Palladium-catalyzed Decarboxylative Coupling Of Cinnamic Acids With Aromatic Sulfinic Acid Sodium Salts

It has been over one century since the discovery of vinyl sulfone compounds. In recent years, with the progress of synthesis and analysis techniques, more and more vinyl sulfone compounds have been synthesized, and their fields of application were expanded. Due to the importance of these compounds, the study of their synthetic methods has attracted wide attention from organic chemists. A vast number of new and efficient approaches have been reported. Due to the need of developing Green Chemistry and the earlier results of our research group, we focused our study on the Pd-catalyzed reaction of benzene sulfonic acid sodium with cinnamic acid to synthesize vinyl sulfone compounds, and a possible mechanisms has been proposed. The major results we have obtained are:(1) By selecting the reaction of cinnamic acid and benzene sulfonic acid sodium salt as the model reaction, a series of optimization experiments are employed to find the optimized conditions. Several reaction parameters such as the temperature, solvent, catalyst and its amount, additives, ligand and its amount were screened. Finally the optimal conditions were established as the following:Pd(OAc)210mol%, Dppb10mol%,2equiv Ag2CO3, lequiv cinnamic acid,2equiv benzene sulfonic acid sodium, DMF as solvent, at75℃for12h in a sealed tube. Phenyl vinyl sulfone was obtained in94%yield.(2) We next set out to explore the scope and the limitation of the reaction. We found that the reaction worked very well for a wide variety of substituted cinnamic acids, affording the desired vinyl sulfones in yields ranging from56%-88%. Groups such as methyl, chloro, bromo, fluoro, methoxy as well as nitro group are well tolerated on the phenyl ring of the cinnamic acids. They can also be placed at the ortho, meta and para positions. As for the substituted aryl sulfinic acid sodium salts, we found methyl, tert-butyl, fluoro, methoxy and bromo-substituted phenyl sulfinic acid sodium salts worked satisfactorily to afford the desired vinyl sulfones in50-88%yield. It is worthwhile to point out that2-methyl cinnamic acid can also participate in the cross coupling with phenyl sulfinic acid sodium salt successfully, furnishing the desired tri-substituted vinyl sulfone in45%yield. However, when we tried to use alkyl sulfinic acid sodium salt such as sodium methanesulfinate as the coupling partner, the reaction became very messy.(3) Based on previous reports, two possible reaction mechanisms were proposed, and the possible mechanism was explored by NMR study. Finally, CO2was confirmed to be removed after the transmetallation step with benzene sulfonic acid sodium salt. In summary, a palladium-catalyzed decarboxylative coupling between the cinnamic acid and benzene sulfonic acid sodium decarboxylation is developed. This reaction was found to give the desired products in higher yields and produce fewer byproducts.