Catalytic Behaviour of Calixarenylphosphanes in Nickel-Catalysed Suzuki–Miyaura Cross-Coupling

Article abstract of DOI:10.1002/ejic.201601351

Two upper-rim-functionalised calix[4]arenes, 5-diphenylphosphino-25,26,27,28-tetra-benzyloxycalix[4]arene and 5-diphenylphosphino-25,26,27,28-tetra-(p-anisyl)methyloxy calix[4]arene, were investigated in the nickel-catalysed cross-coupling of phenylboronic acid with aryl bromides. The catalytic activities are higher than those observed for other triarylphosphanes, notably PPh₃and P(o-tolyl)₃as well as the Buchwald-type ligand o-biph-PPh₂[TOFs up to 2600 mol(ArPh) mol(Ni)^–1h^–1], but the production of large amounts of dehalogenation product could not be avoided, thus strongly contrasting with the classical Pd-catalysed reaction. Good activities were also obtained with aryl chlorides, even at room temperature. The higher efficiency of 1 and 2 relative to that of the Buchwald ligand 3 probably arises from the ease with which these two calix–phosphanes may form appropriate monophosphine complexes before the oxidative addition step.

Full text of DOI:10.1002/ejic.201601351

A Journal of
Accepted Article
Title: Catalytic behaviour of calixarenylphosphanes in nickel-catalysed
Suzuki-Miyaura cross-coupling
Authors: Laure MONNEREAU, David SEMERIL, Dominique MATT,
and Christophe GOURLAOUEN
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To be cited as: Eur. J. Inorg. Chem. 10.1002/ejic.201601351
Link to VoR: http://dx.doi.org/10.1002/ejic.201601351

10.1002/ejic.201601351
European Journal of Inorganic Chemistry
1
GRAPHICAL ABSTRACT
Catalytic behaviour of calixarenylphosphanes in nickel-catalysed Suzuki-
Miyaura cross-coupling
Laure Monnereau, David Sémeril, Dominique Matt, and Christophe Gourlaouen
Br
Ph2P
[Ni] / phosphane
NaH
O
O
O
O
+
dioxane, 100°C
1h
Ph2P
B(OH)2
MeO
OMe
MeO
OMe
TOF : 1110 h-1
TOF : 310 h-1
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10.1002/ejic.201601351
European Journal of Inorganic Chemistry
2
Catalytic behaviour of calixarenylphosphanes in nickel-catalysed
Suzuki-Miyaura cross-coupling
Laure Monnereau,[a] David Sémeril,*[a] Dominique Matt,*[a] and Christophe Gourlaouen[b]
Abstract: Two upper rim-functionalised calix[4]arenes, 5-diphenylphosphino-25,26,27,28-
tetra-benzyloxycalix[4]arene and 5-diphenylphosphino-25,26,27,28-tetra-(p-anisyl)methyloxy
calix[4]arene were investigated in the nickel-catalysed cross-coupling of phenylboronic acid
with aryl bromides. The catalytic activities were higher than those observed with other
triarylphosphanes, notably PPh3, P(o-tolyl)3 as well as the Buchwald-type ligand o-biph-PPh2
(TOFs up to 2600 mol(ArPh) mol(Ni)-1 h-1), but the production of large amounts of
dehalogenation product could not be avoided, thus strongly contrasting with the classical Pd-
catalysed reaction. Good activities were also obtained with aryl chlorides, even at room
temperature. The higher efficiency of 1 and 2 vs. that of the Buchwald ligand 3 probably
arises from the ease with which these two calix-phosphanes may form appropriate
monophosphine complexes before the oxidative addition step.
Keywords: calix[4]arene, bulky phosphanes, Suzuki-Miyaura cross-coupling, homogeneous
catalysis, nickel
___________________
[a] Laboratoire de Chimie Inorganique Moléculaire et Catalyse,
Institut de Chimie UMR 7177 CNRS
Université de Strasbourg
4 rue Blaise Pascal
67070 Strasbourg Cedex (France)
E-mail: dsemeril@unistra.fr; dmatt@unistra.fr
[b] Laboratoire de Chimie Quantique,
Institut de Chimie UMR 7177 CNRS
1 rue Blaise Pascal
Université de Strasbourg
67008 Strasbourg Cedex (France)
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10.1002/ejic.201601351
European Journal of Inorganic Chemistry
3
Introduction
The Suzuki-Miyaura reaction, that is the coupling of an aryl- or vinyl-boronic acid with an
aryl- or vinyl-halide catalysed by a transition metal, has become a major synthetic tool for the
construction of numerous sophisticated intermediates, including natural products, bioactive
molecules, polymers and building blocks for supramolecular chemistry.[1] Suzuki-Miyaura
cross-coupling is commonly achieved with palladium complexes, but inexpensive metals such
as nickel, which like palladium has a good functional group tolerance, can also be used.[2] The
search for improved, nickel-based Suzuki-Miyaura catalysts remains a topic worth exploring,
as this metal is still associated with important drawbacks, one of them being its poorer
selectivity in comparaison with palladium. Typical nickel catalysts for Suzuki-Miyaura cross-
coupling rest on a combination of metal salt and either a monophosphane or a diphosphane.
Phosphanes which have already been employed in Ni-catalysed cross-coupling include
PPh3,[3] [P(m-C6H4-SO3)3]Na3,[4] P(OMe)3,[5] PCy3,[3d, 6] Ph2PCH2CH2OH,[7] Ph2PCH2PPh2,[3c]
Ph2PCH2CH2PPh2,[3c, 8] Ph2P(CH2)4PPh2,[3c] 1,1’-bis-(diphenylphosphino)ferrocene,[2a, 5, 9] 1-
[α-(diphenylphosphino)benzyl]ferrocene,[10] and 1,1'-bis-[α-(diphenylphosphino)benzyl]
ferrocene.[11] Several research groups have further demonstrated the effectiveness in Suzuki-
Miyaura reactions of other nickel/ligand combinations, notably of complexes containing bis-
N-heterocyclic carbene ligands (NHCs) associated with free triphenylphosphanes,[12] as well
as complexes containing hybrid NHC-phosphane[13] or iminophosphorane-phosphane
ligands.[14]
We have recently investigated the properties of some conical calix[4]arenes bearing a
diarylphosphino group tethered at their upper rim. These have been tested in palladium-
catalysed Suzuki-Miyaura[15] and nickel-catalysed Kumada-Tamao-Corriu cross-couplings.[16]
They showed considerably higher activities than conventional triarylphosphanes. This effect
was attributed to the receptor properties of the calixarene moiety towards metal-(π-arene)
moieties, a feature which increases the ligand bulk when the metal binds an incoming ArX
fragment, and therefore favours the formation of key, monoligand intermediates.[17] It should
be mentioned that in addition to their high catalytic efficiency, these receptor phosphanes
exhibit high thermal and chemical stability, making them particularly attractive for utilisation
in homogeneous catalysis. As an extension of our studies on this phosphane class, we have
now assessed, for the first time, two such calixarene-phosphanes[15] (1 and 2, Scheme 1) in
nickel-catalysed Suzuki-Miyaura cross-coupling.
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10.1002/ejic.201601351
European Journal of Inorganic Chemistry
4
Ph2P
Ph2P
MeO
O
O
O
O
O
O
O
O
OMe
MeO
OMe
2
1
Scheme 1. Calixarenylphosphanes 1 and 2.
Results and Discussion
Phosphanes 1 and 2 were tested in the reaction between phenylboronic acid and both aryl
bromides and aryl chlorides (Scheme 2). The catalysts were generated in situ by mixing either
a nickel(0) complex or a nickel(II) salt with the phosphane.
[Ni], monophosphine, base
+
X
(HO)2B
dioxane
R
R
X = Br, Cl
Scheme 2. Suzuki-Miyaura cross-coupling reaction.
We began the study by searching for optimal catalytic conditions (Table 1). For this purpose,
we focussed on the reaction between 4-bromoanisole and phenylboronic acid. The runs were
carried out at 100°C in 1,4-dioxane with ligand 1 and by applying a ArBr/Ni ratio of 1000.
Among the nickel precursors (Table 1, entries 1-4), [Ni(cod)2] led to the highest conversion
(59.5 %; Table 1, entry 4). Under the above conditions, after 1h only 33.6 % of the aryl
bromide was converted into the expected cross-coupling product (Ph-C6H4-p-OMe), the
reaction producing also high amounts of anisole. As previously observed in palladium-
catalysed Suzuki-Miyaura reactions carried out with bulky phosphanes,[15] excess ligand was
required (Table 1, entries 4-8). The best conversions were obtained when 1.5-2 equivalents of
phosphane 1 per Ni were employed. Higher L:Ni ratios led to lower conversions, and
therefore a L:Ni ratio of 2:1 was used in all the following runs. In a third series of tests, we
determined the optimal base (Table 1, entries 4 and 9-12). The highest conversion observed
for 4-bromoanisole (79.3 %) was obtained with tBuOK, however, with this particular base the
reaction resulted mainly in substrate dehalogenation (Table 1, entry 9). NaH turned out to be
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10.1002/ejic.201601351
European Journal of Inorganic Chemistry
5
the most efficient base for forming the cross-coupling product, which was formed in 33.6 %
beside 25.9% of anisole (Table 1, entry 4). Note that the formation of undesirable
dehalogenation products in the presence of nickel catalysts is well documented.[7, 18] Their
formation occurs in competition with the transmetallation step, which has been proposed as
the rate limiting step of the catalytic cycle (vide infra).[19] We further found that the
homocoupling product, namely bis-phenyl, also formed. However, the Ph-Ph:Ar-Ph ratio did
never exceed 20 %.
Table 1. Suzuki-Miyaura cross-coupling of 4-bromoanisole with phenyl boronic acid - search
for optimal conditions.[a]
Conversio
n
Products
Entry Nickel precursor
1/Ni
Base
Ar-Ph (%)
Ar-H (%)
(%)
1
2
[NiCl2Py4]
[Ni(OAc)2]
[NiBr2(dme)]
[Ni(cod)2]
[Ni(cod)2]
[Ni(cod)2]
[Ni(cod)2]
[Ni(cod)2]
[Ni(cod)2]
[Ni(cod)2]
[Ni(cod)2]
[Ni(cod)2]
2/1
2/1
NaH
NaH
NaH
NaH
NaH
NaH
NaH
NaH
tBuOK
28.0
3.6
12.3
2.0
15.7
1.6
3
2/1
19.0
59.5
41.6
59.8
51.8
32.4
79.3
traces
4.6
8.3
10.7
25.9
19.3
26.0
26.5
18.2
61.7
traces
traces
traces
4
2/1
33.6
22.3
33.8
25.3
14.2
17.6
traces
4.6
5
1/1
6
1.5/1
2.5/1
5/1
7
8
9
2/1
[b]
10
11
12
2/1
Cs2CO3
K3PO4
KF
2/1
2/1
2.1
2.1
[a]
Nickel precursor (5 x 10-4 mmol, 0.1 mol %, ArBr/Ni ratio = 103), phosphane 1, ArBr (0.5
mmol), PhB(OH)2 (0.122 g, 1 mmol), dioxane (1.50 mL), base (1.0 mmol), decane (0.05
mL), 100°C, 1 h. The conversions were determined by GC, the calibrations being based on
decane.
[b]
With K2CO3 and Na2CO3 no reaction was observed
Applying the above optimal catalytic conditions and using 0.1 mol % of nickel, the coupling
reaction was carried out with various aryl bromides. For each substrate, except for the
sterically hindered 2-bromoanisole and 2-bromotoluene, moderate to good conversions were
observed after 1 h when operating in dioxane at 100°C (Table 2). In these reactions, 20-43 %
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10.1002/ejic.201601351
European Journal of Inorganic Chemistry
6
of the aryl bromide were transformed into dehalogenated by-product. Aryl bromides bearing
methoxy substituents were more efficiently converted with phosphane 1 than 2 (Table 2,
entries 1-6); conversely, the conversions of those bearing a methyl substituent (o-, m-, or p-
were higher with 2 than 1 (Table 2, entries 11-16). The highest conversion obtained, 94.4 %
(with 2), was obtained with 4-bromotoluene, the yield of coupling product reaching then 53.3
% (Table 2, entry 16). With 2-bromoanisole and 2-bromotoluene, conversions of 88.2 and
77.3%, respectively, could be obtained in 3 h (Table 2, entries 3 and 12). Repeating these runs
over a period of 5 h led, for each substrate, to full substrate conversion without changing
significantly the product distribution.
Table 2. Suzuki-Miyaura cross-coupling of aryl bromides using a ArBr/Ni ratio of 1000.[a]
Products
Ar-Ph (%)
Conversion
(%)
Entry
ArBr
Phosphane
Ar-H (%)
25.9
1
2
3
4
5
6
1 (1 h)
2 (1 h)
1 (3 h)
2 (3 h)
1 (1 h)
2 (1 h)
59.5
33.6
26.8
44.7
37.8
35.7
31.6
MeO
Br
49.3
22.5
OMe
88.2
43.5
Br
72.3
62.0
51.6
34.5
Br
26.3
MeO
20.0
Br
7
8
1 (1 h)
2 (1 h)
75.3
56.2
38.2
28.4
37.1
27.8
9
1 (1 h)
2 (1 h)
1 (3 h)
2 (3 h)
1 (1 h)
2 (1 h)
1 (1 h)
2 (1 h)
79.5
87.9
68.3
77.3
80.1
86.9
88.8
94.4
43.4
50.8
35.0
43.1
42.5
48.3
49.3
53.3
36.1
37.1
33.3
34.2
37.6
38.6
39.5
41.1
Br
10
11
12
13
14
15
Br
Br
Br
16
[a]
[Ni(cod)2] (5 x 10-4 mmol, 0.1 mol %, ArBr/Ni ratio = 103), phosphane (1 x 10-3 mmol, 2
equiv. / Ni), ArBr (0.5 mmol), PhB(OH)2 (0.122 g, 1 mmol), dioxane (1.50 mL), NaH (60
% dispersion in mineral oil) (0.040 g, 1.0 mmol), decane (0.05 mL), 100°C. The
conversions were determined by GC, the calibrations being based on decane.
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10.1002/ejic.201601351
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In order to allow comparison with other triarylphosphanes, the runs carried out for 4-
bromoanisole (under the conditions of Table 2) were repeated with PPh3, P(o-tolyl)3 and o-
biph-PPh2 (3). The two conventional phosphanes PPh3 and P(o-tolyl)3 led to ca. 50 % lower
conversions than calixarenylphosphane 1. With these ligands the chemo-selectivity was
totally reversed with respect to that induced by 1, as the dehalogenated product was here the
major product (Table 3, entries 2 and 3). The Buchwald-type ligand 3 led to performances
similar to those obtained with calixarene 1 (Table 3 entry 4). It should be recalled that
Buchwald's o-biph-P(alkyl)2 phosphanes, are currently among the most efficient ligands for
palladium-catalysed Suzuki-Miyaura cross-coupling.[20]
PPh2
o-biph-PPh2 (3)
Table 3. Suzuki-Miyaura cross-coupling of 4-bromoanisole with phenyl boronic acid –
comparison with other triarylphosphanes.[a]
Products
Conversion
(%)
Entry
Triarylphosphane
Ar-Ph (%)
33.6
Ar-H (%)
25.9
1
2
3
1
59.5
PPh3
23.9
7.1
16.8
P(o-tolyl)3
o-biph-PPh2 (3)
29.9
11.1
18.8
4
51.8
30.4
21.4
[a]
[Ni(cod)2] (5 x 10-4 mmol, 0.1 mol %, ArBr/Ni ratio = 103), phosphane (1 x 10-3 mmol, 2
equiv. / Ni), 4-bromoanisole (0.5 mmol), PhB(OH)2 (0.122 g, 1 mmol), dioxane (1.50 mL),
NaH (60 % dispersion in mineral oil) (0.040 g, 1.0 mmol), decane (0.05 mL), 100°C, 1 h.
The conversions were determined by GC, the calibrations being based on decane.
When raising the ArBr/Ni ratio to 10 000 (Scheme 3 and Table S-1), the catalytic activities
remained high for the whole set of aryl bromides tested. The highest activity was observed in
the catalytic arylation of bromobenzene using ligand 2 (TOF = 2600 mol(ArPh) mol(Ni)-1 h-
1). Interestingly, for all the eight substrates tested, the calixarene-phosphanes showed
performances that were similar or superior to those obtained with o-biph-PPh2 (3). For
example, in the transformation of the sterically hindered 2-bromotoluene, ligand 2 led to a
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10.1002/ejic.201601351
European Journal of Inorganic Chemistry
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TOF of 1110 mol(ArPh) mol(Ni)-1 h-1, vs. 310 mol(ArPh) mol(Ni)-1 h-1 in the presence of 3
(see Table S-1).
Br
OMe
Br
Br
MeO
Br
MeO
1080 (1)
350 (1)
1260 (1)
2030 (1)
Br
Br
Br
Br
1230 (2)
1820 (2)
2600 (2)
1110 (2)
Scheme 3: Highest TOF values observed in the cross-coupling of a series of aryl bromides
with PhB(OH)2 (TOF in mol(ArPh formed).mol(Ni)-1.h-1). Conditions: [Ni(cod)2] (5 x 10-5
mmol, 0.01 mol %), phosphane (2 equiv. / Ni), ArBr (0.5 mmol), PhB(OH)2 (1 mmol),
dioxane (1.50 mL), decane (0.05 mL), 100°C, 1 h. The conversions were determined by GC,
the calibrations being based on decane. The number in parentheses beside the TOF value
designates the corresponding ligand.
Aryl chlorides, which are more challenging reagents, could also efficiently be converted into
biaryls, these reactions being achieved with 1 mol % of nickel at 100°C (Table 4). The
highest conversions were observed with PhCl as well as aryl chlorides substituted by electron-
withdrawing groups. For example, 4-chlorobenzaldehyde led to the formation of 96.8 % of
the expected biaryl in 5 h using ligand 1 (Table 4, entry 7). Interestingly, with activated aryl
chlorides only small amounts (0-14 %) of dehalogenated product were formed (Table 4,
entries 1-10). In contrast, the deactivated substrates 2-chlorotoluene, 4-chlorotoluene and 4-
chloroanisole not only reacted slower, but the proportions of dehalogenated product were
higher (11-29 %) (Table 4, entries 11-16). Using 4-chloroanisole as substrate, the calixarene-
phosphanes were then compared with the three triarylphosphanes PPh3, P(o-tolyl)3 and o-
biph-PPh2 (3), which all resulted in significantly lower conversions and selectivities (Table 4,
entries 15-19). As a general observation, we noted that the proportion of dehalogenated
product decreased when the reaction rate increased. Thus, for example, the proportion of
anisole was considerably lower with 1 (anisole:Ar-Ph = 1:6.9) than with PPh3 (anisole:Ar-Ph
= 1:0.7) (Table 4, entries 15 and 17).
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10.1002/ejic.201601351
European Journal of Inorganic Chemistry
9
Table 4. Suzuki-Miyaura cross-coupling of aryl chlorides using a ArBr/Ni ratio of 100.[a]
Products
Conversion
(%)
Entry
ArBr
Phosphane
Ar-Ph (%)
Ar-H (%)
8.5
1
2
3
4
5
6
1 (5 h)
2 (5 h)
1 (5 h)
2 (5 h)
1 (5 h)
2 (5 h)
95.0
86.5
73.1
80.6
71.3
94.8
96.4
Cl
86.2
13.3
7.1
87.7
O2N
Cl
81.0
9.7
94.8
traces
2.1
NC
Cl
98.5
O
H
7
1 (5 h)
96.8
96.8
traces
Cl
8
2 (5 h)
1 (5 h)
99.5
86.8
99.7
92.3
99.1
55.5
56.7
93.0
93.9
23.7
47.4
57.8
93.7
74.7
87.3
81.1
82.0
40.6
39.7
81.2
64.9
14.0
32.2
44.2
5.8
O
9
12.1
12.4
11.2
17.1
14.9
17.0
11.8
29.0
9.7
Cl
Me
10
11
12
13
14
15
16
17
18
2 (5 h)
1 (16 h)
Cl
2 (16 h)
1 (16 h)
Cl
2 (16 h)
1 (16 h)
2 (16 h)
Cl
MeO
PPh3 (16 h)
P(o-tolyl)3 (16 h)
o-biph-PPh2 (3) (16 h)
15.2
13.6
19
[a]
[Ni(cod)2] (2.5 x 10-3 mmol, 1 mol %, ArBr/Ni ratio = 100), phosphane (5 x 10-3 mmol, 2
equiv. / Ni), ArCl (0.25 mmol), PhB(OH)2 (0.061 g, 0.5 mmol), dioxane (0.75 mL), NaH
(60 % dispersion in mineral oil) (0.020 g, 0.5 mmol), decane (0.025 mL), 100°C. The
conversions were determined by GC, the calibrations being based on decane.
Carrying out the cross-coupling of 4-chloroanisole with NaH in dioxane at 25°C instead of
100°C led mainly to the dehalogenated product (Table 5, entry 1). Using K3PO4 instead of
NaH increased both the conversion (87 % after 24 h, at 25°C) and the production of biaryl (up
to 31.4 %) (Table 5, entry 3). Repeating this run in THF did not improve the chemoselectivity
(Table 5, entry 4).
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10.1002/ejic.201601351
European Journal of Inorganic Chemistry
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Table 5. Suzuki-Miyaura cross-coupling of 4-chloroanisole with phenyl boronic acid at 25°C
- search for optimal conditions.[a]
Products
Ar-Ph (%)
Conversion
(%)
Entry
Solvent
Base
Ar-H (%)
48.6
1
2
3
dioxane
dioxane
dioxane
THF
NaH
64.2
15.6
3.4
tBuOK
K3PO4
K3PO4
46.0
42.6
87.0
31.4
55.6
4
54.4
23.6
30.8
[a]
[Ni(cod)2] (7.5 x 10-3 mmol, 3 mol %), phosphane 1 (15 x 10-3 mmol, 2 equiv. / Ni), 4-
chloroanisole (0.25 mmol), PhB(OH)2 (0.061 g, 0.5 mmol), solvent (0.75 mL), base (0.5
mmol), decane (0.025 mL), 25°C, 24 h. The conversions were determined by GC, the
calibrations being based on decane.
Applying the optimal catalytic conditions inferred from the runs presented in Table 5, the aryl
chlorides were converted in moderate-to-good yields after 24 h reaction time at 25°C (Table
6). The best conversion was obtained for the activated substrate 4-chlorobenzaldehyde (91.6
%, Table 6, entry 4). Except for 4-chlorocyanobenzene and 4-chlorobenzaldehyde, which led
to the corresponding cross-coupling products in 57.4 and 61.8 % yield, respectively (Table 6,
entries 4 and 5), the other aryl chlorides gave dehalogenation compounds as the major
products. Again, calixarene-phosphane 1 turned to be a very performing ligand. Thus, the
conversions with this phosphane were six, three and two times higher than those obtained
with PPh3, P(o-tolyl)3 and o-biph-PPh2 (3), respectively, in the reaction with 4-chloro-anisole
(Table 6, entries 7-9). Phosphane 1 was also the ligand with the better selectivities.
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10.1002/ejic.201601351
European Journal of Inorganic Chemistry
11
Table 6. Suzuki-Miyaura cross-coupling of aryl chlorides carried out at 25°C.[a]
Products
Conversion
(%)
Entry
ArBr
Phosphane
Ar-Ph (%)
25.9
Ar-H (%)
53.9
Cl
1
2
3
1
1
2
79.8
68.8
83.3
O2N
Cl
24.6
57.4
42.2
25.9
NC
Cl
O
Cl
4
5
1
2
91.6
82.8
61.8
25.9
29.8
56.9
H
O
Cl
Me
6
7
8
9
1
87.0
13.4
30.5
39.3
31.4
traces
4.3
55.6
13.4
26.2
30.3
PPh3
Cl
MeO
P(o-tolyl)3
o-biph-PPh2 (3)
9.0
Cl
10
1
1
78.1
36.6
25.8
11.5
52.3
25.1
11
Cl
[a]
[Ni(cod)2] (7.5 x 10-3 mmol, 3 mol %), phosphane (15 x 10-3 mmol, 2 equiv. / Ni), ArCl
(0.25 mmol), PhB(OH)2 (0.061 g, 0.5 mmol), THF (0.75 mL), K3PO4 (0.108 g, 0.5 mmol),
decane (0.025 mL), 25°C, 24 h. The conversions were determined by GC, the calibrations
being based on decane.
Why ligands 1 and 2 resulted in higher activities than phosphane 3, as well as PPh3
and P(o-tolyl)3), is a matter of debate. In previous studies on Pd-catalysed Suzuki-Miyaura
couplings which led to similar observations with these ligands, we tentatively attributed the
higher activities of calixarenylphosphanes to their capacity to behave as both first and second
coordination sphere ligands towards [M(π-arene)] moieties.[21] In the corresponding
intermediates, in which the metal-arene fragment is entrapped in the calixarene, the ligand
crowding is significantly increased with respect to that of related complexes having the metal
exo-bonded. As a result, the proportion of mono-ligated intermediates of the type
[(L)Pd0(X)(Ar)] increases with respect to related bis-phosphane complexes.[15-16]
Monophosphane Pd(0) complexes are known to undergo oxidative addition (the rate
determining step with Pd) faster than the corresponding bis-phosphane complexes. A similar
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10.1002/ejic.201601351
European Journal of Inorganic Chemistry
12
assistance by the cavity may in principle also be operative in the case of nickel, but it is well
accepted that oxidative addition is by far not the rate determining step with this metal.[22] How
could then the presence of the calix-phosphane influence the reaction ? In fact, preliminary
calculations revealed that oxidative addition, transmetallation and reductive elimination are
all easy steps (energy barriers of ca. 9 kcal.mol-1) irrespective of the phosphane used, 1, 2 or
Buchwald phosphane 3. Interestingly, the calculations showed that the step preceding the
oxidative addition, namely the reaction between [Ni(phosphane)2], PhBr, and 2 equiv. of
PhB(OH)ONa resulted in a considerably more stable monophosphane [Ni0(η2-Ar-
X){PhB(OH)(O)Na}2(phosphane)] complex[23] in the case of calix-phosphanes 1 and 2 (ΔG =
- 22.0 kcal.mol-1) than with the biarylphosphane 3 (ΔG = + 3.4 kcal.mol-1) (see SI). Thus, the
higher efficiency of 1 (and 2) vs. that of 3 could merely arise from the relative higher
thermodynamic stability of the initial reactive complex, namely the one formed with the calix-
phosphanes before the oxidative addition step.
Conclusion
In summary, the combination of [Ni(cod)2] with calixarenylphosphanes 1 and 2 efficiently
catalyses the cross-coupling of phenylboronic acid with aryl bromides and aryl chlorides. The
observed activities were significantly higher than those obtained with conventional
triarylphosphanes such as PPh3, P(o-tolyl)3, or that of the bifunctional Buchwald-type
phosphane o-biph-PPh2. In addition, the calixarene-phosphanes resulted in lower amounts of
dehalogenation products when starting from aryl chlorides instead of aryl bromides. The
origin of the better performance of 1 vs. 3 remains a matter of debate. Preliminary
calculations suggest that the higher efficiency of 1 over that of 3 could arise from the easier
formation of the possible monophosphane precursor complex [Ni0(η2-Ar-
Br){PhB(OH)(O)}2(1)]Na2 compared with that of its 3-containing analogue.
Experimental section
General methods. All reations were performed in Schlenk-type flasks under dry nitrogen.
Solvents were dried by conventional methods and were distilled immediately prior to use. The
catalytic solutions were analysed by using a Varian 3900 gas chromatograph equipped with a
WCOT fused-silica column (25 m x 0.25 mm). 5-Diphenylphosphino-25,26,27,28-
tetraalkyloxy-calix[4]arenes 1[15] and 2[15] and [Ni(cod)2][24] were prepared according to
literature procedures.
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10.1002/ejic.201601351
European Journal of Inorganic Chemistry
13
General procedure for nickel-catalysed Suzuki-Miyaura cross-coupling reactions: a
solution of [Ni(cod)2] in dioxane, a solution of the ligand (2 equiv/Ni) in dioxane, arylhalide
(0.5 mmol), phenylboronic acid (0.122 g, 1.0 mmol), base (1.0 mmol), decane (0.05 mL,
internal reference) and an additional amount of dioxane (so that the total reaction volume was
1.5 mL) were introduced in a Schlenk tube. The reaction mixture was heated for 1 h at 100°C.
After cooling to room temperature, a small amount (0.5 mL) of the resulting solution was
passed through a Millipore filter before being analyzed by GC. NMR spectral data of the
resulting coupling products, namely 4-methoxybiphenyl,[25] biphenyl,[25] 4-methylbiphenyl,[25]
4-cyanobiphenyl,[25] 4-nitrobiphenyl,[25] 4-acetyl-biphenyl,[25] 2-methoxybiphenyl,[26] 1-
phenylnaphthalene,[26] 2-methylbiphenyl,[26] 3-methylbiphenyl,[26] biphenyl-4-carbaldehyde[27]
and 2-methoxy-6-phenylnaphthalene[28] were in agreement with those reported in the
literature.
Acknowledgements
The University of Strasbourg is gratefully acknowledged for a grant to L. M.
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