Synthesis of functionalized 4-phenyl-pyridines via electrochemically prepared organozinc reagents

Article abstract of DOI:10.1016/S0040-4020(01)00015-1

The efficient and convenient synthesis of various functionalized 4-phenyl-pyridines 2 is described. The key step of the procedure is the electrochemical formation of aromatic organozinc reagents 1 and their coupling with pyridinium salts. Intermediate 1,4

Full text of DOI:10.1016/S0040-4020(01)00015-1

TETRAHEDRON
Pergamon
Tetrahedron 57 (2001) 1923±1927
Synthesis of functionalized 4-phenyl-pyridines via
electrochemically prepared organozinc reagents
p
Erwan Le Gall, Corinne Gosmini, Jean-Yves Nedelec and Jacques Perichon
 Â
Â
Á
Laboratoire d'Electrochimie, Catalyse et Synthese Organique, LECSO, CNRS, UMR 7582, 2-8 rue Henri Dunant, 94320 Thiais, France
Received 5 October 2000; revised 30 November 2000; accepted 21 December 2000
AbstractÐThe ef®cient and convenient synthesis of various functionalized 4-phenyl-pyridines 2 is described. The key step of the procedure
is the electrochemical formation of aromatic organozinc reagents 1 and their coupling with pyridinium salts. Intermediate 1,4-dihydro-
pyridines are oxidized using mild conditions to provide functionalized 4-phenyl-pyridines in moderate to high overall yields. q 2001
Elsevier Science Ltd. All rights reserved.
1. Introduction
cross-coupling of aromatic compounds our aim was to
achieve the synthesis of various functionalized 4-phenyl-
pyridines. Unfortunately, usual protocols involving nickel-
catalysts and halogenated compounds were inef®cient. We
now report herein a very convenient synthesis of various
4-phenyl-pyridines from functionalized phenyl bromides
and pyridine as starting compounds, and using the cobalt-
catalyzed electrosynthesis of organozinc reagents as the
key-step.
Biaryl compounds and especially 2- and 4-phenyl-pyridines
are of high interest in organic chemistry due to their
pharmaceutical and agrochemical activities1 and several
synthetic procedures including catalytic processes have
appeared during the last years.2 In numerous works devoted
to the cross-coupling of aromatic species, the Suzuki reac-
tion,3 which is a powerful tool in this goal, has been widely
used. For instance, Mitchell et al.4 and Lohse et al.5 have
successfully employed this method, starting from phenyl-
boronic acids and chlorinated heteroaromatic amines in the
presence of palladium, to obtain the cross-coupling products
in moderate to high yields. Other procedures avoiding the
use of expensive aromatic halogenated heterocycles were
developed in order to achieve the synthesis of 4-substituted
pyridines. Comins et al.6 have thus shown that Grignard
reagents are reactive towards pyridinium salts and lead to
4-substituted 1,4-dihydropyridines which can be further
oxidized by ortho-chloranil. However, the scope of this
reaction is limited by the instability7,8 and the high reactivity
of Grignard reagents which discard the presence of electro-
philic groups on starting compounds. This drawback can be
circumvent by the use of organozinc reagents9 as shown by
Shiao et al.10 but despite its high synthetic utility, the use of
dif®cult to handle activated zinc11 makes this procedure
very sensitive.
2. Results and discussion
2.1. Electrosynthesis of organozinc reagents
It has been previously shown that organozinc reagents are
valuable precursors of 4-substituted pyridines but curiously,
we could not ®nd in the literature studies dealing with the
coupling between various functionalized organozinc
reagents and pyridinium salts as outlined in Scheme 1.
Our ®rst investigations were thus devoted to ®nding the
most simple and ef®cient conditions to achieve the electro-
synthesis of functionalized aromatic organozinc reagents. A
typical electrolysis medium was found as a mixture of
acetonitrile, dimethylformamide and pyridine as solvents
and catalytic amounts of cobalt chloride and zinc bromide.
The organozinc compounds were prepared in an undivided
electrochemical cell15 ¯ushed with argon and ®tted with a
zinc rod as the anode and a stainless steel grid as the
cathode. The electrolyses of various phenyl bromides
were carried out at ambient temperature and at constant
current intensity of 0.2 A until a typical charge of 2±2.1 F
per mole of the halide was passed (Scheme 2).
Among the numerous topics developed in this laboratory,
the electrochemical synthesis of organozinc reagents is of
current interest.12,13 These organometallic species are
synthesized in high yields using mild reaction conditions.14
As a part of our program devoted to the electrochemical
Keywords: 4-phenyl-pyridines; pyridinium.
Corresponding author. Tel.: 11-49-781139; fax: 11-49781148;
e-mail: gosmini@glvt-cnrs.fr
p
Yields of organozinc compounds thus obtained were esti-
mated as follows: samples of the electrolysis solutions were
0040±4020/01/$ - see front matter q 2001 Elsevier Science Ltd. All rights reserved.
PII: S0040-4020(01)00015-1

1924
E. Le Gall et al. / Tetrahedron 57 (2001) 1923±1927
Scheme 1.
exposed to iodine crystals then sodium thiosulfate, and
extracted with diethyl ether. Amounts of iodinated
compounds were compared to the amounts of starting
phenyl bromides via an internal standard using gas chroma-
tography. Yields of organozinc compounds are reported in
Table 1.
co-solvent and ligand, we thought that simply adding
methyl chloroformate to the solution of the mixed
copper±zinc organometallic species would lead to the
desired coupling product. It should be mentioned that in
that case the use of preformed pyridinium salts of the
aromatic amines is generally described in the literature.
Methyl chloroformate (1 equiv.) was then added at 08C to
the solution which was allowed to warm to room tempera-
ture during 2 h. The 4-substituted-1,4-dihydropyridine thus
obtained was oxidized without being isolated using silica
gel and oxygen (24±72 h, rt, method A) or using hydrogen
peroxide and acetic acid (12 h, rt, method B) when method
A could not suit. Crude products were puri®ed using silica
gel chromatography with diethyl ether as an eluent to afford
the desired 4-phenyl-pyridines. Results are reported in
Table 2.
The ®rst point to note is that with both electron-donating
and -withdrawing groups satisfactory to high yields are
obtained (54±85%). The electrochemical synthesis of
substituted phenylzinc compounds can be considered as
versatile, with only a few limitations. Thus, when FGˆ
o-CO2Et (entry 3) only homocoupling occurs resulting in
the detection of the sole biaryl product. Also, the failure
observed when FGˆp-NHBOC (entry 17) is likely the
consequence of the high basicity of the organozinc
compound which fast deprotonates this function allowing
only the detection of the reduction product (C6H5±
NHBOC).
Moderate to high overall yields are obtained using this
procedure which can be applied to a wide range of func-
tionalized phenyl bromides. It can be noticed that when an
amino group is involved (entry 16) no coupling products can
be isolated from the reaction mixture. This result may be
explained by the nucleophicity of NH2 which may also react
with methyl chloroformate, and hence numerous side-
products can also be formed.
The numerous bene®ts of this convenient one-pot procedure
have prompted us to investigate the coupling of these
electrogenerated phenylzinc compounds with pyridinium
salts.
2.2. Coupling between mixed copper±zinc organo-
metallic species and pyridinium salts
Electron-donating groups allow the smooth oxidation of
intermediate dihydropyridines whereas with some elec-
tron-withdrawing groups (entries 7±10) the use of hydrogen
A different approach of previously described sequences
allowing the synthesis of 4-phenyl-pyridines was con-
sidered, as outlined in Scheme 3. This approach was direc-
ted towards ®nding the most simple conditions for all the
chemical steps of the sequence. Thus, after completion of
the formation of organozinc species the electrolysis solu-
tions were cooled to 08C. A mixture of cuprous cyanide
(1 equiv.) and lithium chloride (2 equiv.) was then added
and allowed to react during 2±5 min. Longer reaction
times or higher temperatures proved to rise dramatically
the proportions of biaryl compounds when the phenyl
moiety was bearing electron-withdrawing groups and thus
only low yields were obtained.
Table 1. Yields of electrochemically prepared organozinc reagents
Entry
ArBr
(mmol)
FG
Faradic yield (%)
[Charge passed (C)]a
ArZnBr 1 (%)b
1
2
3
4
5
6
7
8
10
10
10
10
10
10
10
10
10
10
10
10
10
10
10
10
10
10
p-CO2Et
m-CO2Et
o-CO2Et
p-COCH3
m-COCH3
p-CN
100 [1950]
90 [2150]
±
75
76
±
100 [1950]
95 [2050]
100 [1950]
95 [2050]
100 [1950]
90 [2150]
95 [2050]
90 [2050]
95 [2050]
55 [3500]
95 [2050]
100 [1950]
100 [1950]
±
68
75
75
73
74
80
80
65
85
70
78
60
54
±
p-CF3
m-CF3
p-F
m-Cl
p-OMe
m-OMe
o-OMe
p-Me
9
10
11
12
13
14
15
16
17
18
Considering the fact that this reaction sequence involves the
formation of a pyridinium ion and that during the ®rst step
pyridine was present in the electrolysis medium, acting as
p-iPr
p-NH2
p-NHBOC
3,4-
95 [2050]
83
a
Faradic yield, based on Faraday's law: Yieldˆ[[nF(m/M)]/(it)]100.
b
Estimated using gas chromatography.
Scheme 2.

E. Le Gall et al. / Tetrahedron 57 (2001) 1923±1927
1925
Scheme 3.
peroxide and acetic acid is required. The low overall yield
observed when two electron-donating groups are on the
phenyl ring (entry 18) seems paradoxical since the related
organozinc compound was obtained in 83% yield. However,
this result can be explained by the fact that the intermediate
1,4-dihydropyridine proved to be very sensitive to oxygen
and fast darkened when adsorbed on silica gel and exposed
to air.
4. Experimental
4.1. Typical procedure
The electrochemical cell ®tted with a zinc rod as the anode
and a stainless steel grid as the cathode was ¯ushed with
argon. Acetonitrile (40 ml), dimethylformamide (5 ml) and
pyridine (5 ml) were added using a syringe. To this solution
were added anhydrous zinc chloride (0.7 g, 0.3 equiv.),
cobalt chloride (0.2 g, 0.13 equiv.) and tetrabutylammo-
nium tetra¯uoroborate in order to rise the conductivity of
the medium. The functionalized phenyl bromide (10 mmol,
1 equiv.) and dodecane (0.2 ml) as internal standard were
added, and a constant current intensity of 0.2 A was applied
until complete disappearance of the starting compound
(ideally 1930 C). The reaction was monitored using gas
chromatography after iodination of the intermediate
organozinc compound. The solution was then cooled to
08C and a mixture of cuprous cyanide (0.9 g, 1 equiv.) and
lithium chloride (0.9 g, 2 equiv.) was added under vigorous
stirring. After 2±5 min, methyl chloroformiate (0.8 ml,
3. Conclusion
In conclusion, the results reported in this paper show that the
electrosynthesis of various functionalized aromatic organo-
zinc reagents and their coupling with pyridine can be
realized in a simple and ef®cient manner. The major
advantage of this method relies on the use of mild and
simple reaction conditions since electrolyses are conducted
at room temperature and that all steps of the reaction
sequence except the oxidation one are carried out one-pot.
Table 2. Yields of functionalized 4-phenyl-pyridines
Entry
FG
Oxidation methoda
2 Overall isolated yield (%)b
Compound
1
2
4
5
6
7
8
9
10
11
12
13
14
15
16
18
p-CO2Et
A
A
A
A
A
B
B
B
B
A
A
A
A
A
±
45
50
39
56
43
44
47
55
66
45
54
53
69
47
±
2a
2b
2c
2d
2e
2f
2g
2h
2i
2j
2k
2l
2m
2n
±
m-CO2Et
p-COCH3
m-COCH3
p-CN
p-CF3
m-CF3
p-F
m-Cl
p-Ome
m-Ome
o-Ome
p-Me
p-iPr
p-NH2
3,4-OCH2CH2O-
A
35
2o
a
Method A: SiO2, O2, rt, 24±72 h; method B: H2O2, CH3CO2H, DMF, rt, 12 h.
Based on starting ArBr.
b

1926
E. Le Gall et al. / Tetrahedron 57 (2001) 1923±1927
1 equiv.) was added using a syringe and the reactional
mixture was allowed to warm over 2 h. The mixture was
then poured into 50 ml of a saturated ammonium chloride
solution. After evaporation of acetonitrile under reduced
pressure, sodium chloride was added to the aqueous layer
which was extracted with 3£100 ml diethyl ether. The
organic layer was dried over magnesium sulfate and evapo-
rated to dryness. The resulting oil was oxidized according to
either method A or B.
4.2.3. 4-Pyridin-4-yl-benzophenone 2c.17 Light brown
1
solid, H NMR, d (ppm): 2.58 (s, 3H), 7.47 (d, Jˆ6.1 Hz,
2H), 7.66 (AB, Jˆ8.4 Hz, 2H), 8.00 (AB, Jˆ8.4 Hz, 2H),
8.63 (d, Jˆ6.1 Hz, 2H). MS, m/z (relative intensity): 197
(M, 23), 182 (M-15, 100), 154 (M-43, 20), 127 (M-70, 9).
4.2.4. 3-Pyridin-4-yl-benzophenone 2d.p Light brown oil,
1H NMR, d (ppm): 2.59 (s, 3H), 7.40±7.58 (m, 3H), 7.76 (d,
Jˆ7.8 Hz, 1H), 7.94 (d, Jˆ7.8 Hz, 1H), 8.15 (s, 1H),
8.60 (broad s, 2H). 13C NMR, d (ppm): 26.54, 121.45,
126.45, 128.77, 129.29, 131.25, 137.63, 138.36, 147.15,
150.58, 197.41. MS, m/z (relative intensity): 197 (M,
36), 182 (M-15, 100), 154 (M-43, 21), 127 (M-70, 7).
HRMS, Exact calcd for C13H11NO: 197.08406; Found:
197.08460.
4.1.1. Oxidation method A. The crude oil was dissolved in
50 ml dichloromethane. Silica gel (10 g) was added to the
solution, the solvent removed under atmospheric pressure
and the adsorbed organic compound was placed in a large
container and allowed to react with air during 24±72 h. The
crude product was puri®ed using silica gel chromatography
with diethyl ether as eluent.
4.2.5. 4-Pyridin-4-yl-benzonitrile 2e.18 Light brown solid,
1H NMR, d (ppm): 7.43 (d, Jˆ5.9 Hz, 2H), 7.68 (AB, Jˆ
2.3 Hz, 2H), 7.71 (AB, Jˆ2.3 Hz, 2H), 8.65 (d, Jˆ5.9 Hz,
2H). MS, m/z (relative intensity): 180 (M, 100), 153 (M-27,
10).
4.1.2. Oxidation method B. The crude oil was dissolved in
10 ml dimethylformamide. To the solution maintained at
room temperature using a large water bath were added drop-
wise 1.5 ml acetic acid and 1.5 ml of a 30% hydrogen
peroxide solution. After 12 h, 50 ml of water were added
to the solution. The resulting mixture was cooled to 08C and
sodium hydroxide pellets were added under vigorous
stirring until pHˆ12. The aqueous layer was extracted
with 3£100 ml dichloromethane, the solvent dried over
magnesium sulfate and evaporated to dryness. The crude
product was puri®ed using silica gel chromatography with
diethyl ether as eluent.
4.2.6. 4-(4-Tri¯uoromethyl-phenyl)-pyridine 2f.p Pale
1
yellow oil, H NMR, d (ppm): 7.58 (d, Jˆ6.0 Hz, 2H),
7.81 (s, 4H), 8.78 (d, Jˆ6.0 Hz, 2H). 13C NMR, d (ppm):
121.57, 125.87, 125.94, 127.26, 141.51, 146.77, 150.24. 19F
NMR, d (ppm): 262.36. MS, m/z (relative intensity): 223
(M, 100), 154 (M-69, 22), 127 (M-96, 5). HRMS, Exact
calcd for C12H8NF3: 223.06088; Found: 223.06130.
4.2.7. 4-(3-Tri¯uoromethyl-phenyl)-pyridine 2g.p Pale
1
yellow oil, H NMR, d (ppm): 7.20±7.81 (m, 6H), 8.52
(broad s, 2H). 13C NMR, d (ppm): 121.32, 123.45, 123.52,
125.36, 125.43, 129.39, 130.00, 138.61, 146.42, 150.01. 19F
NMR, d (ppm): 262.61. MS, m/z (relative intensity): 223
(M, 100), 202 (M-21, 2), 153 (M-70, 3). HRMS, Exact calcd
for C12H8NF3: 223.06088; Found: 223.06110.
4.2. General data and product analysis
Unless otherwise speci®ed, materials were purchased from
commercial suppliers and used without further puri®cation.
Chromatographic separations were realized using Merck 60
ACC (70±200 mesh) silica gel. Gas chromatography
analyses were performed on a 25 m CPSIL-5CB column
using a Varian 3400 CX chromatograph. Mass spectra
were recorded on a Finnigan GC/MS GCQ spectrometer.
1H and 13C NMR spectra were recorded in CDCl3 solutions
on a Bruker AC200 spectrometer. Data are presented as
follows: chemical shift (multiplicity, coupling constants,
number of protons). Compounds which have been
previously described in the literature ®t with the structure
data found in the given references, whereas new compounds
labeled by asterisk (p) have been fully characterized.
4.2.8. 4-(4-Fluoro-phenyl)-pyridine 2h.19 Pale yellow
1
solid, H NMR, d (ppm): 7.00±7.65 (m, 6H), 8.64 (d, Jˆ
5.6 Hz, 2H). MS, m/z (relative intensity): 173 (M, 100), 146
(M-27, 12), 74 (M-99, 4).
4.2.9. 4-(3-Chloro-phenyl)-pyridine 2i.20 Pale yellow oil,
1H NMR, d (ppm): 7.30±7.60 (m, 6H), 8.59 (d, Jˆ5.8 Hz,
2H). MS, m/z (relative intensity): 189 (M, 100), 154 (M-35,
31), 127 (M-62, 19).
4.2.10. 4-(4-Methoxy-phenyl)-pyridine 2j.21 Pale yellow
solid, 1H NMR, d (ppm): 3.84 (s, 3H), 7.00 (AB, Jˆ
8.5 Hz, 2H), 7.47 (d, Jˆ6.0 Hz, 2H), 7.60 (AB, Jˆ8.5 Hz,
2H), 8.62 (broad s, 2H). MS, m/z (relative intensity): 185
(M, 100), 170 (M-15, 15), 115 (M-70, 9).
4.2.1. 4-Pyridin-4-yl-benzoic acid ethyl ester 2a.16 Pale
1
yellow solid, H NMR, d (ppm): 1.30 (t, Jˆ7.1 Hz, 3H),
4.30 (q, Jˆ7.1 Hz, 2H), 7.40 (d, Jˆ5.9 Hz, 2H), 7.55 (AB,
Jˆ8.4 Hz, 2H), 8.30 (AB, Jˆ8.4 Hz, 2H), 8.55 (broad s,
2H). MS, m/z (relative intensity): 227 (M, 85), 199 (M-28,
100), 182 (M-45, 60), 154 (M-73, 20).
4.2.11. 4-(3-Methoxy-phenyl)-pyridine 2k.21 Pale yellow
oil, H NMR, d (ppm): 3.80 (s, 3H), 6.85±7.45 (m, 6H),
1
4.2.2. 3-Pyridin-4-yl-benzoic acid ethyl ester 2b.16 Pale
8.58 (broad s, 2H). MS, m/z (relative intensity): 185 (M,
100), 155 (M-30, 21), 115 (M-70, 11).
1
yellow oil, H NMR, d (ppm): 1.37 (t, Jˆ7.1 Hz, 3H),
4.37 (q, Jˆ7.1 Hz, 2H), 7.40±7.58 (m, 3H), 7.76 (d,
Jˆ7.8 Hz, 1H), 8.06 (d, Jˆ7.8 Hz, 1H), 8.26 (t, Jˆ1.6 Hz,
1H), 8.65 (broad s, 2H). MS, m/z (relative intensity): 227
(M, 65), 199 (M-28, 100), 182 (M-45, 74), 154 (M-73, 33),
127 (M-100, 15).
4.2.12. 4-(2-Methoxy-phenyl)-pyridine 2l.22 Pale yellow
oil, H NMR, d (ppm): 3.75 (s, 3H), 6.82±7.05 (m, 6H),
1
8.53 (d, Jˆ5.6 Hz, 2H). MS, m/z (relative intensity): 185
(M, 100), 170 (M-15, 37), 156 (M-29, 14), 115 (M-70, 38).

E. Le Gall et al. / Tetrahedron 57 (2001) 1923±1927
1927
4.2.13. 4-(4-Tolyl)-pyridine 2m.21 Light brown solid, H
NMR, d (ppm): 2.46 (s, 3H), 7.35 (AB, Jˆ9 Hz, 2H),
7.47±7.66 (m, 4H), 8.67 (broad s, 2H). MS, m/z (relative
intensity): 168 (M-1, 100), 141 (M-27, 10).
1
7. Harvey, S.; Junk, P. C.; Raston, C. L.; Salem, G. J. Org. Chem.
1988, 53, 3134±3140.
8. Parham, W. E.; Jones, L. D.; Sayed, Y. A. J. Org. Chem. 1976,
41, 1184±1191.
9. For a review dealing with synthetic applications of organozinc
reagents see: Knochel, P.; Singer, R. D. Chem. Rev. 1993, 93,
2117±2188.
4.2.14. 4-(4-Isopropyl-phenyl)-pyridine 2n.23 Light brown
solid, 1H NMR, d (ppm): 1.24 (d, Jˆ6.9 Hz, 6H), 2.92 (hept,
Jˆ6.9 Hz, 1H), 7.30 (AB, Jˆ8.2 Hz, 2H), 7.45 (d, Jˆ
5.5 Hz, 2H), 7.48 (AB, Jˆ8.2 Hz, 2H), 8.58 (broad s, 2H).
MS, m/z (relative intensity): 197 (M, 91), 183 (M-14, 100),
167 (M-30, 45).
10. (a) Chia, W. L.; Shiao, M. J. Tetrahedron Lett. 1991, 32,
2033±2034. (b) Shiao, M. J.; Chia, W. L.; Shing, T. L.;
Chow, T. J. J. Chem. Res. (s) 1992, 247.
11. Zhu, L.; Wehmeyer, R. H.; Rieke, R. D. J. Org. Chem. 1991,
56, 1445±1453.
4.2.15. 4-Benzo[1,3]dioxo-5-yl-pyridine 2o.p Light brown
solid, 1H NMR, d (ppm): 5.99 (s, 2H), 6.87 (AB, Jˆ7.9 Hz,
1H), 7.05±7.15 (m, 2H), 7.38 (d, Jˆ5.5 Hz, 2H), 8.56 (d,
Jˆ5.5 Hz, 2H). 13C NMR, d (ppm): 101.44, 107.11, 108.79,
120.89, 121.18, 132.05, 147.89, 148.45, 148.52, 150.02.
MS, m/z (relative intensity): 199 (M, 100), 140 (M-59, 5),
114 (M-85, 6). HRMS, Exact calcd for C12H9NO2:
199.06332; Found: 199.06390.
Â
12. Perichon, J.; Gosmini, C.; Rollin, Y. In Organozinc Reagents;
Knochel, P., Jones, P., Eds.; Oxford University Press: Oxford,
1999; pp 139±156 (Chapter 8).
  Â
13. (a) Gosmini, C.; Lasry, S.; Nedelec, J. Y.; Perichon, J.
Tetrahedron 1998, 54, 1289±1298. (b) Sibille, S.;
  Â
Ratovelomanana, V.; Nedelec, J. Y.; Perichon, J. Synlett
1993, 425±426.
14. (a) Patent application 99/08480, France,
  Â
(b) Gosmini, C.; Rollin, Y.; Nedelec, J. Y.; Perichon, J.
1 July 1999
J. Org. Chem. 2000, 65, 6024±6026.
15. Troupel, M.; Rollin, Y.; Sock, O.; Meyer, G.; Perichon, J.
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