In Situ Allene Formation via Alkyne Tautomerization to Promote [4 + 2]-Cycloadditions with a Pendant Alkyne or Nitrile.

We have explored the net-[4 + 2]-cycloadditions of a variety of aryl (or alkenyl) alkynes. Tautomerization via base-catalyzed alkyne-to-allene isomerization produces a transient allene, which undergoes stepwise cyclization with not only a pendant alkyne but also a nitrile. The operative mechanisms for these reactions were studied by density functional theory and compared with the slower thermal cyclization of the precursor alkyne.