-induced photocatalysis of titanium dioxide can meet these requirements,attracting much attention from scientists around the world which resulted in considerable progresses. However, to achieve this goal by visible-light photocatalysis is still a challenge. In this thesis,we adopt the strategy of dye sensitized titanium dioxide to implement the visible-light photocatalytic selective aerobic oxidation of alcohols. Specifically, two accomplishments have been achieved:(1) Eosin Y was connected with titanium dioxide to capture visible light and led to consequential photocatalytic processes. Importantly, these photocatalytic processes can be neatly incorporated with the redox catalytic cycles of TEMPO which can further induce the oxidation of alcohols under green light LED irradiation. Results of both kinetic studies and kinetic isotope effects indicate that the activation C?-H of alcohol is the rate determining step. This work suggests that the cooperation between visible-light photocatalysis and TEMPO catalysis shows great potential in the execution of challenging oxidative transformations.(2) A quinone dye, alizarin red S, was used as a sensitizer to effectively harvest visible-light to induce further photocatalytic reactions of the direct oxidation of alcohols with ambient air as the terminal oxidant. Titanium dioxide not only provides a platform for alizarin red S photocatalytic reaction, but also plays a redox active role in electron transfer and O2 activation. Activated alcohols such as benzylic alcohols can be transformed to corresponding aldehydes with good to excellent conversions and excellent selectivities. Other representative alcohols such as allylic alcohols,secondary alcohols and primary alcohols can also subject to the standard protocols with good to moderate conversions and selectivities. This discovery suggests that the judicious choice of organic dyes the sensitization of titanium dioxide can be directly used to effectuate selective aerobic oxidation under visible-light irradiation.In summary, visible-light photocatalysis of dye-sensitized titanium dioxide has been successfully carried out the selective formation of aldehydes or ketones from alcohols with dioxygen or air as the terminal oxidant with water as the side product. High efficiency and high selectivities of the products had been achieved, suggesting they are amongst the best heterogeneous photocatalytic systems for organic transformations.