Generation and Reaction of Tungsten-Containing Carbonyl Ylides: [3 + 2]-Cycloaddition Reaction with Electron-Rich Alkenes

Novel tungsten-containing carbonyl ylides 7, generated by the reaction of the o-alkynylphenyl
carbonyl derivatives 1 with a catalytic amount of W(CO)5(thf), reacted with alkenes to give polycyclic
compounds 5 through [3 + 2]-cycloaddition reaction followed by intramolecular C−H insertion of the
produced nonstabilized carbene complex intermediates 8. In the presence of triethylsilane, these tungsten-containing carbene intermediates 8 were smoothly trapped intermolecularly by triethylsilane to give silicon-containing cycloadducts 17 with regeneration of the W(CO)5 species. By this procedure, the scope of alkenes
employable for this reaction was clarified. The presence of the tungsten-containing carbonyl ylide 7c was
confirmed by direct observation of the mixture of o-ethynylphenyl ketone 1c and W(CO)5(thf-d8). Careful
analysis of the intermediate by 2D NMR, along with the observation of the direct coupling with tungsten-183 employing the 13C-labeled substrate, confirmed the structure of the ylide 7c. Examination using (E)- or
(Z)- vinyl ether revealed that the [3 + 2]-cycloaddition reaction proceeded in a concerted manner and that
the facial selectivity of the reaction differed considerably depending on the presence or absence of
triethylsilane. These results clarified the reversible nature of this [3 + 2]-cycloaddition reaction.