Yixiao Pan

Rhodium-Catalyzed Decarbonylative Direct C2-Arylation of Indoles with Aryl Carboxylic Acids

A RhI‐catalyzed direct C2‐arylation of indoles with diversely substituted aryl carboxylic acids has been developed using 2‐pyrimidyl group as an easily installable and readily removable N‐directing group. The reaction proceeded smoothly without the need for any external oxidants under relatively mild conditions to produce the C2‐arylated indoles in high yields with excellent regioselectivity. A range of functional groups in both coupling partners were tolerated regardless of their electronic properties and positions. With the assistance of the 2‐pyrimidyl group, these C2‐functionalized products could further undergo C7‐arylation to give the C7‐aryl indole products. Mechanistic evidence supports that the reaction involves a decarbonylation step, and the carboxylic acids could be activated in situ by treatment with (tBuCO)2O to generate the active anhydrides.

Palladium‐Catalyzed Direct Arylation of Allylamines with Simple Arenes

The Pd(OAc)2‐catalyzed direct CH bond olefination of unreactive arenes with allylamines in the presence of AgOAc was developed. A variety of allylamines including β‐substituted substrates underwent smooth coupling reactions with various arenes to give exclusively the terminal arylation products in high yields with excellent regioselectivities and stereoselectivities. The reaction is compatible with a range of functional groups in both coupling partners. The carbonyl group in the allylamine substrates is critical to catalysis, and the high regio‐ and stereocontrol observed is attributed to coordination between the carbonyl O and Pd atoms.

A versatile rhodium(III) catalyst for direct acyloxylation of aryl and alkenyl C–H bonds with carboxylic acids

Rh(III)-Catalyzed highly regioselective direct acyloxylation of sp2 C–H bonds with carboxylic acids has been developed. The catalytic system consisting of a cationic Rh(III) complex and a silver oxidant allowed a variety of arenes and alkenes bearing different directing groups (not limited to strongly coordinating N-heterocyclic directing groups) to undergo efficient acyloxylation with a broad range of readily available alkyl, alkenyl and aryl carboxylic acids, and a number of valuable functional groups in both coupling partners were well tolerated in the reaction regardless of their electronic properties and positions. This method provides a straightforward and convenient access to various valuable acyloxylated products. Furthermore, the synthetic utility of this protocol was demonstrated by the later-stage functionalization and modification of biologically active compounds. Mechanistic studies reveal the involvement of five-membered rhodacycles as key intermediates.

Metal-free tandem cyclization/hydrosilylation to construct tetrahydroquinoxalines

A one-pot tandem procedure involving cyclization and sequential hydrosilylation of imines and amides under the catalysis of B(C6F5)3 has been developed for the step-economical construction of 1,2,3,4-tetrahydroquinoxalines directly from readily available 1,2-diaminobenzenes, α-ketoesters and low-cost, safe polymethylhydrosiloxane (PMHS). This metal-free approach provides various products in good to excellent yields, and displays a wide range of substrate scope and a high degree of functional group tolerance even to reduction-sensitive moieties. The choice of hydrosilanes is critical to the catalysis, and PMHS has proved to be optimal. Decreasing the amount of PMHS could enable the reaction to stop at the 3,4-dihydroquinoxalin-2(1H)-one stage. The procedure is convenient and scalable, and neither a dried solvent nor an inert atmosphere is required. Moreover, the enantioselective construction of these products was explored, and promising results were achieved.

Rhodium(I)-catalysed decarbonylative direct C–H vinylation and dienylation of arenes

Rh(I)-Catalyzed decarbonylative direct C–H bond vinylation and dienylation of arenes with readily available acrylic acid and (E)-penta-2,4-dienoic acid under chelation assistance have been developed for the first time. A significant effect of the ligand on the reactivity was observed with the bidentate phosphine ligand being optimal. This protocol was efficient under oxidant-free conditions to access synthetically valuable styrenes and 1-aryl-1,3-butadienes in high yields with a broad substrate scope and good functionality tolerance.

Ruthenium(II)-Catalyzed Regioselective C-8 Hydroxylation of 1,2,3,4-Tetrahydroquinolines

Ru(II)-catalyzed chelation-assisted highly regioselective C8-hydroxylation of 1,2,3,4-tretrahydroquinolines has been developed. Various 1,2,3,4-tetrahydroquinolines underwent smooth C8-H hydroxylation with cheap and safe K2S2O8 as the oxidant and oxygen source to furnish the corresponding products in good to excellent yields with high tolerance of the functional groups. The choice of a readily installable and removable N-pyrimidyl directing group is the key to catalysis. Mechanistic studies suggest the involvement of a six-membered ruthenacycle intermediate in the catalytic cycle. The method can also be extended to the direct hydroxylation of other (hetero)arene C-H bonds.