Characterization of two hydroxytrichloropicolinic acids: Application of the one-bond chlorine-isotope effect in 13C NMR

Article abstract of DOI:10.1002/mrc.2196

The structures of 4-hydroxy-3,5,6-trichloropyridine-2-carboxylic acid (1a) and 6-hydroxy-3,4,5-trichloro-2-carboxylic acid (1b) were verified by the NMR analysis of their corresponding methylated and decarboxylated derivatives 2,3,5-trichloro-4-methoxypyr

Full text of DOI:10.1002/mrc.2196

Research Article
Received: 16 April 2007
Revised: 20 December 2007
Accepted: 20 December 2007
Published online in Wiley Interscience: 5 March 2008
(www.interscience.com) DOI 10.1002/mrc.2196
Characterization of two
hydroxytrichloropicolinic acids: application of
the one-bond chlorine-isotope effect in
13C NMR
Nicholas M. Irvine,∗ David H. Cooper and Scott Thornburgh
The structures of 4-hydroxy-3,5,6-trichloropyridine-2-carboxylic acid (1a) and 6-hydroxy-3,4,5-trichloro-2-carboxylic acid (1b)
were verified by the NMR analysis of their corresponding methylated and decarboxylated derivatives 2,3,5-trichloro-4-
methoxypyridine (5) and 3,4,5-trichloro-2-methoxypyridine (8), respectively. The 6-hydroxy isomer (1a) was found to be in
equilibrium with its pyridinone tautomer as evidenced by the formation of significant amounts of 3,4,5-trichloro-1-methyl-6-
oxo-1,6-dihydropyridine-2-carboxylic acid methyl ester (6b) on exhaustive methylation. The one-bond chlorine-isotope effect
was used and shown to be an effective tool for the identification of chlorinated carbons in 13C NMR spectra providing an
c
additional tool for solving structural problems in chlorinated compounds. Copyright ꢀ 2008 John Wiley & Sons, Ltd.
1
Keywords: NMR; H; 13C; picloram; 4-hydroxy-3,5,6-trichloropyridine-2-carboxylic acid; 6-hydroxy-3,4,5-trichloropyridine-2-carboxylic
acid; one-bond chlorine-isotope effect in 13C NMR
Introduction
with a proton-resonance frequency of 400 MHz and a carbon-
resonance frequency of 100 MHz.
Picloram 2 (Fig. 1) is an important commercial herbicide that
has found widespread use in the control of a wide variety of
woody plants and annual and perennial broadleaf weeds.[1] The
synthesis of picloram and its derivatives involves replacement of
thepyridylhydrogenswithchlorineandotherfunctionalgroupsto
give persubstituted pyridines. The replacement of all five pyridine
hydrogens by various substituents in these compounds makes
the identification of the persubstituted products difficult due to
the similarity of their 13C NMR spectra and the resulting difficulty
in establishing substituent regiochemistry. Recently, two such
products isolated from a chlorination process for picolinic acid
gave mass spectra which indicated the addition of three chlorines
and a hydroxyl group to yield tetrasubstituted picolinic acids;
however, a complete structure assignment required definitive
identification of chlorinated and hydroxylated carbons in the
respective 13C NMR spectra. The chlorine-isotope effect provides
a convenient method for the identification of chlorinated carbons
by observation of small differences in the chemical shifts of
carbons bonded to 37Cl relative to those bonded to 35Cl for
chlorinated carbon atoms.[2,3] We describe here the identification
of the two tetrasubstituted picolinic acids 1a and 1b (Fig. 1) using
the chlorine-isotope effect to establish definitively the location of
chlorine atoms on the pyridine ring.
HMBC spectra were obtained in d6-acetone or CDCl3 on a Bruker
DRX-600 instrument with a proton frequency of 600 MHz using
the standard-pulse sequence. The spectra were optimized for a
10 Hz long-range proton–carbon coupling.
High-resolution 13C NMR experiments to detect the chlorine-
isotope effect were carried out in d6-acetone, which had been
deoxygenated by bubbling a stream of nitrogen through the
solution in an NMR tube for 10 min. Experiments were conducted
on a Bruker DRX-600 instrument with a carbon-resonance
frequency of 150 MHz. 13C spectra were obtained using 65 536
datapointsandaspectralwindowof10 593 Hzresultinginadigital
resolution of 0.16 Hz. A total of 3 K transients were accumulated
with a relaxation delay of 10 s. Spectra were processed using
−0.4 Hz exponential and 0.4 Hz Gaussian weighting factors for
resolution enhancement.
General
Trimethylsilyldiazomethane was purchased as 2.0 M solution in
hexanes from Aldrich. Anhydrous solvents such as EtOAc and
CH3OHwerepurchasedfromAldrichinSure/Sealbottlesstoredun-
der an atmosphere of nitrogen. 4-Hydroxy-3,5,6-trichloropyridine-
2-carboxylic acid (1a) and 6-hydroxy-3,4,5-trichloropyridine-2-
carboxylic acid (1b) were obtained from the test-substance in-
ventory at Dow AgroSciences, Indianapolis (TSN101573 for 1a and
TSN101575 for 1b, respectively). Flash column chromatography
Experimental
Spectrometers
∗
Correspondence to: Nicholas M. Irvine, Dow AgroSciences, Indianapolis, IN
46268, USA. E-mail: nmirvine@dow.com
Survey NMR experiments of the hydroxytrichloropicolinic acids
were conducted in d6-DMSO. All other compounds were analyzed
inCDCl3.SpectrawereobtainedonaBrukerDRX-400spectrometer
Dow AgroSciences LLC, 9330 Zionsville Road, Indianapolis, IN 46268, USA
c
Magn. Reson. Chem. 2008; 46: 436–440
Copyright ꢀ 2008 John Wiley & Sons, Ltd.

Characterization of two hydroxytrichloropicolinic acids
3,4,5-Trichloro-6-methoxypyridine-2-carboxylicacid (7)
OH
Cl
NH2
4
3
Cl
Cl
Cl
O
Cl
Cl
O
Cl
Cl
Cl
O
In a similar fashion, 6a (0.85 g, 3.15 mmol) was converted to 7,
isolated as a white solid, 0.75 g (93%). IR cm−1: 1715; 1H NMR
d6-DMSO δ 3.97 (s, 3H); HRMS for C7H4Cl3NO3; measured accurate
mass 254.9255, calculated exact mass 254.9257.
5
2
OH
OH
OH
6
N
N
N
HO
1a
1b
2
2,3,5-Trichloro-4-methoxypyridine (5)
Figure 1. Structures of picloram 2 and picolinic acids 1a and 1b .
Compound 4 (264 mg, 1.03 mmol) was placed in a 25-ml pear-
shaped flask fitted with a reflux condenser. The flask was heated at
190–200 ◦C for 6 min under nitrogen with stirring. The compound
turned to a brown liquid as CO2 bubbled off. The solution was
allowed to cool under nitrogen and the residue purified by silica
gel column chromatography (hexanes in EtOAc, 9 : 1) to give 5, as
a white solid, 157 mg (72%). IR cm−1: 1740; 1H NMR CDCl3 δ 8.22
(s, 1H), 4.06 (s, 3H); HRMS for C6H4Cl3NO; measured accurate mass
210.9357, calculated exact mass 210.9358.
was accomplished using silica gel 60 (230–400 mesh ASTM) from
EM Science. Infrared (IR) data was acquired using a Digilab FTS40
Pro Fourier transform infrared (FTIR) spectrophotometer. High
resolution Fourier mass spectrometry (HRMS) data was acquired
using a Thermo Finnigan light-triggered and quenched Fourier-
transform (LTQ-FT) hybrid linear ion-trap Fourier-transform mass
spectrometer.
Preparation of compounds
3,4,5-Trichloro-2-methoxypyridine (8)
3,5,6-Trichloro-4-methoxypyridine-2-carboxylicacid methyl ester (3)
In a similar fashion, 7 (416 mg, 1.62 mmol) was converted to 8, as
a white solid, 264 mg (77%). IR cm−1: 1740; 1H NMR CDCl3 δ 4.05
(s, 3H), 8.08 (s, 1H); HRMS for C6H4Cl3NO; measured accurate mass
210.9357, calculated exact mass 210.9358.
TMSCHN2 (13.0 ml, 2.0 M solution in hexanes, 26.00 mmol) was
added slowly via syringe to a suspension of 1a (2.00 g, 8.25 mmol)
in EtOAc (30 ml) and CH3OH (10 ml) at room temperature over
10 min. After stirring for a further 15 min the mixture was
concentrated invacuo and the residue purified by silica gel column
chromatography (hexanes in EtOAc, 5 : 1) to give 2.01 g (90%) of 3
as a white solid. IR cm−1: 1740; 1H NMR CDCl3 δ 4.03 (s, 3H), 3.99
(s, 3H); HRMS for C8H6Cl3NO3; measured accurate mass 268.9412,
calculated exact mass 268.9413.
Results and Discussion
Chemical synthesis of 2,3,5-trichloro-4-methoxypyridine (5)
and 3,4,5-trichloro-2-methoxypyridine (8)
In order to characterize the structures of hydroxypyridines 1a and
1b using NMR spectroscopy, each compound was methylated,
thendecarboxylatedtoform2,3,5-trichloro-4-methoxypyridine(5)
and 3,4,5-trichloro-2-methoxypyridine (8), as shown in Schemes 1
and 2, respectively.
The 4-hydroxy acid 1a was first treated with (trimethylsi-
lyl)diazomethane to give methyl ester 3 as the only product
(Scheme 1). Not only was the acid esterified to give the methyl
ester in the 2-position but the 4-hydroxyl group was also methy-
lated. Ester 3 was then converted to picolinic acid 4 (1N NaOH
3,4,5-Trichloro-6-methoxypyridine-2-carboxylic acid methyl es-
ter (6a) and 3,4,5-trichloro-1-methyl-6-oxo-1,6-dihydropyridine-2-
carboxylic acid methyl ester (6b)
TMSCHN2 (13.0 ml, 2.0 M solution in hexanes, 26.00 mmol) was
added slowly via syringe to a suspension of 1b (2.00 g, 8.25 mmol)
in EtOAc (30 ml) and CH3OH (10 ml) at room temperature for
over 10 min. After stirring for a further 15 min the mixture was
concentrated invacuo and the residue purified by silica gel column
chromatography (hexanes in EtOAc, 5 : 1 to 3 : 1) to give 0.92 g
(41%) of 6a as a white solid. IR cm−1: 1741; 1H NMR CDCl3
δ
4.02 (s, 3H), 3.98 (s, 3H); MS m/z 270/272; HRMS for C8H6Cl3NO3;
observed mass 268.9414, calculated mass 268.9413, followed by
0.91 g (41%) of 6b as a white solid. IR cm−1: 1747, 1659; 1H NMR
CDCl3 δ 4.03 (s, 3H), 3.56 (s, 3H); HRMS for C8H6Cl3NO3; measured
accurate mass 268.9412, calculated exact mass 268.9413.
OH
OMe
Me3SiCHN2
Cl
O
Cl
Cl
Cl
Cl
Cl
OMe
CH3OH
EtOAc
(96%)
N
CO2H
N
1a
3
3,5,6-Trichloro-4-methoxypyridine-2-carboxylicacid (4)
NaOH
1N NaOH (25 ml) was added to a suspension of 3 (2.03 g,
7.50 mmol) in CH3OH (25 ml). After stirring for 2 h at room
temperature,thevolumewasconcentratedtohalfvolumeinvacuo
and then water was added until the suspended salt dissolved. The
aqueous solution was extracted with EtOAc once to remove side
products and the organic phase discarded. Concentrated HCl was
then added to the aqueous layer to pH 1. The reaction mixture
was then extracted with EtOAc (×3). The organic layers were dried
(MgSO4), filtered and concentrated to give a crude white solid.
Trituration from hexanes provided 4 as a white solid, 1.85 g (96%).
CH3OH
then HCl
(96%)
OMe
OMe
190-200°C
Cl
Cl
Cl
H
Cl
Cl
Cl
O
OH
- CO2
(72%)
N
N
1
IR cm−1: 2951, 1724; H NMR d6-DMSO δ 4.01 (s, 3H); HRMS for
5
4
C7H4Cl3NO3; measured accurate mass 254.9258, calculated exact
mass 254.9257.
Scheme 1. Synthesis of methoxy pyridine 5.
c
Magn. Reson. Chem. 2008; 46: 436–440
Copyright ꢀ 2008 John Wiley & Sons, Ltd.
www.interscience.wiley.com/journal/mrc

N. M. Irvine, D. H. Cooper and S. Thornburgh
Cl
N
Cl
Cl
Me3SiCHN2
Cl
Cl
O
Cl
Cl
O
Cl
Cl
+
CH3OH
EtOAc
OMe
OMe
MeO
O
HO
N
CO2H
N
1b
6a
easily separated
6b
(41%)
(41%)
NaOH
CH3OH
then HCl
(93%)
Cl
Cl
190-200°C
Cl
Cl
H
Cl
Cl
OH
O
- CO2
(77%)
MeO
N
MeO
N
8
7
Scheme 2. Synthesis of methoxy pyridine 8.
in CH3OH then dilute HCl acid), followed by decarboxylation by
heating to provide the desired tetrasubstituted pyridine 5.
Pyridine 8 was synthesized from 6-hydroxy acid 1b in three
steps as shown in Scheme 2. Acid 1b was first treated with
(trimethylsilyl)diazomethane. However, unlike the methylation of
1a, methylation of 1b resulted in two products, 6a and 6b, as a 1 : 1
mixture easily separated by silica gel column chromatography.
Ester 6a was then converted to carboxylic acid 7 by similar
conditions used in Scheme 1 for ester 3. Decarboxylation of 7
by heating provided the desired tetrasubstituted pyridine 8. The
carbon spectral data of 5 and 8 is summarized in Table 1.
matic region of the 13C NMR spectrum suggest the presence
of a tautomeric exchange process with a rate that is interme-
diate on the NMR timescale. The observed formation of the
N-methylated derivative 6b upon exhaustive methylation indi-
cates the presence of the pyridinone tautomer in equilibrium with
the 6-hydroxypyridine tautomer. This is further supported by the
observation of a second carbonyl absorption at 1686 cm−1 in the
IR spectrum of 1b.
The N-methylated derivative 6b was used to definitively
establish the location of the oxygen atom on the pyridine ring
by identifying the locations of the chlorine atoms utilizing the
one-bond chlorine-isotope effect.[2,3] Careful optimization of the
13C NMR experimental parameters for resolution of chemical-shift
changes in the parts per billion (ppb) range allows observation of a
smallupfieldshiftofabout4 ppbforcarbonsboundto 37Clrelative
to carbons bound to 35Cl. Since the natural abundance of 37Cl is
24.2%and35Clis75.8%, thisresultsintheobservationoftwopeaks
witha3 : 1intensityratioseparatedbyabout0.6 Hz(ata150.9 MHz
13C-resonance frequency as in this work) for each carbon directly
Structure identification of 6-hydroxy-3,4,5-trichloropyridine-
2-carboxylic acid (1b) – the one-bond chlorine-isotope effect
in 13C NMR
The proton spectrum of 1b shows only a broad peak for the
exchangeable OH and CO2H protons and thus is not useful for
identification purposes. The severely broadened lines in the aro-
Table 1. 13C chemical shifts (CDCl3) and proton–carbon couplings in 5 and 8. Individual carbon assignments were made based on chemical shifts,
HMBC, and proton–carbon coupling data, and the observation of chlorine isotope effects
5
8
Carbon
(ppm)
JHC(Hz)
(ppm)
JHC(Hz)
2
148.9
125.5
160.5
125.4
146.9
61.1
15.7 (d)
2.3 (d)
158.9
118.2
141.7
123.7
143.0
55.0
13.2 4.1 (dq)
1.9 (d)
3
4
6.8, 3.4 (dq)
3.4 (d)
9.1
5
3.7
6
190.8 (d)
147.2 (q)
189.1 (d)
147.7 (q)
OCH3
c
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Copyright ꢀ 2008 John Wiley & Sons, Ltd.
Magn. Reson. Chem. 2008; 46: 436–440

Characterization of two hydroxytrichloropicolinic acids
Figure 2. 13C NMR spectrum of 6b in d6-acetone showing the one-bond
chlorine-isotope effect at the chlorinated carbons. Expansions are shown
above each peak to make the small chemical-shift changes resulting
from the one-bond chlorine-isotope effect visible. The carbon-resonance
frequency was 150.92 MHz and the spectral window was 10 600 Hz
using 65.5 K data points for a digital resolution of 0.16 Hz. Resolution
enhancement was applied in the form of −0.4 Hz exponential and 0.4 Hz
Gaussian window functions. Carbons at 142.5, 127.3, and 108.9 ppm can
be seen to exhibit the characteristic peak doubling in a 3 : 1 intensity ratio
indicative of carbons bonded to 35Cl and 37Cl at natural abundance. The
upfield shift of the peak corresponding to C–37Cl is 0.6 Hz for all three
carbons.
Figure 4. 13C NMR spectrum of 8 obtained in CDCl3 with gated-proton
decoupling.
Structure identification of 4-hydroxy-3,5,6-trichloropyridine-
2-carboxylic acid (1a)
As with the 6-hydroxy acid 1b the proton NMR spectrum of
1a shows only broad peaks for the exchangeable OH and CO2H
protonsandisof littlevalueforidentificationpurposes. Thecarbon
spectrum shows no evidence of the exchange process seen in the
correspondingspectrumof1b. Exhaustivemethylationof1afailed
toproduceanyofan N-methylproductanalogoustothatobtained
fromthe6-hydroxyisomerandthusidentificationwasachievedby
examining the decarboxylated derivative 5. The HMBC spectrum
of 5 showed a single correlation of the methyl protons to the
pyridine carbon at 160.5 ppm. Observation of a single correlation
to the pyridine ring confirms that the methyl group is on oxygen
and not on the pyridine nitrogen.
A gated-decoupled 13C spectrum of 5 (Fig. 3) can be compared
to that of the corresponding 6-methoxy isomer 8 (Fig. 4). The
large one-bond C–H couplings of 190.8 and 189.1 Hz identify the
protonated position as C-6 in both structures.[6] The C-2 carbon in
5 is readily identified at 148.9 ppm by the characteristically large
3JH2–C6 coupling constant[6] (15.7 Hz). The absence of long-range
coupling of this carbon to the methyl protons and the HMBC
experiment both show that the methoxy group is attached to
the carbon at 160.5 ppm and not at the C-6 position. This large
downfield chemical shift and the relatively large 6.8 Hz long-
range coupling observed to the proton at C-6 both support the
4-methoxy isomer 5.
bonded to chlorine. The smaller peak corresponding to 13C–37Cl
displays a slight upfield shift attributed to shortening of the C–Cl
bond relative to 13C–35Cl.[4,5]
The HMBC spectrum of 6b shows correlations of the methyl
protons to the two pyridine carbons C-2 and C-6 at 138.4 and
156.7 ppm establishing that methylation had indeed occurred on
thepyridinenitrogenatom.Thehigh-resolution13CNMRspectrum
of 6b is shown in Fig. 2. A chlorine-isotope shift of 0.6 Hz is readily
observed for the carbons at 142.5, 127.3, and 108.9 ppm. Since this
result indicates that neither carbons C-2 or C-6 identified by the
HMBC experiment are chlorinated and since the carboxylic acid
group is known to be at C-2, 6b is established as the 6-keto isomer.
The picolinic acid from which 6b was prepared is thus established
as the 6-OH isomer 1b.
Conclusions
The structures of the persubstituted pyridines 1a and 1b
were determined by the NMR analysis of methylated and
decarboxylated derivatives 5 and 8, respectively. The 6-hydroxy
isomer 1b is in equilibrium with its pyridinone tautomer as
evidenced by the formation of significant amounts of ester 6b
on exhaustive methylation. Evidence for tautomerism with the
pyridinone can also be seen in the extensive line broadening of
the pyridine carbons in the 13C NMR spectrum and the presence
of a second carbonyl absorption at 1686 cm−1 in the IR spectrum
of 1b. In the course of this work, the use of the one-bond chlorine-
isotope effect was shown to be effective for the identification of
chlorinated carbons in 13C NMR spectra and provides a useful tool
for solving structural problems in chlorinated compounds.
Figure 3. 13C NMR spectrum of 5 obtained in CDCl3 with gated-proton
decoupling.
c
Magn. Reson. Chem. 2008; 46: 436–440
Copyright ꢀ 2008 John Wiley & Sons, Ltd.
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N. M. Irvine, D. H. Cooper and S. Thornburgh
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Copyright ꢀ 2008 John Wiley & Sons, Ltd.
Magn. Reson. Chem. 2008; 46: 436–440