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 Pd0 catalyst-allowed better and more reproducible results to be obtained relative to the results achieved by using [Pd2(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]. [ABSTRACT FROM AUTHOR]