Phosphine-catalyzed [4 + 2] annulation and vinylogous addition reactions between 1,4-dien-3-ones and 1,1-dicyanoalkenes

Phosphine-catalyzed [4 + 2] annulation and vinylogous Michael addition reactions between 1,4-dien-3-ones and 1,1-dicyanoalkenes are presented. Under the catalysis of PBu(3) (20 mol %), 1,4-dien-3-ones like styryl ketones with 2-aryl 1,1-dicyanoalkenes as doubly activated alkenes readily undergo a formal [4 + 2] cycloaddition reaction, affording polysubstituted cyclohexanones in satisfactory yield and good diastereoselectivity; with the doubly activated alkenes bearing an acidic methyl or methylene at the 2-position, a vinylogous Michael addition of 1,4-dien-3-ones occurs under the same reaction conditions, giving a non-cyclized multifunctional adduct in good yield. These two phosphine-catalyzed transformations represent atom economical carbon-carbon bond forming reactions capable of rapid construction of molecular complexity. Based on experimental results, formation of the products has been mechanistically rationalized, and a phosphonium activation is proposed.