Catalytic oxidation of primary c-h bonds in alkanes with bioinspired catalysts

Article abstract of DOI:10.2533/CHIMIA.2020.470

Catalytic oxidation of primary C-H bonds of alkanes with a series of iron and manganese catalysts is investigated. Products resulting from oxidation of methylenic sites are observed when hexane (S1) is used as model substrate, while corresponding primary C-H bonds remain unreactive. However, by using 2,2,3,3-tetra-methylbutane (S2) as model substrate, which only contains primary alkyl C-H bonds, oxidation takes place catalytically using a combination of hydrogen peroxide, a manganese catalyst and acetic acid as co-catalyst, albeit with modest yields (up to 4.4 TON). Complexes bearing tetradentate aminopyridine ligands provide the best yields, while a related pentadentate ligand provides smaller product yields. The chemoselectivity of the reaction is solvent dependent. Carboxylic acid 2b is observed as major product when the reaction takes place in acetonitrile, because of the facile overoxidation of the first formed alcohol product 2a. Instead the corresponding primary alcohol 2a becomes dominant in reactions performed in 2,2,2-trifluoroethanol (TFE). Polarity reversal of the hydroxyl moiety arising from the strong hydrogen bond donor ability of the latter solvent accounts for the unusual product chemoselectivity of the reaction. The significance of the current results in the context of light alkane oxidation is discussed.

Full text of DOI:10.2533/CHIMIA.2020.470

470 CHIMIA 2020, 74, No. 6
doi:10.2533/chimia.2020.470
NoN-Noble Metals iN Catalysis
Chimia 74 (2020) 470–477 © V. Dantignana, A. Company, M. Costas
Catalytic Oxidation of Primary C–H Bonds
in Alkanes with Bioinspired Catalysts
Valeria Dantignana, Anna Company*, and Miquel Costas*
Abstract: Catalytic oxidation of primary C–H bonds of alkanes with a series of iron and manganese catalysts is
investigated. Products resulting from oxidation of methylenic sites are observed when hexane (S1) is used as
model substrate, while corresponding primary C–H bonds remain unreactive. However, by using 2,2,3,3-tetra-
methylbutane (S2) as model substrate, which only contains primary alkyl C–H bonds, oxidation takes place
catalytically using a combination of hydrogen peroxide, a manganese catalyst and acetic acid as co-catalyst,
albeit with modest yields (up to 4.4 TON). Complexes bearing tetradentate aminopyridine ligands provide the
best yields, while a related pentadentate ligand provides smaller product yields. The chemoselectivity of the
reaction is solvent dependent. Carboxylic acid 2b is observed as major product when the reaction takes place in
acetonitrile, because of the facile overoxidation of the first formed alcohol product 2a. Instead the corresponding
primary alcohol 2a becomes dominant in reactions performed in 2,2,2-trifluoroethanol (TFE). Polarity reversal
of the hydroxyl moiety arising from the strong hydrogen bond donor ability of the latter solvent accounts for the
unusual product chemoselectivity of the reaction. The significance of the current results in the context of light
alkane oxidation is discussed.
Keywords: Alkane oxidation · Hydrogen peroxide · Iron · Manganese · Primary C–H bonds
Valeria Dantignana obtained her MSc
in Chemistry in 2016 at ‘La Sapienza’
University in Rome. In 2016 she took part
in a Marie Skłodowska-Curie ITN Action
(NoNoMeCat project) and started her PhD
in the QBIS-CAT group at the University of
Girona, under the supervision of Dr. Miquel
Costas and Dr. Anna Company. Her PhD
work is mainly focused on C–H bond oxi-
dation catalyzed by bioinspired complexes
Miquel Costas pursued PhD studies in the
group of Prof. A Llobet at the University
of Girona (UdG). Research work during
his PhD involved scientific stays at Texas
A&M (Prof. D. Barton, 1996), and in Basel
(Prof. A. Zuberbüehler, 1998). He then
moved to the group of Prof. L. Que, Jr, at
the University of Minnesota. He returned
to Spain and was appointed as Professor in
2003. Since 2006, he is group leader in the
and investigation of the reactivity of high valent oxoiron com- IQCC of UdG. His research interests involve biologically inspired
pounds.
design of inorganic reactivity and catalysis and supramolecular
chemistry. He has received the ICREA Academia Award (2008,
Anna Company obtained her BSc in 2013 and 2018), the Prize for Excellence in Research of the RSEQ
Chemistryin2004andherPhDinChemistry (2014) and the Syngenta Lectureship Award (2018). He has been
in 2008 at the University of Girona (UdG, invited professor in the University of Utrecht (2014) and in the
Catalonia). She then joined the group of University of Paris Diderot (2019).
Prof. Driess at the TU Berlin (Germany) as a
2-year postdoctoral researcher with a Marie 1. Introduction
Curie Intra-European Fellowship. She re-
The oxidation of alkane C–H bonds represents a relevant re-
turned to the UdG in 2012 as a ‘Ramón y search topic because it converts abundant and economic hydrocar-
Cajal’ fellow and since April 2018 she is bonsintovaluablecompoundsforfurtherchemicalelaboration.[1–3]
appointed as an Associate Professor. Anna The main challenges of the reaction are the poorly reactive nature
currently leads a research team at IQCC-UdG working on the ac- of alkane C–H bonds, rooted in their high bond dissociation en-
tivation of small molecules and the characterization of high-valent ergy and non-polar character, the control of site selectivity in sub-
species relevant in biology and catalysis. Among other awards, strates containing non-equivalent C–H bonds, and limiting over-
she was the recipient of the 2015 Clara ImmerwahrAward granted oxidation of the first formed oxidation products, since alcohols
by the German Cluster of Excellence UniCat and the 2019 ICREA are more reactive than alkane C–H bonds. Non-porphyrinic iron
Academia Award by Generalitat de Catalunya.
and manganese coordination complexes reproducing basic struc-
*Correspondence: Dr. M. Costas, Dr. A. Company, E-mail: miquel.costas@udg.edu, anna.company@udg.edu
Institut de Química Computacional i Catalisi (IQCC), Departament de Química, niversitat de Girona C/M. Aurelia Capmany 66, Girona (17003), Spain

NoN-Noble Metals iN Catalysis
CHIMIA 2020, 74, No. 6 471
tural aspects of non-heme oxygenases can form high valent metal- methylenic site of butane. Moreover, non-heme model complexes
oxo species with the competence to oxidize aliphatic C–H bonds that could oxidize light alkanes were reported. In particular, Que
in a selective manner under mild experimental conditions.[4–14] and co-workers described the oxidation of butane with a puta-
Oxidation of secondary and tertiary C–H bonds is well docu- tive oxoiron(iv) species, generated by the reaction of [FeII(TpPh2)
mented for a number of these type of catalysts, and competitive (O2CC(O)R)] with molecular oxygen,[39] and Che and co-work-
oxidation of these two type of bonds is customarily observed in ers reported the catalytic oxidation of ethane and propane with
substrates containing the two types.[15–32] In contrast, oxidation a mononuclear ferric complex ([FeII(Me3tacn)(R-acac)Cl]+) and
of primary aliphatic C–H bonds of alkanes has been only seldom oxone (Fig. 1, bottom).[40]
observed. The reaction is particularly interesting when applied to
In this work, we sought to investigate the ability of iron and
light alkanes, which stand as one of the most difficult classes of manganese complexes bearing non-porphyrinic ligands to cata-
substrates in aliphatic C–H oxidation. For example, Shul’pin and lyze the oxidation of primary C–H bonds of alkanes, as a first
co-workers reported the oxidation of unactivated alkanes cata- step towards the development of catalysts for the oxidation of
lyzed by a dinuclear iron complex ([Fe2(HPTB)(µ-OH)(NO3)2] light alkanes.
(NO3)2·CH3OH·2H2O) and pyrazinic acid as co-catalyst, in com-
bination with hydrogen peroxide.[33] In particular the authors were 2. Results and Discussion
able to oxidize methane (TON = 4) and ethane (TON = 21) obtain-
ing the corresponding hydroperoxides as the main products. In 2.1 Reaction Development Employing Hexane (S1) as
subsequent works, the author reported also a study of the oxygen- Model Substrate
ation of alkanes catalyzed by different iron complexes.[34,35] Up to
Catalytic oxidation of hexane (S1), taken as model substrate,
41 TON was obtained in the oxidation of methane, in the presence was investigated first. We screened the catalytic activity of a se-
of pyrazinic acid. In the above mentioned works, the hydroxyl rad- ries of iron and manganese complexes (Fig. 2), which have been
ical and ferroxy radical species were considered as possible oxi- previously described as C–H oxidation catalysts. The choice
dants. Stronger evidence of a possible metal-based oxidation was included a) an exhaustive family of iron and manganese com-
reported by Sorokin and co-workers, who described the first ex- plexes with tetradentate aminopyridine ligands (1 to 16),[4] b)
ample of an oxoiron(iv) complex capable of oxidizing strong C–H bis-bipyridyl manganese complex (17),[41,42] c) bis-picolinate
+
bonds, namely [(TPP)(m-CBA)FeIV(µ-N)FeIV(O)(TPP• )]–.[36] manganese (18)[24,43] as well as corresponding iron (generated in
Using the [(TPP)FeIII(µ-N)FeIV(TPP)] catalyst supported on silica situ) complexes,[44,45] and d) an iron complex with a pentadentate
and m-chloroperbenzoic acid as oxidant, 43.5% yield of formic ligand (19).[46] The first group included complexes bearing the
acid (with respect to m-chloroperbenzoic acid) was obtained in pyridylmethyl-triazacyclononane ligand (1 and 2) and complexes
the oxidation of methane (Fig. 1, top). Of interest, the authors with linear bis-amino-bipyridyl ligands differing in the aliphatic
used 13C-methane to unambiguously demonstrate that formic diamine backbone, and also in the electronic and sterically de-
acid is formed by the oxidation of methane.[37] Recently, Itoh manding properties of the pyridines (5–16). Complexes 3 and
and co-workers reported the oxidation of primary C–H bonds 4, where the methylene groups are deuterated to protect against
of 2,2,3,3-tetramethylbutane (10% yield of alcohol product with oxidative degradation, were also explored.[47,48] The choice of
respect to the iron(iii) content), achieved using a monomeric oxidants included Oxone®, peracetic acid and hydrogen perox-
oxoiron(iv) porphyrin cation radical compound ([FeIV(O)(TMP•+) ide. All of them have previously found utility in C–H oxidation
(Cl)]).[38] This compound was also shown capable of oxidizing the catalysis.
Fig. 1. Oxidation of light alkanes
with [(TPP)FeIII(µ-N)FeIV(TPP)] (top)
and [FeIII(Me3tacn)(R-acac)Cl]+
(bottom).

472 CHIMIA 2020, 74, No. 6
NoN-Noble Metals iN Catalysis
Fig. 2. Catalysts employed in this
work.
We first studied the oxidation of S1 (1 equiv.) using Oxone® and ketones resulting exclusively from methylene oxidation in
as oxygen atom donor (0.2 equiv. single addition as solid) 39% yield (with respect to the oxidant). Again, products resulting
and [M(OTf)2(Pytacn)] (1, 2), [Mn(OTf)2(bpy)2] (17) and from methyl oxidation were not observed.
[Mn(pic)2(H O)2] (18) as catalysts (1 mol%) in acetonitrile solu-
tion at room t2emperature. No oxidation products were observed in 2.2 Reaction Development Employing
any of these reactions. The use of hydrogen peroxide (0.2 equiv. 2,2,3,3-Tetramethylbutane as Model Substrate
added via syringe pump over 30 mins) provided a different out-
While the lack of methyl oxidation products was disappoint-
come. No oxidized products were observed in reactions performed ing, it aligns with previous reports where the oxidation of S1
using manganese catalysts bearing bidentate ligands 17 (entry 1, has been studied. We considered that the lack of primary C–H
Table 1) and 18 (entry 4, Table 1). However, when a combination oxidation products may arise from the lack of oxidation abil-
of Fe(OAc)2 and picolinic acid was used to generate in situ the ity of the metal-oxidants generated in these reactions (presum-
desired iron catalyst,[49] the reaction proceeded in modest yields ably high valent metal-oxos) to break via hydrogen atom transfer
(6%, see supporting information for details, Table S1) providing a (HAT) these particularly strong C–H bonds. Alternatively, it may
mixture of 2-hexanol (1a), 3-hexanol (1b) and the corresponding reflect a much faster and competitive oxidation reactivity against
ketone products (1c and 1d). Unfortunately, no products result- secondary over primary C–H bonds. In order to address this ques-
ing from oxidation of the primary C–H bonds were detected. As tion we studied the oxidation of 2,2,3,3-tetramethylbutane (S2),
a general trend, improved yields were obtained when complexes which contains only primary C–H bonds. Furthermore, due to its
bearing tetradentate aminopyridine ligands, 1, 2 (entries 7 and solid physical state and solubility in conventional solvents, the
10, Table 1) and 7, 8 (entries 13 and 16, Table 1), were employed use of this substrate is technically much simpler than studying
as catalysts; and up to 27% combined yield (4.3 product TON the oxidation of gaseous light alkanes, such as methane, ethane
defined as Σ [oxidation products]/[catalyst]) of products resulting or propane.
from oxidation of methylenic sites was obtained in the case of 7
(entry 13, Table 1).
Oxidation of S2 was initially studied using complexes 7 and
8 as catalysts (1 mol%) and hydrogen peroxide (0.2 equiv., com-
The results could be further improved by performing the reac- bined with acetic acid) or peracetic acid (0.2 equiv.) as oxidant
tions in the presence of acetic acid (AcOH).[51,52] In line with the in acetonitrile solution, at room temperature (Table 2). The reac-
results obtained in the absence of acetic acid, complexes 7 and tions yielded primary alcohol (2a) as minor product (1% with
8 (entries 14 and 17, Table 1) were the most efficient catalysts respect to the oxidant, which corresponds to 0.2 equiv. of 2a/
exhibiting combined yields of alcohol and ketone products of equiv. of catalyst) and the corresponding carboxylic acid (2b)
32% for iron and 40% for manganese, in all cases resulting from as the major reaction product (7–8% with respect to the oxidant,
methylene oxidation. Moreover, iron and manganese complexes which translates to ~ 0.5 equiv. 2b/equiv. of catalyst). It should
bearing the deuterated mcp ligand (3 and 4) were also tested. As be borne in mind that according to stoichiometry each equiv.
pseudobenzylic C–H bonds are susceptible to oxidation, deutera- of 2b requires 3 equiv. of hydrogen peroxide) (entries 1 and
tion of these positions was performed to avoid the possible cata- 5, Table 2). Thus, the reaction takes place but the amount of
lyst self-degradation by pseudobenzylic oxidation. However, no oxidized product is substoichiometric with respect to the cata-
improvement in the yield was observed with the deuterated com- lyst. Performing the reaction in deuterated acetonitrile did not
plexes (entries 19 and 21, Table 1).
improve product yields, discounting the solvent as a competitive
Finally, we tested the use of peracetic acid (PAA) as terminal substrate. However, an improved use of the oxidant was attained
oxidant (Table 1). This oxidant is effective with all the catalysts by using larger concentrations of substrate. For example, in-
tested, and with the single exception of 8 (entry 18, Table 1) it creasing the substrate concentration fivefold, oxidation products
provided the highest yield of oxidation products for the rest of the 2a and 2b were obtained in 4% and 12% yields (respectively, 0.8
catalysts. The best results in terms of product TON were obtained equiv. of 2a and 0.8 equiv. of 2b per equiv. of catalyst) using 7
with 7 (entry 15, Table 1), which provided a mixture of alcohols (entry 3, Table 2).

NoN-Noble Metals iN Catalysis
CHIMIA 2020, 74, No. 6 473
Table 1. Oxidation of hexane (S1)
with hydrogen peroxide (combined
with and without acetic acid) or
peracetic acid.
Entry
Catalyst
17
Oxidant
AcOH
(equiv.)
Yield [%]a Yield [%]a
Product
TONb
1a+1b
1c+1d
1
2
H2O2
H2O2
PAA
H2O2
H2O2
PAA
H2O2
H2O2
PAA
H2O2
H2O2
PAA
H2O2
H2O2
PAA
H2O2
H2O2
PAA
H2O2
PAA
H2O2
PAA
0
2
0
0
2
0
0
2
0
0
2
0
0
2
0
0
2
0
2
0
2
0
0
0
0
0
0
0
3
4
15
0
2.3
0
4
18
1
0
5
0
0
0
6
4
17
5
2.5
1.3
1.5
2.7
0.9
0.6
1.6
4.3
4.8
5.7
1.4
5.6
3.6
4.2
5.4
2.6
3.4
7
4
8
4
7
9
6
15
7
10
11
12
13
14
15
16
17
18
19
20
21
22
2
1
3
0
2
12
11
16
21
6
7
16
16
18
4
8
16
2
24
32
14
20
6
3
4
14
17
10
3
28
aWith respect to the oxidant, determined by GC-FID against an internal standard. Yields are calculated consid-
ering that 2 equiv. of oxidant are necessary for the formation of the ketone products (1c and 1d). bDefined as Σ
[oxidation products]/[catalyst].
While high valent metal oxo species are accepted to be the tions (entries 3 and 7, Table 2), suggest that substrate oxidation
C–H oxidizing species in reactions catalyzed by this class of via the homolytic O–O cleavage path, if present, may be a minor
complexes, we also considered the possibility that the observed contributor to the reaction.
reactivity may be due to the involvement of hydroxyl radicals.
With the aim to improve the preliminary results obtained,
In order to address this question, an experiment was performed we evaluated the effect of different electronic or steric proper-
with complex 19. MePy2tacn is a neutral, pentadentate amino- ties of the ligand, as well as of a different diamine backbone,
pyridine ligand.[46] The corresponding complex has one available on the outcome of the oxidation of S2 (Table 3). The screening
coordination site for peroxide activation. This class of complexes revealed that the change of the cyclohexyldiamine (mcp ligand)
activate hydrogen peroxide via an homolytic O–O cleavage[31,32,52] by the bipyrrolidine backbone (pdp ligand) led to a significant
instead of producing hydroxyl radicals, and because of that it was increase of the yield, with maximum combined values of 26% and
devised as an informative tool to estimate the viability of such 31%, respectively, for 5 (entry 1, Table 3) and 6 (entry 8, Table
a process. Using the best reaction conditions found with 7 (19/ 3) where 2b was the largely dominant product (1.6–1.9 TON of
AcOH/oxidant/substrate ratios 1/200/20/500), product 2a and 2b 2b). Conversely, electronic or steric modification of the nature of
were obtained in 2% and 3% yields (entry 9, Table 2), which cor- the catalysts did not play a relevant role (entries 3–6 and 10–13,
respond to a product TON of 0.6 (0.4 equiv. of 2a and 0.2 equiv. Table 3). Increasing the amount of oxidant employed (from 0.2
of 2b per equiv. of catalyst). The substantially reduced yields, to 1 equiv., entries 2 and 9 in Table 3 and Table S2 in supporting
when compared with catalysts 7 and 8 under analogous condi- information) translated into an increase of TON; up to 3.4 and 4.4

474 CHIMIA 2020, 74, No. 6
NoN-Noble Metals iN Catalysis
Table 2. Oxidation of 2,2,3,3-tetra-
methylbutane (S2) with hydrogen
peroxide (combined with acetic
acid) or peracetic acid.
Entry
Catalyst
7
S2
(equiv.)
Oxidant
AcOH
(equiv.)
Yield
Yield
Product
TONb
[%]a 2a
[%]a 2b
1
2
3
4
5
6
7
8
9
100
200
500
100
100
200
500
100
500
H2O2
H2O2
H2O2
PAA
H2O2
H2O2
H2O2
PAA
H2O2
200
200
200
0
1
2
4
1
1
2
4
0
2
7
8
0.7
0.9
1.6
0.8
0.7
1.3
1.2
0.6
0.6
12
9
8
200
200
200
0
8
14
6
9
19
200
3
aWith respect to the oxidant, determined by GC-FID against an internal standard. Yields are calculated con-
sidering that 3 equiv. of oxidant are necessary for the formation of the carboxylic acid product (2b). Yield 2a
calculated with response factor of 2b. bDefined as Σ [oxidation products]/[catalyst].
TON were obtained respectively with 5 (entry 2, Table 3) and 6 a solid and the concentration of the solutions of this substrate em-
(entry 9, Table 3).
ployed in our reactions are in the order of 0.1–0.5 M. On the other
hand, under the conditions described by Che and co-workers (6.9
bar of pressure of ethane over a 1:1 solvent mixture of water and
acetonitrile), the substrate concentration is not known, but presum-
2.3 Reversal of Chemoselectivity by Employing
Fluorinated Alcohol Solvents
In all the above mentioned reactions, formation of the hydrox- ably smaller. In addition, oxidation of methyl groups in S2 is statis-
ylated product 2a is minimal and instead the major product corre- tically favored over ethane by a 3:1 ratio. Finally, it must be stated
sponds to the overoxidized carboxylic acid 2b. In order to modify that the activity of this class of catalysts falls short when compared
this chemoselectivity, we performed the reactions in a fluorinated with the catalytic oxidation of methane, which is an even more re-
alcohol solvent. These solvents are powerful hydrogen bond do- sistant substrate towards oxidation, catalyzed by [(TPP)FeIII(µ-N)
nors and strongly interact with hydroxyl moieties, causing a po- FeIV(TPP)]-SiO2; in this case the TON reported is about three times
larity reversal that deactivates adjacent C–H bonds against further higher than the one we reached.[36,37]
oxidation.[53–56] Remarkably, the use of TFE led to an important
In conclusion, 5 and 6 (M = Fe and Mn) catalyze oxidation of
change in the chemoselectivity of the reaction. In the case of the primary C–H bonds with peroxides. TON unambiguously support
iron catalyst, a moderate reduction in TON was observed upon catalytic activity, but they are still modest to address light alkane
changing from acetonitrile to TFE (compare entries 1 and 3 with oxidation reactions in a satisfactory manner. Strategies to improve
2 and 4, Table 4). However, the activity of the manganese catalyst the efficiency in the use of the oxidant, for example by increasing
was improved (compare entries 5 and 7 with 6 and 8, Table 4). As the local concentration of substrate or by designing catalysts that
detailed in Table 4, using standard conditions (6/AcOH/oxidant/ generate more electrophilic metal-oxo oxidants, are required and
substrate ratios 1/200/20/500), up to 4.4 TON of products were ob- are currently under exploration.
tained with a 73% selectivity for the alcohol 2a (entry 8, Table 4).
4. Experimental Section
3. Context and Summary
The results of the study show that iron and manganese com- 4.1 Materials
plexes bearing tetradentate aminopyridine ligands can oxidize pri-
Reagents and solvents used were of commercially available re-
mary alkyl C–H bonds in a catalytic manner. While TON should be agent quality unless otherwise stated. Solvents were purchased from
considered as modest, an analysis of literature precedents indicate Scharlab, Acros, Fluorochem or Sigma-Aldrich and used without
that the numbers are in the same order with the state of the art in further purification. Hydrogen peroxide, peracetic acid and acetic
the field. For example, considering the activity of non-heme model acid solutions employed in the oxidation reactions were prepared by
complexes, in the oxidation of ethane Che and co-workers reported diluting commercially available hydrogen peroxide (30% H2O2 solu-
a maximum value for total TON of 3.4.[40] Instead, in the oxida- tion in water, Sigma-Aldrich), peracetic acid (35% peracetic acid so-
tion of propane they obtained a maximum value for total TON of lution in diluted acetic acid, Acros Organics) and glacial acetic acid
20.4, although oxidation is largely dominant in the secondary C–H in the desired solvent. Preparation and handling of air-sensitive ma-
bonds. In fact, the maximum TON for the formation of propionic terials were carried out in an N2 drybox (Jacomex) with O2 and H O
acid was 1.3. On the other hand, direct comparison should be taken concentrations <1 ppm. Complexes 1–16, 18 and 19 were prepar2ed
with care because the experimental conditions and the physical following previously described procedures.[46,47,50,57–66] Products 2a
nature of the substrate are different. 2,2,3,3-tetramethylbutane is and 2b were prepared according to literature protocols.[67–70]

NoN-Noble Metals iN Catalysis
CHIMIA 2020, 74, No. 6 475
Table 3. Oxidation of S2 with
different catalysts.
Entry
Catalyst
H2O2
(equiv.)
Yield
Yield
Product
TONc
[%]a 2a
[%]a 2b
Iron
catalysts
1
2
3
4
5
6
7
5
0.2
1.0b
0.2
0.2
0.2
0.2
0.2
2
<1
1
24
9
2.0
3.4
1.6
1.4
0.8
1.4
0.6
13
9
21
18
9
1
11
15
3
1
1
18
6
1
Manganese
catalysts
8
6
0.2
1.0b
0.2
0.2
0.2
0.2
0.2
2
29
15
18
15
0
2.3
4.4
1.4
1.2
<0.1
2.2
0.3
9
1
10
11
12
13
14
14
10
12
16
4
1
1
traces
1
30
1
1
aWith respect to H2O2, determined by GC-FID against an internal standard. Yields are calculated considering
that 3 equiv. of H2O2 are necessary for the formation of the carboxylic acid product (2b). Yield 2a calculated
with response factor of 2b. bOveroxidation products are formed. cDefined as Σ [oxidation products]/[catalyst].
Table 4. Oxidation of S2 in aceto-
nitrile or 2,2,2-trifluoroethanol.
Entry
Catalyst
5
Solvent
S2
(equiv.)
Yield
Yield
%
2a
Product
TONc
[%]a 2a
[%]a 2b
1
2
3
4
5
6
7
8
MeCN
TFE
100
100
500
500b
100
100
500
500b
2
6
24
6
20
75
39
79
17
67
27
73
2.0
1.6
3.6
2.8
2.3
3.0
3.6
4.4
MeCN
TFE
7
33
9
11
2
6
MeCN
TFE
29
15
39
18
10
5
MeCN
TFE
16
aWith respect to H2O2, determined by GC-FID against an internal standard. Yields are calculated considering
that 3 equiv. of H2O2 are necessary for the formation of the carboxylic acid product (2b). Yield 2a calculated
with response factor of 2b. bOveroxidation products are formed; substrate not totally dissolved. cDefined as Σ
[oxidation products]/[catalyst].

476 CHIMIA 2020, 74, No. 6
NoN-Noble Metals iN Catalysis
[9] R. V. Ottenbacher, E. P. Talsi, K. P. Bryliakov, Chem. Rec. 2018, 18, 78.
[10] W. Sun, Q. Sun, Acc. Chem. Res. 2019, 52, 2370.
4.2 Instrumentation
Oxidation products were identified by comparison of their
GC retention times and GC-MS with those of authentic com-
pounds. GC analyses were performed with an Agilent 7820A gas
chromatograph equipped with an HP-5 capillary column 30m
× 0.32 mm × 0.25 μm and a flame ionization detector. GC-MS
analyses were performed on an Agilent 7890A gas chromato-
graph equipped with an HP-5 capillary column interfaced with
an Agilent 5975C mass spectrometer. NMR spectra were taken
on BrukerDPX300 and DPX400 spectrometers using standard
conditions. Electrospray ionization mass spectrometry (ESI-MS)
experiments were performed on a Bruker Daltonics Esquire 3000
Spectrometer using a 1 mM solution of the analyzed compound.
Elemental analyses were performed using a CHNS-O EA-1108
elemental analyzer from Fisons.
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Asolution(2mL)ofthesubstrate(0.1–0.5M)andthepertinent
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Acknowledgements
The authors thank the Spanish Ministry of Science (CTQ2015-
70795-P to M.C., CTQ2016-77989-P to A.C.), Generalitat de Catalunya
(ICREA Academia Award to M.C. and 2014 SGR 862) and the European
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