Redox Isomerization of Allylic Alcohols to Carbonyl Compounds Catalyzed by Ruthenium(IV) Complexes Containing N-Heterocyclic Ligands in Ionic Liquids

Francisco      J. Suarez; Cristian      Vidal; Joaquin      Garcia-Alvarez

Abstract

Bis-allyl ruthenium(IV) complexes containing N-heterocyclic ligands [Ru(η³:η³-C₁₀H₁₆)Cl₂(κ¹-N-L)] (L = pyrazole (1a), benzimidazole (1b)) are active catalysts in the redox isomerization of allylic alcohols into saturated carbonyl compounds using the ionic liquid [BMIM][BF₄] (BMIM = 1-butyl-3-methylimidazolium) as a non-conventional reaction media in the absence of additional base. Complex 1b has been found to be the most efficient catalyst for the isomerization of a series of primary and secondary allylic alcohols (9 substrates). In addition, we have studied the effect of the counter anion in the ionic liquid finding that non coordinating anions (BF₄^-, PF₆^- and SbF₆^-) led to the best reactions times and conversion, when compared with the coordinating chloride anion (Cl^-). It is important to note that the catalytic system could be recovered through seven catalytic cycles with only partial loss of the initial activity in the first five cycles. Since this catalytic system: i) shows high efficiency and selectivity, ii) involves no presence of phosphines or any base as co-catalyst, iii) proceeds without the formation of by-products (atom economy), and iii) is readily recovered, it can be considered as a genuine example of a Green chemical process.