Ethylene dimerization parallel approach

Figure 7-12. Construction of the correlation diagram for ethylene dimerization with parallel approach. Adaptation of Figure 10.19 from Reference [61] with permission.

A simplest example of a cycloaddition reaction is provided by the dimerization of ethylene with the formation of cyclobutane, i.e., [2 + 2]-cycloaddition. According to the Woodward-Hoffmann rules, the supra-antara route I of the reactants approach is thermally allowed.The parallel supra-supra approach II is unfavored in the electron ground state but must materialize in an excited state. 

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