Iron-Catalysed Selective Aerobic Oxidation of Alcohols to Carbonyl and Carboxylic Compounds

Article abstract of DOI:10.1002/cplu.201600240

A method for aerobic alcohol oxidation catalysed by Fe(NO₃)₃/2,2’-bipyridine/TEMPO has allowed highly selective conversion of primary alcohols into either aldehydes or carboxylic acids in one-step. The oxidation of primary alcohols proceeded selectively to aldehydes, as TEMPO was present in the reaction. Nevertheless, the aldehydes were further oxidized into carboxylic acids as the reaction time was extended. Detailed investigation of the reaction suggested, that the deoxygenation of TEMPO into TMP enabled the auto-oxidation of aldehydes to carboxylic acids, which was initially inhibited in the presence of TEMPO. The procedure was also efficient in oxidation of secondary alcohols when TEMPO was replaced by the less sterically hindered ABNO.

Full text of DOI:10.1002/cplu.201600240

DOI: 10.1002/cplu.201600240
Communications
Iron-Catalysed Selective Aerobic Oxidation of Alcohols to
Carbonyl and Carboxylic Compounds
[
a]
Kalle Lagerblom, Pauli Wrigstedt, Juha Keskivꢀli, Arno Parviainen, and Timo Repo*
A method for aerobic alcohol oxidation catalysed by Fe(NO ) /
Carboxylic acids are highly valuable synthons in chemistry
and readily available through aerobic oxidation of alde-
3
3
2,2’-bipyridine/TEMPO has allowed highly selective conversion
[25–29]
of primary alcohols into either aldehydes or carboxylic acids in
one-step. The oxidation of primary alcohols proceeded selec-
tively to aldehydes, as TEMPO was present in the reaction.
Nevertheless, the aldehydes were further oxidized into carbox-
ylic acids as the reaction time was extended. Detailed investi-
gation of the reaction suggested, that the deoxygenation of
TEMPO into TMP enabled the auto-oxidation of aldehydes to
carboxylic acids, which was initially inhibited in the presence
of TEMPO. The procedure was also efficient in oxidation of sec-
ondary alcohols when TEMPO was replaced by the less sterical-
ly hindered ABNO.
hydes.
So far, direct aerobic oxidation of alcohols into car-
boxylic acids have proven challenging, even though the oxida-
[2]
tion pathway proceeds through aldehyde intermediates.
Herein, we introduce a one-step aerobic alcohol oxidation
procedure catalysed by Fe(NO ) /TEMPO for the selective con-
3
3
version of primary alcohols into either the corresponding alde-
hydes or carboxylic acids (Scheme 1; TEMPO=2,2,6,6-tetra-
Scheme 1. Oxidation of primary alcohols either into aldehydes or carboxylic
acids catalysed by Fe(NO ) /TEMPO in one step.
3 3
Oxidation of alcohols into the corresponding carbonyl and car-
boxylic compounds is among the most studied chemical reac-
[
1,2]
tions.
These transformations are traditionally carried out
[3]
using stoichiometric amounts of oxidants, such as CrO ,
3
[
4]
[5]
MnO2, activated DMSO and hypervalent iodine, which are
all expensive and produce equimolar amounts of hazardous
waste. Lately, environmentally benign oxidation processes
have been the subject of intensive research, including the
metal-catalysed aerobic methods that are now routinely used
for converting alcohols into aldehydes and ketones on a labora-
methyl-piperidyl-1-oxy). Initially, aerobic oxidation of primary
alcohols was investigated using 1-octanol (1a) as a model
compound. Conversion of 1a into octanal (2a) was attempted
in various solvents using 5 mol% of Fe(NO ) /TEMPO catalyst
3
3
formed in situ (for details see Scheme S1 in the Supporting In-
formation). Promising results were obtained in solutions of
MeCN and AcOH, as 1 was converted into 2a in 45 and 48%
yields, respectively (Table 1, entries 1 and 2).
[
6–8]
tory scale.
However, direct aerobic oxidation of primary al-
cohols into carboxylic acids remains a great challenge. To date,
only a handful of noble metals, such as Pd, Ru, Pt, Rh and Au,
The impact of different organic ligands on the oxidation of
1a was then studied (Table 1, entries 3–8; for full details see
Table S3). Of the ligands employed, 2,2’-bipyridine (bpy)
showed the highest catalytic amplification; 1a was converted
into 2a in an excellent 96% yield after 6 h (Table 1, entry 3). A
similar positive effect was detected with 1,10-phenanthroline
and 4,4-di-tert-butyl-2,2-bipyridine ligands (Table 1, entries 4
and 5), whereas 4,4-di-nitro-2,2-bipyridine, di-(2-picolyl)amine
and 1-methylimidazole gave poor outcomes, resulting in low
yields of 2a (Table 1, entries 6–8). Although N-heterocyclic li-
[9–13]
have been reported as catalysts for this transformation.
In the search for novel catalytic systems, Fe has great poten-
tial owing to its fundamental role in enzymatic reactions and
its high catalytic activity in aerobic alcohol oxidation reac-
[
14,15]
tions.
Particularly efficient for alcohol oxidation are the sys-
tems involving simple Fe salts together with NO and nitroxyl
x
[16]
radicals, such as 2,2,6,6-tetramethyl-piperidyl-1-oxy (TEMPO).
These catalytic systems are most often used under homogene-
[
17–21]
ous conditions without any acid or base co-catalysts.
Also,
heterogeneous and ionic-liquid-bound variants have been de-
gands are commonly used in transition-metal-catalysed oxida-
[
22–24]
[7,30–32]
veloped.
A common factor for these Fe-based catalytic
tion chemistry,
such auxiliaries have not attracted sub-
systems is their ability to selectively convert primary and sec-
ondary alcohols into aldehydes and ketones.
stantial interest in Fe/TEMPO-catalysed aerobic oxidation of al-
cohols and therefore their exact role is still undisclosed.
Notably, high reactivity was only observed in AcOH solution
(
Table 1, entry 3). The reactions conducted in other solvents,
[17,21,33]
[a] K. Lagerblom, P. Wrigstedt, J. Keskivꢀli, A. Parviainen, Prof. T. Repo
such as commonly used MeCN
or 1,2-dichloro-
[18,19,34,35]
Department of Chemistry
ethane
(DCE), resulted in significantly lower yields of
University of Helsinki
A.I. Virtasen aukio 1, P.O. Box 55, 00014 Helsinki (Finland)
E-mail: timo.repo@helsinki.fi
2
a (Table 1, entries 9 and 10; for full details see Table S2). This
outcome indicates that the active oxidant is the oxoammoni-
um ion of TEMPO that is formed via acid-catalysed dispropor-
Supporting information for this article can be found under http://
dx.doi.org/10.1002/cplu.201600240.
[36–40]
tionation of the nitroxyl radical.
Furthermore, control ex-
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[a]
Table 1. Screening of the aerobic 1-octanol oxidation conditions
Entry
Metal
precursor
Ligand
Solvent
t
[h]
Yield
[%]
[
b]
[c]
1
2
Fe(NO
Fe(NO
3
3
)
)
3
3
–
–
MeCN
AcOH
23
23
45
48
3
Fe(NO
3
3
)
3
3
AcOH
6
96
4
Fe(NO
)
AcOH
6
94
5
Fe(NO
3
)
3
AcOH
6
93
6
7
Fe(NO
Fe(NO
3
3
)
)
3
3
AcOH
AcOH
6
18
8
Figure 1. Aerobic oxidation of 1-octanol. Reaction conditions:
a) Fe(NO
3
)
3
·9H
2
O/TEMPO/bpy (1:1:1, mol/mol/mol) catalyst formed in situ
23
(
5 mol%). b) Catalyst molar ratio of 1:3:1, TEMPO added in three portions at
0
h, 8 h and 16 h. Yields were determined by GC-FID analysis using the area
8
9
Fe(NO
Fe(NO
3
3
)
)
3
3
AcOH
MeCN
23
23
8
normalization method. Reaction conditions: AcOH (2 mL), 1-octanol
1 mmol), open test tube (20 mL), 258C, 1200 rpm stirring.
(
29
suddenly changed from deep red to brown, and the turnover
rate of 2a into 3a increased significantly, leading to comple-
tion of the reaction in the following 2 h. We reasoned that the
observed colour change and the subsequent rapid oxidation
of aldehyde 2a resulted from quenching of TEMPO, which
1
0
Fe(NO
3
)
3
DCE
23
23
23
23
18
2
[
d]
11
NaNO
2
AcOH
AcOH
AcOH
[
e]
1
1
2
3
Fe(NO
3
)
3
0
[44,45]
then allowed the auto-oxidation to the carboxylic acid 3a.
FeCl
3
2
The hypothesis was confirmed by GC-MS analysis, which
showed complete degradation of TEMPO during the first 16 h
of the reaction. The major decomposition product detected
was 2,2,6,6-tetramethylpiperidine (TMP), which was presumably
3 3 2
[a] Reaction conditions: solvent (2 mL), Fe(NO ) ·9H O/TEMPO/bpy (1:1:1,
mol/mol/mol) catalyst formed in situ (5 mol%), 1-octanol (1 mmol), open
test tube (20 mL), 258C, 1200 rpm stirring. [b] Yields were determined by
GC-FID analysis using the area normalization method. [c] Catalyst was not
[
46,47]
formed via Fe-catalysed NÀO bond cleavage.
Furthermore,
completely soluble. [d] NaNO
2
(15 mol%). [e] Reaction without TEMPO.
the gradual addition of TEMPO (5 mol%) to the oxidation reac-
tion of 1a every 8 hours completely inhibited the oxidation of
the formed aldehyde 2a into carboxylic acid 3a (Figure 1b),
thus supporting the proposed auto-oxidation pathway.
3
+
periments showed that all catalyst components (ligand, Fe
,
À
TEMPO and NO ) were crucial for the catalysis under our con-
The effect of the Fe(NO ) /bpy catalyst on aldehyde oxida-
3
3 3
ditions (Table 1, entry 3 vs. entries 2 and 11–13; for detailed re-
sults see Figure S5).
tion was investigated in the absence of TEMPO. With 5 mol%
of Fe(NO ) /bpy, 2a was converted into 3a in 1 h (Figure 2a),
3
3
As the Fe(NO ) /bpy/TEMPO catalyst exhibited the highest
whereas 12 h was required without the catalyst (Figure 2b).
This outcome demonstrates that Fe(NO ) /bpy catalyses the ox-
3
3
activity in the oxidation of 1a into 2a, this system was investi-
gated in more detail. The studies revealed that 2a was further
converted into octanoic acid (3a) when the reaction time was
prolonged from 6 to 23 h. This outcome was surprising be-
cause TEMPO and other persistent nitroxyl radicals are known
to inhibit the aerobic oxidation of aldehydes into carboxylic
3
3
idation of aldehydes into carboxylic acids without TEMPO and
thus, explains the sudden conversion of 2a into 3a after com-
plete decomposition of TEMPO during the catalytic oxidation
of 1a.
The oxidation of benzylic alcohols was investigated using 4-
methoxybenzyl alcohol (1b) as a model compound. The initial
experiment was conducted applying the same conditions used
for the oxidation of 1a (5 mol% of Fe(NO ) /bpy/TEMPO cata-
[
41–43]
acids.
Gas chromatography–Flame Ionization detector (GC-FID)
analysis of the 1a oxidation reaction (aliquots taken every 2 h)
showed that conversion of the starting alcohol into 2a oc-
curred during the first 6 h, and was followed by a 10 h period
when the oxidation of 2a into carboxylic acid 3a was very
slow (Figure 1a). After 16 h, the colour of the reaction solution
3
3
lyst formed in situ in AcOH and under ambient condition in
air). Under these conditions, 1b was rapidly converted into the
respective aldehyde (2b), but no further oxidation into the car-
boxylic acid (3b) occurred within 23 h. To increase the reactivi-
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Table 2. Substrate scope of the aerobic alcohol oxidation with protocol A.
[
b]
[c]
Entry Substrate
t [h] Product
Conv.
Yield
[
d]
(
sel.) [%]
(isolated) [%]
1
2
1a
1c
1d
1e
6
2a -CHO
97 (99)
>99 (93)
96 (87)
93 (89)
23 3a -COOH
-CHO
3
4
8
2c
99 (98)
99 (95)
97
94
24 3c -COOH
3 2
Figure 2. Aerobic oxidation of octanal. Reaction conditions: a) a) 3) ·9H O/
bpy (1:1, mol/mol) catalyst formed in situ (5 mol%). b) No catalyst. Yields
were determined by GC-FID analysis using the area normalization method.
Reaction conditions: AcOH (2 mL), octanal (1 mmol), open test tube (20 mL),
5
6
6
2d -CHO
99 (94)
95 (94)
93 (88)
89 (85)
26 3d -COOH
258C, 1200 rpm stirring.
-CHO
7
8
6
2e
92 (98)
94 (91)
90
86
24 3e -COOH
-
CHO
9
1
1 f
6
2 f
>99 (96)
94
89
0
24 3 f -COOH
98 (91)
-
CHO
1
1
1g
1h
6
2g
98 (87)
85
76
12
24 3g -COOH
>99 (77)
-CHO
1
3
4
6
2h
99 (90)
96 (76)
89
73
1
24 3h -COOH
1
5
1i
1j
24 4i lactone
99 (63)
62
99
16
7
8
1
2j
2j
>99 (99)
[
a]
1
>99 (>99) >99
1 2
R -C(=O)-R
1
1
8
9
1k
1l
10 2k
72 (99)
>99 (99)
71
99
Figure 3. The effect of catalyst loading on 4-methoxybenzyl alcohol oxida-
tion rate. a) Formation of 4-methoxybenzaldehyde. b) Formation of 4-me-
thoxybenzoic acid. Yields were determined by GC-FID analysis using the
area normalization method. Reaction conditions: AcOH (2 mL), 4-methoxy-
R1-C(=O)-R2
[a]
4
2k
2
2
0
1
26 2l
2l
94 (99)
>99 (99)
93
99
[
a]
1
R
1
-C(=O)-R
2
benzyl alcohol (1 mmol), open test tube (20 mL) fitted with O
08C, 1200 rpm stirring.
2
balloon,
8
[
a] ABNO used as nitroxyl radical. [b] Conversion of alcohol is presented for al-
dehyde and ketone products, conversion of aldehyde is presented for carbox-
ylic acid products. [c] Yields were determined by GC-FID analysis using the
area normalization method. [d] Yield of isolated product. Reaction conditions:
alcohol (1 mmol), AcOH (2 ml), test tube (20 mL), 1200 rpm stirring.
ty, the reaction temperature was raised from ambient to 808C
and an O atmosphere (1 atm) was used instead of air. Now,
2
1
5
b was converted into 2b in 30 min and further into 3b in
h, in 99 and 60% yields, respectively.
The influence of catalyst loading (0.5 to 5 mol%) on the oxi-
dation rate of 1b was examined (Figure 3). Fast and almost
quantitative conversion of the starting alcohol into aldehyde
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2
1
b was observed using amounts of catalyst as low as
mol% (Figure 3a). The formation of 2b was fol-
Table 3. Substrate scope of aerobic alcohol oxidation with protocol B.
lowed by further oxidation into carboxylic acid 3b,
and which was most efficient using 1.5 mol% of cata-
lyst (Figure 3b ). Formation of carboxylic acid was
clearly delayed with higher catalyst amounts because
of the increased amount of TEMPO, which had to be
consumed before the Fe-catalysed auto-oxidation of
the aldehyde initiated (Figure 3b). Likewise with 1a,
full conversion of the alcohol into the aldehyde took
place before the formation of the carboxylic acid
began, thus enabling selective oxidation.
[a]
[b]
Entry Substrate
t [h] Product
Conv.
Yield
[
c]
(sel.) [%]
(isolated) [%]
1
2
1b
0.5 2b -CHO
>99 (99)
79 (96)
99
76
3
3b -COOH
[
[
d]
d]
3
4
1m
1n
1o
0.5 2m -CHO
3m -COOH
>99 (>99) >99 (98)
89 (97)
4
86 (84)
After optimisation of the reaction conditions the
substrate scope was explored. Under the best condi-
tions used for 1-octanol oxidation (Protocol A,
Table 2), undecanol (1c) and 3-phenyl-1-propanol (1d)
were smoothly converted into the corresponding al-
dehydes (2c and 2d, respectively). This was followed
by further conversion into the carboxylic acids (3c
and 3d) in very good yields. Also alcohols bearing
functional groups, such as chlorine (1e), a double
bond (1 f), a triple bond (1g), and ether (1h) were oxi-
dised into the respective aldehydes (2e–h) and fur-
ther to carboxylic acids (3e–h) in good yields. Almost
complete oxidation of alcohols into aldehydes was
observed with all substrates before the formation of
carboxylic products began, thus enabling excellent
control of selectivity and high yields. Oxidation of 1,5-
pentanediol (1i) resulted in the formation of d-valero-
lactone in fair yield.
5
6
0.5 2n -CHO
3n -COOH
>99 (>99) >99
81 (96) 78
3
7
8
0.5 2o -CHO
3o -COOH
>99 (>99) >99
93 (98)
4
91 (87)
9
1
1p
1q
0.5 2p -CHO
3p -COOH
>99 (99)
84 (96)
99
81
0
4
11
0.5 2q -CHO
3q -COOH
>99 (96)
65 (92)
96
60
12
4
1
1
3
4
1r
1s
0.5 2r -CHO
3r -COOH
>99 (99)
96 (95)
99 (97)
91 (88)
3
15
0.5 2s -CHO
3s -COOH
93 (99)
<5 (0)
92
0
16
5
1
1
7
8
1t
0.5 2t -CHO
3t -COOH
>99 (>99) >99 (96)
Oxidation protocol A was also employed for the
conversion of secondary alcohols into ketones. Di-
phenyl methanol (1j) was transformed into diphenyl-
ketone (2j) in excellent yield in 8 h, whereas oxidation
of 2-octanol (1k) and borneol (1l) into the corre-
sponding ketones (2k and 2l, respectively) was slow
and low yielding. To enhance the reactivity, the cata-
lyst system was modified by replacing sterically hin-
dered TEMPO with a less bulky nitroxyl radical, 9-aza-
bicyclo[3.3.1]nonane N-oxyl (ABNO), which has been
proven to be an efficient catalyst in the oxidation of
5
<5 (0)
0
1
20
9
1u
0.5 2u -CHO
quant. (99)
>5 (0)
99
0
5
3u -COOH
[
[
e]
e]
2
2
1
2
1v
1x
1
5
2v -CHO
3v -COOH
96 (96)
<5 (0)
92
0
[
e]
2
2
3
4
1
5
2x -CHO
3x -COOH
95 (86)
<5 (0)
82
0
[e]
[
a] Conversion of alcohol is presented for aldehyde and ketone products, conversion
of aldehyde is presented for carboxylic acid products. [b] Yields were determined by
GC-FID analysis using the area normalization method. [c] Yield of isolated product.
[d] Reaction conducted on a 100 mmol scale. [e] catalyst (3 mol%). Reaction condi-
tions: alcohol (1 mmol), AcOH (2 ml), test tube (20 mL), 1200 rpm stirring. quant. =
quantitative.
[
21,48]
secondary alcohols.
Pleasingly, this change led to
enhanced reactivity and ketones 2j–l were obtained
from the respective alcohols in quantitative yields in
1
to 4 h.
The use of ABNO was also applied to the oxidation
of primary alcohols. This resulted in superior reaction
rates, yet lower selectivities compared with the Fe(NO ) /bpy/
tained in high yields in 0.5–2 h. By extending the reaction time
to 3-4 h, benzylic substrates 1b and 1m–r were further oxi-
dised into the respective carboxylic acids (3b, 3m–r). Partial
amenability to oxidation was observed with 3,4-(methylene-
dioxy)benzyl alcohol 1s as well as with heteroaromatic 1t–
u and allylic alcohols 1v–x, as aldehydes 2s–x were the final
oxidation products. To demonstrate the applicability of the cat-
alyst system for preparative scale synthesis, 100 mmol of 1m
was oxidised into the corresponding aldehyde 2m and carbox-
ylic acid 3m in 98 and 84% yields, respectively.
3
3
TEMPO system (see Table S4).
Notably, applying the conditions optimised for the oxidation
of benzylic primary alcohols (Protocol B, Table 3) to the oxida-
tion of aliphatic primary alcohols resulted in incomplete sub-
strate conversion and poor product selectivities.
Benzylic, heteroaromatic and allylic alcohols were subjected
to oxidation using the conditions optimised for 4-methoxyben-
zyl alcohol (Protocol B, Table 3). With all the substrates (1b,
1
m–x), the corresponding aldehydes (2b, 2m–x) were ob-
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Scheme 2. Proposed mechanism for the catalytic aerobic oxidation of primary alcohols into carboxylic acids.
Based on the pertinent literature, a plausible reaction mech-
anism for conversion of the alcohol and subsequent aldehyde
oxidation is presented in Scheme 2. It is known that TEMPO
placed with ABNO. It was demonstrated that reactions are
easily scalable to ten grams.
disproportionates into the respective hydroxylamine (TEMPOH)
+
and oxoammonium ion (TEMPO ) under acidic conditions Acknowledgements
[
36–38]
(
step 1),
cies in the oxidation of alcohols into aldehydes while simulta-
neously undergoing reduction into TEMPOH
TEMPOH is most likely oxidised back into
the latter of which is reported as the active spe-
We thank the Academy of Finland for funding this research.
[
17,23,39,40,49,50]
(step 2).
Keywords: aerobic oxidation · alcohols · iron · synthetic
methods
III
radical TEMPO by Fe (step 3), which has been proposed to ox-
II [17,19,23]
idise TEMPO derivatives via reduction to Fe .
NO has
3
a catalytic role as a source of NO species that have been sug-
x
II
III [19,23,51]
[1] G. Tojo, M. Fernꢂndez, Oxidations of Alcohols to Aldehydes and Ketones -
A Guide to Current Common Practice, Springer, New York, 2006.
gested to mediate the aerobic oxidation of Fe into Fe .
Although the exact role of bpy is unknown, it plausibly coordi-
nates to Fe and enhances the reactivity by affecting the elec-
[
2] G. Tojo, M. Fernꢂndez, Oxidation of Primary Alcohols to Carboxylic Acids -
A Guide to Current Common Practice, Springer, New York, 2007.
[
52,53]
[3] A. J. Fatiadi, Synthesis 1976, 65–104.
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[
[
[
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H O is formed as a side product.
2
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[
8] Q. Cao, L. M. Dornan, L. Rogan, N. L. Hughes, M. J. Muldoon, Chem.
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hyde is inhibited by the presence of TEMPO, thus allowing
highly selective oxidation of primary alcohols into alde-
[
9] G-J. ten Brink, I. W. C. E. Arends, R. A. Sheldon, Science 2000, 287, 1636–
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[
44,45]
hydes.
However, slow decomposition of TEMPO into TMP,
7
through NÀO bond cleavage (step 4), eventually initiates the
[
[
Fe-catalysed aldehyde auto-oxidation leading to the formation
[
44,45,54–56]
of carboxylic products (steps 5–8).
1286–1295.
In conclusion, we have developed a sustainable, highly effi-
cient and practical Fe(NO ) /bpy/TEMPO-catalysed method for
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[
[
14] R. Crichton in Inorg. Biochem. Iron Metab., Wiley, Chichester, 2002.
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3
3
the selective aerobic oxidation of aliphatic and benzylic pri-
mary alcohols into either aldehydes or carboxylic acids in one-
step. The system is compatible with a wide range of function-
alities including halogens, double bonds, triple bonds, ethers
and esters. Moreover, the method is also effective in the oxida-
tion of secondary alcohols into ketones when TEMPO is re-
[
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Revised: July 4, 2016
794–798.
Final Article published: && &&, 0000
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K. Lagerblom, P. Wrigstedt, J. Keskivꢀli,
A. Parviainen, T. Repo*
&& – &&
Iron-Catalysed Selective Aerobic
Oxidation of Alcohols to Carbonyl and
Carboxylic Compounds
A highly selective aerobic oxidation of
primary alcohols into either aldehydes
or carboxylic acids has been developed.
This mild and practical method used
Fe(NO ) /TEMPO/2,2’-bipyridine as a cata-
dation into carboxylic acids was ach-
ieved in extended reactions where
TEMPO was de-oxygenated into TMP.
The procedure was also efficient for the
oxidation of secondary alcohols when
TEMPO was replaced by the less sterical-
ly hindered ABNO.
3
3
lyst in aerobic conversion of alcohols
into aldehydes (see figure). Further oxi-
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These are not the final page numbers! ÞÞ