Prato Reaction of M3N@Ih-C80 (M = Sc, Lu, Y, Gd) with Reversible Isomerization

The 1,3-dipolar cycloaddition of an azomethine ylide (Prato reaction) with M(3)N@I(h)-C(80) (denoted as M(3)N@C(80); M = Sc, Lu, Y, Gd) was carried out to obtain fulleropyrrolidinebis(carboxylic acid) derivatives as scaffolds for the preparation of various functionalized M(3)N@C(80) materials. The formation of two monoadduct isomers (the [6,6]- and [5,6]-adducts) were detected by HPLC and identified by NMR and vis/NIR spectroscopies. In each Prato reaction with M(3)N@C(80), the initial addition gave a [6,6]-adduct of the I(h)-C(80) cage, and subsequently, a [5,6]-adduct was obtained by complete or partial thermal isomerization via a rearrangement reaction. The reaction rate of the latter thermal conversion of the adducts was dependent on the size of the metal cluster inside C(80), and interestingly, in the reactions of Y(3)N@C(80) and Gd(3)N@C(80), this conversion was found to be reversible for the first time. Detailed kinetic studies provided the enthalpy and entropy barriers for the reactions of the adducts of Lu(3)N@C(80), Y(3)N@C(80), and Gd(3)N@C(80). The utility of the obtained Prato adducts was confirmed by preparation of a highly water-soluble Gd(3)N@C(80) derivative.