Palladium-catalyzed cross-coupling reaction of trialkylbismuthines with 2-haloazines and diazines

Article abstract of DOI:10.3987/COM-13-S(S)114

An efficient method for the cross-coupling reaction of primary trialkybismuth reagents with 2-haloazines and diazines is reported. The reaction functions with pyridines, pyrimidines, pyridazines and pyrazines and tolerates many functional groups. This method gives access to 2-alkylazines and diazines, a class of compounds which is important in medicinal chemistry.

Full text of DOI:10.3987/COM-13-S(S)114

HETEROCYCLES, Vol. 88, No. 2, 2014
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HETEROCYCLES, Vol. 88, No. 2, 2014, pp. 1615 - 1624. © 2014 The Japan Institute of Heterocyclic Chemistry
Received, 29th August, 2013, Accepted, 24th September, 2013, Published online, 27th September, 2013
DOI: 10.3987/COM-13-S(S)114
PALLADIUM-CATALYZED CROSS-COUPLING REACTION OF
TRIALKYLBISMUTHINES WITH 2-HALOAZINES AND DIAZINES†
Pauline Petiot and Alexandre Gagnon*
Department of Chemistry, UQÀM, C.P. 8888, Succ. Centre-Ville, Montréal,
Québec, Canada, H3C 3P8; E-Mail: gagnon.alexandre@uqam.ca
† This contribution is dedicated to Prof. Victor Snieckus on the occasion of his 77th
birthday and for his outstanding contribution to organometallic chemistry.
Abstract – An efficient method for the cross-coupling reaction of primary
trialkybismuth reagents with 2-haloazines and diazines is reported. The reaction
functions with pyridines, pyrimidines, pyridazines and pyrazines and tolerates
many functional groups. This method gives access to 2-alkylazines and diazines,
a class of compounds which is important in medicinal chemistry.
Azines and diazines are ubiquitous in pharmaceutical industry as they can show a wide diversity of
biological activities.1 The introduction of an alkyl group alpha to the nitrogen of an azine or a diazine
allows to screen the binding environment surrounding a bioactive compound2 and to modulate its
biophysical properties such as its lipophilicity, basicity, and permeability.3 Therefore, the development
of efficient and general methods that allow the preparation of 2-alkylazines and diazines is highly
important for organic and medicinal chemistry. The cross-coupling of sp3 alkylmetal species4 with
azines and diazines bearing an halogen at the 2-position is an attractive approach to access the
corresponding 2-alkyl heterocyclic derivatives. However, this type of coupling is notoriously difficult
and only isolated examples involving organoboronic acids,5 organozincs,6 organomagnesium7 and
organotin8 reagents have been reported. In addition, some of these methods suffer from scope limitation
and poor functional group tolerance, or necessitate strict anhydrous conditions or complex and costly
catalysts to prevent the undesired !-hydride elimination pathway. To the best of our knowledge, there is
currently no comprehensive report on the cross-coupling reaction of alkylmetals with 2-halo pyridines,
pyrimidines, pyridazines and pyrazines. Consequently, there is a well justified need to develop a general
procedure to accomplish this transformation.

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Our group has reported over the past years a portfolio of reactions for the formation of C–C,9 C–O,10 and
C–N11 bonds involving organobismuth reagents. Organobismuthanes have found increasing use in bond
formation due to their unique properties and reactivity.12 These reagents, which can be easily prepared
from inexpensive and non toxic bismuth salts, are very tolerant to a wide diversity of functional groups
and thus represent highly attractive species for methodology development.
We recently reported the cross-coupling of trialkylbismuth reagents with aryl and heteroaryl halides and
pseudo halides13 and demonstrated that triphenethylbismuthine reacts smoothly with
2-chloro-3-cyanopyridine 1 to furnish the corresponding coupling product 2 in good yield (Scheme 1).
Interestingly, and contrary to cross-coupling reactions with other alkymetal species, no product coming
from the !-hydride elimination pathway could be detected under these conditions. We would like to
report herein the full scope of the cross-coupling reaction of trialkylbismuthines with 2-halo- and 2-triflyl-
azines and diazines 3, a process which leads to medicinally relevant 2-alkylazines and diazines 4 (Scheme
1).
•
•
•
•
One example only
Pyridine only
CN
Cl
CN
(PhCH2CH2)3Bi
Pd(0)
previous work
Chloride only
N
N
Functional group
tolerance not demonstrated
1
2 (73%)
•
•
11 examples, up to 86% yield
Pyridines, pyridazines,
pyrimidines, pyrazines
Chlorides, bromides, triflates
Good functional group
tolerance
R3Bi
Pd(0)
this work
FG
N
N
FG
N
X
N
R
•
•
4
3
X = Cl, Br, OTf
R = primary
alkyl group
Scheme 1. Synthesis of 2-alkylazines and diazines by cross-coupling reaction involving
trialkylbismuthines (FG = Functional Group)
We began by optimizing the reaction conditions for the cross-coupling reaction between
tri-n-butylbismuthine and 2-chloro-6-methylpyridine 5 (Table 1). Tri-n-butylbismuthine was prepared
in situ by the addition of n-butylmagnesium bromide over bismuth chloride in THF and was not isolated
(see experimental section). The reaction mixture was diluted with DMF to quench any excess Grignard
reagent remaining and the trialkylbismuth reagent thus formed was used directly in the cross-coupling
reaction. This one-pot protocol avoids any cumbersome isolation of the air sensitive trialkylbismuthine.
Using conditions that we previously reported,13 we obtained the desired product 6 in 32% yield (entry 1).
Changing the catalyst for systems that are more suited for cross-coupling reactions with aryl chorides
proved inefficient in our hands (entry 2 and 3). Replacing the base by potassium phosphate proved to be

HETEROCYCLES, Vol. 88, No. 2, 2014
1617
inconsequential (entry 4) and to our surprise, the reaction proceeded even in the absence of base (entry 5).
While these results suggest that the choice of the base is irrelevant, we found that the yield could be
substantially improved upon using cesium carbonate as the base instead of potassium carbonate (entry 6).
The yield was further ameliorated by using 1.5 or 2.0 equivalents of tributylbismuth (entry 7 and 8).
Performing the reaction with 1.1 or 1.5 equivalent of tributylbismuth at 130 ˚C in the presence of cesium
carbonate provided the best yields of the coupling product (entries 9 and 10).
Table 1. Optimization of reaction conditions for the cross-coupling of tri-n-butylbismuthine with
2-chloro-6-methylpyridine 5
n-Bu3Bi (n eq.)
catalyst (5 mol%)
base (2 eq.)
DMF (0.1M), T, overnight
Me
N
Cl
Me
N
5
6
Entry
n-Bu3Bi
Catalyst
Base
T
Yielda
(%)
32
(n equiv)
1.1
(˚C)
110
110
110
110
110
110
110
110
130
130
1
2
Pd(PPh3)4
Pd(Pt-Bu3)2
PEPPSI-IPr
Pd(PPh3)4
Pd(PPh3)4
Pd(PPh3)4
Pd(PPh3)4
Pd(PPh3)4
Pd(PPh3)4
Pd(PPh3)4
K2CO3
K2CO3
K2CO3
K3PO4
N.A.
1.1
6
3
1.1
0
4
1.1
30
5
1.1
34
6
1.1
Cs2CO3
Cs2CO3
Cs2CO3
Cs2CO3
Cs2CO3
54
7
1.5
65
8
2.0
69
9
1.1
70
10
1.5
66
a Isolated yield of pure compound 6.
Using our optimized conditions, we next investigated the scope of the reaction by coupling different
trialkylbismuthines with 2-halo- and 2-triflylazines and diazines bearing a wide range of functional
groups (Table 2). In order to obtain optimal yields, we elected to perform the reaction using 1.5
equivalents of the organobismuth reagent. Firstly, the results demonstrate that the reaction functions
efficiently with pyrazines (entry 1-3, 10), pyrimidines (entry 4 and 5), pyridazines (entry 6) and pyridines
(entry 7-9). Secondly, the reaction furnishes the coupling product in similar yields upon using the
chloro or bromo electrophile, but leads to modest yields when a 2-triflyl substrate is used (entry 8). The
study indicates that functional groups such as ketones (entry 7), nitriles (entry 9), and amines (entry 10)
are well tolerated. These groups can be used for further functionalization of the product, a feature which

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is important for applications in medicinal chemistry. Enolizable groups that would normally not be
tolerated with Grignard reagents, such as a methyl ketone (entry 7), are unaffected in the course of the
reaction. Attempt at performing the reaction directly with Grignard species under iron catalysis led to
the product of addition on the ketone in low yield. While the reaction functions with most primary
linear alkyl groups, we found that cyclopropyl was the only secondary alkyl group capable of undergoing
cross-coupling under these conditions (entry 2). Aryl and heteroaryl cyclopropanes are very useful in
medicinal chemistry since the cyclopropyl group usually shows higher metabolic stability than acyclic
alkyl groups in the presence of cytochromes P450.14
Table 2. Cross-coupling of trialkylbismuthines with 2-halo- and 2-triflylazines and diazines
conditionsa
FG
N
N
FG
N
X
N
R
4
3
Entry
Electrophile
Product
Yieldb
(%)
N
N
1
86
70
N
3a
N
Cl
N
4a
N
Me
N
N
Cl
2
3
3a
4b
N
N
N
Cl
N
85
3a
4c
N
N
N
Cl
N
4
5
68
65
3d
4d
N
N
N
N
Cl
3e
4e

HETEROCYCLES, Vol. 88, No. 2, 2014
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6
7
46
N
N
N
Cl
N
3f
4f
N
Br
N
48
O
O
3g
4g
8
9
24
45
N
OTf
N
3h
4h
NC
NC
N
Cl
N
4i
N
3i
N
N
N
Cl
N
N
10
71
3j
4j
a R3Bi (1.5 equiv), Pd(PPh3)4 (0.05 equiv), Cs2CO3 (2 equiv), DMF, 130 ˚C, overnight b Isolated yield of
pure compounds.
In summary, we have developed a general procedure for the cross-coupling of trialkylbismuth reagents
with 2-halo- and 2-triflylazines and diazines. The method operates under simple conditions using a
simple catalyst. The procedure tolerates many functional groups and the compounds that are generated
are highly valuable in medicinal chemistry. Studies aimed at transferring other substituted and
functionalized alkyl groups are in progress in our laboratories and results will be reported in due course.
EXPERIMENTAL
All reactions were run under argon atmosphere in non flame dried glassware. Unless otherwise stated,
commercial reagents were used without further purification. Grignard reagents were purchased from
Aldrich or prepared using conventional methods with metallic magnesium and were titrated prior use.15
Anhydrous bismuth chloride 99.999% was purchased from Strem Chemicals. Anhydrous solvents were
obtained using a MBRAUN (model MB-SPS 800) encapsulated solvent purification system. The
evolution of reactions was monitored by analytical thin-layer chromatography using silica gel 60 F254

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precoated plates. Flash chromatography was performed employing 230-400 mesh silica (Silicycle)
using the indicated solvent system according to standard techniques.16 Nuclear magnetic resonance
spectra (1H, C) were recorded on a Bruker Avance-III 300MHz spectrometer. Chemical shifts for
13
1H-NMR spectra are recorded in parts per million from tetramethylsilane with the solvent resonance as
the internal standard (chloroform, ! 7.27 ppm, DMSO ! 2.54 ppm). Data are reported as follows:
chemical shift, multiplicity (s = singlet, d = doublet, t = triplet, q = quartet, m = multiplet), coupling
13
constant J in Hz and integration. Chemical shifts for C spectra are recorded in parts per million from
tetramethylsilane using the central peak of deuterochloroform (77.00 ppm) as the internal standard. All
13C spectra were obtained with complete proton decoupling. IR spectra were recorded on a Thermo
Scientific Nicolet 6700 PT-IR from thin films and are reported in reciprocal centimeters (cm-1).
General procedure for the cross-coupling reaction of trialkylbismuthines with 2-halo and
2-triflylazines and diazines
In a round-bottom flask equiped with a condenser and sparged with argon, BiCl3 (144 mg, 0.46 mmol)
was suspended in anhydrous THF (2.3 mL) and cooled to 0 °C. The organomagnesium reagent (1.37
mmol, THF solution) was slowly added dropwise, and the solution was stirred at 0 °C for 5 min, warmed
to rt, and then heated at 65 °C for 30 min to assure complete addition of all three alkyl groups on the
bismuth center. The reaction mixture was cooled to rt and diluted with anhydrous DMF (1 mL). A
DMF solution of the heteroaryl halide (0.31 mmol in 3 mL) was then added, followed by Cs2CO3 (0.6
mmol) and Pd(PPh3)4 (18 mg, 0.015 mmol). The solution was degassed by bubbling argon during 15
min and then stirred at 130 °C for 18 h. The reaction mixture was cooled to rt, diluted with sat. aq.
NaHCO3 (10 mL) and extracted with EtOAc (2 x 10 mL). The combined organic phases were washed
with brine (3 x 10 mL), dried over Na2SO4, filtered, and concentrated under reduced pressure. The
residue was purified by flash chromatography using the indicated solvent system to afford the desired
product. When necessary, the compound was further purified by preparative thin layer chromatography
using the same solvent system. The alcohol derived from the oxidation of the trialkylbismuth is
sometimes observed as a side product.
2-n-Butyl-6-methylpyridine (6)
The general procedure was followed on a 0.39 mmol scale starting from 2-chloro-6-methylpyridine 5.
The crude material was purified on silica gel (20% EtOAc/hexanes) to afford 6 as a colorless oil (38 mg,
66%). Spectral data was identical to literature compound.17 1H-NMR (300 MHz, CDCl3) ! 7.44 (t, J =
6.2 Hz, 1H), 6.92 (d, J = 8.8 Hz, 2H), 2.73 (t, J = 9.1 Hz, 2H), 2.50 (s, 3H), 1.69-1.63 (m, 2H), 1.40-1.33
(m, 2H), 0.91 (t, J = 9.2 Hz, 3H).
2-Methylquinoxaline (4a)
The general procedure was followed on a 0.31 mmol scale starting from 2-chloroquinoxaline 3a. The

HETEROCYCLES, Vol. 88, No. 2, 2014
1621
crude material was purified on silica gel (40% EtOAc/hexanes) to afford 4a as a colorless oil (38 mg,
86%). Spectral data was identical to literature compound.18 1H-NMR (300 MHz, CDCl3) ! 8.72 (s,
1H), 8.07-7.98 (m, 2H), 7.75-7.65 (m, 2H), 2.76 (s, 3H).
2-Cyclopropylquinoxaline (4b)
The general procedure was followed on a 0.31 mmol scale starting from 2-chloroquinoxaline 3a. The
crude material was purified on silica gel (15% EtOAc/hexanes) to afford 4b as a colorless oil (36 mg,
70%). Spectral data was identical to literature compound.19 1H-NMR (300 MHz, CDCl3) ! 8.71 (s,
1H), 8.03 (dd, J = 8.3, 1.2 Hz, 1H), 7.92 (dd, J = 8.3, 1.2 Hz, 1H), 7.70-7.58 (m, 2H), 2.29-2.21 (m, 1H),
1.26-1.23 (m, 2H), 1.18-1.13 (m, 2H).
2-Phenethylquinoxaline (4c)
The general procedure was followed on a 0.31 mmol scale starting from 2-chloroquinoxaline 3a. The
crude material was purified on silica gel (15% EtOAc/hexanes) to afford 4c as a colorless oil (60 mg,
85%). Spectral data was identical to literature compound.20 1H-NMR (300 MHz, CDCl3) ! 8.69 (s,
1H), 8.13 (dd, J = 8.0, 2.0 Hz, 2H), 7.83-7.73 (m, 2H), 7.35-7.26 (m, 5H), 3.42-3.37 (m, 2H), 3.27-3.22
(m, 2H).
2-Phenethylpyrimidine (4d)
The general procedure was followed on a 0.44 mmol scale starting from 2-chloropyrimidine 3d. The
crude material was purified on silica gel (30% EtOAc/hexanes) to afford 4d as a colorless oil (54 mg,
68%). Spectral data was identical to literature compound.20 1H-NMR (300 MHz, CDCl3) ! 8.73 (d, J =
6.3 Hz, 2H), 7.35-7.22 (m, 5H), 7.13 (t, J = 8.9 Hz, 1H), 3.36-3.29 (m, 2H), 3.24-3.18 (m, 2H).
5-Ethyl-2-n-propylpyrimidine (4e)
The general procedure was followed on a 0.35 mmol scale starting from 2-chloro-5-ethylpyrimidine 3e.
The crude material was purified on silica gel (20% EtOAc/hexanes) to afford 4e as a colorless oil (28 mg,
53%): Rf 0.30 (25% EtOAc/hexanes); 1H NMR (300 MHz, CDCl3) ! 8.46 (s, 2H), 2.88 (t, J = 8.1 Hz, 2H),
2.60 (q, J = 8.2 Hz, 2H), 1.85-1.77 (m, 2H), 1.24 (t, J = 7.9 Hz, 3H), 0.96 (t, J = 8.2 Hz, 3H); 13C-NMR
(75 MHz, CDCl3) ! 168.9, 156.4, 133.2, 41.0, 23.3, 22.1, 15.0, 13.9; IR (neat) 3431, 2960, 2926, 1713,
1633, 1466.
3-n-Butyl-6-phenylpyridazine (4f)
The general procedure was followed on a 0.26 mmol scale starting from 3-chloro-6-phenylpyridazine 3f.
The crude material was purified on silica gel (15% EtOAc/hexanes) to afford 4f as a colorless oil (24 mg,
46%). Spectral data was identical to literature compound.21 1H-NMR (300 MHz, CDCl3) ! 8.05 (dd, J
= 8.2, 1.9 Hz, 2H), 7.75 (d, J = 8.8 Hz, 1H), 7.49-7.46 (m, 3H), 7.35 (d, J = 8.8 Hz, 1H), 3.01 (t, J = 7.7
Hz, 2H), 1.82-1.74 (m, 2H), 1.46-1.39 (m, 2H), 0.96 (t, J = 7.3 Hz, 3H).
2-Acetyl-6-phenethylpyridine (4g)

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HETEROCYCLES, Vol. 88, No. 2, 2014
The general procedure was followed on a 0.25 mmol scale starting from 2-acetyl-6-bromopyridine 3g.
The crude material was purified on silica gel (15% EtOAc/hexanes) to afford 4g as a colorless oil (27 mg,
48%): Rf 0.56 (20% EtOAc/hexanes); 1H-NMR (300 MHz, CDCl3) ! 7.91 (dd, J = 7.9, 0.7 Hz, 1H), 7.73
13
(t, J = 7.7 Hz, 1H), 7.35-7.23 (m, 6H), 3.24-3.15 (m, 4H), 2.78 (s, 3H); C-NMR (75 MHz, CDCl3) !
200.8, 160.7, 153.3, 141.4, 136.9, 128.5, 128.4, 126.4, 126.1, 119.0, 39.6, 35.4, 25.8; IR (neat) 3027,
2920, 1697, 1586, 1453, 1355.
2-Ethyl-5-methylpyridine (4h)
The general procedure was followed on a 0.21 mmol scale starting from 2-triflyl-5-methylpyridine 3h.
The crude material was purified on silica gel (20% EtOAc/hexanes) to afford 4h as a colorless oil (6 mg,
24%). Spectral data was identical to literature compound.22 1H-NMR (300 MHz, CDCl3) ! 8.34 (d, J =
1.5 Hz, 1H), 7.41 (dd, J = 7.8, 1.9 Hz, 1H), 7.06 (d, J = 7.9 Hz, 1H), 2.79 (q, J = 7.3 Hz, 2H), 2.29 (s, 3H),
1.28 (t, J = 7.6 Hz, 3H).
6-Ethyl-3-cyanopyridine (4i)
The general procedure was followed on a 0.36 mmol scale starting from 6-chloro-3-pyridinecarbonitrile
3i. The crude material was purified on silica gel (20% EtOAc/hexanes) to afford 4i as a colorless oil (17
mg, 40%). Spectral data was identical to literature compound. 1H-NMR (300 MHz, CDCl3) ! 8.73 (d,
J = 1.6 Hz, 1H), 7.80 (dd, J = 8.1, 2.2 Hz, 1H), 7.21 (d, J = 8.2 Hz, 1H), 2.83 (q, J = 7.6 Hz, 2H), 1.26 (t,
13
J = 7.6 Hz, 3H); C-NMR (75 MHz, CDCl3) 168.1, 152.1, 139.4, 122.2, 117.0, 107.2, 31.8, 13.3; IR
(neat) 3432, 2972, 2933, 2232, 1594, 1484.
2-Ethyl-6-dimethylaminepyrazine (4j)
The general procedure was followed on a 0.32 mmol scale starting from 2-chloro-6-dimethylamine
pyridine 3j. The crude material was purified on silica gel (35% EtOAc/hexanes) to afford 4j as a
1
colorless oil (34 mg, 71%): Rf 0.36 (20% EtOAc/hexanes); H-NMR (300 MHz, CDCl3) ! 7.80 (s, 1H),
7.65 (s, 1H), 3.08 (s, 6H), 2.62 (q, J = 7.6 Hz, 2H), 1.25 (t, J = 7.6 Hz, 3H); 13C-NMR (75 MHz, CDCl3)
! 155.6, 154.5, 130.0, 126.7, 37.4, 28.5, 13.3; IR (neat) 2967, 2933, 1575, 1529, 1423, 1194.
ACKNOWLEDGEMENTS
This work was supported by Natural Sciences and Engineering Research Council of Canada (NSERC)
and by the Université du Québec à Montréal. P.P. is grateful to Université du Québec à Montréal for a
FARE scholarship.
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