Reductive electrolysis of [60]fullerene mono-methanoadducts in THF leads to the formation of bis-adducts in high yields Marcel W. J. Beulen, a Jos´ e A. Rivera, b M. ´ Angeles Herranz, c ´ Angel Mart´ ın-Domenech c Nazario Mart´ ın* c and Luis Echegoyen* a a Department of Chemistry and Center for Supramolecular Science, University of Miami, Coral Gables, Florida 33124, USA. E-mail: echegoyen@miami.edu b Department of Chemistry, Pontifical Catholic University of Puerto Rico, Ponce, Puerto Rico 00731, USA c Departamento de Qu´ ımica Org´ anica, Facultad de Ciencias Qu´ ımicas, Universidad Complutense, E-28040-Madrid, Spain Received (in Columbia, MO, USA) 30th October 2000, Accepted 19th January 2001 First published as an Advance Article on the web 9th February 2001 A new reaction, electrolytically induced adduct transfer between [60]fullerene mono-adducts, leads to bis-adducts with a unique regioisomer distribution. Cyclopropanation of fullerenes by the addition of malonate derivatives (the Bingel reaction) has been widely used for the preparation of fullerene adducts. 1 The groups of Diederich and Echegoyen reported the reverse of this reaction, the retro-Bingel reaction, an electrolytic reduction reaction performed in dichloromethane, which efficiently removes di[alkoxycarbo- nyl]methano (Bingel) adducts to yield the parent fullerene (see Scheme 1). 2 The ‘Bingel–retro-Bingel’ strategy as a protection– deprotection scheme has already found several uses in fullerene chemistry, such as the isolation of enantiomerically pure C 76 2a and of a new C 2v -C 78 bis-adduct, 2b and in the separation of constitutional isomers of C 84 . 2c The groups of Diederich and Echegoyen also described a chemical retro-Bingel reaction (using Mg/Hg, THF, heat) which also efficiently removes the Bingel addends. 3 Very recently we reported that the range of fullerene adducts that can be removed via electrolytic reduction is not limited to di[alkoxycarbonyl]methano adducts (see fullerene 3), but includes those present in structures 1 and 2 (Scheme 1). Controlled potential electrolysis (CPE) in dichloromethane after the first (2) and third (1) electrochemical reduction wave induces the efficient removal of the adducts, to form the parent C 60 . 4 While electrolytic reduction can lead to the efficient removal of the adducts mentioned, it has been shown that methano- adduct formation can also result from the reaction between electrolytically prepared fullerene anions and dihalo-com- pounds, even with dichloromethane. 5 Dichloromethane reacts efficiently with the [60]fullerene trianion to form methano- fullerenes of the type C 60 > (CH 2 ) n , 5b and forms similar adducts with C 84 [C 84 > (CH 2 ) n ] during the reductive retro-Bingel reaction of di[alkoxycarbonyl]methano-adducts of C 84 . 2c In order to avoid these reactions and to further explore the potential and mechanism of the electrolytic methods of fullerene adduct removal, we decided to investigate the electrolysis of compounds 1–3 in THF. In the process we found an electrochemically induced intermolecular reaction that leads to the formation of multiple fullerene adducts. 6 The cyclic voltammograms of the methanofullerenes 1 and 2 in THF are shown in Fig. 1. 7 These compounds exhibit irreversible electrochemistry in THF, similar to that observed in dichloromethane. Compound 1 exhibits, in addition to several reversible electrochemical processes, an irreversible reduction process between the first and third reduction potentials whilst 2 undergoes an initial two-electron reduction, followed by a one- electron reduction. The irreversible behavior presumably results from the cleavage of one of the cyclopropane bonds connecting the addend to C 60 after reduction. 8 Compounds 1 and 2 were subjected to CPE in 0.1 M NBu 4 PF 6 –THF and the products were separated and analyzed. CPE of 1 was performed at ca. 100–150 mV more cathodic than the third, reversible, reduction wave (Fig. 1, arrow a); 2.7 electrons per molecule were discharged and clear changes in the CV and OSWV were observed, indicating that a chemical reaction had taken place. Subsequent re-oxidation at 0 V and purification of the product mixture by column chromatography (eluent: toluene) yielded fullerene products in ca. 91% yield. Analysis of this mixture by HPLC, UV–VIS spectroscopy, and MALDI-TOF spectrometry clearly showed the formation of C 60 as the main product (41%). Thus reductive electrochemistry removes the addend in THF (as also observed in CH 2 Cl 2 ), leading to the formation of C 60 . A second fraction (39%) containing the starting material 1 was also recovered. Inter- estingly and unexpectedly, a third fraction (11%) with a higher polarity was also isolated. Analysis showed this fraction to be composed of the bis-adducts: fullerenes with two spiro- anthraquinone groups attached (MALDI-TOF: m/z 1104). CPE of 2 in THF was performed after the first two-electron reduction wave (Fig. 1, arrow b), and 1.8 electrons per molecule were discharged. Re-oxidation and purification yielded 81% of fullerene products, consisting of 40% C 60 , 27% recovered 2, and 14% of bis-adducts. Adduct removal is also the main Scheme 1 Protective group system for fullerenes: synthesis of methano- fullerenes 1–3 and subsequent adduct removal by reductive electro- chemistry. Fig. 1 Cyclic voltammograms of 1 and 2 in THF. This journal is © The Royal Society of Chemistry 2001 DOI: 10.1039/b008769f Chem. Commun., 2001, 407–408 407