Rhenium-catalyzed didehydroxylation of vicinal diols to alkenes using a simple alcohol as a reducing agent

Article abstract of DOI:10.1021/ja103436v

A new method for the catalytic didehydroxylation of vicinal diols is described. Employing a readily available low-valent rhenium carbonyl complex and a simple alcohol as a reducing agent, both terminal and internal vicinal diols are deoxygenated to olefins in good yield. The optional addition of acid (TsOH, H₂SO₄) provides access to lower reaction temperatures. This new system enables the transformation of a four-carbon sugar polyol into an oxygen-reduced compound, providing promising evidence for its practical application to produce unsaturated compounds from biomass-derived materials.

Full text of DOI:10.1021/ja103436v

Published on Web 07/29/2010
Rhenium-Catalyzed Didehydroxylation of Vicinal Diols to Alkenes Using a
Simple Alcohol as a Reducing Agent
Elena Arceo, Jonathan A. Ellman,* and Robert G. Bergman*
Department of Chemistry, UniVersity of California, and DiVision of Chemical Sciences, Lawrence Berkeley National
Laboratory, Berkeley, California 94720
Received April 22, 2010; E-mail: jonathan.ellman@yale.edu; rbergman@berkeley.edu
Scheme 1. Disproportionation of Diol 1 Catalyzed by Re2(CO)10
Abstract: A new method for the catalytic didehydroxylation of
vicinal diols is described. Employing a readily available low-valent
rhenium carbonyl complex and a simple alcohol as a reducing
agent, both terminal and internal vicinal diols are deoxygenated
to olefins in good yield. The optional addition of acid (TsOH,
H2SO4) provides access to lower reaction temperatures. This new
system enables the transformation of a four-carbon sugar polyol
Table 1. Rhenium-Catalyzed Didehydroxylation of Diol 5 Using
Alcohol 7a, b, or c as a Reductant
into an oxygen-reduced compound, providing promising evidence
for its practical application to produce unsaturated compounds
from biomass-derived materials.
The development of new synthetic technologies for the production
of reduced oxygen-content materials from renewable biomass resources
is a key target in chemistry and chemical engineering research.1 Our
reductant
(mL)
temperature
(°C)
time
(h)
yield 6b
(%)
group is exploring new routes to deoxygenate biomass-derived
polyols,2 and the didehydroxylation of diol moieties is an important
goal in this context. Various stoichiometric methods for carrying out
diol didehydroxylation are synthetically valuable,3 but catalytic methods
are rare.4 In particular, didehydroxylation has been achieved using
Cp*ReO3 as a catalyst, although triphenylphosphine was needed as a
sacrificial reductant.4a We report herein a catalytic method for the
didehydroxylation of vicinal diols to alkenes that employs a readily
available low-valent rhenium carbonyl complex and a simple alcohol
as an environmentally friendly reducing agent.
entrya
catalyst
1
2
3
4
5
3a
3a
3a
3a
3b
7a (4)
7b (4)
7b (5)c
7c (4)
180
170
170
170
170
3.5
4
4
3.5
4
83
82 (82)
83
74
84 (85)
7b (4.5)
a Reactions conducted at 2.5 mmol scale under air. Details in SI.
b Determined by 1H NMR spectroscopy using 1,3,5-trimethoxybenzene
as internal standard. Isolated yield in parentheses. c Further increase in
the volume of reductant resulted in longer reaction times without any
significant improvement in yield.
During preliminary experiments, we observed that heating (4S*,4S*)-
octane-4,5-diol (1) at 180 °C in solvent-free conditions, under air and
in the presence of dirhenium decacarbonyl (3a, 2.5 mol %), resulted
in complete consumption of 1 in 3.5 h to yield the olefin 2 in 50%
yield as determined by NMR spectroscopy (Scheme 1). A droplet of
phase-separated water was also detected in the reaction mixture.
As this system is able to use a vicinal diol as both oxidant and
reductant, we hypothesized that adding a simple alcohol as a
reductant5 might enable selective conversion of the diol moiety to
a C-C double bond without competing diol oxidation. We chose
1,2-tetradecanediol (5) as a substrate for optimization due to the
low volatility of the possible products. When the reaction of 5 (2.5
mmol) and 3a (2.5 mol %) was performed at 180 °C under air and
in the presence of 5-nonanol (7a)6 (9 equiv), the yield of the alkene
was enhanced to over 80% (Table 1, entry 1).
Aliquots of the mixture from the reaction of diol 1 and 3a at
intermediate conversion contained the vicinal diketone 4 (as
identified by 1H, 13C NMR spectroscopy and MS). Although at the
early stages of the reaction the amount of 4 increased along with
the formation of 2, its depletion was subsequently observed leading
to its disappearance when complete conversion of 1 was achieved
(vicinal diketones are unstable under the reaction conditions; see
Supporting Information (SI)). These results suggested that the
transformation might occur through a disproportionation reaction
in which the diol (1) is reduced to generate an alkene (2) and
concurrently acts as a reductant to give oxidized products (4).
Other high-boiling alcohols such as 3-octanol (7b) and 2-octanol
(7c) also gave enhanced yields of the desired product (entries 2-4,
Table 1). The use of 9 to 13 equiv of reductant afforded complete
conversion of 5 in 3.5 to 4 h. In this process, the alcohols 7a-c
generated 1 to 1.5 equiv of the corresponding ketone. We briefly
investigated the use of other low-valent rhenium complexes; while
Cp*Re(CO)3 did not show any activity, BrRe(CO)5 (3b) exhibited
behavior similar to that observed with 3a (entry 5).
Similar results were observed when the terminal diol 1,2-tetrade-
canediol (5; see Table 1) was submitted to the same reaction conditions,
in which the yield of 1-tetradecene (6) was determined to be 49%
(isolated). The diols remained unchanged when heated in the presence
of 3a under air at temperatures lower than 170 °C. When the experiment
was conducted in the absence of O2 (N2 atmosphere) no reaction was
observed after heating at 180 °C for extended periods (24 h).
The use of an alcohol as a reductant also resulted in improved yields
for internal diols. For example, (3R*,4R*)-decane-3,4-diol (8, 2.5 mmol)
afforded the alkene 9 in 82% yield after 2 h at 170 °C in 3-octanol (4.5
mL) with 2.5 mol % of 3a (Scheme 2). The cyclic diol cis-1,2-
cyclohexanediol (10) also provided the corresponding olefin (11) in good
9
11408 J. AM. CHEM. SOC. 2010, 132, 11408–11409
10.1021/ja103436v  2010 American Chemical Society

C O M M U N I C A T I O N S
Scheme 3. Rhenium-Catalyzed Deoxygenation of Erythritol
Scheme 2. Rhenium-Catalyzed Didehydroxylation of the Internal
Diols 8 and 10 Using 3-Octanol as a Reductant
In contrast to the simplicity of the reaction methodology, the
mechanism of the reaction is still unknown. The manifest temper-
ature and oxygen dependence of this transformation suggests an
oxidized rhenium species as the active catalyst, and the requirement
that the diol be capable of achieving a cis disposition for the reaction
to proceed is consistent with the intermediacy of a rhenium diolate
species. Some metal diolates are known to extrude the correspond-
ing alkene on thermolysis.7 Stoichiometric extrusion of olefins has
been reported from high-oxidation-state organorhenium oxides, for
example Re(VII)8 and Re(V),9 and is believed to be operative in
the Cp*ReO3-catalyzed reaction mentioned previously.4a Unlike that
system, the present method tolerates alcohols as reductants and can
be performed in the presence of oxygen which suggests a qualitative
difference in the reaction mechanism. We hypothesize that the
significant effect of the presence of acid on the catalyst performance
might be related to an assisted olefin extrusion by protonation of a
rhenium diolate intermediate.
Table 2. Effect of para-Toluenesulfonic Acid (T) and Sulfuric Acid
(S) as Additives in the Rhenium-Catalyzed Didehydroxylation of 5a
3a
(mol %)
7b
(mL)
acid
(mol %)
temp
(°C)
time
(h)
conv
(%)
yieldb
6 (%)
entry
1
2
3
4
5
6
7
2.5
2.5
1
1
1
2.5
1.25
4
4
3
3
5
5
5
-
155
155
170
155
155
155
155
5
2.5
16
1.5
1.8
2
0
100
80
100
100
100
100
0
T (5)
-
74(76)
46
77
87(83)
82(85)
79
T (2)
T (2)
S (2)
S (2)
2
In conclusion, these results provide a new catalytic system based
on a rhenium-mediated didehydroxylation of vicinal diols using a
simple alcohol as reductant. The addition of acid provides access
to milder reaction temperatures. The practical application to
erythritol strongly suggests that this chemistry could be used to
produce unsaturated compounds from other biomass-derived poly-
ols. Further studies of the mechanism and investigation of other
substrates and additives are in progress.
a Reactions conducted at 2.5 mmol scale under air. Details in SI.
b Determined by 1H NMR spectroscopy using 1,3,5-trimethoxybenzene
as internal standard. Isolated yield in parentheses.
yield. Conversely, the stereoisomer trans-1,2-cyclohexanediol did not react
in the presence of the alcohol or in solvent-free conditions.
In an attempt to decrease the reaction temperature, the use of an
additive was investigated. This is relevant from the perspective of
the application to sugar polyols, since these materials tend to
decompose at high temperatures. The presence of bases such as
tri-n-hexylamine or K2CO3 resulted in longer reaction times or total
obstruction of reactivity (Table S2, Supporting Information).
Conversely, the presence of catalytic quantities of para-toluene-
sulfonic acid (TsOH) or sulfuric acid not only shortened the reaction
times but also permitted the reaction to proceed at lower temper-
atures and with lower catalyst loading.
Acknowledgment. The authors gratefully acknowledge financial
support from the Dow Chemical Co.
Supporting Information Available: Additional information, tables,
and experimental procedures with figures. This material is available
free of charge via the Internet at http://pubs.acs.org.
References
Heating a mixture of 5, 3a, and 7b (1: 0.025: 10) at 155 °C in the
presence of 5 mol % of TsOH resulted in complete conversion of 5 in
2.5 h (entry 2, Table 2, compare to entry 1). Lowering the concentration
of TsOH (2 mol %) and catalyst (1 mol %) resulted in complete
conversion in 1.5 h at 155 °C (entry 4, compare to entry 3). By
increasing the amount of reductant to 12.6 equiv and using 1 mol %
of 3a, 6 was obtained in 87% yield in less than 2 h at 155 °C. The
presence of catalytic quantities (2 mol %) of H2SO4, one of the most
widely used industrial chemicals, resulted also in good yields of the
olefin at 155 °C, even with catalyst loadings as low as 1.25 mol %
(entries 6, 7). The reaction did not proceed in the absence of 3a in the
presence of catalytic quantities of H2SO4 or TsOH.
In order to investigate the scope of this reaction in the context
of biomass-derived materials, meso-erythritol (12), a four-carbon
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promising, in view of the fact that 2,5-dihydrofuran (13) was formed
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Scheme 3. Product 13 was isolated (55% yield; >94 mol % pure) by
fractional distillation from the crude reaction mixture. We believe that
the presence of the acid additive in the mixture favors the initial
cyclization of erythritol to 1,4-anhydroerythritol,2 which then undergoes
didehydroxylate in the presence of 3a and reductant to afford the final
reduced oxygen-content product.
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