Fluoride-Catalyzed [2 + 2] Cycloadditions of Allenoates: Access to Azetidine and Cyclobutene Scaffolds

Here, we report fluoride-catalyzed [2 + 2] cycloaddition reactions of allenoates with either imines or alkynoates, affording azetidine and cyclobutene products featuring a highly (<i>E</i>)-selective exocyclic alkene. Mechanistic studies suggest that the high electrophilicity of allenoate enables 1,4-addition of a weakly nucleophilic fluoride catalyst to generate a 3-fluorobutadienolate intermediate, which subsequently undergoes vinylogous addition with the two π-electrophiles. The final step involves a facile intramolecular 1,4-addition/1,2-elimination sequence, releasing the fluoride catalyst and the four-membered cyclic product.