Evidence for a Concerted [4 + 1]- Cycloaddition between Electron-Rich Carbenes and Electron-Deficient Dienes

Results from the thermal reactions of deuterated dienes 12-15 provide evidence of the concertedness of the [4 + 1]-cycloaddition between dimethoxycarbene and electron-deficient dienes. Other evidence suggests that the main pathway is a concerted [4 + 1]-cycloaddition rather than a cyclopropanation followed by a vinylcyclopropane rearrangement. Ionic pathways can become competitive when steric or geometrical constraints are present.