# Dienes

## Vocabulary

### antarafacial

describes when one $\pi$ system must twist to align like phases of the p orbitals so that two bonding interactions will occur

### Claisen rearrangement

[3,3]-sigmatropic rearrangement that involves the rearrangement of vinyl allyl ethers (phenyl allyl ethers) to unsaturated carbonyls

### conjugated diene

diene with two double bonds that are separated by one single bond

### conrotatory

classification that describes when the two ends of the polyalkene rotate in the same direction during the movement of the molecular orbital to form new $\sigma$ bonds in the cyclization reaction

### Cope rearrangement

[3,3]-sigmatropic rearrangement that involves the rearrangement of 1,5-dienes to a regioisomer, an isomer based on the location of a functional group

### cumulated diene

diene with two successive double bonds without an intervening single bond

concerted reaction in which two or more unsaturated molecules (or parts of the same molecule) combine to form a cyclic product, with a net reduction of bond multiplicity

### Diels-Alder reaction

concerted reaction in which two new $\sigma$ bonds are formed at the expense of two $\pi$ bonds of the conjugated diene and dienophile

### diene

compound containing two double bonds (or a 4 $\pi$-electron system)

### dienophile

compound containing a double bond (or a two $\pi$-electron system)

### disrotatory

classification that describes when the two ends of the polyalkene rotate in opposite directions during the movement of the molecular orbital to form new $\sigma$ bonds in the cyclization reaction

### electrocyclic reaction

reversible reaction that involves ring closure of a conjugated polyalkene to a cycloalkene or ring opening of a cycloalkene to a conjugated polyalkene

reaction of a double (or triple) bond with an electrophile to give a carbocation (or another cationic intermediate) that then reacts with a nucleophile. The reaction breaks a $\pi$-bond and a $\sigma$-bond while simultaneously creating two $\sigma$-bonds.

### endo rule

stereochemical preference for the electron-withdrawing substituent to appear in the endo position rather than the exo position

### frontier molecular orbital theory

concept that the highest-energy occupied orbitals of one molecule interact with the highest-energy unoccupied orbitals of another molecule

### highest occupied molecular orbital (HOMO)

highest energy level that is occupied by electrons

### isolated diene

diene with two double bonds separated by two or more single bonds

### kinetic control

situation in which the reaction to form products is irreversible and the product ratio is determined by the reaction rate

### lowest unoccupied molecular orbital (LUMO)

lowest energy level that is not occupied by electrons

### pericyclic reaction

reaction in which bonds are made or broken in a concerted cyclic transition state

### photochemical change

chemical change initiated by light

### regioselectivity

preference of one direction of chemical bond making or breaking over all other possible directions

### sigmatropic reaction

reaction in which the net result is that one $\sigma$ bond is changed to another $\sigma$ bond in an uncatalyzed intramolecular process

### suprafacial

having like phases of the p orbitals of two molecules on the same side of the $\pi$ systems so that two bonding interactions occur

### thermal condition

condition of a reaction where heat is the deciding factor in the distribution of products

### thermodynamic control

situation in which the reaction to form products is reversible and the product ratio is determined by relative thermodynamic stabilities

### Woodward-Hoffmann rules

set of rules used to rationalize or predict certain aspects of the stereochemical outcome and activation energy of pericyclic reactions