Scope and mechanism of asymmetric C(sp~3)-H/C(Ar)-X coupling reactions: computational and experimental study

摘要

Advances in the efficient palladium-NHC catalysed synthesis of highly enantioenriched 2,3-trans-fused and 2-alkyl indolines via asymmetric C(sp~3)-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(sp~3)-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.