Conjugated polyenes cyclization

The cyclization of conjugated polyenes and the inverse reaction were those processes which provided superb materials 142> leading to the Woodward-Hoffmann rule 51>. 

The cyclization of fully conjugated polyenes containing 2n + 2 jr-electrons (equation 1) was termed electrocydie by Woodward and Hoffmann, who showed that the steric course of such reactions was governed by the rules of orbital symmetryI. 3. 

B. Valence isomerization. The conjugated polyenes in their excited biradical state can lead to intramolecular cyclization in a number of ways giving rise to a number of products. In such cyclization, valence bonds are reorganized without migration of atoms or groups but by migration of c or it electrons only. For example, on excitation of pentadiene, the following products are observed ... 

As films are used e.g. the polymerization product of ethylbenzene and divinylbenzene (33) the copolymer of styrene and butadiene (755) the copolymer of styrene and butadiene mixed with polyethylene (157) a vulcanized or cyclized copolymer of an aromatic vinylcompound and an aliphatic conjugated polyene (2). As a crack resisting matrix is mentioned the copolymer of styrene, divinylbenzene and butadiene with e.g. dioctylphthalate as a plasticizer (176). Other examples are the copolymers of unsaturated aromatic compounds and unsaturated aliphatic compounds (77) and the reaction products of polyolefines and partially polymerized styrene (174). Primary groups can be introduced also with the help of Friedel-Crafts catalyst. Ts. Kuwata and co-workers treated a film of a copolymer of styrene and butadiene with an aluminium-ether complex and ethylenedichloride (79). Afterwards they allowed the film to react with trimethylamine. Another technique is the grafting of e.g. a polyethylene film with styrene (28). 

The thermal and photochemical cyclization of a butadiene bearing different substituents at its terminal carbon atoms is represented by eqs. (3.5.15) and (3.5.16), respectively. Note that for a short conjugated polyene consisting of four or six carbon atoms, conformational interconversion between their transoid (or s-trans) and cisoid (or s-cis) forms takes place readily. 

The best way to understand how orbital symmetiy affects pericyclic reactions is to look at some examples, bet s look first at a group of polyene rearrangements called electrocyclu reactions. An electrocyclic reaction is a pericyclic proce.ss that involves the cyclization of a conjugated polyene. One n bond is broken, the other 7T bonds change position, a new rr bond is formed, and a cyclic compound results. For example, a conjugated triene can be converted into a cyclohe.xa-dicne, and a conjugated diene can be converted into a cyclobutene. 

Electrocyclic reactions involve the cyclization of conjugated polyenes. For example, 1,3,5-hexatriene cyclizes to 1,3-cyclohexadiene on heating. Electrocyclic reactions can occur by either conrotatory or disrotatory paths, depending on the symmetry of the terminal lobes of the tt system. Conrotatory cyclization requires that both lobes rot lte in the same direction, whereas disrotatory cyclization requires that the lobes rotate in oj )posite directions. The reaction course in a specific case can be found by looking at the symmetry of the highest occupied molecular orbital (HOMO). 

An electrocyclic reaction is a pericyclic process in which a conjugated polyene undergoes cyclization. In the process, one it bond is converted into a a bond, while the remaining it bonds all change their location. The newly formed a bond joins the ends of the original TT system, thereby creating a ring. Two examples are shown. 

An electrocyclic reaction [23] is the closure of a conjugated polyene to give a cyclic compound with one less n bond, or the reverse. For the first example, consider the thermal cyclization reaction of E, E-2,4,6-octatriene to cis-5,6-dimethylcyclohexadiene (Eq. 5.13). A a bond forms and new n bonds develop concurrently. 

The achiral triene chain of (a//-rrans-)-3-demethyl-famesic ester as well as its (6-cis-)-isoiner cyclize in the presence of acids to give the decalol derivative with four chirai centres whose relative configuration is well defined (P.A. Stadler, 1957 A. Escherunoser, 1959 W.S. Johnson, 1968, 1976). A monocyclic diene is formed as an intermediate (G. Stork, 1955). With more complicated 1,5-polyenes, such as squalene, oily mixtures of various cycliz-ation products are obtained. The 18,19-glycol of squalene 2,3-oxide, however, cyclized in modest yield with picric acid catalysis to give a complex tetracyclic natural product with nine chiral centres. Picric acid acts as a protic acid of medium strength whose conjugated base is non-nucleophilic. Such acids activate oxygen functions selectively (K.B. Sharpless, 1970). 

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