Correspondence diagram for the cycloaddition of NJ to ethylene

The MOs of the cyclic product are set up in the conventional manner bonding below non-bonding below antibonding, and cr below tt, MOs of the same type being ordered according to the number of nodal planes. The five doubly-occupied MOs are seen to correlate smoothly across the diagram, so no reduction of symmetry below C2v is called for. 

The correspondence diagram for the cycloaddition of ozone and ethylene, which is the first step in the Criegee mechanism of the ozonolysis of olefins, [17], [3, pp. 1067-1070] is illustrated if Fig. 7.6. 

The right side of Fig. 7.6 is identical with that of Fig. 7.5, except for the fact that 3(62) is doubly occupied. The MOs of ozone are easily related to those of N3 the additional occupied MO, n3(ai), is a non-bonding orbital largely localized on the central atom, that is derived from 3, the in-plane member of N3 s degenerate pair of tt orbitals. The required 723(01) 3(62) correspondence implies that the ethylene molecule has to move above or below the molecular plane of O3 along a reaction path that can retain no more than symmetry. 

5 and 7.6 are consistent with the hypothesis, in support of which Huisgen [12,13] has marshalled a great deal of evidence from solvent, substituent and isotope effects, that both types of 1,3-dipolar cycloaddition generally take place in a single step. Stepwise cycloaddition, advocated by Firestone [14, 15], has also been observed [16], but can compete successfully with the concerted 

Cycloaddition of species with triple bonds, which should logically be addressed at this point, will be postponed to later chapters. The reluctance of acetylene to dimerize to cyclobutadiene (CBD) on the ground-state surface follows directly from Fig. 6.2. It is sufficient to note that when two acetylene molecules approach one another in the plane-rectangular (D2/1) orientation, the two additional tt orbitals in acetylene are retained as such in CBD, so they cannot alleviate the forbiddenness of the [ 2g + pathway [5, Fig. 4]. Discussion of the reaction between dioxygen and acetylene to form 1,2-dioxetene and the cycloreversion of tetraalkyl-l,2-dioxetanes to two ketonic fragments has to be postponed until the relation between space and spin symmetry has been introduced in Chapter 9. 

---