Dehydrative Coupling of Benzylic Alcohols Catalyzed by Br?nsted Acid/Lewis Base

Article abstract of DOI:10.1002/ejoc.201900965

Traditional cross-coupling reactions show some disadvantages like the use of organohalides or the production of stoichiometric amounts of waste. The dehydrative homo- or heterocoupling of alcohols therefore arises as an interesting approach for a highly atom-economical formation of carbon–carbon bonds, since water is produced as the only by-product. We herein report a simple and direct, metal-free protocol for the synthesis of olefins by applying catalytic amounts of a sulfonic acid and triphenylphosphane under air. A variety of olefins could be synthesized from benzylic alcohols under relatively mild conditions. Additionally, dehydrative hydroarylation of benzylic alcohols with electron-rich arenes was possible by using only Br?nsted acid under otherwise same reaction conditions. We could show that phosphane additives are essential to overcome oligomerization as main side reaction by the occupancy of the reactive carbocation intermediate.

Full text of DOI:10.1002/ejoc.201900965

A Journal of
Accepted Article
Title: Dehydrative Coupling of Benzylic Alcohols Catalyzed by
Brønsted Acid/Lewis Base
Authors: Marlene Böldl and Ivana Fleischer
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the VoR from the journal website shown below when it is published
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content of this Accepted Article.
To be cited as: Eur. J. Org. Chem. 10.1002/ejoc.201900965
Link to VoR: http://dx.doi.org/10.1002/ejoc.201900965
Supported by

10.1002/ejoc.201900965
European Journal of Organic Chemistry
COMMUNICATION
alcohol activation has been reported in recent years.[12]
Additionally, it was reported that strong Brønsted acids are able
to catalyze dehydrative substitutions, mainly by an SN1
mechanism.[13]
Dehydrative
Coupling
of
Benzylic Alcohols Catalyzed by
Brønsted Acid/Lewis Base
Marlene Böldl,[a] Ivana Fleischer*[a]
When looking on the history of direct dehydrative coupling of
benzylic alcohols, metal catalysts are mainly represented as
activation agents in earlier reports. In 2006, Muzart and co-
workers reported the application of the Wells-Dawson tungsten
heteropolyacid H2P2W18O62 as catalyst for the homocoupling of
1-indanol and of 1-phenylethanol in 1,2-dichloroethane.[14] The
reaction, however, suffered from low yields. Yamamoto et al.
disclosed a Pd(II)-catalyzed heterocoupling of benzylic alcohols
with styrenes shortly afterwards.[15] The desired substituted
Abstract: Traditional cross-coupling reactions show some
disadvantages like the use of organohalides or the production of
stoichiometric amounts of waste. The dehydrative homo- or
heterocoupling of alcohols therefore arises as interesting approach for
a highly atom-economical formation of carbon-carbon bonds, since
water is produced as only byproduct. We herein report a simple and
direct, metal-free protocol for the synthesis of olefins by applying
catalytic amounts of a sulfonic acid and triphenylphosphine under air.
A variety of olefins could be synthesized from benzylic alcohols under
relatively mild conditions. Additionally, dehydrative hydroarylation of
benzylic alcohols with electronrich arenes was possible by using only
Brønsted acid under otherwise same reaction conditions. We could
show that phosphine additives are essential to overcome
oligomerization as main side reaction by occupancy of the reactive
carbocation intermediate.
olefins were obtained in moderate yields by applying
3
equivalents of (CF3CO)2O as additive and PPh3 as ligand. In
addition, catalysts based on Cu,[16] Fe,[17] Ru[18] and Ca[19] were
employed in similar transformations.
Metal-catalyzed couplings:
R1
R1
R2
X
+
+
MX
R2
[M]
[M] = ZnBr, SiR3, SnR3, H, ...
X = Hal, OTf, ...
General approach of dehydrogenative couplings:
Introduction
HO
R4
R3
H
One of the most fundamental subjects in organic chemistry is the
efficient and selective formation of carbon-carbon bonds.
Especially transition-metal catalyzed cross-coupling reactions
emerged as important tool for the construction of sp3-sp2 C-C
bonds and have been widely investigated and developed.[1]
Nevertheless, most of these protocols show disadvantages in
respect to the aspects of green chemistry.[2] Often, additional
preparative steps are necessary, as well as the application of
hazardous reagents and nobel metal catalysts. Also the
production of stoichiometric amounts of waste is unattractive. In
recent years, the direct dehydrative homocoupling of simple
alcohols and heterocoupling of alcohols and alkenes or arenes to
substituted olefins arose as interesting alternatives due to the
advantage of producing water as only byproduct (Scheme 1).[3]
However, since hydroxy groups in alcohols are intrinsically
poor leaving groups, an activation by transformation in a better
one has to occur.[4] This dehydroxylation can be conducted
indirectly by conversion into tosylates,[5] triflates,[6], mesylates,[7}
acetates[8] and halides.[9] Direct substitution of the hydroxy group
is more attractive and can be achieved in SN2 reactions by using
stoichiometric reagents via oxyphosphonium intermediates, as in
the Mitsunobu[10} or Appel reaction.[11] Besides, also catalytic
R2
or
R1 R4
[cat.]
-H2O
OH
+
R3
Ph
R1
Ph
R4
R2
H
R3
[cat.] = W, Pd, Ca, Fe, H+, ...
R2
This report:
OH
Ar1
TsOH•H2O (16 mol%)
PPh3 (2 mol%)
Ar1
Ar1
DCE, 60-100°C, 4 h
(or + Ar2H)
(without PPh3)
or
Ar1
Ar2
Scheme 1. Coupling methods for sp3-sp2 C-C bond formation.
Among the metal-free protocols, Sanz et al. briefly described
dehydrative sp3-sp2 C-C bond formation of benzylic alcohols with
trifluoromethanesulfonic acid as catalyst in context of the
Brønsted acid catalyzed benzylation of 1,3-dicarbonyl
compounds.[20] Nevertheless, only a limited substrate scope and
moderate yields could be obtained. The first metal-free route to
substituted olefins through the direct heterocoupling of benzylic
alcohols with alkenes was reported in 2011 by Yue and co-
workers.[21] Formation of the sp3-sp2 C-C bond could be realized
by application of trifluoromethanesulfonic acid in catalytic
amounts. This report was followed 2014 by a comprehensive
publication on metal-free dehydrative homocoupling of benzylic
alcohols from Xia et al. by applying previously prepared sulfonic
[a]
M. Böldl, Prof. Dr. I. Fleischer
Institute of Organic Chemistry
Eberhard Karls Universität Tübingen
Auf der Morgenstelle 18, 72076 Tübingen
E-mail: ivana.fleischer@uni-tuebingen.de
Homepage: https://uni-tuebingen.de/de/100267
Supporting information for this article is given via a link at the end of
the document.
This article is protected by copyright. All rights reserved.

10.1002/ejoc.201900965
European Journal of Organic Chemistry
COMMUNICATION
acid-functionalized ionic liquids.[13c] The substrate scope was
mainly based on benzhydrols and benzylic alcohols substituted
with halides. Nama and co-workers additionally showed the
application of heterogeneous catalysis on dehydrative coupling
reactions of benzylic alcohols by using zeolithes.[22] In many
reports, the limitations and possible side reaction were not
discussed.
(entry 10). Nevertheless, PPh3 was chosen as additive for
subsequent reactions, due to its low cost and high stability.
Table 1. Initial screening experiments for the homocoupling of benzylic
alcohols.
Additive (2 mol%)
OH
p-TsOH•H2O (16 mol%)
Ph
Ph
Ph
DCE
We developed
a simple, direct metal-free dehydrative
1a
2a
coupling starting from widely available and tractable alcohols by
using easy-to-handle and commercially available catalysts under
mild conditions. The protocol was envisioned to overcome
previous issues like the use of transition metals, additional
preparative steps, stoichiometric amounts of acid or additives.
The reactions are performed with weaker sulfonic acid and
triphenylphosphine as a Lewis base co-catalyst, which was not
reported before. We focused on the investigation of the
homocoupling of benzylic alcohols and the role of the catalyst
components in this reaction.
Yield of 2a
Entry
Additive
T [°C]
t [h]]
[%][a]
1[b]
2[b]
3[b]
4
‒
60
60
18
18
18
18
4
26
73
16
80
80
70
74
75
70
64
PPh3
PPh3
PPh3
PPh3
40
100
100
5
Results and Discussion
6
P(4-OMeC6H4)3
PCy3
100
100
100
100
100
4
7[c]
8
4
In an initial experiment, toluenesulfonic acid and 1-phenylethanol
(1a) dissolved in 1,2-dichloroethane (DCE) and heated to 60 °C
within 18 h under air, which afforded the respective dimer 2a in
26% yield (Table 1, entry 1). Application of 2 mol% of
triphenylphosphine (PPh3) as additive showed the formation of 2a
in 73% yield (entry 2). This was surprising, since in previous
reports, the application of triphenylposphine as additive in
substitutions of alcohols was mainly shown in Mitsunobu or Appel-
type reactions in stoichiometric amounts.[12e, 23]
Subsequent investigations on amounts, concentration,
solvent, acid and temperature were conducted (see supporting
information for further details). A lower reaction temperature of
40 °C diminished the yield to 16% (Table 1, entry 3) whereas a
higher reaction temperature of 100 °C (entry 4) led to the
formation of 2a in 80% yield. Additionally, it was shown that the
reaction is already completed after 4 hours (entry 5). Quantitative
conversion and observation of styrene (3) as intermediate,
indicated in situ dehydration of 1a. The formed styrene can
undergo oligomerization as main side reaction. Addition of
4-tert-butylcatechol as inhibitor could not prevent the
oligomerization nor the homocoupling, which excludes the
possibility of a radical reaction pathway.
TFP
4
9
BINAP
4
10
O=PPh3
4
General reaction conditions: 1a (0.5 mmol, 61 µL), p-TsOH•H2O (16 mol%),
additive (2 mol%), 1,2-dichloroethane (1.5 mL), under air; [a] Yields were
determined via quantitative GC-FID; [b] 1a (1.0 mmol, 121 µL), p-TsOH•H2O
(16 mol%), additive (2 mol%), 1,2-dichloroethane (3 mL), under air [c] under
inert conditions in degassed 1,2-dichloroethane as solvent.
For further mechanistic insights, the reaction progress under
optimized reaction conditions was evaluated (Figure 1). As
already observed, styrene (3) was formed in situ from 1a and
reached a maximum yield of 84% after 20 minutes and was then
continuously converted to dimer 2a. Additionally, an ether
formation from 1a to (oxybis(ethane-1,1-diyl))dibenzene (4) took
place within the first 15 minutes. Compound 4 however, was
consumed quite rapidly within the first 30 minutes. This also
explains the sigmoidal curve of the formation of 2a, which reached
a maximum yield of 80% after 4 h. Leftover styrene was further
consumed, but it did not enhance the yield of 2a.
However, the application of a phosphine conversely appears
to be a necessity for avoiding oligomerization reactions. Hence,
with the optimized reaction conditions in hand, other phosphine
additives were tested (entries 6-10). It could be shown, that an
In order to clarify the influence of the additive, several
mechanistic investigations were conducted (Scheme 2). Firstly, it
was observed, that styrene (3) reacts like 1a smoothly to 2a under
the optimized reaction conditions (Scheme 2A). Without the
addition of PPh3, 2a was only obtained in poor yields (17%), while
3 was fully converted. Hence, in the model reaction of 1a
(Figure 1), the additive must have the most impact on the
transformation of 3 to 2a. Full conversion in both reactions
provides another hint for the role of PPh3 as inhibitor for the
oligomerization.
electron-donating
substituent
on
the
phenylring
of
triphenylphosphine decreased the yield from 80% to 70% (entry
6). Beside phenyl substituted mono-phosphines, also alkane-
substituted phosphines gave 2a in good yields (74%, entry 7) and
heterocyclic tri(2-furyl)phosphine (TFP) showed similar results
(75%, entry 8). Bidentate phosphines like BINAP were as well
applicable to this kind of reaction providing good yields (70%,
entry 9). Interestingly, also triphenylphosphine oxide worked as
suitable additive, however, the yield dropped from 80% to 63%
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10.1002/ejoc.201900965
European Journal of Organic Chemistry
COMMUNICATION
benzylic carbocation (5) generates 2a. As observed before, the
employment of PPh3 resulted in an increase of yield. With this
result in hand we concluded that PPh3 has to interact with
carbocation 5 by catalytically forming a phosphine salt which
allows the attack of a single nucleophile but sterically prevents the
attack of a further one and consequently supresses the formation
of oligomers. In a subsequent experiment, phosphine salt 6 was
applied in the reaction of 1a to 2a (Scheme 2C). As the yield is
comparable to the model reaction (Table 1, entry 5), it supports
the conclusion we have made before.
Furthermore, we followed the progress of the reaction of
product 2a with styrene (3, Scheme 3). Since the product itself is
stable for at least 18 h under the given reaction conditions, we
were wondering, if the addition of 3 would lead to oligomerization.
In the experiment without additive, the amount of 2a initially
increased to 107% of the initially applied 2a due to the
dimerization of 3. However, after only 15 minutes the amount of
2a started to decline. After 24 h, the amount of 2a was 30% lower
than in the beginning. This means that vinylarene 3 indeed
undergoes dimerization to 2a within the first 15 minutes, but
subsequently reacts with 2a to generate higher oligomers.
Contrary to that, 3 was consumed much slower in the presence of
PPh3 and 130% of 2a was obtained after 4 hours. Even after 24
hours the amount of 2a was not lower than at the start of the
reaction. This proves that 3 is directly converted to 2a and PPh3
inhibits the further reaction of 2a and 3. Slow decrease in the
amount of 2a indicates, that the additive cannot completely
suppress oligomerization over a longer period of time.
OH
Ph
Ph
Ph
Ph
Ph
O
4
Ph
1a
2a
3
Figure 1. Reaction progress of the dehydrative coupling of 1a. General reaction
conditions: 1a (1.0 mmol, 121 µL), p-TsOH•H2O (16 mol%), PPh3 (2 mol%),
1,2-dichloroethane (3 mL), 100 °C; Yields were determined via quantitative
GC-FID.
p-TsOH•H2O (16 mol%)
A)
Ph
Ph
Ph
DCE, 100 °C, 4h
3
2a
yield with PPh3 : 75%
yield without additive: 17%
2a
p-TsOH•H2O (16 mol%)
p-TsOH•H2O (16 mol%)
B)
Additive
+
+
Ph
3, 1 eq
Ph
O
Ph
Ph
Ph
Ph
Ph
DCE, 100 °C
DCE, 100 °C, 1 h
higher
oligomers
4
2a
2a, 1eq
yield with PPh3 : 64%
yield without additive: 22%
- H2O
+
+
Ph
OH
1a
Ph
Ph
Ph
5
3
5
Ph
C)
(2 mol%) 6
Br PPh3
OH
TsOH•H2O (16 mol%)
Ph
Ph
Ph
DCE, 100 °C, 4 h
2a, 76%
1a
Scheme 2. Mechanistic investigations. General reaction conditions: A) 3
(1.0 mmol, 115 µL), p-TsOH•H2O (16 mol%), PPh3 (2 mol%),
1,2-dichloroethane (3 mL), 100 °C; B) 4 (1.0 mmol, 113 mg), p-TsOH•H2O
(16 mol%), PPh3 (2 mol%), 1,2-dichloroethane (3 mL), 100 °C; C) 1a (0.5 mmol,
61 µL), p-TsOH•H2O (16 mol%), 6 (2 mol%), 1,2-dichloroethane (1.5 mL);
Yields were determined via quantitative GC-FID.
Scheme 3. Investigation of the oligomerization of 2a with 3. General reaction
conditions: 1a (0.5 mmol, 61 µL), 3 (0.5 mmol, 57 µL), p-TsOH•H2O (16 mol%),
PPh3 (2 mol%), 1,2-dichloroethane (3 mL), 100 °C; Yields were determined via
quantitative GC-FID.
A similar observation was made, when looking at the
transformation of 4 (Scheme 2B). Since 4 is acid labile, 2a was
obtained in 22% yield in the absence of PPh3, with 82%
conversion of 1a and formation of 3 in 10%. This leads to the
assumption that in the model reaction, formation of 4 is reversible
and reformation of 1a, dehydration and subsequent attack on the
Based on these observations, a plausible mechanism of the
dehydrative coupling is depicted in Scheme 4. Unlike shown in
previous publications,[20,21] we exclude the direct formation of 2a
from 4 by reaction with 3 since we could show the reversibility of
the formation of 4 in acidic medium (Scheme 2B). Hence, we
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10.1002/ejoc.201900965
European Journal of Organic Chemistry
COMMUNICATION
propose that dehydration of 1a provides vinylarene 3 and
carbocation 5 in situ, while 5 is stabilized by phosphine to give salt
6. Formation of phosphonium salt 6 and subsequent formation of
7 was investigated in NMR studies (see supporting information for
further details). This carbocation stabilization constitutes a
plausible explanation for the reactivity in this reaction. 7 is
considered as the determining cause for the prevention of
oligomerization due to steric reasons. Proton abstraction from 7
then generates product 2a.
of benzylic alcohols in the homocoupling seems to correlate to
substituents constants, however a precise kinetic analysis is
beyond the scope of this work.
Table 2. Homocoupling of benzylic alcohols: substrate screening.
p-TsOH•H2O (16 mol%)
R2
R2
OH
PPh3 (2 mol%)
DCE, 100 °C
R1
R1
R2
R1
PPh3 (2 mol%)
1
OH
p-TsOH•H2O (16 mol%)
2
2
Ph
Ph
Ph
1a
DCE
Entry
R1
H
R2
H
Product
2a
Time [h]
Yield [%][a]
2a
1
2
4
4
80
77
63
75
91
77
27
75
81
0
+ H+
- H+
PR3
Ph
Br
F
H
2b
2c
- 2 H2O
PR3
3
H
18
4
+
Ph
Ph
Ph
Ph
4
H
2d
2e
7
3
6
- H+
+ 1a
5
I
H
18
12
18
4
oligomers
3
6
Cl
H
2f
4
7
H
Cl
H
2g
2h
2i
Scheme 4. Proposed mechanism for the dehydrative coupling.
8
Me
H
9
Me
H
4
With this mechanism, it is also possible to explain the trend in
the phosphine screening (Table 1, entries 5–10). More electron-
rich phosphines generally show slightly lower yields after 4 h but
also higher amounts of 3 (entries 6,7). This leads to the
assumption that a stronger C-P bond in intermediate 6 slows the
reaction with 3. Application of triphenylphosphine oxide (table 1,
entry 10) shows a faster oligomerization since styrene (3) is
completely consumed. However, a weak coordination to the
carbocation is also possible with the oxide and allows to generate
2a in lower amounts.
10
11
CF3
NO2
2j
4
H
2k
4
0
General reaction conditions: substrate (1.0 mmol), p-TsOH•H2O (16 mol%,
30 mg), PPh3 (2 mol%, 5 mg), DCE (3.0 mL), 100 °C; [a] Isolated yields.
The scope of this reaction was further expanded to C-C
heterocoupling with electron-rich arenes. Similar hydroarylations
of benzylic alcohols and vinylarenes have been shown with
catalysts like gold,[24] calcium,[25] zinc,[26] graphene oxide[27] and
iron (III) porphyrin complexes.[28] In the past, the main focus in the
acid catalyzed heterocoupling of benzylic alcohols was put on
vinylarenes or 1,3-dicarbonyl compounds as nucleophiles.
However, acid-catalyzed dehydrative hydroarylation of benzylic
alcohols with electron-rich arenes is rare. For example,
fluorinated aryl boronic acids were developed as catalysts for this
transformation.[29] In few reports using simple and available
sulfonic acids, either the scope was narrow or a large excess of
nucleophile were used.[30] On the other hand, the Brønsted-acid
catalyzed hydroarylation of vinylarenes with electronrich arenes
was reported.[31]
In subsequent investigations, substrate scope and limitations
of the reactions were studied (Table 2). In general, electron
donating substituents like methoxygroups in ortho-, meta- and
para-position of phenyl, as well as 1-(2-napthyl)ethanol and
heteroaromatic compounds led to inseparable mixtures of various
regioisomeric C-C coupling products. This can be explained by
the fact that the higher electron density in benzylic position
generates a reactive benzylic cation, which cannot provide the
desired head-to-tail dimer selectively. On the other hand, strongly
electron withdrawing substituents as trifluoromethyl (table 2 entry
10) or nitro-groups (entry 11) deactivate the benzylic position and
no product formation takes place.
Thus, electron rich 1,3-dimethoxybenzene (DMB) and
1,3,5-trimethoxybenzene (TMB) were used as nucleophiles to
react with benzylic alcohols. As expected and also as shown in
literature, it is not necessary to add a phosphine or any other
additive. This is due to the fact that TMB and DMB are both very
good C-nucleophiles, which react much faster with the generated
carbocation, than an in-situ formed vinylarene. This inhibits the
oligomerization as side reaction kinetically. Hence, the aryl
compounds were added in excess under the reaction conditions
Nevertheless, halogenated substrates were suitable for this
protocol, due to their “chameleon-like” inductive electron
withdrawing (‒I) and mesomeric electron donating (+M) effects.
Halides in para-position gave good to excellent yields (entries 3-7).
A chloro-substituent in ortho-position however, only provided 27%
yield of 2g (entry 6). Interestingly, also 1-(4-biphenyl)ethanol (1b)
as well as ortho- and para-methylsubstituted phenylethanols (1h,
1i) appear to be reactive and selective enough to generate the
C-C coupling products (entries 2, 9 and 10). Thus, the reactivity
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10.1002/ejoc.201900965
European Journal of Organic Chemistry
COMMUNICATION
applied in the previously described homocoupling reactions just
without the additive (Scheme 5).
alcohols with electron-rich arenes led to a variety of substituted
1,1-diarylylethanes.
Dehydrative hydroarylation starting from 1-phenylethanol (1a)
with TMB yielded the respective product 9a in very good yield
(93%), whereas coupling with DMB gave product 8a in 71% yield
(entry 2), which could be explained by the higher nucleophilicity
of TMB provided by the three electron donating methoxy groups.
This is a general trend which is also visible when applying
benzylic alcohols substituted on the aryl ring. In contrast to the
homocoupling, the hydrooarylation of 1-(2-napthyl)ethanol (1l) to
8l and 9l was carried out in moderate to very good yields, even
though 1l was too reactive in the homodimerization. On the other
hand, methoxy-substituted substrates still underwent various side
reactions due to their higher electron density. Halogenated
substrates worked well, generating the products in good to
excellent yields (8e,f,g and 9e,f,g). Unfortunately, other C-C-
heterocoupling approaches with C-nucleophiles like indoles,
N-methylpyrroles, norbornene or vinylarenes, could not yield the
respective products selectively under these reaction conditions.
Acknowledgements
We are thankful to J. Grammel and R. Kimmich for technical
assistance. Financial support from Fonds der Chemischen
Industrie (Liebig fellowship IF), and the University of Tübingen
(Institutional Strategy of the University of Tübingen: Deutsche
Forschungsgemeinschaft ZUK 63) is gratefully acknowledged.
Keywords: benzylic alcohols • direct dehydrative coupling • acid
catalyzed • sp3-sp2 C-C bond formation• metal-free
[1]
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[2]
[3]
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Hanefeld), Wiley-VCH, Weinheim, 2007.
OH
1
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Chem. Res. 2002, 35, 695-705; d) M. Dryzhakov, E. Richmond, J. Moran,
Synthesis 2016, 48, 935-959.
OMe
Ar
+
p-TsOH•H2O (16 mol%)
Ar
MeO
OMe
DCE, 100 °C, 4 h
R
OMe
[4]
[5]
a) M.B. Smith in March’s Advanced Organic Chemistry (Eds.: Smith, M.
B), 7th ed.; Wiley: Hoboken, NJ, 2013, pp. 500–503; b) J. An, R. M.
Denton, T. H. Lambert, E. D. Nacsa, Org. Biomol. Chem. 2014, 12, 2993;
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2386–2388; b) Y. Ju, D. Kumar, R. S. Varma, J. Org. Chem. 2006, 71,
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Tetrahedron: Asymmetry 2009, 20, 2795–2801.
R = H
R = OMe 9
8
R
R1
OMe
OMe
R1
R = H
R
OMe
R
OMe
R1 = Cl 71% 8g
1 = Cl 93% 9g
R1 = Me 91% 9i
R1 = H 71% 8a
R1 = Ph 80% 8b
R1 = I 74% 8e
R1 = Cl 80% 8f
R = H
R = OMe
R
OMe
R = OMe
R
1 = H 93% 9a
[6]
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R1 = Ph >99% 9b
R1 = I 81% 9e
R1 = Cl >99% 9f
R
OMe
R = H
59% 8l
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Entry for the Table of Contents
COMMUNICATION
Acid Catalysis
LB
Marlene Böldl, Ivana Fleischer*
OH
via:
HA LB
Ar1
Page No. – Page No.
Ar1
Ar1
Ar1
Dehydrative Coupling of Benzylic
Alcohols Catalyzed by Brønsted
Acid/Lewis Base
A combination of Brønsted acid and Lewis base catalyse dehydrative
transformations of benzylic alcohols: homocoupling and hydroarylation.
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