Conjugated systems 1,3-dienes

Conjugare is a Latin verb meaning to link or yoke together and allylic carbocations allylic free radicals and conjugated dienes are all examples of conjugated systems In this chapter we 11 see how conjugation permits two functional units within a molecule to display a kind of reactivity that is qualitatively different from that of either unit alone... 

Allylic carbocations and allylic radicals are conjugated systems involved as reactive intermediates m chemical reactions The third type of conjugated system that we will examine conjugated dienes, consists of stable molecules... 

Whereas simple olefins are not usually made by elimination from halides, conjugated systems are frequently obtained in this way. The cases of a- and j5-halo ketones and their vinylogues have already been covered. Allylic halides may also be eliminated to form dienes, for example, the 2,4-diene (109)... 

Conjugate addition (Sections 10.10 and 18.12) Addition reaction in which the reagent adds to the termini of the conjugated system with migration of the double bond synonymous with 1,4 addition. The most common examples include conjugate addition to 1,3-dienes and to a,(3-unsaturated carbonyl compounds. 

When Diels and Alder published their famous paper in 1928, Diels had been working with related reactions for several years [6]. In 1925, Diels reported the reaction of azodicarboxylic ester (Et0C(0)2CN=NCC(0)0Et) with compounds containing a conjugated diene system. He found that addition of the azodicarboxylic ester occurs at the 1,4-position of the conjugated system as with cyclopentadiene and with butadiene. This work probably led to the famous Diels-Alder reaction. In 1927, Diels and his student Alder published a paper on the reaction of azodicarboxylic ester with styrene. 

Interestingly, benzonitrile oxide does not react with thiirene dioxide 19b even in boiling benzene, whereas the electron-rich diene l-piperidino-2-methyl-l, 3-pentadiene (177) does react under the same reaction conditions to give the expected six-membered [4 + 2] cycloadduct 178, accompanied by sulfur dioxide extrusion and 1,3-hydrogen shift to form the conjugated system 179175 (equation 70). 

Addition to conjugated systems can also be accomplished by any of the other three mechanisms. In each case, there is competition between 1,2 and 1,4 addition. In the case of nucleophilic or free-radical attack, the intermediates are resonance hybrids and behave like the intermediate from electrophilic attack. Dienes can give 1,4 addition by a cyclic mechanism in this way ... 

The double bonds in a conjugated diene are hydroborated separately, that is, there is no 1,4 addition. However, it is not easy to hydroborate just one of a conjugated system, since conjugated double bonds are less reactive than isolated ones. Thexylborane °(48) is particularly useful for achieving the cyclic hydroboration of dienes, conjugated or nonconjugated, as in the formation of 53." ... 

It is of interest to investigate the usefulness of this theory to the chemical change involving the interaction between the conjugated systems 56,62,145). Such a-n interactions are frequently stereoselective. The addition to olefinic double bonds and the a, -elimination are liable to take place with the fraMS-mode 146h The Diels-Alder reaction occurs with the cis-fashion with respect to both diene and dienophile. 

Delocalisation takes place (cf. 1,3-dienes, p. 13), so that an electron-deficient atom results at C3, as well as at C, as in a simple carbonyl compound. The difference between this transmission via a conjugated system, and the inductive effect in saturated system, is that here the effect suffers much less diminution by its transmission, and the polarity at adjacent carbon atoms alternates. 

With unsymmetrical dienes (74a and 74b) and unsymmetrical adducts, the problem of orientation of addition (cf. p. 184) arises. Initial attack will still be on a terminal carbon atom of the conjugated system so that a delocalised allylic intermediate is obtained, but preferential attack will be on the terminal carbon that will yield the more stable of the two possible cations i.e. (75) rather than (76), and (77) rather than (78) ... 

This comparison demonstrates that the exocyclic double bond length is little affected by the cyclic strain, which was also found for the radialenes see Section n.E. However, significant deviations were found for conjugated systems, and the same holds for the linear and branched dienes and polyenes. 

In Section I the sensitivity of conjugated systems to possible photo-dissociation (PD) was mentioned. If this PD is conducted in the ionization chamber of a mass spectrometer, a PDPI/MS (photo-dissociation, photo-ionization/mass spectrometry) can be measured7. The examination of the PDPI/MS spectra of hexa-1,3-, 1,4-, 1,5- and 2,4-dienes using 9.68 and 10.49 eV photo-ionization (PI) is summarized in Table 1. 

In general, one can use a variety of oxidation techniques to form derivatives of dienes and higher polyenes for their analysis however, the information obtained with conjugated systems is muddled by the complexity of products. Also, it is obvious that since the oxidized derivatives contain different functionalities, e.g. epoxides, alcohols, acids etc., the analytical techniques employed should also be variable (see also Scheme 2). 

The reaction is carried out simply by heating a diene or another conjugated system of n bonds with a reactive unsaturated compound (dienophile). Usually the reaction is not sensible to catalysts and light does not affect the course. Depending on the specific components, either carboxylic or heterocyclic products can be obtained. The stereospecificity of the reaction was firmly established even before the importance of orbital symmetry was recognized. In terms of orbital symmetry classification, the Diels-Alder reaction is a k4s + n2s cycloaddition, an allowed process. 

In an electrocyclic ring-closure reaction a new o-bond is formed between the end atoms of a conjugated system of double bonds. Consider the cyclisation of buta-1,3-diene ... 

The addition of selenenyl derivatives to olefins has been shown to be of mechanistic interest and synthetic utility because of the versatility of the selenium functionalities28,133. The possibility of modifying double bonds with seleno derivatives has been applied also to conjugated systems in order to obtain arylseleno dienes, or electron-deficient dienes, both being useful synthetic intermediates or building blocks. 

The isomerization catalysts are hydride complexes, and they can convert the unconjugated dienes or polyenes to conjugated systems through double-bond migration. This process occurs by an M—H addition-elimination process. 

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