Content area
Abstract
1. The antiaromatic instability imparted on the nine membered ring by the presence of a "4n"(pi)-electron (n = 2) periphery was demonstrated several years ago. In contrast, no information was available on the family's pseudo-"4n"(pi) (n = 2), namely the corresponding ketone ( 9 annulenone) whether in the parent state or simple annulated form. In this project we successfully synthesized benzo 9 annulenone 89, studied its acid induced antiaromaticity and characterized structurally as well as mechanistically a number of products resulting from bond reorganization.
2. Nonafulvene, the nine membered analog of parent fulvene, with its basic all-cis array of (pi) centers is well known to represent the size limit beyond which "static" planarity within a (pi) monocycle becomes skeletally inaccessible. The few available nonafulvenes incorporating an unsubstituted ring, i.e. 113 (a,b)('76) undergo (pi)6s electrocyclization to the corresponding cis-dihydroindenes with relative rates which are indicative of increased stability due to enhanced availability of the desired "4n + 2" (pi)-excessive (10(pi)) ring component. Monobenzoid fusion to the nonafulvene skeleton ought to prevent electrocyclization under conditions of minimal electronic perturbation. In this project we successfully prepared and studied a monobenzo derivative 160 and, for comparison purposes, a sterically congested mononaphtho relative 161 as well. Careful NMR examination of these two substances revealed the presence of conformationally visible (161 more than 160) and largely (pi)-localized 9-membered frames.
3. Direct spectroscopic scrutiny of potentially anti-aromatic, predictably unstable, 4n-electron "Huckel" (pi) systems has long been a fundamentally significant aspect of research directed at probing the phenomenon of "aromaticity". Generation and direct NMR (('1)H, ('13)C) examination of the 16-electron (pi)-excessive monohetero and dihetero anthracenyl anions 204a,b and 213a,b,c established the existence of thermally stable frames and the presence of extensive (pi)-delocalization with well developed paratropicity.
4. The induction of helical chirality in 9-membered annulenones 89 and 129 was shown by an ('1)H-NMR study of these compounds in the presence of a chiral shift reagent 233, and enantiomeric resolution of the parent hydrocarbon 126 was exercised by a variety of methods which proved to be unsuccessful due to what is believed to be rapid skeletal interconversion (18 (DELTA)G('(NOT=)) < 7.0 Kcal/mol faster than 126 (DELTA)G('(NOT=)) = 14.1 Kcal/mol) of the 9-membered frames of these substances. (Abstract shortened with permission of author.)
