Reinvestigation of the Organocatalyzed Aerobic Oxidation of Aldehydes to Acids

Article abstract of DOI:10.1021/acs.orglett.9b04193

The organocatalyzed aerobic oxidation of aldehydes to acids was reproduced from the original report. In- and ex-situ analysis of the reaction mixture as the function of time reveals that, unlike the claim in the publication, the aerobic oxidation of aromatic and aliphatic aldehydes leads predominantly to the formation of peracids. The latter are transformed into the corresponding carboxylic acids during the workup procedure. The buildup of peracids in solution poses safety problems that should not be overlooked. This finding has also an influence on the way new catalysts are investigated to improve this reaction as well as on aerobic aldehyde-mediated co-oxidation.

Full text of DOI:10.1021/acs.orglett.9b04193

Letter
Cite This: Org. Lett. XXXX, XXX, XXX−XXX
pubs.acs.org/OrgLett
Reinvestigation of the Organocatalyzed Aerobic Oxidation of
Aldehydes to Acids
Laurent Vanoye,† Mohamed Abdelaal, Kacy Grundhauser, Boris Guicheret, Pascal Fongarland,
†
†
†
†
†
,‡,†
Claude De Bellefon, and Alain Favre-Reguillon*
́
†Universite
Lyon, Laboratoire de Gen
novembre 1918, F-69100 Villeurbanne, France
ie des Proced
es
́
Catalytiques UMR 5285, CNRS-CPE Lyon-UCBL, 43 boulevard du 11
́
́
́
‡
Conservatoire National des Arts et Met
́
iers, EPN 7, 2 rue Conte,
́
75003 Paris, France
*
S Supporting Information
ABSTRACT: The organocatalyzed aerobic oxidation of aldehydes to acids was reproduced from the original report. In- and ex-
situ analysis of the reaction mixture as the function of time reveals that, unlike the claim in the publication, the aerobic oxidation
of aromatic and aliphatic aldehydes leads predominantly to the formation of peracids. The latter are transformed into the
corresponding carboxylic acids during the workup procedure. The buildup of peracids in solution poses safety problems that
should not be overlooked. This finding has also an influence on the way new catalysts are investigated to improve this reaction
as well as on aerobic aldehyde-mediated co-oxidation.
3
he development of methods for the selective and atom-
economic oxidation of organic substrates belongs to the
most challenging fields of contemporary chemical research, in
being generated by PINO. Upon reaction with O , an
2
T
acylperoxy radical B is formed. The abstraction of hydrogen
from NHPI by B regenerated PINO and produced peracid C
(Figure 1). Then, by analogy with the mechanism of the
both the academic and the industrial environment. In terms of
1
7
sustainability, O is a highly attractive option. The importance
Baeyer−Villiger (BV) oxidation, a second step was postu-
2
3b,5,6
of developing better aerobic oxidation catalysts means there is
now an increasing emphasis on this challenge in terms of
reactivity and selectivity. A plethora of homogeneous and
heterogeneous catalysts have been reported for a wide range of
oxidation reactions ranging from alcohol oxidation to C−H
lated.
The peracid C should react with a remaining
aldehyde 2 through a nucleophilic addition to form the Criegee
intermediate D. Rearrangement of D afforded 2 mol of the
corresponding carboxylic acid 3 (Figure 1) through migration
of hydride, whereas migration of the alkyl group gives an
equimolar mixture of carboxylic acid and formate. The ratio of
2
bond functionalization.
Although aerobic oxidation of aldehydes can be performed
hydride to alkyl migration was found to depend critically on
3
4d,6a,8
without catalysts, numerous homogeneous catalysts have been
the structure of aldehydes, temperature, solvent, etc.
In
the case of organocatalyzed oxidation, one can suppose that
the differences lie in the initiation and in the propagation steps,
the products (C and 3) and intermediates (A, B, and D) being
the same.
described for this particular reaction and, in particular, first-row
transition metals. Recently, Y.-B. Kang et al. described the use
4
of an organocatalyst, i.e. N-hydroxyphtalimide (NHPI), for the
5
aerobic oxidation of aldehydes to acids. Alkyl aldehydes were
However, several experimental pieces of evidence suggested
that the proposed mechanism of aerobic oxidation of aldehyde
might be somewhat incorrect: (i) In contrast to ketones, BV
oxidations of aldehydes with peroxyacids are less studied
because a mixture of carboxylic acids and formates are
selectively oxidized into the corresponding carboxylic acids in
3
h at 30 °C in the presence of 1 atm of O with 5 mol % of
2
NHPI, whereas aromatic aldehydes required harsh conditions
5
(
1.5 d, 90 °C, 10 mol % of NHPI, 1 atm of O ). A mechanism
2
was proposed (Figure 1) where a phthalimido-N-oxyl radical
9
generally obtained. The oxidation of hydroxybenzaldehydes
(
PINO) acts as a nonterminating chain propagation radical. In
3b,5,6
this generally accepted mechanism,
the aerobic oxidation
proceeds via a two-step process. The first step is a free radical
chain reaction with acyl radical A as the intermediate, the latter
Received: November 22, 2019
©
XXXX American Chemical Society
A
DOI: 10.1021/acs.orglett.9b04193
Org. Lett. XXXX, XXX, XXX−XXX

Organic Letters
Letter
Figure 2. Aerobic oxidation of benzaldehyde monitored as a function
of time using in-situ ATR FTIR monitoring. Reaction conditions:
benzaldehyde 0.9 M in acetonitrile, O (1 atm), NHPI 4 mol %, room
2
temperature.
Figure S2) leading to the potential incorrect quantification of
reaction components. Furthermore, splitting of the CO acid
5
Figure 1. Y.-B. Kang et al. proposed a mechanism for the
organocatalyzed aerobic oxidation of aldehydes to acids. Reprinted
with permission from American Chemical Society, Copyright 2019.
band through intermolecular hydrogen bonding at 1733 and
−1
1696 cm further complicates the treatment. One way to
overcome these issues could be the treatment of the spectra
14
with a multivariate approach, but this technique requires
pure perbenzoic acid or samples with a known amount of
perbenzoic acid. Nevertheless, two other techniques were
evaluated for quantitative analysis.
and nonactivated benzaldehydes with mCPBA gave the
corresponding phenols in good yield (Dakin oxidation).
ii) The concentration of peracid is supposed to remain low
10
(
since no peracid could be identified in the reaction mixture,
even at low conversion. Maximum yields of 10% to 40% of
peracids have been identified in the crude reaction mixture by
NMR monitoring could be an option (see the Supporting
5
1
13
Information, Figures S3 and S4 for H and C NMR
monitoring as the function of time) using an external lock.
1
NMR during the aerobic oxidation of aliphatic aldehydes using
From H spectra, the oxidation of benzaldehyde into benzoic
4
f,g
Fe(NO ) .
In contrast, synthetic applications of in-situ
acid is not a straightforward process. Clearly, an aromatic
intermediate is building up in the reaction mixture. The
3
3
generated peracids obtained from the aerobic oxidation of
11
13
aldehydes could be found in the literature. As such, a
relatively high concentration of peracids could be expected.
Consequently, we decided to further study this reaction by
specifically trying to quantify the perbenzoic acid formed
during the aerobic organocatalyzed oxidation of benzalde-
emergence of a new signal in the C NMR spectrum of the
crude with a chemical shift of 165.2 ppm corresponding to a
peracid group is also observed as a function of time, and the
intensity of this signal remains high whereas the signal
corresponding to an aldehyde group at 191.9 ppm was no
longer present. However, the NMR monitoring was too
cumbersome and not suited to follow the reaction kinetics of
such a reaction.
5
hyde. For this purpose, a test reaction was designed using
benzaldehyde in acetonitrile at room temperature with O at
2
atmospheric pressure and using NHPI as an organocatalyst
12
5
according to the improved procedure of Y.-B. Kang et al.
Thus, an ex-situ GC analysis was developed because of the
peracid instability. The oxidation of an excess organic sulfide
In-situ ATR FTIR monitoring would theoretically allow
precise quantification of reaction components (benzaldehyde,
perbenzoic acid, and benzoic acid). Figure 2 shows the raw IR
spectrum of aerobic oxidation of benzaldehyde in acetonitrile
15
by peracid into the corresponding sulfoxide and carboxylic
acid was used. The reaction is quantitative within a few
seconds at room temperature whereas hydrogen peroxide or
12
catalyzed by NHPI using optimized conditions. The decrease
of the absorption band of the CO stretching vibration of
benzaldehyde at 1702 cm− is clearly visible as a function of
O does not react with methyl p-tolyl sulfide under those
2
12
conditions.
1
Indeed, after 4 h (Figure 3 lower spectra) GC analysis of the
crude only shows two products (i.e., benzaldehyde and benzoic
acid), but ex-situ titration using p-tolyl sulfide confirms the
presence of a high quantity of peracid in the reaction media.
The concentration of perbenzoic acid in the crude reaction
mixture could be thus quantitatively determined by GC
measurement of the conversion of methyl p-tolyl sulfide into p-
tolyl sulfoxide (and eventually methyl p-tolyl sulfone) (Figure
−
1
time. Concomitantly, the absorption band at 1733 cm , which
corresponds to the superimposition of the absorption band of
CO stretching vibration of NHPI and benzoic acid,
increases. The occurrence of a supplementary absorption
−
1
band at 1761 cm could be attributed to the absorption band
1
3
of the CO stretching vibration of the perbenzoic acid.
Surprisingly, the intensity of such an absorption band remains
high even after 6 h of reaction time.
12
3, upper GC chromatogram). The mass balance was correct
under those conditions. The aerobic oxidation of benzaldehyde
was thus followed as a function of time using ex-situ
monitoring under the previous reaction conditions (Figure 4).
Unfortunately, the reactant, intermediate(s), product(s), and
organocatalyst have overlapping main characteristic peaks in
−
1
the 1800−1600 cm region (see the Supporting Information,
B
DOI: 10.1021/acs.orglett.9b04193
Org. Lett. XXXX, XXX, XXX−XXX

Organic Letters
Letter
1
6
perbenzoic acid. In this procedure, benzoic acid was
suspended in methanesulfonic acid and treated with con-
centrated aqueous hydrogen peroxide. After extraction with
16
CH Cl , perbenzoic acid was obtained with a yield of 80%.
2
2
The ease and improved safety conditions for the synthesis of
perbenzoic acid prompts us to study the preparation of m-
chloroperbenzoic acid (m-CPBA) from m-chlorobenzalde-
17
hyde. With the standard reaction conditions in hand, the
aerobic oxidation of m-chlorobenzaldehyde was thus inves-
tigated. However, a higher concentration of NHPI had to be
used (10 vs 4 mol %), and because of the lower solubility of m-
chlorobenzaldehyde derivatives, precipitation occurs which
further decreases the chemical rate and prevents the correct
determination of the true concentration of the reactant and
products (see the Supporting Information, Figure S5).
Figure 3. Comparison of the GC chromatogram obtained after 4h of
reaction (80% of conversion of benzaldehyde) with and without
quench with methyl p-tolyl sulfide. Reaction conditions: Benzalde-
hyde 0.9 M in acetonitrile, O2 (1 atm), NHPI 4 mol %, room
temperature, cyclohexanecarboxylic acid as internal standard. Sample
was diluted with pure acetonitrile, and quench was performed using
the same volume of methyl p-tolyl sulfide 1 M in acetonitrile (1:1
vol).
Alkyl aldehydes were also subjected to aerobic oxidation
using the same conditions (Figure 5). 2-Ethylhexanal was
Figure 5. Aerobic oxidation of 2-ethylhexanal monitored as the
function of time using ex-situ GC monitoring. Reaction conditions: 2-
ethylhexanal 0.9 M in acetonitrile, O (1 atm), NHPI 4 mol %, room
2
temperature. Quench was performed using methyl p-tolyl sulfide 1 M
in acetonitrile (1:1 vol.). Lines were added to guide the eye.
Figure 4. Aerobic oxidation of benzaldehyde monitored as the
function of time using ex-situ GC monitoring. Reaction conditions:
chosen as a test molecule because 2-ethylhexanoic acid is an
important chemical intermediate used in the production of
synthetic lubricants, film plasticizers for polyvinyl butyral,
stabilizers for PVC as well as oil additives, and functional fluids
Benzaldehyde 0.9 M in acetonitrile, O (1 atm), NHPI 4 mol %, room
2
temperature. Quench was performed using methyl p-tolyl sulfide 1 M
in acetonitrile (1:1 vol.). Lines were added to guide the eye.
18
like automotive coolants. As expected from the work of Y.-B.
5
Using this ex-situ procedure, the same trend already
Kang et al., the oxidation of alkyl aldehydes is much faster
12
observed using in-situ ATR FTIR (Figure 2) and NMR
than that of the aryl aldehydes (Figure 5). In less than 2 h, a
total conversion of 2-ethylhexanal was obtained. As expected,
very high selectivity (78%) for the peracid (2-ethylhexane-
peroxoic acid) was obtained. Before the workup, 2-ethyl-
hexanoic acid was only obtained with a 17% yield. It should be
mentioned that, if the reaction mixture was directly injected in
a GC, a selectivity for carboxylic acid close to 80% could be
found which is comparable to the previous experiment
was obtained. From Figure 4, it can be seen that benzaldehyde
was transformed selectively into perbenzoic acid. After 6 h, the
yield of perbenzoic acid was 83% whereas the yield of benzoic
acid was below 10% (Figure 4). However, it should be noted
that, without using ex situ protocols, a conversion of 93% of
benzaldehyde could be determined by GC and with very high
selectivity toward benzoic acid (>95%). Furthermore, at the
end of the reaction, a workup similar to that by Y.-B. Kang et
3b,4d,6a,8,18a
conducted by us with homogeneous metal catalysts.
5
al. (filtration through a pad of silica gel, washing with EtOAc,
Thus, the low selectivity toward 2-ethylhexanoic acid could
thus be explained by the uncontrolled degradation of 2-
ethylhexaneperoxoic acid in the injector of the gas chromato-
graph instrument or during workup rather than a nonselective
rearrangement of the Criegee intermediate as previously
and concentration by a rotary evaporator under reduced
pressure) provides pure benzoic acid with a yield of 93%
5
similar to the result reported by Y.-B. Kang et al. As such, and
as expected from the low stability of the perbenzoic acid, the
latter was quantitatively transformed into benzoic acid during
workup or ex-situ analysis without quench.
The yield of perbenzoic acid obtained from the aerobic
oxidation of benzaldehyde is similar to the that using the
classical procedure of Swern et al. for the preparation of
3b,4d,6a,8,18a
thought.
If the reaction mixture was kept at room
temperature, after 48 h the residual peracid concentration in
the reaction mixture is below 5%, showing the instability of
peracid in solution and the selectivity toward 2-ethylhexanoic
acid was 80%.
C
DOI: 10.1021/acs.orglett.9b04193
Org. Lett. XXXX, XXX, XXX−XXX

Organic Letters
Letter
Based on these findings, a modified reaction pathway can be
proposed (Figure 6). The free radical chain reactions that lead
to peracid C from aldehyde 2 are the same, consistent with
acidic solution, concentration in vacuo) as well as the analytical
1
5
method ( H NMR) used by Y.-B. Kang et al. does not allow
them to identify such an important intermediate; i.e., the
spontaneous decomposition of peracids leads to the corre-
sponding carboxylic acids.
3
b,5,6,19
literature.
The buildup of peracids in solution poses safety problems
that should not be overlooked. A study on the effect of the
different parameters (catalysts, concentration, solvent, addi-
tives) on the selectivity toward peracids of this important
industrial reaction as well as the development of new methods
for the selective reduction of peracids into the corresponding
carboxylic acids is in due course in our laboratory.
ASSOCIATED CONTENT
Supporting Information
■
*
S
The Supporting Information is available free of charge at
https://pubs.acs.org/doi/10.1021/acs.orglett.9b04193.
Safety, general procedure, in-situ ATR FTIR spectra of
1
the pure products and reaction mixture, ex-situ H and
1
3
C NMR monitoring, aerobic oxidation of m-
chlorobenzaldehyde monitored as the function of time,
reaction scheme of homolysis of perbenzoic acid
induced by NHPI, determination of the concentration
of the peracid by GC (PDF)
AUTHOR INFORMATION
Corresponding Author
■
*
E-mail: afr@lgpc.cpe.fr.
ORCID
Laurent Vanoye: 0000-0001-6029-6390
Alain Favre-Reguillon: 0000-0001-8464-6543
́
Notes
Figure 6. Modified reaction scheme for organocatalyzed aerobic
oxidation of aldehydes to acids. Bold arrows indicates the main
pathway. In· = radical initiator; (a) decomposition is spontaneous in
water, could be catalyzed by organic or metal compounds, or occur
under thermal stress.
The authors declare no competing financial interest.
ACKNOWLEDGMENTS
■
This work was supported by the French Government under
reference ANR-16-CE07-0008 IRSIS “Intensified Reactor
Structures for Intrinsically Secure processes”. The authors
acknowledge the Institut de Chimie de Lyon (ICL) for
providing the ReactIR15 apparatus and “Centre Commun de
The reaction of peracid C with aldehyde 2 forming Criegee
intermediate D is slow and in competition with the free radical
chain oxidation reaction. It should be noted that 10% of
carboxylic acid 3 is formed in solution when more than 90% of
aromatic or aliphatic aldehydes 2 is converted into peracids C
RMN” of Universite Lyon 1 for the full days of NMR analysis.
́
(
Figures 4 and 5).
Furthermore, the formation of carboxylic acid 3 in the
Authors would also like to thank the International Research
Experiences for Students (IRES), sponsored by the National
Science Foundation (NSF) and organized by the Center of
Undergraduate Research of the University of New-Orleans
(UNO), that supported K.G.
reaction mixture could also be explained by the decomposition
of the peracid C. This decomposition is spontaneous in
aqueous solution to give the corresponding acid 3 and
2
0
oxygen, or it could be catalyzed by organic or metal
2
1
compounds. Thermal decomposition of the peracid C due to
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22
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