Efficient CuCl-catalyzed selective and direct oxidation of β- And γ-substituted aliphatic primary alcohols to carboxylic acids

Article abstract of DOI:10.1080/00397910903318690

A new procedure for the selective and direct oxidation of aliphatic primary alcohols having substitution at - and -positions to corresponding carboxylic acids was developed using a catalytic amount of ligand and additive-free CuCl with anhydrous tBuOOH in acetonitrile solvent under very mild reaction conditions. This procedure is very simple and mild and works efficiently without any additives at room temperature.

Full text of DOI:10.1080/00397910903318690

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Efficient CuCl-Catalyzed Selective and
Direct Oxidation of β- and γ-Substituted
Aliphatic Primary Alcohols to Carboxylic
Acids
Sreedevi Mannam a & Govindasamy Sekar a
a Department of Chemistry, Indian Institute of Technology Madras,
Chennai, Tamil Nadu, India
Version of record first published: 25 Aug 2010.
To cite this article: Sreedevi Mannam & Govindasamy Sekar (2010): Efficient CuCl-Catalyzed Selective
and Direct Oxidation of β- and γ-Substituted Aliphatic Primary Alcohols to Carboxylic Acids, Synthetic
Communications: An International Journal for Rapid Communication of Synthetic Organic Chemistry,
40:19, 2822-2829
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Synthetic Communications1, 40: 2822–2829, 2010
Copyright # Taylor & Francis Group, LLC
ISSN: 0039-7911 print=1532-2432 online
DOI: 10.1080/00397910903318690
EFFICIENT CuCl-CATALYZED SELECTIVE AND DIRECT
OXIDATION OF b- AND c-SUBSTITUTED ALIPHATIC
PRIMARY ALCOHOLS TO CARBOXYLIC ACIDS
Sreedevi Mannam and Govindasamy Sekar
Department of Chemistry, Indian Institute of Technology Madras,
Chennai, Tamil Nadu, India
A new procedure for the selective and direct oxidation of aliphatic primary alcohols having
substitution at b- and c-positions to corresponding carboxylic acids was developed using a
catalytic amount of ligand and additive-free CuCl with anhydrous BuOOH in acetonitrile
t
solvent under very mild reaction conditions. This procedure is very simple and mild and
works efficiently without any additives at room temperature.
Keywords: Aliphatic primary alcohols; carboxylic acids; copper catalyst; direct oxidation; selective
oxidation; tertiary butyl hydroperoxide
INTRODUCTION
Oxidation is one of the most fundamental reactions in synthetic organic
chemistry, and a variety of reagents have been developed as oxidants, which is a
testimony to the importance of oxidation reactions. However most of these oxidants
are either toxic, hazardous, or required in large excess. From an economical and
environmental viewpoint, catalytic oxidations are particularly interesting,[1,2] and
major efforts have been devoted over the past few years to the discovery and devel-
opment of efficient procedures employing O2 or hydrogen peroxide as the ultimate
stoichiometric oxidant.[3] However, relatively few general methods exist for the mild
and direct oxidation of alcohols to carboxylic acids. Traditionally, this oxidation
uses a two-step process involving oxidation of the alcohol to an aldehyde followed
by further oxidation to the carboxylic acid.[4] Clearly, it would be advantageous to
have a general, one-step process for the direct formation of carboxylic acids from
primary alcohols under very mild conditions. Excess of pyridinium dichromate
(PDC) in a moist polar solvent such as dimethylformamide (DMF) can successfully
oxidize primary alcohols to carboxylic acids.[5] Similarly, carboxylic acids can also be
obtained with periodic acid (H5IO6) as the stoichiometric oxidant in conjunction
with either CrO3,[6a] or RuO4.[6b] Also, iodoxybenzoic acid (IBX) in dimethylsulfox-
ide (DMSO) was another efficient catalyst, along with N-hydroxysuccinimide for the
Received June 23, 2009.
Address correspondence to Govindasamy Sekar, Department of Chemistry, Indian Institute of
Technology Madras, Chennai, Tamil Nadu 600 036, India. E-mail: gsekar@iitm.ac.in
2822

CuCl-CATALYZED OXIDATION OF ALIPHATIC ALCOHOLS
2823
production of carboxylic acids.[6c] Alternatively, oxidation of primary alcohols can
be achieved with 2,20,6,60-tetramethylpiperidin-1-oxyl (TEMPO) in conjunction with
the co-oxidants such as sodium chlorite (NaClO2)[7a] and sodium hypochlorite
(NaOCl).[7b] Several other reaction conditions included the direct oxidation of
alcohols to carboxylic acids such as Jones conditions,[8] RuO4,[9] RuCl3=
K2S2O8,[10] Ru–Co bimetallic catalyzed oxidation,[11] and Cu(MnO4)2 oxidation.[12]
In most cases, however, the range of substrates tolerated in these aerobic oxidations
is limited to certain classes of alcohols.
Apart from the mentioned reagents, hydrogen peroxide is another convenient
oxidant in terms of economical and environmental concerns, because water is the
only by-product of this oxidative reaction.[13] Although a variety of different cata-
lytic systems for the hydrogen peroxide oxidation of alcohols has been developed,
there is a growing interest in the search for new efficient metal catalysts. Many mol-
ybdenum-,[14] manganese-,[15] iron (Fenton’s reagent)–,[16] and tungsten[17]-based
catalytic systems using hydrogen peroxide have been reported. On the other hand,
the oxidations using hydrogen peroxide catalyzed by copper have not been investi-
gated as much.[18] Recently, Punniyamurthy and Das reported copper-catalyzed oxi-
dation of primary benzylic=aliphatic alcohols to carboxylic acid analogs and ketones
with H2O2 as the source of oxygen using salen ligands at elevated temperatures.[19]
The Lei group used a polymer-supported 2-iodobenzamide as an efficient catalyst
for the oxidation of primary benzylic=aliphatic alcohols to carboxylic acids and
ketones using oxone as the oxidant.[20] Ragauskas and Yang reported an efficient
vanadium-catalyzed selective aerobic oxidation of alcohols into carboxylic acids in
the ionic liquid [bmim]PF6.[21] The very limited availability of reports in the literature
for copper-catalyzed direct oxidation of unactivated aliphatic primary alcohols to
carboxylic acids encouraged us to continue our quest for an efficient, selective,
and mild catalyst for direct alcohol oxidation.
RESULTS AND DISCUSSION
As a part of our ongoing research toward copper-catalyzed oxidation chemis-
try,[22] very recently we reported CuCl-catalyzed direct oxidation of reactive benzylic
and allylic primary alcohols to the corresponding carboxylic acids using anhydrous
tbutyl hydroperoxide at room temperature under ligand-free conditions.[23]
However, the main drawback of this catalytic oxidation is that it failed to oxidize
unreactive aliphatic primary alcohols such as 1-octanol and 1-hexanol to corre-
sponding carboxylic acids. As a continuation of our copper-catalyzed direct alcohol
oxidation, herein we report ligand- and additive-free CuCl-catalyzed selective
oxidation of b- and c-substituted aliphatic primary alcohols to the carboxylic acids
under very mild reaction conditions (Scheme 1). (Although the oxidation of b- and
c-substituted aliphatic alcohols to the corresponding carbonyl is known with copper
complex and additives,[18b] the direct and selective oxidation of b- and c-substituted
aliphatic alcohols to the corresponding carboxylic acid with ligand and additive-free
copper catalyst has not been reported.)
During our initial studies on aliphatic primary alcohol oxidations, we have
chosen cyclohexyl methanol 1 as the model substrate for optimization studies, and
the results are summarized in Table 1. When cyclohexyl methanol 1 was oxidized

2824
S. MANNAM AND G. SEKAR
Scheme 1. Direct oxidation of primary aliphatic alcohols with ligand- and additive-free CuCl to car-
boxylic acid.
with DABCO-CuCl complex and TEMPO, it provided 84% of corresponding
carboxylic acid (Table 1, entry 1) in acetonitrile at room temperature. The same reac-
tion without additive TEMPO also provided the same result (entry 2). Surprisingly,
when the reaction was carried out without DABCO ligand and TEMPO additive,
the oxidation reaction yielded a better result (entry 4). However, without CuCl, no
carboxylic acid formation was observed even after 24 h (entry 5). Replacing the anhy-
drous tBuOOH by aqueous tBuOOH reduced the yield to 51% (entry 6). Then the reac-
tion was screened with several other copper salts, and CuCl turned out to be the best
copper salt (entries 4 and 7–11). Similarly, CH3CN became choice of solvent in the
view of yield and reaction rate after screening different solvents (entries 4 and 12–17).
Having optimized conditions in hand, next we investigated the substrate
scope of this oxidation with a variety of substituted primary aliphatic alcohols,
and the results are summarized in Table 2. Aliphatic acyclic primary alcohols having
Table 1. Optimization studies for the oxidation of cyclohexyl methanol 1 to cyclohexane carboxylic
acid 1a
Time
(h)
Yielda
(%)
Entry
Catalyst
Additive
Oxidant
Solvent
t
5M BuOOH (in decane)
1
2
DABCO-CuCI
DABCO-CuCI
CuCI
TEMPO
CH3CN
CH3CN
CH3CN
CH3CN
CH3CN
CH3CN
CH3CN
CH3CN
CH3CN
CH3CN
CH3CN
EtOAc
THF
7
6
84
85
87
87
00
51
75
63
49
23
72
60
20
67
00
10
51
t
5M BuOOH (in decane)
—
TEMPO
—
t
5M BuOOH (in decane)
3
6
t
5M BuOOH (in decane)
4
CuCI
—
CuCI
6
t
5M BuOOH (in decane)
5
—
—
24
6
t
70% BuOOH
6
t
5M BuOOH (in decane)
7
CuCI2
CuBr
Cul
—
—
5
t
5M BuOOH (in decane)
8
10
12
12
7
t
5M BuOOH (in decane)
9
—
—
t
5M BuOOH (in decane)
10
11
12
13
14
15
16
17
Cu(N03)2
Cu(OTf)2
CuCI
t
5M BuOOH (in decane)
—
—
t
5M BuOOH (in decane)
6
t
5M BuOOH (in decane)
CuCI
CuCI
—
—
6
t
5M BuOOH (in decane)
MeOH
Toluene
DCM
6
t
5M BuOOH (in decane)
CuCI
CuCI
CuCI
—
—
6
t
5M BuOOH (in decane)
24
6
t
5M BuOOH (in decane)
—
Dioxane
aIsolated yield.

CuCl-CATALYZED OXIDATION OF ALIPHATIC ALCOHOLS
2825
Table 2. Selective oxidation of b- and c-substituted aliphatic alcohols to the corresponding carboxylic
t
acids using CuCl and BuOOH as oxidanta
Entry
Alcohols
Products
Time (h)
Yieldb (%)
1
2
n ¼ 6 2
n ¼ 8 3
n ¼ 6 2a
n ¼ 8 3a
24
24
00
00
3
4
6
3
87
91
5
6
20
24
50
35
7
8
16
12
60
65
9
24
59
aReaction conditions: 5 mol% of CuCl, 5 equiv. of 5M tBuOOH (in decane) in MeCN at room temperature.
bIsolated yield. All the products were fully characterized by 1H and 13C NMR, FT-IR, and mass
spectroscopy.

2826
S. MANNAM AND G. SEKAR
substitution at the b-position such as 2-methylpropan-1-ol 5 and 2-ethylhexan-1-ol 7
were oxidized to the corresponding carboxylic acids, isobutyric acid 5a and 2-
ethylhexanoic acid 7a, in 20 and 16 h respectively (entries 5 and 7). When the substi-
tution at the c-position was with a phenyl group such as 3-phenylpropanol 4, the
oxidation took place smoothly in just 3 h and yielded 91% isolated yield under the
present catalytic conditions (entry 4). Similarly, aliphatic cyclic primary alcohols such
as cyclopentyl methanol 8 and cyclohexyl methanol 1 were also oxidized to the
corresponding carboxylic acids, cyclopentane carboxylic acid 8a and cyclohexane
carboxylic acid 1a, in good isolated yields when compared to acyclic aliphatic alco-
hols (entries 3 and 8 versus 5 and 7). Heteroatom-containing alcohol like N-Boc pro-
linol 9 also got oxidized to the corresponding carboxylic acid N-Boc proline 9a with
59% yield and took slightly more time (entry 9). In the case of oxidation of the N-Boc
prolinol 9, the Boc group was unaffected during the oxidation, which clearly indicates
the importance of this catalyst in synthetic organic chemistry. An aliphatic alcohol
having substitution at the c-position such as 3-methylbutan-1-ol 6 was also oxidized
to the corresponding carboxylic acid 6a in 24 h (entry 6). Aliphatic acyclic alcohols
such as 1-octanol 2 and 1-decanol 3 failed to oxidize to the corresponding carboxylic
acids under the present catalytic conditions (Table 2, entries 1 and 2). Using this to
our advantage, we selectively oxidized b-substituted unreactive aliphatic primary
alcohols in the presence of unsubstantiated unreactive aliphatic primary alcohols such
as 1-hexanol and 1-octanol. The detailed mechanistic studies of this selective oxi-
dation reaction, including the reason for selective oxidation of substituted aliphatic
alcohols and increasing the catalytic activity toward oxidation of unsubstantiated
aliphatic alcohols such as 1-octanol and 1-decanol, are under way.
In conclusion, we have developed a new procedure for the direct oxidation of
primary aliphatic alcohols having substitution at b- and c- positions to the corre-
sponding carboxylic acids using catalytic amounts of CuCl and anhydrous tBuOOH
(5 M in decane) in acetonitrile under very mild reaction conditions. This procedure is
very simple and mild and works efficiently without any additives at room tempera-
ture. To the best of our knowledge, this is the first report of ligand- and additive-free
copper-catalyzed direct and selective oxidation of unactivated substituted aliphatic
primary alcohols to carboxylic acids at room temperature.
EXPERIMENTAL
All oxidation reactions were performed under atmospheric conditions. All the
solvents used for the reactions were obtained from Merck, India, and dried by
Vogel’s procedure. Reactions were monitored by thin-layer chromatographic
(TLC) plates (silica gel 60 F254 obtained from Merck) using an appropriate mixture
of ethyl acetate and hexanes. Product purification was done by silica-gel (100–200
mesh) column chromatography using a hexanes and ethylacetate mixture as eluent.
All the products gave satisfactory spectral data.
General Experimental Procedure
tBuOOH (1.23 mL, 5 mmol, 5 M tBuOOH in decane) was slowly added to a sol-
ution of CuCl (4.95 mg, 0.05 mmol) and cyclohexyl methanol 1 (128.21 mg, 1 mmol)

CuCl-CATALYZED OXIDATION OF ALIPHATIC ALCOHOLS
2827
in 2 mL of CH3CN. The resulting reaction mixture was stirred at room temperature
until the disappearance of starting material (monitored by TLC). After completion
of reaction, the solvent was evaporated by rotary evaporator, and water was added
to the resulting crude reaction mixture. The pH of the reaction mixture was adjusted
to 8.0–8.5 with saturated NaHCO3 solution and then extracted into ethyl acetate.
The aqueous layer was acidified to pH 2.0 using 2 N HCl solution and extracted with
ethyl acetate. The organic layer was concentrated and purified by column chromato-
graphy to give carboxylic acid 1a as pure product (123.95 mg, 87% yield). Spectral
data for 1a: Colorless liquid; Rf ¼ 0.25 (1% methanol in ethyl acetate); IR (neat)
1
1700 cmꢀ1; H NMR (400 MHz, CDCl3) d 10.0 (brs, OH), 2.39–2.29 (m, 1H), 1.95
(d, J ¼ 7.2 Hz, 2H), 1.82–1.73 (m, 2H), 1.53–1.41 (m, 2H), 1.38–1.18 (m, 4H). 13C
NMR (100 MHz, CDCl3) d 182.5, 42.9, 28.8, 25.7, 25.3; HRMS (ESI): m=z calcd.
for C7H12O2Na [M þNa]þ: 151.0735; found: 151.0736.
ACKNOWLEDGMENTS
We thank the Department of Science and Technology (DST; Project No. SR=
S1=OC-06=2008), New Delhi, for the financial support. S. M. thanks the Council of
Scientific and Industrial Research, New Delhi, for a senior research fellowship. We
thank the DST, New Delhi, for the funding of the 400-MHz NMR machine to the
Department of Chemistry, IIT, Madras, under the IRPHA scheme and the electro-
spray ionization–mass spectrometry instruments under the Funding for Infrastruc-
ture in Science and Technology program.
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