Imidazolium and Phosphonium Alkylselenites for the Catalytic Oxidative Carbonylation of Amines: Mechanistic Studies

Abstract

The reactions of KSeO2(OCH3) (1) in methanol with [Rmim]Cl (Rmim = 1-alkyl-3-methylimidazolium) at room temperature give imidazolium-based ionic liquids containing methylselenite anion, [Rmim][SeO2(OCH3)] (2a, R = n-C4H9; 3a, R = C2H5; 4a, R = CH3), respectively. Similarly, phosphonium methylselenites, [R4P][SeO2(OCH3)] (6a, R = n-butyl; 7a, R = ethyl) were prepared by reacting 1 with tetraalkylphosphonium bromides. The methoxy groups in imidazolium and phosphonium methylselenites were easily replaced by other alkoxy groups upon interaction with various alcohols. [1,3-dimethylimidazolium][SeO2(OCH3)] (4a) is slowly transformed into [1,3-dimethylimidazolium]2[Se2O5] (5a) in the absence of methanol. An X-ray study reveals that 5a is a dimeric selenium complex consisting of two 1,3-dimethylimidazolium cations and Se2O52- anion. All the ionic liquid compounds containing methylselenite anion show surprisingly high activity for the oxidative carbonylation of aniline, even at temperatures as low as 40 °C, to give diphenylurea in high yield. The effects of molar ratio of aniline to catalyst, temperature, and solvent have been investigated. The plausible mechanism for the carbonylation reaction of aniline based on the 1H and 13C NMR studies using 3a is presented.