Diels-Alder reaction symmetry-allowed process

The Diels-Alder Reaction. A Symmetry Allowed Process 273... 

The reaction is carried out simply by heating a diene or another conjugated system of n bonds with a reactive unsaturated compound (dienophile). Usually the reaction is not sensible to catalysts and light does not affect the course. Depending on the specific components, either carboxylic or heterocyclic products can be obtained. The stereospecificity of the reaction was firmly established even before the importance of orbital symmetry was recognized. In terms of orbital symmetry classification, the Diels-Alder reaction is a k4s + n2s cycloaddition, an allowed process. 

More than one mechanism can account for the experimental observation of the Diels-Alder reaction.521,522,528 However, most thermal [4 + 2]-cycloadditions are symmetry-allowed, one-step concerted (but not necessarily synchronous) process with a highly ordered six-membered transition state.529 Two-step mechanisms with the involvement of biradical or zwitterion intermediates can also be operative.522,528... 

What makes photoexcited lepidopterene and its derivatives undergo adiabatic cycloreversion with so high quantum efficiency The answer to this question must be linked with fact that the formation of lepidopterene from its cycloreversion product A is a highly efficient ground state process, viz. an intramolecular Diels-Alder reaction, which is symmetry-allowed by Woodward-Hoffmann rules. By the same token, the excited state 4jm-2ji cycloreversion of lepidopterene L is a symmetry-forbidden process. Thus, it is... 

While the isomerization of benzene oxides to oxepins occurs spontaneously at low temperature, the analogous mobile valence tautomerization of m-benzene dioxide 143 (Figure 10) to the 10-tt-heterocycle, 1,4-dioxocin, was only evident at temperatures > 50°The latter process is symmetry-allowed and is formally equivalent to a retro-Diels-Alder reaction. The mobile equilibrium at 60°C appeared... 

Therefore, the Diels—Alder reaction (tc" s + tc s cycloaddition) is a thermally allowed process. Under photochemical conditions, this situation is entirely changed. The first excited state of the reactant does not correlate with the first excited state of the product. Rather, it correlates with the upper excited state of the product (Eqn (4.4)). Hence, there is a symmetry-imposed barrier to photochemical reaction of Itch s + tt s] type, due to which the reaction does not proceed under photochemical conditions. 

The cycloaddition of alkenes and dienes is a very useful method for forming substituted cyclohexenes. This reaction is known as the Diels-Alder reaction The concerted nature of the mechanism was generally accepted and the stereospecificity of the reaction was firmly established before the importance of orbtial symmetry was recognized. In the terminology of orbital symmetry classification, the Diels-Alder reaction is a [AUg + lUg] cycloaddition, an allowed process. The transition state for a concerted reaction requires that the diene adopt the s-cis conformation. The diene and substituted alkene (which is called the dienophile) approach each other in approximately parallel planes. The symmetry properties of the n orbitals permit stabilizing interations between C-1 and C-4 of the diene and the dienophile. Usually, the strongest interaction is between the highest occupied molecular orbital (HOMO) of the diene and the lowest unoccupied molecular orbital (LUMO) of the dienophile. The interaction between the frontier orbitals is depicted in Fig. 6.1. 

Most of the ene reactions are concerted and orbital symmetry allowed processes involving all suprafacial transition states of 6e (4jt and 2endo orientations of the electron withdrawing group as depicted in (Fig. 5.1). The addition of the ene to the enophile is stereospecific syn. The TS requires higher activation energy compared to that of Diels-Alder reaction because two cr-electrons of the allylic CT-bond are involved instead of a rr-bond of a diene. 

Assuming that all of the bonds are formed in the same step and only tt electrons are involved, we can use the Htickel Tr-electron approximation to explore the process from a molecular orbital perspective and need examine only those orbitals of the reactants that are directly involved in bond formation. These are called the frontier orbitals and are usually the HOMO of one reactant and the LUMO of another. For the Diels-Alder reaction they are the HOMO of the diene and the LUMO of the dienophile and are as shown in Figure 10.10. The choice of this HOMO-LUMO combination is made to be consistent with the experimental fact that electron-withdrawing groups on the dienophile increase its reactivity and suggest that electrons flow from the diene to the dienophile. Notice that the symmetry properties of these two orbitals are such as to permit the in-phase overlap necessary for a bond formation between the diene and dienophile. In MO terms, the Diels-Alder reaction is classified as symmetry-allowed. 

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