Scope and mechanism of asymmetric C(sp3)–H/C(Ar)–X coupling reactions: computational and experimental study

Abstract

Advances in the efficient palladium–NHC catalysed synthesis of highly enantioenriched 2,3-trans-fused and 2-alkyl indolines via asymmetric C(sp³)–H activation of an unactivated methylene/methyl group are reported. Very high asymmetric inductions (up to 99% ee) were achieved at reaction temperatures ranging from 120 to 160 °C. Factors influencing the efficiency of the reaction (halide, pseudohalide, N-protecting group) were investigated. The reaction pathway and enantioselection were probed by detailed density functional theory (DFT) calculations (M06-L functional). The combined theoretical and experimental study shows that the Pd–NHC catalysed C(sp³)–H arylation proceeds via a concerted metalation–deprotonation (CMD) mechanism. The CMD step is shown by DFT calculations and kinetic isotope effect measurements to be selectivity-determining. A good agreement between experimental enantioselectivities and calculated differences amongst diastereomeric activation barriers is observed.