![]() We will see a few examples of the vehicle for this type of analysis, the orbital correlation diagram, but we won't need to go too deeply the theory.Īromatic transition structure (Dewar-Zimmerman): This is an easy concept to apply to all reaction types, but it's not so easy to understand why it is valid, especially in comparison with the FMO approach that we will generally choose. It correlates all the relevant orbitals in the starting material(s) and product(s). Other important theories and interpretations that will be touched upon include the following:Ĭonservation of orbital symmetry (Woodward-Hoffmann): This is the first theory that successfully explained and predicted the outcome of pericyclic reactions. This allows the interpretation of a molecular interaction to be restricted to an analysis of the interactions between the highest occupied and lowest unoccupied molecular orbitals (HOMOs and LUMOs) of the reacting partners. The underlying principles of pericyclic reactions have emerged in various forms, and we will focus on the frontier molecular orbital (FMO) approach developed by Fukui in the 1950s. Such reactions are usually stereospecific, with the stereochemistry of the starting materials determining that of the products. These are pericyclic reactions in which two pi-components combine to generate a new ring through the formation of two new sigma-bonds. We will consider three major sub-classes of pericyclic reaction, illustrated below in schemes showing both the 'deceptive' mechanistic arrows and a representation of the characterising cyclic transition structure. each starting MO correlates with a product MO that possesses the same symmetry properties. We will see later how Woodward and Hoffmann reached the conclusion that this 'evolution' must take place with the conservation of orbital symmetry, i.e. In a pericyclic reaction, the MOs of the starting material(s) smoothly evolve into the MOs of the product(s). Will soon be obvious that this is deceptive - the bonding changes in a pericyclic reaction might be represented using arrows, but any impression of pairwise movement in one direction is false. Pericyclic reactions are commonly represented using curly arrows, but it Pericyclic reactions are distinct in proceeding through cyclic transition structures in which the participating molecular orbitals (MOs) maintain bonding interactions throughout. A working definition is a reaction that takes place as a continuous,Ĭoncerted reorganisation of electrons. The third distinct reaction class is pericyclic reactions. Such reactions are also driven by structural factors, particularly hypovalency, but proceed without the (necessary) build-up of charge. Radical reactions differ in that they proceed through a stepwise redistribution of The electron pair movements involved in bond making and bond breaking may be synchronous, as in the S N2 reaction, or stepwise, as in electrophilic additions to alkenes, but the driving forces are the same -Įlectron deficiency (electrophilicity, acidity, oxidation potential, etc.) or electron excess (nucleophilicity, basicity, reduction potential). Ionic reactions, the most common, involve charge build-up, with electron pairs moving in one direction under the influence of various structural features. In the broadest sense there are three reaction classes, characterised by the way that the electrons behave in reaction mechanisms. Describing reactions as HOMO–LUMO interactions.Electron distribution in covalent bonds.The π orbitals of allyl (propenyl) anion and cation.Frontier molecular orbitals and interaction diagrams.Mechanistic analysis of pericyclic reactions.General features of pericyclic reactions.Sigmatropic rearrangements Sigmatropic rearrangements.Electrocyclic reactions Electrocyclic reactions.
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