Bis(diphosphaferrocene) palladium(II) dimer complexes as efficient catalysts in the synthesis of arylboronic esters

Article abstract of DOI:10.1016/S0022-328X(01)01146-9

Bis(diphosphaferrocenes)PdCl₂ dimers efficiently catalyse cross-coupling reactions between aryl iodides and pinacol borane in dioxane at 80 °C to afford the corresponding aryl boronic esters derivatives in excellent yields.

Full text of DOI:10.1016/S0022-328X(01)01146-9

Journal of Organometallic Chemistry 640 (2001) 197–199
www.elsevier.com/locate/jorganchem
Bis(diphosphaferrocene) palladium(II) dimer complexes as efficient
catalysts in the synthesis of arylboronic esters
Mohand Melaimi, Fran c¸ ois Mathey, Pascal Le Floch *
Laboratoire ‘‘H e´ t e´ ro e´ l e´ ments et Coordination’’ UMR CNRS 7653, Ecole Polytechnique, 91128 Palaiseau Cedex, France
Received 18 July 2001; accepted 26 July 2001
Abstract
Bis(diphosphaferrocenes)PdCl dimers efficiently catalyse cross-coupling reactions between aryl iodides and pinacol borane in
2
dioxane at 80 °C to afford the corresponding aryl boronic esters derivatives in excellent yields. © 2001 Elsevier Science B.V. All
rights reserved.
Keywords: Arylboronic esters; Catalysis; Diphosphaferrocene; Palladium
6
Aryl boronic esters and acids are very useful syn-
thons in many organic transformations [1]. It has also
been found that they have interesting applications in
biology [2] and for the elaboration of electron transfer
sensors [3]. In organic chemistry, their importance has
been emphasized by the palladium-catalysed Suzuki
cross-coupling reaction, which allows the synthesis of
functionalised biphenyl derivatives [4]. Classical syn-
thetic approaches towards these boronic esters hinge on
transmetallation reactions between organomagnesium
or lithium derivatives with boron compounds [5]. How-
ever, the direct Pd-catalysed cross-coupling reaction
involving the condensation of either tetraalkoxydibo-
ranes or dialkoxyhydroboranes with halogenoarenes
proved to be an interesting alternative [6]. This simple
synthetic sequence, known as the Miyaura coupling,
requires relatively large amounts of catalyst (typically
about 1–5 mol%) and improvements aimed at reducing
the load of catalyst are clearly needed to render this
procedure very attractive. In the course of a study
dealing with the use of diphosphaferrocenes [7] as lig-
ands in homogeneous catalysis [8], we recently showed
that the easily available bis(octaethyldiphosphafer-
to 1.10 ) [9]. This result prompted us to explore the
catalytic activity of this new class of complexes in the
Miyaura coupling. A first series of experiments carried
out with several aryl iodides and pinacol borane as
reactants using the palladium(0) complex 1 as catalyst
(0.1 mol%) only showed a slight improvement. Thus
(phenyl)pinacolboronic ester was obtained in a 75%
yield after 60 h of heating in dioxane using triethy-
lamine as base. Considering that in this type of cou-
pling, [Pd(L )Cl ] (L =chelate ligand) complexes often
2
2
2
yield better results than homoleptic [PdL ] (L=
4
monophosphine) species, we next focused our work on
the use of the diphosphaferrocene palladium(II) com-
plexes such as 2 which is conventionally prepared by
reacting equimolar amounts of the (ocatethyl)-
diphosphaferrocene with the [Pd(COD)Cl ]. These com-
2
plexes were shown not to exist as monomers but as
dimers mainly for structural reasons [10] (Chart 1).
In a first run, a significant improvement was obtained
with only 0.05 mol% of catalyst 2 leading to a quantita-
tive conversion. Interestingly, we found that three con-
secutive batches could be carried out with no significant
loss in product yields and TOF. The synthesis of (2-
furanyl)pinacolboronic ester derivative is representa-
tive. Thus, 2-iodofurane (1.6 mmol), pinacolborane (2.4
rocene) palladium(0) complex
1 could efficiently
catalyse the Suzuki cross-coupling reaction between
phenylboronic acid and various bromoarenes (TON up
mmol) and Et N (4.8 mmol) were successively added at
3
2
(
5 °C to a mixture of catalyst 2 (0.05 mol%) in dioxane
10 ml) and the resulting mixture was heated at 80 °C.
After 48 h of heating, pinacolborane, 2-iodofurane and
*
Corresponding author.
0022-328X/01/$ - see front matter © 2001 Elsevier Science B.V. All rights reserved.
PII: S0022-328X(01)01146-9

198
M. Melaimi et al. / Journal of Organometallic Chemistry 640 (2001) 197–199
Chart 1.
Et N were reloaded and heating was pursued for 48 h.
dimeric diphosphaferrocene Pd(II) based catalysts im-
proves the Miyaura cross-coupling reaction. Though
aryl iodides are usually more expensive than arylbro-
mides, the increased TON (hundred times better than
usual catalysts) renders the use of catalyst such as 2
very attractive. Furthermore aryl-pinacolboronic ester
derivatives were formed in very good yields. Studies
aimed at exploring the catalytic activity of complex
such as 2 in other processes of importance are currently
underway in our laboratories.
3
After a second reload using the same amount of reac-
tants and solvent, an extraction with ether followed by
a flash chromatography afforded the functionalised
furane as a brownish oil in an overall 86% yield (Eq.
(
1)).
(
1)
Whereas it would appear that this coupling is rela-
tively insensitive to steric effects (see Table 1 entries 5
and 6), the nature of the halogen plays a significant
role. Thus, no coupling was observed with aryl bro-
mides. This result is fully rational if we consider that
coordination of diphosphaferrocene, which is a rela-
tively strong p-acceptor ligand, decreases electron den-
sity at the Pd centre and therefore disfavours the initial
oxidative addition step. Additional experiments aimed
at identifying the nature of the active species have been
unsuccessful so far. However, we can reasonably pro-
pose that the dimeric structure of complex 2 is not
maintained in solution and that this dimer serves as a
precursor of a 12 or 14 electron (octaethyldiphosphafer-
rocene)Pd(0) complex which is probably the active spe-
cies. In conclusion, we have showed that the use of
1
. General procedure
All reactions were performed under an inert atmo-
sphere of dry nitrogen by using Schlenk techniques and
dry deoxygenated solvents. Pinacolborane (2.4 mmol),
iodoaryl (1.6 mmol) and Et N (4.8 mmol) were succes-
sively added at 25 °C using a syringe to a solution of
catalyst 2 (1 mg, 0.8 mmol) in dioxane (10 ml). The
resulting mixture was heated in an oil bath at 80 °C and
the progress of the reaction was monitored by gas
chromatography. After 48 h, no traces of the starting
iodoaryl could be detected in the reaction medium.
Pinacolborane (2.4 mmol), iodoaryl (1.6 mmol) and
3
Et N (4.8 mmol) were added and the mixture was
3
further heated for 48 h. After controlling the disappear-
ance of the starting iodoaryl, the reaction was reloaded.
After an additional 48 h heating, the mixture was
cooled to room temperature and the mixture was evap-
orated to dryness. Ether (2×10 ml) was added to allow
Table 1
Cross-coupling reaction between pinacolborane and various aryl io-
dides a
+
−
Entry
Aryl iodide
Yield b (%)
TON
the separation of the Et NH ·I salt and the solution
3
was filtrated. Celite (1 g) was then added and the
solvent was evaporated yielding a pale yellow powder,
which was deposited onto the top of a silicagel column
for chromatography (silicagel was treated with a solu-
1
2
3
4
5
6
7
8
9
Iodobenzene
1-Iodo-4-bromobenzene
p-Iodotoluene
p-Iodoanisole
o-Iodotoluene
o-Iodoanisole
2-Iodothiophene
2-Iodofurane
95
94
95
96
98
89
96
86
92
2862
2839
2866
2884
2966
2693
2901
2581
2760
tion of hexane–Et N (95:5) prior to chromatography).
3
The arylboronic esters were eluted with hexane as
eluent.
2-Iodonaphthalene
1.1. 4,4,5,5-Tetramethyl-2-phenyl-[1,3,2]dioxaborolane
a
Reactions were carried out utilizing one equivalent of aryl iodide
and 1.5 equivalents of pinacolborane with 0.05 mol% catalyst 2 in
Yield: 310 mg (95%). For characterizations, see Ref.
[6c].
refluxing dioxane with three equivalents of Et N as base.
3
b
Isolated yields after three runs.

M. Melaimi et al. / Journal of Organometallic Chemistry 640 (2001) 197–199
199
1
.2. 2-(4-Bromo-phenyl)-4,4,5,5-tetramethyl-[1,3,2]-
1.9. 4,4,5,5-Tetramethyl-2-naphthalen-2-yl-[1,3,2]-
dioxaborolane
dioxaborolane
Yield: 424 mg (94%). For characterizations, see Ref.
6a].
Yield: 373 mg (92%). For characterizations, see Ref.
[6c].
[
1.3. 4,4,5,5-Tetramethyl-2-p-tolyl-[1,3,2]dioxaborolane
Acknowledgements
Yield: 331 mg (95%). For characterizations, see Ref.
6c].
We wish to thank the Centre National de la
Recherche Scientifique and the Ecole Polytechnique for
their financial support.
[
1
.4. 2-(4-Methoxy-phenyl)-4,4,5,5-tetramethyl-[1,3,2]-
dioxaborolane
References
Yield: 359 mg (96%). For characterizations, see Ref.
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998 chapter 2.
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[
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(
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Yield: 333 mg (89%). For characterizations, see Ref.
(
6
(
[6c].
1
.7. 4,4,5,5-Tetramethyl-2-thiophen-2-yl-[1,3,2]-
9268.
[
[
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dioxaborolane
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[6c].
1.8. 4,4,5,5-Tetramethyl-2-furan-2-yl-[1,3,2]-
dioxaborolane
2
862; (f) C. Ganter, C. Glinsb o¨ ckel, B. Ganter, Eur. J. Inorg.
1
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Yield: 267 mg (86%). H-NMR (300 MHz, CDCl ) l
3
3
1
.33 (s, 12H, Me), 6.42 (dd, 1H, JHH=1.62 Hz,
JHH=3.36 Hz, 4-H), 7.06 (d, 1H, JHH=3.36 Hz,
-H), 7.63 (d, 1H, JHH=1.62 Hz, 5-H); C-NMR
50.32 MHz, CDCl ) l 24.5 (s, Me), 83.0 (s, 2-C), 84.0
3
3
3
13
3
(
(
3
s, CꢀO), 110.2 (s, 4-C), 123.1 (s, 3-C), 147.1 (s, 5-C);
[
9] X. Sava, L. Ricard, F. Mathey, P. Le Floch, Organometallics 19
+
EIMS m/z (%): 194 (M , 40), 151 (M-43, 100). Anal.
(
2000) 4899.
Found: C, 62.15; H, 7.83. Calc. for C H BO : C,
1
0
15
3
[10] X. Sava, L. Ricard, F. Mathey, P. Le Floch, Chem. Eur. J. 7 (2001)
3159.
6
1.90; H, 7.79%.