[3 + 2] Cycloadditions of Nitrile Oxides to C₇₀ Affording 4,5‐Dihydroisoxazoles and Their Structure Determination

Abstract

Three isomeric monoadducts 2aA, 2aB, and 2aC of fullerene-corresponding monoadduct 3a of Ce0 was synthesized, iso-70 with 2,4,6-trimethoxybenzonitrile oxide l a were synthe-lated and characterized. In the same manner a mixture of sized and isolated. With a combination of FAB-MS, 13C-and three isomeric monoadducts 2b of C70 with ethoxycarbonyl-'H-NMR spectroscopy the structure of the orientational and nitrile oxide 1 b was obtained and characterized. The regioisomers could be assigned. A precise X-ray structure monoadducts 2c of C70 with 9-anthrylnitrile oxide l c were analysis of the isomer 2aA with the substituent placed above characterized by FAB-MS. the pole could be performed at low temperature (170 K). TheThe lower symmetry of C70 (D5,J compared with C60 (Ill) gives rise to a greater variety of possible cycloadducts in the case of fullerene-70. Cycloaddition reagents are appropriate probes to investigate the reactivity of the different bond types in C70. Apart from Diels-Alder addition reactions ['] 1,3-dipolar cycloadditions of nitrile ylidesL2] and nitrile oxides [3] to C70 have been reported. Because of our experience with the reaction behavior of nitrile oxides towards C60[41 we extended these investigations to C70 in order to obtain information about the regioselectivity of this fullerene in correlation with the degree of pyramidalizations of the fullerene atoms from structural data of the cycloadducts.Similar to fullerene-60 the cycloaddition reactions with C70 take place exclusively on [6,6] bonds. Out of four possible [6,6] bond types of C7" the bonds around the pole region are the reactive ones because of their higher steric train[^.^,^-^]. Three different Diels-Alder monoadducts were isolated [5] and two of them characterized by accurate X-ray structure analysis [5]. Structural data of unsubstituted C70 were obtained by electron diffraction[10] and a powder neutron diffraction study ["]. Moreover, three possible isomeric monoadducts of C70 with nitrile oxides were isolated and characterized by spectro~copy [~]. Adducts of fullerenes with nitrile oxide are referred to in a patent without a detailed description['*]. Synthesis of Cycloadditionsproducts of C7" with Nitrile oxides l a -l cIn order to obtain precise structural data of 1,3-dipolar monoadducts of C70 with nitrile oxides we treated C70 with 2,4,6-trimethoxybenzonitrile oxide l a , ethoxycarbonylnitrile oxide lb, and 9-anthrylnitrile oxide l c and obtained isomeric mixtures of mono-and polyadducts. The nitrile oxides l a and ~c [ '~I were isolated and nitrile oxide lb['4] was generated in situ. To a toluene solution (lop4 M) of C70 a solution of 1.5 equivalents of nitrile oxide l a or l b in toluene was added dropwise at room temperature under argon over a period of 30 min. The nitrile oxide l c was generated in toluene solution of C70 by dehydrochlorination of the corresponding hydroximinoyl in the course of four hours. After a reaction time of 16-24 hours at room temperature the mixture of isomeric cycloadducts were separated by colum...

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