Catalytic Formal [4 + 2] Cycloadditions between Unactivated Allenes and N-Hydroxyaniline Catalyzed by AuCl3/CuCl2/O2

AuCl3-catalyzed formal [4 + 2]-cycloadditions between substituted allenes and N-hydroxyanilines are described. This reaction sequence comprises initial isomerizations of allenes to butadienes under N2 and subsequent oxidations of N-hydroxyanilines to nitrosoarenes under O2. CuCl2 (5 mol %) was added in the second step to increase the oxidation efficiency. The reactions are compatible with various 1,1-di- and 1,1,3-trisubstituted allenes and N-hydroxyaniline derivatives. Our experimental data reveal that the roles of AuCl3 are 3-fold, including catalytic oxidations of N-hydroxyaniline derivatives to nitrosoarenes, isomerizations of alkyl-substituted allenes to dienes, and final nitroso/butadiene [4 + 2] cycloadditions.