The conjugate additions of various amines with alkenes under solvent-free conditions were investigated first (Table 1). The results showed that the reactions catalyzed by the novel ionic liquid went smoothly at room temperature within several min. The methylamine showed extremely high activity for all kinds of electron deficient alkenes with almost complete conversion within 5 min (entries 1, 5, 8). The novel ionic liquid with Fe³⁺ was stable to water and could be used in the methylamine aqueous solution. Also, the ionic liquid was not decomposed in the methylamine aqueous solution and could be reused without loss of activities (entries 1, 5, 8). The yields slightly dropped with the increasing carbon atomicity of the amines because of the steric hindrance (entries 1–4, 5–7, 8–10). The primary amines underwent the single substitution reaction under the reaction conditions. These results indicated the usefulness of the novel ionic liquid for the reactions and the reaction conditions are mild and not sufficient to cause double substitution reaction. The multi-substitution reactions could also be activated when more alkenes and high temperature were applied, then the double substituted products were obtained with high yields. As to the alkenes, the reactivity was affected by the electronic withdrawing groups (EWG) and the steric hindrance also had a certain effect on the reactions.

One property of the novel ionic liquid is the reusability. Thus, the recovery of the catalyst was very convenient. After reactions, the reaction mixture was extracted with ethyl acetate–ethyl ether = 1:1 and the lower phase, the ionic liquid, could be reused easily. The recovered activities were investigated through the reaction of cyclohexamine and methyl acrylate carefully (Figure 1). The yields remained unchanged even after the catalyst had been recycled five times.

It is noteworthy that aromatic amines could not be transformed to the corresponding products under the same reaction conditions (Scheme 2). This result indicated that the present protocol could be applicable to the chemoselective addition of aliphatic amines in the presence of aromatic amines.

In order to investigate the effect of Fe³⁺ on the catalytic activities, ionic liquids with different Fe³⁺ content were utilized as the catalysts for the reaction of cyclohexamine and methyl acrylate (Table 2). It can be seen in Table 2 that both the Lewis acid site Fe³⁺ and the Brønsted acid site H⁺ were very important for the reaction. The catalytic activities were low with complete Lewis acid site Fe³⁺ or the Brønsted acid site H⁺ with long reaction time and low yield. The Fe³⁺ with high charges showed higher attraction for the reactants than the H⁺. The H⁺ attached to the double bond during the catalytic cycle, which was quite useful for the reaction. The catalytic activities reached the peak value when the Fe³⁺ content was 50% with the Lewis to Brønsted acid sites molar ratio of 1:1. Further increasing the Fe³⁺ content reduced the catalytic activities. These results indicated that Fe³⁺ and H⁺ ions cooperated with each other during the catalytic cycle and generated the higher activities than the single acid sites. Here, the probable catalytic procedure was demonstrated in Scheme 3. In the first step, Fe³⁺ cooperated with carbonyl group and diols, which made the hydroxyl groups attack the carbonyl groups more easily. Then the H⁺ attached to the double bond, which resulted in the release of the Fe³⁺. Both acid sites worked during the catalytic procedure, which made the novel ionic liquid more efficient for the reactions with less catalyst amount and higher yield.

A comparative study on the catalytic activities of the novel ionic liquid with the traditional catalysts was carried out using the reaction of cyclohexamine and methyl acrylate (Table 3). From this study it can be concluded that the novel ionic liquid displayed even more activity than the traditional homogeneous catalysts; furthermore it adds the additional advantage of reusability. It clearly shows that this novel catalyst should be considered as one of the best choices for the economically convenient, user-friendly catalyst and for scaling up purposes.

The procedure for synthesizing the novel IL: pyridine (7.9 g, 0.1 mol) and 1,4-butane sulfonate (13.6 g, 0.1 mol) were mixed and stirred magnetically for 72 h at room temperature. Then, a white solid zwitterion was formed. The white solid zwitterion was filtrated and washed repeatedly with ether. After dried in vacuum (110 °C, 1.33 Pa), the white solid zwitterion was obtained in good yield (94%). A stoichiometric amount of sulfuric acid was added to the above obtained zwitterion and the mixture was stirred for 4 h at 60 °C to form the homogeneous liquid phase. Then, Fe(OH)₃ (3.53 g, 0.033 mol) and 10 mL water were added to the liquid phase and the mixture was stirred until the solid was dissolve completely. Here the Fe³⁺ was introduced through the neotration reaction of Fe(OH)₃ and HSO₄⁻. Then, the water was removed by distillation and dried in vacuum (110 °C, 1.33 Pa). The product was formed quantitatively and in high purity with the forms of reddish-brown liquid (Melting point: 20–22 °C, viscosity: 82 × 10⁻³ Pa · s). The IL owned high thermal stability with the decomposition temperature above 200 °C. 1H NMR for the zwitterion (400 MHz, D₂O, TMS): δ 1.74 (q, 2H), 2.11 (t, J_H–H = 7.2 Hz, 2H), 2.90 (t, 2H), 4.60 (t, J_H–H = 7.2 Hz, 2H), 8.01 (d, 2H), 8.49 (t, J_H–H = 7.2 Hz, 2H), 8.80 (d, 2H). IR (KBr): 1037 cm⁻¹ and 907 cm⁻¹ (–SO₃H), 1166 cm⁻¹ (C–N), 3409 cm⁻¹ (O–H). The element analysis: C: 32.6%; H: 4.2%; N: 4.2%; S: 19.4%. The results matched the structure in scheme 1 very well.

Typical procedure for the conjugate addition of amines (Scheme 4) was shown as follows: A mixture of amine (20 mmol), alkene (24 mmol) and the novel ionic liquid (0.04 g, 0.14 mmol) were stirred at room temperature for the certain time as shown in Table 1. The process of the reaction was monitored by GC analysis. The reaction mixture was extracted with ethyl acetate (2 × 20 mL) and the combined extract was dried over anhydrous Na₂SO₄ and evaporated to leave a crude product, which was separated by column chromatography using neutral alumina as stationary phase and petroleum ether/ethyl acetate mixture (95:5) as eluent to give the corresponding product.