Palladium catalysed cross-coupling of (fluoroarene)tricarbonylchromium(0) complexes

Article abstract of DOI:10.1039/a906256d

Fluoroarenetricarbonylchromium(0) complexes were found to undergo Suzuki reactions with arylboronic acids to form biaryltricarbonylchromium(0) complexes in the presence of trimethylphosphine/palladium dibenzylideneacetone and caesium carbonate or caesium

Full text of DOI:10.1039/a906256d

Palladium catalysed cross-coupling of (fluoroarene)tricarbonylchromium(0)
complexes
David A. Widdowson* and Rene´ Wilhelm
Department of Chemistry, Imperial College of Science Technology and Medicine, London, UK SW7 2AY.
E-mail: d.widdowson@ic.ac.uk
Received (in Liverpool, UK) 2nd August 1999, Accepted 29th September 1999
Fluoroarenetricarbonylchromium(0) complexes were found
to undergo Suzuki reactions with arylboronic acids to form
biaryltricarbonylchromium(0) complexes in the presence of
trimethylphosphine/palladium dibenzylideneacetone and
caesium carbonate or caesium fluoride.
The participation of a fluoroarene, albeit as its tricarbonyl-
chromium complex, in a palladium catalysed cross coupling is
unprecedented12 and required careful evaluation. Control
experiments covering the more obvious factors were carried out
and the results are summarised in Table 1. Firstly, the ready
displacement of fluoride in these complexes by other nucleo-
philes,13–16 particularly higher halides, which could then
undergo cross-coupling, required that the system be free of any
source of such nucleophiles. Thus no solvent/additive which
might release higher halide was used and initially, high purity
(99.9%) caesium fluoride was used as the base. In principle
fluoride should be catalytic but attempts to demonstrate this
failed.
The ineffectiveness of tricyclohexylphosphine as ligand (run
3) could be a consequence of steric hindrance between the bulky
tricarbonylchromium unit and the bulky phosphine, and the
smaller trimethylphosphine was used in all subsequent experi-
ments. In the absence of any phosphine, the coupling failed (run
4). Reaction without a base (run 5) also failed but the use of
caesium carbonate (run 6) increased the yield to 61%. A
reaction in the presence of base and phosphine but without
palladium (run 7) gave no coupling. The presence of unchanged
starting material from this reaction removed the possibility that
a borate anion was displacing the fluoride group from the
complex and that the resultant arylborate was the active cross
coupling partner. This result also demonstrated the unreactivity
of neutral phosphine to the fluoro complex, but to reinforce this
conclusion a reaction between trimethylphosphine and fluoro-
benzene complex under cross-coupling conditions was at-
tempted. The complex was recovered unchanged in 98% yield.
This precludes a process of phosphonium salt formation and
participation of this in cross-coupling17,18 as the reaction
sequence.
The Suzuki reaction is one of the most widely used and
successful processes for the synthesis of biaryls and styrenes.1
Although aryl iodides and bromides are the most commonly
used halide partners in the reaction, in some remarkable recent
developments, aryl chlorides, hitherto regarded as inert to
palladium coupling, have been shown to be effective partici-
pants provided that an electron-withdrawing group on the aryl
6
ring2 (including an h -tricarbonylchromium group3) or a basic
phosphine ligand for palladium4–9 is present. In the latter case,
it has been shown that tricyclohexylphosphine and tri-tert-
butylphosphine ligands on the palladium catalyst are partic-
ularly effective, even with electron rich aryl chlorides.4 We
have previously demonstrated the unique synthetic potential of
monofluoroarene complexes in the synthesis of polyfunction-
alised aromatics10,11 and given the ease and efficiency with
which chloroarenetricarbonylchromium complexes undergo
cross-coupling reactions,3 we were prompted to attempt the
Suzuki reaction of the fluoro complexes as a route to highly
functionalised biaryls. We now report the first step towards that
objective, the palladium catalysed cross-coupling of aryl
fluorides.
Using standard conditions for Suzuki coupling,1 tetra-
kis(triphenylphosphine)palladium catalyst, fluorobenzenetri-
carbonylchromium(0) and phenylboronic acid gave the coupled
complex but only in trace amounts (Table 1, run 1). When the
reaction was repeated with tris(dibenzylidenacetone)dipalla-
dium/tricyclohexylphosphine and caesium fluoride as the base
(Fu conditions4), no coupled product was detectable (run 2) but
tris(dibenzylidenacetone)dipalladium/trimethylphosphine (run
3) did produce the biphenyltricarbonylchromium(0) complex in
52% yield.
These experiments provide strong evidence for direct
participation of the fluorobenzene complex in the coupling
process with the implication of an unprecedented oxidative
addition of the C–F bond to the palladium(0) intermediate.19
Whether this is a concerted insertion [Scheme 1, path (a)] or an
addition–elimination sequence [Scheme 1, path (b)] via an exo-
addition20 of palladium to form 1 followed by fluoride loss to
produce 2 cannot be determined at this point. The insertion
product could be active in the catalytic cycle in either neutral (3)
or cationic (2) form.12
Table 1 Suzuki coupling reactions of fluorobenzenetricarbonylchro-
mium(0) and phenylboronic acid
Run Ligand
Palladium
Base (equiv.)
Yield (%)
1a
2
3
4
5b
6
—
Pd(PPh3)4
Pd2(dba)3
Pd2(dba)3
Pd2(dba)3
Pd2(dba)3
Pd2(dba)3
—
Na2CO3 (2.2)
CsF (4)
CsF (4)
CsF (4)
—
Trace
PCy3
PMe3
—
PMe3
PMe3
PMe3
0
52
0
0
61
0
Cs2CO3 (2.2)
Cs2CO3 (2.2)
7
a
b
Toluene as solvent. 4-Methoxyphenylboronic acid replaced phenyl-
boronic acid.
Scheme 1
Chem. Commun., 1999, 2211–2212
This journal is © The Royal Society of Chemistry 1999
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6
Table 2 Suzuki coupling reactions of h -(fluorobenzene)tricarbonylchro-
Notes and references
mium(0)
1 A. Suzuki, in Metal-catalyzed Cross-coupling Reactions, ed. F.
Diederich and P. J. Stang, Wiley-VCH, Weinheim, 1998, p. 49.
2 K.-i. Gouda, E. Hagiwara, Y. Hatanaka and T. Hiyama, J. Org. Chem.,
1996, 61, 7232.
3 W. J. Scott, J. Chem. Soc., Chem. Commun., 1987, 1755.
4 A. F. Littke and G. C. Fu, Angew. Chem., Int. Ed., 1998, 37, 3387.
5 D. W. Old, J. P. Wolfe and S. L. Buchwald, J. Am. Chem. Soc., 1998,
120, 9722.
Run
R1
Yields (%)
6 F. Firooznia, C. Gude, K. Chan and Y. Satoh, Tetrahedron Lett., 1998,
39, 3985.
7 C. Zhang, J. Huang, M. L. Trudell and S. P. Nolan, J. Org. Chem., 1999,
64, 3804.
8 A. F. Little and G. C. Fu, Angew. Chem., Int. Ed., 1999, 38, 2413.
10 J. P. Gilday and D. A. Widdowson, J. Chem. Soc., Chem. Commun.,
1986, 1235.
1
2
3
4
5
6
H
61
87
78
81
79
0a
4-MeO
2-MeO
4-Me
2-Me
4-Br
11 J. P. Gilday and D. A. Widdowson, Tetrahedron Lett., 1986, 27,
5525.
12 E.-I. Negishi and F. Liu, in Metal-catalyzed Cross-coupling Reactions,
ed. F. Diederich and P. J. Stang, Wiley-VCH, Weinheim, 1998, p. 1.
13 K. Kirschke, J. Deutsch and H. J. Niclas, Phosphorus Sulfur Silicon
Relat. Elem., 1996, 117, 293.
14 F. Rose-Munch, E. Rose, A. Semra, L. Mignon, J. Garcia-Oricain and C.
Knobler, J. Organomet. Chem., 1989, 363, 297.
15 M. F. Semmelhack, G. Hilt and J. H. Colley, Tetrahedron Lett., 1998,
39, 7683.
16 V. V. Litvak, P. P. Kun and V. D. Shteingarts, Zh. Org. Khim., 1984, 20,
753.
17 F. E. Goodson, T. I. Wallow and B. M. Novak, J. Am. Chem. Soc., 1997,
119, 12441.
18 M. Sakamoto, I. Shimizu and A. Yamamoto, Chem. Lett., 1995,
1101.
19 H. Yang, H. Gao and R. J. Angelici, Organometallics, 1999, 18,
2285.
a
An intractable oligomeric/polymeric material containing no chro-
mium(0) was produced.
With suitable conditions established, the fluorobenzenechro-
mium complex was coupled with a series of electron-rich
arylboronic acids (Table 2).21 Both moderately hindered (runs
3, 5) and unhindered boronic acids (runs 1, 2, 4) gave good
yields. The polymerisation of 4-bromophenylboronic acid
under the reaction conditions can be attributed to the greater
reactivity of the C–Br bond over the C–F bond.
To further investigate the scope of this process, the more
electron-rich
4-methoxyfluorobenzenetricarbonylchromium
complex was studied.21 The yields (Table 3), with both the
moderately hindered and the unhindered boronic acids, were
comparable to those of the parent complex despite the expected
reduction in reactivity of the alkoxy analogue.
20 M. F. Semmelhack, G. R. Clark, J. L. Garcia, J. J. Harrison, Y.
Thebtaranonth, W. Wulff and A. Yamashita, Tetrahedron, 1981, 37,
3957.
6
6
Table 3 Suzuki coupling reactions of h -(4-fluoromethoxybenzene)-
tricarbonylchromium(0)
21 Typical procedure: A solution of h -(fluorobenzene)tricarbonylchro-
mium(0) (0.300 g, 1.29 mmol), 4-methoxyphenylboronic acid (0.392 g,
2.58 mmol), caesium carbonate (0.925 g, 2.84 mmol), Pd2(dba)3 (5
mol%, 0.060 g, 0.06 mmol) and PMe3 (20 mol%, 0.26 ml, 1 M solution
in toluene) in deoxygenated DME (9 ml) was stirred under reflux for 16
h. Et2O (50 ml) was added and the solution was washed with 10% aq.
NaOH (15 ml), water (15 ml) and brine (15 ml) and dried over MgSO4.
Concentration in vacuo followed by column chromatography (eluent:
6
5% Et2O–hexane) gave h -(4A-methoxybiphenyl)tricarbonylchro-
Run
R1
Yield (%)
mium(0) as a yellow crystalline solid (0.360 g, 87%), mp 58–61 °C
(Found: C, 59.99; H, 3.69. C16H12CrO4 requires C, 60.01; H, 3.78%);
nmax(KBr)/cm21 2986w, 2838w, 1977s, 1961s, 1887s, 1868s, 1609m,
1508m, 1454m, 1279m, 1249m, 1176m, 1020m, 840m, 819m, 661m,
629s, 535m; dH(270 MHz; CDCl3) 7.43 (2H, d, J 8.9), 6.93 (2H, d, J
8.9), 5.64 (2H, dd, J 6.7, 1.0), 5.49 (2H, t, J 6.4), 5.28 (1H, tt, J 6.2, 1.0),
3.83 (3H, s); dC(68 MHz) 233.0, 160.4, 128.6, 128.3, 114.3, 111.2, 93.2,
91.6, 90.9, 55.4; m/z (EI) 320 (M+, 43%), 264 (39), 236 (93), 221 (11),
205 (5), 184 (9), 169 (4), 52 (100) (Observed: M+ 320.0141;
C16H12CrO4 requires 320.0130).
1
2
3
H
2-Me
4-Me
76
77
74
These results clearly raise intriguing mechanistic questions,
which we continue to address. The couplings demonstrate the
versatility of the arenetricarbonylchromium(0) complexes and
add further possibilities to the exploitation of their already
widely established role in synthesis.
Communication 9/06256D
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Chem. Commun., 1999, 2211–2212