Hydrosoluble ruthenium complexes for homogeneous catalytic processes: the role of water in the isomerization of allylic alcohols

The isomerization of allylic alcohols catalysed by metal complexes is an efficient procedure that allows obtaining carbonyl compounds with 100% atom economy. We have shown that the isomerization of linear allylic alcohols from propenol to octenol catalyzed by [RuCp(H2O-O)(PTA)2](CF3SO3) (1) (PTA = 1,3,5-triaza-7-phosphaadamantane) is largely dependent on the amount of water used in the reaction media, as well as on the length of the substrate. Particularly, the results demonstrated that the quantity of water needed to speed up the reaction is far more stoichiometric, indicating that water is not acting as a simple proton carrier. To determine the reasons for this behaviour, a combination of neutron scattering experiments were performed with SANDALS, together with ab initio molecular dynamics (AIMD) simulations. The results revealed that the water molecules in the first solvation sphere of 1 are involved in the expected hydrogen bonds with the nitrogen atoms of the PTA ligand and, interestingly, are also interacting with the lipophilic cyclopentediene. Analogue experiments on solutions of complex [RuCp(2-CH2=CH-CH2-OH)(PTA)2]+ (2), which is a catalytic intermediate of the isomerization of propenol, shed light on the reasons leading to the slowness of this reaction (TOF = 0.5 h-1 at 80ºC): intramolecular water bridges connecting the allylic alcohol to the PTA ligand, that stabilize the complex preventing its evolution along the catalytic cycle (Figure 1).