Stereochemistry of thermal electrocyclic reactions

How can we predict whether conrotatory or disrotatory motion will occur in a given case According to frontier orbital theory, the stereochemistry of an electrocyclic reaction is determined by the symmetry of the polyene HOMO. The electrons in the HOMO are the highest-energy, most loosely held electrons, and are therefore most easily moved during reaction. For thermal reactions, the ground-state... 

We have seen that the stereochemistry of an electrocyclic reaction depends on the mode of ring closure, and the mode of ring closure depends on the number of conjugated 7T bonds in the reactant and on whether the reaction is carried out under thermal or photochemical conditions. What we have learned about electrocyclic reactions can be summarized by the selection rules listed in Table 29.1. These are also known as the Woodward-Hoffmann rules for electrocyclic reactions. 

It turns out that all An systems behave like the four-electron case (thermal reactions conrotatory and photochemical reactions disrotatory) and all 4w + 2 systems behave like the six-electron case (thermal reactions disrotatory and photochemical reactions conrotatory). This generalization is shown in the second half of Table 20.1, which gives the rules for all electrocyclic reactions. Now, if you can count the number of electrons correctly you can easily predict the stereochemistry of any electrocyclic reaction without even working out the molecular orbitals. 

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. 

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. 

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,... 

Predict the stereochemistry of thermal and photochemical electrocyclic reactions. 

It turns out that there is an alternating relationship between the number of electron pairs (double bonds) undei going bond leorganization and the stereochemistry of ring opening or closure. Polyenes with an even number of electron pairs undergo thermal electrocyclic reactions in a conrotatory sense, whereas polyenes with an odd number of electron pairs undergo the same reactions In a disrotatory sense. 

Generalization of either the frontier orbital, the orbital symmetry, or the transition-state aromaticity analysis leads to the same conclusion about the preferred stereochemistry for concerted thermal electrocyclic reactions The stereochemistry is a function of the number of electrons involved. Processes involving 4n + 2 electrons will be disrotatory those involving 4n electrons will be conrotatory for Hiickel transition states. The converse holds for Mobius transition states. 

In Summary Conjugated dienes and hexatrienes are capable of (reversible) electrocyclic ring closures to cyclobutenes and 1,3-cyclohexadienes, respectively. The diene-cyclobutene system prefers thermal conrotatory and photochemical disrotatory modes. The triene-cyclohexadiene system reacts in the opposite way, proceeding through thermal disrotatory and photochemical comotatory rearrangements. The stereochemistry of such electrocychc reactions is governed by the Woodward-Hoffmann rules. 

The effect of the number of n electrons upon the stereochemistry of a reaction is illustrated by the cyclization of a diene system compared to a triene system, as shown above. Although the methyl groups in both compounds have the E configuration, the products have different stereochemistry. Although both reactions are thermal, only the trans isomer results from the diene and only the cis isomer results from the triene. Thermal electrocyclic reactions of systems with 4 n electrons have the opposite stereochemistry to structurally related systems with 4 + 2 ti electrons. Furthermore, the stereochemistry of the thermal and photochemical pericyclic reactions is opposite. Photochemically initiated cychzation of the triene gives the trans isomer, whereas the ds isomer forms in the thermal cyclization. 

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