Z.-W. Mei et al. / Tetrahedron 64 (2008) 10761–10766
10765
1512, 1458, 1441, 1424, 1381, 1333, 1317, 1258, 1180, 1140, 1121,
4.7. Electrochemical set-up and electrochemical procedure
for cyclic voltammetry
1080, 1028, 957, 845 cmꢁ1
;
1H NMR (300 MHz):
d
2.49 (s, 3H),
3.88 (s, 3H), 6.92 (d, J¼9.1 Hz, 2H), 7.94 (d, J¼9.1 Hz, 2H); 13C
NMR (75.5 MHz):
164.4, 183.1.
d
30.2, 55.6, 92.2, 114.2 (2C), 123.8, 132.1 (2C),
Cyclic voltammetry was performed with a home made poten-
tiostat and a wave-form generator, PAR Model 175. The cyclic vol-
tammograms were recorded on a Nicolet 3091 digital oscilloscope.
Experiments were carried out in a three-electrode cell. The working
electrode was a steady gold disk electrode (d¼0.5 mm). The counter
electrode was a platinum wire of ca. 1 cm2 apparent surface area.
The reference was a saturated calomel electrode separated from the
solution by a bridge filled by 2 mL of dichloromethane containing
Bu4NBF4 (0.3 M). Distilled and degassed dichloromethane (15 mL)
containing Bu4NBF4 (0.3 M) was poured into the cell, followed by
4.68 mg (0.03 mmol, 2 mM) of TEMPO (3a). The cyclic voltammetry
was performed at the scan rate of 0.5 V sꢁ1 in the potential range
between 0 and þ1.2 V.
4.3. Time-course for the oxidation of 2a with a 3c-co-oxidants
system
A mixture of 1a (172 mg, 1 mmol), 3c (28 mg, 0. 1 mmol), and
Py$HBr3 (480 mg, 1.5 mmol) in CH2Cl2 (6 mL)–aqueous saturated
NaHCO3 (12 mL) was allowed to react and the aliquots at the pre-
scribed time were analyzed by GC and the selectivity was calculated
based on the peak areas (Fig. 1). Similarly, the time-course of the
oxidation of 7a was achieved by using Py$HBr3 in combination with
various 3a–d (Fig. 2).
Similar experiments were performed from 3b (5.6 mg), 3c
(8 mg), 3d (10 mg), and 3e (11 mg).
4.4. A typical procedure for oxidation of secondary alcohols
to the ketones with poly(4-vinylPy)$HBr3
4.8. General procedure for oxidation of polyhaloalkyl
alcohols to the ketones with 3d–Py$HBr3
A solution of 1a (86 mg, 0.5 mmol) and 3c (28 mg, 0. 1 mmol)
in CH2Cl2 (3 mL) was covered with aqueous saturated NaHCO3
(6 mL). To this biphase mixture was added portionwise poly(4-
vinylpyridinium) tribromide (300 mg) under a vigorous stirring at
room temperature. The stirring was continued at room tempera-
ture until 1a was consumed, for about 2 h as monitored with TLC.
The mixture was filtered off to leave poly(4-vinylpyridine)
(107 mg) and the filtrate was worked up in the usual manner to
give 67 mg (78% yield) of 2a after purification by column
chromatography.
A
solution of 2,2-dichloro-3,3,3-trifluoro-1-(4-methoxy-
phenyl)propanol21 (9b, 288 mg, 1.0 mmol) and 4-(4-CF3C6H4CO2)-
TEMPO (3d, 35 mg, 0.1 mmol) in CH2Cl2 (6 mL) was covered with
aqueous 5% NaHCO3 (12 mL). To this biphase mixture was added
portionwise Py$HBr3 (480 mg, 1.5 mmol) under a vigorous stirring
at room temperature. The mixture was stirred for an additional
1.5 h and the reaction was quenched with aqueous 5% Na2S2O3
(5 mL). The products were extracted with CH2Cl2 and the aqueous
layer was again extracted with AcOEt. Extracts were separately
washed with brine, dried (MgSO4), and concentrated. The com-
bined crude product was purified by column chromatography
(SiO2, hexane–AcOEt 10:1 to 3:1) to give 245 mg (86% yield) of
10b (Rf¼0.65, hexane–AcOEt 5:1); IR (neat): 2845, 1697, 1601,
1574, 1512, 1460, 1425, 1316, 1261, 1207, 1180, 1124, 1045, 1026,
4.5. Preparation of 4-(4-trifluoromethylbenzoyloxy)-
2,2,6,6-tetramethylpiperidine-1-oxyl (3d)
To a solution of 4-hydroxy-2,2,6,6-tetramethylpiperidine-1-oxyl
(1.72 g, 10 mmol) and pyridine (1.62 mL, 20 mmol) in THF (10 mL)
was added dropwise a solution of 4-trifluoromethylbenzoyl chlo-
ride (1.63 mL, 11 mmol) in THF (3 mL) at 0–4 ꢀC. The mixture was
stirred at room temperature overnight and worked up in the usual
manner. The crude product was purified by column chromatogra-
phy (SiO2, hexane–AcOEt 10:1 to 5:1) to give 3.2 g (93% yield) of 3d
as solids; mp 74–75 ꢀC (from hexane) (Rf¼0.55 hexane–AcOEt 3:1);
IR (KBr): 1721, 1585, 1512, 1466, 1412, 1331, 1283, 1242, 1167, 1138,
930, 870, 847, 829, 737, 702, 673 cmꢁ1; 1H NMR (300 MHz):
d 3.91
(s, 3H), 6.97 (d, J¼9.2 Hz, 2H), 8.27 (d, J¼9.2 Hz, 2H); 13C NMR
2
(75.5 MHz):
d
55.5, 78.7 (q, JCF¼31.1 Hz), 113.8 (2C), 121.3 (q,
1JCF¼283.3 Hz), 122.7, 133.4 (2C), 164.7, 181.3; 19F NMR
(282.3 MHz):
d
ꢁ75.2 (s).
4.8.1. Methyl 2,2-dichloro-3-oxo-3-phenylpropanoate (12a)
Yield 79% (Rf¼0.53, hexane–AcOEt 5:1); IR (neat): 2957, 1769,
1746, 1713, 1690, 1597, 1449, 1437, 1252, 1217, 1186, 1015, 864, 824,
1128, 1101, 1067, 1017, 963, 862, 775, 705 cmꢁ1
;
1H NMR, treated
795, 689 cmꢁ1
;
1H NMR (300 MHz):
d
3.87 (s, 3H), 7.45–7.52 (m,
with PhNHNH2 (300 MHz): 1.176 and 1.181 (s, 12H), 1.68 (m 2H),
d
2H), 7.60–7.65 (m, 1H), 8.02–8.07 (m, 2H); 13C NMR (75.5 MHz):
1.94–2.00 (m, 2H), 5.23 (m, 1H), 7.60 (d, J¼8.2 Hz, 2H), 8.03 (d,
d
54.8, 81.6, 128.6 (2C), 130.0 (2C), 130.8, 134.1, 164.5, 183.3. HRMS
J¼8.24 Hz, 2H); 19F NMR, treated with PhNHNH2 (282.3 MHz):
(ESI) calcd for C10H9Cl2O3 (MHþ) 246.9929, found 246.9887 (MHþ).
d
ꢁ63.3 (s). HRMS (ESI) calcd for C17H21F3NO3 (Mþ) 344.1474, found
344.1499 (Mþ).
4.8.2. Methyl 2,2-dichloro-3-oxo-3-(4-bromophenyl)-
propanoate (12b)
4.6. 4-(2,3,4,5,6-Pentafluorobenzoyloxy)-2,2,6,6-tetramethyl-
piperidine-1-oxyl (3e)
Yield 78% (Rf¼0.62, hexane–AcOEt 5:1); IR (neat): 1769, 1746,
1713, 1690, 1584, 1485, 1437, 1398, 1250, 1217, 1184, 1074, 1007, 928,
868, 824, 760, 725 cmꢁ1; 1H NMR (300 MHz):
d 3.89 (s, 3H), 7.63 (d,
Compound 3e was prepared by the reaction of 2,3,4,5,6-penta-
fluorobenzoic acid and 4-hydroxy-2,2,6,6-tetramethylpiperidine-
1-oxyl in the presence of carbon tetrabromide, PPh3, pyridine in
CH2Cl2; mp 109–110 ꢀC (from hexane–AcOEt 10:1) (Rf¼0.64 hex-
ane–AcOEt 3:1); IR (KBr): 1728, 1651, 1526. 1495, 1416, 1368, 1337,
J¼8.8 Hz, 2H), 7.92 (d, J¼8.8 Hz, 2H); 13C NMR (75.5 MHz):
d 55.0.
81.4, 129.6, 129.7, 131.5 (2C), 132.1 (2C), 164.3, 182.7. HRMS (ESI)
calcd for C10H7BrCl2O3 (MHþ) 324.9034, found 324.8992 (MHþ).
4.8.3. Ethyl 2,2-difluoro-3-(4-methoxyphenyl)-3-oxo-
propanoate (14b)
1232, 1177, 1107, 1092, 1069, 1007, 957, 770 cmꢁ1
treated with PhNHNH2 (300 MHz): 1.152 and 1.158 (s, 12H), 1.64
(m 2H), 1.93–1.99 (m, 2H), 5.24 (m, 1H); 19F NMR, treated with
;
1H NMR,
d
Yield 72% (Rf¼0.59, hexane–AcOEt 5:1); IR (neat): 2845, 1771,
1694, 1690, 1600, 1573, 1514, 1464, 1447, 1427, 1395, 1373, 1316,
1269, 1182, 1159, 1122, 1099, 1076, 1026, 924, 910, 847, 791, 712,
PhNHNH2 (282.3 MHz):
d
ꢁ160.6 (m), ꢁ149.0 (m), ꢁ138.9 (m).
HRMS (ESI) calcd for C16H17F5NO3 (Mþ) 366.1129, found 366.1121
(Mþ).
698 cmꢁ1; 1H NMR (300 MHz):
4.38 (q, J¼7.1 Hz, 2H), 6.98 (d, J¼9.1 Hz, 2H), 8.07 (d, J¼9.1 Hz, 2H);
d
1.32 (t, J¼7.1 Hz, 3H), 3.90 (s, 3H),