Pericyclic reactions summary

The thermal favorability of pericyclic reactions can be predicted by looking for a bonding interaction between the HOMO of one reacting partner and the LUMO of the other partner. Pericyclic reactions usually involve the ends of the pi system(s). Therefore, the relative phase of the ends of the pi systems dictate whether a bonding or antibonding interaction results. The ends of the pi system are in phase in the fundamental MO and out of phase in the next higher one-node MO. This phase alternation continues as the energy increases. 

Therefore, if we derive or remember one rule for a pericyclic reaction, then any time an MO phase change is added the rule will reverse. Two reversals cancel each other. For example, 4n face to face (supra-supra) cycloadditions are not thermally allowed. If we add two electrons, we fill the next highest MO, which has a phase reversal. This means An+2 cycloadditions are thermally favored. Thermal electrocyclic reactions of 4n species go conrotatory, whereas thermal 4n+2 electrocyclic reactions go disrotatory. Thermal sigmatropic reactions of 4n species go supra-inversion or antara-retention. Count arrows to tell whether the pericyclic reaction is 4n or 4n + 2. Phase reversals occur between retention/inversion at the migrating center, between antarafacial/suprafacial migration, with 4n vs. 4n+2 electrons, and between thermal and photochemically excited species. 

Thermally allowed 4n + 2 electrocyclic ring openings go disrotatory. 

The rules reverse for every MO phase reversal that occurs in going to 4 electrons, to inversion, to antarafacial, to conrotatory, or to photochemically excited systems. 

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How can you keep straight all the rules about pericyclic reactions The summary information in Tables 30.1 to 30.3 can be distilled into one mnemonic phrase that provides an easy way to predict the stereochemical outcome of any pericyclic reaction ... 

F Summary of Rules for Predicting Thermally Feasible Pericyclic Reactions... 

Kenichi Fukui and Roald Hoffmann won the Nobel prize in 1981 (Woodward died in 1979 and so couldn t share this prize he had already won the Nobel prize in 1965 for his work on synthesis) for the application of orbital symmetry to pericyclic reactions. Theirs is an alternative description to the frontier orbital method we have used and you need to know a little about it. They considered a more fundamental correlation between the symmetry of all the orbitals in the starting materials and all the orbitals in the products. This is rather too complex for our consideration here, and we shall concentrate only on a summary of the conclusions—the Woodward-Hoffmann rules. The most important of these states ... 

We end this chapter with summaries of the three major strategies in the synthesis of heterocycles e ring construction by ionic reactions ring construction by pericyclic reactions... 

A simple summary of rules for pericyclic reactions is as follows ... 

In summary, all pericyclic reactions can be examined simply by writing the HOMO of one component, the LUMO of the other component (where "component" is defined as "separate interacting orbitals" see below), and determining whether, at the geometry being assumed, the orbitals can produce a mixing that is in-pha.se. Often, as we ll explore below, the HOMO and LUMO to be analyzed are within the same molecule, and maybe even in conjugation. 

This way of tabulating the various points in the Dewar theory is somewhat cumbersome. In predicting the preferred transition states for monocyclic pericyclic reactions the only information that is really required is the number of participant electrons. Since we are concerned here only with even electron systems, whether they be charged or electrically neutral, the total number of electrons is therefore 4 or 4n + 2. A concise summary of the Dewar theory is then given by ... 

In summary, the PMO method when extended to photochemical reactions of the G-type predicts a reversal of aromatic properties from ground state to excited state. This justifies the set of rules for photochemical pericyclic reactions given above. Dougherty s analysis also reveals dangers in the use of orbital and state correlation diagrams for assessing photo-pericyclic reactions because the breakdown in the BO approximation leads to a destruction of symmetry. 

How cun you keep straight nil the rules about pericyclic mactions The summary information in Tobies 30.1 -30.3 ran be disUHed into one mnemonic phras that proWdes an eaisy way to predict the stereochemical outcome of any perieyclic reaction"... 

In the [2-i4] pericyclic cycloaddition reaction known as the Diels-Alder reaction, fluonne-containing compounds have been widely used as dienes, dieno-philes, or both Much of the fundamental work, including many comprehensive and systematic studies, was done before 1972, and Hudlicky provides an excellent summary of this work [9] Additional sources for early work in this area are reviews in Organic Reactiom [61] and Fluorine Chemistry Reviews [62]... 

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