Ring carbo-mers: From questionable homoaromaticity to bench aromaticity*
Chunhai Zou, Christine Lepetit, Yannick Coppel, and Remi Chauvin
Laboratoire de Chimie de Coordination du CNRS, UPR 8241, 205 Route de Narbonne, 31700, Toulouse Cédex 4, France
Abstract: The title journey is undertaken at the levels of both theory and experiment. Since 1983, homoaromaticity has been shown to play at most a minor role in the stability of Scott's [N]pericyclyne hydrocarbons - the first ring carbo-mers of cycloalkanes. This statement has been systematically refined for N = 3-6 by using both classical theoretical tools and newly designed tools based on electron localization function (ELF) analysis. The compatibility of the [5]- and [6]-pericyclyne cores with vertex functionalities was established by the synthesis of 20 oxy (carbo-cyclitol) derivatives. The stereoisomeric resolution of two of them has been achieved. Hexaoxy-[6]pericyclynes are actually potential precursors of the carbo-benzenes. Criteria based on density functional theory (DFT) calculations (magnetic, energetic, structural/"electronic") show that the aromaticity of carbo-[N]annulenic species is comparable to that of their parent molecule. This has been challenged by the synthesis of several novel carbo-benzenic molecules with various substitution patterns. The theory-experiment interplay is pursued by considering ring carbo-mers of other conjugated ring systems such as radialenes. The second carbo-mers (butadiyndiyl-expanded rings) of [3]radialene and benzene are also envisioned. Hexaphenyl-carbo2-benzene has been observed by 1H and UV-vis spectroscopy.
Keywords: carbo-mers; homoaromaticity; bench aromaticity; pericyclynes; carbo-cyclitol; DFT; radialene.
*Paper based on a presentation at the 11th International Symposium on Novel Aromatic Compounds (ISNA-11), St. John's, Newfoundland, Canada, 14-18 August 2005. Other presentations are published in this issue, pp. 685-888..