Palladium-catalyzed γ-selective and stereospecific allyl-aryl coupling between acyclic allylic esters and arylboronic acids

Reactions between acyclic (E)-allylic acetates and arylboronic acids in the presence of a palladium catalyst prepared from Pd(OAc)₂, phenanthroline (or bipyridine), and AgSbF₆ (1:1.2:1) proceeded with excellent γ-selectivity to afford allyl-aryl coupling products with E-configuration. The reactions of α-chiral allylic acetates took place with excellent α-to-γ chirality transfer with syn stereochemistry to give allylated arenes with a stereogenic center at the benzylic position. The reaction tolerated a broad range of functional groups in both the allylic acetates and the arylboronic acids. Furthermore, γ-arylation of cinnamyl alcohol derivatives afforded gem-diarylalkane derivatives containing an unconjugated alkenic substituent. The synthetic utility of this method was demonstrated by its utilization in an efficient synthesis of (+)-sertraline, an antidepressant agent. The observed γ-regioselectivity and E-1,3-syn stereochemistry were rationalized based on a Pd(II) mechanism involving transmetalation between a cationic mono(acyloxo)palladium(II) complex and arylboronic acid, and directed carbopalladation followed by syn-β-acyloxy elimination. The results of stoichiometric reactions of palladium complexes related to possible intermediates were fully consistent with the proposed mechanism.