Neutral ionic liquid [hmim]Br as a green reagent and solvent for the mild and efficient dehydration of benzyl alcohols into (E)-arylalkenes under microwave irradiation

Article abstract of DOI:10.1002/ejoc.200800657

A mild and efficient, ionic-liquid-assisted, green protocol for the dehydration of benzyl alcohols into the corresponding (E)-arylalkenes under microwave irradiation has been developed. The method utilizes a neutral and recyclable ionic liquid (1-hexyl-3-methylimidazolium bromide) as a reagent and solvent to cleanly provide a wide range of olefins without the need of harsh and expensive Bronsted/Lewis acids. The method was extended to the efficient conversion of acetylated/benzoylated derivatives of benzyl alcohol into their corresponding (E)-arylalkenes. Wiley-VCH Verlag GmbH & Co. KGaA, 2008.

Full text of DOI:10.1002/ejoc.200800657

FULL PAPER
DOI: 10.1002/ejoc.200800657
Neutral Ionic Liquid [hmim]Br as a Green Reagent and Solvent for the Mild
and Efficient Dehydration of Benzyl Alcohols into (E)-Arylalkenes Under
Microwave Irradiation[‡]
Rakesh Kumar,[a] Abhishek Sharma,[a] Naina Sharma,[a] Vinod Kumar,[a] and
Arun K. Sinha*[a]
Keywords: Ionic liquids / Dehydration / Benzyl alcohol / Microwave-assisted reaction / Alkenes
A mild and efficient, ionic-liquid-assisted, green protocol for
the dehydration of benzyl alcohols into the corresponding
(E)-arylalkenes under microwave irradiation has been devel-
oped. The method utilizes a neutral and recyclable ionic li-
quid (1-hexyl-3-methylimidazolium bromide) as a reagent
and solvent to cleanly provide a wide range of olefins without
the need of harsh and expensive Brønsted/Lewis acids. The
method was extended to the efficient conversion of acety-
lated/benzoylated derivatives of benzyl alcohol into their cor-
responding (E)-arylalkenes.
(© Wiley-VCH Verlag GmbH & Co. KGaA, 69451 Weinheim,
Germany, 2008)
phorus pentachloride, phosphorus pentoxide, copper sul-
fate/toluene, titania catalyst at elevated temperatures, silica
gel/copper sulfate and activated silica gel under solvent-free
conditions.[4] In spite of the availability of the dehydrating
agents described above, a majority of these protocols con-
tinue to be afflicted with perennial limitations like the lack
of generality, the formation of side products, the employ-
ment of expensive, moisture-sensitive and toxic catalysts,
long reaction times and the harshness of Bronsted/Lewis
acids, which not only precludes their use with substrates
possessing sensitive functional groups but also leads to en-
vironmental harm. In particular, the controlled dehydration
of benzyl alcohols into the corresponding olefins including
the immensely important (E)-arylalkene derivatives[1e,i] has
remained a difficult proposition due to the preponderant
tendency of the incipient benzylic carbocation to participate
in competing side reactions leading to the formation of sev-
eral side products[5] including a dimer.[5b,5d,5e] Recently, a
notable report described a pseudourea-mediated dehy-
dration of benzylic alcohols, in which the pseudourea was
formed in situ. However, the protocol involved long reac-
tion times and hazardous metal salts.[6] In addition, a
number of the protocols described above result in the for-
mation of unwanted, toxic, cis isomers, which in turn, re-
quire tedious chromatographic separation owing to the sim-
ilar Rf values of the cis and trans isomers.[7]
Introduction
The dehydration of alcohols is a fundamental and exten-
sively exploited transformation in organic synthesis due to
the immense biological importance and synthetic utility of
the ensuing alkenes.[1] For instance, various methoxylated
(E)-arylalkenes such as α-asarone are known to be active
hypolipidemic agents besides possessing neuroleptic,
antichloretic, antiplatelet and antifungal activities.[1e,1i] In
addition, a number of styrenes and polycyclic arylalkenes
have been recognized as important synthons for various
bioactive stilbenes as well as commercial anti-inflammatory
agents through Heck and hydroformylation approaches,
respectively.[1f,1j,1d] The usual protocols for accomplishing
the above dehydration employ various reagents like mineral
acids, PTSA and oxalic acid.[1a,1b,1c,1g] However, a majority
of the above protocols are limited by their incompatibility
with substrates possessing acid-sensitive functional groups
and a propensity for undesired side reactions.[1g] Accordingly,
there has been a growing impetus to develop new protocols
for the dehydration of alcohols under neutral conditions em-
ploying a variety of reagents including DMSO,[2a,2b] tri-
[2e]
phenylbismuth dibromide–iodine,[2c] PPh3,[2d] ZnCl2
and
metal triflates.[2f] Recently, molecular iodine has also been
reported to catalyze the dehydration of various alcohols.[3] In
addition, the dehydration of alcohols has also been at-
tempted with several other dehydrating agents like phos-
The dehydration of benzyl alcohols through their prior
derivatisation into benzoylated/acetylated counterparts also
constitutes a useful strategy in the multistep synthesis of
complex natural products.[8] However, such transformations
are often carried out with harsh bases, which preclude their
usage in cases of substrates containing sensitive functional
groups.[8]
[‡] IHBT Communication No. 0804.
[a] Natural Plant Products Division, Institute of Himalayan Bio-
resource Technology (C.S.I.R.), Palampur (H.P.) 176061, India
Fax: +91-1894-230433
E-mail: aksinha08@rediffmail.com
Supporting information for this article is available on the
WWW under http://www.eurjoc.org or from the author.
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FULL PAPER
The conventional Lewis/Bronsted catalysts are often gated for their effects on the dehydration of 1a. It is evident
toxic, corrosive and difficult to separate/recover from prod- from Table 1 that the nature of the alkyl chain and anion
ucts, despite their high catalytic activity. A currently rapidly in the ionic liquid play a crucial role in the efficient dehy-
developing area in organic synthesis concerns the design dration of benzyl alcohols. Thus, 1-butyl-3-methylimid-
and usage of catalysts that not only possess high activity azolium hexafluorophosphate and 1-butyl-3-methylimid-
and selectivity but are simultaneously benign to the envi- azolium tetrafluoroborate (Table 1, Entries 2 and 3) pro-
ronment and easily recoverable. In this context, ionic liquids vided a comparatively inferior yield of 1b along with some
have recently attracted considerable interest due to their side products. However, 1-butyl-3-methylimidazolium bro-
several inherent virtues like low vapour pressure, easy re- mide (Table 1, Entry 4) enhanced the yield of 1b to 78%.
cyclability and high thermal stability.[9] In addition, there The best reaction performance was delivered by 1-hexyl-3-
have been some interesting reports wherein the peculiar methylimidazolium bromide (Table 1, Entry 5), which pro-
ability of neutral ionic liquids to effectively promote con- vided the product (1b) in 87% yield (100% conversion) after
ventional acid/base-catalyzed reactions has come to the 7 min of microwave irradiation (Scheme 1). The favourable
fore.[10] Although a few recent reports have also disclosed dependence of the reaction on the presence of the more
the ionic-liquid-promoted dehydration of alcohols, the pro- lipophilic hexyl side chain, which allowed for easier aque-
tocols were either limited to a single substrate like fruc- ous extraction and isolation of the products with improved
tose[11] or required the indispensable presence of an adja- yield, has been observed previously.[16] It was interesting to
cent cyclopropyl moiety.[12]
observe that no reaction occurred when only 1-methylimid-
In view of these concerns, we herein report that a neutral azole (Table 1, Entry 6) was used as the solvent for the de-
and recyclable ionic liquid efficiently acts as a green reagent hydration of 1a, while the treatment of 1a with the well-
and solvent for the dehydration of a diverse range of benzyl known dehydrating agent PTSA/toluene[1h] (Table 1, Entry
alcohols into the corresponding (E)-arylalkenes under mi- 7) under microwave irradiation resulted in a comparatively
crowave irradiation.
Results and Discussion
In continuation of our ongoing efforts toward the devel-
opment of clean synthetic methodologies[13] for the prepa-
ration of various bioactive compounds, we became inter-
ested in developing a mild, efficient and environmentally
Scheme 1. Dehydration of benzyl alcohols and their acetylated/ben-
friendly alternative for the dehydration of benzyl alcohols in
zoylated derivatives.
lieu of the prevalent harsh protocols.[14] It has been widely
recognized that the dehydration of benzyl alcohols is gen-
erally plagued by the extreme susceptibility of the product
alkenes to further polymerization reactions, leading to the
Table 1. Effect of different ionic liquids on the dehydration of 1a
under microwave irradiation.[a]
formation of several side products and tedious purification
steps.[15] We have also recently reported a silica-gel-sup-
ported, solvent-free protocol for the dehydration of substi-
tuted benzyl alcohols into the corresponding arylalkenes.
However, the method was limited by moderate yields of the
arylalkenes.[4g] In view of the remarkable ability of ionic
liquids to efficiently promote a wide range of conventional
acid/base-catalyzed reactions,[10] we became interested in
exploring ionic liquids as green reagents and solvents for
the dehydration of various benzyl alcohols. Initially, 4-
methoxyphenylpropan-1-ol (1a) was heated with commer-
cially available ionic liquid 1-butyl-3-methylimidazolium
chloride under microwave irradiation for 8 min, and the
corresponding (E)-4-methoxyphenylpropene (1b) was ob-
tained in 72% yield along with some side products and un-
reacted 1a. Encouraged by the above success, we employed
various modifications in the reaction conditions, such as
prolonged reaction time and increased reaction tempera-
[a] CEM monomode microwave. Reaction conditions: 1.7 mmol of
ture, to further increase the reaction performance, but to no
1a, 1 mL of ionic liquid, 150 W, 140 °C. [b] 1 mL of 1-methylimid-
avail. Consequently, we shifted our attention to evaluate the
dependence of the above reaction on the composition of the
azole was used in place of the ionic liquid. [c] 0.15 mmol of p-
toluenesulfonic acid, 4 mL of toluene. [d] Isolated yield of 1b after
ionic liquid. A range of ionic liquids (Table 1) were investi- column chromatography.
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Ionic Liquid for the Dehydration of Benzyl Alcohols
lower yield of 1b (46%) along with side products. Surpris- izing the peculiar role of the ionic liquid in efficient dehy-
ingly, the imidazolium-based ionic liquid incorporating the drations. Similarly, the basic ionic liquid [bmim]OH
PTSA moiety (Table 1, Entry 8) provided 1b in an improved (Table 1, Entry 9) provided the expected 1b in only 55%
yield (68%) with less side product formation, thus emphas- yield. In order to evaluate the role of the microwave irradia-
Table 2. Dehydration of benzyl alcohols and their derivatives into corresponding (E)-arylalkenes with 1-hexyl-3-methylimidazolium bro-
mide under microwave.[a]
[a] CEM monomode microwave. Reaction conditions: 1.7 mmol of substrate 1a–25a, ionic liquid (1 mL), 150 W, 140 °C. [b] Based on
NMR spectroscopy. [c] Not detected.
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R. Kumar, A. Sharma, N. Sharma, V. Kumar, A. K. Sinha
FULL PAPER
tion, the dehydration of 1a with 1-hexyl-3-methylimid- radiation by the carbinol-ionic-liquid intermediate could re-
azolium bromide was carried out under conventional heat- sult in the elimination of water to provide the arylalkene
ing at 140 °C for 2 h, and 1b was isolated in 74% yield. In (Figure 1).
addition, the same reaction at room temperature required
prolonged stirring (18 h) to give the expected 1b in 81%
yield.
We subsequently extended the developed method to the
dehydration of a range of substituted benzyl alcohols
(Table 2) to rapidly provide the expected products within 6–
12 min. We generally observed that the product yields were
higher with substrates having a methoxy substitution on the
aromatic ring. The various structurally diverse alcohols
underwent smooth dehydration in good to excellent yields.
We note that the developed method was also suitable for the
efficient dehydration of polycyclic aromatic benzyl alcohols
(Table 2, Entries 5–7, 11, and 14) into the corresponding
olefins, which are important synthons for the synthesis of
various commercial anti-inflammatory agents.[1d] In ad-
dition, the hindered tertiary alcohols (Table 2, Entries 20–
21) also underwent clean dehydration under the developed
reaction conditions. The efficient and neutral nature of this
dehydration protocol further prompted us to extend it to
the conversion of acetylated/benzoylated derivatives of ben-
zyl alcohols into their respective alkenes, due to their im-
mense utility in complex, natural-product synthesis. Conse-
quently, acetylated (Table 2, Entries 16–18) and benzoylated
(Table 2, Entry 19) derivatives of benzyl alcohols were re-
acted under similar reaction conditions, and the corre-
sponding (E)-arylalkenes were obtained in good yields (80–
85%). Our protocol compares very favourably with conven-
tional protocols for the dehydration of acetylated/benzoyl-
ated derivatives of benzyl alcohols into their respective al-
kenes, which involve strong acids and the prolonged heating
of substrates at elevated temperatures.[17]
In the case of the unsymmetrical dialkyl alcohol (Table 2,
Entry 20), a mixture of two alkenes was obtained in a
4:1 ratio, wherein the major product was obtained as per
Zaitzev’s rule. Similarly, the α,β-unsaturated alcohol
(Table 2, Entry 15) was converted into its corresponding bu-
tadiene in 74% yield. However, the β-alcohol (Table 2, En-
try 22) provided the corresponding alkene in lower yield
(22%), while no reaction occurred with the γ-alcohol
(Table 2, Entry 24). Thus, the method showed selective de-
hydration of activated benzyl alcohols over β- and γ-
alcohols.
Figure 1. Proposed mechanism of ionic-liquid-assisted dehydration.
In order to check the recyclability of the ionic liquid,
after completion of the reaction, the product 1b was ex-
tracted with ethyl ether, and the remaining ionic liquid was
reused as such for subsequent cycles. A 5–6% loss in ac-
tivity was observed after three cycles. We note that prior
lyophilisation of the ionic liquid for 30 min allowed it to be
efficiently used for five cycles without any loss of activity.
Conclusions
In conclusion, a mild and efficient protocol for the dehy-
dration of various benzyl alcohols and their acetylated/ben-
zoylated counterparts into the corresponding (E)-arylalk-
enes with a recyclable ionic liquid as a reagent and solvent
under microwave irradiation was developed. The method
allowed hitherto tedious dehydrations to be performed un-
der mild and neutral conditions without any additional
harsh Brønsted/Lewis acids, which not only enhances its
compatibility with substrates possessing acid-sensitive func-
tional groups but also augments its eco-friendly nature. The
remarkable selectivity of the developed method towards the
dehydration of activated benzyl alcohols over β- and γ-
alcohols provides a convenient chemo-selective tool for in-
tricate, multistep, natural-product synthesis.
The method developed herein allows the dehydration of
benzyl alcohols under neutral conditions, which not only
augments its compatibility with acid-sensitive functional
groups but also minimizes the formation of unwanted side
products. Although there is a previous report disclosing the
DMSO-mediated[2a,2b] dehydration of benzylic alcohols un-
der neutral conditions, that methodology was limited by
low to moderate yields and difficulty in removing DMSO
from the organic products.[10a]
Experimental Section
General: All the substrates were either obtained from commercial
sources (Merck or Acros) or synthesized from the corresponding
benzaldehydes/Grignard reagents[4g] or by the reduction of the cor-
responding acetophenones.[1h] The ionic liquids were obtained
either commercially (Merck or Alfa Aesar) or synthesized ([hmim]-
Br) according to the reported method.[9f] The purity of the ionic
liquids was verified by NMR before use. 1H (300 MHz) and 13C
Mechanistically, the ionic-liquid-promoted dehydration
may occur through an initial polarization of the C–O bond
of the carbinol by the imidazolium cation of the ionic li-
quid.[12] Subsequently, an efficient absorption of microwave (75.4 MHz) NMR spectra were recorded with a Bruker Avance-300
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Ionic Liquid for the Dehydration of Benzyl Alcohols
spectrometer.
CEM Discover® focused microwave oven
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(2450 MHz, 300 W) was used.
General Procedure for the Dehydration of Benzyl Alcohols under Fo-
cused Microwave Irradiation. Representative Procedure for the Dehy-
dration of 4-Methoxyphenylpropan-1-ol (1a): A mixture of 4-meth-
oxyphenylpropan-1-ol (1a) (283 mg, 1.7 mmol) and ionic liquid
[hmim]Br (1 mL) was irradiated for 7 min in a 100 mL round-bot-
tomed flask in a focused microwave system (150 W, 140 °C) fitted
with a reflux condenser. After completion of the reaction, the mix-
ture was cooled and extracted with ethyl ether (3ϫ10 mL), and the
ionic liquid was recovered as a residue to be used in the next cycle.
The combined organic layers were washed with water, dried and
evaporated in vacuo. The crude product was purified by column
chromatography on silica gel (60–120 mesh size) with a 1:20 mix-
ture of ethyl acetate and hexane to give 1b (220 mg, 87%) as a
colourless liquid, whose spectroscopic data agreed well with the
reported values.[18f]
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[5]
General Procedure for the Dehydration of 4-Methoxyphenylpropan-
1-ol (1a) under Conventional Heating: A mixture of 1a (283 mg,
1.7 mmol) and ionic liquid [hmim]Br (1 mL) was heated in a
100 mL round-bottomed flask in an oil bath at 140 °C for 2 h. Af-
ter the completion of the reaction, the mixture was worked up as
described above to give the pure product 1b (187 mg, 74%) as a
colourless liquid, whose spectroscopic data agreed well with the
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Acknowledgments
[8]
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R. K., A. S. and V. K. are indebted to the Council of Scientific and
Industrial Research (C.S.I.R.), New Delhi, the University Grants
Commission (U.G.C.), New Delhi, and the Council of Scientific
and Industrial Research (No. IHBT-MLP0025) for financial sup-
port. N. S. is thankful to C.S.I.R. for the award of a diamond jubi-
lee research internship. The authors gratefully acknowledge the Di-
rector of I.H.B.T. Palampur, for his kind cooperation and encour-
agement.
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Received: July 3, 2008
Published Online: October 13, 2008
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