Site-Selective C_(sp)~3-C_(sp)/C_(sp)~3-C_(sp)~2 Cross-Coupling Reactions Using Frustrated Lewis Pairs

摘要

The donor-acceptor ability of frustrated Lewis pairs (FLPs) has led to widespread applications in organic synthesis. Single electron transfer from a donor Lewis base to an acceptor Lewis acid can generate a frustrated radical pair (FRP) depending on the substrate and energy required (thermal or photochemical) to promote an FLP into an FRP system. Herein, we report the C_(sp)~3-C_(sp) cross-coupling reaction of aryl esters with terminal alkynes using the B(C_6F_5)_3/Mes_3P FLP. Significantly, when the l-ethynyl-4-vinylbenzene substrate was employed, the exclusive formation of C_(sp)~3-C_(sp) cross-coupled products was observed. However, when l-ethynyl-2-vinylbenzene was employed, solvent-dependent site-selective C_(sp)~3-C_(sp) or C_(sp)~3-C_(sp)~2 cross-coupling resulted. The nature of these reaction pathways and their selectivity has been investigated by extensive electron paramagnetic resonance (EPR) studies, kinetic studies, and density functional theory (DFT) calculations both to elucidate the mechanism of these coupling reactions and to explain the solvent-dependent site selectivity.