Dipolar Cycloaddition of Diazomethane to Ethene

During the Diels-Alder reaction [1] and in the electrocyclization of cis-1,3,5-hexatriene [2], bonds break and form in a homolytic fashion, with orbitals remaining associated with the same centres throughout the reaction. For such systems, there is a major recoupling of the electron spins. This last takes place most rapidly at or near the transition state. The resonance pattern, taken together with other characteristics, is reminiscent of the spin-coupled description 

An entirely different description emerges for the two 1,3-dipolar cycloaddition reactions that we have studied [3,4]. For such systems, the bond breaking and bond formation involves instead the shifts of well-identifiable orbital pairs, rather than any spin recouplings. Such heterolytic mechanisms, that do not pass through an aromatic structure, now seem to be a likely outcome of studies on other gas-phase concerted 1,3-dipolar cycloaddition reactions. 

Karadakov, D.L. Cooper, T. Thorsteinsson and J. Gerratt, in Quantum Systems in Chemistry and Physics. Volume 1 Basic Problems and Models Systems, ed. A. Hemdndez-Laguna, J. Maruani, R. McWeeny and S. Wilson (Kluwer, Dordrecht, 2000) p. 327. 

Cooper, J. Gerratt, M. Raimondi, M. Sironi and T. Thorsteinsson, Theor. Chim. Acta 85, 261 (1993). 

---

Fig. 15.37. Frontier orbital interactions in the transition state of the 1,3-dipolar cycloaddition of diazomethane to ethene.

---