Electrocyclic reactions orbital symmetry analysis

The prediction on the basis of orbital symmetry analysis that cyclization of eight-n-electron systems will be connotatoiy has been confirmed by study of isomeric 2,4,6,8-decatetraenes. Electrocyclic reaction occurs near room temperature and establishes an equilibrium that favors the cyclooctatriene product. At slightly more elevated temperatures, the hexatriene system undergoes a subsequent disrotatory cyclization, establishing equilibrium with the corresponding bicyclo[4.2.0]octa-2,4-diene ... 

Electrocyclic reactions are not really different from cycloadditions. Figure 20.27 compares the equilibration of 1,3-butadiene and cyclobutene with the 2 + 2 dimerization of a pair of ethylenes. The only difference is the extra o bond in butadiene, and this bond is surely not one of the important ones in the reaction—it seems to be just going along for the ride. Why should its presence or absence change the level of detail av able to us through an orbital symmetry analysis It shouldn t, and in fact, it doesn t. 

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. 

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). 

Analysis of the symmetry properties of hexatriene MO s (Fig. 10.2) follows the same reasoning and leads to a strikingly different conclusion, which is in complete agreement with the experimental observations. Since there are six ir-electrons, 3 is the HOMO, and a bonding interaction will occur only for disrotatory closure. Consideration of orbital symmetries for other it-systems leads to the conclusion that concerted electrocyclic reactions in systems containing 4n + 2 7r-electrons should be disrotatory. 

Generalization of either the frontier orbital, the orbital symmetry, or the transition-state aromaticity analysis leads to the same conclusion about the preferred stereochemistry for concerted thermal electrocyclic reactions The stereochemistry is a function of the number of electrons involved. Processes involving 4n + 2 electrons will be disrotatory those involving 4n electrons will be conrotatory for Hiickel transition states. The converse holds for Mobius transition states. 

Although it is more fruitfiil to constmct a correlation diagram for the detailed analysis of an electrocyclic reaction, there is, nevertheless, an alternative method that also enables us to reach similar conclusions. In this approach, which is extremely simple, our only guide is the symmetry of the highest occupied molecular orbital (HOMO) of the open-chain partner in an electrocyclic reaction. If this orbital has a C2 symmetry, then the reaction follows a conrotatory path, and if it has a mirror plane symmetry, a disrotatory mode is observed. The explanation for this alternative approach is based on the fact that overlapping of wave functions of the same sign is essential for bond formation. 

In their now classic monograph [1], Wooodward and Hoffmann concentrate on three basic types of no mechanism reaction Electrocyclic reactions -notably polyene cyclizations, cycloadditions, and sigmatropic rearrangements. These three reaction types will be taken up in this and the next two chapters from the viewpoint of Orbital Correspondence Analysis in Maximum Symmetry (OCAMS) [2, 3, 4], the formalism of which follows naturally from that developed in Chapter 4. The similarities to the original WH-LHA approach [5, 6], and the points at which OCAMS departs from it, will be illustrated. In addition, a few related concepts, such as allowedness and forbiddenness , global vs. local symmetry, and concertedness and synchronicity , will be taken up where appropriate. 

Analysis of electrocyclic reactions using a variety of methods and the various conclusions that are drawn. A. FMO theory for ring-opening. The LUMOs of the ir systems are compared to the HOMO of the C-C o bond in cyclobutene and 1,3-cyclohexadiene. B. The Hiickel/Mobius approach. C. Using the generalized orbital symmetry rule. Note, as always, that all the methods predict the same outcome. 

The inclusion of the electron correlation energy in the theoretical analysis of chemical reactions becomes a must when electron bond pairs of starting structures are destroyed during reaction. This concerns in the first place the reactions of bond breaking since a calculation in terms of the HF approximation leads to incorrect dissociated states, such as F2 F" + F rather than 2F. Also the reactions proceeding via biradical type structures (see Sect. 1.6 and 8.3) and those forbidden by the orbital symmetry conservation rules fall within the category of such transformations. A typical example is provided by the electrocyclic-type reaction ... 

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