Frontier orbital interactions in Diels—Alder

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. 

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. 

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),...

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. 

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]. 

Most pericyclic reactions, though of course not all, are little influenced by Coulombic forces for example, it is well known that the polarity of the solvent has little effect on the rate of Diels-Alder reactions. We can therefore expect that a major factor influencing reactivity will be the size of the frontier orbital interaction represented by the third term of equation 2-7, p. 27. This is why this chapter is much the largest in this book the most dramatic successes of frontier orbital theory have been the explanations it has given to an amazingly large number of observations in pericyclic chemistry. 

Activation of an alkene to enable addition of carbon radicals may be achieved by complexation of the alkene undergoing addition to an electron-deficient species such as a Lewis acid. This strategy has been used extensively in the activation of dienophiles towards cycloadditions, and the reasons for its efficacy in both cycloaddition and radical addition have the same roots. In a Diels-Alder reaction [4] with normal electron demand, the dominant frontier orbital interaction is between the HOMO of the diene and the LUMO of the dienophile. Complexation of a Lewis acid to the dienophile lowers its LUMO and magnifies the important frontier MO interaction. 

The Diels-Alder cycloaddition reaction of dihydropyran with acrolein was performed in the presence of various H-form zeolites such as H-Faujasites, H-p, H-Mordenites which differ both in their shape selective as well as their acidic properties. The activity of the different catalysts was determined and the reaction products were identified. High 3delds in cycloadduct were obtained over dealuminated HY (Si/Al=15) and Hp (Si/Al=25) compared to HM (Si/Al=10). These results were accounted for in terms of acidity, shape selectivity and microporosity vs mesoporosity properties. The activity and the regioselectivity were then discussed in terms of frontier orbital interactions on the basis of MNDO calculations for thermal and catalyzed reactions by complexing the diene and the dienophile with Bronsted and Lewis acidic sites. From these calculations, Bronsted acidic sites appeared to be more efficient than Lewis acidic sites to achieve Diels-Alder reactions. 

The ethylene-butadiene cycloaddition is a good example to illustrate that symmetry allowedness does not necessarily mean that the reaction occurs easily. This reaction has a comparatively high activation energy, 144 kJ/mol [7-7]. A large number of quantum-chemical calculations has been devoted to this reaction with conflicting results (for recent references, see Ref. [7-18]). It seems, however, that the concerted nature of the prototype Diels-Alder reaction is well established. The reason for the relatively high activation energy is that substantial distortion must occur in the reactants before frontier orbital interactions can stabilize the product. 

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. 

The Diels-Alder reaction (for which Otto Diels and Kurt Alder were awarded together the Nobel Prize in 1950) involves the reaction of a conjugated diene with another group containing a pi bond (referred to as a dienophile since it loves reacting with dienes). In the presence of heat, a diene and a dienophile will combine to give a cyclohexene product. This concerted mechanism is an example of a pericyclic reaction called a [4 -i- 2] cycloaddition since it involves the interaction of a four-electron % system (the diene) with a two-electron 71 system (the dienophile). While many examples of the Diels-Alder reaction can be easily described as a reaction between a nucleophile and electrophile (the approach to be taken here), the mechanism and the regjo- and stereochemistry of the product is usually described by frontier orbital theory in which the HOMO of the diene and the LUMO of the dienophile are matched. 

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