Carbon Monoxide (CO)- and Hydrogen-Driven, Vanadium-Catalyzed Deoxydehydration of Glycols

Article abstract of DOI:10.1021/acscatal.5b02667

Four oxo-vanadium complexes of the type Z⁺LVO₂^- (1-4) have been evaluated for activity as catalysts for the deoxydehydration (DODH) of glycols to olefins with various reductants. Among these, a new complex, [Bu₄N](Salhyd)VO₂ (4), is found to be uniquely effective for the DODH reaction using the practical reductants: hydrogen and carbon monoxide (CO).

Full text of DOI:10.1021/acscatal.5b02667

Letter
pubs.acs.org/acscatalysis
Carbon Monoxide (CO)- and Hydrogen-Driven, Vanadium-Catalyzed
Deoxydehydration of Glycols
Tirupathi V. Gopaladasu and Kenneth M. Nicholas*
Department of Chemistry and Biochemistry, Stephenson Life Sciences Research Center, University of Oklahoma, 101 Stephenson
Parkway, Norman, Oklahoma 73019, United States
S
* Supporting Information
ABSTRACT: Four oxo-vanadium complexes of the type
Z+LVO2 (1−4) have been evaluated for activity as catalysts
−
for the deoxydehydration (DODH) of glycols to olefins with
various reductants. Among these, a new complex, [Bu4N]-
(Salhyd)VO2 (4), is found to be uniquely effective for the DODH reaction using the practical reductants: hydrogen and carbon
monoxide (CO).
KEYWORDS: deoxydehydration, glycol, oxo-vanadium, carbon monoxide, hydrogen
he search for chemical processes to convert renewable
biomass-derived feedstocks to chemicals and fuels has
stimulated efforts to discover and develop new, effective
reactions that transform C−O bonds, enabling the refunction-
alization of these polyoxygenate materials.1 Historically,
dehydration and, to a lesser extent, hydrodeoxygenation
processes have been the most investigated.2
Scheme 2. Optional Catalytic Pathways for
Deoxydehydration
T
More recently, another transformation of polyols, deoxy-
dehydration (DODH), in which two vicinal hydroxyl groups
are eliminated to produce C−C unsaturation (Scheme 1), has
Scheme 1. Oxo-rhenium-Catalyzed Deoxydehdyration
(DODH)
received considerable attention.3 From the initial report of
catalytic DODH by Andrews and Cook, using PPh3 as the
reductant and (C5Me5)ReO3 as catalyst,4 most subsequent
studies have utilized oxo-rhenium compounds as precatalysts
with a growing list of reductants, including H2,5 sulfite,6
secondary7 and benzylic alcohols,8 elements (Zn, Fe, Mn, C),9
and hydroaromatics.10 In these reports, a simplified catalytic
scheme typically has been proposed (Scheme 2) involving three
stages, via either of two different reaction sequences (paths A
and B). In path A, the oxo-metal is first reduced to A, which
condenses with the glycol to give the reduced metal-glycolate
B; in path B, the glycol first condenses with the oxo-metal to
give metal-glycolate C, followed by reduction of C to B. Finally,
the reduced glycolate B undergoes retrocyclization (oxidative
elimination) to produce the olefin and regenerate the oxidized
precatalyst.4,11 Fragmentary experimental results and computa-
tional studies are somewhat conflicting as to which path (A or
B) dominates and which step is turnover-limiting,12 probably
because these issues are both reductant- and catalyst-depend-
ent.
The high cost of rhenium compounds prompted us to seek
more economical catalysts and practical reductants for the
DODH reaction and to develop catalyst structure/activity/
selectivity relationships and deeper mechanistic understanding.
In an initial effort in this regard, we examined various
metavandates and Z+(2,6-pyridine dicarboxylate)VO2− complex
1 as DODH catalysts. The latter was found to be especially
effective for the DODH reaction13,14 and for epoxide
deoxygenation with the reductants PPh3 and sulfite (Scheme
3).15 We note also that others have reported modest DODH
activity for some oxo-molybdenum species with i-PrOH or the
glycol itself as reductants.16
Received: November 24, 2015
Revised: February 9, 2016
© XXXX American Chemical Society
1901
DOI: 10.1021/acscatal.5b02667
ACS Catal. 2016, 6, 1901−1904

ACS Catalysis
Letter
Scheme 3. Oxo-vanadium-Catalyzed Deoxydehydration
Table 1. Oxo-vanadium-Catalyzed DODH Reactions of
a
Styrene Glycol
entry
catalyst
reductant
Na2SO3
conversion (%)
yield (%)
Our continuing interest in the development of more active,
efficient, and economical catalyst/reductant pairs and the
elucidation of the reactivity factors controlling these systems
led us to investigate the DODH efficiency of a structurally
b
1
2
3
4
1
2
3
4
100
100
5
95
b
PPh3
87
H2 (20 atm)
CO (20 atm)
1
25
13
−
related set of oxo-vanadium complexes, Z+LVO2 , with a group
of established and potential reductant candidates. We disclose
herein substantial ligand- and reductant-dependent DODH
reactivity differences among these and the discovery of an
effective catalyst that enables use of the most economical
reductantshydrogen and carbon monoxidefor efficient
DODH.
5
6
7
8
Na2SO3
10
10
5
7
8
PPh3
H2 (20 atm)
CO (20 atm)
trace
trace
5
9
Na2SO3
100
20
24
10
25
26
In order to establish structure−catalytic activity correlations
10
11
12
PPh3
for a set of electronically related oxo-vanadium complexes, we
H2 (20 atm)
CO (20 atm)
100
100
−
selected four Z+LVO2 compounds, 1−4 (Figure 1), for
13
14
15
16
17
18
Na2SO3
100
20
27
10
15
16
33
48
PPh3
benzyl alcohol
Zn
100
20
H2 (20 atm)
CO (20 atm)
100
100
a
Reaction conditions: 160 °C (oil bath), 1.0 mmol glycol, 1.5 mmol
reductant/(20 atm H2 or CO) and 10 mol % of catalyst in 5 mL of
benzene in a sealed thick-walled glass tube or stainless steel reactor.
Conversion and yield determined by gas chromatography with
b
naphthalene as an internal standard. Data from ref 13, with a
reaction time of 72 h.
Figure 1. Oxo-vanadium compounds tested for DODH activity.
catalytic evaluation. Complexes 2−4 share electronic and
H2 or CO as potential reductants (entries 3 and 4). In
comparison, the 8-O-quinoline complex (2) exhibited rather
little diol conversion or styrene formation with any of the four
reagents (entries 5−8). The (triazene−N-O) complex (3)
effected high diol conversion with sulfite, H2, and CO, but only
modest styrene yields with these reductants (entries 9−12).
The sal-hydrazide complex (4), on the other hand, exhibited
low activity and styrene yield with PPh3 (entry 14), but gave
moderate yields (the highest among 1−4) with the practical
gaseous reductants, H2 and CO (entries 17 and 18). No
detectable over-reduction to the alkane occurs using H2.
The scope and effectiveness of the DODH reactions
catalyzed by 4 was examined with two additional substrates
representative of unactivated carbohydrate-derived polyols, 1,2-
hexanediol and (+)-diethyl tartrate. Sulfite, benzyl alcohol, H2
and CO were tested as reductants (Table 2) with these
substrates. These less-reactive glycols generally require longer
reaction times and/or somewhat higher temperature to achieve
high conversion, but generally the yields and DODH selectivity
were similar or better than with styrene diol (entries 5−12 vs
entries 1−4). This is probably the result of the less competitive
side reactions of these unactivated glycols and their olefinic
products. Comparing the DODH reductant efficiencies across
the three substrates, the CO-driven reactions afforded the
highest conversion and yield in each case (entries 4, 8, and 12
vs entries 1−3, 5−7, and 9−11). In fact, the DODH yield/
selectivity with hexanediol (entry 12) is among the highest
−
structural similarities to (dipic)VO2 complex 1, namely, a
d0-V(v) metal center, a negatively charged complex ion, O,N,
(O)-chelated ligands, and a five or six-coordinate geometry.
The 8-hydroxyquinoline and triazene-hydroxylamine derivatives
2 and 3 were prepared and characterized previously.17,18 The
salicylaldehyde hydrazide complex (4) is a new member of the
family possessing this easily assembled unsymmetrical O,N,O-
chelate ligand.19 The ligand for 4 was prepared by condensation
of salicylaldehyde with thiophene carbohydrazide. Heating the
−
hydrazide ligand with Bu4N+ VO2 in acetonitrile provided
yellow complex 4 in good yield, which exhibited appropriate
infrared (IR), nuclear magnetic resonance (NMR), and mass
spectral data (see the Supporting Information (SI)).
The ability of complexes 1−4 to catalyze the DODH of a
model glycol, styrene-1,2-diol, was evaluated by employing a
common set of potential reductants: Na2SO3, PPh3, H2, and
CO. Each reaction was conducted in benzene as the solvent at
160 °C with 1.5 equiv of the solid reductants or 20 atm of the
gaseous ones, and 10 mol % of the respective V-complex for 24
h. Any unreacted diol and the styrene product in each reaction
was identified and quantified by gas chromatography (GC) and
gas chromatography−mass spectroscopy (GC-MS); the results
are summarized in Table 1.20
Although styrene production by [Bu4N](dipic)VO2 (1) with
PPh3 and sulfite reductants is efficient (from prior work),13
little glycol conversion or styrene production was observed with
1902
DOI: 10.1021/acscatal.5b02667
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ACS Catalysis
Letter
Table 2. Substrate and Reductant Survey of DODH
Reactions Catalyzed by 4
Scheme 4. Reaction of 2 with Styrenediol
a
When solutions of 4 are stirred under a CO atmosphere at
room temperature, a gradual color change from yellow to green
occurs (see Scheme 5). By comparison, complexes 1−3 were
Scheme 5. Reaction of 4 with CO
unchanged when exposed to CO under the same conditions.
1H NMR and 13C NMR spectra of the green solution from 4 +
CO show only line broadened peaks of 4. Analysis of the
solution by the Evans method21 confirmed the presence of a
paramagnetic species. An IR spectrum of the solution was also
almost identical to 4, except for an additional medium intensity
band at 1820 cm−1, which is suggestive of a bridging carbonyl.
We interpret these observations as indicating the partial (e.g.,
equilibrium) conversion to a paramagnetic Viv complex. The
identity of this species notwithstanding, the ambient reactivity
of 4 toward CO is nonetheless extraordinary for a high-
oxidation-state, d0-oxo complex.22
The origin of the differing reactivity of 1 and 4 toward CO
and the latter’s efficient catalysis with CO and H2 is presently
unclear. The salhydrazide ligand of 4, being more basic than the
dipicolinate of 1,23 could make it harder to reduce (salhyd)Vv
derivatives but easier to oxidize corresponding Viii species. This
is supported by CV scans of 1 and 4 (CH3CN, 0.1 M
Bu4NPF4), which show the two (irreversible) reduction waves
of 1 to be 200 and 600 mV more facile than for 4. Preliminary
DFT calculations, however, reveal only subtle differences in the
charge distribution and frontier MOs of 1 and 4. The
propensity of oxo-V complexes for 4−6 coordination and the
potential structural diversity of V-glycol species may also factor
into the significantly different catalytic activity of these
complexes.24
a
Reaction conditions: 1.0 mmol glycol, 1.5 mmol reductant or (20 atm
H2 or CO), and 10 mol % catalyst in 5 mL of solvent heated at 160−
180 °C (oil bath) in a sealed thick-walled glass tube or stainless steel
reactor. Conversion and yield determined by gas chromatography
(GC), with naphthalene as the internal standard.
reported for unactivated diols, even in the rhenium-catalyzed
reactions. The tartrate substrate gives fumarate (trans isomer)
highly stereoselectively (>95%, entries 5−8), as the apparent
result of syn−OH elimination. No over-reduction of the
fumarate CC or −CO2Me groups was detected in the H2-
or CO-driven reactions. Although the DODH activity of 4 is
somewhat lower than many reported LReOx/reductant
systems, the yields and selectivity with 4/CO are superior to
most.
Among the significant differences in activity/selectivity
observed for the catalyst/reductant pairs in the above studies,
two stand out, especially (1) the very low activity of complex 2
across all reductants, and (2) the relatively high catalytic
DODH efficiency of 4 when partnered with the most practical
reductants (H2 and CO). To gain some insight into the origin
of these effects, we have conducted experimental and
computational studies to determine if there were notable
differences in the reactivity of 1−4 with the glycols or the
reductants.
The catalytic process for these V-catalyzed DODH reactions
is presumed to follow the basic stages of reduction,
condensation, and oxidative elimination, according to Scheme
2, but determination of the reduction/condensation sequence
and identification of the key intermediates await further study.
Each complex was tested for ambient reactivity with styrene
glycol, with monitoring being done using NMR and visible
color changes. Only 2 clearly reacted, as signaled by a color
1
−
change from yellow to red over 24 h at 20 °C. The H NMR
In conclusion, four ostensibly similar Z+LVO2 complexes
spectrum of the resulting solution exhibited new resonances
shifted ca. 0.6 ppm downfield from styrene diol; the 13C NMR
spectrum showed two new peaks shifted ca. 1.0 ppm from the
free glycol. We tentatively assign these to a Vv-glycolate
derivative, e.g., 5 (see Scheme 4). The reactivity of 5 was
assessed by heating its solution with Na2SO3 (150 °C, 24 h).
After this time, no styrene was detected, suggesting that 5 is a
dead-end intermediate, whose formation could contribute to
the low catalytic activity of 2.
have been shown to have considerably different, reductant-
dependent DODH catalytic activities. Compared to the
pyridine−dicarboxylate complex (1), that is effective with
PPh3/sulfite, the quinoline alcoholate complex 2 shows little
DODH activity with any of the reductants. The triazine
derivative 3 has moderate activity with the set of reductants,
including the economical H2 and CO. The salhydrazide
complex (4) exhibits the highest efficiency with these reagents,
providing the most practical and economical catalyst/reductant
pair to date for the DODH of glycols. Efforts are underway to
identify the origin of these effects and the key reaction
The efficiency of the CO-driven DODH reactions with
catalyst 4 prompted us to examine its direct reaction with CO.
1903
DOI: 10.1021/acscatal.5b02667
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ACS Catalysis
Letter
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ASSOCIATED CONTENT
* Supporting Information
The Supporting Information is available free of charge on the
ACS Publications website at DOI: 10.1021/acscatal.5b02667.
■
S
Experimental procedures, representative gas chromato-
grams, and spectroscopic data for the products (PDF)
AUTHOR INFORMATION
Corresponding Author
*E-mail: knicholas@ou.edu.
Author Contributions
The manuscript was written through contributions of all
authors. All authors have given approval to the final version of
the manuscript.
■
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Notes
The authors declare no competing financial interest.
ACKNOWLEDGMENTS
We are grateful for the support provided by the U.S.
Department of Energy (No. DE-11ER16276).
■
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