Highly Efficient Redox Isomerization of Allylic Alcohols at Ambient Temperature Catalyzed by Novel Ruthenium-Cyclopentadienyl Complexes-New Insight into the Mechanism

A range of ruthenium cyclopentadienyl (Cp) complexes have been prepared and used for isomerization of allylic alcohols to the corresponding saturated carbonyl compounds. Complexes bearing CO ligands show higher activity than those with PPh3 ligands. The isomerization rate is highly affected by the substituents on the Cp ring. Tetra(phenyl)methyl‐substituted catalysts rapidly isomerize allylic alcohols under very mild reaction conditions (ambient temperature) with short reaction times. Substituted allylic alcohols have been isomerized by employing Ru–Cp complexes. A study of the isomerization catalyzed by [Ru(Ph5Cp)(CO)2H] (14) indicates that the isomerization catalyzed by ruthenium hydrides partly follows a different mechanism than that of ruthenium halides activated by KOtBu. Furthermore, the lack of ketone exchange when the isomerization was performed in the presence of an unsaturated ketone (1 equiv), different from that obtained by dehydrogenation of the starting allylic alcohol, supports a mechanism in which the isomerization takes place within the coordination sphere of the ruthenium catalyst. Efficient ruthenium‐catalyzed isomerization of allylic alcohols to the corresponding saturated ketones has been developed. The isomerization catalyzed by [Ru(η5‐Ph4MeCp)(CO)2X] (X=Cl, Br) activated by KOtBu takes place at ambient temperature with short reaction times. Mechanistic studies suggest coordination of the alcohol oxygen to Ru to give a Ru–alkoxide intermediate that undergoes β‐hydride elimination to yield a η3‐ruthenium keto hydride intermediate (A).