4
Tetrahedron
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monomeric form even in the solid state. In the presence of
4.3. General procedure for the stoichiometric oxidation.
concentrated HNO3 as an acidic re-oxidant, 1 is capable of
catalytically oxidizing primary, secondary, benzylic, and
aliphatic alcohols to the corresponding aldehydes and ketones in
moderate to quantitative yields. Our results will expand the
application of nitroso compounds and demonstrate the utility of a
new organocatalyst.
The following oxidation of benzyl alcohol represents the
general procedure. A mixture of nitrosotetrazolium salt (1, 17
mg, 0.049 mmol) and benzyl alcohol (3b, 5.2 µL, 0.050 mmol)
was stirred in MeCN (2.0 mL) at room temperature for 0.5 h, in
the presence of n-cetane (tR = 5.4 min.) as an internal standard.
At intervals, aliquots were analyzed by GC after passing through
a SiO2 column (eluting with CH2Cl2). The yield of 4b (tR = 4.5
min.) was calculated to be 94% based on a calibration curve
using an authentic sample. In the case of benzhydrol, the yield of
4a and 2 were calculated by 1H NMR to be 97% and 57% based
on the peaks of 7.80 ppm (4H of 4a) and 8.19 ppm (2H of 2)
using 1,3,5-trimethoxybenzene (6.08 ppm, 3H) as a standard.
4. Experimental Section
4.1. General
Melting point was measured by a Yanaco MP 50533 and
uncorrected. 1H and 13C NMR spectra were recorded on a Varian
Mercury spectrometer (300 MHz) and a Bruker AVANCE 400
Plus NanoBay spectrometer (1H NMR: 400 MHz, 13C NMR: 100
MHz). IR spectra were obtained on a Jasco FT/IR-200. High
resolution ESI-TOF mass spectroscopy was carried out on a
Waters, Synapt G2 HDMS. Elemental analyses were done with a
Elementar vario EL cube. Electrochemical experiments were
carried out on a BAS ALS/chi 620A electrochemical analyzer.
For cyclic voltammetry, a 3 mm glassy carbon electrode was
used as working electrode. The counter electrode consisted of a
Pt wire and Ag/Ag+ (0.01 M in MeCN / 0.1 M Bu4NClO4) was
used as reference electrode. To remove O2 from the solution, N2
gas was bubbled for 10 min prior to each electrochemical
analysis. Gas chromatography was carried out on a Shimadzu
GC-2014 equipped with a flame-ionization detector and a
capillary column (Agilent DB-WAX, 30 m). X-ray diffraction
data were collected on a Rigaku VariMax RAPID II, Mo-Kα
radiation.
Retention times for the carbonyls 4b–d: tR = 4.5 min. for 4b in
94% yield; tR = 3.9 min. for 4c in 94% yield; tR = 4.0 min. for 4d
in 65% yield.
4.4. General procedure for the catalytic oxidation.
The following oxidation of benzyl alcohol represents the
general procedure. A mixture of nitrosotetrazolium salt (1, 3.6
mg, 0.011 mmol), concentrated HNO3 (16 µL, 0.21 mmol), and
benzyl alcohol (3b, 22 mg, 0.20 mmol) was stirred in MeCN (2.0
mL) at room temperature for 1.3 h in the presence of n-cetane (tR
= 5.4 min.) as an internal standard. At intervals, aliquots were
analyzed by GC after passing through a SiO2 column (eluting
with CH2Cl2). The yield of 4b (tR = 4.5 min.) was calculated to be
93% based on a calibration curve using an authentic sample. In
1
the case of 2-adamantanol (4g), the yield was calculated by H
NMR based on the peaks of 2.63 ppm (2H of 4g) using n-cetane
(0.88 ppm, 6H) as a standard.
1,3-Diphenyltetrazolium-5-hydroxyamide (2) was synthesized
according to the literature.11c Alcohols were purified by
distillation just before use. MeCN was dried by distillation from
CaH2 powder. Other commercially available materials were used
as received. All the reactions were performed under air and
monitored by GC or 1H NMR.
Retention times for the carbonyls 4b–4f: tR = 4.5 min. for 4b
in 93% yield; tR = 3.9 min. for 4c in 75% yield; tR = 4.0 min. for
4d in 40% yield; tR = 6.9 min. for 4e in 16% yield; tR = 2.3 min.
for 4f in 41% yield.
4.5 General procedure for the catalytic oxidation on a 1.0
mmol scale.
4.2 Synthesis of mesoionic nitroso compound 1
Synthesis of 5-nitroso-1,3-diphenyltetrazolium tetrafluoroborate
(1).
A mixture of nitrosotetrazolium salt (1, 17.0 mg, 0.0500
mmol), concentrated HNO3 (76.1 µL, 1.00 mmol), and alcohols
3a or 3g (1.00 mmol) was stirred in MeCN (10.0 mL) at room
temperature for 1.0−1.5 h. The solvent was evaporated under
reduced pressure and the residue was passed through a SiO2
column (eluting with CH2Cl2) to give the corresponding ketones
4a or 4g.
1,3-Diphenyltetrazolium-5-hydroxyamide (2, 51.1 mg, 0.200
mmol) was suspended in CH2Cl2 (0.50 mL). HBF4 (6.3 M, 1.86
mL, 11.6 mmol) was added to the mixture and stirred vigorously
and then CH2Cl2 was evaporated under reduced pressure (100
mmHg). The resulting suspension was cooled at 3–5 °C and
concentrated HNO3 (0.620 mL, 8.14 mmol) was added. The
mixture was stirred vigorously for 10 min while keeping the
temperature constant. The pale green precipitate was filtered
through a glass filter (G4), washed with H2O, THF:CH2Cl2 = 8:2,
and CH2Cl2. The solid was dissolved in MeCN and filtered
through a glass filter (G4). The filtrate was evaporated under
reduced pressure at room temperature to give green crystals of 1
(0.487 mg, 0.143 mmol, 72%). Crystals suitable for X-ray
analysis were obtained by recrystallization from MeCN/CCl4.
Melting point: 110.2–111.2 °C (from MeCN); [Found: C, 46.01;
H, 3.04; N, 20.71. C13H10BF4N5O requires C, 46.05; H, 2.97; N,
20.66%]; δH (400 MHz, MeCN-d3) 7.86 (t, 2H, J = 7.8 Hz, m of
Ph), 7.93–7.99 (m, 3H, p and m of Ph), 8.05 (t, 1H, J = 7.4 Hz, p
of Ph), 8.31 ppm (d, 4H, o of Ph); δC (100 MHz, acetone-d6)
122.2 (m of Ph), 126.5 (m of Ph), 130.8 (o of Ph), 131.0 (o of
Ph), 131.7 (i of Ph), 134.2 (p of Ph), 134.7 (p of Ph), 134.9 (i of
Ph), 160.0 ppm (C+); IR (KBr, cm-1) 3109, 3076, 2921, 2852,
1701, 1618, 1562, 1490, 1333, 1291, 1176, 1123, 1084, 1063,
1041, 999, 767, 680, 419; HRMS (ESI+-TOF): 1–BF4–+2H,
found 254.1047. C13H11N5O requires 254.1042.
Benzophenone 4a. Colorless liquid (180 mg, 0.986 mmol,
98%). δH (300 MHz, CDCl3) 7.49 (t, 4H, J = 7.3 Hz, m of Ph),
7.60 (t, 2H, J = 7.4 Hz, p of Ph), 7.81 ppm (d, 4H, J = 7.2 Hz, o
of Ph).
2-Adamantanone 4g. Colorless crystals (145 mg, 0.968
mmol, 96%). δH (300 MHz, CDCl3) 1.94−2.11 (m, 12H), 2.55
ppm (s, 2H).
4.6. Decomposition of 1 in the catalytic oxidation (Scheme 5).
A mixture of nitrosotetrazolium salt (1, 68 mg, 0.20 mmol),
concentrated HNO3 (0.42 g, 4.0 mmol), and benzyl alcohol (3b,
0.43 g, 4.0 mmol) was stirred in MeCN (40 mL) at room
temperature. The reaction was monitored by TLC until 3b
disappeared completely (4 h). The solvent was evaporated under
reduced pressure and Et2O (c.a. 1 mL) was added. The formed
precipitate was filtered and washed with Et2O to give colorless
crystal of 5 (37 mg, 58%) and a yellow filtrate. The yield of 6,
found in the filtrate, was estimated by 1H NMR to be 28% based