Catalytic deoxydehydration of diols to olefins by using a bulky cyclopentadiene-based trioxorhenium catalyst

Article abstract of DOI:10.1002/cssc.201300364

A bulky cyclopentadienyl (Cp)-based trioxorhenium compound was developed for the catalytic deoxydehydration of vicinal diols to olefins. The 1,2,4-tri(tert-butyl)cyclopentadienyl trioxorhenium (2) catalyst was synthesised in a two-step synthesis procedure. Dirhenium decacarbonyl was converted into 1,2,4-tri(tert-butyl)cyclopentadienyl tricarbonyl rhenium, followed by a biphasic oxidation with H₂O₂. These two new three-legged compounds with a 'piano-stool' configuration were fully characterised, including their single crystal X-ray structures. Deoxydehydration reaction conditions were optimised by using 2 mol % loading of 2 for the conversion of 1,2-octanediol into 1-octene. Different phosphine-based and other, more conventional, reductants were tested in combination with 2. Under optimised conditions, a variety of vicinal diols (aromatic and aliphatic, internal and terminal) were converted into olefins in good to excellent yields, and with minimal olefin isomerisation. A high turnover number of 1400 per Re was achieved for the deoxydehydration of 1,2-octanediol. Furthermore, the biomass-derived polyols (glycerol and erythritol) were converted into their corresponding olefinic products by 2 as the catalyst. In the bulk of it: Bulky 1,2,4-tri(tert-butyl)cyclopentadienyl trioxorhenium was studied as a catalyst for the deoxydehydration of different vicinal diols. Under optimised conditions, a variety of vicinal diols were converted into olefins in good to excellent yields, and with minimal olefin isomerisation. Biomass-derived polyols were also converted into their corresponding olefinic products. Copyright

Full text of DOI:10.1002/cssc.201300364

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DOI: 10.1002/cssc.201300364
Catalytic Deoxydehydration of Diols to Olefins by using
a Bulky Cyclopentadiene-based Trioxorhenium Catalyst
Suresh Raju,[a] Johann T. B. H. Jastrzebski,[a] Martin Lutz,[b] and Robertus J. M. Klein Gebbink*[a]
A bulky cyclopentadienyl (Cp)-based trioxorhenium compound
was developed for the catalytic deoxydehydration of vicinal
diols to olefins. The 1,2,4-tri(tert-butyl)cyclopentadienyl trioxo-
rhenium (2) catalyst was synthesised in a two-step synthesis
procedure. Dirhenium decacarbonyl was converted into 1,2,4-
tri(tert-butyl)cyclopentadienyl tricarbonyl rhenium, followed by
a biphasic oxidation with H2O2. These two new three-legged
compounds with a ‘piano-stool’ configuration were fully char-
acterised, including their single crystal X-ray structures. Deoxy-
dehydration reaction conditions were optimised by using
2 mol% loading of 2 for the conversion of 1,2-octanediol into
1-octene. Different phosphine-based and other, more conven-
tional, reductants were tested in combination with 2. Under
optimised conditions, a variety of vicinal diols (aromatic and
aliphatic, internal and terminal) were converted into olefins in
good to excellent yields, and with minimal olefin isomerisation.
A high turnover number of 1400 per Re was achieved for the
deoxydehydration of 1,2-octanediol. Furthermore, the biomass-
derived polyols (glycerol and erythritol) were converted into
their corresponding olefinic products by 2 as the catalyst.
Introduction
Because of the increasing need for energy and material sour-
ces, biomass is currently considered as an alternative resource
for the sustainable production of chemicals, fuels, and energy.
In this respect, biomass-derived feedstocks, such as sugars and
polyols, are interesting starting materials to replace conven-
tional oil resources,[1] which are further depleting and concomi-
tantly increasing in price. Unlike oil-derived hydrocarbons, the
sugar derivatives are oxygen-rich materials because of the high
abundance of hydroxyl groups. The main challenge in utilising
such oxygen-rich materials is to reduce their oxygen content
and, accordingly, decrease their polarity. In particular, this ap-
proach will yield olefins, which are essential chemicals for both
the polymer and the bulk chemical industries.[2]
tions, rhenium-based catalysts seem best suited because of
the facile bond-forming and bond-breaking tendency of rheni-
um–oxygen bonds.[14] In an early study, Cook and Andrews re-
ported the catalytic deoxydehydration of polyols into olefins
by using Cp*ReO3 as the catalyst (Cp*=1,2,3,4,5-pentamethyl-
cyclopentadienyl).[7] 2 mol% catalyst loading was used in com-
bination with triphenylphosphine as a stoichiometric reductant
to achieve quantitative conversion of phenylethane-1,2-diol
into styrene in chlorobenzene at 908C. Later, Gable and co-
workers used Tp*ReO3 [Tp*=hydrido-tris-(3,5-dimethylpyrazo-
lyl)borate] as an alternative catalytic system by replacing the
Cp* with a Tp* ligand.[15] Nevertheless, the total turnover
number was limited in both cases.
The production of olefins from sugars and sugar-derived
polyols is viable through deoxygenation[3] and dehydration,[4,5]
or combinations of these reactions, such as di-dehydroxyla-
tion[6] or deoxydehydration reactions.[7–12] Although alcohol de-
hydrations are well-known acid-catalysed and high-tempera-
ture transformations,[13] transition-metal-catalysed versions of
these reactions have been studied to a lesser extent. We re-
cently reported rhenium-catalysed dehydration reactions of
benzylic, non-benzylic, and biobased alcohols.[5] Amongst the
transition metal catalysts currently known for these transforma-
The proposed deoxydehydration mechanism of these cata-
lysts involves initial reduction of LReVIIO3 to obtain the active
species, LReVO2 (in which L=ligand). Next, condensation of the
diol substrate with LReVO2 leads to the diolate intermediate,
followed by a thermal cycloreversion through which the olefin
product is released.[7] Herrmann et al. had earlier reported the
formation of ReV diolate intermediates and successive possible
extrusion of olefin products for their Cp*ReO3 catalyst.[16]
Furthermore, Gable and co-workers developed new routes to
synthesise different ReV diolates and investigated the mecha-
nistic pathways of the cycloreversion reactions in detail by
using kinetic studies.[17] One of their findings was that the very
active dioxorhenium (LReVO2) species, formed upon reduction
of the starting trioxo complex, is in equilibrium with its dimeric
form. In addition, over-reduction of the dioxo complex leads to
the formation of catalytically inactive ReIII species. Each of
these events results in catalyst deactivation and lower turnover
numbers per rhenium atom.
[a] S. Raju, Dr. J. T. B. H. Jastrzebski, Prof. Dr. R. J. M. Klein Gebbink
Organic Chemistry & Catalysis
Debye Institute for Nanomaterials Science
Utrecht University
Universiteitsweg 99, 3584 CG Utrecht (The Netherlands)
E-mail: r.j.m.kleingebbink@uu.nl
[b] Dr. M. Lutz
Crystal & Structural Chemistry
The versatile oxidation catalyst CH3ReO3 (MTO) was also
used for deoxydehydration reactions in combination with
Bijvoet Centre for Biomolecular Research
Padualaan 8, 3584 CH Utrecht (The Netherlands)
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a number of different reductants, including molecular hydro-
gen,[8] Na2SO3,[9,10] and alcohols.[11,12] Very recently, independent
works by Yi et al.[11] as well as Shiramizu and Toste[12] showed
the catalytic conversion of polyols into olefins by using MTO
and secondary alcohols as both the reductant and solvent.
In addition, Yi et al. reported that the substrate itself, that is,
glycerol, could be used as the reductant to obtain C3 volatile
products in a distillation set up at 1658C. Shiramizu and Toste
documented the conversion of biomass-derived sugars and
polyols into furan derivatives or aromatics, and linear polyenes,
respectively.
(18H, tBu), and 1.19 ppm (9H, tBu) were observed by using
1H NMR spectroscopy, in addition to a peak at 196.61 ppm in
13C NMR spectroscopy, which corresponded to one unique car-
bonyl environment. Two strong IR vibrational modes were ob-
served at 1886 (nasym) and 2004 cmÀ1 (nsym) for the carbonyl
moiety. Electrospray ionisation mass spectrometry (ESI-MS)
showed an ion with an m/z value of 504.1729, corresponding
to a [M]+ cationic species (calcd m/z=504.1675).
Next, 1 was oxidised to obtain the corresponding trioxorhe-
nium compound, 2, by using a well-established biphasic oxida-
tion method.[20] 1 was refluxed for 15 h in a benzene solution
in the presence of H2O2 (31 equiv., 35% in water) under a nitro-
gen atmosphere to yield the product as a yellow crystalline
material in 73% yield after work-up. A few drops of concen-
trated H2SO4 were added to the reaction mixture to activate
H2O2 for the oxidative decarbonylation of the starting material,
as described by Herrmann and co-workers as well as Wallis and
Kochi.[20,21] Clear downfield shifts were observed for the Cp hy-
drogen (from 5.35 to 6.55 ppm) and the carbon signals in the
NMR analysis, and the disappearance of the carbonyl peak in
the 13C NMR spectrum indicated clean conversion of 1 to 2.
Typical Re=O IR vibrations at 877 (nasym) and 917 cmÀ1 (nsym), as
well as the absence of carbonyl vibrations, further evidenced
the formation of 2. By using ESI-MS, two cationic species were
observed; one at m/z=469.1753 for (M+H)+ (calcd m/z=
469.1683) and one at m/z=532.1820 for (M+Na+CH3CN)+
(calcd m/z=532.1832).
Results and Discussion
In the development of improved deoxydehydration catalysts
based on rhenium, we envisioned the use of robust cyclopen-
tadienyl (Cp)-based catalysts by considering both the electron-
ics and the steric bulk of the ligand system. Amongst a series
of Cp-derived ligands, the Cp* ligand can be compared to the
1,2,4-tri(tert-butyl)cyclopentadienyl ligand in terms of electron-
ics, whereas the latter is bulkier than the former. Herein, we
present the development of the 1,2,4-tri(tert-butyl)cyclopenta-
dienyl-based trioxorhenium complex as a proficient catalyst for
deoxydehydration reactions.
We started our investigation to synthesise 1,2,4-tri(tert-bu-
tyl)cyclopentadienyl trioxorhenium (2) by utilising literature
methods for the two-step synthesis of Cp-based trioxorhenium
complexes (Scheme 1). 1,2,4-Tri(tert-butyl)cyclopentadiene was
synthesised according to the literature procedure reported by
Compounds 1 and 2 crystallized from a 2:1 mixture of di-
chloromethane and hexane at À308C. The molecular structures
exhibit typical three-legged ‘piano-stool’ configurations, similar
to other known Cp-based rhenium compounds (Figure 1). The
OCÀReÀCO angles in 1 are between 89.18(9) and 90.84(7)8, the
CÀC (Cp) distances are in the range 1.420(2)–1.472(2) ꢁ, and
the Re to Cp (centroid) distance is 1.9569(7) ꢁ. The CÀC(Cp)
distances in 2 are in the range 1.408(3)–1.450(2) ꢁ. This indi-
cates a slight decrease in the CÀC bond lengths in the substi-
tuted Cp-ring as a consequence of the oxidation of carbonyl
compound 1 into oxo-compound 2. Such a shortening has pre-
viously been reported in the literature.[20,22] In 2, the Re to Cp
(centroid) distance is 2.0830(8) ꢁ, the Re=O distances are be-
tween 1.7239(15) and 1.7267(14) ꢁ, and the O=Re=O angles
are between 103.86(8) and 105.39(8)8. The orientation of the
tBu groups in 2 is anticipated to be beneficial in shielding the
ReO3 moiety.
We started our catalytical investigations by using 2 for the
deoxydehydration of vicinal diols into olefins with 1,2-octane-
diol as a substrate (Scheme 2). Initial catalytic reactions were
performed with 2 mol% catalyst 2 at 1808C in chlorobenzene
with 1.1 equiv. triphenylphosphine as the reductant under an
inert nitrogen atmosphere, that is, under similar conditions to
those previously reported by Cook and Andrews for Cp*ReO3
in this reaction.[7] A profile of the reaction over time showed
that it proceeded to completion within 15 h, and exclusively
Scheme 1. Synthetic scheme for the formation of 1 and 2.
Sitzmann and co-workers.[18] Similar to the procedure reported
by Patton et al.,[19] an excess of substituted cyclopentadiene
was mixed with dirhenium decacarbonyl and refluxed in neat
conditions under a nitrogen atmosphere to generate the tricar-
bonyl complex. Gradual heating of the reaction mixture from
150 to 2108C led to complete conversion of the rhenium start-
ing material, yielding 42% of the tricarbonylrhenium complex
(1). After a simple work-up by washing away the excess ligand
with cold hexane, 1 was obtained in a pure form according to
NMR spectroscopy. Singlet peaks at 5.35 (2H, CpÀH), 1.37
yielded
octenes
(1-octene/2-octene=1:0.05;
Figure 2).
Performing the reaction under aerobic conditions resulted in
moderate octene formation (60%) at 75% conversion.
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Figure 2. Reaction profile of the deoxydehydration of 1,2-octanediol cata-
lysed by using 2 at 1808C.
Table 1. Optimisation of the reaction temperature for the conversion of
1,2-octanediol into octenes catalysed by 2.[a]
Entry
T
Yield [%][b]
Conversion[b]
t
[8C] 1-octene trans-2-octene cis-2-octene [%]
[h]
1
2
3
4
5
6
7[d]
180 89
150 79
135 94
130 80[c]
120 26
3
3
8.2
1
0
0
>99
>99
>99
86
35
10
15
15
12
15
15
15
38
4.2
1.3
0
0
0
110
6
0
0
100 11
0
18
[a] Reaction conditions: 1,2-octanediol (0.5 mmol), 2 (0.01 mmol), PPh3
(0.55 mmol), PhCl (5 mL). [b] Determined by using GC with mesitylene
(0.5 mmol) as an internal standard. [c] 91% 1-octene at complete conver-
sion after 24 h. [d] No trace of octene after 12 h.
Figure 1. Displacement ellipsoid plot of complex 1 and 2 in the crystal at
50% probability. Hydrogen atoms are omitted for clarity.
tained in the temperature range 100–1208C. No significant im-
provement in conversion was observed at 1008C upon exten-
sion of the reaction time to 38 h. Interestingly, at lower tem-
peratures, isomerisation did not occur and 1-octene was ob-
tained selectively.
Scheme 2. Conversion of 1,2-octanediol into octenes catalysed by 2.
Next, a number of different apolar and polar solvents were
tested for the reaction at their respective reflux temperature.
The reaction performed in toluene resulted in a moderate
yield, whereas in benzene it produced a very low yield
(Table 2, entries 2 and 3). In more polar and coordinating sol-
vents, such as tetrahydrofuran (THF) and acetonitrile, no signifi-
cant reactivity was observed. When the reactions were per-
formed at 1808C in pressure tubes in these solvents, much
better conversions and yields were obtained for the non-polar
solvents, producing >90% octene products after complete
conversion of the substrate (entries 1–3). In this case, 2-octene
isomers were also obtained at a higher trans/cis isomer ratio
(ꢀ2:1) in non-polar solvents, whereas more of the cis isomer
was formed in THF (entry 4). In acetonitrile, no reaction was
observed, even at 1808C (entry 5). It is likely that acetonitrile
coordination to rhenium prevented the reaction from taking
place. Interestingly, another coordinating basic solvent, pyri-
To reduce the formation of 2-octene, the reaction tempera-
ture was lowered. At 150 and 1358C, complete conversion of
1,2-octanediol was observed and octene yields were very good
(Table 1, entries 2 and 3). The amount of 2-octene isomers was
lower when the reaction was performed at 1358C compared to
the amount produced at 1808C. At 1508C, the amount of 2-oc-
tenes (12.4%; trans/cis ratio=0.51/1) was relatively high. This
was attributed to the isomerisation of 1-octene into 2-octenes
under the reaction conditions, that is, at longer reaction times.
When the reaction was performed below the reflux tempera-
ture of chlorobenzene, moderate to low yields were obtained
(Table 1, entries 4–7). At 1308C, 1-octene was obtained in good
yield within 15 h and complete conversion was observed after
24 h (91% 1-octene). Poor conversions and yields were ob-
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Table 2. Solvent optimisation for the deoxydehydration of 1,2-octanediol catalysed by 2.[a]
Entry
Solvent
Yield [%][b,c]
trans-2-octene
Conversion[b,c]
[%]
Yield [%][c,d]
trans-2-octene
Conversion[c,d]
[%]
1-octene
cis-2-octene
1-octene
cis-2-octene
1
2
3
4
5
6
PhCl
94
34
2
trace
trace
2
1.3
0
0
1
0
0
>99
36
16
11
5
89
83
84
84
trace
67
3
3
>99[e]
>99
>99[f]
>99[e]
9
toluene
benzene
THF
CH3CN
pyridine
7.7
4.7
2.3
5.8
3.1
5.3
0
0
8
0
0
83
[a] Reaction conditions: 1,2-octanediol (0.5 mmol), 2 (0.01 mmol), PPh3 (0.55 mmol), solvent (5 mL), 15 h. [b] At reflux temperature for 15 h. [c] Determined
by using GC with mesitylene (0.5 mmol) as an internal standard. [d] At 1808C for 15 h, unless otherwise stated. [e] 12 h. [f] 17 h.
dine, produced a 67% yield of 1-octene at 83% conversion of
1,2-octanediol at 1808C (entry 6), whereas a similar trend of
very poor conversion was observed at 1358C. The reactivity in
pyridine could be of interest for the conversion of very polar
polyol substrates because of solubility reasons.
octane formation (as the over-reduced product) at 35% sub-
strate conversion.[24] Increasing the temperature from 135 to
1808C clearly led to an improved yield (47%) at complete con-
version. Changing the solvent from chlorobenzene to THF
yielded a mixture of octene isomers and octane products in
poor yield at 1358C, whereas at 1808C the activity increased
with an increased formation of 2-octene isomers (entry 6).
By using the secondary alcohol 3-pentanol or the substrate
itself as the reductant, poor yields were obtained (entries 7–8).
At 1808C, moderate yields were obtained in both cases within
15 h. Interestingly, 100% selectivity (50% yield) of 1-octene
was achieved by the consumption of 50% of substrate as the
reductant through hydrogen transfer, as reported by Yi et al.
(entry 8).[11] These experimental observations are in accordance
with the rate-determining step involving the cycloreversion of
the diolate intermediate at elevated temperatures, which was
studied by Gable and co-workers[17] and in recent independent
density functional theory (DFT) calculations by Bi et al., Qu
et al., and Liu et al..[25]
As a next step in protocol optimisation, the use of alterna-
tive reductants to triphenylphosphine was investigated. Other
phosphines such as electron-rich P(nBu)3 and PCy3 or electron-
deficient P(C6F5)3 provided very poor conversions and pro-
duced less than 5% 1-octene at 1358C (Table 3, entries 2A–
4A). Interestingly, the use of these phosphines at 1808C signifi-
cantly improved the overall reactivity (entries 2B and 3B). PCy3
produced an overall octene yield of 94%, including 2-octenes
(14%; trans/cis ratio=0.43:1) at complete conversion of 1,2-oc-
tanediol. P(C6F5)3 produced only 47% 1-octene at 54% conver-
sion over 12 h. A poor yield was obtained with P(nBu)3, which
was attributed to very fast catalyst deactivation (entry 4).[23]
The use of other conventional reductants also resulted in
a poor overall reactivity at 1358C (entries 5A–8A). Sodium sul-
fite resulted in a very poor conversion, which was most likely
because this salt does not dissolve in chlorobenzene (entry 5).
The use of molecular hydrogen (40 bar) resulted in 21% n-
Following these optimisation studies, the substrate scope
was investigated by using a variety of vicinal diols under opti-
mised reaction conditions, that is, 1.1 equiv. PPh3, chloroben-
zene, 1358C (Table 4). Linear ali-
phatic terminal diols were com-
Table 3. Reductant variation for the deoxydehydration of 1,2-octanediol catalysed by 2.[a]
pletely converted into the corre-
sponding terminal olefins with
high selectivities (entries 1–3).
Decanediol and dodecanediol
showed complete conversion
Entry
Reductant
T
[8C]
Yield [%][b]
trans-2-octene
Conversion[b]
[%]
t
[h]
1-octene
cis-2-octene
1
2
3
4
5
6
7
8
A
B
A
B
A
B
A
B
A
B[d]
A
B
A
B[d]
A
B
PPh3
135
180
135
180
135
180
135
180
135
180
135
180
135
180
135
180
94
89
5
80
3
47
trace
8
1.3
3
0
4.2
0
0
1
3
0
9.8
0
>99
>99
5
>99
7
15
12
11
15
15
12
43
24
15
24
15
15
15
12
15
15
PCy3
after 40 h with
a very low
amount of internal olefin forma-
tion. Cis-cyclohexanediol was
converted into cyclohexene in
a low yield (10%), whereas the
corresponding trans-cyclohexa-
nediol did not show any reactivi-
ty (entries 4 and 5). This shows
that the diol needs to be able to
adopt a syn-configuration to be
converted by the rhenium-based
catalyst;[8,10,13a] however, forma-
tion of the diolate intermediate
from a cyclohexanediol substrate
could be hampered by the steric
P(C6F5)3
P(nBu)3
Na2SO3
0
54
<1[c]
17[c]
3
0
0
trace
5
0
1
3.1
0
0
0
0.8
6
0
0
<10
35 (10)
>99 (35)
9
47
16
[e]
H2
21[f] (2[f] +3)
47[f] (12[f] +1.2)
3-pentanol
–
9
43
7
0
0
0
0
50
>99
[a] Reaction conditions: 1,2-octanediol (0.5 mmol), 2 (0.01 mmol), reductant (0.55 mmol), PhCl (5 mL). [b] Deter-
mined by using GC with mesitylene (0.5 mmol) as an internal standard. [c] 1H NMR spectroscopy conversion
[d] Pentadecane was used as internal standard. [e] 40 bar H2 pressure (parentheses results correspond to the re-
action mixture in THF). [f] n-octane yield.
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Table 4. Deoxydehydration of vicinal diols into olefins under optimised conditions catalysed by 2.[a]
Entry
Substrate
Product
Yield [%][b]
trans-2-octene
Conversion[b]
[%]
t
[h]
1-octene
cis-2-octene
1
2
3
94
90
94
1.3
3.7
1.4
1
>99
>99
86
15
40
40
3.5
1
4
5
10
21
12
40
40
trace
6
99
89
>99
>99
>99
40
40
40
7[c]
8
>99
9
49
17
>99
37
38
10
29
[a] Reaction conditions: 1,2-octanediol (0.5 mmol), 2 (0.01 mmol), PPh3 (0.55 mmol), PhCl (5 mL). [b] Determined by using GC with mesitylene (0.5 mmol) as
an internal standard. [c] 10% trans-stilbene.
bulk of 2. The aromatic vicinal diol phenyl-1,2-ethanediol pro-
duced an almost quantitative yield (99%) of styrene (entry 6),
without polymerisation of the product, as was observed before
by us for rhenium-mediated dehydrations towards styrenes.[5]
(R,R)-1,2-diphenyl-1,2-ethanediol selectively produced trans-stil-
bene in a quantitative yield (entry 8), whereas (R,S)-1,2-diphen-
yl-1,2-ethanediol (meso-hydrobenzoin) was fully converted into
a mixture of cis-stilbene (89%) and trans-stilbene (10%) in
40 h.[26] Moreover, 1,4-anhydroerythritol produced 49% 2,5- di-
hydrofuran at full conversion (entry 9). Treatment of the inter-
nal aliphatic diol syn-4,5-octanediol gave a moderate conver-
sion, resulting in only 17% trans-4-octene (entry 10). This ob-
servation could further indicate the sensitivity of 2 to the steric
bulk of the substrate.
Finally, 2 was used in the deoxydehydration of two biobased
polyols, that is, glycerol and erythritol. By using the optimised
reaction conditions (vide supra), glycerol, which is the main
side product from biodiesel production,[27] was selectively con-
verted to the corresponding olefinic product, allyl alcohol, in
91% yield at a substrate conversion of 94% in 26 h. In a similar
reaction, by using 2.2 equiv. of PPh3, erythritol was converted
to 1,3-butadiene (A), with concomitant formation of partial de-
oxydehydration products 3-butene-1,2-diol (B), cis-2-butene-
1,4-diol (C), and trans-2-butene-1,4-diol (D) (Scheme 3).
As shown in Table 5, very poor conversion (20%) and yields
were obtained in chlorobenzene at 1408C over 26 h (entry 1).
Upon increasing the reaction temperature of this heterogene-
ous mixture to 1808C, complete conversion of erythritol pro-
duced 18% butadiene with C (5.8%) and D (2.2%) as side
To probe the activity and stability of catalyst 2, its loading
was lowered to 0.05 mol% to convert 0.985 mmol
1,2-octanediol (0.144 g) at 1808C in 0.5 mL chloro-
benzene. After 59 h, 86% conversion of 1,2-octane-
diol was achieved to yield 72% octenes as a mixture
of 1-octene (62.5%) and 2-octenes (9.5%; trans/cis
ratio=1:2). The total turnover number for 2, based
on the amount of octenes formed, equalled 1400 per
rhenium atom in the reaction. To the best of our
knowledge, this turnover number for 2 is the highest
reported to date for any of the published oxorheni-
um catalyst in a deoxydehydration reaction.
Scheme 3. Conversion of (a) glycerol and (b) erythritol catalysed by 2.
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vacuum. Toluene, THF, and aceto-
nitrile solvents were obtained
Table 5. Conversion of erythritol into butadiene and butenediols catalysed by 2.[a]
from
a
MBraun MB SPS-800
Entry
Solvent
T
[8C]
Yield [%][b]
Conversion[b]
[%]
t
[h]
system, and degassed. Triphenyl-
phosphine was crystallised in eth-
anol and dried under vacuum.
Unless otherwise stated, all other
commercial chemicals were used
without further purification. NMR
spectra were recorded on a Varian
VNMRS400 (400 MHz) instrument
at 298 K.
A
B
C
D
1
2
3
4[c]
PhCl
PhCl
pyridine
3-octanol
140
180
180
170
1.5
2.7
–
4.3
–
0.3
5.8
3.0
–
2.5
2.2
4.9
–
20
>99
60
26
24
15
18
30
67
>99
1.5
[a] Reaction conditions: Erythritol (0.5 mmol), 2 (0.01 mmol), PPh3 (1.1 mmol), solvent (5 mL). [b] Determined by
using GC with mesitylene (0.5 mmol) as an internal standard. [c] Erythritol (0.2 mmol), 2 (0.005 mmol), 3-octanol
(0.67 mL); 7% 2,5-dihydrofuran and yields were determined by using H NMR spectroscopy.
1
IR spectra were recorded by using
a PerkinElmer Spectrum One FTIR
products (entry 2).[28] Also, a homogeneous mixture in pyridine
spectrometer in the range of 650–4000 cmÀ1. ESI-MS spectra were
recorded by using a Waters LCT Premier XE instrument.
resulted in 30% butadiene with butene diols (B=4.3%, C=
3%, and D=4.9%) at 60% conversion of erythritol (entry 3).
Inspired by the work of Shiramizu et al.,[12] who used MTO
(2.5 mol%) as a catalyst in combination with 3-octanol as the
reductant at 1708C to convert erythritol into butadiene, we
also used 2 as a catalyst under these conditions (Table 5,
entry 4).[29] This reaction produced 67% butadiene without the
formation of by-products B–D, but with the formation of 7%
2,5-dihydrofuran (Scheme 4).
Synthesis of 1: In a dried Schlenk tube, Re2(CO)10 (1.0 g, 1.5 mmol)
was charged and degassed under vacuum for 30 min, and an
excess of 1,2,4-tri(tert-butyl)cyclopentadiene (1 mL, 2 mmol) was
added under a nitrogen atmosphere. The resulting mixture was re-
fluxed at 1508C for 30 min. At regular time intervals, the reaction
temperature was increased to reach 2108C in 4 h. The reaction
mixture was maintained at 2108C for 30–45 min to allow for the
complete conversion of the rhenium starting material. After cool-
ing to ambient temperature, a sample of the solidified reaction
mixture was obtained for thin layer chromatography, which
showed complete conversion of the rhenium starting material.
After removing the excess tri(tert-butyl)cyclopentadiene by wash-
ing with cold hexane and drying under vacuum, 1 was obtained as
a bright white solid in 42% yield. Crystals of 1, suitable for single
XRD, were obtained at À308C from
a solution in a 2:1
Scheme 4. Deoxydehydration of erythritol catalysed by using 2, with 3-octa-
nol as the reductant.
1
(1.5:0.75 mL) solvent mixture of dichloromethane/hexane. H NMR
spectroscopy (400 MHz), CDCl3 (7.26 ppm): d=1.19 (s, 9H, tBu),
1.37 (s, 18H, tBu), and 5.35 ppm (s, 2H, CpÀH). 13C NMR spectrosco-
py (400 MHz), CDCl3 (77.16 ppm): d=30.94, 32.21, 33.09, 34.51,
85.74, 113.52, 115.73, and 196.61 ppm. IR [attenuated total reflec-
tance (ATR)/FTIR]: n˜ =1247, 1365, 1463, 1485, 1886, 1900, 2004,
2872, and 2965 cmÀ1; ESI-MS: calcd for [C20H29ReO3]+ =504.1675;
found=504.1729; elemental analysis calcd (%) for C20H29ReO3 (504.
17): C 47.70, H 5.80, O 9.53, and Re 36.97; found: C 45.42, H 9.11, O
11.26, and Re 34.21.
Conclusions
We have shown efficient catalytic deoxydehydration reactions
of vicinal diols to yield olefins by using the bulky Cp-based tri-
oxorhenium catalyst 2. Different phosphine-based and conven-
tional reductants were tested, including the least expensive re-
ductant, molecular hydrogen. Under optimised reaction condi-
tions, different types of vicinal diols are converted to their cor-
responding olefins, with limited isomerisation of the a-olefin
products. The loading of 2 can be reduced to 0.05% to achieve
a turnover number as high as 1400 per rhenium. These data
point at the combined activity and stability of 2 in deoxydehy-
dration reactions, and render 2 a promising lead for the further
development of Cp-based trioxorhenium catalysts, as envi-
sioned by Gable et al.[15a] Preliminary experiments show that 2
may serve as a powerful catalyst for the conversion of bio-
based polyols to olefins. The further development of Cp-based
trioxorhenium catalysts for the improved conversion of polyols
and sugars into olefins, and mechanistic studies of such cata-
lyst systems, are on-going.
Synthesis of 2: Tricarbonyl complex 1 (1 g, 1.98 mmol) was de-
gassed in a dried Schlenk tube by stirring for 30 min under
vacuum, followed by the addition of degassed benzene (50 mL),
and H2O2 (35% in water, 5 mL, 31 equiv.) mixed with concentrated
H2SO4 solution (0.1 mL). The resulting mixture was refluxed for 15 h
at 808C, after which the yellow organic layer was separated from
the water layer by extracting with benzene (3ꢂ25 mL) in open air.
Then, the organic layer was washed with demineralised water (2ꢂ
100 mL), 5% NaHCO3 solution (1ꢂ100 mL), and brine solution (1ꢂ
100 mL), before it was dried over Na2SO4. After the removal of ben-
zene in vacuo, 2 was obtained as a bright yellow crystalline com-
pound in 77% yield, and was further purified by column chroma-
tography with dichloromethane and hexane (1:1) as the eluent
(yield=73%). Crystals of 2 were obtained from a 2:1 mixture of di-
1
chloromethane/hexane (1/0.5 mL) at À308C. H NMR spectroscopy,
CDCl3 (7.26 ppm): d=1.41 (s, 9H, tBu), 1.55 (s, 18H, tBu), 6.55 ppm
(s, 2H, CpÀH); 13C NMR spectroscopy, CDCl3 (77.16 ppm): d=30.11,
32.30, 33.29, 35.18, 110.93, 134.71, and 136.19 ppm. IR (ATR/FTIR):
n˜ =832, 877, 917, 1236, 1369, 1462, 2967 cmÀ1; ESI-MS: calcd for
[C13H29ReO3 +H]+ =469.1683; found=469. 1753; calcd for
[C13H29ReO3 +Na+CH3CN]+ =532.1832; found=532.1820; elemen-
Experimental Section
General: All chemicals, including solvents, were degassed by using
either the freeze–pump–thaw method or degasification under
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tal analysis calcd (%) for C17H29ReO3 (468.17): C 43.66, H 6.25,
O 10.26, Re 39.82; found: C 43.23, H 6.41, O 10.27, Re 39.49.
lations and higher symmetry checking was performed with the
PLATON program.[35]
CCDC 929019 (1) and CCDC 929020 (2) contain the supplementary
crystallographic data for this paper. These data can be obtained
free of charge from The Cambridge Crystallographic Data Centre
via www.ccdc.cam.ac.uk/data_request/cif.
General procedure for catalytic deoxydehydration: Unless other-
wise noted, all reaction mixtures were prepared inside a glove box
under a nitrogen atmosphere or on a conventional Schlenk line in
a 15 mL thick-walled glass pressure tube (Ace) fitted with a Teflon
screw-cap. 1,2-octanediol (0.5 mmol), 2 (0.01 mmol), and mesity-
lene (0.5 mmol) were dissolved in chlorobenzene (5 mL) and mixed
well. Finally, PPh3 (0.55 mmol) was added in the reaction mixture
to avoid over-reduction of 2. Alternatively, stock solutions were
prepared in chlorobenzene. Then, the closed reaction vessel was
brought into a preheated silicone oil bath at 135 or 1808C. After
the reaction, aliquots of the reaction mixture were diluted with
acetone (for olefins) or ethylacetate (for diols). GC measurements
were performed by using a PerkinElmer Autosystem XL gas chro-
matograph equipped with a PerkinElmer Elite-17 column (length=
30 m, internal diameter=0.32 mm, film thickness=0.50 mm), and
Compound 1: C20H29O3Re, formula weight (Fw)=503.63, colourless
block, 0.24ꢂ0.22ꢂ0.18 mm3, monoclinic, space group=P21/c (no.
14), unit cell dimensions: a=10.6140(4), b=11.5980(5), and c=
16.7925(7) ꢁ, angle: b=97.0861(13)8, unit cell volume V=
2051.39(15) ꢁ3, number of formula units per unit cell Z=4, density
calculated from the crystal cell and content Dx =1.631 gcmÀ3
,
linear absorption coefficient m=5.94 mmÀ1. 35746 reflections were
measured at a temperature of 150(2) K up to a resolution of (sin q/
l)max =0.65 ꢁÀ1, and 4682 reflections were unique (Rint =0.017), of
which 4327 were observed [I>2s(I)]. 234 parameters were refined
with no restraints. R1/wR2 [I>2s(I)]=0.0118:0.0282, R1/wR2 [all re-
flections]=0.0140:0.0290, and S=1.028. Residual electron density
between À0.36 and À0.64 eꢁ3.
with
a flame ionisation detector. GC method: 408C, 5 min;
38CminÀ1 to 558C; 208CminÀ1 to 2508C; 2508C, 10 min. All olefinic
products were known compounds and were calibrated against me-
sitylene for quantification.
Compound 2: C17H29O3Re, Fw =467.60, yellow needles, 0.40ꢂ0.24ꢂ
0.14 mm3, monoclinic, space group=P21 (no. 4), unit cell dimen-
sions: a=8.6569(8), b=11.3605(11), and c=9.2162(8) ꢁ, angle: b=
Typical procedure for deoxydehydration with hydrogen as the
reductant: Diol (1 mmol), 2 (0.02 mmol), and mesitylene (1 mmol)
were mixed with 10 mL of chlorobenzene or THF in an autoclave
vessel. The solution was flushed three times with 40 bar H2 pres-
sure and then heated to the aforementioned temperature. Samples
were taken for GC and GC–MS after cooling in an ice bath.
108.164(2)8, V=861.22(14) ꢁ3, Z=2, Dx =1.803 gcmÀ3
,
m=
7.06 mmÀ1. 18827 reflections were measured at a temperature of
150(2) K up to a resolution of (sin q/l)max =0.65 ꢁÀ1. 3900 reflec-
tions were unique (Rint =0.023), of which 3864 were observed [I>
2s(I)]. 200 parameters were refined with one restraint (floating
origin). R1/wR2 [I>2s(I)]=0.0093:0.0234, R1/wR2 [all reflections]=
0.0095:0.0234, and S=1.099. Flack parameter: x=0.007(4).[36] Resid-
ual electron density between À0.44 and 0.22 eꢁ3.
Typical procedure for polyol conversion: Polyol (0.5 mmol), 2
(0.01 mmol), mesitylene (0.5 mmol), and PPh3 (0.55 or 1.1 mmol)
were weighed directly into the reaction tube, and the solvent
(5 mL) was added. The reaction tube was closed and heated in
a preheated oil bath. The mixture was cooled in an ice bath and
homogenised with pyridine. Samples were taken directly by sy-
ringe to analyse for volatiles. Then, the mixture was further diluted
in pyridine to homogenise the mixture for diol and polyol analysis.
Further, acetylation was performed with acetic anhydride (0.5 mL
sample, 1 mL pyridine, 0.3 mL acetic anhydride) at 708C for 20–
30 min to determine the amount of unreacted glycerol. When 3-oc-
tanol was used as the reductant, the catalyst (0.03 mmol) was dis-
solved in 3-octanol (4 mL). 0.67 mL of this stock solution was
added to the reaction tube in which erythritol (0.2 mmol) was al-
ready weighed. After the reaction, the tube was cooled in an ice
bath and 20 mmol mesitylene was added. After mixing, 0.1 mL of
Acknowledgements
This research was performed within the framework of the Catch-
Bio program. The authors gratefully acknowledge support of the
Smart Mix Program of the Netherlands Ministry of Economic Af-
fairs and the Netherlands Ministry of Education, Culture, and
Science. XRD was financed by the Netherlands Organisation for
Scientific Research (NWO). We thank Dr. D. Carmichael (ꢀcole
Polytechnique, France) and Dr. Y. Cabon for the initial supply of
Cp(tBu3)H3 and useful discussions about the synthesis.
1
the mixture was diluted in CDCl3. Yields from H NMR spectroscopy
were calculated based on the aromatic proton signals of
mesitylene.
Keywords: biomass
deoxydehydration · homogeneous catalysis · rhenium
·
cyclopentadienyl
ligands
·
X-ray crystal structure determinations: X-ray reflections were
measured on a Bruker Kappa ApexII diffractometer with sealed
tube and Triumph monochromator (l=0.71073 ꢁ). The intensities
were integrated with the Saint software.[30] Absorption correction
and scaling based on multiple reflections were performed with the
SADABS software.[31] The structure of 1 was solved with the
DIRDIF-08 software,[32] and the structure of 2 was determined by
using the SIR-97 software.[33] Least-squares refinement was per-
formed with the SHELXL-97 software against F2 of all reflections.[34]
Non-hydrogen atoms were refined freely with anisotropic displace-
ment parameters. Hydrogen atoms were located in difference
Fourier maps. The ring hydrogen atoms H3 and H5 in 1 were re-
fined freely by using isotropic displacement parameters; all other
hydrogen atoms were refined with a riding model. Geometry calcu-
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Received: April 19, 2013
Published online on July 10, 2013
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