Molecular orbitals, Diels-Alder reactions

Molecular orbital concepts, in MA copolymerization, 321, 403 Molecular orbitals, Diels-Alder reactions, 140-143 Molecular weights... 

The mechanism of the Diels-Alder reaction is best understood on the basis of a molecular orbital approach To understand this approach we need to take a more detailed look at the rr orbitals of alkenes and dienes... 

A TT MOLECULAR ORBITAL ANALYSIS OF THE DIELS-ALDER REACTION... 

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... 

Molecular orbital calculations indicate that cyclo C-18 carbyne should be relatively stable and experimental evidence for cyclocarbynes has been found [25], Fig. 3B. Diederich et al [25] synthesised a precursor of cyclo C-18 and showed by laser flash heating and time-of flight mass spectrometry that a series of retro Diels-Alder reactions occurred leading to cyclo C-18 as the predominant fragmentation pattern. Diederich has also presented a fascinating review of possible cyclic all-carbon molecules and other carbon-rich nanometre-sized carbon networks that may be susceptible to synthesis using organic chemical techniques [26]. 

Frontier-molecular-orbital Interactions for Carbo-Diels-Alder Reactions... 

The hetero-Diels-Alder reaction of aldehydes 12 with 2-azabutadienes 13 (Scheme 8.5) has been studied using high-level ab-initio multiconfigurational molecular orbital and density functionality calculation methods [28]. 

Keywords abinitio methods, FMO (molecular orbital), hefero-Diels-Alder reaction, transition state structure... 

Dannenberg J. J. The Molecular Orbital Modeling of Free Radical and Diels-Alder Reactions Adv. Mol. Model. 1990 2 1-63... 

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). 

Inagaki, Fujimoto and Fukui demonstrated that ir-facial selectivity in the Diels-Alder reaction of 5-acetoxy- and 5-chloro-l,3-cyclopentadienes, 1 and 2, can be explained in terms of deformation of a frontier molecular orbital FMO [2], The orbital mixing rule was proposed to predict the nonequivalent orbital deformation due to asymmetric perturbation of the substituent orbital (Chapter Orbital Mixing Rules by Inagaki in this volume). 

The FMO of the diene having substituent X at the 5-positions is comprised of three molecular orbitals, namely, jt-HOMO of the diene part, a-orbital of carbon framework, and the nonbonding (n) orbital of X (Scheme 3). The FMO of the diene for Diels-Alder reactions should mainly consist of n-HOMO. The jt-HOMO is antisymmetric with respect to reflection in the plane containing C5 carbon and its substitu-... 

Intramolecular inverse electron-demand Diels-Alder reaction of iV-propargyl-2-(pyrimidin-2-yl)pyrrolidine provides an alternative route to pyridopyrrolizines. For example, heating of 130 to 170 °C in nitrobenzene affords the cyclized product with the loss of HCN <1992JOC3000> (Equation 9). The above reference includes molecular orbital (MO) calculations on relative reactivities in this series. 

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>. 

The Diels-Alder reaction (47t 2ir cycloaddition) is by far the best studied reaction of dienes from both theoretical and experimental viewpoints. Frontier molecular orbital theory predicts three types of Diels-Alder reaction. Structural effects on rate constants show the existence of two types of reaction ... 

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... 

A fundamental understanding of the mechanistic and stereochemical aspects of the Diels-Alder reaction was unfolded during the 1970s. Several theoretical approaches are available nowadays from which Fukui s Frontier Molecular Orbital theory (FMO theory) is most frequently used because of its simplicity49- 53. 

This theory proves to be remarkably useful in rationalizing the whole set of general rules and mechanistic aspects described in the previous section as characteristic features of the Diels-Alder reaction. The application of perturbation molecular orbital theory as an approximate quantum mechanical method forms the theoretical basis of Fukui s FMO theory. Perturbation theory predicts a net stabilization for the intermolecular interaction between a diene and a dienophile as a consequence of the interaction of an occupied molecular orbital of one reaction partner with an unoccupied molecular orbital of the other reaction partner. 

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... 

Lewis-acid catalysis of Diels-Alder reactions (Figure 7.5) in organic solvents leads to an enhancement of the reaction rate, because of the lowering in energy for the lowest unoccupied molecular orbital (LUMO) of the dienophile, and an improvement in the selectivity with specific ligands. 

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