2-Quinoxalinol Based Schiff Base Ligands in Copper(II)-Mediated C-H Activation

Abstract

As environment and energy issues become of increasing concern, so too does our awareness of the impacts of chemical industry. Numerous successful attempts toward “green” or “greener” chemistry have been made. Among these, transition metal mediated homogeneous catalysis is of great interest to us because of its broad capability and fundamental importance. In this dissertation, Cu(II) catalyzed allylic oxidation reactions are investigated. 2-quinoxalinol is introduced as the backbone to a series of new salen ligands to adjust the electronic properties of their Cu(II) complexes. The allylic oxidation of steroids is investigated using tert-butyl hydroperoxide (TBHP) as the oxidant. A variety of D5-steroidal substrates are selectively oxidized to the corresponding enones. Excellent yields are achieved (up to 99% under optimized conditions) while significantly reduced reaction time is required as compared to other current oxidation methods. In addition, simple olefin substrates are also oxidized using the same catalyst. Excellent yields are achieved (up to 99%) within a very short reaction time and with great tolerance for additional functional groups. Using the oxidation of simple alkenes as a probe, mechanistic studies are performed using Raman spectroscopy, cyclic voltammetry, and theoretical calculations. It is believed that the Cu(II) complex binds to TBHP to form the tert-butyl peroxo- Cu(II) complex that undergoes a homolytic cleavage of the O-O bond of the peroxo-. The resulting species will carry out the oxidations of substrates. Besides salen-type ligands, a tridentate Schiff base ligand with 2-quinoxalinol as backbone is also employed to the oxidation reaction. UV-Vis spectroscopy and cyclic voltammetry are employed to study the electronic properties of such tridentate ligand bound Cu(II) complex. It has been found that the tridentate ligand bound Cu(II) complex exhibited different electronic properties from the tetradentate salen-type ligand bound Cu(II) complex. Theoretical calculation provides two different possible reaction pathways leading to the same product exist during the reaction when the tridentate ligand bound Cu(II) complex is used as catalyst. One of the pathways involves the homolytic cleavage of the O-O bond of the peroxo Cu(II) complex, while, the other pathway undergoes a direct H atom abstraction. The experimental results suggest the concurrence of two different reaction pathways.