N-Heterocyclic Carbene-Catalyzed Four-Component Reaction: Chemoselective Cradical-Cradical Relay Coupling Involving the Homoenolate Intermediate

β-Branched carboxylic acid derivatives are privileged structural fragments for synthesis and medicinal chemistry. An important synthetic bottleneck of these molecules is how to introduce sterically demanding side chains. Herein, we report an organocatalytic four-component relay radical coupling to access a broad spectrum of β-tertiary−γ-quaternary carboxylic acid derivatives using simple reagents. This strategy is complementary to the classical closed-shell synthetic approaches and is particularly effective in introducing a quaternary carbon substituent at β-carbon. Mechanistically, relay radical coupling involving N-heterocyclic carbene-derived homoenolates is challenging due to issues associated with site-selectivity and catalyst turnover. In this report, we demonstrate high site control using a triazolium N-heterocyclic carbene catalyst and a transient acyl trapping strategy to prepare diversified carboxylic acid derivatives in a single pot.