## 2 Experimental Section
### General Procedure for Functionalization of Magnetic Nanoparticles with Amine Groups  
The magnetite nanoparticles were prepared following the procedure described in section 2.3 and the same amounts of reagents were used. The prepared magnetite was dispersed in 500 mL ethanol (95%) and sonicated for 60 min. After sonication, the suspension was mechanically stirred and 46.2 mmol of 1-(2-(dimethylamino)ethyl)-3-(3-(trimethyl)propyl)urea or _N_-[3-(trimethyl)propyl]--ethylenediamine dissolved in 100 mL ethanol (95%) were added. The mixture was stirred under N\({}_{2}\) for 36 h. The magnetite nanoparticles were separated from the mixture by application of an external magnetic field, and they were washed twice with ethanol (95%) followed by washing three times with methanol. The washed magnetite nanoparticles were dispersed again in 100 mL methanol and the resulted suspension was sonicated for 30 min. The concentration of the magnetite nanoparticles functionalized with amine groups was 50-54 mg/mL. Before any use, the suspension was sonicated for 10 min.  
General Procedure for Preparing Magnetically Separable Silica Sol-Gel Matrices Functionalized with Amine Groups  
Two hundred milligrams of magnetic nanoparticles modified with 1-butyl-3-(3-trimethoxysilypropyl)-1_H_-imidazol-3-ium chloride were dispersed in 5 mL methanol. This suspension was added to tetraethoxysilane (6 mL, 27.1 mmol) and distilled water (3.2 mL, 177.8 mmol). Then, 0.05 mL of HCl (0.1 N) were added and the mixture was stirred at room temperature for 2 h. After that, 4.6 mmol of 1-(2-(dimethylamino)ethyl)-3-(3-(trimethoxysily)propyl)urea or _N_-[3-(trimethoxysily)propyl]-ethylenediamine dissolved in 2 mL ethanol were added to the mixture. A gellation of the mixture occurred after stirring for 4 h and the wet gel was aged at room temperature for 16 h. The resulting gel was dried under a vacuum of 0.2 mmHg at 60 \({}^{\circ}\)C for 24 h. Typically, 2.4-2.7 g of black xerogel were obtained as a final product.  
General Procedure for the Nitroaldol Reaction Catalyzed by Magnetic Nanoparticles Modified with Amine Groups  
Magnetite nanoparticles containing 0.19 mmol of the desired amine group were separated magnetically from the methanol and re-dispersed directly in 5 mL of a proper solvent. Then, 1.64 mmol of the desired aldehyde and nitroethane (0.17 mL, 2.39 mmol) were added. The resulting mixture was heated at 80 \({}^{\circ}\)C for 16 h. After cooling the mixture to room temperature, the catalyst was separated by applying an external magnetic field, and then the solution was decanted and dried over MgSO\({}_{4}\). The solvent was evaporated and the resulting products were analyzed by spectral methods. When Dimethyl sulfoxide (DMSO) was used as a solvent for the catalytic reaction, it was mixed with 10 mL of water after separation of the catalyst and the products were extracted three times with 20 mLof ether. The combined solutions of ether were washed three times with 15 mL water and then dried over MgSO\({}_{4}\). The ether was evaporated and the products were analyzed by spectral methods.  
General Procedure for the Nitroaldol Reaction Catalyzed by Magnetically Separable Silica Xerogel Modified with Amine Groups  
One hundred milligrams of silica xerogels containing 0.19 mmol of the desired amine groups were dispersed in 5 mL of a proper solvent. 1.64 mmol of the desired aldehyde and mitroethane (0.17 mL, 2.39 mmol) were added to the mixture. The resulting mixture was heated at 80 \({}^{\circ}\)C for 16 h and then cooled to room temperature. After magnetic separation of the catalyst, the solution was decanted and dried over MgSO\({}_{4}\). The solution was concentrated and the resulting products were analyzed by spectral methods. When DMSO was used as a solvent, the products were isolated from the solvent in the same manner as it is described in Section 2.5.