The reaction of cycloaddition of ketene and cyclopentadiene presents experimentally a competing mechanism where the branching ratio between the Woodward-Hoffmann allowed [4+2] and forbidden [2+2] cycloaddition product is 4.56 at -20 °C, but because the minimum energy path misses the [2+2] product altogether, it has been claimed to lie beyond the scope of transition state theory. In this paper, a variational transition state theory study on this reaction is presented. It is found that the minimum energy path affording the [4+2] product travels through a potential energy plateau very close to the minimum energy path that describes the interconversion between both cycloaddition products, allowing the transfer between both and the direct formation of the forbidden [2+2] product, in this way acting as a means to circumvent the Woodward-Hooffmann rules. Within the domain of the competitive canonical unified statistical theory, a value for the branching ratio in very good agreement with experiment is computed. © 2010 Springer-Verlag.
|Journal||Theoretical Chemistry Accounts|
|Publication status||Published - 1 Feb 2011|
- Cycloaddition reaction
- Non-symmetrical bifurcating surfaces
- Variational transition state theory
- Woodward-Hoffmann rules