The [2s + 2a] cycloaddition of ethylene and acetylene has been studied. A transition structure of C2 symmetry was located on the potential surface. Activation energies for the process are also reported.
Andes Hess Jr, B., Schaad, L. J., & Reinhoudt, D. (1986). [2 + 2] Cycloadditions. A concerted pathway for acetylene plus ethylene. International Journal of Quantum Chemistry, 29(3), 345-350. https://doi.org/10.1002/qua.560290308