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.
Boisvert, Luc, Francis Beaumier, and Claude Spino. "Evidence for a Concerted [4 + 1] Cycloaddition between Electron-Rich Carbenes and Electron-Deficient Dienes." Organic Letters. 9.26 (2007): 5361-5363. Print.