Controlled Synthesis of C70 Equatorial Multiadducts with Mixed

Letter Cite This: Org. Lett. 2018, 20, 2328−2332

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Controlled Synthesis of C70 Equatorial Multiadducts with Mixed Addends from an Equatorial Diadduct: Evidence for an Electrophilic Carbanion Shu-Hui Li,† Zong-Jun Li,* Wei-Wei Yang, and Xiang Gao* State Key Laboratory of Electroanalytical Chemistry, Changchun Institute of Applied Chemistry, Chinese Academy of Sciences, 5625 Renmin Street, Changchun, Jilin 130022, P. R. China S Supporting Information *

ABSTRACT: Controlled synthesis of the equatorial tetra-, hexa-, and octaorgano[70]fullerenes with mixed addends was achieved via the reaction of equatorial 7,23Bn2C70 with MeO− and ArCH2Br. The products were structurally characterized by single crystal X-ray diffraction. The regioselectivity of the reaction was studied by in situ vis−NIR and Fukui function analysis. A surprising electrophilic triorgano[70]fullerene carbanion was shown, and an enhanced fluorescence was observed for the mixed octaadducts.

T

MeO-37-o-BrCH2PhCH2C70 (1a, 49%, Figure S1 in Supporting Information (SI) for HPLC) or 7,23,37-Bn3-19-MeOC70 (1b, 46%, Figure S2 for HPLC), respectively, with no notable color change during the reaction. When 2.1 equiv of MeO− were added into the 7,23-Bn2C70 solution in one shot, the color of the solution changed gradually from brown to green in 1.5 h. The color of the solution changed back to brown after ArCH2Br was added, affording the hexaadduct of 7,23-Bn219,44-(MeO)2-27,37-(o-BrCH2PhCH2)2C70 (2a, 53%,) or 7,23,27,37-Bn4-19,44-(MeO)2C70 (2b, 40%) as the major product, and a small amount of the tetra- and octaadducts (Figures S3 and S4 for HPLC). The use of 3.1 equiv of MeO−, however, would not result in the octaadduct as the major product, but mainly, the hexaadduct and a small amount of the tetraadduct (Figure S5 for HPLC) when quenching with oBrCH2PhCH2Br, similar to the case using 2.1 equiv of MeO−. Instead, the octaadduct of 7,23-Bn2-19,44,53-(MeO)3-27,33,37(o-BrCH2PhCH2)3C70 (3a, 41%, Figure S6 for HPLC) or 7,23,27,33,37-Bn5-19,44,53-(MeO)3C70 (3b, 34%, Figure S7 for HPLC) was obtained when 1.1 equiv of MeO− was added into the in situ generated hexaadduct 2, where ArCH2Br was present. The structures of 1a, 2a, and 3a were determined by single crystal X-ray diffraction (1a: CCDC 1440497, 2a: 1440498, and 3a: 1440499). The structures of 1a and 3a contain disordered CS2 solvent molecules, which were removed using the utility SQUEEZE.16 Figure 1 displays the single crystal structures of 1a, 2a, and 3a, along with the Schlegel diagrams. To the best of our knowledge, this is the first time that single crystal structures of the C70 equatorial tetra- and hexaadducts were obtained. The crystals of 1a are composed of a 50:50 mixture of two mirror-image enantiomers, which are designated as 7,19,23,37- and 7,23,44,27-adducts. Compound 2a has C2

he release of the strain energy, which is caused by the deviation from the planarity of sp2 hybridized carbon atoms and constitutes about 80% of the heat of formation of C60, is considered to be the major driving force for the reactivity of curved fullerenes.1 Intriguingly, multifunctionalized C70 derivatives with the general formula of XnC70 (X = Ph, Me, OMe, Br, Cl, H, OOtBu, CF3; n = 8 or 10) typically have the addends at the less curved equatorial belt,2−5 rather than at the more strained polar carbon atoms, as is the case of less functionalized C70 derivatives,3,4b,6 where the equatorial carbons are activated only for the cationic7 and anionic8,9 C70 species. Such a structural preference of X8/10C70 is supported by theoretical calculations,10 and the formation of such compounds is likely associated with the equatorial 7,23-X2C70 adduct.11 However, no report on the synthesis of C70 equatorial multiadducts using 7,23-X2C70 has appeared. In addition, work on mixed functionalized C70 equatorial adducts is limited.5e,12 In fact, the use of mixed addends is interesting, as it may provide more insight into the reaction mechanism by differentiating the addends,13,14 even though the reaction is more complicated than the one involving identical addends. We have recently reported the reaction of mixed methoxylation and benzylation of C60 and C70 with impressive regioselectivity.15 It would be of interest to examine this reaction with 7,23-Bn2C70, as it may help to achieve a controlled synthesis of C70 equatorial multiadducts and reveal more information on the reactivity of the C70 equatorial carbons. Compound 7,23-Bn2C70 was prepared following procedures reported previously.9a Synthesis of C70 equatorial multiadducts was carried out by stepwise addition of MeO− (1.0 M tetra-nbutylammonium hydroxide (TBAOH) in MeOH solution) and a large excess (50-fold) of ArCH2Br (Ar = o-BrCH2Ph or Ph) into the o-DCB solution of 7,23-Bn2C70 at 30 °C under argon. After 7,23-Bn2C70 reacted with 1.1 equiv of MeO− for 1.5 h, the addition of ArCH2Br afforded the tetraadduct of 7,23-Bn2-19© 2018 American Chemical Society

Received: February 26, 2018 Published: April 10, 2018 2328

DOI: 10.1021/acs.orglett.8b00672 Org. Lett. 2018, 20, 2328−2332

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therefore intriguing, as 2a is, in fact, less favored for hexaaddition when starting from the tetraadduct. Compounds 1b, 2b, and 3b were characterized with HRMS (Figures S14, S22, and S30), 1H (Figures S15, S23, and S31) and 13C (Figures S16, S24, and S32) NMR, and UV−vis (Figures S17, S25, and S33) spectroscopic methods. The results indicate that they have the same structures as those of 1a, 2a, and 3a by showing similar NMR and UV−vis spectral patterns. In situ vis−NIR spectroscopy (Figure 2) was performed to probe further into the reaction. A strong absorption band at

Figure 2. In situ vis−NIR spectra of (a) 7,23-Bn2C70 (1.1 × 10−4 M), (b) 7,23-Bn2C70 (1.1 × 10−4 M) after mixing with 1.0 equiv of MeO− for 1.5 h, (c) 7,23-Bn2C70 (1.9 × 10−4 M) after mixing with 1.4 equiv of MeO− for 1.5 h, and (d) 7,23-Bn2C70 (1.8 × 10−4 M) after mixing with 2.1 equiv of MeO− for 1.5 h. The measurements were performed with a 1 cm cuvette in o-DCB at 30 °C under argon.

Figure 1. ORTEP diagrams with 30% ellipsoid probability and Schlegel diagrams of (a) 1a with numbering of C70, (b) 2a, and (c) 3a. Hydrogen atoms and solvent molecules were omitted in the ORTEP diagrams for clarity.

symmetry and is designated as a 7,19,23,37,44,27-adduct. As for 3a, it is composed of a 50:50 mixture of two mirror-image enantiomers, which are designated as 7,19,23,27,33,37,44,53and 7,19,23,27,37,44,48,64-adducts. Notably, the methoxy groups are always located at the para positions with respect to the starting benzyls, while the o-BrCH2PhCH2 groups are at the para sites with respect to the methoxys, indicating that the reaction is initiated with the addition of MeO−, followed by a reaction with alkyl bromide, in agreement with previous results.15 Spectral characterization of these compounds is consistent with the single crystal X-ray analysis. Interestingly, only one regioisomer (1a, 2a, and 3a) was produced for each reaction, which is expected for 1a, as the addition at either C19 or C44, the para carbon with respect to the existing benzyls in 7,23Bn2C70, would result in the same compound. However, the exclusive formation of 2a is quite surprising, as in principle, there exists another possible hexaadduct (2a-II, Figure S8a), in which the second methoxy is at the para carbon with respect to the o-BrCH2PhCH2 group. Indeed, a mixture of hexaadducts was obtained, along with a significant amount of octaadduct(s) when 1.0 equiv of MeO− reacted with the in situ generated tetraadduct (1a) in the presence of o-BrCH2PhCH2Br (Figures S8 and S9). The (RR′)nC70 (n = 6 or 8) obtained in this manner should be formed via an addition to (RR′)n−2C70 precursors, rather than a direct reaction of 7,23-Bn2C70. The 1 H NMR spectrum of the hexaadduct mixture (Figure S35) showed four doublets with equal intensity (4.3−5.0 ppm) due to the two sets of nonequivalent Br-bound methylene protons,17 consistent with the formation of 2a-II with C1 symmetry. The spectrum also exhibited two doublets at 4.90 and 4.38 ppm, but with less intensity, confirming the formation of 2a. The exclusive formation of 2a from 7,23-Bn2C70 is

735 nm and two weak bands at 867 to 990 nm appeared after 1.0 equiv of MeO− was mixed with 7,23-Bn2C70 in o-DCB solution for 1.5 h (Figure 2b). The spectrum is different from that of the monoanions of the 7,23- and 2,5-Bn2C70 mixture (776 and 1062 nm),8 indicating that they were not associated with the monoanionic 7,23-Bn2C70. Instead, the result indicated that a monomethoxylated 7,23-Bn2C70 monoanion, intermediate A (Figure 3), was formed when 1.0 equiv of MeO− was

Figure 3. Schlegel diagrams of intermediates A and B.

mixed with 7,23-Bn2C70, as the anions of singly bonded fullerene intermediates were also capable of strong vis−NIR absorptions.15a,18 Judging from the structures of 1a and 1b, it is rational to assign that the methoxy group is added at either C19 or C44, the equatorial carbon atom para to the starting benzyls. Interestingly, new absorption bands appeared at 924 and 1134 nm when 2.1 equiv of MeO− were added into the 7,23Bn2C70 solution (Figure 2d), which corresponded to a color change of the solution from brown to dark green and indicated explicitly that a new anionic species was formed. Considering that 2a and 2b with C2 symmetry were formed exclusively in this reaction manner, it indicated that the negatively charged 2329

DOI: 10.1021/acs.orglett.8b00672 Org. Lett. 2018, 20, 2328−2332

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B3LYP/6-311G(d) level predicted the carbon atoms para to the methoxy groups in A (C37: −0.073, Figure S41, Table S4) and B (C37: −0.096, C27: −0.096, Figure S42, Table S5) possessed the largest negative charge, consistent with the experimental results. Octaorganoadducts 3a and 3b were obtained by a reaction of the in situ generated 2a and 2b with 1.1 equiv of MeO− in the presence of excessive ArCH2Br. However, the addition of more MeO− would not produce the decaorganoadducts, suggesting that a further addition of MeO− to the heptaorgano[70]fullerene carbanion was unlikely, and probably associated with the shrinkage of the equatorial π-conjugation caused by the multiaddition, which would be unable to hold two acquired electrons. The Fukui function analysis of 2a predicted the largest f k+ of 0.057 at the para carbon atoms with respect to the terminal o-BrCH2PhCH2 group (C53 and C48, Figure S43, Table S6), implying that methoxylation at either C53 or C48 is likely with formation of intermediate C (Scheme 1), which

triorgano[70]fullerene intermediate A could further undergo an electrophilic reaction with MeO− to form the dianionic tetraorgano[70]fullerene intemediate B, where the two methoxy groups were positioned symmetrically at the para carbons of the two starting benzyls. The in situ spectrum of 7,23-Bn2C70 with 1.4 equiv of MeO− (Figure 3c) exhibited a mixed spectral feature of A and B, confirming the conversion of A to B. Even though C60 and C70 are both electron-deficient and are capable of accepting up to six electrons,19 electrophilic addition of anions to the anionic species of C60 and C70 is rare and has been observed only in the gas phase with mass spectrometry for the reaction of C70CN− and C70(CN)3− with CN−, affording C70(CN)22− and C70(CN)42−.20 To the best of our knowledge, this is the first time that an electrophilic anionic fullerene species is observed in the condensed phase. A control experiment with 1,4-Bn2C60 showed absorption bands at 580, 699, and 999 nm when 1.0 equiv of MeO− was added (Figure S37), which corresponded to the 1,4-Bn2-11-MeOC60− with the stable 10π indenyl resonance (Figure S37).15a,b,21 However, no spectral change was observed when 2.1 equiv of MeO− were used (Figure S37), indicating explicitly that the anionic triorgano[60]fullerene analogue is not electrophilic. Notably, similar carbanions with such amazing electrophilicity have been reported for carbenoids, a type of organolithium compound, which have a rich chemistry in organic synthesis.22 Analysis of the electrophilic Fukui function ( f k+) was performed at the B3LYP/6-311G(d) level to understand the most electrophilic site of the C70 cage.23 Calculation on 7,23Bn2C70 predicted that the most electrophilic carbons are the C2 and C58 in the polar region and C19 and C44 in the equatorial region with the two largest f k+ values of 0.043 and 0.036, respectively (Figure S38 and Table S1). The result is reasonable since the polar carbons are highly strained and the most reactive,6a and the presence of only two addends at the equator may have little effect on their reactivity. In the meanwhile, the equatorial addends may have a significant effect on the electronic structure of the vicinal region, thus increasing the reactivity of the less strained equatorial carbons. However, methoxylation at C19 or C44 (A) was predicted to be more stable than methoxylation at C2 or C58 by 1.4 kcal/mol, rationalizing the preferential formation of A that eventually resulted in 1a. Calculation on 7,23-Bn2-19-MeOC70− (A) predicted that the unoccupied carbon atom (C44) para to the benzyl group is the most electrophilic site with the largest f k+ of 0.058 (Figure S39 and Table S2), which nicely agrees with the electrophilicity exhibited by A and the formation of B via the reaction with MeO−. Calculation on 1,4-Bn2-11-MeOC60−, however, predicted that the unoccupied carbon atom (C15) para to the benzyl is not the most electrophilic site in the intermediate, with a much smaller f k+ of only 0.016 (Figure S40 and Table S3), which is consistent with the nonelectrophilicity exhibited by the species with no reaction with MeO−. Notably, the unusual electrophilicity exhibited by the triorgano[70]fullerene anion is likely associated with the unique equatorial πconjugation of C70, which may well disperse the acquired electron(s) through good conjugation, but is absent in the C60 analogue. Benzylation of anionic intermediates A and B with ArCH2Br (Ar = o-BrCH2Ph or Ph) would result in the tetraorgano- (1a or 1b) and hexaorganoadducts (2a or 2b) via an SN2 mechanism,24 which can be well correlated with the NBO charge distribution in a fullerene cage.25 Calculations at the

Scheme 1. Proposed Mechanism for the Formation of Tetra-, Hexa-, and Octaadducts from 7,23-Bn2C70

would produce 3a or 3b via subsequent reaction with ArCH2Br. A proposed reaction mechanism for the formation of the tetra-, hexa-, and octaadducts from 7,23-Bn2C70 is shown in Scheme 1. It has been shown that highly functionalized fullerene derivatives are typically more fluorescent than the parent fullerenes and less functionalized derivatives due to the modification of fullerene π-conjugation.4c,26−28 The same rule applies to the obtained C70 equatorial adducts with the octaadducts (3a and 3b) exhibiting the strongest fluorescence. Figure 4 shows the fluorescence and absorption spectra of 3a, along with photos of 3a under ambient light and UV light (365 nm) in the inset. The fluorescence spectrum was recorded in

Figure 4. Mirror image of the fluorescence (red line, excited at 470 nm) and absorption spectra (black line) of 3a. Inset: photos of 3a under ambient light (left) and UV light at 365 nm (right). 2330

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toluene at room temperature with excitation at 470 nm. The fluorescence and absorption spectra of 3b were obtained under similar conditions and are shown in Figure S33. Both 3a and 3b exhibit a good spectral mirror image between the emission and absorption spectra (625 and 594 nm) with a Stokes shift of 835 cm−1, similar to the result of the isostructural C70Ph8 (617 and 590 nm).29 Interestingly, a fluorescence quantum yield of 0.040 and 0.036 was obtained for 3a and 3b by using C70 (ΦF = 5.4 × 10−4) as a reference,30 which is significantly greater than the values 0.013 and 0.0012 for the isostructural C70Ph831 and C70(CF3)828 with identical addends. Previous work has shown that extensive derivatization4c,27 or addition pattern variance28 may affect the fluorescence of fullerene derivatives by altering the fullerene π-system, while this work indicates that the fluorescence of organofullerenes may be improved by reducing the molecular symmetry. In summary, synthesis of the C70 equatorial multiadducts with mixed addends from 7,23-Bn2C70 was studied. The reaction exhibited good regioselectivity due to the amazing electrophilicity of the triorgano[70]fullerene carbanion, which was further probed with in situ vis−NIR and Fukui function analysis. An enhanced fluorescence was shown for the mixed octaadducts, compared to the analogues with identical addends.

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ASSOCIATED CONTENT

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.orglett.8b00672. Experimental and calculation details, HPLC traces and spectra of new compounds (PDF) Accession Codes

CCDC 1440497−1440499 contain the supplementary crystallographic data for this paper. These data can be obtained free of charge via www.ccdc.cam.ac.uk/data_request/cif, or by emailing [email protected], or by contacting The Cambridge Crystallographic Data Centre, 12 Union Road, Cambridge CB2 1EZ, UK; fax: +44 1223 336033.

AUTHOR INFORMATION

Corresponding Authors

*E-mail: [email protected]. *E-mail: [email protected]. ORCID

Zong-Jun Li: 0000-0002-7944-8371 Xiang Gao: 0000-0002-0624-3279 Present Address †

College of Chemistry and Chemical Engineering, Xiamen University, Xiamen, Fujian 361005, China. Notes

The authors declare no competing financial interest.

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REFERENCES

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ACKNOWLEDGMENTS

This work was supported by the National Natural Science Foundation of China (21472183) and the Jilin Provincial Science & Technology Department (20170101172JC and 20160520128JH). 2331

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