Retro-Bingel reactions

Following the discovery of a bulk fullerene preparation process in 1990, the covalent chemistry of these carbon allotropes has developed at a phenomenal pace. Frontier orbital (LUMO) and tether-directed functionalization concepts have been successfully applied to the regio- and stereoselective preparation of multiple covalent adducts of C60. These have found increasing applications in the construction of functional supramol-ecules. More recently, the sequence of Bingel reaction - retro-Bingel reaction has provided an elegant access to isomerically pure higher fullerenes and, in particular, to pure carbon enantiomers. 

Scheme 7. Optical resolution of ( )-C76 by the Bingel/ retro-Bingel reaction sequence.

The electrochemistry of derivatized Cjq has also been widely investigated [8, 23-28], As observed by electrochemical reduction, derivatization usually decreases the electron affinity of the CgQ-sphere. Typically, cathodicaUy (more negative) shifted waves have been observed by cyclovoltammetry and other methods. Depending on the addend, the shifts range from 30 to 350 mV per adduct with respect to those of pure Cgfl. Reduction of some derivatives resulted in the loss of the addend. In some cases, like the retro-Bingel-reaction (Section 3.2.2), this can also be advantageous. 

The reverse reaction to the Bingel cyclopropanation - the so-called retro-Bingel reaction - was developed by Diederich, Echegoyen and coworkers [70] and opens up the possibility to remove the Bingel-addend completely. This removal was successfully done with Cjq malonates [70, 71], dialkoxyphosphorylmethano[60]-fullerene [72], methano[60]fullerenyl amino acid derivatives [73] and also with... 

The cyclic voltammograms show three reduction waves in the potential window between 0 and —1.3 V versus SCE (Fig. 6.20). The first process is fully reversible, while the second one is chemically irreversible since the dianion is subject to a bond breaking of the cyclopropane ring, known as the retro-Bingel reaction.75 The third... 

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,... 

Figure 1 Bingel and retro-Bingel reactions on C60. Position notation for bis-adducts...

At the beginning of this chapter, we mentioned how the removal of di(alkoxycar-bonyljmethano addends on a C6o core can be achieved by electrolytic reduction to yield the parent [60]fullerene. This so-called retro-Bingel reaction was initially conducted on diethyl l,2-methano[60]fullerene-61,61-dicarboxylate (see Figure 1) leading to > 80 % yield of recovered C60 after controlled potential... 

Figure 22 Selective removal of bis(ethoxycarbonyl)methano addends in mixed C60-bis-adducts by the retro-Bingel reaction.

Figure 30 Preparation of the pure enantiomers of D2-C84 by a retro-Bingel reaction of the fourth and fifth fractions of the bis-adducts, and CD spectra of the diastereo-isomeric bis-adduct (top right) as well as of the fullerene enantiomers (bottom right) in CH2C12. The absolute configurations of the fullerene spheroids reflect the structural drawings, but have not been assigned experimentally. Reprinted from ref. [71] with permission from Wiley-VCH.

Finally, the wide applicability and the experimental simplicity of the retro-Bingel reaction was evidenced in the isolation of pure enantiomers of C76, in the formation of methanofullerene compounds not accessible otherwise, such as ( )-75, and in the separation of constitutional isomers as well as of enantiomers of Cg4. The retro-cyclopropanation reaction seems to be generally selective, and still needs to be exploited in the preparation of unusual derivatives with controlled regiochemistry. 

Figure 1.10. Preparation of the pure enantiomers of D2-C76 by electrochemical retro-Bingel reaction of each of two optically pure, diastereoisomeric adducts with enantiomeric 75 cores, and circular dichroism spectra of the resolved fullerene enantiomers in CH2CI2, including their configurational assignment.

Scheme 1.26. Formation of the D2-symmetric bis-adduct 133 of Di -Cu from C2-symmetric mono-adduct 132 and generation of the parent fullerene by electrochemical retro-Bingel reaction.

V. Controlled Potential Electrolysis as a Synthetic Tool The Shuffle and the Retro-Bingel Reaction... 

V. CONTROLLED POTENTIAL ELECTROLYSIS AS A SYNTHETIC TOOL THE SHUFFLE AND THE RETRO-BINGEL REACTION... 

Clearly, depending on the number of electrons per molecule added during CPE, either removal of the addends [170] or an isomerization reaction was attained. The former reaction was initially called the retro-Bingel reaction [64], since it was the reverse of the Bingel addition of methano addends to Cgo [171]. The generality of the retro-Bingel and isomerization reactions has since been tested using other multiple-malonate adducts of Cgo [172,173[. In fact, the reaction has made it possible to isolate the C21, isomer of C78, a new Cyg bisadduct, and new isomers of C84 [44, 54[. 

Fig. 16. Resolution of inherently chiral fullerenes. Top Kinetic resolution based on the differential reactivity of an optically pure osmium complex towards the enantiomers of 1 2" C76. Hawkins et al. utilized this method also for the resolution of and D2 C 4, Bottom Separation of the enantiomers of H2-C76 by application of the retro-Bingel reaction to each of two optically pure, diastereoisomeric Cyg derivatives having enantiomeric carbon cores...

Already, the retro-Bingel reaction has been of great use in the resolution of the inherently chiral D2-Cy, the enantiomers of which could be assigned by comparison of their experimental to calculated spectra. Hopefully, this will trigger further experimental as well as theoretical studies in the fascinating field of fullerene chirality and ultimately lead to a better understanding of inherently chiral chromophores and their chiroptical properties. 

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