Stereodefined Tandem Addition Reactions of η2-Arenes: A Versatile Route to Functionalized Cyclohexenes

A series of anisoles are complexed by pentaammineosmium(II), and the resulting 5,6- η 2 -anisole complexes are treated with activated olefins or acetals in the presence of triflic acid to form 4 H -anisolium complexes. These intermediates are capable of undergoing inter- or intramolecular nucleophilic addition reactions at C3, and 2-methoxy-1,3-cyclohexadiene complexes are formed. These complexes are readily converted into functionalized cyclohexenones, cyclohexadienes, and cyclohexenes. When BF 3 ·OEt 2 is used, it is possible to form a 4 H -anisolium complex with a pendent boron enolate, which can ultimately undergo intramolecular addition to C1 to form the corresponding [4 + 2] cycloadduct. For cases in which an activated alkyne is added to a C4-alkylated anisole, a migration of the vinyl group occurs, leading to 4-methyl-3-vinylanisoles.