Complementary Base Lowers the Barrier in SuFEx Click Chemistry for Primary Amine Nucleophiles

The sulfur(VI) fluoride exchange (SuFEx) reaction is an emerging scheme for connecting molecular building blocks. Due to its broad functional group tolerance and rather stable resulting linkage, it is seeing rapid adoption in various fields of chemistry. Still, to date the reaction mechanism is poorly understood, which hampers further development. Here, we show that the mechanism of the SuFEx reaction for the prototypical example of methanesulfonyl fluoride reacting with methylamine can be understood as an S_N2-type reaction. By analyzing the reaction path with the help of density functional theory in vacuo and under consideration of solvent and co-reactant influence, we identify the often used complementary base as a crucial ingredient to lower the reaction barrier significantly by increasing the nucleophilicity of the primary amine. With the help of energy decomposition analysis at the transition state structures, we quantify the underlying stereoelectronic effects and propose new avenues for experimental exploration of the potential of SuFEx chemistry.