Catalytic, Kinetic, and Mechanistic Insights into the Fixation of CO₂ with Epoxides Catalyzed by Phenol‐Functionalized Phosphonium Salts

Abstract

Dedicated to Paul Kamer, a great scientist and inspiring person. A series of hydroxy-functionalized phosphonium salts were studied as bifunctional catalysts for the conversion of CO 2 with epoxides under mild and solvent-free conditions. The reaction in the presence of a phenol-based phosphonium iodide proceeded via a first order rection kinetic with respect to the substrate. Notably, in contrast to the aliphatic analogue, the phenol-based catalyst showed no product inhibition. The temperature dependence of the reaction rate was investigated, and the activation energy for the model reaction was determined from an Arrhenius-plot (E a = 39.6 kJ mol À 1). The substrate scope was also evaluated. Under the optimized reaction conditions, 20 terminal epoxides were converted at room temperature to the corresponding cyclic carbonates, which were isolated in yields up to 99 %. The reaction is easily scalable and was performed on a scale up to 50 g substrate. Moreover, this method was applied in the synthesis of the antitussive agent dropropizine starting from epichlorohydrin and phenylpiperazine. Furthermore, DFT calculations were performed to rationalize the mechanism and the high efficiency of the phenol-based phosphonium iodide catalyst. The calculation confirmed the activation of the epoxide via hydrogen bonding for the iodide salt, which facilitates the ring-opening step. Notably, the effective Gibbs energy barrier regarding this step is 97 kJ mol À 1 for the bromide and 72 kJ mol À 1 for the iodide salt, which explains the difference in activity.

Synthesis of Bifunctional Phosphonium Salts Bearing Perfluorinated Side Chains and Their Application in the Synthesis of Cyclic Carbonates from Epoxides and CO2

“…This agrees with previous studies in which catalysts with a phenolic hydroxyl group in the ortho position to the phosphonium moiety show higher activity compared to 2-hydroxyethyl-functionalized salts. [18] Excellent yields of > 90% were achieved with catalysts 8, 9 and 11 when the reaction temperature was increased to 45 °C (Table 1, entries 7-9). To further clarify the role of the hydroxyl group, we prepared an analog to catalyst 9 without this group.…”

“…The bond between the oxygen and hydrogen atom is shorter compared to the previously reported propyl-substituted analogue (0.89(5) Å), while the bond between the hydrogen and iodide counter ion is longer (2.48(5) Å). [18] Subsequently we studied the scope and limitation of catalyst 9 in the conversion of epoxides 1 with CO 2 at room temperature (Scheme 4). Butylene oxide (1a) and propylene oxide (1b) gave the corresponding carbonates 2a and 2b in isolated yields of 93% and 91% respectively.…”

“…The more challenging 1,1-disubstituted terminal epoxide 1h can be transformed to 2h in 81% isolated yield under much milder conditions (45 °C, 1 MPa, 24 h), compared to the conditions published in a previous report (80 °C, 2.5 MPa, 24 h) utilizing a non-fluorous phosphonium salt catalyst. [18] Piperazine derivative 2i is an intermediate in the synthesis of the antitussive dropropizine, [24] Conversion of 1i at 45 °C gave this intermediate in 96% yield. Glycidyl ethers 1j-1o represent a challenging class of substrates.…”

“…[17] Changing the counter anion from Br À to I À allowed the synthesis of various terminal epoxides even at 23 °C. [18] The separation and recycling of homogenous catalysts is generally challenging. In some cases it is possible to separate the catalyst from the cyclic carbonate by distillation.…”

Main-chain/side-chain type phosphine oxide-containing reactive polymers derived from the same monomer: controllable RAFT polymerisation and ring-opening polycondensation

“…As the most attractive member of the epoxide-containing polymer family, poly(glycidyl methacrylate) ( polyGMA) contains a threemembered epoxy group in each repeat unit. Various functional polymers based on GMA and polyGMA can be generated by using controlled polymerisation, [17][18][19] premodification of monomers [20][21][22] or post-polymerisation modification of polyGMA. 23,24 When required, the epoxy groups can be transformed into other substituents via nucleophilic ring-opening reactions.…”