Spatial arrangement of norbornanedicarboxylic acids N-arylimides

Abstract

The effect of substituents on the spatial arrangement of norbornane compounds was investigated. Norbornanedicarboxylic acids N-arylimides with OCH 3 , Cl, NO 2 groups in the ortho-position of the aromatic ring form as conformational isomers with hindered rotation. The specifi c spatial isomerism is confi rmed by the quantum-chemical calculations. The oxidation of bicyclo[2.2.2]oct-2-ene with performic acid is known to afford bicyclo[2.2.2]-octanetrans-2,3-diol, whereas the oxidation of bicyclo[2.2.1]-hept-2-ene under the same conditions is accompanied by Wagner-Meerwein rearrangement and 1,3-hydride shift [1-3].The introduction into the norbornene ring of electronacceptor substituents act as a stabilizing factor fi xing the boat-like conformation of the cyclohexene fragment and thus prevents the hydride shifts and rearrangements [4,5]. In the oxidation of norbornenes containing reactive groups in the endo-position forms as a rule a new lactone ring, whereas the presence of these groups in the exo-position of the norbornene results in normal oxidation products [6,7]. This rule is conserved in the oxidation of norbornene dicarboxylic acids, their anhydrides I, II, and the nitrogen-containing derivatives with the endo-and exo-confi guration [8][9][10]. The epoxidation of exo-compounds proceeds stereoselectively with the formation of an oxirane ring of exo-confi guration, a lactone is formed from the corresponding endo-derivatives [11-13] (Scheme 1).Our investigation of the oxidation of norbornenedicarboxylic acids N-arylimides showed that independent of the endo-or exo-confi guration of the imide ring and also of the electronic nature of the substituents in the ortho-, meta, or para-positions in the N-arylimide fragment only stereoisomeric epoxy or dihydroxy derivatives formed, and no lactonization and Wagner-Meerwein rearrangement was observed [14,15]. The strong electron-acceptor imide moiety decreases the electron density of the π-bond of norbornene and diminishes the ionization of the C-O bond which is the fi rst stage in the Wagner-Meerwein rearrangement, and the low strain of the fi ve-membered imide ring and its chemical inertness prevents the lactonization (Scheme 2).The role of electronic and sterical effects and the positions (ortho-, meta-, para-) of substituents in the formation of the conformational structure of norbornane N-arylimides remains unclear.Our quantum-chemical calculations, spectral analysis, and the consideration of model structures of N-arylimides III-LXVI of bicyclo[2.2.1]hept-5-ene-endo-and -exo-2,3-dicarboxylic acid showed that when in the orthoposition of the aromatic ring were located OCH 3 , Cl, NO 2 groups the large bulk of the substituents hampered the free rotation, and as a result a stable conformational isomer of N-arylimide formed where the benzene ring at the nitrogen atom was perpendicular to the plane of the fi ve-membered heterocycle, and the N-C bond suffered elongation compared to its average length [16]. This fact is confirmed by the presence of two