Deoxydehydration using a commercial catalyst and readily available reductant

Article abstract of DOI:10.1016/j.inoche.2018.11.015

We have explored the use of a commercially available molybdenum compound, ammonium heptamolybdate (AHM) in catalyzing the deoxydehydration (DODH) reaction using sodium sulfite (Na₂SO₃) as reductant. The catalytic reactions were effected at relatively lower temperatures (170–190 °C) yielding the alkenes in moderate yields.

Full text of DOI:10.1016/j.inoche.2018.11.015

Accepted Manuscript
Deoxydehydration using a commercial catalyst and readily
available reductant
Christine A. Navarro, Alex John
PII:
S1387-7003(18)30810-4
DOI:
Reference:
https://doi.org/10.1016/j.inoche.2018.11.015
INOCHE 7179
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Inorganic Chemistry Communications
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1 September 2018
1 November 2018
16 November 2018
Please cite this article as: Christine A. Navarro, Alex John , Deoxydehydration using
a commercial catalyst and readily available reductant. Inoche (2018), https://doi.org/
10.1016/j.inoche.2018.11.015
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ACCEPTED MANUSCRIPT
Deoxydehydration Using a Commercial Catalyst and Readily Available
Reductant
Christine A. Navarro and Alex John*
Chemistry and Biochemistry Department
California State Polytechnic University, Pomona
3801 West Temple Ave., Pomona CA 91768.
Email: ajohn@cpp.edu
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Graphical Abstract for Table of Contents
Abstract
We have explored the use of a commercially available molybdenum compound,
ammonium heptamolybdate (AHM) in catalyzing the deoxydehydration (DODH) reaction using
sodium sulfite (Na2SO3) as reductant. The catalytic reactions were effected at relatively lower
temperatures (170-190 C) yielding the alkenes in moderate yields.
Key Words
Deoxydehydration, Sustainability, Molybdenum, Glycols, Olefins.
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Introduction.
Petroleum as a source of material feedstock is due to its unsustainable nature as well as the
environmentally harmful nature of the refining processes involved. Hence, processes capable of
transforming biomass, a sustainable resource, into renewable substitutes for petrochemicals are in
high demand.1 The deoxydehydration (DODH) reaction is particularly unique in this respect, as it
is a single step transformation that converts a vicinal diol into an olefin moiety, which is a highly
versatile functional group.2 The reaction is catalyzed by highvalent oxometal (MOx) catalysts
and requires a stoichiometric reductant since the reaction is a formal reduction. Early work3 and
prominent catalytic methods are based primarily on oxo-rhenium (ReOx) compounds including
commercially available ones like MeReO3, NH4ReO4, NaReO4, Cp*ReO3 etc.4 A downfall to
using these catalysts is their exorbitant cost which renders them unsuitable for bulk industrial
processes. The use of supported rhenium catalysts has also been widely explored to take advantage
of their recyclability properties.5 Given the high cost of rhenium, homogeneous and supported
catalysts based on other less expensive metals, especially V6 and Mo7, have also been explored in
+

the DODH reaction. Oxo-vanadium complexes of the type Z LVO2 developed by Nicholas and
coworkers6a based on the dipicolonate ligand are notable for their high reactivity, although at
higher catalyst loadings. Apart from these homogeneous complexes, commercially available

metavanadate (VO3 ) compounds were also found to be active as demonstrated in the conversion
of glycerol to allyl alcohol in the absence of an external reductant.8 Molybdenum catalysts studied
thus far also include homogeneous complexes, supported catalysts, and those based on commercial
compounds like ammonium heptamolybdate (AHM).7 For example, the Fristrup group first
reported AHM catalyzed DODH of glycols in dodecane or 1,5-pentanediol as the solvent at 195-
220 C.7b Representative diols including glycerol were converted to the corresponding alkenes in
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up to 45 % yield (Figure 1(a)). Detailed investigation of the catalytic mixture revealed that the diol
substrate was acting as a sacrificial reductant in this system. To circumvent the loss of diol as a
reductant, the same group later reported on the use of AHM to convert glycols into olefins using
isopropanol as both the solvent and reductant at 240-250 C over 12-18 h.7c Under these conditions,
1,2-decanediol was converted to 1,2-decene in 49 % yield; a higher yield (55 %) was obtained in
the presence of strong base (15 mol % Bu4NOH) (Figure 1(b)). In the case of C6 diols, 1,2-hexane
diol (46 % 1-hexene) and 3,4-hexanediol (42 % 3-hexene), the addition of Bu4NOH (15 mol %)
resulted in alkene yields as high as 77 % and 69 % respectively.
The reductant being a crucial component of the DODH reaction, its scope has been
developed to include phosphines,3a elemental metals,9 alcohols,10 hydroaromatic compounds,11
reducing gases such as H2 and CO,6b and inorganic salts etc.12 The choice of the reductant is critical
as it affects the overall efficiency and practical appeal of the reaction. Although secondary alcohols
and PPh3 exhibit superior reactivity, they are relatively expensive and result in side products that
need to chromatographically/laboriously separated and/or recycled. The diol itself can also be used
as a reductant; however this leads to the formation of aldehyde side products (from oxidative
cleavage) that subsequently form acetal side-products by engaging the diol substrate.7b An
inorganic salt such as sodium sulfite (Na2SO3) is a benign reductant and its oxidized byproduct
2-
2-
(SO4 ) is nontoxic and recyclable. Sulfite (SO3 ) is comparable to other reductants
thermodynamically when looking at the H for the DODH of polyols.12 The Nicholas group
demonstrated that sulfite-driven deoxydehydration of polyols to olefins catalyzed by MeReO3 and
-
Z+ReO4 derivatives (Z = Na, Bu4N) to be an efficient process yielding the olefins in moderate to
good yields (2560 %).12
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Figure 1. Ammonium heptamolybdate (AHM) in deoxydehydration (DODH) of glycols.
In light of prior successes documented in effecting the deoxydehydration reaction employing the
cheap and commercially available ammonium heptamolybdate (AHM) using alcohols (including
the diol substrate) as reductant, we became interested in expanding the potential of this catalyst.
Specifically, we were intrigued by the possibility of pairing AHM with the readily available
reductant, Na2SO3. Herein, we report our findings on deoxydehydration reaction of several glycols
catalyzed by AHM (5 mol %) using Na2SO3 as an economical and benign reductant at 170190
C (Figure 1(c)).
Results and Discussion:
We began our investigation by screening different reductants (1.5 equiv.) in the
prototypical conversion of styrene glycol to styrene in toluene as solvent at 170-190 C (Scheme
1 & Table 1). Specifically, PPh3, 3-octanol, and inorganic reductants such as Na2SO3, KI, Fe,
NaHSO3 etc. were screened initially in the DODH reaction at 5 mol % loading of AHM (Table 1,
entry 1-6). Both PPh3 and Na2SO3 were found to be more active (31 & 18 % respectively) than 3-
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octanol (2 %) and NaHSO3 (10 %). The use of Fe filings and KI furnished higher yields (23 % &
24 %) but signals characteristic of an aldehyde (benzaldehyde) and acetal were detected in the
reaction mixture upon 1H NMR analysis. The detection of these side products is suggestive of the
diol substrate acting as the reductant.7b When the reaction temperature was increased to 190 C, a
marginal improvement (23 %) in styrene yield was observed with Na2SO3 while it dropped with
PPh3 (19 %) as reductant (Table 1, entries 7 & 8). However, styrene yield dropped (9 %) upon
further increase (210-220 C) in the reaction temperature (Table 1, entry 10). The benign reductant
Na2SO3 was favored over other competing reductants. Although isopropanol has been
demonstrated to be effective as a solvent and reductant in the AHM catalyzed DODH reaction at


higher temperatures (240-250 C) we did not detect any alkene in our reactions, even at 190 C
(Table 1, entry 9).
Scheme 1. AHM catalyzed deoxydehydration driven by Na2SO3.
Various glycols were then tested to explore the substrate scope under the catalytic
conditions developed. The DODH of aliphatic glycols provided moderate yields when subjected
to the reaction conditions (170 C, 1.5 mmol Na2SO3 and 5 mol% AHM) for 24h. Specifically,
1,2-octanediol produced 6 % of 1-octene (Table 1, entry 11) while the longer chain 1,2-decnediol
produced 1-decene in 11 % yield (Table 1, entry 13). Increasing the reaction temperature (190 C)
resulted in enhanced reactivity yielding 1-octene and 1-decene in 13 % and 21 % yields
respectively (Table 1, entry 12 & 14). To achieve more insight into the pathway of the DODH, we
chose a stereospecific acyclic substrate (+)-diethyl L-tartrate. When (+)-diethyl L-tartrate
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underwent sulfite driven DODH with 5 mol% AHM at 170 C, diethyl fumarate which is the
corresponding trans-alkene was produced at 14% yield, which increased to 20 % at 190 C (Table
1 entry 15 & 16). Preferential generation of the trans-alkene product in this reaction has been
suggested to be an indication of cis-elimination during the DODH process. In the resulting reaction
mixture traces of diethyl maleate as well as considerable amounts of ethanol were also
Table 1. AHM-catalyzed DODH of vicinal diols.a
T (C)
entry
substrate
reductant
yieldb (%)
1.
2.
PPh3
Na2SO3
3-octanol
Fe filings
KI
31
18
2
3.
170
4.
23
24
10
19
23
-
5.
6.
NaHSO3
PPh3
7.
8.
Na2SO3
iPrOH
190
9.c
10.d
11.
12.
Na2SO3
210-220
170
9
6
Na2SO3
190
13
13.
14.
170
190
11
21
Na2SO3
Na2SO3
15.
16.
17.
18
170
190
190
170
170
14
20
16
13
15
PPh3
KI
19
NaHSO3
20.
Na2SO3
190
5
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aReaction conditions: 1.0 mmol glycol, 1.5 mmol reductant and 5 mol% of AHM in toluene (ca.
5 mL) in a sealed thick walled pressure tube at 170 or 190 C (oil bath/aluminum metal block).
bYields were determined by 1H NMR spectroscopy using 1,3,5-trimethoxybenzene as an internal
standard. cUsing iPrOH as solvent and reductant. dCarried out in a sand bath, yield averaged over
two runs.
1
detected by H NMR spectroscopy. The formation of ethanol under the reaction conditions
suggests that ester hydrolysis is prevalent under the catalytic conditions. This is in stark contrast
with earlier studies using perrhenate catalysts {MeReO3 & (Bu4N)ReO4} which did not detect any
ester hydrolysis and presumably is due to the acidic (+NH4) counter ion in the catalyst. Epoxides
have also been deoxygenated to alkenes using the CH3ReO3/Na2SO3 system, and more recently
using a MoO2(dtc)2 {dtc = dithiocarbamate} catalyst using a secondary alcohol (2,3-dimethyl-3-
pentanol) as a sacrificial reductant.13 However, when styrene oxide was subjected to the catalytic
conditions using AHM/Na2SO3 system, only trace amounts (5 %) of the olefinic product was
detected suggesting that epoxides are not suitable substrates for the reaction (Table 1, entry 20).
Encouraged by the promising results reported by the Fristrup group on the beneficial effect
of adding a strong base (15 mol % of Bu4NOH) in improving yields as well as product selectivity,
we tested catalytic outcomes in the presence of bases such as NaOAc and pyridine. Unfortunately,
these additives were seen to hinder catalytic activity, and produced 1-decene in lower yields (Table
2). The inorganic base, NaOAc, was found to have a more detrimental effect on the reaction yield
as compared to the organic base, pyridine.
Table 2. Comparison of additives in the DODH of 1,2-decanediol to 1-decene.a
entry
additive
NaOAc
pyridine
pyridine
mol % yieldb (%)
1
2
3
50
25
50
<1
15
12
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aReaction conditions: 1.0 mmol glycol, 1.5 mmol Na2SO3 and 5 mol% of AHM in 5 mL of
toluene at 190 C (oil bath) in a sealed thick-walled pressure tube. bYield determined by 1H
NMR spectroscopy using 1,3,5-trimethoxybenzene as an internal standard.
The mechanism for transition metal catalyzed deoxydehydration reactions are still under
active investigation. In case of molybdenum catalyzed reactions, computational investigations
suggest that either the reduction step or alkene extrusion can be rate-limiting.14,7c A plausible
mechanism for AHM catalyzed deoxydehydration driven by Na2SO3 as a reductant could involve
(a) an initial condensation of the glycol with the oxo-molybdenum species to generate the Mo(VI)-
2-
glycolate intermediate. The glycolate could then be (b) reduced by SO3 to yield the Mo(VI)-
glycolate which then (c) releases the olefin product and regenerates the high-valent oxo-
molybdenum species (Figure 2).
Figure 2.Proposed mechanism for AHM catalyzed sulfite driven deoxydehydration.
Conclusion
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In summary, we explored the potential of using Na2SO3 as a readily available and benign reductant
in effecting AHM catalyzed deoxydehydration of glycols to olefins. Catalytic reactions were
performed at relatively milder conditions (170-190 C) compared to other related studies (195-250
C), and resulted in moderate yields (up to 23 %). Substrates featuring ester functionality
underwent hydrolysis under the reaction conditions in contrast to earlier reports with rhenium
catalysts. Efforts are underway in our lab to expand the scope of molybdenum catalyzed
deoxydehydration reactions.
Acknowledgments
Funding for this work was provided by the Chemistry & Biochemistry Department & College of
Science at California State Polytechnic University, Pomona. Funding from National Science
Foundation (MRI-1040566) is also acknowledged.
Experimental Section
General Procedures: All reagents including AHM, diols, PPh3, Na2SO3 etc. were obtained
commercially and used without further purification. ACS grade solvents were purchased and used
with further purification as well. All 1H and 13C{1H} spectra were collected on a Varian 400-MR
spectrometer and calibrated to the residual protonated solvent at  7.27 for deuterated chloroform
(CDCl3).
Representative procedure for deoxydehydration reactions.
A flame dried thick-wall Chemglass high-pressure tube was charged with the styrene glycol (0.140
g, 1.01 mmol), AHM (0.061 g, 0.05 mmol), reductant (1.5 mmol) and toluene (ca. 5 mL) followed
by a Teflon-coated stir bar. The pressure tube was sealed with a Teflon screw cap and then placed
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in a pre-heated oil bath or aluminum heating block which are both equipped with a digital
temperature controller. The pressure tube was heated at 170C or 190 C for 18-24h with stirring.
The resulting reaction flask is then pulled out of the oil bath and allowed to cool to room
temperature. At this point, 1,3,5-trimethoxybenzene (0.010 g) was introduced into the flask and
the mixture stirred at room temperature to get a uniform solution. An aliquot was taken by pipette
and analyzed by 1H NMR spectroscopy using 1,3,5-trimethoxybenzene as an internal standard.
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Highlights
1. Use of commercially available catalyst
2. Cheap and benign reductant
3. Relatively mild reactions conditions.
4. Moderate yields
14