(0) catalyst leading to metalated intermediate 6. In the next
step, the arylboronic acid 5 undergoes the transmetalation
step with 6 to obtain intermediate 7. The Pd(II) species 7 is
then reduced in the first reductive elimination step leading
to the monoarylated product 8 and Pd(0). If the diffusion
process of the Pd(0) core away from the coupling product 8
is faster than the oxidative addition step, the regenerated Pd-
(0) catalyst is expected to undergo oxidative addition mainly
with another molecule of the more reactive 2,7-dihalofluorene
4 starting material, leading to a second molecule of the
monoarylated coupling product 11 (path B, hereafter referred
to as mono) as the major coupling product. If the oxidation
addition process occurs faster than the diffusion process, the
regenerated Pd(0) catalyst will undergo oxidative addition
preferably with the monosubstituted fluorene 8, leading to
the disubstituted product 12 (path A, hereafter referred to as
di). Therefore, this specific reaction represents a very suitable
model for the mechanism of the coupling reaction.
Moreover, it was observed that in controlled Suzuki-type
cross-couplings with “preferential oxidative addition” 1,4-
dihalobenzenes underwent a much slower coupling than their
1,2- and 1,3-analogues.18 These results indicate that the
oxidative addition of the regenerated Pd(0) into the para-
carbon-halogen bond, although faster than the diffusion of
the Pd(0) to another monomer, occurs more slowly than the
similar oxidative addition for the 1,2- and 1,3-disubstituted
analogues and represents the rate-limiting step. For complete
conversions, systems with spatially more separated reactive
sites need longer reaction times and higher temperatures.18
Comparing the distance of the two reactive sites of 1,2-
dihalobenzenes (about 3.6 Å) and 2,7-dihalofluorene 4 (about
11.2 Å), the regenerated Pd(0) has to migrate over a much
longer distance for a “preferential oxidative addition” step
for fluorene monomers.
Therefore, an investigation of the above outlined model
reaction for 9,9′-disubstituted fluorenes, which represent
well-suited monomers toward semiconducting polyfluorenes,
seems to be very attractive. If the Pd(0) species can
preferentially “migrate” also over the “large” distance of
approximately two benzene rings, a controlled chain-growth
polymerization of fluorene monomers should be possible.
With the already reported Pd2(dba)3/P(t-Bu)320 as the catalyst
system, K3PO4 as the base, and THF as solvent, cross-
coupling reactions of 2,7-dihalofluorenes 4 with 1 equiv of
either benzeneboronic acid (C6H5B(OH)2) or 4-tert-butyl-
benzeneboronic acid (4-t-BuC6H4B(OH)2) have been carried
out (Table 1).
Excellent ratios of diarylfluorenes 12a to monoarylfluo-
renes (11, R′ ) H) in good overall yields of ca. 90% were
observed for couplings both of 2,7-dibromo-9,9′-bis(2-
ethylhexyl)fluorene 4a with X ) Br, Br-F2/6-Br, Table 1,
entry 1, and 2,7-diiodo-9,9′-bis(2-ethylhexyl)fluorene 4b with
X ) I, I-F2/6-I, Table 1, entry 3, and benzeneboronic acid.
Table 1. Suzuki-Type Cross-Couplings of Dihalofluorenes and
Benzeneboronic Acidsa
smb,e monoc,e did,e mono/
entry
dihalide
boronic acid
(%)
(%)
(%)
di
1
2
3
4
Br-F2/6-Brf C6H5B(OH)2
52.0
99.3
46.5
48.0 0:100
0.7
53.5 0:100
0.0
h
i
Br-F2/6-Br 4-t-BuC6H4B(OH)2
I-F2/6-Ig
I-F2/6-I
C6H5B(OH)2
4-t-BuC6H4B(OH)2 100.0
a Reaction conditions (not optimized): dihalide (1 equiv), boronic acid
(1 equiv), Pd2(dba)3 (1.5%), P(t-Bu)3 (6%), K3PO4 (3 equiv), THF (5 mL),
80 °C, 5 days. b Starting material. c Monosubstituted product. d Disubstituted
product. e Detected by GC-MS. f 2,7-Dibromo-9,9′-bis(2-ethylhexyl)fluo-
rene (4a, X ) Br). g 2,7-Diiodo-9,9′-bis(2-ethylhexyl)fluorene (4b, X )
I). h Benzeneboronic acid. i 4-tert-Butylbenzeneboronic acid.
An approximate 1:1 ratio of disubstituted product and
remaining starting material (because only 1 equiv of aryl-
boronic acid is used) indicates that the reaction proceeds
exclusively via the “preferential oxidative addition pathway”
(path A). Obviously, the reaction of 2,7-dihalofluorenes 4
with 1 equiv of 4-tert-butylbenzeneboronic acid (Table 1,
entries 2 and 4) leads to almost no conversion of the
dihalofluorenes. This should result from the “deactivating”,
electron-donating character of the tert-butyl substitutent (for
synthesis and characterization of 4a/b with X ) Br/I and
12a/b with R′ ) H, t-Bu, see the Supporting Information).
To summarize, this work demonstrates that a “preferential
oxidative addition” via a strictly intramolecular motion of
the regenerated Pd(0) catalyst is observed for extended 2,7-
substituted fluorene monomers. In such a process, 2,7-
diarylfluorene and 2,7-dihalofluorene as starting materials
are the exclusive reaction products of a coupling of 1 equiv
of 2,7-dihalofluorene and 1 equiv of arylboronic acid. These
results show that a controlled, intramolecular Pd insertion
in Suzuki-type cross-coupling reactions is also possible across
the “large” distance of the fluorene monomer.
Acknowledgment. We are grateful to Ju¨rgen Do¨necke
(BU Wuppertal) for GC-MS measurements. S.K.W is a
recipient of a fellowship from the Cusanuswerk for which
he is grateful.
Supporting Information Available: Representative pro-
cedures and characterization data of the new compounds.
This material is available free of charge via the Internet at
(18) Dong, C.-G.; Hu, Q.-S. J. Am. Chem. Soc. 2005, 127, 10006.
(19) Sinclair, D. J.; Sherburn, M. S. J. Org. Chem. 2005, 70, 3730.
(20) Littke, A. F.; Dai, C.; Fu, G. C. J. Am. Chem. Soc. 2000, 122, 4020.
OL061476B
Org. Lett., Vol. 8, No. 18, 2006
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Weber, Sven K.
