Chapter 30: Orbitals and Organic Chemistry: Pericyclic Reactions
A third kind of reaction
Almost every reaction in this course so far has belonged to one of two families. Polar reactions move a pair of electrons from a nucleophile to an electrophile, and radical reactions move single electrons. Both proceed in steps, and both pass through species you can draw and sometimes trap: a carbocation, an alkoxide, a radical. Pericyclic reactions are the third family, and they behave differently in every one of those respects. Three features define them. The reaction is concerted, meaning it has exactly one transition state, so reactants become products without stopping anywhere. The…
Building the π orbitals, and counting nodes
Everything in this chapter is decided by the shape of one orbital, so the orbitals have to be built first. The rule that generates them is short: when n atomic orbitals are combined, exactly n molecular orbitals come out. Nothing is lost and nothing is created. Start with the simplest case. Ethylene has two parallel p orbitals, one on each carbon, and therefore two π molecular orbitals. Adding the two p orbitals with their lobes in phase gives a bonding orbital, written π, in which electron density is built up between the nuclei. Combining them out of phase gives an antibonding orbital,…
The frontier orbitals, and what a photon actually does
A molecule with six π electrons has three filled orbitals, and in principle all three take part in a reaction. In practice one of them dominates, and the simplification is what makes the analysis tractable by hand. The reasoning is the same one that governs any interaction between orbitals: two orbitals interact strongly when they are close in energy and weakly when they are far apart. The filled orbital that lies highest is closest in energy to the empty orbitals of whatever it is reacting with, so its interaction is the strongest. That orbital is the highest occupied molecular orbital, the…
Closing a polyene into a ring
Look at what changes when butadiene closes to cyclobutene. Butadiene has two π bonds; cyclobutene has one π bond and a new σ bond joining the carbons that were the ends of the chain. Nothing else has altered. That transformation - a conjugated polyene becoming a cyclic compound with one fewer π bond and one more σ bond, or the reverse - is an electrocyclic reaction. The electron count is the first thing to establish, because everything else depends on it. Count the electrons in the π system that is reorganizing, not the electrons in the molecule. Butadiene's reorganizing system is four…
Which way do the ends turn?
For the two ends of a polyene to bond, the p orbitals on the terminal carbons have to swing out of the plane of the π system and point at each other. Each end rotates about the bond joining it to the rest of the chain, and there are only two possibilities. If both ends rotate in the same sense - both clockwise, or both counterclockwise - the motion is conrotatory. If they rotate in opposite senses, it is disrotatory. A useful image: conrotatory is two wheels on a shared axle turning together; disrotatory is the two covers of a book opening. The stereochemical consequence is immediate and is…
Two π systems, one ring: the Diels–Alder reaction
In a cycloaddition, two separate π systems come together and two new σ bonds form simultaneously, converting them into a ring. The Diels-Alder reaction between a conjugated diene and an alkene is the most important example in all of organic synthesis, because it builds a six-membered ring and up to four stereocenters in one operation from simple starting materials. The bracket notation names the reaction by electron count. In a [4+2] cycloaddition, one partner contributes four π electrons and the other contributes two. The diene supplies four from its two π bonds; the alkene, called the…
Suprafacial addition, stereospecificity, and endo selectivity
Because the two partners approach in parallel planes and both bonds form at once, each molecule reacts on one face only. Addition to a single face is called suprafacial; addition across both faces, which would require the array to twist, is antarafacial. A thermal Diels-Alder reaction is suprafacial on both components, and that simple geometric fact has consequences worth spelling out separately. The first is stereospecificity on the dienophile. Since both bonds form to the same face of the alkene, its two carbons never rotate relative to one another. Substituents that were cis across the…
Moving a σ bond along a π system
The third family looks stranger than the other two on first sight, because nothing is obviously gained or lost. A σ bond breaks in one place and a new σ bond forms in another, the π bonds shift along to accommodate it, and the molecule ends with the same numbers of each kind of bond it started with. What has changed is where they are - the connectivity is different, and often the product is considerably more stable. The naming system is the part most worth getting right, since it is mechanical and questions depend on it. Find the σ bond that breaks and label both of its atoms number 1, one on…
One transition state, two famous reactions
Among sigmatropic rearrangements, the [3,3] shifts are by far the most useful, and two of them have names. The Cope rearrangement takes a 1,5-diene and converts it into an isomeric 1,5-diene. The σ bond between carbons 3 and 4 breaks, a new σ bond forms between carbons 1 and 6, and both π bonds shift inward. Count the electrons: two from the σ bond and four from the two π bonds, giving six, so a thermal suprafacial process is allowed. In the parent case, 1,5-hexadiene rearranging to itself, the reaction is invisible because the product is identical to the starting material; substituted cases…
Sunlight, skin, and an enzyme that only holds a shape
The properties that make pericyclic reactions unusual in a flask make them unusual in a cell as well. They need no acid, no base, no nucleophile, and no metal. Two of the best-known examples in human biochemistry make the point. The first is the synthesis of vitamin D, and it is genuinely uncatalyzed. The starting material is 7-dehydrocholesterol, a cholesterol relative carrying a conjugated diene in ring B of the steroid, and it sits in the membranes of skin cells. When ultraviolet light in sunlight reaches it, that diene - part of a six-electron array spanning ring B - undergoes an…
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