Pericyclic reactions component analysis

A component analysis of pericyclic reactions proceeds by the following steps, which are illustrated for each of the classes of reaction in Figure 12.4 ... 

Figure 12.4. Procedure for general component analysis illustrated for each of the three types of pericyclic reactions.

Pericyclic reactions are described in Chapter 12 as a special case of frontier orbital interactions, that is, following Fukui [1]. However, the stereochemical nomenclature supra-facial and antarafacial and the very useful general component analysis of Woodward and Hoffmann [3] are also introduced here. 

The conclusion we may draw from this analysis is that in pericyclic reactions of these kinds we shall always be able to discover the inherent symmetry of the interaction topologically by considering the system as being made up of suitable components, even when there is no actual symmetry maintained in the molecule as a whole. We shall therefore be able to analyze the situation in terms of the... 

This opens up the possibility of a systematic investigation of pericyclic reactions not only for model cases of parent unsubstituted systems, but for inclusion if zwitterionic contributions also enable the analysis of the eventual mechanistic changes induced by the polar substitution. As an example, the push-pull substituted Diels-Alder system will be analysed, in which the diene component is substituted in position 1 by a donor, and dienophilic component in position 6 by an acceptor substitution. In order to avoid the problems with the relative wieght of individual limiting structures of the intermediate (Eq. 30), the coulombic integrals modelling the substitution in the HMO wave function were arbitrarily set to a = 3/ and a = — 3) so that there is sufficient polarity in the system to secure the approximation of the intermediate by pure zwitterionic structure Z,. 

Pericyclic reactions, 161-174 cheletropic, 165-166 component analysis, 167-168 rule for, 168... 

As is the case for other pericyclic reactions, the selection rules for a thermal [i, ] sigmatropic reaction are reversed for the photochemical reaction. If irradiation of a 1,5-hexadiene produces the electronically excited state of one and only one of the two allyl components, then the HOMO of one component is (/f3, and the HOMO of ihe other component is suprafacial-suprafacial reaction (Figure 11.46) is forbidden (as is the antarafacial-antar-afacial pathway), but the antarafacial-suprafacial and suprafacial-antarafacial pathways are allowed (Figure 11.47). Analysis of higher sigmatropic reactions shows that the selection rules also reverse with the addition of a carbon-carbon double bond to either of the n systems. Thus, the [3,5] sigmatropic reaction is thermally allowed to be suprafacial-antarafacial or antarafacial-suprafacial and photochemically allowed to be suprafacial- suprafacial or antarafacial-antarafacial. Two of these reaction modes are illustrated in Figure 11.48. 

Although it is more fruitful to construct a correlation diagram for the detailed analysis of a pericyclic reaction, there is, nevertheless, an altemative method that also enables us to reach similar conclusions. It is an easy and extremely simple approach that is based on the interaction of the frontier orbitals, i.e., the highest occupied molecular orbital (HOMO) and the lowest unoccupied molecular orbital (LUMO) of the components that are involved in a pericyclic reaction. 

This is a very powerful rule, and it is especially useful when there are several components to a pericyclic reaction. With several components it is often difficult to identify the appropriate HOMOs and LUMOs for an FMO analysis, and difficult to quickly write an orbital or state correlation diagram. In such cases, aromatic transition state theory, or the generalized orbital symmetry rule, are the easiest approaches for analyzing the reaction. It is your decision as to which works best for you. 

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