Synthesis of C60-fused tetrahydrothiophene derivatives via nucleophilic cycloaddition of thiocyanates

Article abstract of DOI:10.1039/b807394e

Nucleophilic cycloaddition of thiocyanates 1a-e with C₆₀ in the presence of 1,8-diazabicyclo[5.4.0]undec-7-ene afforded C₆₀-fused 2-iminotetrahydrothiophene derivatives 2a-e and methanofullerenes 3a-d. The product distributions were highly sensitive to the substrates employed. The 2-iminotetrahydrothiophene derivatives 2a-e could be further manipulated by hydrolysis and acetylation to give 2-oxotetrahydrothiophene derivatives 4a-e and 2-acetamidotetrahydrothiophene derivatives 5a-e. A possible reaction mechanism for the formation of products 2a-e and 3a-d was proposed. The Royal Society of Chemistry.

Full text of DOI:10.1039/b807394e

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www.rsc.org/obc | Organic & Biomolecular Chemistry
Synthesis of C60-fused tetrahydrothiophene derivatives via nucleophilic
cycloaddition of thiocyanates†
Guan-Wu Wang,*a,b Jia-Xing Lia and Yu Xua
Received 1st May 2008, Accepted 2nd June 2008
First published as an Advance Article on the web 26th June 2008
DOI: 10.1039/b807394e
Nucleophilic cycloaddition of thiocyanates 1a–e with C60 in the presence of
1,8-diazabicyclo[5.4.0]undec-7-ene afforded C60-fused 2-iminotetrahydrothiophene derivatives 2a–e and
methanofullerenes 3a–d. The product distributions were highly sensitive to the substrates employed.
The 2-iminotetrahydrothiophene derivatives 2a–e could be further manipulated by hydrolysis and
acetylation to give 2-oxotetrahydrothiophene derivatives 4a–e and 2-acetamidotetrahydrothiophene
derivatives 5a–e. A possible reaction mechanism for the formation of products 2a–e and 3a–d was
proposed.
Introduction
Results and discussion
Since fullerenes became available in a macroscopic amount
in 1990, various effective methods1 have been developed to
synthesize an impressive number of fullerene derivatives, which
may find application in biology and material science.2 Among
them sulfur-containing fullerene derivatives demonstrate inter-
esting electronic properties.3 Fullerene diads and triads bear-
ing oligothiophene/polythiophene4 and tetrathiofulvalene/p-
extended tetrathiofulvalene5 have been intensively studied. Sul-
fur reagents such as sulfones,4b,6 sultines,7 stabilized sulfonium
ylides,8 a,b-unsaturated thiocarbonyl compounds,9 o-thioquinone
methide,10 a thiocarbonyl ylide,11 heterocyclic masked 1,3-dipoles
5-imino-1,2,4-thiadiazolidine-3-ones,12 disulfides,13 SO3,14 H2S15
and sulfur itself16 have been employed in fullerene chemistry.
Recently a new protocol for the synthesis of sulfur-containing
dihydro-pyrrolo C60 derivatives was reported by Elemes and co-
workers,17 utilizing the 1,3-dipolar cycloaddition reaction of C60
with sulfide-bearing imines of glycine esters. We synthesized sulfur-
containing C60 derivatives by the reaction of C60 with CS2 and
amino acid ester hydrochlorides in the presence of triethylamine
(Et3N).18 Isothiocyanates generated in situ from amino acid ester
hydrochlorides and CS2 in the presence of Et3N were believed to
undergo subsequent nucleophilic cycloaddition with C60. However,
the reaction of thiocyanates with C60 has not been reported until
now. In continuation of our interest in fullerene chemistry,18,19
herein we disclose the nucleophilic cycloaddition of thiocyanates
with C60, affording C60-fused 2-iminotetrahydrothiophene
derivatives, and further transformations to other fullerene
derivatives.
Thiocyanates have been used in the synthesis of thiazole
derivatives.20 However, reactions employing the a-carbon of these
compounds as a nucleophilic site are really rare.21 We thus
investigated the feasibility of the nucleophilic attack of the a-
carbon of thiocyanates on C60 under basic conditions.
We chose the reaction of C60 with 1-(4-methoxyphenyl)-2-
thiocyanatoethanone (1a) as a model reaction to optimize the
reaction conditions (Scheme 1).
By using triethylamine, pyridine, 4-dimethylaminopyridine
(DMAP) or triethylenediamine (DABCO) as a base, the reaction
afforded products 2a and 3a (Scheme 1) in very low yields with
over 90% recovered C60 (entries 1–4, Table 1). To our delight, the
reaction proceeded rapidly to give products 2a and 3a in 65% total
yield in 10 min when 1 equiv. of 1,8-diazabicyclo[5.4.0]undec-7-ene
(DBU) was used. Product 3a was obtained as the major product
with a yield of 44%, being more than twice that (21%) of product
2a (entry 5, Table 1). The structures of products 2a and 3a were
assigned as a C60-fused 2-iminotetrahydrothiophene derivative and
a methanofullerene derivative, respectively (vide infra). Reducing
the amount of DBU to 0.5 equiv., the total yield of the two
products was almost the same with little change in the product
distribution (entry 6, Table 1). Interestingly, further reducing the
quantity of DBU to 0.2 equiv. led to the preferred formation of
adduct 2a (entry 7, Table 1). The product ratio of adduct 2a vs.
adduct 3a increased to nearly 3 : 1 when 0.1 equiv. of DBU was
employed (entry 8, Table 1). Nevertheless, an unsatisfactory result
was obtained in terms of total yield and reaction time when 0.05
equiv. of DBU was applied (entry 9, Table 1). The above data
demonstrated that the major product (adduct 2a vs. adduct 3a)
could be altered by adjusting the amount of DBU used.
Being interested in the product type of adduct 2a, we extended
the reaction shown in Scheme 1 by replacing substrate 1a
with various types of thiocyanate and using the appropriate
amount of DBU. Other 1-aryl-2-thiocyanatoethanones (1b and
1c), thiocyanatoacetate (1d) and thiocyanatomalonate (1e) were
explored for their reactions with C60 in the presence of DBU. It
was found that the reaction of C60 with thiocyanates 1b–e gave
aHefei National Laboratory for Physical Sciences at Microscale and
Department of Chemistry, University of Science and Technology of China,
Hefei, Anhui 230026, P. R. China. E-mail: gwang@ustc.edu.cn
bState Key Laboratory of Applied Organic Chemistry, Lanzhou University,
Lanzhou, Gansu 730000, P. R. China
† Electronic supplementary information (ESI) available: Spectral data of
products 2a–e, 3c, 4a–e and 5a–e, and NMR spectra of representative
products 2a, 3c, 4a and 5a. For ESI see DOI: 10.1039/b807394e
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Scheme 1
Table 1 Product yields and recovered C60 for the reaction of C60 with 1a under different reaction conditions a
Entry
Base
Equiv. of base
Time
Yield of 2a b
Yield of 3a b
Recovered C60
1
2
3
4
5
6
7
8
9
Et3N
1
1
1
1
24 h
24 h
24 h
24 h
10 min
1 h
4 h
4 h
12 h
2%
2%
2%
3%
2%
3%
91%
94%
90%
91%
19%
17%
25%
49%
74%
Pyridine
DMAP
DABCO
DBU
DBU
DBU
2%
3%
1
21%
25%
34%
31%
13%
44%
39%
21%
11%
4%
0.5
0.2
0.1
0.05
DBU
DBU
a 36.0 mg (0.05 mmol) of C60, 20.7 mg (0.1 mmol) of 1a were used, the reaction was conducted at room temperature. b Isolated yield.
both tetrahydrothiophene derivatives 2b–e and methanofullerene
derivatives 3b–d except for 1e, which afforded 2e exclusively
(Scheme 2).
was employed, tetrahydrothiophene derivative 2e was obtained
selectively.
The structures of products 2a–e were fully established by their
1
MS, H NMR, 13C NMR, FT-IR and UV–vis spectral data. It
should be noted that the very low solubility of 2c precluded its
13C NMR measurement. The ESI mass spectra of 2a–e showed
1
the correct molecular ion peaks. The H NMR spectra of 2a–
=
e displayed a broad singlet at 10.54–11.33 ppm for the NH
group. In the 13C NMR spectra of 2a, 2b, 2d and 2e, the imine
carbon and the two sp3-carbons of the C60 cage appeared at 173.12–
178.52 ppm, 76.60–79.91 ppm and 69.74–71.94 ppm, respectively.
More than forty-nine peaks for 2a, 2b and 2d and twenty-nine
peaks for 2e were observed in the range 153.46–132.01 ppm,
consistent with the C1 (2a, 2b and 2d) and Cs (2e) symmetry of their
molecular structures. The IR spectra of 2a–e showed absorptions
Scheme 2
The yields of products 2a–e and 3a–d, along with recovered C60,
for the reaction of C60 with thiocyanates 1a–e are listed in Table 2.
As seen from Table 2, when R = H and Rꢀ = Ar, the yields of
products 2a–c were more than 30%, while the yields of products
3a–c were less than 12%, and the electronic properties of the
substituent on the phenyl ring of the Rꢀ group had little effect on
the product yields. However, when R = H and Rꢀ = CO2C(CH3)3
(1d), product 2d was obtained in a lower yield (13%), while the
Bingel-type adduct 3d was isolated in a slightly higher yield
(18%). To our surprise, when diethyl 2-thiocyanatomalonate (1e)
at 3272–3278 cm−1 and 1622–1626 cm−1 due to the C NH group.
=
The UV–vis spectra of 2a–e exhibited a peak at 428–429 nm,
which is a diagnostic absorption for a cycloadduct of C60 at the
[6,6]-junction.
Products 3a, 3b and 3d are known compounds, and their
identities were confirmed by comparison of their spectral data
with those reported in the literature.8a Product 3c was fully
characterized by its MS, 1H NMR, 13C NMR, FT-IR and UV–vis
spectral data. The MS (+ESI) of 3c showed its molecular ion peak
at m/z 872. Its 1H NMR spectrum exhibited a singlet at 5.54 ppm
for the methine proton and two doublets at 8.37 and 7.62 ppm for
the aromatic protons. In the 13C NMR spectrum of 3c, the peaks
at 187.49 and 43.75 ppm were assigned to the carbonyl carbon
and methine carbon, the two sp3-carbons of the C60 cage appeared
at 71.89 ppm, and twenty-eight peaks including overlapped ones
were observed in the range 147.57–129.52 ppm due to the fifty-
eight sp2-carbons of the C60 skeleton and six aromatic carbons,
consistent with the Cs symmetry of its molecular structure.
Table 2 The yields of products 2a–e and 3a–d for the reaction of C60 with
1a–e in the presence of DBU at room temperaturea
Substrate
Time (h)
Yield of 2 b
Yield of 3b
Recovered C60
1a
1b
1c
4
4
4
12
12
31%
31%
30%
13%
32%
11%
11%
12%
18%
0
49%
47%
50%
60%
64%
1dc
1ed
a Unless specified, the molar ratio of C60 : 1 : DBU = 1 : 2 : 0.1. b Isolated
=
Just as the C S bond of the thioamide moiety in our previously
yield. c Molar ratio of C60 : 1d : DBU = 1 : 2 : 0.2. d Molar ratio of C60
1e : DBU = 1 : 2 : 1.
:
synthesized fullerene derivative was moisture sensitive,18 the C N
=
bond of the thioimidate unit in products 2a–e was prone to
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hydrolysis. C60-fused 2-iminotetrahydrothiophene derivatives 2a–
e could be converted to C60-fused 2-oxotetrahydrothiophene
derivatives 4a–e in excellent yields in a mixture of CS2–THF–
H2O in the presence of p-toluenesulfonic acid (PTSA) at room
temperature (Scheme 3). The isolated yields for products 4a, 4b,
4c, 4d and 4e were 98%, 95%, 96%, 92% and 93%, respectively.
of the low-field sp3-carbon and the upfield shift of the high-field
sp3-carbon of the C60 skeleton.
A possible reaction mechanism for the formation of products
2a–e and 3a–d is shown in Scheme 5. The deprotonation of
thiocyanate 1 by DBU generates carbanion 6, which attacks C60
to yield intermediate 7. The resulting fullerenyl anion 7 could go
through two pathways: in path a, the fullerenyl carbanion attacks
≡
the C N bond of the SCN group to afford the five-membered
heterocyclic anion 8, which is then protonated to give the final
product 2; in path b, intramolecular nucleophilic addition of the
fullerenyl carbanion to the carbon connected with the SCN group
affords the Bingel-type adduct 3, accompanied by extrusion of the
SCN− anion.
Scheme 3
To demonstrate the utilization of 2-iminotetrahydrothiophene
derivatives 2a–e, the acetylation of the imine group was suc-
cessfully exploited to generate C60-fused 2-acetamidotetrahydro-
thiophene derivatives 5a–e by the reaction of 2a–e with acetyl
chloride in the presence of pyridine (Scheme 4). The isolated yields
for products 5a, 5b, 5c, 5d and 5e were 94%, 95%, 95%, 92% and
92%, respectively.
Scheme 5
Conclusions
A novel procedure for the synthesis of tetrahydrothiophene-
fused fullerene derivatives through nucleophilic cycloaddition of
thiocyanates with C60 has been established for the first time. The
current protocol provides easy access to tetrahydrothiophene-
fused fullerene derivatives that may be of biological importance,
and are difficult to synthesize by other methods.
Scheme 4
The solubility of the resulting hydrolyzed products 4a–e and
N-acetylated products 5a–e in CS2 and CDCl3 was improved
dramatically over that of adducts 2a–e, facilitating their NMR
measurements.
Experimental
Procedure for the synthesis of compounds 2a–e and 3a–d by the
reaction of C60 with thiocyanates 1a–e
Compounds 4a–e and 5a–e were also fully characterized by
their MS, 1H NMR, 13C NMR, FT-IR and UV–vis spectral data.
Compared with compounds 2a–e, products 4a–e and 5a–e lacked
A mixture of C60 (36.0 mg, 0.05 mmol), thiocyanate 1a (1b, 1c,
1d, or 1e) and DBU in the desired amounts was dissolved in
toluene (25 mL) and stirred at room temperature. The reaction
was monitored by TLC and stopped at the designated time. After
evaporation of the toluene, the residue was separated on a silica
gel column with CS2, then CS2–ethyl acetate as the eluent, to give
unreacted C60, product 2a (2b, 2c, 2d, or 2e) and product 3a8a (3b8a,
3c, or 3d8a), respectively. The yields, along with the recovered C60,
are listed in Table 2.
the absorptions at 3272–3278 cm−1 in their IR spectra and the
1
=
broad singlet in their H NMR spectra for the NH group. A new
peak at 2.52–2.55 ppm for the methyl group was observed in the
1H NMR spectra of 5a–e. In the 13C NMR spectra of 4a–e, the
=
carbonyl carbon derived from the hydrolysis of the C NH group
appeared at 191.72–194.60 ppm. In the 13C NMR spectra of 5a–e,
besides the corresponding peaks of 2a–e, new peaks were found at
182.09–185.00 ppm and 24.80–25.95 ppm due to the acetyl group.
It is interesting to note that the chemical shift difference of the two
sp3-carbons of the C60 cage in compounds 4a–e (67.14–69.77 and
79.53–83.27 ppm) and 5a–e (68.65–71.04 and 79.01–82.31 ppm)
increased to 11.72–13.50 ppm and 10.06–11.27 ppm, respectively,
from 6.64–8.08 ppm for products 2a–e, due to the downfield shift
Preparation of compounds 4a–e by the hydrolysis of compounds
2a-e
Compound 2a (or 2b–e, 0.02 mmol) was added to a mixture of
CS2–THF–H2O (15 mL, 10 : 1 : 0.1) and p-toluenesulfonic acid
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(0.04 mmol) and the reaction mixture was vigorously stirred at
room temperature for 12 h. After evaporation of the solvent, the
residue was separated on a silica gel column with CS–ethyl acetate
as the eluent to give product 4a (or 4b–e).
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Preparation of compounds 5a–e by the acetylation of compounds
2a–e
A mixture of 2a (or 2b–e, 0.02 mmol) and pyridine (0.08 mmol)
was dissolved in CS2 (15 mL), then to the solution was added
acetyl chloride (0.08 mmol) and the reaction was stirred at room
temperature for 1 h. After evaporation of excess CS2, the residue
was separated on a silica gel column with CS2–ethyl acetate as the
eluent to give product 5a (or 5b–e).
Acknowledgements
The authors are grateful for the financial support of the Na-
tional Natural Science Foundation of China (nos. 20572105
and 20621061) and National Basic Research Program of China
(2006CB922003).
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