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A property of aromatic compounds is their preference for reaction with electrophiles by substitution rather than by addition. A number. of substituted cyclopentadienes have been prepared by electrophilic substitution of the cyclopentadienide anion, a 6pi-electron aromatic species. When the substituents are electron- withdrawing groups, the result is increased acidity of the cyclopentadienes, and deactivation of their anions towards further electrophilic attack. Reaction of cyclopentadienide anion with acyl chlorides gives rise to 1,2-diacyicyclopentadienes, which appear to exist, in the solid phase and in non-polar solvents almost entirely as the hydrogen-bonded enol tautomers, which are 2-acyl-6-hydroxyfulvenes. These enols have considerable acidity, since the negative charge of their anions is delocalised (See abstract in thesis for diagram). The time-averaged structure of the enol molecule is symmetrical, and it is not possible spectroscopically to distinguish the hydrogen atom at C-3 from that at C-5. The electrophilic substitution of the 2-acyl-6-hydroxy- fulvenes or their anions is of interest because of the deactivating influence of the acyl groups, together with the fact that, there are three different possible sites for electrophilic substitution, viz. 0-5(5), C-4 and 0. Nitrated, brominated and azo-coupled derivatives have been prepared from 2-benzoyl-6-phenyl-6-hydroxyfulvene and 2-acetyl-6-raethyl-6-hydroxyfulvene. Bromination and nitration give C-4 substituted products; the monobrominated derivatives readily undergo further bromination at C-3 and C-5, since the presence of one bromine atom has little, if any, deactivating effect. The reaction between 1,2-dibenzoyl- or 1,2-diacetylcyclopenta-dienide anion and aryl diazonium salts gives products substituted at G-5j which are tautomeric with 2,3-diacylcyclopentadienone arylhydrazones. An O-substituted compound has been obtained from the' reaction of dibenzoylcyclopentadienide anion with methyl chloroformate. Attempts to prepare triacylated cyclopentadienes by the action of acyl chlorides (without a Friedel-Crafts catalyst), or by Vilsmeier formylation of 2-acyl-6-hydroxyfulvenes, were unsuccessful. Some possible pathways to diacylcyclocyclopentadienylides and pyridinium diacylcyclopentadienylides were explored, but yielded no ylides. However, triphenylarsonium 3,4-dibenzoylcyclopenta-dienylide was prepared by the reaction of 2-benzoyl-6-phenyl-6- hydroxyfulvene with triphenylarsine oxide in acetic anhydride, Triphenylarsonium 3,4-dipivaloylcyclopentadienylide was similarly prepared from 2-pivaloyl-6-t-butyl-6-hydroxyfulvene. Linn and Sharkey reported the reaction of 2-benzoyl-6- phenyl-6-hydroxyfulvene with hydrazine and with hydroxylamine to give the compounds 1,4-di]phenyl-2H-cyclopenta[d]pyridazine and 1,4-diphenylcyclopentaC d]-2,3-oxazine. (See abstract in thesis for diagram). Analogues have been prepared from 2-acetyl-6-methyl-6-hydroxy- fulvene. These heterocyclic systems are iso-pi-electronic with azulexie. The 2H,-cyclopenta[d]pyridaaines were found to resemble azulenes in their reaction with electrophiles on the five- membered ring, Brominated and formylated derivatives were prepared, the substituents occupying the 5- or 7-position, Protonation takes place at C-5 or G-7 and the resulting methylene group is reactive. Condensation with carbonyl compounds such as p-dimethyl- aminobenaaldehyde and diphenylcyclopropenone was found to occur readily. The cyclopenta[d]-2,3-oxazines' appear to be' somewhat less reactive towards electrophiles than the 2H-cyclopentaCd]pyrid- azines, and formylation was successful, Brominated derivatives were nevertheless obtained. It is concluded, on the basis of their reactions, and the K.M.R, coupling constants of the five- membered ring protons of the brominated derivatives, that a lesser degree of n-electron delocalisation exists in the cyclo- penta[d]-2,3-oxazines than in the 2-substituted cyclopenta[d]-pyridazines. The appendix to this thesis contains an account of some reactions of the cyclononatetraenide anion, including the attempted preparation of ylides incorporating the 10 n-electron cyclonona-tetraenide ring.
Thesis, PhD Doctor of Philosophy
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