Dihydroxybenzene-derived ILs as Halide-Free, Single-component Organocatalysts for CO₂ Insertion Reactions

Graphical Abstract

Magic insertion! We report the sustainable, halide-free synthesis of dihydroxybenzene based ILs which displayed an unprecedent activity as single component catalyst for the selective synthesis of cyclic organic carbonates upon CO₂ insertion reaction in terminal and internal epoxides (6 examples, up to 98 % yield and >99 % selectivity). An unusual mechanistic pathway is also proposed.

Abstract

A series of dihydroxybenzene-derived ILs was synthesised via a halide-free, eco-friendly methodology and fully characterized. Their activity as single component catalyst towards synthesis of cyclic organic carbonates (COCs) via CO₂ insertion into terminal epoxides was evaluated, observing that methyltrioctylammonium hydroquinolate, [N₁₈₈₈][HYD], was the most active catalyst in the proposed optimized conditions ([N₁₈₈₈][HYD] 10 % mol, T=120 °C, t=6 h, p⁰(CO₂)=2.0 MPa, 12 examples, conversion >99 %, yield up to 98 %). Interestingly, [N₁₈₈₈][HYD] was also an active catalyst for CO₂ insertion reactions with cyclohexene oxide (CHO), observing formation of both the COC and polycarbonate product. It is proposed that for p⁰(CO₂)≥1.0 MPa, the catalytically active species is the hemicarbonate derivative of the hydroquinolate anion, active towards epoxide ring opening via an unusual hemicarbonate-alkoxide pathway.