New alkyl derivatives phosphine sulfonate (P-O) ligands. Catalytic activity in Pd-catalysed Suzuki-Miyaura reactions in water

Article abstract of DOI:10.1039/b707590c

Two novel bis(o-methoxyphenyl) phosphinoalkylsulfonate (P-O) ligands have been prepared through a new and sustainable synthetic route; they are air stable as well as water soluble and have been applied in Pd-catalysed Suzuki-Miyaura cross-coupling reactions in neat water in conjunction with microwave heating. The Royal Society of Chemistry.

Full text of DOI:10.1039/b707590c

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New alkyl derivatives phosphine sulfonate (P–O) ligands. Catalytic activity in
Pd-catalysed Suzuki–Miyaura reactions in water†‡
Eduardo J. Garc´ıa Sua´rez,a Aurora Ruiz,a Sergio Castillo´n,*b Werner Oberhauser,c Claudio Bianchinic and
Carmen Claver*a
Received 18th May 2007, Accepted 21st May 2007
First published as an Advance Article on the web 8th June 2007
DOI: 10.1039/b707590c
Two novel bis(o-methoxyphenyl) phosphinoalkylsulfonate
(P–O) ligands have been prepared through a new and sustain-
able synthetic route; they are air stable as well as water soluble
and have been applied in Pd-catalysed Suzuki–Miyaura cross-
coupling reactions in neat water in conjunction with micro-
wave heating.
Chelating anionic P–O ligands constitute a class of ligands that
have been studied much less than their diphosphine counterparts.
Oligomerization and polymerization reactions are efficiently catal-
ysed by metal complexes with anionic P–O ligands.1 The palladium
Scheme 1 Synthesis of ligands 1 and 2.
The palladium(II) complex 1a with ligand 1 was synthesised by
catalysed copolymerisation of carbon monoxide and ethene gives
perfectly alternating polyketones with diphosphine ligands while
non-alternating polyketones with P–O ligands are obtained.2
We present here the synthesis of two novel diaryl alkyl phosphine
sulfonate ligands through a new and clean synthetic protocol. This
kind of ligand, containing an alkyl backbone, can be modified
both in the length of the aliphatic chain and in the nature
of the substituents on the aryl moieties. Synthetic routes for
related phenyl derivatives reported in the literature involve the
use of ammonia as solvent3 or sulfones as reactants.4 Indeed, all
attempts to synthesise phosphinoalkylsulfonate ligands without
either ammonia or sulfones were unsuccessful.5
Ligands 1 and 2 were synthesised by the reaction of butyllithium
with bis(o-methoxy)phenylphosphine in THF,6 followed by reac-
tion of the lithium salt with the appropriate bromoalkylsulfonate
derivative as shown in Scheme 1. The ligands were isolated as
zwitterions in 54% and 43% yield, respectively. Both ligands are
water soluble and air stable.
2
adding [Pd2(l-Cl)2{g-1,g -C8H12OMe}2]7 to a solution of ligand 1
as its sodium salt in CH2Cl2 (Scheme 2).
Scheme 2 Synthesis of Pd(II) complex 1a.
The structure of 1a was determined by X-ray crystallography.†
An ORTEP drawing of the complex is shown in Fig. 1. The com-
plex exhibits a distorted square-planar geometry. The phosphine
sulfonate ligand coordinates to palladium in a bidentate fashion.
Evidence for the presence of a P–H bond in either ligand was
confirmed by low temperature 31P NMR spectroscopy, showing
1
the characteristic JPH coupling constants of 545 Hz for ligand 1
and 533 Hz for ligand 2.
aDept. de Qu´ımica Inorga´nica, Facultat de Qu´ımica, Univesitat Rovira I
Virgili, C/Marcel.li Domingo S/N, 43007 Tarragona, Spain. E-mail:
carmen.claver@urv.cat; Fax: +34 977 559563; Tel: +34 977 559574
bDept. de Qu´ımica Orga´nica, Facultat de Qu´ımica, Univesitat Rovira I
Virgili, C/Marcel.li Domingo S/N, 43007 Tarragona, Spain
cIstituto di Chimica dei Composti Organometallici (ICCOM-CNR), Area
di Ricerca di Firenze, Via Madonna del Piano 10, 50019 Sesto Fiorentino,
Italy
† CCDC reference number 640968. For crystallographic data in CIF or
other electronic format see DOI: 10.1039/b707590c
‡ Electronic supplementary information (ESI) available: Synthetic proce-
dures and characterization for 1, 2 and 1a; X-ray data for 1a. See DOI:
10.1039/b707590c
Fig. 1 ORTEP drawing of complex 1a. Hydrogen atoms are omitted for
clarity. Thermal ellipsoids are shown at the 30% probability level.
Very few examples of metal complexes containing a direct M–
O–SO2 bond are reported in the literature due to the poor ligating
properties of the sulfonate group. The Pd(1)–O(4) bond length
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a
Table 1 Palladium catalysed S–M cross-coupling of aryl bromides R1–Br and boronic acids R2–B(OH)2
Entry
R1
R2
Conversionb(%) [P–O] 1
Conversionb(%) [P–O] 2
1
2
3
4
5
6
7
8
4-AcC6H4
4-AcC6H4
4-AcC6H4
4-AcC6H4
C6H5
C6H5
4-MeOC6H4
4-MeOC6H4
Ph
1-naphthyl
4-FC6H4
4-MeOC6H4
4-FC6H4
4-MeOC6H4
4-FC6H4
>99
>99
>99
>99
>99
>99
60
>99
>99
>99
>99
>99
>99
71
4-MeOC6H4
90
90
a Conditions; 0.05 mol% Pd(OAc)2, T = 80 ◦C, t = 2 h, Solvent H2O (3 ml), R1–Br (1.5 mmol, R2–B(OH)2 (1.72 mmol), L : Pd (1 : 1), K2CO3 (3 mmol).
b Conversion determined by GC and NMR spectroscopy.
Table 2 Palladium catalysed S–M cross-coupling of aryl bromides R1–Br and boronic acids R2–B(OH)2 promoted by microwave heatinga
Entry
[P–O]
t/min
Pd (mol%)
R1
R2
Conv. (%)b
1
2
3
4
5
6
1
1
1
2
2
2
5
5
10
10
5
0.05
0.05
0.05
0.05
0.05
0.025
4-AcC6H4
4-AcC6H4
4-MeOC6H4
4-MeOC6H4
4-AcC6H4
4-AcC6H4
1-naphthyl
F-C6H4
1-naphthyl
1-naphthyl
Ph
87
96
72
86
90
90
5
Ph
a Conditions: microwave heating with cooling, cat. Pd(OAc)2/[P–O], Pd : L (1 : 1), T = 150 ◦C, (300 W), Solvent H2O (3 ml), R1–Br (1.5 mmol), R2–B(OH)2
(1.72 mmol), K2CO3 (3 mmol). b Conversion determined by GC.
˚
especially in water,14 some experiments with the present catalysts
were performed in conjunction with microwave heating.
Table 2 summarizes the results obtained when the Suzuki–
Miyaura reaction with aryl bromides was promoted in neat water
by microwave heating. Under these conditions, high conversions
were obtained even in a very short time. When electron-poor
aryl bromides were used conversions up to 96% were obtained in
only 5 min (entries 1–2 and entries 5–6) using either ligand. With
electron-rich aryl bromides, longer times (10 min) were required
to achieve similar conversions (entries 3–4). As already shown
(Table 1, entries 7–8), ligand 2, containing a larger alkyl chain, is
slightly more active than ligand 1 (entries 3–4). Further evidence
for this ligand effect is provided by entry 6 that shows how a good
conversion can be obtained even with very small amounts of Pd
complex (0.025 mol%).
is 2.161(2) A, which is comparable to the analogous distance in
known palladium structures containing a {O2S–O−} group.2b,8
Due to their solubility in water and their stability to oxygen,
ligands 1 and 2 can be used in aqueous catalysis.
The Suzuki–Miyaura coupling reaction (Scheme 3) is one of the
most useful methods for the formation of C–C bonds.9 Due to the
high solubility of arylboronic acids in water and the low toxicity of
reagents and by-products as compared to other protocols,9,10 the
Suzuki coupling is an ideal reaction to be performed in water. Over
the last few years, several reports have appeared describing Suzuki
coupling reactions catalysed by water-soluble Pd systems.9a,11,12
In the light of the results obtained with aryl bromides, ligands
1 and 2 were tested with the much less reactive and less expensive
4-chloroacetophenone substrate. The results obtained are shown
in Table 3.
Scheme 3 Suzuki–Miyaura cross-coupling reaction.
We present here the application of compounds 1 and 2 as catalyst
precursors for the Suzuki–Miyaura cross-coupling reaction. To the
best of our knowledge, no diaryl alkyl phosphine sulfonate anionic
ligand has ever been employed in such a reaction in neat water.
In Table 1 we report the results obtained for the Suzuki–Miyaura
reaction between aryl bromides and aryl boronic acids in neat
water and open to the air.
Excellent conversions were achieved under these conditions
when either 4-bromoacetophenone or bromobenzene was used for
the cross-coupling reaction (entries 1–6), using only 0.05 mol% of
palladium. Good conversions were obtained with 4-bromoanisole
(entries 7–8).
Table 3 Palladium catalysed S–M cross-coupling of aryl chlorides R1–Cl
and boronic acids R2–B(OH)2 promoted by microwave heatinga
Entry [P–O] Pd (mol%) R1
R2
Conv. (%)b
1
2
3
4
5
1
1
1
2
2
1.0
2.0
1.0
0.2
0.2
4-AcC6H4 Ph
45
60
45
30
4-AcC6H4 1-naphthyl
4-AcC6H4 4-MeOC6H4
4-AcC6H4 Ph
4-AcC6H4 2-formylphenyl 56
a Conditions: microwave heating with cooling, cat. Pd(OAc)2/[P–O], Pd/L
◦
(1/1), T = 150 C, (300 W), Solvent H2O (3 ml), t = 10 min, R1–
It is well known that microwave technology may be a suc-
cessful technique for chemical synthesis.13 Since this technique
may shorten the reaction times of Suzuki–Miyaura couplings,
Cl (1.5 mmol), R2–B(OH)2 (1.72 mmol), K2CO3 (3 mmol). b Conversion
determined by GC.
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Conversions of up to 60% were achieved using ligand 1 with
2 mol% of Pd complex (entry 2), while for ligand 2 conver-
sions up to 56% were obtained with 0.2 mol% of Pd complex
(entry 5).
In conclusion, two new diaryl alkyl phosphine sulfonate ligands
have been synthesized as zwitterions. The structure of complex 1a
confirms the coordination of the phosphine sulfonate ligand in
a bidentate fashion. Both ligands 1 and 2 have been successfully
used in the palladium-catalysed Suzuki–Miyaura cross-coupling
reaction of aryl bromides as well as aryl chlorides in neat and
non-deareated water.
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Acknowledgements
10 Boronic Acids: Preparation and Applications in Organic Synthesis and
Medicine, ed. D. G. Hall, VCH, Weinheim, 2005.
We thank the Spanish Government (CTQ2004–04412/BQU,
Consolider Ingenio 2010, CSD2006–0003) and the Generalitat
de Catalunya (2005SGR007777 and Distinction for Research
Promotion, 2003 C.C.) for financial support. The Network of
Excellence IDECAT (contract NMP3-CT-2005–516972) is also
thanked for financial support to European integration. We thank
also the polymers group (Q.A.Q.O.) of the Universitat Rovira i
Virgili for the facilities to perform the microwave experiments.
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14 Microwave-assisted Suzuki–Miyaura coupling, for example: N. E.
Leadbeater, Chem. Commun., 2005, 2881.
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The Royal Society of Chemistry 2007
Dalton Trans., 2007, 2859–2861 | 2861
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