Clean protocol for deoxygenation of epoxides to alkenes: Via catalytic hydrogenation using gold

Article abstract of DOI:10.1039/d0cy01695k

The epoxidation of olefin as a strategy to protect carbon-carbon double bonds is a well-known procedure in organic synthesis, however the reverse reaction, deprotection/deoxygenation of epoxides is much less developed, despite its potential utility for the synthesis of substituted olefins. Here, we disclose a clean protocol for the selective deprotection of epoxides, by combining commercially available organophosphorus ligands and gold nanoparticles (Au NP). Besides being successfully applied in the deoxygenation of epoxides, the discovered catalytic system also enables the selective reduction N-oxides and sulfoxides using molecular hydrogen as reductant. The Au NP catalyst combined with triethylphosphite P(OEt)3 is remarkably more reactive than solely Au NPs. The method is not only a complementary Au-catalyzed reductive reaction under mild conditions, but also an effective procedure for selective reductions of a wide range of valuable molecules that would be either synthetically inconvenient or even difficult to access by alternative synthetic protocols or by using classical transition metal catalysts. This journal is

Full text of DOI:10.1039/d0cy01695k

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Clean protocol for deoxygenation of epoxides to
alkenes via catalytic hydrogenation using gold†
Cite this: DOI: 10.1039/d0cy01695k
*
Jhonatan L. Fiorio
and Liane M. Rossi
The epoxidation of olefin as a strategy to protect carbon–carbon double bonds is a well-known procedure
in organic synthesis, however the reverse reaction, deprotection/deoxygenation of epoxides is much less
developed, despite its potential utility for the synthesis of substituted olefins. Here, we disclose a clean
protocol for the selective deprotection of epoxides, by combining commercially available
organophosphorus ligands and gold nanoparticles (Au NP). Besides being successfully applied in the
deoxygenation of epoxides, the discovered catalytic system also enables the selective reduction N-oxides
and sulfoxides using molecular hydrogen as reductant. The Au NP catalyst combined with triethylphosphite
P(OEt)3 is remarkably more reactive than solely Au NPs. The method is not only a complementary Au-
catalyzed reductive reaction under mild conditions, but also an effective procedure for selective reductions
of a wide range of valuable molecules that would be either synthetically inconvenient or even difficult to
access by alternative synthetic protocols or by using classical transition metal catalysts.
Received 28th August 2020,
Accepted 3rd November 2020
DOI: 10.1039/d0cy01695k
rsc.li/catalysis
ReIO2(PPh3)2,19–21 and an oxoruthenium complex together
with PPh3, H2, or Na2SO3 as reductants.22 These methods lack
Introduction
The carbon–carbon double bond of alkenes is not often
described as central functional group of organic
versatility due to the intolerance towards other reducible
functional groups, reaction are mainly conducted using
stoichiometric amounts of reagents, and the catalysts suffer
from low activity, low atom efficiency, and tedious work-ups
with moisture-sensitive reaction conditions.21,23–25 Therefore,
the development of efficient catalytic systems for the chemo-
and stereoselective deoxygenation of epoxides with readily
available catalysts and practical reductants such as H2 under
mild reaction conditions to the corresponding alkenes, is
notably lacking in the arsenal of techniques available to
synthetic organic chemists.
Gold-based catalysts are plausible candidates aiming high
selectivity in the reductive deoxygenation of epoxides into
alkenes, due to their low propensity to reduce carbon–carbon
double bonds. Pure gold demonstrates very low or no activity
in hydrogenations, but the combination of gold nanoparticles
(Au NPs) with ligands has disclosed the successful
application of Au NPs as selective catalysts for hydrogenation
reactions.26–33 The activating effect associated to added
ligands is a result of the cooperative character that the ligand
plays upon adsorption on metal's surface decreasing the
unfavorable energetic barrier to H2 homolytic dissociation on
Au surface,34 by triggering a new channel to H2 heterolytic
dissociation on the ligand/Au interface.31,33,35–37 So far, the
catalytic systems formed by Au/ligand have already been
employed in the selective hydrogenation of alkynes to
alkenes,31,38–40 aldehydes to alcohols,28,30,41 including allylic
alcohols.26,27
a
synthesis,1,2 but it can be useful to obtain alcohols,
aldehydes, amines, diols, and epoxides, and more complex
molecules via reactions including olefin metathesis,
hydroformylation, oxidative cleavage, Diels–Alder reaction,
and Heck coupling reaction among others.3 Due to the
usually high number of steps in synthetic organic routes,
reliable protection–deprotection strategies for the carbon–
carbon double bond may find wide applications. Most
methodologies suffer the disadvantages of harsh reagents or
reaction conditions, or low chemoselectivity.4–6 Protecting
group strategies for alkenes include the epoxidation of
alkenes, followed by its regeneration using a range of
reducing condition, but the deoxygenation of epoxides in the
presence of other more reactive and reducible functional
groups remains a major challenge owing to a lack of
chemoselectivity.7–11 Efficient reduction of epoxides can be
done by using stoichiometric amounts of Li,12 metal halides/
Amberlyst15,13
diazomalonate/Rh2OAc4,17
oxorhenium complexes
SmI2,14,15
Me2PhSiLi,16
ZrCl4/NaI,18
high
including MeReO3
dimethyl
valent
and
Departamento de Química Fundamental, Instituto de Química, Universidade de
São Paulo, Av. Prof. Lineu Prestes, 748, 05508-000, São Paulo, SP, Brazil.
E-mail: lrossi@iq.usp.br
† Electronic supplementary information (ESI) available. See DOI: 10.1039/
d0cy01695k
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To date, several heterogeneous catalysts have been
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recovered by filtration and thoroughly washed twice with hot
water (60 °C), acetone and dried in vacuum. The catalysts
contain 3.5 wt% of gold as determined by FAAS and the mean
diameter was 2.3 0.6 nm. The non-magnetic and magnetic
catalysts behave similarly and the magnetic version was used
on the recycling studies to facilitate catalyst recovery.
reported for deoxygenation reactions using H2, CO/H2O or
alcohols as reductants. These catalysts, however, fall shortly
due to the lack of selectivity for alkenes and limited scope of
substrates, especially when it comes to stereospecific
deoxygenation of epoxides, usually a mixture of cis and trans
alkenes are obtained.9,24,42–47 Metals, such as Pd and Pt, are
well-known to catalyze the ring-opening (hydrogenolysis) of
epoxides into the corresponding alcohols,48–56 or when the
alkene is formed, the over hydrogenation of the product is
difficult to prevent.8,57
Herein, we demonstrate that the Au NP catalyst combined
with triethylphosphite, P(OEt)3, is remarkably more reactive
than solely Au NPs for the deoxygenation of epoxides to
alkenes. We achieved excellent catalytic activity for a broad
scope of substrates while keeping high selectivity, e.g., intact
carbon–carbon double bonds due to the lack of reactivity of
gold to reduce the alkene.
Catalytic deoxygenation reactions. A typical procedure for
the deoxygenation reactions is as follows: substrate (1
mmol), ligand (0.02 mmol), Au/SiO2 (1 mol%) and 2 mL of
solvent were added to a modified Fischer–Porter 100 mL
glass reactor in a glove box. The reactor was removed from
the glovebox and purged five times with H2, leaving the
vessel at 6 bar. The resulting mixture was vigorously stirred
using Teflon-coated magnetic stir bar, temperature was
maintained at the desired temperature with an oil bath on
a hot stirring plate connected to a digital controller (ETS-D5
IKA). After the desired time, the catalyst was removed by
centrifugation and the products were analyzed by GC with
an internal standard to determine the conversion of
substrate and the selectivity for the product. Usually, the
conversion was constantly monitored by GC, and to
determine the isolated yield of the hydrogenated products,
the crude reaction mixture was purified by silica gel column
chromatography (hexane/AcOEt mixtures) to afford the
corresponding final product. All the experiments were
performed multiple times to confirm reproducibility, no
significant difference was observed in the conversion and
selectivity among the runs.
Recycling experiment. A magnetic version of the catalyst
was used for the recycling experiments, as previously
described.31 After each catalytic experiment under the above
typical reaction conditions, the catalyst was recovered by
putting a magnet on the wall of the reactor the used catalyst
was washed two times with isopropyl alcohol, dried in
vacuum and subsequently used in the next reaction. The
product obtained after magnetic separation was analyzed by
GC with an internal standard to determine the conversion
and selectivity.
Scale-up hydrogenation of trans-stilbene oxide. Trans-
stilbene oxide (20 mmol), Au/SiO2 (0.054 mol%),
triethylphosphite (0.054 mol%), and 10 mL of isopropyl
alcohol were added to a 75 mL stainless steel reactor (Series
5000 multiple-reactor system, Parr Instrument Company).
After sealing the reactor and degassing internal air, H2 was
introduced until reaching the pressure of 30 bar. Then, the
mixture was stirred (1000 rpm) at 100 °C for 24 h. The crude
reaction mixture was purified by silica gel column
chromatography (hexane/AcOEt mixtures) to afford the
corresponding final product.
Product analysis of catalytic experiments. GC analyses
were carried out with a Shimadzu GC-2010 equipped with a
RTx-5 column (30 m × 0.25 mm × 0.25 mm) and a FID
detector. Method: Ti = 40 °C, Tf = 200 °C, 17 min. The
conversion and selectivity were determined based on peak
areas of GC calibrated with an internal standard, usually
biphenyl. The internal standard was added to the diluted
Experimental section
General information
All the reagents used for the support and catalyst preparation
were of analytical grade, purchased from Sigma-Aldrich and
used as received. Tetrachloroauric(III) acid was purchased as
a 30 wt% aqueous solution in dilute HCl (Sigma-Aldrich).
Commercial reagents were purchased from Sigma Aldrich,
and used as received. The glass reactor was thoroughly
cleaned with aqua regia (HCl : HNO3 = 3 : 1 v/v), rinsed with
copious pure water, and then dried in an oven prior to use.
Gold content in the catalyst was measured by FAAS analysis,
on a Shimadzu AA-6300 spectrophotometer using an Au
hollow cathode lamp (Photron). Metal leaching into the
supernatant solution was measured by inductively coupled
plasma measurements, performed on a Spectro Arcos ICP-
OES. TEM analyses were performed with a JEOL 2100.
Catalyst samples for TEM were prepared by sonicating the
catalyst powder in propan-2-ol. A drop of the resulting
dispersion was placed on a carbon-coated copper grid (Ted
Pella, Inc.). The histogram of nanoparticle size distribution
was obtained from the measurement of about 200 particles.
Experimental procedures
Preparation of Au/SiO2 and Au/Fe3O4@SiO2. Gold
nanoparticles supported on silica nanospheres (SiO2) or
silica-coated magnetic nanoparticles (Fe3O4@SiO2) both
obtained by means of a reverse microemulsion were prepared
by the deposition–reduction method reported elsewhere.31
Briefly, in a typical method 1.0 g of the amino-modified
silica-based support was added to aqueous solution of
HAuCl4 (20 mL, 1.3 × 10−2 mol L−1) under continuous
stirring. After 30 min, the mixture was centrifuged and the
recovered solid was dispersed in 10 mL of water.
Subsequently, under vigorous stirring an as-prepared cold
aqueous NaBH4 solution (10 mL, 0.2 mol L−1) was dropwise
added. After further stirring for 20 min, the solid was
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organic phase after separation of the catalyst, and then the
total amount of detected products and reactant was
calculated according to the following equations.58
In fact, the catalytic activity of the Au/SiO2 catalyst varied
significantly
depending
on
the
identity
of
the
organophosphorus ligand added to the reaction mixture
(Table 1). The addition of alkyl-phosphine ligands to gold,
such as PCy3 and PnBu3 (Table 1, entries 7 and 9), increased
the conversion to 80–90%. In contrast, the addition of aryl-
phosphine PPh3 (Table 1, entry 6) resulted in the same
conversion as pure gold (23%). Additionally, phosphites were
also tested as ligand, since the oxygen atom insulates the
phosphorus (and therefore the metal to which it is attached)
from steric and electronic effects arising from substituent
modification.59 While the triphenylphosphite (Table 1, entry
10) was detrimental to reactivity, alkyl phosphites (Table 1,
entries 10 and 11) afforded an improved reactivity of the
gold catalyst. In contrast, tertiary phosphine oxide or
organophosphate ligands did not show the same activating
effect of the parent ligands (Table 1, entries 12–14).
½Substrateꢀin − ½Substrateꢀout
Conversion ð%Þ ¼
× 100
½Substrateꢀin
½Productꢀout
Seletivity ð%Þ ¼
× 100
½Substrateꢀin × conversion ðfractionalÞ
where [Substrate]in, [Substrate]out, [Product]out represent the
inlet concentration of the analyzed substrate and the outlet
concentrations of substrate and product, as determined by
the GC analysis.
Results and discussion
A systematic survey of parameters such as the effect of the
amount of added ligand, solvent, temperature and H2
pressure led to the optimized reaction conditions (Table S1–
S4†). Lowering the amount of added phosphite from 2 to 1
mol% molar equivalent did not decrease the catalytic activity
(>99% conversion) and selectivity to the corresponding
alkene, but a decrease in the reaction conversion to 75% was
noticed after further reducing the amount of ligand to 0.5
mol% (Table S1†). A series of blank experiments (Table 1,
entries 3–4) indicate that the ligand is not acting as a
reductant as previously reported for rhenium21 and
molybdenum11 catalysts. Even the stoichiometric reaction of
1a with P(OEt)3 (1 mmol of ligand and 1 mmol of epoxide)
showed no conversion after 24 h. In summary, the Au NP-
catalyzed reaction can be conducted with only 1 mol% of
P(OEt)3 in i-PrOH. Most of the catalytic tests were conducted
at 100 °C and 6 bar of H2, but the hydrogenation reaction
can be conducted at lower temperature and pressure.
The selective deoxygenation of styrene epoxide 1a was chosen
as benchmark reaction to investigate the effect of the
organophosphorus ligand on the catalytic activity of gold
nanoparticles supported on silica. The Au/SiO2 catalyst
without ligand reached 23% of conversion (Table 1, entry 1)
after 6 h of reaction (1 mmol of epoxide, 1 mol% of Au, 2
mL of i-PrOH, 100 °C,
conditions, triethylphosphite (P(OEt)3),
6
bar of H2). Under similar
commercially
a
available, cheap and quite stable to air-phosphite, was added
as an auxiliary ligand to gold, promoting full conversion of
styrene epoxide 1a (Table 1, entry 2). The desired
deoxygenate product was delivered with 90% isolated yield.
Table 1 Reaction optimization studiesa
The timepoint studies illustrate that the catalyst system
Au NP and P(OEt)3 affords higher reaction rates when
compared to the reaction without ligand (Fig. 1a). Moreover,
the catalyst was easily separated from the reaction mixture,
washed, and reused after adding new portions of substrate
and P(OEt)3. By running kinetic curves, no loss of catalytic
efficiency was noticed in five successive reactions (Fig. 1a).
To gain insight into the catalytic active species, a hot
filtration test was performed (Fig. 1b). No conversion took
place after the Au/SiO2 catalyst was filtered off, indicating
that the catalysis was heterogeneous in nature, and the
catalytic activity and selectivity is not related to the formation
of homogenous species. To corroborate with this, no gold
content was detected by ICP-AES analysis of the reaction
product. A homogeneous gold catalyst, such as Au(PPh3)Cl,
was tested under similar conditions and was not able to
catalyze the hydrogenation either. Comparing the
morphology and size of as-prepared catalyst (Fig. 1c) with the
recycled catalyst (Fig. 1d) by TEM images revealed only a
slight increase of the mean diameter size from 2.3 0.6 nm
to 3.2 1 nm after five recycles.
Conversion of Selectivity to
Entry Ligand
1a (%)
2a (%)
1
No ligand
23
100
0
>99
>99 (90%)
—
2
P(OEt)3
P(OEt)3
P(OEt)3
3b
4c
5
0
—
PPh3 instead of P(OEt)3
PCy3 instead of P(OEt)3
Dppe instead of P(OEt)3
PBu3 instead of P(OEt)3
P(OPh)3 instead of P(OEt)3
P(OMe)3 instead of P(OEt)3
P(OBu)3 instead of P(OEt)3
OP(OEt)3 instead of P(OEt)3
OPCy3 instead of P(OEt)3
PPh3O instead of P(OEt)3
23
86
89
94
12
81
86
21
24
25
>99
>99
>99
>99
>99
>99
>99
>99
>99
>99
6
7
8
9
10
11
12
13
14
a
Reaction conditions: 1 mmol of epoxide, 1 mol% of Au, 2 mol% of
ligand (when added), 2 mL of i-PrOH, 100 °C, 6 bar of H2, 6 h.
Conversion and selectivity were determined by GC analysis using
biphenyl as an internal standard. Isolated yield is shown in
b
c
parenthesis. Au catalyst was not added. Reaction was performed
under N2 atmosphere.
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Fig. 1 a) Timepoint study and recycles of Au/SiO2 magnetic catalyst with and without P(OEt)3 as ligand for the hydrogenation of styrene epoxide
1a; b) hot filtration test (black dots/traces represent the reaction conducted in the presence of Au/SiO2 catalyst and red dots/traces after catalyst
removal); c) TEM image and size distribution of fresh Au/SiO2 catalyst (scale bar, 100 nm) and d) after the fifth reuse (scale bar, 25 nm). Reaction
conditions: 1 mmol of epoxide, 1 mol% of Au, 1 mol% of P(OEt)3, 2 mL of i-PrOH, 100 °C, 6 bar of H2, 6 h.
The catalytic system (Au/SiO2 + P(OEt)3) was able to
catalyze the deoxygenation of a wide range of epoxides
(Table 2), affording the corresponding alkenes in good to
excellent yields, while hydrogenation of the alkene products
did not occur. Styrene oxide substituted with methoxy group
2b was completely converted into the desired alkene. Benzylic
epoxides could also be deoxygenated with yield over 75% of
alkene (2c and 2d). Aliphatic epoxides were converted into
the corresponding alkenes in high yields (2e–2j). Importantly,
halides, hydroxy, ketone and alkene moieties remained intact
during the deoxygenation of epoxides (2f–h, 2m, 2n, and 2i).
Deoxygenation of α,β-epoxy carbonyl compounds occurred
smoothly to the unsaturated α,β-carbonyl compound 2o. The
Encouraged by these results, we became interested in the
deoxygenation of series of heterocyclic N-oxides to
N-heteroarenes, an important class of heterocycles key
structures in wide range of pharmacologically and
a
a
biologically relevant natural products.60 Deoxygenation of a
representative selection of heterocyclic N-oxides using H2 as
reductant revealed to be compatible with the catalytic system
affording the corresponding reduced products in good to
excellent yields. Notably, substrates possessing reducible
functionalities such as halides, cyano, and hydroxyl were
also chemoselectively reduced to the corresponding
N-heterocycles (Table 3). Furthermore, to investigate the
scope of the deoxygenation ability of the present catalytic
system, we next applied this system to deoxygenation of
sulfoxides (Table 4). Phenyl, benzylic, and aliphatic
sulfoxides were converted into the corresponding sulfides in
yields over 70%.
deoxygenation proceeded in
a
stereospecific fashion:
trans-stilbene oxide to trans-stilbene (2p) and cis-epoxides to
cis-alkenes (2q and 2r). Moreover, 25-hydroxy-4,5-epoxy-
cholestan-3-one, undergo the reaction in good yield,
furnishing to 25-hydroxycholestenone (2s), the biologically
active compound responsible for the regulation of calcium
and phosphate ion concentration in cells.
The utility of this protocol was also demonstrated under
scaled-up conditions (eqn (1)); 1o (20 mmol; 3.92 g) was
successfully converted to the alkene, 2o (90% isolated yield;
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a
Table 2 Chemoselective reduction of epoxides to alkenes catalyzed by Au/SiO2 and P(OEt)3
a
Reaction conditions: 1 mmol of epoxide, 1 mol% of Au, 1 mol% of P(OEt)3, 2 mL of i-PrOH, 100 °C, 6 bar of H2. Conversion and selectivity
were determined by GC (selectivity was >99%). Isolated yields in parenthesis. b 8 bar. c Determined by H NMR. d 2 mol% of Au, 10 bar of H2.
1
Table 3 Selective deoxygenation of pyridine N-oxides catalyzed by Au/
a
SiO2 and P(OEt)3
Table 4 Hydrogenation of sulfoxidesa
a
Reaction conditions: 1 mmol of epoxide, 1 mol% of Au, 1 mol% of
P(OEt)3, 2 mL of i-PrOH, 100 °C, 6 bar of H2. Conversion and
a
selectivity were determined by GC (selectivity was >99%). Isolated
Reaction conditions: 1 mmol of sulfoxide, 0.01 mmol of Au, 1
mol% of P(OEt)3, 2 mL of i-PrOH, 100 °C, 10 bar of H2.
b
yields in parenthesis. 8 bar.
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3.21 g), importantly, no side product of isomerization was
detected. A turnover number (TON) of 1800 was reached with
a productivity of approximately 75 mol mol−1 h−1, and the
catalyst can be recycled as demonstrated above.
this useful protocol, as well as continue with more detailed
mechanism studies.
Conflicts of interest
The authors declare no competing interests.
Acknowledgements
The authors are grateful to INCT-Catálise/FAPESC/CNPq/
CAPES (CNPq 465454/2014-3; 444061/2018-5) and FAPESP
(2016/16738-7, 2018/26253-6) for financial support, and CNPQ
and CAPES for fellowships.
Although the mechanistic aspects of our gold catalyzed
hydrogenation reactions remain somewhat speculative at this
time, hydrogen activation might be occurring via a heterolytic
pathway that happens due to a cooperative effect among the
gold surface, the ligand and, possibly, the substrate. The
concerted mechanism leading to heterolytic activation of H2
on gold surface has already been proposed to be effective in
the hydrogenation of carbonyl groups using gold
catalysts.26,27,35 The hydrogenation resulted from a combined
effect of the ligand employed, usually secondary phosphine
oxides, the gold surface catalyst, and also the substrate
(α,β-unsaturated aldehydes), which was hydrogenated via
the heterolytic cleavage of H2 and its concerted addition
to the CO bond to form the corresponding alcohol.
The activity was governed by the basicity between ligand
and reactant and the sites available on the nanoparticle
surface. Here, we have expanded the practical applicability
of gold catalysts possibly via a catalytic cycle where H2 is
activated at the ligand–gold NP surfaces interface leading
to the heterolytic splitting of H2 molecules at mild
conditions; most important for epoxide to alkene
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