DIENES Dienes are classified as either conjugated, isolated or cumulated, depending on the structural relationship of the double bonds. Cumulated dienes have the double bonds sharing the central carbon. The simplest cumulated diend has the common name allene.
If an orbital picture of allene is made, the central carbon must have sp hybridization in order to share a p orbital with each of the outer carbons, which are hybridized. The two groups must lie in perpendicular planes, so the molecule is not planar. One pair of C-H bonds is in the plane of the paper, while the other pair are in a plane perpendicular to the plane of the paper. geometry, a 1,3-disubstituted allene has a non-superimposable mirror chiral center is present.
Verification of the added stability of conjugated dienes comes from studies of heats of hydrogenation and heats of combustion, which are lowere that the theoretical amount for two double bonds.
For example, Addition of HBr to 1,3-butadiene produces 3-bromo-1-butene and 1-bromo-2-butene The products are a direct result of allylic cation formation. One of the end carbons is protonated to produce an allylic cation. It has resonance over the other end carbon of the three carbon allylic system. Bromide binds to either end to produce both products. Unlike many reactions which give mixtures of products, the outcome of this one depends on temperature. At low temperatures, the 1,2 product is favored, because it forms faster.
The pathway leading to the 1,2 product is lower energy, possibly because the bromide is nearby after the protonation step. At higher temperatures, the 1,4 product is favored, because it is thermodynamically more stable, having the more substituted double bond, along with the fact that the reaction is reversible. Allylic
halides ionize easily, and given enough energy (heat), the 1,2 product goes back to the allylic cation, which can get over the higher energy pathway to the 1,4 product, which lies at a lower final energy.
Draw structures for all products formed on treatment of each of the compounds below with HCl at and circle the major product formed at Diels-Alder Reaction: This can be viewed as a 1,4 addition of an alkene (called the dienophile) to a conjugated diene, producing a cyclohexene derivative. The simplest case involves 1,3-butadiene and ethene, but does not give a good product yield.
Conjugated dienes exist in two conformation called s-cis and s-trans. They interconvert freely in open-chain dienes, by single bond rotation, but may not be able to in some cyclic cases. For the Diels-Alder reaction to occur, the s-cis conformation needs to be present, so the two carbons of the alkene can connect to both of them simultaneously.
The reaction occurs in a single step during which the pi electrons reorganize through a cyclic transition state, forming two new single bonds to the ends of the diene. It is one of the reactions among a larger class called cycloaddition, and cycloaddition falls into a broader class called pericyclic - reactions which involve no intermediate, only a cyclic reorganization of electrons.
Good yields of Diels-Alder products are obtained when the dienophile (alkene) has electron-withdrawing groups, often carbonyl groups, ester groups or cyano groups. Maleic anhydride is often used as a dienophile.
Alkynes are often used as the dieneophile, but need electron withdrawing groups to enhance the reaction. 1,4-cyclohexadiene derivatives are produced.
If this product was treated with more 1,3-butadiene and heat, another ring would form. Which of the two alkenes would be the one to react? Draw the structure of this product.
Cyclic dienes provide a good synthetic route to bridged bicyclic compounds. When two stereoisomers are possible usually the preferred product is the one which has the unsaturated groups of the dienophile facing the alkene bridge, away from the saturated bridge, of the bicyclic coupound. This is known as the endo isomer.
Another stereochemical detail is the stereospecificity with regard to the dienophile. Since the reaction occurs in a single step, groups that are cis in the dienophile stay cis in the product, and trans in the dienophile remain trans in the product.