Trapping and Analysing Wheland–Meisenheimer σ Complexes
from the starting materials and the presence of new signals that
agree with those of structure WM8.
the reaction mixture in CH2Cl2 (Scheme 4). Note that this
is the first example of the evolution of WM complexes to
neutral products, probably favoured by the possibility, for
WM8, of losing water.
Finally, when DNTP was added to a solution of WM8
in CD3CN, the WM9 complex was formed in a slow process
(about 2 weeks for quantitative conversion; Scheme 5),
which confirmed our previous conclusion on the reversibil-
ity of the formation of WM complexes.
{[7-(2,4-Dipyrrolidin-1-yl-4,5-dihydro-1,3-thiazol-4-ylium-5-yl)-6-ni-
tro-3-oxido-2,1,3-benzoxadiazol-4(7H)-ylidene](oxido)amino}oxid-
anide (WM8): Orange solid. Yield: 79 mg, 80%. M.p. Ͼ178.8 °C
(dec.). TLC (CHCl3/MeOH, 9:1, v/v): Rf = 0.44. UV/Vis (CH2Cl2):
1
λmax (ε) = 485 (33628 m–1 cm–1) nm. H NMR (600 MHz, CDCl3,
25 °C): δ = 2.07–2.23 (m, 7 H), 2.31–2.38 (m, 1 H), 3.38–3.46 (m,
1 H), 3.50–3.58 (m, 1 H), 3.71–3.84 (m, 3 H), 3.87–3.95 (m, 1 H),
3
3
4.17–4.27 (m, 2 H), 5.52 (d, JH,H = 2.8 Hz, 1 H), 6.01 (d, JH,H
=
2.8 Hz, 1 H), 9.02 (s, 1 H) ppm. 1H NMR (600 MHz, CD3CN,
25 °C): δ = 1.97–2.12 (m, 8 H), 3.42 (t, JH,H = 6.9 Hz, 2 H), 3.64–
3.79 (m, 4 H), 4.14 (t, JH,H = 6.9 Hz, 2 H), 5.51 (d, 3JH,H = 2.7 Hz,
1 H), 5.92 (br. d, 1 H), 8.77 (s, 1 H) ppm. 1H NMR (600 MHz,
[D6]DMSO, 25 °C): δ = 1.88–2.08 (m, 7 H), 2.14–2.21 (m,1 H),
3.33–3.39 (m, 1 H), 3.44–3.51 (m, 1 H), 3.61–3.73 (m, 4 H), 4.05–
4.17 (m, 2 H), 5.63 (d, 3JH,H = 2.7 Hz, 1 H), 6.13 (d, 3JH,H = 2.7 Hz,
1 H), 8.63 (s, 1 H) ppm. 13C NMR (100.57 MHz, [D6]DMSO,
25 °C): δ = 174.5, 172.6, 149.9, 132.7, 122.0, 111.0, 109.6, 59.6,
51.8, 51.76, 51.3, 50.0, 35.6, 25.5, 24.8, 24.6, 23.9 ppm. MS (ESI+):
m/z = 450 [M + H]+.
Conclusions
The reactions between 2,4-dipyrrolidin-1-yl-1,3-thiazole
and 4,6-dinitrobenzofuroxan or 4,6-dinitrotetrazolopyrid-
ine quantitatively produced covalent complexes that are
contemporaneously a Wheland and Meisenheimer interme-
diate of the two main reactions of the aromatic substrates:
a nucleophilic and electrophilic substitution reaction. The
reactions occur with high diastereoselectivity, and this is ex-
plained by considering the particular approach of the two
partners. The particular stability at room temperature of
these complexes allowed single crystals to be obtained that
are suitable for X-ray diffraction analysis; the analyses con-
firmed their structures and revealed some interesting details
that explain the behaviour of WM complexes. Furthermore,
exchange of the electrophilic partner in WM8 with DNTP
led to WM9, which also confirmed our previous conclusion
on the reversibility of the formation of WM complexes. Fi-
nally, the complex obtained with DNBF as the electrophilic
partner easily eliminated water with re-aromatization of
both rings to afford an unusual substitution product, a fur-
azan derivative.
{[5-(2,4-Dipyrrolidin-1-yl-4,5-dihydro-1,3-thiazol-4-ylium-5-yl)-6-ni-
trotetrazolo[1,5-a]pyridin-8(5H)-ylidene](oxido)amino}oxidanide
(WM9): Orange solid. Yield: 84 mg, 88%. M.p. Ͼ206.2 °C (dec.).
TLC (CHCl3/MeOH, 9:1, v/v): Rf = 0.42. UV/Vis (CH2Cl2): λmax
(ε) = 481 (14529 m–1 cm–1) nm. 1H NMR (600 MHz, CDCl3, 25 °C):
δ = 1.95–2.45 (m, 8 H), 3.33–3.40 (m, 2 H), 3.64–3.70 (m, 2 H),
3.79–3.88 (m, 1 H), 4.00–4.07 (m, 1 H), 4.26–4.33 (m, 2 H), 6.17
(br. s, 1 H), 7.00 (br. s, 1 H), 9.02 (s, 1 H) ppm. 1H NMR
(600 MHz, CD3CN, 25 °C): δ = 2.10–2.20 [m, 7 H (partially
eclipsed by water signal)], 2.26–2.32 (m, 1 H), 3.21–3.27 (m, 1 H),
3.32–3.39 (m, 1 H), 3.53–3.59 (m, 1 H), 3.62–3.69 (m, 1 H), 3.76–
3.84 (m, 1 H), 3.85–3.91 (m, 1 H), 4.15–4.26 (m, 2 H), 6.05 (br. s,
3
1
1 H), 7.02 (d, JH,H = 2.1 Hz, 1 H), 8.79 (s, 1 H) ppm. H NMR
(600 MHz, [D6]DMSO, 25 °C): δ = 1.80–1.97 (m, 4 H), 1.97–2.13
(m, 3 H), 2.17–2.28 (m, 1 H), 3.11–3.21 (m, 1 H), 3.37–3.45 (m, 1
H), 3.48–3.58 (m, 1 H), 3.58–3.68 (m, 1 H), 3.73–3.85 (m, 2 H),
4.13–4.22 (m, 1 H), 4.22–4.32 (m, 1 H), 6.32 (br. s, 1 H), 7.23 (d,
3JH,H = 1.8 Hz, 1 H), 8.65 (s, 1 H) ppm. 13C NMR (150.82 MHz,
[D6]DMSO, 25 °C): δ = 174.6, 173.0, 148.0, 131.2, 118.7, 109.7,
61.9, 56.1, 52.2, 51.7, 51.2, 50.2, 25.5, 24.7, 24.5, 23.9 ppm. MS
(ESI+): m/z = 434 [M + H]+, 456 [M + Na]+.
Experimental Section
Caution: 4,6-Dinitrobenzofuroxan (DNBF) is a powerful explosive
with a sensitivity level comparable to that of dry picric acid. Conse-
quently, all preparations and manipulations of compounds contain-
ing the DNBF moiety were carried out only on a small scale
(Ͻ0.1 g) behind suitable protective shielding. 2,4-Dipyrrolidin-1-yl-
1,3-thiazole[40](6), DNBF,[41] and 4,6-dinitrotetrazolopyridine[42]
(DNTP) were prepared and purified as described in the literature.
Synthesis of 4-(2,4-Dipyrrolidin-1-yl-1,3-thiazol-5-yl)-5,7-dinitro-
2,1,3-benzoxadiazole (7): Aluminium oxide (0.200 g) was added to
a solution of complex WM8 (20.0 mg, 0.045 mmol) in CH2Cl2
(15 mL). Immediately, the solution turned from an intense orange-
yellow colour to violet. The reaction was monitored by TLC analy-
sis (eluent: CHCl3/MeOH, 9:1, v/v) and the mixture stirred with a
magnetic stirring bar until disappearance of the starting reagent
(about 10 min); then the mixture was filtered, and the aluminium
oxide was washed with CH2Cl2 (3ϫ10 mL). After removal of the
solvent in vacuo, pure 7 was obtained (0.016 g, 77 %). M.p.
Ͼ300 °C (dec.). TLC (CHCl3/MeOH, 9:1. v/v): Rf = 0.82. UV/Vis
(CH2Cl2): λmax (ε) = 585 (17125 m–1 cm–1) nm. 1H NMR (600 MHz,
CDCl3, 25 °C): δ = 1.72–1.82 (m, 1 H), 1.95–2.06 (m, 1 H), 2.06–
2.24 (m, 6 H), 2.56–2.65 (m, 1 H), 2.78–2.89 (m, 1 H), 3.58 (t, J =
6.9 Hz, 2 H), 3.87 (t, J = 6.9 Hz, 2 H), 3.90–3.99 (m, 1 H), 4.07–
4.15 (m, 1 H), 9.53 (s, 1 H) ppm. 13C NMR (100.57 MHz, CDCl3,
25 °C): δ = 170.2, 169.2, 151.8, 143.6, 132.6, 128.9, 121.7, 121.4,
111.2, 52.9, 50.4, 50.3, 49.9, 26.4, 25.7, 25.1, 24.2 ppm. MS (ESI+):
m/z = 432 [M + H]+.
Typical Procedure for the Synthesis of the σ Complexes WM8 and
WM9: A solution of 2,4-dipyrrolidin-1-yl-1,3-thiazole (6; 49.1 mg,
0.22 mmol) in CH3CN (5 mL) was added, at room temperature,
to a solution of DNBF (49.7 mg, 0.22 mmol) in CH3CN (5 mL).
Immediately after mixing, the solution turned from pale yellow to
a more intense orange-yellow and an orange solid formed (in the
case of the reaction with DNTP the solution, immediately after
mixing, turned from pale yellow to deep red and, after a few sec-
onds, the solution became orange, and an orange solid precipi-
tated). The solid was collected by filtration, washed with a small
amount of cold CH3CN and recrystallized from CH3CN/CH2Cl2
(1:1, v/v). The melting-point analysis of this solid produced a grad-
ually darkening above 178.8 °C (206.2 °C for the solid precipitated
from the reaction with DNTP). The crystals were analysed by
NMR, ESI-MS and X-ray diffraction. The reaction was also car-
ried out in CD3CN, and the solid was removed by filtration. The
1H NMR spectrum of the solution showed the absence of signals
Crystal Data for WM8: Suitable crystals obtained from the concen-
tration of a solution of CH3CN/CH2Cl2 (1:1, v/v). C17H19N7O6S,
Eur. J. Org. Chem. 2012, 1123–1129
© 2012 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
www.eurjoc.org
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