Synthesis of fluorene-based oligomeric organoboron reagents via Kumada, Heck, and Stille cross-coupling reactions

Article abstract of DOI:10.1021/jo0602470

Boronic pinacol ester group is not reactive in Kumada, Heck and Stille coupling reaction conditions. Fluorene-based sophisticated organoboron compounds were synthesized by means of Palladium catalyzed Kumada, Heck and Stille cross-coupling reactions from

Full text of DOI:10.1021/jo0602470

terminal groups for further metal mediated cross-coupling
reactions are still few.6
Synthesis of Fluorene-Based Oligomeric
Organoboron Reagents via Kumada, Heck, and
Stille Cross-Coupling Reactions
Organoboron compounds are key intermediates of Suzuki
coupling reaction, which have been widely used in the synthesis
of conjugated molecules. In general, arylboron reagents are
prepared from lithium or Grignard reagents and trialkyl borates.
However, the yield of lithium or Grignard reagents dramatically
decreases with increasing molecular weight.6b Moreover, it is
difficult to make the lithium reagents through lithium-halogen
exchange while other reactive groups presence. Kumada, Stille
and Heck cross-coupling reactions normally work well in the
anhydrous condition, in which boronic acid or ester groups
should not take part in the reaction. In fact, Stille coupling
reaction has been used to synthesize conjugated building blocks
carrying boronic ester group for subsequent Suzuki coupling
reaction.7-9 Of the conjugated systems, fluorene-based materials
have attracted particular interest in recent years due to their
superior light-emitting properties. 1e,3 Therefore in current work,
we focus on the synthesis of various 2′-aryl-9′,9′-dioctyl-fluoren-
7′-yl-4,4,5,5-tetramethyl-[1,3,2] dioxaborolanes and fluorene-
based MCOs end-capped with boronic pinacol esters via
Kumada, Stille or Heck coupling reactions started from halo-
fluorenyl boronic pinacol esters.
Xiaojie Zhang, Hongkun Tian, Qin Liu, Lixiang Wang,
Yanhou Geng,* and Fosong Wang
State Key Laboratory of Polymer Physics and Chemistry,
Changchun Institute of Applied Chemistry, Graduate School of
Chinese Academy of Sciences, Chinese Academy of Sciences,
Changchun 130022, P. R. China
yhgeng@ciac.jl.cn
ReceiVed February 7, 2006
We first selected compounds 1a and 1b as substrates to test
the validity of three types of cross-coupling reactions in
preparation of boronic reagent. Mono-dehalogenation of 2,7-
dibromo/iodo-9,9-dioctylfluorene with n-butyllithium (n-BuLi)
followed by treatment with tri(i-propyl) borate and 2 M aqueous
HCl in succession yielded corresponding boronic acids,10 which
were refluxed with pinacol in tetrahydrofuran (THF) to afford
boronic esters 1a and 1b in a two-step yield of 75% and 54%,
respectively (Scheme 1). Gas chromatography-mass spectro-
metric (GC-MS) measurements revealed that the purity of 1a
and 1b was 98.89% and 96.91%, respectively, and main
impurity was 9′,9′-dioctyl-fluoren-2′-yl -4,4,5,5-tetramethyl-
[1,3,2] dioxaborolane.
Boronic pinacol ester group is not reactive in Kumada, Heck
and Stille coupling reaction conditions. Fluorene-based
sophisticated organoboron compounds were synthesized by
means of Palladium catalyzed Kumada, Heck and Stille
cross-coupling reactions from halofluorenyl boronic esters.
Conjugated materials represent one of the most investigated
classes of advanced materials in recent years because of their
potential applications in electronics, photonics, and optoelec-
tronics.1 In contrast to conjugated polymers, monodisperse
conjugated oligomers (MCOs) are characterized by well-defined
structures and superior chemical purity.1a-c These intrinsic
features are imperative for establishment of structure-property
relationship and high performance optoelectronics. Meanwhile,
MCOs are also well-defined building blocks of supramolecular
systems and block copolymers.2-4 Various rod-coil or rod-rod
block cooligomers have been reported.2,5 However, MCOs with
The results of Kumada, Heck and Stille reactions of com-
pounds 1a and 1b with thienylmagnesium bromide, p-vinylbi-
phenyl and three tributylstannyl reagents are listed in Table 1.
Kumada 11 reaction of 1a with thienylmagnesium bromide gave
compound 2 in a yield of 86%. Considering the Grignard reagent
is also an organic base, this indicates that boronic ester group
is stable and not reactive in the absence of water, even in the
presence of strong bases. However, the reaction of 1b (X)I)
in the same condition only afforded a mixture of 2 and starting
1
material 1b with ∼52% 2 estimated by H NMR and HPLC,
(1) (a) Martin, R. E.; Diederich, F. Angew. Chem., Int. Ed. 1999, 38,
1350. (b) Mu¨llen, K.; Wegner, G. Electronic Materials: The Oligomer
Approach; Wiley-VCH: Weinheim, New York, 1998. (c) Schwab, P. F.
H.; Smith, J. R.; Michl, J. Chem. ReV. 2005, 105, 1197. (d) Kraft, A.;
Grimsdale, A. C.; Holmes, A. B. Angew. Chem., Int. Ed. 1998, 37, 402. (e)
Neher, D. Macromol. Rapid Commun. 2001, 22, 1365.
even the reaction time was increased to 48 h. Reason to induce
low yield is unclear yet. With triethylamine as the organic base
and Pd(OAc)2 as the catalyst, Heck coupling reactions12 of 1a
(2) Hoeben, F. J. M.; Jonkheijm, P.; Meijer, E. W.; Schenning, A. P. H.
J. Chem. ReV. 2005, 105, 1491 and the references therein.
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P.; Katsis D.; Chen, S. H. J. Am. Chem. Soc. 2003, 125, 14032.
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S. H. Chem. Mater. 2003, 15, 542.
(6) (a) Grisorio, R.; Mastrorilli, P.; Nobile, C.; Romanazzi, G.; Suranna,
G.; Meijer, E. Tetrahedron Lett. 2004, 45, 5367. (b) Geng, Y. H.;
Trajkovska, A.; Katsis D.; Ou, J. J.; Culligan, S. W.; Chen, S. H. J. Am.
Chem. Soc. 2002, 124, 8337.
(7) Yamamoto, Y.; Seki, T.; Nemoto, H. J. Org. Chem. 1989, 54, 4734.
(8) Malan, C.; Morin, C. Synlett 1996, 167.
(5) (a) Wang, H.; Wang, H. H.; Urban, V. S.; Littrell, K. C.; Thiyagarajan,
P.; Yu, L. J. Am. Chem. Soc. 2000, 122, 6855. (b) Li, W.; Wang, H.; Yu,
L.; Morkved, T. L.; Jaeger, H. M. Macromolecules 1999, 32, 3034. (c)
Tsolakis, P. K.; Kallitsis, J. K. Chem. Eur. J. 2003, 9, 936. (d) Peeters, E.;
Van Hal, P. A.; Meskers, S. C. J.; Janssen, R. A. J.; Meijer, E. W. Chem.
Eur. J. 2002, 8, 4470.
(9) Lehmann, U.; Schlu¨ter, A. D. Eur. J. Org. Chem. 2000, 3483.
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8685.
(11) Hayashi, T.; Konishi, M.; Kobori, Y.; Kumada, M.; Higuchi, T.;
Hirotsu, K. J. Am. Chem. Soc. 1984, 106, 158.
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10.1021/jo0602470 CCC: $33.50 © 2006 American Chemical Society
Published on Web 04/26/2006
4332
J. Org. Chem. 2006, 71, 4332-4335

SCHEME 1. Synthesis of Compounds 2-5
3.90 mmol), and Pd(PPh3)4 (135 mg, 0.120 mmol) in 80 mL of
anhydrous DMF and Toluene (1:1) was stirred for 24 h at 85
°C. The mixture was cooled to room temperature then poured
into a large amount of water for extraction with methylene
chloride. The organic extracts were washed with brine before
dried over Na2SO4. Upon evaporating off the solvent, the residue
was purified with column chromatography on silica gel with
petroleum ether:ethyl acetate (16:1) as the eluent to afford 2
(1.80 g, 77%) as a light yellow oil. 1H NMR (300 MHz,
CDCl3): δ(ppm) 7.84 (d, J ) 7.56 Hz, 1H), 7.11-7.78 (m,
3H), 7.59-7.65 (m, 2H), 7.42 (dd, J ) 7.20 Hz, J ) 1.05 Hz,
1H), 7.33 (dd, J ) 10.2 Hz, J ) 1.02 Hz, 1H), 7.15 (m, 1H),
2.02-2.05 (m, 4H), 1.43 (s, 12H), 1.06-1.20 (m, 20H), 0.81-
0.98 (m, 6H), 0.55-0.71 (m, 4H). 13C NMR (75 MHz,
CDCl3): δ(ppm) 151.1, 149.0, 144.1, 142.6, 139.5, 132.8, 132.6,
127.8, 127.0, 123.8, 123.5, 121.9, 119.5, 119.2, 118.0, 82.7,
76.4, 76.0, 75.6, 54.2, 39.2, 30.8, 28.9, 28.2, 23.9, 22.6, 21.6,
13.0, 12.6. Molecular Mass: Calcd for C39H55BO2S: 598.4016.
Found: 598.4020 (MALDI-TOF MS). (HPLC: 95.67%).
and 1b with p-vinylbiphenyl gave similar yield (55% and 50%,
respectively). In Stille reactions,13 2-tributylstannyl thiophene
and 5-tributylstannyl-2, 2′-bithiophene gave the better yield than
p-tributylstannylbiphenyl. The relative low yield from p-
tributylstannylbiphenyl can be attributed to its lower reactivity.
13b
Although aryl iodine is more reactive than aryl bromine in
palladium-catalyzed Stille reactions as reported in the reference,13b
the compound 1b gave inferior yield than 1a in Stille reactions.
We found out that the Stille reactions of 1b gave more
deiodonation byproduct. The dehalogenation reaction was also
observed by Nemoto in the preparation of boronic ester via Stille
reaction.7
B. From 1a by Kumada reaction. A solution of thienylmag-
nesium bromide (1.1 mL, 0.8 M, 0.88 mmol), 1a (0.50 g, 0.84
mmol), and Pd(dppf)Cl2 (7.0 mg, 0.0086 mmol) in 10 mL of
anhydrous THF was stirred for 48 h at room temperature. The
mixture was poured into a large amount of water for extraction
with methylene chloride. The organic extracts were washed with
brine and dried over Na2SO4. Upon evaporating off the solvent,
the residue was purified with column chromatography on silica
gel with petroleum ether:ethyl acetate (16:1) as the eluent to
afford 2 (0.42 g, 86%) as a light yellow oil. (HPLC: 96.30%).
Based on aforementioned results of coupling reactions, two
monodisperse fluorenyl/bithienyl cooligomers carrying two
boronic ester end-capping groups were synthesized by means
of Stille reaction. As shown in Scheme 2, the reactions of 1a
with organic tin derivatives 6 and 8 afforded compounds 7 and
9 in a yield of 37% and 45%, respectively, after purified by
column chromatography and recrystallization.
7′-(2,2′-bithien-5-yl)-9′,9′-dioctyl-fluoren-2′-yl-4,4,5,5,-tet-
ramethyl-[1,3,2]dioxaborolane (3). The procedure for the
synthesis of 2 from 1a by Stille reaction was followed to prepare
3 from 1a and 5-tributylstannyl-2,2′-bithiophene in a yield of
Thienyl-terminated conjugated segments can be further func-
tionalized or polymerized for building block copolymers or
supramolecular system.14 Compounds 2 and 3 with one boronic
ester group can be used to synthesize monodisperse conjugated
oligomers with thiophene end groups. As an example, compound
3 reacted with 2,7′′-dibromo-[9,9,9′,9′,9′′,9′′-hexahexyl]-7, 2′;7′,2′′-
terfluorene (10) in a Suzuki coupling15 condition yielded
bithienyl-capped pentafluorene 11 in a yield of 60% after
chromatography, as shown in Scheme 3.
In summary, we have demonstrated that boronic pinacol ester
group is not reactive in Kumada, Heck and Stille coupling
reaction conditions. Various fluorene-based organoboron com-
pounds have been synthesized from 2-bromo or iodo-fluorenyl
boronic pinacol ester via Kumada, Heck and Stille coupling
reactions. Our results provide a new protocol to synthesize
sophisticated arylboron compounds and MCOs with reactive
end-capping groups as building blocks/intermediates for con-
struction of complicated conjugated system.
1
79%. H NMR (300 MHz, CDCl3): δ(ppm) 7.85 (d, J ) 7.59
Hz, 1H), 7.71-7.78 (m, 3H), 7.58-7.64 (m, 2H), 7.33 (d, J )
3.72 Hz, 1H), 7.26-7.27 (m, 2H), 7.21 (d, J ) 3.81 Hz, 1H),
7.06-7.09 (m, 1H), 2.01-2.08 (m, 4H), 1.43 (s, 12H), 1.07-
1.23 (m, 20H), 0.80-0.90 (m, 6H), 0.61-0.71 (m, 4H). 13C
NMR (75 MHz, CDCl3): δ(ppm) 152.6, 150.5, 144.3, 143.9,
141.1, 137.9, 136.9, 134.2, 133.7, 129.3, 128.3, 127.9, 125.0,
124.9, 124.7, 124.0, 120.9, 120.3, 119.4, 84.1, 77.8, 77.6, 77.4,
74.0, 55.6, 40.6, 32.2, 30.3, 29.6, 28.3, 27.3, 25.3, 24.1, 23.0,
17.9, 14.4, 14.0. Anal. Calcd. for C43H57BO2S2: C, 75.85; H,
8.44. Found: C, 75.63; H, 8.11. Molecular Mass: Calcd for
C43H57BO2S2: 680.3893. Found: 680.3808 (MALDI-TOF MS).
5,5′-Bis[9′,9′-dioctyl-fluoren-2′-yl-4,4,5,5,-tetramethyl-[1,3,2]-
dioxaborolane]-2,2′-bithiophene (7). In absence of light, a
solution of 5,5′-bis(tributylstannyl)-2,2′ -bithiophene (6) (0.74
g, 1.0 mmol), 1a (1.20 g, 2.10 mmol) and Pd(PPh3)4 (70 mg,
0.060 mmol) in 30 mL of anhydrous DMF and Toluene (1:1)
was stirred for 24 h at 85 °C. The mixture was cooled to room
temperature then poured into a large amount of water for
extraction with methylene chloride. The organic extracts were
washed with brine and dried over Na2SO4. Upon evaporating
off the solvent, the residue was purified with column chroma-
tography on silica gel with petroleum ether:ethyl acetate (8:1)
as the eluent followed by recrystallization with petroleum ether
gave pure 7 (0.44 g, 37%). 1H NMR (300 MHz, CDCl3):
δ(ppm) 7.85 (d, J ) 7.69 Hz, 2H), 7.72-7.78 (m, 6H), 7.60-
7.66 (m, 4H), 7.36 (d, J ) 1.88 Hz, 2H), 7.26 (d, J ) 1.90 Hz,
Experimental Section
7′-(Thien-2-yl)-9′,9′-dioctyl-fluoren-2′-yl-4,4,5,5,-tetramethyl-
[1,3,2]dioxaborolane (2).
A. From 1a by Stille reaction. In absence of light, a solution
of 2-tributylstannyl thiophene (1.45 g, 3.90 mmol), 1a (2.32 g,
(13) (a) McKean, D. R.; Parrinello, G.; Renaldo, A. F.; Stille, J. K. J.
Org. Chem. 1987, 52, 422. (b) Bao, Z.; Chan, W. K.; Yu, L. J. Am. Chem.
Soc. 1995, 117, 12426. (c) Espinet, P.; Echavarren, A. Angew. Chem., Int.
Ed. 2004, 43, 4704.
(14) Crouch, D. J.; Skabara, P. J.; Lohr, J. E.; McDouall, J. J. W.; Heeney,
M.; McCulloch, I.; Sparrowe, D.; Shkunov, M. Coles, S. J.; Horton, P. N.;
Hursthouse, M. B. Chem. Mater. 2005, 17, 6567.
(15) Miyaura, N.; Suzuki, A. Chem. ReV. 1995, 95, 2457.
J. Org. Chem, Vol. 71, No. 11, 2006 4333

TABLE 1. Palladium Catalyzed Cross-coupling Reactions of the Compounds 1A and 1b
a Yields after purification. b THF, refluxing 28 h. c DMF/toluene (1/1 v/v), 85 °C, 24 h. d DMF, NEt3, tris (o-tolyl)phosphine, 110 °C, 24 h. e A mixture
of 1b and 2 with ∼52% 2.
SCHEME 2. Synthesis of Monodisperse Conjugated Oligomers7 and 9
1
SCHEME 3. Synthesis of Monodisperse Conjugated
Oligomer11
followed to prepare 9 as a yellow solid in a yield of 45%. H
NMR (300 MHz, CDCl3): δ(ppm) 7.82 (d, J ) 7.53 Hz, 2H),
7.69-7.75 (m, 6H), 7.61-7.63 (m, 4H), 7.57 (s, 4H), 7.34 (d,
J ) 3.66 Hz, 4H), 7.24 (d, J ) 3.81 Hz, 4H), 2.04-2.06 (m,
12H), 1.43 (s, 24H), 1.08-1.24 (m, 60H), 0.81-0.86 (m, 18H),
0.66 (m, 12H). 13C NMR (75 MHz, CDCl3): δ(ppm) 152.7,
152.2, 150.5, 144.3, 144.2, 144.0, 141.0, 140.7, 137.0, 134.3,
133.7, 133.4, 129.3, 125.1, 124.9, 124.1, 121.0, 120.6, 120.3,
119.4, 84.1, 77.8, 77.4, 77.0, 55.7, 40.8, 40.7, 32.2, 30.4, 29.6,
25.4, 24.1, 23.0, 14.5, 14.1. Anal. Calcd. for C115H152B2O4S4:
C, 79.00; H, 8.76. Found: C,78.74; H, 8.86. Molecular Mass:
Calcd for C115H152B2O4S4: 1747.0759. Found: 1747.0728
(MALDI-TOF MS).
2H), 2.04-2.06 (m, 8H), 1.43 (s, 24H), 1.08-1.24 (m, 40H),
0.81-0.86 (m, 12H), 0.66 (m, 8H). 13C NMR (75 MHz,
CDCl3): δ(ppm) 152.6, 150.5, 144.3, 144.0, 141.1, 137.0, 134.3,
133.7, 129.3, 127.9, 124.8, 124.1, 121.0, 120.3, 119.4, 84.1,
77.8, 77.4, 77.0, 55.7, 40.6, 32.2, 30.4, 29.6, 25.3, 24.1, 23.0,
14.5. Anal. Calcd. for C78H108B2O4S2: C, 78.37; H, 9.11.
Found: C, 78.11; H, 9.25. Molecular Mass: Calcd for
C78H108B2O4S2: 1194.7875. Found: 1195.5976 (MALDI-TOF
MS).
Compound 11. In absence of light, a solution of 2,7′′-
dibromo-[9,9,9′,9′,9′′,9′′- hexahexyl] -7,2′;7′,2′′-terfluorene (10)
(0.42 g, 0.36 mmol), 3 (0.54 g, 0.80 mmol), NaHCO3 (1.00 g,
12.0 mmol)and Pd(PPh3)4 (18 mg, 0.16 mmol) in 30 mL of
anhydrous THF and 10 mL water was refluxed for 28 h. The
mixture was cooled to room temperature then poured into a large
amount of water for extraction with methylene chloride. The
Compound 9. The procedure for the synthesis of 7 was
4334 J. Org. Chem., Vol. 71, No. 11, 2006

organic extracts were washed with brine and dried over Na2SO4.
Upon evaporating off the solvent, the residue was purified with
column chromatography on silica gel with petroleum ether:
methylene chloride (8:1) as the eluent to afford 11 (0.45 g, 60%)
Found: 2105.1782 (MALDI-TOF MS). (HPLC: 97.25%).
Acknowledgment. This work is supported by NSFC (Nos.
20423003, 20474063, and 20521415), Hundreds Talents Pro-
gram of Chinese Academy of Sciences, and Distinguished
Young Scholar Foundation of Jilin Province (No. 20040101).
1
as a light yellow solid. H NMR (300 MHz, CDCl3): δ(ppm)
7.62-7.89 (m, 30H), 7.36 (d, J ) 3.78 Hz, 2H), 7.27-7.29
(m, 4H), 7.24 (d, J ) 3.70 Hz, 2H), 7.08-7.11 (m, 2H), 2.14-
2.21 (m, 20H), 1.05-1.28 (m, 80H), 0.80-0.90 (m, 46H). 13
C
Supporting Information Available: Experimental procedure
and characterization for all intermediates and compounds not
NMR (75 MHz, CDCl3): δ(ppm) 152.2, 152.1, 144.4, 141.1,
141.0, 140.8, 140.4, 140.2, 138.0, 136.8, 133.2, 128.3, 126.6,
125.1, 124.7, 123.9, 121.9, 120.6, 120.4, 77.8, 77.4, 77.0, 70.5,
55.7, 40.8, 32.2, 31.9, 30.4, 30.1, 29.6, 24.3, 23.0, 14.4. Anal.
Calcd. for C149H186S4: C, 85.00; H, 8.90. Found: C, 84.95; H,
8.62. Molecular Mass: Calcd for C149H186S4: 2103.3437.
1
included in Experimental Section, H NMR spectra of all com-
pounds, GC-MS spectra of key intermediates and UV-vis
absorption spectra of compounds 7, 9, and 11. This material is
available free of charge via the Internet at http://pubs.acs.org.
JO0602470
J. Org. Chem, Vol. 71, No. 11, 2006 4335