A. Dewan et al. / Tetrahedron Letters 52 (2011) 2563–2565
2565
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chloride to benzaldehyde (Table 3, entry 17). No over-oxidized
product was observed in any of the alcohol studied. Moreover,
the active oxidant could be regenerated by addition of appropriate
amount of benzyltrimethylammonium chloride and 30% H2O2 and
reused in further oxidation reaction.22b
In summary, we have developed an efficient method for the
selective and rapid oxidation of benzylic alcohols to the corre-
sponding aldehydes and ketones under solvent less condition at
room temperature. Synthesis of this oxidant is very simple and
starting materials used are commercially available and cheap.
The oxidant is also very stable hence, can be stored and handled
very easily. The reaction time is very fast and no over-oxidized
product was observed. After the reaction, the active oxidant can
be regenerated and reused for further oxidation reaction hence
minimizing the waste generation.
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22. (a) General procedure for oxidation of benzylic alcohol: 1-phenylethanol
(0.122 gm,
1.0 mmol),
vanadium
polyoxometalates
[C7H7N(CH3)3]3V10O28H3Á3H2O] (0.073 g, 0.05 mmol) and PTSAÁH2O (0.095 g,
0.5 mmol) were mixed at room temperature, ground thoroughly in a mortar
until the mixture became homogeneous. The yellow colour of the mixture
became green instantaneously on grinding, indicating progress of the reaction.
The reaction mixture was diluted with ether (30 mL) and washed with water
(20 mL). The organic layer was washed with brine (3 Â 20 mL), dried over
anhydrous Na2SO4, concentrated, and finally purified by column
chromatography on silica, using ethyl acetate/petroleum ether (1:9) as the
eluent to obtain acetophenone in 97% yield. 1H NMR (CDCl3, 300 MHz): d 7.9–
8.2 (m, 2H), 7.4–7.7 (m, 3H), 2.6 (s, 3H); 13C NMR (CDCl3, 75.5 MHz): d 197.4,
136.9, 134.2, 129.3, 129.9, 26.5; IR (KBr): 1688 cmÀ1
.
(b) Regeneration of the oxidant: The volume of the aqueous extract (20 mL) was
reduced to one fourth under reduced pressure and 30% H2O2 (2 mL) and
benzyltrimethylammonium chloride (0.046 mg, 0.25 mmol) was added and
stirred at room temperature for one hour. A pale yellowish coloured precipitate
appeared. The precipitate was recovered by filtration under suction and dried
under vacuum over fused CaCl2. The regenerated oxidant was used for
oxidation of 1-phenylethanol following the above mentioned procedure and
found to be effective for three cycles, yields: 92% (cycle-1), 87% (cycle-2), (82%
cycle-3).
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