Pericyclic reactions factors

You can interpret the stereochemistry and rates of many reactions involving soft electrophiles and nucleophiles—in particular pericyclic reactions—in terms of the properties of Frontier orbitals. This applies in particular to pericyclic reactions. Overlap between the HOMO and the LUMO is a governing factor in many reactions. HyperChem can show the forms of orbitals such as HOMO and LUMO in two ways a plot at a slice through the molecule and as values in a log file of the orbital coefficients for each atom. 

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 ... 

The effect of water molecules on pericyclic reactions can also be compared with the effects of Lewis acids on these reactions. The enhanced polarization of the transition state in these reactions would lead to stronger hydrogen bonds at the polar groups of the reactants, which will result in a substantial stabilization of the transition states in the same way Lewis acids do. A computer-simulation study on the Diels-Alder reaction of cyclopentadiene by Jorgensen indicated that this effect contributes about a factor of 10 to the rates.7... 

The actual rates of thermally-allowed pericyclic reactions vary vastly, and frontier-orbital theory (14, 15, 16) has proven to be the primary basis for quantitative understanding and correlation of the factors responsible. It is therefore of interest to find the dominant frontier orbital interactions for the group transfer reactions hypothesized to occur. 

The photochemistry of conjugated polyenes has played a central role in the development of modern molecular photochemistry, due in no small part to its ultimate relevance to the electronic excited state properties of vitamins A and D and the visual pigments, as well as to pericyclic reaction theory. The field is enormous, tremendously diverse, and still very active from both experimental and theoretical perspectives. It is also remarkably complex, primarily because file absorption spectra and excited state behavior of polyene systems are strongly dependent on conformation about the formal single bonds in the polyene chain, which has the main effect of turning on or off various pericyclic reactions whose efficiencies are most strongly affected by conformational factors. 

At this point, it is appropriate to draw a parallel with the straightforward MO explanations for the aromaticity of benzene using approaches based on a single closed-shell Slater determinant, such as HMO and restricted Hartree-Fock (RWF), which also have no equivalent within more advanced multi-configuration MO constructions. The relevance of this comparison follows from the fact that aromaticity is a primary factor in at least one of the popular treatments of pericyclic reactions Within the Dewar-Zimmerman approach [4-6], allowed reactions are shown to pass through aromatic transition structures, and forbidden reactions have to overcome high-energy antiaromatic transition structures. 

The factors that control if and how these cyclization and rearrangement reactions occur in a concerted manner can be understood from the aromaticity or lack of aromaticity achieved in their cyclic transition states. For a concerted pericyclic reaction to be thermally favorable, the transition state must involve An + 2 participating electrons if it is a Hiickel orbital system, or 4 electrons if it is a Mobius orbital system. A Hiickel transition state is one in which the cyclic array of participating orbitals has no nodes (or an even number) and a Mobius transition state has an odd number of nodes. 

Ionic, radical and pericyclic reactions are the three main groups of organic reactions. Every organic chemist has to be able to recognize each of these types of reaction, and know something of their mechanisms and the factors that affect how well they work in organic synthesis. 

In connection with Eq. (22), yet another important factor differentiates our approach from usual quantum chemical analyses of reaction mechanisms. This difference concerns the fact that while a quantum chemical approach is in principle independent of any external information (all participating species appear automatically as various critical points on the PE hypersurface), in our model that is more closely related to classical chemical ideas some auxiliary information about the structure of the participating molecular species is required. This usually represents no problem with the reactants and the products since their structure is normally known, but certain complications may appear in the case of intermediates. This complication is not, however, too serious since in many cases the structure of the intermediate can be reasonably estimated either from some experimental or theoretical data or on the basis of chemical intuition. Thus, for example, in the case of pericyclic reactions that are of primary concern for us here, the intermediates are generally believed to correspond to biradical or biradicaloid species with the eventual contributions of zwitterionic structures in polar cases. 

Many pericyclic reactions are stereospecific and, because they have to be run at temperatures higher than ambient, are very robust. It is somewhat surprising that there are very few examples of pericyclic reactions being run at scale, especially in light of our understanding of the factors that control the stereochemical course of the reaction either through the use of a chiral auxiliary or catalyst (Chapter 26). 

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 factors that make ene and retro-ene reactions proceed are nicely illustrated by a synthesis of enantiopure, isotopically labeled acetic acid CH(D)(T)C02H, a useful compound for studying the mechanisms of enzyme-catalyzed reactions. One ene and one retro-ene reaction occur in this synthesis. The ene reaction is driven by formation of a new tr bond at the expense of a C=C tt bond the retro-ene reaction is driven by the formation of a C=0 77 bond. Note that both pericyclic reactions proceed stereospecifically, even at the very high temperatures required for them to proceed ... 

Most pericyclic reactions, though of course not all, are little influenced by Coulombic forces for example, it is well known that the polarity of the solvent has little effect on the rate of Diels-Alder reactions. We can therefore expect that a major factor influencing reactivity will be the size of the frontier orbital interaction represented by the third term of equation 2-7, p. 27. This is why this chapter is much the largest in this book the most dramatic successes of frontier orbital theory have been the explanations it has given to an amazingly large number of observations in pericyclic chemistry. 

The oxy-Cope rearrangement can be thermally induced (equation 223, path a) but this process competes with an other well-established, concerted pericyclic reaction, i.e. the /1-hydroxyolefin retro-ene cleavage (path h)-- -. However, it was found that the oxy-Cope rearrangement can be accelerated under base-catalysis conditions (e.g. in the presence of potassium alkoxides) by a factor of I O - (the so-called anionic oxy-Cope rearrangement , path This base-induced acceleration is attributable to a dramatic decrease in the... 

Abstract The integration of conservation of orbital symmetry and the orbital overlap effect serves as a powerful tool to reliably predict the stereochemical course of pericyclic reactions as exemplified in this chapter. The orbital overlap factor has been discussed with a variety of examples such as the thermal fragmentations of cyclopropanated and cyclobutanated r .v-3,6-dimethyl-3,6-dihydropyridazine, and [1,5] sigmatropic shifts in c/.v-2-alkenyl-1 -alkylcyclopropanes and civ-2-alkenyl-1 -alkylcyclobutanes. 

Mock et al. [47] used the complexation of alkylammonium ions by Cucurbituril (53) to catalyze 1,3-dipolar cycloadditions of ammonium substituted alkynes 54 and alkyl azides 55. This pericyclic reaction is accelerated by a factor of 5.5 x 10 under the catalytic influence of 53. 

According to the Woodward-Hoffinann notation [3+2]-cycloaddition reactions are 7t s + Ti s pericyclic reactions. They enable, usually under thermal reaction conditions, the selective construction of carbo- and heterocyclic ring systems. Secondary orbital interactions as well as other factors controlling diastereofacial discrimination, regiochemistry or endotexo selectivity have been discussed in detail in the literature. " For many [3+2] cycloadditions it is not easy to predict, whether the 1,3-dipole functions as the donor or the acceptor component in these HOMO/LUMO-controlled conversions. " ... 

Application of this method to pericyclic reactions led to the generalization that thermal reactions take place via aromatic or stable transition states whereas photochemical reactions proceed via antiaromatic or unstable transition states. This is the case because a controlling factor in photochemical processes is conversion of excited state reactants into ground state products. Thus, the photochemical reactions convert the reactants into the antiaromatic transition states that correspond to forbidden thermal pericyclic reactions and so lead to corresponding products. 

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