Regioselective Hydroarylation of Internal and Terminal Alkenes With N-Aryl Pyrazoles Enabled by Lanthanum Catalysts.

Catalytic C─H alkylation of pyrazoles with internal alkenes represents an ideal strategy for constructing functionalized pyrazoles. However, such a transformation has remained underexplored to date, likely due to the inherently low reactivity of internal alkenes and the lack of suitable catalysts. In this study, we describe a lanthanum-catalyzed highly regioselective C─H alkylation of N-aryl pyrazoles with internal alkenes. The key to success of this unprecedented transformation is the use of a cationic imidazolin-2-iminato lanthanum catalyst, whose large ionic radius and less steric hindrance provide an open coordination environment that facilitates the coordination of sterically demanding internal alkenes to the metal center and subsequent alkene migratory insertion. Besides internal alkenes, 1,1-disubstituted styrenes and terminal alkenes were amenable to the reaction. This protocol offers a straightforward and atom-efficient route for the synthesis of a new family of N-((ortho-alkyl)aryl) pyrazole derivatives (94 examples, up to 99% yield, >19:1 rr). The scale-up synthesis and further transformation to hedgehog enzyme inhibitor analog highlight the potential application of this method. Combined experimental and computational studies elucidated the reaction mechanism and the influence of metal ion size on the turnover-limiting alkene migratory insertion step.