Diels-Alder reaction orbital correlation diagram

As an example, we shall discuss the Diels-Alder reaction of 2-methoxybuta-l,3-diene with acrylonitrile. Figure 3-7 gives the reaction equation, the correlation diagram of the HOMOs and LUMOs, and the orbital coefficients of the correlated HOMO and LUMO. 

Figure 1.1. Orbital correlation diagram illustrating the distinction between normal electron demand (leftside) and inverse electron demand (right side) Diels-Alder reactions.

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). 

We have emphasized that the Diels-Alder reaction generally takes place rapidly and conveniently. In sharp contrast, the apparently similar dimerization of olefins to cyclobutanes (5-49) gives very poor results in most cases, except when photochemically induced. Fukui, Woodward, and Hoffmann have shown that these contrasting results can be explained by the principle of conservation of orbital symmetry,895 which predicts that certain reactions are allowed and others forbidden. The orbital-symmetry rules (also called the Woodward-Hoffmann rules) apply only to concerted reactions, e.g., mechanism a, and are based on the principle that reactions take place in such a way as to maintain maximum bonding throughout the course of the reaction. There are several ways of applying the orbital-symmetry principle to cycloaddition reactions, three of which are used more frequently than others.896 Of these three we will discuss two the frontier-orbital method and the Mobius-Huckel method. The third, called the correlation diagram method,897 is less convenient to apply than the other two. 

The photochemical dimerization of unsaturated hydrocarbons such as olefins and aromatics, cycloaddition reactions including the addition of 02 ( A ) to form endoperoxides and photochemical Diels-Alders reaction can be rationalized by the Woodward-Hoffman Rule. The rule is based on the principle that the symmetry of the reactants must be conserved in the products. From the analysis of the orbital and state symmetries of the initial and final state, a state correlation diagram can be set up which immediately helps to make predictions regarding the feasibility of the reaction. If a reaction is not allowed by the rule for the conservation of symmetry, it may not occur even if thermodynamically allowed. 

Figure 7.15 An orbital correlation diagram for the Diels-Alder reaction. The if/A and y/n orbitals at the left are for ethylene, while the others at the left are for butadiene. The orbitals on the right are for the product.

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... 

Diels-Alder reaction, 169-170 aromatic TS, 151 benzyne, 160 butadiene + ethylene, 169 diastereoselectivity, 292 interaction diagram, 169 orbital analysis, 169-170 orbital correlation diagram, 198, 201 reverse demand, 169 substituent effects, 169-170 Diethyl tartrate, 11 Difluorocarbene ( CF2), 115... 

Orbital Correlation Diagrams. We shall begin with an allowed reaction, the ubiquitous Diels-Alder. 

Fig. 6.16 Orbital correlation diagram for the Diels-Alder reaction...

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]. 

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... 

MOs, while tlie two 7t c orbitals lead to the tt and tt MOs. In the initial stage of (he dimerization, the interaction between two ethylencs is weak so that 7t+ and tt. lie far below the n+ and tt levels, so that only 7t+ and rr are occupied. Of the a orbitals of cyclobutane described earlier, only those related to the tt., 7t1 and nl levels by symmetry are shown in Figure 11.1. Not all the occupied MOs of the reactant lead to occupied orbitals in the product. In particular, tt. correlates with one component of the empty set in cyclobutane. The tt+ combination ultimately becomes one component of the filled set in cyclobutane. So the reaction is symmetry forbidden. The reader should carefully compare the correlation diagram for ethylene dimerization here with the Ho + O2 reaction in ITgure 5.8. flie two correlation diagrams are very similar, as they should be, since in this instance the spatial dfstributions of tt and n " are similar to those of and respectively, in H2. These two reactions are probably the premier examples of symmetry-forbidden reactions. A related symmetry-allowed example is the concerted cycloaddition of ethylene and butadiene, the Diels-Alder reaction. We shall not cover the orbital symmetry rules for organic, pericyclic reactions. There are several excellent reviews that the reader should consult.But it should be pointed out that the orbital symmetry rules have stereochemical implications in terms of the reaction path and products formed. The development of these rules by Woodward and Hoffmann... 

Construct a MO correlation diagram similar to Figure 11.66 (page 737) to show that the photochemical Diels-Alder reaction is forbidden by the principles of orbital symmetry. 

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... 

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