Orthopalladated phosphinite complexes as high-activity catalysts for the suzuki reaction

Article abstract of DOI:10.1039/b303657j

The synthesis of a range of phosphinite ligands PR₂(OAr) (R = Ph, ⁱPr), their simple complexes with palladium(II) and their palladacyclic complexes has been investigated. The crystal structure of one of the palladacyclic complexes, [{Pd(μ₂-Cl)} {κ²-P, C-PⁱPr₂(OC₆H₂-2,4-^t Bu₂)} ₂], has been determined. The palladacyclic complexes show extremely high activity in the Suzuki coupling of aryl bromide substrates with phenylboronic acid and can also be used with alkylboronic acid substrates. A comparison of the phosphinite-based catalysts with equivalent phosphite- and phosphine-based systems highlights their superior activity. The orthometallation of the phosphinite ligand in the pre-catalyst appears to be crucial for optimal activity. While the phosphinite palladacycles are only moderately active in the coupling of activated and non-activated aryl chloride substrates, their tricyclohexylphosphine adducts prove to be highly active in the coupling of the deactivated substrate, 4-chloroanisole. This high activity compared with other palladacyclic systems is explained in terms of catalyst longevity. The orthometallated precatalysts appear to undergo a reductive activation process to generate zerovalent active catalysts via reductive elimination of the orthometallated ring with a phenyl introduced by the boronic acid. This implies that the true active catalysts contain 2-arylated ligands. Catalysts formed with such 2-arylated ligands tend to show markedly higher activity than their parent ligands.

Full text of DOI:10.1039/b303657j

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Orthopalladated phosphinite complexes as high-activity catalysts
for the Suzuki reaction†
Robin B. Bedford,*a Samantha L. Hazelwood (née Welch),a Peter N. Hortonb and
Michael B. Hursthouseb
a School of Chemistry, University of Exeter, Exeter, UK EX4 4QD. E-mail: r.bedford@ex.ac.uk
b Department of Chemistry, EPSRC National Crystallography Service,
University of Southampton, Highfield Road, Southampton, UK SO17 1BJ
Received 1st April 2003, Accepted 6th June 2003
First published as an Advance Article on the web 22nd September 2003
The synthesis of a range of phosphinite ligands PR2(OAr) (R = Ph, iPr), their simple complexes with palladium()
and their palladacyclic complexes has been investigated. The crystal structure of one of the palladacyclic complexes,
[{Pd(µ2-Cl){κ2-P,C-PiPr2(OC6H2-2,4-tBu2)}2], has been determined. The palladacyclic complexes show extremely
high activity in the Suzuki coupling of aryl bromide substrates with phenylboronic acid and can also be used with
alkylboronic acid substrates. A comparison of the phosphinite-based catalysts with equivalent phosphite- and
phosphine-based systems highlights their superior activity. The orthometallation of the phosphinite ligand in the
pre-catalyst appears to be crucial for optimal activity. While the phosphinite palladacycles are only moderately active
in the coupling of activated and non-activated aryl chloride substrates, their tricyclohexylphosphine adducts prove to
be highly active in the coupling of the deactivated substrate, 4-chloroanisole. This high activity compared with other
palladacyclic systems is explained in terms of catalyst longevity. The orthometallated precatalysts appear to undergo
a reductive activation process to generate zerovalent active catalysts via reductive elimination of the orthometallated
ring with a phenyl introduced by the boronic acid. This implies that the true active catalysts contain 2-arylated
ligands. Catalysts formed with such 2-arylated ligands tend to show markedly higher activity than their parent
ligands.
complexes and their use in the Suzuki coupling reactions of
both aryl bromide and aryl chloride substrates.
Introduction
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 Traditional palladium-
based catalysts such as [Pd(PPh3)4] or catalysts formed in situ
from a triarylphosphine and an appropriate palladium source
suffer from two major limitations. The first is that these
catalysts have to be used in comparatively high loadings (typi-
cally a few mol% Pd). This is problematic because currently for
application in fine chemicals and pharmaceuticals processes the
maximum acceptable palladium contamination of the product
is in the low ppm region. This necessitates expensive clean-up
procedures. In addition the high cost of palladium may make
the use of coupling chemistry prohibitively expensive. The
second major problem is that traditional catalysts show, at best,
limited activity with aryl chlorides – particularly deactivated
(electron rich) aryl chlorides – due to the high C–Cl bond
strength.2 These substrates are of particular interest as they
tend to be cheaper and more readily available than their brom-
ide or iodide counterparts. Therefore there has been major
interest in developing catalysts that can be used in very
low loadings and ideally that activate aryl chloride substrates.
Palladacyclic complexes have played a particular role in this
Results and discussion
regard.3 We have found that orthopalladated triarylphosphite
Synthesis of phosphinite ligands and complexes
or phosphinite complexes of the types 1 and 2 show excellent
activity in the Suzuki coupling of aryl bromides,4 and that the
tricyclohexylphosphine adducts 3 are highly effective for the
coupling of deactivated aryl chloride substrates.5 We now
report in full the synthesis of orthopalladated phosphinite
The phosphinite ligands 4 are synthesised by reaction of the
appropriate phenols with chlorophosphines in the presence of
triethylamine in toluene at reflux temperature (Scheme 2). In the
case of the triarylphosphinite ligands 4a–i (Table 1) simple
Scheme 1 The Suzuki biaryl coupling reaction.
† Based on the presentation given at Dalton Discussion No. 6, 9–11th
September 2003, University of York, UK.
Scheme 2 (i) ClPR3 , Et3N, toluene, 18 h.
2
D a l t o n T r a n s . , 2 0 0 3 , 4 1 6 4 – 4 1 7 4
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 3
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Table 1 Selected data for phosphinite ligands
Compound no.
R1
R2
R3
Yield (%)
31P–{1H} NMR δ (ppm)a
∆δ compared with R2P(OPh) (ppm)
4a
4b
4c
4d
4e
4f
4g
4h
4i
4j
4k
4l
4m
4n
4o
4p
4q
4r
H
H
H
H
Me
Me
tBu
tBu
Ph
H
H
H
H
Me
Me
tBu
tBu
Ph
H
Ph
Ph
Ph
Ph
Ph
Ph
Ph
Ph
Ph
iPr
iPr
iPr
iPr
iPr
iPr
iPr
iPr
iPr
97
96
97
99
95
95
98.5
98
96
89
92
97
94
94
95
96
96
82
111.2
111.8
111.4
110.9
110.1
110.3
108.6
108.5
113.3
149.0
149.6
149.4
148.9
144.5
144.3
138.5
138.4
151.5
0
0.6
0.2
Ϫ0.3
Ϫ1.1
Ϫ0.9
Ϫ2.6
Ϫ2.7
2.1
Me
Et
tBu
H
Me
H
tBu
H
H
0
0.6
0.4
Me
Et
tBu
H
Ϫ0.1
Ϫ4.5
Ϫ4.7
Ϫ10.5
Ϫ10.6
2.5
Me
H
tBu
H
a CDCl3, 121.5 MHz, 25 ЊC.
filtration of the cooled reaction mixture to remove precipitated
triethylamine hydrochloride followed by removal of the solvent
gives the products cleanly. In the case of the diisopropyl-
phosphinite ligands 4j–r it is first necessary to add petroleum
ether (bp = 60–80 ЊC) to ensure complete precipitation of the
ammonium salt. If this step is not taken then the product
ligands are contaminated by triethylamine hydrochloride,
which prevents the formation of palladacyclic complexes.6
Table 1 shows the 31P–{1H} NMR data for the range of phos-
phinite ligands synthesised. When the 31P chemical shifts of the
ligands are compared then it can be seen that those for the
iPr2P(OAr) ligands are shifted downfield with respect to those
for the analogous Ph2P(OAr) ligands. A closer inspection shows
that while variation in the substitution of the 4-position of the
aryloxide ring has little or no effect on the 31P signal, increase in
the steric bulk of alkyl groups in the 2-position leads to an
upfield shift. This trend is particularly pronounced in the case
of the diisopropylphosphinites. By contrast the introduction of
a 2-phenyl substituent leads to a downfield shift. In general,
variation in size of simple triarylphosphites has very little effect
on the chemical shift as exemplified by P(OPh)3 and P(OC6H3-
2,4-tBu2)3 which have 31P NMR δ values of 129 and 131 ppm
respectively. By contrast variation in the size of substituents in
phosphines can have a profound effect. It seems that in the case
of phosphinites an intermediate position is adopted; steric
variation leads to significant change in δ, but in a predictable
fashion.
Having synthesised a range of phosphinite ligands we then
turned our attention to generating a representative range of
palladium adducts and palladacyclic complexes. Providing
there is an ethyl or larger group in either the 2- or 4-position of
the aryloxide residue, then complexes with orthopalladated
PPh2(OAr) ligands can be readily formed by heating the ligand
with one equivalent of palladium chloride in toluene (Scheme
3). If the largest group in either the 2- or 4-position is a methyl
then it becomes necessary to use [PdCl2(NCMe)2] as the
palladium source. The palladacyclic complex of the diisopro-
pylphosphinite ligand 4q, complex 2i, is best prepared by heat-
ing the ligand with palladium dichloride in 1,4-dioxane. The
31P NMR spectra of the complexes with orthopalladated
PPh2(OAr) ligands, complexes 2a–h, all show two peaks in the
range 151.1–157.5 ppm indicating that both the cis and trans
isomers are present, as seen with orthopalladated triaryl-
phosphites.7,4c The diisopropylphosphinite complex 2i shows
Scheme 3 Conditions: (i) PdCl2, toluene, ∆, 18 h. (ii) [PdCl2(NCMe)2],
THF, ∆, 18 h. (iii) PdCl2, 1,4-dioxane, ∆, 18 h. (iv) 0.5 [PdCl2(NCMe)2],
CH2Cl2, r.t. 4 h. (v) 0.5 [PdCl2(NCPh)2], toluene, ∆, 18 h.
two peaks at δ 203.4 and 202.7 ppm in the 31P NMR spectrum
again indicating the presence of both cis and trans isomers.
The crystal structure of complex 2i has been determined and
the molecule is shown in Fig. 1. As can be seen it adopts the
trans-configuration in the solid state. The P1–Pd1 and C1–Pd1
bond lengths and the P1–Pd1–C1 bond angle are essentially
identical to those reported previously for complex 1 (Y =
OC6H3-2,4-tBu2).4a By contrast both of the Pd–Cl bond lengths
of 2i are longer and the Cl1–Pd1–Cl1i angle substantially more
obtuse.
As well as palladacyclic complexes we also produced the non-
orthometallated complexes cis-[PdCl2(4)2], 5, by reaction of the
D a l t o n T r a n s . , 2 0 0 3 , 4 1 6 4 – 4 1 7 4
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that obtained with the previous best catalyst – one formed in
situ from the tetraphosphine ligand 7.10 High activities are also
seen with the sterically hindered substrates 2-bromotoluene and
2-bromo-m-xylene (entries 13 and 14).
The diisopropylphosphinite-containing catalyst 2i generally
shows higher activity than the diphenylphosphinite-containing
analogue 2a, presumably due to the greater electron donor
ability of the dialkylphosphinite ligand and/or its greater steric
bulk. The observation that 2a seems to show little difference
in activity in the coupling of phenylboronic acid with either
4-bromoacetophenone or 4-bromoanisole at the same catalyst
loadings (compare entries 1 and 2 with 3 and 4) indicates that,
in this case, the oxidative addition of the aryl bromide is prob-
ably not the rate-determining step in the catalytic cycle. Com-
paring entries 3 and 7 it can be seen that decreasing the steric
profile of the metallated ring has a deleterious effect on the
performance of the catalysts, with complex 2e showing much
lower activity. Interestingly, complex 2e still shows substantially
higher activity than the notionally related complex 8 in the
same reaction under the same conditions.11
Fig. 1 Molecular structure of complex 2i. Selected interatomic lengths
(Å) and angles (Њ): Pd1–C1 2.012(2), Pd1–P1 2.1686(6), Pd1–Cl1
2.4313(6), Pd1–Cl1i 2.4332(6); P1–Pd1–C1 80.24(7), P1–Pd1–Cl1i
95.83(2), C1–Pd1–Cl1 97.72(7), Cl1–Pd1–Cl1i 86.13(2), P1–Pd1–Cl1
176.87(2), C1–Pd1–Cl1i 175.73(7).
appropriate ligands with [PdCl2(NCMe)2] in dichloromethane
at room temperature. The synthesis of complex cis-5a using this
method has been reported previously.8 When the reaction of 4h
with [PdCl2(NCMe)2] is left to stir for 4 hours the complex 5b
is obtained exclusively as the cis isomer, as indicated by the
presence of two ν(Pd–Cl) peaks at 328 and 307 cmϪ1 in the IR
spectrum. By contrast, we have previously found that when this
reaction is left for only 1 hour then the product is exclusively
trans.8 This indicates that the kinetic product is the trans isomer
while the thermodynamic product is the cis isomer. The latter is
preferred electronically as the π-acidic phosphinite ligands are
trans to the π-basic chlorides. Under the longer reaction condi-
tions the complexes 5c and 5d are formed as mixtures of
isomers. Previously we found that reactions between [PdCl2-
(NCMe)2] and ligands 4j and 4q stirred for only 1 hour gave
mixtures of several compounds.8 The o-biphenyl substituted
triarylphosphinite ligand 4i reacts with [PdCl2(NCMe)2] to
give only the trans isomer, presumably due to the bulk of the
ligand. The synthesis of the diisopropyl-phosphinite-contain-
ing analogue 5f is achieved by heating the ligand 4r with
[PdCl2(NCPh)2] in toluene at reflux temperature.
In contrast with the extraordinary results obtained with the
deactivated aryl bromide 4-bromoanisole, complex 2i proves to
be a very poor catalyst when 4-chloroanisole is used as a
substrate. However when the non-activated or activated
aryl chloride substrates 4-chlorotoluene, 4-chloronitrobenzene,
4-chlorobenzaldehyde or 4-chloroacteophenone are employed
then reasonable activity results at 1 mol% Pd loading (entries
17–19). In these cases caesium carbonate was used as the base
and DMA as the solvent. Even under these modified con-
ditions the performance of the catalyst 2i with the deactivated
substrate 4-chloroanisole is poor (entry 16). Gibson, Cole-
Hamilton and co-workers have prepared the structurally related
complex 9a and tested its activity in the Suzuki coupling of
both aryl bromides and chlorides.12 In both instances it appears,
from a comparison of their results with those in Table 2, that
the replacement of the oxygen in the palladacyclic ring of com-
plexes of the type 2 with a methylene group is deleterious to the
overall performance of the catalysts. In addition the authors
found that when 4-chlorobenzaldehyde is used as a substrate in
the coupling with phenylboronic acid then the product is con-
taminated with substantial amounts of 1-(4-chlorophenyl)-1-
phenylmethanol and 1,4-biphenyl-1-phenylmethanol. We do
not observe these side-products when complex 2i is used as the
catalyst.
Phosphinite complexes as catalysts for the Suzuki reaction
(a) The coupling of aryl boronic acids. Orthopalladated di-
phenyl- and diisopropyl-phosphinite complexes are excellent
catalysts for the Suzuki coupling of aryl bromide substrates at
very low catalyst loadings.4b Table 2 summarises the coupling of
a range of aryl bromides with phenylboronic acid. In all cases
the reactions were performed in toluene with potassium carb-
onate acting as base. These conditions have not been optimised.
The complexes 2a and 2i prove to be extremely active catalysts
for both the ‘easy to couple’ substrate 4-bromoacetophenone
and the more challenging, electronically deactivated substrate
4-bromoanisole. To the best of our knowledge, the activity
shown by the catalyst mixture of 2i and one equivalent of
added ligand 4q (entry 12) is the highest reported for any
Suzuki reaction – giving over five times higher turnover number
(TON, mol product/mol Pd) than for the analogous reaction
catalysed by palladium complexes of the phosphine ligands 6.9
By contrast, the triarylphosphite-containing palladacycle 1a
shows a maximum TON of 58.5 million in this reaction.4c More
importantly the maximum TON with the electronically deactiv-
ated substrate 4-bromoanisole (entry 9) is 3.5 times higher at a
lower temperature and after a much shorter reaction time than
In order to perform a more accurate comparison of phos-
phinite ligands with analogous benzylphosphine-containing
systems and with related triarylphosphite ligands, P(OAr)3, and
also to see whether orthometallation is important, we studied
the coupling of 4-bromoanisole with phenylboronic acid
catalysed by a range of catalysts. These catalysts fall into three
classes: preformed dimeric palladacycles of the types 1 and 2,
preformed complexes of the type [PdCl2(L)2] and catalysts
formed in situ from palladium bis(dibenzylideneacetone) and
D a l t o n T r a n s . , 2 0 0 3 , 4 1 6 4 – 4 1 7 4
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Table 2 Suzuki coupling of phenylboronic acid with aryl bromides and chlorides catalysed by orthopalladated phosphinite complexes.a
Loading
(mol% Pd)
Conversion
(%)a
Entry
Aryl halide
Catalyst
Product
TON
1
2
0.0001
0.00001
90
20
900,000
2,000,000
3
4
0.0001
0.00001
85
26
850,000
2,600,000
5
6
0.0001
0.00001
91
42
910,000
4,200,000
7
0.0001
29
290,000
8
9
0.0001
0.00001
100
87.5
1,000,000
8,750,000
10
0.00001
50
5,000,000
11
12
0.000001b
100
47.5
100,000,000
475,000,000
0.0000001b
13
14
0.001
100
100,000
48,000
0.001
48
15
16
1.0
6
6
6
6
1.0c
17
1.0c
76
76
18
1.0c
56
56
19
20
1.0c
1.0c
100
31
100
31
a Reaction conditions: aryl halide (10.0 mmol), PhB(OH)2 (15.0 mmol), K2CO3 (20 mmol), toluene (30 mL), 130 ЊC (external temp., internal temp.
approx 110 ЊC), 18 h. b 24 h reaction time. c Cs2CO3, DMA, 110 ЊC.
the appropriate ligand, L. The known ligand PPh2(CH2Ph),
10a, the new ligand PiPr2(CH2Ph), 10b, and the complexes cis-
[PdCl2{PR2(CH2Ph)}2], 11 (a: R = Ph, b: R = iPr) were prepared
as shown in Scheme 4.
The complex 9b was prepared by a literature method.13 We
were unable to prepare an analogous complex with R = Pr
using either the method for the formation of 9a or that for the
synthesis of 9b.
i
4-Bromoanisole was chosen as the test substrate for the
Suzuki coupling because it is electronically deactivated and
therefore gives a better measure of optimal catalyst perform-
ance than more activated examples. The coupling reactions
were performed at several catalyst loadings in the range 0.001–
0.00001 mol% Pd in order to ascertain the maximum TON that
these systems can give. The maximum TONs obtained are
Scheme 4 Conditions: (i) Mg, Et2O, 0 ЊC to r.t., 2 h. (ii) ClPR2, 0 ЊC to
r.t., 18h. (iii) [PdCl2(NCMe)2], CH2Cl2, r.t., 30 min.
D a l t o n T r a n s . , 2 0 0 3 , 4 1 6 4 – 4 1 7 4
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summarised in Fig. 2. As can be seen in all cases, regardless of
the ligand, the preformed complexes [PdCl2(L)2] fared least
well. Much better results are obtained when the catalysts are
formed in situ from palladium bis(dibenzylideneacetone) and
two equivalents of the ligand. Dibenzylideneacetone (dba) can
be a highly tenacious ligand for Pd(0), which competes effect-
ively with incoming substrates.14 Indeed we previously found
[Pd2(dba)3] to be a poor precursor for PCy3-containing catalysts
for the Suzuki coupling of aryl chlorides.15 It is possible
that here the ligands used are sufficiently π-acidic to compete
effectively with dba-coordination, thus displacing it from the
coordination sphere and limiting competitive inhibition. Chan-
ging from the Pd-dba/2L systems to preformed phosphite-
containing palladacycles of the type 1 does not give a particu-
larly large increase in performance. By contrast a huge increase
in activity is observed for the orthopalladated phosphinite
ligands. This demonstrates that, in the case of the phosphinite
ligands, orthometallation of the pre-catalyst is highly import-
ant. The possible role of the orthometallation is discussed later.
ligands should be better. However if the ligands are too π-acidic
then the palladium centre may become too electron deficient,
tipping the rate-determining step to oxidative addition. Indeed,
unlike with the phosphinite-containing palladacycle 2a, which
does not show an enormous difference in activity between
4-bromoanisole and 4-bromoacetophenone under identical
conditions, there are two orders of magnitude difference in
maximum TON with these substrates when the phosphite-
containing complex 1a is used as a catalyst.4c This suggests that,
in the case of triarylphosphite-based palladacycles, oxidative
addition is rate-determining. Thus the phosphinite catalysts
represent the right balance of electronics – sufficiently π-acidic
to facilitate both the nucleophilic attack of the boronate16
and the reductive elimination of the product without unduly
disfavouring oxidative addition.
(b) The coupling of alkyl boronic acids. There has recently
been increasing interest in the use of alkylboronic acids in
coupling reactions.17 We wondered whether the orthopalladated
phosphinite complexes 2 would act as effective catalysts in such
processes. A brief solvent/base optimisation study was per-
formed for the use of complex 2i in the coupling of butyl-
boronic acid and 4-bromoanisole, the results of which are
summarised in Fig. 3. As can be seen, the best conversion is
obtained with K3PO4 in 1,4-dioxane and these conditions were
used for the remainder of the study, the results of which are
summarised in Fig. 4. For comparison purposes, selected data
obtained previously with P(OAr)3 and PCy3 are included.4c
Fig. 3 Solvent/base optimisation in the coupling of n-butylboronic
acid with 4-bromoanisole. Conditions: 4-BrC6H4OMe (10.0 mmol),
nBuB(OH)2 (15.0 mmol), base (20.0 mmol), solvent (30 mL), 2i
(0.5 mol% Pd), 100 ЊC, 18 h. Conversion to 4-BuC6H4OMe determined
by GC (hexadecane internal standard). NMP is N-methylpyrollidone.
It is apparent that the performance of catalysts based on the
diisopropylphosphinite ligand 4q is highly dependent on the
palladium source, with only the preformed palladacyclic com-
plex 2i showing good activity. By contrast both of the more
π-acidic ligands PPh2(OAr), 4h, and P(OAr)3 (Ar = C6H3-2,4-
tBu2) are tolerant of palladium source, with no real change in
activity observed with varying catalyst precursors. It appears
that as the π-acidity of the PR2(OAr) ligands increases (R = iPr
< Ph < OAr) then so does the activity. However steric factors
can be equally important as demonstrated by the good activity
observed with PCy3 since this ligand is far less π-acidic than the
others used but is sterically hindered.
Fig. 2 Suzuki coupling of PhB(OH)2 with 4-BrC6H4OMe with a
variety of catalyst precursors and ligands. Conditions are the same as
those used in Table 2 for aryl bromides.
It is also apparent that the phosphinite-based palladacycles
show far higher activity than related phosphite complexes, at
least under these conditions. Since it seems that the rate-deter-
mining step is not oxidative addition of the aryl bromide, it
might be anticipated that the use of π-acidic ligands would
increase the rate of catalysis which implies triarylphosphite
D a l t o n T r a n s . , 2 0 0 3 , 4 1 6 4 – 4 1 7 4
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cases an induction time of up to about 1 hour is observed. As
can be seen both of the adducts formed from the complexes 2a
and 2i show excellent activity – the TON obtained by 24 hours
is nearly 30,000 in both cases – but their overall performance is
not as great as the adducts formed from the phosphite com-
plexes 1a and 1c. Interestingly it appears that the catalysts that
show highest turn-over frequencies (TOF = TON/t) show the
lowest overall conversion. The rate-determining step is almost
certainly oxidative-addition of the aryl chloride as evidenced by
the fact that the more electron-rich the palladium centre formed
in situ from the palladacycles, the higher the initial rates of
catalysis. Therefore the catalyst resting state is a zerovalent spe-
cies, assuming a Pd(0)/Pd() cycle is operative (vide infra).18 The
greater the π-acidity of the co-ligand, the greater the longevity
of the catalyst, due to increased stability of the zerovalent rest-
ing state. Therefore, it seems that the overall performance of the
catalyst is not governed by an increase in the rate of catalysis,
but rather by a hugely enhanced catalyst lifetime. By contrast,
under identical conditions, the PCy3-containing amine-based
palladacyclic catalyst 12, shows a lifetime of only ≈ 30 minutes
and a total conversion of 4%.5a
Fig. 4 Suzuki coupling of n-butylboronic acid with 4-bromoanisole.
Conditions: 4-BrC6H4OMe (10.0 mmol), nBuB(OH)2 (15.0 mmol),
K3PO4 (20.0 mmol), 1,4-dioxane (30 mL), catalyst (0.5 mol% Pd), 100
ЊC, 18 h. Conversion to 4-BuC6H4OMe determined by GC (hexadecane
internal standard).
The possible role of orthometallation in the formation of active
catalyst
It is apparent from the data above that phosphinite pallada-
cycles are outstanding catalysts for the Suzuki coupling of
aryl bromide substrates, while tricyclohexylphosphine adducts
are excellent for aryl chloride coupling reactions. There
has recently been considerable discussion on the subject of
whether coupling reactions catalysed by palladacycles follow a
‘classical’ Pd(0)/Pd() pathway or rather whether the Pd–C
bond is maintained and a Pd()/Pd() pathway is operative.18
In general the weight of evidence is pointing towards a classical
Pd(0)/Pd() manifold, with active, zerovalent catalysts formed
in situ either thermally or by a reductive process in which
the metallated aryl group reacts with one of the coupling
partners.3,18
In order to establish whether catalysts of the type 2 are likely
to form Pd(0) or Pd() active catalysts, we investigated the reac-
tions of complex 2e with components of the Suzuki reaction.
No reaction was observed between 2e and 4-bromoaceto-
phenone in toluene at reflux temperature. By contrast when 2e
was reacted with phenylboronic acid (4 molar equivalents/Pd)
in the presence of K2CO3 (5 molar equivalents/Pd) in toluene at
reflux temperature the reaction mixture rapidly turned black –
within seconds of reaching about 50 ЊC. After 24 hours at reflux
temperature the mixture was filtered to remove the inorganic
solids and the resultant solution was analysed by GC/MS. This
showed a peak corresponding to the ligand PPh2(OC6H4-2-Ph),
4i. Hydrolysis of the reaction mixture followed by GC/MS
analysis indicated the presence of 2-phenylphenol. A plausible
mechanism of decomposition which results in the liberation of
the ligand 4i and palladium metal is shown in Scheme 5. Similar
ring-opening processes have been observed in the formation of
active catalysts for the Suzuki reaction based on imine and
amine palladacycles and for the formation of an active Stille
coupling catalyst from the palladacycle 13.15,19,20 Far from being
stable, it appears that palladacyclic Pd–C bonds are highly
Phosphine adducts of phosphinite palladacycles as catalysts for
the Suzuki coupling of aryl chlorides
While the phosphinite palladacycles 2 show outstanding activ-
ity in the Suzuki coupling of aryl bromides, their performance
with aryl chlorides is somewhat less impressive. We recently
found that the tricyclohexylphosphine adducts of pallada-
cycles, 3, show, to the best of our knowledge, the highest activ-
ity yet reported in such reactions.5 The application of PCy3
adducts of the phosphinite-containing palladacycles was exam-
ined. Fig. 5 shows the time-dependent study of the reaction
between the electronically deactivated substrate 4-chloroanisole
and phenylboronic acid, catalysed by PCy3 adducts formed
in situ. For comparison purposes the data for the catalysts
formed in situ from 1a and 1c and PCy3 are included. In all
Fig. 5 Suzuki coupling of 4-chloroanisole with phenylboronic acid.
Conditions: 4-ClC6H4Me (10.0 mmol), PhB(OH)2 (15.0 mmol), Cs2CO3
(20.0 mmol), catalyst (0.001 mol% Pd), 1,4-dioxane (30 mL)
hexadecane (internal standard, 0.204 mmol). Conversion to 4-
methoxybiphenyl determined by GC.
D a l t o n T r a n s . , 2 0 0 3 , 4 1 6 4 – 4 1 7 4
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Scheme 5
reactive under catalytic conditions. We have recently exploited
this reactivity in the rhodium catalysed, intermolecular ortho-
arylation of phenols.21
The putative activation process outlined in Scheme 5 yields
a low-coordinate palladium(0) species which contains a new
2-arylated phosphinite ligand. It is possible that the 2-aryl
group helps stabilise the palladium centre by a π-interaction.
Such stabilisation has previously been suggested by Buchwald
as being an important feature in the high activity associated
with palladium complexes of the ligands 6.22 Even if there is no
specific stabilisation by π-coordination of the secondary aryl
ring, the increase in size compared with the parent phosphinite
may be anticipated to be beneficial in the Suzuki reaction. For
this reason we undertook a comparison of phosphinite-, phos-
phite- and phosphine-containing catalyst systems with ECH2-
(C6H4-2-Ph) (E = O, CH2) substituents. The syntheses of the
phosphite ligand P(OC6H4-2-Ph)3, 14, and the phosphines
PR2(CH2C6H4-2-Ph), 15 (a: R = Ph, b: R = iPr) are outlined in
Scheme 6, as is the synthesis of the palladium dichloride
adducts of the phosphines, the complexes 16.
Fig. 6 Suzuki coupling of PhB(OH)2 with 4-BrC6H4OMe with a
variety of catalyst precursors and ligands. Conditions are the same as
those used in Table 2 for aryl bromides.
unsubstituted counterparts. A major exception is seen with the
catalyst formed in situ from [Pd(dba)2] and the phosphine
PiPr2(CH2C6H5), 10b, which shows by far the highest activity
under these conditions, while the catalysts formed in situ from
the 2-phenylated analogue PiPr2(CH2C6H4-2-Ph), 13b, does not
perform particularly well. At present we are unable to explain
this exception. Another general trend that can be observed is
that the catalysts formed in situ from [Pd(dba)2] and two
equivalents of the ligands (Fig. 6, odd catalyst nos.) tend to
show higher activity than the preformed catalysts of the form
[PdCl2(L)2] containing the same ligands.
Conclusions
Palladacycles based on orthopalladated phosphinite ligands are
extremely active catalysts for the Suzuki coupling of aryl brom-
ides; these catalysts can also be used successfully for the Suzuki
coupling of alkyl boronic acids. While these catalysts are not
particularly useful for the coupling of aryl chloride substrates,
their tricyclohexylphosphine adducts, formed in situ, show very
high activity. The high TONs obtained in the coupling of aryl
chlorides appears to be as a result of high catalyst longevity,
brought about by the stabilisation of the Pd(0) resting state by
the π-acidic phosphinite ligands. This, coupled with the ease of
synthesis of both the ligands and complexes from relatively
inexpensive, commercially available precursors makes pallada-
cyclic phosphinite complexes one of the foremost catalysts of
choice for the Suzuki reaction.
1
–
Scheme 6 Conditions: PCl3, NEt3, toluene, ∆, 18 h. (ii) Mg, Et2O,
3
Experimental
0 ЊC to r.t., 2 h. (iii) ClPR2, 0 ЊC to r.t., 18 h. (iv) [PdCl2(NCMe)2],
CH2Cl2, r.t., 30 min.
General
The Suzuki reaction chosen for study was again the coupling
of the deactivated aryl bromide 4-bromoanisole with phenyl-
boronic acid. The catalysts studied were the preformed [PdCl2-
(L)2] complexes and catalysts formed in situ from [Pd(dba)2] and
two equivalents of the appropriate ligands. The results of the
study are summarised in Fig. 6. In general it can be seen that
the 2-phenylated ligands show much higher activity than their
All reactions and manipulations of air-sensitive materials were
carried out under nitrogen either in a glove-box or using stand-
ard Schlenk techniques. Solvents were dried and freshly distilled
prior to use. All other chemicals were used as received. The
ligands 4h,l and 4q,8 the complexes 5a, [PdCl2(NCMe)2],23
[PdCl2(NCPh)2]24 and [Pd(dba)2]25 were prepared according to
literature methods. GC analyses were performed on a Varian
D a l t o n T r a n s . , 2 0 0 3 , 4 1 6 4 – 4 1 7 4
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3800 GC fitted with a 25 m CP Sil 5CB column and data were
recorded on a Star workstation.
PiPr2(OPh), 4j. Pale yellow oil, 89%. Found: C, 69.1; H, 9.1.
Calc. for C12H19OP: C, 68.55; H, 9.10%. NMR (CDCl3): δH (300
MHz) 7.38 (dd, 2H, JHH = 7.2 Hz, JHH = 2.0 Hz, OPh), 7.25
3
4
(m, 1H, H4), 7.05 (m, br, 2H, OPh), 2.08 (apparent dh, 2H,
Syntheses
2
3JHH = 7.0 Hz, JPH = 2.5 Hz, CH(CH3)2), 1.35 (dd, 6H,
3JHH = 7.0 Hz, 3JPH = 10.7 Hz, CH(CH3)2), 1.30 (dd, 6H, 3JHH
=
General method for the synthesis of PPh2(OAr) ligands. In a
Schlenk tube under an atmosphere of nitrogen were placed the
appropriate dried (toluene azeotrope) phenol, (34 mmol)
chlorodiphenylphosphine (6.0 mL, 33.4 mmol), toluene (50
mL) and triethylamine (7.0 mL, 50.0 mmol). The resultant mix-
ture was then heated at reflux temperature overnight, allowed to
cool to room temperature and the precipitated Et3NϩHClϪ
removed by filtration through a pad of Celite. The precipitate
was washed with toluene (2 × 10 mL) and the combined organic
fractions were then evaporated to dryness in vacuo yielding the
phosphinite ligands which were not purified further.
PPh2(OPh), 4a. White solid, 97% yield. Found: C, 77.8; H,
5.3. Calc. for C18H15OP: C, 77.69; H, 5.41%. NMR (CDCl3):
δH (300 MHz) 7.09 (tt, 1H, 3JHH = 7.3 Hz, 4JHH ≈ 2.0 Hz, OPh),
7.21 (d, 2H, 3JHH = 8.5 Hz, OPh), 7.32 (td, br, 2H, 3JHH = 8.0 Hz,
4JHH = 2.0 Hz, OPh), 7.46 (m, 6H, PPh2), 7.68 (td, 4H, JHH = 7.7
Hz, JHH = 1.9 Hz, PPh2) ppm. δP (121.5 MHz) 111.2 ppm.
PPh2(OC6H4-4-Me), 4b. Pale yellow oil, 96% yield. NMR
(CDCl3): δH (300 MHz) 7.88 (m, br, 4H, PPh2), 7.55 (m, 6H,
PPh2), 7.20 (s, br, 2H, OAr), 7.02 (dd, 2H, J = 2.0 Hz, J =
8.1 Hz, OAr), 2.52 (s, 3H, CH3) ppm. δP (121.5 MHz) 111.8
ppm.
7.2 Hz, 3JPH = 15.9 Hz, CH(CH3)2) ppm. δP (121.5 MHz) 149.0
ppm.
PiPr2(OC6H4-4-Me), 4k. Yellow oil, 92%. NMR (CDCl3):
3
δH (300 MHz) 7.11 (dd, 2H, JHH = 7.0 Hz, J ≈ 0.5 Hz, OAr),
7.06 (dd, 2H, 3JHH = 7.3 Hz, J = 1.6 Hz, OAr), 2.37 (s, 3H, CH3),
1.95 (apparent dh, 2H, 3JHH = 7.0 Hz, 2JPH = 2.4 Hz, CH(CH3)2),
3
3
1.25 (dd, 6H, JHH = 6.9 Hz, JPH = 10.7 Hz, CH(CH3)2), 1.17
3
3
(dd, 6H, JHH = 7.2 Hz, JPH = 15.9 Hz, CH(CH3)2) ppm.
δP (121.5 MHz) 149.6 ppm.
PiPr2(OC6H4-4-tBu), 4m. Pale yellow oil, 94%. NMR
(CDCl3): δH (300 MHz) 7.38 (dd, 2H, 3JHH = 7.0 Hz, J = 2.2 Hz,
3
OAr), 7.15 (dd, 2H, JHH = 6.7 Hz, J = 1.7 Hz, OAr), 2.04
(apparent dh, 2H, 3JHH = 7.0 Hz, 2JPH = 2.3 Hz, CH(CH3)2), 1.43
t
3
3
(s, 9H, Bu), 1.31 (dd, 6H, JHH = 7.0 Hz, JPH = 10.7 Hz,
3
3
CH(CH3)2), 1.25 (dd, 6H, JHH = 7.3 Hz, JPH = 15.9 Hz,
CH(CH3)2) ppm. δP (121.5 MHz) 148.9 ppm.
PiPr2(OC6H4-2-Me), 4n. Yellow oil, 94%. NMR (CDCl3):
δH (300 MHz) 7.42 (dd, 1H, 3JHH = 6.0 Hz, JHH = 1.6 Hz, OAr),
7.20 (m, 2H, OAr), 6.95 (apparent t, 1H, 3JHH = 6.0 Hz, OAr),
2.35 (s, 3H, CH3), 2.05 (apparent dh, 2H, 3JHH = 7.0 Hz, 2JPH
=
2.6 Hz CH(CH3)2), 1.27 (dd, 6H, 3JHH = 7.2 Hz, 3JPH = 10.4 Hz,
3
3
PPh2(OC6H4-4-Me), 4c. Pale yellow oil, 97% yield. NMR
(CDCl3): δH (300 MHz) 7.75 (m, 4H, PPh2), 7.50 (m, 6H, PPh2),
7.20 (s, 4H, br, OAr), 2.68 (q, 2H, 3JHH = 7.0 Hz, CH2CH3), 1.35
CH(CH3)2), 1.15 (dd, 6H, JHH = 7.3 Hz, JPH = 15.7 Hz,
CH(CH3)2) ppm. δP (121.5 MHz) 144.5 ppm.
PiPr2(OC6H4-2,4-Me2), 4o. Yellow oil, 95%. NMR (CDCl3):
δH (300 MHz) 7.37 (dd, 1H, J = 3.5, J = 1.3 Hz, H3, OAr), 7.07
(d, br, 2H, 3JHH = 7.4 Hz, OAr), 2.44 (s, 3H, CH3), 2.42 (s, 3H,
3
(t, 3H, JHH = 7.0 Hz, CH2CH3) ppm. δP (121.5 MHz) 111.4
ppm.
PPh2(OC6H4-4-tBu), 4d. White solid, 99% yield. NMR
(CDCl3): δH (300 MHz) 7.62 (m, 4H, PPh2), 7.4 (m, 6H, PPh2),
3
2
CH3), 2.12 (apparent dh, 2H, JHH = 7.1 Hz, JPH = 2.8 Hz,
CH(CH3)2), 1.38 (dd, 6H, JHH = 6.9 Hz, JPH = 10.7 Hz,
3
3
3
3
3
7.30 (dd, 2H, JHH = 6.6 Hz, J = 2.0 Hz, OAr), 7.07 (dd, 2H,
CH(CH3)2), 1.30 (dd, 6H, JHH = 7.2 Hz, JPH = 15.6 Hz,
CH(CH3)2) ppm. δP (121.5 MHz) 144.3 ppm.
3JHH = 6.9 Hz, JHH = 1.6 Hz, OAr), 1.34 (s, 9H, Bu) ppm.
5
t
PiPr2(OC6H4-2-tBu), 4p. Pale yellow oil, 96%. NMR
δP (121.5 MHz) 110.9 ppm.
PPh2(OC6H4-2-Me), 4e. Pale yellow oil, 95% yield. NMR
(CDCl3): δH (300 MHz) 7.90 (m, 4H, PPh2), 7.55 (m, 6H, PPh2),
7.35 (d, 1H, JHH = 7.5 Hz, OAr), 7.32 (d, 1H, JHH = 7.4 Hz),
7.15 (m, 2H, OAr), 2.55 (s, 3H, CH3) ppm. δP (121.5 MHz)
110.1 ppm.
3
(CDCl3): δH (300 MHz) 7.46 (m, 1H, JHH = 7.3, J = 1.7 Hz,
OAr), 7.31 (m, 2H, OAr), 7.04 (m, 1H, OAr), 2.25 (apparent dh,
3
3
3
2
2H, JHH = 7.1 Hz, JPH = 3.1 Hz, CH(CH3)2), 1.70 (dd, 6H,
3JHH = 7.0 Hz, 3JPH = 11.2 Hz, CH(CH3)2), 1.62 (dd, 6H, 3JHH
=
3
t
5.9 Hz, JPH = 14.0 Hz, CH(CH3)2), 1.53 (s, 9H, Bu) ppm.
δP (121.5 MHz) 138.5 ppm.
PPh2(OC6H3-2,4-Me2), 4f. Pale yellow oil, 95% yield. NMR
(CDCl3): δH (300 MHz) 7.78 (m, 4H, JHH = 7.7 Hz, JHH
3
PiPr2(OC6H4-2-Ph), 4r. Yellow-orange oil, 82%. Found: C,
75.9; H, 8.15. Calc. for C18H23OP: C, 75.50; H, 8.10%. NMR
(CDCl3): δH 7.67 (m, 2H), 7.54 (m, 3H), 7.45 (m, 3H), 7.16 (m,
=
1.7 Hz, PPh2), 7.50 (m, 6H, PPh2), 7.11 (s, br, 1H, OAr), 7.05
(apparent d, br, 2H, 3JHH ≈ 8.0 Hz, OAr), 2.41 (s, br, 6H, CH3)
ppm. δP (121.5 MHz) 110.3 ppm.
1H), 1.95 (apparent dh, 2H, JHH = 6.5 Hz,2JPH = 2.1 Hz,
3
PPh2(OC6H4-2-tBu), 4g. White solid, 98.5% yield. NMR
(CDCl3): δH (300 MHz) 7.65 (m, 4H, PPh2), 7.43 (m, 6H, PPh2),
3
3
CH(CH3)2), 1.24 (dd, 6H, JHH = 7.0 Hz, JPH = 9.6 Hz,
CH(CH3)2), 1.15 (dd, 6H, JHH = 7.0 Hz, JPH = 15.4 Hz,
CH(CH3)2) ppm. δP (121.5 MHz) 151.5 ppm.
3
3
3
7.35 (dd, 1H, JHH = 8.0 Hz, JHH = 1.0 Hz, OAr), 7.15 (d, 1H,
3JHH = 6.4 Hz, OAr), 7.13 (d, 1H, 3JHH = 6.3 Hz, OAr), 7.00 (td,
1H, 3JHH = 6.4 Hz, JHH = 1.0 Hz, OAr), 1.39 (s, 9H, tBu) ppm.
δP (121.5 MHz) 108.6 ppm.
General method for the synthesis of the orthopalladated com-
plexes of the ligands PPh2(OAr) with groups larger than methyl
in the 2- or 4-position(s). In a Schlenk tube under an atmosphere
of nitrogen, were placed palladium dichloride (0.25 g, 1.40
mmol) the appropriate phosphinite ligand (1.40 mmol) and
toluene (30 mL). The reaction mixture was heated at reflux
temperature overnight, allowed to cool and then the solvent
was removed in vacuo. The residue was extracted with di-
chloromethane (30 mL) and filtered through a pad of Celite.
Ethanol was added to induce precipitation of the product
which was collected by filtration and recrystallised from
CH2Cl2/EtOH.
PPh2(OC6H4-2-Ph), 4i. Yellow oil, 96%. Found: C, 81.4; H,
5.5. Calc. for C24H19OP: C, 81.34; H, 5.40%. NMR (CDCl3):
δH 7.55 (m, 6H), 7.18 (m, 5H), 7.35 (m, 8H) ppm. δP (121.5
MHz) 113.3 ppm.
General method for the synthesis of PiPr2(OAr) ligands. In a
Schlenk tube under an atmosphere of nitrogen were placed the
appropriate dried (toluene azeotrope) phenol, (32 mmol)
chlorodiisopropylphosphine (5.0 mL, 31.4 mmol), toluene (80
mL) and triethylamine (5 mL, 35.9 mmol). The resultant mix-
ture was then heated at reflux temperature overnight, allowed to
cool to room temperature and petroleum ether (50 mL) added.
The precipitated Et3NϩHClϪ was removed by filtration through
a pad of Celite and washed with petroleum ether (3 × 10 mL)
and the combined organic fractions were then evaporated to
dryness in vacuo yielding the phosphinite ligands which were
not purified further.
[{Pd(µ-Cl){κ2-P,C-PPh2(OC6H2-2,4-tBu2)}}2], 2a. Yellow
solid, 82% yield. Found: C, 59.2; H, 5.9. Calc. for C52H60O2P2-
Pd2Cl2: C, 59.10; H, 5.90%. Two isomers obtained in a 1.15 : 1
ratio. NMR (CDCl3): both isomers (integrations approximate)
δH (300 MHz) 7.95 (m, 8H, PPh2), 7.85 (m, 8H, PPh2), 7.605 (s,
br, 2H, OAr), 7.595 (s, br, 2H, OAr), 7.49 (m, 12H, PPh2), 7.40
(m, 12H, PPh2), 7.06 (s, br, 4H, OAr), 1.39 (s, br, 36H, tBu), 1.35
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t
t
(s, 18H, Bu); 1.28 (s, 18H, Bu) ppm. δP (121.5 MHz) 155.2
(major isomer), 154.7 (minor isomer) ppm.
6.80 (s, br, 2H, OAr), 6.78 (2, br 2H, OAr), 2.15 (s, 6H, CH3)
ppm. δP 153.3 ppm.
[{Pd(µ-Cl){κ2-P,C-PPh2(OC6H3-4-tBu)}}2], 2b. Yellow
solid, 43% yield. Found: C, 55.9; H, 5.01. Calc. for C44H44-
O2P2Pd2Cl2: C, 55.60; H, 4.67%. Two isomers obtained in a
1.25 : 1 ratio. NMR (CDCl3): major isomer δH (300 MHz)
[{Pd(µ-Cl){κ2-P,C-PPh2(OC6H3-2-Me)}}2], 2h. Yellow
solid, 78.5%. Two isomers obtained in a 1.38 : 1 ratio. NMR
(CDCl3): major isomer δH (300 MHz) 7.56 (s, br, 2H, OAr), 7.55
(m, 8H, PPh2), 7.47 (m, 12H, PPh2), 6.88 (s, 2H, OAr), 6.86 (s,
2H, OAr), 2.29 (s, br, 6H, CH3) ppm. δP (121.5 MHz) 152.6
ppm. Minor isomer δH 7.93 (m, 12H, PPh2), 7.86 (m, 8H, PPh2),
6.73 (m, 2H, OAr), 6.70 (m, 4H, OAr), 2.62 (s, 6H, CH3) ppm.
δP 151.9 (s) ppm.
3
7.62 (d, br, 2H, JHH = 8.2 Hz, OAr), 7.53 (m, 8H, PPh2),
7.49 (m, 12H, PPh2), 7.06 (s, br, 4H, OAr), 1.33 (s, 18H,
tBu) ppm. δP (121.5 MHz) 153.9 ppm. Minor isomer δH 7.96 (m,
3
8H, PPh2), 7.84 (m, 12H, PPh2), 7.61 (d, 2H, JHH = 10.0 Hz,
OAr), 6.81 (s, br, 4H, OAr), 1.27 (s, 18H, Bu) ppm. δP 154.4
t
ppm.
Synthesis of [{Pd(ꢀ-Cl){ꢁ2-P,C-PiPr2(OC6H2-2,4-tBu2)}}2], 2i.
In a Schlenk tube under an atmosphere of nitrogen were
placed, palladium dichloride (0.25 g, 1.40 mmol), the phos-
phinite ligand 4p (0.451 g, 1.40 mmol) and 1,4-dioxane (30 mL).
The reaction mixture was heated at reflux temperature over-
night, allowed to cool and then the solvent was removed in
vacuo. The residue was extracted in dichloromethane (30 mL),
filtered through a pad of Celite and ethanol was then added to
induce precipitation of the product. The crude product was
recrystallised from CH2Cl2/EtOH to give 2i as an orange solid,
0.31 g, 47% yield. Found: C, 52.1; H, 7.2. Calc. for C40H68O2-
P2Pd2Cl2: C, 51.85; H, 7.40%. Two isomers obtained in a 1.2 : 1
ratio. NMR (CDCl3): major isomer δH (300 MHz) 7.03 (s, br,
4H, OAr), 2.50 (apparent dh, 4H, 3JHH = 7.4 Hz, 2JPH = 2.1 Hz,
CH(CH3)3), 1.35 (dd, 12H, JHH = 7.3 Hz, JPH = 9.5 Hz,
CH(CH3)2), 1.32 (s, 18H, tBu), 1.28 (s, 18H, tBu), 1.25 (dd, 12H,
3JHH = 7.3 Hz, 3JPH = 12.3 Hz, CH(CH3)2) ppm. δP (121.5 MHz)
203.4 ppm. Minor isomer δH 7.67 (s, br, 2H, OAr), 7.53 (d, br,
2H, OAr), 2.40 (apparent dh, 4H, 3JHH = 7.3 Hz, 2JPH = 1.9 Hz,
CH(CH3)3), 1.53 (s, 18H, tBu), 1.42 (dd, 6H, 3JHH = 7.3 Hz, 3JPH
= 9.6 Hz, CH(CH3)2), 1.39 (dd, 12H, 3JHH = 7.3 Hz, 3JPH = 13.1
Hz, CH(CH3)2), 1.40 (s, 18H, tBu) ppm. δP 202.7 ppm.
[{Pd(µ-Cl){κ2-P,C-PPh2(OC6H3-2-tBu)}}2], 2c. Yellow
solid, 49%. Found: C, 55.6; H, 4.7. Calc. for C44H44O2P2Pd2Cl2:
C, 55.60; H, 4.67%. Two isomers obtained in a 1.6 : 1 ratio.
NMR (CDCl3): major isomer δH (300 MHz) 7.73 (apparent t, br,
3
2H, JHH = 5.0 Hz, OAr), 7.53 (m, 12H, PPh2), 7.48 (m, 8H,
PPh2), 7.07 (s, br, 2H, OAr), 7.05 (s, br, 2H, OAr), 1.33 (s, br,
18H, Bu) ppm. δP (121.5 MHz) 155.7 ppm. Minor isomer
t
δH 7.95 (m, 12H, PPh2), 7.86 (m, 8H, PPh2), 6.98 (apparent t,
3
2H, JHH = 5.0 Hz, OAr), 6.83 (m, 4H, OAr), 1.49 (s, br, 18H,
tBu) ppm. δP 155.0 ppm.
[{Pd(µ-Cl){κ2-P,C-PPh2(OC6H3-4-Et)}}2], 2d. Yellow
solid, 86%. Found: C, 54.0; H, 4.05.Calc. for C40H36O2P2Pd2Cl2:
C, 53.71; H, 4.06%. Two isomers obtained in a 1.36 : 1 ratio.
NMR (CDCl3): major isomer δH (300 MHz) 7.64 (s, br, 2H,
OAr), 7.54 (m, 8H, PPh2), 7.47 (m, 12H, PPh2), 6.98 (s, br, 4H,
3
3
3
OAr), 2.52 (q, 4H, JHH ≈ 7.0 Hz, CH2CH3), 1.17 (t, br, 6H,
3JHH ≈ 7.0 Hz, CH2CH3) ppm. δP (121.5 MHz) 154.1 ppm.
Minor isomer δH 7.94 (m, 12H, PPh2), 7.86 (m, 8H, Ph ring),
7.40 (m, 2H, OAr), 6.82 (s, 2H, OAr), 6.80 (s, 2H, OAr), 2.61
3
3
(q, 4H, JHH = 6.9 Hz, CH2CH3), 1.18 (t, 6H, JHH = 6.9 Hz,
CH2CH3) ppm. δP 153.4 ppm.
General method for the synthesis of the orthopalladated com-
plexes of the ligands PPh2(OAr) with only methyl or smaller
groups in the 2- or 4-position(s). In a Schlenk tube under an
atmosphere of nitrogen were placed bis(benzonitrile)dichloro-
palladium (0.50 g, 1.30 mmol), the appropriate phosphin-
ite ligand (1.4 mmol) and THF (30 mL). The reaction mixture
was heated at reflux temperature overnight, allowed to cool
and then the solvent was removed in vacuo. The residue was
extracted with dichloromethane (30 mL) and filtered through a
pad of Celite. The filtrate was concentrated under reduced
pressure and ethanol was added to induce precipitation. The
product was collected by filtration and recrystallised from
CH2Cl2/EtOH.
General method for the synthesis of the complexes [PdCl2(4)2],
5. A solution of [PdCl2(NCMe)2] (0.50 g, 1.93 mmol) and
the appropriate phosphinite ligand (3.85 mmol) in dichloro-
methane (30 mL) was stirred at room temperature for 4 hours.
The solvent was then removed under reduced pressure and the
solid residue recrystallised from dichloromethane/hexane.
cis/trans-[PdCl2{PiPr2(OPh)}2], 5c. Cream solid, 61%.
Found: C, 48.9; H, 6.1. Calc. for C24H38Cl2O2P2Pd: C, 48.22; H,
6.41%. Two isomers obtained in a 1.1 : 1 ratio. NMR (CDCl3):
major isomer δH (300 MHz) 7.44 (dd, 2H, 3JHH = 7.0 Hz, J ≈ 1.0
Hz, OPh), 7.00 (dd, 4H, 3JHH = 7.5 Hz, JHH = 1.5 Hz, OPh), 6.76
(d, br, 4H, 3JHH = 7.5 Hz, OPh), 2.43 (apparent dh, 4H, 3JHH
=
7.1 Hz, 2JPH = 2.1 Hz, CH(CH3)3), 1.36 (dd, 12H, 3JHH = 6.9 Hz,
[{Pd(µ-Cl){κ2-P,C-PPh2(OC6H4)}}2], 2e. Cream solid,
80%. Found: C, 51.85; H, 3.2. Calc. for C36H28O2P2Pd2Cl2: C,
51.58; H, 3.37%. Two isomers obtained in a 1.13 : 1 ratio. NMR
(CDCl3): both isomers (integrations approximate) δH (300 MHz)
7.89 (m, 8H, PPh2), 7.66 (m, 12H, PPh2), 7.47 (t, 2H,3JHH = 7.0
Hz, OPh), 7.45 (m, 8H, PPh2), 7.32 (m, 12H, PPh2), 7.23 (t, 2H,
3JHH = 7.0 Hz OPh); 7.10 (m, 4H, OPh), 6.93 (m, 4H, OPh), 6.85
3JPH = 7.3 Hz, CH(CH3)2, major isomer), 1.30 (dd, 12H, 3JHH
=
3
6.9 Hz, JPH = 7.3 Hz, CH(CH3)2). δP (121.5 MHz) 163.8 ppm.
3
Minor isomer δH 7.71 (dd, 2H, JHH = 6 Hz, J ≈ 2.0 Hz, OPh),
7.37 (dd, 4H, 3JHH = 7.5 Hz, J ≈ 1.4 Hz, OPh), 6.85 (d, br, 4H,
3
3JHH = 7.5 Hz, OPh), 2.83 (apparent dh, 4H, JHH = 7.0 Hz,
2JPH = 2.1 Hz, CH(CH3)3), 1.46 (dd, 12H, 3JHH = 7.0 Hz, 3JPH
=
7.2 Hz, CH(CH3)2), 1.41 (dd, 12H, 3JHH = 7.0 Hz, 3JPH = 7.2 Hz,
CH(CH3)2) ppm. δP 164.0 ppm. IR (KBr) ν(Pd–Cl) 362, 310,
3
(d, br, 4H, JHH = 6.0 Hz, OPh) ppm. δP (121.5 MHz) 157.5
(minor isomer), 154.8 (major isomer) ppm.
335 cmϪ1
.
[{Pd(µ-Cl){κ2-P,C-PPh2(OC6H3-2,4-Me2)}}2], 2f. Yellow
solid, 67%. Found: C, 54.1; H, 4.5. Calc. for C40H36O2P2Pd2Cl2:
C, 53.71; H, 4.06%. Two isomers obtained in a 1.27 : 1 ratio.
NMR (CDCl3): major isomer δH (300 MHz) 7.53 (m, 8H, PPh2),
7.48 (m, 12H, PPh2), 6.69 (s, br, 4H, OAr), 2.23 (s, br, 12H,
CH3) ppm. δP (121.5 MHz) 151.9 ppm. Minor isomer δH 7.93
(m, 12H, PPh2), 7.84 (m, 8H, PPh2), 7.20 (m, 4H, OAr), 2.15 (s,
br, 12H, CH3) ppm. δP 151.1 ppm.
[PdCl2{PiPr2(OC6H3-2,4-tBu2)}2], 5d. Colourless solid,
78%. Found: C, 59.1; H, 8.3. Calc. for C40H70Cl2O2P2Pd: C,
58.43; H, 8.58%. Two isomers obtained in a 1.34 : 1 ratio. NMR
3
(CDCl3): major isomer δH (300 MHz) 8.16 (dd, 2H, JHH
=
8.0 Hz, J = 2.0 Hz, OAr), 7.19 (d, 2H, J = 2.0 Hz, OAr), 6.99 (d,
2H, 3JHH = 8.0 Hz, OAr), 2.77 (apparent dh, 4H, 3JHH = 7.0 Hz,
2JPH = 2.0 Hz, CH(CH3)3), 1.61 (dd, 12H, 3JHH = 7.2 Hz, 3JPH
=
3
3
10.1 Hz, CH(CH3)2), 1.57 (dd, 12H, JHH = 7.2 Hz, JPH = 9.9
[{Pd(µ-Cl){κ2-P,C-PPh2(OC6H3-4-Me)}}2], 2g. Yellow
solid, 77%. Two isomers obtained in a 1.34 : 1 ratio. NMR
(CDCl3): major isomer δH (300 MHz) 7.74 (m, 2H, OAr), 7.55
(m, 8H, PPh2), 7.42 (m, 12H, PPh2), 6.87 (m, 4H, OAr), 2.3 (s,
br, 6H, CH3) ppm. δP (121.5 MHz) 154.1 ppm. Minor isomer δH
7.94 (m, 8H, PPh2), 7.85 (m, 12H, PPh2), 7.35 (m, 2H, OAr),
Hz, CH(CH3)2), 1.35 (s, br, 18H, Bu), 1.27 (s, br, 18H, Bu
major isomer) ppm. δP (121.5 MHz) 143.8 ppm. Minor isomer
δH 8.08 (d, 2H, 3JHH = 8.0 Hz, OAr), 7.30 (dd, 2H, 3JHH = 8.0 Hz,
J = 2.0 Hz, OAr), 6.92 (s, br, 2H, OAr), 2.42 (apparent dh, 4H,
t
t
2
3JHH = 7.0 Hz, JPH = 2.0 Hz, CH(CH3)3, minor isomer), 1.46
(dd, 12H, 3JHH = 7.3 Hz, 3JPH = 10.2 Hz, CH(CH3)2), 1.42 (dd,
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3
3
br, 12H, JHH = 7.3 Hz, JPH = 9.8 Hz, CH(CH3)2), 1.24 (s, br,
warm to room temperature and stirred for a total of 2 hours.
t
t
18H, Bu), 1.14 (s, br, 18H, Bu) ppm. δP 142.6 (minor isomer)
The supernatant liquid was removed from the unreacted Mg
with a filter cannula, cooled to 0 ЊC and the appropriate chloro-
phosphine (0.02 mol) was added dropwise. The mixture was
left to stir at room temperature overnight and then filtered
through a pad of Celite which was washed with diethyl ether.
The combined solution was evaporated to dryness under
reduced pressure to give the phosphines which were not purified
further.
ppm. IR (KBr) ν(Pd–Cl) 372, 342, 320 cmϪ1
.
trans-[PdCl2{PPh2(OC6H4-2-Ph)}2], 5e. Orange solid, 18%.
Found: C 65.1; H, 4.3. Calc. for C48H38O2P2PdCl2: C, 65.92; H,
4.01%. NMR (CDCl3): δH (300 MHz) 7.58 (m, 12H) 7.40 (m,
16H), 7.21 (m, 10H) ppm. δP (121.5 MHz) 108.5 ppm. IR (KBr)
ν(Pd–Cl) 374 cmϪ1
.
Synthesis of trans-[PdCl2{PiPr2(OC6H3-2-Ph)}2], 5f. A mix-
ture of [PdCl2(NCMe)2] (0.25 g, 0.965 mmol) and 2-phenyl-
phenyl diisopropylphosphinite, 4r, (0.55 g, 1.93 mmol) in tolu-
ene (40 mL) was heated at reflux temperature overnight. The
solution was allowed to cool and filtered through a pad of
Celite which was washed with dichloromethane. The volatiles
were removed from the combined solution under reduced pres-
sure and the residue recrystallised from CH2Cl2/EtOH to give
the product as a yellow solid, 23%. Found: C, 58.1; H, 5.9. Calc.
for C36H46O2P2PdCl2: C, 57.65; H, 6.18%. NMR (CDCl3):
δH (300 MHz) 7.72 (m, 2H, Ph), 7.64 (m, 4H, Ph), 7.52 (m, 2H,
PPh2(CH2C6H4-2-Ph), 15a. Cream solid, 74.1%. Found: C,
84.9, H; 5.8. Calc. for C25H21P: C, 85.20; H, 6.00%. NMR
(CDCl3): δH (300 MHz) 7.36 (m, 4H), 7.26 (complex m, br,
15H), 3.45 (m, 2H, CH2) ppm. δP (121.5 MHz) Ϫ8.11 ppm.
PiPr2(CH2C6H4-2-Ph), 15b. White solid, 81%. Found: C,
79.9; H, 8.6. Calc. for C19H25P: C, 80.25; H, 8.86%. NMR
(CDCl3): δH (300 MHz) 7.34 (m, 4H, Ph), 7.21 (m, 5H, Ph), 3.40
3
2
(m, 2H, CH2), 2.20 (apparent dh, 2H, JHH = 6.0 Hz, JPH
=
2.0 Hz, CH(CH3)2), 1.21 (dd, 6H, 3JHH = 6.7 Hz, 3JPH = 9.2 Hz,
3
3
CH(CH3)2), 1.15 (dd, 6H, JHH = 6.7 Hz, JPH = 13.4 Hz,
CH(CH3)2) ppm. δP (121.5 MHz) Ϫ6.54 ppm.
3
Ph), 7.19 (m, 1H, Ph), 1.96 (apparent dh, 4H, JHH = 6.5 Hz,
2JPH = 2.1 Hz, CH(CH3)3), 1.20 (dd, 12H, 3JHH = 5.9 Hz, 3JPH
=
General method for the synthesis of the phosphine complexes
[PdCl2{PR2(CH2C6H4-2-R)}2], 11 (R ؍ H) and 16 (R ؍ Ph).
A solution of [PdCl2(NCMe)2] (0.50 g, 1.93 mmol) and the
appropriate phosphine, (3.85 mmol) in dichloromethane
(10 mL) was stirred at room temperature for 30 minutes after
which time, MeOH (10 mL) was added and the solution con-
centrated in vacuo to induce precipitation of the product which
was then recrystallised from CH2Cl2/MeOH.
10.2 Hz, CH(CH3)2), 1.11 (dd, 12H, 3JHH = 5.9 Hz, 3JPH = 14.8
Hz, CH(CH3)2) ppm. δP (121.5 MHz) 142.1 ppm. IR (KBr)
ν(Pd–Cl) 369 cmϪ1
.
Synthesis of PiPr2(CH2Ph), 10b. Magnesium shavings (0.30 g,
12.30 mmol) were activated by stirring with a seed crystal
of iodine, under an atmosphere of nitrogen overnight. Et2O
(50 mL) was added, the mixture was cooled to 0 ЊC and benzyl
chloride (1.65 g, 13.10 mmol) was added dropwise to the
cooled solution. The resultant solution was allowed to warm
to room temperature and stirred for a total of 2 hours. The
supernatant liquid was removed from the unreacted Mg with
a filter cannula, cooled to 0 ЊC and chlorodiisopropylphosphine
(2.0 mL, 12.5 mmol) was added dropwise. The mixture was
left to stir at room temperature overnight and then filtered
through a pad of Celite which was washed with diethyl ether.
The combined solution was evaporated to dryness under
reduced pressure to give the product as a pale yellow solid,
2.31 g, 89%. Found: C, 75.3; H, 10.0. Calc. for C13H20P: C,
74.97; H, 10.16%. NMR (CDCl3): δH (300 MHz) 7.22 (d, 2H,
3JHH = 6.8 Hz, Ph), 7.18 (dd, 2H, 3JHH = 5.0 Hz, 3JHH = 6.8 Hz,
cis-[PdCl2{PPh2(CH2Ph)}2], 11a. Yellow solid, 61%. Found:
C, 61.9, H; 4.2. Calc. for C38H34P2PdCl2: C, 62.53; H, 4.69%.
NMR (CDCl3): δH (300 MHz) 7.58 (m, br, 6H), 7.44 (m, br,
6H), 7.28 (m, 4H), 7.10 (m, 8H), 7.02 (m, 2H), 6.92 (m, 4H),
3.97 (m, 4H, CH2) ppm. δP (121.5 MHz) 29.6 ppm. IR (KBr)
ν(Pd–Cl) 302, 330 cmϪ1
.
cis-[PdCl2{PiPr2(CH2Ph)}2], 11b. Cream solid, 45%. Found:
C, 53.2; H, 7.0. Calc. for C26H42P2PdCl2: C, 52.58; H, 7.13.
3
NMR (CDCl3): δH (300 MHz) 7.55 (d, br, 4H, JHH = 7.0 Hz,
Ph), 7.38 (m, br, 6H, Ph), 4.28 (m, 4H, CH2), 2.32 (apparent dh,
4H, 3JHH = 6.0 Hz, 2JPH = 2.1 Hz, CH(CH3)3), 1.25 (dd, br, 12H,
3
3JHH = 6.5 Hz, JPH = 9.9 Hz, CH(CH3)2), 1.20 (dd, br, 12H,
3JHH = 6.5 Hz, 3JPH = 14.5 Hz, CH(CH3)2) ppm. δP (121.5 MHz)
38.9 ppm. IR (KBr) ν(Pd–Cl) 301, 325 cmϪ1
.
3
Ph), 7.04 (t, 1H, JHH = 5.0 Hz, Ph), 4.43 (s, 2H, CH2), 1.61
cis-[PdCl2{PPh2(CH2C6H4-2-Ph)}2], 16a. Orange solid,
(apparent dh, 2H, 3JHH = 7.0 Hz, 2JPH = 2.1 Hz, CH(CH3)2), 1.02
(dd, 6H, 3JHH = 6.8 Hz, 3JPH = 9.8 Hz, CH(CH3)2), 0.94 (dd, 6H,
3JHH = 6.8 Hz, 3JPH = 14.2 Hz, CH(CH3)2) ppm. δP (121.5 MHz)
Ϫ7.9 ppm.
69%. Found: C, 68.6; H, 4.2. Calc. for C50H42P2PdCl2: C, 68.08;
5
H, 4.80%. NMR (CDCl3): δH (300 MHz) 8.47 (dd, 2H, JHH
=
2.0 Hz, 3JHH ≈ 7.0 Hz), 7.42 (m, 14H), 7.20 (m,12H), 7.08 (d, br,
4H, JHH ≈ 7.0 Hz), 6.80 (d, br, 2H, J = 2.0 Hz), 6.55 (m, 4H),
3
4.13 (m, 4H, CH2) ppm. δP (121.5 MHz) 31.0 ppm. IR (KBr)
ν(Pd–Cl) 374 cmϪ1
.
Synthesis of P(OC6H4-2-Ph)3, 14. To a solution of dried
(toluene azetrope) 2-phenylphenol (29.26 g, 0.17 mol) in tolu-
ene (80 mL) was added PCl3 (5.0 mL, 57.0 mmol) dropwise and
then triethylamine (25.0 mL, 0.18 mol) dropwise. The mixture
was heated at reflux temperature overnight, allowed to cool and
then filtered through a pad of Celite to remove the precipitated
triethylamine hydrochloride. The solvent was removed under
reduced pressure to yield the crude product as a brown oil
which was then purified by column chromatography (silica,
hexane/dichloromethane 1 : 1). The title product was obtained
as a white solid, 12.45 g, 42.5%. NMR (CDCl3): δH (300 MHz)
7.40 (m, 12H), 7.35 (m, 3H), 7.18 (m, 6H), 6.92 (m, 6H) ppm. δP
(121.5 MHz) 130.5 ppm.
cis-[PdCl2{PiPr2(CH2C6H4-2-Ph)}2], 16b. Yellow solid, 40%.
Found: C, 61.85; H, 6.1. Calc. for C38H56P2PdCl2: C, 61.17; H,
6.75%. NMR (CDCl3): δH (300 MHz) 7.42 (m, br, 8H) 7.31 (m,
3
br, 10H), 3.65 (m, 4H, CH2) 2.25 (apparent dh, 4H, JHH
=
6.5 Hz, 2JPH = 2.2 Hz, CH(CH3)3), 1.24 (dd, 12H, 3JHH = 6.5 Hz,
3JPH = 9.5 Hz, CH(CH3)3), 1.15 (dd, 12H, 3JHH = 6.5 Hz, 3JPH
=
13.5 Hz, CH(CH3)3) ppm. δP (121.5 MHz) 41 ppm. IR (KBr)
ν(Pd–Cl) 378 cmϪ1
.
Catalysis
Suzuki coupling of aryl bromides with phenylboronic acid
(Table 2, Fig. 2, Fig. 6). In a three-necked flask under an atmos-
phere of nitrogen were placed the appropriate aryl bromide
(10.0 mmol), phenylboronic acid (1.83 g, 15.0 mmol), K2CO3
(2.76 g, 20.0 mmol) and toluene (30 mL total, including
catalyst/added ligand solution). The correct amount of catalyst/
added ligand was added as a toluene solution made up by
multiple volumetric dilutions of a stock solution. The mixture
was then heated at reflux temperature for 18 h, cooled in an ice
General method for the synthesis of the phosphines PR2-
(CH2C6H4-2-Ph), 15. Magnesium shavings (0.49 g, 0.02 mol)
were activated by stirring with a seed crystal of iodine, under
an atmosphere of nitrogen overnight. Dry degassed diethyl
ether (50 mL) was added, the mixture was cooled to 0 ЊC and
2-phenylbenzylbromide (5.00 g, 0.02 mol) was added dropwise
to the cooled solution. The resultant solution was allowed to
D a l t o n T r a n s . , 2 0 0 3 , 4 1 6 4 – 4 1 7 4
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bath, quenched with aqueous HCl (2 M, 100 mL), extracted
with dichloromethane (3 × 100 mL), dried (MgSO4) and
evaporated to dryness. Hexadecane (0.068 M in CH2Cl2, 3.00
mL) and dichloromethane (5–7 mL, to ensure complete dis-
solution) were added. The conversion to coupled product was
then determined by GC analysis.
Acknowledgements
We thank EPSRC for funding and Johnson Matthey for fund-
ing and the loan of palladium salts.
References and notes
1 For recent reviews see: (a) N. Miyaura and A. Suzuki, Chem. Rev.,
1995, 95, 2457; (b) S. P. Stanforth, Tetrahedron, 1998, 54, 263;
(c) A. Suzuki, J. Organomet. Chem., 1999, 576, 147.
2 For a discussion see: V. V. Grushin and H. Alper, Chem. Rev., 1994,
94, 1047.
Suzuki coupling of aryl chlorides with phenylboronic acid
(Table 2). As above but using DMA (N,N-dimethylacetamide)
as solvent and Cs2CO3 as base. The reaction temperature was
110 ЊC.
3 For a review see: R. B. Bedford, Chem. Commun., 2003, 1787.
4 (a) D. A. Albisson, R. B. Bedford, S. E. Lawrence and P. N. Scully,
Chem. Commun., 1998, 2095; (b) R. B. Bedford and S. L. Welch,
Chem. Commun., 2001, 129; (c) R. B. Bedford, S. L. Hazelwood,
M. E. Limmert, D. A. Albisson, S. M. Draper, P. Noelle Scully,
S. J. Coles and M. B. Hursthouse, Chem. Eur. J., 2003, accepted.
5 (a) R. B. Bedford, C. S. J. Cazin and S. L. Hazelwood, Angew.
Chem., Int. Ed., 2002, 41, 4120; (b) R. B. Bedford, S. L. Hazelwood
and M. E. Limmert, Chem. Commun., 2002, 2610.
6 Attempts to prepare palladacyclic complexes in the presence of
triethylamine hydrochloride lead to the formation of ‘ate’ complexes
of the form [PdCl3{PR2(OAr)}]HNEt3.
7 A. Albinati, S. Affolter and P. S. Pregosin, Organometallics, 1990, 9,
379.
Suzuki coupling of 4-bromoanisole with n-butylboronic acid
(Fig. 4). In a three-necked flask under an atmosphere of nitro-
gen were placed 4-bromoanisole (1.87 g, 10.0 mmol), butyl-
boronic acid (1.56 g, 15.0 mmol), K3PO4 (4.24 g, 20.0 mmol)
and 1,4-dioxane (30 mL total, including catalyst/added ligand
solution). The catalyst was added as a dioxane solution made
up by multiple volumetric dilutions of a stock solution. The
mixture was heated at reflux temperature for 18 h, cooled in an
ice bath, quenched with aqueous HCl (2 M, 100 mL) and
extracted with CH2Cl2 (3 × 100 mL). The combined extracts
were dried (MgSO4) and then the solvent was removed on a
rotary evaporator. Hexadecane (0.068 M in CH2Cl2, 3.00 mL)
and dichloromethane (5–7 mL, to ensure complete dissolution)
were added. The conversion to coupled product was then
determined by GC analysis.
8 R. B. Bedford, S. L. Hazelwood, M. E. Limmert, J. M. Brown,
S. Ramdeehul, A. R. Cowley, S. J. Coles and M. B. Hursthouse,
Organometallics, 2003, 22, 1364.
9 J. P. Wolfe, R. A. Singer, B. H. Yang and S. L. Buchwald, J. Am.
Chem. Soc., 1999, 121, 9550.
10 M. Feuerstein, D. Laurenti, C. Bougeant, H. Doucet and
M. Santelli, Chem. Commun., 2001, 325.
11 R. B. Bedford, S. M. Draper, P. N. Scully and S. L. Welch, New J.
Chem., 2000, 24, 745.
12 S. Gibson, D. F. Foster, G. R. Eastham, R. P. Tooze and D. J.
Cole-Hamilton, Chem. Commun., 2001, 779.
13 A. D. Ryabov, A. V. Eliseev, E. S. Sergeyenko, A. V. Usatov,
L. I. Zakharkin and V. N. Kalinin, Polyhedron, 1989, 8, 1485.
14 For leading references see: C. Amatore, A. Jutland and A. Thuilliez,
J. Organomet. Chem., 2002, 643–644, 416.
15 R. B. Bedford, C. S. J. Cazin, S. J. Coles, T. Gelbrich, P. N. Horton,
M. B. Hursthouse and M. E. Light, Organometallics, 2003, 22, 987.
16 The process is sometimes referred to as ‘transmetallation’.
17 For a recent review see: S. R. Chemler, D. Trauner and S. J.
Danishefsky, Angew. Chem., Int. Ed., 2001, 40, 4544.
18 Pd()/Pd() pathways have previously been suggested for certain
coupling reactions. For a discussion of this topic see: W. A.
Herrman, V. P. W. Böhm and C.-P. Reisinger, J. Organomet. Chem.,
1999, 576, 23.
Time-dependant study on the coupling of phenylboronic acid
with 4-chloroanisole (Fig. 5). In a two-necked flask under an
atmosphere of nitrogen were placed 4-chloroanisole (1.42 g,
10.0 mmol), phenylboronic acid (1.83 g, 15.0 mmol), Cs2CO3
(6.56 g, 20,0 mmol), hexadecane (0.068 M in 1,4-dioxane, 3 mL
0.204 mmol, internal standard), and 1,4-dioxane (26 mL). The
mixture was then heated to 100 ЊC and the catalyst mixture was
added as 1,4-dioxane (1.00 mL) solution made up to the correct
concentration by multiple volumetric dilutions of a stock solu-
tion. The temperature was maintained at 100 ЊC for 120 min
and 0.2 mL aliquots were taken at regular intervals. These
samples were quenched in aqueous HCl (2 M, 0.5 mL), the
mixture extracted with toluene (3 × 1 mL), the combined
organic extracts dried (MgSO4) and then the conversion to
coupled product was determined by GC.
19 (a) R. B. Bedford and C. S. J. Cazin, Chem. Commun., 2001, 1540;
(b) R. B. Bedford, C. S. J. Cazin, M. B. Hursthouse, M. E. Light,
K. J. Pike and S. Wimperis, J. Organomet. Chem., 2001, 633, 173.
20 J. Louie and J. F. Hartwig, Angew. Chem., Int. Ed. Engl., 1996, 35,
2359.
21 R. B. Bedford, S. J. Coles, M. B. Hursthouse and M. E. Limmert,
Angew. Chem., Int. Ed., 2003, 42, 112.
22 J. P. Wolfe, R. A. Singer, B. H. Yang and S. L. Buchwald, J. Am.
Chem. Soc., 1999, 121, 9550.
23 D. Wilhelm, J.-E. Bäckvall, R. E. Nordberg and T. Norin,
Organometallics, 1985, 4, 1296.
24 G. K. Anderson and M. Lin, Inorg. Synth., 1990, 28, 60.
25 M. F. Rettig and P. M. Maitlis, Inorg. Synth., 1990, 28, 110.
26 R. Hooft, COLLECT, Data collection software, Nonius BV, Delft,
The Netherlands, 1998.
27 Z. Otwinowski and W. Minor, Methods in Enzymology: Macro-
molecular Crystallography, Part A, ed. C. W. Carter, Jr. and R. M.
Sweet, Academic Press, New York, 1997, vol. 276, pp. 307–326.
28 G. M. Sheldrick, Acta Crystallogr., Sect. A, 1990, 46, 467.
29 G. M. Sheldrick, University of Göttingen, Germany, 1997.
X-Ray structure determination of complex 2i
Suitable crystals were selected and data collected on a Bruker
Nonius KappaCCD Area Detector at the window of a Bruker
Nonius FR591 rotating anode (λMo-Kα = 0.71073 Å) driven by
COLLECT26 and DENZO27 software at 120 K. Structures
were determined in SHELXS-9728 and refined using SHELXL-
97.29 Crystal data: C21H36Cl3OPPd, M = 548.22, monoclinic,
a = 10.63170(10), b = 22.1407(3), c = 11.3297(2) Å, β = 112.61Њ,
U = 2461.91(6) Å3, T = 120(2) K, space group P21/c (no. 14),
Z = 4, µ(Mo-Kα) = 1.154 mmϪ1, 15633 reflections measured,
4331 unique (Rint = 0.056) which were used in all calculations.
Final R1 = 0.0283, wR2 = 0.0690 [F 2 > 2σ(F 2)], R1 = 0.0336,
wR2 = 0.0715 (all data).
CCDC reference number 207369.
See http://www.rsc.org/suppdata/dt/b3/b303657j/ for crystal-
lographic data in CIF or other electronic format.
D a l t o n T r a n s . , 2 0 0 3 , 4 1 6 4 – 4 1 7 4
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