Pericyclic, 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. 

Reformatsky reaction has covered aspects of this topic (20b). If analogous pericyclic transition states are involved in these condensations, the added stereochemical control element imposed on the condensation by the imine geometry should provide a more well-defined set of transition states than for the analogous aldehyde condensations. The four diastereomeric chair and boat transition states for ( )- and (Z)-enolates with ( )-imines are illustrated in Scheme 15. 

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. 

The thermally induced [6 + 4] cycloaddition of 2,4,6-cycloheptatrien-l-one (tropone) (1) with a variety of diene pikers has played a central role in the development of the mechanistic precepts and preparative utility of the entire class of higher-order pericyclic processes. The relatively well-defined reaction parameters for these cycloadditions render diem particularly well-suited for the rqiid assembly of relatively complex polycyclic carbon arrays. 

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-... 

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