Synthesis of bicyclo[4.1.0]heptenes via palladium-catalyzed intramolecular coupling-cyclization of 3-(cyclohexa-2,4-dienyl)pentane-2,4-dione with β-styryl bromides

Palladium-catalyzed intramolecular coupling-cyclization of 3-(cyclohexa-2,4-dienyl)pentane-2,4-dione with β-styryl bromides proceeds in regio- and stereoselective fashions to give 2-styryl-substituted bicyclo[4.1.0]hept-3-enes in good yields. The formation of the bicyclo[4.1.0]hept-3-enes was rationalized by starting with oxidative addition of Pd(PPh₃)₄ to the carbon-bromine bond of β-styryl bromide (RBr) to give Pd(R)BrLn. Reaction of Pd(R)BrLn with Ag ₂CO₃ generates a cationic RPd(II)Ln species. Chelation of the RPd(II)Ln cation to both the O-enolate of the ketone and the proximal double bond followed by attack of the enolate on the activated double bond gave a bicyclic η¹-allylpalladium intermediate. The postulated bicyclic η¹-allylpalladium intermediate leads to 2-styryl-substituted bicyclo[4.1.0]hept-3-enes with exclusive regio- and stereoselectivities after reductive elimination.