Synthesis of new styrylarenes via Suzuki-Miyaura coupling catalysed by highly active, well-defined palladium catalysts

Article abstract of DOI:10.1039/c3dt52063c

An efficient synthetic route for well-defined palladium(0) complexes [Pd(η²-dba)(PPh₃)₂] (2), [Pd(η²-dba)(PCy₃)₂] (3) and their crystallographic structures is reported. This is the first crystallographic characterization of palladium complexes coordinated with one dibenzylideneacetone and two phosphines. A highly effective, fully controlled method for selective synthesis of mono- (5-9) and distyrylarenes (10-15) via Suzuki-Miyaura coupling is described. ? The Royal Society of Chemistry 2013.

Full text of DOI:10.1039/c3dt52063c

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Synthesis of new styrylarenes via Suzuki–Miyaura
coupling catalysed by highly active, well-defined
palladium catalysts†
Cite this: Dalton Trans., 2013, 42, 15535
Received 30th July 2013,
Accepted 9th August 2013
DOI: 10.1039/c3dt52063c
Mariusz Majchrzak,* Sylwia Kostera, Maciej Kubicki and Ireneusz Kownacki
www.rsc.org/dalton
An efficient synthetic route for well-defined palladium(0) com-
plexes [Pd(η2-dba)(PPh3)2] (2), [Pd(η2-dba)(PCy3)2] (3) and their
crystallographic structures is reported. This is the first crystallo-
graphic characterization of palladium complexes coordinated
with one dibenzylideneacetone and two phosphines. A highly
effective, fully controlled method for selective synthesis of
mono- (5–9) and distyrylarenes (10–15) via Suzuki–Miyaura coup-
ling is described.
Scheme 1 General scheme of Suzuki–Miyaura coupling.
The Suzuki–Miyaura cross-coupling reaction appears to be an
extremely useful method in the organic synthesis of com-
pounds containing a carbon–carbon bond, e.g. biaryl com-
pounds.1 The formation of a C–C bond is of vital importance
in preparative organic processes from molecular, model
chemistry to macromolecular and nanomaterial science.
In general, in this kind of reaction, organoboronic acid is
coupled with an organic halide or triflate in the presence of a
catalyst and a mixture of less polar or polar solvents and base,2
as shown in Scheme 1.
The reactions of aryl halides or halide equivalents with
various nucleophiles have been mainly catalysed by a palla-
dium system. Phosphine and carbenes are well known ligands
for this process.3–8 The character of the active catalytic species
when using palladium–phosphine systems is essential.
Two equivalents of phosphine are usually present in the
catalyst precursor or just added into the mixture. It appears
that they stay on the metal during the reaction. It has been
suggested that a monophosphine species could be used as a
real catalyst.
Moreover, bulky and electron-rich phosphines have a ten-
dency to improve the reaction (through chemo- and regioselec-
tivity and yielding).9–18
This method turned out to be an effective and really practi-
cal way to produce non-natural and natural products, e.g. poly-
mers, dendrimers, peptides or amino acids.19–23
In this communication we present an efficient, stereo-
selective method for the preparation of π-conjugated mono-
and distyryl-arylene derivatives through the Suzuki–Miyaura
coupling (SM) catalyzed by well-defined, highly active, three
coordinated palladium(0) complexes.
Commercially available Pd(PPh3)4, Pd(OAc)2, Pd(dba)2 and
Pd2(dba)3·CHCl3 are generally used as precursors of Pd(0) cata-
lysts with or without the addition of phosphine ligands.
However, it should be mentioned that the catalytic activities
of Pd(0) catalysts generated ‘in situ’ from these palladium com-
pounds are not always the same, and it is advisable to test all
of them in order to achieve efficient catalytic reactions.
These well-defined catalysts are readily obtainable from bis-
(dibenzylideneacetone) palladium(0) (1) on treatment with tri-
cyclohexylphosphine or triphenylphosphine (the best
molar ratio is [Pd] : [PR3] = 1 : 2.02) which results in the for-
mation of mononuclear bis(phosphine)(dibenzylideneacetone)
palladium(0) – [Pd(η2-dba)(PR3)2] active species with very good
yields, e.g. for R = Ph3 – 98% (2) and PCy3 – 95% (3), as shown
in Scheme 2.
Faculty of Chemistry, Adam Mickiewicz University, Umultowska89b, 61-614 Poznań,
Poland. E-mail: mariusz.majchrzak@amu.edu.pl; Fax: +48 61 8291509;
Tel: +48 61 82191407
†Electronic supplementary information (ESI) available: Experimental and
analytical details, synthesis of 2 and 3, procedure for catalytic tests, synthesis of
The phosphine was added to a dark purple Pd(dba)2 solid
and both substrates were dissolved in toluene to give a yellow-
orange solution of the final species. The solid state structures
of 2 and 3 were determined by X-ray crystallography. As far as
mono- (5–9) and distyryl arenes (10–15), crystal data collection and structure
refinement. CCDC 844578 and 844579. For ESI and crystallographic data in CIF
or other electronic format see DOI: 10.1039/c3dt52063c
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Scheme 2 Synthesis of well-defined palladium(0) species.
Fig. 2 A perspective view of complex 3 with labelling scheme. Ellipsoids are
drawn at the 33% probability level, hydrogen atoms are omitted for clarity.
Selected geometrical parameters (Å, °) Pd1–X1 2.052(5), Pd1–C2 2.172(5),
Pd1–C3 2.154(5), Pd–P1 2.3832(17), Pd–P2 2.3641(14), X1–Pd1–P1 124.60(17),
X1–Pd1–P2 119.68(18), P1–Pd1–P2 115.61(6).
Fig. 1 A perspective view of 2 with labelling scheme. Ellipsoids are drawn at
the 50% probability level, hydrogen atoms are shown as spheres of arbitrary
radii. Selected geometrical parameters (Å, °): Pd1–X1 2.047(5), Pd1–C2 2.190(5),
Pd1–C3 2.160(6), Pd–P1 2.3427(14), Pd–P2 2.3313(14), X1–Pd1–P1 127.00(12),
X1–Pd1–P2 118.04(12), P1–Pd1–P2 114.88(5).
Scheme 3 Pd(0) catalyzed Suzuki–Miyaura coupling of aryl bromines with
boronic acids.
we know this is the first crystallographic characterization of
palladium complexes coordinated with one dibenzylideneace-
tone and two phosphines. Fig. 1 and 2 show configurations of
two three co-ordinated Pd(0) complexes. Deep yellow (2) and structure of 3 there are voids which are filled by the disordered
orange (3) crystals were obtained from a toluene–hexane solvent (toluene) molecules.
mixture which was slowly concentrated at room temperature.
The relevant geometric parameters for both complexes are natively unsaturated complexes. Our palladium precursors
presented in Table 3, ESI.† [Pd(dba)(PR3)2] are 16 electron species and they have one free
In both complexes the Pd atoms have a trigonal coordi- coordination place.
The well-known oxidative addition step occurs with coordi-
nation, taking the middle point of the double C2vC3 bond
In order to optimize the reaction conditions with respect
(X1) as the coordination centre. The Pd atoms are only slightly to activity, selectivity, catalyst loading, time and temperature
out of the plane created by the three coordination centres, P1, the model reaction of bromobenzene (or 4-bromostyrene
P2 and X1, the deviations are 0.041(2) Å in 3 and 0.038(2) Å 0.5 mmol, both of them after redistillation) and 4-vinylphenyl-
in 2.
boronic acid (or phenylboronic acid 0.5 mmol after recrystalli-
In both complexes only one of the double bonds is involved zation from water) with a molar ratio of 1 : 1, was used
in the coordination. This difference is reflected in the double according to Scheme 3.
bond lengths: those involved in coordination (C2vC3 of
The study of two different substrate combinations allowed
1.425(7) Å in 3 and 1.395(7) Å in 2) are significantly longer us to confirm that the first system was much more effective
than those which are not taking part in such an interaction and efficient than the other. The reason for that is the influ-
(C10vC11 of 1.313(8) Å in 3, 1.292(7) Å in 2).
ence of the different substituent in the ‘para’ position on the
The conformations of the organic ligands, defined by the distribution of electron density in the ring and thus on the
dihedral angles between the terminal phenyl rings, are also reactivity of the vinyl group. After many catalytic tests, we
similar; the dihedral angles are 33.4(3)° in 3 and 27.1(3)° in 2. know that the boronic acid fragment –B(OH)2 (donating group)
The central conjugated chains are approximately planar (max. increases the reactivity of the group –CHvCH2 in the process.
deviation is 0.153(4) Å in 3 and 27.1(3) in 2). In the crystal We compared two different well defined catalysts built of
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Table 1 Optimizing Suzuki–Miyaura coupling reaction conditions
Pd(dba)(PPh3)2 – 2
Entry Catalyst amount (mol%) Temp. (°C) Time (h) Yielda (%)
1
2
3
4
5
1
40
60
90
40
60
90
40
60
90
40
60
90
40
60
90
8
3
1.5
12
31b, 33c
82b, 84c
88b, 94c
29b, 31c
81b, 83c
86b, 92c
54b, 56c
82b, 84c
87b, 90c
16c
Scheme 4 Synthesis of styryl-derivative arenes in the presence of 2 or 3.
0.5
0.2
0.1
0.01
5
2
18
10
8
24
24
24
24
24
24
In one case, we identified on the basis of MS spectra, trace
amounts of the side compound 4. The reaction is carried out
in the presence of complex 2 (1 mol%) at 90 °C (see Table 1,
entry 1).
For the full control of the synthesis of the new π-conjugated
styrylarene derivatives having an aromatic unit we additionally
made several catalytic examinations of the following reaction,
see Scheme 4.
In the synthesis of the new olefins we used a biphasic
solvent system of toluene (0.25 M solution concentration for
halide)–ethanol (2 mL)–base (3 mL, 2 M solution) and the new
palladium catalyst (see Fig. 1 and 2). Since the reactive sub-
strate contains the styryl group, the reaction conditions were
optimized to eliminate the possible effects of polymerization.
All catalytic details are presented in Table 2 below.
Using the new palladium complex significantly reduced the
temperature and duration of the reaction. Furthermore, the
reaction proceeds efficiently with a lower amount of catalyst,
which makes it more economical. Notably, during the course
of the reaction the system was homogeneously yellowish in
colour and there was ‘no reduction’ to the black species of the
palladium. These reaction parameters have a significant
impact on the total control of the process.
37c
65c
15c
22c
31c
Pd(dba)(PCy3)2 – 3
Entry Catalyst amount (mol%) Temp. (°C) Time (h) Yielda (%)
6
1
40
60
80
40
60
80
40
60
80
40
60
80
40
60
80
8
3
1
8
6
3
18
6
54b, 58c
89c
98c
55c
87c
97c
79
7
0.5
0.2
0.1
0.01
8
85
3
96c
66c
79c
95c
20c
34c
46c
9
24
18
4
24
24
24
10
a Determined by GC and GCMS methods. b Na2CO3. c K2CO3; open
systems under Ar and air were examined.
The 1H NMR spectrum of 2-(4-vinylphenyl)thiophene (8)
(Fig. 3) below shows us a clean, new olefin isolated using a
simple, convenient method of flash filtration (glass filter–
silica gel–celite).
various types of phosphine characterized by steric effects influ-
Signals A, B, C and their coupling constants (from 10.9 Hz
encing the stability and reactivity of the catalyst; data are col- to 17.6 Hz) come from a typical vinyl group on aromatic com-
lected in Table 1.
pounds. A small amount of anthracene was observed during
the catalysis with 9-bromoanthracene. That is why part of the
When the catalyst amount was reduced to 0.01 mol% and
the temperature was at 40 °C, a longer reaction time was synthesis was performed in the presence of the weaker base
required (24 h). The use of the catalyst in a lower amount
(0.01%) gave a low yield of the desired product. On the other
Na2CO3 and the reaction time was prolonged.
Next, we selected more active catalysts from this palladium
hand, the use of a higher amount (1 mol%) of palladium cata- system and used them in preliminary catalytic tests in order to
lyst (3) reduced the reaction time to 1 hour (Table 1). The
results presented in the table show that it is possible to effec-
choose the best conditions for the selective, controlled syn-
thesis of distyrylarene derivatives from commercially available
tively reduce the temperature to 70 °C. We also used a little substrates, Scheme 5.
more catalyst 3 (1.5 mol%) at 60 °C and achieved a very good
yield of 94% after a short time (60 min).
In order to avoid the formation of by-products, the stoichio-
metric ratio of 1 : 2 of dibromoaryl to boronic acid derivative
At the same time the first combination of substrates was used.
appeared to be much better for obtaining a product with a
yield of over 98% for the third complex. An important factor in
To maintain a homogeneous system an appropriate amount
of toluene was used in the synthesis. Typically, the crude
raising the efficiency of the reaction was the kind of applied mixture was extracted with water and a few crystals of sodium
base. The most appropriate was potassium carbonate (K2CO3),
with which it is possible to get nearly 100% conversion of sub-
strates to 4-vinylbiphenyl (5).
chloride to disperse the slurry. The mixture was left over mag-
nesium sulfate for four to five hours. The final products were
purified from the mixture of dichloromethane–hexane by
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Table 2 Catalytic test of 4-vinylphenylboronic acid with aryl bromines via Suzuki-coupling reaction
Br–[Ar] Ar
Catalysis system [Pd]–base
2–K2CO3
Temp. (°C)
Time (h)
Molar ratio [Br] : [acid] : [cat.]
1 : 1 : 1 × 10−2
Conversion Br–[Ar]a (%)
90
70
90
70
2
7
2
4
2
44
97
67
>99
21
3–K2CO3
1 : 1 : 5 × 10−3
2–K2CO3
1 : 1 : 1 × 10−2
18
2
>99
65
3–K2CO3
1 : 1 : 5 × 10−3
4
>99
2–Na2CO3
2–K2CO3
3–K2CO3
2–K2CO3
3–K2CO3
90
90
70
90
70
2
18
2
7
2
4
2
20
2
8
1 : 1 : 1 × 10−2
1 : 1 : 1 × 10−2
1 : 1 : 5 × 10−3
1 : 1 : 1 × 10−2
1 : 1 : 1 × 10−2
29
82
64
93
85
>99
28
90
43
>99
2/K2CO3
3–K2CO3
90
70
2
8
2
1 : 1 : 1 × 10−2
1 : 1 : 5 × 10−3
57
92
89
3.5
>99
a Determined by GC and GCMS methods; open systems under Ar and air were examined.
Fig. 3 1H NMR spectrum of 8 in CDCl3 at 25 °C.
Fig. 4 1H NMR spectrum of 11 in CDCl3 at 25 °C.
Compounds 5–15 were isolated and fully characterized
spectroscopically.
In conclusion, two very active and selective, mononuclear
phosphine palladium(0) complexes (2, 3) were synthesized
(with very good yields, e.g. for 2 – 98% and for 3 – 95%) and
their structures were solved by spectroscopic and X-ray
methods. These complexes appeared to be efficient homo-
genous catalysts in the Suzuki coupling of vinylphenyl boronic
acid with mono- and dibromoarenes. A highly selective
method for the synthesis of new π-conjugated olefins was pres-
ented. The reactivity of the products is now under investi-
Scheme 5 Synthesis of new distyrylarenes via the Suzuki–Miyaura coupling
reaction.
precipitation and dried under a high vacuum for 15 minutes at gation. All the obtained compounds can be used as starting
15–17 °C. materials for the synthesis of new symmetrical, molecular-
In this way a few new π-conjugated 10–15 organic com- derivatives and stereo-regular-polymeric materials with unique
pounds were synthesized with yields of almost 100%. The clear mechanical, thermal and optoelectronic properties. Further
proton NMR spectrum of the final distyryl-conjugated product results on the use of these compounds for the synthesis of
is presented above, see Fig. 4. Because of the potential distri- large, high-molecular π-conjugated organic compounds will be
bution, the olefin should be kept at low temperatures. reported in due time.
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Acknowledgements
We gratefully acknowledge financial support from the Ministry
of Science and Higher Education (Poland) (Project no. NN204
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