Configurational assignment of long-chain alkylated pyridinium aldoxime bromides by Noesy experiments

Article abstract of DOI:10.3184/030823408X318424

A configurational study of long-chain N-alkylated pyridinium aldoxime derivatives and their positional isomers has been carried out by two-dimensional ¹H-¹H NOESY (nuclear Overhauser effect spectroscopy). Cross peak intensities were observed to be enhanced with increasing mixing time. Mixing times longer than 250 ms result in increasing contribution of spin diffusion that produces unrealistic hydrogen-hydrogen distances. The results of NOE measurements showed significant enhancement in the intensity of iminyl proton resonances on irradiation of hydroxyl proton resonances and vice versa. The chemical shift difference between hydroxyl proton resonances and iminyl proton resonances was found to be ～4 ppm for syn and ～5 ppm for anti configurations. The study reveals that these compounds exist in the E configuration i.e. the syn form in solution. The syn isomer predominates; the anti isomer amounts are 3% (2-oxime), 4% (3-oxime) and 6% (4-oxime).

Full text of DOI:10.3184/030823408X318424

252 RESEARCH PAPER
MAY, 252–255
JOURNAL OF CHEMICAL RESEARCH 2008
Configurational assignment of long-chain alkylated pyridinium aldoxime
bromides by Noesy experiments
Mamta Sharma*, A.K. Gupta and S.K. Raza
Synthetic Chemistry Division, Defence Research and Development Establishment Gwalior-474002, M.P. India
A configurational study of long-chain N-alkylated pyridinium aldoxime derivatives and their positional isomers
has been carried out by two-dimensional 1H–1H NOESY (nuclear Overhauser effect spectroscopy). Cross peak
intensities were observed to be enhanced with increasing mixing time. Mixing times longer than 250 ms result
in increasing contribution of spin diffusion that produces unrealistic hydrogen-hydrogen distances. The results of
NOE measurements showed significant enhancement in the intensity of iminyl proton resonances on irradiation of
hydroxyl proton resonances and vice versa. The chemical shift difference between hydroxyl proton resonances and
iminyl proton resonances was found to be ~4 ppm for syn and ~5 ppm for anti configurations. The study reveals
that these compounds exist in the E configuration i.e. the syn form in solution. The syn isomer predominates; the
anti isomer amounts are 3% (2-oxime), 4% (3-oxime) and 6% (4-oxime).
Keywords: 1-D DIFNOE, 2-D NOESY, NꢀDON\ODWHGꢁS\ULGLQLXPꢁDOGR[LPHVꢂꢁꢃꢀK\GUR[\LPLQRPHWK\OS\ULGLQLXPꢁPHWK\OꢁFKORULGH
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JOURNAL OF CHEMICAL RESEARCH 2008 253
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&ꢀꢋꢁꢄꢈꢊꢍꢆꢑꢁSSPꢅꢁVKRZHGꢁFRUUHODWLRQꢁZLWKꢁWKHꢁ+ꢀꢋꢁSURWRQꢁꢄꢑꢆꢌꢍꢁSSPꢅꢆꢁ
7KLUGO\ꢁ WKHꢁ GHVKLHOGHGꢁ &ꢀꢊꢁ ꢄꢈꢊꢍꢆꢈꢅꢁ VKRZHGꢁ DWWDFKPHQWꢁ ZLWKꢁ +ꢀꢊꢁ
ꢄꢐꢆꢋꢁSSPꢅꢆꢁ&ꢀꢍꢁDQGꢁ&ꢀꢏꢁVKRZHGꢁDWWDFKPHQWꢁZLWKꢁDVVLJQHGꢁ+ꢀꢍꢁDQGꢁ
H-3 at 127.4 ppm and 125.6 ppm in the f2 dimension of the HSQC
N
R
R
(a)
(b)
Fig. 1 (a) N-Alkyl pyridinium aldoxime bromides, showing
the structure of anti forms, R is an alkyl group having general
formula CnH2n+1 where n varies from 2 to 16, –CH=NOH group
present at 2, 3 or 4 positions of the pyridinium ring; (b)
structure of syn forms.
Table 1 The 1H chemical shifts of 2, 3, 4 substituted N-dodecylpyridinium aldoximes bromides
Position
PA-I
PA-II
J/Hz
PA-III
J/Hz
Syn
J/Hz
Anti
J/Hz
Syn
Anti
J/Hz
Syn
Anti
J/Hz
2
3
–
–
–
–
9.25
–
8.71
8.14
9.04
8.33
–
–
8.2
6.1
6.0
–
9.60
–
8.81
8.22
9.08
7.76
–
–
7.6
5.9
6.2
–
9.02
8.22
–
8.22
9.02
8.42
6.6
6.6
9.30
8.51
–
8.51
9.30
7.91
5.6
5.5
8.39
8.55
8.10
9.05
8.78
8.0
7.8
6.9
6
8.32
8.65
8.16
9.2
7.5
7.9
7.0
5.9
–
4
5
6.6
6.6
–
5.5
5.6
6
7
–
8.18
8
9
13.13
4.73
1.79
0.85
–
12.84
4.6
–
–
7.5
–
12.25
4.61
1.92
0.85
–
12.79
–
–
12.82
4.55
1.90
0.85
–
13.5
–
–
–
–
–
10
11
19*
7.6
7.6
6.6
7.3
7.3
6.6
7.2
7.2
6.6
PA-I = N-Dodecyl-2-(hydroxyiminomethyl)pyridinium bromide.
PA-II = N-Dodecyl-3-(hydroxyiminomethyl)pyridinium bromide.
PA-III = N-Dodecyl-4-(hydroxyiminomethyl)pyridinium bromide.
*H-12 to H-18 appeared in the range of 1.25–1.09 ppm.
Table 2 The 13C{1H} chemical shifts of 2, 3, 4 substituted N-dodecylpyridinium aldoximes bromides
Position
PA-I
PA-II
PA-III
Syn
Anti
Syn
Anti
Syn
Anti
2
3
146.7
125.6
145.1
127.4
145.9
141.4
–
146.9
128.0
145.4
129.6
146.1
137.4
–
142.5
133.6
141.4
128.1
144.3
143.3
–
142.5
133.5
141.4
128.1
144.3
143.3
–
144.9
124
148.3
124
144.9
124
148.3
124
4
5
6
144.9
145.1
–
144.9
145.1
–
7
8
9
–
–
–
–
–
–
10
11
12
13
14
15
16
17
18
19
57.9
30.3
31.2
28.7
28.7
28.5
28.3
25.3
22.0
13.8
58.6
30.2
31.2
28.7
28.6
28.5
28.3
25.3
22.0
13.8
61.2
30.6
31.3
28.8
28.7
28.6
28.4
25.4
22.1
13.9
61.2
30.6
31.3
28.8
28.7
28.6
28.4
25.4
22.1
13.9
60.3
30.5
30.9
28.0
28.6
28.5
28.3
25.3
21.9
13.8
60.3
30.5
30.9
28.0
28.6
28.5
28.3
25.3
21.9
13.8

254 JOURNAL OF CHEMICAL RESEARCH 2008
I
II
Fig. 3 1H–13C HSQC NMR spectrum (partial) of N-dodecyl-2-
(hydroxyiminomethyl)pyridinium bromide.
III
Fig.4 (a)1HNMRspectrumofN-octyl-3-(hydroxyiminomethyl)
pyridinium bromide (b) NOE Difference Spectrum of N-octyl-
3-(hydroxyiminomethyl)pyridinium bromide resulting from
irradiation of the CH proton.
Fig.2 1HNMRspectrumofN-dodecyl-3-(hydroxyiminomethyl)
pyridinium bromide (I), N-dodecyl-4-(hydroxyiminomethyl)
pyridinium bromide (II), N-dodecyl-2-(hydroxyiminomethyl)
pyridinium bromide (III) (high intensity signals are for syn form
and low intensity signals are for the anti form) showing proton
signals for syn (S) and anti (A) form of compounds.
2D-NOESY experiments (intramolecular interactions)
7KHꢁ VSDWLDOꢁ FRQQHFWLYLW\ꢁ RIꢁ 2+ꢁ DQGꢁ +ꢁ SURWRQVꢁ DQGꢁ RWKHUꢁ FORVHO\ꢁ
SODFHGꢁSURWRQVꢁRIꢁS\ULGLQLXPꢁULQJꢁDQGꢁDON\OꢁFKDLQꢁKDVꢁEHHQꢁVWXGLHGꢁ
E\ꢁꢃ'ꢀ12(6<ꢁ105ꢁH[SHULPHQWꢆ18 This also depicts the orientation
RIꢁDON\OꢁFKDLQꢁDQGꢁR[LPHꢁJURXSꢁZLWKꢁUHVSHFWꢁWRꢁWKHꢁS\ULGLQLXPꢁULQJꢆꢁ
In NꢀGRGHF\OꢀꢃꢀꢄK\GUR[\LPLQRPHWK\OꢅS\ULGLQLXPꢁ EURPLGHꢁ &+ꢁ LVꢁ
close to OH, H-10, H-11, H-12 and H-6 is present near to the H-10,
H-11 and H-12 protons. In NꢀGRGHF\OꢀꢏꢀꢄK\GUR[\LPLQRPHWK\Oꢅ
S\ULGLQLXPꢁEURPLGHꢁ&+ꢁLVꢁFORVHꢁWRꢁ2+ꢇꢁ+ꢀꢃꢁDQGꢁ+ꢀꢃꢓ+ꢀꢋꢁDUHꢁFORVHꢁWRꢁ
H-10, H-11 and H-12 in space. In NꢀGRGHF\OꢀꢊꢀꢄK\GUR[\LPLQRPHWK\Oꢅ
S\ULGLQLXPꢁEURPLGHꢁ&+ꢁLVꢁFORVHꢁWRꢁ2+ꢁDQGꢁ+ꢀꢃꢓ+ꢀꢋꢁDUHꢁFORVHꢁWRꢁ+ꢀꢈꢌꢇꢁ
H-11 and H-12 in space. Seven of NꢀGRGHF\OꢀꢃꢄK\GUR[\LPLQRPHWK\Oꢅ
S\ULGLQLXPꢁEURPLGHꢁDQGꢁIRXUꢁFURVVꢁSHDNVꢁRIꢁNꢀGRGHF\OꢀꢊꢄK\GUR[\ꢀ
LPLQRPHWK\OꢅS\ULGLQLXPꢁ EURPLGHꢁ DUHꢁ VKRZQꢁ LQꢁ )LJVꢁ ꢍDꢇꢁ DQGꢁ ꢍEꢁ
UHVSHFWLYHO\ꢆꢁ 6HYHQꢁ KLJKꢁ LQWHQVLW\ꢁ FURVVꢁ SHDNVꢁ VKRZLQJꢁ FRUUHODWLRQꢁ
EHWZHHQꢁ SURWRQVꢁ RIꢁ NꢀGRGHF\OꢀꢃꢀꢄK\GUR[\LPLQRPHWK\Oꢅꢁ S\ULGLQLXPꢁ
EURPLGHꢁ ZHUHꢁ WDNHQꢁ WRꢁ SORWꢁ WKHꢁ JUDSKꢆꢁ $OOꢁ WKHꢁ 12(6<ꢁ 105ꢁ
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7KHꢁ 12(6<ꢁ H[SHULPHQWVꢁ ZHUHꢁ GRQHꢁ DWꢁ GLIIHUHQWꢁ PL[LQJꢁ WLPHVꢁ
(50, 100, 150, 200, 250, 300, 350, 400 and 450 ms). Decrease in
WKHꢁFURVVꢁSHDNꢁLQWHQVLW\ꢁZDVꢁREVHUYHGꢁLQꢁWKHꢁRUGHUꢁRIꢁ&+ꢀ+ꢀꢈꢌ!+ꢀ
VSHFWUXPꢁ UHVSHFWLYHO\ꢆꢁ &ꢀꢒꢁ DWꢁ ꢈꢊꢈꢆꢊꢁ SSPꢁ VKRZHGꢁ FRUUHODWLRQꢁ ZLWKꢁ
H-7 (8.78 ppm). C-7, H-7 of the antiꢁIRUPꢁZHUHꢁVKLHOGHGꢁPRUHꢁWKDQꢁ
in the syn form due to the shielding effect of the lone pair of electrons
RQꢁ QLWURJHQꢇꢁ DSSHDUHGꢁ DWꢁ ꢈꢏꢒꢆꢊꢁ SSPꢁ DQGꢁ ꢐꢆꢈꢐꢁ SSPꢁ UHVSHFWLYHO\ꢆꢁ
7KHꢁVDPHꢁVKLHOGLQJꢁHIIHFWꢁZDVꢁREVHUYHGꢁLQꢁWKHꢁRWKHUꢁLVRPHUVꢇꢁWKHꢁꢃꢁ
DQGꢁꢏꢁVXEVWLWXWHGꢁR[LPHVꢁEHLQJꢁVKRZQꢁLQꢁ7DEOHꢁꢈꢆꢁ&ꢀꢒꢁDQGꢁ+ꢀꢒꢁRIꢁ
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chemical shift scale and 7.91 ppm of the proton chemical shift scale.
DIFNOE NMR experiments
$OOꢁ WKHꢁ FRPSRXQGVꢁ ZHUHꢁ VXEMHFWHGꢁ WRꢁ WKHꢁ ',)12(ꢁ H[SHULPHQWꢆꢁ
NꢀRFW\OꢀꢏꢀꢄK\GUR[\LPLQRPHWK\OꢅS\ULGLQLXPꢁEURPLGHꢁZDVꢁFKRVHQꢁDVꢁ
model compound for presentation of results. In these measurements,
WKHꢁLUUDGLDWLRQꢁRIꢁWKHꢁR[LPHꢁK\GUR[\OꢁSURWRQꢁUHVXOWHGꢁLQꢁWKHꢁVLJQL¿FDQWꢁ
HQKDQFHPHQWꢁ RIꢁ WKHꢁ LPLQRꢁ PHWKLQHꢁ SURWRQꢁ UHVRQDQFHꢆꢁ 6LPLODUO\ꢁ LQꢁ
reverse experiments irradiation of the imino methine proton caused
HQKDQFHPHQWꢁRIꢁWKHꢁK\GUR[\OꢁSURWRQꢁUHVRQDQFHꢁDVꢁVKRZQꢁLQꢁ)LJꢆꢁꢊꢆꢁ
6LPLODUꢁUHVXOWVꢁZHUHꢁREWDLQHGꢁIRUꢁDOOꢁRIꢁWKHꢁLVRPHULFꢁFRPSRXQGVꢁRIꢁ
HWK\OꢁWRꢁFHW\OꢁS\ULGLQLXPꢁDOGR[LPHꢁEURPLGHVꢁHYHQꢁZLWKꢁWKHꢁYDULDWLRQꢁ
RIꢁ WKHꢁ SRVLWLRQꢁ RIꢁ WKHꢁ K\GUR[\LPLQRꢁ PHWK\Oꢁ PRLHW\ꢆꢁ 7KHꢁ UHVXOWꢁ RIꢁ
WKLVꢁ VWXG\ꢁ LQGLFDWHVꢁ WKDWꢁ WKHꢁ K\GUR[\Oꢁ DQGꢁ &+ꢁ JURXSVꢁ DUHꢁ LQꢁ FORVHꢁ
SUR[LPLW\ꢇꢁ ZKLFKꢁ SURYLGHVꢁ VWURQJꢁ HYLGHQFHꢁ WKDWꢁ WKHVHꢁ FRPSRXQGVꢁ
exist in the E/SynꢁFRQ¿JXUDWLRQꢁLQꢁ'062ꢀG6 solution.
ꢋꢀ+ꢀꢈꢌ!&+ꢀ&+ꢀꢈꢈ!+ꢀꢋꢀ+ꢀꢈꢈ!&+ꢀ+ꢀꢈꢃ!+ꢀꢋꢀ+ꢀꢈꢃ!&+ꢀ2+ꢁ
LQꢁ
NꢀGRGHF\OꢀꢃꢀꢄK\GUR[\LPLQRPHWK\Oꢅꢁ S\ULGLQLXPꢁ EURPLGHꢆꢁ &URVVꢁ
SHDNVꢁREWDLQHGꢁIRUꢁ&+ꢁDQGꢁ2+ꢁSURWRQVꢁRIꢁWKHꢁR[LPLQRꢁJURXSꢁVKRZHGꢁ
ORZꢁ LQWHQVLW\ꢁ GXHꢁ WRꢁ WKHꢁ ODUJHꢁ GLVWDQFHꢁ EHWZHHQꢁ WKHPꢆꢁ 7KHVHꢁ WZRꢁ
groups are at the same side of the double bond. The build-up curves
ꢄ)LJꢆꢁ ꢋꢅꢁ RIꢁ WKHꢁ LQWHQVLWLHVꢁ RIꢁ DOOꢁ WKHꢁ 12(ꢁ SHDNVꢁ VKRZHGꢁ WKHꢁ
characteristic parabolic shape. The maximum NOE intensities are
obtained in the 100–150 ms range of mixing time. Above mixing

JOURNAL OF CHEMICAL RESEARCH 2008 255
(a)
(b)
Fig. 5a NOESY Spectrum of N-dodecyl-2-(hydroxyiminomethyl)pyridinium bromide in DMSO solvent at a concentration of 20 mM.
The numbering of the atom correspond to that shown in the structure. 5b NOESY spectrum of N-dodecyl-4-(hydroxyiminomethyl)
pyridinium bromide in DMSO solvent at a concentration of 20 mM. The numbering of the atoms corresponds to that shown in the
structure. Expansion of contours is also shown.
OH G difference is more for the anti than the syn structures.
Similar observations are reported for the oximes of aliphatic
DQGꢁDOLF\FOLFꢁDOGHK\GHV19 also.
WLPHꢁRIꢁꢃꢍꢌꢁPVꢁVSLQVꢁVWDUWHGꢁJHWWLQJꢁGLIIXVHGꢆꢁ2QO\ꢁ¿YHꢁPL[LQJꢁWLPHVꢁ
of 50, 100, 150, 200 and 250 ms are taken to plot the graph.
Conclusion
$OOꢁWKHꢁ1ꢀDON\ODWHGꢁS\ULGLQLXPꢁDOGR[LPHVꢁZHUHꢁIRXQGꢁWRꢁEHꢁ
in the syn form. There is no effect of the long-chain and the
SODFHPHQWꢁRIꢁWKHꢁR[LPHꢁJURXSꢁDWꢁS\ULGLQLXPꢁULQJꢇꢁZKHWKHUꢁLWꢁ
LVꢁDWꢁꢃꢇꢏꢁRUꢁꢊꢁSRVLWLRQꢇꢁRQꢁWKHꢁFRQ¿JXUDWLRQꢁRIꢁWKHꢁK\GUR[\Oꢁ
LPLQRꢁ JURXSꢆꢁ 7KLVꢁ DOZD\Vꢁ UHPDLQVꢁ LQꢁ WKHꢁ syn form, that is
the OH and H protons are at same side of the double bond.
'HFUHDVHꢁLQꢁFURVVꢁSHDNꢁLQWHQVLW\ꢁLQꢁ12(6<ꢁ105ꢁVSHFWUDꢁLVꢁ
the indication of increase of inter protonic distances in the
molecule. The CH proton of the syn form appeared at a high
IUHTXHQF\ꢁYDOXHꢁꢄaꢐꢁSSPꢅꢁWKDQꢁWKHꢁ&+ꢁSURWRQꢁRIꢁWKHꢁanti form
ꢄaꢒꢁSSPꢅꢆꢁ7KLVꢁLVꢁGXHꢁWRꢁWKHꢁVKLHOGLQJꢁHIIHFWꢁRIꢁWKHꢁORQHꢁSDLUꢁ
electrons of nitrogen of the oxime group. The difference in
FKHPLFDOꢁVKLIWVꢁRIꢁ2+ꢁDQGꢁ&+ꢁSURWRQVꢁIRUꢁORQJꢀFKDLQꢁDON\ODWHGꢁ
S\ULGLQLXPꢁDOGR[LPHVꢁLVꢁIRXQGꢁWRꢁEHꢁꢊꢆꢏꢍꢇꢁꢏꢆꢑꢃꢁDQGꢁꢊꢆꢊꢌꢁSSPꢁ
for syn and 4.66, 5.03 and 5.59 ppm for the anti form of 2,
ꢏꢇꢁꢊꢁVXEVWLWXWHGꢁR[LPHꢁGHULYDWLYHVꢁUHVSHFWLYHO\ꢆꢁ7KLVꢁ&+ꢁDQGꢁ
:HꢁWKDQNꢁ'Uꢆꢁ5ꢆꢁ9LMD\DUDJKDYDQꢇꢁ'LUHFWRUꢁ'5'(ꢇꢁ*ZDOLRUꢇꢁ
for his keen interest, encouragement and fruitful discussion.
Received 18 February 2008; accepted 15 April 2008
Paper 08/5102
doi: 10.3184/030823408X318424
References
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6ZHGHQꢇꢁSSꢆꢃꢏꢏ±ꢃꢏꢒꢆ
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H6-H10
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H6-H11
CH-H10
25
CH-H11
20
H6-H12
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ꢁꢈꢃꢁ *ꢆ(ꢆꢁ+DZNHVꢇꢁ.ꢆꢁ+HUZLJꢁDQGꢁ-ꢆ'ꢆꢁ5REHUWVꢇꢁJ. Org. Chem., 1974, 39,1017-
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ꢁꢈꢋꢁ 9DOHQWLQꢁ $QDQLNRYꢇꢁ =HOLQVN\ꢁ ,QVWLWXWHꢁ RIꢁ 2UJDQLFꢁ &KHPLVWU\ꢇꢁ 5XVVLDQꢁ
$FDGHP\ꢁRIꢁ6FLHQFHꢇꢁ0RVFRZꢇꢁBruker Biospin report 2005, pp.41-43.
ꢁꢈꢒꢁ -ꢆꢁ(SVWHLQꢇꢁ-ꢆ-ꢆꢁ.DPLQVNLꢇꢁ1ꢆꢁ%RGRUꢇꢁ5ꢆꢁ(QHYHUꢇꢁ-ꢆꢁ6RZDꢁDQGꢁ7ꢆꢁ+LJXFKLꢇꢁ
J. Org. Chem., 1978, 43(14), 2816-2821.
CH-H12
CH-OH
15
10
5
0
0
50
100
150
200
250
mixing time in ms
Fig. 6 Build-up curves of the seven most intense intraproton
tr-NOE cross-peaks. (a) CH-OH; (b) H6-H12; (c)CH–H12; (d)
H6–H11; (e) CH–H11; (f) H6–H10; (g) CH–H10. The parabolic
behaviourisobservedforallthepeaksforN-dodecyl-2-(hydroxy-
iminomethyl)pyridinium bromide.
18 N. Heimer, S. Del, E. Rico and W.R. Carper, Magn. Reson. Chem., 2004,
42, 71-75.
19 G.G. Kleinspehn, J.A. Jung and S.A. Studnlarz, J. Org. Chem., 1967, 32,
460.