Catalytic reductive homocoupling of 9-bromofluorene

Article abstract of DOI:10.1016/j.tetlet.2007.07.120

A number of organometallic compounds and inorganic salts, including the group 8 metal carbonyls M₃(CO)₁₂ and salts MCl₃ (M = Fe, Ru, Os), were tested for their catalytic activity in the reductive coupling of 9-bromofluorene. Among them, FeCl₃ was found to show excellent activity (TOF = 1960). The reaction is believed to proceed via a radical mechanism.

Full text of DOI:10.1016/j.tetlet.2007.07.120

Tetrahedron Letters 48 (2007) 6669–6670
Catalytic reductive homocoupling of 9-bromofluorene
Venugopal Shanmugham Sridevi and Weng Kee Leong*
Department of Chemistry, National University of Singapore, 3 Science Drive 3, Singapore 117543, Singapore
Received 13 June 2007; revised 9 July 2007; accepted 19 July 2007
Available online 27 July 2007
Abstract—A number of organometallic compounds and inorganic salts, including the group 8 metal carbonyls M3(CO)12 and salts
MCl3 (M = Fe, Ru, Os), were tested for their catalytic activity in the reductive coupling of 9-bromofluorene. Among them, FeCl3
was found to show excellent activity (TOF = 1960). The reaction is believed to proceed via a radical mechanism.
Ó 2007 Elsevier Ltd. All rights reserved.
The reductive homocoupling of 9-bromofluorene (RBr)
to 9,90-bifluorenyl (RR) has been reported to be effected
by a variety of reducing agents in stoichiometric
amounts.1 Recently, we reported that 0.1 mol %
Ru3(CO)12 can efficiently catalyse the reductive homo-
coupling of RBr to RR via transfer of the bromine to
the solvent (Scheme 1).2 In order to find a cheaper and
better catalyst, other group 8 metal carbonyls and metal
salts were tested and the results are reported herein.
It is clear from Table 1 that in comparison to
Ru3(CO)12, RuCl3 had very low efficiency (entries 2
and 4, respectively). Interestingly, IrCl3 also showed
activity albeit the efficiency was low (entry 5). On the
other hand, Fe3(CO)12 and FeCl3 (entries 1 and 3) were
as active as Ru3(CO)12; the activity for FeCl3 was clearly
the best at a 0.1 mol % catalyst loading (entries 6–10).
The homocoupling of the substituted bromofluorene,
2-nitro-9-bromofluorene also occurred under FeCl3
catalysis (entry 12). However, the reaction failed with
9-phenyl-9-bromofluorene, presumably because of steric
reasons. The reaction also failed for compounds with
bromine substitutions on the aromatic rings, for exam-
ple, 2-bromofluorene and 2,7-dibromofluorene.
A number of organometallic compounds (Fe3(CO)12,
Os3(CO)12, Cp2TiCl2, Cp2ZrCl2, Cp2TiZnCl, Cp2Zr-
ZnCl and (C9H7)Ru(CO)2Cl) and inorganic salts (FeCl3,
RuCl3, OsCl3, IrCl3, NiCl2 and CuCl2) were tested for
their catalytic activity in the reductive coupling of 9-
bromofluorene. Of these, only the group 8 metal carbo-
nyls M3(CO)12 and salts MCl3 (M = Fe, Ru, Os), as well
as IrCl3, showed activity (Table 1).
Both Os3(CO)12 and OsCl3 showed moderate efficiencies
(entries 8 and 10). We utilized the slower rate of reaction
with Os3(CO)12 to monitor the progress of the reaction
1
by H NMR spectroscopy and found an induction per-
iod of about 40 min. We had earlier ruled out the possi-
bility of heterogeneous catalysis (ruthenium metal did
not exhibit any catalytic activity),2 and also found that
an equimolar reaction between Os3(CO)12 and RBr
afforded a 90% recovery of Os3(CO)12. This suggests
that the induction period is probably related to the for-
mation of the catalytically active tetraosmium analogue
of Ru4(l3-OMe)(l3-OH)(l-Br)2(CO)10, viz., Os4(l3-
OMe)(l3-OH)(l-Br)2(CO)10.2 However, attempts to
isolate intermediates from a stoichiometric reaction of
RBr with FeCl3, Fe3(CO)12 or Os3(CO)12 were
unsuccessful.
Br
Br
Toluene
1/2
+
Δ
,
140 oC
Ru3(CO)12
CH3
RBr
RR
Scheme 1.
Keywords: Reductive coupling; Catalysis; Bifluorenyl; 9-Bromo-
fluorene.
Addition of a small amount of TEMPO to the FeCl3-
catalysed reaction led to complete inhibition (entry
11). This is probably an indicative that the reaction
*
Corresponding author. Tel.: +65 6516 5131; fax: +65 6779 1691;
e-mail: chmlwk@nus.edu.sg
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doi:10.1016/j.tetlet.2007.07.120

6670
V. S. Sridevi, W. K. Leong / Tetrahedron Letters 48 (2007) 6669–6670
Table 1. Catalytic runs for the reductive coupling of RBr
Run
Substrate:catalyst mole ratio
Catalyst
Reaction time (h)
Yield (% of RR)
TON
TOF (hꢀ1
)
1
2
3
4
100:1
100:1
100:1
100:1
Fe3(CO)12
Ru3(CO)12
FeCl3
2
1.5
0.5
2.5
20
6
24
2
2.5
7
0.5
2
5.5
0.5
18
2
98
98
78
98
1
37
49
52
196
0.4
1.9
5
2.7
495
364
114
1960
70
38
0
46
800
300
79a
98
RuCl3
1
37
30a
64a
99
91
80
98 (96a)
14
21
0
83
16
5
100:1
IrCl3
30
64
6
7
8
9
10
1000:1
1000:1
1000:1
1000:1
1000:1
Fe3(CO)12
Ru3(CO)12
Os3(CO)12
FeCl3
990
910
800
980
140
210
0
830
1600
1800
OsCl3
11b
12c
13
1000:1
1000:1
10,000:1
FeCl3
FeCl3
FeCl3
6
18
a Isolated yields.
b In the presence of TEMPO (0.1 mol %).
c For RBr = 2-nitro-9-bromofluorene. Product identified by 1H NMR (DMSO-d6): d 5.35 (s, C–H), 6.9–8.3 (m, aromatic). Lit. values:3 1H NMR
(DMSO-d6): d 5.45 (s, C–H), 6.8–8.4 (m, aromatic).
follows a radical pathway, unlike in the carbonyl cluster
case. In all the reactions showing activity, 1H NMR
analyses of the reaction mixtures showed that 4-bromo-
toluene was produced in a 2:1 ratio with respect to biflu-
orenyl (Scheme 1). No 2- or 3-bromotoluene, nor benzyl
bromide, were observed among the products. Although
the precise mechanism is not clear, it does not involve a
simple halogen atom transfer.
Supplementary data
Experimental procedures and characterization data for
the catalytic runs, and the method for determination
of the induction period, are provided. Supplementary
data associated with this article can be found, in the on-
line version, at doi:10.1016/j.tetlet.2007.07.120.
In conclusion, we have found that in addition to the me-
tal carbonyl Ru3(CO)12, the other group 8 carbonyls, as
well as some inorganic salts, are also active in the cata-
lytic reductive homocoupling of 9-bromofluorene to give
bifluorenyl. In particular, a very cheap and environmen-
tally friendly catalyst, FeCl3, showed excellent activity.
References and notes
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Its catalytic activity probably involves
pathway.
a radical
Acknowledgements
2. Sridevi, V. S.; Leong, W. K.; Zhu, Y. Organometallics 2006,
25, 283–288.
3. Romanelli, M. N.; Teodori, E.; Scapecchi, S.; Dei, E. Eur.
J. Med. Chem. 1992, 27, 53–56.
This work was supported by an A*STAR grant (Re-
search Grant No. 012 101 0035) and one of us (V.S.S.)
thanks ICES for a Research Scholarship.