Pyrido[1,2-a][1,2,4]triazol-3-ylidenes
yield after crystallization from acetone. Mp 185-187 °C.1H NMR
(600 MHz, CD3OD) δ 1.72 (t, J ) 7.2 Hz, 3 H), 2.81 (s, 1 H),
4.70 (q, J ) 7.8 Hz, 2 H), 7.25 (d, J ) 6.6 Hz, 1 H), 7.77-7.80
(m, 1 H), 7.84 (d, J ) 9.0 Hz, 1 H). 13C NMR (150 MHz, CD3OD)
δ 14.4, 17.8, 50.2, 113.7, 118.1, 135.1, 137.6, 139.1, 149.8. MS
(ESI+) m/z 161 [M - BF4 - H]+. Anal. Calcd for C9H12BF4N3: C,
43.41; H, 4.86; N, 16.87. Found: C, 43.26; H, 4.52; N, 16.44.
The Alternative Preparation of the Pyrido[1,2-a][1,2,4]triazol-
2-ium salts 12b,c. Compounds 12b and 12c were prepared
following the same procedure as described above for 12a, using
13a and 13b as the starting materials, respectively. Tetrafluoroborate
acid (50% in diethyl ether) was used as the N-alkylating agent in
place of triethyloxonium tetrafluoroborate. Compounds 12b and 12c
were obtained in 86% and 81% yields, respectively.
catalysts in catalytic benzoin condensation and transesterification
at 25 °C. The catalytic activity was largely dependent on the
steric and electronic nature of the R1 and R2 substituent of the
triazolium salt. The triazolium-catalyzed benzoin condensations
are proposed to undergo the “traditional” Breslow mechanism
rather than the dimer pathway. Further investigations into other
versions of organocatalysis, organometallic catalysis, as well
as the related reaction mechanisms are currently underway and
will be reported in due course.
Experimental Section
General Remarks. 1H NMR spectra were obtained with a Bruker
AV-300, a Varian Inova-400, or a Varian Inova-600 spectrometer,
while 13C NMR spectra were recorded with a Bruker AV-300, a
General Procedure for the Preparation of the Pyrido[1,2-
a][1,2,4]triazol-2-ium salts 12d-g. 2-Phenylpyrido[1,2-a][1,2,4]-
triazol-2-ium hexafluorophosphate (12d): A dry, argon-flushed
Schlenk tube equipped with a reflux condenser was charged with
1-(pyridin-2-yl)-2-phenylhydrazine 13c (3.7 g, 20 mmol), am-
monium hexafluorophosphate (3.26 g, 20 mmol), trimethyl ortho-
formate (4.4 mL, 40 mmol), and dry THF (30 mL). After being
heated at 80 °C for 10 h, the mixture was cooled to room
temperature and concentrated in vacuo. The resulting residue was
recrystallized from acetone to give the triazolium salt 12d as
1
Varian Inova-400, or a Varian Inova-600. The H chemical shifts
were measured relative to tetramethylsilane as the internal reference,
while the 13C NMR chemical shifts were recorded with THF-d8,
CDCl3, or CD3OD as the internal standard. Elemental analyses were
performed with a CARLO ERBA1106 instrument. The mass spectra
(ESI) were obtained by using a Finnigan-LCQDECA spectrometer,
and the GC-MS spectra were recorded with an Agilent 6890-5973
machine.
Materials. Unless otherwise noted, all reagents were obtained
from commercial suppliers and used without further purification.
Benzaldehyde, benzyl alcohol, and isopropenyl acetate were freshly
distilled under reduced pressure prior to use. Solvents were dried
by refluxing for at least 24 h over CaH2 (CH2Cl2) and sodium/
benzophenone (THF or diethyl ether), and freshly distilled prior to
use. Unless otherwise indicated, all syntheses and manipulations
were carried out under dry N2 atmosphere. The pyridylhydrazine
13a and 13b were synthesized according to the procedure reported
by Lien et al.20 The pyridylhydrazine derivates 13c, 13d, 13e, and
13f were prepared by the literature procedure.21
General Procedure for the Preparation of the Pyrido[1,2-
a][1,2,4]triazol-2-ium salts 12a-c. 2-Methylpyrido[1,2-a][1,2,4]-
triazol-2-ium tetrafluoroborate (12a): A flame-dried round-
bottomed flask equipped with a reflux condenser was charged with
trimethyloxonium tetrafluoroborate (1.78 g, 12 mmol), 1-(pyridin-
2-yl)hydrazine 13a (1.09 g, 10 mmol), and chlorobenzene (30 mL).
The mixture was then stirred for 30 min, followed by addition of
trimethyl orthoformate (2.2 mL, 20 mmol). After being heated at
110 °C for 10 h, the reaction mixture was concentrated in vacuo.
The resulting residue was recrystallized from acetone to give the
triazolium salt 12a as colorless crystals in 86% yield. Mp 171-173
°C. 1H NMR (600 MHz, CD3OD) δ 4.41 (s, 3 H), 7.43 (t, J ) 7.2
Hz, 1 H), 7.85-7.87 (m, 1 H), 7.98 (d, J ) 9.6 Hz, 1 H), 8.73 (d,
J ) 7.8 Hz, 1 H). 13C NMR (150 MHz, CD3OD) δ 40.5, 116.4,
119.6, 127.1, 134.9, 149.2. MS (ESI+) m/z 133 [M - BF4 - H]+.
Anal. Calcd for C7H8BF4N3: C, 38.05; H, 3.65; N, 19.02. Found:
C, 37.96; H, 3.53; N, 19.29.
2-Ethyl pyrido[1,2-a][1,2,4]triazol-2-ium tetrafluoroborate
(12b): This compound was prepared following the same procedure
as described above for 12a, using 13a as the starting material and
triethyloxonium tetrafluoroborate as the N-alkylating agent. Com-
pound 12b was obtained as colorless crystals in 91% yield after
crystallization from acetone. Mp 112-114 °C. 1H NMR (600 MHz,
CD3OD) δ 1.70 (t, J ) 7.2 Hz, 3 H), 4.71 (q, J ) 7.2 Hz, 2 H),
7.43 (t, J ) 6.6 Hz, 1 H), 7.84-7.87 (m, 1 H), 8.00 (d, J ) 10.2
Hz, 1 H), 8.70 (d, J ) 7.8 Hz, 1 H). 13C NMR (150 MHz, CD3OD)
δ 14.4, 50.2, 116.4, 119.6, 127.2, 134.9, 149.3. MS (ESI+) m/z
147 [M - BF4 - H]+. Anal. Calcd for C8H10BF4N3: C, 40.89; H,
4.29; N, 17.88. Found: C, 40.65; H, 4.08; N, 17.71.
1
colorless crystals in 75% yield. Mp 203-205 °C. H NMR (600
MHz, DMSO-d6) δ 7.52 (t, J ) 7.2 Hz, 1 H), 7.71 (t, J ) 7.8 Hz,
1 H), 7.77 (t, J ) 8.4 Hz, 2 H), 7.94-7.96 (m, 1 H), 8.09 (d, J )
7.8 Hz, 2 H), 8.21 (d, J ) 9.0 Hz, 1 H), 8.87 (d, J ) 7.2 Hz, 1 H),
11.46 (s, 1 H). 13C NMR (150 MHz, DMSO-d6) δ 115.2, 118.8,
121.7, 126.6, 130.4, 131.4, 134.5, 135.2, 135.3, 147.3. MS (ESI+)
m/z 195 [M - PF6 - H]+. Anal. Calcd for C12H10F6N3P: C, 42.24;
H, 2.95; N, 12.32. Found: C, 42.56; H, 3.01; N, 12.29.
5-Methyl-2-phenylpyrido[1,2-a][1,2,4]triazol-2-ium hexafluo-
rophosphate (12e): This compound was prepared following the
same procedure as described above for 12d, using 13d as the starting
material. Compound 12e was obtained as a white solid in 64% yield
after crystallization from acetone. Mp 274-276 °C. 1H NMR (600
MHz, DMSO-d6) δ 2.83 (s, 3 H), 7.39 (d, J ) 6.6 Hz, 1 H), 7.72
(t, J ) 7.2 Hz, 1 H), 7.79 (t, J ) 8.4 Hz, 2 H), 7.89-7.92 (m, 1
H), 8.08 (d, J ) 9.0 Hz, 1 H), 8.17 (d, J ) 7.8 Hz, 2 H), 11.56 (s,
1 H). 13C NMR (150 MHz, DMSO-d6) δ 17.7, 112.5, 117.1, 121.5,
130.4, 131.3, 134.8, 135.4, 136.4, 147.6. MS (ESI+) m/z 209 [M
- PF6 - H]+. Anal. Calcd for C13H12F6N3P: C, 43.96; H, 3.41; N,
11.83. Found: C, 44.30; H, 3.66; N, 11.85.
2-(2-Isopropylphenyl)pyrido[1,2-a][1,2,4]triazol-2-ium hexaflu-
orophosphate (12f): This compound was prepared following the
same procedure as described above for 12d, using 13e as the starting
material. Compound 12f was obtained as colorless crystals in 82%
yield after crystallization from acetone. Mp 214-215 °C. 1H NMR
(400 MHz, DMSO-d6) δ 1.20(d, J ) 6.8 Hz, 6 H), 2.86-2.93 (m,
1 H), 7.52-7.58 (m, 2 H), 7.61 (d, J ) 8.0 Hz, 1 H), 7.75 (d, J )
3.6 Hz, 2 H), 7.94-7.98 (m, 1 H), 8.19 (d, J ) 9.2 Hz, 1 H), 8.88
(d, J ) 6.8 Hz, 1 H), 11.33 (s, 1 H). 13C NMR (100 MHz, DMSO-
d6) δ 24.1, 27.6, 115.6, 119.0, 127.1, 127.4, 127.5, 128.0, 132.8,
133.8, 134.8, 137.8, 145.2, 147.7. MS (ESI+) m/z 238 [M - PF6]+.
Anal. Calcd for C15H16F6N3P: C, 47.01; H, 4.21; N, 10.96. Found:
C, 46.71; H, 4.41; N, 11.24.
2-(2-Isopropylphenyl)-5-methylpyrido[1,2-a][1,2,4]triazol-2-
ium hexafluorophosphate (12g): This compound was prepared
following the same procedure as described above for 12d, using
13f as the starting material. Compound 12g was obtained as a white
solid in 77% yield after crystallization from acetone. Mp 296-297
1
°C. H NMR (400 MHz, DMSO-d6) δ 1.21 (d, J ) 7.2 Hz, 6 H),
2.81 (s, 3 H), 2.89-2.93 (m, 1 H), 7.39 (d, J ) 6.8 Hz, 1 H),
7.52-7.57 (m, 1 H), 7.63 (d, J ) 8.0 Hz, 1 H), 7.76 (d, J ) 4.0
Hz, 2 H), 7.89-7.93 (m, 1 H), 8.05 (d, J ) 9.2 Hz, 1 H), 11.33 (s,
1 H). 13C NMR (100 MHz, DMSO-d6) δ 18.1, 24.1, 27.7, 112.9,
117.4, 127.5, 127.6, 128.0, 132.9, 133.9, 135.1, 135.8, 137.0, 145.3,
148.1. MS (ESI+) m/z 252 [M - PF6]+. Anal. Calcd for
2-Ethyl-5-methylpyrido[1,2-a][1,2,4]triazol-2-ium tetrafluo-
roborate (12c): This compound was prepared following the same
procedure as described above for 12a, using 13b as the starting
material and triethyloxonium tetrafluoroborate as the N-alkylating
agent. Compound 12c was obtained as colorless crystals in 65%
J. Org. Chem. Vol. 73, No. 21, 2008 8263