In the past few decades, processes that involve transition-metal catalysis have represented am ajor parto f the synthetic chemist'st oolbox. Recently,t he interesth as shifted from the well-established cross-coupling reactions to CÀHb ond functionalization, thus making it ac urrent frontier of transition-metal-catalyzed reactions. Constant progress in this field has led to the discoveryo fe nantioselective methods to generate and control various types of stereogenic elements, thereby demonstrating its high value to generate scalemic chiral molecules.The present review is dedicated to enantioselective Pd 0-catalyzed CÀHa ctivation,w hich may be considered as an evolution of Pd 0-catalyzed cross-couplings, with af ocus on the different chiral ligands and catalysts that enable theset ransformations.
A heterogeneous reusable palladium(II)-bis(N-heterocyclic carbene) catalyst was prepared and shown to catalyze the intramolecular C(sp 3 )À H activation/cyclization of N-alkyl-2-bromoanilines furnishing indolines. This new catalytic system was based on a bis-imidazolium ligand immobilized on a spaced crosslinked polystyrene support. The iodide ligands on the catalyst played a central role in the efficiency of the process occurring through a "release and catch" mechanism. The heterogeneous nature of the catalyst was further exploited in the design of a continuous-flow protocol that allowed a more efficient recovery and reuse of the catalyst, as well as a very fast and safe procedure.
In the past years, Pd0‐catalyzed C(sp3)−H activation provided efficient and step‐economical methods to synthesize carbo‐ and heterocycles via direct C(sp2)−C(sp3) bond formation. We report herein that a 1,4‐Pd shift allows access to N‐heterocycles which are difficult to build via a direct reaction. It is shown that o‐bromo‐N‐methylanilines undergo a 1,4‐Pd shift at the N‐methyl group, followed by intramolecular trapping by C(sp2)−H or C(sp3)−H activation at another nitrogen substituent and remote C−C bond formation to generate biologically relevant isoindolines and β‐lactams. The product selectivity is influenced by the employed ligand, with NHCs favoring the product of remote C−C coupling against products arising from direct C−C coupling and N‐demethylation.
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