1,4-Alkylcarbonylation of 1,3-Enynes to Access Tetra-Substituted Allenyl Ketones via an NHC-Catalyzed Radical Relay

Reactions involving allenyl ion intermediates have been widely applied in the synthesis of functionalized allenes, but reactions involving allenyl radicals have been less studied and limited successful examples have been realized mainly by transition metal catalysis. We herein demonstrate the generation of allenyl radicals by N-heterocyclic carbene (NHC) organocatalysis and their applications in the three-component radical relay 1,4-alkylcarbonylation of 1,3-enynes without metal participation. This strategy could accommodate a collection of different alkyl radical precursors such as CF3I, alkyl halides, cycloketone oxime esters, and aliphatic carboxylic acid derived redox-active esters, enabling a convenient pathway to access a range of synthetically challenging tetra-substituted allenyl ketones with high regioselectivity. The key success of this protocol relied on the Csp-C(O)sp(2) radical-radical coupling of the allenyl radicals with the NHC-bound ketyl radicals, constructing the allenyl ketone motifs in a highly efficient radical reaction pathway.