Copper(II)-Mediated 2-Quinoxalinol Salens in Alcohol Oxidation and C-H Activation: A Greener Approach
Weerasiri, Weerasiri Arachchige Kushan
Type of Degreedissertation
DepartmentChemistry and Biochemistry
MetadataShow full item record
Reduction of waste production in chemical processes is the idea of greener chemistry by using less toxic or benign methods and eliminating purification steps. Because of this, using a catalytic rather than stoichiometric method is much preferred. In this dissertation, 2-quinoxalinol salen (salqu) copper(II) complexes are used in selective oxidation of alcohols and in C-H functionalization. The oxidation of propargylic alcohols was investigated using tert-butyl hydroperoxide (TBHP) as the oxidant. This catalytic system produced excellent yields (up to 99 %) of α,β-acetylenic carbonyl compounds from a wide variety of substrates within a short reaction time (1 h). This catalytic protocol can also be used with propargylic alcohols that contain alkyl groups in the α-position, which can be difficult to oxidize selectively using other available oxidation methods. Excellent selectivity was achieved for the oxidation of propargylic alcohols over that of isolated hydroxy groups, triple bonds or propargylic methylene groups. The work continued using a water-soluble version of salqu copper(II) complex considering the principles of green and sustainable chemistry. First, the water-soluble salqu copper(II) complex was designed and then synthesized in multistep organic synthesis. It was found that this catalytic system is capable of performing selective oxidation of propargylic, benzylic and allylic alcohols in water effectively. This reaction constitutes of a rare example of propargylic alcohol oxidation in water, and this process is considered greener through the elimination of the use of hazardous organic solvents, toxic metals, and high catalyst loading. The oxidation occurs in good yields (up to 99 %) with excellent selectivity for propargylic, benzylic and allylic alcohols, and the reaction is thought to go through a radical pathway. It was also found that the water-soluble salqu copper(II) complex is an efficient catalyst for the selective functionalization of benzylic and allylic C-H bonds to the corresponding carbonyl compounds in the presence of oxidant TBHP. The reactions proceed with very low catalyst loading (1 mol %) in water and good yields (up to 96 %) can be achieved for a wide variety of substrates. Excellent selectivity was also achieved with this catalytic protocol for the activation of benzylic and allylic C(sp3)-H bonds. Allylic C-H activations catalyzed by 2-quinoxalinol salen copper(II) complex was also investigated under molecular oxygen to see the potential of possible replacement of TBHP with molecular oxygen. It was found that TBHP can not be substituted completely by dioxygen, but it is possible to reduce the amount of TBHP by introducing high pressures of oxygen.