Symmetry rules

Correlation diagrams can be constructed in an analogous fashion for the disrotatory and conrotatory modes for interconversion of hexatriene and cyclohexadiene. They lead to the prediction that the disrotatory mode is an allowed process whereas the conrotatory reaction is forbidden. This is in agreement with the experimental results on this reaction. Other electrocyclizations can be analyzed by the same method. Substituted derivatives of polyenes obey the orbital symmetry rules, even in cases in which the substitution pattern does not correspond in symmetiy to the orbital system. It is the symmetry of the participating orbitals, not of the molecule as a whole, that is crucial to the analysis. 

Is the reaction concerted As was emphasized in Chapter 11, orbital symmetry considerations apply only to concerted reactions. The possible involvement of triplet excited states and, as a result, a nonconcerted process is much more common in photochemical reactions than in the thermal processes. A concerted mechanism must be established before the orbital symmetry rules can be applied. 

Because a [1,5] sigmatropic rearrangement involves three electron pairs (two ir bonds and one cr bond), the orbital-symmetry rules in Table 30.3 predict a suprafacial reaction. In fact, the 1,5] suprafacial shift of a hydrogen atom across... 

The last decade has witnessed an unprecedented strengthening of the bone between theory and experiment in organic chemistry. Much of this success may be credited to the development of widely applicable, unifying concepts, such as the symmetry rules of Woodward and Hoffmann, and the frontier orbital thee>ry of Eukui. Whereas the the ore tical emphasis had historically been on detailed structure and spectroscopy, the new methods are de signe d to solve pre)blems e>f special importance to organic chemists reactivity, stereochemistry, and mechanisms. 

Orbital symmetry rules for inorganic reactions from perturbation theory. R. G. Pearson, Top. Curr. Chem., 1973, 41, 75-112 (53). 

In this method, the orbital symmetry rules are related to the Hiickel aromaticity rule discussed in Chapter 2. Huckel s mle, which states that a cyclic system of electrons is aromatic (hence, stable) when it consists of 4n + 2 electrons, applies of course to molecules in their ground states. In applying the orbital symmetry principle, we are not concerned with ground states, but with transition states. In the present method, we do not examine the molecular orbitals themselves but rather the p orbitals before they overlap to form the MO. Such a set of p orbitals is called a basis set (Fig. 15.5). In investigating the possibility of a concerted reaction, we put the basis sets into the position they would occupy in the transition state. Figure 15.6 shows this for both the... 

There is evidence that the reactions can take place by all three mechanisms, depending on the structure of the reactants. A thermal [ 2, + 2s] mechanism is ruled out for most of these substrates by the orbital symmetry rules, but a [ 2s + mechanism is allowed (p. 1072), and there is much evidence that ketenes and certain other linear molecules in which the steric hindrance to such an approach is... 

Dihydrothiophene-1,1-dioxides (42) and 2,17-dihydrothiepin-1,1-dioxides (43) undergo analogous 1,4 and 1,6 eliminations, respectively (see also 17-38). These are concerted reactions and, as predicted by the orbital-symmetry rules (p. 1067), the former is a suprafacial process and the latter an antarafacial process. The rules also predict that elimination of SO2 from episulfones cannot take place by a concerted mechanism (except antarafacially, which is unlikely for such a small ring), and the evidence shows that this reaction occurs by a non-concerted pathway.The eliminations of SO2 from 42 and 43 are examples of cheletropic reactions, which are defined as reactions in which two a bonds that terminate at a single atom (in this case the sulfur atom) are made or broken in concert. 

Although the orbital-symmetry rules predict the stereochemical results in almost all cases, it is necessary to recall (p. 1070) that they only say what is allowed and what is forbidden, but the fact that a reaction is allowed does not necessarily mean that the reaction takes place, and if an allowed reaction does take place, it does not necessarily follow that a concerted pathway is involved, since other pathways of lower energy may be available.Furthermore, a forbidden reaction might still be made to go, if a method of achieving its high activation energy can be found. This was, in fact, done for the cyclobutene butadiene interconversion (cis-3,4-dichloro-cyclobutene gave the forbidden cis.cis- and rran.y, ra i -l,4-dichloro-1,3-butadienes,... 

In any given sigmatropic rearrangement, only one of the two pathways is allowed by the orbital-symmetry rules the other is forbidden. To analyze this situation we first use a modified frontier-orbital approach. We will imagine that in the transition state the migrating H atom breaks away from the rest of the system, which we may treat as if it were a free radical. 

The orbital symmetry rules also help us to explain, as on pages 1083 and 1433, the unexpected stability of certain compounds. Thus, 102 could, by a thermal [1,3] sigmatropic rearrangement, easily convert to toluene, which of course is far more stable because it has an aromatic sextet. Yet 102 has been prepared and is stable at dry ice temperature and in dilute solutions. ... 

The mechanisms of these reactions are not completely understood, although relief of strain undoubtedly supplies the driving force. The reactions are thermally forbidden by the orbital-symmetry rules, and the role of the catalyst is to provide low-energy pathways so that the reactions can take place. The type 1 reactions are the reverse of the catalyzed [2 + 2] ring closures discussed at 15-61. The following... 

Woodward and Hoffmann presented an orbital symmetry rule for pericychc reactions [12, 13]. 

Recently, a symmetry rule for predicting stable molecular shapes has been developed by Pearson Salem and Bartell" . This rule is based on the second-order, or pseudo, Jahn-Teller effect and follows from the earlier work by Bader . According to the symmetry rule, the symmetries of the ground state and the lowest excited state determine which kind of nuclear motion occurs most easily in the ground state of a molecule. Pearson has shown that this approximation is justified in a large variety of inorganic and small organic molecules. 

We now move to the predictions based on the symmetry rule. Comparing the lowest excitation energies (the sixth column) with the results concerning the symmetry reduction obtained on the basis of the dynamic theory (the last column), we can draw a very clear-cut criterion for the symmetry reduction ... 

The symmetries of the lowest excited states listed in Table 1 are nothing but the symmetries to which the most soft second-order bond distortions belong. It is seen that the types of symmetry reduction predicted using the symmetry rule are in complete agreement with those obtained on the basis of the dynamic theory. 

Next, we consider the application of the symmetry rule to the prediction of the geometrical structures of some other related systems. 

Sevilla et The analysis of the hyperfine spectra of these radicals has revealed that the cation radical should have D2H symmetry, whereas the molecular symmetry of the anion radical should be lower than the apparently-full molecular symmetry, i. e., We now examine the molecular symmetries of the anion and cation radicals of heptafulvalene, together with those of its 5-membered analogue, fulvalene PCXI) using the symmetry rule. 

The lowest excited state of azulene is predicted to possess 2 symmetry, which is in agreement with the result obtained using the symmetry rule. A recent vibrational analysis of the longest wave-length absorption band in the electronic spectrum of azulene indicates that the lowest-excited state would possess C2 symmetry . ... 

Fulvalene and heptafulvalene are predicted, in agreement with the result obtained using the symmetry rule, to suffer a symmetry reduction 2v in their lowest excited states. The longest wave-length electronic absorption bands of these molecules are expected to be relatively broad. This seems to be what is observed . On the other hand, the lowest excited state of sesquifulvalene is predicted not to undergo symmetry reduction, which again supports the prediction based on the symmetry rule. 

The mechanism of reaction of elec- 12 trophiles with some o-bonded (25) organotransition ions Symmetry rules for chemical reac- 9 tions (20)... 

Orbital symmetry rules and the 16 mechanism of inorganic reactions (25) TT-Complex mechanism of catalysis 14... 

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