[2-PHOTOCYCLOADDITION OF CYCLIC ENONES TO C-60(2])

The photocycloaddition of cyclic enones to C-60 is a general reaction. The addition was initially explored using a crown ether-tagged C-60 m olecule which allows convenient monitoring of the reaction course by e lectrospray ionization mass spectroscopy (ESI-MS) after complexation w ith 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 exce ss. A series of experiments in which the concentration of reagents and reaction time were systematically varied established that adduct form ation occurs only upon light absorption by the enones. Thus, excitatio n at 532 nm or in the UV range using low enone: C-60 ratios did not le ad to adduct formation. ESI-MS analysis showed that monoaddition was f avored at short irradiation times, and that up to seven enone units co uld be added to a single fullerene molecule upon longer exposure to li ght. Competitive experiments using 12 different enones indicated that the best product yields were obtained, in decreasing order, with 3-met hyl-2-cyclohexen-1-one, isophorone, 2-cyclohepten-1-one, and 2-cyclohe xen-1-one. Products isolated from reactions carried out on a larger sc ale were subjected to spectroscopic analysis (H-1 NMR, C-13 NMR, IR, U V). 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 una mbiguously identified as cis- and trans-fused [2 + 2] cycloadducts. In the case of 2-cyclohexen-1-one, the cis-fused C-60 adduct is the majo r product, while in the case of 3-methyl-2-cyclohexen-1-one the trans isomer predominates. C-13 NMR spectra indicated addition occurred acro ss [6,6] pyracyclene bonds of the fullerene to give ring-closed struct ures with two sp(3) fullerene carbons. These racemic [2 + 2] adducts c ould 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 att ributed to an additional chromophore in the trans isomers, which accor ding to MM2 calculations is a C-2 symmetric chiral skewed fullerene mo iety. He-3-NMR analysis of the product mixture from photoaddition of 3 -methyl-2-cyclohexen-1-one and 2-cyclohepten-1-one to C-60 containing a He-3 atom (He-3 @ C-60) led to the appearance of two new peaks at ca . -9.3 ppm relative to gaseous He-3, consistent with formation of two monoadduct diastereomers with [6,6]-closed structures. The mechanism o f the reaction presumably involves addition of enone triplet excited s tates to ground state fullerenes, via triplet 1,4-biradical intermedia tes, as in typical enone-alkene photocycloadditions. The fact that eno ne triplets are not quenched by triplet energy transfer to the fullere nes, which would be highly favorable energetically, is rationalized by poor coupling of the chromophores and Marcus theory.