Rainer Haag and Armin de Meijere Abstract

The chemistry of oligounsaturated five-membered carbocycles has fascinated chemists for more than 100 years, since Thiele published the first synthesis of the aromatic cyclopentadienyl (Cp) anion 1 [1], In the last couple of decades, knowledge about fully unsatu- 

Cyclopentadienide (Cp) 1 is well known as one of the most frequently used ligands in organometallic chemistry. In addition, the cyclopentadienide anion 1 has always been quoted as a classic example of Hiickel aromaticity, to demonstrate along with benzene and the cydoheptatrienyl cation the validity of the (4n + 2) -electron rule. In contrast, a simple and stable cyclopentadienyl cation of the type 1+ remains to be elusive [5]. With the highly unstable neutral cyclobutadiene and the cydoheptatrienyl anion, 1+ shares the character- 

Modern Arene Chemistry. Edited by Didier Astruc 

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