Nucleophilic Cyclopropanation - Bingel Reaction

Gold colloids were observed both on the sides and at the ends of the nanotubes, indicating sidewall and termini modification. To confirm further the derivatiza-tion of the SWNTs, a perfluorinated marker was introduced by transesterification to allow the nanotubes to be probed by 19F NMR spectroscopy and XPS [160], 

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The reaction of Cjq with silylated nucleophiles [47] requires compounds such as silyl ketene acetals, silylketene thioacetals or silyl enol ethers. It proceeds smoothly and in good yields in the presence of fluoride ions (KF/18-crown-6) (Scheme 3.10). The advantage of the latter synthesis is the realization of the cyclopropanation under nearly neutral conditions, which complements the basic conditions that are mandatory for Bingel reactions. Reaction with similar silyl ketene acetals under photochemical conditions and without the use of F does not lead to methanofullerenes but to dihydrofullerene acetate [48]. 

The stabilization of reaction intermediates RCjq and RC q to form dihydrofullerene derivatives can also be achieved by intramolecular nucleophilic substitutions (SjJ), if R contains a leaving group. As shown by Bingel [31], the generation of a carbon nucleophile by deprotonation of a-halo esters or a-halo ketones leads to a clean cyclopropanation of Cjq. 

The Bingel-Hirsch Reaction One of the most valuable preparative methods applicable to fullerenes is the Bingel-Hirsch reaction. This is, in detail, the nucleophilic attack of a bromomalonate on the double bond of a fuUerene in the course of which a cyclopropane ring is generated. The reaction exclusively yields the meth-anofullerene with a bridged (6,6)-bond. It has first been described by C. Bingel in 1993, and A. Hirsch subsequently optimized the conditions, so today it is one of the best controllable reactions of fullerenes and can be performed with a multitude of reagents and substrates. 

The most used nucleophilic synthesis reaction to form fuUerenes is cyclopropanation. The original method was developed by Bingel, and it employs the generation of a carbon nucleophile starting from a-halo esters in the presence of a base (such as NaH), and the subsequent addition to After the addition of the anions of a-halo ester, an intramolecular substitution of the halide takes place with the intermediate fullerene anion, giving the corresponding methanofullerene derivative (Scheme 2.1e). Further modification of this method consist of preparing the a-halomalonate in situ. 

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