The examination of chiral X-type ligands in Pd(II)-catalyzed enantioselective oxdiatative transformations

Abstract

Palladium catalysis has been utilized extensively in organic chemistry for the synthesis of complex molecules. Despite its abundant use and many successful applications, there remain challenging transformations, specifically with developing new chiral centers. The aim of this research was to explore the underdeveloped, weakly coordinating X-type ligands and their applicability in enantioselective reactions. The electrophilic catalyst, generated by the coordination of sulfonic or phosphoric acid ligands, was utilized to explore underfunctionalized starting materials, such as unactivated alkenes and aryl C-H bonds. Herein we report two Pd II-catalyzed enantioselective transformations: oxidative amination and 1,2-carboamination. The Wacker-type oxidative amination was accomplished with good yields and modest enantioselectivity in the synthesis of chiral indolines and a cyclic carbamate. Substantial loss in enantioselectivity was seen with ortho-substituted anilines. The 1,2-carboamination coupled a mild, directing group facilitated C-H activation on a series of aryl ureas with a subsequent chiral C-N bond formation. Electron-rich, para-substituted aryl ureas provided the highest consistent yields and enantioselectivities. Electron deficient substrates provided little reactivity and substitutions at the ortho- and meta-positions gave inconsistent results. To our knowledge these transformations mark the first enantioselective examples of Pd II-catalyzed oxidative transformations utilizing chiral sulfonic acid ligands.