N,N,O-Coordinated tricarbonylrhenium precatalysts for the aerobic deoxydehydration of diols and polyols

Article abstract of DOI:10.1039/d0cy00618a

Rhenium complexes are well known catalysts for the deoxydehydration (DODH) of vicinal diols (glycols). In this work, we report on the DODH of diols and biomass-derived polyols using L⁴Re(CO)₃as precatalyst (L⁴Re(CO)₃= tricarbonylrhenium 2,4-di-tert-butyl-6-(((2-(dimethylamino)ethyl)(methyl)amino)methyl)phenolate). The DODH reaction was optimized using 2 mol% of L⁴Re(CO)₃as precatalyst and 3-octanol as both reductant and solvent under aerobic conditions, generating the active high-valent rhenium speciesin situ. Both diol and biomass-based polyol substrates could be applied in this system to form the corresponding olefins with moderate to high yield. Typical features of this aerobic DODH system include a low tendency for the isomerization of aliphatic external olefin products to internal olefins, a high butadiene selectivity in the DODH of erythritol, the preferential formation of 2-vinylfuran from sugar substrates, and an overall low precatalyst loading. Several of these features indicate the formation of an active species that is different from the species formed in DODH by rhenium-trioxo catalysts. Overall, the bench-top stable and synthetically easily accessible, low-valent NNO-rhenium complex L⁴Re(CO)₃represents an interesting alternative to high-valent rhenium catalysts in DODH chemistry.

Full text of DOI:10.1039/d0cy00618a

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DOI: 10.1039/D0CY00618A
ARTICLE
N,N,O- coordinated Tricarbonylrhenium Precatalysts for the
Aerobic Deoxydehydration of Diols and Polyols
Jing Li, a Martin Lutz b and Robertus J. M. Klein Gebbink *a
Received 00th January 20xx,
Accepted 00th January 20xx
Rhenium complexes are well known catalysts for the deoxydehydration (DODH) of vicinal diols (glycols). In this work, we
report on the DODH of diols and biomass-derived polyols using L4Re(CO)3 as pre-catalyst (L4Re(CO)3 = tricarbonylrhenium
2,4-di-tert-butyl-6-(((2-(dimethylamino)ethyl)(methyl)amino)methyl)phenolate). The DODH reaction was optimized using 2
mol% of L4Re(CO)3 as pre-catalyst and 3-octanol as both reductant and solvent under aerobic conditions, generating the
active high-valent rhenium species in situ. Both diol and biomass-based polyol substrates could be applied in this system to
form the correspoing olefins with modrate to high yield. Typical features of this aerobic DODH system include a low tendency
for the isomerization of aliphatic external olefin products to internal olefins, a high butadiene selectivity in the DODH of
erythritol, the preferential formation of 2-vinylfuran from sugar substrates, and an overall low precatalyst loading. Several
of these features indicate the formation of an active species that is different from the species formed in DODH by rhenium-
trioxo catalysts. Overall, the bench-top stable and synthetically easily accessible, low-valent NNO-rhenium complex
L4Re(CO)3 represents an interesting alternative to high-valent rhenium catalysts in DODH chemistry.
DOI: 10.1039/x0xx00000x
the development of this type of Cp’Re complexes is that the
overall yield of the synthesis of the final Cp’ReO3 complexes is
rather low (4% to 49% starting from Cp’H/Re), mainly because
the trioxo-rhenium species decomposes under the oxidizing
conditions of its synthesis from Cp’Re(CO)3.13 A possible way to
solve this problem is to use Cp’Re(CO)3 as a pre-catalyst to form
the corresponding Cp’ReO3 species in situ during catalysis in the
presence of oxygen. Yet, on the basis of the work of Ellman and
Bergman, Cp*Re(CO)3 is not active towards the
deoxydehydration of diols under aerobic conditions.12
For this reason, we were interested in other low-valent Re-
complexes that would allow for an oxidative activation to
provide in-situ access to high-valent Re-trioxo complexes for
DODH catalysis. Accordingly, we have started to investigate Re-
complexes derived from anionic NNO-type ligands. Such ligands
provide an anionic 6e– platform that is isoelectronic to Cp’-type
ligands, and potentially coordinated to Re in a facial fashion
similar to Cp’ ligands. Here, we report on the synthesis and
characterization of novel low-valent (NNO)Re(I) tricarbonyl
complexes, in which NNO represents an open chain bis-amino-
Introduction
Due to the increased consumption of fossil hydrocarbon
resources, the development of alternative and sustainable
feedstocks is highly recommended.1,2 Biomass-derived
feedstocks are considered as attractive and renewable
resources for a sustainable chemical industry.3,4 One of the
many challenges in biomass conversion and valorization is the
defunctionalization of oxygen-rich molecular components
obtained after depolymerization of cellulosic biomass.5
Deoxydehydration (DODH) reactions, which formally combine
deoxygenation6,7 and dehydration,8 can efficiently remove
vicinal hydroxyl groups to form the corresponding olefin. The
successful application of DODH reactions on oxygen-rich
biomass components would provide access to a variety of low-
volume, high-value chemicals.9–11
High-valent rhenium complexes are known as catalysts for the
DODH of diols and polyols,9 while low-valent rhenium
complexes, e.g., Re2(CO)10 and BrRe(CO)5, can also initiate
DODH reactions in the presence of oxygen.12 Among the high-
valent rhenium complexes, Cp-based trioxorhenium complexes
(Cp’ReO3, Cp’ = substituted cyclopentadienyl) show high
catalytic activity and DODH product selectivity. A drawback in
phenol
ligand
(NNO
=
2-(((2-
(dimethylamino)ethyl)(methyl)amino)methyl) phenol), and on
the use of these complexes as precatalysts in the aerobic DODH
of vicinal diols and polyols.
a. Organic chemistry and Catalysis, Debye Institute for Nanomaterials Science,
Utrecht University, Universiteitsweg 99, 3584CG, Utrecht, The Netherlands.
b. Crystal and Structural Chemistry, Bijvoet Centre for Biomolecular Research,
Faculty of Science, Utrecht University, Padualaan 8, 3584 CH, Utrecht, The
Netherlands.
† Footnotes relating to the title and/or authors should appear here.
Electronic Supplementary Information (ESI) available: synthesis and
characterization of LRe(CO)3 complexes and general catalysis procedures are
included. See DOI: 10.1039/x0xx00000x
Results and discussion
Synthesis and characterization
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The NNO-type ligands (HL) were synthesized via direct reductive conformation, in L4Re(CO)3 it is intermediate between a boat
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DOI: 10.1039/D0CY00618A
amination of N,N,N’-trimethylethylenediamine with a phenolic and twist conformation as indicated by the deviation of N1 and
aldehyde using NaBH(OAc)3 (1.5 equiv.) in dichloroethane Re1 from the least-squares plane of the phenyl ring of the
(Scheme 1).14 The preligand HL was then reflux with BrRe(CO)5 phenolate donor (Table 2). Further characterization of the
and an excess amount of NaOAc in toluene to form the complexes included 1H NMR, 13C NMR, IR, ESI-MS, and
corresponding LRe(CO)3 complexes.15 After recrystallization, all elemental analysis (see Electronic Supplementary Information).
LRe(CO)3 complexes were isolated as needle-like crystals in
>70% Re-based yield. The complexes synthesized in this way are
summarized in Figure 1.
Figure 2. Molecular structures of L1Re(CO)3, L3Re(CO)3, and L4Re(CO)3 in the crystal.
Displacement ellipsoids are drawn at the 50% probability level. Hydrogen atoms are
omitted for clarity.
Scheme 1. Synthesis of NNO-based tricarbonylrhenium complexes: a. NaBH(OAc)3 (1.5
equiv.), dichloroethane, r.t., overnight, N2; b. NaOAc (10 equiv.), toluene, reflux, 3 h, N2.
Table 1 Selected bond lengths [Å], angles and torsion angles [°] in complexes L1Re(CO)3
L1Re(CO)3, L3Re(CO)3, and L4Re(CO)3.
L1Re(CO)3
2.111(3)
2.235(4)
2.278(4)
1.330(5)
109.4(3)
39.9(6)
L3Re(CO)3
2.122(2)
2.238(3)
2.275(3)
1.309(4)
110.1(3)
43.3(5)
L4Re(CO)3
2.144(3)
2.247(3)
2.262(3)
1.354(4)
108.7(3)
-65.1(5)
-60.0(4)
Re1-O1
Re1-N1
Re1-N2
C1-O1
C7-N1-C8
C1-C6-C7-N1
N1-C8-C9-N2
57.5(5)
58.8(4)
Table 2 Deviations of N1 and Re1 [Å] from the least-squares plane of the phenyl ring (O1,
C1-C7) in complexes L1Re(CO)3 L1Re(CO)3, L3Re(CO)3, and L4Re(CO)3.
L1Re(CO)3
0.819(4)
0.0427(2)
L3Re(CO)3
0.872(3)
0.1875(4)
L4Re(CO)3
1.017(3)
1.7545(4)
N1
Re1
Figure 1. (NNO)tricarbonylrhenium complexes reported in this work.
The structures of L1Re(CO)3, L3Re(CO)3, and L4Re(CO)3 were In the IR spectrum of BrRe(CO)3, two absorption signals at 1956
established by X-ray crystal structure determination (Figure 2). and 2023 cm-1 were observed (Table 3). In case of the NNO-
Selected geometrical parameters are given in Table 1. ligand coordinated tricarbonylrhenium complexes and
Diffraction-quality crystals of L1Re(CO)3 and L3Re(CO)3 were CptttRe(CO)3, three absorption bands at 1851-1889, 1873-1921,
obtained by diffusion of diethyl ether into acetonitrile solutions and 2007-2016 cm-1 were observed. These absorption bands
of the complexes at room temperature. Diffraction-quality
crystals of L4Re(CO)3 were obtained by recrystallization from
Et2O/hexane (1:2) at –30 C. In all three complexes nitrogen
are attributed to the symmetric and asymmetric (CO)
stretches. The similarity of IR spectra indicate that the NNO-
ligand coordinated tricarbonylrhenium complexes have a
similar structure in the solid state, that is, a octahedral
geometry structure with the tripodal N,N,O-ligand coordinated
to rhenium in a facial manner.
o
atom N1 is chiral, but the compounds crystallize as racemates
in centrosymmetric space groups, respectively. They showcase
the overall octahedral geometry of the complexes discussed
here with the tripodal N,N,O-ligand coordinated to rhenium in
a facial manner. Due to the chelating bonding mode the bond
angles around Re deviate from the perfect 90° and range from
79.66(14)-96.98(17)° in L1Re(CO)3, 79.51(10)-96.66(13)° in
L3Re(CO)3, and 80.03(12)-96.17(15)° in L4Re(CO)3. The five-
membered chelate rings in L1Re(CO)3 and L3Re(CO)3 are best
described as twist conformations, while in L4Re(CO)3 the ring
adopts an envelope conformation. The six-membered chelate
rings in L1Re(CO)3 and L3Re(CO)3 are in an envelope
2 | J. Name., 2012, 00, 1-3
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then heated at 180 oC in a closed pressure tube. The amount of
Table 3 Solid-state infrared  (CO) data of penta/tricarbonylrhenium complexes.
View Article Online
DOI: 10.1039/D0CY00618A
O2 present in the reaction mixture was approximately 18 equiv.
entry
[Re]
Infrared  (CO) (cm-1)
1956, 2023
1
on the basis of L1Re(CO)3. A sample was taken for H NMR
1
2
3
4
5
6
7
BrRe(CO)5
L1Re(CO)3
L2Re(CO)3
L3Re(CO)3
L4Re(CO)3
L5Re(CO)3
CptttRe(CO)3
analysis after the mixture was heated for 3 h. 71% of 1-octene
had formed, as well as dehydration products (2-/3-octene)
derived from 3-octanol (the latter products are not derived
from isomerization of 1-octene, vide infra). The ratio of 1-
octene and 2-/3-octene was 3.23:1. In order to reach full
conversion of 1,2-octanediol, the mixture was heated for
another 3 h, after which 96% of 1-octene had formed. The ratio
of 1-octene and 2-/3-octene after 3 h was 1.71:1, which can be
explained on the basis of the continued dehydration of the 3-
octanol solvent/reductant while diol DODH reaches close to full
conversion (Table 4, entry 2). When applying BrRe(CO)5 as the
pre-catalyst under these conditions for 3 h, 1,2-octanediol was
fully converted forming 94% of 1-octene, as well as a significant
amount of 2-/3-octene (ratio 0.97:1; entry 1). BrRe(CO)5
therefore seems to promote both the deoxydehydration of 1,2-
octandiol and the dehydration of 3-octanol without a particular
preference. The tendency of L1Re(CO)3 to promote the
dehydration of 3-octanol is significantly lower as compared to
BrRe(CO)5.
1877, 1918, 2014
1859, 1886, 2007
1884, 1921, 2016
1851, 1894, 2010
1855, 1873, 2007
1889, 1911, 2007
Besides, a trend in the energies of the IR absorptions was
observed in relation to the electronic properties of the
substituents on the phenol ring. Figure
3 shows the
wavenumbers of the IR absorptions plotted against the
Hammett constants (p) of the para-substituents.16 Although
electronic effects of the ortho-substituents may also be
involved for L4Re(CO)3 and L5Re(CO)3, the overall trend clearly
shows that the more electron-donating para-substituents give
rise to lower energies of the CO vibrations. In other words, the
CO ligands are more strongly coordinated to the more electron-
rich rhenium centers.
2040
p-tBu
p-NO2
p-OMe
H
p-Cl
Table 4 Penta/tricarbonylrhenium initiated DODH of 1,2-octanediol in 3-octanol.a
2000
1960
1920
1880
1840
1800
1760
entry
1
2
[Re]
BrRe(CO)5
L1Re(CO)3
t
yield b
94%
71%
96%
83%
95%
13%
56%
13%
92%
21%
65%
8%
n(1-) : n (2-/3-)c
0.97:1
3.23:1
1.71:1
2.11:1
1.5:1
3 h
3 h
6 hd
3 h
6 hd
3 h
6 hd
6 h
3 h
3 h
6 hd
3 h
6 hd
3 h
3
4
5
L2Re(CO)3
L2Re(CO)3
L3Re(CO)3
0.89:1
2.77:1
0.74:1
2.52:1
1.23:1
2.71:1
0.62:1
0.62:1
14:1
6
7
8
L3Re(CO)3
L4Re(CO)3
L5Re(CO)3
-0.5
0
0.5
1
σp
Figure 3. Relation between the Hammett parameter (p) of the phenolate para-
substituent and the IR wavenumbers of the CO vibrations for LRe(CO)3 complexes.
9
CptttRe(CO)3
L4Re(CO)3
8%
56%
10e
Catalytic evaluation
a) Reaction conditions: 1,2-octanediol (0.5 mmol), rhenium pre-catalyst (0.01
mmol, 2 mol%), 3-octanol (5 mL), 180 oC (temperature of oil bath), air. b) 1-Octene
yield determined by 1H NMR using mesitylene (0.5 mmol) as an internal standard.
c) Ratio of 1-octene and 2-/3-octene product isomers. d) The reaction mixture was
heated in a closed pressure tube with air for 3 h, then the reaction was cooled to
room temperature, a sample was taken from the reaction mixture. After sampling
the reaction mixture, the pressure tube was closed again and heated for another
3 h, in which case more air entered the reaction system. e) Instead of 5 mL, 1 mL
of 3-octanol was used. Conversion of 1,2-octanediol was 56%.
With the new NNO-Re complexes in hand, we started our
catalytic investigation by applying the parent L1Re(CO)3 complex
(2 mol%) to the deoxydehydration of 1,2-octanediol using
triphenylphosphine (PPh3) as reductant and chlorobenzene as
solvent. Running the reaction under oxygen-free conditions
gave no 1-octene formation, while 12% of 1-octene was formed
under aerobic conditions. This result indicated that an active
DODH species, likely a high-valent rhenium species, can be
formed starting from L1Re(CO)3 in the present of oxygen. Since
secondary alcohols have been reported as both reductants and
solvents for Re-catalyzed DODH reactions,12 we also
investigated the L1Re(CO)3 initiated DODH of 1,2-octanediol
using 3-octanol as both reductant and solvent: 0.5 mmol of 1,2-
octanediol was dissolved in 5 mL of 3-octanol, 2 mol% of
L1Re(CO)3 was used as pre-catalyst (Table 4). The mixture was
After this initial study, the other NNO-based tricarbonylrhenium
complexes were also investigated using the same DODH
conditions (Table 4). With an electron-donating methoxy group
at the para-position of the phenol moiety (complex L2Re(CO)3),
a slightly higher 1-octene yield (83%) was obtained after 3 h
(entry 3). Upon full 1,2-octanediol conversion after 6 h, the 1-
octene yield was 95% (entry 4). With an electron-withdrawing
This journal is © The Royal Society of Chemistry 20xx
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nitro substituent at the para-position of the phenol (complex was selected as the pre-catalyst for further invesViteiwgaArttiioclenOanlninde
L3Re(CO)3), a 1-octene yield of only 13% was achieved after 3 h optimization of DODH reactions of diols and polyols.
DOI: 10.1039/D0CY00618A
(entry 5). After having taken a sample for analysis, and exposing As the next step in the protocol optimization, different alcohol
the reaction mixture to more air, the 1-octene yield increased reductants, including primary and secondary alcohols, were
to 56% after closing the reaction tube again and heating it for investigated (Table 5). No 1-octene was detected when 1-
another 3 h. When this same reaction was carried for 6 h butanol and 2-butanol were used as reductant and solvent
without opening the reaction tube (entry 6), the 1-octene yield (entries 1 and 2). Surprisingly, with 3-pentanol and 2,4-
was only 13%, i.e. the same as heating the mixture for 3 h. This dimethyl-3-pentanol, which have been reported as good
result indicates that the activation of L3Re(CO)3 was not reductants in high-valent rhenium-catalyzed DODH
complete after heating for 3 h. After exposing the reaction reactions,12,17–21 only 14% and trace amounts of 1-octene were
mixture to more oxygen/air, L3Re(CO)3 was further activated, formed, respectively (entry 3 and 4). When using 3-pentanol, a
resulting in an enhanced formation of 1-octene, as well as an small amount (4%) of 2-pentene was observed. Other
increased 1-octene to 2-/3-octene product ratio (change from secondary alcohols, such as 2-octanol and 4-octanol, were also
0.89:1 to 2.77:1). In case of the bis(tert-butyl)-substituted investigated. When using 2-octanol as reductant and solvent,
complex L4Re(CO)3, full conversion of 1,2-octanediol was 88% of 1-octene was formed from 1,2-octanediol, and 49% (=
achieved after 3 h, achieving a 1-octene yield of 92% at a 2.52:1 n2-octene/n1,2-octanediol) of 2-octene was observed (entry 5). When
ratio of 1-octene and 2-/3-octene (entry 7). For the o,p- 4-octanol was used, only 20% of 1-octene was formed, together
dichlorinated complex L5Re(CO)3, only 21% of 1-octene was with 4% (= n3-/4-octene/n1,2-octanediol) of 3-/4-octene (entry 7).
formed after heating for 3 h, and 65% after heating the mixture Besides, 1,2-octanediol itself could also be used as reductant; in
for another 3 h after exposure to air (entry 8). As a comparison, this case 32% of 1-octene was formed (entry 8). Based on these
CptttRe(CO)3 (Cpttt = 1,2,4-tri-tert-butylcyclopentadienyl) was results, it was decided to stick with 3-octanol as the reductant
also investigated under these reaction conditions, providing and solvent in further investigations.
only 8% of 1-octene with or without additional exposure to air
and extending the reaction time. Besides, the 1-octene to 2-/3-
Table 5 L4Re(CO)3 catalyzed DODH of 1,2-octanediol using different alcohol reductants.a
octene product ratio was 0.62:1.
entry
secondary alcohol
1-butanol
2-butanol
3-pentanol
2,4-dimethyl-3-pentanol
2-octanol
yield b
n.d. c
n.d. c
14%
trace
88%
92%
20%
32%
b.p. of solvent d
116-118 oC
98 oC
114-115 oC
139-140 oC
177-182 oC
174-176 oC
174-176 oC
Interestingly, this series of experiments shows the preference
of the NNO-based tricarbonylrhenium complexes, upon
exposure to air, to engage in diol deoxydehydration over
secondary alcohol dehydration. The concentration of 1,2-
octanediol in these experiments was much lower than that of 3-
octanol. However, more deoxydehydration product from 1,2-
octanediol was formed than dehydration product from 3-
octanol. A higher concentration of 1,2-octanediol was tested
(entry 10), by decreasing the amount of 3-octanol from 5 to 1
mL. In this case, 56% of 1-octene was formed at 56% of 1,2-
octanediol conversion, i.e. full selectivity for olefin formation.
The ratio of 1-octene and 2-/3-octene in this experiment was
14:1. This result strengthens the notion that DODH is kinetically
favoured for all LRe(CO)3 pre-catalysts, although dehydration of
the secondary alcohol reductant is always observed to a
significant extent.
These initial catalysis experiments also show the beneficial
effect of electron-donating substituents at the phenol moiety
on both the overall DODH product formation and the DODH vs
dehydration product ratio. A likely explanation for this
observation is the stabilization of the high-valent ReVII center in
the anticipated trioxo-complexes by the more electron-rich
donor ligands, even though a more electron-rich Re-center
would also lead to more firmly bound CO-ligands which could
hamper oxidation of the low-valent carbonyl complexes. The
catalysis data shown in Table 4 are in line with the trend in CO
vibrational energies depicted in Figure 3. Based on these
observations, the di(tert-butyl) substituted complex L4Re(CO)3
1
2
3
4
5
6
7
8
3-octanol
4-octanol
none
a) Reaction conditions: 1,2-octanediol (0.5 mmol), L4Re(CO)3 (0.01 mmol, 2 mol%),
secondary alcohol (5 mL), 180 C (temperature of oil bath), 3h, air. b) 1-Octene
yield, determined by 1H NMR using mesitylene (0.5 mmol) as an internal standard.
c) n.d.= no 1-octene was detected. d) All solvents were purchased from Sigma-
Aldrich; boiling points are reported on the basis of the corresponding Safety Data
Sheet.
o
Following these optimization studies, the substrate scope using
a variety of vicinal diols and polyols was investigated under
optimized conditions, i.e., using 0.5 mmol of substrate, 2 mol%
o
of L4Re(CO)3, 5 mL of 3-octanol, 180 C, under air in a closed
pressure tube (Table 6). The aliphatic vicinal diol 1,2-
dodecanediol gave 93% of 1-dodecene. No olefin isomers of the
1-dodecene product were detected, lending further credit to
the notion that the formation of 2-octene and 3-octene results
from alcohol dehydration, instead of 1-octene isomerization
when using 3-octanol as the reductant. For the previously
reported Cp’Re trioxo catalyzed DODH reactions, the formation
of internal olefin isomers derived from the terminal olefin
product were detected at reaction temperatures ≥135oC,
o
o
typically in 2-15% yield at 135 C, and 4-29% yield at 180 C.13
Accordingly, in situ activation of pre-catalyst L4Re(CO)3 does not
provide access to a species active in olefin isomerization.
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The aromatic vicinal diol 1-phenyl-1,2-ethanediol gave 83% using 5 mol% of MTO as catalyst (Figure 5).20,30 In our system,
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yield of styrene at full conversion (entry 3). Interestingly, cis- the yield of fumaric acid and fumarates is slightly lower, albeit
1,2-cyclohexanediol gave 62% yield of 1-cyclohexene under that a lower pre-catalyst loading of 2 mol% is used.
these reaction conditions. This cyclic vicinal diol is known as a
quite challenging substrate for rhenium-catalyzed DODH
reactions. For example, when using the CptttRe(CO)3 catalyst
Table 6 L4Re(CO)3 catalyzed DODH of diols and polyols.a
only 10% of 1-cyclohexene forms at 21% substrate conversion.22
Inspired by these results, we moved to biomass-based
substrates, such as 1,4-anhydroerythritol, glycerol, erythritol,
and DL-threitol.23,24 When glycerol was used as substrate, 97%
of allyl alcohol was formed (entry 5). The DODH activity of
precatalyst L4Re(CO)3 for this substrate is competitive with
respect to the activities that can be attained with high-valent
rhenium trioxo catalysts.21 When erythritol and DL-threitol
were used as substrate, 78% and 64% of 1,3-butadiene was
formed, respectively (entry 7 and 8). On the other hand, 1,4-
anhydroerythritol gave 80% of 2,5-dihydrofuran (entry 6). The
latter observations suggest that the DODH of C4 tetraols using
L4Re(CO)3 does not proceed via dehydrative cyclisation to form
a cyclised diol as intermediate.
Given the interest in the catalytic formation of 1,3-butadiene
from biomass, the activity and selectivity of precatalyst
L4Re(CO)3 was compared to that of other catalysts reported in
the field (Figure 4). The butadiene yield achieved with L4Re(CO)3
competes with the highest reported yields for this reaction
using either CpttReO3 or MTO as catalyst, i.e. compared to
catalysts in which Re is already in a high-oxidation trioxo form
from the start of the reaction.18,21,22 For catalysts included in this
comparison that operate starting from a low oxidation state
under oxidative conditions, L4Re(CO)3 gave a higher total yield
compared to BrRe(CO)5. Among all these Re-systems, L4Re(CO)3
gave the highest 1,3-butadiene selectivity, i.e. 78% of 1,3-
butadiene yield and only 2% of 2,5-dihydrofuran byproduct,
though the total yield of volatile products was not as high as for
the CpttReO3 and MTO catalysts.
entry
1
substrate
product
yield b
92%
2
93%
83%
3c
4
62%
5
6
97%
80%
7
8
(
)
78% (2%)
67% (6%) d
(
)
64% (6%)
81%
9 e
10 f,g
46%
Next, two biomass-based acids, L-(+)-tartaric acid25 and mucic
acid26-28, were investigated using L4Re(CO)3. Since 3-octanol was
also used as solvent, the deoxydehydration product of these
two substrates was a mixture of acid and mono/di-octyl ester.
L-(+)-tartaric acid gave 81% of fumaric acid and fumarates
(entry 9), while mucic acid gave 46% of muconates. Remarkably,
when using Re2(CO)10 as pre-catalyst, no muconates were
formed from mucic acid.29 Su and Zhang claimed that this is
probably due to the poor tolerance of Re2(CO)10 to the
carboxylic acid groups. Our results clearly indicate that once
ligated by an N,N,O-type ligand the low-valent Re center does
not suffer from the presence of these functions groups.
Although 3-pentanol was not a very efficiency reductant for the
deoxydehydration of 1,2-octanediol, it provided a good overall
activity in case of the deoxydehydration of L-(+)-tartaric acid;
83% of a mixture of fumaric acid and fumarates was obtained at
a) Reaction conditions: substrate (0.5 mmol), L4Re(CO)3 (0.01 mmol, 2 mol%), 3-
octanol (5 mL), 180 oC (temperature of oil bath), 3 h, air. Unless otherwise noted,
the conversion of substrate was >98%. b) Determined by 1H NMR using mesitylene
(0.5 mmol) as an internal standard. c) reaction time: 1.5 h. d) BrRe(CO)5 (2 mol%)
was used as pre-catalyst. e) The product was a mixture of fumaric acid and
fumarates. f) reaction time: 6h. g) The product was a mixture of muconates.
o
155 C for 12 h (Figure 4). In previous work, L-(+)-tartaric acid
was converted to 87% of fumarates while using 12 mol% of
HReO4 as catalyst, and 97% of fumarates were formed while
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analyse the MTO-catalyzed DODH and claim that MTO releases
View Article Online
Ref. 18
methane to form a ReVII alkoxide species with the help of a
DOI: 10.1039/D0CY00618A
100
90
80
70
60
50
40
30
20
10
0
Ref. 21
secondary alcohol at high temperature, and that the ReVII
alkoxide species is the actual catalyst. Interestingly, the same
authors also reported that Re2(CO)10 is not active for the
deoxydehydration of glycerol at 140 oC in air with 2,4-dimethyl-
3-pentanol as reductant and solvent. Since in our current
investigations, a secondary alcohol was also used as reductant,
we were curious if similar Re-alkoxide active species would be
formed and would lead to related product profiles in the DODH
of sugar substrates. D-mannose, D-glucose, and D-galactose
were then investigated using L4Re(CO)3 under optimized
conditions (Table 7). Comparing the performance of L4Re(CO)3
to MTO and CpttReO3 clearly shows that for each sugar
substrate the major product with L4Re(CO)3 is 2-vinylfuran
instead of furan. This observation may be taken as an indication
that L4Re(CO)3 operates through a different active species
compared to the Re-trioxo catalysts. In addition, no black solid
was observed for the deoxydehydration of simple vicinal diols
in our system. The highest total yield in furans for L4Re(CO)3 was
achieved using D-mannose as substrate (32%), which is quite
similar to the MTO catalyst but somewhat lower than the
CpttReO3 catalyst. For the other two sugars, the yields achieved
with L4Re(CO)3 do not compete with those obtained with MTO.
Ref. 22
CptttReO3
CpttReO3
L4Re(CO)3
BrRe(CO)5
MTO
1,3-butadiene
2,5-dihydrofuran
Figure 4 Comparison of DODH reactions of erythritol catalyzed/initiated by different Re-
complexes.18, 21, 22
Table 7 DODH of sugars with different rhenium catalysts/pre-catalysts.
sugar
[Re] (x mol%)
MTO (2.5)18
total yield
(2-VF:furan)
25%
conditions
D-glucose
3-pentanol (0.05 M)
155 oC, N2
Figure 5 DODH of L-(+)-tartaric acid 20,30
(1:1.8)
CpttReO3 (2.0)21
L4Re(CO)3 (2.0)
MTO (2.5)18
trace
3-pentanol (0.05 M)
155 oC, N2
3-pentanol (0.1 M)
155 oC, air
3-pentanol (0.05 M)
155 oC, N2
3-pentanol (0.05 M)
155 oC, N2
3-pentanol (0.1 M)
155 oC, air
3-pentanol (0.05 M)
155 oC, N2
3-pentanol (0.05 M)
155 oC, N2
3-pentanol (0.1M)
155 oC, air
On basis of the results above, we have discovered a rhenium
system which could be applied to the deoxydehydration of diols
and polyols with quite good olefin yields. Sugars, which in a way
can be labelled as complicated polyol substrates, are quite
challenging substrates for Re-catalyzed DODH reactions. MTO
has been reportedly used for the deoxydehydration of sugars,
e.g., D-mannose, D-allose, D-glucose, and D-galactose. 2-
vinylfuran (2-VF) and furan were the main products in these
reactions, with furan being the major product in most cases.18
For the CpttReO3-catalyzed deoxydehydration of sugars, 2-
vinylfuran and furan were also found to be the main products,
but the product distribution was not the same as for MTO,21
which indicates that these systems might operate through a
different active species. In a very recent report, Marrone,
d'Alessandro and co-workers explored the deoxydehydration of
glycerol using a series of rhenium sources (such as MTO, ReO3,
Re2O7, IReO2(PPh3)2,ReCl5, and ReI3) as catalyst and secondary
alcohols as reductant.31 For the latter reactions, an induction
period was observed and a black solid separated from the
reaction mixtures. When used in a next catalytic run, this black
solid was found to be active In DODH without an induction
period. The authors proposed that the black solid is the active
species in these DODH reactions. Besides, they specifically
6%
(1.8:1)
32%
(1:3.0)
14%
(no 2-VF)
11%
(2.0:1)
30%
(1:2.3)
39%
(2.3:1)
32%
D-galactose
D-mannose
CpttReO3 (2.0)21
L4Re(CO)3 (2.0)
MTO (2.5)18
CpttReO3 (2.0)21
L4Re(CO)3 (2.0)
(1.5:1)
Conclusions
A series of LRe(CO)3 complexes (LH = substituted 2-(((2-
(dimethylamino)ethyl)(methyl)amino)methyl)phenol) were
investigated as pre-catalyst for the deoxydehydration of diols
and polyols under aerobic conditions and using a secondary
alcohol as reductant and solvent. The 2,4-di-tert-butyl-
substituted complex L4Re(CO)3 was selected as the pre-catalyst
6 | J. Name., 2012, 00, 1-3
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for further investigation after initial catalytic screening of the 11
series. Both aliphatic and aromatic vicinal diols, as well as
biomass-derived glycerol and polyols can be converted using 12
L4Re(CO)3 to the corresponding olefins with good yields. The
application of sugar substrates to these reaction conditions 13
leads to the preferential formation of 2-vinylfuran. This unusual
product distribution indicates that the active species generated
from L4Re(CO)3 could be different from the ones generated by 14
Re-trioxo systems.
L. J. Donnelly, S. P. Thomas, and J. B. Love, Chem. Asian J.
2019, 14, 3782.
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DOI: 10.1039/D0CY00618A
E. Arceo, J. A. Ellman, and R. G. Bergman, J. Am. Chem. Soc.
2010, 132, 11408.
S. Raju, C. A. M. R. Van Slagmaat, J. Li, M. Lutz, J. T. B. H.
Jastrzebski, M. E. Moret, and R. J. M. Klein Gebbink,
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D. G. Lonnon, S. B. Colbran, and D. C. Craig, Eur. J. Inorg.
Chem. 2006, 2006, 1190.
Compared to the Cp-based trioxo rhenium complexes 15
previously reported by us, the new NNO-based Re(CO)3
precatalysts are much more easy to handle. Not only are the 16
NNO-ligands easy to prepare and modify, the low-yielding 17
oxidation from Re(I) to Re(VII) of the (pre)catalyst is avoided and
the (NNO)Re(CO)3 systems are air stable at room temperature 18
while the Cp’ReO3 systems need to be stored at low
temperature or under N2. In view of these considerations, the 19
new NNO-based Re-tricarbonyl precatalysts are interesting
alternatives for high-valent Re-trioxo systems in DODH 20
chemistry.
M. Toganoh, N. Harada, and H. Furuta, Polyhedron 2013, 52,
1153.
C. Hansch, A. Leo, and R. W. Taft, Chem. Rev. 1991, 91, 165.
J. Yi, S. Liu, and M. M. Abu-Omar, ChemSusChem 2012, 5,
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M. Shiramizu, and F. D. Toste, Angew. Chem. 2012, 124,
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S. Raju, J. T. B. H. Jastrzebski, M. Lutz, and R. J. M. Klein
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Conflicts of interest
23
24
25
26
27
Y. Nakagawa, T. Kasumi, J. Ogihara, M. Tamura, T. Arai, and
K. Tomishige, ACS Omega 2020, 5, 2520.
S. Tazawa, N. Ota, M. Tamura, Y. Nakagawa, K. Okumura,
and K. Tomishige, ACS Catal. 2016, 6, 6393.
J. Fu, E. S. Vasiliadou, K. A. Goulas, B. Saha, and D. G.
Vlachos, Catal. Sci. Technol. 2017, 7, 4944.
H. Zhang, X. Li, X. Su, E. L. Ang, Y. Zhang, and H. Zhao,
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R. T. Larson, A. Samant, J. Chen, W. Lee, M. A. Bohn, D. M.
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There are no conflicts to declare.
Acknowledgements
JL acknowledges financial support through the Chinese
Scholarship Council and Utrecht University. The X-ray
diffractometer has been financed by the Netherlands
Organization for Scientific Research (NWO).
28
29
J. Lin, H. Song, X. Shen, B. Wang, S. Xie, W. Deng, D. Wu, Q.
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