Aerobic epoxidation catalysed by transition metal substituted polyfluorooxometalates

Article abstract of DOI:10.1039/c6dt01928e

First row transition metal substituted polyfluorooxmetalates with quasi Wells-Dawson structures and a nitro terminal ligand, [NaH₂M(NO₂)W₁₇F₆O₅₅]^q-, were used as catalysts for the aerobic epoxidation of cyclic alkenes. The Cu(NO₂) analog combined the best traits of conversion and selectivity. Some C-C bond cleavage was also observed and cis isomers reacted preferentially without stereochemical inversion indicating an oxygen atom to double bond concerted reaction.

Full text of DOI:10.1039/c6dt01928e

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Aerobic epoxidation catalysed by transition metal
substituted polyfluorooxometalates
Cite this: DOI: 10.1039/c6dt01928e
Received 15th May 2016,
Accepted 19th July 2016
Marco Bugnola and Ronny Neumann*†
DOI: 10.1039/c6dt01928e
www.rsc.org/dalton
First row transition metal substituted polyfluorooxmetalates with substituted arenes, although under certain conditions epox-
quasi Wells–Dawson structures and
nitro terminal ligand, ides were formed.7 Based on the further observation that the
[NaH2M(NO2)W17F6O55
q−, were used as catalysts for the aerobic substitution of fluorine into a polyoxometalate scaffold can
a
]
epoxidation of cyclic alkenes. The Cu(NO2) analog combined the lead to better stabilization of higher valent oxidation states,8
best traits of conversion and selectivity. Some C–C bond cleavage we were interested in exploring the possibility of using O2 for
was also observed and cis isomers reacted preferentially without the formation of epoxides from alkenes using a transition
stereochemical inversion indicating an oxygen atom to double metal substituted polyfluorooxometalate catalyst of the quasi
bond concerted reaction.
Wells–Dawson structure, Qq[NaH2M(NO2)W17F6O55] with nitro
ligands, where M = Mn, Fe, Co, and Cu and Q is trioctylmethyl
ammonium. The M–NO2 species acts as an oxygen donor, and
can be recovered using O2 according to Scheme 1.7
The use of ground state molecular oxygen as a reagent for the
oxidative transformations of hydrocarbons such as alkenes,
alkanes and arenes is a topic with broad implications in the
field of green chemistry.1 In the area of the transformation of
alkenes one can enumerate various reactions depending on
the product selectivity and presence of a co-reagent. Notable
examples include oxidative hydration to yield ketones, the
Wacker reaction, and other Pd catalysed reactions,2 epoxidation
typically in the presence of a reducing agent,3 and carbon–
carbon bond cleavage reactions that may often occur via an
epoxide intermediate.4 As concerns epoxidation, oxiranes
being important industrial intermediates,5 the use of a co-
reducing agent is less attractive from a green chemistry/atom
economy point of view. The so-called liquid phase “direct
epoxidation” reaction of alkenes with O2 in the absence of any
reducing agent and through careful exclusion of an autoxida-
tion type mechanism has been mostly studied in the context of
a dioxygenase mechanism where the O–O bond is cleaved to
yield a higher valent metal-oxo species active for oxygen trans-
fer. Success in the endeavour has been mostly with ruthenium
based catalysts.6 An alternative approach for a direct epoxi-
dation reaction is to use a metal activated oxygen donor that
can then be recovered by O2. Indeed we recently demonstrated
that Co–NO2 and Cu–NO2 species substituted into either a
Wells–Dawson or a “sandwich” type polyoxometalate com-
pound were reactive for C–C bond cleavage of mostly alkenyl
The various polyfluorooxometalates were synthesized by
preparing K9[NaH2Zn(H2O)W17F6O55] and then carrying out
metathetical exchange of Zn(II) with Fe(III), Mn(II), Co(II), and
Cu(II) to yield the corresponding potassium salts of [NaH2M
(H2O)W17F6O55]q− 8,9
. ‡ The sodium cation was replaced by the
trioctylmethyl ammonium cation and then treated with
gaseous NO2 as previously reported for other polyoxometa-
lates7 to yield Qq[NaH2M(NO2)W17F6O55].§ The 19F NMR
spectra of Qq[NaH2M(NO2)W17F6O55] showed no peaks indicat-
ing that in all likelihood the compounds were paramagnetic,
Scheme 1 The aerobic epoxidation of alkenes catalysed by a transition
metal substituted polyfluorooxometalate with a NO2 terminal ligand. W
– black; O – red; Na – yellow; F – green; First row transition metal –
magenta; N – blue. The quaternary ammonium cation is not shown.
Department of Organic Chemistry, Weizmann Institute of Science, Rehovot, 76100,
Israel. E-mail: ronny.neumann@weizmann.ac.il
†R. N. is the Rebecca and Israel Sieff Professor of Organic Chemistry.
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Note that the aqua substituted compounds, Qq[NaH2M
(H2O)W17F6O55] in the absence of any NO•2 source showed no
conversion of cis-cyclooctene. The results show that the Fe(III)
substituted compound was the least selective with the for-
mation of a large amount of additional products. The Co(II),
Mn(II) and Cu(II) showed similar reaction profiles. In
nitroethane as solvent conversions were high, >90, but along
with the formation of epoxide, significant amounts of 1-nitro-
cyclooctene were formed due to a competing nitration reac-
tion. Interestingly about 10 mol% of product was the result of
a C–C bond cleavage reaction to yield 1,8 octanedialdehyde
with no formation of carboxylic acid derivatives; autooxidation
is inhibited. Reactions catalysed by Qq[NaH2M(NO2)W17F6O55
]
Fig. 1 IR spectra of Q9[NaH2CuII(NO2)W17F6O55] (blue) and Q9[NaH2-
with the exception of Fe(III) were significantly more selective to
the formation of epoxide with the best results, 76 mol%
selectivity, obtained with the Cu(II) compound, Q9[NaH2-
CuII(H2O)W17F6O55] (red) deposited on Ge plates.
with the exception of the Cu(II)– NO•2 where spin coupling CuII(NO2)W17F6O55].
between the unpaired electron of Cu(II) and the NO2 ligand
In order to further evaluate the reactivity of Q9[NaH2-
would yield a diamagnetic singlet. The IR spectra, Fig. 1, of CuII(NO2)W17F6O55], a mixture of cis- and trans-cyclododecene
the four compounds were identical with four peaks characteri- was chosen as substrate. The effect of the temperature can be
stic of the polyfluorooxometalate framework between 700 seen in Fig. 2, where it can be noticed that the selectivity and
and 1000 cm−1 as can also be observed for the analogous yield to the epoxide was quite high especially at 130 °C and the
aqua species and at 1289 and 1325 cm−1 attributable to N–O catalytic activity was significant, ∼40 turnovers in 2 h.
vibrations at νN–O(assym) and νN–O(sym)
.
Further resolution of the results taking into account the
As previously demonstrated for other polyoxometalates,7 presence of the cis- and trans-isomers of cyclododecene
the Qq[NaH2M(NO2)W17F6O55] compounds can also be pre-
pared in situ under aerobic conditions using nitroethane as
solvent where Qq[NaH2M(H2O)W17F6O55] catalyses the for-
mation of NO•2 and CH3COOH; NO•2 then reacts with
Qq[NaH2M(H2O)W17F6O55] to yield Qq[NaH2M(NO2)W17F6O55].
This method has the disadvantage that NO•2 is continuously
formed during the reaction leading to its excess in the solu-
tion. In order to make an initial appraisal of the reactivity of
the various transition metal substituted polyfluorooxometa-
lates a comparative series of reactions was carried out using
cis-cyclooctene as substrate, Scheme 2 and Table 1.
Fig. 2 The effect of temperature on the aerobic epoxidation of cyclo-
dodecene. Reaction conditions:
5
mM Qq[NaH2CuII(NO2)W17F6O55];
Scheme 2 Reaction products for the aerobic epoxidation of cis-
400 mM cyclododecene; acetonitrile; 2 bar O2; t – 2 h; internal standard
cyclooctene.
dodecane.
Table 1 Epoxidation of cis-cyclooctene catalysed by Qq[NaH2M(H2O)W17F6O55
]
Epoxide, GC yield% Dialdehyde, NMR yield %
Nitroalkene, GC yield %
Others, GC yield%
M-PFOM
A
B
A
B
A
B
A
B
Fe(III)
Co(II)
Mn(II)
Cu(II)
36.8
47.9
44.5
50.0
44.5
13.6
15.8
29.6
5.1
13.5
0.2
1.4
0.9
0
0
23.0
28.6
8.7
13.5
11.3
31.8
37.2
29.5
9.4
9.5
8.3
0
0
0
0
0
0
Reaction conditions: A – 5 mM Qq[NaH2M(H2O)W17F6O55]; 200 mM cyclooctene; nitroethane; 2 bar O2; T – 110 °C; t – 2 h. B – 5 mM Qq[NaH2M
(NO2)W17F6O55]; 200 mM cyclooctene; acetonitrile; 2 bar O2; T – 110 °C; t – 2 h. Amounts are given as yields as determined by GC-FID analysis
using dodecane as internal standard. Others – various oxygenated compounds that were not identified.
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Table 2 Oxidation of alkenes
Epoxide,
Aldehyde,
GC yield %
Nitroalkene,
GC yield%
M-PFOM
Substrate
GC yield %
Mn(II)
4-Octene
1-Decene
1-Dodecene
1-Octene
4.5
1.2
1.0
0.3
5.2
1.5
1.7
0.8
0.6
0.6
0.6
1.2
Cu(II)
Fe(III)
Co(II)
4-Octene
1-Decene
1-Dodecene
1-Octene
8.5
1.2
1.0
1.0
3.0
1.5
1.7
2.8
0.9
0.6
0.6
1.4
4-Octene
1-Decene
1-Dodecene
1-Octene
1.7
0.7
0.7
0.2
2.5
2.6
2.4
0.2
0.6
0.5
0.7
0.3
4-Octene
1-Decene
1-Dodecene
4-Octene
8.9
0.9
0.4
4.5
2.0
2.2
0.2
5.2
3.0
1.4
0.3
0.6
Fig. 3 Reaction profile for the epoxidation of cis- and trans-cyclodo-
decene. Reaction conditions:
389 mM cyclooctene; nitroethane; 2 bar O2; T – 110 °C; internal stan-
dard dodecane.
5
mM Qq[NaH2CuII(H2O)W17F6O55];
5 mM Qq[NaH2Cu(NO2)W17F6O55]; 200 mM cyclooctene; acetonitrile;
2 bar O2; T – 110 °C; t – 2 h; internal standard dodecane.
initially present in a 2.8 : 1 cis to trans ratio reveals that cis-
cyclododecene initially reacts much faster than trans-cyclodo-
decene, Fig. 3 The initially slight increase in the concentration
of trans-cyclododecene, barring experimental error, may indi-
cate some isomerization of the cis isomer to the trans isomer.
The reaction is almost complete after 2 hours, whereupon
there is only a small change in the relative amount of the
oxides present. The formation of 1,12-dodecanedialdehyde
and 1-nitrocyclododecene combined represent about 25% of
the product yield after 4 h. One can conclude from this experi-
ment that cis-cyclododecene oxide is formed from cis-cyclo-
dodecene, pointing towards a fast oxygen transfer from the
Cu–NO2 moiety to the double bond of the less sterically hin-
dered isomer. C–C bond cleavage occurs already from the
beginning of the reaction and this is in contrast to our pre-
vious research using polyoxometalates that do not contain
fluorine atoms where such C–C bond cleavage did not occur.7
Attempts were made to react terminal alkenes and a rep-
resentative internal alkene, 4-octene, Table 2. The conversions
Scheme 3 Oxidation of chalcone by Q9[NaH2CuII(NO2)W17F6O55].
Reaction conditions: 5 mM Qq[NaH2M(NO2)W17F6O55]; 200 mM sub-
strate; acetonitrile; 2 bar O2; T – 110 °C; t – 2 h; internal standard
dodecane.
Indeed, only a slight amount of C–C bond cleavage was
observed.
Conclusions
observed were quite low with a selectivity of 2–3 : 1 for oxi- First row transition metal substituted polyfluorooxometalates
dation (epoxidation and C–C bond cleavage) versus nitration. of the quasi Wells–Dawson structure with a nitro terminal
A comparative reaction with a “sandwich” type polyoxometa- ligand on the transition metal Q9[NaH2MII(NO2)W17F6O55].
late, Q12[WZnCuII(NO2)2(W9O34)2] showed no conversion of (M = Cu(II), Mn(II) and Co(II)) are shown to be oxygen donors to
terminal alkenes.
alkenes. The Cu(II) analogue was the most reactive and selec-
A conclusion that can be drawn from the low reactivity of tive and research was therefore focussed on Q9[NaH2CuII(NO2)
the terminal linear alkenes versus the much higher reactivity of W17F6O55]. Notably, using O2 as terminal oxidant, cyclic
the cyclic alkenes is that the oxidation reaction is likely alkenes such as cyclododecene and cyclooctene are catalyti-
between an electrophilic Cu(II)–NO2 and an alkene that reacts cally epoxidised. The cis/trans mixture of cyclododecene reveals
as a nucleophile. This is also confirmed by the difference in that the epoxidation of the cis isomer in preference to the
conversion between internal alkenes and terminal ones. In epoxidation of the trans isomer presumably due to the higher
order to support this conclusion, chalcone, a β-keto alkene accessibility to the double bond in a M–NO(O)⋯= interaction.
and a weak nucleophile, but one that was expected to easily The reaction occurs with retention of the geometry of the
undergo C–C bond cleavage as a phenyl substituted substrate,6 double bond pointing towards a concerted mechanism. The
was reacted with Q9[NaH2CuII(NO2)W17F6O55], Scheme 3. polyfluorooxmetalates are more reactive than their recently
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reported non-fluorinated counterparts.7 This can be associated
with a higher electrophilicity of oxygen atom of the NO•2 ligand
due to the higher electron withdrawing properties of F versus O
trans to the M–NO2 moiety. Despite this improved reactivity,
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This research was supported by the Israel Science Foundation
grant #763/14.
Notes and references
‡K9[NaH2Zn(H2O)W17F6O55] (2 g) of were dissolved in 20 mL buffer acetate at
pH = 5 and heated to 80 °C. A solution of Cu(OAc)2·2H2O (0.2 g in 2 mL of
acetate buffer pH = 5) was added. The colour turned from coloress to green.
After stirring at 80 °C for 2 h the solution was cooled to RT and filtered. The
product was obtained from the filtrate by addition of ∼1 mL of a saturated solu-
tion of KCl and recrystallized from distilled water obtaining 1.5 g of product.
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§Qq[M(H2O)H2F6NaW17O55
]
was prepared by extracting 0.7
g Kq[M(H2O)
H2F6NaW17O55 in 20 mL water with
]
8
or 9 equivalents of trioctylmethyl-
ammonium chloride in dichloromethane. Then 300 mg Q9[M(H2O)
H2F6NaW17O55] were dissolved in 1 mL of DCM in a 50 mL flask. The flask was
closed with a septum and the air was evacuated by three freeze–pump–thaw
cycles. At 77 K, NO2(g) was introduced for 1–2 seconds. The solvent was evapor-
ated with gentle flow of Ar to yield oily products.
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