Phosphine and arsine adducts of N-donor palladacycles as catalysts in the Suzuki coupling of aryl bromides

Article abstract of DOI:10.1039/b304053d

Triarylphosphine and arsine adducts of imine- and amine-based palladacycles have been produced and the crystal structures of three examples have been determined, as has the structure of a parent imine-based palladacycle. The complexes were tested in the Suzuki coupling of an electronically deactivated aryl bromide and the phosphine adducts were found to show much greater activity than the parent palladacycles. Triarylphosphine adducts are preferable to trialkylphosphine adducts as they not only show higher activity but they are also more easily synthesised.

Full text of DOI:10.1039/b304053d

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Phosphine and arsine adducts of N-donor palladacycles
as catalysts in the Suzuki coupling of aryl bromides
Robin B. Bedford,*a Catherine S. J. Cazin,a Simon J. Coles,b Thomas Gelbrich,b
Michael B. Hursthouseb and Véronique J. M. Scordiaa
a School of Chemistry, University of Exeter, Exeter, UK EX4 4QD. E-mail: r.bedford@ex.ac.uk
b EPSRC National Crystallography Service, Department of Chemistry,
University of Southampton, Highfield Road, Southampton, UK SO17 1BJ
Received 10th April 2003, Accepted 10th July 2003
First published as an Advance Article on the web 29th July 2003
Triarylphosphine and arsine adducts of imine- and amine-based palladacycles have been produced and the crystal
structures of three examples have been determined, as has the structure of a parent imine-based palladacycle. The
complexes were tested in the Suzuki coupling of an electronically deactivated aryl bromide and the phosphine
adducts were found to show much greater activity than the parent palladacycles. Triarylphosphine adducts are
preferable to trialkylphosphine adducts as they not only show higher activity but they are also more easily
synthesised.
Suzuki coupling of aryl bromides and the results of this study
are presented below.
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 There has recently been
considerable interest in the development 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. 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 sub-
strates.2 We demonstrated that the pincer complexes 2 and the
orthopalladated triarylphosphite and phosphinite complexes 3
show good to excellent activity in such couplings,3 whilst Cole-
Hamilton and co-workers showed that the phosphine-based
complexes 4 can also be used.4 Activity is not limited to ortho-
metallated phosphorus donor systems. We,5 and later others,6
showed that the N-donor palladacycle 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,7 while Nájera has shown
that the related oxime-containing complexes 7 can even activate
deactivated aryl chlorides in the presence of water and tetra-
butylammonium bromide.8 Dupont and Monteiro have shown
that orthometallated thioether complexes of the type 8 can also
be used.9 Tricyclohexylphosphine adducts of both imine- and
amine-based palladacycles, complexes 9a and 10a, show very
good activity when aryl chlorides are used as substrates.10 Very
recently we have found that similar adducts formed between
complexes of the types 3 and 11 with tricyclohexylphosphine,
as well as other di- and tri-alkylphosphines, show amongst
the highest activities yet reported in aryl chloride coupling
reactions.11
Scheme 1 The Suzuki biaryl coupling reaction.
We have previously investigated the possibility of using
silica-supported analogues of the imine-based complex 5a as
recyclable catalysts for the Suzuki reaction.12 During the course
of that study we noticed that addition of a triphenylphosphine
ligand to a solid-supported imine-based palladacycle substan-
tially increases the catalytic activity of the complex. We there-
fore decided to explore further the effect of phosphine ligands
on the activity of palladated imine and amine catalysts in the
D a l t o n T r a n s . , 2 0 0 3 , 3 3 5 0 – 3 3 5 6
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 bond lengths (Å) and angles (Њ) for the complex
[{Pd(µ-TFA)(κ2-N,C-C6H4CH2NMe2)}2], 6c
Results and discussion
Synthesis and characterisation of the complexes
The literature methods for the production of 6a and b12,13 were
Pd1–C1
Pd1–N1
Pd1–O1
Pd1–O4
1.948(4)
2.030(3)
2.175(3)
2.049(3)
Pd2–C16
Pd2–N2
Pd2–O2
Pd2–O3
1.956(4)
2.021(3)
2.055(3)
2.165(3)
extended to the synthesis of the analogous complex [{Pd-
(µ-TFA)(κ2-N,C-C H CH᎐NPh)} ], 6c, from Pd(TFA) and
᎐
6
4
2
2
N-benzylidene aniline in 57% yield (Scheme 2). The crystal
structure of 6c was determined and the molecule is shown in
Fig. 1, while selected data are given in Table 1. The palladium
centres adopt approximately square planar coordination
geometries, with only one of the two possible isomers, the trans
isomer, evident. The structure is very similar to those reported
previously for the complexes 6a and b,13,10b with essentially
identical Pd–C, Pd–N, and Pd–O bonds to 6a and only slightly
shorter Pd–O (trans to C) compared with 6b.
C1–Pd1–N1
C1–Pd1–O1
C1–Pd1–O4
N1–Pd1–O1
N1–Pd1–O4
O1–Pd1–O4
81.19(15)
175.26(13)
92.04(14)
98.24(12)
173.01(13)
88.65(11)
C16–Pd2–N2
C16–Pd2–O2
C16–Pd2–O3
N2–Pd2–O3
N2–Pd2–O2
O2–Pd–O3
80.89(15)
92.08(14)
174.95(13)
99.12(12)
172.97(12)
87.87(12)
parent dimers with triphenylphosphine in dichloromethane
(Scheme 2), according to the method used previously for the
synthesis of 9b.12 Similarly the phosphine and arsine adducts
10b–e were prepared by reaction of the dimeric complexes 5
with the appropriate ligands in dichloromethane. The 31P NMR
spectra of complexes 9c and d show singlets at δ 42.3 and 42.2
ppm, respectively, very similar to that reported for 9b.12 The
spectra of the complexes 10b, d–f show singlets in the range
1
δ 39.9–43.0 ppm. The H NMR spectra of the adducts 10b–f
reveal that the proton on the carbon ortho to the palladated
carbon always appears at the low field end of the peaks associ-
ated with the metallated ring, in the range 6.18–6.44 ppm. This
range is at higher field compared with those reported for the
analogous complexes with PCy2(o-biphenyl), PCy3 and PtBu3
ligands (δ 6.89, 7.11 and 7.42 ppm, respectively).10b The com-
plexes 10b, d–f show a doublet in the range δ 4.00–4.10 ppm
for the methylene protons of the orthometallated N,N-dimethyl-
benzylamine with JPH couplings in the range 1.5 to 2.2 Hz and
a doublet in the range δ 2.72–2.88 ppm with JPHs in the range
2.4 to 2.75 Hz corresponding to the N-methyl groups. By com-
parison complex 10c shows singlets at δ 4.08 and 2.80 ppm,
respectively, for these two environments. Further evidence that
the observed couplings to the methylene and methyl protons of
10f are due to phosphorus was obtained by running a 1H-{31P}
NMR spectrum which showed the two environments as singlets.
The similarities in the NMR data indicate that the stereo-
chemistries must be essentially identical in all cases. The struc-
tures of three of the phosphine adducts, namely complexes 9d,
10d and f, were determined unequivocally by single crystal
X-ray analysis and the molecules are shown in Figs. 2–4 while
selected data are given in Table 2. The crystal structure of 10f
shows poor R-factors as a result of stacking faults and twin-
ning. In all cases it can be seen that only the isomer in which the
phosphorus is trans to the N-donor is obtained. The Pd–C bond
lengths are essentially identical while the Pd–P are very similar.
Comparing the imine- and amine-containing complexes 9d and
10d it can be seen that the Pd–N bond length of the latter is
marginally longer.
Scheme 2 Conditions: (i) Pd(TFA)2, THF, 60 ЊC, 4 h. (ii) 2.2 PR3 or
AsPh3, CH2Cl2, r.t., 20 min.
Fig. 1 The molecular structure of [{Pd(µ-TFA)(κ2-N,C-C6H4CH᎐
᎐
NPh)}2], 6c. One of the CF3 groups exhibits rotational disorder over
two sites around C30, one of which is omitted for clarity.
Fig. 2 The molecular structure of [Pd(TFA)(κ2-N,C-C6H4CH᎐NPh)-
The triphenylphosphine adducts of the complexes 6, com-
plexes 9c and d, are synthesised in good yields by reaction of the
᎐
(PPh3)], 9d.
D a l t o n T r a n s . , 2 0 0 3 , 3 3 5 0 – 3 3 5 6
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solvent and K2CO3 as base in order to allow comparison with
previous studies in the literature. The results from the catalytic
studies are summarised in Table 3.
As can be seen (Table 3, entry 1) complex 6a shows good
activity, with the TON obtained being somewhat higher than
those recorded previously under similar conditions (102 000–
136 000).7 By contrast, the highest TON reported in this
reaction is 8.75 million with 3c acting as catalyst under essen-
tially identical conditions.3c The addition of triphenylphosphine
is clearly beneficial, with a jump in TON to 480 000 (entry 2).
Interestingly, changing from an orthometallated ketimine to
aldimine precursor (compare entries 2 and 3) leads to an even
greater enhancement in activity. Similarly when the imine is
replaced with an orthometallated N,N-dimethylbenzylamine
ligand then greater activity results (entry 5). This system also
has the advantage that the orthometallated ligand is com-
mercially available and inexpensive. Again it is obvious that the
triphenylphosphine ligand is necessary to maintain optimum
activity, with the parent dimer 5b showing substantially lower
conversion (entry 6). The nature of the anionic ‘X’ ligand
also seems to play an important role in catalyst performance:
changing from TFA to Cl appears to be highly deleterious as
exemplified by comparing the result obtained with 10b with that
with 10f (entries 5 and 7). While counter-ion effects are
observed when related PCy3-containing adducts are used in
the coupling of aryl chlorides, they are nowhere near as
pronounced.10b
Fig. 3 The molecular structure of [Pd(TFA)(κ2-N,C-C6H4CH2NMe2)-
{P(C6H4-4-CF3)3}], 10d.
Since the incorporation of a phosphine ligand leads to
substantial enhancement in activity, we next examined the
effect on performance of changing this ligand. The results are
summarised in entries 8–12. As can be seen the PCy3 complexes
9a and 10a show somewhat poorer activity than their triphenyl-
phosphine counterparts, despite the fact that both of these
complexes are excellent pre-catalysts for the coupling of aryl
chlorides.10 One explanation for this is that the rate-determining
step in the aryl bromide coupling may not be oxidative addition
of the aryl halide, as is the case at least with deactivated aryl
chlorides, but rather either nucleophilic attack of the base-
activated boronate at the palladium centre,14 or reductive
elimination of the product. If this is the case then far from
being advantageous, the presence of good σ-donor but poor
π-acid ligands such as PCy3 would be expected to be deleterious
to the rate. Triphenylarsine is a significantly better π-acid than
PPh3 and therefore may be expected to show good activity,
however the activity observed is somewhat lower (entry 10).
In order to examine more subtle variations in electronic
effects we investigated the use of the palladacyclic adducts 10d
and e which contain P(C6H4-4-CF3)3 and P(C6H4-4-OMe)3
ligands, respectively (entries 11 and 12). Interestingly both show
poorer activity than the PPh3-containing analogue 10b under
these conditions, with the more electron-deficient complex 10d
proving the least effective. In order to perform a more accurate
comparison we next followed the course of the coupling of
4-bromoanisole with phenylboronic acid catalysed by 10b, d
and e at 0.1 mol% Pd and 80 ЊC. The plots of conversion against
time obtained are shown in Fig. 5. As can be seen, under these
conditions the most-electron rich system, complex 10e, shows
the lowest overall conversion, the PPh3-containing complex 10b
is slightly better and the most electron-deficient pre-catalyst,
complex 10d, gives by far the highest performance. Interestingly
this system seems to have the lowest initial rates of con-
version, but the highest lifetime. It is this slightly increased
longevity that leads to the greatest conversion under these
conditions.
Fig. 4 The molecular structure of [PdCl(κ2-N,C-C6H4CH2NMe2)-
(PPh3)], 10f.
Table 2 Selected bond lengths (Å) and angles (Њ) for the complex
[Pd(TFA)(κ2-N,C-C6H4CH᎐NPh)(PPh3)], 9d and [Pd(X)(κ2-N,C-
᎐
C6H4CH2NMe2)(PR3)], 10dؒ(0.5MeOH) (X = TFA, R = C6H4-4-CF3)
and 10f (X = Cl, R = Ph)
9d
10dؒ(0.5MeOH)
1.999(2)
2.2495(5)
2.1457(18)
2.1417(15)
10f
Pd–C
Pd–P
Pd–N
Pd–X
2.003(3)
2.2489(9)
2.122(3)
2.124(2)
1.972(9)
2.268(3)
2.106(10)
2.153(8)
C–Pd–N
C–Pd–P
C–Pd–X
P–Pd–N
P–Pd–X
N–Pd–X
81.67(12)
93.41(10)
172.80(11)
175.00(8)
92.85(7)
82.48(8)
94.12(7)
174.64(7)
176.32(5)
91.13(5)
92.25(7)
82.9(4)
94.8(3)
170.4(4)
176.8(3)
94.1(2)
88.0(4)
92.12(10)
Catalytic studies
In order to standardise the catalytic studies, all of the catalyst
systems were tested in the same reaction, namely the Suzuki
coupling of phenylboronic acid with 4-bromoanisole. This
bromide was chosen since catalysts that show very high turn-
over numbers (TONs) with more easy to couple bromides such
as 4-bromoacetophenone often show quite poor activity with
this bromide. This is because the para-methoxy function elec-
tronically deactivates the substrate. Therefore this substrate is a
good indicator of optimal catalyst performance. The reaction
conditions were not optimised, but rather toluene was used a
Fig. 6 shows the effect on the reaction of increasing the ratio
of the triarylphosphines to Pd in the coupling of 4-bromo-
anisole with phenylboronic acid. As can be seen there is not
a particularly large effect with the optimum ratio of P : Pd
varying depending on the phosphine. This is in marked contrast
to the application of PCy3 adducts of amine- or phosphite-
D a l t o n T r a n s . , 2 0 0 3 , 3 3 5 0 – 3 3 5 6
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Table 3 Suzuki coupling of 4-bromoanisole with phenylboronic acida
Entry
1
Catalyst
Conversion (%)
64
TON (mol product/mol cat)
320 000
2
3
48
82
480 000
820 000
4
5
70
73
700 000
730 000
6
7
10
15
100 000
150 000
8
40
400 000
9
10
11
65
48
35
650 000
480 000
350 000
12
51
510 000
a Conditions: 4-bromoanisole (2.0 mmol), phenylboronic acid (3.0 mmol), K2CO3 (4.0 mmol), toluene (20 mL), reflux, 17 h.
based palladacycles in the coupling of 4-chloroanisole with
phenylboronic acid.10b,15 In these cases the addition of greater
than 2 : 1 or 1 : 1 of PCy3 : Pd, respectively, is highly deleterious.
This suggests that while a low-coordinate active catalyst is
required for chloride coupling reactions, this is less important
with aryl bromides.
D a l t o n T r a n s . , 2 0 0 3 , 3 3 5 0 – 3 3 5 6
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advantage compared with far more inexpensive triphenyl-
phosphine analogues with bromide substrates. In addition, the
much lower air-sensitivity of triphenylphosphine compared
with tricyclohexylphosphine makes adducts with this ligand far
more attractive catalysts in aryl bromide couplings.
Experimental
General
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.
Pd(TFA)2,18 N-benzylidene aniline19 and complexes 6a,13 b,12
9a,10b b12 and 10a,10b f20 were prepared according to literature
methods. GC analyses were performed on a Varian 3800 GC
fitted with a 25m CP Sil 5CB column and data were recorded on
a Star workstation.
Fig. 5 Plots of conversion against time for the Suzuki coupling of
4-bromoanisole with phenylboronic acid at 80 ЊC. Catalysts: ᭜ =
[Pd(TFA)(κ2-N,C-C6H4CH2NMe2){P(C6H4-4-CF3)3}], 10d;
᭿
=
[Pd(TFA)(κ2-N,C-C6H4CH2NMe2)(PPh3)], 10b; ꢀ = [Pd(TFA)(κ2-N,C-
C6H4CH2NMe2){P(C6H4-4-OMe)3}], 10e. Conditions: 4-bromoanisole
(10 mmol), phenylboronic acid (15 mmol), K2CO3 (20 mmol), toluene
(30 mL), hexadecane (internal standard, 0.20 mL), 80 ЊC.
Syntheses
Synthesis of [{Pd(ꢀ-TFA)(ꢁ2-N,C-C H CH᎐NPh)} ], 6c. A
᎐
6
4
2
solution of Pd(TFA)2 (1.346 g, 4.05 mmol) and N-benzylidene
aniline ( 0.961 g, 5.30 mmol) in THF (80 mL) was warmed at
60 ЊC for 4 h. The resultant solution was filtered through celite,
the solvent removed on a rotary evaporator and the crude
product recrystallised from CH2Cl2/EtOH. Yellow solid, 0.92 g,
57%.
General method for the synthesis of triarylphosphine/arsine
adducts. A solution of the appropriate dimeric complex
(0.13–0.66 mmol) and triarylphosphine/arsine (2.2 equivalents)
in dichloromethane (20–30 mL) was stirred at room temper-
ature for 20 min. The solution was filtered through celite,
ethanol (30 mL) was added and the solution concentrated on
a rotary evaporator to induce precipitation. The product was
collected by filtration and recrystallised from either CH2Cl2/
EtOH or CH2Cl2/MeOH.
Fig. 6 Effect of adding extra PR3 to the catalysts [Pd(TFA)(κ2-N,C-
C6H4CH2NMe2)(PR3)] in the coupling of 4-bromoanisole with
phenylboronic acid, conditions as in Fig. 5, 17 h. R = C6H4-4-CF3, dark;
C6H4-4-OMe, mid; Ph, light.
[Pd(TFA)(ꢁ2-N,C-C H CMe᎐NiPr)(PPh )], 9c. Yield 66%
᎐
6
4
3
(CH2Cl2/EtOH). Found: C, 58.05; H, 4.4; N, 2.0. Calc. for
C33H25F3NO5PPd: C, 58.00; H, 4.55; N, 2.18%. NMR (CDCl3):
δH (300 MHz) 7.78 (m, 6H, PPh3), 7.39 (m, 9H, PPh3), 7.23 (d,
br, 1H, metallated ring), 6.88 (m, 1H, metallated ring), 6.48 (m,
2H, metallated ring), 4.31 (m, 1H, CH(CH3)2), 2.42 (s, 3H,
Why do the phosphine adducts 9 and 10 show higher cata-
lytic activity than the parent dimeric complexes 5 and 6? We
have previously demonstrated that while complex 9b shows no
reaction with aryl bromides, stoichiometric reaction with excess
PhB(OH)2 and base leads to the loss of the orthometallated
imine ligand via reductive elimination with a phenyl ligand.12
We postulated that this process probably generates low
coordinate, monophosphine species ‘Pd(PR3)’. Such species
would presumably be partially stabilised by the reversible
coordination of weak ligands such as the eliminated imine or
solvent. A similar loss of a ‘coupled’ ligand is seen in the
reaction of complex 10a with PhB(OH)2 and base under
catalytic conditions.10 Low coordinate monophosphine com-
plexes have been demonstrated to be active in coupling
reactions,16 in particular Beller has recently shown that zero-
valent diene monophosphine complexes [Pd(diene)(PR3)] act as
high activity catalysts in the Suzuki coupling of aryl chlorides.17
Therefore it seems likely that the true role of the ortho-
metallated amine and imine ligands is a sacrificial one which
leads to the liberation of low coordinate, high-activity catalysts.
3
CH3), 1.40 (d, br, 6H, JHH = 6.8 Hz, CH(CH3)2) ppm. δP (121
MHz) 42.3 ppm.
[Pd(TFA)(ꢁ2-N,C-C H CH᎐NPh)(PPh )], 9d. Yield 90%
᎐
6
4
3
(CH2Cl2/EtOH). Found: C, 59.9; H, 3.6; N, 2.0. Calc. for
C33H25F3NO5PPd: C, 59.88; H, 3.81; N, 2.12%. NMR (CDCl3):
δH (300 MHz) 8.21 (d, br, 1H, JPH = 6.3 Hz, N᎐CH), 7.79 (m,
᎐
6H), 7.35 (m, 15H), 6.99 (m, 1H, metallated ring), 6.65 (ddd, br,
metallated ring), 6.53 (ddd, br, 1H, metallated ring) ppm. δP
(121 MHz) 42.2 ppm.
[Pd(TFA)(ꢁ2-N,C-C6H4CH2NMe2)(PPh3)], 10b. Yield 56%
(CH2Cl2/EtOH). Found: C, 56.5; H, 4.2; N, 2.1. Calc. for
C29H27F3NO2PPd: C, 56.55; H, 4.42; N, 2.27%. NMR (CDCl3):
δH (300 MHz) 7.73 (m, 6H, PPh3), 7.42 (m, 9H, PPh3), 7.01 (d,
3
3
br, 1H, metallated ring), 6.87 (ddd, 1H, JHH = 7.4 Hz, JHH
=
7.2 Hz, 4JHH = 0.9 Hz, metallated ring), 6.42 (m, 1H, metallated
ring), 6.35 (m, 1H, metallated ring), 4.06 (d, br, 2H, CH2), 2.76
(d, 6H, 4JPH = 2.6 Hz, Me) ppm. δP (121 MHz) 42.6 ppm.
Conclusions
In summary triarylphosphine adducts of amine- and imine-
based palladacycles show much greater activity than the parent
dimers in the Suzuki coupling of a deactivated aryl bromide.
While tricyclohexylphosphine-containing adducts perform
well in the Suzuki coupling of aryl chlorides,10 they show no
[Pd(TFA)(ꢁ2-N,C-C6H4CH2NMe2)(AsPh3)], 10c. Yield 50%
(CH2Cl2/MeOH). Found: C, 52.6; H, 4.05; N, 1.9. Calc. for
C29H27AsF3NO2Pd: C, 52.78; H, 4.12; N, 2.12%. NMR (CDCl3):
3
δH (300 MHz) 7.62 (d, 6H, JHH = 6.9 Hz, ortho H of AsPh3),
7.38 (m, 9H, AsPh3), 7.05 (dd, 1H, 3JHH = 7.6 Hz, 4JHH = 0.9 Hz,
D a l t o n T r a n s . , 2 0 0 3 , 3 3 5 0 – 3 3 5 6
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Table 4 Crystal data and refinement parameters for complexes 6c, 9d, 10d and 10f
6c
9d
10d
10f
Empirical formula
Formula weight
Crystal system
Space group
C30H20F6N2O4Pd2
799.28
C33H25F3NO2PPd
661.91
Monoclinic
P21/c
C32.50H24F12NO2.50PPd
833.90
C27.35H27Cl1.30NO0.25PPd
557.15
Triclinic
Triclinic
Triclinic
¯
¯
¯
P1
P1
P1
a/Å
b/Å
c/Å
α/Њ
9.6340(2)
11.3047(3)
14.3380(3)
68.0360(15)
85.2730(15)
88.4090(9)
1443.27(6)
2
9.911(2)
29.932(6)
9.907(2)
90
104.20(3)
90
2849.3(10)
4
1.543
10.08040(10)
11.74440(10)
14.4457(2)
74.0690(10)
83.9980(10)
83.5620(10)
1629.27(3)
2
9.4871(19)
13.833(3)
15.203(4)
64.56(2)
86.38(3)
73.77(3)
1726.2(7)
2
β/Њ
γ/Њ
Volume/Å3
Z
Density (calc)/Mg mϪ3
Absorption coefficient/mmϪ1
Reflections
1.839
1.324
17171
1.700
0.721
33547
1.072
0.696
8990
0.759
14475
Independent reflections
R(int)
5041
0.0904
0.0404,
0.1069
5317
7436
0.1007
0.0366,
0.0955
5746
0.0868
0.0429,
0.1154
0.0753
0.1043,
0.2581
Final R1, wR2 [F 2 > 2σ(F 2)]
3
3
metallated ring), 6.87 (ddd, 1H, JHH = 7.4 Hz, JHH = 8.4 Hz,
4JHH = 1.9 Hz, metallated ring), 6.44 (m, 2H, metallated ring),
4.08 (s, 2H, CH2), 2.80 (s, 6H, CH3) ppm.
atmosphere of nitrogen were placed 4-bromoanisole (1.87 g,
10.0 mmol), phenylboronic acid (1.83 g, 15.0 mmol), K2CO3
(2.76 g, 20.0 mmol), hexadecane (0.20 mL, 0.68 mmol, internal
standard) and toluene (30 mL). The mixture was heated to
80 ЊC and then the appropriate catalyst was added as a solution
in toluene (1.00 mL), prepared by volumetric dilution. The
temperature was maintained at 80 ЊC for 24 h and aliquots
(0.2 mL) 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
over MgSO4 and then the conversion to coupled product was
determined by GC.
[Pd(TFA)(ꢁ2-N,C-C6H4CH2NMe2){P(C6H4-4-CF3)3}], 10d.
Yield 60% (CH2Cl2/MeOH). Found: C, 46.0; H, 2.9; N, 1.5.
Calc. for C31H24F12NO2PPd: C, 46.88; H, 2.95; N, 1.71%. NMR
(CDCl3): δH (300 MHz) 7.84 (dd, 6H, 3JHH = 8.2 Hz, 3JPH = 11.4
Hz, ortho H of PR3), 7.67 (dd, 6H, 3JHH = 8.2 Hz, 4JPH = 1.5 Hz,
3
4
meta H of PR3), 7.05 (dd, 1H, JHH = 7.4 Hz, JHH = 1.2 Hz,
3
3
metallated ring), 6.92 (dd, 1H, JHH = 7.4 Hz, JHH = 8.6 Hz,
3
3
metallated ring), 6.46 (ddd, 1H, JHH = 7.9 Hz, JHH = 8.5 Hz,
4JHH = 1.5 Hz, metallated ring), 6.18 (ddd, 1H, JHH = 7.3 Hz,
3
4JHH = 0.6 Hz, J = 6.1 Hz, metallated ring), 4.09 (d, 2H, 4JPH
=
X-Ray structure determinations
4
2.1 Hz, CH2), 2.75 (d, 6H, JPH = 2.5 Hz, CH3) ppm. δP (121
MHz) 42.4 ppm. δF (282 MHz) Ϫ75.7 (s, 3F, TFA), Ϫ63.8 (s, 9F,
ArCF3) ppm.
Data were collected by means of combined phi and omega
scans on a Bruker-Nonius Kappa CCD area detector situated
at the window of a rotating anode (λMo Kα = 0.71073 Å). The
structures were solved by direct methods, SHELXS-97 and
refined using SHELXL-97.21 Hydrogen atoms were included in
the refinement, but thermal parameters and geometry were
constrained to ride on the atom to which they are bonded. The
data were corrected for absorption effects using SORTAV.22
Crystal data for the structures are given in Table 4.
[Pd(TFA)(ꢁ2-N,C-C6H4CH2NMe2){P(C6H4-4-OMe)3}], 10e.
Yield, 86% (CH2Cl2/MeOH). Found: C, 54.8; H, 4.6; N, 1.65.
Calc. for C32H33F3NO5PPd: C, 54.44; H, 4.71; N, 1.98%. NMR
(CDCl3): δH (300 MHz) 7.61 (dd, 6H, 3JPH = 11.2 Hz, 3JHH = 8.9
Hz, ortho H of PR3), 6.99 (dd, 1H, 3JHH = 7.5 Hz, 4JHH = 1.1 Hz,
3
4
metallated ring), 6.87 (dd, 7H, JHH = 8.9 Hz, JPH = 1.7 Hz,
meta H of PR3 and 1 H of metallated ring (obscured)), 6.45
(ddd, br, 1H, 3JHH = 7.5 Hz, metallated ring), 6.37 (ddd, br, 1H,
CCDC reference numbers 208239–208242.
See http://www.rsc.org/suppdata/dt/b3/b304053d/ for crystal-
lographic data in CIF or other electronic format.
3JHH = 6.97 Hz, JHH = 1.1 Hz, metallated ring), 4.00 (d, 2H,
4
2JHH = 1.5 Hz, CH2), 3.80 (s, 9H, OCH3), 2.72 (d, 6H, 4JPH = 2.4
Hz, CH3) ppm. δP (121 MHz) 40.1 ppm.
Acknowledgements
Catalysis
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.
General method for the Suzuki coupling of 4-bromoanisole
with phenylboronic acid (Table 3). To a mixture of 4-bromo-
anisole (0.358 g, 2.0 mmol), PhB(OH)2 (0.366 g, 3.0 mmol) and
K2CO3 (0.524 g, 4.0 mmol) in toluene (19 mL) was added the
catalyst as a toluene solution (1.00 mL) made up to the correct
concentration by multiple volumetric dilutions of a stock
solution. The resultant mixture was then heated at 110 ЊC for
17 h, cooled and quenched with HCl(aq) (2 M, 40 mL). The
organic layer was removed and the aqueous layer was extracted
with toluene (3 × 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
toluene (6 mL), hexadecane (0.068 M in CH2Cl2, 1.00 mL,
internal standard) was added and the conversion to product
determined by GC.
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