Stille-type aryl-aryl cross-coupling catalysis using triarylphosphine ligands with electron-rich Fe(ii)-alkynyl substituents

Article abstract of DOI:10.1039/c1nj20480g

The synthesis of four new phosphane ligands featuring electron-rich Fe(ii) "(η²-dppe)(η⁵-C₅Me ₅)FeCC-" substituents in para and meta position(s) on the aryl rings (1-4) is reported along with those of the corresponding PdCl ₂(PAr₃)₂ precatalysts (7-10). These precatalysts have then been tested in a Stille-type aryl-aryl cross coupling reaction. It is shown that these new ligands survive the reaction conditions and perform at least as well as the classic triphenylphosphine ligand for this transformation.

Full text of DOI:10.1039/c1nj20480g

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LETTER
Stille-type aryl–aryl cross-coupling catalysis using triarylphosphine
ligands with electron-rich Fe(II)-alkynyl substituentsw
a
a
a
ab
´
Guillaume Grelaud, Ayham Tohme, Gilles Argouarch, Thierry Roisnel and
a
´ ´
Frederic Paul*
Received (in Montpellier, France) 3rd June 2011, Accepted 19th September 2011
DOI: 10.1039/c1nj20480g
The synthesis of four new phosphane ligands featuring electron-
2
5
R
rich Fe(II) ‘‘(g -dppe)(g -C5Me5)FeC C-’’ substituents in para
and meta position(s) on the aryl rings (1–4) is reported along
with those of the corresponding PdCl2(PAr3)2 precatalysts
(7–10). These precatalysts have then been tested in a Stille-type
aryl–aryl cross coupling reaction. It is shown that these new
ligands survive the reaction conditions and perform at least as well
as the classic triphenylphosphine ligand for this transformation.
Judicious spatial organisation of electron-releasing or sterically-
demanding groups on phosphine ligands has fostered the
development of very active palladium-based catalysts for
various cross-coupling reactions.1–3 We therefore wondered if
incorporation of the bulky and electron-releasing organometallic
‘‘(Z2-dppe)(Z5-C5Me5)FeCRC-’’ fragment4 on the periphery of
the classic triphenylphosphine ligand could lead to similar effects.
While the use of phosphane ligands featuring ferrocenyl groups
in catalysis has precedence,5–14 the use of phosphane ligands
bearing electron-rich group 8 transition metal alkynyls as sub-
stituents has never been attempted so far.15 In particular, the
chemical stability of our organometallic fragment during the
catalytic reaction was a key question. However, selected reports
about similar s-metallated alkynes in related reactions were
suggestive that this organoiron functional group might resist
typical cross-coupling conditions.16–18 In addition, the fair stabi-
lity of this particular organometallic endgroup under its oxidized
Fe(III) state19,20 might be exploited to switch the activity of a
catalyst using redox chemistry,21–23 since Fe(III) substituents will
evidently exert a much diminished electron-releasing effect.
In order to investigate these points we have synthesized the
triarylphosphine metallo-ligands 1–4 (Scheme 1) and the
corresponding palladium(II) precatalysts. These precatalysts
Scheme 1
were then used in an aryl–aryl Stille-type cross-coupling
reaction.24,25
The organometallic ligands 1–4 were synthesized from the
corresponding organic phosphine precursors26 presenting per-
ipheral alkyne groups and from the Fe(II) chloride complex
(Z2-dppe)(Z5-C5Me5)FeCl (5) in two steps, following a classic
activation-deprotonation reaction (eqn (1) and (2)).27 They
were fully characterized28 and X-ray structures were also
obtained for 1 and 3 (Fig. 1).29
a
´
Sciences Chimiques de Rennes, UMR CNRS 6226, Universite de
Rennes 1, Campus de Beaulieu, 35042 Rennes Cedex, France.
E-mail: frederic.paul@univ-rennes1.fr
b Centre de diffractometrie, UMR CNRS 6226, Universite´ de Rennes
1, Campus de Beaulieu, 35042 Rennes Cedex, France.
w Electronic supplementary information (ESI) available: Final atomic
positional coordinates, with estimated standard deviations, bond
lengths and angles and anisotropic thermal parameters. CCDC
755886 (1) and 755887 (3). For ESI and crystallographic data in
CIF or other electronic format see DOI: 10.1039/c1nj20480g
From these new ligands, the precatalysts 7–10 were readily
obtained from the 1,5-cyclooctadiene Pd(II) precursor 630
(eqn (3)). The targeted complexes 7–10 were isolated with high
yields as brown to orange solids and spectroscopically charac-
terized (ESIw).31 These compounds exhibit the characteristic
c
2740 New J. Chem., 2011, 35, 2740–2742
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selected so as to obtain an incomplete conversion with the
classic Pd(II) precursor (PPh3)2PdCl2 (14) in order to be able to
evaluate small changes in the yield when using 7–10. Thus,
4-iodoanisole (11), which proved more sluggish to react than
para-bromonitrobenzene (compare entries 1 and 2 in Table 1)
and 4-(tri-n-butylstannyl)toluene (12) were selected as reactants.
The resulting 4-methoxy-40-methylbiphenyl (13) which forms
during the reaction was isolated after purification by column
chromatography each time. Under the reaction conditions
retained, it is formed in 49% yield when 14 is used as a
precatalyst (Table 1, entry 2). The side products formed during
this catalytic reaction were not identified nor even quantified, but,
4,40-ditolyl was always detected as a minor product (r10%).
The data in Table 1 indicate that better yields are obtained
for all the runs involving the new organometallic ligands 1–4
in place of triphenylphosphine under the usual conditions.
This clearly reveals the beneficial effect of the bulky and
electron-rich ‘‘(Z2-dppe)(Z5-C5Me5)FeCRC’’ substituent(s)
for the catalysis.4 Further, comparison of the yields obtained
with the precatalysts containing the monofunctional ligands 1
and 3 (entries 3 and 6) suggests a beneficial electron-releasing
effect originating from a mesomeric contribution, since a lower
yield is obtained when the organometallic substituent is in
meta-position relative to the phosphorous atom, itself coordi-
nated to the reacting site. Then, when increasing the number of
these substituents appended to the phosphine ligand from one
to three, only a modest effect on the yield of 13 is stated for
ligands with para substituents, i.e. when progressing from
7 to 8 (entries 3 and 5). However, a much more pronounced
effect is stated for substituents in the meta position, i.e. when
progressing from 9 to 10 (entries 6 and 7). This 17% increase
in yield cannot be solely ascribed to the cumulated electronic
effects of the three organoiron substituents because at least a
similar improvement should have been stated between 7 and 8.
More likely, the observed improvement originates in part
from the increase in steric bulk around the phosphorus atom
specifically in 4. Indeed, while the cone angles of the new
phosphines 1–3 might be relatively similar, as indicated by
the comparable Cipso–P–Cipso angles derived from their X-ray
structures, a significant increase in the cone angle can be
anticipated for 4, based on molecular models.33 Such a ligand
might therefore better stabilize low coordinated Pd(0) species
in the catalytic cycle, explaining thereby the increased catalytic
activity stated with 10.2
Fig. 1 ORTEP representations of the complexes 1 (a) and 3 (b) with
displacement ellipsoids at the 50% probability level.
Table 1 Effect of the precatalyst type on the coupling of 4-iodo-
anisole (11) and 4-(tri-n-butylstannyl)toluene (12)
Entry
Precatalyst (ligand)
E
0 a/V
Yieldb [%]
1
2
3
4
5
6
7
14 (PPh3)
14 (PPh3)
7 (1)
/
/
80c
49
61
52
65
54
71
ꢀ0.10
ꢀ0.10
+0.09e
ꢀ0.13
ꢀ0.10
7 (1)d
8 (2)
9 (3)
10 (4)
a
Values in
V vs. SCE. Conditions: CH2Cl2 solvent, 0.1 M
[NnBu4][PF6] supporting electrolyte, 20 1C, Pt electrode, sweep rate
0.100 V sꢀ1
.
Yield of isolated product (ꢁ2%). para-Bromonitro-
b
c
benzene was used instead of para-iodoanisole (11). [(Z5-C5H5)2Fe][PF6]
(2 eq. vs. precatalyst) added to the reaction medium. e In THF instead of
CH2Cl2.
d
n
CRC stretch near 2040 cmꢀ1, as well as two singlets in 31P NMR,
near 100 ppm and 25 ppm in a 1:2 or 1:6 ratio. The latter signal
which corresponds to the pendent phosphine group evidences that
coordination took place at this position since it is significantly
downfield-shifted relative to its position for the free ligands 1–4,
around ꢀ4 ppm. All these complexes show also a characteristic
oxidation process near ꢀ0.11 V vs. SCE in dichloromethane,
typical of the metal-centered Fe(III/II) oxidation of the organo-
metallic substituent(s) (Table 1).27,32
ð4Þ
ð3Þ
We next tested the Pd(II) precatalysts 7–10 along with CuI as
a cocatalyst in the Stille-type cross-coupling recently developed
by Baldwin and coworkers (eqn (4)).24,25 In this reaction,
unsymmetrical biaryls are formed from aryl bromides and aryl
stannanes reactants. The reaction was run in N,N-dimethyl-
formamide (DMF), since this solvent allows for the mildest
conditions. Also, operating conditions and substrates were
Finally, an attempt to switch the activity of the catalyst by
using a ligand with oxidized Fe(III) substituent(s) was made
(entry 4).23,34 To this aim, the precatalyst containing 1 (7)
was oxidized with two equivalents of ferricinium hexafluoro-
phosphate and the catalytic run was repeated. The yields
in 13 were only slightly smaller than those under the standard
conditions (compare entries 3 and 4). This statement is
c
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not surprising since we could check a posteriori, using
[(Z2-dppe)(Z5-C5Me5)FeCRCPh][PF6] (15[PF6]) to model
the Fe(III) substituents, that the reaction medium was certainly
reducing them back to Fe(II) ones during the early reaction
course.35 Along the same lines, when checking if the added
ferricinium salt did not exert any detrimental effect on the
catalyst activity we have also stated that ferricinium was
reduced back to ferrocene during the run, further evidencing
the reducing nature of the reaction medium at 50 1C. Obviously,
the medium is too reducing to allow for the survival of any
Fe(III) ‘‘[(Z2-dppe)(Z5-C5Me5)FeCRC-]+’’ substituent under
the operating conditions.
13 S.-Y. Liu, M. J. Choi and G. C. Fu, Chem. Commun., 2001,
2408–2409.
14 A. Jacob, B. Milde, P. Ecorchard and H. Lang, J. Organomet.
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C. Lapinte, Organometallics, 2005, 24, 5464–5478.
20 F. Paul and C. Lapinte, Coord. Chem. Rev., 1998, 178–180,
431–509.
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K. Meyer, P. Mehrkhodavandi and P. L. Diaconescu, J. Am.
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´ ´ ´
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In conclusion, we report in this letter the synthesis of new
Pd(II) precatalysts 7–10 from the original phosphine ligands
1–4 and show that these precatalysts can be successfully used
to catalyze aryl–aryl Stille-type cross-coupling reactions. Like-
wise to ferrocenyl substituents, it is thus shown for the first
time that redox-active group 8 transition metal alkynyls, such
as ‘‘(Z2-dppe)(Z5-C5Me5)FeCRC’’ fragments can be used as
a functional group when designing ligands for catalytic trans-
formations. In the particular cross coupling reaction used,
these new organometallic substituents, by virtue of their
electron-releasing power and/or steric bulk, allow to perform
better with 1–4 than with triphenylphosphine. However,
attempts to exploit the redox bistability of these ligands to
control the activity of the catalyst in this transformation were
frustrated by the reducing nature of the reaction medium.
Efforts are underway to test these new redox-active metallo-
ligands in other catalytic reactions.
28 In a Schlenk tube, 5 (0.625 g, 1 mmol), KPF6 (0.221 g, 1.2 mmol)
and (4-ethynylphenyl)diphenylphosphine (0.344 g, 1.2 mmol) were
dissolved in THF (15 mL) and MeOH (15 mL) and stirred over-
night at room temperature. After removal of the solvents, the dark
brown residue was extracted with dichloromethane, concentrated,
reprecipitated with pentane and dried in vacuo to give the corres-
ponding vinylidene complex as an orange solid. This solid was then
stirred for 1 h in THF in the presence of excess DBU (0.44 mL,
3 mmol). Removal of the solvent in vacuo, purification by column
chromatography and washing of the resulting solid with pentane
afforded the acetylide complex 1 (0.63 g, 72%) as an orange
powder. (Calcd for C56H53FeP3: C: 76.89%, H: 6.11%; Found:
C: 76.62%, H: 6.27%). m/z: 874.2711 (M+, 55%) [calc: 874.2709].
Acknowledgements
G.G. thanks Region Bretagne for a scholarship. The ANR
Blanc program is acknowledged for financial support (ANR
2010 BLAN 719).
n
max(KBr)/cmꢀ1 2048 s (CRC). dP(81 MHz; C6D6; H3PO4) 101.4
(2P, s, dppe), ꢀ4.2 (1P, s, PPh2). dH(200 MHz; C6D6; Me4Si) 7.98
(4H, Har, m), 7.48–7.04 (30H, m, Har), 2.62 (2H, m, CH2 dppe) 2.12
(2H, m, CH2 dppe), 1.52 (15H, m, C5Me5). CV (CH2Cl2, 0.1 M
n-Bu4NPF6): E1/2 = ꢀ0.14 V.
Notes and references
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%
29 Crystal data for 1: C56H53P3Fe, MZ= 874.74, P1 triclinic space
group, a (A) = 10.9096(6), b (A) = 11.1052(5), c (A) = 20.2561(10),
a(1) = 102.897(3), b(1); = 94.369(3), g(1) = 107.657(2), V (A3) =
2251.97(19), Z = 2, R = 0.063, Rw = 0.110, GOF = 1.017. Crystal
data for 3: C56H53P3Fe, MZ= 874.74, P21/n monoclinic space
group, a (A) = 12.2780(4), b (A) = 21.9742(10), c (A) =
16.8487(7), b(1) = 103.994(2), V (A3) = 4410.9(3), Z = 4, R =
0.050, Rw = 0.0.094, GOF = 1.031.
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35 When 15[PF6] (83 mg; 0.1 mmol) is heated in DMF (5 mL) at 50 1C
during 12 h in the presence of 4-tolyl tributylstannane 12 (1 eq.)
and CsF (2 eq.), the Fe(II) complex 15 can be recovered in more
than 50% yield after chromatographic separation.
c
2742 New J. Chem., 2011, 35, 2740–2742
This journal is The Royal Society of Chemistry and the Centre National de la Recherche Scientifique 2011