Palladium-catalyzed carbon-carbon bond formation and cleavage of organo(hydro)fullerenes

Article abstract of DOI:10.1002/chem.200900022

Palladium-catalyzed reactions of organo-(hydro)fullerenes, new C-C bond formation, and cleaving processes for organo-(hydro)fullerenes were reported. These reactions caused the introduction of various organic fragments and a hydrogen atom to the fullerene

Full text of DOI:10.1002/chem.200900022

DOI: 10.1002/chem.200900022
Palladium-Catalyzed Carbon–Carbon Bond Formation and Cleavage of
OrganoACTHNUGRTENUNG(hydro)fullerenes
Masakazu Nambo and Kenichiro Itami*[a]
Nanocarbons such as fullerenes and carbon nanotubes
chemistry of nanocarbons using transition-metal catalysts.[13]
Herein we report several Pd-catalyzed reactions of organo-
have attracted the interest of scientists from many disci-
plines due to their unique structures and properties.[1] The
chemical modification of fullerenes has proven to be a
promising approach for the preparation of new nanocarbon-
based materials, providing the opportunity to tune the prop-
erties of these interesting materials. A number of privileged
reactions for the functionalization of fullerenes have
emerged.[1] Cyclopropanation with a-halomalonates,[2]
Diels–Alder-type [4+2] cycloadditions with 1,3-dienes,[3]
[3+2] cycloaddition with azomethine ylides,[4] nucleophilic
addition with organolithium and organomagnesium re-
agents,[5] multifold addition with organocopper reagents,[6]
radical additions,[7] and electrophilic additions[8] are the rep-
resentative synthetic methods that have contributed signifi-
cantly to the development of this field. The chemical modifi-
cation of carbon nanotubes[9] or graphene sheets[10] is anoth-
er emerging area of extensive research. However, despite
these significant advances, the synthetic toolbox for fuller-
ene functionalization remains comparatively limited. Thus a
novel synthetic methodology is required for the develop-
ment of new nanocarbon-based materials.
AHCTUNGTREG(NNUN hydro)fullerenes. In addition to anticipated coupling reac-
À
tions, unexpected new C C bond forming and cleaving pro-
cesses were also uncovered during this study.
As our initial foray into the area of nanocarbon chemistry,
we developed a new organoboron-based functionalization of
C60 catalyzed by RhI or PdII complexes (Scheme 1).[13] These
reactions enable the introduction of various organic frag-
ments and a hydrogen atom to the fullerene surface in a
highly regioselective and mono-addition selective manner.[13]
A notable feature of the resultant hydrofullerenes 1 is that
À
they possess an acidic C H bond reflecting the highly conju-
gated fullerene backbone.[14] For example, Fagan and Evans
have experimentally determined the pKa of tBuC60H to be
5.7 (in DMSO at 258C) by voltammetric techniques.[14a]
Geerlings has also identified several highly acidic organo-
Transition-metal catalysis has been a fundamental tool in
organic synthesis, enabling a variety of chemical transforma-
tions that are otherwise difficult or impossible to achieve
with traditional methods.[11] In many instances, remarkable
levels of reactivity/selectivity control in chemical transfor-
mations can be accomplished. To explore new directions in
metal catalysis and nanocarbon chemistry,[12] we recently ini-
tiated a program aimed at developing new functionalization
[a] M. Nambo, Prof. Dr. K. Itami
Department of Chemistry
Graduate School of Science
Nagoya University, and
PRESTO, Japan Science and Technology Agency
Chikusa, Nagoya 464-8602 (Japan)
Fax : (+81)52-788-6098
E-mail: itami@chem.nagoya-u.ac.jp
Supporting information for this article is available on the WWW
under http://dx.doi.org/10.1002/chem.200900022.
Scheme 1. Organo
tionalization.
ACHTUNGTREN(NUGN hydro)fullerenes 1 as key molecules in fullerene func-
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COMMUNICATION
A
attack of fullerenyl anion at 4-position might be due to
À
the ample precedents demonstrating that Pd catalysts can
steric reasons. The mechanism of the key C60 allyl bond for-
mation may be either 1) nucleophilic attack of fullerenyl
anion on the allyl ligand on Pd, or 2) its attack on the Pd
À
functionalize a range of acidic C H bonds of organic mole-
cules,[15] we surmised that such catalytic transformations
À
À
should also be possible at the C H bonds of organo-
center followed by subsequent C60 allyl bond-forming re-
ductive elimination from diorganopalladium(II) species.
(hydro)fullerenes 1 (Scheme 1).
À
As a first model reaction of C H bond functionalization
Having developed an efficient protocol for the allylation
of 1, we examined the Pd-catalyzed allylation reaction.[15] In
early experiments, the use of methallyl methyl carbonate (2)
was found to be optimal as a source of allylic electro-
reaction, we next investigated the C H bond arylation reac-
À
tion of 1 using aryl halides as electrophilic partners.[15,20]
After extensive screening, a catalytic system producing the
desired arylation product was identified. The treatment of
1a, C6H5I, LiOH, and Pd/PCy2(o-biphenyl) catalyst in 1,2-
Cl2C6H4 at 1208C afforded the arylation product 4 in 23%
isolated yield (Scheme 3). Similar to the previous allylation
reaction, arylation occured exclusively at the 4-position re-
sulting in the 1,4-isomer. The arylation also took place with
1b, albeit less efficiently (6: 6% yield).
phile.[15–17] As for the Pd catalyst, the Pd/P
ACHTUNGTRENNUNG
ACHTUNGTRENNUNG
CTHUNGTRENNUNG
(10 mol%; dba=trans,trans-dibenzylideneacetone) and P-
À
(OPh)3 (80 mol%) furnished the C H bond allylation prod-
uct 3 in 85% isolated yield (Scheme 2).[18] The 13C NMR
À
Scheme 3. Pd-catalyzed C H bond arylation of 1 and discovery of two
unexpected reactions.
À
Scheme 2. Pd-catalyzed C H bond allylation of organo
1a.
G
À
Although C H bond arylation products can be obtained
from 1 under Pd catalysis, the reaction efficiency is low in
comparison to that of allylation. During our investigation to
identify the mass balance in these arylation reactions, we
discovered two unexpected reactions that take place in par-
allel and are also catalyzed by Pd.
In the case of the reaction with 1a, the interesting fuller-
ene dimer 5 (ArC60C60Ar) was produced in 46% yield
(Scheme 3).[21] Dimer 5 was not obtained in the absence of
spectrum of 3 showed 55 signals for sp2-hybridized carbon
atoms (though there exist some overlapped peaks), indicat-
ing the C1-symmetric structure (1,4-isomer) of the product.
The magnetic non-equivalence of two hydrogen atoms be-
tween C60 core and the vinyl group (d=3.64 and 3.73 ppm)
in the 1H NMR spectrum also indicates the 1,4-isomeric
structure.[19] However, without an X-ray crystal structure
analysis, the proposed structure is tentative and we cannot
completely rule out other isomers at this stage.
À
Pd, providing evidence that the observed C H bond dimeri-
1
zation is catalyzed by the Pd complex. The H NMR spec-
A possible mechanism for this catalytic allylation reaction
is also depicted in Scheme 2. We believe that the catalytic
cycle is initiated by the reaction of [Pd0(L)n] (L=ligand)
trum and LC-MS analyses indicate that 5 consists of two iso-
mers. Because of the bulkiness of the aryl group, it is likely
that 5 results from dimerization at positions remote from
the aryl groups. Thus, the 1,4-1’,4’-isomer and the 1,4-1’,11’-
isomer are the possible two isomers obtained in the reaction
(Scheme 4). As highly indicated by theoretical calcula-
tions,[22] the corresponding isomers containing a 1,2-linkage
of a pivot carbon and an aryl-attached carbon should be
II
[16]
À
with 2 generating (p-allyl)Pd OMe species, which then
deprotonates the acidic hydrogen atom of 1a. The reaction
of the thus-generated delocalized fullerenyl anion and cat-
ionic (p-allyl)PdII species would lead to the coupling prod-
ucts 3 with the regeneration of [Pd0(L)n]. The preferential
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K. Itami and M. Nambo
phenylphosphino)ethane (dppe), PCy3 (Cy=cyclohexyl),
and P
ACHTUNGTRENNUNG
resulted in somewhat higher yield of C60 (entries 2–5), P
ACHTUNGTRENNUNG
tol)3 (o-tol=ortho-tolyl) was found to be a far superior
ligand, yielding C60 in 79% yield (entry 6). Concomitant
with production of C60, the alkynyl group and hydrogen
atom were released from the fullerene core as the terminal
alkyne (54% yield). Other potential alkyne-derived prod-
ucts, such as enyne or diyne, were not obtained, as judged
by the GC analysis of reaction mixture. Considering the fact
that the related Pd
ACHTNUGTNERNU(GN OCOCF3)2/PACHTNUTREGNNG(UN o-Tol)3 and PdCl2/PACHTUNGTRENNUNG(o-Tol)3
systems possess virtually no activity (entries 7 and 8), the ex-
ceptionally high catalytic activity of Pd
interesting.
ACHUTGTNERN(NGU OAc)2/PCAHTUNGTRNE(NUGN o-Tol)3 is
The driving force of this reaction is evidently the forma-
tion of stable, pristine C60. Shown in Scheme 5 is our current
Scheme 4. Possible isomers of fullerene dimer 5.
very unstable. However, we cannot completely rule out such
isomers at this stage.
In the case of the reaction using 1b, the corresponding
dimer was not formed along with the arylation product 6.
Rather, pristine C60 was produced in 26% yield (Scheme 3).
Although some retro-reactions of functionalized fullerene
derivatives have been reported,[23] the present reaction rep-
resents the first example using organoACTHNUTRGNEUNG(hydro)fullerenes with
the aid of transition-metal catalyst.
À
This C C bond-cleaving process is intriguing also from
the point of view of transition-metal catalysis.[24] Therefore,
to gain a better grasp of reaction, the conditions were fur-
ther optimized (Table 1). In early experiments, it was identi-
fied that iodobenzene and base (LiOH) are spectators in the
production of C60. The treatment of 1b with a catalytic
À
Scheme 5. A possible mechanism for the Pd-catalyzed C C bond cleav-
age of alkynylACTHNUTRGNEU(GN hydro)fullerene 1b.
amount of PdACHTUNGTRENNUNG(OAc)2 in toluene at 1008C for 12 h gave C60
in 2% yield (entry 1). While the addition of PPh3, 1,2-bis(di-
working model for a possible Pd-catalyzed pathway. Molecu-
lar modeling (DFT) of the alkynyl(hydro)fullerene suggests
[a]
À
Table 1. Pd-catalyzed C C bond cleavage of 1b.
AHCTUNGTRENNUNG
that the alkynyl group and the hydrogen atom at the surface
of C60 are perfectly synperiplanar (0.18). Thus, the coordina-
tion of alkynyl group to Pd would bring the basic acetate
moiety and acidic hydrogen atom in close proximity, thereby
À
À
promoting the concerted bond formation (H OAc, Cfullerene
À
À
À
Cfullerene
,
Calkynyl Pd) and cleavage (Cfullerene H, Cfullerene
À
Calkynyl, Pd OAc) to release pristine C60. Subsequent proto-
depalladation of thus-obtained alkynyl–PdOAc with HOAc
would produce the terminal alkyne and regenerate the Pd-
Entry
Pd precursor
Ligand
Yield (C60) [%][b]
1
2
3
4
Pd
Pd
Pd
Pd
Pd
Pd
Pd
N
–
2
7
5
ACHUTNGREN(NUG OAc)2/PACHTUGNTREN(NUNG o-Tol)3 catalyst. However, pathways initiated by
PPh3
Ph2PCH2CH2PPh2
PCy3
P
P
P
–
Pd0 cannot be ruled out at this moment.
In summary, we have established that Pd catalysts enable
4
À
5
G
18
79
<1
<1
<1
several
C H
bond
transformations
of
organo-
6[c]
7
ACHTUNGTRENNUNG
(hydro)fullerenes. The allylation reaction can be a versatile
ACHTUNGTRENNUNG
and general method to functionalize fullerenes. Subsequent
transformations of thus-formed allylated fullerenes might be
achieved by various methods including the well-established
olefin metathesis reaction. Although the arylation reaction
is still in its infancy, the two new reactions found while in-
8
9
ACHTUNGTRENNUNG[PdCl2(PACHTUNGTRENNUNG
–
P
ACHTUNGTNER(NUNG o-tol)3
[a] Conditions: 1b, (1 equiv), Pd (20 mol%), ligand (Pd/P=1:2), toluene,
1008C, 12 h. [b] Determined by HPLC using C70 as an internal standard.
[c] The yield of 1-undecyne was determined to be 54% yield.
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Homogeneous Catalysis
COMMUNICATION
À
À
À
vestigating the arylation reaction are intriguing. The C H
bond dimerization reaction might contribute in the genera-
tion of new fullerene-assembled materials that are otherwise
Keywords: C C cleavage · C C coupling · fullerenes ·
homogeneous catalysis · palladium
À
difficult to make. The C C bond-cleaving reaction may find
use in the “deprotection” of fullerenes, assuming an alkynyl-
ACHTUNGTRENNUNG
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ACHTUNGTRENNUNG
transition-metal catalysis for fullerene functionalization, but
also unlocks opportunities for markedly different strategies
in nanocarbon synthesis.
Experimental Section
À
Procedure for Pd-catalyzed C H bond allylation of 1a (Scheme 2): A
20 mL glass Schlenk flask containing a magnetic stirring bar was flame-
dried under vacuum and filled with argon after cooling to room tempera-
ture. [Pd2ACHTUNGTRENNUNG(dba)3]·CHCl3 (1.7 mg, 1.6 mmol), PACHTUNGTRNEN(UGN OPh)3 (3.1 mL, 12 mmol),
and dry o-dichlorobenzene (2.0 mL) were added at room temperature to
this flask under a stream of argon. After stirring the mixture at this tem-
perature for 30 min, 1a (13.8 mg, 15 mmol) and methallyl methyl carbon-
ate (2: 3.8 mg, 29 mmol) were added to the flask under a stream of argon.
After stirring at 808C for 18 h, the mixture was cooled to room tempera-
ture. The mixture was passed through a pad of silica gel with copious
washings with toluene (~30 mL). The filtrate was concentrated (~5 mL)
and subjected to preparative recycling HPLC equipped with a Buckyprep
column (eluent: toluene) to afford the coupling product (3: 12.4 mg,
85%) as dark brown solid. 1H NMR (270 MHz, CDCl3): d=0.89 (t, J=
7.0 Hz, 3H), 1.20–1.50 (m, 12H), 1.65–1.80 (m, 2H), 2.11 (s, 3H), 2.77 (t,
J=7.8 Hz, 2H), 3.64 (d, J=13.0 Hz, 1H), 3.73 (d, J=13.0 Hz, 1H), 5.12
(s, 2H), 7.47 (d, J=8.4 Hz, 2H), 8.21 ppm (d, J=8.4 Hz, 2H); 13C NMR
(67.8 MHz, CDCl3): d=14.13, 22.70, 24.97, 29.36, 29.40, 29.57, 29.61,
31.55, 31.92, 35.77, 50.20, 59.31, 61.50, 117.56, 127.06, 129.54, 137.11,
138.42, 138.60, 138.80, 139.26, 140.53, 140.90, 142.08, 142.23, 142.59,
142.65, 142.74, 143.09, 143.14, 143.22, 143.26, 143.28, 143.85, 143.88,
143.94, 143.96, 144.07, 144.18, 144.25, 144.32, 144.40, 144.44, 144.48,
144.55, 144.67, 144.90, 144.95, 144.96, 145.07, 145.13, 145.53, 145.60,
146.61, 146.91, 146.94, 146.96, 147.03, 147.10, 147.24, 147.62, 148.65,
148.70, 148.82, 150.96, 151.98, 157.16, 157.69 ppm; HRMS (ESI-TOF, neg-
ative): m/z calcd for C79H30: 978.2348, found 978.2354.
À
Procedure for Pd-catalyzed C C bond cleavage of 1b (Table 1, Entry 6):
A 50 mL glass Schlenk flask containing a magnetic stirring bar was
flame-dried under vacuum and filled with argon after cooling to room
temperature. PdACHTUNGTRENNUNG(OAc)2 (1.3 mg, 6 mmol), PACHUTNGTREN(NUNG o-Tol)3 (3.7 mg, 12 mmol), and
dry toluene (20 mL) were added at room temperature to this flask under
a stream of argon. After stirring the mixture for 30 min, 1b (26.2 mg,
30 mmol) was added under a stream of argon. After stirring at 1008C for
12 h, the reaction mixture was cooled to room temperature. The yield of
C60 was determined to be 79% by HPLC analysis (Buckyprep column;
toluene as eluent; flow rate 0.5 mLminÀ1; UV detection at 326 nm) of
this crude mixture using C70 as an internal standard. Thereafter, the mix-
ture was passed through a pad of silica gel with copious washings with
toluene (~30 mL) and the solvent was evaporated. The yield of 1-un-
ACHTUNGTRENNUNG
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Acknowledgements
This work was supported in part by the PRESTO program of Japan Sci-
ence and Technology Agency, and a grant-in-aid for scientific research
from MEXT and JSPS. M.N. is a recipient of the JSPS Predoctoral Fel-
lowships for Young Scientists. We thank Dr. Olivier Coutelier and Dr.
Jean Bouffard (Nagoya University) for critical comments and discussion.
Chem. Eur. J. 2009, 15, 4760 – 4764
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K. Itami and M. Nambo
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À
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Published online: February 19, 2009
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