New sonochemical reactions of the C60 fullerene with amino alcohols yielding morpholine–C60 adducts

Abstract A selective and efficient method for the synthesis of previously unknown mono-adducts of the C60 fullerene with fused morpholine moieties has been developed based on the reaction of C60 with amino alcohols, including biogenic amines (2-aminoethanol, 2-amino-1-phenylethanol, noradrenaline, and adrenaline), in a mixed solvent (toluene/DMF) at room temperature under air and ultrasonication. The radical anion C60 •‒ (g = 2.0004 and ΔH 1/2 = 3.4 G) was detected as a key intermediate by the EPR method upon the synthesis of the C60–morpholine adduct. This intermediate results due to the one-electron transfer from 2-aminoethanol onto the C60 core.

The ultrasonic vibrations were generated using the ultrasound UZDNT-2T with 22 kHz working frequency and 20 W power and amplitude 20 lm ( Figure 1). The generator has a piezoelectric transducer with a submersible titanium waveguide (the diameter of its irradiating surface is 12 mm). It was immersed in the solution to a depth of 2 cm. The reactions were performed in a glass reactor (100 Â 35 mm) with a thermostatic jacket to maintain the constant temperature (rjvyanyaz nevgehaneha). In special cases, the reaction mixture was saturated with argon to perform the reactions under anaerobic conditions.
The reaction products were analyzed by HPLC chromatograph Altex (model 330, USA) with a UV detector (k max ¼ 340 nm), Buckyprep Waters 4.6 Â 250 mm column at 30 C (toluene was the mobile phase; the flow rate was 1.0 mL Â min -1 ). The mixtures were separated on a metal preparative column Cosmosil Buckyprep Waters (10 Â 250 mm) at $20 C (toluene was an eluent; the flow rate was 3.0 mL Â min -1 ). The UV-visible spectra were registered in CHCl 3 (l ¼ 1 and 0.1 cm) with spectrometer Perkin Elmer Lambda 750. The FT-IR spectra were registered in CHCl 3 with the use of spectrometer Vertex 70 V (Bruker). One-dimensional ( 1 H and 13 C) and two-dimensional (COSY, HSQC and HMBC) NMR spectra of compounds were recorded on Bruker Avance 400 spectrometer (400.13 MHz for 1 H and 100.62 MHz for 13 C) and Bruker Avance II 500 HD Ascend spectrometer (500.17 MHz for 1H and 125.77 MHz for 13 C); CDCl 3 þ CS 2 (3:1) was used as a solvent and Me 4 Si as the internal standard. Mass spectra were measured by MALDI TOF/TOF methods on Bruker Autoflex III spectrometer with laser desorption and registration of positive ions in the reflective mode (elemental sulfur was a matrix). The EPR measurements were performed with spectrometer Bruker EMX plus 10/12 with -diapason (Bruker Co., Ltd, Germany, $9.45 GHz) at 77 K (the diameter of the quartz ampoules is 4 mm).
Original dark purple solutions became dark brown. Further, the solutions were stirred using a magnetic stirrer for additional 7 hours and then passed through the column filled with a small layer of silica gel ($4 cm). The reaction products were extracted with preparative HPLC. After removing the solvent in vacuo, the dark brown powder products 2a, 2d, and 2f-i were obtained.

EPR measurements
A solution of 1a (9,9 mL, 0.16 mmol) in DMF (2 mL) in argon atmosphere was added to a solution of C 60 (30 mg, 0,04 mmol) in toluene (10 mL). The prepared mixture was sonicated for 2 min. The solution changed original purple color to dark green. The resulting solution was immediately transferred into the EPR ampoule (preliminarily treated with argon), frozen in liquid nitrogen, and placed to the spectrometer cell. The EPR spectra were recorded at 77 K.

Results and discussion
Optimal experimental conditions for the reaction between amino alcohols and C 60 have been selected using available 2-aminoethanol 1a as an example (Scheme 1). The reactions between C 60 and 1a (the molar ratio of 1:4) using a mixed  Table 1). The decrease in the ratio of initial reagents to 60 :1a ¼ 1:1 sharply reduces the yield of the target product (entries 1-3, Table 1). Changing the ratio of initial reagents, viz., increasing the content of 1a relative to C 60 , decreases the selectivity due to the attachment of additional molecules of 1a to the C 60 core (entry 5, Table 1). Carrying out the reaction in argon atmosphere does not significantly change the yield of 2a (42%) (entry 6, Table 1), thus indicating that O 2 does not have a decisive influence on the course of the reaction. Below the room temperature, the reaction rate decreases (for example, 10 , entry 7, Table 1). At higher temperatures, e.g., 40-50 C product 2a is not formed (entry 8, Table 1). Syntheses of 2a were performed in the presence of DMF. Without DMF and ultrasonication, there is no reaction (entries 9 and 10, Table 1). The use of o-DCB (1,2-dichlorobenzene) instead of toluene do not decrease the yield of 2a (entry 11, Table 1). Replacement of DMF with DMSO and MeCN as co-solvents lead to worse results (entry 12 and 13, Table 1). A prolongation of the ultrasonic exposure up to 3 hours with additional stirring for 5 hours decreases in the selectivity of the reaction. Carrying out the reaction only under ultrasonic irradiation for 1 hour provides 2a in 32% yield (entry 14, Table 1).
Isolated and chromatographically purified product 2a is a dark brown solid. The structure 2a was confirmed by 1 D ( 1 H, 13 Figures S11-S12, ESI). The maximum at 428 nm is peculiar for mono-cycloadducts of C 60 having the cycle annulated via the closed  bond. [38,42,43,46,47] The IR spectrum of 2a contains the bands attributed to the fullerene skeleton at 524 and 575 cm À1 , bands of the CH 2 groups at 2851-2955 cm À1 (stretching) and 1462 cm À1 (deformation), stretching vibrations associated with C-O bonds at 1018-1117 and 1267 cm À1 , the C-N bond at 1378 cm À1 . The N-H bond absorbs at 726 cm À1 (deformation vibrations) and a very weak stretching vibration is registered at 3400 cm À1 (Figure S13, ESI).
We have investigated the generality of our sonochemical reaction under the selected optimal conditions. The electron-acceptor effect of substituents at the N atom (phenyl group) and at arelative to the N atom (carboxyl group) reduces nucleophilicity of the nitrogen atom that significantly affects its reactivity and do not afford appropriate cycloadducts 2b and 2c (Scheme 2).
However, if the phenyl substituent is located at the aatom relative to the O atom (2-amino-1-phenylethanol, 1d), the reaction proceeds with a moderate yield of cycloadduct 2d (42%, Scheme 2). The presence of a more electron-donor carbonyl group, as compared to the phenyl group, at arelative to the O atom (aminoacetic acid, 1e) does not favor the occurrence of the Table 1. Screening of the optimal conditions for the reaction of C 60 with 2aminoethanol (Scheme 1). a Entry C 60 :1a molar ratio Yield (%) reaction with fullerene (compound 2e, Scheme 2). However, when reducing the carbonyl group with NaBH 4 , the reaction proceeds with the formation of the morpholine adduct 2a in 40% yield, thus demonstrating that the electronic effects of substituents influence the course of the reaction.
Amino alcohols with an electron donating group (at the N atom in 1f and at a-C relative to the oxygen atom, 1g) in the reactions with fullerene showed higher reactivity and led to the corresponding products 2f and 2g with 56 and 52% yields, respectively (Scheme 2). Compound 2g has been previously synthesized in the lower yield (only 35%). [26] Thus, ultrasonication is more efficient than thermal or catalytic activation.
Expanding the synthetic potential of the reaction under study, we have scrutinized the synthetic utility of our method. Hydroxyl and amine groups are simultaneously present in numerous biologically active natural compounds, e.g., catecholamines (adrenaline, noradrenaline), which are formed in animal or plant organisms and possess high biological activity acting as neurotransmitters and hormones. We have succeeded in our attempt to covalently bind noradrenaline and adrenaline with the fullerene core under the studied reaction conditions: the corresponding cycloadducts 2h and 2i were obtained in 43 and 48% yields, respectively (Scheme 2). The presence of catechol fragment in the molecules of represented biogenic amines did not interrupt the reaction under study since the reaction between C 60 and pyrocatechol is not observed under the studied reaction conditions. Hydrophilic 1d, 1g-1i in the composition of the resulting fullerene cycloadducts 2d, 2g-i become hydrophobic. The 1 H and 13 C NMR spectra of known compound 2f well agree with those described in the literature. [] Compositions of compounds 2d and 2g-i were deduced from MALDI TOF/TOF mass-spectra (ESI). Structures of these new compounds were identified using one-( 1 H, 13 C) and two-dimensional (COSY, HSQC, HMBC) NMR, UV-vis spectroscopic techniques. For example, in the HMBC experiments on adduct 2i, the signals of methylene protons, appeared as the multiplet at d H $3.14 ppm, correlate with the sp 3 -hybridized carbon atom of the fullerene skeleton ( 6 ) at d C $75.88 ppm, carbon atoms of the methylene group at d C $47.37 ppm, and carbon atoms of the methyl group at d C $29.93 ppm. The signal of the methyl group at d H $2.38 ppm correlates with the sp 3 -hybridized carbon atom of the fullerene cage ( 5 ) at d C $72.36 ppm ( Figure  S28, ESI). The UV-vis spectra of 2d and 2g-i contain maxima at 428-432 nm, which are peculiar for 60 mono-cycloadducts annulated via the closed [6-6]-bond. [38,42,43,46,47] Based on the experimental results and our previous investigations into the mechanisms of C 60 sonochemical reactions with diols [38,39] and diamines, [42,43] we have proposed a mechanism for the sonochemical reaction between amino alcohols and fullerene using the simplest adduct 1a as an example (Scheme 3). Initially, in sonochemical reaction between the C 60 fullerene and 2-aminoethanol 1a, a green solution was formed. This coloring is characteristic for radical anion C 60 -(intermediate ff, Scheme 3). The EPR Scheme 2. Synthesis of C 60 -morpholine adducts 2b-i.
spectrum of this green solution confirms the formation of the radical anion C 60 -(g ¼ 2.0004 and DH 1/2 ¼ 3.4 G; Figure S29, ESI) and agrees well with our previous work and the literature data. [42,[48][49][50] The recombination of intermediate A and the amine radical cation initiates the formation of bipolar ion B. The intramolecular proton transfer from nitrogen to the fullerene carbon atom results in the formation of a neutral product C (Scheme 3), which was the final product in the previous synthetic works. [23,24] Herewith, its further transformation were not studied. For the title reaction of the present work, product C is intermediate and undergoes further attack by radicals originating from DMF sonolysis.
It was previously noted that the studied reaction does not occur in the absence of DMF and without ultrasound exposure. We have obtained the similar experimental results earlier when studying sonochemical reaction between diamines and C 60 . [42,43] The chemical effects of ultrasound are associated with cavitation, which generates high concentrations of extremely reactive radicals and ions formed in solution. [51] The DMF sonolysis produces radicals ( 3 , CH 2 N(CH 3 )C(O)H, C(O)N(CH 3 ) 2 ) (Scheme 3), which were identified using a radical trap (N-tert-butyl-a-phenylnitrone). [52] Probably, these radicals abstract hydrogen atoms from intermediate C to form biradical D, which converts to product 2a upon subsequent intramolecular cyclization.

Conclusion
In conclusion, we have developed a new sonochemical method to produce previously unknown fused morpholine monodducts of C 60 based on the reaction between fullerenes and 2-aminoethanol, 2-amino-1-phenylethanol, amino-2propanol, noradrenaline, and adrenaline in 46, 42, 52, 43, and 48% yields, respectively. The undoubted advantage of the combined effect of ultrasound and DMF as compared to thermal and catalytic methods for activation of chemical reactions is the possibility to simultaneously activate two functional groups (N-H and O-H) thus affording previously unknown compounds. The use of ultrasound to synthesize known compounds permits to combine mild reaction conditions (room temperature, air) with a significant increase in the yield of products. A probable mechanism for the formation of final cycloadducts is proposed. The key intermediate, radical anion C 60 -(g ¼ 2.0004 and DH 1/2 ¼ 3.4 G) was detected by EPR method in the reaction of 2-aminoethanol with fullerene.

Disclosure statement
No potential conflict of interest was reported by the authors.

Funding
The structural studies of compounds were performed in "Agidel" Collective Usage Center at the Institute of Petrochemistry and Catalysis (Russian Academy of Sciences) in accordance with the Federal Program (FMRS-2022-0077 and FMRS-2022-0078). EPR spectra were recorded using the equipment of CCU "Spektr" (IMCP, UFRC of RAS).