IONIC HYDROGENATION OF ALKYNES BY HOTF AND CP(CO)(3)WH

Alkynes can be hydrogenated at room temperature by an ionic hydrogenat ion method using triflic acid (CF3SO3H) as the proton donor and a tran sition metal hydride (Cp(CO)(3)WH) as the hydride donor. Reaction of P hC=CH with HOTf and Cp(CO)(3)WH gives ethylbenzene as the final produc t 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 trip le bond of the alkyne. Hydrogenation of PhC=CMe by HOTf and Cp(CO)(3)W H similarly produces propylbenzene as the ultimate product. Along with vinyl triflates, additional intermediates observed in this reaction w ere the cis and trans isomers of the beta-methylstyrene complex [Cp(CO )(3)W-(eta(2)-PhHC=CHCH3)](+)[OTf](-). Hydrogenation of n-butylacetyle ne to n-hexane does occur upon reaction with HOTf/Cp(CO)(3)WH, but is very slow. In the absence of metal hydrides, 2-methyl-1-buten-3-yne re acts with HOTf to give the vinyl triflate CH2=CMeC(OTf)=CH2, but react ion with HOTf and Cp(CO)(3)WH gives Me(2)C=C(OTf)Me. The key character istics 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 t he strong acid. Cp(CO)(3)WH meets these requirements, but HSiEt(3), wh ile an effective hydride donor, is decomposed by HOTf on the time scal e of these alkyne hydrogenation reactions.