Stereochemistry electrocyclic reactions

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 ... 

Electrocyclic reactions of 1,3,5-trienes lead to 1,3-cyclohexadienes. These ring closures also exhibit a high degree of stereospecificity. The ring closure is normally the favored reaction in this case, because the cyclic compound, which has six a bonds and two IT bonds, is thermodynamically more stable than the triene, which has five a and three ir bonds. The stereospecificity is illustrated with octatrienes 3 and 4. ,Z, -2,4,6-Octatriene (3) cyclizes only to cw-5,6-dimethyl-l,3-cyclohexadiene, whereas the , Z,Z-2,4,6-octa-triene (4) leads exclusively to the trans cyclohexadiene isomer. A point of particular importance regarding the stereochemistry of this reaction is that the groups at the termini of the triene system rotate in the opposite sense during the cyclization process. This mode... 

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. 

A striking illustration of the relationship between orbital symmetry considerations and the outcome of photochemical reactions can be found in the stereochemistry of electrocyclic reactions. In Chapter 11, the distinction between the conrotatory and the disrotatory mode of reaction as a function of the number of electrons in the system was... 

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,... 

Thermal and photochemical electrocyclic reactions always take place with opposite stereochemistry because the symmetries of the frontier orbitals are always different. Table 30.1 gives some simple rules that make it possible to predict the stereochemistry of electrocyclic reactions. 

Thermal and photochemical cycloaddition reactions always take place with opposite stereochemistry. As with electrocyclic reactions, we can categorize cycloadditions according to the total number of electron pairs (double bonds) involved in the rearrangement. Thus, a thermal Diels-Alder [4 + 2] reaction between a diene and a dienophile involves an odd number (three) of electron pairs and takes place by a suprafacial pathway. A thermal [2 + 2] reaction between two alkenes involves an even number (two) of electron pairs and must take place by an antarafacial pathway. For photochemical cyclizations, these selectivities are reversed. The general rules are given in Table 30.2. 

A pericyclic reaction is one that takes place in a single step through a cyclic transition state without intermediates. There are three major classes of peri-cyclic processes electrocyclic reactions, cycloaddition reactions, and sigmatropic rearrangements. The stereochemistry of these reactions is controlled by the symmetry of the orbitals involved in bond reorganization. 

Explain the ideas relating to frontier orbitals when considering the stereochemistry of the electrocyclic reactions of dienes and trienes. 

The basis of electrocyclic reactions can be considered in terms of the molecular orbitals involved. By considering the phasing of the molecular orbitals it is possible to say whether a reaction can proceed (only orbitals of the same phase can overlap and bond) and to predict the stereochemistry of the reaction. This approach is called the frontier orbital model. 

The observed stereochemistry of electrocyclic reactions is codified in the following table, where the total number of electrons (N) involved in the major bonding changes is expressed as a multiple (4n, or not a multiple (4n + 2), of four. 

Electrocyclic reactions, 163, 165 butadienes to cyclobutenes, 164-165 component analysis, 168 stereochemistry, 165 Electron... 

The cyclization step of Equation 28-8 is a photochemical counterpart of the electrocyclic reactions discussed in Section 21-10D. Many similar photochemical reactions of conjugated dienes and trienes are known, and they are of great interest because, like their thermal relatives, they often are stereospecific but tend to exhibit stereochemistry opposite to what is observed for formally similar thermal reactions. For example,... 

As a first example of an electrocyclic reaction illustrating stereochemistry, let us take the pair of conrotatory cyclobutene openings, showing that the reactions are stereospecific. 

In this example the ring system is compatible with the allowed stereochemistry the disrotatory equilibrium between 4.55 and 4.56 has no problems. On the other hand, rings can constrain or even prevent allowed electrocyclic reactions. In the cyclobutene 4.57, for example, the ring fusion... 

In this primer, Ian Fleming leads you in a more or less continuous narrative from the simple characteristics of pericyclic reactions to a reasonably full appreciation of their stereochemical idiosyncrasies. He introduces pericyclic reactions and divides them into their four classes in Chapter 1. In Chapter 2 he covers the main features of the most important class, cycloadditions—their scope, reactivity, and stereochemistry. In the heart of the book, in Chapter 3, he explains these features, using molecular orbital theory, but without the mathematics. He also introduces there the two Woodward-Hoffmann rules that will enable you to predict the stereochemical outcome for any pericyclic reaction, one rule for thermal reactions and its opposite for photochemical reactions. The remaining chapters use this theoretical framework to show how the rules work with the other three classes—electrocyclic reactions, sigmatropic rearrangements and group transfer reactions. By the end of the book, you will be able to recognize any pericyclic reaction, and predict with confidence whether it is allowed and with what stereochemistry. 

An electrocyclic reaction is the formation of a new o-bond across the ends of a conjugated 7T-system or the reverse. They thus lead to the creation or destruction of one a-bond. Hexatrienes 1 can cyclise to six-membered rings 2 in a disrotatory fashion but we shall be more interested in versions of the conrotatory cyclisation of pentadienyl cations 3 to give cyclopentenyl cations 4. The different stereochemistry results from the different number of rt-electrons involved.1... 

In electrocyclic reactions the end carbons of the conjugated system must rotate for the p orbitals on these carbons to begin to overlap to form the new carbon-carbon sigma bond. The preference for the stereochemistry of the rotation in these reactions can be understood by examination of the new orbital overlap in the HOMO as the rotation occurs. For the formation of the new sigma bond to be favorable, rotation must occur so that the overlap of the orbitals forming this bond is bonding in the HOMO. 

What made his proposal so convincing was that the stereochemistry of the endiandric acid D is just what you would expect from the requirements of the Woodward-Hoffmann rules. The first step from the precursor is an 87t electrocyclic reaction, and would therefore be conrotatory. 

The stereochemistry of an electrocyclic reaction is determined by the symmetry of the polyene HOMO. 

Thermal and photochemical electrocyclic reactions always take place with opposite stereochemistry. 

Predict the stereochemistry of thermal and photochemical electrocyclic reactions. 

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