Pericyclic reactions defined

There are several general classes of pericyclic reactions for which orbital symmetry factors determine both the stereochemistry and relative reactivity. The first class that we will consider are electrocyclic reactions. An electrocyclic reaction is defined as the formation of a single bond between the ends of a linear conjugated system of n electrons and the reverse process. An example is the thermal ring opening of cyclobutenes to butadienes ... 

Woodward, R.B., and Hoffman, R. MThe Conservation of Orbital Symmetry Verlag Chemie GmbH Weinheim, 1970. Pericyclic reaction terminology defined in this text is used in the present paper. 

The second mechanism, due to the permutational properties of the electronic wave function is referred to as the permutational mechanism. It was introduced in Section I for the H4 system, and above for pericyclic reactions and is closely related to the aromaticity of the reaction. Following Evans principle, an aromatic transition state is defined in analogy with the hybrid of the two Kekule structures of benzene. A cyclic transition state in pericyclic reactions is defined as aromatic or antiaromatic according to whether it is more stable or less stable than the open chain analogue, respectively. In [32], it was assumed that the in-phase combination in Eq. (14) lies always the on the ground state potential. As discussed above, it can be shown that the ground state of aromatic systems is always represented by the in-phase combination of Eq. (14), and antiaromatic ones—by the out-of-phase combination. 

There are a number of examples of pericyclic reactions for which the interaction diagram is not simply connected. We define a non-simply connected pericyclic system as one in which in the interaction diagram at least one basis orbital is connected to more than two others. An example is shown with its interaction diagram in Equations 11.37-11.38. 

Another anomalous cycloaddition is the insertion of a carbene into an alkene. 6-Electron cheletropic reactions (p. 28) are straightforward allowed pericyclic reactions, which we can now classify with the drawings 3.47 for the suprafacial addition of sulfur dioxide to the diene 2.179 and its reverse. Similarly, we can draw 3.48 for the antarafacial addition of sulfur dioxide to the triene 2.180 and its reverse. The new feature here is that one of the orbitals is a lone pair, which is given the letter co to distinguish it from o- and n-bonds, with suprafacial and antarafacial defined by the drawings 3.45 and 3.46, which apply to all sp3 hybrids and p orbitals, filled or unfilled. 

First, pericyclic reactions are defined, and an example of their unusual stereochemical selectivity is presented. A theoretical treatment of pericyclic reactions requires examination of the MOs for the conjugated molecules that participate in these reactions, so MO theory for these compounds is developed next. Then a theoretical explanation for the selectivity and stereochemistry observed in each of the three classes of pericyclic reactions is presented, along with a number of common examples of reactions of each kind. 

In addition, Dewar demonstrated that the classification of pericyclic reactions has nothing to do with symmetry. The nature of the reaction is defined by the AO overlap topology in a pericyclic transition state and not by the MO symmetry. 

Fulvenes, like their trpericyclic reactions. A reasonably well-defined reactivity profile of these systems has emerged as the result of extensive scrutiny of the cycloaddition behavior of the fulvene nucleus. To a large extent, fulvenes undergo concerted cycloadditions to dienes as either the 6ir or lit participant and the factors governing which of these reactivities is expressed in a particular circumstance has been elucidated employing fiontier molecular orbital considerations. ... 

The single-parameter X-model is now extended to a parametric description of complex reactions with an arbitrary number of reaction parameters. Let p( 3) be the number of reaction partn s (reactants, products or intermediates) the reaction lattice is then isomorphic to the lattice Pip + 1) 2 with a diagram of a higher dimensional cube (6.32). Accordin y, the dynamic sublattice is isomorphic to P(p) = 2 and thus contains at least one element of the non-roechanistic dimension A (see Ch. "Generalized reaction lattice"). Ck>nsequently, the choice of the reaction path is no longer unique - in contrast to the sin e-parameter X model for pericyclic reactions with a well defined reaction path (via an aromatic or antiaromatic transition state.). The formal algebraic description of... 

The terms aromatic and antiaromatic have been extended to describe the stabilization or destabilization of TRANSITION STATES of PERICYCLIC REACTIONS. The hypothetical reference structure is here less clearly defined, and use of the term is based on application of the Huckel (4n+2) rule and on consideration of the topology of orbital overlap in the transition state. Reactions of molecules in the ground state involving antiaromatic transition states proceed, if at all, much less easily than those involving aromatic transition states. 

In fact, cations frequently disobey Baldwin s rules. Other well-defined exceptions to Baldwin s rules include pericyclic reactions and reactions in which second-row atoms such as sulfur are included in the ring. This 5-endo-... 

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 other definition is due to Hashimoto [164] who define the same reference in the form formally similar to the one derived from one-determinantal wave fimction. The similarity index (116) was then applied to a series of selected pericyclic reactions (both forbidden and allowed) and its variation with the systematic variation of the reaction coordinate q> for both types of reference standard is discussed in the study [165]. Since all the details of this discussion can again be found in this stud we remind here only the most important results. They concern, above all, the form of the dependence g(cp) vs. 9. An example of such a dependence, for the case of simple 2+2 ethene dimerization, is given in Fig. 14, but practically the same type of dependence is observed for remaining reactions as well. Also the change of the reference standard has only negligible effect and the most important difference between the Kutzelnigg s and Hashimoto s type of reference standard is the systematic vertical shift of the curves conditioned by the evident change in the values of similarity indices for 9 = 0 and 9 = ti/2 (117). 

We then present ab initio molecular orbital theory. This is a well-defined approximation to the full quantum mechanical analysis of a molecular system, and also the basis of an array of powerful and popular computational approaches. Molecular orbital theory relies upon the linear combination of atomic orbitals, and we introduce the mathematics and results of such an approach. Then we discuss the implementation of ab initio molecular orbital theory in modern computational chemistry. We also describe a number of more approximate approaches, which derive from ab initio theory, but make numerous simplifications that allow larger systems to be addressed. Next, we provide an overview of the theory of organic TT systems, primarily at the level of Hiickel theory. Despite its dramatic approximations, Hiickel theory provides many useful insights. It lies at the core of our intuition about the electronic structure of organic ir systems, and it will be key to the analysis of pericyclic reactions given in Chapter 15. 

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