Pericyclic reactions bond changes

The best way to understand how orbital symmetry affects pericyclic reactions is to look at some examples. Let s look first at a group of polyene rearrangements called electrocyclic reactions. An electrocyclic reaction is a pericyclic process that involves the cycli/ation of a conjugated polyene. One 7r bond is broken, the other 7t bonds change position, a new cr bond is formed, and a cyclic compound results. For example, a conjugated triene can be converted into a cyclohexa-diene, and a conjugated diene can be converted into a cyclobutene. 

Diels-Alder reactions are found to be little influenced by the introduction of radicals (cf. p. 300), or by changes in the polarity of the solvent they are thus unlikely to involve either radical or ion pair intermediates. They are found to proceed stereoselectively SYN with respect both to the diene and to the dienophile, and are believed to take place via a concerted pathway in which bond-formation and bond-breaking occur more or less simultaneously, though not necessarily to the same extent, in the transition state. This cyclic transition state is a planar, aromatic type, with consequent stabilisation because of the cyclic overlap that can occur between the six p orbitals of the constituent diene and dienophile. Such pericyclic reactions are considered further below (p. 341). 

Antisymmetric matrix, non-adiabatic coupling, vector potential, Yang-Mills field, 94-95 Aromaticity, phase-change rule, chemical reaction, 446-453 pericyclic reactions, 447-450 pi-bond reactions, 452-453 sigma bond reactions, 452 Aromatic transition state (ATS), phase-change rule, permutational mechanism, 451-453... 

Intrinsic symmetry of reacting orbitals Third, in making bonding models, we noted that it is not always necessary to use all the symmetry elements of the molecule. We may be able to pick out one or more of a number of symmetry elements that will give the desired information. We shall find examples of this procedure in applications of the pericyclic theory. We also need another idea, already introduced in Section 10.4, namely that in some circumstances it is appropriate to use a symmetry element that is not strictly, but rather only approximately, a correct symmetry element of the molecule. The reason we can do this is that in pericyclic reactions we shall focus on those orbitals in the molecule that are actually involved in the bonding changes of interest, which we... 

For many years, pericyclic reactions were poorly understood and unpredictable. Around 1965, Robert B. Woodward and Roald Hoffmann developed a theory for predicting the results of pericyclic reactions by considering the symmetry of the molecular orbitals of the reactants and products. Their theory, called conservation of orbital symmetry, says that the molecular orbitals of the reactants must flow smoothly into the MOs of the products without any drastic changes in symmetry. In that case, there will be bonding interactions to help stabilize the transition state. Without these bonding interactions in the transition state, the activation energy is much higher, and the concerted... 

A theory of pericyclic reactions stating that the MOs of the reactants must flow smoothly into the MOs of the products without any drastic changes in symmetry. That is, there must be bonding interactions to help stabilize the transition state, (p. 692)... 

Since pericyclic reactions proceed with a cyclic reorganization of bonding electron pairs, it is necessary to evaluate changes in the associated MOs that take place in going from reactants to products. These orbitals may be classified by two independent symmetry operations a mirror plane (m) perpendicular to the functional plane and bisecting the molecule, and a twofold axis of rotation (C2). 

Identifying pericyclic reactions takes care. They may be executed under acidic, basic, or neutral conditions, just like polar reactions. Many reactions involve several polar steps as well as one pericyclic step. Also, sometimes it s quite difficult to figure out the relationship between the starting materials and the products because of the extensive changes in bonding patterns that often occur with pericyclic reactions. 

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