Sulfuric Acid-Promoted Oxidation of Benzylic Alcohols to Aromatic Aldehydes in Dimethyl Sulfoxide: An Efficient Metal-Free Oxidation Approach

Article abstract of DOI:10.1055/s-0037-1609149

An efficient metal-free oxidation of benzylic alcohols to Aaromatic aldehydes is described. Heating a solution of the benzylic alcohol in DMSO in the presence of H ₂ SO ₄ afforded the corresponding aldehyde in excellent yield. This oxidation reaction, which proceeds with a short reaction time and no side products, is akin to the Pfitzner-Moffatt oxidation, but without the need for N, N ′-dicyclohexylcarbodiimide.

Full text of DOI:10.1055/s-0037-1609149

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©
Georg Thieme Verlag Stuttgart · New York
2018, 29, A–E
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Synlett
E. Sheikhi et al.
Letter
Sulfuric Acid-Promoted Oxidation of Benzylic Alcohols to
Aromatic Aldehydes in Dimethyl Sulfoxide: An Efficient
Metal-Free Oxidation Approach
Ehsan Sheikhi*a
Mehdi Adib*a
OH
O
H SO , (prot. A: 1 equiv; prot. B: 0.5 equiv), DMSO
2
4
Ar
reflux
Ar
H
Morteza Akherati Karajabada
Seyed Jamal Addin Goharib
Ar: Ph, 4-MeC6H4, 3-MeC6H4,
naphthyl, 4-ClC6H4, 2-ClC6H4,
14 derivatives
each one by two protocols
yields of protocol A: 80–95%
yields of protocol B: 78–92%
4
-BrC6H4, 3-MeOC6H4,
i
a
2,5-(MeO)2C6H3, 4-PrC6H4,
School of Chemistry, College of Science, University of Tehran,
3
4
,4-(MeO)2C6H3, 4-(Me2N)C6H4,
-O2NC6H4, 3-O2NC6H4
PO Box 14155-6455, Tehran, Iran
ehsan.sheikhi@gmail.com (E. Sheikhi)
madib@khayam.ut.ac.ir (M. Adib)
b
Department of Chemistry, Imam Hossein University, Tehran,
1. Metal-free oxidation of benzyl alcohols
2. DMSO as oxidant and O-atom transfer
Iran
3
4
. Efficient development for Pfitzner–Moffat oxidation
. No side products formation
+
13g
+
13h
Received: 17.10.2017
Accepted after revision: 18.12.2017
Published online: 29.01.2017
H O /Br/H ,
azobenzene/Na /O ,
NaIO /γ-Fe O @HAp-
4 2 3
2
2
2
13i
Mn(salophen)OAc, and 3,6-di(pyridin-2-yl)-1,2,4,5-tetra-
zine/visible light.1
3j
DOI: 10.1055/s-0037-1609149; Art ID: st-2017-d0770-l
Among the most important oxidation methods are
DMSO-based systems, such as DMSO/DCC/H (Pfitzner–
Abstract An efficient metal-free oxidation of benzylic alcohols to
aromatic aldehydes is described. Heating a solution of the benzylic alco-
hol in DMSO in the presence of H SO afforded the corresponding alde-
hyde in excellent yield. This oxidation reaction, which proceeds with a
short reaction time and no side products, is akin to the Pfitzner–Moffatt
oxidation, but without the need for N,N′-dicyclohexylcarbodiimide.
+
1
4
Moffatt oxidation), DMSO/SO ·Py (Parikh–Doering oxida-
3
2
4
tion),15 DMSO/oxalyl chloride (Swern oxidation) , DM-
16
17
SO/Ac O (Albright–Goldman oxidation), DMSO/P O (On-
2
2
5
18
odera oxidation), and DMSO/phenyl dichlorophosphate
(
Liu oxidation).19
Key words benzylic alcohols, aromatic aldehydes, sulfuric acid,
dimethyl sulfoxide, oxidation
However, some of these methods have significant disad-
vantages, such as the use of toxic and moisture-sensitive
oxalyl chloride, the need for low temperatures to avoid
Pummerer rearrangement, the requirement for anhydrous
conditions for the Swern oxidation, the removal of urea in
the Pfitzner–Moffatt oxidation, and the competitive forma-
tion of the methylthiomethyl ether byproducts for all of
them. Consequently, it is still desirable to develop an effi-
cient and simple method for the oxidation of primary
alcohols.
Oxidation of primary alcohols to aldehydes is an im-
portant functional-group transformation in organic synthe-
sis. Many protocols and a broad range of oxidizing reagents
have been reported for this reaction, including chromium-
1
based systems [CrO /H SO (Jones oxidation), CrO ·py2
3
2
4
3
2
(
Collins oxidation), pyridinium chlorochromate (Corey oxi-
dation), or pyridinium dichromate]; hypervalent iodine
reagents [Dess–Martin periodinane or o-iodoxybenzoic ac-
TEMPO-based systems [TEMPO/polymer-bound
haloate (I), TEMPO/Br /NaNO , TEMPO/[CuBr (bipy)],
TEMPO/1,3-dibromo-5,5-dimethylhydantoin/NaNO ,
TEMPO/HBr/tert-butyl nitrite,
imim-TEMPO] X /[imim-CO H] X /NaNO ] (imim-CO = 1-
carboxymethyl-3-methylimidazolium chloride); the Co-
rey–Kim reagent, the Oppenauer reagent, manganese di-
oxide; or tetrapropylammonium perruthenate (Ley–Grif-
3
4
5
In 2002, Li and co-workers described the HBr-catalyzed
oxidation of benzylic alcohols with DMSO. They claimed
that among H SO , H PO , CeCl , TsOH, and aqueous HBr,
6
7
id ];
8
a
8b
8c
2
2
2
2
4
3
4
3
8
d
only the reaction catalyzed by HBr gave a pure product in
2
8
e
8f
20
TEMPO/HCl/NaNO2, or
high yield. They proposed a carbocation-based mecha-
+
–
+
–
[
nism for this reaction, and they observed a decrease in the
reaction rate with electron-withdrawing or bulky groups
and an increase in the reaction rate with electron-donating
groups. Although, the effects of electron-withdrawing and
electron-donating groups were in agreement with the pro-
posed mechanism, the notable effects observed with bulky
groups were inconsistent with the formation of a planar
carbocation. In addition, the reaction between HBr and
DMSO forms bromodimethylsulfonium bromide as an acti-
vated sulfonium ion, whereas other acids cannot form this
2
2
2
8
g
9
10
1
1
8
g,12
fith reagent).
In recent years, several other strategies have
been reported that use reagents such as O2 and an
1
3a
amphiphilic resin-dispersion of palladium nanoparticles,
13b
13c
Au-Cu/SiO ,
Ru(PPh )(OH)-salen complex,
Ru-CHNAP-
2
3
8g
MgO, [Ru(bbp)(pydic)] (pydic = pyridine-2,6-dicarboxylic
13e
13d
13f
acid), K [OsO (OH) ]/chloramine-T, an Ag–NHC complex,
2
2
4
©
Georg Thieme Verlag Stuttgart · New York — Synlett 2018, 29, A–E

B
Synlett
E. Sheikhi et al.
Letter
reagent.21 It therefore seems likely that the reaction pro-
ceeds through activation of alcohol 1 by bromodimethyl-
sulfonium bromide (3), followed by elimination of dimethyl
sulfide (Scheme 1). In this mechanistic rationale, benzylic
alcohols bearing bulky groups should show a decrease in
reaction rate.
To investigate the reaction conditions for acid-promoted
oxidation of benzylic alcohols 1 to the corresponding
aromatic aldehydes 2, we selected benzyl alcohol (1a) as a
model substrate, and we examined the effects of several
acids and various temperatures, times, solvents and
amounts of the acid (Table 1). Initially, benzyl alcohol 1a
was treated with H SO (1 equiv) in DMSO at room tem-
perature, but no reaction was observed after five hours (Ta-
ble 1, entry 1). When this reaction was performed at 100 °C,
benzaldehyde (2a) was obtained in 30 and 35% yields after
one and two hours, respectively. The yield of 2a did not im-
prove after five hours (entry 2). When the reaction was car-
ried out at 150 °C, 2a was obtained in 43, 46, and 50% yields
after one, two, and five hours, respectively (entry 3). How-
ever, by carrying out the reaction under reflux conditions,
2
4
O–
Br
S+ + 2 HBr
S+ Br–
–
H2O
3
H
H
(
3)
S+ Br–
Ar
OH
ArCHO
–
HBr Ar
O
2
– HBr, Me S
1
4
2
Scheme 1 More probable mechanism for the HBr-catalyzed oxidation
of benzylic alcohols in DMSO
2a was obtained in 94% yield after one hour, and the yield
However, the question still remained as to whether oth-
er acids might react with benzylic alcohols under different
reaction conditions to effect oxidation. Here, we describe an
acid-catalyzed approach for the oxidation of benzylic
alcohols to produce the corresponding aldehydes.
did not change after longer periods (entry 4). We also tested
the reaction in the presence of two and three equivalents of
H SO , but the yield of 2a did not significantly improve (en-
2
4
tries 5 and 6). On using 0.5 equivalents of H SO , the yield of
2
4
2a after one hour decreased to 40%, but this increased to 90
and 91% after two and five hours, respectively (entry 7).
Table 1 Screening of Conditions for the Acid-Promoted Oxidation of Benzyl Alcohol (1a) to Benzaldehyde (2a)a
OH
O
acid, solvent
Ph
Ph
H
1a
2a
Entry
Acid (equiv)
Solvent
Temp (°C)
Yieldb (%) of 2a after 1 h Yieldb (%) of 2a after 2 h Yieldb (%) of 2a after 5 h
1
2
3
4
5
6
7
8
9
H
H
H
H
H
H
H
H
H
2
2
2
SO
SO
SO
4
4
4
(1.0)
(1.0)
(1.0)
(1.0)
(2.0)
(3.0)
(0.5)
(1.0)
(1.0)
DMSO
DMSO
DMSO
DMSO
DMSO
DMSO
DMSO
DMSO
DMSO
DMSO
DMSO
t-BuOH
DMF
r.t.
NRc
30
NR
35
46
94
95
93
90
63
57
13
15
NR
NR
NR
NR
NR
NR
NR
NR
35
50
94
95
95
91
63
59
17
23
NR
NR
NR
NR
NR
NR
NR
100
150
43
2
SO
4
4
4
reflux
reflux
reflux
reflux
reflux
reflux
reflux
reflux
reflux
reflux
reflux
reflux
reflux
reflux
reflux
94
2
SO
SO
SO
PO
PO
94
2
93
2
4
4
3
40
3
2
51
22
10
11
12
13
14
15
16
17
18
TsOH (1.0)
MsOH (1.0)
trace
trace
NR
NR
NR
NR
NR
NR
NR
H
H
H
H
H
H
H
2
2
2
2
2
2
2
SO
SO
SO
SO
SO
SO
SO
4
4
4
4
4
4
4
(1.0)
(1.0)
(1.0)
(1.0)
(1.0)
(1.0)
(1.0)
THF
MeCN
CHCl
3
toluene
2
H O
a
b
c
Reaction conditions: BnOH (1a; 1 mmol), acid (1.0 or 0.5 equiv), solvent (3 mL).
Isolated yield.
NR = no reaction.
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E. Sheikhi et al.
Letter
Other acids such as H PO , H PO , TsOH, or MsOH were
OH
OH
O
3
4
2
3
DCC, DMSO, H+
H2SO4, DMSO
Pfitzner–Moffat
oxidation
studied. Although, H PO and H PO led to 63 and 57%
+ DCU
3
4
2
3
R
R
R
H
yields after two hours (entries 8 and 9), only very low
amounts of product were detected when using TsOH or
MsOH (entries 10 and 11). These results can be explained
by the difference in the acidity of these acids. For an addi-
tional control test, we examined the reaction in various sol-
O
Our study
R
H
Scheme 2 Comparison of Pfitzner–Moffat oxidation with our method
vents such as t-BuOH, DMF, THF, MeCN, CHCl , toluene, and
3
water. However, all these solvents were inappropriate for
this transformation (entries 12–18). Thus, optimal condi-
tions for the oxidation of benzyl alcohol (1a) to benzalde-
hyde (2a) were determined to be 1.0 or 0.5 equivalents of
H SO in boiling DMSO for one or two hours, respectively
To demonstrate the generality and scope of this proto-
col, we examined the reactions of various benzylic alcohols
1 in the presence of H SO in DMSO. These reactions pro-
2
4
ceeded cleanly under reflux conditions to afford the corre-
sponding aromatic aldehydes 2, and no side reactions were
observed (Table 2). All the reactions went to completion af-
2
4
(Table 1, entries 4 and 7).
1
This oxidation approach is similar to the Pfitzner–Mof-
ter convenient reaction times. H NMR analyses of the reac-
fatt oxidation, but without the need for dicyclohexylcarbo-
diimide (DCC); consequently, the difficulty of removing the
dicyclohexylurea (DCU) byproduct is avoided (Scheme 2).
tion mixtures clearly indicated the formation of the corre-
sponding aldehydes 2 in excellent yields. All the products
1
13
were characterized on the basis of their H and C NMR
spectroscopic data.22
Table 2 Oxidation of Benzylic Alcohols to Aromatic Aldehydes by the H SO /DMSO System
2
4
OH
O
H2SO4, (prot. A: 1 equiv; prot. B: 0.5 equiv), DMSO
reflux
Ar
Ar
H
1
2
Entry
Alcohol
BnOH
Aldehyde
PhCHO
Product
Protocol Aa
Protocol Bb
Time
c
c
Time
60
Yield (%)
Yield (%)
1
2
2a
2b
94
120
90
CHO
OH
60
60
93
93
120
92
CHO
OH
3
2c
120
90
CHO
OH
4
5
6
7
8
2d
2e
2f
60
60
60
60
55
91
94
92
95
90
125
125
130
120
110
92
91
88
91
89
CHO
OH
Cl
Cl
CHO
OH
Cl
Cl
CHO
OH
2g
2h
Br
Br
MeO
MeO
CHO
OH
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Synlett
E. Sheikhi et al.
Letter
Table 2 (continued)
Entry
9
Alcohol
Aldehyde
Product
Protocol Aa
Time
Protocol Bb
Time
c
c
Yield (%)
Yield (%)
MeO
MeO
CHO
CHO
OH
OH
2i
2j
40
50
93
89
85
90
88
MeO
MeO
MeO
MeO
1
0
1
110
OMe
OMe
CHO
OH
1
2k
2l
35
65
90
93
70
87
90
N
N
OH
CHO
12
120
CHO
CHO
OH
OH
1
3
4
2m
2n
90
85
80
84
165
145
78
84
O2N
O2N
O N
2
O2N
1
a
b
c
Reaction conditions: alcohol (1 mmol), H
Reaction conditions: alcohol (1 mmol), H
Isolated yield.
2
SO
4
(1 mmol), DMSO (3 mL), reflux.
4
(0.5 mmol), DMSO (3 mL), reflux.
2
SO
A proposed mechanism for this reaction is provided in
As shown in Table 2, alcohols with electron-donating
groups were oxidized faster, and the presence of ortho-sub-
stituents on the alcohol did not affect the reaction rate.
These results are in agreement with a carbocation-based
mechanism. In addition, the catalytic behavior of the sulfu-
ric acid can be explained by our proposed mechanism.
In conclusion, we have developed an efficient and
metal-free approach for the oxidation of benzylic alcohols
to the corresponding aromatic aldehydes by a H SO /DMSO
system under reflux conditions. This oxidation procedure
extends the Pfitzner–Moffatt oxidation, doing away with
the need to add DCC and, consequently, eliminating the
need to remove DCU. In addition, this protocol has notable
advantages such as readily available and inexpensive chem-
icals, short reaction times, a simple procedure and workup,
high yields of products, and lack of byproduct formation.
Scheme 3. At first, treatment of benzylic alcohol 1 with
H SO yields a carbocation intermediate 5 through removal
2
4
of H O. This intermediate is readily converted into the
2
alkoxysulfonium ion intermediate 6 through a nucleophilic
–
addition reaction of DMSO. Next, in the presence of HSO ,
4
intermediate 6 is deprotonated to give the sulfur ylide 7.
Finally, in a five-membered-ring transition state, the sulfur
ylide 7 undergoes elimination of dimethyl sulfide (DMS) to
form the corresponding aldehyde 2 through deprotonation
of the benzylic hydrogen.
2
4
OH
CH2–
S+
H
H
O
H2SO4
Ar
1
Ar
O
– DMS Ar
H
7
2
H2O
HSO4–
H
H
HSO4–
H
Funding Information
Ar CH2+ –O S+
5
S+
Ar
O
6
This research was supported by the Research Council of University of
Tehran.
)(
Scheme 3 Proposed reaction mechanism
©
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E. Sheikhi et al.
Letter
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(
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(
1
9
8% H SO (1 mmol) in DMSO (3 mL) was stirred for the appro-
2 4
priate time under reflux conditions. The mixture was then
cooled to r.t., and brine (4 mL) was added. The organic phase
was extracted with CH Cl (6 mL), and the organic layer was
(
1976, 133. (c) Lou, J.-D.; Xu, Z.-N. Tetrahedron Lett. 2002, 43,
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2
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dried (Na SO ), filtered, and concentrated under reduced pres-
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4
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purity, and did not require further purification by distillation or
column chromatography.
(
Protocol B: A mixture of benzylic alcohol (1 mmol) and 98%
H SO (0.5 mmol) in DMSO (3 mL) was stirred for the appropri-
2
4
ate reaction time under reflux conditions. The reaction mixture
was then worked up as described in Protocol A.
©
Georg Thieme Verlag Stuttgart · New York — Synlett 2018, 29, A–E