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entree SiliaCat

Selective Hydrogenation of Nitroarenes Using SiliaCat Pt0

Functionalized anilines are important intermediates in various industries such as pharmaceuticals, polymers, and dyes. Simple aromatic amines are generally obtained by catalytic hydrogenation of nitroarene compounds with various heterogeneous commercial catalysts (supported nickel, copper, cobalt) including Pt/C. Yet, the selective reduction of a nitro group with H2 when other reducible groups are present in the same molecule is generally not feasible with these catalytic materials and requires the use of advanced heterogeneous catalysts. Using SiliaCat Pt0 exhibits chemoselective catalytic activity for the hydrogenation reaction of a series of substituted nitro compounds under remarkably mild conditions, namely at room temperature with 1 bar H2 in a simple hydrogen balloon, using a modest 0.5 mol % catalyst amount.

Solvent and Catalyst Concentration Effects

The best results were obtained using methanol as solvent at 0.1 M concentration with respect to substrate. Even if the use of EtOAc results in high selectivities, the reaction times are generally much longer. Complete conversion is obtained after 1 hour in hexane using 0.5 or 0.1 mol % catalyst, but the selectivity to 4-chloroaniline was generally low.
 
Solvent and Catalyst Concentration Effects
SiliaCat Pt0
(mol %)
Time
(h)
Solvent
(M)
Yield (%)
Product Aniline
1.0
0.5
MeOH (0.01 M)
92
8
0.5
0.5
MeOH (0.01 M)
87
13
0.2
1
MeOH (0.01 M)
84
13
0.1 2 MeOH (0.01 M) 90 10
0.5 4 EtOAc (0.01 M) 55 0.5
1.0 4 EtOAc (0.01 M) 75 1
1.0 1 THF (0.01 M) 45 17

 

SiliaCat Pt0 Reusability and Leaching

The reusability test of SiliaCat Pt0 was studied using 4-chloronitrobenzene as substrate under the optimal reaction conditions identified above. Reusing the catalyst in 7 consecutive cycles did not result in any loss of catalytic activity and leaching of Pt and Si (assessed by ICP-MS) was minimal. Complete substrate conversion was obtained even after the seventh cycle, with 99% selectivity. The selectivity of the reaction even improves with each subsequent cycle going from 84% in the first run up to 99% in run 7. The positive-feedback phenomenon of effective selectivity in consecutive reaction cycles is probably attributed to the silica matrix alkylation.

HydrogenationNitrobenzenReaction.gif

 

SiliaCat Pt0 Reusability and Leaching
Reusability
Yield (%)
Leaching (ppm)
Product Aniline Pt Si
1st
84
12
0.20
1.20
2nd
89
11
0.04
0.40
3rd
90
10
0.02
0.08
4th
92
8
0.17
0.10
5th 98 2 0.01 0.10
6th 99 1 0.01 0.12
7th 99 1 0.01 0.08

 

SiliaCat Pt0 vs Competitive Catalysts

Other commercially available Pt heterogeneous catalysts [Pt/C, Pt/SiO2 and Reaxa Pt(0)EnCat40] were tested in the selective reduction of 4-chloro-nitrobenzene. In comparison to other Pt(0) heterogeneous catalysts, the SiliaCat Pt0 catalyst proved to be much more reactive, with complete conversion after 0.5 h with just 0.5 mol %. Furthermore, selectivity was significantly higher with only 4% aniline formed as by-product. No secondary product was observed in solution.

SiliaCat Pt0 vs Competitive Catalysts
Catalyst
Mol %
Pt/C
Pt/SiO2
Reaxa Pt(0)EnCat40 wet
Reaxa Pt(0)EnCat40 dry
5 1 0.5 5 1 0.5 5 1 0.5 5 1 0.5
Time (h)
1
1 1
1
2 2
1
2
2
0.5
2
2
Product (%)
82
65 43
84
88 48
75
78
72
87
90
86
Aniline (%)
14
4 0
13
10 2
18
14
12
13
10
13
Exp. conditions: 2 mol substrate in 20 mL MeOH under hydrogen balloon at room temperature.

 

Substrate Scope and Selectivity

The Hydrogenation of different nitro compounds, including those nitro compounds containing different functionalities, was attempted to demonstrate the selectivity of SiliaCat Pt0 catalyst in a wide range of reactions. The material was tested under hydrogen balloon, at room temperature conditions in methanol solvent with 0.5 – 1 mol % Pt catalyst.

Note: look at our publication in Adv. Synth. Catal., 2011, 353, 1306-1316 for more examples.
Substrate Scope and Selectivity
Substrate
SiliaCat Pt0 (mol %) Solvent (M)
Time (h)
Conversion (%) Selectivity (%)
Structure #1
0.5 MeOH (0.05 M)
1
100 98 (5% pyrene)
Structure #2
0.5 MeOH (0.05 M)
1
98 100
Structure #3
0.5 MeOH (0.1 M)
1
100 100
Structure #4
0.5 / 1.0 MeOH (0.1 M)
2
100 / 100 98 / 100
Structure #5
0.5 MeOH (0.1 M)
1
100 100
Structure #6
0.5 MeOH (0.1 M)
1
100 95
Structure #7
0.5 MeOH (0.07 M)
2
100 100
1 If 0.5 mol % was used only one NH2 group was reduced. If 1 mol % was used, both nitro groups were reduced.

Substrate Structures

HydrogenationNitroAreneSubstrate1.gif
HydrogenationNitroAreneSubstrate2.gif
HydrogenationNitroAreneSubstrate3.gif
HydrogenationNitroAreneSubstrate4.gif
1
2
3
4

 

     
HydrogenationNitroAreneSubstrate1.gif
HydrogenationNitroAreneSubstrate1.gif
HydrogenationNitroAreneSubstrate1.gif
5
6
7

 

Conclusion of Selective Hydrogenation of Nitroarenes

The hydrogenation of different nitro compounds and the selective hydrogenation of different nitro compounds in the presence of different functionalities, including reducible carbonyl, amide, ester, amine and halide groups was achieved with SiliaCat Pt0 catalyst in methanol at room temperature and under 1 bar H2 pressure. Given the broad applicability of Pt-based catalysts to widely different chemical reactions, it is envisaged that these catalysts, now commercially available, will be used in numerous fields of chemical synthesis as well as in energy generation applications.

Selective Hydrogenation of Nitroarenes Typical Experimental Procedure

Conventional Experimental Conditions

Reaction - Typical reactions are performed on a 2 mmol scale. The substrate is dissolved in 20 mL of MeOH and then treated with 0.1 – 1 mol % of SiliaCat Pt0 catalyst. The mixture is degassed twice, replacing the vacuum by hydrogen each time. The reaction mixture, connected to a balloon of hydrogen, is stirred at room temperature until it shows maximum conversion.

Work-up - Once the reaction is complete (determined by TLC or GC-MS), the catalyst is filtered off and washed with EtOH or MeOH. The filtrate is concentrated to give a crude product, and the conversion to the desired product is determined by GC/MS analysis.

Reusability - To reuse the catalyst, after completion of the reaction remove the catalyst by filtration, rinse with MeOH/THF solvents and dry under vacuum.

 

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  1. SiliaCat DPP-Pd heterogeneous catalyst

    SiliaCat® Heterogeneous Catalysts DPP-Pd

    The significant costs associated with precious metal catalysts and their undesired tendency to remain in organic products has generated interest for an increase in reactivity and ways to recover and reuse these metals. SiliaCat DPP-Pd is a unique diphenylphosphine palladium (II) heterogeneous catalysts made from a leach-resistant organoceramic matrix.
  2. SiliaCat® Heterogeneous Catalysts TEMPO

    SiliaCat® Heterogeneous Catalysts TEMPO

    SiliaCat® TEMPO is a heterogeneous catalyst/reagent made from a proprietary class of organosilica-entrapped radicals suitable for the selective oxidation of delicate substrates into valued carbonyl derivatives.

  3. SiliaCat® Heterogeneous Catalysts Pd0

    SiliaCat® Heterogeneous Catalysts Pd0

    SiliaCat Pd0 is a new series of patent-protected sol–gel-entrapped Pd nanocatalysts. It is made from highly dispersed Pd nanoparticles (uniformly in the range 4.0–6.0 nm) encapsulated within an organosilica matrix.
  4. SiliaCat® Heterogeneous Catalysts Pt0

    SiliaCat® Heterogeneous Catalysts Pt0

    SiliaCat Pt0 is made of organosilica physically doped with nanostructured platinum (0), and is both stable and efficient. This catalyst was successfully prepared by a novel and simple sol-gel route.
  5. SiliaCat® complete heterogeneous catalysts kit

    SiliaCat® complete heterogeneous catalysts kit

    Containing : SiliaCat DPP - Pd, Pd0, Pt0 and TEMPO

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