Palladium-Catalyzed C-H Activation Induced Enantioselective Route to Fused Indolines

Significance

The synthesis of highly enantioenriched trans-fused indolines 2 is reported via Pd-catalyzed C-H activation in an unactivated methylene. The nucleophilic carbene NHC-HI proved to be the best precursor to generate the most efficient palladium-NHC catalyst for this transformation. Carrying out the reaction at lower catalyst loading (5 mol%), shorter reaction time (3 h) and higher temperature (160 ˚C) led to lower yields of products with minor effect on the enantiomeric ­excess. The reaction was successful with cyclohexyl- and cycloheptyl-substituted substrates 1 (n = 2, 3) but failed with cyclopentyl derivatives 1 (n = 1). A good range of substituents R was shown to be tolerated. The absolute configuration of the fused indolines 2 was determined by X-ray diffraction analysis. A mechanism was suggested without evidence.