Diels-Alder reactions orbital overlaps

Woodward and Hoffmann pointed out that the Diels-Alder reaction involved bonding overlap of the highest-occupied molecular orbital (HOMO) on the diene and the lowest-unoccupied molecular orbital (LUMO) on the dienophile. Display the HOMO for 2-methoxybutadiene. Where is it localized Display the LUMO for acrylonitrile. Where is it localized Orient the two fragments such that the HOMO and LUMO best overlap (A clearer picture is provided by examining-the HOMO map for 2-methoxybutadiene and the LUMO map for acrylonitrile.) Which product should result ... 

Lewis acids catalyze Diels-Alder reactions. Do they enhance overlap between diene and dienophile orbitals and/ or do they reduce the HOMO/LUMO energy difference ... 

An important contribution for the endo selectivity in the carho-Diels-Alder reaction is the second-order orbital interaction [1], However, no bonds are formed in the product for this interaction. For the BF3-catalyzed reaction of acrolein with butadiene the overlap population between Cl and C6 is only 0.018 in the NC-transi-tion state [6], which is substantially smaller than the interaction between C3 and O (0.031). It is also notable that the C3-0 bond distance, 2.588 A, is significant shorter than the C1-C6 bond length (2.96 A), of which the latter is the one formed experimentally. The NC-transition-state structure can also lead to formation of vinyldihydropyran, i.e. a hetero-Diels-Alder reaction has proceeded. The potential energy surface at the NC-transition-state structure is extremely flat and structure NCA (Fig. 8.6) lies on the surface-separating reactants from product [6]. 

Endo products result from Diels-Alder reactions because the amount of orbital overlap between diene and dienophile is greater when the reactants lie directly on top of one another so that the electron-withdratving substituent on the dienophile is underneath the diene. In the reaction of 1,3-cyclopentadiene with maleic anhydride, for instance, the following result is obtained ... 

The diene must adopt what is called an s-cis conformation, meaning "cis-like" about the single bond, to undergo a Diels-Alder reaction. Only in the s-cis conformation are carbons 1 and 4 of the diene close enough to react through a cyclic transition state. In the alternative s-trans conformation, the ends of the diene partner are too far apart to overlap with the dienophile p orbitals. 

As applied to cycloaddition reactions the rule is that reactions are allowed only when all overlaps between the HOMO of one reactant and the LUMO of the other are such that a positive lobe overlaps only with another positive lobe and a negative lobe only with another negative lobe. We may recall that monoalkenes have two n molecular orbitals (p. 9) and that conjugated dienes have four (p. 36), as shown in Figure 15.1. A concerted cyclization of two monoalkenes (a 2 -f- 2 reaction) is not allowed because it would require that a positive lobe overlap with a negative lobe (Fig. 15.2). On the other hand, the Diels-Alder reaction (a 2 -f 4 reaction) is allowed, whether considered from either direction (Fig. 15.3). 

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. 

In contrast with exo (top) facial selectivity in the additions to norbomene 80 [41], Diels-Alder reaction between isodicyclopentadiene 79 takes place from the bottom [40] (see Scheme 32). To solve this problem, Honk and Brown calculated the transition state of the parent Diels-Alder reaction of butadiene with ethylene [47], They pointed ont that of particular note for isodicyclopentadiene selectivity issue is the 14.9° out-of-plane bending of the hydrogens at C2 and C3 of butadiene. The bending is derived from Cl and C4 pyramidalization and rotation inwardly to achieve overlap of p-orbitals on these carbons with the ethylene termini. To keep the tr-bonding between C1-C2 and C3-C4, the p-orbitals at C2 and C3 rotate inwardly on the side of the diene nearest to ethylene. This is necessarily accompanied by C2 and C3 hydrogen movanent toward the attacking dienophile. They proposed that when norbomene is fused at C2 and C3, the tendency of endo bending of the norbomene framework will be manifested in the preference for bottom attack in Diels-Alder reactions (Schane 38). 

Diels-Alder reactions are found to be little influenced by the introduction of radicals (cf. p. 300), or by changes in the polarity of the solvent they are thus unlikely to involve either radical or ion pair intermediates. They are found to proceed stereoselectively SYN with respect both to the diene and to the dienophile, and are believed to take place via a concerted pathway in which bond-formation and bond-breaking occur more or less simultaneously, though not necessarily to the same extent, in the transition state. This cyclic transition state is a planar, aromatic type, with consequent stabilisation because of the cyclic overlap that can occur between the six p orbitals of the constituent diene and dienophile. Such pericyclic reactions are considered further below (p. 341). 

The mechanism of the Diels-Alder reaction involves a-overlap of the n-orbitals of two unsaturated systems. One molecule must donate electrons, from its highest occupied molecular orbital (HOMO), to the lowest unoccupied molecular orbital (LUMO) of the other. Also, the two interacting orbitals must have identical symmetry i.e. the phases of the terminal p-orbitals of each molecular orbital must match. There are two possible ways for this to happen the HOMO of the diene combining with the LUMO of the dienophile, and the LUMO of the diene with the HOMO of the dienophile (Figure 7.1). 

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... 

The interactions of the occupied orbitals of one reactant with the unoccupied orbitals of the other are described by the third term of the Klopman-Salem-Fukui equation. This part is dominant and the most important for uncharged reaction partners. Taking into account that the denominator is minimized in case of a small energy gap between the interacting orbitals, it is clear that the most important interaction is the HOMO-LUMO overlap. With respect to the Diels-Alder reaction, one has to distinguish between two possibilities depending on which HOMO-LUMO pair is under consideration. The reaction can be controlled by the interaction of the HOMO of the electron-rich diene and the LUMO of the electron-poor dienophile (normal electron demand) or by the interaction of the LUMO of an electron-poor diene and the HOMO of an electron-rich dienophile (inverse electron demand cf Figure 1). 

The distance between the two terminal carbon atoms at the TS (central point on the IRC segment) is already 2.291 A, which results in very perceptible changes in the orbital shapes, spin-coupling pattern and overlaps between neighbouring orbitals. Orbital /i (see the central column of orbitals in Fig. 5) becomes less distorted towards the orbital at the other terminal carbon, /g, and this is reflected in a decrease in their overlap (see Fig. 7). Orbitals /2 and /3 (and their symmetry-related counterparts, /5 and /4) attain shapes which are very similar to those of /2 from the TS of the Diels-Alder reaction (see the central column of orbitals in Fig. 1) and of a SC orbital for benzene [8-10]. These changes are accompanied by a tendency towards equalization of the nearest-... 

L in Scheme 11.3) departs. Nucleophilic addition to the intermediate benzyne (step D) is readily explained by perturbative MO arguments. The extra and orbitals of benzyne are compared to those of ethylene in Figure 11.7. The aromatic n system is not involved in the special properties of benzyne. The third benzyne n bond is due to the overlap in fashion of the two sp2 hybrid orbitals which lie in the nodal plane of the intact 6 electron system. Two factors contribute to a very low LUMO for benzyne. First, the sp2 hybrid orbitals are lower in energy than the 2p orbitals from which the ethylene orbitals are constructed. Second, the intrinsic interaction between the two sp2 orbitals is less than the normal / cc since the orbitals have less p character and are tipped away from each other. The low LUMO of benzyne makes the molecule a strong Lewis acid, susceptible to attack by bases, and a reactive dienophile in Diels-Alder reactions, as we shall see later. 

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-... 

In [4 + 2]-cycloadditions the diene reacts in the cisoid conformation,530 which is essential for p-orbital overlapping in the diene and the dienophile and to form the new double bond in the six-membered ring. Diels-Alder reactions are regioselec-tive. Of the two possible isomers formed in the cycloaddition of nonsymmmetri-cally substituted reactants [Eq. (6.87)], usually one predominates depending on the structure of the reacting molecules and reaction conditions ... 

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