Concerted Pericyclic Electrocyclic Reactions

In an electrocyclic reaction both ends must rotate 90° to convert a sigma bond into a pi bond or the reverse. The direction of rotation can be identical, for example, both clockwise or both counterclockwise, which is called conrotatory (con). They can rotate in opposite directions, either both in or both out, which is disrotatory (dis). Dashed arrows will indicate orbital movement. 

Stereochemical labels on the cyclobutene confirm that this 4-electron electrocyclic ring opening is conrotatory. Another way to view the reaction is from the closure of the diene. The ends of the HOMO of the diene must rotate con to create a bonding overlap for the new sigma bond. Because of microscopic reversibility, opening and closure follow the same route if the opening is conrotatory, then the closure will likewise be con. 

Chapter 12 Qualitative Molecular Orbital Theory Pericyclic Reactions 

Once we decide con or dis, there is still another decision to be made, since each rotational mode has two ways it can happen. With alkyl groups, steric effects dominate the conrotation that does not bump the alkyl groups is preferred. 

With the conrotatory cyclobutene ring opening, secondary electronic effects contribute. Electron-releasing groups prefer to rotate outward, and electron-withdrawing groups prefer to rotate inward. 

---

The formation of cyclic nitrones (150) from pericyclic mechanism. Kinetic and computational studies have provided evidence for the involvement of a novel pseudo-pericyclic electrocyclization in the conversion of o-vinylphenyl isocyanates into quinolin-2-ones. " Such reactions have also provided evidence of torquoselectivity in a 6jt system. Hash vacuum thermolysis of triazoles (151) has been found to afford dihydroquinolines (155), presumably by generation of a-oxoketenimines (152) which can undergo a [1,5]-hydrogen shift to the o-quinoid imines (153)7(154) and subsequent electrocyclization (see Scheme 57). 

Pericyclic processes comprise a broad and important class of concerted reactions of both theoretical and practical interest. These transformations, which are especially useful in the construction of carbon-carbon bonds,93 include electrocyclic reactions, sigmatropic rearrangements, and cycloadditions. Because they are not typically subject to general acid-general base chemistry but can be highly sensitive to strain and proximity effects, they are attractive targets for antibody catalysis. 

The concerted reactions presented in this chapter are called pericyclic reactions. They are divided into electrocyclic reactions, cycloaddition reactions, and sigmatropic rearrangements. Some occur when energy is supplied in the form of heat others require light energy to occur. Most have strict stereochemical requirements. 

That the reaction occurs as expected between the HOMO of azide (which is, after all, an anion) and the LUMO of the electrophilic nitrile is confirmed by reactions with aryl nitriles ArCN with sodium azide. The yields are all close to quantitative but the reaction is faster when R is an electron-withdrawing group. Not everyone agrees that the reaction is concerted as the approach of the two linear molecules looks very hindered. However, a stepwise addition of azide ion 92 followed by cyclisation 93 looks, if anything, worse as two negatively charged atoms must attack each other in the cyclisation 93. However, this too is pericyclic (electrocyclic) and not ionic. 

Pericyclic reactions are concerted processes that occur by way of a cyclic transition state in which more than one bond is formed or broken within the cycle. The classic example of such a process is the Diels-Alder cycloaddition reaction, one of the most common and useful synthetic reactions in organic chemistry. Cycloaddition reactions, sigmatropic rearrangements and electrocyclic reactions all fall into the category of pericyclic processes, representative examples of which are given in Schemes 3.1-3.3. This chapter will discuss these reactions and their use in synthesis. 

The Rules for the stereospecificity apply only to pericyclic reactions which are concerted. The Rules apply neither to non-concerted reactions nor to those that are not pericyclic. Pericyclic reactions are those which involve a monocyclic transition state having a conjugated array of interacting orbitals, one per atom. Three types of pericyclic processes have been recognized electrocyclic reactions, cycloadditions, and sigmatropic shifts. 

Pericyclic reactions proceed via a concerted process with a cyclic transition state, and they are classified as cydoad-drtion reactions, electrocyclic reactions, and sigmatropic rearrangements. 

Pericyclic reactions were defined in 1969 by R. B. Woodward and R. Hoffmann as reactions in which all first-order change.s in bonding relationships take place in concert on a closed curve that is, as concerted reactions in which all bonds are formed or brokeasimultaneously around a circle. This definition arose from the systematic study of the conservation of orbital symmetry in a series of reactions electrocyclic reactions, cycloadditions, sigmatropic shifts, cheletropic reactions, and group transfer and elimination reactions. Excellent reviews on the historical evolution of pericyclic reactions have been published. ... 

Pericyclic reactions are the ones where the electrons rearrange through a closed loop of interacting orbitals, snch as in the electrocyclization of 1,3,5-hexatriene (88). Lemal pointed ont that a concerted reaction could also take place within a cyclic array, bnt where the orbitals involved do not form a closed loop. Rather, a disconnection occnrs at one or more atoms. At this disconnection, nonbonding and bonding orbitals exchange roles. Such a reaction has been termedpseudopericyclic. 

Many reactions involve a cyclic transition state. Of these, some involve radical or ionic intermediates and proceed by stepwise mechanisms. Pericyclic reactions are concerted, and in the transition state the redistribution of electrons occurs in a single continuous process. In this chapter, we will consider several different types of pericyclic reactions, including electrocyclic transformations, cycloadditions, sigmatropic rearrangements, and the ene reaction. 

Reactions in which more than one pair of electrons move simultaneously in a concerted manner, rather than sequentially, are called pericyclic reactions. These can be divided into three principal types, namely electrocyclic, sigmatropic and cycloaddition. 

In pericyclic reactions, there is a concerted movement of electrons. There are three principal types of pericyclic reactions, namely, electrocyclic, cycloaddition and sigmatropic. 

Electrocyclic A type of pericyclic reaction in which there is a concerted cyclisation of a polyene, or the reverse, which results in ring opening. 

---