Spectral assignment and proton transfer studies of N-(R-salicylidene)-1-amino-1-deoxy- d -sorbitols

Full text of DOI:10.1002/mrc.4261

Letter ‐ spectral assignment
Received: 16 February 2015
Revised: 8 April 2015
Accepted: 20 April 2015
Published online in Wiley Online Library: 12 June 2015
(wileyonlinelibrary.com) DOI 10.1002/mrc.4261
Spectral assignment and proton transfer studies
of N-(R-salicylidene)-1-amino-1-deoxy-D-sorbitols
A. Szady-Chełmieniecka,a P. Ossowicz,a W. Schilfb and Z. Rozwadowskia*
machine (BRUKER BioSpin, Rheinstetten, Germany) using triple-
resonance inverse probehead. For single scan one-dimensional
Introduction
Schiff bases (1–9), derivatives of various o-hydroxyaldehydes and
1-amino-1-deoxy-D-sorbitol, are optically active compounds that
can be studied as potential ligands for various metal complexes.[1,2]
The properties of the Schiff base complexes and especially their
use as enantioselective catalysts[3–6] are related to the structure of
the ligands; therefore, the knowledge about factors crucial for struc-
ture of ligands like hydrogen bond and proton transfer processes
can be important for the possible applications of studied
compounds.
In this report, we present studies concerning intramolecular
hydrogen bonding and tautomeric equilibrium as well as full as-
signment of 1H and 13C NMR spectra and 15N NMR data of
N-(R-salicylidene)-1-amino-1-deoxy-D-sorbitols, which can be useful
in synthesis of various optically active metal complexes[1,2] or chiral
N-acyloxazolines.[7] Schiff bases, derivatives of natural product like
D-sorbitol, can be especially useful in synthesis of new chiral com-
plexes because of the presence of carbon chain with several hy-
droxyl groups and possible formation of polydentate complexes.
Additionally, recent studies concerning Schiff bases, derivatives of
glucosamine, and their complexes deal usually with ring form of
glucose moiety,[8,9] only few deal with chain form.[1,2]
proton spectra, π/2 pulse was applied; for multi-pulse experiments,
usually 30° flip angle was used. one-dimensional 13C NMR spectra
were obtained using 30° pulse with broadband proton decoupling.
For proton spectra, the acquisition time of 4 s without additional re-
laxation delay was used. The carbon NMR spectra were recorded
with acquisition time about 1.5s and relaxation delay of 1 s. The
typical spectral widths of 15, 220, and 400 ppm were used for pro-
ton, carbon, and nitrogen spectra, respectively. All two-dimensional
spectra (gCOSY, gHSQC, and gHMBC) usually were acquired with
2048 data points for t2 and 256 for t1 increments. The long-range
coupling correlation measurements (gHMBC) were optimized for
nJ = 8 Hz for both carbon and nitrogen experiments. For all
two-dimensional experiments, the linear prediction and zero filling
procedures were applied. The following weighting functions were
used: squared sine bell for f1 and f2 domains in gCOSY, Gaussian
in f1 and squared sine bell in f2 for gHMBC, and Gaussian in f2
1
and f2 for gHSQC measurements. The H and 13C chemical shifts
were referred to internal TMS, and 15N chemical shifts were
referred to external nitromethane as a standard according to
Internation Union of Pure and Applied Chemistry (IUPAC) recom-
mendation. The standard Bruker software was used for acquisition
of spectra and data processing. Typical concentration of the
samples was 0.1M.
Experimental
Materials
Results and discussion
All salicyladehydes used and 1-amino-1-deoxy-D-sorbitol were pur-
chased from Sigma-Aldrich Poznań, Poland, and the methanol was
purchased from Chempur Piekary Śląskie, Poland.
1
The full assignments of H signals of aromatic moiety as well as
sugar moiety for studied compounds 1–9 in DMSO solutions are
given in Table 1.
Synthesis
The proton signals with the highest frequencies, assigned to the
proton donor group, are observed in the range δ = 13.33–14.58
(Table 1) and indicate the presence of a medium strong hydrogen
bond.[10] The most deshielded signals are of low intensity and
mostly broad. For all compounds studied, imine signals at room
temperature were in the range from 8.25 to 8.69 ppm. The proton
chemical shifts of sugar moiety have changed only slightly with
Schiff bases (1–9) (Fig. 1) were prepared according to the procedure
described in Ref. [7] in methanol solution. The crude products were
recrystallized from methanol. Compounds (1) and (6) are already
known and characterized[7]; the other studied Schiff bases have
not been synthesized before. Melting point of these new com-
pounds, elemental analysis, tables with FTIR-spectra description,
UV-Vis bands as well as specific rotation are available in the
Supporting information.
*
Correspondence to: Z. Rozwadowski, Department of Inorganic and Analytical
Chemistry, West Pomeranian University of Technology, Szczecin, Al. Piastów 42,
Szczecin 71-065 , Poland. E-mail: zroz@zut.edu.pl
Measurements
1
The H and 13C NMR spectra were recorded on Bruker DPX-400
a Department of Inorganic and Analytical Chemistry, West Pomeranian University
of Technology, Szczecin, Al. Piastów 42, Szczecin 71-065, Poland
spectrometer (BRUKER BioSpin, Rheinstetten, Germany) at room
temperature (25 °C) in DMSO-d6 solution. The 15N indirect correla-
tion measurements have been performed on Bruker DRX-500
b Institute of Organic Chemistry, Polish Academy of Sciences, ul. Kasprzaka 44,
Warsaw 01-224, Poland
Magn. Reson. Chem. 2015, 53, 849–852
Copyright © 2015 John Wiley & Sons, Ltd.

A. Szady-Chełmieniecka et al.
Figure 1. Studied N-(R-salicylidene)-1-amino-1-deoxy-D-sorbitols (1–9).
the change of substituents on aromatic ring (DELTAδH up to
~0.1ppm). Larger change of the chemical shifts (up to 0.4ppm)
was observed for OH signals at position 2′.
The full assignments of 13C signals for studied compounds 1–9 in
DMSO solutions are given in Table 2. The values of 13C chemical
shifts of aromatic moiety for compounds (1), (8), and (9) were sim-
ilar to those observed for glucosamine Schiff bases in DMSO.[9]
The chemical shift of the C-2 carbons, the most sensitive for the
position of the proton transfer equilibrium, was in the range from
δ =154.70ppm for N-(5-methoxysalicylidene)-1-amino-1-deoxy-D-
sorbitol (4) to 178.60 ppm for N-(5-nitrosalicylidene)-1-amino-1-
deoxy-D-sorbitol (2) and was similar to those observed for other Schiff
bases, derivatives of various aromatic o-hydroxyaldehydes.[11–13] For
compounds (1), (3), (4), and (7), values of δC-2 about 160 ppm sug-
gested localization of the hydrogen at oxygen atom of OH group,
while for compounds (2), (5–6), and (8–9), values above ~165 ppm
might indicate the presence of proton transfer equilibrium (Fig. 2).
To estimate the equilibrium constants for the proton transfer
process in the Schiff base and hence mole fraction of the NH form,
the equation proposed in Ref. [14] has been used:
δ ꢀ δOH
K ¼
δNH ꢀ δ
where δ is the observed chemical shift and δOH and δNH are chem-
ical shifts for the pure OH and NH forms, respectively. As a refer-
ence, values of δOH and of δNH, 152.2[11] and 180.4 ppm,[12]
respectively, have been used. Detailed description of calculation
of the χNH values is available in the Supporting information. The
estimated mole fractions of the NH form (χNH) on the basis of the
δC-2 chemical shift are given in Table 3.
The established χNH values suggested the presence of proton
transfer equilibrium for almost all the compounds studied (except
4). For compounds 2, 5, 6, 8, and 9, χNH values were equal to or
even larger than 0.5. The strongest influences on the position of
the proton transfer equilibrium have shown to be electron-
withdrawing nitro group at position 5, methoxy or hydroxyl group
at position 4, and presence of the halogen atoms at positions 3
and 5. Similar effect of substituent at position 5 was observed for
Schiff bases, derivatives of benzylamine.[13]
In the glucose moiety, the largest variation of the chemical shifts
was observed for C-1′ and C-2′ carbons. Smaller values at δC-1′ and
δC-2′ were found for these compounds, where proton transfer equi-
librium has been suggested, for example, (2), (5–6), and (8–9).
The 15N chemical shifts were in the range from ꢀ83.9 ppm for N-(5-
methoxysalicylidene)-1-amino-1-deoxy-D-sorbitol (4) to ꢀ141.9 ppm
for N-(4-methoxysalicylidene)-1-amino-1-deoxy-D-sorbitol (5)(Table4).
Almost all δN values are typical for Schiff bases with OH…N intramo-
lecular hydrogen bond where proton transfer exists,[15] similar to
those observed for glucosamine Schiff bases[9] as well as for deriva-
[14]
tives of benzylamine.[13] Using the equation proposed by Ref.
,
the position of the proton transfer equilibrium has been also esti-
mated taking into account the values of 15N chemical shifts (Table 3).
The value ꢀ83.1 ppm has been taken as a reference value for pure
OH form while ꢀ243.9 ppm for the pure NH form.[16]
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Magn. Reson. Chem. 2015, 53, 849–852

Proton transfer studies of N-(R-salicylidene)-1-amino-1-deoxy-D-sorbitols
Table 2. 13C chemical shifts (ppm) of N-(R-salicylidene)-1-amino-1-deoxy-D-sorbitols (1–9) in DMSO-d6 solution
Compound
Position
6
1
2
3
4
5
7
1′
2′
3′
4′
5′
6′
(1)
(2)
(3)
(4)
(5)
(6)
(7)
(8)
(9)
118.68
113.36
118.42
119.08
111.63
111.09
119.50
116.73a
116.25
161.23
178.60
158.66
154.70
168.67
167.16
160.75
166.35
166.12
116.62
123.11
116.29
117.16
101.33
102.96
118.93
116.73a
115.77
132.21
118.18
131.63
133.25
131.44
114.85
133.45
133.61
130.55
134.37
130.51
166.64
167.82
166.51
166.29
165.13
165.29
165.56
166.09
166.17
61.37
54.82
61.63
61.81
58.29
58.98
61.00
55.72
56.00
72.26
70.88
72.30
72.29
72.22
72.31
72.09
71.45
71.49
70.06
70.20
70.03
70.05
70.02
69.98
70.10
70.15
70.16
71.64
71.14
71.68
71.67
71.52
71.61
71.48a
71.42
71.42
71.50
71.40
71.50
71.50
71.47
71.48
71.48a
71.01
71.02
63.43
63.37
63.43
63.43
63.42
63.42
63.43
63.39
63.40
129.34
132.92
132.81
126.80 (19.95)
151.43 (55.56)
105.40
118.55
163.84 (55.15)
162.20
106.45
132.00
121.18
138.39
102.65
133.09
125.29
aOverlapped.
close to 0, while for (2), (5), and (6), χNH values are between 0.2 and
0.4. However, these values were much smaller than calculated based
on 13C chemical shifts. Because of discrepancy of the χNH values es-
timated based on 13C and 15N chemical shifts, the position of the
proton transfer equilibrium was also evaluated based on UV-Vis re-
sults (Table 3). The presence of two bands, low-energy band at
~400 nm and high-energy band ~300 nm, for all N-(R-salicylidene)-
1-amino-1-deoxy-D-sorbitols indicated the existence of the proton
transfer equilibrium[17,18] (Table S1 in the Supporting information)
and allowed calculating the mole fractions of the NH form (Table 3).
The description of the equilibrium constants calculation and χNH
values of the studied N-(R-salicylidene)-1-amino-1-deoxy-D-sorbitols
is available in the Supporting information.
Figure 2. Proton transfer equilibrium in Schiff bases.
Table 3. Estimated mole fraction of the NH form (χNH) of compounds
studied
Compound
(χNH)
Comparison of the data from Table 3 has shown better agree-
ment between χ values estimated based on 13C chemical shift
and evaluated by UV-Vis spectroscopy than estimated based on
15 N chemical shift values for almost all compounds studied. The ex-
tremely large variation of the χ values for 8 and 2 has shown the
sensitivity of the δ15 N not only to position of the proton transfer
equilibrium but also to substituent effects. Hence, δC-2 values were
a better tool in preliminary estimation of position of the proton
transfer equilibrium than δN values. Larger differences between
both values ( χ up to 0.3) were observed for compounds where
mole fraction of the NH form was larger or equal to 0.5. Different
values of χ calculated based on δC-2, δN, and UV-Vis data clearly
have shown that chemical shifts can be useful only for preliminary
estimation of the mole fraction of the NH form.
Taking into account the χ values estimated by UV-Vis spectros-
copy, the nitro group at position 5 or halogens at positions 3 and
5 have the strongest influence on proton transfer equilibrium. Sub-
stituents at position 5 or at 3 and 5 have influence on acidity of the
phenolic group through mesomeric and steric effects. Smaller ef-
fect was observed for compounds with substituents at position 4
where both effects caused a change in the basicity of the nitrogen
atom. Similar influence of the substituents on position of the proton
transfer equilibrium was observed for N-(R-salicylidene)-
glucamines[9] and N-(R-salicylidene)-methylamines[11] or N-(5-R-
salicylidene)-benzylamines.[13]
13C NMR
15N NMR
UV-Vis
1
2
3
4
5
6
7
8
9
0.3
0.9
0.2
0
0
0.2
0
0.2
0.6
0.1
0.1
0.3
0.3
0.2
0.8
0.8
0
0.6
0.5
0.3
0.5
0.5
0.4
0.3
0
0
—
Table 4. 15N chemical shifts (ppm) of N-(R-
salicylidene)-1-amino-1-deoxy-D-sorbitols (1–8) in
DMSO-d6 solution
Compound
δ
(1)
(2)
(3)
(4)
(5)
(6)
(7)
(8)
ꢀ90.4
ꢀ110.2
ꢀ87.6
ꢀ83.9
ꢀ141.9
ꢀ132.9
ꢀ92.2
All studied compounds were optically active, so their optical
properties, especially stability of the molar rotation, are also impor-
tant. The specific and molar rotations [α] of compounds studied in
DMSO are summarized in Table S2 and available in the
Supporting information. Molar rotation was stable in time, and no
signs of racemisation were observed. The values of [α] were in the
range from ꢀ62.7 for compound (9) to ꢀ1.98 for compound (1).
ꢀ94.4
For compounds (1), (3–4), and (7–8), the 15N NMR chemical shifts
have suggested position of the equilibrium strongly shifted towards
the OH form and the mole fraction of the NH form was equal to 0 or
Magn. Reson. Chem. 2015, 53, 849–852
Copyright © 2015 John Wiley & Sons, Ltd.
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A. Szady-Chełmieniecka et al.
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Figure 3. Dependence of the molar rotation on position of the proton
transfer equilibrium for Schiff bases (1–9).
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Supporting information
Additional supporting information may be found in the online ver-
sion of this article at the publisher’s web site.
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wileyonlinelibrary.com/journal/mrc
Copyright © 2015 John Wiley & Sons, Ltd.
Magn. Reson. Chem. 2015, 53, 849–852