Development of Organocatalysts for the Synthesis of Heterocyclic Compounds through CO2 Insertion Reactions

Abstract

A series of novel Ionic Liquids (ILs) consisting of tetraalkyl-phosphonium or ammonium cations and phenol or polyhydroxybenzene counteranions were synthesized following a metal- and halogen-free methodology previously reported by our group. The IL synthetic methodology included two steps: an initial trialkylamine or trialkylphosphine quaternization step using dimethylcarbonate (DMC) as the methylating agent, obtaining the corresponding trialkylmethyl-phosphonium or ammonium methylcarbonate ILs. Methylcarbonate ILs underwent metathesis reaction in presence of acidic counteranions precursors (e.g. phenol, catechol, resorcinol, hydroquinone): this latter reaction is irreversible due to the formation of methylcarbonic acid as by-product, which is unstable in standard conditions and decomposes to CO2 and MeOH. All the synthesized ILs were characterized by NMR. These ILs were further evaluated as novel organocatalysts for CO2 insertions reactions with terminal epoxides, e. g. styrene oxide (SO). All the tested ILs displayed catalytic activity towards epoxide activation/CO2 insertion in relatively mild conditions (T = 80 °C, p(CO2) = 1 - 20 bar of, t = 18h) with conversions up to 76 % and selectivities up to 99 % for Trioctylmethylammonium-Hydroquinolate (TOMA-Hyd.) ILs (10 % mol.). TOMA-Hyd, TPMP-Hyd. (Triphenylmethylphosphonium-Hydroquinolate) and TBMP-Hyd. (Tributylmethylphosphonium-Hydroquinolate) were also tested as catalyst for CO2 insertion with a propargyl alcohol to obtain cyclic α-alkylidene carbonates using the same experimental condition as epoxide insertion (T = 80 °C, p(CO2) = 20 bar; conversion: 86%, selectivity: 98%). Additionally, it has been previously demonstrated by our group that phosphonium ILs can act as latent phosphorous ylide precursors, therefore were tested in Wittig vinylation conditions with benzaldehyde. The results obtained confirm that the phosphonium cation has an ylide behavior and that it is therefore capable of transferring both the methyl group and the other alkyl groups, that possess acidic α-hydrogens, on benzaldehyde by this reactivity.