J. M. Khurana, B. M. Kandpal / Tetrahedron Letters 44 (2003) 4909–4912
4911
The progress of the reaction was monitored by TLC
using benzene/ethyl acetate (90:10 v/v) as eluent. The
starting material disappeared after 1 h and two new
spots were observed on TLC analysis corresponding to
benzoin and benzil. The reaction was allowed to con-
tinue until only one product was observed on TLC (2.5
h). The reaction was quenched with water (100 mL)
and the carbon tetrachloride layer was separated. The
(
(
2)
3)
aqueous layer was extracted with CCl (2×10 mL). The
4
combined organic layers were dried over anhyd.
MgSO , filtered and concentrated on a rotary evapora-
4
tor to afford a yellow solid which was recrystallized
from ethanol and was identified as benzil (0.92 g 94%)
by mp, mixed mp, IR and NMR spectra.
The reaction of substrates 1h–j (1 g, n mmol) were
carried out with NBS in the presence of dry pyridine
The mechanism of oxidation of alcohols with NBS has
not been clearly established although two interpreta-
tions have been advanced and it is generally accepted
(
according to the molar ratios shown in Table 1) as
above. In the reactions of aliphatic 1,2-diols (1 g, n
mmol) with NBS in the presence of dry pyridine, the
temperature of the oil bath was maintained at 60–70°C.
All the reactions were worked up as detailed above.
The reaction of tartaric acid was not quenched with
water owing to the high solubility of the product in
water. The carbon tetrachloride was evaporated and the
product was extracted by triturating with ether.
15
that a positive halogen is the attacking species. We
believe that the hydrobenzoins react with NBS to form
a hypobromite which readily loses hydrogen bromide to
form the carbonyl product (Eq. (4)). 4,4%-Dichloro-
hydrobenzoin dimethyl ether and 4,4%-dimethylhydro-
benzoin dimethyl ether did not undergo any oxidation
with NBS even after prolonged reaction times under
otherwise identical reaction conditions and the starting
materials were recovered unchanged due to the non-
availability of hydroxyl hydrogens as (required by the
mechanism proposed) above. The oxidations are
undoubtedly proceeding via the corresponding acyloins
as has been confirmed by the presence of acyloins when
the progress of various reactions was monitored. It
appears from the available data that the initial oxida-
tion of 1,2-diols is faster than the oxidation of corre-
sponding acyloins. Benzopinacole did not undergo any
oxidation with NBS under these conditions as expected.
Acknowledgements
B.M.K. thanks CSIR, New Delhi, India for a Junior
Research Fellowship.
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1
6
trap. A mixture of hydrobenzoin 1a (1.0 g, 4.67
mmol) and NBS (2.5 g, 14.01 mmol) in dry CCl (50
4
mL) was refluxed in an oil bath maintained at 80–85°C.