Elucidating the ruthenium-mediated conversion of aryl alkynes to alkoxy(benzyl)carbene and benzyl carbonyl complexes

Metal vinylidenes are key intermediates in the activation of terminal alkynes. Previous studies concerning ruthenium η⁶-arene complexes showed how the elusive vinylidenes are often transformed into more stable alkoxy(alkyl)carbene complexes upon reaction with alcohols, highlighting their electrophilicity. We reinvestigated the reactivity of terminal alkynes and alcohols with ruthenium(II) η⁶-arene precursors and we found out new aspects of the formation and the reactivity of the alkoxy(carbene)complexes. First, the reactivity of ruthenium complexes bearing different η⁶-arene, phosphane, halide co-ligands on the activation process of a series of arylalkynes have been examined. Under optimized conditions, a series of alkoxy(benzyl)carbene complexes of general formula [RuCl{C(OR’)CH₂(4-C₅H₄R)}(PR’’₃)(η⁶-arene)]⁺ were obtained. Five compounds were isolated in 82–96 % yield and they were characterized by spectroscopic techniques and X-ray diffraction in three cases. Notably, these carbene complexes are the predominant reaction products even in presence of a large molar excess of water in the mixture for short reaction times.

In fact, DFT calculations on a model system showed that the vinylidene intermediate, resulting from the Ru/alkyne interaction, is preferentially attacked by MeOH instead of water. The subsequent formation of carbonyl complexes was assessed in various conditions by IR and NMR and four unprecedented and comparatively rare benzyl carbonyl complexes of general formula [Ru{CH₂(4-C₆H₄R)}(CO)(PPh₃)(η⁶-arene)]⁺ are reported, including the crystal structure of one example. Next, an unprecedented reactivity study on selected alkoxy(benzyl)carbene complexes was carried out. Joint experimental and computational results indicate that these benzyl carbonyl complexes may actually arise from the reaction of the carbene complexes with water, a reactivity pathway that has never been considered in previous studies on the Ru-mediated hydrolytic cleavage of alkynes.