Diels-Alder HOMO-LUMO interactions

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. 

Diels-Alder cycloadditions involving norbomene 57 [34], benzonorbomene (83), 7-isopropylidenenorbomadiene and 7-isopropylidenebenzonorbomadiene (84) as dienophiles are characterized as inverse-electron-demand Diels-Alder reactions [161,162], These compounds react with electron-deficient dienes, such as tropone. In the inverse-electron-demand Diels-Alder reaction, orbital interaction between the HOMO of the dienophile and the LUMO of the diene is important. Thus, orbital unsymmetrization of the olefin it orbital of norbomene (57) is assumed to be involved in these top selectivities in the Diels-Alder cycloaddition. 

Fig. 6.2. HOMO-LUMO interactions rationalize regioselectivity of Diels-Alder reactions.

The endo selectivity in many Diels-Alder reactions has been attributed to attractive secondary orbital interactions. In addition to the primary stabilizing HOMO-LUMO interactions, additional stabilizing interactions between the remaining parts of the diene and the dienophile are possible in the endo transition state (Figure 3). This secondary orbital interaction was originally proposed for substituents having jr orbitals, e.g. CN and CHO, but was later extended to substituents with tt(CH2) type of orbitals, as encountered in cyclopropene57. 

FIGURE 1. The HOMO/LUMO interactions in Diels-Alder reactions with normal and inverse electron demand... 

It is also pertinent that there are two HOMO-LUMO interactions possible between butadiene and ethylene, one in which die HOMO is diat of die diene, which acts as die electron donor, and one in which the HOMO is that of the olefin, which would be die electron donor. A normal Diels-Alder reaction is one in which die diene is electron rich and acts as the electron donor and the... 

The essential features of the Diels-Alder reaction are a four-electron n system and a two-electron it system which interact by a HOMO-LUMO interaction. The Diels-Alder reaction uses a conjugated diene as the four-electron n system and a it bond between two elements as the two-electron component. However, other four-electron it systems could potentially interact widi olefins in a similar fashion to give cycloaddition products. For example, an allyl anion is a four-electron it system whose orbital diagram is shown below. The symmetry of the allyl anion nonbonding HOMO matches that of the olefin LUMO (as does the olefin HOMO and the allyl anion LUMO) thus effective overlap is possible and cycloaddition is allowed. The HOMO-LUMO energy gap determines the rate of reaction, which happens to be relatively slow in this case. 

Why do the Diels-Alder reactions with both normal and inverse electron demand occur under relatively mild conditions And, in contrast, why can [4+2]-cycloadditions between ethene or acetylene, respectively, and butadiene be realized only under extremely harsh conditions (Figure 15.1) Equation 15.2 described the amount of transition state stabilization of [4+2]-cycloadditions as the result of HOMO/LUMO interactions between the 7T-MOs of the diene and the dienophile. Equation 15.3 is derived from Equation 15.2 and presents a simplified estimate of the magnitude of the stabilization. This equation features a sum of two simple terms, and it highlights the essence better than Equation 15.2. 

Figure 7-16. HOMO-LUMO interaction in the Diels-Alder reaction.

There are two important variations for the Diels-Alder reaction and both are synthetically useful. In one, the HOMO-LUMO interactions that drive the reaction change due to heteroatoms and/or substituents on the diene and/or alkene units. In such cases, the reaction is driven by the LUMOdiene-HOMOalkene interaction rather than the HOMOdiene-LUMOaikene interaction. In the second variation, the reversible nature of the Diels-Alder reaction is exploited. Cycloadducts are produced that can be manipulated and then a retro-Diels-Alder reaction generates a new diene or alkene. Both of these reactions will be examined in this section. 

The Diels—Alder reaction may also be analyzed by a similar consideration of the molecular orbitals of butadiene and ethene. In this case, there are two possible HOMO—LUMO interactions. Since the phases of the 1,4-lobes of the HOMO of butadiene match with those in the LUMO of ethene, the [7r" s + TT s] cycloaddition is thermally allowed. We reach a similar conclusion by considering the symmetry of LUMO of butadiene and the HOMO of ethene (Figure 4.10). However, on energetic grounds the latter interaction will make a smaller contribution than the former. 

Since 1,3-DPCA reactions involve Trr-electrons from the 1,3-dipole and 2TT-electrons from the dipolarophiles, it may be considered as symmetry-allowed [tt" s + TT s] cycloaddition resembling Diels—Alder reaction. There is HOMO—LUMO interaction in which either reactant can be the electrophilic or nucleophilic component (Figure 5.12). 

When a diene and dienophile approach each other in the Diels-Alder reaction, the important orbital interactions are between the HOMOs and LUMOs. As Figure 20.20 shows, there are two possible HOMO-LUMO interactions. 

FIGURE 20.20 The two possible HOMO-LUMO interactions in the prototypal Diels-Alder reaction between butadiene and ethylene. 

FIGURE 20.21 The orbital overlaps in the two HOMO—LUMO interactions of the Diels-Alder reaction. Either of the HOMO-LUMO interactions can lead to product. 

What about the photochemical Diels-Alder reaction The observation that this reaction is most uncommon leads us to the immediate suspicion that there is something wrong with it. Usually, the absorption of a photon will promote an electron from the HOMO to the LUMO. In this case, the lower energy HOMO-LUMO gap is that in the diene partner. Absorption of light creates a new photochemical HOMO for the diene, 3, and now the HOMO-LUMO interaction with the dienophile partner involves one antibonding overlap. Both new bonds cannot be formed at the same time (Rg. 20.22). So this photochemical Diels-Alder reaction is said to be forbidden by orbital symmetry. ... 

When both 1,3-dipole and dipolarophile are unsymmetrical, two products are possible. The formation of major product can be predicted by consideration of their TSs. The most stable TS will provide the major product. The stabihty of the TS is controlled by both electronic and steric factors. Therefore, the regioselectivity of a 1,3-DPCA reaction is determined by the steric and electronic properties of the substituents attached to 1,3-dipole and dipolarophile. The FMO theory may also be applied to analyze the regioselectivity of 1,3-DPCA reaction [107]. A relatively stronger donor-acceptor interaction between HOMO and LUMO and lowest dipole moment favors the TS. The HOMO and LUMO of a 1,3-dipole are similar to that of a diene in a Diels-Alder reaction. The interactions of HOMO or LUMO of a dipole with a LUMO or HOMO of a dipolarophile depend on their electron donor and electron acceptor property. The orbital interactions of HOMO and LUMO of dipole and dipolarophile are shown in Fig. 3.11. 

Focusing on HOMO-LUMO interactions can aid our understanding of many organic reactions. Its early development is attributed to Professor Kenichi Fukui (Kyoto), and its application to Diels-Alder reactions constitutes but one part of the Woodward-Hoffmam rules proposed by Professors R. B. Woodward (Harvard) and Roald Hoffmann (Cornell). 

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