Acylzirconocene chloride: Formation of carbocycles by palladium-catalyzed cascade reaction

Acylzirconocene chloride complex as an acyl group donor reacts with ω-carbonyl α,β-enones or with bis-enones to give carbocyclic compounds under 10 mol% Pd(OAc)₂-catalyzed conditions, and each reaction was accelerated by the addition of a stoichiometric amount of Me₂Zn. The formation of the carbocycles from ω-carbonyl α,β-enones was considered to be a result of a series of reactions; (i) the formation of Pd(II)-intermediate by an electron transfer from the Pd(0)-catalyst to an α,β-enone function in an initial step, (ii) an acyl group transfer from the acylzirconocene complex to the Pd(II)-intermediate (transmetalation), (iii) the reductive elimination of Pd(0)-metal, and (iv) an intramolecular addition of metal enolate to ω-carbonyl group. On the other hand, the reaction of bis-enones with acylzirconocene chloride under the identical condition afforded reductive cyclization product, bicyclo[3.3.0] octane derivatives, in which acyl group from acylzirconocene complex was not incorporated.