Tips: All reactions were carried out with 10 mol% catalyst loading for 2.0 h. [a] 4.0 h at 50 °C ; [b] 70 °C; [c] [BDMAEEH][MeOAc] with 10 wt.% H₂O.

Based on the results of our study, a plausible mechanism is proposed inFigure 2 ( Taking [BDMAEEH][MeOAc] as the catalyst) . Firstly, [BDMAEEH][MeOAc] (1 ) has the free tertiary amine group to interact with H₂S to release nucleophilic SH⁻ (2 ). Secondly, the hydrogen proton of the PILs will form strong hydrogen bonding with the oxygen atom of styrene oxide after the addition of substrate to enlarge the C-O bond of styrene oxide (1a ), which will be conducive to the ring-opening attack of the nucleophile sulfhydryl group (1a ). Due to the basic condition is provided by (1 ), the ring-opening reaction is mainly affected by steric hindrance ^38-40. That is to say, the secondary carbon with less substitution (#1 ) is vulnerable by the sulfhydryl group. Finally, the proton in (2 ) can transfer from the nitrogen atom and form the target product (1b ) or (1c ) in the formation of equilibrium at a low energy barrier ⁴¹. Water extraction was adopted to separate the product from the system (Figure S13 ). Within expectation, the target product was concentrated in the lower phase. NMR analysis of the lower phase shows that almost all catalysts can be separated from the system (Figure S14 ), which provides a good way for the recycling of PILs catalyst with a green method (more details in “Regeneration of PILs ”).

Figure 2 . The plausible mechanism for the in-situ conversion of H₂S mediated in [BDMAEEH][MeOAc].

To figure out the reaction mechanism of the H₂S addition reaction, DFT-based theoretical calculations were further carried out to analyze the bond length and transition state of the structures (more details please see supporting information). All calculations were performed using the Gaussian 09 program. The B3LYP/6-31g(d) method has been used for structure optimizations and subsequent frequency calculations. As shown inFigure 3 , H₂S was firstly activated by [BDMAEEH][MeOAc] to generate complex Rea (0.0 kcal/mol). After the addition of styrene oxide, the binding energy is -3.8 kcal/mol. Notably, there are two different active sites of the epoxy group to be attacked by nucleophilic SH⁻. According to calculation results, the energy barriers of TS-1band TS-1c are 14.7 kcal/mol and 17.2 kcal/mol, respectively, demonstrating that 1b is a more favorable transition state to generate the product, which is in good agreement with the experimental results. The enthalpy changes of the H₂S addition reaction are -22.8 kcal/mol and -26.0 kcal/mol for route 1c and route 1b , respectively, which is reasonable for the reaction to proceed readily at such a mild condition ⁴².

Figure 3 . The results of theoretical calculation (numbers in pink, and blue in the units of kcal/mol, and Å, respectively.)

With the optimized condition in hand, diverse epoxides were selected to examine the substrate scope in the presence of [BDMAEEH][MeOAc] at 50 °C for 2.0 h (Figure 4 ). It is found that good to excellent conversion was acquired in almost all cases. Interestingly, the regioselectivity is achieved when the structure of the substrate is methine of the epoxy group connected directly with a conjugated carbon (The substrates were divided into two parts, the epoxide connected directly with a conjugated carbon as part A and other ones aspart B ). For cycloaddition reaction in basic conditions, steric hindrance usually plays a key role to get the desired group connected to the methylene of the epoxy group ^40,43. The less sterically crowded and electron-poor methylene carbon of epoxides is more likely to be attacked, resulting in only one product. However, when methine of the epoxy group is directly connected to a conjugated carbon, the carbocations obtained by nucleophilic -SH attacking the two active sites are both relatively stable, so there are two corresponding products. Exceptionally, for 2-methyl-2-phenyloxirane, it is the quaternary carbon rather than methine that is directly connected to the conjugated carbon. Since the steric hindrance of the quaternary carbon position is significantly increased, it is not conducive to -SH attack, resulting in only one product. The conversion of 2,3-diphenyloxirane is 72%, which is lower than that of other substrates in Part A, probably due to the large steric hindrance on both sides of the epoxy group. For all of the systems, TLC was utilized to separate the product with EA/PE as eluent. NMR and ESI-MS data of all products are presented in supporting information (Figures S15 ~ S83 ).

Figure 4 . Investigation of epoxide substrate scope catalyzed by [BDMAEEH][MeOAc].

To explore the catalytic ability of [BDMAEEH][MeOAc], the kinetic behavior of the H₂S addition reaction with styrene oxide was investigated at 30 ºC with 10 mol% catalyst loading. The conversion dynamics of styrene oxide were found to be almost quantitatively performed within 20 min, exhibiting an ultrafast kinetic process (Figure 5 ). In addition, FT-IR was also adopted to investigate qualitatively the reaction process (Figure 6 ). The whole reaction system was monitored by the method of intermittent sampling with the H₂S addition reaction going on, and the time interval was 5 minutes. Entry 1 in Figure 6is the infrared signal of the mixture of styrene oxide and [BDMAEEH][MeOAc] (The FT-IR spectrums of pure [BDMAEEH][MeOAc], (1a ), (1b ), and (1c ) were presented inFigure S84 , respectively). It is found that the typical absorption bands at 2571 cm^-1 and 3376 cm^-1 appear after the reaction with H₂S, which can be attributed to the stretching vibration of -OH and -SH, respectively.³ Moreover, the infrared signals of -OH and -SH were enhanced with the reaction proceeding. Both the quantitative and qualitative conversion illustrate that these PILs have a highly efficient catalytic activity in the conversion of H₂S.