Modular access to 9,9-spirobifluorenes by oxidative coupling using molybdenum pentachloride

Article abstract of DOI:10.1055/s-0033-1338297

The strong oxidizing agent molybdenum pentachloride was used for an efficient direct C-C bond formation of 9,9-diarylfluorenes to the corresponding 9,9-spirobifluorenes. Thus, a versatile method that is compatible with labile groups, such as iodo moieties

Full text of DOI:10.1055/s-0033-1338297

1160▌
PRACTICAL SYNTHETIC PROCEDURES
pMracoticdalusylnathreticAprcoccedeusress to 9,9-Spirobifluorenes by Oxidative Coupling Using
Molybdenum Pentachloride
Oxidative Coupling of Aryls Using MoCl5
Simon Trosien, Dieter Schollmeyer, Siegfried R. Waldvogel*
Institute of Organic Chemistry, Johannes Gutenberg University Mainz, Duesbergweg 10–14, 55128 Mainz,
Germany
Fax +49(6131)3926777; E-mail: waldvogel@uni-mainz.de
PSP
Received: 07.03.2013; Accepted: 11.03.2013
No248
Abstract: The strong oxidizing agent molybdenum pentachloride was used for an efficient direct C–C bond formation of 9,9-diarylfluo-
renes to the corresponding 9,9-spirobifluorenes. Thus, a versatile method that is compatible with labile groups, such as iodo moieties, was
established. By this approach important building blocks for light emitting polymers were synthesized in high yields.
Key words: molybdenum pentachloride, oxidative coupling, spiro compounds, biaryls, C–C bond formation
R3
R3
R2
MoCl5, CH2Cl2
23 °C, 4–8 h
R2
R1
R1
R1
R1
79–96%
Scheme 1 Direct oxidative coupling of 9,9-diarylfluorenes to the corresponding spirobifluorenes
In the past decades, the unique electronic structure of spi- Molybdenum pentachloride is a versatile and easily avail-
robifluorenes turned out to be an important feature for op- able oxidizing agent.11 Because of its strong oxidative
toelectronic applications1 such as solar cells,2 light- power and fast coupling reaction several labile moieties
emitting devices,3 and field-effect transistors.4 Further- are tolerated.12 In many cases, Lewis acids are added for
more, the molecular vertex can generate a chiral cavity, intercepting hydrogen chloride formed during the
which can be exploited for molecular recognition.3g,5 Ad- reaction13 or for providing a preorientation of the substrate
ditionally, the rigid skeleton can be functionalized to form by a precoordination14 to enhance the performance of the
chiral ligands for transition metal catalysts.6 Analogues of Mo(V) reagent. MoCl5 is often considered as a compara-
natural products containing spirobifluorenes were report- ble reagent to hypervalent iodine reagents like phenyl-
ed as well.7
iodine bis(trifluoroacetate) (PIFA).11 But in several
examples MoCl5 yields superior results.11a,b,15 The molyb-
denum salts generated during the reaction can determine
the stereoselectivity of the coupling,16 cause domino oxi-
dations,17 or induce subsequent reductions by a redox
play.18
The most prominent method for synthesizing 9,9-spiro-
bifluorenes represents the protocol by Clarkson and
Gomberg. 9-(Biphen-2-yl)fluoren-9-ols undergo a Friedel–
Crafts type reaction.8 The fluorenol derivative was ob-
tained by addition of a biphenyl Grignard reagent to the
corresponding fluorenone. However, the method requires Herein, we report a convenient synthesis of highly func-
sophisticated 2-halobiphenyls as starting materials. In ad- tionalized 9,9-spirobifluorenes (Scheme 1), which toler-
dition, the Friedel–Crafts reaction occurs in the position ates a variety of functionalities and is complementary to
para to the electron-releasing substituent limiting the existing methods. Diiodinated and dibrominated species
scope. Direct oxidative methods for the formation of the are obtainable, which can be used as building blocks for
aryl–aryl bond using DDQ/MeSO3H or DDQ/FeCl3 mix- polymers with crucial optoelectronic features.
tures were reported, but are established only on two spiro-
bifluorene derivatives.9 An elegant Friedel–Crafts
approach was established by Zhou et al. starting from 2,2′-
diarylbenzophenones;10 however, the latter have to be pre-
pared in several steps.
Scope and Limitations
The starting materials are easily accessible from the cor-
responding fluorenones by a double Friedel–Crafts type
reaction or by an initial Grignard addition followed by a
Friedel–Crafts type reaction (Scheme 2, and Supporting
Information).
SYNTHESIS 2013, 45, 1160–1164
Advanced online publication: 21.03.2013
0
0
3
9
-
7
8
8
1
1
4
3
7
-
2
1
0
X
DOI: 10.1055/s-0033-1338297; Art ID: SS-2013-Z0189-PSP
© Georg Thieme Verlag Stuttgart · New York

PRACTICAL SYNTHETIC PROCEDURES
Oxidative Coupling of Aryls Using MoCl5
1161
R2
coupling reaction to the diiodinated spirobifluorene 4. Us-
ing the conditions reported, neither DDQ/MeSO3H nor
DDQ/FeCl3 could form the desired and valuable building
block 4 as effectively as MoCl5 (entries 6–8). The dichlo-
rodicyanoquinone-based reagent mixtures cause protode-
iodination as a dominant side reaction. The physical
properties of 4 and its partially and fully deiodinated spe-
cies are very similar. Consequently, the content was deter-
mined in the product mixture. The formation of the
diiodinated compound was verified by X-ray analysis of a
suitable single crystal (Figure 1).
O
Friedel–Crafts
R2
type reaction
R1
R1
R1
R1
Grignard
addition
R3
R3
Friedel–Crafts
R2
type reaction
HO
R1
R1
R1
R1
Scheme 2 Synthesis of starting materials
The commonly used reagent mixture consisting of molyb-
denum pentachloride with titanium tetrachloride provides
in the oxidative coupling reaction to 1 inferior yields than
MoCl5 as sole reagent (Table 1, entries 1, 2). The addition
of titanium tetrachloride leads to the formation of a spar-
ingly soluble precipitate with the substrate (entry 3). The
spiro product 1 is generated in excellent yield if reaction
times are prolonged (entry 4). Application of ferric chlo-
ride at analogous reaction conditions resulted in complete
decomposition of the starting material and no traces of 1
Figure 1 Molecular structure of 4 determined by X-ray analysis of a
could be detected (entry 5). The unique and superior char- suitable single crystal
acter of MoCl5 as reagent is demonstrated in the oxidative
The screening of the oxidative coupling conditions indi-
Table 1 Comparison of Different Coupling Conditions
cated that MoCl5 is capable to construct effectively the
spirobifluorene skeleton. Subsequently, the scope of sub-
strates was elucidated (Table 2). A variety of 9,9-diphe-
nylfluorenes with differently substituted phenyl groups
were subjected to the oxidative coupling protocol. As
found in previous studies, a 1,2-dimethoxy substitution
pattern is very beneficial for the oxidative coupling pro-
cess since a precoordination of MoCl5 prior to electron
transfer is anticipated.11g This was underlined by the ex-
cellent yield of 1 and 4, respectively. If only three elec-
tron-releasing groups are present in the substrate, product
2 can still be obtained in 86% yield (Table 2, entry 1). If
only one coupling moiety is activated, no intramolecular
C–C bond formation is observed (entry 3). When the cor-
responding diarylfluorene is treated with MoCl5, even af-
ter 14 hours no coupling process is observed, whereas
chlorination reactions of the starting material occur.
OMe
OMe
OMe
MeO
OMe
R
MeO
MeO
oxidative
coupling
R
MeO
R
R
1: R = H
4: R = I
Entry
Conditions
Result
1
MoCl5/TiCl4, 2 h, 23 °C
MoCl5, 2 h, 23 °C
64%a (1)
78%a (1)
2
b
3
TiCl4, 2 min, 23 °C
MoCl5, 5 h, 23 °C
–
4
96%a (1)
<1%c (4)
34%c (4)
60%c (4)
92%a (4)
Iodo and bromo substituents are splendid leaving groups
for transition metal catalysis. Consequently, spirobifluo-
renes exhibiting two of such functionalities are very use-
ful building blocks for polymer synthesis.19,20
5
FeCl3, 5 h, 23 °C
6
DDQ, FeCl3, 1 h, 0 °C
7
DDQ, MeSO3H, 1 h, 0 °C
MoCl5, 4 h, 23 °C
By the oxidative coupling of the corresponding substrates
the 2,7-dibromo- and 3,6-dibromospirobifluorenes 3 and
5 are accessible in excellent to good yields, respectively
(Table 2, entries 3, 4). The coupling protocol is not only
compatible with labile moieties on the aromatic core but
8
a Isolated yield.
b Formation of a sparingly soluble precipitate.
c Determined by 1H NMR spectroscopy.
© Georg Thieme Verlag Stuttgart · New York
Synthesis 2013, 45, 1160–1164

1162
S. Trosien et al.
PRACTICAL SYNTHETIC PROCEDURES
also useful for substrates with larger side chains (entry 5).
This tetrakisoctyloxylated spirobifluorene 6 was em-
ployed as building block for light emitting polymers. The
synthesis reported for 6 involves seven steps with a total
yield of 7% starting from 2,7-dibromofluorenone, cate-
chol, and 1-bromooctane.20 In our work, the synthesis of 6
is accomplished in three steps with an overall yield of
71% starting from the same chemicals.
Table 2 Scope of the Coupling Reaction to Spirobifluorenes
R3
R3
R2
R2
MoCl5, CH2Cl2
R1
23 °C, 4–8 h
R1
R1
R1
The scope for oxidative coupling reaction yielding five-
membered ring systems is not limited to spirobifluorenes
and can be expanded to substrates exhibiting a geminal di-
substitution with veratryl moieties (Table 3). By our pro-
tocol, the 9,9-dimethylfluorene 7 was synthesized in very
good yield (Table 3, entry 1). Even alkyl-substituted am-
ides are useful substrates, which are usually prone to an
oxidative overfunctionalization of the alkyl group.21 The
spiro[indole-3,9′-fluorene] 8 was obtained in 77% yield
(entry 2). The required starting material is made in a sin-
gle step from the N-methylisatine (see the Supporting In-
formation).
Entry
Product
Time (h) Yield (%)
OMe
OMe
1
8
86
MeO
2
OMe
OMe
Table 3 Oxidative Coupling Reaction of Analogue Architectures
OMe
OMe
2
14
–
MoCl5
CH2Cl2
OMe
OMe
MeO
MeO
MeO
MeO
23 °C
1–5 h
R2
R2
10
R1
R1
OMe
OMe
Br
Entry
Product
Time (h) Yield (%)
MeO
OMe
3
5
94
MeO
Br
OMe
MeO
1
5
90
MeO
3
OMe
7
OMe
OMe
OMe
MeO
MeO
MeO
MeO
4
4
79
2
1
77
Br
O
N
Br
Me
5
8
O-Oct
O-Oct
Br
Oct-O
In conclusion, a reliable and an easy to perform synthetic
access to 9,9-spirobifluorenes was established. The oxida-
tive coupling reaction represents the key step and was
conducted by the powerful reagent MoCl5 at room tem-
perature. The cyclization was compatible with a variety of
labile groups, for example, iodo moieties and amides. The
yield for the coupling reaction for all derivatives is in the
range of 77 to 96%. By this approach a valuable building
block for light emitting polymers was synthesized in ex-
cellent yield.
5
4
92
Oct-O
Br
6
Synthesis 2013, 45, 1160–1164
© Georg Thieme Verlag Stuttgart · New York

PRACTICAL SYNTHETIC PROCEDURES
Oxidative Coupling of Aryls Using MoCl5
1163
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Commercial reagents were used as supplied. All reagents were used
of analytical grades. Solvents were dried if necessary by standard
methods. Flash chromatography was performed on silica gel (40–
60 μm, Merck, Darmstadt, Germany) by using mixtures of cyclo-
hexane with EtOAc or CH2Cl2 with MeOH as eluents. Silica gel 60
sheets on glass (F254, Merck, Darmstadt, Germany) were used for
TLC. Melting points were determined by a Melting Point Apparatus
B-545 (Büchi, Flawil, Switzerland) and are uncorrected. Microanal-
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1
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recorded at 25 °C by using a Bruker AC 300 instrument (Analyt-
ische Messtechnik, Karlsruhe, Germany). All 1H NMR experiments
are reported in δ units, parts per million (ppm) downfield from TMS
(internal standard) and were measured relative to the signal for re-
sidual CHCl3 (7.26 ppm) or DMSO-d5 (2.50 ppm) in the deuterated
solvent. All 13C NMR spectra are reported in ppm relative to CDCl3
(77.16 ppm) or deuterated DMSO (39.52 ppm), and all were ob-
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1
tained by H decoupling. Mass spectra and high-resolution mass
spectra were obtained by using a QTof Ultima 3 (Waters, Milford,
Massachusetts) apparatus employing ESI. All X-ray analysis data
were collected on a STOE IPDS-2T diffractometer at –80 °C using
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2,7-Diiodo-2′,3′,6′,7′-tetramethoxy-9,9-spirobifluorene (4);
Typical Procedure
2,7-Diiodo-9,9-bis(3,4-dimethoxyphenyl)fluorene (109.8 mg, 0.16
mmol) was added to a mixture of MoCl5 (132 mg, 0.48 mmol) and
CH2Cl2 (8 mL) and stirred at 23 °C for 4 h under an argon atmo-
sphere. Then, sat. aq NaHCO3 (10 mL) was added and the reaction
mixture was stirred for additional 2 min. The organic layer was sep-
arated and the aqueous layer was extracted with CH2Cl2 (3 × 50
mL). The combined organic fractions were dried (MgSO4), and the
solvent was evaporated. The crude product was purified by flash-
column chromatography (eluent: CH2Cl2) to yield the desired spiro-
bifluorene 4 as a colorless solid (100.1 mg, 92%). For analytical
purposes the product can be recrystallized from CH2Cl2–MeOH by
slow evaporation of the solvent; mp 258.1 °C.
1H NMR (300 MHz, CDCl3): δ = 7.69 (dd, J = 8.1, 1.4 Hz, 2 H),
7.54 (d, J = 8.1 Hz, 2 H), 7.23 (s, 2 H), 7.03 (d, J = 1.4 Hz, 2 H),
6.14 (s, 2 H), 5.29 (s, 2 H), 4.03 (s, 6 H), 3.65 (s, 6 H).
13C NMR (75 MHz, CDCl3): δ = 151.1, 150.1, 149.1, 140.6, 139.4,
137.3, 135.0, 133.4, 122.1, 107.3, 102.8, 94.0, 65.6, 56.6, 56.5.
HRMS-ESI (+): m/z calcd for C29H22I2O4 + Na [M + Na]+:
710.9505; found: 710.9503.
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Acknowledgment
Support by H.C. Stark GmbH, Goslar, Germany, by a donation of
MoCl5 is highly appreciated.
Supporting Information for this article is available online at
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http://www.thieme-connect.com/ejournals/toc/synthesis.SnuIomprifgtooatrinSugpIoi
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nfomartit
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