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120
Chemistry Letters 2000
An Efficient and Selective Oxidation of Benzylic Alcohols to the Corresponding
Carbonyl Compounds under Solvent-Free Conditions
Abdol Reza Hajipour,* Shadpour E. Mallakpour,* Iraj Mohammadpoor-Baltork,† and Sepideh Khoee
Pharmaceutical Research Laboratory, College of Chemistry, Isfahan University of Technology, Isfahan 84156, I.R. Iran
†Department of Chemistry, Isfahan University, Isfahan, 81744, I. R. Iran
(Received October 18, 1999; CL-990893)
15 mL CC14 was added to the reaction mixture and after vigor-
ous stirring, the mixture was filtered off, then the solvent was
evaporated and the residue separated by column chromatogra-
phy on silica gel (eluent: CC14). One of the extracted materials
was the carbonyl compound which produced during the reac-
tion. The second one in all reactions was benzyl alcohol and no
sign of the starting alcohol was observed. According to the
above results, we have proposed the following mechanism:
The Cr(IV) that is produced in the initial step is not stable
and this species is capable of a further one-electron oxidation
1-Benzyl-4-aza-1-azoniabicyclo[2.2.2]octanedichromate is a
useful reagent for selective oxidation of benzylicalcohols and α-
hydroxy ketones to the corresponding carbonyl compounds. The
reaction was carried out under solvent-free conditions without
or in the presence of a catalytic amount of aluminum chloride.
Oxidation of organic compounds under non-aqueous and
aprotic conditions has an important role in the modern organic
synthesis. For this purpose, some new oxidizing reagents have
been prepared.1-8 Unfortunately most of them suffer at least from
one of the following disadvantages: 1) high cost of preparations,
2) long reaction time, 3) hygroscopisity, 4) high acidity, 5) insta-
bility, 6) no selectivity, 7) photosensitivity, 8) dangerous proce-
dures for their preparation and 9) tedious work-up procedures.
In continuous to our previous works6−8 here, we wish to
introduce an efficient, mild and rapid method for selective oxi-
dation of alcohols to their corresponding carbonyl compounds
by using 1-benzyl-4-aza-1-azoniabicyclo[2.2.2]octane dichro-
mate, which is readily prepared by reaction of an aqueous solu-
tion of l-benzyl-4-aza-1-azoniabicyclo[2.2.2]octane chloride 2
with CrO3 in 3M solution of HCl at room temperature as
shown in Scheme 1.
step as well as reagent 5 cleavage. It is believed that a part of
the substrate is oxidized via a free radical intermediate result-
ing from oxidation by Cr(IV).10 Scheme 3 also includes Cr(V)
as a participant in the mechanism:
During the reaction, DABCO was collected as a by-product,
which remained on the filter paper after filtration and its struc-
The resulting orange powder which can be stored for
months without loss of activity is soluble in acetonitrile, acetone
and N,N-dimethylformamide and slightly soluble in chloroform,
ethylacetate and dichloromethane, but is not soluble in carbon
tetrachloride, n-hexane and diethylether. This new oxidant has
several advantages: 1 ) In comparing with the previously report-
ed method,9 does not require a large excess of reagent and long
reaction time. 2) Because of the low reactivity of aliphatic alco-
hols, only benzylic alcohols could be converted to the correspon-
ding carbonyl compounds, 3) Due to its mild behavior, no fur-
ther oxidation to the carboxylic acid was observed.
The oxidation reaction was performed via two methods. In
method A, one equimolar of different alcohols and reagent
were mixed in a mortar and ground with a pestle until TLC
showed complete disappearance of starting alcohols. Following
the reaction by TLC shows that by disappearing the starting
alcohols spots, the carbonyl compounds spots appear. As well
as this spot, a new spot was also detected, which appeared
lower than the starting alcohol. Adding a large excess of
reagent not only can not dissolve this problem but also rein-
force this spot. In order to investigate the reaction mechanism.
1
ture was confirmed by H-NMR. The time required to com-
plete these reactions varied between 10 to 30 min. To acceler-
ate the reaction rate, a catalytic amount of AlC13 (0.1 mole
ratio of oxidant) was added to the reaction mixture (method
B). By adding this catalyst to the reaction media the reaction
time reduced from 15-30 min to 30-60 s and the benzyl alcohol
spot which was produced as a by-product, disappeared.
According to these observations here we propose another
mechanism (Scheme 4). AlC13 may be linked to the two oxy-
gens of the chromate and increased the positive charge and
reactivity of the Cr(VI). This behavior accelerates the reaction
rate and reduces the time of reaction.
On the other hand, the presence of AlC13 in the reaction
media acts as an H2O attracting group and avoided the H2O
molecules to attack to the reductive reagent 8 and consequent-
ly produced Al(OH)3 and a very stable complex i.e. l-benzyl-4-
aza-1-azoniabicyclo[2.2.2]octane chloride 2 and no sign of
Copyright © 2000 The Chemical Society of Japan