STEREOCHEMICAL AND KINETIC STUDIES ON SOME DERIVATIVES OF DIELS-ALDER ADDUCTS

Abstract

Conformational studies about the C-N bond of arylimide derivatives of Diels-Alder adducts such as 2,7- dihydroxynaphthalene- maleic anhydride, 2,6 - dihydroxynaphtha1ene - maleic anhydride and 6-bromo-2-naphtho1-maleic anhydride adduct have been very useful in the assignment of configurations of these adducts.Rotation about the aryl C-N bond in ortho-substituted arylimides is restricted and non-planar conformations exist. PMR studies on these aspects have been very useful in solving structural problem. The spectra of o-tolylimide and 1-naphthy1imide derivatives of the isomeric adducts. have diagnostic merits in the configurationa1 assignment of the adducts. E1ectrophi1ic addition to the olefinic bond has remained a subject of active interest for quite long. The stereochemistry of methoxybromination of some Diels-Alder adducts in the presence of yellow lead oxide/silver nitrate has been studied and assigned with the help of PMR spectroscopy. 6,6-Dimethy1fu1vene -p-benzoquinone adduct, 6,6-dipheny1fu1vene -p-benzoquinone adduct, 6,6-dimehty1fulvene-p- naphthoquinone adduct, 6,6-dipheny1fulvene-p- naphthoquinone adduct and 1',4'-diacetoxy benzonorbornadiene derivatives undergo exo-cis addition to yield 2-methoxy-3-bromo product whereas 1',4'-diacetoxybenzonorbornadiene gives the rearranged 2- methoxy -7-bromo product. cis- Addition of methoxybromine and exoorientation are indicated by coupling constants. The modified Karplus relationship has been taken into consideration while analysing the influence of electronegative substituents in evaluating coupling constants. The 2-methy1anthracenep- benzoquinone adduct leads to the formation of trans-product. The bromine atom and the methoxy group of the methoxybromo adduct are present in trans-l,2-diaxial positions in the chair type of eye 1ohexadione ring. The results of kinetic studies on the reactions of 2,3-Ccyc1openten- 3',5'-diyl] -exo-N- (2,4-dinitrophenoxy) succinimide with hydroxide ion and some neutral amines, viz, morpholine, eye Iohexy1 amine, piperidine and n-butylamine in 80-20 acetonitrile water (v/v) at 35 ± 0.1 C are reported. Kinetic studies have been carried out under the conditions of excess of nucleophile over substrate. The second order rate coefficients, k. or k obtained by dividing the pseudo first A _r r order rate coefficients by [Nul .. .. or CNu3 are almost ef feetlve constant with morpholine, eye 1ohexy1 amine and n-butylamine. However, k. or k increases with increased [Nul• . .. or [Nul in -A -r effective the case of piperidine and hydroxide ion indicating a possible base catalysis. Studies on solvent effect reveal the polar nature of the transition state. The effect of pH on the rate seems to be negligible for the reactions with morpholine, eye 1ohexy1 amine and n-butylamine, but in the case of piperidine the rate increases with increased pH, indicating a possible catalysis by hydroxide ion. The rections have been carried out at various temperatures ranging from 25 to 45 C and the activation parameters have been calculated. Entropy of activation values are large and negative indicating the congested nature of the transition state. The site of nucleophilic attack has been identified to be the nitroactivated aromatic carbon attached to the oxygen atom and the cleavage of C-0 bond occurs. From the analysis of experimental data, the rate determining formation of zwitterionic c-complex in the case of morpholine, eye 1ohexy1 amine and n-butylamine is suggested, whereas in the case of piperidine and hydroxide ion the rate determining decomposition of the intermediate complex is indicated. An attempt has been made to account for the observed difference in reactivity between the endo and exo isomers for 2,3-Ccyclopenten-3',5'-diy1] N-(2,4-dinitrophenoxy) succinimide with various nuc1eophi1es.