Imidazolium and phosphonium alkylselenites for the catalytic oxidative carbonylation of amines: Mechanistic studies

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 Se₂O₅^2- 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.