Fe(TAML)Li/(diacetoxyiodo)benzene-mediated oxidation of alcohols: Evidence for mild and selective C-O and C-C oxidative cleavage in lignin model transformations

Article abstract of DOI:10.1002/ejoc.201301093

A novel combination of Fe(TAML)Li and (diacetoxyiodo)- benzene for the oxidation of primary and secondary alcohols at 25 °C in acetone is reported. In view of the interesting ability of this system to selectively cleave specific types of C-C bonds of elaborated alcohols, the application of this novel combination to the oxidative cleavage of lignin model molecules was investigated. Considering the numerous supported versions of the oxidant as well as the mild conditions employed, the developed methodology appears to be a promising lignin depolymerization strategy.

Full text of DOI:10.1002/ejoc.201301093

FULL PAPER
DOI: 10.1002/ejoc.201301093
Fe(TAML)Li/(diacetoxyiodo)benzene-Mediated Oxidation of Alcohols:
Evidence for Mild and Selective C–O and C–C Oxidative Cleavage in Lignin
Model Transformations
François Napoly,[a] Ludivine Jean-Gérard,[a] Catherine Goux-Henry,[a] Micheline Draye,*[b]
and Bruno Andrioletti*[a]
Keywords: Homogeneous catalysis / Oxidation / Renewable resources / Biomass / Sustainable chemistry / Iron
A novel combination of Fe(TAML)Li and (diacetoxyiodo)-
benzene for the oxidation of primary and secondary alcohols
at 25 °C in acetone is reported. In view of the interesting abil-
ity of this system to selectively cleave specific types of C–C
bonds of elaborated alcohols, the application of this novel
combination to the oxidative cleavage of lignin model mole-
cules was investigated. Considering the numerous supported
versions of the oxidant as well as the mild conditions em-
ployed, the developed methodology appears to be a promis-
ing lignin depolymerization strategy.
advantages of DAIB are its high solubility in organic sol-
vents (in comparison with PhIO) and, despite its poor
atom-economy, its numerous recyclable versions.[3b]
Preliminary studies carried out in our group led us to
consider the use of an unprecedented combination of an
iron tetraamido macrocyclic complex Fe(TAML)Li[6] and
DAIB to (selectively) oxidize primary and secondary
alcohols under mild conditions. To the best of our knowl-
edge, Fe(TAML)Li has never been combined with oxidants
other than hydrogen peroxide or tert-butyl hydroperoxide.[7]
In view of the interesting results obtained during the evalu-
ation of the scope of the transformation, lignin model oxi-
dation was undertaken, revealing promising features for the
oxidative degradation of lignin.
Introduction
Oxidation of alcohols is a pivotal reaction in organic syn-
thesis and many sets of conditions can be found in the lit-
erature to achieve this transformation.[1] Whereas the oxi-
dation of secondary alcohols generally affords ketones, pri-
mary alcohols can be oxidized to either the aldehydes or
the corresponding carboxylic acids. However, methods in-
volving nontoxic metals that allow the selective formation
of either the aldehyde or the carboxylic acid are scarce.[2]
Hypervalent(III) iodine reagents have drawn considerable
interest for the development of mild and selective oxidation
reactions and are now recognized as the oxidant of choice
for many organic transformations.[3] Among such reagents,
(diacetoxyiodo)benzene [PhI(OAc)2, DAIB] particularly at-
tracted our attention because of its valuable oxidative prop-
erties.[3b] Indeed, DAIB has already been described for the
oxidation of alcohols to the corresponding aldehydes and
carboxylic acids with the concomitant use of metal com-
plexes,[4] however in rather low efficiencies, and requiring
the use of solvents that are not environmentally friendly
such as dichloromethane. Of note, the use of 2,2,6,6-
tetramethylpiperidine-1-oxyl (TEMPO) has also been re-
ported, however, it is rather expensive.[5] Among others, the
Results and Discussion
After screening several combinations of “classical”
metallo complexes with various oxidants (see the Support-
ing Information), we decided to use the combination of an
iron tetraamido macrocyclic complex Fe(TAML)Li pion-
eered by Collins[6] (Figure 1) and DAIB in acetone for the
oxidative transformation of veratryl alcohol 1. Indeed, this
unique combination appeared to be the most promising in
terms of efficiency (conversion, reaction time, etc.) for the
transformation of 1.
Reactions in anhydrous acetone were carried out first
(Table 1). Control experiments revealed that both the oxi-
dant and the catalyst were required; no conversion was ob-
served in the absence of either one (Table 1, entry 1). Using
1 mol-% catalyst, veratryl alcohol 1 was converted into the
corresponding veratraldehyde 2 in 60% in one hour using
2 equiv. DAIB (Table 1, entries 2 and 3). Extending the re-
action time to four hours allowed the formation of 2 in an
[a] Institut de Chimie et Biochimie Moléculaire et
Supramoléculaire, UMR CNRS 5246, Université Claude
Bernard Lyon 1, Bâtiment Curien (CPE)
43 Boulevard du 11 novembre 1918, 69622, Villeurbanne Cedex,
France
E-mail: bruno.andrioletti@univ-lyon1.fr
www.icbms.fr
[b] Laboratoire de Chimie Moléculaire et Environnement,
Université de Savoie,
Campus scientifique, Le Bourget du Lac Cedex 73376, France
E-mail: micheline.draye@univ-savoie.fr
Supporting information for this article is available on the
WWW under http://dx.doi.org/10.1002/ejoc.201301093.
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FULL PAPER
Having in hand an interesting method for selectively oxi-
dizing a primary alcohol to the corresponding aldehyde in
anhydrous acetone, the effect of the water content was in-
vestigated (Table 2).
Table 2. Optimization of the oxidation of veratryl alcohol with
Fe(TAML)Li and DAIB.
Figure 1. Fe(TAML)Li structure.
increased yield (79%; Table 1, entry 4). The catalyst loading
was then increased to 5 mol-% (entries 5 and 6), affording 2
in 78% yield and 86% conversion in the presence of 2 equiv.
DAIB (entry 6). Thus, the same amount of veratraldehyde
2 was formed by using 1 mol-% catalyst in four hours (en-
try 4) or 5 mol-% Fe(TAML)Li in one hour (entry 6). This
yield was further improved by increasing the reaction time
to four hours (Table 1, entry 7). However, in an effort to
develop cost-effective transformations, we chose to evaluate
the scope of the combination of 1 mol-% Fe(TAML)Li/
2 equiv. DAIB for the mild oxidation of alcohols under an-
hydrous conditions.
Entry Water content [%]
Conv. [%][a]
Yield [%][a]
2
3
1
2
3
control[b]
0
60
100
100
100
100
100
0
0
0
45
59
0
5
57
53
39
4
5
10
20
50
0
5 (trace) 95 (94)
7 (trace) 93 (92)
Table 1. Optimization of the oxidation of veratryl alcohol with
Fe(TAML)Li and DAIB in anhydrous acetone.
6
7[c]
39
60
[a] Determined by GC/MS analysis relative to an internal standard
(diphenyl ether). All samples were derivatized by using BSA. The
indicated GC yield corresponds to an average yield based on dupli-
cate experiments. Isolated yields are indicated in parentheses.
[b] Two control experiments were carried out in commercial acet-
one with the catalyst or DAIB alone. [c] PhIO was used as the oxi-
dant.
As observed in anhydrous acetone, the synergistic effect
of the catalyst and the oxidant was confirmed with control
experiments, with no oxidation of the alkyl side chain of
veratryl alcohol being observed with DAIB or the catalyst
alone. When 5% water was used as cosolvent in the reac-
tion, veratryl alcohol was fully converted into the corre-
sponding aldehyde 2 in 53% yield, in conjunction with the
corresponding carboxylic acid in a promising 45% yield
(Table 2, entry 3). Further optimization revealed that a
minimum of 20% water was needed to insure excellent
selectivity towards the acid (Table 2, entries 3–6). The con-
ditions developed thus allow the complete and selective
conversion of veratryl alcohol into carboxylic acid in the
presence of water.
Entry Fe(TAML)Li DAIB Reaction time Conv.[a] Yield of 2[a,b]
[mol-%]
[equiv.]
[h]
[%]
[%]
1
2
3
4
5
6
7
control[c]
–
–
1
1
1
5
5
5
1.0
2.0
2.0
1.0
2.0
2.0
1
1
4
1
1
4
45
60
80
73
86
98
42
57
79
67
78
93
[a] Determined by GC/MS analysis relative to an internal standard
(diphenyl ether). All samples were derivatized by using BSA. The
indicated GC yield corresponds to an average yield based on dupli-
cate experiments. [b] Veratric acid 3 was never observed under these
conditions. [c] Two control experiments were carried out in anhy-
drous acetone with the catalyst or DAIB alone.
Interestingly, the oxidation appeared to occur at a higher
rate in the presence of water. Indeed, in contrast to the re-
sults obtained under anhydrous conditions, the presence of
Interestingly, after one hour, complete selectivity toward water allowed complete conversions of the starting material
veratraldehyde was observed (Table 1, entry 3), albeit with in less than one hour. In addition, the proportion of water
limited conversion (60%). To improve the conversion, the also proved to dramatically change the kinetics of the reac-
reaction time was extended to 2, 3 and 4 hours (for the re- tion. Indeed, in the presence of a 1:4 water/acetone mixture
sults obtained after 4 h, see Table 1, entry 4) affording the (Table 2, entry 5) the turnover frequency (TOF) was esti-
corresponding aldehyde in 68, 74, and 79%, respectively. It mated to be 1880 h–1, and 5520 h–1 in the presence of a 1:1
is noteworthy that no veratric acid was detected under these water/acetone mixture (Table 2, entry 6). A similar in-
reaction conditions. Extending the reaction time further did fluence of the water content on the outcome and the rate
not result in any significant improvement.
of the transformation had previously been observed for
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Mild and Selective Oxidation of Alcohols
iron(III) porphyrin complex catalyzed oxygenation reac- Table 3. Oxidation of various alcohols with Fe(TAML)Li and
tions using DAIB as the terminal oxidant.[8] Current
DAIB in anhydrous acetone.
hypothesis favors the formation of iodosylbenzene in situ
by partial hydrolysis of DAIB with water. PhIO then reacts
faster with the iron complex than DAIB under anhydrous
conditions. To verify this hypothesis, the efficiency of PhIO
as the oxidant under anhydrous conditions was confirmed
by the complete conversion of veratryl alcohol into the cor-
responding aldehyde (39%) and carboxylic acid (60%)
(Table 2, entry 7).
The conditions thus developed allow the complete and
selective conversion of veratryl alcohol into either the alde-
hyde or the carboxylic acid depending on the presence or
not of water. Based on the excellent results obtained during
the optimization on veratryl alcohol 1, we decided to inves-
tigate the scope of this selective oxidation reaction on other
alcohols.
Oxidation of alcohols under anhydrous conditions was
initially studied. Primary alcohols were tested first. Benzylic
alcohol afforded moderate conversions (Table 3, entry 1),
and unreactive aliphatic alcohols also appeared to be diffi-
cult substrates to oxidize using our set of conditions
(Table 3, entries 2 and 3). In contrast, secondary alcohols
were all converted into the corresponding ketones in good
to excellent yields. Indeed, apart from the aliphatic 2-oc-
tanol, which was converted into 2-octanone in a modest
yield of 51% (Table 3, entry 4), secondary benzylic alcohols
(entries 5 and 6) and cyclohexanol (entry 7) were efficiently
[a] Determined by GC/MS analysis relative to an internal standard
oxidized to the corresponding ketones in almost quantita-
(diphenyl ether). All samples were derivatized by using BSA, except
for entries 4, 6, and 7. GC yield corresponds to an average yield
tive yields.
Further studies were devoted to the oxidation of alcohols based on duplicate experiments. Isolated yields are indicated in pa-
rentheses. [b] GC yield could not be calculated because the corre-
under the optimized aqueous conditions developed in
sponding GC peak coeluted with PhI, the reduced form of DAIB.
Table 2. As previously observed, primary alcohols proved
to be more difficult to oxidize than secondary alcohols
(Table 4). Among the three primary alcohols tested, benz- and veratric acid 3 in a significant global yield of 18%
ylic alcohol afforded the best results, with a conversion of (Table 4, entry 8). These two products may arise from an
75%, albeit with limited selectivity (Table 4, entry 1). The oxidative C(benzyl)–C bond cleavage reminiscent of that
role of the methoxy group on the performance of the trans- observed with lignin peroxidase. Hence, it can be antici-
formation appeared to be crucial. Indeed, the selectivity pated that the combination of Fe(TAML)Li and DAIB gen-
and the efficiency were both improved when methoxy erates a high valent iron complex that oxidizes the β-O-4
groups were present on the substrate. This observation is model molecule 4 to the corresponding radical cation 4+·
consistent with the established beneficial role of electron- (Scheme 1) by single-electron transfer. Bonds β to the aren-
donating groups on oxidation processes. Indeed, kinetic ium radical are then easily cleaved.[10] The main cleavage
studies revealed that, in constrast to veratryl alcohol, the (Route 1) involves Cα–H bond scission because of the in-
oxidation of benzaldehyde to benzoic acid is slower than creased acidity of the radical cation compared with its neu-
the oxidation of benzylic alcohol to benzaldehyde (see the tral form. This cleavage leads to the formation of ketone 5
Supporting Information, p. S26). Because the catalyst ac- in 31% yield. Conversely, cleavage of the Cα–Cβ bond
tivity is lost after approximately 30 seconds of reaction (Route 2) generates veratraldehyde 2 (and further veratric
(TOF = 5520 h–1; TON = 184), a mixture of benzaldehyde acid 3, 3%) in 15% yield along with an unfavorable primary
and benzoic acid was obtained.
carbon radical, which further polymerizes.[11]
As expected, the oxidation of various aliphatic and benz-
Facing this intriguing reactivity, we decided to investigate
ylic secondary alcohols under similar conditions resulted in further this transformation, which could be of great interest
the formation of the expected ketones in almost quantita- for the valorization of lignin, a biosourced alternative for
tive yields (Table 4, entries 5–7). The use of more elaborate aromatic chemicals (Figure 2).[9c]
alcohols proved to be particularly interesting. Benzylic sec-
Considering the promising result obtained with the β-O-
ondary alcohol 4, one of the main β-O-4 scaffolds present 4 molecule 4,[12] we sought to use our mild oxidative condi-
in lignin polymers,[9] was oxidized to the corresponding tions on other representative lignin-inspired linkages con-
ketone 5 with concomitant formation of veratraldehyde 2 taining benzylic and aliphatic C–C, C–O and C–H bonds.
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Table 4. Oxidation of various alcohols with Fe(TAML)Li and DAIB in acetone/H2O.
[a] Determined by GC/MS analysis relative to an internal standard (diphenyl ether). All samples were derivatized by using BSA, except
for entries 4, 6, 7. The indicated GC yield corresponds to an average yield based on duplicate experiments. Isolated yields are indicated
in parentheses.
The following study only focuses on the use of the aqueous
conditions because any potential application to lignin poly- O-4 linkages, the latter one being of interest, because it is
mer would necessarily involve the presence of water. present as part of the 8- and 5-membered ring such as the
Among the chosen model linkages are the β-1 and the α-
Scheme 1. Proposed mechanism for the cleavage of the β-O-4 linkage of lignin inspired model 4.

Mild and Selective Oxidation of Alcohols
Figure 2. Example of a softwood lignin structure.
benzodioxocin and phenylcoumaran structures. Ferulic es- tions, we were pleased to observe the oxidative cleavage of
ter was also studied as a model compound inspired from the C–O bond, which furnished the aldehyde and the corre-
coniferyl alcohol – one of the three building blocks of the sponding carboxylic acid in 35% global yield (Table 5, en-
lignin macromolecule – but also a product arising from the try 2). As already proposed,[13] and by analogy with the β-
disruption of some linkages of lignin during the depolyme- O-4 oxidative cleavage, under oxidative conditions, 6 is con-
rization process.
verted into the corresponding radical cation 6+·. The latter
The α-O-4 model linkage 6 was thus submitted to our might undergo proton loss to produce an α-phenoxybenzyl
oxidative conditions. Using the optimized aqueous condi- radical that further reacts with dioxygen to form a hemi-
Table 5. Oxidation of lignin model molecules with Fe(TAML)Li and DAIB.
[a] Determined by GC/MS analysis relative to an internal standard (diphenyl ether). All samples were derivatized by using BSA. The
indicated GC yield corresponds to an average yield based on duplicate experiments. Isolated yields are indicated in parentheses.
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Scheme 2. Proposed reaction pathway for the α-O-4 oxidative cleavage.
acetal. The hemiacetal is subsequently hydrolyzed to proved successful on most of them, with particularly inter-
veratraldehyde 2 and veratric acid 3 in 22 and 13% yield, esting outcomes on the α-O-4 linkage; (4) the methoxy
respectively (Scheme 2).
groups that are extensively present in lignin appeared to be
The β-1 linkage was also investigated, and application of essential to ensure efficient cleavage of C–C or C–O bonds.
the oxidative conditions also proved successful with diol 7, However, the current shortcomings of this strategy are the
which underwent C(benzyl)–C(benzyl) bond cleavage, af- low selectivity for some of the studied linkages and the
fording the corresponding aldehyde 8 and carboxylic acid 9 concomitant formation of stoichiometric amounts of
in 36 and 39% yield, respectively (Table 5, entry 3). It iodoarene during the organic process. The somewhat low
should be noted that DAIB is known to be an excellent selectivity may be circumvented by optimization of the li-
reagent for the cleavage of 1,2-diols, although it has rarely gand design and efforts in our group are currently devoted
been used to this end.[14] To illustrate, in the absence of to the development of a supported version of the ligand
catalyst, the aldehyde and the carboxylic acid are formed in that should enhance the stability of the complex as well as
80 and 12% yields, respectively. Thus, the presence of the facilitate its recycling.
catalyst results in a higher formation of the acid at the ex-
pense of the aldehyde. Next, we became interested in the
oxidative transformation of ferulic ester derivative 10. Ex-
Experimental Section
cellent conversions were obtained by using aqueous acet-
one, with the major product being veratraldehyde, which
was formed in 40% yield (Table 5, entry 4). Noticeably, a
side product that was identified as acetylated diol 11 (14%
isolated yield) was also detected by GC–MS analysis and
1H NMR spectroscopy. The acetylated diol results from
acetate-mediated ring opening of the corresponding epox-
ide, with the acetate anion originating from DAIB.
General Procedure for the Oxidation of Alcohols/Lignin Model
Compounds under Anhydrous Conditions: A dry, inert Radleys tube
was charged under argon atmosphere with the substrate (1 mmol),
diphenyl ether (0.1–0.25 equiv.), Fe(TAML)Li (1 mol-%), and an-
hydrous acetone (5 mL). The solution was placed on a Radleys
carrousel and thermostated at 25 °C. Iodobenzene diacetate
(2 mmol) was then added to the mixture and the tube was stirred
for 1 or 4 h. Sat. aq. sodium sulfite (5 mL) was then added to the
mixture, which was extracted with ethyl acetate (3ϫ 5 mL). The
obtained solution was dried on 3 Å molecular sieves and analyzed
by following the procedures detailed in the Supporting Infor-
mation.
Conclusions
The present study reports on the development of a new
catalytic system composed of Fe(TAML)Li and DAIB as
the oxidant. The related oxidation reactions can be carried
out in an environmentally friedly solvent. Its application
to the oxidation of primary and secondary alcohols was
presented, and the original ability to cleave C–C and C–O
bonds permitted the oxidative transformation of lignin
model molecules. The study highlights several important
features: (1) a good selectivity in the oxidation reaction of
General Procedure for the Oxidation of Alcohols and Lignin Model
Compounds in the Presence of Water: A Radleys tube was charged
with the substrate (1 mmol), diphenyl ether (0.1–0.25 equiv.),
Fe(TAML)Li (1 mol-%), and acetone/water in different pro-
portions (5 mL). The solution was placed on a Radleys carrousel
and thermostated at 25 °C. Iodobenzene diacetate (2 mmol) was
added and the solution was stirred for 1 h. Water (5 mL) was added
to the mixture, which was extracted with ethyl acetate (3ϫ 5 mL).
primary alcohols can be obtained depending on the pres- The obtained solution was dried on 3 Å molecular sieves and ana-
lyzed by following the procedures detailed in the Supporting Infor-
mation.
ence or not of water; (2) Fe(TAML)Li, which has only been
mentioned once for the oxidation of lignin,[15] associated
with a hypervalent(III) iodine reagent exhibits good cata-
lytic activities for the oxidative cleavage of lignin model
linkages; (3) an extensive study has been carried out on four
Supporting Information (see footnote on the first page of this arti-
cle): Experimental procedures, copies of NMR spectra correspond-
ing to all reported compounds, as well as the GC–MS analyses of
different linkages of lignin, and the developed conditions all the reactions.
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Mild and Selective Oxidation of Alcohols
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Acknowledgments
F. N. deeply acknowledges the Rhône-Alpes region for financial
support. B. A. thanks the French Agence Nationale de la Recher-
che (ANR) (ANR 12 CDII 45 0001 02 “CHEMLIVAL”) for partial
financial support.
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June 5, 2001.
Received: July 23, 2013
Published Online: November 28, 2013
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