pH-Dependent Selective Transfer Hydrogenation of α,β-Unsaturated Carbonyls in Aqueous Media Utilizing Half-Sandwich Ruthenium(II) Complexes

Abstract

Half-sandwich ruthenium(II) PTA complexes bearing the 1,2-dihydropentalenyl (C8H9 -, Dp) and indenyl (C9H7 -, Ind) ancillary ligands have been synthesized and characterized using multinuclear NMR spectroscopy and X-ray crystallography. The complexes DpRu(PTA)(PPh3)Cl, DpRu(PTA)2Cl, IndRu(PTA)(PPh3)Cl, and [IndRu(PTA)2(PPh3)]Cl were obtained in good to excellent yields. The solid-state structures of these compounds exhibit piano stool geometries with η5-coordination of the indenyl and dihydropentalenyl moieties. DpRu(PTA)2Cl is water-soluble (S 25 ° C = 43 mg/mL), while the mixed phosphine compounds are slightly soluble in acidic solutions. The Ru−H complexes, Cp‘Ru(PTA)(PPh3)H (Cp‘ = Ind, Cp), have been synthesized in good yield and spectroscopically and structurally characterized. The ruthenium hydrides undergo an H/D exchange reaction with CD3OD with relative rates CpRu(PTA)(PPh3)H ≫ IndRu(PTA)(PPh3)H > CpRu(PTA)2H. The air-stable Cp‘Ru(PTA)(PR3)Cl complexes (Cp‘ = Cp, Dp, Ind; PR3 = PPh3 or PTA) exhibit activity in the regioselective transfer hydrogenation of α,β-unsaturated carbonyls in aqueous media with HCOONa, HCOOH, or isopropanol/Na2CO3 serving as the hydrogen source. They were found to be effective in the selective reduction of the carbonyl functionality of cinnamaldehyde and the CC bond of benzylidene acetone and chalcone. IndRu(PTA)(PPh3)Cl was less active than Cp‘Ru(PTA)(PPh3)Cl (Cp‘ = Cp or Dp). Results of the transfer hydrogenation of unsaturated substrates using CpRu(PTA)(PPh3)H are also reported.

Mass spectrometry and UV‐VIS spectrophotometry of ruthenium(II) [RuClCp(mPTA)2](OSO2CF3)2 complex in solution

“…Therefore, the effect of chloride was studied in detail and it was found that by greatly increasing the chloride concentration the solution of I can be stabilized. 30 This also supports our explanation for the stabilization of I in isotonic NaCl solution. The stabilization of I might be important for the use of the compound in medical practice.…”

Alkyne activation by half-sandwich ruthenium(II) complexes bearing the water-soluble phosphane 1,3,5-triaza-7-phosphaadamantane (PTA)

“…Among water-soluble phosphanes, particular attention is recently being paid to the cage-like tertiary phosphane 1,3,5-triaza-7-phosphatricyclo[3.3.1.1 3,7 ]decane (PTA) (Fig. 1) [5] whose high solubility in water has been ascribed to the extensive participation of its three nitrogen atoms in hydrogen interactions with water molecules.…”

Evaluation of catalytic activity of [RuClCp(dmoPTA)(PPh3)](OSO2CF3) in the isomerization of allylic alcohols in water (dmoPTA=3,7-dimethyl-1,3,7-triaza-5-phosphabicyclo[3.3.1]nonane)

“…Large efforts have been done in the last years to develop a useful and general process to isomerise allylic alcohols in water [2] [8].…”