Synthesis of [70]azafulleroids: Investigations of azide addition to C70

Article abstract of DOI:10.1021/ja963063j

Treatment of C₇₀ with (2-methoxyethoxy)methyl azide gives rise to three out of a possible six triazoline isomers, showing chemoselectivity as well as regioselectivity. The major product arose from addition of azide to the double bond of C₇₀ possessing the greatest local curvature. Selective thermolysis of the least stable triazoline, the one resulting from addition to the least curved [6,6] double bond, allowed solution of the structure of the three isomers. Thermolysis of the triazoline isomers produced mixtures of C₇₀, azafulleroids, and fulleroaziridines.

Full text of DOI:10.1021/ja963063j

J. Am. Chem. Soc. 1997, 119, 943-946
943
Synthesis of [70]Azafulleroids: Investigations of Azide
Addition to C70
Cheryl Bellavia-Lund and Fred Wudl*
Contribution from the Departments of Chemistry and Materials, Institute for Polymers and
Organic Solids, UniVersity of California, Santa Barbara, California 93106-5090
ReceiVed September 3, 1996X
Abstract: Treatment of C70 with (2-methoxyethoxy)methyl azide gives rise to three out of a possible six triazoline
isomers, showing chemoselectivity as well as regioselectivity. The major product arose from addition of azide to
the double bond of C70 possessing the greatest local curvature. Selective thermolysis of the least stable triazoline,
the one resulting from addition to the least curved [6,6] double bond, allowed solution of the structure of the three
isomers. Thermolysis of the triazoline isomers produced mixtures of C70, azafulleroids, and fulleroaziridines.
Introduction
The chemistry of C60 has been extensively investigated.1 In
comparison, there have been far fewer studies on the function-
alization of C70.2-5 Cycloadditions to C70 present a greater
challenge than to C60; the lower symmetry of the former can,
in principle, give rise to a large number of isomers. C70 has
five distinct types of carbon atoms6 (Figure 1) and eight different
types of C-C bonds, four of which are [6,6] bonds. Similar to
C60, cycloaddition to C70 occurs across the [6,6] bond, giving
rise to a minimum of four possible isomers. The 1,3-dipolar
addition of azide could, in principle, give rise to a maximum
of six triazoline isomers, where addition across a [6,6] bond
between two different types of carbon atoms gives rise to a pair
of regioisomers3,7 (Scheme 1).
Figure 1. Five distinct sets of carbon atoms of C70 are labeled a-e.
findings led us to investigate the formation of [70] fullerotria-
zoline(s) and their conversion to [70]azafulleroid(s), and here
we report on the chemo- and regioselective addition of an alkyl
azide to C70.13
A number of cycloaddition reactions of C60 have been the
subject of intense investigation in our group over the past five
years. Of these, the addition of SEM8 and MEM azide9,10 have
been of particular interest because the latter led to the synthesis,
in bulk quantity, of the heterofullerenes (C59N)210 and C59HN.11
The reaction of alkyl azides with C60 leads to the formation of
a thermally labile triazoline which, upon loss of nitrogen, affords
an azafulleroid (cf. 5, 6, and 8). The [60]azafulleroids are
attractive because to date they are the only fullerene derivatives
which provide chemospecific cycloaddition reactions.9,12 These
Results and Discussion
Treatment of C70 with an excess of (2-methoxyethoxy)methyl
azide (MEM-N3) in o-dichlorobenzene (ODCB) at room tem-
perature for 48 h afforded, upon chromatography, one band
which contained a mixture of triazoline isomers 1, 2, and 3 (32%
yield, 60% based on consumed C70). Analysis of the mixture
by high-performance liquid chromatography (HPLC) showed
the apparent formation of only two isomers (62:38). However,
1H NMR spectroscopy revealed the presence of three com-
pounds, two with Cs symmetry and one with C1 symmetry
(Scheme 1); of these, it was later established that 2 and 3 were
inseparable by HPLC. The percentage of 3 obtained in the
product mixture was inversely proportional to the temperature
of the reaction. Interestingly, when a mixture of 1, 2, and 3
was heated to 55 °C overnight, 1H NMR spectroscopic analysis
revealed the complete disappearance of 3, with only 1, 2, 6,
and 7 remaining (cf. Scheme 3). HPLC analysis confirmed this
observation as well as revealing a small amount of C70 and trace
amounts of other products, too minute to characterize. The
fortunate, selective, thermal conversion of 3 allowed the
separation and complete characterization of both 1 and 2.
After separation of 1 and 2 by preparative HPLC, each was
subjected to thermolysis. A dilute toluene solution of each
isomer was heated to 80 °C until the disappearance of the
triazoline was observed by TLC (Scheme 2a,b). In each case
1 or 2 gave rise to three separable bands by column chroma-
X Abstract published in AdVance ACS Abstracts, January 15, 1997.
(1) Taylor, R. The Chemistry of Fullerenes; World Scientific: Singapore,
1995.
(2) Hawkins, J. M.; Meyer, A.; Solow, M. J. Am. Chem. Soc. 1993, 115,
7499-7500.
(3) Smith, A. B., III; Strongin, R.; Brard, L.; Furst, G.; Romanow, W.;
Owens, K.; Goldschmidt, R. J. Am. Chem. Soc. 1995, 117, 5492-5502
and references within.
(4) Meier, M.; Poplawska, M.; Compton, A.; Shaw, J.; Selegue, J.; Guarr,
T. J. Am. Chem. Soc. 1994, 116, 7044-7048.
(5) Seiler, P.; Herrmann, A.; Diederich, F. HelV. Chim. Acta 1995, 78,
344-354.
(6) Taylor, R.; Hare, J. P.; Abdul-Sada, A. K.; Kroto, H. W. J. Chem.
Soc., Chem. Commun. 1990, 1423.
(7) Hassner, A. J. J. Org.Chem. 1968, 33, 2684.
(8) Prato, M.; Chan Li, Q.; Wudl, F.; Lucchini, V. J. Am. Chem. Soc.
1993, 115, 1148-1150.
(9) Hummelen, J. C.; Prato, M.; Wudl, F. J. Am. Chem. Soc. 1995, 117,
7003-7004.
(10) Hummelen, J. C.; Knight, B.; Pavlovich, J.; Gonza´lez, R.; Wudl,
F. Science 1995, 269, 1554-1556.
(11) Keshavarz-K., M.; Gonza´lez, R.; Hicks, R.; Srdanov, G.; Srdanov,
V.; Collins, T.; Hummelen, J.; Bellavia-Lund, C.; Pavlovich, J. ; Wudl, F.;
Holczer, K. Nature 1996, 383, 147-150.
(13) While this work was in progress, a publication on the use of a [70]-
bisazafulleroid appeared (Nuber, B.; Hirsch, A. J. Chem. Soc., Chem.
Commun. 1996, 1421).
(12) Gro¨sser, T.; Prato, M.; Lucchini, V.; Hirsch, A.; Wudl, F. Angew.
Chem., Int. Ed. Engl. 1995, 34, 1345.
S0002-7863(96)03063-6 CCC: $14.00 © 1997 American Chemical Society

944 J. Am. Chem. Soc., Vol. 119, No. 5, 1997
BellaVia-Lund and Wudl
Scheme 1. Six Possible Triazoline Isomers as a Result of
Addition of MEM Azide to C70 and Their Point Group
Assigments
N-methylene protons. The 13C NMR spectrum contained a
minimum of 60 sp2 resonances between 155 and 128 ppm. The
1H NMR spectra of aziridines 4 and 7 were much alike; the
difference was that the spectrum of 4 had its resonances shifted
downfield relative to 7. This observation was in accord with
the theoretical deshielding effect in C70, which increases as
substituents get closer to the pole.4 Further structural evidence
was seen in the 13C NMR spectra, where aziridine 4 had two
distinctive sp3 resonances at 79.6 and 79.7 ppm and two sp2
resonances at 150.8 and 145.8 ppm with a relative intensity of
1 carbon each. Aziridine 7 had only one sp3 resonance at 73.5
ppm and four sp2 resonances at 151.9, 149.1, 142.2, and 146.9
ppm, respectively, with a relative intensity of 1 carbon. An
unusual feature of compound 7’s NMR spectra was that the
N-methylene proton resonance and the N-methylene carbon
resonance were considerably broader than those of the other
derivatives.
Analogous to C60 chemistry,12 azafulleroids can be produced
directly from C70 by treatment with 1.25 equiv of MEM-N3 in
refluxing ODCB (180 °C). Column chromatography of the
reaction mixture afforded two bands as the major products, after
separation from C70 and higher fullerene adducts. The two
fractions were subjected to HPLC analysis, where the first band
showed formation of two azafulleroids 5 and 6 (49:51) and the
second band consisted of aziridines 4 and 7 (60:40), respectively.
Azafulleroid 8 was not observed, in accordance with previously
reported diazomethane cycloaddition reactions of C70.3
Conclusion
At moderate temperatures the addition of (2-methoxyethoxy)-
methyl azide (MEM azide) to C70 gives rise to three triazoline
isomers, where addition occurs to a greater extent across the
[a,b] bond, the bond with greatest curvature, with preferential
formation of 1. Addition across the lower strain [c,c] bond also
occurs but to a lesser extent, producing 3. As a result and in
accord with theory,14 structures C1(d,c), C1(c,d), and Cs(e,e)
(Scheme 1) are not observed. Also, by the same principle,
azafulleroid 8 is not observed. At 55 °C triazoline 3, a
tography (silica gel, toluene): C70 (64%), azafulleroid 5 or 6
(23%), respectively, and aziridine 4 (5%). Since 1 and 2 gave
rise to the same aziridine, both isomers must arise from addition
across the same double bond and are regioisomers. Therefore,
1 and 2 must result from addition across the [a,b] bond since
their Cs symmetry has already been established. Differentiation
of 1 and 2 and characterization of 3 were accomplished by
thermolysis of a solution of 2 and 3 in the same manner as
described above (Scheme 3). The mixture of 2 and 3 afforded
two different aziridines (4 and 7). However, only one azaful-
leroid (6) was formed, indicating that the MEM-bearing nitrogen
atom of the nitrogen triad of 2 and 3 is attached to the same
carbon atom in both compounds.
Thin layer chromatography Rf values and UV/vis absorption
were used initially to differentiate azafulleroids from aziridines.8
An overlapped spectrum of 4, 5, and C70 suggested that 5 was
an azafulleroid, having a UV/vis absorption pattern nearly
identical with that of C70, with only subtle differences. In
contrast, the absorption maxima in 4 had a slight hypsochromic
shift relative to C70.
chemoisomer of regioisomers 1 and 2, was readily and
selectively transformed to azafulleroid 6 and aziridine 7 as well
as a small quantity of C70, showing the varied stability of the
isomers. In comparing the thermal stability of 1, 2, and 3, the
reduced stability of 3 is very likely due to the higher strain in
its polar region; i.e., 1 and 2 have a lower total strain energy
than 3 because, in the former, two of the polar carbon atoms
are pyramidal and introduction of sp3 carbons in the poles is
expected to alleviate some of the polar strain of the C70 cluster.
At the higher temperature of 180 °C, we observed that the
preference of addition across the bond containing the greatest
local curvature (a-b) (reflected in formation of 4), as opposed
to addition across the bond with the shortest bond order (c-c)
(reflected in formation of 7), decreased from 4:1 to 3:2. This
result reflects that (1) at this temperature the MEM azide also
Fortunately the characterization of these labile C70 derivatives
did not turn out to be as difficult as we assumed, because the
1
simplicity of the H NMR spectra in combination with low-
temperature pyrolysis (see Schemes 2 and 3) allowed complete
characterization of the three triazolines observed.
The following set of spectroscopic experiments confirms the
1
above assignments. A combination of H and 13C NMR of 5
was indicative of an azafulleroid with Cs symmetry, whereas
azafulleroid 6 showed a 1H NMR spectrum containing a doublet
of doublets (AB pattern) at 4.92 ppm, corresponding to the
(14) Henderson, C. C.; Rohlfing, C. M.; Gillen, K. T.; Cahill, P. A.
Science 1994, 264, 397.

Synthesis of [70]Azafulleroids
J. Am. Chem. Soc., Vol. 119, No. 5, 1997 945
Scheme 2. Thermolysis of MEM-Triazolinesa
a (a) Thermolysis of 1. b Thermolysis of 2.
Scheme 3. Thermolysis of MEM-Triazoline Isomer 3
149.2, 148.9, 148.6, 148.5, 148.0, 147.3, 147.2, 147.1 (1C), 146.9,
146.1, 146,0, 143.9, 143.7, 143.5, 143.2, 140.6, 140.3, 140.0, 138.0,
133.6, 133.4, 131.9, 131.2, 101.4 (bridgehead C), 78.0, 74.0 (bridgehead
C), 72.2, 68.0, 59.0 ppm; -FAB-MS (chloroform/NBA) m/z 971 (M)+,
840 (C70)+. 2: UV/vis (toluene) λmax 534, 462, 400, 382, 338, 312,
286 nm; IR (DRIFT) 2976 (m), 2904 (m), 1453 (m), 1444 (m), 1437
(s), 1374 (w), 1153 (m), 1120 (m), 1092 (m), 1070 (m), 1050 (m), 820
(w), 806 (w), 777 (w), 674 (w), 640 (w), 579 (w), 536 (w), 469 (w),
decomposes to MEM nitrene, which is expected to exhibit lower
selectivity due its higher energy content, and (2) at more elevated
temperature the selectivity of the 1,3-dipolar addition is also
expected to decrease. This study is the first step toward a full
understanding of addition of azide to C70 and allows for easy
characterization of the subsequent transformation to azafulle-
roids. The addition of MEM azide to C70 shows a wider range
of selectivity than previously reported reactions of C70 such as
osmylation2 and addition of diazomethane.3
1
431 (w), 420 (m) cm-1; H NMR (CS2/D2O, 500 MHz) 5.51 (s, 2H),
3.66 (t, J ) 4.8 Hz, 2H), 3.39 (t, J ) 4.8 Hz, 2H), 3.18 (s, 3H) ppm;
13C NMR (CS2/D2O, 125 MHz) 155.1, 151.0, 150.7 (1C), 150.4, 150.3,
150.2, 150.1, 149.4, 148.9, 148.7, 148.2, 148.1, 147.3, 147.0, 146.9,
146.8, 146.7 (1C), 146.4, 145.6, 145.5, 143.6, 143.2, 143.1, 142.9,
140.1, 139.8, 139.4, 137.1, 133.2, 132.2, 131.6, 130.9, 130.8, 101.1
(bridgehead C), 77.2, 73.3 (bridgehead C), 71.6, 67.3, 58.4 ppm; -FAB-
MS (chloroform/NBA), m/z 971 (M)+, 840 (C70)+.
Experimental Section
[6,6]-(1-N-MEM-triazoline)-C70 1 and 2. To a degassed solution
of C70 (150 mg, 0.18 mmol) in HPLC grade ODCB (10 mL) was added
MEM-N3 (330 mg, 2.5 mmol, 14 equiv). The resulting mixture was
stirred at 50 °C for 48 h under nitrogen. The crude product was
chromatographed over a silica gel column (toluene as eluant) and
concentrated to afford 67 mg of unreacted C70 (45%) and 55 mg of a
7:3 mixture of 1 and 2 (32%; 60% based on consumed C70). The
isomers were separated by HPLC (semipreparative Cosmosil Buckyprep
column, UV/326 nm, 50% CHCl3, 50% hexane, 5 mL/min; retention
times 21.82 and 23.82 min, respectively). 1: UV/vis (toluene) λmax
532, 462, 400, 340, 312, 286 nm; IR (DRIFT) 2912 (m), 2860 (m),
2816 (w), 1501 (m), 1457 (s), 1359 (w), 1258 (m), 1215 (w), 1143
(m), 1112 (s), 1070 (s), 1042 (s), 941 (w), 876 (w), 811 (m), 778 (w),
709 (m), 697 (m), 641 (m), 577 (m), 548 (w), 467 (w) cm-1; 1H NMR
(CS2/D2O, 500 MHz) 6.02 (s, 2H), 3.89 (t, J ) 4.8 Hz, 2H), 3.63 (t, J
) 4.8 Hz, 2H), 3.39 (s, 3H) ppm; 13C NMR (CS2/D2O, 125 MHz) 153.9,
151.3, 151.1 (1C), 150.8, 150.7, 150.5, 150.4, 150.0, 149.6, 149.4,
A General Procedure for the Decomposition of N-MEM-fulle-
rotriazoline to N-MEM-azafulleroid and N-MEM-fulleroaziridine.
A dilute, degassed solution of triazoline 1 (10 mg, 10.3 mmol) in toluene
(30 mL) was heated overnight at 80 °C under nitrogen with stirring.
The disappearance of triazoline and the concomitant formation of three
products, C70 (5 mg, 64%), fulleroaziridine 4 (0.4 mg, 5%), and
azafulleroid 5 (2 mg, 23%), were detected by TLC (toluene; silica gel).
The fractions were collected after chromatography (toluene, silica gel),
concentrated, and precipitated with hexane. The precipitate was
centrifuged, washed with hexane (3 × 10 mL), and dried in Vacuo.
[6,6]-(1-N-MEM-fulleroaziridine)-C70 4: UV/vis (toluene) λmax 574,
464, 378, 330, 286 nm; IR (DRIFT) 2948 (m), 2890 (m), 1477 (s),
1383 (m), 1315 (w), 1154 (s), 1119 (s), 1025 (w), 999 (w), 807 (m),

946 J. Am. Chem. Soc., Vol. 119, No. 5, 1997
BellaVia-Lund and Wudl
681 (w), 678 (w), 675 (m), 644 (w), 589 (m), 571 (w), 559 (w), 543
2H), 3.35 (s, 3H) ppm; 13C NMR (CS2/D2O, 125 MHz) 154.2, 150.0,
149.6, 149.5, 149.2, 149.1, 148.7, 147.6, 147.58, 147.4, 147.35, 147.2,
147.1, 147.0, 146.9, 146.8, 146.7, 146.5, 146.4, 146.3, 146.26, 146.2,
145.7, 145.6, 145.2, 144.9, 144.0, 143.5, 143.4, 143.2, 143.1, 143.0,
142.6, 142.5, 142.3, 142.1, 141.7, 141.5, 140.7, 140.3, 139.9, 139.3,
138.6, 135.5, 134.5, 134.4, 134.3, 132.6, 130.9, 130.0, 129.6, 129.2,
128.8, 128.0, 127.6, 127.3, 126.7, 126.6, 125.5, 124.4, 123.2, 121.5,
82.7, 71.3, 67.3, 57.8 ppm; -FAB-MS (chloroform/NBA) m/z 943-
(m)+, 840 (C70)+. [6,6]-(1-N-MEM-fulleroaziridine)-C70 7: UV/vis
(toluene) λmax 450, 376, 316, 288 nm; IR (DRIFT) 2959 (m), 2920
(m), 2893 (m), 1462 (s), 1386 (w), 1156 (s), 1119 (m), 1092 (w), 1015
(w), 803 (m), 687 (w), 680 (w), 672 (m), 651 (m), 586 (m), 564 (w),
1
(w), 538 (m), 453 (w) cm-1; H NMR (CS2/D2O, 500 MHz) 4.76 (s,
2H), 4.16 (t, J ) 4.8 Hz, 2H), 3.68 (t, J ) 4.8 Hz, 2H), 3.40 (s, 3H)
ppm; 13C NMR (CS2/D2O, 125 MHz): 153.9, 150.8 (1C), 150.7, 150.7,
150.1, 150.1, 148.9, 148.6, 148.6, 148.1, 148.0, 147.9, 147.5, 147.5,
147.1, 147.0, 146.6, 145.8 (1C), 145.6, 145.3, 143.7, 143.6, 143.4,
143.4, 143.0, 142.7, 142.2, 141.8, 140.9, 136.8, 132.9, 131.9, 130.5,
130.5, 130.4, 130.2, 127.2, 82.3, 79.7 (bridgehead C), 79.6 (bridgehead
C), 72.1, 69.2, 58.6 ppm; -FAB-MS (chloroform/NBA), m/z 943 (M)+,
841 (C70)+. [5,6]-(1-N-MEM-azafulleroid)-C70 5: UV/vis (toluene)
λmax 468, 392, 336, 288 nm; IR (DRIFT) 2969 (m), 2928 (m), 2889
(w), 1598 (w), 1578 (w), 1426 (m), 1446 (s), 1325 (w), 1244 (w), 1125
(s), 816 (m), 687 (m), 583 (m),554 (m), 504 (m) cm-1; 1H NMR (CS2/
D2O, 500 MHz) 5.27 (s, 2H), 4.29 (t, J ) 4.8 Hz, 2H), 3.77 (t, J ) 4.8
Hz, 2H), 3.45 (s, 3H) ppm; 13C NMR (CS2/D2O, 125 MHz) 151.8,
150.4, 149.8, 148.5, 148.3 (1C), 148.0, 147.8, 147.7, 147.5, 147.2,
147.0, 146.8, 146.0, 145.7, 144.8, 144.3, 144.1, 144.0, 143.9, 143.5,
143.3, 143.2, 142.7, 141.6, 139.6, 139.5, 136.5 (1C), 135.3, 134.8 (1C),
132.2, 132.2, 131.2, 130.9, 128.1, 128.0 (1C), 127.9, 84.8, 72.2, 68.4,
58.6 ppm; -FAB-MS (chloroform/NBA) m/z 943 (M)+, 840 (C70)+.
[5,6]-(1-N-MEM-azafulleroid)-C70 6: UV/vis (toluene) λmax 468, 384,
340, 286 nm; IR (DRIFT) 2945 (m), 2890 (m), 1557 (w), 1442 (s),
1397 (w), 1108 (m), 809 (w), 688 (m), 650 (w), 631 (m), 581 (m), 561
(m), 531 (w), 459 (m) cm-1; 1H NMR (CS2/D2O, 500 MHz) 4.95 (2H,
AB, δA ) 4.97, δB ) 4.92, JAB ) 9.5 Hz), 4.02 (2H; dt, AB(M2), δA)
4.02, δB ) 4.01, JAB ) 2 Hz, JAM ) JBM ) 5 Hz), 3.61 (t, J ) 5 Hz,
1
529 (w), 456 (w) cm-1; H NMR (CS2/D2O, 500 MHz) 4.69 (s, 2H),
4.11 (t, J ) 4.6 Hz, 2H), 3.70 (t, J ) 4.6 Hz, 2H), 3.44 (s, 3H) ppm;
13C NMR (CS2/D2O, 125 MHz) 151.9 (1C), 150.8, 149.4, 149.1 (1C),
148.5, 148.4, 148.2, 147.9, 147.8, 147.7, 147.1, 146.92 (1C), 146.86,
146.6, 146.6, 145.7, 145.5, 145.1, 144.6, 144.3, 144.1, 143.4, 142.2
(1C), 131.7, 131.4, 131.4, 130.8, 82.7, 73.5 (bridgehead C), 71.9, 69.0,
58.5 ppm; -FAB-MS (chloroform/NBA), m/z 943 (M)+, 840 (C70)+.
Acknowledgment. We thank the National Science Founda-
tion for support through the MRSEC and Grants DMR-9122536
and DMR-9500888.
JA963063J