Delocalisation dienes

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) ... 

The reaction of the diene with a free radical produces an allyl radical having unpaired electron delocalised over more than one carbon atom. The allyl free radical can undergo 1, 2, or 1, 4 addition. 

The deamination of the allylic 3-aminocholest-4-enes is rather remarkable in that the products are not derived from an intermediate delocalised allylic cation 14) [20]. Other cholest 4-en-3-yl derivatives solvolyse to give the allylic cation (14) irrespective of the original C(3)"Configuration, and the cation affords a characteristic mixture of cholesta-3,5-diene and cholest-4-en-3a yl and -SjS-yl derivatives (p. 381). The 3(S-amine (10), however, gave only cholest-4-en 3 -ol (ii) on deamination, whereas the 3a-amine (12) gave cholesta-2,4-diene (13) [20], Both amines therefore react in the same... 

It is generally accepted that, in the polymerisation of dienes on lanthanide catalysts, the growing chain is attached to the transition metal by an 7t-allyl bond and that the chain growth occurs by incorporation of the monomer via the metal-carbon o-bond. In the case of neodymium catalysts, the delocalised 7t-allyl type structure of the terminal unit has been observed by spectroscopic methods [8, 26, 28, 58-60]. The results reported in these papers show that the relative contents of cis-l,A- and tri2ns-1,4-units in polydienes depend on the type of solvent used, the polymerisation temperature, structure of diene monomer, and the composition of lanthanide-based catalysts. These data can be interpreted in terms of the concept of isomerisation equilibrium between anti- and syn-forms of n-allyl terminal unit. One of the arguments in favour of the existence of this isomerisation... 

The p.e. spectrum of diazocyclopentadiene has been recorded and analysed, using ab initio SCF MO calculations [70]. It was concluded that a diene structure provided a more appropriate picture of the five-membered ring than a delocalised structure and that there was a large amount of C-N ir-bonding interaction [70]. ... 

Once we have proven the existence of bond alternation, we must prove the existence of p-electron delocalisation (i.e. conjugation). In this case, we use vibrational intensities (see section on Intensity Spectroscopy" above), and compare the data derived from butadiene, the shortest trans-diene. Let us consider the electro-optical parameters which describe the change... 

The alkoxy ir -allylic derivatives underwent alkoxyl exchange very readily when treated with the appropriate alcohol containing a little mineral acid (10" M)(e.g. methoxy-ethoxy and vice versa). These reactions are believed to go via an intermediate carbonium ion stabilised by the delocalised electron system of the tt-allylic group. Methoxy tt-allylic complexes derived from 2,5-dimethylhexa-2,4-diene, cyclohepta-1,3-diene and cycloocta-1,3-diene on heating lost methanol irreversibly to give a8-unsaturated tr-allylic complexes e.g. cyclohepta-1,3-diene first gave a tt-methoxycycloheptenyl and then a TT -cycloheptadienyl complex. 

This affirmation has been tested by ESR spectroscopy and product analysis of the reactions of pure NO and of NO-air mixtures with alkenes containing electron-withdrawing and electron-releasing substituents as well as dienes [22], fatty acids [24] and P-carotene [25]. It has been shown that ARs can be detected in the reaction of NO with all these compounds. In the complete absence of air, no ESR spectra in alkenes were obtained. Elowever, when some quantity of air was introduced into the NO gas stream, ESR spectra appeared. Pure NO therefore does not add directly to alkenes containing acceptor or donor substituents or to dienes. The thermodynamic stabilisation due to electron delocalisation (Equation 3.18) was not sufficient to alter... 

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