π-Lewis-Base-Catalyzed Asymmetric Vinylogous Umpolung Reactions of Cyclopentadienones and Tropone

We disclose that the carbonates of 4-hydroxy-2-cyclopentenones can form π-allylpalladium-based 1,2-carbodipoles, which isomerize to interesting η²-Pd⁰-cyclopentadienone complexes. Compared with the labile parent cyclopentadienone, the HOMO energy of the related η²-complex was significantly raised via the back-bonding of Pd⁰ as a π-Lewis base, rendering the uncoordinated C=C bond an electron-richer dienophile in inverse-electron-demand aza-Diels–Alder-type reactions with diverse 1-azadienes. The vinylogous (aza)Morita–Baylis–Hillman or cross Rauhut–Currier addition to (imine)carbonyls or activated alkenes, respectively, was also realized to afford chiral [4+2] or [2+2] cycloadducts, respectively, after trapping the re-generated π-allylpalladium species. New C₁-symmetric ligands from simple chiral sources were developed, exhibiting high stereoselectivity even with racemic substrates via an unusual dynamic kinetic resolution process. Besides, tropone could be similarly activated by a Pd⁰ complex.