Aerobic oxidation of benzylic halides to carbonyl compounds with molecular oxygen catalyzed by tempo/kno2 in aqueous media

Article abstract of DOI:10.1080/00397910903040245

Benzylic halides were successfully oxidized to the corresponding aldehydes and ketones in good to excellent yields in aqueous media with molecular oxygen as oxidant in the presence of catalytic amounts of TEMPO (2,2,6,6- tetramethylpiperidyl-1-oxy) and potassium nitrite (KNO2).

Full text of DOI:10.1080/00397910903040245

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Aerobic Oxidation of Benzylic Halides
to Carbonyl Compounds with Molecular
Oxygen Catalyzed by TEMPO/KNO in
2
Aqueous Media
a b
a
b
a
b
Qifa Liu
, Ming Lu , Fei Yang , Wei Wei , Feng Sun ,
a
b
Zhenbang Yang & Sufeng Huang
a
Chemical Engineering College, Nanjing University of Science and
Technology, Nanjing, China
b
Nanjing Cosmos Chemical Co., Nanjing, China
Published online: 12 Mar 2010.
To cite this article: Qifa Liu , Ming Lu , Fei Yang , Wei Wei , Feng Sun , Zhenbang Yang & Sufeng
Huang (2010): Aerobic Oxidation of Benzylic Halides to Carbonyl Compounds with Molecular Oxygen
Catalyzed by TEMPO/KNO in Aqueous Media, Synthetic Communications: An International Journal for
2
Rapid Communication of Synthetic Organic Chemistry, 40:8, 1106-1114
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1
Synthetic Communications , 40: 1106–1114, 2010
Copyright # Taylor & Francis Group, LLC
ISSN: 0039-7911 print=1532-2432 online
DOI: 10.1080/00397910903040245
AEROBIC OXIDATION OF BENZYLIC HALIDES TO
CARBONYL COMPOUNDS WITH MOLECULAR
OXYGEN CATALYZED BY TEMPO/KNO IN
2
AQUEOUS MEDIA
1
,2
1
2
1
Qifa Liu, Ming Lu, Fei Yang, Wei Wei, Feng Sun,
2
1
Zhenbang Yang, and Sufeng Huang
Chemical Engineering College, Nanjing University of Science and
Technology, Nanjing, China
2
1
2
Nanjing Cosmos Chemical Co., Nanjing, China
Benzylic halides were successfully oxidized to the corresponding aldehydes and ketones in
good to excellent yields in aqueous media with molecular oxygen as oxidant in the presence
of catalytic amounts of TEMPO (2,2,6,6-tetramethylpiperidyl-1-oxy) and potassium
2
nitrite (KNO ).
Keywords: Benzylic halide; oxidation; potassium nitrite; TEMPO
INTRODUCTION
The oxidation of benzylic halides to the corresponding aldehydes and ketones
is an important fundamental reaction and of great significance in both laboratory
and industrial synthetic chemistry. In many cases, direct oxidation of benzylic
halides to aldehydes and ketones affords a more convenient synthesis than the oxi-
dation of the corresponding alcohols. Of these, the best classical methods are the
[
1]
[2]
[3]
Sommelet and Hass–Bender reactions, but others such as the Kr o¨ hnke reaction
[4]
and Kornblum oxidation are still commonly employed. Other procedures, such as
oxidations with certain amine N-oxide,
periodate–dimethylformamide,
[
(IBX),
metal nitrates,
[5–7]
[8]
N-alkoxypyridinium salts, sodium
[9]
[10]
selenium compounds,
[12]
o-iodoxybenzoic acid
[13]
11]
MnO under neutral conditions,
[
chromium-based reagents,
have all been developed for this transformation. Because of the
and
2
14–17]
deterioration of the environment, the oxidation of benzylic halides with environmen-
[
18–21]
[22–24]
tally benign oxidants such as hydrogen peroxide
has emerged quietly. In fact, Hashemi and Beni
and molecular oxygen
have reported a very simple
[
23]
and convenient procedure for oxidation of primary and secondary aryl=alkyl halides
to carbonyl compounds, which uses molecular oxygen and CuCl=Kieselguhr as cata-
[24]
lyst. Itoh and co-authors have also reported the photo-oxidation of aryl bromides
Received March 11, 2009.
Address correspondence to Ming Lu, Chemical Engineering College, Nanjing University of Science
and Technology, Nanjing 210094, China. E-mail: luming302@126.com
1
106

AEROBIC OXIDATION OF BENZYLIC HALIDES
1107
Scheme 1. TEMPO=KNO
2
-catalyzed oxidation of 4-isopropylbenzyl bromide by O
2
in aqueous media.
with mesoporous silica FSM-16 using dioxygen as the oxidant. Although these meth-
ods have the advantage that oxygen is used as the oxidant, volatile organic solvents
such as acetone, hexane, or dichloromethane are also used. This defeats the aim of
reducing the environmental burden of volatile organic contaminants. Therefore, the
introduction of a new, economical, convenient, and ecofriendly method for oxi-
dation of organic halides is still in demand.
In continuation of our ongoing program to develop organic-solvent- and
[
transition-metal-free catalytic systems for chloromethylation and oxidation reac-
25]
[
tions, herein we report a highly efficient catalytic system consisting of KNO and
17]
2
2,2,6,6-tetramethylpiperidyl-1-oxy (TEMPO) for the aerobic oxidation of benzylic
halides to aldehydes and ketones with molecular oxygen in aqueous media. Our
inspiration arises from the results of the NaNO =TEMPO systems for aerobic alco-
2
[26–29]
hol oxidation.
The oxidation of 4-isopropylbenzyl bromide (1a, 0.50 mol) was first carried out
by vigorously stirring the two-phase system (1a and 15 mol H O) at reflux tempera-
2
ture in the presence of 5 mol% KNO and 3 mol% TEMPO under an O atmosphere
2
2
(Scheme 1). The oxidation proceeded smoothly, and 72.5% isolated yield of 4-isopro-
pylbenzaldehyde (2a) was obtained with 76.3% conversion after 3.5 h (Table 1,
entry 1). Increasing the amount of H O to 30 mol, 99.0% conversion and 84.2% iso-
2
lated yield were obtained at the same interval (entry 2). Under the present reaction
conditions, overoxidation product (4-isopropylbenzoic acid) was not observed, and
the main by-product was 4-isopropylbenzyl alcohol, which could be fully converted
into 2a with yield of 92% by prolonging the reaction time to 4.0 h (entry 3). However,
Table 1. Optimization of the reaction conditions for oxidizing 4-isopropylbenzyl bromide to 4-isopropyl-
a
benzaldehyde
b
c
Entry
KNO
2
(mmol)
TEMPO (mmol)
H
2
O (mol)
Time (h)
Con. (%)
Yield (%)
1
2
3
4
5
25.0
25.0
25.0
25.0
—
15.0
15.0
15.0
—
15.0
30.0
30.0
30.0
30.0
3.5
3.5
4.0
4.0
4.0
76.3
99.0
99.7
99.5
99.2
72.5
84.2
92.0
d
59.5
e
15.0
19.0
a
Reaction conditions: 4-isopropylbenzyl bromide (0.5 mol), reflux, under O
Conversions are based on the GC with area normalization.
2
atmosphere.
b
c
Isolated yields.
d
30 g cumenic acid was obtained from the distillated residue.
The main product (76%) was 4-isopropylbenzyl alcohol.
e

1108
Q. LIU ET AL.
a
Table 2. Catalytic aerobic benzylic halide oxidation in aqueous media
b
Entry
1
Organic halide
Product
Time (h)
4
Yield (%)
92
2
3
5
5
89
91
4
5
3.5
4
95
92
c
6
7
8
10
4
70
90
85
5
9
6
81
1
1
0
1
3
2
87
89
d
1
2
3
1.5
24
15
e
1
49
(
Continued )

AEROBIC OXIDATION OF BENZYLIC HALIDES
1109
Table 2. Continued
b
Entry
Organic halide
Product
Time (h)
Yield (%)
f
1
1
4
5
4
86
15
93
1
6
7
1
3
87
94
1
18
1
90
1
2
9
0
4
90
89
12
21
3.5
92
g
—
2
2
2
3
6
nd
h
—
24
—
a
Reaction conditions: halide (0.5 mol), KNO
under O atmosphere. In the case of chloride, 10 mol% KBr (50 mmol) was added.
Isolated yield.
2
(25 mmol), TEMPO (15 mmol), H
2
O (30 mol), reflux,
2
b
c
Partial polymerization occurred with vacuum distillation.
d
1
-(Dimethoxymethyl)4-methoxybenzene (23%) and 4,4-oxy-bis-(methylene)diphenol (28%) were
formed as two by-products.
e
4
5% of starting material was recovered.
f
5
-Bromo-thiophene-2-aldehyde was formed as major by-product in 4.5%.
g
An intractable mixture.
h
No oxidation reaction was observed.
in the absence of TEMPO, further oxidation of 2a to 4-isopropylbenzoic acid
occurred (entry 4). The KNO was also crucial for achieving good yields of 2a
2
because the yield decreased to 19% (entry 5) in the absence of KNO but increased
2
to 92% (entry 3) in the presence of KNO . Further experiments demonstrated the
2
oxidation reaction could not be carried out under anhydrous conditions. At lower
ꢀ
temperatures, such as 70 or 90 C, this oxidation reaction proceeded very slowly.
Therefore, the optimal reaction conditions were as follows: 0.5 mol of bromide,

1110
Q. LIU ET AL.
3 mol% of TEMPO, 5 mol% of KNO , 30 mol of H O, at reflux and under an O
2 2 2
atmosphere.
Under these conditions, various benzyl halides were converted into their corre-
sponding aldehydes (Table 2, entries 1–13). Benzyl chlorides were much less reactive
than the corresponding bromides, but the reaction activity could be enhanced signifi-
cantly by addition of a catalytic amount of 10 mol% KBr. The electron-neutral benzyl
halides could be easily oxidized to the corresponding aldehydes and afford good to
excellent isolated yields. However, electron-rich 4-methoxybenzyl chloride gave only
1
1
5% isolated yield of 4-methoxybenzaldehyde (entry 12) with two by-products:
-(dimethoxymethyl)-4-methoxybenzene (23%) and 4,4-oxy-bis-(methylene)diphenol
(28%). When substrates that possess a strongly electron-withdrawing nitro group
were used, the yield was somewhat less than the yields for the other reactions, and
4
2
5
5% of the starting material was recovered after 24 h (entry 13). The oxidation of
-chloromethylthiophene gave the desired thiophene-2-carbaldehyde along with
-bromo-thiophene-2-aldehyde as major by-product (entry 14). As for the oxida-
tion of 5-(bromomethyl)-2-chloropyridine, a longer reaction time (15 h) was
needed to obtain full conversion and 93% isolated yield of 6-chloronicotinaldehyde
(
entry 15).
The corresponding ketones were obtained from the secondary benzylic halides
Table 2, entries 16–21). The reaction rate of the secondary halides was obviously
(
faster than that of primary halides. Because of the presence of a double bond, cinna-
myl chloride was oxidized to give an intractable mixture (Table 2, entry 22), and no
efforts were made to isolate the resulting aldehydes from the mixture. The oxidation
of 2-bormoethylbenzene, an aliphatic halide, could not be achieved in the catalytic
process (Table 2, entry 23).
A possible overall mechanism for the oxidation of bromides, similar to that of
[26–29]
alcohol, proposed by Liu and coworkers, is shown in Scheme 2. Hydrolysis of
starting material 1 yielded the corresponding alcohol 2 and HBr. The oxidation of
the resulting alcohol into carbonyl compounds was realized by oxoammonium cat-
[
30–32]
ion, an active oxidant of alcohols,
cycle I by oxidizing TEMPOH, the reduced form of TEMPO, with Br , and the latter
which could be continuously obtained in
2
was reduced into HBr, which initially came from the hydrolysis of bromides. The
reoxidation of HBr to Br by NO is shown in cycle II. NO was reduced to NO
2
2
2
Scheme 2. Proposed overall mechanism.

AEROBIC OXIDATION OF BENZYLIC HALIDES
1111
when it completed the oxidation of HBr. The oxidation of NO into NO could be
2
easily performed with molecular oxygen (cycle III). The hydrolysis of halides was
a reversible process, and increasing the amount of water used could lead to a faster
reaction rate. In the oxidation of benzylic chlorides, addition of catalytic amount of
KBr was helpful to this reaction by producing the HBr, which played an essential
role in the catalytic cycle. It looks likely that the hydrolysis of the halides was the
rate-controlling step. Secondary halides hydrolyzed faster than primary ones, so that
they were oxidized faster.
CONCLUSION
In summary, we have found a useful method for aerobic oxidation of benzylic
halides directly to the corresponding aldehydes and ketones in aqueous media under
an O atmosphere with catalytic amounts of TEMPO and KNO . Water was used as
2
2
both reagent and solvent, and no organic solvents were used in the whole process.
Although the reaction does not work for aliphatic halides, lack of transition metals,
simple reaction conditions, and good to excellent yields make this method an attract-
ive synthetic tool for the oxidation of benzylic halides to corresponding aldehydes or
ketones.
TYPICAL EXPERIMENTAL PROCEDURES
The typical experimental procedure is exemplified by the oxidation of 4-
isopropylbenzyl bromide in a 1000-mL, round-bottomed flask equipped with a
mechanic stirrer, a thermometer, and a reflux condenser. The bromide (0.5 mol),
TEMPO (15 mmol), and KNO (25 mmol) were stirred vigorously at reflux in H O
2
2
(30 mol) under an O atmosphere for 4 h. The reaction progress was monitored using
an Agilent6890 gas chromatograph equipped with an HP-5MS capillary column
2
(
30 m ꢁ 0.32 mm ꢁ 0.25 mm ID) and a hydrogen flame detector. The gas chromato-
ꢀ
graphy (GC) conditions were as follows: injection port temperature was set at 280 C
ꢀ
and detector temperature at 300 C. Inlet pressures of nitrogen gas and hydrogen gas
were 65 KPa and 80 Kpa, respectively, and the amount of the test specimen was
ꢀ
0
for 2 min, then ramped to 165 C at 7 C=min, again up to 250 C at 25 C=min, fol-
lowed by 15 min at 250 C. After completion, the organic layer was separated,
washed with 20 wt% aqueous Na S O solution and saturated aqueous NaHCO
3
.1 mL. The temperature program started at 80 C and maintained this temperature
ꢀ
ꢀ
ꢀ
ꢀ
ꢀ
2
2
3
solution to remove any trace of oxidant and TEMPO, dried over anhydrous Na SO ,
2
4
and finally distilled under vacuum to give pure 4-isopropylbenzaldehyde (bp:
8
authentic sample.
ꢀ
1
6–87 C=5 mmHg) in 92% yield identified by H NMR and comparison to its
ANALYTICAL DATA OF NEW PRODUCTS
4,4-Oxy-bis-(methylene)diphenol (12)
ꢀ
ꢀ
1
Mp 33.5–35 C, bp: 124–126 C=5 mmHg. H NMR (CDCl , 500 MHz): d 4.46
3
(
4H, s, ꢂCH ꢂ), 6.89 (4H, d, J ¼ 14.5, ArH), 7.28 (4H, d, J ¼ 14.5, ArH), 9.52 (2H,
2

1112
Q. LIU ET AL.
þ
þ
þ
þ
s, ꢂOH). MS (m=z): 230 (M ), 229 (M ꢂ 1), 198 (M ꢂ 32), 153 (M ꢂ 77), 91
þ
þ
(
M ꢂ 139), 77 (M ꢂ 153). Elemental analysis: C, 73.03%; H, 6.13%; calculated
from C H O . Found: C, 72.90%; H, 6.12%.
1
4
14
3
6-Chloronicotinaldehyde (15)
ꢀ
ꢀ
1
Mp: 69.5–71.0 C, bp: 117–119 C=5 mmHg. H NMR (CDCl , 500 MHz): d
3
7
.52 (H, d, J ¼ 8.2, 5-PyH), 8.15 (H, q, J ¼ 10.4, 4-PyH), 8.87 (H, d, 2-PyH), 10.14
þ
þ
þ
þ
(H, s, ꢂCHO). MS (m=z): 142 (M þ 1), 141 (M ), 113 (M ꢂ 28), 107 (M ꢂ 34),
þ
þ
7
calculated from C H ClNO. Found: C, 50.79%; H, 2.85%; Cl, 24.99%.
9 (M ꢂ 62), 52 (M ꢂ 89). Elemental analysis: C, 50.91%; H, 2.85%; Cl, 25.05%
6 4
3-Methyl-2-acetylbenzofuran (2l)
ꢀ
ꢀ
1
Mp: 23.5–24.5 C, bp: 102–104 C=5 mmHg. H NMR (CDCl , 500 MHz): d
3
2
.57 (3H, s, ꢂCH ), 2.59 (3H, s, ꢂCH ), 7.28 (H, m, J ¼ 14.6, ArH), 7.46 (H, m,
3
3
J ¼ 15.3, ArH), 7.48 (H, d, J ¼ 8.0, ArH), 7.60 (H, d, J ¼ 7.9, ArH). MS (m=z):
þ
þ
þ
þ
þ
1
74 (M ), 173 (M ꢂ 1), 159 (M ꢂ 15), 131 (M ꢂ 43), 103 (M ꢂ 71), 77
þ
(
M ꢂ 97). Elemental analysis: C, 75.84%; H, 5.79% calculated from C H O .
1
1
10
2
Found: C, 75.64%; H, 5.78%.
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