Prato reaction

The Prato reaction in fullerene chemistry describes the functionalization of fullerenes and nanotubes with azomethine ylides in a 1,3-dipolar cycloaddition.^[1] The amino acid sarcosine reacts with paraformaldehyde when heated at reflux in toluene to an ylide which reacts with a double in a 6,6 ring position in a fullerene in a 1,3-dipolar cycloaddition to yield a N-methylpyrrolidine derivative or pyrrolidinofullerene or pyrrolidino3,4:1,2 [60]fullerene in 82% yield.

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This method is also used in the functionalization of single wall nanotubes ^[2]. When the amino acid is modified with a glycine chain the resulting nanotubes are soluble in common solvents such chloroform and acetone. Another characteristic of the treated nanotubes is their larger aggregate dimensions compared to untreated nanotubes.

Just as in other fullerene reactions like the Bingel reaction or Diels-Alder reactions this reaction can be reversed. A thermal retro-cycloaddition of a pyrrolidinofullerene with a strong dipolarophile such as maleic acid and a catalyst such as Wilkinson's catalyst or copper triflate in 1,2-dichlorobenzene at reflux 8 to 18 hours regenerates the pristine C60 fullerene ^[3]. The dipolarophile is required in a 30 fold excess and traps the ylide driving the reaction to completion. The N-methyl-pyrrolidine derivative reacts poorly (5% yield) and for a successful reaction the nitrogen ring also requires substitution in the α-position with methyl, phenyl or carboxylic ester groups.

In an alternative method a nanotube addition is performed with the N-oxide of trimethylamine and LDA ^[4] at reflux in tetrahydrofuran with an efficiency of 1 functional group in 16 nanotube carbon atoms. When the amine also carries an aromatic group such as pyrene the reaction takes place even at room temperature because this group preorganizes itself to the nanotube surface prior to reaction by pi stacking.

In one application a liquid fullerene is obtained when the pyrrolidone substituent is a 2,4,6-tris(alkyloxy)phenyl group ^[5] although a small amount of solvent is still required.

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