Deoxygenation of biomass-derived feedstocks: Oxorhenium-catalyzed deoxydehydration of sugars and sugar alcohols

Article abstract of DOI:10.1002/anie.201203877

Turn sugar into oil: The deoxygenation reaction of sugar moieties is important for the conversion of biomass into chemicals and fuels. The methyltrioxorhenium-catalyzed deoxydehydration reaction was successfully applied to this purpose using another alcohol as solvent/reductant. The reaction was highly stereospecific, affording linear polyene products from C ₄-C₆ sugar alcohols and aromatic compounds from C ₄-C₆ sugars. Copyright

Full text of DOI:10.1002/anie.201203877

Angewandte
Chemie
DOI: 10.1002/anie.201203877
Biomass Conversion
Deoxygenation of Biomass-Derived Feedstocks: Oxorhenium-
Catalyzed Deoxydehydration of Sugars and Sugar Alcohols**
Mika Shiramizu and F. Dean Toste*
The conversion of renewable cellulosic biomass into fuels and
chemicals has attracted increased attention with the growing
[(Cp*Ru(CO) ) ] for deoxygenation of diols and epoxides
2 2
using H as reductant to produce alkanes and hydrocracking
2
[
1]
[10]
demand for sustainability. However, one fundamental
challenge is that saccharides, the major component of
cellulosic biomass, are highly oxygen-rich when compared
with the majority of current commodity chemicals and fuels.
The polyol structure also generally presents poor solubility in
organic solvents, thermal instability, and limited scope for
functionalization, making its chemical manipulation difficult.
Therefore, efficient deoxygenation reactions of sugars and
sugar derivatives need to be developed. Current methods are
products.
While these methods are effective for simple
vicinal diols and appear to lay solid foundations in the context
of biomass deoxygenation, no system has been reported to
have a general efficiency on polyols. The only sugar alcohol
employed in this reaction to date is erythritol (C4 sugar
alcohol) and product yields were moderate (21–62%) after
[
7b,8]
long reaction times (12–100 h).
To the best of our knowl-
edge, there is no report of DODH reaction of larger (C and
5
C ) sugar alcohols, which can be readily obtained by hydro-
6
[
2]
dominated by the high-temperature pyrolysis, acid-cata-
genation of naturally abundant sugars such as xylose (the
major hemicellulose component) and glucose (cellulose
component). Moreover, the direct DODH reaction of sac-
charides is unprecedented, although it could constitute
a major advance towards sustainable chemical production.
Based on this background, we sought to develop a more-
efficient DODH protocol capable of deoxygenating the
challenging polyol substrates. We focused our attention on
the alcohol transfer hydrogenation system because of the
significant advantage, that the oxidized alcohol (ketone or
aldehyde) can be readily hydrogenated if reductant recycling
is necessary. In addition, whereas only large secondary
alcohols (such as 5-nonanol, 3-octanol, and 2-octanol) have
been used as DODH reductants thus far, we were interested
in examining the use of other inexpensive/bio-derived alco-
hols. We noted that [Re (CO) ] and [BrRe(CO) ] catalysts
[3]
[4]
lyzed dehydration, and hydrogenolysis reactions. A much
less developed deoxygenation pathway is the deoxydehydra-
tion (DODH) reaction, which removes two adjacent hydroxy
groups from vicinal diols to afford alkenes (Scheme 1).
2
10
5
[8]
employed in the original report of Bergman and Ellman
required air and high temperature for activation. We postu-
lated that the actual active catalyst may be an oxidized
Scheme 1. A general scheme for DODH reaction.
[
11]
rhenium species,
and that consequently the oxorhenium
compounds could constitute superior catalysts for this reac-
tion.
In the initial experiments to evaluate the viability of
oxorhenium compounds, 1,4-anhydroerythritol (1) was used
Existing examples of catalytic DODH reactions use high-
[
5]
[6]
valent oxorhenium complexes, and employ PPh , H , or
3
2
[
7]
Na SO3 as reductants. The Bergman and Ellman groups
2
demonstrated a hydrogen-transfer-type DODH reaction
using [Re (CO) ] and [BrRe(CO) ] as catalyst in conjunction
as
a model substrate (Table 1). When 2.5 mol% of
[Re (CO) ] was used, 2,5-dihydrofuran (2) was obtained in
2
10
5
2
10
[
8]
[12]
with a secondary alcohol as solvent/reductant, and Fer-
nandes and Sousa reported the oxorhenium-catalyzed deox-
> 90% yield
(methyltrioxorhenium; MTO) was used in place of
[Re (CO) ], similarly excellent yield was obtained
(entry 2). The difference in reactivity between these catalysts
was revealed when the alcohol was changed to 1-butanol,
a typical biomass-derived alcohol. While no reaction was
observed with [Re (CO) ] (entry 3), three other oxorhenium
compounds catalyzed the formation of 2 in approximately
in 3-octanol (entry 1). When [CH ReO ]
3 3
[9]
ygenation of styrene oxides in the absence of a reductant.
More recently, Srivastava and co-workers utilized
a
2
10
[13]
[
*] M. Shiramizu, Prof. F. D. Toste
Department of Chemistry University of California, Berkeley
Berkeley, CA 94720 (USA)
2
10
[
14]
7
0% yield (entries 6–8). We selected MTO as our catalyst
E-mail: fdtoste@berkeley.edu
of choice based on its simple ligand-free structure and the
ease of handling as a crystalline solid, as opposed to an
[
**] This work is supported by the Energy Biosciences Institute. The
authors thank Prof. R. G. Bergman for useful discussion.
[15]
aqueous solution, which is the commercial form of HReO4.
Supporting information for this article is available on the WWW
under http://dx.doi.org/10.1002/anie.201203877.
Other alcohols (for full details see the Supporting Informa-
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Table 1: Comparison of rhenium catalysts and alcohols.
producing 6 in slightly lower yield (6, 72%; 2, 15%). The
reaction temperature could be decreased to approximately
1
508C (6, 79%; 2, 6% at 1558C, 2.5 h). We also tested HReO
4
[8]
to examine if Brønsted acidity increases the yield of 2, but
[
a]
[a]
both product distribution and total yield were similar to MTO
Entry Re Catalyst
t [h] Alcohol
Yield [%]
Conv. [%]
(6, 73%; 2, 7% at 1558C, 5.5 h). When dl-threitol (10) was
1
2
3
4
5
6
7
8
9
1
[Re (CO) ]
1.5
1.5
1.5
3-octanol
3-octanol
1-butanol
1-butanol
1-butanol
1-butanol
1-butanol
1-butanol
ethanol
91
92
0
20
25
68
66
70
0
100
100
0
54
62
100
100
100
0
2
10
subjected to the optimized reaction conditions, 6 was obtained
in a similar yield (81%) and the minor product (13%) was
[CH ReO ]
3
3
[Re (CO) ]
2
10
1
,4-anhydroethreitol (11) instead of 2 (Scheme 4). In both
[ReO(PPh ) Cl ] 1.5
3
2
3
[NH ReO ]
1.5
1
1
1
1
4
4
[ReIO (PPh ) ]
2
3 2
[
b]
[HReO4]
[CH ReO ]
3
3
[
c]
[CH ReO ]
3
3
0
[CH ReO ]
1
3-pentanol 95
100
3
3
1
[
a] Yields and conversions were determined by H NMR spectroscopy
Scheme 4. DODH reaction of dl-threitol.
using mesitylene as an internal standard. [b] 77% solution in water.
c] Same result was obtained when anhydrous alcohol was used under N2
[
atmosphere.
cases, compounds 7, 8, and 9 were not observed, thus
suggesting that a) the diol substrate needs to adopt the
cis conformation (assuming 2 is produced via 1), and b) ter-
minal diols and cyclic, internal cis diols (e.g. moiety 1) are
more reactive toward DODH than acyclic internal diols.
To understand the mechanism of this process, we com-
bined the reductant alcohol (3-pentanol), MTO, and alkyne
tion) were tested in the reaction using MTO; unfortunately
small alcohols (ꢀ 3 carbon atoms), such as ethanol, were
ineffective (entry 9). For larger (ꢁ 5 carbon atoms) alcohols,
various alcohols could be used including amyl alcohols, which
can be produced by fermentation, but secondary alcohols
were found to be generally more favorable than primary
[16]
[
21]
ligand and obtained a stable organorhenium(V) compound
[
17]
[22]
[23]
alcohols (entry 10).
12
along with 3-pentanone, at 1558C (Scheme 5).
We began our investigation by applying this DODH
system to glycerol (3), a by-product of biodiesel (fatty acid
esters) production from oil-based feedstocks and considered
[18]
an important platform chemical for bio-based materials. To
our delight, allyl alcohol (4) was obtained in excellent yield
(
Scheme 2). Encouraged by this result, we then tested
Scheme 5. The reduction of MTO by 3-pentanol.
Whereas binding of diol 1 to 12 occurred immediately at
V
Scheme 2. DODH reaction of glycerol.
room temperature to afford Re complex 13, extrusion of
[
24]
olefin 2 required heating of 13 to 1558C (Scheme 6a). The
extent of binding to 12 was dependent on the diol structure.
Although trans-hexanediol showed no evidence of binding
(Scheme 6b), combining meso-2,3-butanediol 14 (1.2 equiv)
and 12 in C D produced 15 but only in 56% conversion
erythritol (5), a C sugar alcohol that can be obtained by the
4
[
19]
fermentation of glucose
or by the decarbonylation of
(Scheme 3). 1,3-Butadiene (6), an industrially
important rubber precursor, was obtained in 89% yield, with
,5-dihydrofuran (2) as a minor product (11%). The experi-
[
20]
pentoses
6
6
(observed molar ratio 12/14/15 = 0.8:1.1:1; Scheme 6c, top).
When 1 (1.2 equiv) was subsequently added to this mixture,
all rhenium compounds (12 and 15) were exclusively con-
verted into 13, thus resulting in a solution of 13, 14, and the
excess 1 (observed molar ratio 13/14/1 = 1:1.2:0.2; Scheme 6c,
bottom). The diol exchange from 15 to 13 indicates that this
binding is reversible. Moreover, rhenium complex 12 exhib-
ited DODH catalytic activity that is virtually identical to
MTO (Scheme 7). Although the effect of the 3-hexyne ligand
should be considered, these results provide support for
methyldioxorhenium(V) as the catalytically relevant species.
Based on these observations, a plausible catalytic cycle is
depicted in Scheme 8. We postulate that the product selec-
tivity originates from the relative rate of diol coordination to
2
ments were generally conducted under N as a precaution;
2
however, the reaction could be conducted in air, albeit
V
Scheme 3. DODH reaction of erythritol.
the Re species and/or the alkene extrusion.
2
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Scheme 9. DODH reaction of C sugar alcohols.
5
Scheme 6. The diol binding to 12.
Scheme 10. DODH reaction of C sugar alcohols.
6
Scheme 7. Rhenium complex 12 as a DODH catalyst.
[26]
precursor, was obtained in 54% yield in both cases when
high temperature and short reaction time were employed.
To further diversify the scope of biomass-derived chem-
icals accessible through DODH, we applied our reaction to
[27]
inositols, a class of natural carbohydrate. On one hand, one
can imagine the formation of benzene by three consecutive
DODH reactions, given the appropriate stereochemistry or
fast isomerization. On the other hand, based on the knowl-
edge garnered from xylitol DODH reactions, a cationic
species may readily form after two DODH events. Driven by
aromatization, the net loss of water would yield a phenol
moiety. As both benzene and phenol are stable, high-volume
chemicals conventionally produced only from petroleum
source, their bio-derived alternatives would be highly attrac-
tive. When different inositol isomers were examined, benzene
and phenol were indeed obtained as a mixture, in the total
[
28]
Scheme 8. A proposed catalytic cycle for MTO-catalyzed alcohol-driven
DODH reaction.
yields of 24-96% (Scheme 11). High yields of benzene were
obtained not only from allo-inositol (23) but also from d-
chiro-inositol (24) and muco-inositol (25). In contrast, myo-
inositol (26), bearing only one cis-diol group, gave low yields
for both benzene and phenol. This result suggests that there is
an isomerization process to afford a cis-1,2-diol intermediate
To extend our method to C5 sugar alcohols, we first
investigated xylitol (17). In the initial experiments under
previously employed reaction conditions, (E)-5-penta-1,3-
diene ethers (e.g. 16) were obtained in low yields (10–20%).
Considering that three equivalents of alcohol are required for
this transformation, lowering the concentration of xylitol
improved the yield to 61% (Scheme 9). d-Arabinitol (18) and
ribitol (19) also gave the E isomer 16. The presumed cis-diol
stereochemical requirement of the DODH reaction suggests
that an E–Z isomerization process is involved. We propose
the Lewis acid catalyzed formation of a pentadienyl cation
and its trapping by the alcohol solvent, thus favoring the
[
25]
more-stable E conformation. Gratifyingly, d-sorbitol (21)
and d-mannitol (22), C sugar alcohols derived from glucose
6
and fructose, also underwent clean DODH reaction
Scheme 11. Deoxygenation of inositols to aromatics. [a] 3-octanol was
(
Scheme 10). (E)-hexatriene (20), an interesting polymer
used instead of 3-pentanol.
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at the third DODH step, which could proceed through an
oxorhenium-assisted mechanism (see the Supporting Infor-
mation).
We are currently investigating the application of this
method to sugars, the most-direct feedstock from biomass but
arguably the most-challenging substrate owing to their
thermal instability as well as the complexity associated with
the equilibrium between multiple isomeric forms (furanose/
pyranose, aldose/ketose, a isomer/b isomer); this equilibrium
is a critical consideration given the cis-diol specificity of our
oxorhenium-catalyzed DODH reaction. When we first exam-
ined tetroses to assess the effect of aldose functionality, both
d-erythrose (27) and l-threose (28) afforded furan (through
a DODH reaction and subsequent dehydration) in compara-
ble yields (Scheme 12). The high reactivity of 28 in our
Scheme 13. Deoxygenation of hexoses to 2-vinylfuran and furan.
applied it to sugars and sugar alcohols with a view to biomass
conversion. Oxorhenium compounds, namely MTO, showed
much higher activity than the previously reported rhenium
carbonyl catalysts and enabled the DODH reaction to be
applied to unstable polyol substrates. In addition, mechanistic
V
insights were acquired by studying the isolated Re species.
Besides being cis-diol specific, the DODH reaction provided
a strong differentiation among diol moieties and this stereo-
specificity was the key to obtaining the high yields. Although
secondary alcohols were more favorable as reductants, we
demonstrated the viability of the use of bio-derived alcohols
such as 1-butanol in this reaction. Sugar alcohols yielded
linear polyene products, possible feedstock for polymers and
fuels. When DODH was combined with a subsequent dehy-
dration reaction, stable aromatic compounds with a low
oxygen content were obtained from sugars (benzene, phenol,
and furan moieties). Future work will include increasing the
efficiency of sugar DODH, the application to polysaccha-
rides, and immobilization/recycling of the catalyst.
Scheme 12. Deoxygenation of tetroses to furan.
DODH process can have two plausible explanations;
a) DODH of the C1 and C2 hydroxy groups, b) the epime-
rization of the C2 hydroxy group via erythrulose. The DODH
of the internal diol (C2 and C3 hydroxy groups) in the open-
chain form is unlikely because 28 would then yield (E)-4-
hydroxybut-2-enal, which cannot re-cyclize because of the
trans geometry. Although this experiment alone could not
specify the exact DODH pathway, it suggested that the cis-
stereochemistry requirement of DODH is not necessarily
stringent for sugar substrates when the epimerization on C1
and C2 hydroxy groups can provide access to the desired
product. Similar to the inositol case, once the first DODH
reaction occurs, the resulting alkene directs the dehydration
reaction to produce a stable compound (in this case furan)
given the appropriate structure. This appears a very effective
strategy to achieve a good selectivity in the DODH-based
sugar deoxygenation reaction. The DODH reaction of
pentoses (ribose, lyxose, arabinose, xylose) produced 2-
Received: May 19, 2012
Published online: && &&, &&&&
Keywords: biomass · deoxydehydration · deoxygenation ·
.
rhenium · sustainable chemistry
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[
[
In summary, we have developed an oxorhenium-catalyzed
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4
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6
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13
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4
2
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Information.
[25] When penta-1,4-dien-3-ol was heated in 3-octanol with
2.5 mol% of MTO, (E)-3-(penta-2,4-dienyloxy)octane was
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1
[
[
[
[
12] As most products were volatile, yields and conversions were
1
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inhibited when a significant amount of water was initially
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1
present. No conversion of 1 was observed in 3-octanol/H O = 5:1
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2
or 1-butanol/H O = 10:1 with 2.5 mol% of MTO, 1708C, 1 h.
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2
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[
17] Secondary alcohols also underwent dehydration to produce
alkenes (e.g. cis- and trans-pentenes from 3-pentanol). However,
this reaction did not interfere with the desired DODH reaction.
Angew. Chem. Int. Ed. 2012, 51, 1 – 6
ꢀ 2012 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
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Communications
Biomass Conversion
M. Shiramizu,
F. D. Toste*
&&&& — &&&&
Deoxygenation of Biomass-Derived
Feedstocks: Oxorhenium-Catalyzed
Deoxydehydration of Sugars and Sugar
Alcohols
Turn sugar into oil: The deoxygenation
reaction of sugar moieties is important
for the conversion of biomass into
chemicals and fuels. The methyltrioxo-
rhenium-catalyzed deoxydehydration
reaction was successfully applied to this
purpose using another alcohol as sol-
vent/reductant. The reaction was highly
stereospecific, affording linear polyene
products from C –C sugar alcohols and
4
6
aromatic compounds from C –C sugars.
4
6
6
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ꢀ 2012 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
Angew. Chem. Int. Ed. 2012, 51, 1 – 6
These are not the final page numbers!