Direct oxidative carboxylation of terminal olefins to cyclic carbonates by tungstate assisted-tandem catalysis

Abstract

Tungstate catalysts are well established for olefin epoxidation reactions, while their catalytic activity for CO₂ insertion in epoxides is a more recent discovery. This dual reactivity of tungstate prompted the present development of a catalytic tandem process for the direct conversion of olefins into the corresponding cyclic organic carbonates (COCs). Each of the two steps was studied in the presence of the ammonium tungstate ionic liquid catalyst – [N_8,8,8,1]₂[WO₄] – obtained via a benign procedure starting from ammonium methylcarbonate ionic liquids. The catalytic epoxidation first step was optimised on 1-decene as model substrate, using H₂O₂ as benign oxidant, [N_8,8,8,1]₂[WO₄] as catalyst and phosphoric acid as promoter affording quantitative conversion with 92% selectivity towards decene oxide. Unfortunately, the addition of CO₂ from the start (auto-tandem catalysis) gave low yields of decene carbonate (<10%). On the contrary, the addition of 1 atm CO₂ and tetrabutyl ammonium iodide after completion of the epoxidation first step without any intermediate work-up (assisted-tandem catalysis) afforded a 94% yield in decene carbonate. The protocol could be scaled up to a 10 gram scale. The scope of the reaction was demonstrated for primary aliphatic olefins with different alkyl chain lengths (C₆–C₁₆), while cyclic and aromatic activated olefins such as cyclohexene and styrene suffered from the formation of undesired overoxidation products in the first step.