Nodal properties 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... 

The ease with which the reaction proceeds is directly related to the property or behaviour of these particular MO s connecting these to the phenomena of orientation or stereoselection. The electron distribution (valence-inactive population) plays a leading role in the interaction between the particular orbitals, HO, LU, and SO, in usual molecules, no matter whether they are saturated or unsaturated, and determines the orientation in the molecule in the case of chemical interaction. In that case, the extension and the nodal property of these particular MO s decide the spatial direction of occurrence of interaction. 

Fukui recognized the importance of the symmetry properties of HOMOs and LUMOs perhaps for the first time in connection with the Diels-Alder reaction. According to his Nobel lecture [50], however, it was only after the appearance of the papers by Woodward and Hoffmann in 1965 that he became fully aware that not only the density distribution but also the nodal property —that is, symmetry— of the particular orbitals have significance in. .. chemical reactions. ... 

Analysis of a reaction by frontier orbital theory has additional benefits, particularly for predicting reactivity and stereochemistry. Woodward and Hoffman pointed out "that electrocyclic reactions followed the stereochemistry dictated by the symmetry, or nodal properties of the HOMO of the polyene".This concept of orbital symmetry will be important for discussions of all pericyclic reactions. Of particular importance is the difference in energy between the HOMO one Ji system and the LUMO of a second Jt-system, because this will be used to predict reactivity in pericyclic reactions (see below). 

Even in highly uns5unmetrical molecules the above picture can be used to visualize the course of chemical reactions, and to draw conclusions about mechanism. It is necessary to have information about the nodal properties of the valence shell molecular orbitals. For many purposes, bond orbitals and anti-bond orbitals will suffice. 

Woodward and Hoffmann s analysis of this set of reactions [1, pp. 38-45] is too familiar to require more than a brief recapitulation The first member of the series, s-cz5-butadiene cyclobutene, will be discussed in some detail, and the Rules for its higher homologs will be shown to follow directly from the nodal properties of the polyenes. 

Important is also the fact that, as a rule, the localization of frontier orbitals at the interacting centers leads to the narrowing of the gap between energy levels of the orbitals and thereby to the enhancement of orbital interactions [3]. If the symmetry (nodal) properties of the frontier MO s of reactants are such that an effective overlapping of the orbitals is impossible, then the mechnism described cannot be realized for the given geometry of approach. This conclusion constitutes the basis for the principle of stereoelectronic selection of appropriate reaction pathways. 

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. 

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