Polymer biquinolyl-containing complexes of Pd(ii) as efficient catalysts for cyanation of aryl and vinyl halides with K4Fe(CN)6

Article abstract of DOI:10.1039/c6nj02345b

A catalytic system for cyanation of aryl and vinyl halides with K₄Fe(CN)₆ based on a structurally tunable and nontoxic polymer backbone of polyamic type with biquinolyl fragments in the polymer chain capable of coordination to Pd^II ions is developed. The catalyst is eligible for thermal and microwave activation; in the latter case the reaction time is dramatically decreased. Cyanation of vinyl bromides occurs stereoselectively, and the configuration of the starting alkene is retained; even for Z-isomers the impact of configuration inversion is less than 5%. The polymer-based Pd catalyst is applicable for one-pot multi-step synthesis of the precursors of mesogenic structures of biphenyl type. Consecutive cross-coupling and cyanation reactions can be performed in the presence of the same portion of catalyst, in the same solvent, without isolation of intermediate products.

Full text of DOI:10.1039/c6nj02345b

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Polymer biquinolyl-containing complexes of Pd(II)
as efficient catalysts for cyanation of aryl and
vinyl halides with K4Fe(CN)6
Cite this:DOI: 10.1039/c6nj02345b
Oleg M. Nikitin,ab Olga V. Polyakova,a Petr K. Sazonov,a Alexander V. Yakimansky,c
Mikhail Ya. Goikhman,c Irina V. Podeshvoc and Tatiana V. Magdesieva*a
A catalytic system for cyanation of aryl and vinyl halides with K4Fe(CN)6 based on a structurally tunable
and nontoxic polymer backbone of polyamic type with biquinolyl fragments in the polymer chain
capable of coordination to PdII ions is developed. The catalyst is eligible for thermal and microwave
activation; in the latter case the reaction time is dramatically decreased. Cyanation of vinyl bromides
occurs stereoselectively, and the configuration of the starting alkene is retained; even for Z-isomers
the impact of configuration inversion is less than 5%. The polymer-based Pd catalyst is applicable for
one-pot multi-step synthesis of the precursors of mesogenic structures of biphenyl type. Consecutive
cross-coupling and cyanation reactions can be performed in the presence of the same portion of
catalyst, in the same solvent, without isolation of intermediate products.
Received (in Montpellier, France)
27th July 2016,
Accepted 26th October 2016
DOI: 10.1039/c6nj02345b
www.rsc.org/njc
(biQ) fragments in the main chain of the polymer backbone
provide an opportunity for coordination to transition metal ions,
Introduction
Polymer backbones containing organic moieties capable of e.g., Ni(II),13,14 Cu(I),15–18 Pd(II),17,19 yielding polymer complexes
coordination to transition metal ions can be considered as which show excellent efficiency in electrocatalytic aerobic oxidation
polydentate macroligands which can be applied for the creation of aliphatic alcohols15 and amines16 to corresponding carbonyl
of homo- and heteropolymetallic catalytic systems. The usage compounds as well as in catalysis for C–C bond formation
of polymer catalysts provides important advantages such as (Suzuki–Miyaura19 and Sonogashira17,18 couplings).
easy separation from the reaction mixture, the possibility for
The present paper describes the further development of this
immobilization on various supports as well as recycling. A research and is aimed at elaboration of an efficient catalytic
bulky polymer ligand is prone to stabilize the nonequilibrium process of cyanation of aryl and vinyl halides with K4Fe(CN)6
states of the coordination unit which are active in catalysis. All using Pd–polymer complexes of the aforementioned type
this provides a precondition for increasing interest in this type (Scheme 2). Cyanation can be considered as one of the most
of catalytic systems and many efficient catalytic processes have synthetically important reactions since nitriles are valuable
been already elaborated using this approach (see, e.g., ref. 1–8). synthons in organic chemistry20,21 as well as constitute key
Polymers of polyamic type are widely used in various practical components of dyes, pharmaceuticals, herbicides, etc.22 Currently,
areas since they are easily accessible via facile synthetic procedures;9–11 K4Fe(CN)6 is the reagent of choice in the majority of publications on
the simplest are commercially available. Recently,12 new polymers this topic [see ref. 23 and references cited therein], since its first
of this type (PA, Scheme 1) which exhibit excellent thermal presentation in ref. 24.
stability and good solubility in organic solvents were synthesized.
Advantages of this reagent are very significant. K4Fe(CN)6 is
The presence of the CH2-group between two aromatic rings non-toxic and cheap; all cyanide ions can be transferred to the
ensures plasticity, and imide fragments in the copolymer are halide providing an atom-economy strategy; the slow release of
responsible for the tensile strength of the polymer. The biquinolyl cyanide ions from K4Fe(CN)6 allows minimizing catalyst
deactivation.25,26 K4Fe(CN)6 utilization requires Pd or Cu catalysts
a Department of Chemistry, M.V. Lomonosov Moscow State University,
Leninskie Gory, 1/3, Moscow, 119991, Russia. E-mail: tvm@org.chem.msu.ru;
Tel: +7 (495) 939-30-65
b A.N. Nesmeyanov Institute of Organoelement Compounds,
Russian Academy of Science, Vavilova St, 28, Moscow, 119991, Russia
c Institute of Macromolecular Compounds, Russian Academy of Science, V.O.,
Bol’shoi pr. 31, St. Petersburg, 199004, Russia
and many catalytic systems have been already elaborated. The
overwhelming part of publications describe catalytic cyanation of
aryl halides (see ref. 23 and 27 and references cited therein)
(mainly, iodides and bromides; examples of efficient cyanation of
chlorides are rare28–31). As catalysts, Pd complexes with various
phosphine ligands (which are very efficient but toxic and
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Scheme 1 The structure of the polymer ligands.
Scheme 2 General scheme of aryl and vinyl cyanation.
expensive) are commonly used;24,31–35 however, the current electrochemical approach seems to be more convenient, due to
emphasis is gradually shifted to Pd nanocomposites36–45 and the simplicity of the instrumental control over the reaction
polymer-bound Pd catalysts.46–48 Copper containing catalysts are course and the absence of additional nucleophilic anions,
cheaper but usually less efficient, require harsh reaction conditions which may play the role of extra ligands at the metal center
and usually have lower functional group tolerance.49–51 Recently, and be harmful for catalysis.
combined Pd–Cu catalytic systems have been suggested for
cyanation of aryl halides and have turned out to be rather static regime (I = 1 mA) in NMP solution containing a polymer
efficient for iodides and activated bromides.43,44
(the amount of the polymer was calculated with respect to the
The dissolution of the Pd anode was performed in a galvano-
As concerns cyanation of vinyl halides with K4Fe(CN)6, only required molar concentration of biQ units). The amount of
few publications are available and the scope of halides is electricity passed through the solution was estimated to provide
mainly limited to various styryl bromides.50,52,53 Cyanation of the metal : biQ equimolar ratio. The composition of the catalytic
aryl and vinyl halides usually requires elevated temperature. centres obtained on the polymer chain was proved using cyclic
Currently, more and more publications are appearing in which voltammetry in combination with spectral methods and elemental
microwave irradiation is used instead of commonly applied analysis19 indicating that biQPdL2 coordination units (L = NMP) are
heating (see, e.g., ref. 54–61). The use of microwave irradiation formed. Pd content in the polymer can be determined by cyclic
in homogeneous transition-metal catalyzed reactions leads to voltammetry. The characteristic peak corresponding to the PdII/0
reduction of the reaction time producing high yields, higher redox couple in biQPdL2 coordination units appears at a potential of
selectivity and increased lifetime of the catalyst.
ꢀ0.45 V63 (vs. Ag/AgCl/KCl). A comparison of peak current values for
Thus, creation of Pd catalysts without toxic ligands which the PdII/0 redox couple and for biQ reduction (ꢀ1.02 V) provides an
will be efficient for cyanation of both aryl halides (including estimation of the amount of catalytic units in the solution.
cheap aryl chlorides) and vinyl halides and which are eligible
for thermal and microwave activation is still a challenge.
The thus obtained catalytic system was tested in the cyanation
of aryl and vinyl halides.
Cyanation of aryl halides
Results and discussion
One of the significant advantages of polymer-based catalysts is
Pd–polymer complexes based on polyamic acids containing the possibility for fine-tuning of the structure of the polymer
biquinolyl coordinating units can be obtained via Pd anode backbone to provide an optimal configuration and coordination
dissolution in the presence of the polymer ligand, as well as by environment of the catalytic center. To evaluate catalytic activity
the addition of a Pd salt, as it has been previously described.19 and to choose the best catalytic system, three polymer ligands
The efficiency of catalytic systems obtained via two different were tested (Scheme 1). All of them are co-polymers which differ
approaches was previously tested in coupling reactions, and in the amount of biquinolyl units as well as in the presence of the
comparable yields of the products were obtained.19 Since electro- phenylenediamine fragments which increase the conformational
chemical Pd(0) deposition is quite a routine procedure62 the rigidity of the polymer. The reaction was performed in NMP
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Table 1 The influence of the polymer type on the yield of the nitrile (K4Fe(CN)6, 0.04 mmol; p-NO2C6H4Br, 0.2 mmol; Pd–PA (0.3 mol% of Pd); Na2CO3,
1 mmol; 2 ml of NMP, 8 h, 100 1C)
Pd–PAI
90/91
Pd–PAII
52/55
Pd–PAIII
21/26
Pd–biQa
14/16
Pd–bPyb
11/12
PdCl2
24/25
c
Catalyst
ArCN, %/conversion, %
a
b
8 h, 0.3 mol% of Pd[biQ(COOHex)2]Cl2 (biQ(COOHex)2 = dihexyl ester of 2,20-biquinolyl-4,40-dicarboxylic acid). 8 h, 0.3 mol% of Pd(bPy)Cl2
c
(bPy = 2,20-bipyridine). 17 h, 1 mol% of PdCl2.
Table 3 Effect of water on the yield of the nitriles (K4Fe(CN)6, 0.04 mmol;
solution (which turned out to be the best solvent for this type of
polymers; it is also applicable both for microwave activation
and for heating). First, para-NO2C6H4Br was selected as the
ArHal, 0.2 mmol; Pd–PAI (0.3 mol% of Pd), Na2CO3, 1 mmol; 2 ml of
H2O–NMP mixture, 100 1C)
model substrate and heating at 100 1C was used for activation.
The catalyst was dissolved in NMP and its amount was calculated
to provide 0.3 molar% of Pd. The reaction products were analyzed
using a GC-MS system. The results obtained for three different
polymers and for ligandless PdCl2 taken for comparison are given
in Table 1.
NMP/H2O (v/v)
ArCN yield/conversion, % 10/— t, h 9/1
t, h 7/3 t, h
1
2
3
4
C6H5Br
73/73 12
82/84 12
17/23 20
12/12 20
81/84 12
6/8 12
3/5 12
p-OHC-C6H4Br
p-H3C-C6H4Br
C6H5Cl
89/90 12
49/50 20
39/40 20
As follows from Table 1, the maximal yield of the nitrile was
obtained using PAI. This polymer is less rigid as compared to
PAIII and contains biQ fragments in each monomer unit. This can be found in the literature.41,53,67,68 To clarify the situation,
result is in line with the previously reported data when the same the influence of water content was tested on a series of substrates
catalytic systems were used in Suzuki coupling.19 Ligandless PdCl2 including aryl bromides with electron-poor and electron-rich
gave poor results though the reaction time was doubled. The substituents as well as chlorobenzene. The results obtained
catalytic efficiency obtained for PdCl2 in the presence of similar indicate that the optimal NMP/H2O ratio for our catalytic
N-containing ligands of low-molecular weight (bPy and biQ) was system is 9/1 (see Table 3). A further increase in water content
higher but still sufficiently lower than that for the polymeric dramatically decreases the yield of nitriles. One of the probable
catalytic system. This emphasized the key role of the polymer reasons might be a decrease in the solubility of the polymeric
ligand in the catalytic reaction.
For the screening of the reaction conditions, a series of decreasing the availability of the catalytic centers.
experiments were performed varying the nature of the base. The
As follows from the tables presented above, PdPAI exhibits
complex which leads to its coil-to-globule transition, thus
role of the base is crucial to facilitate the reduction of the Pd(II) sufficiently high catalytic activity in the cyanation of aryl bromides,
intermediate to catalytically active Pd(0)64,65 as well as to provide but for aryl chloride the yield is moderate. The reaction was
a gradual release of the CN groups.25 It was found that, among performed at an elevated temperature (100 1C) and for a prolonged
the common inorganic bases, application of Na2CO3 gave better reaction time (12–20 h). However, it was still not sufficient to
yield of the nitrile (Table 2). K2CO3 is less efficient, in spite provide complete conversion of the starting halide. To overcome
of similar basicity. Thus, Na2CO3 was used in all subsequent the problem, microwave activation was tested. The use of micro-
experiments.
wave irradiation in homogeneous transition metal-catalyzed
Several experiments were conducted with some amount of reactions is known to lead to remarkable reduction in the
water added into the reaction system to investigate its influence reaction time producing better yields and increased lifetime
on catalytic efficiency. Rather inconsistent data on this subject of the catalyst. Microwave activation has been successfully used
can be found in the literature. It was shown that less than ca. in palladium-catalyzed cyanation of aryl halides in the previous
5% of water did not influence the cyanation66 but addition of reports.54,56,57,61
some amount of water can increase the solubility of hexa-
Application of the other type of activation (microwave irradiation
cyanoferrate in organic solvents. Meanwhile, it has been demon- instead of prolonged heating) required new experiments for
strated that hydrolysis of cyanide occurs to generate HCN which optimization of the reaction conditions (Table 4). Iodobenzene
can compete with aryl halides for oxidative addition to Pd(0) was taken as a model substrate. Two different polymers were
catalyst and thus impair the catalysis.26 Some examples of tested (PAI and PAIII) and PAI showed better results, as it has
Pd-catalyzed cyanation with K4Fe(CN)6 in aqueous solution been previously observed under activation by heating. The bases
were also varied and Na2CO3 was the best again. NMP, a micro-
wave absorbing polar solvent, was used. However, water addition
to NMP solution turned out to be harmful decreasing the yield of
benzonitrile significantly (Tables 4 and 5). Furthermore, the best
results were obtained when the salts were preliminary dried in
Table 2 The influence of the base nature on the yield of the nitrile
(K4Fe(CN)6, 0.04 mol; p-NO2C6H4Br, 0.2 mmol; Pd–PAI (0.3 mol% of
Pd); base, 1 mmol; 2 ml of NMP, 100 1C)
Base
Na2CO3 K2CO3 NaOAc K3PO4 KF
vacuum and anhydrous solvent was used.
Comparison of the results obtained for halobenzenes under
Time, h
8
8
12
12
12
ArCN, %/conversion, % 90/91
62/62
55/57
6/8
6/20 MW irradiation and under prolonged heating (see Table 5),
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Table 4 Optimization of the reaction conditions for MW-assisted cyanation of iodobenzene (K4Fe(CN)6, 0.04 mmol; C6H5I, 0.2 mmol; Pd–PAI (0.3 mol%
of Pd); Na2CO3, 1 mmol; NMP, MW (70 W), 20 min)
Catalyst
PdPAI
96/99
Base
NMP/H2O
10/—
PdPAIII
41/42
PdbiQa
3/3
PdbPyb
Na2CO3
96/99
K2CO3
61/64
AcONa
32/33
9/1
ArCN/conversion, %
o1/1
96/99
63/64
a
b
PdbiQ(COOHex)2Cl2 was used as the catalyst (0.3 mol%, 20 min). PdbPyCl2 was used as the catalyst (0.3 mol%, 20 min).
nitriles were determined using 1H and 13C NMR. The chemical
shifts of the signals of a- and b-protons in the bromides and
nitriles with Z- and E-configurations differ significantly. This
allows reliable control on the stereoselectivity of the process as
well as the estimation of the conversion degree. Additional
information can be obtained from the 13C NMR signals of the
nitrile carbon which are different for Z- and E-nitriles.
The first experiments were performed with E-bromostyrene.
They demonstrated (Table 6) that with both ligand-free PdCl2
and PdCl2 in the presence of similar N-containing ligands of
low-molecular weight (biPy and biQ) the reaction is very slow,
as it has been observed also for aryl halides. Addition of 10% of
water to NMP was found to have a profound effect on the yield
of the nitrile. 12 h under common heating at 100 1C or 20 min
under MW irradiation resulted in quantitative conversion of
E-bromostyrene to the corresponding nitrile.
Table 5 Halobenzene cyanation under MW irradiation (70 W, 20 min) and
under prolonged heating (100 1C) (K4Fe(CN)6, 0.04 mmol; PhHal,
0.2 mmol; PdPAI (0.3 mol% of Pd); Na2CO3, 1 mmol; 2 ml of NMP or
H2O–NMP mixture)
ArCN/conversion, %
MW irradiation, 20 min
Heating, 100 1C
a
NMP NMP/H2O = 9/1 PdCl2
Time, h NMP/H2O = 9/1
C6H5Cl 46/46 9/10
C6H5Br 76/76 54/59
2/3
27/29
20
15
8
49/50
81/84
96/97
C6H5I
96/99 63/64
a
Ligand-free PdCl2 (1 mol%) in NMP was used as the catalyst.
with the other reaction parameters identical, showed that, for
almost quantitative yield of benzonitrile to be obtained for
phenyl iodide, 20 min and 8 h are required, respectively. This
emphasizes the benefits of microwave activation in comparison
with traditional heating methods and confirms that the polymer
catalyst is stable under MW irradiation. The benzonitrile yields
obtained for phenyl bromide and phenyl chloride after 20 min
under MW irradiation are lower (76% and 46%, respectively) but
incomplete conversion indicates that the yield can be increased
by increasing the reaction time. To obtain similar yields in the
case of thermal activation, prolonged heating of the reaction
mixtures under optimized conditions is necessary (15 h and
20 h, respectively). To emphasize the role of the polymer ligand,
both ligand-free PdCl2 and PdCl2 in the presence of similar
N-containing ligands of low-molecular weight (biPy and biQ)
were also tested as catalysts in the same reaction conditions. The
dramatically lower yields (Tables 4 and 5) obtained confirmed the
importance of the elaborated catalytic system.
It was interesting to investigate the cyanation of vinyl
bromides containing substituents at the a- and b-carbon atoms
(Table 7). Cyanation of these substrates has not been investigated
before; scarce publications are focused on styryl bromides with
substituents in the aromatic ring.50,52,53
As follows from Table 7 (entry 7), cyanation of a-bromostyrene
gave low yields of the nitrile, under both thermal and MW
activation. In contrast to the other investigated compounds, the
starting bromide was not detected in the reaction mixture but up
to 20% of elimination product (alkyne) was present. One can
suggest that a side-reaction of alkene or alkyne polymerization
might take place. Dibromostylbene produced the corresponding
dinitrile with good yield (entry 6) when two equivalents of
K4Fe(CN)6 were taken. The brominated methyl ester of cinnamic
acid gave good yield of the nitrile under prolonged heating
whereas under MW irradiation the yield was low (Table 7, entry 5).
Incomplete consumption of the starting bromide in the latter
case indicates that the probable reason is a low reaction rate
rather than side-reactions.
Cyanation of vinyl halides
Cyanation of vinyl halides is much less investigated as compared
to their aryl counterparts. Besides, the stereoselectivity issue
arises. Having encouraging results for aryl halides, it was
interesting to test the elaborated catalytic system for cyanation
of vinyl bromides. Common heating and MW irradiation were
applied. In a typical experimental procedure, a mixture of vinyl
bromide, K4Fe(CN)6, Na2CO3 and PdPAI (0.3 mol% of Pd) in
NMP (or in NMP/H2O = 9/1 v/v) was heated at 100 1C under Ar
for the required period of time under intensive stirring.
MW-activated reactions were performed in anhydrous NMP,
with the same ratio of the reactants; the reaction time was
20 min. After the standard workup procedure, the products
were analyzed using GC-MS (with the internal standard). Z–E
Table 6 E-Bromostyrene cyanation under MW irradiation (70 W, 20 min)
and under prolonged heating (100 1C, 12 h) (K4Fe(CN)6, 0.04 mmol;
E-bromostyrene, 0.2 mmol; Pd–PAI (0.3 mol% of Pd); Na2CO3, 1 mmol;
2 ml of NMP–H2O mixture)
Heating, 100 1C NMP/H2O ratio
MW irradiation
Conditions
10/— 9/1 7/3
9/1
9/1
10/— NMP
Nitrile yield/ 79/81 99 55/60 9/11a 10/12b 20/21c 100
conversion, %
a
PdbiQ(COOHex)2Cl2 was used as the catalyst (0.3 mol%, 12 h).
b
c
PdbPyCl2 was used as the catalyst (0.3 mol%, 12 h). Ligand-free
configurations of the starting bromides and the corresponding PdCl2 (1 mol%) in NMP was used as the catalyst, 17 h.
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Table 7 Cyanation of vinyl bromides under MW irradiation (70 W, 20 min)
and under prolonged heating (100 1C, 14 h) (K4Fe(CN)6, 0.04 mmol; VinBr,
0.2 mmol; Pd–PA (0.3 mol%); Na2CO3, 1 mmol; 2 ml of NMP)
present work provides extremely high stereoselectivity even
for Z-isomers.
Yield of VinCNa, %
One-pot multi-step transformations
No. Vinyl bromide
Heating MW irradiation
Synthesis of complicated organic molecules usually requires a
large number of reaction steps involving separation and refining
procedures. For economical reasons, it is necessary to reduce
the number of steps, and one-pot synthesis is the best strategy.
The applicability of the polymer-based Pd catalyst discussed in
the present work for one-pot multi-step synthesis of the pre-
cursors of mesogenic structures of biphenyl type was tested.
Previously, we demonstrated that our catalytic system is
efficient in Suzuki–Miyaura coupling.19 Thus, cross-coupling
and cyanation can be performed in the presence of the same
catalyst. This enables catalysis of two different consecutive
reactions in one-pot syntheses, in the same solvent, without
separation of intermediate products (Scheme 3). However, this
requires optimization and fine-tuning of reaction conditions
with respect to the activity and selectivity of the catalyst for each
reaction step.
As starting compounds, 4-iodo- and 4-chloro-bromobenzenes
were taken. For catalysis, the PdPAI complex was taken since
this polymer ligand provides maximal efficiency in both coupling
and cyanation reactions (see above). Suzuki–Miyaura coupling
was performed first, to obtain the maximal selectivity. In this case
we can be sure that only one halogen atom will be substituted in
the first step for the aryl moiety (after one halogen atom is
replaced, the product containing the phenyl ring with donor
substituent becomes less active for further transformation as
compared to the starting dihalide). Commonly, a small excess of
boronic acid is used but we had to avoid it, to prevent substitu-
tion of the second halogen atom. If cyanation is performed first,
formation of a dinitrile by-product can be expected (due to the
activating effect of the electron-withdrawing CN-group). Thus, the
possibility for selective substitution of the most active halogen
atom for the aryl fragment was tested first. In the next step,
K4Fe(CN)6 was added to the reaction mixture obtained in the first
step without any purification; no additional portions of the
catalyst were used to provide substitution of the second halide
for the nitrile group.
1
2
3
4
5
6
7
8
Z-PhCHQCHBr
E-PhCHQCHBr
Z-PhCHQCPhBr
E-PhCHQCPhBr
Z-PhCHQC(COOMe)Br
Z-PhBrCQCPhBr + 2 eq. K4Fe(CN)6 82 (84)
PhBrCQCH2
Ph2CQCPhBr
99
99
97
100
85 (89)
99
100
97
90 (100)
52 (53)
76 (79)
24 (100)
49 (52)
5 (100)
5 (8)
a
The conversion is given in parentheses for the cases with incomplete
transformation.
Reliable determination of the configurations of the starting
bromide and the corresponding cyanation product was performed
based on the NMR spectra, by comparison with the literature.
Thus, E- and Z-cinnamonitriles can be distinguished due to the
difference in the chemical shifts of the doublet signals corres-
ponding to the a-protons (at 5.89 and 5.45 ppm, respectively69). In
the case of 2-phenylcinnamonitrile, the signals of the E- and Z-
isomers in 1H NMR spectra are overlapped and the determination
of the configuration of the product was based on 13C NMR data, on
the signals corresponding to the nitrile carbon atom which are well
distinguished (chemical shifts are 120 ppm for the E-isomer
and 118 ppm for the Z-isomer70). Differentiation between
E- and Z-nitriles of 2-carbomethoxy cinnamon acid and deter-
mination of their relative amounts are more convenient to
perform considering the singlets corresponding to the protons
of the methyl group (chemical shifts are 3.76 ppm and 3.91 ppm
for the E- and Z-isomer71).
In all cases E-bromides were stereoselectively converted to
the corresponding nitriles with the retention of the starting
alkene configuration. As concerns the Z-isomers, the traces
(less than 5%) of the E-nitrile were detected in some cases.
These results are in line with previously reported data50,53
where cyanation of Z-styryl bromides provided a mixture of
Z- and E-isomers (up to 20% of E-nitriles). The explanation
given in ref. 53 seems reasonable. In the case of Z-styryl bromide,
the NC–Pd–styryl intermediate may undergo b-elimination to
form the alkyne–Pd–CN intermediate which can lead to a mixture
of both Z- and E-isomers. Contrarily, the thermodynamically
more stable E-bromide forms E-nitrile in a straightforward way.
Detection of the alkyne in the reaction mixture (see above) serves
as an indirect confirmation of the presented explanation. However,
it should be emphasized that the polymer catalyst suggested in the
To optimize the reaction conditions, Suzuki coupling was
tested first. A number of boronic acids with different substituents
were applied; the reaction was performed in NMP at 100 1C, using
PdPAI (0.3 mol% of Pd) in solution, with Na2CO3 as a base. The
results obtained are given in Table 8.
Usually, the reaction is performed at higher temperatures
(130 1C and more). To make the reaction more selective, the applied
Scheme 3 Consecutive one-pot cross-coupling and cyanation of aryl dihalides.
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Table 8 The yields of 4-halobiphenyls in Suzuki–Miyaura coupling (ArHal,
0.2 mmol; ArB(OH)2, 0.2 mmol; PdPAI (0.3 mol% of Pd); Na2CO3, 1 mmol;
2 ml of NMP, 100 1C, 6–10 h)
Table 10 The yield of products in two-step one-pot synthesis (RHal,
0.2 mmol; ArB(OH)2, 0.2 mmol; K4Fe(CN)6, 0.04 mmol; PdPAI (0.3 mol% of
Pd); Na2CO3, 2 mmol; 2 ml of NMP, 100 1C)
ArHal
Time, h
ArB(OH)2
Biphenyl, %
Ar in
ArB(OH)2
Yield of
Yield of
ArHal
Time, h biphenyl, % Ar-C6H4-CN, %
p-I-C6H4-Br
p-I-C6H4-Br
p-I-C6H4-Br
p-I-C6H4-Br
p-Cl-C6H4-Br
p-Cl-C6H4-Br
p-Cl-C6H4-Br
p-Cl-C6H4-Br
6
8
8
PhB(OH)2
499
499
499
95
499
98
p-CH3-C6H4B(OH)2
p-tBu-C6H4B(OH)2
p-CH3O-C6H4B(OH)2
PhB(OH)2
p-CH3-C6H4B(OH)2
p-tBu-C6H4B(OH)2
p-CH3O-C6H4B(OH)2
p-I-C6H4-Br
p-I-C6H4-Br
Ph
6 + 15
499
499
10 + 24 499
89
88
76
74
p-CH3O-C6H4 8 + 15
8
p-Cl-C6H4-Br Ph
p-Cl-C6H4-Br p-CH3O-C6H4 10 + 24
10
10
10
10
98
96
96
Recyclability of the catalyst was tested in the reaction of
4-iodobromobenzene with phenylboronic acid followed by
one-pot cyanation with K4Fe(CN)6. Though the efficiency of the
immobilized catalyst was gradually decreased, it was sufficiently
active during at least three cycles. The yields of 4-cyanobiphenyl
were 87%, 85% and 68% for three consecutive cycles (the total
reaction time for the second and third cycles was increased from
30 to 36 h).
The possibility of Pd leaching from the immobilized polymer
catalyst was also investigated. For this purpose, the first step
(Suzuki–Miyaura coupling) was performed for 4-iodobromo-
benzene as the model substrate as described above using the
immobilized PdPAI complex. After the reaction mixture was
heated at 100 1C for 6 h, the immobilized catalyst was taken
off and the cyanating agent was added. The reaction mixture
was heated again for 24 h at 100 1C. GC-MS analysis of the
products showed the presence of 4-biphenyl bromide (98%) and
no cyanation product was detected. This indicated that the
PdPAI polymer catalyst is strongly immobilized on the graphite
surface and heterogeneous catalysis has been performed.
temperature was 100 1C. As follows from Table 8, this allowed
selective substitution of the more active halogen. The reaction
time for iodo- and chloro-bromobenzenes was different (6–8 h
and 10 h, respectively).
The products of the Suzuki coupling were isolated and subjected
to the cyanation procedure, to optimize the reaction time. The results
are given in Table 9. As follows from the data presented, 15 h for
bromides is enough to get a good yield of the cyanated biphenyls.
For the corresponding chlorides, the reaction time was increased to
24 h and the nitriles were obtained with moderate yields. Hence, the
reaction time for these compounds should be prolonged.
Using these data, one-pot consecutive cross-coupling and
cyanation reactions were performed. After allowing the time
required for arylation, a probe for GC-MS analysis was taken (to
test the completion of the coupling) and the cyanating agent
with a new portion of the base was added to the reaction
mixture which was subjected to additional heating at the same
temperature for 15–24 h. After the work-up procedure, the
products were analyzed using GC-MS. The yields of cyanated
biphenyls are given in Table 10.
Unfortunately, after the work-up procedure the polymer
catalyst becomes insoluble and it cannot be used twice. To
overcome the problem, the immobilized form of the catalyst
was applied. To prepare the immobilized catalyst, the graphite
fabric was placed in the solution of PdPAI in NMP for 2 h; the
graphite fabric impregnated with the catalyst (0.1 mol% of Pd)
was taken off, dried and rinsed with MeCN to remove non-
coordinated Pd ions and dried in vacuum. Afterwards, it was
used for the one-pot two-step reaction described above.
After the reaction came to completion, the immobilized
polymer catalyst was taken from the solution, rinsed and used
again without purification with new portions of the reactants.
Conclusion
Thus, it was demonstrated that polyamic acids with biquinolyl
fragments in the polymer backbone capable of coordination to
Pd(II) ions can be used to prepare catalysts for cyanation of aryl
and vinyl halides with K4Fe(CN)6. The structurally tunable and
nontoxic polymer backbone allows creating an efficient catalytic
system which can be applied in solution and in an immobilized
form on the graphite fabric; cyanation of aryl halides (including
chlorides) and vinyl bromides is possible. The catalyst is not
destroyed under microwave irradiation. This allows performing
cyanation not only under common heating but under MW
activation as well; in the latter case the reaction time is dramatically
decreased. The yields of nitriles are 40–90%. Cyanation of vinyl
bromides occurs stereoselectively, and the configuration of the
starting alkene is retained; even for Z-isomers, the impact of
configuration inversion is less than 5%. The polymer-based Pd
catalyst is applicable for one-pot multi-step synthesis of the
precursors of mesogenic structures of biphenyl type. Consecutive
cross-coupling and cyanation reactions can be performed in the
presence of the same portion of catalyst, in the same solvent,
without isolation of intermediate products. The immobilized
catalyst can be reused for at least three runs; however, the
reaction time for the second and third runs should be increased.
Table 9 The yields of cyanated biphenyls (ArBr, 0.2 mmol; K4Fe(CN)6,
0.04 mmol; PdPAI (0.3 mol% of Pd); Na2CO3, 1 mmol; NMP, 100 1C)
Yield of
Ar-C6H4-CN, %
Ar-C6H4-Hal
Time, h
Conversion, %
Ph-C6H4-Br
15
15
15
15
24
24
24
24
87
86
82
75
76
81
48
63
87
88
83
76
77
82
52
65
p-CH3-C6H4-C6H4-Br
p-Bu-C6H4-C6H4-Br
p-CH3O-C6H4-C6H4-Br
Ph-C6H4-Cl
p-CH3-C6H4-C6H4-Cl
p-Bu-C6H4-C6H4-Cl
p-CH3O-C6H4-C6H4-Cl
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Paper
Experimental
PhCN. EI-MS, m/z (I, %): 103 [M+] (100); 76 [M+–CN] (32).
+
Q
PhC(H) C(H)CN. EI-MS, m/z (I, %): 129 [M ] (100); 103
The reaction mixtures obtained after the cyanation reactions
were analyzed using a ‘‘Pegasus 4D’’ (LECO) GC/MS spectrometer
(ionization energy 70 eV, a DB-5MS quartz capillary column (30 m),
temperature mode: 50 1C (2 min)–280 1C (15 min), heating rate
20 deg minꢀ1).
[M+–CN] (28); 77 [M+–C2H2–CN] (11).
+
Q
PhCH CPhCN. EI-MS, m/z (I, %): 205 [M ] (100); 179
[M+–CN] (14), 178 [M+–H–CN] (16).
+
Q
PhC(CN) CH2. EI-MS, m/z (I, %): 129 [M ] (100); 103
[M+–CN] (45).
1H and 13C NMR spectra were recorded using a Varian VXR
400 in CDCl3. Chemical shifts were measured relative to TMS.
MW irradiation was performed using LG Intellowave MS-2322G,
and the power used was 70 W.
N-Methylpyrrolidone (NMP, Aldrich, spectroscopic quality)
was stirred over CaH2 for 3 h and distilled under reduced pressure. 1H and 13C NMR spectrum
+
Q
Ph2C CPhCN. EI-MS, m/z (I, %): 281 [M ] (100); 255
[M+–CN] (31), 178 [M+–CN–Ph] (28).
+
Q
PhCH C(COOMe)CN. EI-MS, m/z (I, %): 187 [M ] (100); 161
[M+–CN] (78), 128 [M+–COOMe] (47).
Polyamide ligands (PA) were synthesized as previously
described.12 The corresponding Pd-complexes were synthesized
using preparative electrolysis with a Pd anode as described
previously.8,19 p-Iodobromobenzene and p-bromochlorobenzene
were synthesized according to the literature protocol;72 (Z)- and
(E)-1-bromo-1,2-diphenylethylene were synthesized as described
in ref. 73. Other investigated aryl and vinyl halides were com-
mercially available from Aldrich and Acros Organics and used as
received: iodobenzene (PhI, 99%, Aldrich), bromobenzene (PhBr,
Z99.5%, Aldrich), chlorobenzene (PhCl, 99.8%, Sigma-Aldrich),
Z- and E-bromostyrene (97%, Sigma-Aldrich), a-bromostyrene
(97%, Sigma-Aldrich), (Z)-1,2-dibromo-1,2-diphenylethylene (97%,
Sigma-Aldrich), bromotriphenylethylene (98%, Sigma-Aldrich),
1-bromo-4-nitrobenzene (4-NO2C6H4Br, 99%, Aldrich), para-
bromoanisole (para-CH3OC6H4Br, 98%, Acros Organics), sodium
carbonate (Na2CO3, 98%, Reachem), potassium carbonate (98%,
Reachem), sodium acetate (NaOAc, 98%, Reachem), tripotassium
phosphate (K3PO4, Z99.5%, Aldrich), potassium ferrocyanide
(K4Fe(CN)6, Z99.5%, Aldrich).
1
Q
E-PhCH CHCN. NMR H (400 MHz, CDCl3): d, ppm: 7.46–
7.38 (6H, m), 5.89 (1H, d, J = 16.7 Hz); lit.: 7.48–7.37 (6H, m),
5.87 (1H, d, J = 16.4 Hz).69
1
Q
Z-PhCH CHCN. NMR H (400 MHz, CDCl3): d, ppm: 7.83–
7.78 (2H, m), 7.48–7.41 (3H, m), 7.13 (1H, d, J = 12.0 Hz), 5.45
(d, J = 12.1 Hz, 1H); lit.: d, ppm: 7.84–7.76 (2H, m), 7.48–7.42
(3H, m), 7.13 (1H, d, J = 12.0 Hz), 5.45 (1H, d, J = 12.0 Hz).69
1
Q
E-PhCH CPhCN. NMR H (400 MHz, CDCl3): d, ppm: 7.39–
7.29 (m, 9H), 7.16–7.10 (m, 2H); NMR 13C (100 MHz, CDCl3): d,
ppm: 144.2, 133.5, 132.7, 129.9, 129.8, 129.3, 129.1, 128.9,
128.6, 120.2, 114.4, lit.: NMR 1H (CDCl3, 400 MHz): d 7.40–
7.31 (m, 6H), 7.30–7.18 (m, 3H), 7.18–7.12 (m, 2H); NMR 13C: d
144.1, 133.5, 132.6, 129.8, 129.7, 129.2, 129.0, 128.8, 128.5,
120.1, 114.3.70
Z-PhCH CPhCN. NMR 1H (400 MHz, CDCl3): d, ppm: d
Q
7.88–7.83 (m, 2H), 7.67–7.62 (m, 2H), 7.52 (s, 1H), 7.50–7.37 (m,
6H) NMR 13C (100 MHz, CDCl3): d, ppm: 142.3, 134.5, 133.8,
130.6, 129.24, 129.20, 129.0, 128.9, 126.0, 118.0, 111.7; lit.: NMR
1H (CDCl3): d, ppm = 7.92–7.86 (m, 2H), 7.70–7.65 (m, 2H), 7.53
(c, 1H), 7.50–7.36 (m, 6H); NMR 13C (CDCl3): d, ppm: 142.2, 134.5,
133.7, 130.5, 129.24, 129.18, 129.0, 128.9, 126.0, 118.0, 111.7.70
General procedure for cyanation of organic halides under heating
E-PhCH C(COOMe)CN. NMR 1H (400 MHz, CDCl3): d,
ppm: 8.22 (1H, s), 7.96–7.91 (2H, m), 7.56–7.42 (3H, m), 3.93
A mixture of 1 ml of NMP, organic halide (0.2 mmol), 14.7 mg of
K4Fe(CN)6 (0.04 mmol), 106 mg of Na2CO3 (1 mmol) and 1 ml of
the PdPAI solution in NMP (0.6 mmol of Pd) was deaerated
with Ar flow. Afterwards, the reaction mixture was placed in a
hermetic vessel of 10 ml volume and vigorously stirred using a
shaker (J-KEM Scientific, 200 rpm) under an Ar atmosphere at 100 1C
for several hours. After cooling to room temperature, 50 ml of water
saturated with sodium chloride was added to the reaction mixture in
NMP and the organic products were extracted with diethyl ether (3 ꢁ
20 ml). The ether extract was washed with water, dried over sodium
sulfate and analyzed using gas chromatography-mass spectro-
metry (GC-MS, ‘‘Pegasus 4D’’, LECO) with naphthalene as an
internal standard. The products were characterized by comparing
their MS and 1H, 13C NMR spectra with those found in the
literature. Configurations of the starting vinyl bromides and
Q
1
(3H, s); lit.: NMR H (CDCl3), d, ppm: 8.21s (1H, s), 7.97–7.92
(2H, m), 7.56–7.43 (3H, m), 3.90 (3H, s).74
General procedure for cyanation under microwave irradiation
The reaction mixture was prepared as described above. Instead
of heating, it was subjected to MW irradiation (70 W) for 3 min
periods which were alternated with cooling. The yields of nitriles
were determined as described above.
One-pot synthesis of cyanated biphenyls
A mixture of aryl dihalogenide (0.2 mmol), ArB(OH)2 (0.2 mmol),
106 mg of Na2CO3 (1 mmol) and 2 ml of the PdPAI solution in
NMP (0.6 mmol of Pd) obtained via anode dissolution as
described in ref. 8 and 19 was deaerated with Ar flow. After-
wards, the reaction mixture was placed in a hermetic vessel of
8 ml volume and vigorously stirred using a shaker (J-KEM
1
nitriles were determined using H, 13C NMR spectra.
Mass-spectra for cyanation products
4-NO2C6H4CN. EI-MS, m/z (I, %): 148 [M]+ (41), 102 [M–NO2]+ Scientific, 200 rpm) under an Ar atmosphere at 100 1C for several
(100), 76 [M–NO2–CN]+ (26).
hours. After cooling to room temperature, the vessel was opened
4-MeOC6H4CN. EI-MS, m/z (I, %): 133 [M+] (100); 107 and a sample for GC-MS analysis was taken. Afterwards, 14.7 mg
[M+–CN] (37); 90 [M+–CN–OMe] (42).
of K4Fe(CN)6 (0.04 mmol) and 106 mg of Na2CO3 (1 mmol) were
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added and the solution was deaerated with Ar again. Then the
p-MeO-C6H4-C6H4-CN. EI-MS, m/z (I, %): 210 [M+] (100); 184
vessel was hermetically closed and heated at 100 1C under stirring [M+–CN] (49); 169 [M+–CN–Me] (51); 153 [M+–CN–OMe] (26).
for the required period. After the reaction came to completion,
50 ml of water saturated with sodium chloride was added to the
reaction mixture in NMP and the organic products were extracted
Acknowledgements
This work was supported by the Russian Foundation for Basic
Research (Project No. 15-33-50364-mol) and Russian President
Foundation (Project No. MK-4875.2016.3).
with diethyl ether (3 ꢁ 15 ml). The ether extract was washed
with water, dried over Na2SO4, and analyzed using GC-MS, with
naphthalene as an internal standard. The products were charac-
terized by comparing their MS and 1H, 13C NMR spectra with those
found in the literature.
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