| Abstract: | The thermal isomerization of vinylcyclopropane to cyclopentene was discovered in 1960 and soon recognized as the simplest known example of a 1,3] sigmatropic shift. Experimental observations for the parent rearrangement and for isomerizations shown by substituted systems suggest that diradical transition structures are involved; recent theoretical treatments of the reaction find no minima corresponding to diradical intermediates. The common dichotomy opposing concerted versus diradical and thus necessarily stepwise mechanisms appears inappropriate. The reaction of vinylcyclopropane may involve four energetically concerted paths traversed by different conformational forms of nearly isoenergetic diradical species leading through four isometric diradical transition structures to cyclopentene. © 1998 John Wiley & Sons, Inc. J Comput Chem 19: 222–231, 1998 |
