Fine-Bubble-Based Strategy for the Palladium-Catalyzed Hydrogenation of Nitro Groups: Measurement of Ultrafine Bubbles in Organic Solvents

Abstract

Fine bubbles of hydrogen were employed as a new reaction medium for the autoclave-free gas–liquid–solid multiphase hydrogenation of nitro groups on a multigram scale. Furthermore, ultrafine bubbles were examined by nanoparticle-tracking analysis in organic solvents.

Key words

microbubbles - ultrafine bubbles - hydrogenation - heterogeneous catalysis - green chemistry - nitro compounds

• References and Notes

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Although UFBs appear to be specific to water and aqueous solutions according to a first report,^6a ‘surface UFBs’ have been observed in solvents such as formamide, ethylammonium nitrate, and propylammonium nitrate, but not in propylene carbonate or dimethyl sulfoxide.^6b To date it is thought that bubbles generated in nonaqueous solutions are not stable and disappear rapidly:
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• 13 Hydrogenation of Nitroarenes 1 by a H₂-FB-Based Strategy; General Procedure The hydrogenation was carried out in a 100 mL vial equipped with an FB generator, without additional stirring. Nitroarene 1 (10 mmol) was dissolved in AcOEt (80 mL) and the solution was warmed to 30 °C. By using the FB generator (MA3-FS), H₂-FB were introduced into the reactor in the presence of Pd on alumina spheres (0.5% Pd, 2–4 mm, 0.3 mmol, 3 mol%) at a H₂ flow rate of 5 mL/min. Samples of the reaction mixture were removed periodically to permit monitoring of the progress of the reaction by GC analysis. When the hydrogenation reaction was complete, the AcOEt was evaporated in vacuo to afford the desired aniline 2 with good to excellent purity. GC analyses: SHIMADZU GC-2010, capillary column: GL Sciences TC-17; He = 0.80 MPa, H₂ = 0.50 MPa, air = 0.50 MPa, flow rate: 1.4 mL/min, T _inj = 250 °C, T _det = 250 °C, T _i = 100 °C, T _f = 250 °C, rate = 10 °C/min; Nitrobenzene (1a, CAS Reg. No.: 98-95-3; t _R = 5.3 min), aniline (2a, CAS Reg. No: 62-53-3; t _R = 5.5 min) (see Supporting Information).
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• 16 The reactivity in H₂O was lower than that in AcOEt despite the larger number of UFBs observed in H₂O (Table 1, entry 1 vs. 7). Because the reaction rate of the hydrogenation was proportional to the concentration of dissolved hydrogen, the reactivity decreased in H₂O, in which H₂ gas has a low solubility. In addition, the solubility of the hydrophobic substrate should also affect the reaction rate and, consequently, the reactivity in H₂O would be reduced.