Woodward-Hoffinann

In contrast to pentadienyl anions, heptatrienyl anions have been found to cyclize readily to cycloheptadienyl anions. The transformation of heptatrienyl anion to cyclo-heptadienyl anion proceeds with a half-life of 13 min at — 13°C. The Woodward-Hoffinann rules predict that this would be a conrotatory closure. ... 

The reverse reaction, closure of butadiene to cyclobutene, has also been explored computationally, using CAS-SCF calculations. The distrotatory pathway is found to be favored, although the interpretation is somewhat more complex than the simplest Woodward-Hoffinann formulation. It is found that as disrotatory motion occurs, the singly excited state crosses the doubly excited state, which eventually leads to the ground state via a conical intersection. A conrotatory pathway also exists, but it requires an activation energy. 

Such 2 + 2 cycloadditions also confirm the generality of Woodward-Hoffinann rule. 

Thus, the apparent paradox lies in the fact that radical and radical-ion electrocyclic reactions are all forbidden in the Woodward-Hoffinann sense because the symmetry of the singly occupied molecular orbital (SOMO) changes... 

The ene reaction is rather different in orbital terms. For the Woodward-Hoffinann description of the reaction we must use the two electrons of the C-H bond to replace the two electrons of the double bond in the Diels-Alder reaction, but we must make sure that all the orbitals are parallel, as shown. 

The Conservation of Orbital Symmetry (Woodward-Hoffinann Rules)... 

According to Pearson, whether the bonds being broken in the reagents correspond (after symmetrization) to the bonds being formed in the product is an adequate and equivalent criterion for the reaction to be allowed or forbidden in the sense of Woodward-Hoffinann. Apart from simplicity, the efficiency of the rule of bond symmetry is also ensured by the fact that it is equally applicable to it- and cr-bonds. Hence, its predictions are equally applicable to both the full-face ethylene dimerization (Scheme 4a) or to the Diels-Alder reaction (Scheme 4b) and to addition reaction, e.g., the addition of molecular hydrogen to ethylene or butadiene. 

Furthermore, it is taken into account in the formulation of the principle of correspondence that the experimental confirmation of predictions in the sense of Woodward-Hoffinann extends much further than for unsubstituted molecules [25]. A kind of symmetry control without symmetry is observed in nature, e.g., the addition of propylene to ethylene obeys the same limitation with respect to orbital symmetry as the full-face ethylene dimerization. 

Hence, nature itself suggests the concepts of pseudosymmetry . The greater the difference between pseudosymmetry and symmetry, the weaker is the role of limitations in the sense of Woodward-Hoffinann and vice versa. The physical meaning of the correspondence principle is the postulation of the existence of not only qualitative but also quantitative correlation between pseudosymmetry as a measure of the quality and quantity of symmetry on the one hand and the polymerization rate on the other. 

Note that two stereoisomeric products that are consistent with the Woodward-Hoffinann rules can be obtained. The Woodward-Hoffmann rules allow you to predict the stereochemical relationship between substituents derived from the same component. They do not allow you to predict the relationship between substituents derived from different components. Guidelines for predicting the latter kind of relationship will be discussed shortly. 

Principle of correspondence. Reactions that, other conditions being equal, are in better correspondence with respect to symmetry in the sense of Woodward-Hoffinann proceed at higher rates. 

Optical purity, by NMR, 13, 14 Orbital correlation diagrams, 196-203 cycloaddition reactions, 197-196 Diels-Alder, 198 ethylene -E ethylene, 198 electrocyclic reactions, 198-200 butadienes, 199 hexatrienes, 199 limitations, 203 photochemical, 201 Woodward-Hoffinann, 197 Orbital energies, see also Energies, orbital degeneracy, 27, 90 Orbital interaction theory, 34-71 diagram, 40, 42, 47 limitations, 69-71 sigma bonds, 72-86 Orbitals... 

This reaction is allowed by the Woodward-Hoffinann rules. Both interactions are bonding. The same prediction would be arrived at if the HOMO of allyl cation and LUMO of 1,3-buta-diene were the orbitals considered. 

According to the Woodward-Hoffinann notation [3+2]-cycloaddition reactions are 7t s + Ti s pericyclic reactions. They enable, usually under thermal reaction conditions, the selective construction of carbo- and heterocyclic ring systems. Secondary orbital interactions as well as other factors controlling diastereofacial discrimination, regiochemistry or endotexo selectivity have been discussed in detail in the literature. " For many [3+2] cycloadditions it is not easy to predict, whether the 1,3-dipole functions as the donor or the acceptor component in these HOMO/LUMO-controlled conversions. " ... 

According to the Woodward-Hoffinann rules, the four n electrons, on which we are focusing, occupy the SS and SA orbitals from the beginning to the end of the reaction. This corresponds to low energy at the beginning of the reaction (R) but is very unfavorable at its end (P), because the unoccupied AS orbital is lower in the energy scale. 

For many years, pericyclic reactions were poorly understood and unpredictable. Around 1965, Robert B. Woodward and Roald Hoffmann developed a theory for predicting the results of pericyclic reactions by considering the symmetry of the molecular orbitals of the reactants and products. Their theory, called conservation of orbital symmetry, says that the MOs of the reactants must flow smoothly into the MOs of the products without any drastic changes in symmetry. In that case, there will be bonding interactions to help stabilize the transition state. Without these bonding interactions in the transition state, the concerted cyclic reaction cannot occur. Conservation of symmetry has been used to develop rules to predict which pericyclic reactions are feasible and what products will result. These rules are often called the Woodward-Hoffinann rules. 

In addition, Tsuda and Oikawa carried out molecular orbital calculations of the electronic structures in the excited states of poly(vinyl cinnamate). They based their calculations on the reaction of intermolecular concerted cycloaddition that take place according to the Woodward-Hoffinann s rule. This means that the cyclobutane ring formation takes place if a nodal plane exists at the central double bond in the lowest unoccupied MO(LLUMO) and not in the highest occupied MO(HOMO) of the ground state cinnamoyloxy group. This is within the picture of Huckel MO or Extended Huckel MO theory. The conclusion is that the concerted cycloadditions occur favorably in the lowest triplet state Ti and in the second excited singlet state S2. 

A Summary of the important discoveries made by these authors (Woodward-Hoffinann symmetry rules). 

Cycloadditions of ketenes and alkenes have been shown to have synthetic utility for the preparation of cyclobutanones. The stereoselectivity of ketene-alkene cycloaddition can be analyzed in terms of the Woodward-Hoffinann rules. To be an allowed process, the [27t + 27t] cycloaddition must be suprafacial in one component and antarafacial in the other. An alternative description of the transition state is a [271 -1- (271 + 271 )] addition. Figure 6.6 illustrates these transition states. The ketene, utilizing its low-lying LUMO, is the antarafacial component and interacts with the HOMO of the alkene. The stereoselectivity of ketene cycloadditions can be rationalized in terms of steric effects in this transition state. Minimization of interaction between the substituents R and R leads to a cyclobutanone in which these substituents are cis. This is the... 

The concerted mechanism is allowed by the Woodward-Hoffinann rules. The transition state involves the n electrons of the alkene and enophile and the a electrons of the C-H bond. 

Similarly for 2 + 2 cycloadditions the 2g + 2g mechanism is preferred for thermal reaction, whereas the alternative 2g + 2g mechanism dominates under the conditions of the photochemical reaction. The reaction mechanism which is preferred for a given type of reaction and reaction conditions (thermal vs. photochemical initiation) by the Woodward-Hoffinann rules is called symmetry-allowed, whereas the remaining alternative process is synunetiy-forbidden. 

This original intuitive formulation is not, however, completely satisfactory. This is due to the fact that the above simple approach is not completely general, since its applicability is restricted only to electrocyclic and sigmatropic reactions but not to cycloadditions. For that reason the studies speared early after the formulation of Woodward-Hoffinann rules, attempting to place these rules on a more sound theoretical basis. The most important of these alternative approaches is the technique of the so-called correlation diagrams [16,23-25]. 

In cormection with such examples of the violation of the Woodward-Hoffinann rules it is necessary to say, however, that the products are not usually formed in these reactions by the symmetry-forbidden concerted mechanism but by the nonconcerted stepwise process involving the participation of appropriate biradical or zwitterionic intermediates [39-42]. 

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