Regiodivergent Palladium-Catalyzed Alkene Difunctionalization Reactions for the Construction of Methylene Cyclobutanes and Methylene Cyclopentanes.

ABSTRACT

Regiodivergent palladium-catalyzed alkene difunctionalization reactions between diethyl malonate and 1,5-dienes bearing a triflate group at C2 are described. Use of tris(2,4-di-tert-butylphenyl)phosphite as a ligand leads to 4-exo-cyclization/functionalization to afford malonate-substituted methylene cyclobutanes. In contrast, the 1,2-bis(diphenylphosphino)benzene ligand provides methylene cyclopentanes via 5-endo-cyclization/functionalization. The five-membered ring-forming reactions occur via anti-carbopalladation of the enolate nucleophile, whereas four-membered ring-forming reactions proceed through syn-4-exo-migratory insertion of the tethered alkene, followed by C(sp3)-C(sp3) bond-forming reductive elimination from an (alkyl)Pd(II)(malonate) complex.