Synthesis and multinuclear NMR investigation of some mesoionic 3-phenyl-5-imino-1,2,3,4-oxatriazole imines

Article abstract of DOI:10.1016/j.molstruc.2003.08.017

Six new derivatives of mesoionic 3-phenyl-5-imino-1,2,3,4-oxatriazole imine have been obtained and investigated by ¹H, ¹³C, ¹⁴N, ¹⁵N and ¹⁷O NMR techniques. The ¹⁵N and ¹⁷O NMR s

Full text of DOI:10.1016/j.molstruc.2003.08.017

Journal of Molecular Structure 687 (2004) 23–33
www.elsevier.com/locate/molstruc
Synthesis and multinuclear NMR investigation of some mesoionic
-phenyl-5-imino-1,2,3,4-oxatriazole imines
3
Jaros l⁄ aw Ja z´ wi n´ ski , Olga Staszewska-Krajewska
*
Institute of Organic Chemistry, Polish Academy of Sciences, 01-224 Warszawa, ul. Kasprzaka 44/52, Poland
Received 17 June 2003; revised 11 August 2003; accepted 13 August 2003
Abstract
Six new derivatives of mesoionic 3-phenyl-5-imino-1,2,3,4-oxatriazole imine have been obtained and investigated by H, C, N,
1
13
14
15
N
and O NMR techniques. The N and O NMR studies reveal that mesoionic oxatriazoles imines exist in a cyclic, ‘mesoionic’ form. Two
stable conformers of mesoionic 3-phenyl-5-imino-1,2,3,4-oxatriazole imines, E and Z; in comparable concentrations, have been detected for
1
7
15
17
1
the first time in the solution by the use of low temperature NMR experiments. H NMR techniques and band shape analysis were applied for
estimation of Z –E interconversion barrier of mesoionic 3-phenyl-5-methylimino-1,2,3,4-oxatriazole imine 6 and 3-phenyl-5-benzylimino-
1
,2,3,4-oxatriazole imine 10. Some one-bond J( N– C) coupling constants have been measured using N enriched oxatriazole and
15
13
15
1
tetrazole, as a means of the C5–N 6 bond characterisation.
q 2003 Elsevier B.V. All rights reserved.
2
17
Keywords: Oxatriazole; Mesoionic compound; Nitrogen NMR; O NMR; E–Z interconversion
2
2
1
. Introduction
conjugated bond. Character of the C5–N 6 (C5–C 6)
bond implies the possibility of hindered rotation around this
bond, and existence of two stable rotamers of a compound
Mesoionic oxatriazoles, thiatriazoles and tetrazoles, due
(
cal’ 1,2,3,4-thiatriazolium-5-methylides, for example 12,
Fig. 2a,b). Restricted rotation was observed for ‘symmetri-
to interesting structure, attract attention of chemistry and
physical chemistry. The derivatives of mesoionic 1,2,3,4-
thiatriazole imine 1 (Fig. 1) have been investigated as
multicentre ligands [1–3]. Compounds containing mesoio-
nic 1,3-diphenyl-5-imino-1,2,3,4-tetrazole moiety were
investigated as high-energy molecules [4–6]. Recently,
derivatives of mesoionic 5-phenylimino-1,2,3,4-oxatriazole
imine 4 have been examined as nitrogen oxide carrier agent
in pharmacology [7–10].
1
2
R ¼ R ¼ CN, where two CN groups are non-equivalent
1
3
15
and give two signals on C and N NMR spectra. On the
other hand, simply one set of NMR signals, deriving from
one conformer only, was observed for ‘unsymmetrical’
1
2
methylides (R – R ; e.g. 13).
Ab initio MO calculations [17] suggest that one
conformer of a mesoionic 5-phenylimino-1,2,3,4-thiatria-
zole imine 1 should exist in a solution in minority, less than
The C5–N6 bond of a mesoionic aminide exhibits a
partial double bond character. The C5–N6 bond lengths of
various thiatriazole imines cover the range from 1.262 to
1%, due to energy difference between both forms. On the
contrary, low energy difference between conformers is
expected for mesoionic 5-imino-1,2,3,4-oxatriazole imine;
thus both form of an oxatriazole, being in equilibrium,
should exist in the solution. However, no experimental
proof was found in course of previous investigations of
oxatriazoles [18].
˚ ˚
.290 A [11,12]. Similar bond length, of 1.272 A, is
1
observed for oxatriazole imine 4 [13]. These bonds are
comparable with carbon–nitrogen bond of imines or acyclic
2
amides [14]. The C5–C 6 bond lengths of thiatriazolium-5-
methylides 12 and 13 (Fig. 1) cover the range from 1.398 to
2
2
Ring-chain rearrangement (Fig. 2c) is expected for all
mesoionic compounds [19,20]. Such rearrangement has
been found for sydnonimines [21–23] and isothiosydnoni-
mines [24] only. X-ray study of oxatriazole imine 4 exhibits
an interesting fact that the C5–O1 bond is longer than
˚
.414 A [15,16], i.e. a range of C(sp )–C(sp ) single,
1
*
Corresponding author. Tel.: þ48-22-632-0904; fax: þ48-22 þ 632-
6
681.
E-mail address: jarjazw@icho.edu.pl (J. Ja z´ wi n´ ski).
0022-2860/$ - see front matter q 2003 Elsevier B.V. All rights reserved.
doi:10.1016/j.molstruc.2003.08.017

24
J. Ja z´ wi n´ ski, O. Staszewska-Krajewska / Journal of Molecular Structure 687 (2004) 23–33
Fig. 1. The compounds discussed in the present paper. For all of compound the same numbering scheme is adopted for the comparison purposes.
the C–O bond in furan [13]. Authors conclude that the
oxatriazole 4 tends to adopt an open-chain structure. It is
noteworthy that pharmacological activity and nitrogen
donating properties of mesoionic oxatriazoles are similar
to those observed for 3-morpholino-sydnonimine, which
undergoes ring-chain rearrangement [25].
aromatic, N–CH –R group, with potential pharmacological
2
interest; (ii) examination of hindered rotation around C5–
–
N 6 bond and founding whether two stable conformers of a
compound exist; (iii) establishing if mesoionic 5-imino-
1,2,3,4-oxatriazole imine exists as a linear or a cyclic
valence tautomer.
The present work has been done for a following reasons:
i) synthesis of a new set of mesoionic 5-imino-1,2,3,4-
(
oxatriazole imines, especially compounds with non-
2. Experimental
2
.1. NMR measurements
All NMR measurements were carried out on BRUKER
1
13
Avance DRX 500 spectrometer. H NMR, C NMR,
COSY, HSQC and HMBC experiments were performed
using standard Bruker software with either a 5 mm dual
1
13
H/ C or 5 mm triple broadband inverse gradient probes.
Chemical shifts were referred to internal TMS
1
(
(
d ¼ 0:00 ppm) for H and the central peak of acetone-d
6
1
3
d ¼ 29:8 ppm) for C. Digital resolutions were 0.2 Hz per
1
13
point in the H and 0.5 Hz per point in the C NMR spectra.
Typically, a relaxation delay of 1 s, an acquisition time of
1
ca. 1 s and pulse flip angle of 458 were applied for C NMR
3
experiment. Carbon-13 chemical shifts have been taken
3
1
from 1D C NMR spectra; HSQC and HMBC 2D spectra
were used as a means of signals assignments only.
1
5
13
One-bond N– C coupling constants were taken from
C NMR spectra, with parameter values of 0.1 s for a
1
3
relaxation delay, an acquisition time of ca. 9 s, and a sweep
width of ca. 1500 Hz, giving FID resolution of ca. 0.055 Hz
per point, and, after zero-filling, a spectral resolution of
Fig. 2. Equilibriums potentially expected for mesoionic compounds in a
solution. (a), (b) equilibriums between E and Z form for free mesoionic
base and salt, being a consequence of hindered rotation around C5–N6 or
1
5
2
0.01 Hz per point. The samples 100% N enriched at N6
position were used.
C5–C bond; (c) hypothetical ring-chain valence tautomerism for
mesoionic 3-phenyl-5-alkylimino-1,2,3,4-oxatriazole imine.

J. Ja z´ wi n´ ski, O. Staszewska-Krajewska / Journal of Molecular Structure 687 (2004) 23–33
25
1
The N NMR chemical shifts were referred with respect
5
experimental values of T ; conformers molar fractions p1
2
to the signal of external neat nitromethane (d ¼ 0 ppm). For
and p2; chemical shifts of both forms and guess value of
exchange rate, k: Both spectra, calculated one and
experimental one, have been compared on the plot. In the
next step the values of chemical shifts and normalization
factor were readjusted, the exchange rate k was changed in
order to obtain the spectrum similar to experimental one.
Then calculations were repeated. As a quantitative means
of spectra fitting, mean squared error was used. On this
1
5
15
direct N NMR measurement a dedicated 10 mm N probe
was utilized. A frequency of 50.689 MHz was applied with
typical parameters values of 3 s for a relaxation delay, 1.6 s
for an acquisition time, a flip angle of 308 and about 300–
1000 scans depending on the sample concentration. The
solutions with concentration of ca. 1 M were used (saturated
solutions); about 1% of Cr(acac) per one mole of substrate
3
molecule was added as a relaxation reagent.
1
way, it was possible to obtain the exchange rate k with
21
4
The N NMR measurements were recorded at
6.136 MHz using a 5 mm broad band probe; with a typical
relaxation delay of 0.001 s, an acquisition time of 0.2 s,
accuracy of ^0.2 s
.
3
The normalization factor was adjusted by the comparison
of signals integral intensities for calculated and experimen-
tal spectra. The initial population ratios of conformers,
p1/p2, have been calculated using signal integral intensities
from a spectrum recorded at low temperature, usually
253 K. The population ratios were recalculated for each
temperature by Boltzman’s equation, p1=p2 ¼ expð2DE=
spectral resolution of 1 Hz per point, flip angle 908 and ca.
4000 scans. Typically, diluted solutions of mesoionic
heterocycles with concentration of 0.1 M, were used. The
1
4
spectrometer software was applied for the N signals
deconvolution.
1
7
The O NMR spectra were taken at 67.818 MHz using a
0 mm broad band probe with typical parameters values of
.001 s for a relaxation delay, 0.02 s for an acquisition time,
RTÞ: Initial T relaxation time values were calculated on the
2
1
0
basis of signals half-width taken from the low temperature
spectrum (253 K). Then, T2 was estimated for each
experiment applying the reference line method [26] with
a flip angle of 908 and about 7000 scans. FID resolution of
6 Hz/point was applied, giving, after zero-filling, a spectral
2
the use of central signal of residual acetone-d multiplet on
6
resolution of 13 Hz per point. The solvent signal was used as
secondary reference; dacetone ¼ 573:4 ppm with respect to
water signal dwater ¼ 0 ppm.
each spectrum as the reference line. Its half-width was found
by deconvolution procedure using Bruker spectrometer
software. Chemical shift drift, of ca. 0.4 Hz K2 ; was
readjusted manually, during line shape calculation, in order
to obtain the best fit of experimental and calculated line.
1
1
13
15
Temperature-variable H, C and N NMR spectra
were recorded on a Bruker Avance DRX 500 instrument,
equipped with BVT 3000 temperature control unit giving
temperature stability of 0.1 K, and with the use of a probe
#
#
DH and DS have been obtained by a least-squares
adaptation of the rate constant and corresponding tempera-
1
13
21
21
fitted with thermocouple (5 mm dual H/ C probe, 5 mm
triple broadband inverse gradient probe or a dedicated
tures to a straight line of the form lnðkT Þ ¼ bT þ c;
#
#
where DH ¼ 28:31b and DS ¼ 8:31(c 2 23.76). Free
1
0 mm N probe). Temperatures, from 233 to 318 K, have
5
#
1
energy of activation, DG ; has been calculated by the
#
#
#
been read from the instrument panel; no further measures
for more precise temperature determinations for standard
samples were taken. The measurements for band shape
analysis have been made using 5 mm triple broadband
inverse gradient probe; the temperature reads have been
corrected by means of a calibration curve based on methanol
chemical shift thermometer.
equation DG ¼ DH 2 TDS [26].
Standard deviations were obtained from least-square
2
1
procedures [26] assuming inaccuracy of ^0.2 s
for
interconversion rate k and ^0.2 K for a temperature.
Temperature control unit allows obtaining a temperature
stability of ^0.1 K. A systematic error of ca. ^1.5 K
between two different series of experiments arises from
equipment setting itself. The temperature scale for a set of
experiments can be shifted by ^1.5 K, whereas the
temperature differences within the set are given with
accuracy of ^0.1 K. A systematic error provides an
2
.2. NMR data treatments
All data treatments were performed with Mathcad 2001
1
21 21
#
Professional program, running on a personal computer. H
NMR measurements at various temperatures, and the
complete band shape method were applied for estimation
of E–Z interconversion rate for oxatriazoles 5, 6 and 10.
The signals of either CH or CH groups were used for
inaccuracy of ^0.5, ^1 and ^1.3 J mol
K
for DG ;
#
#
DH and DS ; respectively, i.e. gives the values comparable
or slightly greater than standard deviations obtained from
least-square methods.
3
2
calculations, assuming an uncoupled two-site, A and B,
exchange model [26]. The spectra were recorded at various
temperatures, at least six measurements for each sample,
below and above of coalescence temperature. Interconver-
sion rate k has been estimated for each temperature by
means of band shape analysis. Typically, a spectrum
was calculated using a normalization factor, estimated
2.3. Synthesis
All thiosemicarbazides were obtained from commer-
cially available isothiocyanates and phenylhydrazine, using
slightly modified, published procedure [27]. Typically,
3
phenyhydrazine (5.5 g, 5 cm , 51 mmol) was dissolved
3
in ethyl ether (100 cm ), then suitable isothiocyanate

26
J. Ja z´ wi n´ ski, O. Staszewska-Krajewska / Journal of Molecular Structure 687 (2004) 23–33
(
51 mmol) was added. The mixture was stirred for ca.
5 min, then the solution was cooled in ice bath, the
2.5. Mesoionic 3-phenyl-5-(4-methoxyphenyl)imino-
1,2,3,4-oxatriazole imine 5
1
precipitate of thiosemicarbazide were filtered, washed
with cold ethyl ether, and dried in vacuo. If the product
did not precipitate, then the solvent was evaporated in
vacuo, residual oil was diluted with small amount of ethyl
ether and cooled overnight in refrigerateur. Next day
crystals were filtered, washed with cold ether and dried in
vacuo. Thiosemicarbazides were used for further reaction
without additional purification. Following thiosemicarba-
zides have been obtained in this manner: 1,4-diphenyl-3-
thiosemicarbazide (11.2 g, 90%); 1-phenyl-4-(4-methoxy-
phenyl)-3-thiosemicarbazide (13.4 g, 97%); 1-phenyl-4-
methyl-3-thiosemicarbazide (6.6 g, 72%); 1-phenyl-4-
ethyl-3-thiosemicarbazide (5.7 g, 57%); 1-phenyl-4-(n-
butyl)-3-thiosemicarbazide (6.2 g, 55%); 1-phenyl-4-ben-
zyl-3-thiosemicarbazide (10.4 g, 80%); 1-phenyl-4-allyl-3-
1-phenyl-4-(4-methoxyphenyl)-3-thiosemicarbazide
(14.9 g, 58 mmol) and sodium nitrite (6 g, 87 mmol) was
used. Crystallization of crude mesoionic base from ethanol
yielded deep red crystals of 5 (8.9 g, 57%), m p. 113–
þ
115 8C. (Found C, 62.52; H, 4.55; N, 20.96%; M 268.
C H N O requires C, 62.68; H, 4.51; N, 20.88%; M
14 12 4 2
268.28).
2.6. Mesoionic 3-phenyl-5-methylimino-1,2,3,4-oxatriazole
imine 6
1-phenyl-4-methyl-3-thiosemicarbazide (3 g, 17 mmol)
was suspended in ethanol (50 cm ). The mixture was cooled
3
3
on ice-bath (0 8C); then butyl nitrite (9 g, 10 cm , 87 mmol)
1
5
thiosemicarbazide (5.7 g, 57%). N enriched 1-phenyl-
4
was added. The mixture was saturated with hydrogen
chloride, obtained from concentrated hydrochloric acid
-methyl-3-thiosemicarbazide was obtained from
N-methylisothiocyanate, which has been prepared from
1
5
3
3
(20 cm ) and concentrated sulfuric acid (30 cm ), and
stirred at room temperature for ca. 30 min. Ethanol and an
excess of butyl nitrite were evaporated in vacuo; the residue
1
5
N-methylamine (HCl salt) by means of reaction
with CS and Pb(CH COO) , followed by steam distilla-
tion [28].
2
3
2
3
was dissolved in ethanol (100 cm ) and acidified with a few
Oxatriazole 4 has been obtained originally from 1,4-
diphenylthiosemicarbazide by nitrosation with butylnitrite
drops of hydrochloric acid. Small amount of activated
carbon was added to the solution; the mixture was heated for
a few minutes, filtered and cooled in an ice-bad. A large
volume of diethyl ether was added stepwise to a cold, stirred
filtrate. A salt of the mesoionic oxatriazole slowly
precipitated; the mixture was cooled overnight in refriger-
ator; then a white solid of salt was filtered and dried in vacuo
giving a salt of mesoionic compound (2 g, 57%). The salt
[
29]. However, making use of sodium nitrite seems to be a
cheaper and simpler way of synthesis. Oxatriazole 5 was
also obtained by this manner. Application of sodium nitrite
seems to be a general method of synthesis of oxatriazoles
with aromatic group attached to ‘exocyclic’ nitrogen atom,
that are 4 and 5. However, the remaining oxatriazoles, 6–
3
1
0, need to use butylnitrite as nitrosation agent.
was dissolved in water (50 cm ) and alkalized with a few
grams of sodium carbonate. The mixture was extracted three
times with methylene chloride; extract was dried over
sodium carbonate, filtered and evaporated in vacuo. The
resulting yellow residue was dissolved in warm n-hexane,
the solution was heated with small amount of activated
carbon for a few minutes, filtered, concentrated in vacuo and
cooled overnight in a refrigerator, at ca. 210 8C. Next day
yellow crystals of the free mesoionic base were filtered and
2.4. Mesoionic 3-phenyl-5-phenylimino-1,2,3,4-oxatriazole
imine [30] 4
1,4-diphenylthiosemicarbazide (14 g, 58 mmol) was
3
suspended in ethanol (350 cm ), cooled (210 8C) on ice
bath, and acidified with concentrated hydrochloric acid
(
3
21 cm ). The solution of sodium nitrite (6 g, 87 mmol) in
dried in vacuo (0.7 g, 4 mmol, 24%), m p 61–63 8C. (Found
þ
3
water (90 cm ) was added stepwise to the suspension of
thiosemicarbazide. Then the reaction mixture was stirred at
room temperature for ca. 15 min; subsequently a large
C, 54.89; H, 4.66; N, 31.80%; M , 176. C
8
H
8
N
4
5
O requires
1
C, 54.54; H, 4.58; N, 31.80%; M, 176.18). N-enriched
oxatriazole has been obtained in the similar manner, by the
1
use of N-labelled thiosemicarbazide.
5
3
excess of water (ca. 400 cm ) was added, and the mixture
Compounds 7–10 were obtained in a similar way
was filtered in order to removal of insoluble polymeric
material. The filtrate was alkalized with anhydrous sodium
carbonate (ca. 35 g) and cooled overnight. Next day red
crystals was filtered, washed with water and dried in vacuo.
Crystallization from ethanol or n-hexane yielded red
crystals of 4 (12 g, 88%). The compound was identified
2.7. Mesoionic 3-phenyl-5-ethylimino-1,2,3,4-oxatriazole
imine 7
1-phenyl-4-ethyl-3-thiosemicarbazide (3.3 g, 17 mmol)
yielded slightly gray powder of hydrochloride of 7 (2.5 g,
65%). Alkalization of salt and crystallization of free base
from n-hexane yielded yellow crystals of free mesoionic
base (1.3 g, 41%), m p 34.5–36.0 8C. (Found C, 56.57; H,
by means of NMR data, elemental analyses and MS.
þ
(Found C, 65.71; H, 4.14; N, 23.68%; M 238.
C H N O requires C, 65.54; H, 4.23; N, 23.52%; M
1
3 10 4 2
2
38.25).

J. Ja z´ wi n´ ski, O. Staszewska-Krajewska / Journal of Molecular Structure 687 (2004) 23–33
27
þ
13
C NMR data are taken from Ref. [18] and given for the
comparison purposes: DMSO-d , 303 K, C5: 162.4. N3-Ph:
5
.28; N, 29.61 %; M , 190. C H N O requires C, 56.83; H,
9
10 4
5
.30; N, 29.46%; M, 190.21).
6
0 0 0 0
134.0 (C1 ); 121.6 (C2 ); 130.3 (C3 ); 133.8 (C4 ). N6-Ph:
145.1 (C1 ); 122.9 (C2 ); 128.9 (C3 ); 122.5 (C4 ).
00 00 00 00
2.8. Mesoionic 3-phenyl-5-(n-butyl)-imino-1,2,3,4-
oxatriazole imine 8
2
1
.13. Mesoionic 3-phenyl-5-(4-methoxyphenyl)imino-
,2,3,4-oxatriazole imine 5
1-phenyl-4-(n-butyl)-4-3-thiosemicarbazide (3.7 g,
7 mmol) yielded slightly gray powder of hydrochloride
1
salt of oxatriazole 8 (2.0 g, 46%). Alkalization of salt and
1
0
H NMR: Acetone-d , 253 K, N3–Ph: 8.23, 8.19* (H2 );
6
crystallization of free base from n-hexane yielded yellow
crystals of free mesoionic base 8 (1.2 g, 32%), m p 39–
0
0
00
7
6
2
.80 m (H3 ); 7.86 m (H4 ). N6-R: 7.31, 7.22* (H2 ); 6.87,
.90* (H3 ); 3.76, 3.75 (CH ). Acetone-d /CF COOH,
00
3
6
3
þ
3
C H N O requires C, 60.53; H, 6.47; N, 25.67%; M
9.5 8C. (Found C, 60.53; H, 6.59; N, 25.72%; M 218.
0
0
0
53 K, 8.47, 8.39* (H2 ); 8.01 m (H3 ); 7.89 m (H4 ).
00
00
1
1 14 4
N6–R: 7.65, 7.57* (H2 ); 7.12*, 7.08 (H3 ); 3.81*, 3.79
a
2
18.26).
a
a
CH ). NH: 12.95*( ), 12.76( ), CF COOH: 14.19( ).
(
3
3
1
3
C NMR: Acetone-d , 253 K, C5: 164.7*, 163.6. N3-Ph:
6
2.9. Mesoionic 3-phenyl-5-allylimino-1,2,3,4-oxatriazole
imine 9
0
0
0
1
1
1
35.3d (C1 ); 122.1, 122.0* (C2 ); 130.9 (C3 ); 134.3,
00
0
00
00
34.2* (C4 ). N6-R: 139.7, 139.0* (C1 ); 124.4d (C2 );
0
0
14.6*, 114.5 (C3 ); 156.3*, 155.9 (C4 ); 55.1d (CH ).
3
3.5 g (17 mmol) of 1-phenyl-4-allyl-3-thiosemicarbazide
3.5 g, 17 mmol) gave slightly gray powder of hydrochlo-
Acetone-d /CF COOH, 253 K, C5: 170.8*. 170.5. N3–Ph:
3
6
(
ride salt (2.4 g, 60%). Alkalization of salt and crystallization
0
b
0
0
1
1
1
33.7, 133.6* (C1 ); 123.2( ), 123.1* (C2 ); 132.0d (C3 );
0
00
b
37.2d (C4 ). N6-R: n.d. (C1 ); 123.3*( ), 124.4 (C2 );
00
of free base from n-hexane yielded yellow crystals of free
mesoionic base 9 (1.5 g, 44%), m p 37–39 8C. (Found C,
00
00
15.9*, 115.7 (C3 ); 160.1*, 159.5 (C4 ); 55.8*, 55.7
(CH3).
þ
5
requires C, 59.40; H, 4.98; N, 27.71%; M 202.22).
9.28; H, 4.79; N, 27.91%; M þ H, 203. C H N O
1
0 10 4
2.14. Mesoionic 3-phenyl-5-methylimino-1,2,3,4-
oxatriazole imine 6
2.10. Mesoionic 3-phenyl-5-benzylimino-1,2,3,4-
oxatriazole imine 10
1
0
H NMR: Acetone-d , 318 K, N3–Ph: 8.11 br (H2 ); 7.74
6
0 0
H3 ); 7.80 (H4 ). N6–R: 3.03 (CH ). Acetone-d , 253K,
3 6
(
1
-phenyl-4-benzyl-3-thiosemicarbazide (4.4 g, 17 mmol)
0
0
0
N3–Ph: 8.10*, 8.06 (H2 ), 7.71 m (H3 ); 7.77 m (H4 ). N6-
R: 2.95*; 2.91 (CH ). Acetone-d /CF COOH, 318 K, N3–
gave hydrochloride salt of 10 (2.6 g, 52%). Alkalization of
salt and crystallization of free base from n-hexane yielded
yellow crystals of free mesoionic base 10 (1 g, 24%), m p
3
6
3
0
0
0
Ph: 8.36 br (H2 ); 7.91 m (H3 ); 8.03 (H4 ). N6–R: 3.45
(CH ). Acetone-d /CF COOH, 253 K, N3-Ph: 8.42, 8.36*
þ
3
6
3
6
2
2
6–68 8C (Found C, 66.79; H, 4.69; N, 22.12%; M þ H,
0
0
0
(H2 ); 7.92 m (H3 ); 8.04 m (H4 ). N6–R: 3.44 (CH ). NH:
3
53. C H N O requires C, 66.66; H, 4.79; N, 22.21%; M,
4
1
4
12
a
a
a
1.20*( ). 10.92( ); CF COOH: 13.8( ).
1
3
52.28).
1
3
C NMR: Acetone-d , 243 K, C5: 165.9*, 162.1. N3-Ph:
6
0
0
0
1 13
.11. H and C NMR data for oxatriazoles 4–10
135.4, 135.3 (C1 ); 121.8d (C2 ); 130.8d (C3 ); 134.1*,
0
2
1
34.0 (C4 ). N6-CH : 35.7*, 33.7. Acetone-d /CF COOH,
3
6
3
0
0
3
03 K, C5: 172.9. N3–Ph: 134.0 (C1 ); 123.3 (C2 ); 131.9
Asterisk denotes a signal of a major conformer.
Abbreviations: d—two signals with chemical shift differ-
ence less than 0.1 ppm; br—broad signal; m—unresolved
0 0
C3 ); 136.9 (C4 ). N6–CH : 31.7 br.
3
(
a
multiplet; n.d.—signal no detected; ( ) data from a spectrum
2.15. Mesoionic 3-phenyl-5-ethylimino-1,2,3,4-oxatriazole
imine 7
b
taken at 233 K; ( ) signal assignment can be opposite.
Chemical shifts (ppm) were referred to internal TMS
1
1
0
(
d ¼ 0:00 ppm) for H and to the central peak of acetone-
H NMR: Acetone-d
0
6
, 253K, N3–Ph: 8.14, 8.11 (H2 ),
0
1
3
d (d ¼ 29:8 ppm) for C.
7.82 m (H3 ); 7.76 m (H4 ). N6-R: 3.32, 3.27 (CH
1
(H2 ); 7.88 (H3 ); 7.99 (H4 ). N6–R: 3.84 (CH
2
); 1.15,
.12 (CH ). Acetone-d /CF COOH, 303 K, N3-Ph: 8.34 br
6
3 6 3
0
0
0
2.12. Mesoionic 3-phenyl-5-phenylimino-1,2,3,4-
oxatriazole imine 4
2
); 1.44
COOH, 253 K, N3–Ph: 8.53*, 8.41
(CH
). Acetone-d
/CF
H2 ); 8.03 m (H3 ); 7.92 m (H4 ). N6–R: 3.84 (CH ); 1.40
3
6
0
3
0
0
(
(CH
2
1
0
0
0
a
a
a
H NMR: Acetone-d , 253 K, N3–Ph: 8.22*, 8.20 (H2 ),
6
3
). NH: 10.95*( ), 10.67( ). CF
3
COOH 14.0( ).
C NMR: Acetone-d , 243 K, C5: 160.8, 164.9. N3–Ph:
6
0
0
0
0
13
7.85 m (H3 , H4 ). N6–Ph: 7.30 m (H2 , H3 ); 7.01 (H4 ).
Acetone-d /CF COOH, 353 K, N3–Ph 8.49*, 8.41 (H2 );
0
0
0
135.4, 135.3 (C1 ); 121.8d (C2 ); 130.8 (C3 ); 134.1, 133.9
6
3
0
0
0
0
7
.93 (H3 ); 8.05 (H4 ). N6–Ph: 7.78 (H2 ); 7.31–7.70 m
0
(C4 ). N6–R: 43.9, 41.8 (CH ); 16.7, 16.6 (CH ). Acetone-
2
3
0
0
d /CF COOH, 303 K, C5: 172.4. N3–Ph: 134.1 (C1 ); 123.3
6 3
(
H3 , H4 ).

28
J. Ja z´ wi n´ ski, O. Staszewska-Krajewska / Journal of Molecular Structure 687 (2004) 23–33
0
0
0
00
(
C2 ); 131.9 (C3 ); 137.0 (C4 ). N6–R: 41.7 (CH ); 14.0
2
126.9d (C4 );52.8, 50.6 (CH ). Acetone-d /CF COOH,
2
6
3
0
0
(
CH3).
303 K, C5: 172.7. N3–Ph: 134.1 (C1 ); 123.3 (C2 ); 131.9
0 0 00 00
(C3 ); 137.0 (C4 ). N6–R: 135.6 (C1 ); 129.1 (C2 ); 129.8
(C3 ); 129.4 (C4 ); 49.7 (CH ).
Molar fractions of the major conformer in the solution
0
0
00
2.16. Mesoionic 3-phenyl-5-(n-butyl)-imino-1,2,3,4-
oxatriazole imine 8
2
1
one can calculated on the basis of signal integrations on H
NMR spectra (253 K). Energy differences between forms,
1
0
H NMR: Acetone-d , 253 K, N3–Ph: 8.13, 8.10 (H2 );
6
0
0
21
calculated using Arrhenius equation (J mol , at 253 K), are
7
1
.44 m (H3 ); 7.80 m (H4 ); N6–R: 3.29*, 3.23 (NCH );
2
.50 m (NCH CH ); 1.37 m (CH CH ); 0.89, 0.88 (CH ).
3
given in parenthesis. The suitable values for free mesoionic
bases (acetone-d solution), are: 0.63 (1150) for 4, 0.61
(900) for 5, 0.54 (340) for 6, 0.53 (270) for 7, 0.53 (270) for
8, 0.52 (130) for 9 and 0.51 (90) for 10. The values for
mesoionic salts (acetone-d /CF COOH solution) are: 0.77
2
2
2
3
0
Acetone-d /CF COOH, 253 K, N3–Ph: 8.38, 8.32* (H2 );
6
3
6
0
0
7
.89 m (H3 ); 8.00 m (H4 ). N6–R: 3.79 m (NCH ); 1.76 m
2
a
NCH CH ); 1.45 m (CH CH ); 0.92 (CH ). NH 10.85*( ),
(
2
a
2
2
3
3
a
1
0.59( ). CF COOH 14.52( ).
3
6
3
13
C NMR: Acetone-d , 253 K, C5: 164.7, 160.7. N3–Ph:
6
(2500) for 4, 0.69 (1700) for 5, 0.61 (940) for 6, 0.65 (1300)
for 7, 0.65 (1300) for 8, 0.65 (1300) for 9 and 0.69 (1700)
for 10.
0
0
0
1
35.5d (C1 ); 121.9*, 121.8 (C2 ); 130.8d (C3 ); 134.0*,
0
1
33.9 (C4 ). N6–R: 49.2, 47.1 (NCH ); 34.1*, 34.0
2
(
d /CF COOH, 253 K, C5: 171.3, 171.1*. N3–Ph: 132.8,
NCH CH ); 20.9d (CH CH ); 14.2d (CH ). Acetone-
2 2 2 3 3
6
3
0
0
0
0
1
N6–R: 44.9, 43.6* (NCH ); 30.1*, 29.9 (NCH CH ); 19.1*,
32.7 (C1 ); 122.1, 122.0* (C2 ); 130.8 (C3 ); 135.8 (C4 ).
3. Results and discussion
2
2
2
1
9.0 (CH CH ); 12.7 (CH ).
3
3.1. Synthesis
2
3
2
.17. Mesoionic 3-phenyl-5-allylimino-1,2,3,4-oxatriazole
imine 9
All mesoionic 3-phenyl-5-imino-1,2,3,4-oxatriazole imi-
nes were obtained from suitable thiosemicarbazide, via
nitrosation by butyl nitrite in ethanol with presence of
gaseous hydrochloride, and by alkalization of salt with
sodium bicarbonate. This method was also used originally
for synthesis of 3-phenyl-5-phenylimino-1,2,3,4-oxatriazole
imine 4 [13]. However, the oxatriazoles 4 and 5, i.e. the
compounds with N Ar group at C5 position, we obtained by
nitrosation of thiosemicarbazide with sodium nitrite, which
seems to be a simpler method of synthesis.
1
0
H NMR: Acetone-d , 318 K, N3–Ph: 8.12 br (H2 );
6
0
0
7
.74 m (H3 ); 7.80 m (H4 ). N6–R: 5.99 m (–CHy); 5.27 br
yCH ); 5.02 br (yCH ); 3.99 br (–CH –). Acetone-d , 253
(
2
2
2
6
0
0
0
K, N3yPh: 8.14 m (H2 ); 7.77 m (H3 ); 7.83 m (H4 ). N6–
R: 5.97 m (–CHy); 5.30, 5.25 (yCH ); 5.03, 5.01 (yCH );
2
2
2
3
N3yPh: 8.41, 8.36* (H2 ); 7.91 m (H3 ); 8.03 m (H4 ).
.94, 3.90 (–CH –). Acetone-d /CF COOH, 253 K,
0
2
6
3
0
0
N6yR: 6.04 m (–CHy); 5.50, 5.48* (yCH ); 5.34*, 5.32
2
Mesoionic 3-phenyl-5-arylimino-1,2,3,4-oxatriazole
imines containing aromatic R group directly bonded to
a
a
(
COOH 10.2( ). C NMR: Acetone-d , 243 K, C5: 165.6,
yCH ); 4.46 m (–CH –). NH: 10.97( ), 10.24*( ). CF
2 2 3-
a
13
2
N 6 atom, 4 and 5, are relatively stable. In contrast,
6
0
0
2
oxatriazoles 6–10, with CH -R group attached to N 6
1
61.6. N3–Ph: 135.3, 135.2 (C1 ); 121.9, 121.8 (C2 );
0
2
0
1
30.8d (C3 ); 134.1, 134.0 (C4 ). N6–R: 137.6, 137.2
–CHy); 114.1, 113.9 (yCH ); 51.6, 49.4 (–CH –).
atom, are unstable, and decompose within 24 h at room
temperature. These compounds can be stored at dry ice
temperature for a long time. All of these compounds form
colour crystals, deep red (4 and 5, R ¼ Ar) or yellow (6–10,
(
Acetone-d /CF COOH, 303 K, C5: 172.6. N3–Ph: 134.1
2
2
6
3
0
0
0
0
(
C1 ); 123.3 (C2 ); 131.9 (C3 ); 136.9 (C4 ). N6–R: 131.7
–CHy); 119.4 (yCH ); 47.5 (–CH ).
(
R ¼ CH R). The salt of these compound are white or
2
2
2
slightly yellow. Generally, the salts of mesoionic oxatria-
zole imines are more stable than free bases.
2.18. Mesoionic 3-phenyl-5-benzylimino-1,2,3,4-
oxatriazole imine 10
1 13
.2. H and C NMR
3
1
0
H NMR: Acetone-d , 318 K, N3–Ph: 8.14 br (H2 ); 7.74
6
0
0
00
00
1
13
H and C NMR data for mesoionic oxatriazoles 4–10,
(
H3 ); 7.81 (H4 ). N6–R: 7.47 (H2 ); 7.31 (H3 ); 7.20
0
0
(
C4 ); 4.56 (CH ). Acetone-d , 253 K, N3–Ph: 8.18 m
2
taken at various temperatures, are given in the experimental
part of work. The compounds were investigated both as free
bases (acetone-d6 solutions) and as salts (acetone-d6
solutions with addition of trifluoroacetic acid). The signal
assignments were done by means of common NMR
procedures: coupling constant pattern, NOE experiments,
2D COSY, HSQC and HMBC spectra.
6
0 0 0 00
(
H2 ); 7.71 m (H3 ); 7.84 m (H4 ). N6–R: 7.44 m (H2 );
.33 m (H3 ); 7.23 m (H ); 4.55, 4.51 (CH ). Acetone-
0
0
00
7
d /CF COOH 253 K, N3–Ph: 8.44, 8.36* (H2 ); 7.9m
2
0
6
3
0 0 00 00 00
(
H3 ); 8.1m (H4 ). N6–R: 7.5m (H2 ); 7.45 m (H3 , H4 );
a
a
a
5
.02 br (CH ). NH: 11.65*( ), 11.36( ). CF COOH 14.4( ).
2 3
13
C NMR: Acetone-d , 243 K, C5 166.1, 161.9. N3–Ph:
6
0
0
0
0
00
1
135.3d (C1 ); 121.9d (C2 ); 130.8 (C3 ); 134.1, 134.0 (C4 ).
00
N6–R: 141.9, 141.6 (C1 ); 127.7d (C2 ); 128.7d (C3 );
The H NMR spectra of a free mesoionic base, acquired
at low temperature, contain two sets of signals, resulting
00

J. Ja z´ wi n´ ski, O. Staszewska-Krajewska / Journal of Molecular Structure 687 (2004) 23–33
29
1
Fig. 3. H NMR spectra of mesoionic 3-phenyl-5-ethylimino-1,2,3,4-oxatriazole imine 7 at various temperatures.
from two forms of a compound, Z and E. Broad signals are
observed at room temperature, 303 K, and only one set of
signals is observed at 318 K. Molar ratio of two conformers,
estimated from signal integral intensities, varies from 1:0.7
as a major component in the solution. According to ab initio
MO calculation of electronic energy [17], a conformer with
R group at Z position in relation to oxygen atom is
preferred. However, all calculations were made using as an
input the simplified structures, containing hydrogen atoms
instead of groups at 3 and 6 positions, with no consideration
of solvent effects.
1
to 1:1 The H NMR spectra of 7 (R ¼ CH CH ) at various
2
3
temperatures are shown as an example (Fig. 3). The same is
1
3
true for C NMR spectra. At low temperature, one can see
two sets of signals, while the average signals are observed at
high temperature (318 K).
1
3.3. N, N and O NMR
4
15
17
1
Protonation of a free base results in changes of all H and
C chemical shifts. As an example, one can consider NMR
1
3
14
The N, N and O NMR data (Table 1) are the most
15
17
1
5
spectra of oxatriazole 6 (R ¼ CH ), taken at 318 K. The
diagnostic ones. A N spectrum of a mesoionic 3-phenyl-5-
imino-1,2,3,4-oxatriazle imine, taken on at 318 K, includes
four signals, at ca. 220 (N2), 273 (N3), 2150 (N4) and
2220 ppm (N6). The signal assignments are obvious by
using simple comparison with previously collected data for
3
1
high frequency shift of ca. 0.3 ppm was observed for H
signals of N3–Ph group; the larger change, of 0.5 ppm, was
1
found for CH group. The low field shift of the C signal, of
3
3
1
3
ca. 10 ppm, was noted for the C5 atom, while remaining
C
1
5
signals move from 1 to 3 ppm only. The shift of the C5
signal is larger than that observed for the methyl group,
probably due to fact that the C5 atom is attached directly to
the centre of protonation via conjugated CN bond
similar structures, and by using selectively N labelled
1
5
compounds [18]. On N spectra, at low temperature, there
are two sets of signals, arising from two conformers of a
mesoionic compound (Fig. 4).
The spectra of mesoionic salts, taken at low temperature,
contain also two sets of signals. The molar ratio of
conformers ranges from 1:0.3 to 1:0.6. At low temperature,
Addition of trifluoroacetic acid to a solution of free
mesoionic base causes a significant shift of the N2 and N6
signals, of ca. þ15 and 280 ppm, respectively. The similar
1
15
on H NMR spectra, one can observe separately the N6–H
signals of both forms, and the signal of CF COOH.
behaviour of N signals has been observed during protona-
tion of thiatriazole imine, 1 [1,2], proving the proton location
1
3
15
1
The signal dispersion, defined as a chemical shift
difference between E and Z form, is large for the atoms
neighbouring to the N6, while the effect vanishes with
increasing the distance from the N6 atom. Oxatriazole 8
at the N6 atom. The one-bond J( N– H) coupling observed
for the N6 signalis also an evidence of protonation site.
1
13
15
Similarly to H and C experiments, the N spectra of
oxatriazole salts contain the signals of two forms of a
1
3
15
compound, E and Z. The signal dispersion on N NMR
(
R ¼ n–Bu), is an example: C signal dispersion of
2
.1 ppm is observed for N–CH signal, while less than
2
1
.1 ppm are found for the remaining C signals of n-Bu
spectra varies from 1 to ca. 10 ppm.
14
3
0
A
N NMR spectra of oxatriazolium free bases, 4–10,
group. The largest dispersion, of 4 ppm, is noted for the
signals of C5 atom. Dispersions are observed also for the
contain broad signals, with half-width in order of thousands
Hz, with exception of one narrow signal of the N3 atom, of
ca. 100 Hz. The signal of N3 atom consists of three
components: the peaks of two rotamers of oxatriazole, and
1
3
C signals of N3-Ph group, located far from N6 atom.
Our NMR data do not allow choosing which form of
a compound, E or Z; is more stable, and which one exists
the signal of N , dissolved in a solvent [31]. Deconvolution
2

30
J. Ja z´ wi n´ ski, O. Staszewska-Krajewska / Journal of Molecular Structure 687 (2004) 23–33
Table 1
14
15
N, N and O NMR data for some mesoionic 3-phenyl-5-imino-1,2,3,4-oxatriazole imines
17
R group
O1
N2
N3
N4
N6
Remarks
Mesoionic oxatriazole (free bases)
b
a
4
c
d
c,e
f
C
6
H
5
216.6; 220.8
273.5; 274.1
272.4 (450)
274.0; 274.9 (140)
2149.6; 2139.6
2143.3
2139.2; 2150.3
2210.8; 2220.2
2210.0
2210.8; 2219.3
b
2
303 (O1) (900) 217.6*; 222.7
18.2
5
6
p–C
6
H
4
(OCH
3
)
CH
3
308 (750)
b
217.0*; 223.6 (1500) 273.0; 274.2 (110)
17.6; 223.4 274.1; 275.2
217.8*; 224.4 (2200) 273.1*; 274.4 (90)
2142.2*; 2150.9 (1230) 2226.1*; 2223.9 (580)
2231.5; 2239.4
c
f
2
2143.6; 2151.6
2143.1*; 2150.9 (870)
7
8
9
1
CH
CH
CH
2
2
2
CH
(CH
-CH ¼ CH
–C
3
308 (820)
b
2210.8*; 2216.6 (580)
2222.0; 2226.2 (920)
f
2
)
2
CH
3
217.1*; 219.8 (2000) 273.2*; 273.7 (110) 2143.2; 2148.9 (1160)
216.5*; 223.0 (1250) 272.8; 274.0 (100)
216.8*; 223.0 (2000) 272.9; 274.1 (120)
f
2
310 (800)
310 (900)
2142.1*; 2150.0 (1360) 2217.6; 2225.1 (660)
2142.8*; 2150.2 (1800) 2219.0; 2226.3 (750)
f
0 CH
2
6
H
5
Salts of mesoionic oxatriazole
b
g
g
i
4
5
6
C
6
H
5
25.4*; 25.0
25.6*; 25.8
21.6*; 23.0
269.0*; 269.3
269.1*; 269.4
268.2*; 267.5
269.6*; 268.9
268.5*; 267.9
268.4*; 267.8
268.4; 267.5
268.1; 267.2
2140.8*; 2135.9
2141.6*; 2137.1
2141.0*; 2138.3
2143.0*; 2140.3
2141.7*; 2138.4
2141.2*; 2138.3
2141.2; 2137.9
2140.8; 2137.8
2282.8*; 2280.3
h
2284.5
p–C
6
H
4
(OCH
3
)
CH
3
2306.9*; 2304.3
2309.2*; 2306.5
2290.1*; 2287.8
2291.7; 2293.8
2296.1; 2293.8
2293.7; 2291.6
g
i
2
4.0*; 25.1
7
8
9
1
CH
CH
CH
2
2
2
CH
(CH
–CH ¼ CH
2 C
3
22.9*; 24.0
22.4*; 23.5
22.0; 23.2
21.2; 22.6
i
2 2 3
) CH
i
2
g
0 CH
2
6 5
H
1
5
17
N chemical shifts (ppm) referred to external neat nitromethane (d ¼ 0 ppm). O chemical shifts (ppm) referred to external neat water (d ¼ 0 ppm). The
1
4
17
3
N and O line widths are reported in parenthesis [Hz] as the full width at half the vertical height of the signal. A relaxating agent, Cr(acac) was added to all
15
of the samples used for N NMR measurements. A signal of major component, if identified, is marked with asterisks.
a
b
c
d
e
f
15
Some N NMR data for 4 were published in Ref. [18].
Not measured.
Measurement carried out in CD
3
OD solution, at 253 K.
solution, at 303 K. Data taken from Ref. [18].
group at 44 ppm; with line width of 780 Hz.
The measurements were carried out in acetone-d solution either at 253 K ( N NMR) or at 303 K ( N and O NMR).
OD solution with addition of CF COOH, at 253 K.
Second signal not detected, probably due to broadening.
Measurement carried out in acetone-d solution with addition of CF
Measurement carried out in dmso-d
6
17
O signal of OCH
3
15
14
17
6
g
h
i
Measurement carried out in CD
3
3
6
3
COOH, at 253 K.
15
14
Fig. 4. (a) N NMR spectrum of oxatriazole 6, at 233 K. Two set of signals result from two forms, Z and E; of the compound (b) N NMR spectrum of 6, at
1
4
3
03 K. The narrow signal at ca: 274 ppm indicates location of positive charge within molecule. (c) deconvolution of N signal at 274 ppm. The signal
contains three components: a signal at 273.8 ppm, with linewith of 58 Hz, a signal at 274.9 ppm (59 Hz), and the small signal at 272.2 ppm (34 Hz), caused
by the presence of N , dissolved in sample [31]. Bold trace means a calculated spectrum. (d) experimental spectrum.
2

J. Ja z´ wi n´ ski, O. Staszewska-Krajewska / Journal of Molecular Structure 687 (2004) 23–33
13
31
1
3.4. N– C one bond coupling, J( N– C)
5
1
15
13
procedure reveals that overlapped signals of N3 atoms have
1
4
half-width from 34 to 59 Hz. (Fig. 4). A N NMR signal
line-width depends on the symmetry of chemical environ-
ment of atom under consideration, on a molecule reorienta-
tional correlation times, and on electric field gradient at the
nucleus. The latter depends on density of electric charge
1
5
13
One bond N– C coupling constants have been applied
sporadically as a parameter describing a character of the
C5–N6 bond of a mesoionic molecule [36–38]. A small
number of data did not provide general conclusions. As a
1
15
13
[
to produce narrow signals on N NMR spectrum. Because
32]. Thus, the nuclei having a positive charge are expected
4
contribution to these investigations, the J( N– C)
coupling constants across the C5–N6 bond for mesoionic
tetrazole 2 and oxatriazole 6 have been measured. Available
reference data [36,38,39] concerning mesoionic hetero-
cycles are quoted for the comparison purposes (Table 2).
1
1
4
of a few factors controlling N signal width, any
conclusions on charge distribution within molecule should
be taken with care. However, for a rigid molecule contain-
ing a similar type of nitrogen atoms, one can assume that the
presence of narrow peak indicates the location of positive
charge. In the case of mesoionic oxatriazoles, 4–10,
positive charge is located mainly at N3 atom. These findings
agree with previously obtained results for mesoionic
thiatriazoles and tetrazoles [33].
1
The absolute value of J(N6–C5) coupling constant for a
mesoionic free base cover the range from ca. 10 Hz for the
tetrazole 2 to ca. 16 Hz for one conformer of oxatriazole 6
(Table 2). The value of 11.3 Hz was found for oxadiazole 14
[36], an example of another class of mesoionic heterocycles,
acetylsydnonimines.
1
7
1
2
O NMR spectra of oxatriazoles 4–10 contain one signal
Imines, (R )(R )CyNR, are expected to be a reference
model of mesoionic C5–N6 bond. The suitable coupling
constant across the NyC bond does not exceed a value of
5 Hz [40]. The largest value of 14.5 Hz is observed for
imine 15 containing heteroatoms attached to the carbon
at ca. 300 ppm. In some cases the signal can be deconvoluted.
Forexample, thesignalof6,at308 ppm, resultsintwosignals
at 305 and 308 ppm, with half-widths of 610 and 450 Hz,
1
7
respectively.The Ochemicalshiftsof4–10arecomparable
with the values observed for aromatic oxazole [34].
1
5
17
N and O NMR results clearly indicate that mesoionic
-phenyl-5-imino-1,2,3,4-oxatriazole imines adopts a cyc-
Table 2
3
1 15 13
Absolute values of J( N- C) coupling constants for some mesoionic
compounds and related structures.
lic, ‘mesoionic’ form (Fig. 2c). A hypothetical linear valence
tautomer of a 5-imino-1,2,3,4-oxatriazole imine contains a
1
Number of compound J( N– C) [Hz]
15
13
1
nitroso group, which is expected to have a O NMR signal
7
Solvent,
1
5
and R group
temperature
within the range from 650 to 700 ppm [34], and N NMR
signal within the range from 155 to 200 ppm [35]. In fact,
1
1
J(N6–C5) J(N6-R)
1
5
17
there are not N and O NMR signals on the spectra of
oxatriazoles 4–10, fitting these ranges. Instead of that, the
observed chemical shift pattern is comparable with the NMR
data of analogous compounds, derivatives of mesoionic
thiatriazole 1. On the other hand, our NMR data cannot
exclude occurrence of the linear form of oxatriazole in low
concentration, being in equilibrium with the cyclic form.
a
a
1
12.6
24.0
26.9
N6–CPh 4.9
N6–CPh 13.3
N6–CPh 15.7
303 K, acetone-d
6
1 (HCl salt)
303 K, acetone-d6
a
1
1
303 K, acetone-d
6
N6–CH
N6–CH
N6–CH
3
3
3
5.3
0.3
9.5
b
2
2
10.3
26.9
303 K, dmso-d
303 K, dmso-d
6
3
(CF COOH salt)
6
with CF
3
COOH
b,c
6
16.1
N6–CH 1.2
3
243 K, CDCl3
1
13
c
b
1
1
1
1.9
5.8
1.4
1.0
It is obvious that H and C NMR experiments reveal
the existence of two components in the solution, but does
not provide an answer which kind of equilibrium is
observed, i.e. equilibrium between E and Z form, or an
equilibrium between cyclic and linear forms of a compound.
d
N6–CH
3
, 1
243 K, acetone-d
6
6
6(CF
3
COOH salt)
30.1
N6–CH
3
9.0
243 K, acetone-d
3
with CF COOH
c
30.1
11.3
23.7
14.5
23.5
30
1
5
17
e
1
1
1
1
1
4
N6–CO 0.9
N6–CO 9.1
r.t., CD
r.t., CD
3
OD
OD
In contrast, N and O NMR techniques offer insight into a
backbone of a mesoionic heterocycle, and allow denoting
the type of equilibrium.
e
4 (HCl salt)
3
f
5
g
6
7
(HCl salt)
For the comparison purposes, some NMR measurements
at various temperatures of thiatriazole 1 and tetrazole 2 have
h
CDCl
dmso-d6
3
h
17 (HCl salt)
23
1
5
been made. The N NMR spectra of thiatriazole 1, taken at
various temperatures, from 213 to 318 K, contain only one
set of signal, deriving from one form only. Some broadening
a
Data from Ref. [37].
b
Only residual signal splitting is observed; coupling constant was
estimated by signal deconvolution using spectrometer software.
c
Major component.
1
1
of H signal has been observed on H NMR spectrum;
however, the signals of second conformer were not noted at
d
1
Singlet; maximum value of J was estimated assuming that J does not
1
1
13 K. Similarly, H NMR spectrum of 2, taken at low
exceed the signal half width.
e
Data from Ref. [36].
2
temperature, 253 K, contains only one signal of CH group.
f
3
Data from Ref. [40].
g
Presumably, the concentrations of less stable conformers are
too low to be detected.
Data from Ref. [38].
h
Data from Ref. [39].

32
J. Ja z´ wi n´ ski, O. Staszewska-Krajewska / Journal of Molecular Structure 687 (2004) 23–33
1
atom [35]. The values of J(NC) with similar magnitude
have been also noted for compounds containing formally
single, but conjugated carbon nitrogen bond, such as
aromatic amines and amides.
Table 3
E–Z interconversion rates and activation parameters for oxatriazoles 6 and
1
0
a
b
T (K)
T (K)
6
10
Protonation or methylation of mesoionic aminides,
2
1
21 21
(s
21
21 21
(s
1
occurring at N6 atom, increases J(NC) coupling; the
compounds 1, 2, 6 and 14 are the examples. The explanation
k (s
)
k
)
k (s
)
k
)
1
of J(NC) increasing is not obvious, and cannot be attributed
300.5
299.3
302.4
305.4
308.5
311.5
314.6
317.6
320.6
–
–
9.9
13.9
19.0
26.1
35.2
47.5
64.2
85.7
9.3
3
3
03.0
05.5
14.0
20.2
29.1
39.9
54.9
72.3
103.4
12.1
17.4
25.2
34.2
47.1
62.1
88.9
13.1
17.9
24.5
33.1
44.6
60.2
80.3
simply to an increase in the CN bond order. It is known from
X-ray data, that methylation or protonation occurring at the
N6 atom decrease C–N bond order, at least for derivatives
308.0
10.5
313.0
3
1
of thiatriazole [11,12] 1. It seems that increasing of J(NC)
is caused by the effective removal of the electron lone-pair
3
3
15.5
18.0
1
through protonation [40]. The magnitude of J(NC) across
Activation parameters
another bond, N6–R, varies from ca. 0 to 1.2 Hz for the
N6–CH and N6–COCH bonds, and reach the value of ca.
r
20.999 20.999
68(2.5) 68(2.5)
85(1.3) 85(1.3)
21
21
#
21
DG (kJ mol
#
)
67.6(0.5) 67.7(0.5)
79.0(0.3) 78.8(0.3)
3
3
21
DH (kJ mol
#
DS (Jmol
)
21 21
5
Hz for the N–Ph bond. The protonation at the N6 atom
1
K
)
57(4)
57(4)
38(1)
37(1)
results in an increase of J(NC), up to the values from ca. 5
to ca. 14 Hz.
The 1,3,4-thiadiazolium-5-anilide [39] 17 seems to be an
E–Z interconversion rates k and k2 have been obtained from H NMR
1
1
#
#
data by complete band shape analysis. DH and DS have been obtained by
a least-squares adaptation of the rate constant and corresponding
1
exception (Table 2). The J(N6–C5) coupling constant of
3
compounds mentioned so far. In addition, protonation of
2
1
21
temperatures to a straight line of the form lnðkT Þ ¼ bT þ c; where
0.1 Hz exceeds much the values found for mesoionic
#
#
DH ¼ 28.31b and DS ¼ 8.31(c-23.76) [26]. Free energy of activation,
#
#
#
#
DG ; has been calculated by the equation DG ¼ DH 2 TDS [26].
1
this compound decrease the magnitude of J(NC) coupling.
Standard deviations, given in parentheses have been obtained by a least-
21
squares procedure assuming errors of ^0.2 s for k and ^0.2 K for T;
The existence of such an exception, and untypical change of
NMR parameter upon protonation, suggests the necessity of
further investigation in this field.
with no consideration of systematic error provided by spectrometer.
a
temperature read from the instrument panel.
b
corrected temperature with the use of spectrometer calibration curve,
based on methanol chemical shift termometer.
3.5. NMR measurements at various temperatures
and estimation of activation parameters
the barrier found for mesoionic oxatriazoles. For example,
21
#
the values of DG are 85 and 96 kJ mol
for (CH3)2-
CyNCH3 and (CH ) CyNPh, respectively [42]. Some
heteroatoms attached to imine carbon atom decrease the
1
3 2
H NMR measurements at various temperatures and the
complete band shape analysis were applied for oxatriazoles
#
interconversion barier; for example the DG values of 78,
5
, 6 and 10. Satisfactory reproducing of experimental
2
1
5
9and 52.4 kJ mol were observed for (CH S) CyNCH ,
3 2 3
spectra was obtained for oxatriazoles 6 and 10; no good fit
of experimental signals and calculated one have been
obtained for oxatriazole 5. Presumably, the shape of NMR
signal reflects two mechanisms, i.e. rotation around C5–N6
and OCH3 bonds, and two-site exchange model is not
adequate. The results are summarized in Table 3 and on
Fig. 5, and in the experimental part.
(
[
CH O) CyNCH and (CH ) NCHyNCH Ph, respectively
3 2 3 3 2 2
42,43]. Low interconversion barrier in this case has been
explained in terms of a mechanism proceeding through
#
21
The similar values of DG ; 68 kJ mol , were found for
-phenyl-5-methylimino-1,2,3,4-oxatriazole 6 and 3-phe-
3
nyl-5-benzylimino-1,2,3,4-oxatriazole 10. The barrier of
A ! B conversion, k; is equal to the barrier of B ! A
2
1
conversion, k ; within experimental error; in fact the
energy difference between rotamers is very low and does not
2
1
exceed a value of ca. 0.4 kJ mol . One can consider either
imine double CyN bond or amide single C–N bond as two
extreme models of mesoionic C5–N6 bond. It is believed
that E–Z interconversion of imine bond occurs via so called
‘
lateral shift mechanism’, a linear transition state in which
the p bond remains intact and the nonbonded electron pair
on nitrogen rehybridized to a p orbital [41]. The barrier
of E–Z interconversion, attributed to this mechanism,
3
Fig. 5. The signal of oxatriazole 6(CH group) at various temperatures:
experimental spectra (left traces) and calculated spectra (right traces).
2
1
ranges from 84 to 96 kJ mol , i.e. it is much greater than

J. Ja z´ wi n´ ski, O. Staszewska-Krajewska / Journal of Molecular Structure 687 (2004) 23–33
33
[15] J. Slowikowska, J. Jazwin
´
ski, L. Stefaniak, G.A. Webb, J. Mol. Struct.
a polar transition state in which unsharing of the electrons of
#
442 (1998) 175.
16] J. Ja z´ wi n´ ski, L. Stefaniak, S. Ishikawa, M. Yamaguchi, J. Lipkowski,
the p bond has occurred [44]. The value of DG observed
for mesoionic oxatriazole are comparable with values found
[
[
J. Slowikowska, G.A. Webb, J. Mol. Struct. 344 (1995) 227.
17] J.W. Wiench, L. Stefaniak, A. Tabaszewska, G.A. Webb, Electr.
J. Teoret. Chem., 2 (1997) 71; J.W. Wiench, personal communication;
O. Staszewska-Krajewska, PhD Thesis, Institute of Organic Chem-
istry PAS, Warsaw, 1999.
2
1
for amides; for example the value of 74 kJ mol
observed for CH C(yO)N(CH ) [45]. Summarizing this
was
3
3 2
part of work one can conclude that E–Z interconversion of
mesoionic oxatriazolium aminides occurs via rotation
around a polar CyN bond, similarly to amides or imines
with heteroatoms, and not via linear transition state. It seems
[
18] W. Bocian, J. Ja z´ wi n´ ski, L. Stefaniak, G.A. Webb, Khim. Geterotsikl.
Soedin., 9, (1995) 1260. (engl. trans. Chem. Heterocycl. Comp., 9
(
1995) 1103).
that the CyN bond of imines, (RX) CyNR, where X ¼ O, S
1
finding agrees with the results of J(N6–C5) coupling
constant measurements.
[19] C.A. Ramsden, Mesoionic heterocycles, in Comprehensive Organic
Chemistry, 4, 1979, pp. 1172.
2
or NH, are good model of mesoionic C5–N6 bond. This
[20] G. Newton, C.A. Ramsden, Tetrahedron 38 (1982) 2965.
[
[
21] L. Stefaniak, Tetrahedron 33 (1977) 2571.
22] L. Stefaniak, M. Witanowski, G.A. Webb, Bull. Pol. Ac.: Chem. 29
(
1981) 497.
[
[
[
23] J.D. Roberts, G.A. Webb, L. Stefaniak, M. Witanowski, Bull. Pol.
Ac.: Chem. 34 (1986) 81.
4
. Conclusions
24] W. Bocian, L. Stefaniak, J.W. Wiench, G.W. Webb, Magn. Reson.
Chem. 34 (1996) 453.
1
The N, N and O NMR reveal that mesoionic
4
15
17
25] H. Kankaanranta, E. Rydell, A.S. Petersson, P. Holm, E. Moilanen, T.
Corell, G. Karup, P. Vuorinen, S.B. Pedersen, A. Wennmalm, T.
Metsa-Ketela, Brit. J. Pharmacol. 117 (1996) 401.
26] J. Sandstr o¨ m, in Dynamic NMR Spectroscopy, Academic Press,
London, 1982, pp. 14, 86–91 and 109–112.
oxatriazole imines exist in a cyclic, ‘mesoionic’ form.
Protonation of mesoionic oxatriazolium imines occurs at the
exocyclic nitrogen atom, N6. Both of the form, free
mesoionic bases and its salts exist in a solution as two
relatively stable rotamers, E and Z; depending on the
[
[
[
27] K. Jensen, B. Anthoni, B. Kr a¨ g, C. Larsen, C. Pedersen, Acta Chem.
Scand. 22 (1968) 1.
n
position of exocyclic N R group. The value offree activation
21
energy, DG
values observed for imines containing heteroatoms attached
28] D. Worrell, J. Am. Chem. Soc. 50 (1928) 1456.
#
2
¼ 68 kJ mol , is comparable with the
[29] R. Hanley, W. Ollis, C. Ramsden, J. Chem. Soc., Perkin Trans. I
1979) 736.
98 K
(
[
30] SinceUPAC nomenclature of mesoionic compounds is troublesome,
we adopt in the present paper simplified system from Comprehensive
Heterocyclic Chemistry, K. Potts (ed) Pergamon Press, Oxford,
to carbon atom, (RX) C ¼ NR, where X is an oxygen or
2
1
nitrogen atom. The measurements of J(N6–C5) coupling
constant for tetrazole 2 and oxatriazole 6 are in agreement
with this founding.
6(1984) 598
[31] M. Witanowski, W. Sici n´ ska, Spectroscopy 9 (1991) 55.
[
32] M. Witanowski, L. Stefaniak, G.A. Webb, in: G.A. Webb (Ed.),
Nitrogen NMR Spectroscopy, Annual Report on NMR Spectroscopy,
vol. 11B, Academic Press, London, 1981.
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