The importance of imidazolium substituents in the use of imidazolium-based room-temperature ionic liquids as solvents for palladium-catalyzed telomerization of butadiene with methanol

Room-temperature ionic liquids are used as cosolvents in the palladium-catalyzed telomerization of butadiene with methanol. The basic catalyst is palladium(II) acetate with either triphenylphosphine or sodium diphenylphosphinobenzene-3-sulfonate (TPPMS), which was reacted with butadiene and methanol at 85°C. It was found that the addition of several equivalents of 1,3-dialkylimidazolium salts per palladium leads to complete deactivation of the catalyst. It was suspected that the deactivation of the catalyst was due to the formation of highly stable palladium imidazolylidene complexes, but no direct evidence of this species was obtained. On the basis of this hypothesis, the use of pyridinium and 1,2,3-trialkylimidazolium salts was studied in an attempt to limit the formation of such species. It was found, particularly for the 1,2,3-trialkylimidazolium salts, that highly active and selective systems for the telomerization of butadiene with methanol were obtained. Furthermore, a biphasic catalytic system composed of palladium(II) acetate, TPPMS, heptane, and 1-butyl-2,3-dimethylimidazolium bis(trifluoromethyl)sulfonyl)imidate, [BMMI][TF₂N], was used. In this case, after the telomerization reaction, the methoxyoctadiene products were recovered by simple decantation and the ionic liquid phase reintroduced to the reactor. In this way the catalyst was recycled four times (to total turnover number of 10 000) with little loss of activity.