Phosphine-Catalyzed Crossed Rauhut-Currier-Type Coupling and [3 + 2]-Cycloaddition of 2-Alkylidene-3-oxindoles with Alkenes and Allenes to Access β,γ-Unsaturated Enones and Polycyclic Oxindoles.

Abstract

Dihydroazepino[1,2-a]indole diones are tricyclic N-acyl-2-alkylidene-3-oxindole enones that readily engage in tertiary phosphine-catalyzed intermolecular coupling reactions with acceptor-substituted alkenes. In these reactions, the tricyclic α-substituted enones undergo an α-alkylation with concomitant formation of a quaternary stereocenter, as well as the installation of a new double bond within the seven-membered azepane ring. The organocatalytic reaction constitutes a special case of the crossed intermolecular Rauhut-Currier reaction as the presence of the α-substituent in the enones prohibits the formation of an α,β-unsaturated product, but instead, skipped β,γ-unsaturated enones are obtained. With allene carboxylates as alternative coupling partners, the crossed Rauhut-Currier-type reaction competes with a regioselective allene to enone [3 + 2]-cycloaddition, and the product selectivity can be controlled by the choice of the phosphine organocatalyst.