Synthesis of N,N-di(arylmethylidene)arylmethanediamines by flash vacuum pyrolysis of arylmethylazides

Article abstract of DOI:10.1016/j.tet.2004.06.082

Flash vacuum pyrolysis of arylmethylazides 7a-d gave 2,4-diazapentadienes 5a-d in high yield (76-92%). The thermal cyclization of 5a-d gave cis-imidazolines 1a-d, further heating or Swern oxidation of 1a-d gave dehydrogenated products, imidazoles 2a-d.

Full text of DOI:10.1016/j.tet.2004.06.082

Tetrahedron 60 (2004) 6581–6584
Synthesis of N,N-di(arylmethylidene)arylmethanediamines
by flash vacuum pyrolysis of arylmethylazides
*
Chin-Hsing Chou, Li-Tse Chu, Shao-Jung Chiu, Chin-Fan Lee and Yao-Teng She
Department of Chemistry, National Sun Yat-Sen University, Kaohsiung 80424, Taiwan, ROC
Received 19 March 2004; revised 24 May 2004; accepted 1 June 2004
Abstract—Flash vacuum pyrolysis of arylmethylazides 7a-d gave 2,4-diazapentadienes 5a-d in high yield (76–92%). The thermal
cyclization of 5a-d gave cis-imidazolines 1a-d, further heating or Swern oxidation of 1a-d gave dehydrogenated products, imidazoles 2a-d.
q 2004 Elsevier Ltd. All rights reserved.
1. Introduction
microwave irradiation or heating with hexamethyldisila-
zane.6 Recently, we have synthesized 5a-d by the flash
vacuum pyrolysis of their corresponding arylmethylazides
7a-d. We report here the results of this work.
Amarine 1a is a very useful precursor for organic synthesis,
which was found to give the corresponding imidazole 2a by
dehydrogenation.1 1a can rearrange to 2,3,5,6-tetra-phenyl-
pyrazine (3) by irradiation,2 and converts to ligand 4,3 which
has been employed for enantioselective synthesis.4 Amarine
1a can be prepared by cyclization of the N,N-di(arylmethyl-
idene)arylmethanediamine 5a with a strong base or under
thermal condition. Compound 5a was formed previously by
condensation of imine 6a, which was generated in liquid
ammonia with benzaldehyde5 (Scheme 1). Synthesis of 5
and 1 from arylaldehydes has also been accomplished by
2. Results and discussion
Arylmethylazides 7a-d were prepared from the reported
method.7 FVP of 7a-d at 450–550 8C and ca. 1£1022 Torr,
gave presumably gaseous nitrenes 8a-d as the primary
pyrolysis products. 1,2-Hydrogen shift of 8a-d would
give imines 9a-d, which then underwent a condensed
Scheme 1.
Keywords: Arylmethylazides; Pyrolysis; N,N-Di(arylmethylidene)arylmethanediamines.
*
Corresponding author. Tel./fax: þ886-7-5253915; e-mail address: ch-chou@mail.nsysu.edu.tw
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doi:10.1016/j.tet.2004.06.082

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C.-H. Chou et al. / Tetrahedron 60 (2004) 6581–6584
products and 5d cyclized directly to give 2d, we did not
isolate 2b and 1d.
We also found that FVP of 7c,d gave not only the condensed
trimers 5c,d as the major products, but also a small amount
of dimers 10c,d. We propose the mechanism for the
formation of 10c as shown in Scheme 3. The formation of
10d follows the same type of mechanism.
It is noteworthy that the yields of imidazoles 2 from cis-
imidazolines 1 can be improved by performing Swern
oxidation. For instance, the yield of 2c can be increased to
52% by Swern oxidation of 1c (Scheme 4) as compared to
31% from simple heating of 1c (Table 1).8
Scheme 2.
trimerization reaction to give 5a-d in high yield (76–92%).
Thermal isomerization of compounds 5a-d under
90–120 8C for 4–6 h gave cis-imidazolines 1a-d. Further
heating of 1a-d at 140–160 8C under low pressure condition
(ca. 1£1021 Torr) gave dehydrogenated products, imida-
zoles 2a-d. The route to the pyrolysis products and
thermally isomerized products from 8a-d are summarized
in Scheme 2 and the yields of each product are listed in
Table 1. Since further heating of 1b gave only decomposed
Scheme 4.
Table 1. Products and yields from FVP of 7a-d, and from thermolysis of the
resulting compounds
Entry
Starting materials
Products (yields, %)
5a-d
1a-d
2a-d
1
2
3
4
7a
7b
7c
7d
92
78
86
76
84
79
63
—
55
—
31
33
Figure 1. Crystal structure of 5a.
Pure 5a can be recrystallized from CH2Cl2. The structure of
5a was analyzed by X-ray crystallography and shown as
Figure 1.9a Pure 2c was recrystallized from ethyl acetate and
n-hexane. The structure of 2c was analyzed by X-ray
crystallography and shown as Figure 2.9b
Figure 2. Crystal structure of 2c. S2/S2a and S3/S3a are the disordered sites
for S/CH disorder within the thiophene rings.
Scheme 3.

C.-H. Chou et al. / Tetrahedron 60 (2004) 6581–6584
6583
3. Conclusion
product was purified by flash chromatography on silica gel
using 2:1 n-hexane/ethyl acetate to give 0.90 g (52%)
imidazole 2c.
In conclusion, the flash vacuum pyrolysis of arylmethyl-
azides 7a-d is a new and efficient method to generate
2,4-diazapentadienes 5a-d, further heating of 5a-d can
induce ring cyclization to give cis-imidazolines 1a-d. We
also found the yield of dehydrogenated products, imidazoles
2a-d from 1a-d, can be improved by performing Swern
oxidation.
4.5. Spectral data of products
4.5.1. N,N-Di(phenylmethylidene)phenylmethane
diamine (5a). IR (CDCl3, cm21) 3085, 2844, 1639, 1579.
1
Mp 105–107 8C. H NMR (300 MHz, CDCl3): d 8.57 (s,
2H), 7.83–7.86 (m, 4H), 7.18–7.53 (m, 11H), 5.97 (s, 1H).
13C NMR (75 MHz, CDCl3): d 160.5, 141.6, 135.9, 130.9,
128.6, 128.5, 128.4, 127.7, 127.1, 92.6. MS (FAB) m/z (%)
299 [(Mþ1)þ, 5.0]. (Lit. 5a Mp 101–102 8C).
4. Experimental
4.1. General
4.5.2. N,N-Di[(2-furanyl)methylidene](2-furanyl)meth-
anediamine (5b). IR (CDCl3, cm21) 1635. Mp 118–
Infrared spectra were recorded with a FTS-175/185 IR
1
1
spectrophotometer. H and 13C NMR spectra were carried
119 8C. H NMR (300 MHz, CDCl3): d 8.41 (d, J¼0.6 Hz,
2H), 7.56 (d, J¼1.2 Hz, 2H), 7.39 (d, J¼0.6 Hz, 1H), 6.88
(d, J¼3.3 Hz, 2H), 6.51 (q, J¼1.8 Hz, 2H), 6.39 (d, J¼
3.3 Hz, 1H), 6.35 (q, J¼1.8 Hz, 1H), 6.13 (s, 1H). 13C NMR
(75 MHz, CDCl3): d 152.7, 151.5, 150.5, 145.3, 142.5,
115.8, 111.8, 110.4, 107.8, 84.1. MS (LR, 70 eV) m/z (%)
out in CDCl3 or acetone-d6 in a Varian VXR-300 NMR
spectrometer. Chemical shifts are reported in parts per
million (ppm) downfield from tetramethylsilane (TMS).
Mass spectra were recorded with a VG QUATTRO 5022
spectrometer. The X-ray structures were analyzed by a
RIGAKU AFC7S diffractoneter.
5a
268 (Mþ, 11.34). (Lit. Mp 116–117 8C).
4.2. General pyrolysis procedure10
4.5.3. N,N-Di[(2-thienyl)methylidene](2-thienyl)methane-
1
diamine (5c). IR (neat, cm21) 3166, 2252, 1691, 1626. H
NMR (300 MHz, acetone-d6): d 8.75 (s, 2H), 7.65 (d, J¼
8.5 Hz, 2H), 7.46 (dd, J¼6.5, 1.5 Hz, 2H), 7.39 (dd, J¼8.5,
1.5 Hz, 1H), 7.14 (dd, J¼8.5, 6.0 Hz, 2H), 7.08 (d, J¼
5.5 Hz, 1H), 6.99–7.01 (m, 1H), 6.23 (s, 1H). 13C NMR
(75 MHz, acetone-d6): d 155.5, 146.9, 143.4, 133.2, 131.0,
128.6, 127.5, 126.3, 125.2, 87.3. HRMS Calcd for
C15H12N2S3: 316.0163, found: 316.0165.
The furnace was maintained at temperatures in the range
400–450 8C. A sample for pyrolysis was placed into the
sample chamber and the system was evacuated to ca.
1022 Torr. During the pyrolysis CDCl3 was deposited into
the cold trap through a side arm. After the pyrolysis was
completed, nitrogen was introduced into the system, the
liquid-nitrogen-cooled trap was warmed to room tempera-
ture and all the FVP products were collected. At the exit of
the horizontal fused quartz tube, the pure products 5a-d
were obtained without purification. These products were
analyzed by 1H, 13C NMR, IR and Mass. The percent yields
4.5.4. N,N-Di[(3-thienyl)methylidene](3-thienyl)methane
diamine (5d). IR (neat, cm21) 2927, 2254, 1692, 1635. 1H
NMR (300 MHz, CDCl3): d 8.53 (s, 2H), 7.70 (t, J¼1.5 Hz,
2H), 7.65 (d, J¼5.0 Hz, 2H), 7.30–7.33 (m, 4H), 7.13–7.14
(m, 1H), 5.93 (s, 1H). 13C NMR (75 MHz, CDCl3): d 155.2,
142.9, 140.3, 129.6, 126.5, 126.4, 126.1, 126.0, 122.1, 89.0.
HRMS Calcd for C15H12N2S3: 316.0163, found: 316.0161.
1
were determined from H NMR.
4.3. Heating of 2,4-diazapentadienes 5a-d and
cis-imidazolines 1a-d
4.5.5. cis-2,4,5-Triphenylimidazoline (1a). IR (CDCl3,
cm21) 2834, 2347, 1636. Mp 128–130 8C. 1H NMR
(300 MHz, CDCl3): d 7.97 (d, J¼8.4 Hz, 2H), 7.53–7.46
(m, 4H), 7.03–6.90 (m, 10H), 5.41 (s, 2H). 13C NMR
(75 MHz, CDCl3): d 164.5, 138.6, 131.2, 129.5, 128.6,
127.6, 127.4, 127.3, 126.8, 70.6. MS (LR, 70 eV) m/z (%)
298 (Mþ, 8.3). (Lit.5a Mp 127–128 8C).
A sample of FVP products 5a-d was placed into the sample
chamber of the bulb-to-bulb distillation, and the whole
system was maintained at a pressure of 1022 Torr. The
sample chamber was heated to 120 8C for 5 h to give 1a-d.
Further heating of 1a-d at 140–160 8C gave dehydro-
genated products, imidazoles 2a-d. These crude products
were purified using preparative TLC or column chroma-
tography on silica gel.
4.5.6. cis-2,4,5-Tri(2-furanyl)imidazoline (1b). IR
(CDCl3, cm21) 3124, 2936, 2873, 1633. Mp 111–112 8C.
1H NMR (300 MHz, CDCl3): d 7.51 (d, J¼0.9 Hz, 1H), 7.19
(t, J¼0.6 Hz, 2H), 7.14 (d, J¼3.3 Hz, 1H), 6.52 (q, J¼
1.8 Hz, 1H), 6.17 (q, J¼2.1 Hz, 2H), 6.04 (d, J¼3.0 Hz,
2H), 5.38 (s, 2H), 5.00 (br, 1H). 13C NMR (75 MHz,
CDCl3): d 156.2, 151.9, 144.7, 144.2, 141.8, 112.9, 111.9,
110.1, 107.2, 63.3. MS (LR, 70 eV) m/z (%) 268 (Mþ, 38.2).
(Lit.5a Mp 115–116 8C).
4.4. Swern oxidation of imidazoline 1c
A stirred solution of oxalyl chloride (0.79 mL, 1.6 equiv.)
in CH2Cl2 (20 mL) at 278 8C was treated with DMSO
(1.37 mL, 3.2 equiv.) in CH2Cl2 (10 mL), dropwise over
5 min. After 10 min, imidazoline 1c (1.75g, 5.53 mmol) in
CH2Cl2 (10 mL) was added over 10 min, followed by
triethylamine (3.9 mL, 5 equiv.), dropwise over 10 min. The
mixture was stirred with gradual warming overnight, and
then the reaction was quenched with water. The organic
phase was washed with brine, dried over MgSO4, filtered,
and concertrated to give the crude product. The crude
4.5.7. cis-2,4,5-Tri(2-thienyl)imidazoline (1c). IR (neat,
cm21) 1716, 1699, 1627. Mp 127–128 8C. 1H NMR
(300 MHz, acetone-d6): d 7.75 (d, J¼4.0 Hz, 1H), 7.67 (d,

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C.-H. Chou et al. / Tetrahedron 60 (2004) 6581–6584
J¼5.0 Hz, 1H), 7.17–7.19 (m, 1H), 7.11–7.12 (m, 2H),
6.76–6.78 (m, 4H), 5.65 (s, 2H), 3.20 (br, 1H). 13C NMR
(75 MHz, acetone-d6): d 160.3, 144.2, 134.9, 130.5, 129.4,
128.4, 126.9, 126.0, 125.3. HRMS Calcd for C15H12N2S3:
316.0163, found: 316.0165. (Lit.6a Mp 125–126 8C).
References and notes
1. (a) Kyrides, L. P.; Zienty, F. B.; Steahly, G. W.; Morrill, H. L.
J. Org. Chem. 1947, 12, 577–586. (b) Strain, H. H. J. Am.
Chem. Soc. 1927, 49, 1558–1571. (c) Matsuura, T.; Ito, Y.;
Saito, I. Bull. Chem. Soc. Jpn 1973, 46, 3805–3809.
2. Matsuura, T.; Ito, Y. Bull. Chem. Soc. Jpn 1975, 48,
3669–3674.
4.5.8. 2,4,5-Triphenylimidazole (2a). IR (KBr, cm21
)
3028, 2926, 1786, 1647. Mp 274–275 8C. 1H NMR
(300 MHz, acetone-d6): d 11.73 (br, 1H), 8.13–8.15 (m,
2H), 7.15–7.68 (m, 13H). 13C NMR (75 MHz, acetone-d6):
d 148.8, 138.6, 136.2, 131.2, 130.4, 130.1, 129.7, 129.6,
129.5, 129.3, 129.2, 129.1, 128.9, 128.9, 128.2, 127.0.
HRMS: Calcd for C21H16N2: 296.1313, found: 296.1317.
(Lit.11 Mp 270–273 8C).
3. Corey, E. J.; Kuhnle, F. N. M. Tetrahedron Lett. 1997, 38,
8631–8634.
4. Williams, O. F.; Bailar, Jr., J. C. J. Am. Chem. Soc. 1959, 81,
4464–4469.
5. (a) Hunter, D. H.; Sim, S. K. Can. J. Chem. 1972, 50,
669–677. (b) Sprung, M. M. Chem. Rev. 1940, 26, 297–338.
6. (a) Uchida, H.; Shimizu, T.; Reddy, P. Y.; Nakamura, S.; Toru,
T. Synthesis 2003, 1236–1240. (b) Uchida, H.; Tanikoshi, H.;
Nakamura, S.; Reddy, P. Y.; Toru, T. Synlett 2003,
1117–1120.
4.5.9. 2,4,5-Tri(2-thienyl)imidazole (2c). IR (neat, cm21
)
1
3304, 2927, 1736, 1716, 1687. Mp 248–249 8C. H NMR
(300 MHz, acetone-d6): d 7.64–7.65 (m, 1H), 7.59 (br, 2H),
7.51–7.52 (m, 1H), 7.35 (br, 2H), 7.11–7.24 (m, 3H), 6.97
(br, 1H). 13C NMR (75 MHz, acetone-d6): d 143.1, 138.5,
135.1, 134.6, 127.9, 129.7, 128.6, 127.4, 125.3. HRMS
Calcd for C15H10N2S3: 314.0006, found: 314.0009.
7. Bertozzi, C. R.; Bednarski, M. D. J. Org. Chem. 1991, 56,
4326–4329.
8. Dubowchik, G. M.; Padilla, L.; Edinger, K.; Firestone, R. A.
J. Org. Chem. 1996, 61, 4676–4684.
9. (a) CCDC 223876 refers to the deposit number of the
supplementary crystallographic data for 5a. These datas can
be obtained free of charge via http://www.ccdc.cam.ac.uk/
conts/retrieving.html (b) CCDC 223877 refers to the deposit
number of the supplementary crystallographic data for 2c.
10. Chou, C. H.; Trahanovsky, W. S. J. Am. Chem. Soc. 1986, 108,
4138–4144.
4.5.10. 2,4,5-Tri(3-thienyl)imidazole (2d). IR (neat, cm21
)
2985, 2252, 1731, 1693. Mp 244–246 8C. 1H NMR
(300 MHz, acetone-d6): d 7.96 (t, J¼1.5 Hz, 1H), 7.73 (d,
J¼1.5 Hz, 1H), 7.55–7.57 (m, 3H), 7.48 (dd, J¼5.0, 3.0 Hz,
2H), 7.30 (dd, J¼5.0, 1.0 Hz, 2H). 13C NMR (75 MHz,
acetone-d6): d 143.4, 135.0, 133.7, 128.4, 127.3, 126.9,
126.5, 122.7, 122.3. HRMS Calcd for C15H10N2S3:
314.0006, found: 314.0003.
11. Boyle, J. T. A.; Grundon, M. F.; Scott, M. D. J. Chem. Soc.,
Perkin Trans. 1 1976, 207–212.
Acknowledgements
We thank the National Science Council of the Republic of
China for financial support.