The application of semiconductors as oxidants in synthetic organic chemistry.

Abstract

Oxidation of alcohols is a vital transformation in synthetic organic chemistry as evidenced by their numerous applications in natural product synthesis. However, the traditional oxidants employ hazardous, toxic and malodorous reagents. A welcome addition to the field of alcohol oxidation is the emergence of tandem coupling reactions in which the oxidized alcohol is immediately trapped by an appropriate nucleophile. In previous research within the group, the synthesis of quinoxalines using a photocatalyzed tandem coupling approach was demonstrated. However, an extension of this research to other tandem coupling reactions was unsuccessful due to the high redox potential of the active oxidizing species. Thus, the immediate goal of this project was to develop a photocatalyzed oxidative system that was selective and high yielding. The assembling of this photooxidative system began with the choice of TiO2 and ZnO as the photocatalysts, Alizarin Red S as the dye, silver (I) ions as the electron acceptor and 2,2,6,6-tetramethylpiperidine-1-oxyl radical (TEMPO) as the active oxidizing species. A test reaction was conducted using the dye sensitized TiO2/silver/TEMPOand dye sensitized ZnO/silver/TEMPO systems. While the dye sensitized ZnO system afforded a good yield the dye sensitized TiO2 system produced only a trace amount of product. Thus, a full study was conducted on a range of alcohols using the developed dye sensitized ZnO system. In addition, the oxidation of alcohols using the dye sensitized ZnO system could also be scaled-up with notable success. The developed system was applied to a one-pot tandem Wittig reaction which unfortunately was unsuccessful. The dye sensitized ZnO/silver/TEMPO system was however successful when applied to a pseudo- tandem Wittig reaction. Subsequently, an electron paramagnetic resonance (EPR) study was conducted using the dye sensitized TiO2/silver/TEMPO and the dye sensitized ZnO/silver/TEMPO system. The results of the EPR study supported the proposed mechanism for the dye sensitized ZnO system and revealed a ‘break’ in the photooxidative chain for the dye sensitized TiO2 system which accounted for the low yields obtained.

As part of our interest in developing new photocatalyzed oxidative systems, attention was directed towards the application of impure diamondoid powder as a potential tandem coupling reagent. Using the diamondoid powder, the synthesis of highly conjugated quinoxalines was effected in moderate to excellent yields.