The catalytic activity of ruthenium( IV) ([Ru(h 3 :h 3-C 10H 16)Cl 2L]; C 10H 16=2,7-dimethylocta-2,6-diene-1,8-diyl, L=pyrazole, 3-methylpyrazole, 3,5-dimethylpyrazole, 3-methyl-5-phenylpyrazole, 2-(1H-pyrazol-3-yl)phenol or indazole) and ruthenium(II) complexes ([Ru(h 6-arene)Cl 2(3,5-dimethylpyrazole)]; arene=C 6H 6, p-cymene or C 6Me 6) in the redox isomerisation of allylic alcohols into carbonyl compounds in water is reported. The former show much higher catalytic activity than ruthenium(II) complexes. In particular, a variety of allylic alcohols have been quantitatively isomerised by using [Ru(h 3 :h 3-C 10H 16)Cl 2(pyrazole)] as a catalyst; the reactions proceeded faster in water than in THF, and in the absence of base. The isomerisations of monosubstituted alcohols take place apidly (10-60 min, turn-over frequency= 750-3000 h -1) and, in some cases, at 35 8C in 60 min. The nature of the aqueous species formed in water by this complex has been analysed by ESI-MS. To analyse how an aqueous medium can influence the mechanism of the bifunctional catalytic process, DFT calculations (B3LYP) including one or two explicit water molecules and using the polarisable continuum model have been carried out and provide a valuable insight into the role of water on the activity of the bifunctional catalyst. Several mechanisms have been considered and imply the formation of aqua complexes and their deprotonated species generated from [Ru(h 3 :h 3-C 10H 16)Cl 2(pyrazole)]. Different competitive pathways based on outer-sphere mechanisms, which imply hydrogen-transfer processes, have been analysed. The overall isomerisation implies two hydrogen-transfer steps from the substrate to the catalyst and subsequent transfer back to the substrate. In addition to the conventional Noyori outer-sphere mechanism, which involves the pyrazolide ligand, a new mechanism with a hydroxopyrazole complex as the active species can be at work in water. The possibility of formation of an enol, which isomerizes easily to the keto form in water, also contributes to the efficiency in water. © 2012 Wiley-VCH Verlag GmbH&Co. KGaA, Weinheim.
|Journal||Chemistry - A European Journal|
|Publication status||Published - 18 Jun 2012|
- Allylic alcohols
- Cooperating ligands
- Ruthenium water chemistry