Bis methanofullerene

Interestingly, the analytical chromatogram showed that the cis-2 isomer had completely disappeared and the e bis-adduct had become the predominant isomer in the newly formed product mbrture (e 57% trans-3 31% trans-4 8% cis-3 4%). These unexpected results, recently reported [13], led us to conduct similar experiments starting with some of the other bis-adduct isomers. Thus the pure cis-3, e, trans-4, and trans-3 bis-methanofullerenes were submitted to the exact same protocol le CPE around -1.2 V vs Ag, followed by oxidative CPE at 0 V and product analysis using chromatographic techniques. These four regioisomers were recovered in 75-90% yields and did not show any isomerization. Therefore, isomeric conversion upon le CPE occurs only for the cis-2 bis-methanofullerene. 

These observations prompted us to analyze the behavior of the bis-methanofullerenes upon 2e CPE and these experiments led to several interesting observations. The CPE experiments were conducted around -1.55 V vs Ag, but in every case they had to be manually interrupted before the current had decreased to background levels. This is a very important point, which actually leads to other observations that are described in detail in the next section, under the title oi retro-BingeF. Suffice it to say at this point that CPE at the potential corresponding to the second voltammetric reduction peaks were interrupted after determining, via coulometry, that the amount of charge passed corresponded to 2e per molecule. 

In order to test die generality of diis electrolytic reaction, we also investigated die exhaustive electrolysis of the bis-methanofullerene adducts at the second reduction potential. Recall diat in the previous section we interrupted the second electrolysis of the bis-adducts, and did not allow them to go to completion, since the current levels were high after 2e electrolysis. If electrolysis of any of the bis-adducts is conducted around -1.5 V vs Ag, and allowed to proceed to completion (to background current levels), a total... 

The electrochemical properties of all these bis-adducts have been studied in reasonable detail, using either cyclic voltammetry (CV) or controlled potential electrolysis (CPE) [8], Reductive electrolysis of ester-containing methanofullerenes results in the removal of the adducts in a versatile and useful reaction, initially called the retro-Bingel reaction (Figure 1). Additional work resulted in the discovery of an intramolecular electrochemically induced isomerization of C6o-bis-adducts. Exhaustive reduction with one electron per molecule resulted in seven regio-isomers regardless of which pure bis-adduct regioisomer was electrolyzed. Recently, it has been observed that, in addition to the malonates, electrochemical reduction of other methano-adducts can also lead to removal of the addends,... 

First to be checked was a set of methanofullerene derivatives 289a-c, readily available via addition of substituted diarylcarbenes (generated in situ) to one of the double bonds of fullerene 59 followed by a conventional transformation of the carboxamido groups of the side chains (Scheme 4.88). Bis-succinoyl-... 

Scheme 7-1 (A) Position notation for bis-adducts of Cgg according to Hirsch et al. [10]. (B) Products isolated from the second Bingel addition to bis(diethoxycarbonyl)methanofullerene [C6i(COOEt)2l [10]. a) Diethyl 2-bromomalonate, NaH, toluene, 20°C.

Hirsch and co-workers subsequently discovered that the yield of hexakis-adducts such as 8 or 9 can be greatly increased and their separation from side products substantially facilitated, if 9,10-dimethylanthracene (DMA) is used as a template (Scheme 7-2) [14, 15]. Anthracene derivatives such as DMA are well known to undergo reversible Diels-Alder additions with fullerenes at ambient temperamre [16-18]. After addition to the fullerene, the template directs diethyl malonate addends in the Bingel addition tegio-selectively into e positions, ultimately yielding the hexakis-adducts with a pseudo-octahedral, all-e addition pattern. The templated activation of e 6-6 bonds (bonds between two six-membered rings) is also efficient starting from C o mono-adducts, and several examples are shown in Scheme 7-2. Thus, bis(alkynyl)methanofullerene 10 [19] reacted with diethyl 2-bromomalonate/DBU (8 equiv) and DMA (12 equiv) to provide hexakis-... 

The most convincing evidence for structure 8.36, i. e., the [6,6]-closed isomer was provided recently by an X-ray investigation carried out jointly by Diederich s group at ETH Zurich and by Gross and coworkers at the Universite Louis Pasteur in Strasbourg. They found that the 61,61-bis[4-(trimethylsilyl)buta-l,3-diynyl]-l,2-di-hydro-l,2-methanofullerene[60] (8.37) has bond lengths that are only compatible with a [6,6]-closed structure (Anderson et al., 1994b). 

Various methanofullerenes containing suitable substituents at C(61) undergo reactions to other products by means of those groups. Such processes are described in papers from the groups of Diederich and Wudl (see above and Wooley et al., 1993). These reactions are, however, outside the scope of this book, except the work on bis(phenethylamino succinate)C6o, obtained from the corresponding 4,4 -disub-stituted diphenyldiazomethane. Such compounds are claimed to inhibit an HIV-1 protease on the basis of the hypothesis that fullerene[60] has approximately the same diameter as the cylinder that describes the active site of that protease. These investigations were conducted by Wudl s group in collaboration with pharmaceutical chemists (Friedman et al., 1993 Sijbesma et al., 1993). 

We have shown that fullerenes containing bis-ester methano-adducts undergo unusual transformations when reduced electrolytically. Mono- as well as multiple-adducts can easily be removed if enough electrons are supplied to the corresponding methanofullerenes, in a reaction that we have called the retro-Bingel. The removal of one adduct from Ceo occurs in high yield, about 80%, while removal from C70 occurs in 70% yield. Removal of two adducts from C60 occurs in over 75% yield, and removal of four adducts in 15%. 

The first example was reported in 1994 by Diederich, who electropolymerized a bis(l,3-dialkyne)methanofullerene to obtain conductive films [51]. Nowadays, several research groups are engaged in the synthesis and study of D-C polymers, especially in the search for more efficient organic photovoltaic solar devices [52]. 

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