Switched role of fullerene in the Diels–Alder reaction: facile addition of dienophiles to the conjugated fullerene diene moiety

Abstract

The isolated cyclopentadiene moiety in a fullerene multi-adduct derivative reacts readily with dienophiles to form [4 + 2] cycloaddition products.

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Cycloaddition reactions are the most popular functionalization method in fullerene chemistry.¹ A number of cycloaddition patterns have been reported and numerous fullerene derivatives have been prepared through the cycloaddition reactions.² Among them the [2 + 4] Diels–Alder reaction has been extensively used to prepare stable fullerene adducts.^3,4 For example, several C₆₀-TTF dyads, triads and polyads have been prepared, which exhibit exciting electron and energy transfer properties.⁵ Both stepwise⁶ and concerted mechanisms⁷ have been observed for such reactions depending on the electronic structure of the diene substrates. Due to the electron deficient nature of fullerenes, double bonds on the pristine fullerene skeleton act as the 2π electron electrophile in all the known Diels–Alder reactions.¹ In their study of cage-opening reactions, Rubinet al. reported the first example of a diene moiety in a skeleton modified fullerene derivative acting as the 4π electron nucleophile, which remains as the only example so far.⁸

We have been studying the chemistry of fullerene-mixed peroxides⁹ and prepared various skeleton modified fullerene derivatives such as C₅₉ fulleroids.¹⁰ As a result of the cage modification, these fullerene derivatives exhibit quite different chemical properties from pristine fullerene in some cases.^9,10 Here we report the skeleton modified oxahomofullerene2 acting as a 4π electron nucleophile and the characterization of the resulting adduct by single crystal X-ray analysis.‡

The cage-opened diketone fullerene derivative 1 was prepared from C₆₀ in three steps as we have reported previously.^9b In an effort to couple the two carbonyl groups and close the opening, we treated 1 with p-hydroquinone in the presence of potassium carbonate (Scheme 1). Compound 2 was obtained in good yield as indicated by TLC. During the work-up procedure, the mixture was evaporated at around 60 °C by using a rotavap. Compound 3a was detected from the residue, apparently the oxidized byproduct quinone added to 2 under the conditions. To prepare pure 2, the bulky 2-tert-butyl-1,4-hydroquinone was then used as the reducing agent. Unlike the p-quinone, the tert-butylquinone did not react with 2, thus pure 2 was isolated in 88% yield.

Scheme 1 Reduction of fullerene dione 1 and subsequent Diels–Alder cycloaddition reactions with dienophiles.

The facile formation of the Diels–Alder adduct 3a prompted us to further investigate the reaction of 2 towards other dienophiles. Under similar conditions, compounds 3b, 3c, 3d and 3e were obtained from N-phenylmaleimide, maleic anhydride, styrene and methylacrylate respectively (Scheme 1). Only one stereoisomer was observed in all these reactions. Yields of compounds 3 vary from 21% for the N-phenylmaleimide adduct 3b to 87% for the styrene adduct 3d. Steric hindrance should be responsible for the relatively low yield of 3b.

Structural assignments of the cycloaddition products 3 were deduced from their spectroscopic data. The ESI-HRMS spectra gave the molecular ion as the most prominent signal.

The ¹H NMR spectra showed the expected proton coupling pattern for these C₁ symmetric fullerene derivatives. Chemical shifts of the hemiketalfullerenecarbon in compounds 2 and 3 appear in the range from 107.6 to 108.0 ppm, which are essentially the same within experimental error. The number of sp³fullerene carbons changed from 6 for the precursor 2 to 8 for cycloadducts 3. These data strongly suggest a Diels–Alder reaction product, but give no information as to whether compounds 3 adopt the endo- or exo-geometry. X-Ray diffraction results were needed to assign the stereochemistry conclusively.

Attempts at growing single crystals succeeded for the p-quinone adduct 3a by slow evaporation of its solution in a mixture of toluene and ethanol at r.t.‡ The structure in Fig. 1 shows the quinone ring tilted towards the newly formed double bond on the central pentagon. The dihedral angle between the quinone ring and the plane containing the newly formed double bond within the central pentagon is 62°. The space-filling model in Fig. 1 indicates the tert-butyl groups are just far enough from the quinone ring not to introduce any steric hindrance. The bond distance between the quinone and the fullerenecarbon is slightly shorter near the hemiketal side (1.575 Å) than the one near the tert-butylperoxogroup (1.607 Å). Both steric and electronic effects may contribute to this difference.

Fig. 1 X-Ray crystal structure of compound 3a. Ellipsoids were drawn at the 50% level. For clarity hydrogen atoms were not shown on the ball-stick model. Colour scheme: white = hydrogen, grey = carbon, dark grey = oxygen.

Compounds 3b and 3c should follow the same endo-stereochemistry as that of 3a since they are all cyclic dienophiles. The presence of the epoxidegroup adjacent to the central pentagon may be another driving force in favor of the endo-addition pattern besides the normal π–π interaction observed for other Diels–Alder reactions. The unsymmetric styrene and methyl acrylate exhibit good regio-selectivity besides stereo-selectivity in the formation of adducts 3d and 3e. In light of the X-ray structure of 3a, compounds 3d and 3e should be assigned as the endo-adducts as well. Electronic effect directs the phenyl and the methoxycarbonyl groups to the hemiketal side. Steric effects also favor such a geometry. Thus the most likely structure for compounds 3d and 3e is the one shown in Fig. 2.

Fig. 2 Structure of compounds 3d and 3e.

The mechanism for the formation of 2 from 1 probably involves single electron transfer as the key step. Under basic conditions, the deprotonated hydroquinone could donate an electron to 1 to form the radical anion intermediate A with the negative charge centered on the oxygen atom and the radical on the pentagon. The nucleophilic oxygen anion in A then attacks the adjacent carbonyl carbon forming the hemiketal moiety in intermediate B. The epoxide moiety may be formed through homolytic cleavage of the peroxo bond as indicated in Scheme 2. Alternatively, intermediate B may accept another electron and the tert-butoxylgroup is cleaved as the anion instead of the radical.

Scheme 2 Proposed mechanism for the reduction of fullerene dione 1 with hydroquinone.

In summary, the two adjacent diketonegroups in fullerene-mixed peroxide1 couple into a hemiketal moiety upon reduction with hydroquinone. The hemiketaloxygen atom connected to the cyclopentadiene acts as an electron donating group and facilitates facile addition of dienophiles to the isolated pentagon. The results demonstrate that fullerene skeleton modification can dramatically change the reactivity of the cage such as the present switch from electrophile to nucleophile. Further work is under way to prepare fullerene skeleton modified compounds and to explore their chemical and physical properties.

This work is supported by NNSFC (Grants 20632010, 20521202, and 20772004) and the Major State Basic Research Development Program (2006CB806201)

Notes and references

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Footnotes

† Electronic supplementary information (ESI) available: Experimental details and selected spectra for new compounds. CCDC 703259. For ESI and crystallographic data in CIF or other electronic format see DOI: 10.1039/b817300a

‡ Compound2: tert-butylhydroquinone (403 mg, 2.43 mmol) and potassium carbonate (133 mg, 0.96 mmol) was added to a stirred solution of compound 1 (540 mg, 0.49 mmol) in toluene (54 mL) at 30 °C. After 8 min, the organic layer was directly chromatographed on a silica gel column eluting with toluene/petroleum ether/AcOEt (10 : 10 : 1). The first band was a trace amount of unreacted material 1. The second band, compound 2, was collected and evaporated, then washed with methanol and dried (445 mg, 0.43 mmol, 88%). Characterization data: ¹H NMR (400 MHz, CDCl₃): δ 5.06 (s, 1H), 1.42 (s, 9H), 1.40 (s, 9H), 1.36 (s, 9H). ¹³C NMR (100 MHz, CDCl₃): all signals represent 1C except noted. δ 153.16, 151.21, 149.77 (2C), 149.33, 148.93, 148.46, 148.16, 148.10, 148.96 (2C), 147.59, 147.48, 147.33, 147.21, 147.11 (2C), 147.02, 146.91 (2C), 146.82, 146.80, 146.34, 146.23, 146.16, 146.03, 146.94, 145.67, 145.64, 145.21, 145.16, 144.94, 144.79, 144.66, 144.35, 143.98, 143.95, 143.30, 143.17, 143.10, 142.84, 142.63, 142.28, 142.17, 142.04, 141.83, 140.80, 140.40, 140.18, 139.52, 131.93, 128.66, 126.68, 107.73 (1C, sp³), 84.55 (1C, sp³), 81.81 (1C–(CH₃)₃), 81.36 (1C–(CH₃)₃), 81.30 (1C–(CH₃)₃), 78.77 (1C, sp³), 71.30 (1C, sp³), 70.62 (1C, sp³), 26.78 (3CH₃), 26.55 (6CH₃), 26.46 (3CH₃). FT-IR (microscope): 3390, 2978, 2926, 2853, 1709, 1644, 1589, 1458, 1387, 1364, 1288, 1261, 1243, 1191, 1155, 1111, 1094, 1051, 1016, 908. ESI-MS: m/z (rel. int.) 1059 (100, M + Na⁺). ESI-HRMS C₇₂H₂₇O₉ (M − 1, 100) calc. 1035.1655, found 1035.1656. Compound3a: Benzoquinone (200 mg, 1.85 mmol) was added to a stirred solution of compound 2 (40 mg, 0.04 mmol) in toluene (6 mL) at 50 °C. After 30 min, the organic layer was chromatographed on a silica gel column eluting with toluene/petroleum ether/AcOEt (5 : 5 : 1). The first band was unreacted benzoquinone. The second band, compound 3a, was collected and evaporated (31 mg, 0.03 mmol, 70%). Characterization data: ¹H NMR (400 MHz, CDCl₃): δ 7.08 (d, J = 10.4 Hz, 1H), 6.92 (d, J = 10.4 Hz, 1H), 6.90 (s, 1H), 4.77 (d, J = 8.8 Hz, 1H), 4.47 (d, J = 8.8 Hz , 1H), 1.41 (s, 9H), 1.35 (s, 9H), 1.27 (s, 9H). ¹³C NMR (100 MHz, CDCl₃): all signals represent 1C except as noted; δ 193.74, 192.02, 153.32, 149.62, 149.06, 148.95, 148.89, 148.78 (2C), 148.69, 148.57, 148.50, 148.47, 148.41, 148.26, 148.24, 148.22, 148.08, 147.98, 147.92, 147.76, 147.74, 147.66, 147.60, 147.56, 147.28, 146.91, 146.57, 145.68, 145.51, 145.41, 145.34, 145.28, 145.15, 144.82, 144.03, 143.99, 143.91, 143.84, 143.78, 143.41 (2C), 142.90, 142.58, 142.39, 142.26, 141.97, 141.95, 140.75, 140.55, 139.34, 139.20, 137.51, 135.35, 131.61, 130.55, 107.58 (1C, sp³), 89.06 (1C, sp³), 88.25 (1C, sp³), 85.27 (1C, sp³), 83.66 (1C, sp³), 82.27 (1C-(CH₃)₃), 82.09 (1C-(CH₃)₃), 81.92 (1C-(CH₃)₃), 77.29 (1C, sp³), 68.78 (1C, sp³), 56.64 (1C, sp³), 55.75 (1C, sp³), 51.56 (1C, sp³), 27.08 (3CH₃), 26.70 (3CH₃), 26.66 (3CH₃). FT-IR (microscope): 3506, 3347, 2977, 2931, 1680, 1616, 1502, 1470, 1455, 1388, 1365, 1287, 1246, 1190, 1142, 1103, 1085, 1044, 1026, 1007, 956, 864, 756. ESI-MS (Bruker Esquire): m/z (rel. int.) 1162 (100, M + NH₄⁺). ESI-HRMS C₇₈H₃₁O₁₁ (M − 1, 100) calc. 1143.1866, found 1143.1864.

Crystal data for 3a: C₈₇H₄₆O₁₂, M_w = 1283.24 g mol⁻¹, T = 123(2) K, monoclinic, space groupP2₁/c, unit cell dimensions: a = 15.358(3), b = 13.053(3), c = 28.534(6) Å, β = 98.99 (3)°, V = 5650(2) ų. Z = 4, D_c = 1.509 Mg m⁻³, μ = 0.1 mm⁻¹. Reflections collected/unique 44193/12855 [R(int) = 0.0642]. Final R indices [I > 2σ(I)] R₁ = 0.0637, wR₂ = 0.1136.

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