Retro-cyclopropanation reactions

This chapter presents an up-to-date account of the redox properties of the pristine fullerenes and a large number of their derivatives as revealed by electrochemical studies in solution. The description here is as exhaustive as possible, although not completely comprehensive due to the large number of reports on the subject that have appeared over the years. A section on electrosynthesis of fullerene derivatives is included, with special emphasis on the retro-cyclopropanation reaction, a reaction that has led to the formation of novel derivatives as well as... 

In another study involving C78, a pure sample of the C2v isomer was prepared using the cyclopropanation-retro-cyclopropanation reaction sequence [44, 64]. This reaction scheme consists of a controlled potential electrolytic (CPE) reduction of a previously synthesized cyclopropane derivative of the isomer, leading to removal of the cyclopropane moiety (s), (see Sect. 6.1.5). A pure sample of the D3 isomer was obtained by high performance Kquid chromatography (HPLC) as previously described [49, 65]. The redox behavior of both isomers, in DCM at room temperature, reveals that their cathodic electrochemistry is indeed very similar (although not identical) in this solvent [44]. The first two reductions are easier for the D3 isomer by 60 and 100 mV, respectively, while the third and fourth reductions are nearly identical for the two... 

At this point, it is important to indicate that a very large number of C-bridged cyclopropanated fullerene derivatives undergo irreversible reduction processes leading to the removal of the addend and recovery of the pristine parent fullerene. The process has been advantageously used in electrosynthetic procedures, and thus a separate section covering the electrochemically induced retro-cyclopropanation reaction is presented later in this chapter (see Sect. 6.1.5.2). A number of other C-bridged cyclopropanated derivatives will be discussed there. 

Two of the three pairs of diastereomers reduce at potentials more positive than those of the parent [76] fullerene, while the other pair is actually more difficult to reduce. Interestingly, in contrast with similar Ceo and C70 derivatives, the di-and trianions of these C76 monoadducts are much more chemically stable. In fact, the retro-cyclopropanation reaction (see Sect. 6.1.5.2 below), which removes the malonate addend, occurs in very low yields... 

The only electrochemical study on derivatives of Cg4 describes the use of the Bingel-retro-Bingel protocol, now known as the retro-cyclopropanation reaction (see Sect. 6.1.5.2), to isolate the two major isomers of pure Cg4 by removing bis(phenylbutyl)malonate addends from mono and bis-adducts of Cg4 (see (56) in Fig. 22) [54]. CV profiles in DCM (+0.12 M TBAPFg) of two different monoadducts and four different... 

Spiromethanofullerenes that exhibit retro-cyclopropanation reactions under reductive electrochemistry. 

The usefulness of the retro-cyclopropanation reaction is even more remarkable than previously anticipated. It was questioned whether this reaction allowed the selective removal of a Bingel-type addend while leaving addends of a different type unaffected. A variety of mixed bis-adducts such as those shown in Fig. 29, were prepared, all of which contained a bis(ethoxycarbonyl)methano group [182]. In all cases, CPE led to the selective removal of the Bingel addend in over 60% yield, while the other one was retained, confirming that the reaction may be used in a synthetic protection-deprotection protocol to prepare novel fullerene derivatives. 

Figure 21 Isomerization and retro-cyclopropanation reactions of bis-Bingel adducts of [60]fullerene induced electrolytically.

Figure 25 Cyclic voltammograms in CH2Cl2(- -0.1 M Bu4NPF6) at 100 mV s-1 for (a) (—) trans-l fullerene-dibenzo[18]crown-6 (( )-37) (b) ( ) after 2e CPE and reoxidation. Reprinted from Chemical Retro-cyclopropanation Reactions in Methanofullerenes Effect of the 18-crown-6 moiety, 1, M. A. Herranz, et al. 299-303, copyright 2003, with permission from Elsevier.

In this section we present the use of the retro-cyclopropanation reactions for the synthesis of fullerene derivatives which are not available by other routes, and for the isolation of isomers and enantiomers of the higher fullerenes. 

Retro-cyclopropanation Reaction in the Preparation of Optically Active C76 Enantiomers... 

Retro-cyclopropanation Reaction in the Isolation of Pure C2v- C78 and a new C2-symmetric Bis-adduct... 

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. 

Angeles Herranz, M. et al. Chemical retro-cyclopropanation reactions in methanoMlerenes Effect of the 18-crown-6 moiety. J. Supramol Chem. 1, 299-303, 2001. 

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