1835 Pure Appl. Chem., Vol. 77, No. 11, pp. 1835–1850, 2005. DOI: 10.1351/pac200577111835 © 2005 IUPAC Cycloproparenyl anions: From models to real systems* Mirjana Eckert-Maksic ‡ and Zoran Glasovac Division of Organic Chemistry and Biochemistry, Rudjer Boskovic Institute, P.O. Box 180, HR-10002 Zagreb, Croatia Abstract: An overview of our recent work on cycloproparenyl anions is given. Preparation, the electronic structure, and the properties of the progenitor of the series, cyclopropabenzenyl anion, are discussed. It is shown that the cyclopropabenzenyl anion is by ca. 145 kJ mol –1 more stable than the parent cyclopropenyl anion according to results of the MP2/6-31+G(d) calculations. This finding was attributed to a delicate balance of two opposing effects: (a) propensity of the aromatic ring to alleviate unfavorable 4π electron interaction within the three-membered ring by the anionic resonance effect and (b) a pyramidalization of the an- ionic center, which tends to maximize the s-character of the lone pair. We have also shown that stability of the cyclopropabenzenyl anion could be considerably enhanced by substitu- tion of the aromatic ring with fluorine and cyano groups, as well as by a linear extension of the aromatic backbone. Finally, the impact of the fusion of additional cyclopropenyl ring to the benzene moiety to acidity of the benzylic position in cyclopropabenzene is discussed. Keywords: Cycloproparenes; gas-phase acidity; solvent effects; substituent effects; strain. INTRODUCTION Fusion of a cyclopropene ring to an aromatic moiety generates a family of experimentally and theo- retically interesting compounds known as cycloproparenes [1]. Their chemistry is characterized by an interplay of two antagonistic effects: aromaticity, which is generally known to stabilize compounds, and angular strain, which destabilizes them. Their juxtaposition leads to some new and sometimes unex- pected novel features, leading to a plethora of fascinating compounds [1]. It is interesting to mention for historical reasons that the parent member of these compounds—cyclopropabenzene—was first re- ported by Perkin in his seminal paper entitled “Synthetical formation of closed-chains in the aromatic series. Part I. On some derivatives of hydrindonaphthene and tetrahydronaphthalene” more than a cen- tury ago [2]. In the same paper, Perkin described synthesis of less strained representatives of cycloalkarenes— indene and tetralin—while synthesis of more strained representatives—cyclobutabenzene and cyclo- propabenzene—was achieved more than 60 years later, apparently because of their lesser stability due to the angular strain. Thus, cyclobutabenzene was prepared by Cava and Napier in 1958 [3], and the first synthesis of unsubstituted cyclopropabenzene was published by Vogel and coworkers [4]. These discoveries initiated tremendous interest from both experimental and theoretical chemists in their physicochemical properties [1]. Most of the theoretical work in this area has been focused on the struc- *Paper based on a presentation at the 17 th International Conference on Physical Organic Chemistry (ICPOC-17), Shanghai, China, 15–20 August 2004. Other presentations are published in this issue, pp. 1807–1921. ‡ Corresponding author: E-mail: mmaksic@emma.irb.hr