First regioselective c-2 lithiation of 3- and 4-chloropyridines

Article abstract of DOI:10.1002/1099-0690(200102)2001:3<603::AID-EJOC603>3.0.CO;2-2

We have shown that the BuLi/LiDMAE reagent promotes the clean and regioselective C2 lithiation of 3- and 4-chloropyridines, while other reagents such as LDA or BuLi/TMEDA lead to classical ortho lithiation products or mixtures of regioisomers. The method was successfully applied to the preparation of various reactive 2,3- and 2,4-disubstituted pyridines.

Full text of DOI:10.1002/1099-0690(200102)2001:3<603::AID-EJOC603>3.0.CO;2-2

FULL PAPER
First Regioselective C-2 Lithiation of 3- and 4-Chloropyridines
Sabine Choppin,[a] Philippe Gros,[a] and Yves Fort*[a]
Keywords: Chloropyridines / Lithiation / Regioselectivity / Complexation
We have shown that the BuLi/LiDMAE reagent promotes the
gioisomers. The method was successfully applied to the pre-
clean and regioselective C2 lithiation of 3- and 4-chloropyri- paration of various reactive 2,3- and 2,4-disubstituted pyrid-
dines, while other reagents such as LDA or BuLi/TMEDA
ines.
lead to classical ortho lithiation products or mixtures of re-
Introduction
Table 1. Reaction of 1 with lithiating reagents[a]
Chloropyridines are particularly versatile compounds. In-
deed, their CϪCl bond makes them suitable for subsequent
SNAr[1] or organometallic coupling reactions,[2] while the
presence of acidic protons on the pyridine ring offers addi-
tional sites for the introduction of new functional groups.
Thus, the design of methods allowing for the metallation of
chloropyridines with retention of the CϪCl bond is of great
synthetic value. Recently, we reported that the BuLi/Me2N-
(CH2)2OLi (BuLi/LiDMAE)[3] reagent performed the re-
gioselective C-6 lithiation of 2-chloropyridine with com-
plete retention of the CϪCl bond.[4] This unprecedented
high level of CϪCl bond tolerance by a BuLi-containing
reagent[5] prompted us to further investigate the lithiation
of 3- and 4-chloropyridines. We now report that the BuLi/
LiDMAE superbase promotes the clean regioselective C-2
metallation of 3- and 4-chloropyridines, opening a route to
new reactive 2,3- and 2,4-disubstituted pyridines.
[a]
Reaction performed on 2.7 mmol of 1. Ϫ [b] Isolated yields after
purification on a Chromatotron.
In agreement with previous work, it was shown that
LDA[6a,6b] leads to exclusive lithiation at C-4 and 3 was isol-
ated in 80% yield. On the other hand, the BuLi/TMEDA
complex[7] led mainly to the C-2-substituted product 2a
(60%). However, the latter reaction was found to be less
selective. Indeed, careful analysis of the reaction products
revealed the presence of 3 as well as a significant amount
of the addition product 4 (9 and 20%, respectively).
The results of the BuLi/LiDMAE reaction is in sharp
contrast to those obtained above. Indeed, after a short pre-
liminary study (not reported here), we were pleased to find
that clean and exclusive C-2 lithiation of 1 could be ob-
tained by performing the metallation at Ϫ60 °C in hexane
with 3 equiv. of the basic reagent. Under these conditions,
no side-products were detected and 2a was isolated as a
single product in 80% yield. It is noteworthy that no addi-
tion products were detected, thus underlining the high basi-
city/nucleophilicity ratio of BuLi/LiDMAE.
Results and Discussion
Selective Lithiation and Functionalisation of 3-
Chloropyridine (1)
The reaction of 3-chloropyridine with various lithiating
agents was investigated. The reactivity of our BuLi/LiD-
MAE basic system was compared to those of classic re-
agents by performing some relevant experiments found in
the literature (Table 1).
[a]
`
´
´
Synthese Organique et Reactivite, UMR CNRS-UHP 7565,
´
´
´
Faculte des Sciences, Universite Henri Poincare
The fact that this reaction was particularly clean and re-
gioselective strongly encouraged us to further investigate
the lithiation of 4-chloropyridine (5).
`
Nancy I, B. P. 239, 54506 Vandoeuvre-Les-Nancy, France
Fax: (internat.) ϩ 33-3/83404558
E-mail: Yves.Fort@sor.uhp-nancy.fr
Eur. J. Org. Chem. 2001, 603Ϫ606
WILEY-VCH Verlag GmbH, 69451 Weinheim, 2001
1434Ϫ193X/01/0203Ϫ0603 $ 17.50ϩ.50/0
603

S. Choppin, P. Gros, Y. Fort
FULL PAPER
Table 3. Preparation of C-2-substituted 3- and 4-chloropyridines[a]
Selective Lithiation and Functionalisation of 4-Chloro-
pyridine (5)
For comparison, the lithiation of 5 was performed with
the BuLi/LiDMAE basic system as well as with classic
lithiating reagents (Table 2).
Table 2. Reaction of 5 with lithiating reagents[a]
[a] Reaction performed on 2.7 mmol of 1. Ϫ [b] Isolated yields after
purification on a Chromatotron.
[a] Reactions performed on 2.7 and 2 mmol of 1 and 5, respectively.
Surprisingly, BuLi/LiDMAE led to lithiation exclusively
at C-2 of 5. This unprecedented selectivity strongly con-
trasted with the classical C-3 lithiation found with LDA[6a]
or BuLi/TMEDA.[8] Furthermore, while the introduction of
a substituent at C-2 had been performed only by means of
bromine/magnesium exchange on 2-bromo-4-chloropyrid-
ine,[9] our method simply involved the parent 4-chloropyrid-
ine. Note that the conditions previously determined for the
lithiation of 1 led to 6a only in 50% yield without recovery
of starting 5. These differences could be attributed to a
lower thermal stability of the 4-chloro-2-lithiopyridine at
Ϫ60 °C which was then more prone to undergo degradation
(see below). Finally, after adjusting the conditions, we
found that the temperature of the metallation reaction had
to be decreased (Ϫ78 °C), and that the amount of base had
to be increased (4 equiv.) to obtain 6a in a very good 85%
isolated yield.
Ϫ
[b] GC yields relative to an internal standard. Ϫ [c] Isolated yields
[d]
after column chromatography on silica gel. Ϫ
10 equiv. were
[e]
used. Ϫ
In parentheses, deuterium content determined by 1H
NMR.
As shown, the functional derivatives 2b؊h and 6b؊h
were prepared in acceptable to very good yields. The deu-
teration experiments led to high rates of deuterium incorp-
oration, an indication of the formation of chloro-2-lithiopy-
ridines as intermediates. The method was found to be par-
ticularly efficient for the preparation of compounds 2f؊h
and 6f؊h which have to be considered as useful reactive
precursors for further functionalisations. It is worth noting
that 4-chloropyridine generally led to good but lower GC
yields than 3-chloropyridine, once again underlining the
lower stability of the corresponding lithiated intermediate.
As a general trend, all the prepared products appeared to
be sensitive,[10] as shown by deviations between the GC and
the isolated yields obtained after purification on silica gel.
All attempts to optimise the isolated yields by other puri-
fication methods, such as chromatography on alumina or
distillation, proved to be unsuccessful. After having demon-
strated the synthetic usefulness of our methodology, we
turned to the explanation of the observed selectivities.
Synthesis of C-2-Substituted Chloropyridines
In order to illustrate the versatility and synthetic value of
our new methodology for the preparation of C-2-substi-
tuted 3- and 4-chloropyridines, we have examined the con-
densation of various representative electrophiles (see
Table 3).
Origin of the Selectivities
We thought that the exclusive lithiation at C-2 of 3-chloro-
pyridine (1) was probably a result of a strong cooperative
604
Eur. J. Org. Chem. 2001, 603Ϫ606

First Regioselective C-2 Lithiation of 3- and 4-Chloropyridines
FULL PAPER
and 2-dimethylaminoethanol were distilled before use. All other re-
agents were used as received. Hexane, THF, and xylene were dis-
tilled and stored over sodium wire before use.
complexation of lithium by BuLi/LiDMAE aggregates, and
pyridine nitrogen and chlorine atoms (Scheme 1). Such a
strong complexation was hardly conceivable with the BuLi/
TMEDA complex, thus allowing for the formation of the
C-4 isomer in significant amounts when this reagent was
used.
General Procedure for C-2 Functionalisation of 3-Chloropyridine (1):
A solution of 2-(dimethylamino)ethanol (0.72 g, 8 mmol) in hexane
(5 mL) was cooled to ca. Ϫ5 °C and BuLi (10 mL, 16 mmol) was
added dropwise under nitrogen. After 30 min at 0 °C, the solution
was cooled to Ϫ60 °C and a solution of 3-chloropyridine (0.306 g,
2.67 mmol) in hexane (5 mL) was added dropwise. After 1 h of
stirring, the orange solution was treated dropwise with a solution
of the appropriate electrophile (10.67 mmol) in THF (20 mL). The
reaction medium was then allowed to warm slowly to room temper-
ature (1 h) and the mixture was hydrolysed at 0 °C with H2O
(20 mL). The aqueous layer was then extracted with dichlorome-
thane (20 mL). The organic layer was dried (MgSO4) and the solv-
ents were evaporated under reduced pressure. The crude product
was analysed by GC and purified by column chromatography. (3-
Chloro-2-pyridyl)trimethylsilane (2a),[11] 3-chloro-2-pyridyl methyl
sulfide (2c),[1b] and 2-bromo-3-pyridyl chloride (2g)[12] were found
to be identical (spectroscopic data) to authentic samples.
Scheme 1
With 4-chloropyridine (5), a similar complexation path-
way could be postulated. However, the absence of addi-
tional complexation of lithium by the chlorine atom in this
case offered a weaker stabilisation of the lithium aggregates.
This stabilisation was probably favoured by a decrease in
the temperature of the metallation reaction from Ϫ60 to
Ϫ78 °C, thus promoting the selective functionalisation at
C-2 of 5 (Scheme 2). This interpretation agreed with our
previous observations during the metallation of 2-methoxy-
pyridine and pyridine.[3a,3b]
3-Chloro-[2-2H]pyridine (2b): 201 mg (66%), colourless oil; eluent:
hexane/AcOEt (95:5). Ϫ 1H NMR (400 MHz, CDCl3): δ ϭ 7.25
(dd, J ϭ 8.0 and 4.40 Hz, 1 H), 7.7 (d, J ϭ 8.0 Hz, 1 H), 8.5 (d,
J ϭ 4.70 Hz, 1 H). Ϫ 13C NMR (100 MHz, CDCl3): δ ϭ 122.9,
131.8, 139.7, 150.7, 152.8, 153.5, 154.2. Ϫ MS (EI); m/z (%): 115
(21) [Mϩ ϩ 1], 114 (86) [Mϩ], 79 (100), 51 (20).
1-(3-Chloro-2-pyridyl)-2,2-dimethyl-1-propanol (2d): 320 mg (60%),
white solid, m.p. 67Ϫ68 °C; eluent: hexane/AcOEt (75:25). Ϫ 1H
NMR (400 MHz, CDCl3): δ ϭ 0.95 (s, 9 H), 3.75 (d, J ϭ 9.7 Hz,
1 H), 4.90 (d, J ϭ 9.9 Hz, 1 H), 7.15 (dd, J ϭ 8.0 and 4.6 Hz, 1
H), 7.65 (dd, J ϭ 8.0 and 1.2 Hz, 1 H), 8.50 (dd, J ϭ 4.6 and
1.1 Hz, 1 H). Ϫ 13C NMR (100 MHz, CDCl3): δ ϭ 25.8, 37.8, 75.7,
123.1, 131.0, 137.1, 146.2, 158.0. Ϫ MS (EI); m/z (%): 199.00 (1)
[Mϩ], 168 (2), 166 (6), 151 (4), 146 (2), 143 (85), 142(100), 127 (4),
114 (13), 107 (4), 89 (2), 87 (3), 78 (34), 63 (3), 57 (27), 51 (21). Ϫ
HRMS: calcd. for C10H14ClNO requires 199.0764, found 199.0762.
Scheme 2
(3-Chloro-2-pyridyl)(phenyl)methanone (2e): 400 mg (69%), yellow
oil; eluent: hexane/AcOEt (75:25). Ϫ 1H NMR (400 MHz, CDCl3):
δ ϭ 7.40 (dd, J ϭ 8.3 and 4.8 Hz, 1 H), 7.50 (t, J ϭ 7.7 Hz, 2 H),
7.6 (t, J ϭ 8.4 Hz, 1 H), 7.8 (m, 3 H), 8.55 (dd, J ϭ 4.6 and 1.3 Hz,
1 H). Ϫ 13C NMR (100 MHz, CDCl3): δ ϭ 125.3, 128.5, 129.4,
130.1, 133.9, 135.0, 137.8, 146.9, 154.3, 192.4. Ϫ MS (EI); m/z (%)
217 (14) [Mϩ], 216 (22) [Mϩ Ϫ 1], 191 (16), 189 (48), 154 (24), 105
(100), 77 (66), 51 (15). Ϫ HRMS: calcd. for C12H8ClNO requires
217.0294, found 217.0290.
Conclusion
In summary, we have shown that BuLi/LiDMAE pro-
moted the clean lithiation at C-2 of 3-chloropyridine and
an unprecedented lithiation at C-2 of 4-chloropyridine. The
retention of the CϪCl bond obtained in the reaction opens
the way to new reactive functional pyridines. Synthetic ap-
plications are now under investigation.
2-Iodo-3-pyridyl Chloride (2f): 422 mg (66%), yellow solid, m.p.
1
82Ϫ84 °C; eluent: hexane/AcOEt (90:10). Ϫ H NMR (400 MHz,
Experimental Section
CDCl3): δ ϭ 7.30 (dd, J ϭ 7.9 and 4.6 Hz, 1 H), 7.75 (dd, J ϭ 7.9
and 1.7 Hz, 1 H), 8.25 (dd, J ϭ 4.0 and 1.6 Hz, 1 H). Ϫ 13C NMR
(100 MHz, CDCl3): δ ϭ 121.4, 123.5, 136.9. 138.1, 148.6. Ϫ MS
(EI); m/z (%): 239 (48) [Mϩ], 238 (33), 126 (82), 113 (19), 111 (95),
85 (19), 76 (100), 62 (8), 51 (27). Ϫ HRMS: calcd. for C5H3ClIN
requires 238.9000, found 238.9001.
1H and 13C NMR spectra were recorded with a Bruker spectro-
meter at 400 and 100 MHz, respectively, with TMS as internal
standard and CDCl3 as solvent; J values are given in Hz. Ϫ GC
analyses were performed (with an internal standard) with a Shim-
adzu GC-14A apparatus using an HP1 20-m column and temper-
ature programming. Ϫ GC/MS (EI) spectra were recorded with an
HP5871 spectrometer. Ϫ Melting points were performed with a
Tottoli apparatus and are uncorrected. Ϫ HRMS analyses were
performed by the Service Central d’Analyse du CNRS (Vernaison,
France). Ϫ Commercially available hexane solutions of BuLi (1.6
2,3-Dichloropyridine (2h): 237 mg (60%), white solid, m.p. 77Ϫ79
°C; eluent: hexane/AcOEt (90:10). Ϫ 1H NMR (400 MHz, CDCl3):
δ ϭ 7.25 (dd, J ϭ 7.9 and 4.8 Hz, 1 H), 7.7 (dd, J ϭ 7.9 and 1.6 Hz,
1 H), 8.3 (dd, J ϭ 4.8 and 1.6 Hz, 1 H). Ϫ 13C NMR (100 MHz,
CDCl3): δ ϭ 123.1, 130.5, 138.6, 147.1, 149.0. Ϫ MS (EI); m/z (%):
) were used. 4-Chloropyridine (5) was prepared by neutralisation 148 (62%) [Mϩ], 147 (100), 114 (31), 112 (98), 85 (13), 76 (33),
(K2CO3) of 4-chloropyridine hydrochloride. 3-Chloropyridine (1)
51 (10).
Eur. J. Org. Chem. 2001, 603Ϫ606
605

S. Choppin, P. Gros, Y. Fort
FULL PAPER
General Procedure for C-2 Functionalisation of 4-Chloropyridine (5):
A solution of 2-(dimethylamino)ethanol (0.72 g, 8 mmol) in hexane
(5 mL) was cooled to ca. Ϫ5 °C and BuLi (10 mL, 16 mmol) was
added dropwise under nitrogen. After 30 min at 0 °C, the solution
was cooled to Ϫ78 °C and a solution of 4-chloropyridine (0.299 g,
2 mmol) in hexane (5 mL) was added dropwise. After stirring for
1 h, the orange solution was treated dropwise with a solution of the
appropriate electrophile (10 mmol) in THF (20 mL). The reaction
medium was then allowed to warm slowly to room temperature
(1 h) and the mixture was hydrolysed at 0 °C with H2O (20 mL).
The aqueous layer was then extracted with dichloromethane
(20 mL). The organic layer was dried (MgSO4) and the solvents
were evaporated under reduced pressure. The crude product was
analysed by GC and purified by column chromatography. (4-
Chloro-2-pyridyl)trimethylsilane (6a)[9] and (4-chloro-2-pyridyl)-
(phenyl)methanone (6e)[13] were found to be identical (spectro-
scopic data) to authentic samples.
2-Bromo-4-pyridyl Chloride (6g):[15] 189 mg (49%), yellow oil; elu-
ent: hexane/AcOEt (90:10). Ϫ H NMR (400 MHz, CDCl3): δ ϭ
7.30 (dd, J ϭ 5.3 and 1.8 Hz, 1 H), 7.55 (d, J ϭ 1.83 Hz, 1 H), 8.3
(d, J ϭ 5.32 Hz, 1 H). Ϫ 13C NMR (100 MHz, CDCl3): δ ϭ 123.2,
127.2, 142.4, 145.3, 150.6. Ϫ MS (EI); m/z (%): 192 (24) [Mϩ], 191
(19), 112 (81), 106 (8), 81 (74), 76 (100), 75 (43), 51 (57).
1
2,4-Dichloropyridine (6h):[15] 131 mg (44%), yellow oil; eluent: hex-
1
ane/AcOEt (90:10). Ϫ H NMR (400 MHz, CDCl3): δ ϭ 7.25 (d,
J ϭ 5.3 and 1.7 Hz, 1 H), 7.35 (d, J ϭ 1.83 Hz, 1 H), 8.3 (d, J ϭ
5.32 Hz, 1 H). Ϫ 13C NMR (100 MHz, CDCl3): δ ϭ 122.8, 124.3,
145.7, 150.1, 152.2. Ϫ MS (EI); m/z (%): 149 (36) [Mϩ], 147 (58),
114 (30), 112 (100), 85 (35), 76 (74), 62 (32), 51 (54).
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4-Chloro-[2-2H]pyridine (6b): 132 mg (58%), colorless oil; eluent:
hexane/AcOEt (95:5). Ϫ 1H NMR (400 MHz, CDCl3): δ ϭ 7.30 (s,
2 H), 8.50 (s, 1 H). Ϫ 13C NMR (100 MHz, CDCl3): δ ϭ 124.2,
142.5, 151.2, 151.9, 152.6, 154.6. Ϫ MS (EI); m/z (%): 115 (24) [Mϩ
ϩ 1], 114 (75) [Mϩ], 79 (100), 51 (27).
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[3c]
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`
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[4]
[5]
4-Chloro-2-pyridyl Methyl Sulfide (6c): 204 mg (64%), colorless oil;
S. Choppin, Ph. Gros, Y. Fort, Org. Lett. 2000, 2, 803Ϫ805.
Reaction of BuLi with halopyridines leads to addition products
1
eluent: hexane/AcOEt (95:5). Ϫ H NMR (400 MHz, CDCl3): δ ϭ
´
with untimely loss of the chlorine atom, see: F. Trecourt, F.
2.55 (s, 3 H), 7.00 (dd, J ϭ 5.3 and 1.9 Hz, 1 H), 7.20 (d, J ϭ
2.0 Hz, 1 H), 8.30 (d, J ϭ 5.32 Hz, 1 H). Ϫ 13C NMR (100 MHz,
CDCl3): δ ϭ 13.3, 120.9, 143.6, 149.9, 161.8. Ϫ MS (EI); m/z (%):
159 (67) [Mϩ], 158 (58), 113 (77), 82 (32), 78 (100), 76 (51), 73
(15), 51 (47). Ϫ HRMS: calcd. for C6H6ClNS requires 158.9909,
found 158.9910.
´
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´
´
´
F. Marsais, F. Trecourt, P. Breant, G. Queguiner, J. Heterocycl.
1-(4-Chloro-2-pyridyl)-2,2-dimethyl-1-propanol (6d): 203 mg (51%),
viscous yellow oil; eluent: hexane/AcOEt (80:20). Ϫ 1H NMR
(400 MHz, CDCl3): δ ϭ 0.95 (s, 9 H), 4.05 (d, J ϭ 7 Hz, 1 H), 4.35
(d, J ϭ 7 Hz, 1 H), 7.2 (dd, J ϭ 5.3 and 2 Hz, 1 H), 7.25 (s, 1 H),
8.45 (d, J ϭ 5.3 Hz, 1 H). Ϫ 13C NMR (100 MHz, CDCl3): δ ϭ
26.1, 36.3, 80.4, 122.5, 122.9, 143.6, 148.7, 162.0. Ϫ MS (EI); m/z
(%): 199 (1) [Mϩ], 184 (1), 166 (2), 113 (17), 77 (22), 56 (25), 50
(20). Ϫ HRMS: calcd. for C10H14ClNO requires 199.0764, found
199.0764.
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[11]
[12]
[13]
2-Iodo-4-pyridyl Chloride (6f):[14] 335 mg (70%), yellow oil; eluent:
hexane/AcOEt (90:10). Ϫ 1H NMR (400 MHz, CDCl3): δ ϭ 7.3
(dd, J ϭ 5.3 and 1.8 Hz, 1 H), 7.75 (d, J ϭ 1.84 Hz, 1 H) 8.25 (d,
J ϭ 5.32 Hz, 1 H). Ϫ 13C NMR (100 MHz, CDCl3): δ ϭ 117.7,
123.6, 134.5, 144.4, 151.0 MS (EI); m/z (%): 239 (96) [Mϩ], 127
(13), 114 (32), 76 (100), 51 (6).
[14]
[15]
Received July 11, 2000
[O00360]
606
Eur. J. Org. Chem. 2001, 603Ϫ606