Palladium-Catalyzed Asymmetric Allylic Alkylation/α-Iminol Rearrangement:A Facile Access to 2-Spirocyclic-Indoline Derivatives

We report an unprecedented Pd-catalyzed asymmetric allylic alkylation of 1-(indol-2-yl)cyclobutanols followed by anα-iminol rearrangement.High yields with excellent chemo-,regio-,diastereo-,and enantioselectivities have been realized,affording a wide range of enantioenriched 2-spirocyclicindolines bearing two contiguous stereocenters.The facial selectivity of the ensuing rearrangement is controlled by the subtle difference of the substituents on the all-carbon quaternary stereogenic center formed in the allylic alkylation step.Nonactivated racemic terminal allylic alcohols are utilized as efficient electrophiles via kinetic resolution pathways for the first time in Pd-catalyzed asymmetric allylic alkylation.The role of Et3B additive is pivotal to activating allylic alcohols toward the formation of π-allylpalladium species and suppressing N/O allylic alkylation of indole with enhanced C3-nucleophilicity.Electrospray ionization high-resolution mass spectrometry(ESIHRMS)experiments provided strong evidence for the existence of the key nucleophilic boron anionic species,which fully accounts for the essential role of the Et_(3)B additive.The study of the mechanism indicates that the real catalytically active species is an electronic π-cinnamyl-palladium complex coordinated by two phosphoramidite ligands,which is consistent with the observed nonlinear effect and control experiments and is further confirmed by X-ray structure analysis.