3He NMR as a sensitive probe of fullerene reactivity: [2 + 2] photocycloaddition of 3-methyl-2-cyclohexenone to C70.

The [2 + 2] photoadditions of 3-methyl-2-cyclohexenone to C70 and 3He@C70 have been studied by a combination of HPLC chromatography and FAB-MS, as well as IR and 1H and 3He NMR spectroscopies. The total yield of the mixture of monoadducts was 55% (67% on the basis of the recovered C70). The use of 3He NMR was especially powerful in determining the regioselectivity of the photoaddition reaction of enone to C70. Results of the 3He NMR experiments conducted on the product mixture implicate the two [6,6] bonds closest to the poles of the fullerene (C1-C2 and C5-C6) in the photoaddition process. This reaction mode is analogous to that of most thermal addition reactions to C70. Separation and characterization of the product mixture shows that eight distinct monoadducts are formed in the photoaddition, namely, the four diastereomeric adducts to the C1-C2 and C5-C6 bonds of the C70 cage, each consisting of cis- and trans-fused isomers in a ratio of 2:3. The major mode of photoaddition, accounting for 65% of the product mixture, involves addition to the C1-C2 bond of the ovoid fullerene. Mechanistic implications of these findings are discussed.