First synthesis of a new fullerene oximes bis(9-[hydroxo]-1-[1′-hydroxymino-2′-oxo-2′-alkyl-ethyl])1,9-dihydro-[C60-Ih][5,6]fullerenes via reaction of C60 with ketones and NaNO2 promoted by HCl

ABSTRACT A mixture of cis-2 and cis-3 isomers of new fullerene derivatives bis(9-[hydroxo]-1-[1′-hydroxymino-2′-oxo-2′-alkyl-ethyl])-1,9-dihydro-[C60-Ih][5,6]fullerenes have been selectively synthesized with high yields at the reaction of C60 fullerene with ketones Me2CO, EtMeCO, i-PrMeCO, PhMeCO in the presence of NaNO2 and HCl.


Introduction
Oxime derivatives of [60]fullerene are prospective as complexation reagents, physiologically active compounds and precursors for further chemical functionalization leading to fullerene derivatives with desired properties (1)(2)(3)(4)(5). At the same time, the works on the synthesis of fullerene oximes are limited. For example, the reaction of C 60 with nitroalkanes (RCH 2 NO 2 ; R D H, Me, CH 2 CH 2 COOEt) in the presence of trimethylamine Et 3 N and trimethylsilylchloride Me 3 SiCl (¡20 C, Ar, 30 min) results in the intermediate isooxolidine cycloadduct of C 60 , which then converts into b-hydroxyoxime of C 60 fullerene (34-65% yields) via its treatment with tetraethylammonium chloride (¡20 C, Ar, 60 min) and neutralization by HCl (6). However, this synthetic methodology requires cooling and involves formation of two intermediate products. One-pot room-temperature synthesis of b-hydroxyoximes of fullerene C 60 (34-46% yields, 2 hrs) has been performed by using of nitroalkane excess (C 60 :RCH 2 NO 2 D 1:20; R D H, Me, CO 2 Et, CH 2 CH 2 CO 2 Et, CH 2 CH 2 CO 2 Bu t , Bn) (7). Chemical modification of cyclopropane derivative of C 60 with oxylamine group has been used to attach diverse oligosaccharides to fullerene cage via strong oxime bonds (yields of the target compounds are not provided) (8). To our knowledge hitherto not been obtained fullerene oximes containing a carbonyl group.

Materials and methods
We used a commercial C 60 (99.9%), produced by arc discharge between graphite electrodes (Institute of Organometallic Chemistry. G.A. Razuvaev, RAS, Nizhny Novgorod). The solvent CCl 4 was purified according to the procedure (9). NaNO 2 , acetone, acetophenone, methyl ethyl ketone and methyl isopropyl ketone were analytical grade and used as purchased. High performance liquid chromatography (HPLC) was performed on Hewlet Packard, series 1090, with a UV detector (λ max D 340 nm), Buckyprep Waters 4.6 £ 250 mm column at a temperature of 30 C, toluene as a mobile phase, flow rate 1.0 mL/min. The components of the mixture were separated using a Cosmosil Buckyprep Waters semicolumn 250 £ 10 mm at room temperature; mobile phase: toluene; flow rate: 2.0 mL/min. UV-Visible spectra were recorded on a Perkin Elmer Lambda 750 spectrometer (l D 0.1; 1 cm). IR spectra have been recorded by a Vertex 70 V spectrometer (Bruker) in CHCl 3 . The NMR spectra of 1 H and 13 C were recorded on Bruker Avance-400 spectrometer (operating frequency of 400.13 MHz and 100.62). CDCl 3 was used as a solvent (d 77.1 ppm), internal standard-(CH 3 ) 4 Si. Mass spectra were obtained on the Bruker MALDI TOF/TOF Autoflex-III instrument with a laser desorption and registration of positive and negative ions in the reflective mode. The elementary sulfur S n was used as a matrix.
General procedure for the synthesis of the C 60 bisadducts 1-4 To solution of C 60 in CCl 4 (30 mg, 0.04 mmol) were sequentially added ketones (Me 2 CO, EtMeCO, i-PrMeCO, PhMeCO) (0.4 mol), NaNO 2 (0.1 mol), and HCl (0.2 mol). Immediately after the addition of HCl, were observed vigorous evolution of brown gas and the deep purple solution became dark brown and white precipitate is formed. The reaction solution was mixed in 30 min and then neutralized by CaC? 3 . The liquid phase was separated by filtration and analyzed by HPLC. Reaction kinetics was monitored by the decrease of the C 60 peak up to the 100% conversion. HPLC of the solution after reaction contains the single broadened peak with the retention time equal to 6.2 min. The reaction products were isolated by semipreparative HPLC with toluene as an eluent. Unfortunately, the replacement of toluene by the polar eluent CH 3 CN:H 2 O D 4:1 also did not allow separating the regioisomeric bisadducts. Additional purification was successfully performed by water washing (3 £ 50 mL). When water added, two liquid layers aroused, so by-products were removed to the water layer. A liquid organic residue was dried under anhydrous MgSO 4 , and toluene was distilled off. The obtained precipitates 1-4 were dried in vacuo.

Analysis oxime groups of the 1 adduct
Distilled water (10 mL) and concentrated HCl (0.5 mL) was added to solid yellow adduct 1 (37 mg). This mixture was heated in 30 min at reflux of liquid phase. Then solid was washed by water to pH D 7 and dried in vacuum. This way, solid powder 7 was obtained (Scheme 3).
Interaction of C 60 with acetone has been used to define the optimal ratio of regents (Table 1, entry 1-5). The highest yield of fullerene oxime 1 (isolated yield 95%) is achieved at the molar ratio C 60 :Me 2 CO:NaNO 2 :HCl D 0.01:100:25:50. When other carbonyl compounds are involved in the reaction under study (a-and b-diketones, cyclic ketones, aliphatic aldehydes, or carboxylic acids), fullerene oximes are not produced (Table 1, entry 9-14).
Contents of 1 compound have been found from MALDI-TOF/TOF mass-spectra. The structure of compound 1 has been determined by 1 H NMR and 13 C NMR spectra. The 1 H NMR spectrum of 1 has the broadened signals of the protons of the methyl group in the range d 2.34 and OH groups of oxime moiety at 6.50 ppm, which correspond to E-isomers. In addition, the proton signals in the range of 8-10 ppm correspond to Z-isomers are registered. The ratio of the signals from E-and Zisomers is estimated as 17:1. In the 1 H NMR spectrum, the signals of hydrogen atoms of OH addends are absent. We explain this absence with their intra-and intermolecular interactions. In contrary, the signals of hydrogen atoms of oxime moieties are registered at 6.5 ppm due to their According to theoretical considerations, there are minimally eight such regioisomers for the C 60 bisadducts (10). However, in the 13 C spectrum, we do not observe single signals but deal with the sets of the signals of the CHO, CHN, C-O, and C-C groups of the isomers of the synthesized compound 1, which are chromatographically inseparable. In the HMBC experiments, the signals of methyl groups (d H 2.34) have the cross-peaks with carbon atoms of the CHO (d C 185-193) and CHN (d C 162-167) groups. Nevertheless, the above NMR data obtained well agree with the known data on the fullerene adducts (6,7). The UV-Vis spectrum of the solution 1 in CCl 4 has the maximum at 258 and the bent at 420 nm ( Figure 1) that is typical for cis-2 and cis-3 fullerene bisadducts (11)(12)(13)(14). This test is universal, especially if CDO is conjugated with another double bond (in the case of 1, it is CDN). Thus, we have confirmed the presence of CDO group in 1 by appearance of the maxima 225, 273 and 459 nm in the UV-Vis spectrum of the caustic soda solution of hydrazone 6 ( Figure 2). Presence of oxime moiety in 1 was confirmed by formation of diketone 7 in the reaction of acidic hydrolysis of 1 (16) (Scheme 3).
The diketone was identified by NMR ( 1 H, 13 C) and mass-spectrometry. The 1 H spectrum of 7 contains the signals of methyl protons at 2.6 ppm. The 13 C spectrum of 7 has signals of carbon atoms in the range 130-152 ppm, CDO group at 189-193 ppm, CH 3 groups at 27-29 ppm as well as sp 3 -C atoms of C 60 at 95-98 and 67-69 ppm. The contents of 7 were deduced from mass-spectrum, which

Funding
The work was financially supported by Russian Foundation for Basic Research (project 16-03-00822 A).