π‐Lewis‐Base‐Catalyzed Asymmetric Vinylogous Umpolung Reactions of Cyclopentadienones and Tropone
Xing‐Xing Yang, Ru‐Jie Yan, Guang‐Yao Ran, Chen Chen, Jing‐Fei Yue, Xiao Yan, Qin Ouyang, Wei Du, Ying‐Chun Chen
Abstract
Abstract We disclose that the carbonates of 4‐hydroxy‐2‐cyclopentenones can form π‐allylpalladium‐based 1,2‐carbodipoles, which isomerize to interesting η 2 ‐Pd 0 ‐cyclopentadienone complexes. Compared with the labile parent cyclopentadienone, the HOMO energy of the related η 2 ‐complex was significantly raised via the back‐bonding of Pd 0 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 C 1 ‐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 Pd 0 complex.