Ionic Hydrogenation of Alkynes by HOTf and Cp(CO)₃WH

Alkynes can be hydrogenated at room temperature by an ionic hydrogenation method using triflic acid (CF₃SO₃H) as the proton donor and a transition metal hydride (Cp(CO)₃WH) as the hydride donor. Reaction of PhC≡CH with HOTf and Cp(CO)₃WH gives ethylbenzene as the final product in high yield. Intermediates observed in this reaction are the vinyl triflate CH₂=C(Ph)(OTf) and the geminal ditriflate Ph(CH₃)C(OTf)₂, which result from the addition of 1 or 2 equiv of HOTf to the C≡C triple bond of the alkyne. Hydrogenation of PhC≡CMe by HOTf and Cp(CO)₃WH similarly produces propylbenzene as the ultimate product. Along with vinyl triflates, additional intermediates observed in this reaction were the cis and trans isomers of the β-methylstyrene complex [Cp(CO)₃W-(η²-PhHC=CHCH₃)]⁺[OTf]^_. Hydrogenation of n-butylacetylene to n-hexane does occur upon reaction with HOTf/Cp(CO)₃WH, but is very slow. In the absence of metal hydrides, 2-methyl-l-buten-3-yne reacts with HOTf to give the vinyl triflate CH₂=CMeC(OTf)=CH₂, but reaction with HOTf and Cp(CO)₃WH gives Me₂C=C(OTf)Me. The key characteristics required for the metal hydride used in these hydrogenations are the ability to donate hydride in the presence of strong acid, and the absence of rapid decomposition of the hydride through reaction with the strong acid. Cp(CO)₃WH meets these requirements, but HSiEt₃, while an effective hydride donor, is decomposed by HOTf on the time scale of these alkyne hydrogenation reactions.