Frontier orbital interactions in Diels—Alder reaction

Fig. 11.12. Frontier orbital interactions in Diels-Alder reactions.

Frontier Orbital Interactions In Diels—Alder Reaction... 

FIGURE 4.12 Frontier molecular orbital interactions in Diels—Alder reactions. 

As it happens, the frontier orbital interactions in the Diels-Alder cycloaddition shown above are like those found in the middle drawing, i.e., the upper and lower interactions reinforce and the reaction proceeds. The cycloaddition of two ethene molecules (shown below), however, involves a frontier orbital interaction like the one on the right, so this reaction does not occur. 

Secondary orbital interactions have also been invoked to explain regiochemistry as well as stereochemistry. Whereas 1 -substituted dienes sometimes have only a small difference between the coefficients on C-l and C-4 in the HOMO, they can have a relatively large difference between the coefficients on C-2 and C-3. Noticing this pattern, Alston suggested that the regioselectivity in Diels-Alder reactions may be better attributed, not to the primary interactions of the frontier orbitals on C-l and C-4 that we have been using so far, but to a... 

They react easily with electrophiles and add nucleophiles at C-6. In cycloaddition reactions they may react as 2jt, 4n, or 6 i compounds. According to frontier orbital considerations they readily react with electron-deficient dienophiles (e.g., silenes) in Diels-Alder reactions this is due to the strong interaction between the fulvene HOMO and dienophile LUMO [9]. Although the n and n orbitals of silenes are generally 1-2.5 eV higher in energy than is the case for the alkene congeners [10] a normal [4+2] cycloaddition behaviour for 3 is observed in earlier works [3-5]. 

Orbital energies and approximate wavefunctions for the HOMO S of benzonor-bomadiene and 7-isopropylidenebenzonorbomadiene have been obtained from their p.e. spectra, and the differential reactivity of the systems in Diels-Alder reactions with inverse electron demand has been discussed in terms of frontier orbital analysis. Spectra for syn- and anti-7-norborneol show that in the sy -isomer the n-bond is ca. 0.2 eV more difficult to ionize, a result that is interpreted as arising from H-bond-induced stabilization of the tt-bond. Differential orbital interactions are ruled out by the finding that the n-n difference is the same for both of the analogous methyl ether derivatives. 

According to Alder s Endo rule (1930) Endo isomers are major products in Diels-Alder reactions. These results look surprising on the grounds that Endo-isomers are less stable for steric reasons. But, Endo-rule can be rationalized on the basis of frontier orbital theory. Endo-transition state is stablized by secondary interactions in comparison to exo-isomers in which secondary interactions are absent. This facts makes endo-isomers more stable, i.e.,why they are major-products in Diels-Alder reactions. 

The same conclusions are drawn by analysis of the frontier orbitals involved in cycloadditions. For the most common case of the Diels-Alder reaction, which involves dienophiles with electron-attracting substituents, the frontier orbitals are l/2 of the diene (which is the HOMO) and n of the dienophile (which is the LUMO). Reaction occurs by interaction of the HOMO and LUMO, which can be seen from the illustration below to be allowed. 

When both the 1,3-dipoIe and the dipolarophile are unsymmetrical, there are two possible orientations for addition. Both steric and electronic factors play a role in determining the regioselectivity of the addition. The most generally satisfactory interpretation of the regiochemistry of dipolar cycloadditions is based on frontier orbital concepts. As with the Diels-Alder reaction, the most favorable orientation is that which involves complementary interaction between the frontier orbitals of the 1,3-dipole and the dipolarophile. Although most dipolar cycloadditions are of the type in which the LUMO of the dipolarophile interacts with the HOMO of the 1,3-dipole, there are a significant number of systems in which the relationship is reversed. There are also some in which the two possible HOMO-LUMO interactions are of comparable magnitude. 

In certain cases, multiple frontier orbital interactions must be considered. This is particularly true of cycloaddition reactions, such as the Diels-Alder reaction between 1,3-butadiene and ethene. 

Fig. 8.1 Frontier-orbital interaction for carbo-Diels-Alder reactions, (a) The interaction of a dienophile with a low-energy LUMO, in the absence and presence of a Lewis acid (LA),...

According to frontier molecular orbital theory (FMO), the reactivity, regio-chemistry and stereochemistry of the Diels-Alder reaction are controlled by the suprafacial in phase interaction of the highest occupied molecular orbital (HOMO) of one component and the lowest unoccupied molecular orbital (LUMO) of the other. [17e, 41-43, 64] These orbitals are the closest in energy Scheme 1.14 illustrates the two dominant orbital interactions of a symmetry-allowed Diels-Alder cycloaddition. 

The chemical reactions through cyclic transition states are controlled by the symmetry of the frontier orbitals [11]. At the symmetrical (Cs) six-membered ring transition state of Diels-Alder reaction between butadiene and ethylene, the HOMO of butadiene and the LUMO of ethylene (Scheme 18) are antisymmetric with respect to the reflection in the mirror plane (Scheme 24). The symmetry allows the frontier orbitals to have the same signs of the overlap integrals between the p-or-bital components at both reaction sites. The simultaneous interactions at the both sites promotes the frontier orbital interaction more than the interaction at one site of an acyclic transition state. This is also the case with interaction between the HOMO of ethylene and the LUMO of butadiene. The Diels-Alder reactions occur through the cyclic transition states in a concerted and stereospecific manner with retention of configuration of the reactants. 

The main stabilization in reactions with activated reaction partners, viz. when one partner is electron-rich and the other electron-poor, arises through interaction between the donor HOMO and the acceptor LUMO which are much closer in energy than the acceptor HOMO and the donor LUMO. Figure 2 illustrates which interactions between the frontier orbitals cause the main stabilization in normal, neutral and inverse Diels-Alder reactions. For example, the main stabilization in the reaction between an electron-rich diene and an electron-poor dienophile stems from the interaction of the diene HOMO with the dienophile LUMO. 

The reactivity and selectivity of the Diels-Alder reaction can be understood in terms of Frontier Molecular Orbital (FMO) theory which evolved during studies of the role of orbital symmetry in pericyclic reactions by Woodward and Hoffmann58 and, independently, by Fukui59. FMO theory explains the driving force of a reaction between two compounds by the efficiency with which the molecular orbitals of the two partners overlap. This orbital interaction is maximized when their energy separation is small. FMO theory further states that the two most important interacting orbitals are the Highest Occupied... 

Orbital Interaction Analysis. An orbital interaction diagram for the Diels-Alder reaction is shown in Figure 12.5a. The geometry of approach of the two reagents which ensures a maximum favorable interaction between the frontier MOs (dashed lines) preserves a plane of symmetry at all separations. The MOs are labeled according to whether they are symmetric (S) or antisymmetric (A) with respect to reflection in the plane. Simultaneous overlap of both HOMO-LUMO pairs is a necessary feature of all peri-... 

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