Selective oxidation of activated alcohols by the combination of H 2O2, AcOH and NaBr: An efficient metal-free alternative

Article abstract of DOI:10.1055/s-0030-1259538

The ternary HAcOH-NaBr combination has been found to be extremely effective for the oxidation of benzyl alcohol and its derivatives. Comparative studies indicated that the addition of NaBr offered significant improvement in product yields and selectivity towards aldehyde formation. Georg Thieme Verlag Stuttgart New York.

Full text of DOI:10.1055/s-0030-1259538

LETTER
555
Selective Oxidation of Activated Alcohols by the Combination of H O , AcOH
2
2
and NaBr: An Efficient Metal-Free Alternative
S
X
elective
O
xidatio
i
n
of
A
c
n
tivate
d
A
lc
o
g
hols yi Qi,* Jing Wang, Liwei Zheng, Lin Qi
School of Chemistry and Environment, Beijing University of Aeronautics and Astronautics, Beijing 100191, P. R. of China
Fax +86(10)82313981; E-mail: qixy@buaa.edu.cn
Received 29 October 2010
So far significant progress has been made in the alcohol
oxidation. Nevertheless from a practical viewpoint, there
is still large room for improvement in many reported cas-
Abstract: The ternary H O –AcOH–NaBr combination has been
found to be extremely effective for the oxidation of benzyl alcohol
and its derivatives. Comparative studies indicated that the addition
2
2
of NaBr offered significant improvement in product yields and se- es. For example, the deactivation of catalyst normally
lectivity towards aldehyde formation.
constitutes a challenging issue in the catalytic oxidation of
alcohols. In addition, the methods for preparation and syn-
thesis of the catalysts used to date usually involve a series
of demanding, laborious and/or expensive procedures.
Therefore, the development of more economic and effi-
cient reaction systems is eagerly expected to be made in
this important field.
Key words: alcohols, selective oxidation, aldehydes, ketones
The oxidation of alcohols is one of the fundamental chem-
ical transformations to obtain desirable aldehyde and ke-
1
,2
tone compounds. In general, traditional methods for
performing such a process involve the use of stoichiomet- Herein, we present the informative results of the selective
ric oxygen donors such as chromium(VI) and manga- oxidation of a number of activated alcohols by the metal-
nese(VII) reagents, which inevitably generates copious free combination of hydrogen peroxide, acetic acid and
amounts of industrial metal waste and environmental pol- sodium bromide and show that a remarkably enhanced
lutions. For both economic and environmental reasons, conversion of alcohols was achieved only in the coexist-
there is an increasing need for the reaction systems that ence of H O , AcOH and NaBr. The alcohol oxidation ex-
2
2
employ clean oxidants, e.g., O and H O and operate un- periments were carried out in a 25-mL stirred batch glass
2
2
2
der benign reaction conditions.
reactor fitted with a water-cooled reflux condenser; the
conversion and selectivity data were determined by gas
chromatography. It is well known that reaction parameters
such as reaction temperature and reaction time play a cru-
cial role in liquid-phase selective oxidations of organic
substrates. Hence, the study was undertaken to probe the
effect of these two parameters on conversion and selectiv-
ity in the oxidation of a representative alcohol (benzyl al-
cohol) in the presence of NaBr or without NaBr, in an
attempt to ascertain the reaction capability of the H O –
Until now, there has been much literature concerning aer-
obic oxidations of alcohols, mainly using group eight met-
al catalysts. Ruthenium compounds have been mostly
3
investigated in the oxidation of a variety of alcohols. Be-
4
sides ruthenium, palladium and certain transition metals,
5
6
7
8
9
e.g., chromium, manganese, cobalt, copper, gold, bi-
1
0
metallic ruthenium-copper (Ru-Cu-HT) and palladium-
1
1
gold have been shown to be capable of catalyzing aero-
bic oxidation of alcohols despite the requirement of base
or aldehyde additives in some of these reaction systems.
2
2
AcOH–NaBr system.
As shown in Figure 1 (A), the H O –AcOH–NaBr system
2
2
As for H O , ruthenium-based catalysts are generally not
2
2
is highly effective in the oxidation of benzyl alcohol. At
as low as 30 °C (4 h), 54% conversion of benzyl alcohol
was obtained with 94% selectivity to benzaldehyde. As
the temperature was varied over the range 30–70 °C, a
sharp increase in the conversion of benzyl alcohol took
place, resulting in a 92% conversion with 87% selectivity
at 50 °C. The conversion plateau then arrived above 50 °C
while the selectivity to benzaldehyde was decreased from
preferred for the alcohol oxidation because of their high
H O -decomposition activity. In contrast, first-row transi-
2
2
tion metals have been reported to be effective in the oxi-
2
dation of alcohols with H O as an oxidant. Two research
2
2
1
2
groups reported that titanium silicalite-1 (TS-1, MFI)
could effect H O -oxidation of alcohols to aldehydes, ke-
2
2
tones, and/or carboxylic acids. The success of TS-1 has
greatly stimulated extensive investigation of alcohol oxi-
dations with H O in the presence of other redox mi-
8
6% (60 °C) to 75% (70 °C). At the same time, more ben-
2
2
1
3
14
zoic acid (another oxidized product) was formed at higher
temperature. The efficiency of the H O –AcOH–NaBr
croporous materials, such as VS-1 (MFI), Ti-beta,
AFI-V (Cr) aluminophosphate molecular sieves (with
2
2
1
5
3+
16
system was further evidenced by the variation of benzyl
alcohol conversion with reaction time as shown in
Figure 1 (B). In a reaction period of 0.5 hour (60 °C), 65%
conversion of benzyl alcohol was achieved with 92% se-
lectivity to benzaldehyde. Only a slight increase in benzyl
alcohol conversion was observed beyond the reaction
TBHP as the oxidant), Fe /montmorillonite-K10 and
zeolite-encapsulated metal (Cr, Ni and Cu) complexes.17
SYNLETT 2011, No. 4, pp 0555–0558
x
x.
x
x.
2
0
1
1
Advanced online publication: 08.02.2011
DOI: 10.1055/s-0030-1259538; Art ID: W17410ST
©
Georg Thieme Verlag Stuttgart · New York

5
56
X. Qi et al.
LETTER
time of two hours over which 92% conversion was ob- ing that Br was a key intermediate species involved in the
2
tained with 88% selectivity. The oxidation of benzyl alco- oxidation of benzyl alcohol; (3) substitution of acetic acid
hol was also carried out by the binary H O –AcOH in the H O –AcOH–NaBr combination by other solvent
2
2
2
2
counterpart (60 °C, 4 h). The results show that compared molecules such as methanol, acetonitrile and N,N-dimeth-
to a 95% conversion with 86% selectivity obtained by the ylformamide led to a dramatic decrease in the conversion
H O –AcOH–NaBr system, the binary reagent H O – of benzyl alcohol, and only about 5% conversion was ob-
2
2
2
2
AcOH afforded only a 21% conversion with 26% selectiv- tained for these three solvent systems; (4) a considerable
ity (benzoic acid was the remaining product). Such a huge decrease in the conversion of benzyl alcohol was also ob-
difference in reactivity was indicative of the unique reac- served by the replacement of bromide with chloride or io-
tion feature of the H O –AcOH–NaBr system.
dide, and 35% and 10% conversions were obtained by
H O –AcOH–NaCl and H O –AcOH–NaI, respectively;
2
2
2
2
2
2
(
5) much lower or higher concentrations of NaBr (than 0.1
M) in the H O –AcOH–NaBr system each resulted in a
2
2
noticeably diminished conversion of benzyl alcohol; (6)
no appreciable consumption of NaBr was found during
the oxidation of benzyl alcohol because post-reaction ad-
dition of aqueous AgNO solution into the reaction system
3
prompted a quantitative precipitation of AgBr. In all of
these oxidizing systems, the same amount of H O was
2
2
used and added in one single portion unless otherwise
specified (for details see supporting information).
In summary, the concomitant presence of H O , AcOH
2
2
and NaBr seems clearly to be a prerequisite for the high
efficiency in the oxidation of benzyl alcohol. As H O has
2
2
q
q
a higher redox potential E
(E
= 1.77 V)
a,H2O2/H2O
a,H2O2/H2O
q
than HBrO (E
- = 1.33 V) and the latter reagent
a,HBrO/Br
–
very easily reacts with Br to afford Br in acidic media,
2
oxidation of alcohols can be effected by Br added directly
2
1
8
to or generated in situ from the reaction systems. Such
alcohol oxidations have been confirmed to go through a
mechanism in which the rate-determining step involves a
hydride transfer by Br from the OH-bearing carbon fol-
2
lowed by a rapid proton removal from the oxygen (per-
haps in a partly concerted action).19 Also based on the
observations here, the reaction steps involved in this oxi-
dative transformation can be summarized in Scheme 1.
Therefore, the nature of the H O –AcOH–NaBr system
2
2
for the oxidation of benzyl alcohol can be described as be-
low: in the medium of acetic acid, the bromide ion is first
–
converted to HBrO–BrO by acquiring a single oxygen
Figure 1 (A) Effect of reaction temperature on benzyl alcohol oxi- atom from hydrogen peroxide; the newly formed HBrO–
–
dation by the H O –AcOH–NaBr system: reaction time: 4 h, benzyl
2
2
BrO then reacts with bromide ion to yield bromine. As
for the last step in the overall reaction scheme, the forma-
tion of benzaldehyde involves the bromine-mediated hy-
dride ion abstraction as mentioned above. This was
accompanied by regeneration of bromide ion and initia-
tion of another reaction cycle, which suggests that the
whole reaction process is somewhat catalytic in the bro-
mide ion.
alcohol (5.32 mmol), benzyl alcohol–H O (1:2), AcOH (5 mL),
2
2
[
NaBr] (0.1 M). (B) Effect of reaction time on benzyl alcohol oxida-
tion by the H O –AcOH–NaBr system: reaction temperature: 60 °C,
2
2
benzyl alcohol (5.32 mmol), benzyl alcohol–H O (1:2), AcOH (5
2
2
mL), [NaBr] (0.1 M)
Moreover, some additional observations were made as
follows: (1) the presence of bromine (Br ) in the reaction
2
system was demonstrated by a light red coloration which
developed during the oxidation of benzyl alcohol by the
H O –AcOH–NaBr system; (2) the addition of one equiv-
2
2
alent of cyclohexene (relative to benzyl alcohol, 5.32
mmol) as the competing substrate into the reaction system
deeply suppressed the oxidation of benzyl alcohol, with
the conversion of benzyl alcohol being decreased from
OH
H2O2
Br–
O
Br–
+ Br–
HBrO
Br2
AcOH
AcOH
AcOH
9
5% to 16%. Furthermore, the formation of 1,2-dibro- Scheme 1 Reaction steps involved in the oxidation of benzyl alco-
mocyclohexane was detected by GC–MS analysis, imply- hol by the H
O –AcOH–NaBr system
2
2
Synlett 2011, No. 4, 555–558 © Thieme Stuttgart · New York

LETTER
Selective Oxidation of Activated Alcohols
557
a
Table 1 Oxidation of Various Alcohols by H O –AcOH–NaBr
2
2
The scope of the H O –AcOH–NaBr system can be illus-
2
2
trated by Table 1 in which oxidations of several other al-
coholic substrates were also investigated under the same
conditions. Just as was the case for benzyl alcohol, most
of its derivatives, viz. activated alcohols could readily un-
dergo oxidation to the corresponding aldehydes or ke-
tones with excellent conversions and selectivities.
However, the same high efficiency was not observed for
the oxidation of aliphatic and alicyclic alcohols. As shown
in Table 1, a low or moderate conversion was obtained for
the oxidation of five selected alcohols, 1-pentanol, 2-pen-
tanol, 2-heptanol, cyclopentanol and cyclohexanol. As a
result, this limited reactivity renders the H O –AcOH–
Substrate
Product
Temp Conv. Selectivity
°C) (mol%) (mol%)
(
OH
O
85.6b
6
0
95.3
OH
O
O
O
6
0
90.0
100
OH
OH
6
6
6
0
0
0
92.0
95.9
91.6
100
2
2
NaBr combination impractical for the oxidation of these
aliphatic and alicyclic alcohols.
100
To summarize, we have discovered a very simple, inex-
pensive, efficient metal-free reaction system in which a
variety of activated alcohols can be easily transformed
into the corresponding carbonyl compounds under mild
Cl
Cl
99.4b
conditions using H O as the ultimate oxidizing agent.
OH
OH
O
O
2
2
And we are in the process of optimizing the reaction con-
ditions in order to expand its synthetic scope for oxida-
tions of alcohols and other organic substrates in the future.
7
6
7
0
0
0
93.8c
96.3
47.7
97.7b
F
F
Supporting Information for this article is available online at
http://www.thieme-connect.com/ejournals/toc/synlett.
OH
O
98.4b
Cl
Cl
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