Electrochemically induced retro-cyclopropanation reactions

The efficient removal of methano addends from fullerenes using reductive electrochemistry is a versatile and useful reaction, which was initially called the retro-Bingel reaction since it was observed for the first time on bis(alkoxycarbonyl)methano adducts, commonly known as Bingel adducts. Since its initial discovery, this reaction has been successfully employed in the separation of enantiomers and constitutional isomers of fullerenes, which in some cases were otherwise not accessible. Even more interesting than the retro-Bingel reaction was the electrochemically induced isomerization that bis-methano adducts of C-60 exhibit, and which was referred to as the "shuffle". The combination of both reactions gave interesting results during the electrolysis of tetrakis-, pentakis-, and hexa-kis-Bingel adducts of C-60. The selective removal of Bingel-type addends, while leaving other addends undisturbed, was also investigated. More recently, it was observed that in addition to bis(alkoxycarbonyl)methano adducts, electrochemical reduction of other methano adducts, such as spiromethanofullerenes, could also result in removal of the methano addends, thus the reaction was more generally denoted as retro-cyclo-propanation. In some of the spiromethanofullerenes investigated, and depending on the solvent used, an electrochemically induced intermolecular addend transfer was observed, which provided a regioisomeric distribution that differed significantly from that obtained by a synthetic route. We have recently shown that singly bonded dimers are formed as intermediates during retro-cyclopropanation reactions. In this microreview, we describe the general observations that led to the initiation of these exciting electrochemical investigations, review the differences between the chemical and electrochemical methodologies in retro-cyclopropanation reactions and explore the mechanisms involved during addend removal. ((C) Wiley-VCH Verlag GmbH & Co. KGaA, 69451 Weinheim, Germany, 2004).