Electrocyclic Reactions14-9

Electrocyclic reactions involve the ring closure (or opening) of a single conjugated di-, tri- or polyene.

These reactions belong to the class of reactions called pericyclic, because they exhibit transition states with a cyclic array of nuclei and electrons.

The ring closure of cis-1,3,5-hexatriene takes place thermally. The ring opening of cyclobutene (a strained system) also takes place thermally. Both reactions can be reversed by irradiation with ultraviolet light.

Electrocyclic reactions are concerted and stereospecific.

The thermal isomeration of cis-3,4-dimethylcyclobutene gives only the cis,trans product:

The thermal isomeration of trans-3,4-dimethylcyclobutene gives only the trans,trans product:

In the thermal cyclobutene ring opening, the rehybridization from sp3 to sp2 is accomplished by both atoms rotating either clockwise or counterclockwise.

This mode of reaction is called a conrotary process.

The photochemical closure of butadiene to cyclebutene occurs with the opposite stereochemistry found in the ring opening. If one carbon rotates clockwise, the other rotates counterclockwise.

The mode of movement is called disrotatory.

In the case of the thermal ring closure of trans,cis,trans- and cis,cis,trans-Octatriene, the process is disrotatory:

However, the photochemical ring closure of trans,cis,trans-octatriene is conrotatory.

The stereocontrol of these and many other electrocyclic transformations is governed by the symmetry properties of the relevant molecular orbitals and is summarized by the Woodward-Hoffmann rules:

Polymerization of Conjugated Dienes: Rubber14-10

1,3-Butadiene can form cross-linked polymers.1,2-Butadiene can be polymerized at C1 and C4 to yield polyethenylethene (polyvinylethylene) or at C1 and C4 to yield trans-polybutadiene, cis-polybutadiene or a mixed polymer.

Because the product of butadiene polymerization is unsaturated, irradiation or radical initiators can cause additional linkages between individual polymer chains.

These substances are called cross-linked polymers.

Cross-linking generally increases the density and hardness of polymeric materials.

In addition, cross-linking increases the elasticity of polymeric materials, a property characteristic of rubbers.

Synthetic rubbers are derived from poly-1,3-dienes.2-Methyl-1,3-butadiene (isoprene), when polymerized using a Ziegler-Natta catalyst, results in a synthetic rubber called polyisoprene of almost 100% Z configuration.

2-Chloro-1,3-butadiene can be polymerized into an elastic, heat- and oxygen-resistant polymer having trans double bonds called neoprene.