HOMO highest occupied Diels-Alder reactions

The Diels-Alder reaction is believed to proceed m a single step A deeper level of understanding of the bonding changes m the transition state can be obtained by examining the nodal properties of the highest occupied molecular orbital (HOMO) of the diene and the lowest unoccupied molecular orbital (LUMO) of the dienophile... 

AMI semi-empirical and B3LYP/6-31G(d)/AMl density functional theory (DFT) computational studies were performed with the purpose of determining which variously substituted 1,3,4-oxadiazoles would participate in Diels-Alder reactions as dienes and under what conditions. Also, bond orders for 1,3,4-oxadiazole and its 2,5-diacetyl, 2,5-dimethyl, 2,5-di(trifluoromethyl), and 2,5-di(methoxycarbonyl) derivatives were calculated <1998JMT153>. The AMI method was also used to evaluate the electronic properties of 2,5-bis[5-(4,5,6,7-tetrahydrobenzo[A thien-2-yl)thien-2-yl]-l,3,4-oxadiazole 8. The experimentally determined redox potentials were compared with the calculated highest occupied molecular orbital/lowest unoccupied molecular orbital (HOMO/LUMO) energies. The performance of the available parameters from AMI was verified with other semi-empirical calculations (PM3, MNDO) as well as by ab initio methods <1998CEJ2211>. 

In the course of investigation of reactivity of the mesoionic compound 44 (Scheme 2) the question arose if this bicyclic system participates in Diels-Alder reactions as an electron-rich or an electron-poor component <1999T13703>. The energy level of the highest occupied molecular orbital (HOMO) and lowest unoccupied molecular orbital (LUMO) orbitals were calculated by PM3 method. Comparison of these values with those of two different dienophiles (dimethyl acetylenedicarboxylate (DMAD) and 1,1-diethylamino-l-propyne) suggested that a faster cycloaddition can be expected with the electron-rich ynamine, that is, the Diels-Alder reaction of inverse electron demand is preferred. The experimental results seemed to support this assumption. 

In 1960, Yates and Eaton (192) demonstrated that Lewis acids can dramatically accelerate the Diels-Alder reaction. In principle, any transformation wherein coordination of a Lewis acid may reduce the gap between the highest occupied molecular orbital (HOMO) and lowest unoccupied molecular orbital (LUMO) of a given set of reactants should be susceptible to Lewis acid catalysis. Indeed, numerous important carbon-carbon bond-forming organic transformations have been shown to be amenable to rate acceleration by a Lewis acid. In many cases, the use... 

As applied to cycloaddition reactions the rule is that reactions are allowed only when all overlaps between the highest occupied molecular orbital (HOMO) of one reactant and the lowest unoccupied molecular orbital (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 mono-alkenes have two n molecular orbitals (p. 10) and that conjugated dienes have four (p. 38), as shown in Fig. 15.2. A concerted cyclization of two monoalkenes (a [2 + 2]-reaction) is not allowed because it would require that a positive lobe overlap with a negative lobe (Fig. 15.3). On the other hand, the Diels-Alder reaction (a [4 + 2]-reaction) is allowed, whether considered from either direction (Fig. 15.4). 

Dienes that contain electron-donating groups (activated dienes) are more reactive in Diels-Alder reactions than unsubstituted or electron-deficient dienes. In molecular orbital formalism, the substituents on the diene perturb the tT-electron density to cause an increase in the energy of the highest occupied molecular orbital (HOMO Figure 1). In a normal-demand Diels-Alder reaction this results in an increase in the interaction between the HOMO of the diene and the LUMO (lowest unoccupied molecular orbital) of the dienophile. This interaction, in turn, lowers the transition state energy of the reaction. Similar arguments have also been used to explain the increased reactivity of activated dienes towards heterodienophiles such as aldehydes. 

The two new a bonds that are formed in a Diels-Alder reaction result from a transfer of electron density between the reactants. Molecular orbtial theory provides an insight into this process. When electrons are transferred between molecules, we must use the HOMO (highest occupied molecular orbital) of one reactant and the LUMO (lowest unoccupied molecular orbital) of the other because only an empty orbital can accept electrons. It doesn t matter whether we use the HOMO of the dienophile and the LUMO of the diene or the HOMO of the diene and the LUMO of the dienophile. We just need to use the HOMO of one and the LUMO of the other. 

The cycloaddition of alkenes and dienes is a very useful method for forming substituted cyclohexenes. This reaction is known as the Diels-Alder reaction The concerted nature of the mechanism was generally accepted and the stereospecificity of the reaction was firmly established before the importance of orbtial symmetry was recognized. In the terminology of orbital symmetry classification, the Diels-Alder reaction is a [AUg + lUg] cycloaddition, an allowed process. The transition state for a concerted reaction requires that the diene adopt the s-cis conformation. The diene and substituted alkene (which is called the dienophile) approach each other in approximately parallel planes. The symmetry properties of the n orbitals permit stabilizing interations between C-1 and C-4 of the diene and the dienophile. Usually, the strongest interaction is between the highest occupied molecular orbital (HOMO) of the diene and the lowest unoccupied molecular orbital (LUMO) of the dienophile. The interaction between the frontier orbitals is depicted in Fig. 6.1. 

The Diels-Alder reaction is driven by the energy differences between the highest occupied molecular orbital (HOMO) of the... 

Figure 24.1 shows the molecular orbitals (MOs) for both ethene and 1,3-butadiene and identifies the highest occupied molecular orbitals (the HOMOs) and the lowest unoccupied molecular orbitals (the LUMOs) for both compounds. The MOs shown include the p-orbitals on each carbon atom. How are these molecular orbitals constructed First, only the p-orbitals of the diene and the alkene are used because these orbitals are involved in the Diels-Alder reaction. An alkene has two jt-electrons, so it will have two n-molecrdar orbitals. The two orbitals cannot be of the same energy, and the two electrons will reside, spin paired (see Chapter 3, Section 3.1.2), in the lowest energy orbital. 

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