Pericyclic reactions general rules

It is a general rule for pericyclic reactions and predicts if a given pericyclic reaction will be allowed under a given set of conditions. The rule is as follows ... 

Applying these rules in pericyclic reactions it has been shown and a generalization given that thermal reactiom occur via aromatic transition states while photochemical reactions proceed via antiaromatic transition state. A cyclic transition state is considered to be aromatic or isoconjugate with the corresponding aromatic system if the member of conjugated atoms and that of the n... 

In previous sections we have seen how the CM model may be utilized to generate reaction profiles for ionic reactions, and it is now of interest to observe whether the same general principles may be applied to the class of pericyclic reactions, the group of reactions that is governed by the Woodward-Hoffmann (1970) rules. In other words, the question we ask is whether the concept of allowed and forbidden reactions may be understood within the CM framework. 

The general rule for all pericyclic reactions was formulated by Woodward and Hoffmann ([3], p. 169). A ground-state pericyclic change is symmetry allowed if the total number of (An + 2)s and (Am)a components is odd. 

It is generally a good idea to try to draw the dashed lines to make as many suprafacial components as you can, for it is possible to simplify the rule further when you do. If the total number of electrons involved is a (4n+2) number, the all-suprafacial reaction will be allowed, as in 3.21. This can be made to apply to a high proportion of the pericyclic reactions you will ever come across, and especially to cycloadditions, as we saw with a preliminary... 

The so-called aromaticity rules are chosen for comparison, as they provide a beautiful correspondence with the symmetry-based Woodward-Hoffmann rules. A detailed analysis [92] showed the equivalence of the generalized Woodward-Hoffmann selection rules and the aromaticity-based selection rules for pericyclic reactions. Zimmermann [93] and Dewar [94] have made especially important contributions in this field. 

Classification based on the Number of Electrons General Rule for Pericyclic Reactions... 

Formulated in the most general way, the Woodward-Hoffmann rule for thermal pericyclic reactions states ... 

The Woochvard-Huflimnn rules are a general expression of this. A pericyclic reaction which is entirely suprafacial is allowed thermally if 4n + 2 electrons are involved (Hiickel transition state), but forbidden for 4n electrons. If there is one antarafacial component, the reaction will be allowed thermally if 4n electrons are involved (Mobius transition state), but forbidden for 4n + 2 electrons. For photochemical reactions, these rules are reversed. Roald Hoffmann shared the Nobel prize for Chemistry with Kenichi Fukui in 1981 for his contribution to this concept Robert Burns Woodward had already won the prize in 1965. 

A great deal of effort has been expended to model the transition structures of concerted pericyclic reactions. All of the major theoretical approaches, semiempirical MO, ab initio MO, and DFT have been applied to the problem and some comparisons have been made. The conclusions drawn generally parallel the orbital symmetry rules in their prediction of reactivity and stereochemistry and provide additional insight into substituent effects. 

A generalization allows easy application of the Rules to any concerted pericyclic reaction involving two-electron bonds for the allowed reaction there must always be an odd number of suprafacial (as opposed to antarafacial) uses of bonds. Here, suprafacial is defined as utilizing both atoms of a C7 bond with retention of configuration or both with inversion with a Trbond, the p orbitals must be used from the same face of the bond. An antarafacial use of a cr bond would give retention at one atom and inversion at the other with a tt bond, the p orbitals must be used from opposite faces of the bond. 

The selection rules for cycloadditions (Table 11.1) apply to pericyclic reactions described as cycloadditions, but the rules may be generalized concise form as follows " ... 

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