Download RheniumCatalyzed Deoxydehydration of Diols and Polyols

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DOI: 10.1002/cssc.201402987
Rhenium-Catalyzed Deoxydehydration of Diols and Polyols
Johannes R. Dethlefsen and Peter Fristrup*[a]
The substitution of platform chemicals of fossil origin by biomass-derived analogues requires the development of chemical
transformations capable of reducing the very high oxygen content of biomass. One such reaction, which has received increasing attention within the past five years, is the rheniumcatalyzed deoxydehydration (DODH) of a vicinal diol into an
alkene; this is a model system for abundant polyols like glycer-
ol and sugar alcohols. The present contribution includes
a review of early investigations of stoichiometric reactions involving rhenium, diols, and alkenes followed by a discussion of
the various catalytic systems that have been developed with
emphasis on the nature of the reductant, the substrate scope,
and mechanistic investigations.
1. Introduction
While the majority of oil, coal, and gas is used for energy production, the realization of an economy completely independent of fossil resources also requires biomass-based substitutes
for polymers, medicine, pesticides, and so forth.[1] The evergrowing world population makes it questionable to use arable
land for the growth of plastic bags instead of food,[2] but vast
amounts of non-edible, low-value products (e.g., glycerol and
sugars) are accessible today,[3, 4] and their transformation into
platform chemicals can provide an economic incentive for the
construction of biorefineries (analogues to petroleum refineries).
The direct substitution of fossil resources with bio-based resources gives rise to several complications and challenges.
While the depolymerization of biomass raw materials seems
similar in nature to the well-known cracking of crude oil, the
differences in elemental content cannot easily be overcome:
Fossil feedstocks primarily contain carbon and hydrogen, but
the oxygen content in most biomass raw materials is very
high, and chemical transformations capable of reducing it are
thus in demand. Although it is not always necessary to transform biomass into platform chemicals identical to the ones of
fossil origin (bioplastic is likely to be different from petroleumbased plastic), a reduction of its very high oxygen content is
indeed desirable.[5] Reactions that increase the oxygen content
of organic compounds have been studied in detail during the
20th century (epoxidation, dihydroxylation, etc.), but transformations that are capable of reducing the oxygen content
have, on the other hand, only recently begun to receive attention, and in spite of the recent surge in activity in this area,
only a few reactions are known. One of these is the rheniumcatalyzed deoxydehydration (DODH) of vicinal diols into the
corresponding alkenes in the presence of a reductant, which
[a] Dr. J. R. Dethlefsen, Dr. P. Fristrup
Department of Chemistry
Technical University of Denmark
Kemitorvet 207
DK-2800 Kgs. Lyngby (Denmark)
E-mail: [email protected]
2014 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
has received increasing attention during the past five years
(Scheme 1). The virtues of the reaction are the simultaneous
removal of two oxygen atoms and the formation of a carbon–
carbon double bond that can be readily functionalized.
Scheme 1. Rhenium-catalyzed deoxydehydration of a vicinal diol into an
alkene in the presence of a reductant (referred to as “red”); R1, R2 = alkyl,
aryl, or H.
The DODH can be regarded not only as a reverse dihydroxylation—it has been referred to as the didehydroxylation[6]—but
also as an overall, single-step dehydration and deoxygenation,
and is illustrated in the modified van Krevelen diagram below
(Figure 1). Van Krevelen diagrams are traditionally used for assessing the maturity and elemental content of fossil feedstocks,[7] but we find that it is equally useful for discussing
transformations of bio-based resources; in the diagram, red,
blue, and green arrows represent deoxygenation, dehydration,
and DODH reactions, respectively. Focusing on the valorization
of glycerol, its transformation into allyl alcohol could in principle occur in two different two-step processes: (a) dehydration
to hydroxyacetone followed by deoxygenation or (b) deoxygenation to 1,2-propanediol followed by dehydration. However,
both of these approaches suffer from the possibility of forming
a mixture of products (e.g., deoxygenation to both 1,2- and
1,3-propanediol) as well as the risks of either excessive deoxygenation to propanols or excessive dehydration to acrolein.[8]
The DODH of glycerol, on the other hand, can only yield the
desired allyl alcohol, which cannot react further.
Focusing on glucose, the van Krevelen diagram illustrates
that the DODH reaction is not only a short cut but also an entirely different strategy. It is well established that glucose, or
the isomer fructose, can undergo either three-fold dehydration
to 5-hydroxymethylfurfural,[9] retro-aldol cleavage and hydride
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Figure 1. Van Krevelen diagram illustrating the positions of glycerol and glucose as well as the products obtained by their deoxygenation ( ), dehydration ( ), and DODH ( ). The vertical black arrow represents the hydrogenation of glucose to sorbitol (structure omitted for clarity). Glucose and
lactic acid have identical positions in the diagram.
!
!
!
shift to lactic acid,[4] and, after reduction to its corresponding
sugar alcohol sorbitol, three-fold DODH to hexatriene.[10] These
three conversions into possible precursors for bioplastics are
Johannes Rytter Dethlefsen was born
in Denmark in 1984. He received his
M.Sc. within inorganic chemistry in
2008 and his Ph.D. on semiconductor
nanocrystals in 2011 from the University of Copenhagen under the supervision of Associate Professor Anders
Døssing and with external stays with
Professor Peter C. Ford at the University of California, Santa Barbara. Since
2012, he has done postdoctoral work
on catalyzed biomass transformations
with Associate Professor Peter Fristrup
at DTU.
Peter Fristrup was born in Aalborg,
Denmark in 1977. He completed his
chemistry studies at the Technical University of Denmark (DTU), Lyngby with
a Ph.D. in 2006 on selective homogeneous catalysis in asymmetric synthesis
under the guidance of Profs. David
Tanner and Per-Ola Norrby. After postdoctoral work (2006–2007) with Prof.
R. Madsen at DTU and W. A. Goddard III at CalTech (2008) he joined the
faculty at DTU as assistant professor in
2009. He was promoted to his current position as associate professor in 2012. His current research interests are focused on the use
of homogeneous and heterogeneous catalysis for the conversion
of biomass to chemical building blocks.
2014 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
also represented in the van Krevelen diagram; based on the
distinctly different positions in the diagram, the polymers derived from these chemicals are likely to have markedly different
properties, and the DODH reaction can therefore also be regarded as a novel route to oxygen-free plastic precursors.
With respect to the possible substrates, three fundamentally
different kinds of vicinal diols are typically employed, namely
(1) aliphatic diols, in which none of the hydroxyl groups are
benzylic, (2) benzylic diols, in which at least one hydroxyl
group is benzylic (e.g., 1-phenyl-1,2-ethanediol, referred to as
styrenediol, and hydrobenzoin, referred to as stilbenediol), and
(3) biomass-derived polyols, in which at least one hydroxyl
group is adjacent to two hydroxyl groups (e.g., glycerol, erythritol, and sorbitol). Although one or both alcohol groups in the
diol in Scheme 1 could be tertiary—for example, 2,3-dimethyl2,3-butanediol (pinacol)—this complication is rarely relevant,
as tertiary alcohols are not present in carbohydrates. Similarly,
benzylic diols are not obvious model compounds for biomassderived polyols but they have, nevertheless, received significant attention in mechanistic investigations, as their DODH
proceeds at lower temperatures than the DODH of aliphatic
diols.
The rhenium-catalyzed DODH has been studied by several
groups since it gained renewed interest in 2009.[11] It has been
highlighted twice,[12, 13] been the subtopic of two reviews,[14, 15]
and recently both the uncatalyzed and transition-metal-catalyzed DODH were treated in a book chapter;[16] within the past
two years, more than ten papers dealing with mechanistic investigations[17–19] and the development of new rhenium-based
catalysts,[20, 21] reductants,[22, 23] substrates,[24, 25] and catalysts
based on other elements[26–28] have been published, and we
therefore think that the time has come for a dedicated Minireview. The present contribution is divided into three sections:
In the first section, the relevant stoichiometric reactions involving rhenium, diols, and alkenes are reviewed, while the second
section deals with the catalytic reactions, including mechanistic
investigations; in the third section, the future scope of the reaction is discussed together with recent research in the use of
other cheaper catalysts.
Some related rhenium-catalyzed reactions, for example, epoxidation of alkenes,[29, 30] deoxygenation of epoxides,[31, 32] and
dehydration of alcohols,[33] are beyond the scope of this Minireview, and will only be included in the discussion if they are relevant for the DODH of diols.
2. Stoichiometric Reactions
It is generally accepted that the rhenium-catalyzed DODH in
Scheme 1 relies on three steps: (1) condensation of a diol and
an oxorhenium complex (oxidation state + V or + VII), (2) oxidative extrusion of an alkene from a rhenium(V) diolate under
formation of an oxorhenium(VII) complex, (3) and reduction of
rhenium(VII) to rhenium(V). These three individual reactions
are reviewed in this section.
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2.1. Formation of rhenium diolates
Although the ambiguous term “diolate” can cover
both a singly and a doubly deprotonated diol, rhenium complexes containing the monovalent diolate
ion, for example, ReO3(Hpinacolate),[34] are rare, and
the term “rhenium diolate” will therefore be used ex- Scheme 4. Formation of dimeric rhenium(VI) diolate.[37]
clusively for a rhenium center chelated by a doubly
deprotonated diol (as well as a number of auxiliary ligands, e.g., oxide or methyl). Among the various ways to preepoxide,[42] and CH3ReO2(stilbenediolate), which was prepared
pare rhenium diolates, the condensation of an oxorhenium
from the diol.[19] Since only the light-yellow compounds were
complex and a neutral diol molecule (Scheme 2) is particularly
purified by vacuum sublimation, the red color is presumably
relevant for the catalytic process so this will be the main focus
due to traces of impurities, possibly compounds similar to the
red–brown dimeric rhenium(VI) diolate reported by Herrmann
and co-workers (Scheme 4).[37]
A final route to rhenium(V) diolates is cycloaddition of an
alkene to an oxorhenium(VII) complex (Scheme 5); this route
was developed in 1993–94 by Gable and Phan,[43, 44] who
Scheme 2. Reversible condensation of an oxorhenium complex and a diol;
auxiliary ligands have been omitted.
here. Apart from the condensation of CH3ReO3 and o-catechol
(a diol incapable of undergoing DODH),[35, 36] this reaction was
first employed by Herrmann and co-workers in 1991 for the
preparation of the light-yellow CH3ReO2(pinacolate) in
CH2Cl2.[37] The reversibility of the condensation was later demonstrated for the analogues CH3ReO2(perfluoropinacolate).[38]
In addition to the preparation of rhenium(VII) diolates, the
condensation reaction is also applicable to the preparation of
rhenium(V) diolates: In 1994, Gable[39] showed that various
diols could condensate with in situ-generated Cp*ReO2 (Cp* =
C5(CH3)5)—formed by addition of a slight excess of PPh3 to
Cp*ReO3 in THF—to form the corresponding purple rhenium(V) diolates; these had previously been prepared by reacting the monovalent diolate ion and a rhenium(V) complex
with labile ligands.[40] The condensation was found to be
reversible—addition of excess meso-2,3-butanediol to
Cp*ReO(OCH2CH2O) resulted in diol–diolate exchange—and
this observation was later confirmed by Shiramizu and Toste.[10]
In 1996, Espenson and co-workers[41] showed that the three
rhenium(VII) diolates CH3ReO2(pinacolate), CH3ReO2(styrenediolate), and CH3ReO2(stilbenediolate) could be prepared by
the reaction between CH3ReO3 and the corresponding epoxides (which, at least formally, can be regarded as dehydrated
diols) 2,3-dimethyl-2-butene oxide, styrene oxide, and cis-stilbene oxide (Scheme 3). All three rhenium diolates, which were
purified by vacuum sublimation, were reported to be light
yellow, which is contrary to the reported deep, red color of
both CH3ReO2(styrenediolate), which was prepared from the
Scheme 3. Formation of a rhenium diolate by reaction between an epoxide
and an oxorhenium complex.
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Scheme 5. Reversible cycloaddition of norbornene to Cp*ReO3.
showed that reaction between Cp*ReO3 and norbornene, norbornadiene, and trans-cyclooctadiene (but not the cis isomer)
led to the formation of the corresponding Cp*ReO(diolate).
The cycloaddition was shown to be reversible, that is, the rhenium(V) diolates were shown to undergo cycloreversion, or cycloelimination, upon heating. The cycloreversion, which is usually referred to as “alkene extrusion” today, is dealt with below.
2.2. Extrusion of alkenes
The extrusion of alkenes from rhenium(V) diolates (Scheme 6)
is a general reaction that proceeds upon heating, irrespective
of the nature of both the diolate and the auxiliary ligands; the
extrusion is accompanied by a change in oxidation state of the
rhenium center of + 2, thus excluding the possibility of extruding alkenes from rhenium(VII) diolates. The reaction was dis-
Scheme 6. Thermally induced alkene extrusion from a rhenium(V) diolate.
covered in 1987, where the red–violet rhenium(V) diolate
Cp*ReO(OCH2CH2O) was shown to eliminate ethylene and form
Cp*ReO3 when heated to 150 8C under vacuum or refluxed in
toluene in the absence of oxygen.[40] If the formed alkene is
conjugated (e.g., styrene), the extrusion can occur at temperatures as low as 50 8C, while unconjugated alkenes require temperatures of at least 80 8C.[45] The mechanism of the extrusion,
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which has been extensively studied by Gable and coworkers,
will be discussed in the rest of this section.
Contrary to the extrusion of alkenes from rhenium(V) diolates, the reverse reaction, cycloaddition of alkenes to an oxorhenium(VII) complex (Scheme 5), is not general: A study of
the extrusion of ethylene, 2-butene, bicyclo[2.2.2]oct-2-ene,
and norbornene from the corresponding Cp*ReO(diolate) complexes revealed that alkene strain had little or no effect on the
activation enthalpies and entropies; the cycloaddition of alkenes to Cp*ReO3, on the other hand, was only possible for
strained alkenes, namely norbornene, norbornadiene, and
trans-cyclooctene (possibly explaining why the cis isomer did
not react).[44] This study was corroborated in 1995 by an investigation of the effect of methyl and phenyl substituents on the
rate of alkene extrusion from various Cp*ReO(diolate) complexes.[45] While phenyl-substituted alkenes were extruded significantly faster than methyl-substituted ones, the results were
mainly used in an attempt to discern between the different extrusion mechanisms (Scheme 7), namely the stepwise heterolytic or homolytic bond cleavage, the reverse (2+2) cycloaddition (a stepwise CH2 migration), and the reverse (3+2) cycloaddition (a concerted mechanism).
Scheme 7. Overview of the proposed alkene extrusion mechanisms: (a) the
stepwise heterolytic or homolytic bond cleavage, (b) the reverse (3 + 2) cycloaddition (a concerted mechanism), and (c) the reverse (2 + 2) cycloaddition (a stepwise CH2 migration).
The activation enthalpies and entropies as well as the secondary deuterium kinetic isotope effect (KIE) for the extrusion of
alkenes from Cp*ReO(diolate) were only consistent with the
stepwise CH2 migration.[44] No positive evidence was, however,
obtained, and the study was therefore corroborated by a determination of the activation enthalpies and entropies for the extrusion of methyl- and phenyl-substituted alkenes,[45] which
suggested that a flat ring structure inhibited the extrusion, and
a Hammett study of the extrusion of substituted styrenes,[46]
which showed a buildup of electron density on the reacting
carbon; both studies were in agreement with the CH2 migration pathway.
A more elaborate determination of the primary (13C/12C) and
secondary (2H/1H) KIEs at both the a and the b positions for
the extrusion of p-methoxystyrene from Tp’ReO(diolate) (Tp’ =
hydridotris(3,5-dimethylpyrazolyl)borate) revealed that it fol 2014 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
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lowed a highly asynchronous, concerted pathway with most
bond cleavage at the a carbon.[47] The apparent contrast to
previous results was explained by a Hammett study that revealed that two different mechanisms were in competition,
thus indicating a highly flexible transition state structure. Interestingly, the suggestion of a highly asynchronous but concerted pathway was also the results of the intense investigations
into the mechanism of the osmium-catalyzed dihydroxylation,[48] thus suggesting similar mechanisms for the two opposite reactions.
The use of Tp’ instead of Cp* had previously been shown to
result in very similar activation enthalpies and entropies for the
alkene extrusion, thus suggesting that the extrusion proceeds
through similar pathways in spite of the substantially different
electronic properties and steric demands of the two ligands.[49]
The recently proposed alkene extrusion from a rhenium(III)
diolate[19] will be discussed in Section 3.2.
2.3. Reduction of rhenium(VII) to rhenium(V)
The redox chemistry of rhenium complexes is very rich, so the
focus will be on the reduction of rhenium(VII) to rhenium(V) by
reductants that have been employed in the catalytic DODH.
The reduction of a rhenium(VII) diolate to a dimeric rhenium(VI) diolate has already been mentioned (Scheme 4);[37] the
nature of the reductant was not resolved. A monomeric rhenium(VI) bisdiolate has been prepared by Sillanp and coworkers,[34] who reduced the rhenium(VII) hydrogendiolate ReO3(Hdiolate) by PPh3 in the presence of one equivalent of diol
and isolated the red ReO(diolate)2. For more elaborate discussions on the topic, see Refs. [50] and [51].
Before the renewed interest in the catalytic DODH in 2009,
the use of PPh3 as reductant for oxorhenium(VII) complexes
was the most prevalent choice, illustrated by the aforementioned in situ generation of Cp*ReO2 by addition of a slight
excess of PPh3 to a solution of Cp*ReO3 in THF.[39] In addition
to being a reductant, PPh3 can also act as a ligand: Addition of
two equivalents of PPh3 to CH3ReO3 in diethyl ether led to the
precipitation of the red–brown, mixed-valence complex
CH3ReVO2(PPh3)2·CH3ReVIIO3 with an oxygen bridge between the
two rhenium centers,[52] whereas the addition of six equivalents
allowed the isolation of only the orange rhenium(V) entity, that
is, CH3ReO2(PPh3)2.[19] Herrmann and co-workers[53] were able to
trap the in situ-generated CH3ReO2, formed by reduction of
CH3ReO3 by polymer-bound PPh3 at 25 8C in toluene, through
coordination of various alkynes to the rhenium center. At room
temperature, the reduction of CH3ReO3 by PPh3 occurs almost
immediately upon mixing, but at 0 8C, the complex
CH3ReO3(PPh3) is stable for approximately 30 min; irradiation of
this complex (l = 405 nm) results in a photoinduced redox reaction with a quantum yield of 0.09.[54]
Secondary alcohols have evolved to become the most sustainable choice of reductant (see Section 3.2), but the investigation of their reduction of CH3ReO3 in the absence of a diol
has been hampered by excessive reduction.[10, 19] That said,
Toste and Shiramizu[10] were, analogous to Herrmann and coworkers,[53] able to trap in situ-generated CH3ReO2—formed by
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reduction of CH3ReO3 by 3-pentanol at 155 8C in CHCl3—
through coordination of 3-hexyne to the rhenium center.
3. The Catalytic DODH Reaction
As mentioned before, there is a consensus that the DODH proceeds in three steps: condensation of the diol and an oxorhenium(VII) or oxorhenium(V) complex, reduction of rhenium(VII)
to rhenium(V), and extrusion of the alkene from the rhenium(V)
diolate accompanied by reformation of the rhenium(VII) complex. The two main points of dispute are (1) the sequence of
the condensation and reduction and (2) the identification of
the rate-limiting step. The two fundamentally different pathways that can be envisioned were originally outlined by Nicholas and co-workers[55] and are shown in Scheme 8.
The reason for the two disputes is undoubtedly that both
are critically dependent on the nature of the reductant. In
Table 1, the various approaches to the rhenium-catalyzed
DODH have been compiled; with that basis, this section will
contain a comparison of the reactions with emphasis on
choice of catalyst, reductant, and substrate. Mechanistic investigations have been carried out for some of the systems, and
they will be discussed concurrently.
Scheme 8. The two fundamentally different pathways for the rhenium-catalyzed DODH of a vicinal diol driven by oxidation of a reductant (referred to
as “red”); X = CH3, O , Cp*, or Tp’. The possible coordination of additional
nucleophiles to the rhenium center has been omitted for clarity.
Table 1. Rhenium-based catalysts and reductants.
Entry
Catalyst[a]
Reductant
Substrate (alkene yield)
Ref.
1
Cp*ReO3
PPh3
benzylic diol (~ 100 %), erythritol (80 % butadiene)
[56]
2
CH3ReO3
H2
epoxide (95 %), aliphatic diol (60 %)
[11]
3
Bu4NReO4
CH3ReO3
Na2SO3
benzylic diol (71 %), aliphatic diol (90 %), erythritol (27 % butadiene, 6 % 2,5-dihydrofuran)
benzylic diol (59 %), aliphatic diol (80 %)
[42, 55]
4
Re2(CO)10
28 alcohol
aliphatic diol (87 %), erythritol (62 % 2,5-dihydrofuran; only cooled to room temp.)
[6]
5
CH3ReO3 or ReO4
none
3-octanol
1-heptanol
glycerol (96 %),[b] erythritol (46 %)[b]
glycerol (74 %)[b]
glycerol (55 %),[b] erythritol (58 %)[b]
[59]
6
CH3ReO3
28 alcohol
aliphatic diol (95 %), glycerol (90 % allyl alcohol), erythritol (89 % 1,3-butadiene),
dL-threitol (81 % 1,3-butadiene), sorbitol (54 % E-hexatriene)
[10]
7
NH4ReO4
CH3ReO3
BnOH
BnOH
3-octanol
aliphatic diol (50 %), monoglyceride (80-90 %)
glycerol (47 %)
glycerol (70 %)
[22]
8
ReOx-C
H2
BnOH
tetralin
aliphatic diol (39 %), aliphatic diol (56 %)
aliphatic diol (52 %)
aliphatic diol (40 %)
[20]
9
CpttReO3
PPh3
3-octanol
aliphatic diol (94 %), benzylic diol (99 %), glycerol (91 %)
erythritol (67 % 1,3-butadiene)
[21]
10
CH3ReO3
HReO4
3-pentanol
1-butanol
mucic acid (57 % trans,trans-muconic acid)
diester of mucic acid (94 % diester of trans,trans-muconic acid)
[24]
11
CH3ReO3
3-pentanol
mucic acid (99 % trans,trans-muconic acid)
[25]
12
NH4ReO4
Zn, Fe, Mn, or C
aliphatic diol (64-69 %), benzylic diol (Zn: 46 %), diethyl tartrate (Zn: 85 % diethyl fumarate),
monoglyceride (Zn: 51 %)
[23]
[a] Cp* = C5(CH3)5 ; Cptt = 1,2,4-tri(tert-butyl)cyclopentadienyl radical. [b] Yields were reported as “yield of volatile products”.
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3.1. Initial studies: PPh3, H2, and Na2SO3 as reductants
The first rhenium-catalyzed DODH was designed in 1996 by
Cook and Andrews (Table 1, entry 1),[56] whose objective was to
develop a method that could reduce the oxygen content of
carbohydrates. They showed that styrenediol was quantitatively converted into styrene in chlorobenzene at 90 8C using PPh3
as reductant and Cp*ReO3 as catalyst, thus relying on the work
by Gable on condensation[39] and extrusion.[45] The extrusion
was concluded to be the rate-limiting step. In addition to this
transformation, they showed that not only erythritol and 3butene-1,2-diol but also cis-2-butene-1,4-diol could be converted into 1,3-butadiene. The observation that cis-2-butene-1,4diol isomerized to 3-butene-1,2-diol was corroborated in 1998
by Espenson and Gordon,[57] who investigated the CH3ReO3catalyzed isomerization (or 1,3-transposition, see Scheme 9) of
allylic alcohols, and in 2013 by Shiramizu and Toste, who expanded the scope of the isomerization to diols, in which the
hydroxyl groups were separated by two carbon–carbon
double bonds (e.g., 2,4-hexadiene-1,6-diol).[24]
Scheme 9. Rhenium-catalyzed 1,3-transposition of allylic alcohols in benzene
at room temperature; in the case R = n-C5H11, the equilibrium concentrations
of the primary and secondary alcohols are ~ 1:2.[57]
After the publication of the Cook and Andrews paper, no
new contributions to the rhenium-catalyzed DODH of diols
were made until 2009, where Abu-Omar and co-workers
showed that CH3ReO3 catalyzed the H2-driven transformation
of vicinal diols (and epoxides) into the corresponding alkenes
(Table 1, entry 2).[11] The reaction was carried out in THF in an
autoclave pressurized with 5–20 bar of H2 and heated to
150 8C for 0.5–16 h. The advantage of using H2 as the reductant is that the only byproduct is water, which is already an inevitable product, but the attainment of a high alkene yield
was hampered by hydrogenation of the alkene to the undesired alkane, the value of which is inherently lower due to its
more difficult functionalization. In addition, the method was
not directly applicable to biomass-derived polyols like erythritol that decomposed under the reaction conditions. Abu-Omar
and co-workers initially proposed that the reduction occurred
before the condensation (thus favoring the right-hand side of
Scheme 8), but a subsequent DFT study of the reaction by Lin
and co-workers[58] revealed that the energy for the reduction
of CH3ReO3 to CH3ReO2 by H2 was significantly higher than for
the reduction of the rhenium(VII) diolate to the corresponding
rhenium(V) diolate (182 vs. 128 kJ mol1), and it was therefore
concluded that the pathway shown in the left-hand side of
Scheme 8 was indeed the correct one. The calculation was performed on styrene oxide (instead of styrenediol), and the ratedetermining step was the addition of the epoxide to CH3ReO3,
which resulted in the formation of the corresponding rhenium(VII) diolate. That said, the formation of the rhenium(VII)
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diolate from the condensation of the diol and CH3ReO3 was reported to be a fast equilibrium, thus possibly making another
step rate-determining in catalytic cycles involving diols instead
of epoxides.
In an attempt to employ an inexpensive catalyst with low
toxicity, strong reducing potential, and potential recyclability,
Nicholas and co-workers developed a DODH system driven by
oxidation of sodium sulfite, Na2SO3 (Table 1, entry 3).[42, 55] The
reaction was carried out in benzene, chlorobenzene, THF, or
CH3CN heated to 150–160 8C using CH3ReO3 or various
perrhenate salts as catalysts, and it was applicable to aliphatic
diols, benzylic diols, and erythritol, although the reaction of
the latter was very slow. The different activity of CH3ReO3 and
the perrhenate salts was ascribed to different solubilities and
coordination of the solvent to the Lewis-acidic CH3ReO3.
A tentative investigation of the individual steps in the sulfite-driven, CH3ReO3-catalyzed DODH of styrenediol under stoichiometric conditions revealed that the extrusion of styrene
was the rate-limiting step,[42] and this conclusion was confirmed and corroborated in a subsequent DFT study[18] on the
DODH of ethylene glycol: The reduction of CH3ReO3 was found
to occur before the condensation between the rhenium(V)
complex and ethylene glycol, and the extrusion was calculated
to be the rate-determining step, although its activation energy
was critically dependent on the coordination of auxiliary ligands like water and NaSO3 to CH3ReO(OCH2CH2O). The electrophilicity
of
the
rhenium
complexes
CH3ReO3,
CH3ReO2(OCH2CH2O), CH3ReO2, and CH3ReO(OCH2CH2O) was
underlined by the free energy changes upon coordination of
a water molecule: These were calculated to be + 15, + 28,
44, and 45 kJ mol1, respectively, and at least the rhenium(V) complexes are thus prone to being ligated by nucleophiles like the inevitably generated water, the solvent (e.g.,
THF), or the reductant (e.g., PPh3 or alcohols).
3.2. Secondary alcohols as reductants
Of particular interest is the use of a secondary alcohol as the
reductant (Table 1, entry 4), which was developed by Bergman
and Ellman in 2010.[6] The alcohol, which was also used as solvent, was typically heated to 155–180 8C for 1–4 h, and, in addition to aliphatic diols, which were converted to the corresponding alkenes in yields up to 87 %, erythritol underwent
a single DODH, forming 2,5-dihydrofuran in a yield of 62 %; the
formation of 1,3-butadiene (bp 4.4 8C) cannot be excluded as
the reaction mixture was only cooled to room temperature.
The employed catalysts were Re2(CO)10 and BrRe(CO)5, but the
reaction only proceeded in the presence of air, thus indicating
that higher oxidation states of rhenium (possibly + V and + VII)
were involved in the catalytic cycle. This hypothesis was later
confirmed independently by Abu-Omar and co-workers[59] and
Toste and Shiramizu,[10] who showed that various rhenium(VII)
compounds, CH3ReO3, NH4ReO4, and others, were equally efficient, even when the reaction was conducted under N2. Bergman and Ellman observed that while cis-1,2-cyclohexanediol
was converted to cyclohexene in a yield of 72 %, the trans
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isomer did not react at all; this observation was also confirmed
by Abu-Omar and Toste.
One virtue of using a secondary alcohol as the reductant is
the possibility of using the diol itself as the reductant. This approach was pursued by Abu-Omar and co-workers (entry 5),[59]
who, in an open distillation setup, heated neat glycerol
(Scheme 10) or erythritol and a rhenium(VII)-based catalyst
Not only the secondary alcohols 3-octanol and 3-pentanol
were tested for the DODH of 1,4-anhydroerythritol; cheaper alcohols that could possibly be derived from sustainable sources
(and be more suited for the dissolution of carbohydrates) were
also tested, and, surprisingly, 1-butanol was a much better reductant than 2-butanol (70 % vs. 0 % yield), whereas 3-pentanol
was better than 2-pentanol which, in turn, was better than 1pentanol (alkene yields of 91 %, 78 %, and 51 %, respectively). Ethanol was completely unreactive.
The mechanism of the alcohol-driven CH3ReO3-catalyzed DODH of 1,4-anhydroerythritol was investigatScheme 10. Rhenium-catalyzed DODH of glycerol to allyl alcohol driven by oxidation of
ed by Wang and co-workers in 2013.[17] Although
glycerol itself to 1,3-dihydroxyacetone; the reaction was performed in neat glycerol at
both pathways shown in Scheme 8 were found to be
165 8C.[59]
wrong, the new proposed mechanism is similar to
the right-hand side of Scheme 8 (i.e., reduction
(CH3ReO3 or a perrhenate salt) to 165 8C for several hours. For
before condensation), the main difference being that CH3ReO3
the DODH of glycerol, the highest yield (reported as “yield of
is reduced to CH3ReO(OH)2 instead of CH2ReO2, which lowers
volatile products”, which include not only allyl alcohol but also
the highest energy barrier from 189 kJ mol1 to 164 kJ mol1
acrolein, propanal, and probably water) was 96 %. For erythri(i.e., the rate-determining step is the reduction). The left-hand
tol, the yield of volatile products (2,5-dihydrofuran, trans-2-buside, where the condensation occurs before the reduction, was
tenal, furan, and water) was 46 %; the products were collected
discarded due to the very high barrier (222 kJ mol1) for the reover an ice bath, and the formation of 1,3-butadiene can
duction of the rhenium(VII) diolate. The first step in this distherefore not be excluded. While CH3ReO3 appeared to be
carded pathway, condensation between CH3ReO3 and the diol,
more active than the perrhenate salts, the effects of the addiwas calculated to have a barrier of 169 kJ mol1, but as this
tives NaCl, KCl, HCl, and NH4Cl were just as important, albeit
contradicted the reversible condensation under ambient condinot easily rationalizable: The yield of volatile products in an extions (cf. discussion in Section 2.1), Wang and co-workers conperiment using 2 mol % of NaReO4 and 1.5 mol % of NH4Cl was
cluded that the entropic penalties were overestimated and
that a more correct value was 112 kJ mol1.
more than twice as high as that in an experiment using
2 mol % of NH4ReO4 and 1.5 mol % of NaCl. A KIE of 2.4 was
Indirect evidence for the proposed intermediate
CH3ReO(OH)2 may have been obtained by Gable in 2003:[60] In
observed when glycerol deuterated at the carbon atoms was
employed, whereas glycerol deuterated at the oxygen atoms
the related Tp’ReO3-catalyzed deoxygenation of cis-stilbene
did not give rise to any KIE, thus suggesting that cleavage of
oxide and styrene oxide driven by oxidation of PPh3, the paththe CH bond is involved in the rate-determining step.
way was proposed to proceed via Tp’ReO2, but only
The applicability of the alcohol-driven DODH to a number of
Tp’ReO(OH)2, which displayed the same catalytic activity as
biomass-derived sugars and sugar alcohols was demonstrated
Tp’ReO3, could be isolated.
by Toste and Shiramizu (Table 1, entry 6).[10] After demonstratIn 2013, Toste and Shiramizu (Table 1, entry 10) showed that
ing the viability of using CH3ReO3 as the catalyst for the DODH
mucic acid underwent two successive DODHs to form trans,of the aliphatic diol 1,4-anhydroerythritol to 2,5-dihydrofuran
trans-muconic acid (Scheme 11). Using the dibutyl ester of
(92 %) driven by the oxidation of the solvent 3-octanol at
mucic acid, 1-butanol as the reductant, and HReO4 as the catalyst, they obtained a yield of 94 % after 6 h at 170 8C; employ170 8C, they showed that, under the same conditions, glycerol
ing a one-pot, two-step strategy, subsequent hydrogenation of
was converted to 90 % of allyl alcohol, while erythritol was
the diester of trans,trans-muconic acid afforded the diester of
converted to 89 % of 1,3-butadiene and 11 % of 2,5-dihydrofuradipic acid in a yield of 62 %. Su and Zhang (Table 1,
an. Using 3-pentanol as reductant and solvent, it was possible
entry 11)[25] improved the procedure and obtained almost
to convert the two hexitols sorbitol and mannitol to 1,3,5-hexatriene in 54 % yield, and the three pentitols xylitol, arabinitol,
quantitative yields by lowering the temperature to 120 8C and
and ribitol underwent two successive DODHs to form 2,4-penusing CH3ReO3 as catalyst and 3-pentanol as reductant. Their
tadiene-1-ol in yields of 61 %, 43 %, and 33 %, respectively.
DFT calculations of the mechanism were similar to those of
Based on stoichiometric reactions between the in situ-generatWang,[17] the most important addition being that the second
ed rhenium(V) complex mentioned in Section 2.1 and various
DODH was much faster than the first.
diols, it was concluded that the
reduction
of
CH3ReO3
to
CH3ReO2 occurred before the
condensation between CH3ReO2
and the diol; whether the ratedetermining step was reduction
Scheme 11. Rhenium-catalyzed DODH of mucic acid to trans,trans-muconic acid driven by the oxidation of an alor extrusion could not be deter- cohol and subsequent hydrogenation to adipic acid; the acids are found as a mixture of the free acid and the
mined.
esters of the reducing alcohol.[24, 25]
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Both pathways in Scheme 8 rely on the ability of the oxorhenium complexes to cycle between the oxidation states + V and
+ VII, but a fundamentally different mechanism involving an
oxorhenium(III) diolate has recently been proposed by AbuOmar and co-workers.[19] for the CH3ReO3-catalyzed DODH of
stilbenediol driven by oxidation of 3-octanol. The conclusion,
which is supported by an observed inability of the rhenium(V)
diolate CH3ReO(stilbenediolate) to extrude stilbene, is contradictory to every other paper on the subject (see Section 2.2),
and more work is therefore needed to confirm it or provide an
alternative explanation of the results.
3.3. Other reductants and catalysts
In addition to developing the use of Na2SO3 as the reductant,
Nicholas has proved that benzyl alcohol (Table 1, entry 7),[22]
tetralin (entry 8),[20] and the elements Zn, Fe, Mg, and C
(entry 12)[23] are efficient. In addition, Nicholas and Jentoft developed the first example of a heterogeneous rhenium catalyst,
namely carbon-supported perrhenate (entry 8).[20] Notwithstanding the qualities of many of the catalytic reactions, the
turn-over numbers (TONs) are typically low (< 200), but Klein
Gebbink and co-workers (entry 9)[21] have obtained a TON of
1400 for the DODH of 1,2-octanediol driven by the oxidation
of PPh3 in chlorobenzene at 180 8C using the bulky CpttReO3 as
catalyst (Cptt = 1,2,4-tri(tert-butyl)cyclopentadienyl), thus giving
hope for economically viable transformations with rhenium
catalysts.
4. Summary and Outlook
The transformation of glycerol to allyl alcohol, erythritol to 1,3butadiene, sorbitol to 1,3,5-hexatriene, and mucic acid to
adipic acid are the most prominent examples on the employment of the rhenium-catalyzed DODH for the conversion of
biomass-derived polyols to platform chemicals for the existing
chemical industry. Notwithstanding the high yields and purities
that have been attained, the reactions are not on the verge of
becoming economically viable due to problems regarding
both the catalyst and the reductant.
With respect to the reductant, the very efficient PPh3 is too
expensive, while the very cheap H2 is too efficient, resulting in
reduction to metallic rhenium. Alcohols appear to represent
a better balance between price and efficiency, but they need
to be recycled in order to make the process sustainable. In addition, secondary alcohols like 3-pentanol and 3-octanol are
not good at dissolving polyols, the concentrations of which
have to be, consequently, very low.
With respect to the catalyst, elements other than the scarce
rhenium, which, as a byproduct of molybdenum, has an extremely volatile price,[61] have been suggested as better catalysts for the large-scale DODH of glycerol, namely vanadium[27]
and molybdenum.[26, 28]
Nicholas and co-workers[27] reported that various vanadium(V) complexes, and in particular (Bu4N)VO2(dipic)
(H2dipic = dipicolinic acid), catalyzed the DODH of aliphatic
and benzylic diols and even the diester of tartaric acid to the
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corresponding alkenes in yields as high as 97 %. The reaction
was most efficiently driven by the oxidation of PPh3 and was
conducted in benzene or chlorobenzene at 150–170 8C over
24–96 h.
In a paper primarily concerned with the preparation of new
dioxomolybdenum(VI) complexes,[26] it was noted that some of
the new complexes catalyzed the deoxygenation of styrene
oxide driven by oxidation of PPh3, but the attempted DODHs
of diols in toluene at 110 8C using PPh3 as reductant were relatively unsuccessful: The highest yield was 55 %, and only 28 %
of OPPh3 was formed. The reaction was further elucidated by
Fristrup and co-workers,[28] who discovered that although
a number of molybdenum(VI) compounds, most notably the
cheap and commercially available (NH4)6Mo7O24·4 H2O, catalyzed the DODH of aliphatic diols and glycerol, the catalysts
were only able to oxidize the substrates (i.e., the aliphatic diol
or glycerol) themselves (Scheme 12). Although the stoichiometry of this reaction inevitably limits the maximum amount of
formed alkene to 50 % of the added amount of substrate, the
cheapness of a diol like glycerol can justify the sacrificial oxidation of half of it; this has already been proposed by Abu-Omar
and co-workers.[59]
Scheme 12. Molybdenum-catalyzed DODH of a vicinal diol into an alkene
driven by the oxidative deformylation of the diol itself into formaldehyde
and the aldehyde with one less carbon atom.[28]
In addition to the transition metal-catalyzed DODH, the
transformation can also be carried out without a catalyst: Glycerol can, for instance, react with one equivalent and formic
acid forming allyl alcohol in a yield of 80 % (Scheme 13).[62]
Scheme 13. Catalyst-free conversion of glycerol into allyl alcohol by reaction
with formic acid.[62]
Although the rhenium-catalyzed DODH has evolved dramatically in recent years, there are still significant challenges remaining with respect to reductant and catalyst. Only time will
tell whether the chemical community is able to overcome
these challenges and develop the methodology to a level
where large-scale biomass conversion becomes economically
viable. Recent developments based on cheap and earth-abundant metals such as vanadium and molybdenum gives some
hope for the future utility of the DODH reaction.
Acknowledgements
The research was supported by a Sapere Aude research leader
grant (P.F.) from the Danish Council for Independent Research,
Grant no. 11-105487.
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Keywords: biomass · catalysis · deoxydehydration · polyols ·
rhenium
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Received: September 12, 2014
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J. R. Dethlefsen, P. Fristrup*
&& – &&
Rhenium-Catalyzed Deoxydehydration
of Diols and Polyols
2014 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
Taking a shortcut! Recent developments in the rhenium-catalyzed deoxydehydration reaction are reviewed. Although the reaction is formally a deoxygenation and a dehydration, the use of
transition metal catalysis opens up new
possibilities. The focus is on recent developments of the substrate scope, alternative reductants, and mechanistic
investigations.
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