Molecular orbitals symmetry-allowed reactions

Let us now examine the Diels-Alder cycloaddition from a molecular orbital perspective Chemical experience such as the observation that the substituents that increase the reac tivity of a dienophile tend to be those that attract electrons suggests that electrons flow from the diene to the dienophile during the reaction Thus the orbitals to be considered are the HOMO of the diene and the LUMO of the dienophile As shown m Figure 10 11 for the case of ethylene and 1 3 butadiene the symmetry properties of the HOMO of the diene and the LUMO of the dienophile permit bond formation between the ends of the diene system and the two carbons of the dienophile double bond because the necessary orbitals overlap m phase with each other Cycloaddition of a diene and an alkene is said to be a symmetry allowed reaction... 

In absence of a catalyst, simple olefins are essentially fixed in their bonding configurations reaction paths to interconversions through molecular collisions, fusions, and disassociations are apparently closed because of orbital symmetry restrictions, as proposed by Hoffman and Woodward 8°). Mango 8 has postulated that in the presence of certain transition metal catalysts, these orbital symmetry restraints are lifted, allowing bonds to flow freely and molecular systems to interchange. Thus, the conservation of molecular orbital symmetry is a key function of the catalyst. 

Eq. 5). The essential feature of a symmetry-allowed reaction is the symmetry-match of occupied molecular orbitals in A with those in B and a similar match of unoccupied A and B orbitals. Thus a band of bonds (oc-... 

Another test of orbital symmetry comes from the reaction of methyl cations with ketones. ICR experiments have shown at least 10 different pathways for reaction of CD with acetone. One of the minor pathways produces CsHf cation. Thermodynamically it is plausible that the neutral products are formaldehyde and molecular hydrogen, as equation 10 depicts for the orbital-symmetry allowed decomposition of the... 

In the ground state, if the symmetry of MOs of the reactant matches that of the products that are nearest in energies, then reaction is thermally allowed. However, if the symmetry of MOs of the reactant matches that of the product in the first excited state but not in the ground state, then the reaction is photochemically allowed (Figure 1.12). When symmetries of the reactant and product molecular orbitals differ, the reaction does not occur in a concerted manner. It must be noted that a symmetry element becomes irrelevant when orbitals involved in the reaction are aU symmetric or antisymmetric. In conclusion, we can say that in pericyclic transformations, symmetry properties of the reactants and products remain conserved. 

If the symmetries of the orbitals of the reactants and products match, a concerted symmetry-allowed reaction occurs. If the symmetries of the reactants and products do not match, the reaction is symmetry forbidden (or sometimes, symmetry disallowed ). Symmetry-allowed reactions occur under reasonably mild conditions because the molecular orbitals can be smoothly transformed from reactant to product. Symmetry-forbidden reactions may actually occur, but they require much higher energies than symmetry-allowed reactions, and they occur by a different mechanism. They are not concerted, usually generate intermediates in multistep reactions, and are not stereospecific. 

MOLECULAR ORBITALS IN PERICYCLIC REACTIONS Symmetry-Allowed Reactions... 

According to frontier molecular orbital theory (FMO), the reactivity, regio-chemistry and stereochemistry of the Diels-Alder reaction are controlled by the suprafacial in phase interaction of the highest occupied molecular orbital (HOMO) of one component and the lowest unoccupied molecular orbital (LUMO) of the other. [17e, 41-43, 64] These orbitals are the closest in energy Scheme 1.14 illustrates the two dominant orbital interactions of a symmetry-allowed Diels-Alder cycloaddition. 

Each reaction species must have molecular orbitals available and with the correct symmetry to allow bonding. These will be called frontier orbitals composed of the highest occupied molecular orbital (HOMO) and the lowest unoccupied molecular orbital (LUMO). In addition to their involvement in bonding between species, these orbitals are of considerable interest in that they are largely responsible for many of the chemical and spectroscopic characteristics of molecules and species and are thus important in analytical procedures and spectroscopic methods of analysis [5-7],... 

If the reverse back reaction is prevented or is forbidden by other considerations, the energy remains stored in the photoproducts. Some simple photorearrangement reactions which are governed by Woodward-Hoffman rules have been found useful. These rules provide the stereochemical course of photochemical rearrangement based on symmetry properties of the highest occupied molecular orbital (HOMO) and the lowest unoccupied molecular orbital (LUMO) of the molecule (Section 8.6). A reaction which is photochemically allowed may be thermally forbidden. Front the principle of microscopic reversibility, the same will be true for the reverse reaction also. Thermally forbidden back reaction will produce. ble - photoproducts. Such electrocyclic rearrangements are given in . ..ure... 

---