Steps ahead in understanding the catalytic isomerization mechanism of linear allylic alcohols in water: dynamics, bonding analysis and crystal structure of a 2-allyl-intermediate.

Abstract

The isomerization of 1-penten-3-ol into 3-pentanone catalyzed by [RuCp(H2O-κO)(PTA)2](CF3SO3) (1CF3SO3) (PTA = 1,3,5-triaza-7-phosphaadamantane) was studied and two water soluble ruthenium catalyst reaction intermediates were characterized. The main intermediate, the complex [RuCp(exo-2-1-penten-3-ol)(PTA)2](CF3SO3)·2H2O (exo- 2CF3SO3·2H2O) was isolated and characterized by NMR in solution and by single-crystal X-ray diffraction in solid state, constituting the first example of a fully characterized complex containing a coordinated 2-allylic alcohol and the first crystal structure for a water-soluble metal complex containing a 2-allyl ligand. NMR and Eyring analysis show the crucial involvement of water molecules both in the transformation of allylic alcohol into a ketone as well as in the concomitant isomerization of the exo-coordinated substrate into the endo conformer. DFT structure and bonding analyses are used to assess the relative stabilities of the isomers and how the metal drives the electronic distribution on the substrate.