The synthesis of poly-nitrile aromatic and oligopyridine ligands via palladium-catalyzed cyanation of aryl halides

Article abstract of DOI:10.1055/s-2005-872113

Modification of Seller's palladium-catalyzed cyanation procedure for simple aromatic halides leads to a versatile and rapid route to complex multi-nitrile aryl and oligopyridyl ligands that improves on known literature methods. By heating the reagents in the high boiling solvent mesitylene to reflux temperatures at ambient pressure, we have observed the conversion of halogenated precursors to the corresponding nitrile compounds. The resulting compounds can be precipitated from CH₂Cl₂ solutions of the reaction mixtures and isolated as pure compounds in moderate to high yields. The current approach offers a safer alternative to the pressure tube method, as it does not involve the use of KCN at high pressures. Georg Thieme Verlag Stuttgart.

Full text of DOI:10.1055/s-2005-872113

PAPER
2683
The Synthesis of Poly-Nitrile Aromatic and Oligopyridine Ligands Via
Palladium-Catalyzed Cyanation of Aryl Halides
Synthesis
aof Poly-Nit
c
r
ile Aromat
q
ic and
O
ligo
u
pyridines
L
i
e
gands line M. Veauthier, Christin N. Carlson, Gavin E. Collis, Jaqueline L. Kiplinger, Kevin D. John*
Chemistry Division, MS J582, Los Alamos National Laboratory, Los Alamos, NM 87545, USA
E-mail: kjohn@lanl.gov
Received 8 March 2005; revised 26 April 2005
During the course of our research in this field, we became
interested in aromatic and oligopyridyl ligands that con-
tain multiple nitrile groups (Figure 2, complexes 1–5).
Examination of known literature procedures indicates that
Abstract: Modification of Beller’s palladium-catalyzed cyanation
procedure for simple aromatic halides leads to a versatile and rapid
route to complex multi-nitrile aryl and oligopyridyl ligands that im-
proves on known literature methods. By heating the reagents in the
high boiling solvent mesitylene to reflux temperatures at ambient Potts condensation methods have been used to prepare 4¢-
cyano-2,2¢:6¢,2¢¢-terpyridine (1)7 (overall yield; 4% in five
pressure, we have observed the conversion of halogenated precur-
sors to the corresponding nitrile compounds. The resulting com-
pounds can be precipitated from CH2Cl2 solutions of the reaction
cols have been used to access 6,6¢¢-dicyano-2,2¢:6¢,2¢¢-
mixtures and isolated as pure compounds in moderate to high
yields. The current approach offers a safer alternative to the pres-
steps) and Ochiai’s8 N-oxide addition/elimination proto-
terpyridine (4)9 (overall yield; 46% in 2 steps). Not sur-
prisingly, the more difficult substitution patterns of 5-
cyano-2,2¢-bipyridine (2)10, 5,5¢-dicyano-2,2¢-bipyridine
(3)11 and 1,3,5-tricyanobenzene (5)12 (overall yields; 46%
in two steps for 2, 23–54% in two steps for 3, 40% in two
steps for 5) have necessitated the use of harsh amide de-
hydration procedures to obtain the desired nitrile group.
Deterred by the complexity of the many different synthet-
ic methods required for preparation of these nitrile
ligands, we investigated whether a common simplified
procedure would be feasible.
sure tube method, as it does not involve the use of KCN at high pres-
sures.
Key words: palladium-catalyzed cyanation, aryl halide, bipyridine,
terpyridine, nitrile
The nitrile group is used frequently by organic chemists as
a means to access a diverse range of functionalities (i.e.,
carboxylic acids/esters, aldehydes, amines and
amidines).1 Compounds containing cyano substituents are
also utilized in a variety of commercial applications in-
cluding herbicides,2 drug intermediates,3 and materials
where the electronic nature of the nitrile group is critical
to the intrinsic properties of these materials, such as non-
linear optical (NLO) response.4 Our primary interest in ni-
trile compounds stems from their direct application in the
synthesis of the novel ytterbium mixed-valence 4¢-cyano-
terpyridine adducts5 and in formation of actinide ketimido
complexes6 that can be prepared by reaction of aromatic
nitrile ligands with actinide bisalkyl or bisaryl precursors
(Figure 1).
N
N
N
NC
NC
CN
N
N
N
N
2
3
CN
1
NC
CN
N
NC
N
N
CN
CN
4
5
Figure 2 Target poly-nitrile ligands synthesized in this work.
R
Since Rosenmund–Von Braun13 first discovered that aryl
bromides/iodides could be transformed into aryl nitriles,
albeit with stoichiometric copper(I) cyanide, progress in
this area has been directed at investigating metal catalyzed
systems. Palladium-mediated cyanations14,15 have re-
ceived the most attention as this metal allows access to a
wide range of conditions and also tolerates many func-
tional groups. Of these reports, there is one example
where a 4-chloro-2,2¢:6¢,2¢¢-terpyridine ruthenium com-
plex is converted to the 4-cyano-2,2¢:6¢,2¢¢-terpyridine ru-
thenium derivative using palladium with Zn(CN)2 as the
nitrile source.16 Here, the terpyridine ligand is not avail-
able for further binding. Unfortunately, there are no re-
ported conditions for halide cyanation of non-coordinated
complex heterocyclic ligands. Furthermore, we have fore-
seen that certain coupling reaction conditions {e.g.
N
N
Ph
Ph
N
An
Yb
N
CN
N
R
(A)
(B) An = U, Th; R = Me, Ph, CH2Ph
Figure 1 Lanthanide (A) and actinide (B) complexes employing
aryl-nitrile functionalities.
SYNTHESIS 2005, No. 16, pp 2683–2686
Advanced online publication: 04.08.2005
x
x.
x
x
.
2
0
0
5
DOI: 10.1055/s-2005-872113; Art ID: M01305SS
© Georg Thieme Verlag Stuttgart · New York

2684
J. M. Veauthier et al.
PAPER
CuCN,13c Zn(CN)2,16,17 and K4[Fe(CN)6]15} may not be
appropriate for heterocyclic precursors given that these
ligands (1–5) will be applied in magnetic materials. Thus
we developed a simple and convenient approach for the
synthesis of poly-nitrile complex ligands using palladi-
um-mediated catalysis.
N
N
Pd(OAc)2/dpppe
TMEDA, KCN
N
N
N
N
mesitylene
reflux, 16 h
Cl
CN
6
1
85%
Scheme 1
Initially, we chose Beller’s palladium-catalyzed
procedure14 to convert simple aryl chlorides to aryl nitriles
because the method tolerated a variety of functionalized
aromatic and heteroaromatic species. The reaction of
commercially available 4¢-chloro-2,2¢:6¢,2¢¢-terpyridine
(6) (using Beller’s protocol14) with Pd(OAc)2/dpppe as the
active catalyst in the presence of TMEDA in a pressure
tube containing toluene heated to 160 °C gave discourag-
ing results. Analysis of the crude reaction mixture by GC–
MS indicated that 4¢-cyano-2,2¢:6¢,2¢¢-terpyridine (1)
formed in less than 10% yield, while the majority of the
material was unreacted 4¢-chloroterpyridine (6).
This method was used to provide 4 and 5 in moderate to
high yields (Table 1). Bipyridine derivatives 2 and 3 were
prepared starting from 5-bromo-2,2¢-bipyridine (7) and
5,5¢-dibromo-2,2¢-bipyridine (8) respectively, which are
both accessed in two steps from commercially available
materials.19 The cyanation of 7 and 8 (Scheme 2) gave 2
and 3 directly in 72% and 67% yield respectively. The
overall yields for 2 and 3 from synthesized precursors are
56% and 64%, respectively, which is a moderate improve-
ment form previous methods (Table 1).
We considered that the solubility of the ligand might be a
factor in the poor conversion of the chloride to the nitrile
goup,18 and decided to use conventional reflux conditions
with the high boiling solvent mesitylene, thereby elimi-
nating the need for pressure tube conditions. Accordingly,
using the same amounts of reagents as before, the reaction
was conducted at ambient pressures and reflux tempera-
tures under an inert atmosphere (Scheme 1). The organic
precursor appeared to be completely soluble at reflux tem-
perature. GC–MS analysis of the reaction mixture after 16
hours revealed that 4¢-chloroterpyridine (6) had been
completely consumed and the reaction was clean, afford-
ing 4¢-cyanoterpyridine (1) as the main product. The pure
product 1 was precipitated as a white powder from a
CH2Cl2 solution of the reaction mixture in 85% yield. No
further purification is necessary.
Pd(OAc)2/dpppe
TMEDA, KCN
N
N
R'
R
R
R'
mesitylene
reflux, 16 h
N
N
7 R = H; R' = Br
8 R, R' = Br
2 R = H; R' = CN 72%
3 R, R' = CN 67%
Scheme 2
In conclusion, a simple modification of the palladium-
mediated cyanation protocol developed by Beller and co-
workers15 provides a convenient and safe synthetic route
to mono-, di-, and tri-substituted aryl nitriles in moderate
to high yields. The method appears to tolerate a wide va-
riety of precursors and works well for pyridyl ligands ha-
logenated at the ortho-, meta- and para-positions. This
Table 1 Summary of Cyanations of N-Heterocyclic/Aryl Halides as Compared to Previously Reported, Multi-Step Literature Preparations
Entry
1
Precursor
Product
Yielda (%)
85 (1)
Previously reported yieldb (%)
4 (5)7
N
N
N
N
N
N
Cl
CN
2
3
4
56 (2)b
64 (2)b
70 (1)
46 (2)10
N
N
NC
NC
Br
N
N
N
23–54 (2)11
46 (2)9
N
N
Br
N
Br
CN
N
N
N
N
Br
Br
N
Br
NC
NC
N
CN
5
50 (1)
40 (2)12
CN
Br
CN
Br
a Yields reported are from this work.
b Yields reported are overall yields based on the availability of commercial starting materials. Value in parentheses refers to the total number
of synthetic steps required to obtain the final product.
Synthesis 2005, No. 16, 2683–2686 © Thieme Stuttgart · New York

PAPER
Synthesis of Poly-Nitrile Aromatic and Oligopyridine Ligands
2685
The aqueous slurry from the first filtration step was extracted with
CH2Cl2 (2 × 15 mL) and this was combined with the undissolved
portion. The product 2 was isolated as an off-white powder. Yield:
0.170 g (0.940 mmol, 72%); mp 147–148.5 °C (Lit.10 148–150 °C).
modified procedure uses the identical catalytic system re-
ported by Beller with the exception that toluene (in a
sealed tube at 160 °C) is substituted with mesitylene at
ambient pressure and reflux temperatures. This is a nota-
ble safety advance since it does not require the heating of
a sealed system containing KCN.
IR (mineral oil): 2233 (C≡N) cm–1.
1H NMR (400.13 MHz, CD2Cl2, 25 °C): d = 8.92 (br d, J = 2.0 Hz,
1 H, H6), 8.70 (br dt, J = 4.5, 1.0 Hz, 1 H, H6¢), 8.59 (d, J = 8.3 Hz,
1 H, H3), 8.47 (d, J = 8.0 Hz, 1 H, H3¢), 8.09 (dd, J = 8.3, 2.0 Hz, 1
H, H4), 7.87 (td, J = 8.0, 7.6, 1.7 Hz, 1 H, H4¢), 7.39 (ddd, J = 7.4,
4.5, 1.0 Hz, 1 H, H5¢).
13C{1H} NMR (100.61 MHz, CD2Cl2, 25 °C): d = 159.6, 154.7,
152.5, 150.1, 140.7, 137.7, 125.5, 122.3, 121.1, 117.6 (C≡N), 110.0.
All reactions were carried out at atmospheric pressures unless noted
otherwise (600 mTorr at Los Alamos, NM, elevation 7200 ft). 4¢-
Chloro-2,2¢:6¢,2¢¢-terpyridine [TPY-Cl] (Acros), 6,6¢¢-dibromo-
2,2¢:6¢,2¢¢-terpyridine [TPY-(Br)2] (Aldrich), 1,3,5-tribromoben-
zene (Aldrich), KCN and NaCN (Fisher), palladium(II) acetate
[Pd(OAc)2] (Strem), 1,5-bis(diphenylphosphino)pentane [dpppe]
(Aldrich), mesitylene (Acros), CH2Cl2 (Acros), CD2Cl2 (Cambridge
Isotopes), HPLC grade water (Aldrich), and pentane (Acros) were
used as received. N,N,N¢,N¢-Tetramethylethylenediamine [TME-
DA] (Acros) was passed through an activated column of alumina
prior to use. 5-Bromo-2,2¢-bipyridine and 5,5¢-dibromo-2,2¢-bipyri-
dine were prepared according to the literature procedure.19 Caution:
Cyanide salts are extremely toxic. All reactions should be quenched
with water and the aqueous wash made basic before disposal.
5,5¢-Dicyano-2,2¢-bipyridine (3)
5,5¢-Dicyano-2,2¢-bipyridine was prepared according to the same
method as for TPY-CN from 5,5¢-dibromo-2,2¢-bipyridine (0.400 g,
1.28 mmol), NaCN (0.124 g, 2.54 mmol), Pd(OAc)2 (0.036 g, 0.16
mmol), dpppe (0.15 g, 0.34 mmol), TMEDA (0.13 mL, 0.86 mmol)
and mesitylene (5 mL). Product isolation varied slightly from that
of TPY-CN due to the partial solubility of the product in mesitylene.
The aqueous slurry from the first filtration step was extracted with
CH2Cl2 (2 × 15 mL) and this was combined with the undissolved
portion. The product 3 was isolated as an off-white powder. Yield:
0.176 g (0.859 mmol, 67%); mp 268–269 °C (Lit. 275.4–276.2
°C,11a 284–285 °C,11b 269–271 °C11c).
NMR spectra were recorded at ambient temperature (25 °C) on a
Bruker Avance 400 MHz spectrometer. IR Spectra were recorded
on a Thermo-Nicolet FT-IR module instrument Magna 760 spec-
trometer at 4 cm–1 resolution as mineral oil mulls. GC–MS data
were obtained with a Hewlett Packard 6890 gas chromatograph
configured with a DB-5 column (0.32 mm ID, 30 m) in series with
an HP 5973 quadrupole mass detector. The column was operated at
a flow rate of 0.6 mL/min and ramped between 70–230 °C at a rate
of 20 °C/min.
IR (mineral oil): 2231 (C≡N) cm–1.
1H NMR (400.13 MHz, CD2Cl2, 25 °C): d = 8.97 (d, J = 2.0, 0.8 Hz,
2 H, H6, H6¢), 8.63 (d, J = 8.0, 0.8 Hz, 2 H, H3, H3¢), 8.15 (dd, J =
8.0, 2.0 Hz, 2 H, H4, H4¢).
13C{1H} NMR (100.61 MHz, CD2Cl2, 25 °C): d = 157.5, 152.7,
141.2, 122.1, 117.1 (C≡N), 111.2.
4¢-Cyano-2,2¢:6¢,2¢¢-Terpyridine (1)
TPY-Cl (0.535 g, 2.00 mmol), KCN (0.130 g, 2.00 mmol),
Pd(OAc)2 (0.009 g, 0.04 mmol), dpppe (0.035 g, 0.080 mmol),
TMEDA (0.06 mL, 0.4 mmol) and mesitylene (5 mL) were loaded
into a 25 mL round bottom flask equipped with a stir bar. The flask
was fitted with a reflux condenser and the whole apparatus was
purged under Ar for 10 min before initiating reflux. The contents
were heated to reflux temperatures under Ar for 16 h during which
time the color of the reaction became dark brown. The reaction was
allowed to cool yielding a gray-colored gel. HPLC grade water (10
mL) was added to the reaction flask and the slurry was stirred for 10
min. The slurry was filtered through a medium sintered glass frit
providing a gray solid, which was then dissolved in CH2Cl2 (30 mL)
and the solution was filtered through a medium sintered glass frit.
The solvent was removed under vacuum using a rotatory evaporator
and the powder was rinsed with pentane (3 × 20 mL) on a sintered
glass frit and dried under vacuum for 12 h (at 50 mTorr) to yield 1
as a white powder. Yield: 0.439 g (1.70 mmol, 85%); mp 168–169
°C (Lit.7 168.5–169 °C).
6,6¢¢-Dicyano-2,2¢:6¢,2¢¢-terpyridine (4)
Dicyanoterpyridine was prepared according to the same method as
for TPY-CN from 6,6¢¢-dibromo-2,2¢:6¢,2¢¢-terpyridine (0.782 g,
2.00 mmol), KCN (0.260 g, 4.00 mmol), Pd(OAc)2 (0.018 g, 0.080
mmol), dpppe (0.070 g, 0.16 mmol), TMEDA (0.12 mL, 0.80
mmol) and mesitylene (5 mL). Product isolation varied slightly
from that of TPY-CN due to the partial solubility of the product in
mesitylene. The aqueous slurry from the first filtration step was ex-
tracted with CH2Cl2 (2 × 15 mL) and this was combined with the un-
dissolved portion. The product 4 was isolated as an off-white
powder. Yield: 0.394 g (1.39 mmol, 70%); mp 239–241 °C (Lit.9b
230–232 °C).
IR (mineral oil): 2237 (C≡N) cm–1.
1H NMR (400.13 MHz, CD2Cl2, 25 °C): d = 8.84 (dd, J = 8.0, 1.0
Hz, 2 H, H3, H3¢¢ or H5, H5¢¢), 8.55 (d, J = 8.0 Hz, 2 H, H3¢, H5¢),
8.00–8.10 (overlapping m, 3 H, H4, H4¢, H4¢¢), 7.77 (dd, J = 8.0, 1.0
Hz, 2 H, H3, H3¢¢ or H5, H5¢¢).
13C{1H} NMR (100.61 MHz, CD2Cl2, 25 °C): d = 157.3, 153.6,
138.6, 138.1, 133.2, 128.5, 124.2, 122.3, 117.4 (C≡N).
IR (mineral oil): 2238 (C≡N) cm–1.
1H NMR (400.13 MHz, CD2Cl2, 25 °C): d = 8.72 (overlapping s, dd,
4 H, H6, H3¢, H5¢, H6¢¢), 8.62 (dd, J = 8.0 Hz, 2 H, H3, H3¢¢), 7.91
(dt, 2 H, H4, H4¢¢ or H5, H5¢¢), 7.42 (ddd, 2 H, H4, H4¢¢ or H5, H5¢¢).
1,3,5-Tricyanobenzene (5)
Tricyanobenzene was prepared according to the same method as for
TPY-CN from 1,3,5-tribromobenzene (0.631 g, 2.00 mmol), KCN
(0.390 g, 6.00 mmol), Pd(OAc)2 (0.027 g, 0.12 mmol), dpppe
(0.105 g, 0.240 mmol), TMEDA (0.18 mL, 1.2 mmol) and mesity-
lene (5 mL). Product isolation varied slightly from that of TPY-CN
due to the partial solubility of the product in mesitylene. The aque-
ous slurry from the first filtration step was extracted with CH2Cl2
(2 × 15 mL) and this was combined with the undissolved portion.
The product was isolated as a tan powder. Yield: 0.151 g (0.990
mmol, 50%); compound 5 decomposed above 240 °C (no mp data
reported in the literature).
13C{1H} NMR (100.61 MHz, CD2Cl2, 25 °C): d = 157.3, 154.8,
150.0, 137.7, 125.4, 123.0, 122.9 (C≡N), 121.6, 117.6.
5-Cyano-2,2¢-bipyridine (2)
5-Cyano-2,2¢-bipyridine was prepared according to the same meth-
od as for TPY-CN from 5-bromo-2,2¢-bipyridine (0.305 g, 1.30
mmol), NaCN (0.064 g, 1.3 mmol), Pd(OAc)2 (0.013 g, 0.058
mmol), dpppe (0.053 g, 0.12 mmol), TMEDA (0.08 mL, 0.5 mmol)
and mesitylene (5 mL). Product isolation varied slightly from that
of TPY-CN due to the partial solubility of the product in mesitylene.
Synthesis 2005, No. 16, 2683–2686 © Thieme Stuttgart · New York

2686
J. M. Veauthier et al.
PAPER
IR (mineral oil): 2240 (C≡N) cm–1.
1H NMR (400.13 MHz, CD2Cl2, 25 °C): d = 8.16 (s, 3 H, aromatic).
13C{1H} NMR (100.61 MHz, CD2Cl2, 25 °C): d = 138.9 (CH),
116.1 (CC≡N), 114.8 (CC≡N).
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Transactinium Science Postdoctoral Fellowship Program (J.M.V.,
C.N.C.), and Dr. Paul Plieger for helpful discussions.
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Synthesis 2005, No. 16, 2683–2686 © Thieme Stuttgart · New York