Electrocyclic 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 and Hoffmann define as electrocyclic reactions the formation of a single bond between the termini of a system containing k n electrons, and the reverse process(89) ... 

An electrocyclic reaction is defined as a concerted closure of a ring starting from a conjugated open system in which a k orbital is converted into a o orbital. It is therefore an example of intramolecular cycloaddition or retroaddition. 

Fig. Z The three types of radical electrocyclic reaction, as defined by Bischof [8] and Haselbach et al. [9] The notation A-C is taken from Ref [9]. Reactions of type C are essentially unknown, whereas tyi s A and B are both orbital- and state-symmetry forbidden...

This is an example of a Diels-Alder reaction. This is an electrocyclic reaction where no charges are involved. While no charges are involved, electron pairs do move and their movement can be illustrated using arrow pushing. The mechanism, illustrated below, involves aligning cyclopentadiene (a diene) with methyl vinyl ketone (a dienophile) such that all three double bonds define a six-membered... 

Finally, the solvent dependence results for two electrocyclic reactions are mentioned. Electrocyclic reactions are generally defined as reactions involving the concerted cyclization of an n r-electron system to an (n — 2) r -I- 2electron system, or the reverse process [90, 113, 818]. 

In 1965 R.B. Woodward and Roald Hoffmann 2> defined electrocyclic reactions as the two following processes ... 

In the PMO method, we analyze an electrocyclic reaction through the following steps (1) Define a basis set of 2p-atomic orbitals for all atoms involved (li for hydrogen atoms). (2) Then connect the orbital lobes that interact in the starting materials. (3) Now let the reaction start and then we identify the new interactions that are occurring at the transition state. (4) Depending upon the number of electrons in the cyclic array of orbitals and whether the orbital interaction topology corresponds to a Huckel-type system or Mobius-type system, we conclude about the feasibility of the reaction under thermal and photochemical conditions. 

An electrocyclic reaction is defined as the thermal or photochemical conversion of an acyclic conjugated system into a ring system by formation of a o bond between the ends of the conjugated system in a concerted process, or the reverse of this reaction. These reactions are reversible in nature. 

The key concept for the formulation of the rules in question is the concept of the transition vector (see Sect. 1.3.3.1). This vector may be regarded as a quite short, albeit finite, portion of the part of the reaction path whose beginning lies at the transition state point. A displacement of the system in the direction defined by the transition vector lowers its potential energy. Figure 1.10 shows as an example the form of the transition vector for the electrocyclic reaction of disrotatory rearrangement of the cis-Dewar benzene into benzene [49]. 

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. ... 

The chemical aspects of cycloaddition (i+j) reactions have been carefully defined and discussed (Huisgen et al., 1964 Rhoads, 1963 Warrener and Bremner, 1966). Here, they shall simply be regarded as intermolecular electrocyclic changes involving two or more charged or... 

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