Pericyclic reaction kinds

Most organic reactions take place by polar mechanisms, in which a nucleophile donates two electrons to an electrophile in forming a new bond. Other reactions take place by radical mechanisms, in which each of two reactants donates one electron in forming a new bond. Both kinds of reactions occur frequently in the laboratory and in living organisms. Less common, however, is the third major class of organic reaction mechanisms—pericyclic reactions. 

The following synthesis of dienones occurs readily. Propose a mechanism to account for the results, and identify the kind of pericyclic reaction involved. 

Karahanaenone, a terpenoid isolated from oil of hops, has been synthesized by the thermal reaction shown. Identify the kind of pericyclic reaction, and explain how karahanaenone is formed. 

Dienes and polyenes have been a subject of great interest due to their important role in biology, materials science and organic synthesis. The mechanism of vision involves cis-trans photoisomerization of 11 -civ-retinal, an aldehyde formed from a linear polyene. Moreover, this kind of molecule exhibits high linear and non-linear electrical and optical properties. Short polyenes are also involved in pericyclic reactions, one of the most important classes of organic reactions. 

The conclusion we may draw from this analysis is that in pericyclic reactions of these kinds we shall always be able to discover the inherent symmetry of the interaction topologically by considering the system as being made up of suitable components, even when there is no actual symmetry maintained in the molecule as a whole. We shall therefore be able to analyze the situation in terms of the... 

Crudely, but adequately for now, we may state rule governing which cycloadditions can take place and which not. A thermal pericyclic cycloaddition is allowed if the total number of electrons involved can be expressed in the form (4n+2), where n is an integer. If the total number of electrons can be expressed in the form 4n it is forbidden. Another way of saying the same thing is that reactions with an odd number of curly arrows are allowed and those with an even number are forbidden. This rule needs to be qualified, as we shall see shortly, and in due course in Chapter 3 made more precise, along with the rules for all the other kinds of pericyclic reaction, in one all-encompassing rule. For now, we need to introduce the rule for photochemical pericyclic cycloadditions. 

First, pericyclic reactions are defined, and an example of their unusual stereochemical selectivity is presented. A theoretical treatment of pericyclic reactions requires examination of the MOs for the conjugated molecules that participate in these reactions, so MO theory for these compounds is developed next. Then a theoretical explanation for the selectivity and stereochemistry observed in each of the three classes of pericyclic reactions is presented, along with a number of common examples of reactions of each kind. 

Tills needs some explanation. A component is a bond or orbital taking part in a pericyclic reaction as a single unit. A double bond is a n2 component. The number 2 is the most important part of this designation and simply refers to the number of electrons. The prefix 7C tells us the type of electrons. A component may have any number of electrons (a diene is a n4 component) but may not have mixtures of % and c electrons. Now look back at the rule. Those mysterious designations (4 q + 2) and (4r) simply refer to the number of electrons in the component where q and rare integers. An alkene is a n2 component and so it is of the (4 q + 2) kind while a diene is a nA component and so is of the (4r) kind. 

This ring opening is clearly pericyclic—the electrons go round in a ring, and the curly arrows could t>e drawn either way—but it is neither a cycloaddition (only one 7t system is involved) nor a sigmatropic rearrangement (a a bond is broken rather than moved). It is, in fact, a member of the third and last kind of pericyclic reaction, an electrocyclic reaction. 

We have now seen a large number of rules, expressed differently for each kind of pericyclic reaction. Learning them would seem to impose a considerable burden, but Woodward and Hoffmann saved us from this effort by rewriting them in one all-encompassing rule that applies to all thermal pericyclic reactions 20... 

The Diels-Alder reaction of a diene and a dienophile has become one of the most powerful carbon-carbon bond-forming processes [81]. In normal Diels-Alder reactions of an electron-poor dienophile with an electron-rich diene, the main interaction is between the HOMO of the diene and the LUMO of the dienophile. Coordination of a Lewis acid to the dienophile reduces its frontier orbital energies, and this increases the rate of the reaction. Regio- and stereoselectivity are also markedly affected by the Lewis acid. Recent extensive studies on the design of chiral Lewis acids have led to fruitful results in the control of the stereochemistry of a variety of pericyclic reactions. Several chirally modified Lewis acids have been developed for the asymmetric Diels-Alder reaction [82,83] and spectacular advances have recently been achieved in this area. Various kinds of polymer-supported chiral Lewis acid have also been developed. Polymer-supported A1 Lewis acids such as 62 have been used in the Diels-Alder reaction of cyclopentadiene and methacrolein (Eq. 20) [84] as has polymer-supported Ti alkoxide 63 [84]. These Ti catalysts are readily prepared and have high activity in the Diels-Alder reaction. 

What kind of pericyclic reaction could happen under these reaction conditions ... 

It is less common to find two pericyclic reactions of the same kind coupled together but the Alder ene reaction and the oxo ene reaction can both be catalysed by Lewis acids under the same conditions. A simple example is the combination of the exocyclic alkene 157 with acrolein. The intermediate unsaturated aldehyde 158 cyclises stereoselectively to form a new carbocyclic ring 159. The intermediate 158 is perfectly stable so the tandem sequence is convenient rather than necessary.22... 

---