pericycle

Examples of pericycle in the following topics:

• A Useful Mnemonic Rule

  • Before pericyclic reactions can be put to use in a predictable and controlled manner, a broad mechanistic understanding of the factors that influence these concerted transformations must be formulated.
  • The simplest, albeit least rigorous, method for predicting the configurational path favored by a proposed pericyclic reaction is based upon a transition state electron count.
  • In most of the earlier examples, pericyclic reactions were described by a cycle of curved arrows, each representing a pair of bonding electrons.
  • Since pericyclic reactions proceed by a cyclic reorganization of bonding electron pairs, it is necessary to evaluate changes in the associated molecular orbitals that take place in going from reactants to products.

• Some Examples of Pericyclic Reactions

  • Before reviewing representative examples of various types of pericyclic reactions, the previous caution that a given transformation be truly concerted must be emphasized again.
  • With this caveat in mind, extensive lists of pericyclic reactions may be assembled, and their rationalization by the previously noted mnemonic or orbital analysis is both remarkably successful and instructive.

• Perplexing Features of Pericyclic Reactions

  • The examples of pericyclic reactions presented here provide ample evidence of their usefulness in constructing or modifying complex molecules, often with a high degree of stereospecificity.
  • However, in contrast to the general applicability of most common ionic reactions, pericyclic reactions often display a marked sensitivity to small structural changes.

• A Useful Mnemonic for Regioselectivity

  • An outstanding treatment of pericyclic reactions, including lots of challenging questions with answers, has been provided by Henry Rzepa.
  • To reach his Imperial College home page, click on the following link (http://www.ch.ic.ac.uk/local/organic/pericyclic/).
  • Rzepas pericyclic reaction site will be found under Teaching, Training and Internet Innovation Highlights.
  • If you have trouble finding this site, try the menu bar that will be presented when you click the following link (http://www.ch.ic.ac.uk/local/organic/pericyclic/i.html).

• Pericyclic Reactions

  • Since reactions of this kind often proceed by nearly simultaneous reorganization of bonding electron pairs by way of cyclic transition states, they have been termed pericyclic reactions.
  • The four principle classes of pericyclic reactions are termed: Cycloaddition, Electrocyclic, Sigmatropic, and Ene Reactions.
  • Although some pericyclic reactions occur spontaneously, most require the introduction of energy in the form of heat or light, with a remarkable product dependence on the source of energy used.

• Non-Ionic Reactions

  • Here we shall consider two other classes of organic reactions: Free-Radical Reactions & Pericyclic Reactions.
  • The term "pericyclic reaction" encompasses a large and varied group of concerted thermal and photochemical transformations.
  • In contrast to ionic reactions, both free radical and pericyclic reactions may occur in the gas phase, as well as in solution in various solvents.

• Theoretical Models for Pericyclic Reactions

  • Such symmetry characteristics play an important role in creating the orbital diagrams used by Woodward and Hoffmann to rationalize pericyclic reactions.
  • The original approach of Woodward and Hoffmann involved construction of an "orbital correlation diagram" for each type of pericyclic reaction.
  • Two related methods of analyzing pericyclic reactions are the transition state aromaticity approach, and the frontier molecular orbital approach.

• Stereochemical Notations

  • One characteristic shared by most pericyclic reactions, and noted in many cases described above, is their stereospecificity.
  • Since these configurational change notations are not appropriate for pericyclic reactions, new designations are needed.
  • Suprafacial examples of these pericyclic transformations are shown below.

• Meristems

  • Meristematic tissues are found in many locations, including near the tips of roots and stems (apical meristems), in the buds and nodes of stems, in the cambium between the xylem and phloem in dicotyledonous trees and shrubs, under the epidermis of dicotyledonous trees and shrubs (cork cambium), and in the pericycle of roots, producing branch roots.

• Cascade Reaction Sequences

  • The second example shows a sequence of thermal pericyclic reactions.