Synthesis of a 4-Vinyltetrahydrocarbazole by Palladium-Catalyzed Asymmetric Allylic Alkylation of Indole-Containing Allylic Carbonates

Palladium-catalyzed intramolecular asymmetric allylic alkylation was used for the first time to prepare 4-vinyltetrahydrocarbazole 3. Suitable indole-containing allylic carbonates were synthesized and cyclized in the presence of a chiral catalyst formed in situ from Pd(OAc)(2) and members of the PhthalaPhos ligand library (i.e., 1,1-bi-2-naphthol-monophosphites possessing a phthalic acid diamide group). The use of a stable and readily available palladium source such as Pd(OAc)(2)?reduced in situ by the phosphite to form the Pd-0 catalyst?allowed better and more reproducible results to be obtained relative to the results achieved by using [Pd-2(dba)(3)CHCl3] (dba = dibenzylideneacetone) and [Pd(allyl)Cl](2). A ligand screening led to identification of the best ligand [(S)-L1], which, after optimization of the reaction parameters, gave product 3 with 75%ee. Remarkably, the reaction turned out to be stereodivergent: by using ligand (S)-L1, the absolute configuration of product 3 was found to depend on the geometry of the substrate's double bond [(E)-substrate(S)-3, (Z)-substrate(R)-3].