Di- And tri-alkylphosphine adducts of S-donor palladacycles as catalysts in the Suzuki coupling of aryl chlorides

Article abstract of DOI:10.1039/b407922c

The reaction of tricyclohexylphosphine with the S-based palladacycle [{Pd(μ-OAc)(k²-S, C-C₆H₄CH ₂SMe)}₂] gives several products, regardless of stoichiometry, one of which, [Pd(k¹-OAc)(n¹-C ₆H₄CH₂SMe)(PCy₃)₂], has been characterised crystallographically. Despite this, catalysts formed in situ from di- and tri-alkylphosphines and [{Pd(μ-OAc)(k²-S,C-C ₆H₄CH₂SMe)}₂] show excellent activity in the Suzuki coupling of a range of deactivated, non-activated and activated aryl chloride substrates.

Full text of DOI:10.1039/b407922c

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F U L L P A P E R
Di- and tri-alkylphosphine adducts of S-donor palladacycles as
catalysts in the Suzuki coupling of aryl chlorides
Robin B. Bedford,*a Catherine S. J. Cazin,a Michael B. Hursthouse,b Mark E. Lightb and
Ve´ronique J. M. Scordiaa
a Department of Chemistry, University of Exeter, Exeter, UK, EX4 4QD.
E-mail: r.bedford@ex.ac.uk; Fax: +44 (0) 1392 263434
b EPSRC National Crystallography Service, Department of Chemistry, University of
Southampton, Highfield Road, Southampton, UK, SO17 1BJ
Received 25th May 2004, Accepted 14th September 2004
First published as an Advance Article on the web 22nd October 2004
The reaction of tricyclohexylphosphine with the S-based palladacycle [{Pd(l-OAc)(j2-S,C-C6H4CH2SMe)}2] gives
1
several products, regardless of stoichiometry, one of which, [Pd(j1-OAc)(g -C6H4CH2SMe)(PCy3)2], has been
characterised crystallographically. Despite this, catalysts formed in situ from di- and tri-alkylphosphines and
[{Pd(l-OAc)(j2-S,C-C6H4CH2SMe)}2] show excellent activity in the Suzuki coupling of a range of deactivated,
non-activated and activated aryl chloride substrates.
There has recently been considerable interest in the devel-
Introduction
opment of new, high-activity catalysts that can be used in low
loadings in such reactions, and palladacyclic complexes have
played a significant role in this regard.2 The area was initiated
by Beller, Herrmann and co-workers, who demonstrated that
the palladacyclic complex 1 acts as a good catalyst in the
coupling of aryl bromide substrates.3 We demonstrated that the
pincer complexes 2 and the orthopalladated triarylphosphite
and phosphinite complexes 3 show good to excellent activity in
such couplings,4 whilst Cole-Hamilton and co-workers showed
that the phosphine-based complexes 4 can also be used.5
The coupling of aryl halides with aryl boronic acids, the Suzuki
reaction (Scheme 1), is one of the most powerful and versatile
methods for the synthesis of biaryls.1
Scheme 1 The Suzuki biaryl coupling reaction.
Activity is not limited to orthometallated phosphorus donor
systems. We,6 and later others,7 showed that the N-donor pal-
ladacycle 5a can also be used in aryl bromide coupling reactions.
Of particular note, Milstein and co-workers demonstrated that
the orthometallated imine complex 6a shows excellent activity,8
while Na´jera has shown that the related oxime-containing
complexes 7 can even activate deactivated aryl chlorides in the
presence of water and tetrabutylammonium bromide (TBAB).9
Tricyclohexylphosphine adducts of both imine- and amine-
based palladacycles, complexes 8a and 9a, show very good
activity when aryl chlorides are used as substrates.10 Recently we
have found that similar adducts formed between complexes of
the types 3 and 10 with tricyclohexylphosphine, as well as other
di- and tri-alkylphosphines, show amongst the highest activity
11,4d,e
yet reported in aryl chloride coupling reactions.
Dupont and Monteiro have shown that the orthometallated
thioether complexes 11a–d and related S-based palladacycles
can be used not only for the coupling of aryl bromides but also,
in some cases, aryl chlorides.12 Again the addition of TBAB is
necessary for the reaction to proceed. In view of the substantial
increase in activity in the Suzuki coupling of aryl chlorides on
forming trialkylphosphine adducts of phosphite-, imine- and
amine-based palladacycles, we were interested to see whether
a similar enhancement in activity is observed on formation of
phosphine adducts of thioether-based palladacycles. The results
of this study are presented below.
3 8 6 4
D a l t o n T r a n s . , 2 0 0 4 , 3 8 6 4 – 3 8 6 8
T h i s j o u r n a l i s
T h e R o y a l S o c i e t y o f C h e m i s t r y 2 0 0 4
©

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chosen, PCy3, PtBu3, PR2(o-biphenyl) (R = Cy, tBu) have all been
shown to give good activity in the Suzuki coupling of aryl chlo-
ride substrates.14 The reaction conditions were not optimised,
but rather 1,4-dioxane was used as solvent and Cs2CO3 as base in
order to allow comparison with results obtained previously with
related imine- and amine-based palladacyclic systems.10 In the
first instance we focussed on the coupling of phenyl boronic acid
with 4-chloroanisole since this is an electronically deactivated
(electron rich) aryl chloride and thus provides a challenge for
the catalysts.
Fig. 2 shows the results of a study on varying the type and
amount of di- or trialkylphosphine added to complex 11e at
0.1 mol% Pd loading. Comparing the trialkylphosphines PCy3
and PtBu3 it can be seen that the optimum systems give similar
conversions, however the optimum Pd : P ratio is lower in the
case of PCy3 than it is for PtBu3. In both cases, the higher
phosphine levels employed lead to a reduction in catalytic
Results and discussion
Synthesis of a tricyclohexylphosphine adduct
We chose to use the complex [{Pd(l-OAc)(j2-S,C-C6H4CH2-
SMe)}2], 11e, as the S-based palladacyclic precursor because
the parent thioether is commercially available and the complex
synthesis facile. The reaction of complex 11e with PCy3 does
not give a single clean species, regardless of whether the ratio
of PCy3 : Pd is 1 : 1, 2 : 1 or 2.2 : 1. In all cases the 31P NMR
spectrum of the crude reaction mixture shows a number of peaks
in the range 38–53 ppm, indicative of PCy3 coordination to
Pd(II) centres. In the case of higher phosphine loadings, one of
the singlets at 52 ppm predominates, suggesting the formation
of a trans-bis-phosphine adduct 12 rather than the expected
monophosphine adduct 13 (Scheme 2). This is in contrast
with amine-, imine- and phosphite palladacycles, all of which
form a mono-PCy3 adduct, regardless of the amount of added
phosphine.10,11 Crystals grown from a crude reaction mixture
were analysed by single crystal X-ray diffraction and confirmed
activity, as observed previously with both amine- and phosphite-
10b,4d
based palladacycles,
presumably due to over-coordination
the presence of the new bis-phosphine complex [Pd(j1-OAc)(g -
C6H4CH2SMe)(PCy3)2], 12. The molecular structure is shown
in Fig. 1. The Pd–C bond length is not significantly different
1
of the active catalyst. In contrast when PCy2(o-biphenyl) is
used then, under all the ratios examined, addition of extra
phosphine leads to an increase in activity. This is again in line
10b
1
with the observations with amine-based palladacycles. When
to that found previously in the related complex [Pd(I)(g -
the bulkier dialkylphosphine PtBu2(o-biphenyl) is used, then
C6H4CH2SMe)(PPh3)2],13 while both the P–Pd bond lengths are
longer in the case of the PCy3 adduct.
very poor activity is observed.
Fig. 2 The effect of varying the P : Pd ratio on the Suzuki coupling
of 4-chloroanisole with phenylboronic acid catalysed by the 11e/PR3
mixtures (0.1 mol% Pd loading). Left to right: PR3 = PCy3 (grey
bars), PCy2(o-biphenyl) (dark grey bars), PtBu3 (white bars) and
PtBu2(o-biphenyl) (black bars). Conditions: 4-chloroanisole (1.0 mmol),
phenylboronic acid (1.5 mmol), Cs2CO3 (2.0 mmol), 1,4-dioxane
(10 mL), 100 ◦C, 18 h. Conversion to coupled product, based on
chloroanisole, determined by GC (hexadecane standard).
Scheme 2 (i) PCy3, CH2Cl2, r.t., 30 min.
The data tend to imply that the optimum active catalyst,
when both tricyclohexylphosphine and tri-tert-butylphosphine
are used, is low coordinate and that over coordination effectively
‘switches off’ the catalysis. Indeed a recurrent feature of chloride
coupling catalysts is the observation that in many cases the
active catalysts are probably mono-phosphine complexes.15 With
ligands such as PCy2(o-biphenyl) it is possible that, regardless
of the amount of added phosphine, the formation of low-
coordinate phosphine adducts is favoured. Such a preference
for mono-coordination may be due in part to a secondary p-
coordination of the aromatic ring to the palladium centre.16
Indeed, Fink and co-workers recently demonstrated such an
interaction between this ligand and a Pd(0) centre.17 In this
scenario excess phosphine would simply act to force any
dissociation equilibria back to the formation of complex and
thus increase catalyst longevity. By contrast, the use of an excess
of a phosphine that can have no secondary interactions may
tip the balance in favour of higher-coordinate complexes, thus
diminishing activity.
1
Fig. 1 The molecular structure of [Pd(j1-OAc)(g -C6H4CH2SMe)-
(PCy3)2], 12. Thermal ellipsoid set at 30% probability. Selected inter-
◦
˚
atomic lengths (A) and angles ( ): Pd1–C37 1.995(4), Pd1–O1 2.124(3),
Pd1–P1 2.3677(15), Pd1–P2 2.3651(15); C37–Pd1–P1 90.76(13),
C37–Pd1–P2 90.85(13), C37–Pd1–O1 179.11(16), P1–Pd1–P2 166.24(4),
O1–Pd1–P1 89.79(10), O1–Pd1–P2 88.78(10).
Catalytic studies
Since we were not able to cleanly generate mono- or bis-PCy3
adducts of the complex 11e, all the catalysts used were prepared
in situ from 11e and added phosphines. The phosphine ligands
When the coupling of 4-choloroanisole with phenylboronic
acid is repeated with 11e/PCy3 (Pd : P 1 : 3) at 0.01 mol%
Pd then no activity is observed. In contrast PCy3 adducts of
D a l t o n T r a n s . , 2 0 0 4 , 3 8 6 4 – 3 8 6 8
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Table 1 Suzuki coupling of aryl chlorides with aryl boronic acids catalysed by mixtures of complex 11e and three equivalents of PCy3 or PCy2(o-
biphenyl)a
Entry
Aryl chloride
Aryl boronic acid
11e loading (mol% Pd)
Phosphine
Product
Conv.b (%)
1
2
3
4
5
6
7
8
0.1
0.1
0.3
0.3
0.01
0.01
0.1
0.1
PCy3
PCy2(o-biphenyl)
PCy3
PCy2(o-biphenyl)
PCy3
PCy2(o-biphenyl)
PCy3
47.5
62
21.5
66
0
0
72.5
61.5
PCy2(o-biphenyl)
9
10
0.1
0.1
PCy3
PCy2(o-biphenyl)
80
87
11
12
13
14
15
16
0.1
0.1
0.01
0.01
0.001
0.001
PCy3
PCy2(o-biphenyl)
PCy3
PCy2(o-biphenyl)
PCy3
PCy2(o-biphenyl)
100
100
17
89
4
3
17
18
0.1
0.1
PCy3
PCy2(o-biphenyl)
75
75
19
20
0.1
0.1
PCy3
PCy2(o-biphenyl)
38
33
a Conditions as in Fig. 2, with 1.0 mmol of aryl chloride and 1.5 mmol aryl boronic acid. b Conversion to coupled product, based on chloroanisole,
determined by GC (hexadecane standard).
amine- and imine-based palladacycles show good activity at this
catalyst loading under the same conditions,10 indicating that
while trialkylphosphine adducts of S-based palladacycles are
good, they are not as good as related N-based palladacycles in
aryl chloride Suzuki couplings.
In the absence of added phosphine the palladacycle 11e shows
no activity under the conditions employed here, even when the
loading is increased to 1 mol% Pd. This lack of activity is in
contrast to the use of complexes of the type 11 in the Suzuki
coupling of 4-chloroanisole which show some limited activity
(13% conversion at 4 mol% Pd).12 However in the latter case
the reaction required the presence of TBAB, as is also observed
with nearly all simple N-based palladacycles.2 Given that N- and
P-based palladacyclic Suzuki catalysts rapidly generate Pd(0) by
nucleophilic attack of the aryl boronic acid at the palladium
A possible mechanism for its formation is shown in Scheme 3
starting from complexes of the types 12 or 13 in which there are
one or two phosphine ligands and the sulfur is coordinated or
not respectively. This implies that the active catalysts are low-
coordinate Pd(0) species and that the reaction follows a ‘clas-
sical’ Pd(0)/Pd(II) pathway rather than a Pd(II)/Pd(IV) cycle.2
Since similar active catalysts have been suggested when phos-
phine adducts of N-based palladacycles are used as pre-catalysts,
it may be expected that the S-based analogues should show a
similar activity, which they do not. The lower activity observed
here may be due to a slower induction process or to competitive
coordination of the eliminated thioether 14. Studies to elucidate
the basis of this observation are ongoing in our group.
centre, followed by reductive elimination of the aryl and the
4e,10
orthometallated ligand,
it seems likely that a similar process
is operative in the case of S-based palladacycles (see below). The
active catalysts would then be essentially ‘ligandless’ palladium
supported by the TBAB. Indeed such species have been shown
to be active in the Suzuki coupling of aryl chloride substrates.18
In order to determine whether PCy2(o-biphenyl) adducts of
the complex 11e generally perform better than PCy3-containing
analogues we compared the performances of catalysts formed
in situ with these phosphines with a range of aryl chloride
substrates. The P : Pd ratios were maintained at 3 : 1 for both
phosphines used and the results are summarised in Table 1.
As can be seen, in most cases the PCy2(o-biphenyl) ligand
did indeed perform better than tricyclohexylphosphine. As
mentioned earlier, the activity shown with all substrates is not
as high as that shown with N- or P-based palladacycles.
As referred to earlier, both N- and P-based palladacycles have
been shown to undergo ring-opening processes in Suzuki and
Stille couplings to generate palladium zero complexes. In order
to determine whether a similar process occurs with thiother-
based palladacycles, we examined the reaction of an 11e/PCy3
mixture with phenylboronic acid in the presence of Cs2CO3.
Heating the mixture in 1,4-dioxane led to the rapid (minutes)
formation of palladium black. After 18 hours the main isolable
product (62.5% yield) was shown to be the coupled thioether 14.
Scheme 3 Putative catalyst activation mechanism from 11e/PCy3
mixtures.
Conclusions
In summary we have found that, as for amine-, imine-, and
phosphite-based palladacycles, the addition of di- and tri-
alkylphosphines to thioether-based palladacycles gives catalysts
that are far more active than the precursor complexes in the
Suzuki coupling of aryl chloride substrates. The activity shown
3 8 6 6
D a l t o n T r a n s . , 2 0 0 4 , 3 8 6 4 – 3 8 6 8

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by the S-based palladacycles is good, but not as high as that
shown by either N- or P-based analogues.
50 mL), the combined organic layers were washed with water,
dried (MgSO4), filtered and the solvent removed under reduced
pressure. The residue was dissolved in dichloromethane (5 mL),
hexadecane (0.068 M in CH2Cl2, 1.00 mL, internal standard)
was added and the conversion to product determined by GC.
Experimental
General methods
Reaction of a mixture of 11e/PCy3 with PhB(OH)2
All reactions and manipulations of air-sensitive materials were
carried out under nitrogen either in a glove-box or using
standard Schlenk techniques. Solvents were dried and freshly
distilled prior to use. All other chemicals were used as received.
Complex 11e was prepared according to a literature method.19
Catalytic reactions were performed on a Radleys Carousel
ReactorTM. This consists of twelve ca. 45 ml tubes which are
fitted with screw-on Teflon caps that are equipped with valves
for the introduction of inert gas and septa for the introduction of
reagents. The twelve reaction tubes sit in two stacked aluminium
blocks, the lower one fits on a heater-stirrer and can be
maintained at a constant temperature with a thermostat, while
the upper block has water circulating which cools the top of the
tubes, allowing reactions to be performed at reflux temperature.
GC analyses were performed on a Varian 3800 GC fitted with
a 25 m CP Sil 5CB column and data were recorded on a Star
workstation.
A mixture of complex 11e (0.734 g, 1.202 mmol), PCy3 (0.734 g,
2.617 mmol), PhB(OH)2 (0.737 g, 6.044 mmol) and Cs2(CO3)
(2.953 g, 9.063 mmol) in 1,4-dioxane (40 mL) was heated
at 100 ◦C for 18 hours. The cooled reaction mixture was
filtered through Celite and then silica which was washed with
acetone and the solvent was removed from the combined
organic fractions under reduced pressure. The crude product
was purified by column chromatography (silica, pentane : diethyl
ether 50 : 50) to give 2-methylsulfanylmethyl-biphenyl, 14, as a
1
yellow oil (0.322 g, 62.5%). NMR H (300.13 MHz, CDCl3)
d ppm: 1.88 (s, 3H, –S–CH3); 3.58 (s, 2H, –CH2–S); 7.15–7.36
(m, 9H, Ph). 13C (75.47 MHz, CDCl3) d ppm: 16.0 (–CH3); 36.3
(–CH2–); 127.3 (CH, Ph); 127.5 (CH, Ph); 127.8 (CH, Ph); 128.5
(2 CH, Ph); 129.8 (2 CH, Ph); 130.4 (CH, Ph); 130.6 (CH, Ph);
135.9 (Cquat, Ph); 141.4 (Cquat, Ph); 142.5 (Cquat, Ph). HRMS
m/z Found: 214.0814 (M+); Calcd. For C14H14S: 214.0816.
Reaction of complex 11e with PCy3
Acknowledgements
A solution of complex 11e (0.300 g, 0.50 mmol) and PCy3
(0.306 g, 1.09 mmol) in dichloromethane (10 mL) was stirred
at room temperature for 30 min. 31P NMR spectroscopy showed
it to be a mixture of compounds. Ethanol (20 mL) was added
We thank the EPSRC, the Institute of Applied Catalysis and the
University of Exeter for funding and Johnson Matthey for the
loan of palladium salts.
◦
and the resultant solution was cooled at −10 C for a week to
References
give a small quantity of crystals suitable for X-ray analysis.
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X-Ray structure determination of complex 12
Data were collected on a Bruker Nonius Kappa CCD area
detector diffractometer with a rotating anode following standard
procedures. The structures were solved via direct methods20 and
refined by full matrix least squares on F2. Non-hydrogen atoms
were refined anisotropically and hydrogen atoms were placed
in idealised positions and refined using a riding model. The
structure contains 2 symmetry related voids of 749 A containing
unresolvable solvent, this was treated using the squeeze method
lowering the R1 value by ca. 3% and removing several high
difference peaks.21
2 For a recent review see: R. B. Bedford, Chem. Commun., 2003, 1787.
¨
3 M. Beller, H. Fischer, W. A. Herrmann, K. Ofele and C. Brossmer,
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Lawrence and P. N. Scully, Chem. Commun., 1998, 2095; (c) R. B.
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M. B. Hursthouse, Dalton Trans., 2003, 4164.
3
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5 S. Gibson, D. F. Foster, G. R. Eastham, R. P. Tooze and D. J. Cole-
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6 D. A. Albisson, R. B. Bedford and P. N. Scully, Tetrahedron Lett.,
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7 (a) S. Iyer and C. Ramesh, Tetrahedron Lett., 2000, 41, 8981; (b) I. P.
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41, 179.
Crystal data. Yellow block, C46H78O2P2PdS, Mr = 863.48,
T = 298(2) K, monoclinic, space group P21/n, a = 11.322(5),
◦
3
˚
˚
b = 28.220(5), c = 17.963(5) A, b = 90.311(5) , V = 5739(3) A ,
qcalc = 0.999 g cm −3, l = 0.443 mm−1, Z = 4, reflections collected:
20403, independent reflections: 8928 (Rint = 0.0539), final R
indices [I > 2rI]: R1 = 0.0585, wR2 = 0.1503, R indices (all
data): R1 = 0.0843. wR2 = 0.1599.
CCDC reference number 241083.
See http://www.rsc.org/suppdata/dt/b4/b407922c/ for cry-
stallographic data in CIF or other electronic format.
10 (a) R. B. Bedford and C. S. J. Cazin, Chem. Commun., 2001, 1540;
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15 See: J. P. Stambuli, M. Bu¨hl and J. F. Hartwig, J. Am. Chem. Soc.,
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Catalysis
General method for the Suzuki coupling of aryl chlorides
with phenylboronic acid. To a mixture of the appropriate aryl
chloride (1.0 mmol), aryl boronic acid (0.183 g, 1.5 mmol)
and Cs2CO3 (0.651 g, 2.0 mmol) was added the appropriate
phosphine and complex 11e both as solutions (1–3 mL) made
up by multiple volumetric dilutions of stock solutions in 1,4-
dioxane and then sufficient 1,4-dioxane was added to bring
the total solvent volume to 10 mL. The resultant mixture was
then heated at 100 ◦C for 18 h, cooled and quenched with
HCl(aq) (2 M, 20 mL). The organic layer was removed and
the aqueous layer was extracted with dichloromethane (3 ×
D a l t o n T r a n s . , 2 0 0 4 , 3 8 6 4 – 3 8 6 8
3 8 6 7

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18 (a) R. B. Bedford, M. E. Blake, C. P. Butts and D. Holder, Chem.
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3 8 6 8
D a l t o n T r a n s . , 2 0 0 4 , 3 8 6 4 – 3 8 6 8