Fullerenes Bingel reaction

Figure 15.8 The Bingel reaction for the modification of fullerenes involves the in situ formation of a reactive halogen species in the presence of the strong base DBU. The cyclopropanation product can be used to create many bioconjugates.

Figure 15.9 The reaction of the amine-blocked derivative of 3-hydroxypropylamine with ethylmalonyl chloride gives an ethylmalonate-protected-amine compound, which can be used in the Bingel reaction to create an amine group on a fullerene surface. Reaction with Cfl in the presence of I2 and DBU gives the cyclopropanation product that can be deprotected with TFA to yield the free amine.

Figure 15.10 Fullerene-PCBM derivatives can be prepared using reactive diazo intermediates, which yield a cyclopropanation product similar to the Bingel reaction derivatives.

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. 

As well as the Bingel reaction and its modifications some more reactions that involve the addition-elimination mechanism have been discovered. 1,2-Methano-[60]fullerenes are obtainable in good yields by reaction with phosphorus- [44] or sulfur-ylides [45,46] or by fluorine-ion-mediated reaction with silylated nucleophiles [47]. The reaction with ylides requires stabilized sulfur or phosphorus ylides (Scheme 3.9). As well as representing a new route to l,2-methano[60]fullerenes, the synthesis of methanofullerenes with a formyl group at the bridgehead-carbon is possible. This formyl-group can be easily transformed into imines with various aromatic amines. 

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... 

Preparation of mono-adducts of fullerene - for studies on electrostatic interactions - was undertaken by cyclopropanation of fullerene with appropriately functionalised malonic esters 1 (Bingel reaction) to form 2. Coupling with the tert-butyl protected oligoamide-amino-dendron 3 and subsequent hydrolysis lead to the water-soluble fullerene dendron 5, which can carry up to nine negative charges after depro to nation. After association with the zinc complex of cytochrome C, photoinduced electron transfer (PET) from the redox protein to the fullerene can be accomplished, which was studied by fluorescence spectroscopy. 

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 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.

Scheme 16. Bingel reaction of bismalonate TB derivative 72 with fullerene C60. Reaction conditions (i) C60,12, DBU, 0°C...

Some representative examples of fullerene-porphyrin dyads are shown in Scheme 9. In other examples, porphyrin analogs such as phthalocyanines and subphthalocyanines have been used for the construction of efficient dyads. Again, the most straightforward approach for their synthesis involved 1,3-dipolar cycloaddition of the appropriate azomethine ylides to C60 [203-205]. Also, with the aid of the Bingel reaction, other phthalocyanine-fullerene systems have been prepared [206,207] with the most prominent example being the one that contains a flexible linker possessing an azacrown subunit [208]. The novelty of this dyad can be found in the nature of the linker that could, in principle, induce conformational changes in the multicomponent system when certain ions (e.g., alkaline ions) are present. As a direct consequence this would potentially allow an external control over the electronic interactions between the phthalocyanine and fullerene units. 

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.

Scheme 8.13 The Bingel reaction of fullerenes under mechanochemical conditions.

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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