Activation of the Vinylic C−Cl Bond by Complexation of Fe(CO)₃: Palladium-Catalyzed Coupling Reactions of (η⁴-Chlorodiene)tricarbonyliron Complexes^†

The reactions of 1-chloro- or 2-chloro-substituted (η4-1,3-cyclohexadiene)Fe(CO)3 complexes with terminal alkynes under the catalysis of PdCl2(PPh3)2/CuI at 45−60 °C led to the formation of the corresponding Sonogashira coupling products in 64−97% yields. The complexes also underwent smoothly the intermolecular Heck reactions with activated alkenes under the catalysis of Pd(OAc)2/PPh3 at 60 °C to afford the expected coupling products in 39−83% yields. On the other hand, the corresponding uncomplexed molecule 2-chloro-1,3-cyclohexadiene remained inert under the same reaction conditions. These results clearly demonstrated the strong activation effect of Fe(CO)3 on the oxidative cleavage of the vinylic C−Cl bond.