Studies Of Sulfur-Directed Palladium-Catalyzed C-H Bond Activation

The transformation of C-H bonds common existed in organic molecules to the C-C bonds or C-X bonds via C-H activation, which could avoid substrate limitation, shorten the reaction steps and improve the efficiency of the synthesis, has become an important method in modern organic synthesis. In recent years, the C-H bond activation reaction has gradually been applied to the synthesis of important natural products, drugs and functional materials, which was owing to the excellent atom economy, broad substrate scope and good reaction efficiency. The introduction of directing groups, which could improve the regioselectivity and efficiency of the reaction, remains a hot topic in the metal-catalyzed C-H bond activation. Sulfoxides and sulfides are common functional groups, of which the lone pair electrons could coordinate with transition metals. In this paper, we describe the use of sulfoxide (sulfide) as directing groups in the palladium-catalyzed C-H bond activation as follows:1. The Pd-catalyzed ortho-olefination of arenes applying sulfoxides as remote directing groups.We reported the Pd(Ⅱ)-catalyzed ortho-olefination of arene C-H bonds using sulfoxides as directing groups. The C-H activation was attributed to the five or six-membered palladium intermediates, which was also efficient in the remote C-H bond olefination. The palladium intermediates in the reaction were isolated and the corresponding structures were revealed by the single crystal structure analysis, through which we demonstrated that the sulfur in the sulfoxide coordinated to the palladium catalyst to promote the subsequent C-H bond activation. In addition, the directing group could be easily transformed into other functionalities or successfully removed, which is synthetically useful.2. The synthesis of sulfur-bridged polycycles via Pd-catalyzed dehydrogenative cyclization.Sulfur-bridged heterocycles have drawn much attention due to their potential biological activity, pharmaceutical significance, and wide application in functional materials. The traditional synthesis of sulfur-bridged polycycles often met with the substrate limitation, in which the presence of halogen functionality or its analogo as was necessary. In this work, we have developed an efficient sulfoxide-directed C(sp2)-H/C(sp2)-H coupling reaction, which would promote the straightforward synthesis of five-, six-, and seven-membered sulfur-bridged polycycles. In addition, a sulfur-bridged six-membered pyrene-thienoacene compound was synthesized by this method and excellent performance of photoluminescence quantum yield was observed.3. Thioether-directed acetoxylation of C(sp2)-H bond of arenes by palladium catalysis.Thioether is one of the most important functional group, which exists in many functional compounds and natural products. We developed the Pd-catalyzed C(sp2)-H bond acetyl oxidation reaction directed by thioethers. Both benzyl sulfides and phenethyl sulfides could react with PhI(OAc)2 to afford the site-selectively acetoxylated products in good yields. A wide range of functionalities, including -CH3,-F, -Cl, -Br, -CF3, -CN, -OCF3 and -COOMe were tolerated. The directing groups could be further transformed into synthetically useful functional groups or successfully removed.