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.