Convenient and Efficient Pd-Catalyzed Regioselective Oxyfunctionalization of Terminal Olefins by Using Molecular Oxygen as Sole Reoxidant

摘要

The evolution of selective oxidation routes that use molecular oxygen (O2) is one of the ultimate goals of present-day chemical research.[1] In particular, the Pd-catalyzed Wacker oxidation and acetoxylation of terminal olefins have received much attention because of useful protocols for direct oxyfunctionalization at the C1 and C2 positions.[2] These processes, however, require cocatalysts, such as copper,[3] heteropolyacids,[4] or benzoquinone,[5] or reducing agents[6] to facilitate the reoxidation of the Pd0 species by O2 and prevent its precipitation into inactive metals. Current research has focused on the development of promising methodologies that do not rely on these cocatalysts because of practical and environmental concerns.[7] However, few examples of oxyfunctionalization systems using O2 as the sole reoxidant have appeared, and these Pd-catalyzed oxidation reactions often suffer from low activities and a limited substrate scope.[8], [9] We disclose that PdCl2 in combination with DMA as the solvent is an extremely simple and highly efficient catalytic system for the Wacker oxidation of terminal olefins. This catalytic system allows for an efficient dioxygen-coupled turnover without the need for additional cocatalysts or reducing reagents; it is tolerant toward long-chain and functionalized olefins and is also applicable to Wacker-type intramolecular cyclization. Furthermore, it facilitates acetoxylation of the terminal olefins, thus affording selectively linear allylic acetates. An oxygen atom can be selectively incorporated at the C1 or C2 position of terminal olefins by using an appropriate nucleophile, namely, H2O or AcOH (Scheme 1). The work-up procedure is also straightforward, and the solution containing the catalyst can be recycled without any loss of activity and selectivity.