Synthesis of bifluorenes via cobalt halide radical coupling

Article abstract of DOI:10.1016/j.tetlet.2011.03.127

Reaction of in situ generated lithium fluorenide salts with cobalt chloride results in formation of the corresponding bifluorenes. The scope of fluorenes that can be coupled include those containing substituents amenable to polymerization for application

Full text of DOI:10.1016/j.tetlet.2011.03.127

Tetrahedron Letters 52 (2011) 3261–3265
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Tetrahedron Letters
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Synthesis of bifluorenes via cobalt halide radical coupling
⇑
Malik H. Al-Afyouni, Tiffany A. Huang, Fernando Hung-Low, Christopher A. Bradley
Department of Chemistry and Biochemistry, Texas Tech University, Lubbock, TX 79409-1061, United States
a r t i c l e i n f o
a b s t r a c t
Article history:
Reaction of in situ generated lithium fluorenide salts with cobalt chloride results in formation of the cor-
responding bifluorenes. The scope of fluorenes that can be coupled include those containing substituents
amenable to polymerization for application to poly(bifluorene) synthesis and alkyl substituted fluorenes
for use in the preparation of bimetallic transition metal complexes. Based on experiments using a radical
trap, the reaction mechanism involves reduction of cobalt and subsequent fluorenyl radical coupling.
Overall, this study demonstrates a simple and effective method for preparation of hindered tertiary
and quaternary C–C bonds using a readily available and inexpensive coupling agent.
Received 7 February 2011
Revised 21 March 2011
Accepted 23 March 2011
Available online 12 April 2011
Keywords:
Bifluorenes
Cobalt
Ó 2011 Elsevier Ltd. All rights reserved.
Fluorene
Radical coupling
1. Introduction
Here, we report an optimized method based on these studies for
preparation of a series of substituted bifluorenes. The cobalt in-
Fluorenes are versatile building blocks for materials science and
provide ubiquitous ligand frameworks for transition metals. For in-
stance, 9,90-dialkyl substituted fluorene monomers can be coupled
to give fluorene polymers and copolymers with applications to
OLEDs and other optical devices.1–3 Fluorenes are also readily
deprotonated and serve as anionic ancillary ligands, primarily sup-
porting Group IV metals for olefin polymerization.4
duced coupling provides a straightforward method to synthesize
hindered tertiary and quaternary C–C bonds selectively.
2. Results and discussion
Our initial strategy for preparation of fluorenyl cobalt(II) halide
complexes consisted of salt metathesis of cobalt chloride with the
substituted lithium fluorenide, a method which has ample prece-
dence in related pentamethylcyclopentadienyl (Cp⁄) chemistry.13
Reaction of isolated lithium 9-isopropylfluorenide (1) with cobalt
chloride in diethyl ether gives a purple color change (Eq. 1); upon
workup, a white solid is obtained. The compound was identified as
9,90-di-isopropyl-9,90-bifluorene (2), based on 1H and 13C NMR
spectroscopy, elemental analysis, and X-ray crystallography. Upon
examining the crude reaction mixture by 1H NMR spectroscopy,
the 9-isopropylfluorene starting material was detected as a minor
secondary product of the reaction (<5%). Though substituted ind-
enes have been coupled in low to moderate yields using Ni14 or
Cu15 halides, we were surprised the presumed bulky fluorene rad-
ical intermediate would couple so selectively, as opposed to simply
abstracting a hydrogen from solvent (vide infra).
Synthesis of bifluorene units as an extension of fluorenes for
materials and ligand applications has received little attention,5
perhaps due to current synthetic access to bifluorenes. As a repre-
sentative example, though coupling of 9-bromofluorene proceeds
cleanly and in high conversion with Ru carbonyl or Fe halide cata-
lysts, further synthetic scope of the reaction has not been ex-
plored.6 Coupling of 9-substituted fluorenes has been
accomplished using (g
5-fluorenyl)Ti(IV) complexes,7 but the com-
pounds must be isolated prior to induced coupling to attain rea-
sonable yields and the Ti compounds must be prepared by use of
toxic Sn precursors. Yields in this and other bifluorene syntheses
are not ideal or multiple steps are required to prepare fluorene pre-
cursors.8–12 A simple, versatile method of converting substituted
fluorenes to the corresponding bifluorenes would be an ideal strat-
egy to provide access to the desired monomers/ligands.
Our group hoped to explore substituted fluorenes as ligands in
late transition metal complexes, specifically with cobalt. In the
course of attempted synthesis of cobalt(II) fluorenyl halide
complexes, selective radical coupling to form bifluorenes was
consistently observed during typical salt metathesis conditions.
⇑
Corresponding author. Tel.: +1 806 742 0022; fax: +1 806 742 1289.
E-mail address: chris.bradley@ttu.edu (C.A. Bradley).
ð1Þ
0040-4039/$ - see front matter Ó 2011 Elsevier Ltd. All rights reserved.
doi:10.1016/j.tetlet.2011.03.127

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M. H. Al-Afyouni et al. / Tetrahedron Letters 52 (2011) 3261–3265
Table 1
Optimization of coupling conditions with fluorenea
Entry
Conditions
4:3 Ratiob
1
2
3
4
5
6
7
8
9
1 equiv CoCl2, 20 mL THF
1 equiv CoCl2, 20 mL Et2O
0.5 equiv CoCl2, 20 mL Et2O
2 equiv CoCl2, 20 mL Et2O
1 equiv CoCl2, 40 mL Et2O
1 equiv CoCl2, 10 mL Et2O
1 equiv CoI2, 20 mL Et2O
1 equiv Co(acac)2, 20 mL Et2O
1 equiv CoCl2, 20 mL Et2O, 12 h
1:2.2
1:0.4
1:2.1
1:0.4
1:0.4
1:0.7
1:1.6
0:1
Figure 1. Molecular structure of 2 with 30% probability ellipsoids. Hydrogen atoms
are omitted for clarity.
1:0.4
X-ray quality crystals of 2 were grown from pentane at ꢀ35 °C
and further support C–C bond coupling (Fig. 1). The quaternary
C(9)–C(25) bond formed during the reaction is elongated
(1.638(2) Å) and of similar length to analogous C–C single bonds
in isolated and structurally characterized bifluorenes.7,16 The iso-
propyl groups are aligned approximately anti with respect to one
another, likely due to steric repulsion of the bulky substituents.
To explore the generality of the coupling, isolated lithium flu-
orenide was exposed to similar conditions. The less hindered fluo-
rene coupled to give a ratio of bifluorene/fluorene of 2:1 based on
1H NMR spectroscopy (CDCl3). Presumably, upon oxidation of the
fluorenide unit, hydrogen abstraction becomes more competitive
with the less sterically hindered radical. Based on the observed
coupling with isolated lithium fluorenide salts, reaction conditions
were explored to optimize yield of bifluorene (Table 1) using in situ
generated lithium salts in the presence of a slight excess of butyl
lithium, for convenience and to permit facile manipulation on a
Schlenk line.
a
See Section 4 or Supplementary data for more details on reaction conditions.
Ratios determined by 1H NMR spectroscopy (CDCl3).
b
and 4). The reaction also seems relatively insensitive to the amount
of solvent present, as dramatic changes in product ratios upon
reducing or increasing the amount of ether are not observed. The
nature of the cobalt source is important, as use of CoI2 diminishes
the amount of bifluorene produced (entry 7) while nonhalide con-
taining cobalt sources, such as Co(acac)2 (acac = acetylacetonate),
result in no detectable amounts of bifluorene (entry 8). Under our
optimized conditions, fluorene can be coupled in respectable con-
versions (>70% based on fluorene) and isolated analytically pure
by recrystallization, sublimation, or by rinsing of crude product with
cold pentane. Under none of these coupling conditions is the pres-
ence of bifluorenylidene detected, a common product of reductive
couplings of fluorenones, based on NMR spectroscopy.18
As shown in Table 1, reaction in THF proceeds to give a 1:2.2 mix-
ture of bifluorene/fluorene. Though coupling is observed, the pres-
ence of resonances in CDCl3 at 1.40–1.80 and 3.80 ppm are
consistent with ring opening of THF.17 Furthermore, the oily consis-
tency of the final product mixture suggested the use of ether as the
solvent of choice in all subsequent experiments. Aside from solvent,
the amount of CoCl2 is critical, as less than 1 equiv results in a re-
duced bifluorene/fluorene ratio. Excess cobalt chloride appears to
have little effect on the amount of bifluorene produced (entries 2
Using the optimal coupling conditions of the least sterically hin-
dered fluorene, the substrate scope with 9-substituted fluorenes
was explored. Fluorenes, containing both smaller and bulkier sub-
stituents, as well as those with less electron donating groups were
investigated (Table 2). All coupled to form bifluorenes, in most cases
with excellent conversions (>90% relative to fluorene starting mate-
rials), based on 1H NMR spectroscopy, after stirring with CoCl2 at
ambient temperature. Each in situ generated fluorenide reacted in
high conversion to the corresponding bifluorene after 1 h, with the
Table 2
Synthesis of 9-substituted bifluorenesa
R
Fluorene compound number
B:F ratiob
Bifluorene compound number
H
3
5
7
8
10
12
14
71:29 (60)
91:9 (79)
94:6 (85)
98:2 (91)
95:5 (83)
89:11(76)
50:50 (25)
4
6
2
9
11
13
15
Me
iPr
nBu
Ph
CH2Ph
SiMe3
c
a
b
c
See Section 4 or Supplementary data for more details on reaction conditions.
Ratios determined by 1H NMR spectroscopy; isolated yields, relative to fluorene starting material in parentheses.
Reaction time: 16 h.

M. H. Al-Afyouni et al. / Tetrahedron Letters 52 (2011) 3261–3265
3263
exception of 14. The extended reaction time may result from the per-
sistence of the fluorenyl radical, due to either the presence of a less
electron rich substituent or the group’s steric bulk. In most cases,
due to almost complete conversion to the bifluorene, products can
be purified by simply washing with cold pentane to remove minor
amounts of the substituted fluorene starting material or by recrys-
tallization in the appropriate hydrocarbon or ethereal solvent.
Extension of the coupling to substrates with peripheral groups on
the benzo rings of the fluorenes was also investigated. In particular,
2,7-dihalofluorenes were targeted to produce bifluorenes contain-
ing substituents amenable to coupling polymerization, a method of-
ten employed in the preparation of polyfluorenes.1,19 When X = Br or
Cl, the corresponding bifluorenes 17 and 19 can be produced in low
conversion (Eq. 2), with bifluorene/fluorene ratios of 1:2 for 17 and
3:7 for 19. The reactions also require extended reaction times (48 h)
relative to the 9-substituted fluorene substrates.
The coupled product 22 has been characterized by 1H and 13C
NMR spectroscopy (THF-d8), combustion analysis, and EI mass
spectrometry. The presence of a hydrogen on the 9-position is
confirmed by the peak at 4.67 ppm in the 1H NMR spectrum
of 22. Unlike the other substituted bifluorenes prepared in this
study, 22 exhibits restricted rotation about the newly formed
C–C bond, as 8 methyl and 4 Benzo resonances are observed
in the 1H NMR spectrum at ambient temperature. Interestingly,
one of the aromatic C–H resonances, identified based on an
HSQC spectrum, displays
5.75 ppm. The dramatic change of only one set of hydrogens
likely results from orientation of that hydrogen over the sys-
a significant upfield shifting to
p
tem of the benzo ring of the other half of the bifluorene unit
as seen in the X-ray structure of 22 (Fig. 2, Supplementary data).
Proximity of hydrogen atoms to such
p systems are well estab-
lished to give rise to such effects.22 The 13C NMR spectrum sim-
ilarly indicates the number of aliphatic and aromatic resonances
consistent with a molecule of C2 symmetry and speaks to the
highly congested nature of the coupled product.
X
X
X
X
1) nBuLi
2) CoCl2
Et2O
- LiCl
X-ray quality crystals of 22 were grown by slow evaporation
from THF at ambient temperature and confirm C–C bond coupling
(Fig. 2). The quaternary C–C bond formed during the reaction is less
elongated (1.558(3) Å) when compared to the 9-substituted 2.
Also, note the two fluorene units adopt a gauche type arrangement
in the solid state, consistent with the static structure suggested by
the solution NMR spectroscopic data.
To demonstrate use of the cobalt mediated coupling as a prac-
tical large scale method of bifluorene synthesis, 8 was coupled
on a 10 g scale. This particular substrate was chosen as it most clo-
sely mimics alkyl substituted fluorenes currently used to prepare
poly(fluorenes).1 Exposure to standard CoCl2 coupling conditions
followed by workup gives a yield of 76% (7.6 g of 9). Hence, the co-
balt coupling appears amenable to scaleup to amounts useful for
monomer or ligand syntheses.
0.5
X
X
X = Cl, 16
X = Br, 18
X = Cl, 17
X = Br, 19
ð2Þ
As an alternative and higher yielding method of incorporating
halide substituents on bifluorenes, 9 can be further functionalized
by aromatic bromination with NBS at ambient temperature to give
bifluorene 20 (Eq. 3).20 Formation of the functionalized bifluorene,
based on 1H and 13C NMR spectroscopy as well as an X-ray diffrac-
tion data set that establishes connectivity (Supplementary data),
suggests postmodification of 9-substituted bifluorenes after cobalt
coupling could be a successful strategy for synthesis of halide-
substituted bifluorene monomers.
After establishing the generality and large scale utility of the
reaction, the mechanism was of interest, primarily to circumvent
coupling and permit access to the desired cobalt fluorenyl com-
plexes. One potential mechanism involves one electron reduction
of Co(II) to Co(I), based on reaction stoichiometry (Table 1, entries
2 and 4) thereby generating a fluorenyl radical (Scheme 1). This
Br
Br
Br
Br
NBS
nBu
nBu
radical is stabilized by the
p
system of the aryl groups,23 permit-
nBu
nBu
HBr
CH3COOH
6 hrs
ting a lifetime adequate for coupling of two radicals to occur. The
20
9
ð3Þ
The alkyl substituted fluorene 21, a fragment found to have
great utility as an ancillary ligand support in Group IV olefin poly-
merization,21 was also explored as a substrate in the cobalt medi-
ated coupling (Eq. 4). Under standard conditions, 21 can be coupled
to give the corresponding bifluorene 22 in a 95:5 ratio relative to
21. As seen in the coupling of fluorenes with 9-substitution, protec-
tion of that position by encumbering groups, in this case distal to
the 9-position, permits greater selectivity in coupling relative to
the parent fluorene (vide supra).
1) nBuLi
0.5
2) CoCl2
Et2O
- LiCl
22
21
Figure 2. Molecular structure of 22 with 30% probability ellipsoids. Hydrogen
atoms are omitted for clarity.
ð4Þ

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M. H. Al-Afyouni et al. / Tetrahedron Letters 52 (2011) 3261–3265
4. Experimental26
4.1. Preparation of 9,90-di-isopropyl-9,90-bifluorene (2)
A 100 mL Schlenk flask was purged with nitrogen and charged
with 1.00 g (4.8 mmol) of 9-isopropylfluorene 7. Diethyl ether
(25 mL) was then added via cannula and the solution was stirred
until all solid dissolved. The solution was cooled to ꢀ78 °C, and
3.3 mL (5.3 mmol) of 1.6 M nbutyllithium was added slowly by syr-
inge. The solution was then allowed to stir at ambient temperature
for one hour upon which the solution turned bright red. The reac-
tion was cooled again to ꢀ78 °C, and against a nitrogen counter-
flow, 623 mg (4.8 mmol) of cobalt(II) chloride was added to the
solution and the reaction was allowed to stir at ambient tempera-
ture for 1 h. Saturated aqueous solutions of NaCl (20 mL) and
NH4Cl (20 mL) were then added and the aqueous phase was ex-
tracted with 3 ꢁ 100 mL portions of dichloromethane. The organic
phase was dried using magnesium sulfate, filtered, and solvent was
removed in vacuo to give a white solid. The product was recrystal-
lized from diethyl ether to give 846 mg (85% based on 7) of 2 as a
white solid. Anal. Calcd for C32H30: C, 92.70; H, 7.30. Found: C,
92.44; H, 7.22. 1H NMR (chloroform-d): d = 0.46 (d, 7 Hz, 12H,
CHMe2), 3.00 (sept, 7 Hz, 2H, CHMe2), 7.10 (m, 8H, Benzo), 7.27
(m, 4H, Benzo), 7.59 (d, 7 Hz, 4H, Benzo). 13C NMR (chloroform-
d): d = 20.89 (CHMe2), 34.76 (CHMe2), 64.31 (CCHMe2), 118.90,
125.78, 126.80, 127.44, 142.22, 147.98 (Benzo).
Scheme 1. Proposed mechanism of bifluorene formation.
Acknowledgments
We graciously thank the Welch Foundation (Grant D1707) for
financial support of this research. Texas Tech University is
acknowledged for startup funding. The X-ray data was collected
on a Bruker Apex II instrument purchased through an internal
Texas Tech Office of Research Services (ORS) equipment grant.
We would also like to acknowledge Professors Tamara Hanna
and Louisa Hope-Weeks for assistance with X-ray crystallography.
fluorenyl radical could also abstract a hydrogen from the solvent to
reform the starting material. Alternatively, formation of
cobaltocene analogue followed by reductive coupling to give
Co(0), while possible, is unlikely, given the stability of related
bis(indenyl) cobalt complexes.24
a
9,10-Dihydroanthracene was used as a nonprotic radical trap,25
as it does not react with lithium fluorenide or CoCl2 under standard
reaction conditions. Addition of 1.5 equiv of 9,10-dihydroanthra-
cene during the coupling of fluorene gives only small amounts of
bifluorene 4 (<10%), along with significant amounts of anthracene
as detected by mass spectrometry and 13C NMR spectroscopy. The
generation of anthracene is highly suggestive of radical formation
during the reaction.
Supplementary data
Supplementary data (additional experimental details, including
1H and 13C NMR spectra of selected compounds, and X-ray data for
2, 19, and 21) associated with this article can be found, in the on-
line version, at doi:10.1016/j.tetlet.2011.03.127.
References and notes
3. Conclusions
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Such reactions are in progress.

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