Electrocyclic reactions examples

Certain polyenes and cyclic compounds can be interconverted through a pericyclic process known as an electrocyclic reaction. Examples include the 1,3-butadiene-cyclobutene and 1,3-cyclohexadiene-l,3,5-hexatriene interconversions (Figs. 20.5 and 20.16). 

The final class of reactions is those characterized by cyclic transition states in which there is continuous overlap of a cyclic array of orbitals. These processes take place in a single step, without the intervention of any intermediate species. They may combine multiple components to introduce new rings, as in the Diels-Alder and other cycloaddition reactions, or they may take the form of ring-opening and ring-closing processes—the electrocyclic reactions. Examples are presented in Table 3. 

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 ... 

We have now considered three viewpoints from which thermal electrocyclic processes can be analyzed symmetry characteristics of the frontier orbitals, orbital correlation diagrams, and transition-state aromaticity. All arrive at the same conclusions about stereochemistiy of electrocyclic reactions. Reactions involving 4n + 2 electrons will be disrotatory and involve a Hiickel-type transition state, whereas those involving 4n electrons will be conrotatory and the orbital array will be of the Mobius type. These general principles serve to explain and correlate many specific experimental observations made both before and after the orbital symmetry mles were formulated. We will discuss a few representative examples in the following paragraphs. 

There are also examples of electrocyclic processes involving anionic species. Since the pentadienyl anion is a six-7c-electron system, thermal cyclization to a cyclopentenyl anion should be disrotatory. Examples of this electrocyclic reaction are rare. NMR studies of pentadienyl anions indicate that they are stable and do not tend to cyclize. Cyclooctadienyllithium provides an example where cyclization of a pentadienyl anion fragment does occur, with the first-order rate constant being 8.7 x 10 min . The stereochemistry of the ring closure is consistent with the expected disrotatory nature of the reaction. 

Scheme 13.1. Some Examples of Photochemical Cycloaddition and Electrocyclic Reactions...

Scheme 13.1 lists some example of photochemical cycloaddition and electrocyclic reactions of the type that are consistent with the predictions of orbital symmetry considerations. We will discuss other examples in Section 13.4. 

The latter product is an example of a product formed by a concerted, photochemically allowed, electrocyclic reaction. A hydrogen-atom migration from a cyclopropyldimethyl... 

This is an example of an electrocyclic reaction, and involves rotation of the terminal methylene groups either in the same way ( conrotatory ) or in opposite ways ( disrotatory ). 

The best way to understand how orbital symmetry affects pericyclic reactions is to look at some examples. Let s look first at a group of polyene rearrangements called electrocyclic reactions. An electrocyclic reaction is a pericyclic process that involves the cycli/ation of a conjugated polyene. One 7r bond is broken, the other 7t bonds change position, a new cr bond is formed, and a cyclic compound results. For example, a conjugated triene can be converted into a cyclohexa-diene, and a conjugated diene can be converted into a cyclobutene. 

The most striking feature of electrocyclic reactions is their stereochemistry. For example, (2 ,4Z,6 )-2,4,6-octatriene yields only c/s-5,6-dimethyl-l,3-cyclo-hexadiene when heated, and (2 ,4Z,6Z)-2,4,6-octatriene yields only trnns-5,6-dimethyl-l,3-cyclohexadiene. Remarkably, however, the stereochemical results change completely when the reactions are carried out under what are called photochemical, rather than thermal, conditions. Irradiation, or photolysis,... 

Electrocyclic reaction (Section 30.3) A unimolecular peri-cyclic reaction in which a ring is formed or broken by a concerted reorganization of electrons through a cyclic transition state. For example, the cyciization of 1,3.5-hexatriene to yield 1,3-cyclohexadiene is an electrocyclic reaction. 

As is the case for [2 + 2] cycloaddition reactions (15-61), certain forbidden electrocyclic reactions can be made to take place by the use of metallic catalysts." An example is the silver ion-catalyzed conversion of tricyclo[4.2.0.0. ]octa-3,7-diene to cyclooctatetraene " ... 

Vitamin D chemistry provided some of the first examples of both thermal and photo electrocyclic reactions<170 173) ... 

Eastman, R. H., 158, 166 Eaton, P. F., 460 Eigen, M., 80 Eisenberg, W., 125 Electrocyclic addition, 46 Electrocyclic reaction rules, 339 Electrocyclic reactions, 402,408 4n-examples, 408 (4n + 2)-examples, 410 Electron impact spectroscopy, triplet energy, 220-223 Electronic energy transfer, 267 Electronic integral, 21 Electronic transitions /-a ,16 n -Mr, 16... 

A classical topic for illustration of the above principles is the analysis of the behaviour of the system trimethylene cyclopropane methylene + ethylene (11). Ring closure of trimethylene is a simple example of an electrocyclic reaction (8a) ... 

Electrocyclic reactions are examples of cases where n-electron bonds transform to sigma ones [32,49,55]. A prototype is the cyclization of butadiene to cyclobutene (Fig. 8, lower panel). In this four electron system, phase inversion occurs if no new nodes are formed along the reaction coordinate. Therefore, when the ring closure is disrotatory, the system is Hiickel type, and the reaction a phase-inverting one. If, however, the motion is conrotatory, a new node is formed along the reaction coordinate just as in the HC1 + H system. The reaction is now Mobius type, and phase preserving. This result, which is in line with the Woodward-Hoffmann rules and with Zimmerman s Mobius-Hiickel model [20], was obtained without consideration of nuclear symmetry. This conclusion was previously reached by Goddard [22,39]. 

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. 

The simplest example of an electrocyclic reaction involving 4n electron system is the thermal opening of cyclobutenes to 1,3 butadienes. The reaction can be done thermally or photochemically and under either conditions, it is stereospecific. 

From this it should not be inferred that all electrocyclic reactions under thermal conditions proceed in a conrotatory manner and under irradiation they proceed in a disrotatory fashion. This is clear from the following example ... 

Although a few other acyclic examples of stereospecific isomerisation of hexatrienes are known, specially in the field of natural product like steroid chemistry, the commonest reactions of this type are in cyclic hexatrienes. Cyclooctatriene and cyclooctatetraene are systems in which the electrocyclic reaction goes very readily and they show an interesting trend. 

Many other examples of contrasting behaviour have been discovered. For example all-cis-cyclodecapentaene (VII) photochemically equilibrate at low temperatures with trans 9, 10 dihydronapthalene by a conrotatory six electron electrocyclic reaction but it is converted thermally into cis-9, 10 dihydronaphthalene by disrotatory closure. 

The concepts of electron-transfer catalysis and so-called hole-catalysis [1] are closely related. It is now generally accepted that many organic reactions that are slow for the neutral reaction system proceed very much more easily in the radical cation. Although hole-catalysis is now well documented experimentally [2], there is surprisingly little mention of the corresponding reductive process, in which a reaction is accelerated by addition of an electron to the reacting system. Although the concept of electron-catalysis is not as well known as hole-catalysis, there are experimental examples of electrocyclic reactions that proceed rapidly in the radical anion, but slowly or not at all in the neutral system [3], For reasons that will be outlined below, we can expect that, in many cases, difficult or forbidden closed-shell reactions will be very much easier if an unpaired electron is introduced into the system by one-electron oxidation or reduction. Thus, if a neutral reaction A - B proceeds slowly or not at all, the radical cation (A" -> B" ) or radical anion (A" B" ) may be facile... 

As related to Diels-Alder reaction this also represents a 6p electron electrocyclic reaction. Like Diels-Alder reaction the ene reaction is reversible. For example, the produce) 1-pentene of the ene reaction between ethene and propene, gives back ethene and propene on decomposition at 400° C. 

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