Tellurorhodamine Photocatalysts in Aerobic Oxidation Reactions: Development of Synthetic Methods and Reaction Mechanisms

Abstract

Aerobic oxidation photocatalysis involves the use of oxygen as an oxidant species, coupled to a light driven catalyst to achieve useful chemical transformations. Catalytic aerobic oxidation reactions can be defined as either oxidase-like, in which O2 acts as an electron donor/acceptor but is not incorporated into the reaction products, or oxygenase-like where O2 acts as an electron donor/acceptor and is incorporated into the reaction products. A multitude of inorganic catalysts have been developed that can achieve a wide scope of oxidase-like or oxygenase-like reactivity, however, it is rare for a catalyst to be capable of both. We report the use of a series of tellurium-containing rhodamine derivatives that are capable of both oxidase-like and oxygenase-like aerobic oxidative transformations. Initially developed for use as photodynamic therapy agents, tellurorhodamine dyes, like their oxygen-containing predecessors, can sensitize 1O2, a reactive oxygen species (ROS) with improved reactivity towards organic substrates. It was observed that following generation of self-sensitized 1O2, tellurorhodamine dyes react with 1O2, oxidizing Te(II) to Te(IV). We have shown that Te(IV) acts as a mild oxidizing agent that is reduced back to Te(II) upon oxidation of various substrates. In our initial studies of the tellurorhodamine catalytic cycle, we showed that one tellurorhodamine dye, substituted with a mesitylene moiety, could oxidize thiols to their corresponding disulfides under mild conditions. The proposed mechanism of thiol oxidation involves a substitution mechanism in which the first step is the hydration of Te(IV)-containing telluroxide to Te(IV) dihydroxy tellurane. This precedes iterative substitution of thiol to Te, followed by elimination of the hydroxyl ligands as water. Reductive elimination of disulfide returns Te(II). We also showed that tellurhodamine dyes can oxidize organosilanes and phosphines to their corresponding silanol and phosphine oxide derivatives in oxygenase-like mechanisms. This was the first instance that tellurorhodamines have been shown to possess oxygenase-like reactivity. Silanes and phosphines were catalytically oxidized in the presence of O2, water, light, and the tellurorhodamine photocatalyst. Various kinetic, thermodynamic, and computational experiments were conducted to elucidate the mechanisms of silane and phosphine oxidation. We initially proposed that silanes could be oxidized by the Te(IV) dihydroxy tellurane in a similar substitution mechanism to thiol oxidation. Following additional experiments however, we propose that instead silane adds directly to the telluroxide-bound oxygen atom followed by a hydride transfer from the silane to Te. Reductive elimination of silanol then returns Te(II). Phosphine oxidation proceeds through a tellurium-centered mechanism, in which a Te-P bond