Dehydrogenative Formation of a (η⁴-Enone)ruthenium(0) Complex as a Key Intermediate in the Catalytic Isomerization of Allylic Alcohol to Ketone

(η4-Enone)ruthenium(0) complexes, Ru[η4-CH2CHC(R)O](PEt3)3 (R = H (3), Me (4)), are prepared by the reactions of Ru(η4-1,5-COD)(η6-1,3,5-COT) (1)/PEt3 and Ru(η4-1,3,5-COT)(PEt3)3 (2) with allylic alcohols such as allyl alcohol and 3-buten-2-ol, respectively. Hydrogen transfer from external allyl alcohol to the coordinated η4-3-buten-2-one ligand in 4 leads to the formation of 2-butanone and 3, suggesting the importance of such a (η4-enone)ruthenium complex in the ruthenium-catalyzed isomerization of allylic alcohol.