Diaryl Ketone Synthesis via Carbonylation of Diaryliodonium Salts Promoted by Palladium-Zinc System

Abstract

The palladium-catalyzed carbonylation of diaryliodonium salts has been investigated. In the presence of zinc and a catalytic amount of palladium compounds, diaryliodonium salts were easily carbonylated to give a mixture of mono and double carbonylation products namely diaryl ketones and diaryl a-diketones in good yields under mild conditions. Biaryls were also formed as a by-product.
2 [Ar-I-Ar]X + CO Zn →Pd-cat ArCOAr, ArCOCOAr+2ArI ZnX₂
Diphenyliodonium bromide was used for examination of various factors in the reaction. Catalytic activity of transition metals decreased in the order: Pd>Rh>Ni, and the activity of palladium species was in the order: Pd(OAc)>[Pd(acac)₂]>[Pd(dba)₂]>PdCl₂>[PdCl₂(PPh₃)₂]>Pd-black (Tablel). Zinc was the most effective reducing agent and other metals such as Sn and Cu were only slightly active (Table 2). Various kinds of solvents were satisfactory for the reaction, but in alcoholic solvents larger amounts of benzoates were formed (Table 3). Biphenyl appeared to be formed in preference to the carbonylation products at higher reaction temperature (Table 4). Increase of the carbon monoxide pressure brought about an increase of selectivity in the double carbonylation product (Table 5).
Diaryliodonium salts([Ar₂I]X:Ar=Ph-, 4-CH₃C₆H₄-, 4-t-C₄H₉C₆H₄-, 4-ClC₆H₄-, 4-BrC₆H₄-, X=Cl, Br, 1, BF₄)were effectively transformed to diaryl ketones (38-68%) and diaryl cx-diketones (4-20%) under the atmospheric carbon monoxide pressure with the palladium-zinc system. Effect of substituents on the reaction showed that the electron. withdrawing groups decreased the yields of the carbonylation products and the electrondonating groups increased them (Table 6).
The carbonylation is considered to proceed as follows. The initial oxidative addition of a diaryliodonium salt to palladium(0) yields the arylpalladium species. Disproportionation of the arylpalladium halide by zinc leads to the diarylpalladium species, which then reacts with carbon monoxide to form an acylarylpalladium and diacylpalladium compounds. The diaryl ketone and diaryl ex-diketone are formed by reductive elimination, respectively.