An efficient didehydroxylation method for the biomass-derived polyols glycerol and erythritol. Mechanistic studies of a formic acid-mediated deoxygenation

Article abstract of DOI:10.1039/b907746d

An efficient 1,2-deoxygenation method, involving an unexpected mechanism, was found for simple diols and for biomass-derived polyols (glycerol and erythritol) that results in the conversion of the 1,2-dihydroxy group to a carbon-carbon double bond.

Full text of DOI:10.1039/b907746d

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An efficient didehydroxylation method for the biomass-derived polyols
glycerol and erythritol. Mechanistic studies of a formic acid-mediated
deoxygenationw
Elena Arceo,a Peter Marsden,a Robert G. Bergman*b and Jonathan A. Ellman*a
Received (in College Park, MD, USA) 17th April 2009, Accepted 5th May 2009
First published as an Advance Article on the web 13th May 2009
DOI: 10.1039/b907746d
An efficient 1,2-deoxygenation method, involving an unexpected
mechanism, was found for simple diols and for biomass-derived
polyols (glycerol and erythritol) that results in the conversion of
the 1,2-dihydroxy group to a carbon–carbon double bond.
Earlier studies8 reported substantial charring during the
treatment of glycerol with formic acid. We have now found
that the simple expedient of protecting this reaction from air
provides a much more tractable process. Specifically, treating
glycerol with formic acid while directing a stream of nitrogen
through the reaction mixture completely eliminates charring,
which is mainly responsible for the formation of undesired
products. The method thus substantially improves the yield
and selectivity of the process. A description of our reaction
conditions and full experimental details are presented in the
supplementary information.w
By continuously removing the sensitive product alcohol
(the most volatile component of the reaction mixture) by
distillation, the high temperature can be maintained while
allowing the reaction to proceed nearly to completion,
resulting in a yield of 80% after purification by distillation
over K2CO3. Besides protecting the product from atmospheric
oxidation, the nitrogen bubbling facilitates distillation of the
alcohol.
Biomass is an abundant and readily available feedstock that
has great potential as a renewable source of chemical
intermediates.1 Whereas the preparation of useful chemicals
from petroleum has historically been accomplished by adding
heteroatom functionality to unfunctionalized starting materials,
the conversion of biomass commonly requires the removal of
oxygen atoms from the natural precursors. In this context,
the deoxygenation of polyhydroxy compounds, especially
biomass-derived carbohydrates, is an important target.
Glycerol (1) is the main byproduct in the triglyceride
transesterification process for biodiesel manufacture. The
recent market expansion of biofuels has made glycerol
attractive as a starting material for the synthesis of more
valuable chemicals.2 We describe in this paper an improved
and efficient method for the formic acid-mediated conversion
of glycerol into allyl alcohol (3) (Fig. 1). Allyl alcohol is a
large-scale industrial chemical, presently produced from
propene, which is used as a starting material in the manufacture
of various polymers and other allyl compounds, and has also a
great potential as a precursor of 3-hydroxypropionic acid,3 an
important building block for the development of bio-refinery
processes. Ours is a solvent-free procedure that employs
formic acid, a high-volume commodity chemical. Formic acid
has recently shown significant potential as a low-cost renew-
able chemical because of its occurrence in bio-oils from
biomass pyrolysis4 and because of its promising role in
hydrogen storage through the fixation of CO2 with H2.5
Glycerol is known to undergo proton-catalyzed dehydration
reactions in the presence of solid acid catalysts6 and in super-
critical aqueous media7 to afford the unsaturated aldehyde
acrolein, usually with low selectivity. In the course of our
investigation of the acid-catalyzed dehydration of glycerol, we
observed production of allyl alcohol instead of the aldehyde.
With respect to the mechanism of this reaction, our
first hypothesis was that it involved initial acid-induced
dehydration of glycerol to acrolein via a 1,2-hydrogen shift
pathway, followed by reduction of the aldehyde carbonyl
group consistent with the well-documented role of formic acid
as a hydride donor.9 In order to test this hypothesis, we carried
out isotopic labeling experiments using d5-glycerol (1-d5) and
perdeuterated formic acid (2-d2) (Scheme 1). The incorporation
of deuterium in the final product was monitored by 2H-NMR.
First, d5-glycerol was treated with undeuterated formic acid
and, surprisingly, no protons were incorporated from the
supposed reducing agent formic acid into the product alcohol.
Instead, the allyl alcohol product was completely deuterated at
all carbon positions (3-d5). When 2-d2 was employed in the
a Department of Chemistry, University of California, Berkeley,
Berkeley, CA 94720-1460, USA. E-mail: jellman@berkeley.edu;
Fax: (+1) 510-642-8369
b Department of Chemistry, University of California, Berkeley and
Division of Chemical Sciences, Lawrence Berkeley National
Laboratory, Berkeley, CA 94720-1460, USA.
E-mail: rbergman@berkeley.edu; Fax: (+1) 510-642-7714
w Electronic supplementary information (ESI) available: Experimental
details. See DOI: 10.1039/b907746d
Fig. 1 Formic acid-mediated deoxygenation of the biomass-derived
polyols glycerol and erythritol.
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Scheme
3
Mechanism proposed for the formic acid-mediated
Scheme 1 Isotopic labeling experiments.
didehydroxylation.
reaction with non-labeled 1, no deuterium was incorporated in
the final product (3), except for the exchange of deuterium into
the hydroxyl group (3-d1) and the formyl proton of the minor
allyl ester 4-d1. Thus, there is no hydride transfer from formic
acid, ruling out the intervention of the expected acrolein
intermediate.
one-step procedure that directly affords the alkenes in high
yield and purity.
We suggest that these reactions proceed through an
orthoester-type intermediate, inspired by Eastwood’s proce-
dure for the conversion of vicinal diols to olefins using triethyl
orthoformate and acetic anhydride11 and by the thermal
decomposition observed for 2-acetoxy-1,3-dioxolanes.12
A proposed mechanism for the formic acid-mediated dide-
hydroxylation of glycerol (eqn (1)) is given in Scheme 3.
We concluded from the above experiments that formic acid
was somehow inducing a direct removal of two hydroxy
groups (didehydroxylation reaction) from glycerol. To examine
the generality of this process, we studied the reaction of several
simple diols (Scheme 2) following the same procedure.
The reaction was found applicable in the case of 1,2-diols
such as 1,2-octanediol (5) and 1,2-decanediol (6) which
smoothly underwent this conversion to the corresponding
alkenes in yields over 90%. Cyclic diols including
cis-1,2-cyclopentanediol (7), cis-1,2-cyclohexanediol (8) and
cis-1,2-cyclooctanediol (9) afforded the corresponding
cyclic olefins in high yields. Trihydroxy compounds such as
1,2,3-hexanetriol (10) and 1,2,6-trihydroxyhexane (11) led as
well to the corresponding alkenes 17 and 18.
ð1Þ
We suggest that the reaction involves the transient formation
of a cyclic carbocation intermediate (c, Scheme 3) which is
resonance-stabilized by the two oxygen atoms vicinal to the
cationic center. The preparation and characterization of
1,3-dioxolan-2-ylium ions from alpha glycols has been
previously described.13 The most plausible route from the
1,3-dioxolan-2-ylium carbocation to the observed product
involves attack of the carbonyl oxygen of formic acid to
provide a 2-acyloxy-1,3-dioxolane (d, Scheme 3), which then
undergoes thermal carbon dioxide elimination and hydrogen
transfer to give an olefin and formic acid.
Deoxygenation of diols to produce olefins is a useful
synthetic transformation that typically requires initial
conversion of the vicinal diol into an activated group, which
is subsequently transformed into an olefin in a further step.10
The formic acid-mediated didehydroxylation provides
a
The fact that cis-1,2-cyclopentanediol (7) reacted cleanly
with formic acid at high temperature to yield cyclopentene
(14), while trans-1,2-cyclopentanediol did not form the olefin,
suggested that the formic acid-mediated deoxygenation is a
highly stereospecific syn elimination, which is in accordance
with the pathway proposed above. In order to confirm this
supposition, we subjected two stereoisomeric internal acyclic
1,2-diols to the reaction. Thus, treatment of (3R*,4R*)-
decane-3,4-diol (19) and (3S*,4R*)-decane-3,4-diol (21) with
formic acid (2) proceeded readily, generating the trans (20) and
cis (22) isomers of 3-decene respectively (Scheme 4). The diol
substrates and the deoxygenation products have magnetically
different protons that lead to specific and distinguishable
Scheme 2 Formic acid-mediated didehydroxylation of diols and
triols.
Scheme 4 Stereospecific deoxygenation of diols 19 and 21.
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Scheme 5 Synthesis and thermal transformation of 2-acetoxy-1,3-
dioxolane 24.
patterns. Both reactions were stereospecific as observed by
NMR analysis of the crude distillates, in which no signals
(o1%) due to the alkene with the opposite stereochemistry
were detected in the spectra.
Scheme 6 Synthesis of 2,5-dihydrofuran (27) from erythritol (25) and
possible intermediates 26 and 28 (pathways a and b, respectively).
The transformation of erythritol (25) to 2,5-dihydrofuran (27)
is a good example of how this reaction could offer an
efficient approach to the conversion of biomass to value-added
chemicals.
In order to demonstrate the capability of thermal transfor-
mation of the cyclic orthoesters of type b (Scheme 3) to
generate a double bond, the synthesis of a 2-acyloxy-1,3-
dioxolane from a 2-alkyloxy-1,3-dioxolane was carried out.
1,2-Decanediol (6) was used as a model system for this study.
The 2-ethoxy-1,3-dioxolane derivative 23 was prepared and
subsequently converted12 to the 2-acetoxy-1,3-dioxolane 24
(Scheme 5). When a mixture of dioxolanes 23 and 24 was
carefully heated, the 2-acetoxy-1,3-dioxolane 24 was trans-
formed into the olefin 13 at 40–48 1C, while compound 23 was
unaffected.
In conclusion, we have uncovered an improved selective
one-pot, one-reagent deoxygenation technique for converting
vicinal diols to unsaturated systems in high yields, and provide
evidence for an unexpected concerted mechanism for the
critical bond-changing processes in this transformation. Our
preliminary results with inexpensive biomass-derived polyols
suggest that the reaction of polyhydroxy compounds with
formic acid will be a valuable alternative for the manufacture
of reduced oxygen content products.
The above results suggest that the formic acid-mediated
didehydroxylation might be extendable to carbohydrates,
an economically and environmentally important target.
Preliminary results with sugar polyols are very promising.
Erythritol (25) is a four-carbon sugar alcohol that is accessible
through yeast or fungal fermentation of glucose from corn
starch (Fig. 1). The reaction of 25 with formic acid
at 210–220 1C, following the procedure described above for
glycerol (1), gave a distillate that contained 2,5-dihydrofuran
(27) along with water and formic acid. A 39% yield of the pure
dihydrofuran 27 was isolated after purification by fractional
distillation (Scheme 6).
The authors gratefully acknowledge Dow Chemical Co. and
Director, Office of Science, of the U.S. Department of Energy
under Contract No. DE-AC02-05CH11231 (to R.G.B.
and J.A.E.) for support of this work and the postdoctoral
scholarship support of the Fulbright Program and the
Ministry of Science and Innovation (Spain) (to E.A.).
Notes and references
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To investigate the course of this reaction, cis-1,4-anhydro-
erythritol (26) was prepared by acid-catalyzed cyclo-
dehydration of erythritol with p-toluenesulfonic acid14 in 88%
yield. Formic acid-mediated didehydroxylation of the cyclic
product 26 afforded the corresponding olefin 27 in 87%
yield after purification by redistillation. The reaction of
3,4-dihydroxybutene (28) with formic acid was also investi-
gated, since it is a reasonable potential intermediate in the
conversion of erythritol to 2,5-dihydrofuran. If deoxygenation
of the vicinal primary and secondary hydroxy groups (through
an orthoester type intermediate) occurred, the diol 28 could
possibly provide the product 27 by acid-catalyzed cyclo-
dehydration (Scheme 6, path b). However, as shown in
Scheme 6, 28 did not yield 2,5-dihydrofuran when submitted
to our standard formic acid-mediated didehydroxylation
conditions, pointing to path a as the most plausible reaction
pathway.
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