[2 + 2] Photocycloaddition of Cyclic Enones to C60

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• 作者：David I. Schuster ; Jingrong Cao ; Nikolas Kaprinidis ; Yunhui Wu ; Anton W. Jensen ; Qingyi Lu ; Hui Wang ; and Stephen R. Wilson
• 刊名：Journal of the American Chemical Society
• 出版年：1996
• 出版时间：June 19, 1996
• 年：1996
• 卷：118
• 期：24
• 页码：5639 - 5647
• 全文大小：543K
• 年卷期：v.118,no.24(June 19, 1996)
• ISSN：1520-5126

文摘

The photocycloaddition of cyclic enones to C₆₀ is ageneral reaction. The addition was initially exploredusing a crown ether-tagged C₆₀ molecule which allowsconvenient monitoring of the reaction course byelectrosprayionization mass spectroscopy (ESI-MS) after complexation with metalions, usually K⁺. A Hanovia 450-W lampwas used to excite a benzene solution of fullerene and enone, with thelatter in large excess. A series of experimentsin which the concentration of reagents and reaction time weresystematically varied established that adduct formationoccurs only upon light absorption by the enones. Thus, excitationat 532 nm or in the UV range using low enone:C₆₀ ratios did not lead to adduct formation. ESI-MSanalysis showed that monoaddition was favored at shortirradiationtimes, and that up to seven enone units could be added to a singlefullerene molecule upon longer exposure to light.Competitive experiments using 12 different enones indicated thatthe best product yields were obtained, in decreasingorder, with 3-methyl-2-cyclohexen-1-one, isophorone,2-cyclohepten-1-one, and 2-cyclohexen-1-one. Productsisolatedfrom reactions carried out on a larger scale were subjected tospectroscopic analysis (¹H NMR, ¹³C NMR, IR,UV).Two monoadducts were formed from 2-cyclohexen-1-one, its 3-methylanalog, and 2-cyclohepten-1-one, which couldbe separated by HPLC on a Buckyclutcher column. In the first twocases, these products were unambiguouslyidentified as cis- and trans-fused [2 + 2]cycloadducts. In the case of 2-cyclohexen-1-one, thecis-fused C₆₀ adductis the major product, while in the case of 3-methyl-2-cyclohexen-1-onethe trans isomer predominates. ¹³CNMRspectra indicated addition occurred across [6,6] pyracyclene bonds ofthe fullerene to give ring-closed structureswith two sp³ fullerene carbons. These racemic [2 +2] adducts could be enantiomerically resolved on a chiralHPLC column. Much larger optical rotations were found for thetrans than for the cis isomers. Additionallong-wavelength bands were also found in the UV absorption and CD spectra ofthe trans isomers. These chiropticaleffects are attributed to an additional chromophore in thetrans isomers, which according to MM2 calculations isaC₂ symmetric chiral skewed fullerene moiety.³He-NMR analysis of the product mixture fromphotoaddition of3-methyl-2-cyclohexen-1-one and 2-cyclohepten-1-one to C₆₀containing a ³He atom(³He@C₆₀) led to theappearanceof two new peaks at ca. -9.3 ppm relative to gaseous³He, consistent with formation of two monoadductdiastereomerswith [6,6]-closed structures. The mechanism of the reactionpresumably involves addition of enone triplet excitedstates to ground state fullerenes, via triplet 1,4-biradicalintermediates, as in typical enone-alkenephotocycloadditions.The fact that enone triplets are not quenched by triplet energytransfer to the fullerenes, which would be highlyfavorable energetically, is rationalized by poor coupling of thechromophores and Marcus theory.