Catalytic dehydration of benzylic alcohols to styrenes by rhenium complexes

Article abstract of DOI:10.1002/cssc.201000055

The oxygen content of biomass-based materials can be reduced by selective dehydration of hydroxyl groups. As a first step towards biomass-based chemicals, rhenium-based catalysts are shown to be active in the dehydration of various benzylic alcohols to styrene moieties. The turnover frequencies are superior to the benchmark catalyst sulfuric acid, without sacrificing any selectivity.

Full text of DOI:10.1002/cssc.201000055

DOI: 10.1002/cssc.201000055
Catalytic Dehydration of Benzylic Alcohols to Styrenes by Rhenium
Complexes
Ties J. Korstanje, Johann T. B. H. Jastrzebski, and Robertus J. M. Klein Gebbink*[a]
The catalytic dehydration of (poly)alcohols represents a facile
route to (functionalized) olefins. Current dehydration methods
use strong acids, such as sulfuric acid or p-toluenesulfonic acid
(pTSA),[1] or solid acids, such as zeolites[2] or metal oxides.[3,4]
The major problems with these methods are their low selectivi-
Scheme 1. Rhenium-catalyzed dehydration of 1,2,3,4-tetrahydronaphthol (re-
action 1).
ty and low functional group tolerance. Therefore a selective,
widely useable dehydration method is highly desirable. Such a
method might prove useful in the dehydration of biomass-
based compounds, which are very rich in hydroxyl groups. De-
hydration of both the (hemi)cellulosic components of biomass
as well as aromatic lignin components could provide very in-
teresting building blocks for the chemical industry.[5,6]
ing olefin 2 as the single product. Lowering the amount of cat-
alyst to 1 mol% also resulted in complete conversion after
30 min.
To date only a few alcohol-to-olefin dehydration reactions
catalyzed by homogeneous catalysts have been reported. Cata-
lysts based on ruthenium[7,8] have proven active in the com-
bined dehydration/hydrogenation of diols, and also palladium-
Next, different commercially available rhenium complexes
were tested for their activity in reaction 1. A blank reaction
gave low conversion after 3 days of reaction time, yielding a
mixture of both olefin and ether. Re2(CO)10 did show some ac-
tivity, yet poor selectivity, yielding a 1:1 mixture of olefin and
ether after 2 days. Rhenium metal showed a much higher ac-
tivity as well as selectivity, pushing the reaction to full conver-
sion after 22 h, with good selectivity (87% towards the olefin).
The rhenium(VII) complexes MTO, Re2O7, and HReO4 performed
excellent in this dehydration reaction: all gave full conversion
after 1 h and selectively yielded the olefin.
[9]
and zinc-based[10] catalysts have been reported for the dehy-
dration of alcohols. A remarkable non-metal-based dehydration
of glycerol and erythritol that makes use of formic acid was re-
cently reported by Ellman et al.[11]
High-valent rhenium complexes have also shown promising
activity in dehydration reactions. Multiple methods were re-
ported: Methyltrioxorhenium(VII) (MTO; CH3ReO3) showed
good results at room temperature in the dehydration of vari-
ous alcohols,[12] while in the presence of hydrogen at higher
temperature and pressure epoxides were deoxygenated in
good and diols in moderate yield.[13] Cp*ReO3 performed very
well in the presence of PPh3 in the deoxygenation of various
diols and polyols,[14] however, phosphine oxide was obtained
as a byproduct in quantitative amounts. Herein, we report on
a study into various rhenium-based catalysts for the dehydra-
tion of alcohols to olefins under mild conditions. We initially
focused on the dehydration of various benzylic alcohols.
Following the original procedure by Zhu and Espenson,[12]
the first attempts using MTO as a catalyst for the dehydration
of 1,2,3,4-tetrahydronaphthol 1 to 1,2-dihydronaphthalene 2
(Scheme 1) at room temperature gave surprisingly poor results.
The yield of 2 using benzene as a solvent was only 4%, while
71% was originally reported. In our hands, the highest yield of
2 obtained was 10%, using THF as solvent, a reaction time of
3 days, and 10 mol% MTO as catalyst at room temperature.
Using the same procedure at elevated temperature, however,
gave very good results. In toluene at 1008C, complete conver-
sion was obtained after as little as 30 min reaction time, yield-
Because excellent results were obtained with reaction 1, the
more challenging dehydration reaction of 1-phenylethanol 4
to styrene 5 and the corresponding ether 6 was investigated
(reaction 2; Table 1). No reaction was observed in a blank ex-
Table 1. Catalytic activity of rhenium complexes and classic catalysts in
the dehydration of 1-phenylethanol 4 to styrene 5 and the corresponding
ether 6 (reaction 2).[a]
Catalyst
Amount
[mol%]
Conversion[b]
[%]
Yield of
5
Yield of
[b] [%]
TOF[c]
[b] [%]
6
[hÀ1
]
–
–
0.5
1
1
1
1
0.5
1
1
2.5
5
0
92
35
0
0
0
13.1
3.3
0.6
19.1
36.7
431
25.5
0
52.2
3.4
Re2(CO)10
Re metal
ReCl3O(PPh3)2
ReIO2(PPh3)2
MTO
64
23
7
81
89
98
56
0
28
16
10
19
11
1
38
0
5
17
>99
>99
>99
94
0
>99
96
Re2O7
HReO4 (aq)
NBu4ReO4
H2SO4
39
85
pTSA
1
[a] T. J. Korstanje, Dr. J. T. B. H. Jastrzebski, Prof. R. J. M. Klein Gebbink
Organic Chemistry & Catalysis
[a] Reaction conditions: 2 mmol 1-phenylethanol, 0.01–0.1 mmol catalyst,
250 mL pentadecane (internal standard), 10 mL toluene, 1008C. [b] GC
conversion or selectivity after 24 h. [c] Based on the consumption of 1-
phenylethanol in the first 30 min
Faculty of Science, Utrecht University
Padualaan 8, 3584 CH Utrecht (The Netherlands)
Fax: (+31)30 2523615
E-mail: r.j.m.kleingebbink@uu.nl
ChemSusChem 2010, 3, 695 – 697
ꢀ 2010 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
695

periment. The use of Re2(CO)10 gave good conversion and se-
lectivity within 24 h of reaction time, while rhenium metal
showed poor conversion. For both catalysts, however, the turn-
over frequency (TOF) was very low. Two rhenium(V) com-
pounds were also tested in this reaction, and while the mono-
oxo species ReCl3O(PPh3)2 showed very low conversion, the
dioxo species ReIO2(PPh3)2 showed full conversion, good selec-
tivity, and a reasonable TOF. Rhenium(VII) compounds also
pushed the reaction to completion, while giving good to excel-
lent selectivities towards styrene. MTO and HReO4 showed
comparable TOFs in the order of 30 hÀ1, while Re2O7 showed a
much higher TOF of 431 hÀ1 and pushed the reaction to com-
pletion within only 1 h. The latter TOF is superior to results ob-
tained with both 2.5 mol% sulfuric acid and 5 mol% p-toluene-
sulfonic acid as a catalyst, which showed TOFs of 52.2 and
3.4 hÀ1, respectively. More interesting is the selectivity of Re2O7
compared to the conventional catalysts: Re2O7 yielded styrene
almost exclusively while sulfuric acid only yielded 39% of sty-
rene, with many styrene dimerization and oligomerization
products. Also, p-toluenesulfonic acid yielded only 85% of sty-
rene, with the remainder mostly being dimers and oligomers.
It can clearly be concluded that in the dehydration of 1-phe-
nylethanol Re2O7 outperforms the classical acidic catalysts in
both activity and selectivity.
conversion using the rhenium catalyst. With Re2O7, TOFs above
800 hÀ1 were achieved under these conditions. Only the pri-
mary non-benzylic alcohol 2-phenylpropan-1-ol (entry 8) did
not react with either catalyst. In comparison, it is clear that
Re2O7 shows superior reaction rates and TOFs for all substrates
at lower catalyst loadings, while conversions and selectivities
after 24 h are comparable to sulfuric acid.
To explore the influence of electronic effects on the dehy-
dration reaction, different para-substituted 1-phenylethanol
substrates were used in the dehydration reaction catalyzed by
either Re2O7 or sulfuric acid (Table 3). In both cases a clear cor-
relation between the electron-donating properties of the sub-
stituent and the activity in the dehydration reaction was ob-
served: more electron-donating substituents greatly enhance
Table 3. Substituent effect on the dehydration of para-substituted 1-phe-
nylethanols catalyzed by 0.5 mol% Re2O7 and 2.5 mol% H2SO4.[a]
R
Re2O7
H2SO4
Conversion[b] Olefin
TOF[c] Conversion[b] Olefin
TOF[c]
[%]
yield[b] [%] [hÀ1
]
[%]
yield[b] [%] [hÀ1
]
OMe >99
77
97
67
0
792
392
101
0
>99
>99[d]
>99
0
11
42[d]
58
0
151
52
28
0
H
Cl
CN
NO2
>99
>99
0
Given these results, the substrate scope of the best-perform-
ing catalyst, Re2O7, was investigated. Various benzylic alcohols
were dehydrated by using 0.5 mol% Re2O7, and its perfor-
mance was compared to 2.5 mol% sulfuric acid as catalyst
(Table 2). All secondary and tertiary alcohols were converted
completely into the corresponding olefins within 24 h except
for cyclohexyl phenylmethanol (entry 6), which showed 91%
0
0
0
92
42
5
[a] Reaction conditions: 2 mmol para-substituted 1-phenylethanol,
0.01 mmol Re2O7 or 0.05 mmol H2SO4, 250 mL pentadecane (internal stan-
dard), 10 mL toluene, 1008C. [b] GC conversion or yield after 24 h.
[c] Based on the consumption of starting material in the first 15 min,
average of 2 experiments. [d] After 6 h.
Table 2. Dehydration of various substrates catalyzed by 0.5 mol% Re2O7 and 2.5 mol% H2SO4.[a]
the reaction, while substrates
with more electron-withdrawing
substituents do not react at all.
These observations suggest the
involvement of carbenium ions
in the reaction mechanism of
both catalysts. Further work to
explore the reaction mechanism
using Re2O7 in more detail is on-
going. A comparison of the two
catalysts again shows that Re2O7
is outperforming sulfuric acid in
both activity and selectivity,
except when using 1-(4-nitro-
phenyl)ethanol as substrate. This
substrate did not react using
Re2O7, while it reacted slowly
with sulfuric acid.
Entry
Substrate
Re2O7
H2SO4
Conversion[b]
[%]
Olefin
yield [%]
TOF[c]
[hÀ1
Conversion[b]
[%]
Olefin
yield [%]
TOF[c]
]
[hÀ1
]
1
2
1
4
>99
>99
>99
98
>800
431
>99
>99
>99
39
154
52
3
4
5
6
>99
>99
>99[e]
91
>99 (1:13)[d]
464
54
>99
>99
>99[f]
>99
>99 (1:10)[d]
32
31
87
24
>99
90[e]
>99
76[f]
>800
164
84 (3:1)[g]
92 (3:1)[g]
7
8
>99
>99 (1:13)[d]
404
0
>99
93 (1:11)[d]
13
0
Interestingly 1-(4-methoxyphe-
nyl)ethanol, which was trans-
formed into the corresponding
styrene rapidly and selectively
when using Re2O7, resembles the
p-coumaryl alcohol fragment
0
0
0
0
[a] Reaction conditions: 2 mmol substrate, 0.01 mmol Re2O7 or 0.05 mmol H2SO4, 250 mL pentadecane (internal
standard), 10 mL toluene, 1008C. [b] GC conversion or yield after 24 h. [c] Based on the consumption of starting
material in the first 15 min, average of 2 experiments. [d] cis:trans ratio. [e] 15 min reaction time. [f] 1 h reaction
time. [g] (Cyclohexylidenemethyl)benzene/(cyclohexenylmethyl)benzene ratio
696
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sealed with a septum and the mixture was heated with magnetic
stirring to 100C for 24 h. Samples for GC analysis were taken by sy-
ringe and filtered over neutral alumina, using acetone as eluent.
(and other methoxy-substituted phenethyl alcohols) known to
be present in softwood lignin and has been used as a lignin
model compound in previous research.[15,16]
In conclusion, we have found that rhenium-based catalysts
are very effective in the dehydration of benzylic alcohols.
Re2O7 (0.5 mol%) was shown to perform better in this reaction
than the current benchmark catalyst, sulfuric acid (2.5 mol%).
In all cases the TOFs of Re2O7 were superior to those of sulfuric
acid, without sacrificing selectivity for the olefin products. The
rhenium-based protocol has shown an excellent activity and
selectivity in the dehydration reaction of a methoxy-substitut-
ed phenylethyl alcohol and holds great promise for the dehy-
dration of other lignin-based alcohols. This protocol might also
be of interest in the field of synthetic organic chemistry for the
conversion of readily available benzylic alcohols into styrene
moieties. Current research efforts in our laboratory are aimed
at broadening the substrate scope of this dehydration reaction
towards biomass-based substrates, and at understanding the
underlying mechanism.
Acknowledgements
This research was performed within the framework of the Catch-
Bio program. The authors gratefully acknowledge support
through the Smart Mix Program of the Netherlands Ministry of
Economic Affairs and the Netherlands Ministry of Education, Cul-
ture and Science.
Keywords: alcohols · dehydration · elimination · rhenium ·
styrenes
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4 equivalents of
Typical procedure for the rhenium-catalyzed dehydration of alco-
hols to olefins: Catalyst (0.01 or 0.02 mmol) was dissolved or sus-
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Typical procedure for the sulfuric acid-catalyzed dehydration of al-
cohols to olefins: Substrate (2 mmol) and pentadecane (250 mL)
were dissolved in toluene (10 mL). Sulfuric acid (4.9 mg, 2.7 mL,
0.05 mmol) was added by using a Finnpipette. The reactor was
Received: February 26, 2010
Revised: March 18, 2010
Published online on May 12, 2010
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