Functionalization of fullerenes involves chemical modifications that enhance their solubility, stability, and reactivity for various applications. The most common reactions include cycloadditions, nucleophilic additions, and radical additions, each yielding distinct derivatives such as fullerols, fullerene esters, and other adducts. These reactions exploit the unique electronic structure of fullerenes, particularly the strained double bonds in their carbon framework, which act as electron-deficient alkenes.

Cycloadditions are among the most widely used methods for fullerene functionalization. The [2+2], [3+2], and [4+2] cycloadditions are particularly notable. The [4+2] Diels-Alder reaction is frequently employed, where a diene reacts with the fullerene to form a cyclohexene ring. For example, cyclopentadiene reacts with C60 to yield a stable cycloadduct. The [3+2] cycloaddition, such as the reaction with diazo compounds, forms fulleropyrrolidines. Azomethine ylides, generated in situ from aldehydes and amino acids, undergo 1,3-dipolar cycloaddition to produce N-substituted fulleropyrrolidines. The [2+2] cycloaddition is less common but can be achieved under photochemical conditions or with highly reactive intermediates like benzyne.

Nucleophilic additions exploit the electron-deficient nature of fullerenes. The Bingel reaction is a classic example, where a nucleophilic carbanion, generated from a bromomalonate ester in the presence of a base like sodium hydride or DBU, attacks the fullerene to form a cyclopropanated derivative. Another prominent nucleophilic addition is the reaction with organolithium or Grignard reagents, which add across a double bond to form fullerene anions that can be further derivatized. For instance, methylmagnesium bromide reacts with C60 to form a monoadduct, which can be protonated or alkylated. Amines also participate in nucleophilic additions, yielding amino-functionalized fullerenes. The reaction with secondary amines, such as morpholine, proceeds via a single-electron transfer mechanism, producing aminofullerenes.

Radical reactions are another key pathway for fullerene functionalization. Perfluoroalkyl radicals, generated from perfluoroalkyl iodides under UV irradiation or thermal conditions, add to fullerenes to form perfluoroalkylated derivatives. Similarly, aryl radicals, produced from diazonium salts or peroxides, undergo addition to yield arylfullerenes. The radical addition often results in multiple adducts due to the high reactivity of fullerene toward radicals. Hydrogenation of fullerenes, achieved via hydroboration or Birch reduction, produces fulleranes (hydrogenated fullerenes) with varying degrees of saturation. The degree of hydrogenation can be controlled by reaction conditions, yielding products like C60H18 or C60H36.

Key derivatives synthesized through these reactions include fullerols, fullerene esters, and halogenated fullerenes. Fullerols, or polyhydroxylated fullerenes, are produced by reacting C60 with aqueous NaOH in the presence of a phase-transfer catalyst like tetrabutylammonium hydroxide, followed by acidification. This yields C60(OH)n, where n typically ranges from 12 to 24, depending on reaction conditions. Fullerols are highly water-soluble and exhibit antioxidant properties. Fullerene esters are synthesized via esterification of fullerols with acyl chlorides or carboxylic acids under standard conditions. For example, reaction with acetic anhydride yields C60(OCOCH3)n. Halogenated fullerenes, such as C60F36 or C60Cl24, are obtained by direct halogenation. Fluorination is achieved using elemental fluorine at elevated temperatures, while chlorination employs chlorine gas or sulfuryl chloride. These halogenated derivatives serve as precursors for further functionalization via nucleophilic substitution.

Another important class of derivatives is the thiofullerenes, formed by reacting fullerenes with sulfur-containing compounds. For instance, treatment with elemental sulfur at high temperatures yields C60Sx, where x varies based on stoichiometry. Thiol-ene reactions with alkyl thiols under UV irradiation produce alkylthiofullerenes. Oxidative functionalization is also possible, such as epoxidation with meta-chloroperoxybenzoic acid (mCPBA) to form fullerene epoxides, which can be further ring-opened with nucleophiles.

The regiochemistry of fullerene functionalization is influenced by the addition pattern. Monoadditions typically occur at the 6,6-ring junction (between two hexagons), while multiple additions follow specific rules like the octahedral or equatorial addition patterns. For example, the Bingel-Hirsch bis-adduct predominantly forms trans-1 or e-isomers due to steric and electronic factors. The choice of solvent, temperature, and catalyst further dictates the product distribution. Polar solvents like toluene or o-dichlorobenzene are commonly used due to the poor solubility of fullerenes in most solvents.

Quantitative studies reveal that reaction kinetics and thermodynamics play crucial roles in functionalization. The first addition to C60 is typically the fastest, with subsequent additions becoming progressively slower due to decreased reactivity of the remaining double bonds. For instance, the second Bingel addition is approximately ten times slower than the first. The redox properties of fullerenes also influence reactivity. Electrochemical studies show that reduced fullerenes (C60n-, n=1-6) exhibit enhanced nucleophilicity, facilitating further functionalization.

In summary, the chemical functionalization of fullerenes encompasses cycloadditions, nucleophilic additions, and radical reactions, each offering distinct pathways to derivatives like fullerols, esters, and halogenated adducts. These modifications expand the utility of fullerenes beyond their native form, enabling tailored properties for diverse applications. The synthetic strategies emphasize precise control over regioselectivity and stoichiometry, leveraging the unique reactivity of the fullerene core.