[2 + 2] Photocycloaddition of cyclic enones to C₆₀

The photocycloaddition of cyclic enones to C₆₀ is a general reaction. The addition was initially explored using a crown ether-tagged C₆₀ molecule which allows convenient monitoring of the reaction course by electrospray ionization mass spectroscopy (ESI-MS) after complexation with metal ions, usually K⁺. A Hanovia 450-W lamp was used to excite a benzene solution of fullerene and enone, with the latter in large excess. A series of experiments in which the concentration of reagents and reaction time were systematically varied established that adduct formation occurs only upon light absorption by the enones. Thus, excitation at 532 nm or in the UV range using low enone:C₆₀ ratios did not lead to adduct formation. ESI-MS analysis showed that monoaddition was favored at short irradiation times, and that up to seven enone units could be added to a single fullerene molecule upon longer exposure to light. Competitive experiments using 12 different enones indicated that the best product yields were obtained, in decreasing order, with 3-methyl-2-cyclohexen-1-one, isophorone, 2-cyclohepten-1-one, and 2-cyclohexen-1-one. Products isolated from reactions carried out on a larger scale were subjected to spectroscopic analysis (¹H NMR, ¹³C NMR, IR, UV). Two monoadducts were formed from 2-cyclohexen-1-one, its 3-methyl analog, and 2-cyclohepten-1-one, which could be separated by HPLC on a Buckyclutcher column. In the first two cases, these products were unambiguously identified as cis- and trans-fused [2 + 2] cycloadducts. In the case of 2-cyclohexen-1-one, the cis-fused C₆₀ adduct is the major product, while in the case of 3-methyl-2-cyclohexen-1-one the trans isomer predominates. ¹³C NMR spectra indicated addition occurred across [6,6] pyracyclene bonds of the fullerene to give ring-closed structures with two sp³ fullerene carbons. These racemic [2 + 2] adducts could be enantiomerically resolved on a chiral HPLC column. Much larger optical rotations were found for the trans than for the cis isomers. Additional long-wavelength bands were also found in the UV absorption and CD spectra of the trans isomers. These chiroptical effects are attributed to an additional chromophore in the trans isomers, which according to MM2 calculations is a C₂ symmetric chiral skewed fullerene moiety. ³He-NMR analysis of the product mixture from photoaddition of 3-methyl-2-cyclohexen-1-one and 2-cyclohepten-1-one to C₆₀ containing a ³He atom (³He@C₆₀) led to the appearance of two new peaks at ca. -9.3 ppm relative to gaseous ³He, consistent with formation of two monoadduct diastereomers with [6,6]-closed structures. The mechanism of the reaction presumably involves addition of enone triplet excited states to ground state fullerenes, via triplet 1,4-biradical intermediates, as in typical enone-alkene photocycloadditions. The fact that enone triplets are not quenched by triplet energy transfer to the fullerenes, which would be highly favorable energetically, is rationalized by poor coupling of the chromophores and Marcus theory.