Differentiation of Isomers Resulting from Bisaddition to C60 Using 3He NMR Spectrometry

3 He NMR spectrometry has been used to examine bisaddition to C 60 containing an encapsulated 3 He atom ( 3 He@C 60 ) using three types of reactions: (1) cyclopropanation with diethyl bromomalonate and base to give dicarbethoxymethanofullerenes (Bingel−Hirsch reaction), (b) addition of azomethine ylides to give N -methylfulleropyrrolidines (Prato−Wudl reaction), and (c) reduction to give C 60 H 4 . 3 He NMR spectra of crude reaction mixtures in all three series showed well-separated resonances for each of the bisadducts, spread out over more than 2 ppm. The major isomeric bisadducts from the first two reactions were separated and characterized, and the 3 He NMR spectra of the individual major bisadducts from 3 He@C 60 were obtained (five isomers from reaction 1, four isomers from reaction 2). Although the absolute chemical shifts in the two series of bisadducts differ, the relative chemical shifts are significantly but not perfectly correlated. Bisadducts with appended ligands on opposite hemispheres of C 60 ( 7, 6, and 5 ) tend to have 3 He NMR resonances downfield of the bisadducts with appended ligands on the same hemisphere (bisadducts 2 and 3 ). Unfortunately, the equatorial isomer 4, a major product from the Hirsch addition reaction, was not obtained in sufficient quantity for study from the Prato reaction. Well-separated resonances were also seen for the isomeric compounds of composition 3 He@C 60 H 4 from reaction of the fullerene with diimide, but the individual isomers were not separated. The very large differences in the 3 He NMR chemical shifts of the isomeric bisadducts in all three reactions demonstrate that the magnetic field felt by the 3 He atom due to the ring currents in the residual π-system is extremely sensitive to the pattern of ligation on the C 60 surface. These results provide further support for the assertion that 3 He NMR is a very sensitive probe of patterns of chemical addition to fullerenes, and suggest that this technique will prove to be generally useful in determining the ratio as well as the identity of isomeric fullerene bisadducts.