Oxidation of aromatic alkenes and alkynes catalyzed by a hexa-acetonitrile iron(ii) ionic complex [Fe(CH3CN)6][BF4]2

Article abstract of DOI:10.1039/c8nj02226g

Oxidative cleavage of aromatic alkenes and oxidation of alkynes have been catalyzed by a simple Fe(ii) complex [Fe(CH₃CN)₆][BF₄]₂ using H₂O₂ as an oxidant at room temperature. A free radical mechanism has been proposed involving the peroxy (OOH) radical to account for our experimental findings.

Full text of DOI:10.1039/c8nj02226g

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ISSN 1144-0546
PAPER
Jason B. Benedict et al.
The role of atropisomers on the photo-reactivity and fatigue of
diarylethene-based metal–organic frameworks
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Oxidation of Aromatic Alkenes and Alkynes Catalyzed by a Hexa-
acetonitrile Iron(II) Ionic Complex [Fe(CH CN) ][BF ]
3
6
4 2
Received 00th January 20xx,
Accepted 00th January 20xx
a
a
b
b
c
Chew Pheng Yap , Jing Kuang Ng , Sherzod Madrahimov , Ashfaq A. Bengali , Tsz Sian Chwee and
*a
Wai Yip Fan
DOI: 10.1039/x0xx00000x
Oxidative cleavage of aromatic alkenes and oxidation of alkynes have been catalyzed by a simple Fe(II) complex
Fe(CH CN) ][BF using H as an oxidant at room temperature. A free radical mechanism has been propsoed involving
the peroxy (OOH) radical to account for our experimental findings.
www.rsc.org/
[
3
6
]
4 2
2 2
O
various styrene derivatives in acetonitrile using [Fe(salen)Cl]
(salen = N,N′-ethylenebis(salicylideneaminato)) and excess of
Introduction
2 2
H O in acetonitrile. The iron salen complexes appeared to
The oxidation of alkenes and alkynes to carbonyl compounds
1
, 2
plays a pivotal role in organic synthesis. Oxidative cleavage
of alkenes is industrially performed by ozonolysis, based on
have better stability if they are anchored onto clay. Simple iron
salts in the presence of other ligands were also used to oxidise
2
1
3
hazardous ozone as the oxidant. With the growing interest
-5
aromatic alkenes. If pyridine-2-carboxylic acid (pic) ligand
was used, acetals were observed to be the main products
6
for future clean energy via biomass valorisation , methodology
2
2
while the use of pyridine-2,6-dicarboxylic acid (dipic) ligand
in the presence of iodosylbenzene converts alkenes to
for oxidative cleavage of unsaturated CƔC double bonds
attracted wide research attention. Despite the use of
2
3
aldehydes slowly. Shaikh et al found that a simple iron salt,
FeCl 6(H O), without the presence of ligands, could be used to
stoichiometric or more than stoichiometric amounts of strong
7
oxidants, such as potassium permanganate (KMnO ) , meta-
3
⋅
convert trans-stilbene to its carboxylic acid in the presence of
2
4
8
chloroperoxybenzoic acid (m-CPBA) , iodosylbenzene (PhIO) ,
9
o
1
0, 11
12, 70% t-butyl hydrogen peroxide at 80 C. However when
sodium periodate (NaIO )
and osmium tetroxide (OsO4)
obtained excellent cleavage product yields, many of these
chemical oxidants are also toxic and contribute to the
4
1
3
hydrogen peroxide was used instead, no products were
observed.
,
1
4
Herein, we wish to report a hexaacetonitrile-coordinated Fe(II)
generation of wasteful by-products.
In the last decade, introduction of second- and third-row
complex, [Fe(CH
catalyst for the oxidation of aromatic alkenes and alkynes to
carbonyl compounds in the presence of H under ambient
3 6 4 2
CN) ][BF ] (Complex I) that act as an efficient
1
5-19
transition metal catalysts containing Au, Pd, Re and Ru
the presence of milder oxidants such as sodium hypochlorite
NaOCl), peracetic acid (CH CO H), hydrogen peroxide (H O )
, in
2 2
O
and environmentally-benign conditions. With this iron
complex, the catalytic process does not require the presence
of any other ligands, solid-state anchors, harsh oxidants and
elevated temperatures. Two other iron complexes have also
been used for comparison purposes. It will be shown that
catalysis proceeds via the peroxy radical (OOH) generated by
the Fenton reaction.
(
3
3
2 2
and oxygen gas (O ) have been widely studied for cleavage of
2
various alkenes. Although effective, these precious rare-earth
metals are expensive and less-abundant, thus there remains
much interest in developing more sustainable and greener
oxidation protocols using first-row transition metals such as
iron, which are usually cheaper and more environmentally
friendly.
Iron-centered compounds have also been reported as catalysts
Results and Discussion
2
as well. Pitchumani et al demonstrated the cleavage of
0
Scheme 1. The three Fe(II) catalysts used in this work.
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Three Fe(II) complexes
I,
II and III have been used as catalysts
serves as a precursor for the
preparation of II while complex III is commercially-available.
Complex was prepared in high yields from the oxidation of
iron powder using nitrosyl tetrafluoroborate (NOBF ) in
acetonitrile. An interesting feature of complex lies in the
observation of two (CN) infrared bands at 2313 cm and
(
Scheme 1). Complex I
Table 1. Optimization of the oxidative cleavage of styrene to benzaldehyde reaction
catalysed by Fe(II) complexes
I
4
I
-
1
ν
-1 24-26
2286 cm .
Despite the octahedral symmetry for which only
one
ν
(CN) band is expected, Fermi resonance coupling
Catalyst
Loading
(%)
Yield of
[
a]
2 2
H O
between the C
≡
stretch of the same symmetry and similar energy resulted in
N stretching and first overtone of the C–C Entry
Solvent
Catalyst
Benzaldehyde
Equiv.
[b]
(%)
2
two vibrational bands being observed. Complex
7
I
was then
1
2
3
4
5
6
7
8
9
–
I
–
3
–
3
3
3
–
used to react stoichiometrically with Triphos (1,1,1-
tris(diphenylphosphinomethyl)ethane) in acetonitrile at room
temperature to afford II. Although the structure of II has been
reported, we have successfully grown suitable crystals of II and
reproduced the X-ray structural analysis (see Supplementary
Information). As shown in Scheme 1, the Triphos ligand in II
exerts a chelating effect on the Fe(II) centre while three
acetonitrile ligands are still retained to form an overall trigonal
anti-prismatic complex.
10
10
10
10
–
CH
3
CN
I
78
31
15
12
12
15
18
–
II
III
DMF
DMSO
THF
I
10
3
EA
1
0
1
2
DCM
1
1
CHCl
3
–
The room-temperature oxidative cleavage of styrene to
1
5
25
56
80
benzaldehyde was used as the test case with H O as the
2
2
13
14
3
CH CN
I
3
oxidant and complex
yield was optimized by varying the experimental parameters
see Table 1). By comparing entries 1–3, the oxidation process
clearly requires the presence of both complex and H O . The
calculated yield in entry 3 corresponded to a turnover number
TON) of 5.9 with respect of 1 mol of styrene. Apart than Other than acetonitrile, we have used solvents of different
benzaldehyde, formaldehyde is expected to be the other dielectric constants for the catalytic process. In solvents with
carbonyl product resulting from oxidative cleavage of styrene. low (entries 8–11), the catalysis slowed down tremendously
Indeed,
small signal at ~9.8 ppm attributable to and did not even occur in dichloromethane or chloroform. The
formaldehyde was detected in the H NMR spectrum. main cause is likely the immiscibility of H
However, quantitative analysis of CH O is difficult as it can solvents. On the other hand, solvents with high
I
as the catalyst. The benzaldehyde NMR
30
[
a] Catalytic runs involved 1 mmol of styrene and 3 mL of solvent. Reaction
(
mixture stirred at ambient temperature for 24 h. [b] Conversion and yield
I
1
2
2
determined by H NMR using nitromethane as internal standard.
(
ε
ε
a
1
2
O
2
with such
such as DMF
ε
2
either form paraformaldehyde or undergo significant and DMSO also decrease the catalytic yield. In this case, the
evaporation even at room temperature. Thus, the yield of the solvent molecule itself may have acted as a strong donor
oxidative cleavage reaction is much more reliably determined ligand for Fe(II) coordination and hence hindered the catalytic
from benzaldehyde quantitation. Traces of benzoic acid (<1% process. The effect of Fe(II) catalytic loading was also
yield) have also been detected as a result of further oxidation investigated. Comparing entry 3 with entries 12–14, the
of benzaldehyde.
product yield was found to be proportional to the catalytic
When the catalytic efficiency of complexes
I
to III was mole loading. However, a 10% mole loading offers a good
compared in acetonitrile solvent (entries 3 to 5), all three balance in terms of absolute product yield and TON. For
catalysts were capable of oxidizing styrene to benzaldehyde, example, although a very high mole loading of (entry 14)
albeit with differing yields. However, complex II could not be afforded a better absolute yield of benzaldehyde, the TON is
I
31
recovered as the P NMR spectrum recorded upon completion actually lower than that when 10% was used. Thus the results
of catalysis showed extensive dissociation of the Triphos ligand of our optimization studies have led us to adopt the
from the iron centre. Similarly, when the ν
(CO) bands of III experimental conditions of entry 3 for further substrate scope
were monitored via infrared spectroscopy, they appeared to studies.
have disappeared even during the earlier stages of catalysis.
Table 2. Oxidative cleavage of alkenes to its corresponding carbonyls catalysed by I
These observations suggest that the complexes used are only
in the form of a pre-catalyst, and how facile their conversion is
to the active catalyst in solution may have been one of the
rate-determining steps in the process. In our case, complex
I
which contains more labile acetonitrile ligands could have
undergone the conversion much faster and thus able to
generate more products over the same catalytic duration.
2
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Entry
[b]
[c]
yields above 40% and TON from 6 to 10. However, the
aliphatic alkene (entry 10) remained unreacted, even when the
reaction was extended to a few days at slightly elevated
[
a]
Substrate
Product (Yield %)
TON
temperatures (50°C–60°C). The aromatic alkenes that undergo
oxidative cleavage generate two carbonyl products, all of
1
8
which were detected by NMR spectroscopy. Even for substrate
1d, a small amount of the highly volatile acetaldehyde was
detected via its weak quartet NMR signal at 9.7 ppm. The
cleavage process appears to be the major reaction pathway as
we are unable to detect any other significant products. The
catalytic efficiency tends to decrease slightly as a function of
the steric bulk of the substrate although oxidative cleavage of
triphenylethylene (entry 6) still proceeds with reasonable TON.
Oxidation of alkynes by non-heme iron catalysts and
2
3
4
5
7
10
7
2
8
hydroperoxides have been previously reported by Ryu et al.
To expand the scope of the substrates tested, the optimized
reaction conditions for alkene oxidation were also applied to
the oxidation of some alkyne substrates using I as the catalyst.
The results are shown in Table 3. Similarly, only aromatic
alkynes are successfully oxidized while the aliphatic alkynes
remain unreactive. The terminal alkyne 1k was oxidized to
benzaldehyde (71%) while the internal alkyne 1l was oxidized
to a 1,2-diketone product 2l (77%). It appears that the
oxidation reaction has occurred for terminal alkynes but not
for internal alkynes. However, the following section will
describe how benzoic acid is formed from over-oxidation of a
diketo product rather than the acid being a direct product of
the oxidation process. After varying the experimental
parameters, it turns out that the optimized conditions used for
alkene oxidation also gave the highest TON for alkyne
oxidation.
8
6
7
6
7
8
9
8
8
–
1
0
–
[a] Reaction conditions: Substrate (1 mmol), catalyst 1 (10 mol %) and H
2 2
O (3
equiv.) as the oxidant in 3 mL of solvent stirred at ambient temperature for
1
2
4 h. [b] Conversion and yield determined by H NMR using nitromethane as
internal standard. For entry 2, two benzaldehyde molecules are formed per
mol of substrate. [c] TON are rounded to the nearest integer.
The oxidation of different alkenes has been carried out with
complex
I as the catalyst and the results are summarized in
Table 2. Aromatic alkene substrates (entries 1–9) were
oxidized to their respective aldehyde or ketone products with
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2
, 23
Table 3. Oxidation of alkynes by complex I
on the catalytic process (entry 4, Table 4).
In this case, a
complete arrest of oxidation was observed. We have also
repeated the experiment on the stilbene substrate and
likewise did not detect any oxidation products (entry 5, Table
[a]
[b]
Entry
1
Substrate
Product (Yield %)
TON
7
4
).
Table 4. Oxidation of proposed intermediates catalysed by I
2
3
7
7
Entry
[b]
[a]
Substrate
Product (Yield %)
TON
8
1
2
4
7
5
5
6
–
–
–
–
3
4
5
5
0
0
–
–
[
a] Reaction conditions: Substrate (1 mmol), catalyst 1 (10 mol %) and H
2 2
O (3
equiv.) as the oxidant in 3 mL of solvent stirred at ambient temperature for
1
2
4 h. [b] Conversion and yield determined by H NMR using nitromethane as
internal standard.
[
a] Reaction conditions: Substrate (1 mmol), catalyst 1 (10 mol %) and H
2 2
O (3
Mechanistic Studies
equiv.) as the oxidant in 3 mL of solvent stirred at ambient temperature for
1
2
4 h. [b] Conversion and yield determined by H NMR using nitromethane as
Some studies to understand the mechanism of alkene and
alkyne oxidation by our Fe(II) complexes have bene carried
out. In previous studies, alkene oxidation by some iron
complexes under different experimental conditions afforded
internal standard.
It is well-known that Fe(II) ions are able to induce the hydroxyl
2
9-32
33-36
epoxides
or diols
as the major product. Initially we
(
OH) and peroxy (OOH) free radical formation from hydrogen
3
7
hypothesized that epoxides and/or diols have also been
generated as intermediates in our system before converting
fully to the final carbonyl products. This hypothesis was tested
by subjecting 1q and 1r (entries 2 and 3 in Table 4) to oxidation
under the same reaction conditions as styrene 1a. If oxidative
cleavage were to proceed via either or both intermediates,
their product yield should be the same or even higher than
that for styrene under the same duration. However, their
respective yields were determined to be only approximately
half that of 1a. The results suggest that, at least under our
experimental conditions, oxidative cleavage of aromatic
alkenes may have occurred without prior formation of either
peroxide via the Fenton reaction (Scheme 2). One or both
radicals generated from this redox activity can serve to initiate
the alkene or alkyne oxidation process.
1
the epoxide or diol. Moreover, the H signals characteristic of
1
or 1r were not observed in the H NMR spectrum of styrene
1q
throughout the oxidation period.
As the oxidative cleavage of alkenes may involve the
participation of free radicals, the free radical TEMPO was
added into the reaction mixture of styrene to assess its effect
4
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Scheme 3. Proposed reaction pathway for alkyne oxidation following the same free
radical pathway proposed for alkenes.
Here the terminal alkyne substrate is chosen as an example.
The over-oxidation of phenylglyoxylic acid to benzoic acid is
also shown in the subsequent step.
Scheme 2. Proposed free-radical mechanism for the oxidative cleavage of alkenes
A similar radical pathway as seen for the alkene substrates can
be applied to alkyne oxidation. The OOH radical first adds to
(
styrene as the example)
A mechanism involving the very reactive OH radical for these the alkyne triple bond to generate a vinyl radical intermediate
reasons is disfavored. A diol product or intermediate is (Scheme 3). This intermediate then reacts with hydrogen
expected to be formed via successive addition of the OH peroxide to form intermediate . The 1,2-diketone product is
6
7
radical onto the alkene double bond. However as mentioned finally generated via the elimination of a water molecule. For
earlier, diols were not detected under our experimental terminal alkynes however, further oxidation of the product can
conditions. In addition, the hydroxyl radical is also observed to also occur. Following the same mechanism, the expected
react with the acetonitrile solvent itself to form some as-yet product resulting from 1k oxidation is phenylglyoxal. This
unknown products. Hence its high reactivity may have aldehyde is oxidized further in the presence of hydrogen
38
rendered it much less useful for specifically oxidising the peroxide to phenylglyoxylic acid and subsequently undergoes
alkenes or alkynes. decarboxylation to form benzaldehyde. Further oxidation of
When a mechanism was derived involving the less-reactive benzaldehyde finally affords benzoic acid, the major product
peroxy radical, we find that it is more consistent with our detected in the NMR spectrum.
experimental findings (Scheme 2). Upon decomposition of the Finally, the oxidation of aliphatic alkenes and alkynes by our
hydrogen peroxide into OOH, the radical may also oxidise the iron complexes was unsuccessful. We believe that the addition
3
+
iron catalyst to give a [Fe ]-OOH intermediate which goes on of the peroxy radical onto the unsaturated bonds of the
to react with the alkene substrate. Alternatively, the free OOH aliphatic systems may have been endothermic. The expected
radical reacts with the alkene to form the first step of oxidative carbon-centre radical generated from the reaction suffers
cleavage. From Table 2, it is noted that there is a small from the absence of aromatic delocalization and thus could
decrease in product yield from 1a (78%), 1b (67%) to 1f (59%) have decomposed to regenerate the precursors. Currently the
as the number of phenyl rings on the substrate increases. As reaction pathways are also studied with the aid of
an alkene with di- or tri-substituted phenyl groups is no longer computational chemistry to support our hypothesis.
planar, this trend suggests that steric factors can exert an
influence on the oxidative cleavage reaction. However, the
Conclusions
observed decrease in yield is not dramatic, hence it is plausible
that the first step of oxidation most likely involves the double
The present work illustrates the capability of inexpensive Fe(II)
bond addition of the free OOH radical instead of the bulkier
3 6 4 2
catalysts, in particular [Fe(CH CN) ][BF ] in effecting the
3
Fe ]-OOH intermediate to form
+
[
a
benzylic radical
oxidative cleavage of aromatic alkenes and oxidation of
alkynes at room temperature using hydrogen peroxide as the
oxidant. Unfortunately, the process could not be extended to
aliphatic substrates. Further work using epoxide and diol as
substrates and TEMPO as a radical scavenger lend support to
the role of the peroxy radical generated via the Fenton
reaction in converting the alkenes and alkynes to their
respective carbonyl products.
intermediate 4 although the contribution of the latter cannot
be ruled out definitively.
In the subsequent step, the benzylic radical intermediate
4
reacts with another molecule of hydrogen peroxide to afford
intermediate . This is followed by the elimination of H
molecule and the cleavage of a C–C bond to yield the aldehyde
products 2a and 3a This proposed reaction pathway
5
2
O
.
eliminates the need for an epoxide or diol intermediate, which
is consistent with our experimental data.
Experimental Section
Chemicals. Iron powder (
tetrafluoroborate (NOBF
dicarbonyl iodide (CpFe(CO)
≥99.99% trace metals basis), nitrosyl
4
)
(95%), Cyclopentadienyliron
2
I) (98%), glacial acetic acid
(
≥
99%), hydrogen peroxide solution (30% w/w in H2O),
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styrene (99.9%), cis-stilbene (96%), trans-β-methylstyrene removed under reduced pressure. The resulting product was
(
(
(
99%), trans-
97%), trans-4,4’-dibromostilbene (95%), triphenylethylene purification.
99%), -phenylcinnamonitrile (99%), 1-hexene (97%), Oxidation of epoxide and diol. The oxidation was carried out
α-methylstyrene (99%), 1,1-diphenylethylene analysed using 1H NMR spectroscopy without further
α
phenylacetylene (98%), diphenylacetylene (98%), 1-hexyne in the same way as the oxidation of alkenes and alkynes as
(97%), 3-hexyne (99%), 2,2,6,6-tetramethylpiperidine 1-oxyl described above.
(TEMPO) (98%), styrene oxide (97%) and 1-phenyl-1,2- TEMPO Reactions. 2,2,6,6-tetramethylpiperidine 1-oxyl
ethanediol were purchased from Sigma-Aldrich. CD3CN-d3 (TEMPO) was used as a radical trap to test for the presence of
99.8 atom % D), CDCl (99.8 atom % D) and DMSO-d6 (99.8 radicals during the reaction. [Fe(CH CN) ][BF (48 mg, 0.1
atom D) were purchased from Cambridge Isotope mmol) and 3 equiv. TEMPO (469 mg, 3 mmol) was first
Laboratory Inc. All commercial chemicals were directly used dissolved in 1 mL acetonitrile followed by the addition of
without any purification. styrene or stilbene (1 mmol, 115 L). 3 equiv. H (3 mmol,
General Procedures. All Fourier-transformed infrared (FT-IR) 307 L) was added dropwise into the solution. The solution
spectra were recorded using a Shidmazu IR Prestige-21 was stirred for 24 h at room temperature. The resulting
(
3
3
6
4 2
]
%
ꢀ
2 2
O
ꢀ
-
1
1
Fourier-transformed IR spectrometer (1000 – 4000 cm at 1 product was then analysed using H NMR spectroscopy.
-1
cm resolution) using a 0.05 mm path length CaF2 cell for
liquid samples. Electrospray ionization-mass spectrometry (ESI-
MS) was conducted using a Finnigan MAT LCQ spectrometer.
All proton and carbon nuclear magnetic resonance (1H and
Conflicts of interest
In accordance with our policy on Conflicts of interest please
ensure that a conflicts of interest statement is included in your
manuscript here. Please note that this statement is required
for all submitted manuscripts. If no conflicts exist, please state
that “There are no conflicts to declare”.
13C NMR) spectra were recorded using a Bruker AV300 MHz
instrument. Single crystal X-ray structural studies were
performed on Bruker-AXS Smart Apex CCD Single-Crystal
Diffractomers. Data were collected at 100(2) K using graphite
monochromated Mo K
evaluated using SMART CCD system.
Synthesis Procedures. [Fe(CH CN)
α
radiation (
λ
= 0.71073 Å) and
3
6
][BF
4
]
2
(Complex
I). Fe Acknowledgements
powder (111.8 mg, 2 mmol) was stirred in 10 mL of acetonitrile
followed by the addition of NOBF (233.6 mg), 1 mmol). Rapid
evolution of nitric oxide was observed and removed by passing
through the flask. The resulting solution was stirred for 72 h
under N at room temperature. Subsequently, the solution was
This publication was made possible by funding through an
NPRP award (8-094-1-018) from the Qatar National Research
Fund (member of Qatar Foundation). We also acknowledge
the National University of Singapore for participating through
research grant 143-000-656-597.
4
N
2
2
filtered and the solvent was removed under vacuum
conditions to yield an off-white solid. The solid was further
-1
dried under vacuum for 1 h. Yield: 71%; IR (DMSO, cm ): 2313,
References
2
+
2
283 (
51.05. Found: m/z = 152.6.
CN) (Triphos)][BF
47.6 mg, 0.1 mmol) was first dissolved in 10 mL acetonitrile
followed by the addition of 1,1,1-
tris(diphenylphosphinomethyl)ethane (Triphos) (62.5 mg, 0.1
mmol). The resulting red solution was stirred for 2 h under N
ν
CN); ESI-MS for [Fe(CH
3
CN)
6
]
Calculated: m/z =
1
.
A. Gonzalez-de-Castro and J. Xiao, Journal of the
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1
[Fe(CH
3
3
4 2 3 6 4 2
] (Complex II). [Fe(CH CN) ][BF ]
2.
(
2
014, 4, 2182-2209.
3
.
.
R. Criegee, Angewandte Chemie International Edition in
English, 1975, 14, 745-752.
2
at room temperature. Subsequently, the solvent was being
removed under vacuum conditions giving a red residue. The
residue was re-dissolved in acetonitrile and crystallization of
the complex II by slow diffusion of diethyl ether was carried
out. The obtained bright red crystals were filtered off and
washed with small amount of hexane to afford a yield of 83.1
4
G. Gatwood and G. Murphy, J. Chem. Educ, 1969, 46
A103.
,
5
.
.
F. Leh, J. Chem. Educ, 1973, 50, 404.
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6
2
+
7.
mg (85%). ESI-MS for [Fe(CH
01.1.
Oxidation of Alkenes and Alkynes. [Fe(CH
.1 mmol) was first dissolved in 3 mL acetonitrile followed by
the addition of the alkene or alkyne (1 mmol). 3 equivalents of
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3 3
CN) (Triphos)] : 401.84, found:
5
4
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3
2
O
2
2
ꢀ
the solution. The resulting dark-coloured solution was stirred
for 24 h at room temperature. The reaction was quenched
with 20 mL of water, and ethyl acetate was used for
extraction. The organic layer was obtained and the solvent was
1
0.
6
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Entry for table of contents
A simple Fe(II) catalyst that catalyses the oxidation of aromatic alkenes and alkynes under ambient
conditions.