Orbital correlation diagrams cycloaddition reactions

When the orbitals have been classified with respect to symmetry, they can be arranged according to energy and the correlation lines can be drawn as in Fig. 11.10. From the orbital correlation diagram, it can be concluded that the thermal concerted cycloadditon reaction between butadiene and ethylene is allowed. All bonding levels of the reactants correlate with product ground-state orbitals. Extension of orbital correlation analysis to cycloaddition reactions involving other numbers of n electrons leads to the conclusion that the suprafacial-suprafacial addition is allowed for systems with 4n + 2 n electrons but forbidden for systems with 4n 7t electrons. 

Orbital correlation diagrams are useful for cycloadditions and electrocyclic reactions but not for sigmatropic rearrangements since no element of symmetry is preserved. 

Figure 14.1. (a) Orbital correlation diagram for nls + n2s dimerization of two olefins to form a cyclobutane. (b) Orbital correlation diagram for n4s + n2s cycloaddition of a diene and an olefin (the Diels-Alder reaction). 

A key step in one route to the synthesis of hexamethyl Dewar benzene is the cycloaddition of 2-butyne to tetramethylcyclobutadiene (stabilized by A1 cation). Using the parent compounds (no methyls), develop a Woodward-Hoffmann orbital correlation diagram for the reaction and determine whether the reaction is thermally allowed. 

Cycloaddition reactions, 162-165, 197-198 component analysis, 168 Diels-Alder, 162, 198 ethylene + ethylene, 198 orbital correlation diagram, 198 stereochemistry, 162-163 Cycloalkanols, synthesis, 277 Cyclobutadiene barrier, 91 ground state, 91 point group of, 5 self-reactivity, 97 SHMO, 151 structure, 309-310 Cyclobutane... 

Since orbital correlation diagrams for more cycloaddition reactions are more complex, we will not deal further with this approach. 

In view of the demonstrated stereospecificity of at least some cation radical Diels-Alder reactions, it is at least possible that these reactions, like the neutral Diels-Alder, are true pericyclic reactions, i.e., they may occur via a concerted cycloaddition. The results of a variety of calculations, however, make clear that the cydoadditions must at least be highly non-synchronous, so that the extent of the formation of the second bond, which completes the cyclic transition state, is no more than slight [55, 56]. If the cation radical Diels-Alder reaction is nevertheless interpreted as pericyclic and the concept of orbital correlation diagrams is applied to them, it emerges that the cycloaddition is symmetry allowed if the ionized (cation radical) component is the dienophile, but forbidden if it is the diene [39, 55], The former mode of reaction has been referred to as the [4-1-1] mode, and the latter as the [3 -t- 2] mode. Interestingly, the great majority of cation radical Diels-Alder reactions thus far observed seem to represent the formally allowed [4-1-1] mode. An interesting case in point is the reaction of l,l -dicyclohexenyl with 2,3-dimethylbutadiene (Scheme 24) [57]. 

The orbital correlation diagram for the [4 + 2] cycloaddition of singlet oxygen is shown in Figure 7.55 for the reaction... 

The orbital correlation diagrams put forth by Woodward and Hoffmann336 for electrocyclic and sigmatropic reactions (Section 6.1.2) and for cycloaddition reactions (Section 6.1.5) are well known and the details of their construction are not reiterated here. We show only the case of the [2S + 2S] cycloaddition of two ethene molecules to cyclobutane as an example (Figure 4.34). 

Optical purity, by NMR, 13, 14 Orbital correlation diagrams, 196-203 cycloaddition reactions, 197-196 Diels-Alder, 198 ethylene -E ethylene, 198 electrocyclic reactions, 198-200 butadienes, 199 hexatrienes, 199 limitations, 203 photochemical, 201 Woodward-Hoffinann, 197 Orbital energies, see also Energies, orbital degeneracy, 27, 90 Orbital interaction theory, 34-71 diagram, 40, 42, 47 limitations, 69-71 sigma bonds, 72-86 Orbitals... 

The case of 2+2 cycloaddition of two alkene units can serve as an example. This reaction was classified as a forbidden thermal reaction on the basis of an orbital correlation diagram (Fig. 11.2) that showed that the ground-state molecules would lead to an excited cyclobutane, and would therefore involve a prohibitive energy requirement. 

The construction of an orbital correlation diagram requires the retention of at least one symmetry element along the reaction pathway, so none can be drawn for +7r2a]-cycloaddition. Whether or not one can be constructed for... 

The Woodward-Hoffmann mles for cycloaddition reactions can be explained from frontier orbital interactions, orbital correlation diagram and aromatic transition... 

Let us consider the orbital symmetry properties of the reactants and products for the Diels-Alder cycloaddition reaction of butadiene and ethylene into cyclohexene (Fig. 3.5). The addition of the diene and dienophile takes place face to face, where diene assumes 5-cA-conformation. The reactants, TS and product maintain symmetry (plane of symmetry) among their orbitals during the course of cycloaddition. To understand this fact, an orbital correlation diagram [31] is constracted by arranging the orbitals with respect to their energy content, or correlation lines are... 

Construct an orbital correlation diagram for the broadside 2 -h 2 cycloaddition of two acetylenes to form cyclobutadiene (IX). Is the reaction likely to proceed... 

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