Imidazole Based Ruthenium(IV) Complexes as Highly Efficient Bifunctional Catalysts for the Redox Isomerization of Allylic Alcohols in Aqueous Medium: Water as Cooperating Ligand

Bis-allyl ruthenium­(IV) complexes containing 1,3 azole
β-N-H protic ligands [Ru­(η3:η3-C10H16)­Cl2L] (C10H16 = 2,7-dimethylocta-2,6-diene-1,8-diyl) (L = imidazole (1a), benzimidazole (1b)), and N-methylimidazole (1c) are highly active precatalysts
in the redox isomerization of allylic alcohols into carbonyl compounds
in aqueous medium and in the absence of base. A wide series of primary
and secondary allylic alcohols can be isomerized into the corresponding
carbonyl compounds. Remarkably, complex 1b has been found
to be the most efficient catalyst reported to date for the isomerization
of 1-octen-3-ol in water leading to a turnover frequency (TOF) value
of 60 000 h–1. Moreover, catalyst 1b can be recycled remaining active up to seven cycles. Density functional
theory (DFT) calculations give evidence that the hydroxo complexes
derived from 1a–c species can be
formed in aqueous solution and that they can act as the catalytic
active species in a bifunctional catalyzed process. This study demonstrate
that in water the participation of the β-N-H protic group of
the 1,3-azole ligands in the bifunctional catalysis is not required,
provided that a water molecule can act as cooperating ligand.