Der pH-Wert: ein Selektivitätsschalter für die Metallkomplex-Katalyse in wäßrigen Lösungen – die Sulfonatophenylphosphan-Ru- Komplex-katalysierte Hydrierung ungesättigter Aldehyde

Abstract

Die Gleichgewichtsverteilung der wasserlöslichen Rutheniumhydride [HRuCl(tppms)3] und [H2Ru(tppms)4] (tppms = (3‐Sulfonatophenyl)diphenylphosphan) bei der Umsetzung mit H2 hängt vom pH‐Wert ab. Damit kann die Selektivität der Hydrierung von Zimtaldehyd durch Verändern des pH‐Wertes zugunsten der Reduktion an der C = O‐ oder an der C = C‐Gruppe eingestellt werden (siehe unten).

Homogeneous hydrogenation of aqueous hydrogen carbonate to formate under mild conditions with water soluble rhodium(I)- and ruthenium(II)-phosphine catalysts

“…14, 857-859 (2000) and it is known from our earlier studies 8 4 ], and the effect of excess TPPMS, both in the case of the ruthenium 5 and the rhodium catalyst, is consistent with the catalytic role of higher phosphine-coordinated complexes. It is important to mention that in our reactions formate was never produced in excess to the initial bicarbonate, i.e.…”

Theoretical Investigation of the Selective CC Hydrogenation of Unsaturated Aldehydes Catalyzed by [{RuCl2(mtppms)2}2] in Acidic Media

“…One of the most important characteristics of aqueous systems is that water does not behave as a mere solvent but can play an active role in the reactions. − The pH of the solution can alter the presence of the different catalytically active hydride species, − whereas water molecules can coordinate to the metal centers, as observed on many occasions, can take part in proton-transfer reactions, and can form hydrogen bonds with different parts of the complex catalysts. − These effects give rise to different mechanistic possibilities for reactions compared to the possibilities in organic solvents.…”

“…One of the most interesting hydrogenation reactions is the selective reduction of α,β-unsaturated aldehydes. − In aqueous solution, the ruthenium(II) complex of m tppms (( m -sulfonatophenyl)diphenylphosphine) was tested thoroughly by Joó and co-workers, for the hydrogenation of cinnamaldehyde (3-phenylpropenal). , The reactions were run at elevated temperatures (80 °C) and atmospheric hydrogen pressure, and selective reduction of the different double bonds was obtained, depending on the pH of the reaction medium. In acidic solutions the product of the reaction was exclusively dihydrocinnamaldehyde, formed by the saturation of the olefinic double bond, while in basic solution selective hydrogenation of the CO bond took place to yield cinnamyl alcohol.…”

“…These observations were explained by the distribution of the catalytically active hydride species that are present in the solution as a function of the pH. On the basis of potentiometric and NMR results ( 1 H and 31 P), under ambient pressure in acidic media [RuHCl( m tppms) 3 ] was detected as the major species, whereas under neutral and basic conditions the dominant ruthenium species was [RuH 2 ( m tppms) 4 ]. , Thus, [RuHCl( m tppms) 3 ] was thought to be active in the catalysis of the CC double bond and inactive in the reduction of the CO functionality, while [RuH 2 ( m tppms) 4 ] was considered to have the opposite selectivity.…”

Promoter effect of bicarbonate in hydrogenation of cinnamaldehyde catalyzed by a water-soluble Ru(II)-phosphine complex

“…At pH ≥ 8 hydrogenation yielded unsaturated alcohol while at pH ≤ 5 the exclusive product was saturated aldehydethe pH even could be used as a selectivity switch. [16,17] A detailed study later showed that depending on the pH and the hydrogen pressure in solutions of In a Schlenk-flask, NaHCO 3 (16.8 mg, 0.2 mmol), [{RuCl 2 (mtppms) 2 } 2 ] (10.0 mg, 0.005 mmol) and mtppms (32.0 mg, 0.08 mmol) were dissolved in water (7.5 mL) under an argon atmosphere followed by addition of cinnamaldehyde (50 µL, 0.4 mmol). Argon was replaced by hydrogen and the solution was stirred at 50 °C for the desired reaction time.…”