Unexpected De-arylation of a pentaaryl fullerene

Article abstract of DOI:10.1021/ol900224w

A triphenylamine-derived pentaaryl fullerene undergoes an unusual oxidative dearylation under basic conditions to give tetraarylated epoxy fullerene in high yield. The structure of the product was confirmed by single crystal X-ray diffraction. A mechanism is proposed to account for the loss of the addend and the subsequent formation of the epoxide group.

Full text of DOI:10.1021/ol900224w

ORGANIC
LETTERS
2
009
Vol. 11, No. 6
389-1391
Unexpected De-Arylation of a Pentaaryl
Fullerene
1
Simon Clavaguera, Saeed I. Khan, and Yves Rubin*
Department of Chemistry and Biochemistry, UniVersity of California, Los Angeles,
California 90095-1569
rubin@chem.ucla.edu
Received February 2, 2009
ABSTRACT
A triphenylamine-derived pentaaryl fullerene undergoes an unusual oxidative dearylation under basic conditions to give tetraarylated epoxy
fullerene in high yield. The structure of the product was confirmed by single crystal X-ray diffraction. A mechanism is proposed to account
for the loss of the addend and the subsequent formation of the epoxide group.
We have been studying novel approaches to promote greater
control over the morphology of the bicontinuous conducting
polymer-fullerene network in bulk heterojunction photovol-
Pentafunctionalized fullerenes are interesting targets be-
cause their molecular structure (badminton shuttlecock-
shaped) can promote self-assembly into one-dimensional
1
1,6
taic devices. In the course of our syntheses of a series of
stacks. They are formed by the highly regioselective and
pentaarylated fullerene derivatives (shuttlecocks) inspired
from Nakamura’s work, we encountered the unexpected
efficient addition of organocopper reagents, prepared by
transmetalation between a Grignard reagent and CuBr·SMe ,
2
2
dearylation reaction of pentaadduct 1 bearing five tripheny-
lamine addends, which afforded epoxy tetraaryl fullerene 2.
While this tetraadduct epoxide pattern is commonly encoun-
to the fullerene C60. Additionally, electron-donating aromatic
addends such as triphenylamine (TPA) have been widely
used in molecular dyads and triads involving C60 to form
light energy harvesting systems or molecular electronic
3
,4
tered in secondary amine and peroxide additions to C60
,
7
,8
devices.
this is the first time that such an adduct is formed by
5
The initial goal of this work was to investigate the
influence of a hole transport molecule on the photovoltaic
properties of a pentafunctionalized fullerene. It has been
oxidative loss of one addend from a pentaadduct.
(
1) Kennedy, R. D.; Ayzner, A. L.; Wanger, D. D.; Day, C. T; Halim,
M.; Khan, S. I.; Tolbert, S.; Schwarz, B.; Rubin, Y. J. Am. Chem. Soc.
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2) Sawamura, M.; Kawai, K.; Matsuo, Y.; Kanie, K.; Kato, T.;
Nakamura, E. Nature 2002, 419, 702–705.
3) (a) Isobe, H.; Nakanishi, W.; Tomita, N.; Jinno, S.; Okayama, H.;
2
(5) For a recent study of a related phenylcarbazole adduct, see: (a) Zhang,
(
X.; Matsuo, Y.; Nakamura, E. Org. Lett. 2008, 10, 4145–4147.
(6) Matsuo, Y.; Nakamura, E. Chem. ReV. 2008, 108, 3016–3028
.
(
(7) (a) Williams, R. M.; Zwier, J. M.; Verhoeven, J. W. J. Am. Chem.
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T.; Nakanishi, W.; Lemi e` gre, L.; Nakamura, E. J. Org. Chem. 2005, 70,
I. Chem. ReV. 1998, 98, 2527–2548
.
4
3
826–4832. (c) Isobe, H.; Tomita, N.; Nakamura, Org. Lett. 2000, 2, 3663–
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Chem. Lett. 2007, 36, 20–21
.
(
4) (a) Jia, Z.; Zhang, X.; Zhang, G.; Huang, S.; Fang, H.; Hu, X.; Li,
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.
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10.1021/ol900224w CCC: $40.75  2009 American Chemical Society
Published on Web 02/26/2009

Scheme 1. Synthesis of Pentaadduct 1 and Subsequent
De-Arylation Providing Tetraadduct Epoxide 2
shown that the addition of small hole-transporting molecules,
like TPA, to bulk heterojunction solar cells based on
conjugated polymers and fullerenes results in an increased
9
power conversion efficiency.
Compound 1 was prepared by Grignard reagent formation
from 4-bromo-N,N-diphenylaniline, which was transmeta-
2
lated in situ with CuBr·SMe and added to C60 (Scheme 1).
For this particular reaction, the quenching step is critical and
has to be performed with degassed solutions under argon.
We observed an unusually facile tendency for 1 to convert
to epoxide 2 during the workup process if the quenchings
were not rigorously done under inert atmosphere.
Figure 1. (a) X-ray structure of 2 showing dimeric pairs with tight
C···O contacts. Hydrogen and included solvent are omitted. (b)
Packing diagram of 2 viewed down the c axis showing the
3
pseudohexagonal arrangement of fullerenes with C -symmetric
helical arrangement of 2 at the vertices of the pseudohexagon.
In a controlled synthesis of epoxide 2, deprotonation of
the cyclopentadiene hydrogen of compound 1 with potassium
tert-butoxide resulted in a color change of the reaction
mixture from red orange to dark-red for the corresponding
12,13
with excess MeLi, followed by hydrolysis.
treatment of the dodecaadduct C60Me
followed by exposure to molecular oxygen afforded the
penta-oxygenated product C60Me (OH)Ph O(OH) as con-
Furthermore,
5
HPh H with KH
5
O
5 2
5
1
4
anion. Reaction with O
symmetric epoxide 2 as a single product in 91% yield after
purification on silica gel (CS /CH Cl 9:1). The oxidation
2 s
in the absence of light gave the C -
firmed by an X-ray structure.
Red-colored cubic crystals of 2 suitable for single crystal
X-ray diffraction analysis were obtained by slow diffusion
of cyclohexane into a THF solution of 2 containing also 5%
of trifluoroacetic acid, which was necessary to induce clean
crystallization. The crystals belong to the hexagonal crystal
2
2
2
of 1 cleanly gives epoxy[60]fullerene 2, without benzo[b]-
furan products nor polyoxygenated derivatives, unlike the
exposure of C60Ph
fluorophenyl derivative to air.
5 5
Cl, C60Ph H, or the corresponding p-
10,11
15
Additional epoxidation of
system and the space group of R 3j . The unit cell comprises
a double bond in the central cyclopentadiene unit can also
lead to several polyoxygenated products. The propensity of
the cyclopentadiene system to oxidize is illustrated with the
nine pairs of molecules stacked in a head-to-head manner
forming dimers related by a center of inversion (Figure 1).
The closest intermolecular contact between the two
partners of a dimeric pair is 2.893 Å for the O1-C82′
X-ray crystal structure of C
s
-symmetric bisepoxyfullerene
C60Me
5
O
2
OH, which was produced in the reaction of C60Cl
6
(
12) Al-Matar, H.; Hitchcock, P. B.; Avent, A. G.; Taylor, R. Chem.
Commun. 2000, 1071–1072
.
(
9) (a) Yang, C. H.; Qiao, J.; Sun, Q. J.; Jiang, K. J.; Li, Y. L.; Li,
(13) Al-Matar, H.; Abdul-Sada, A. K.; Avent, A. G.; Fowler, P. W.;
Y. F. Synth. Met. 2003, 137, 1521–1522. (b) Hendrickx, E.; Kippelen, B.;
Thayumanavan, S.; Marder, S. R.; Persoons, A.; Peyghambarian, N. J. Chem.
Phys. 2000, 112, 9557–9561.
Hitchcock, P. B.; Rogers, K. M.; Taylor, R. J. Chem. Soc., Perkin Trans.
2 2002, 53–58
.
(14) (a) Nakamura, E.; Tahara, K.; Matsuo, Y.; Sawamura, M. J. Am.
Chem. Soc. 2003, 125, 2834–2835. (b) Matsuo, Y.; Tahara, K.; Sawamura,
(
10) (a) Darwish, A. D.; Avent, A. G.; Birkett, P. R.; Kroto, H. W.;
Taylor, R.; Walton, D. R. M. J. Chem. Soc., Perkin Trans. 2 2001, 1038–
044. (b) Avent, A. G.; Birkett, P. R.; Darwish, A. D.; Kroto, H. W.; Taylor,
R.; Walton, D. R. M. Chem. Commun. 1997, 1579–1580
11) For a nonepoxidized fulvene-type tetrakisadduct, see: (a) Murata,
Y.; Shiro, M.; Komatsu, K. J. Am. Chem. Soc. 1997, 119, 8117–8118
M.; Nakamura, E. J. Am. Chem. Soc. 2004, 126, 8725–8734.
3
1
(15) X-ray data for 2·(C
6 12
H )
1.4: Red cube (0.12 × 0.12 × 0.12 mm )
j
.
from THF/cyclohexane. Space group R3; a ) 46.5271(19) Å; b )
3
(
46.5271(19) Å; c ) 26.061(2) Å; V ) 48857.77(5) Å ; Z ) 18; T ) 100(2)
.
K; Nref ) 26874; R
1
) 0.0717; R ) 0.1907.
W
1390
Org. Lett., Vol. 11, No. 6, 2009

distance. This distance is much smaller than the sum of the
van der Waals radii for carbon and oxygen (3.22 Å). Another
particular feature of this dimer is in the relative orientation
of two TPA phenyl rings closest to the fullerenes. The phenyl
rings C13-C14-C15-C16-C17-C18 and C68-
C69-C70-C71-C72-C73 are both tilted with a 17.43°
angle (centroid-centroid distance: 12.918 Å) and placed in
a face-to-face manner as to embrace the adjacent fullerene
moiety, acting as tweezers.
The pairs of tetraadduct epoxide 2 pack in the ab plane
of the unit cell in an extended deformed hexagonal geometry
occupying the edges of a pseudohexagon (Figure 1b). The
3
fullerene cores assemble themselves into C -symmetric
helices at the vertices of the pseudohexagon. Centroid-
centroid distances between these C60 cores are 10.132 Å,
corresponding to carbon-carbon close contacts of 3.442 Å
(
C115-C125). Most of the voids in this packing structure
are occupied by the substituent phenyl rings and two located
molecules of cyclohexane solvent per fullerene.
The mechanism involved in the transformation of 1 to 2
can be rationalized by the pathway in Figure 2. First, single
-
electron transfer (SET) from the fullerenyl anion 1 to
molecular oxygen generates the superoxide anion and the
•
11
corresponding fullerene cyclopentadienyl radical 1 . Radical
•
1
is trapped by another molecule of oxygen to give peroxyl
•
•
radical 3 . Peroxyl radical 3 subsequently undergoes cy-
clization with the nearest TPA arm to give the resonance-
•
stabilized spiro peroxide radical 4 .
•
The intermediate radical 4 is analogous to the well-
Figure 2. Proposed mechanism for the dearylation of pentaadduct
1 to tetraadduct 2.
established species formed in the quenching of peroxidic
radicals by phenols like 2,6-di-tert-butyl-4-cresol (BHT).
Spiro peroxide radical 4 undergoes homolytic C-C bond
1
6
•
X-ray structure unambiguously shows that this compound
bears four triphenylamine addends and an epoxide ring
arranged around one pentagon of the fullerene. A mechanism
explaining the loss of one triphenylamine arm and the
subsequent formation of an epoxide has been proposed. We
are exploring the scope of this reaction on related structures.
cleavage and ensuing rearomatization of the TPA group to
form the resonance-stabilized radical 5 . Several examples
•
of rearrangement of alkoxy radical via spirocyclic intermedi-
1
7
ates into stabilized carbon radicals have been reported.
•
Radical 5 undergoes epoxide formation to give 2 via OsO
bond fragmentation and intramolecular cyclization with
release of the 4-(diphenyl-amino) phenoxy radical. The
phenoxy radical may be ultimately oxidized to quinoid cation
Acknowledgment. This work was supported by research
and instrumentation grants NSF-CHE-0527015, NSF-CHE-
6, although we have not be able to identify any TPA-derived
0
9
617052 and ONR-N00014-04-1-0410 (YR), NSF-CHE-
974928 (NMR), NSF-CHE-9871332 (X-ray).
molecular byproduct in this reaction.
In summary, adduct 1 dearylates surprisingly easily under
basic aerobic conditions to give epoxide 2 in high yield. An
Supporting Information Available: Experimental sec-
tion, spectroscopic and crystallographic data for compounds
and 2. This material is available free of charge via the
Internet at http://pubs.acs.org.
(
16) (a) Pryor, W. A.; Gu, J. T.; Church, D. F. J. Org. Chem. 1985, 50,
1
1
85–189. (b) Ingold, K. U. Acc. Chem. Res. 1969, 2, 1–9.
(
17) (a) Banks, J. T.; Scaiano, J. C. J. Phys. Chem. 1995, 99, 3527–
ˇ
3
1
531. (b) Hartung, J.; Gottwald, T.; Spehar, K. Synthesis 2002, 11, 1469–
498.
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