Flipping the Metalation of 4-Dimethylaminopyridine: Steric Repulsion versus London Dispersion Attraction.

Abstract

Non-covalent interactions, including the coordination of an organolithium reagent by a directing group and the steric hindrance from substituents, play a crucial role in determining the selectivity of metalation reactions. Here, we demonstrate the effective utilization of steric interactions for flipping the lithiation of 4-dimethylaminopyridine (DMAP). Introduction of a Me3Si substituent to the position 1 of DMAP or simple complexation with t-BuLi allows selective C3-lithiation, due to the steric hindrance of a C2-H bond by the bulky moiety at the pyridine nitrogen. This simple approach creates a convenient way to achieve the selective C3-functionalization of DMAP. In contrast, the utilization of an even bulkier i-Pr3Si substituent leads to exclusive C2-functionalization due to the dispersion interactions with organometallic bases. For the first time, it is demonstrated that the i-Pr3Si moiety can serve as a directing group, providing a new type of directed ortho-metalation effect.