Activation of Molecular Oxygen and Selective Oxidation with Metal Complexes

Selective oxidation with molecular oxygen is one of the most appealing approaches to functionalization of organic molecules and, yet at the same time, one of the most challenging reactions facing organic synthesis due to poor selectivity control. Molecular oxygen is a green and inexpensive oxidant, producing water as the only byproduct in oxidation. Not surprisingly, it has been used in the manufacturing of many commodity chemicals in the industry. It is also nature’s choice of oxidant and drives a variety of oxidation reactions critical to life and various other biologic processes. While the past decades have witnessed great progress in understanding, both structurally and mechanistically, how nature exploits metalloenzymes, i.e., monooxygenases and dioxygenases, to tackle some of the most challenging oxidation reactions, e.g., methane oxidation to methanol, there are only a small number of well-defined, man-made metal complexes that have been reported to enable selective oxidation with molecular oxygen of compounds more relevant to fine chemical and pharmaceutical synthesis.

In the past 10 years or so, our laboratories have developed several transition metal complexes and shown that they are capable of catalyzing selective oxidation under 1 atm of O₂. Thus, we have shown that an Fe(II)-bisimidazolidinyl-pyridine complex catalyzes selective oxygenation of C–H bonds in ethers with concomitant release of hydrogen gas instead of water, and when the iron center is replaced with Fe(III), selective oxidative cleavage of C═C bonds of olefins becomes feasible. To address the low activity of the iron complex in oxidizing less active olefins, we have developed a Mn(II)-bipyridine complex, which catalyzes oxidative cleavage of C═C bonds in aliphatic olefins, C–C bonds in diols, and carboxyl units in carboxylic acids under visible light irradiation. Light is necessary in the oxidation to cleave an off-cycle, inactive manganese dimer into a catalytically active Mn═O oxo species. Furthermore, we have found that a binuclear salicylate-bridged Cu(II) complex enables the C–H oxidation of tetrahydroisoquinolines as well as C═C bond cleavage, and when a catalytic vitamin B1 analogue is brought in, oxygenation of tetrahydroisoquinolines to lactams takes place via carbene catalysis. Still further, we have found that a readily accessible binuclear Rh(II)-terpyridine complex catalyzes the oxidation of alcohols, and being water-soluble, the catalyst can be easily separated and reused multiple times. In addition, we recently unearthed a simple protocol that allows waste polystyrene to be depolymerized to isolable, valuable chemicals. A cheap Brønsted acid acts as the catalyst, activating molecular oxygen to a singlet state through complexation with the polymer under light irradiation, thereby depolymerizing the polymer.