Base-Induced Mechanistic Variation And Application In Palladium-Catalyzed Carbonylation

Herein, using triethylamine and sodium ethanolate as base respectively, the mechanisms and application of palladium-catalysed carbonylation of aryl iodides were investigated. We proposed a new mechanism when sodium ethanolate employed as base according to the results of apparent kinetics.To verify the proposed mechanism, we carried out a series of kinetics experiments and DFT computation. Furthermore, this new method was applied in sythesis of esters including tert-butyl ester and r-butyl thioester, which are difficult to prepare by conventional method.Firstly, carbonylation of aryl iodides was achieved usingπ-acid phosphine-olefin ligand and Pd(MeCN)2Cl2 as catalyst while triethylamine used as base. It was proved that alcoholysis was the rate-limiting step by the data of kinetics experiments as carbonylation of ethyl 4-iodobenzoate was employed as model reaction. Howerver, lower yields was obtained for electron-rich aryl iodides.Subsequently, carbonylation of 1-iodo-4-methoxybenzene was carried out usingπ-acid phosphine-olefin ligand and PdCl2 (MeCN)2 or PdCl2(PPh3)2 as catalysts in the presence of triethylamine and sodium ethanolate in ethanol independently. Only 24% GC yield of product was obtained after 6 hours and no byproduct was detected in triethylamine system, while 83% yield of product was detected and a small amount of anisole was detected by GC in sodium ethanolate system. Inspired by the data above, we speculated that different base induced different mechanisms for the carbonylation of aryl iodides.In order to study the generality of the mechanism, common catalyst Pd(PPh3)2Cl2 was used as catalyst and detailed mechanism was examined in the presence of sodium ethanolate when the carbonylation of 1-iodo-4-methoxybenzene as a model reaction. We presented a new mechanism and afforded probability by palladium complex experiments. Kinetics data were detected by in situ IR and rate-determining step was figured out.Encouraged by the mechanism study and understanding of sodium ethanolate induced carbonylation, we attempted to achieve various carbonylation in the presence of different sodium alkoxides. Ethoxylcarbonylation was carried out smoothly at room temperature and balloon pressure in the presence of sodium ethanolate. Iso-propyl esters and tert-butyl ester were prepared under 10 bar CO with more than 60% yield. Moreover, we are pleased to note that the thiocarbonylation with varied NaSR and ArI could smoothly take place under high-pressure CO. Best of all, this is the first example in the literature for direct synthesis of sterically hindered thioesters, such as ArCOS'Bu.