Palladium-catalyzed oxidation of unsaturated hydrocarbons using molecular oxygen.

Oxidation reactions are central components of organic chemistry, and modern organic synthesis increasingly requires selective and mild oxidation methods. Although researchers have developed new organic oxidation methods in recent years, the chemistry community faces continuing challenges to use "green" reagents and maximize atom economy. Undoubtedly, with its low cost and lack of environmentally hazardous byproducts, molecular oxygen (O(2)) is an ideal oxidant. However, relatively limited methodologies are available that use O(2) efficiently in selective organic transformations. Recently, the use of metal catalysts coupled with the reduction of O(2) has become an attractive approach for aerobic oxidation. In particular, Pd complexes have shown great potential for the development of versatile aerobic reactions because of their ability to directly couple O(2) reduction. As a result, these complexes have attracted tremendous research attention and afford new opportunities for selective oxidation chemistry. In this Account we highlight some of our progress toward the synthetic goal to functionalize the unsaturated hydrocarbons largely through the appropriate choice of Pd catalysts and O(2). We have focused on developing simple and efficient methods to construct new carbon-carbon and carbon-heteroatom bonds with O(2) as the oxidant and/or reactant. We have demonstrated Pd-catalyzed oxidation of carbon-carbon double bonds, Pd-catalyzed oxidation of carbon-carbon triple bonds, and Pd-catalyzed oxidative cross-coupling reactions of alkenes and/or alkynes with high selectivity. O(2) plays a critical role in the success of these transformations. Most of the reactions can tolerate a range of functional groups, and some can occur under aqueous conditions. Depending on the specific process, we propose several mechanistic scenarios that describe the in situ generation of different intermediates and discuss the plausible reaction pathways. These methods provide new strategies for the green synthesis of diverse 1,2-diols, carbonyls, lactones, conjugated dienes, trienes, and aromatic rings. These products have potential applications in natural product synthesis, materials science, and bioorganic chemistry. Given our new mechanistic understanding, we are optimistic that additional Pd-catalyzed aerobic oxidative transformations will be developed that are both more economical and environmentally friendly.