Diels-Alder reactions atomic orbital coefficients

The explanation of the regiospecificity of Diels-Alder reactions requires knowledge of the effect of substituents on the coefficients of the HOMO and LUMO orbitals. In the case of normal electron demand, the important orbitals are the HOMO on the diene and the LUMO on the dienophile. It has been shown that the reaction occurs in a way which bonds together the terminal atoms with the coefficients of greatest magnitude and those with the coefficients of smaller magnitude [18]. The additions are almost exclusively cis and with only a few exceptions, the relative configurations of substituents in the components is kept in the products [19]. 

Calculations performed at the HF/3-21G level indicated smaller energy gaps between the HOMOs of the aforementioned electron-rich dienophiles and the LUMOs of the quinone ketals, as can be expected for inverse electron-demand Diels-Alder reactions under FMO control [141]. Regiochemical controls observed with quinone ketals such as 76a were well corroborated by the relative magnitudes of the atomic coefficients of the frontier orbitals. The highest coefficients at C-5 of the quinone ketal LUMO and at C-2 of the electron-rich alkenes would indeed promote bond formation between these centers. The results of calculations on other quinone ketals were, however, rather vague [141]. 

In frontier oibital terms, the regiochemistry is governed largely by the atomic orbital coefficients at the termini of the reaction pailners, which are tered by the substituents. Hence, in normal Diels-Alder reactions a diene substituent at C-1 has the tendency to direct the addition of a carbonyl-conjugated al-kene towards the ortho product (X), whereas a substituent at C-2 favors the para product (XI). The... 

In order to answer the question first posed in Chapter 1 and repeated above, we begin by ignoring the substituents and counting only those parts of the conjugated system directly involved in the reaction. (We shall return to the crucial role of the substituents later in the chapter.) Thus the Diels-Alder reaction is simplified to that of butadiene reacting with ethylene the former component has four rc-electrons and the latter two, and these are the only electrons directly involved, as we can see from the curly arrows. Such a reaction is called a [4 + 2] cycloaddition. We now examine the signs of the coefficients of the frontier orbitals on the atoms which are to become bonded (Fig. 4-1). We are not yet concerned with the magnitude of the coefficients of the frontier orbitals, and therefore in this section all orbitals are drawn the same size, so as not to... 

More readily identifiable geometrical factors probably outweigh the contribution of the frontier orbitals in the remarkable reaction 6.47 between tetracyanoethylene and heptafulvalene giving the adduct 6.49 (see p. 261). The HOMO coefficients for heptafulvalene 6.420 (see p. 347) are highest at the central double bond, but a Diels-Alder reaction, with one bond forming at this site is impossible. The best reasonable possibility for a pericyclic cycloaddition, from the frontier orbital point of view, would be a Diels-Alder reaction across the 1,4-positions (HOMO coefficients of -0.199 and 0.253), but this evidently does not occur, probably because the carbon atoms are held too far apart. This is well-known to influence the rates of Diels-Alder reactions cyclopentadiene reacts much faster than cyclohexadiene, which reacts much faster than cycloheptatriene (see p. 302). The only remaining reaction is at the site which actually has the lowest frontier-orbital electron population, the antarafacial reaction across the 1, f-positions, which have HOMO coefficients of —0.199. 

In frontier orbital terms the regioselectivity of Diels-Alder reactions is mainly controlled by the atomic orbital coefficients of the frontier orbitals of both reaction partners. The regioselectivily observed for substituted dienes with a, )-unsaturated carbonyl compounds is shown below. With substituents at C-1 of the diene the 3,4-disubstituted cyclohex-1-enes are preferably obtained. The formation of 1,4-disubstituted cyclohex-1-enes is favored in cycloaddition reactions of C-2-substituted dienes. In cycloaddition reactions of electron-poor alkenes with 1,3-disubstituted dienes generally a preference for the 1,3,4-trisubstituted cyclohex-1-enes is observed. 

Regioselectivity An increase in the regioselectivity in the catalyzed reactions can also be explained on the basis of FMO interactions. More effective overlap of the orbitals in the transition state of a Diels—Alder reaction takes place when the reacting compounds are oriented in such a way that the atom with the largest coefficient in the dienophile interacts preferentially with the... 

In this reaction, formation of exo adduct predominates. This is in contrast to the Diels—Alder reaction, which gives endo adduct as the major product. Moreover, the [6 + 4] cycloaddition takes place in preference to a Diels—Alder [4 + 2] cycloaddition (both are thermally allowed). Such a situation is known as periselectivity and is explained by the fact that the coefficients of the frontier molecular orbitals of the LUMO of the tropone are highest at atoms C-2 and C-7. It has been found that the ends of conjugated systems have the largest coefficients in the frontier orbitals, and in accordance with the orbital symmetry rules, pericyclic reactions make use of the longest part of such systems. However, such reactions have to be permissible by the geometry of the molecule. 

Secondary orbital interactions of alkenes bearing substituents having a n-bond lead to a preference for the endo product in the Diels-Alder reaction. This reaction is regioselective due to interactions of orbital coefficients on the orbitals of the reacting atoms. Substituents at Cl and C4 of the diene move in a disrota-tory manner and opposite, relative to the incoming alkene. 

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