Boron-Catalyzed Dehydrative Friedel-Crafts Alkylation of Arenes Using β-Hydroxyl Ketone as MVK Precursor

Article abstract of DOI:10.1002/adsc.202001269

Boron-catalyzed environmentally benign dehydrative Friedel-Crafts alkylation of indole/pyrrole and aniline derivatives with β-hydroxyl ketones has been developed for the first time. This method provides an efficient and green replacement of toxic and unst

Full text of DOI:10.1002/adsc.202001269

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DOI: 10.1002/adsc.202001269
Boron-Catalyzed Dehydrative Friedel-Crafts Alkylation of Arenes
Using β-Hydroxyl Ketone as MVK Precursor
Htet Htet San,a, b Jie Huang,a Seinn Lei Aye,c and Xiang-Ying Tanga,*
a
School of Chemistry and Chemical Engineering and Hubei Key Laboratory of Bioinorganic Chemistry and Materia Medica,
Huazhong University of Science and Technology, 1037 Luoyu Road, Wuhan 430074, People’s Republic of China
E-mail: xytang@hust.edu.cn
Department of Industrial Chemistry, Yadanabon University, Amarapura Township, Mandalay Region, 05063, Myanmar
b
c
Environment and Water Studies Department, University of Yangon, Kamayut Township, Yangon, 11041, Myanmar
Manuscript received: October 15, 2020; Revised manuscript received: December 22, 2020;
Version of record online: December 30, 2020
Supporting information for this article is available on the WWW under https://doi.org/10.1002/adsc.202001269
ketone moiety is an active functionality in the presence
Abstract: Boron-catalyzed environmentally benign
of a Lewis acid or Brønsted acid.[3] For example, in the
dehydrative Friedel-Crafts alkylation of indole/
acid catalysed/promoted reaction of pyrrole with
pyrrole and aniline derivatives with β-hydroxyl
acetone and 4-hydroxy-2-butanone, ketones are acti-
ketones has been developed for the first time. This
vated and double Friedel-Crafts reactions are dominant
method provides an efficient and green replacement
instead of the dehydration process[3d] (Scheme 1, b). To
of toxic and unstable methyl vinyl ketone (MVK)
date, examples for the combination of dehydration and
by safer and cheaper β-hydroxyl ketone. The
further transformations of in situ formed MVK are
reaction features easy operation, wide substrate
extremely limited to the Friedel-Crafts reaction of
scope and significantly, only water is formed as by-
phenol derivatives,[4] and the yields and selectivities
product.
are not very good, probably due to the elevated
[4c]
°
reaction temperature (100 C) (Scheme 1, c).
To our great surprise, the dehydrative Friedel-Crafts
Keywords: Boron-catalyzed; Dehydrative Friedel-
reaction of indoles with 4-hydroxy-2-butanone for the
Crafts alkylation; β-hydroxyl ketones; MVK
direct synthesis of 4-(1H-indol-3-yl)butan-2-ones,
Methyl vinyl ketone (MVK) is a versatile feeding
stock in organic synthesis, for example, it is widely
used as Michael acceptor in Robinson annulations and
Baylis-Hillman reactions, and also in the synthesis of
many natural products and drug molecules,[1] such as
Biperiden,[1a] Caulersin,[1b] Vinclozolin[1c] and Vitamin
A acetate.[1d] However, its flammability, high toxicity,
and pungent odour bring trouble with transportation
and storage. On the other hand, it should be noted that
MVK can easily undergo polymerization, resulting in
low yield and by-products which are not easily to be
purified. Thus, it is very important to find a bench-
stable replacement of MVK. The utilization of 4-
hydroxy-2-butanone as the precursor of MVK in an
one-pot reaction should be an ideal choice. However,
to develop a reaction that compatible with the
Scheme 1. Challenges in one-pot usage of 4-hydroxy-2-buta-
none as MVK precursor.
dehydration of 4-hydroxy-2-butanone is quite challeng-
ing, not only due to that harsh reaction conditions are
required in the conversion of 4-hydroxy-2-butanone to which are important intermediates to access many
MVK (Scheme 1, a),[2] but also because of that the useful indole alkaloids,[5] such as benzo[a]
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carbazoles,[5a] β-carbolines,[5b] (�)-debromoflustramide ization. To the best of our knowledge, there is no
B,[5e] (�)-debromoflustramine B and E,[5e] has not been report for the dehydrative Friedel-Crafts alkylation of
successfully achieved yet (Scheme 2). It is probably indoles/pyrroles with β-hydroxyl ketones as MVK-type
due to that in the presence of a Brønsted or Lewis acid precursors.
catalyst, the reaction prefers to undergo double
Initially, indole 1a and β-hydroxy ketone 2a were
Friedel-Crafts alkylation to afford bisindole products.[6] chosen as model substrates to optimize the reaction
Therefore, the development of an efficient catalytic conditions. When the reactions were performed in the
system, which is compatible with dehydration of 4- presence of metal Lewis acids such as FeCl3, ZnCl2
°
hydroxy-2-butanone to MVK and subsequent chemo- and AlCl3 at 60 C in DCE, the reactions were quiet
selectively MVK transformation, is highly desirable.[7] complex and 3a was obtained in very low yields
During the last decades, Lewis acids or Brønsted (Table 1, entries 1–3).[15] When BF3 ·Et2O was em-
acids catalyzed Friedel-Crafts alkylation reaction has ployed as Lewis acid catalyst, 3a was obtained in 45%
gained much attention because it provided an efficient yield (Table 1, entry 4). Brønsted acids CF3COOH and
method for the facile contruction of diversfied CÀ C TsOH showed better catalytic activity and delivered
and CÀ X (X=heteroatom) bonds.[8] Particullarly, the the desired product in 54% and 58% yields (Table 1,
dehydrative Friedel-Crafts alkylation is of great inter- entries 5 and 6). However, PhCOOH and HCl could
esting, not only because of the readily available barely catalyze this transformation to give only trace
starting materials were used, but also due to that only amount of desired product (Table 1, entries 7 and 8).
water was formed as by-product. On the other hand, Interestingly, in the presence of 5 mol% B(C6F5)3 ·H2O,
electron-deficient boryl derivatives have been inten- the desired product 3a was obtained in 70% yield after
sively investigated in Frustrated Lewis pair chemistry[9]
and other useful transformations either as Lewis
Table 1. Screening of the reaction conditions.a
acid[10] or Brønsted acid.[11] Recently, Talor and co-
workers reported arylboronic acid catalyzed dehydra-
tive esterification and C-alkylation using benzylic
alcohols as the electrophile. The group of Prof. Zhang
reported an interesting ortho-functionalization of
phenol with α-diazoesters using B(C6F5)3 as a bifunc-
tional catalyst.[12] Our group also observed that α-aryl
entry
cat.
sol.
x
yield (%)b
α-dizaoesters could undergo OÀ H bond insertions
under the bifunctional catalysis of B(C6F5)3 ·H2O
complex.[13] Moreover, we reported the first example
for azide insertion of α-aryl α-dizaoesters catalysed by
B(C6F5)3.[14] To continue our reserach interesting, we
envisaged that B(C6F5)3 ·H2O may be an efficient
Brønsted catalyst for dehydration of 4-hydroxy-2-
butanone. Herein, we report a dehydrative Friedel-
Crafts alkylation of indole and pyrrole derivatives with
4-hydroxy-2-butanone. This method has great advan-
tages in replacement of MVK or other α, β-unsaturated
ketones due to their instability and trend to polymer-
1
2
3
4
5
6
7
8
FeCl3
ZnCl2
AlCl3
BF3 ·Et2O
CF3COOH
TsOH
PhCOOH
HCl
DCE
DCE
DCE
DCE
DCE
DCE
DCE
DCE
DCE
DCE
DCE
DCE
Hexane
PhCl
Toluene
THF
1
1
1
1
1
1
1
1
1
2
2
2
2
2
2
2
2
2
2
20c
15c
25c
45c
54
58
trace
trace
70
9
B(C6F5)3 ·H2O
B(C6F5)3 ·H2O
10
11
12
13
14
15
16
17
18
19
88
d
–
N.R.
60e
51
70
45
B(C6F5)3 ·H2O
B(C6F5)3 ·H2O
B(C6F5)3 ·H2O
B(C6F5)3 ·H2O
B(C6F5)3 ·H2O
B(C6F5)3 ·H2O
B(C6F5)3 ·H2O
B(C6F5)3 ·H2O
N.R.
N.R.
75f
MeCN
DCE
DCE
68g
a) Reactions were carried out on a 0.2 mmol scale under air in
°
solvent (2 mL) at 60 C for 24 h.
b) Yields of isolated products.
c) Complex mixture was formed.
d) Without catalyst.
e)
°
Reaction was performed at 40 C.
f) 3 mol% catalyst was added.
g) 7 mol% catalyst was added. THF=tetrahydrofuran. DCE=
dichloroethane. PhCl=chlorobenzene. MeCN=acetonitrile.
Scheme 2. The first dehydrative Friedel-Crafts reaction of
indoles using 4-hydroxy-2-butanone as MVK precursor.
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24 h (Table 1, entry 9). Notably, the yield of 3a respectively, while the electronic properties of the
increased to 88% when 2 equiv. of β-hydroxyl ketone substitutes had no significant effect on the reaction
was added (Table 1, entry 10). In the absence of a outcomes. For indolyl substrates with C5 substituents,
catalyst, no reaction took place (Table1, entry 11). the reactions all went on smoothly to give correspond-
°
°
Conducting the reaction at 40 C instead of 60 C, the ing products 3g–3i in moderate yields. It had to been
yield of 3a was decreased to 60% (Table 1, entry 12). mentioned that for substrate bearing a 5-aldehyde
A solvent screen showed that DCE was the most group, relatively low yield of 3h was obtained
suitable solvent for this reaction (Table 1, entries 13– probably due to that the activation of aldehyde with
17). Decreasing or increasing the loadings of catalyst, borane complex would result in unwanted side reac-
the reactions furnished diminished yields of product tions. Substrates with C6 and C7 functionalities were
3a in both cases (Table 1, entries 18 and 19).
also harmonious with this reaction, and furnished the
With the optimal reaction conditions in hand desired products 3j–3o in moderate yields. Specially,
(Table 1, entry 10), we focused our attention to C3 substituted indole was also found to be a good
evaluating the substrate scope of the reaction by substrate and furnished 2-substituted indole product 3p
varying the structure of indoles (Scheme 3). For in 79% yield. N-Methyl, Bn and Ts protected indoles
reactions of C4 substituted indoles with β-hydroxyl were also suitable for this transformation and provided
ketone 2a under the standard reaction conditions, the expected 3-substituted indoles 3q–3s in 60–70%
products 3b–3f were obtained in 55%-72% yields, yields, respectively. For N,N-dimethylaniline and N,N-
diethylaniline derivatives, the reactions also proceeded
smoothly to give the para-functionalized products 3t
and 3u in 60% and 63% yields, respectively. Notably,
for pyrrole substrate, double functionalization took
place and product 3v was obtained in 62% yield. It
should be noted that when anisole was employed as
substrate, no reaction take place under the standard
reaction conditions, probably due to the strong inter-
action of oxygen atom with the boryl complex will
reduce its catalytic activity.
To determine the generality of β-hydroxyketones,
we next examined a series β-hydroxyketones with N-
methylindole under the standard reaction conditions.
The β-hydroxyketones were synthesized by the aldol
reaction of corresponding ketones and aldehydes. As
can be seen in Scheme 4, for both electron-donating
and electron-withdrawing groups on the phenyl group,
the reactions all proceeded smoothly to afford the
desired products 4a–4i in 56–75% yields. It is worth
noting that R4 could be phenyl groups, and correspond-
ing products 4h and 4i were delivered in moderate to
good yields.
To gain a deeper understanding of the mechanism,
several control experiments were conducted. As shown
in Scheme 5, The reactions of indole with ethyl vinyl
ketone (EVK) and chalcone in the present of B
(C6F5)3 ·H2O gave corresponding ketone products 3w
and 4j in excellent yields, indicating the unsaturated
ketone should be the intermediate of the reaction.[10o]
While treatment of indole with acetophenone or
acetone under the standard reaction conditions, no
reaction took place, representing that the ketone
products 3 is not active to react with indoles[6] in the
presence of B(C6F5)3 ·H2O.
On the basis of the mechanistic studies, NMR
experiments (SI) and previous reports,[10o,11g and j,16] we
propose a putative reaction mechanism for the present
dehydrative Friedel-Crafts alkylation in Scheme 6.
Initially, in the presence of in the presence of B
Scheme 3. Substrate Scope of indole and electron-rich arene
derivatives.a,b
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Scheme 6. Plausible mechanism.
β-hydroxyl ketones instead of activation of the ketone
moiety, thus avoiding the direct use of highly toxic
MVK. Moreover, aniline and pyrrole derivatives are
also suitable substrates for this method. Further studies
to gain more mechanistic insight and application of
this methodology in organic synthesis are currently
underway in our laboratory.
Scheme 4. Substrate Scope of β-hydroxyketones.a,b
Experimental Section
A 10 mL reaction tube was equipped with a magnetic stir bar,
followed by the addition of B(C6F5)3 ·H2O (5.1 mg, 5 mol%)
and DCE (2 mL). Subsequently 0.2 mmol of indole/pyrrole
derivatives and β-hydroxyketones (2 equiv, 0.4 mmol) were
added into the reaction tube. The reaction mixture was stirred at
°
60 C for 24 h (monitored by TLC) then the mixture was
concentrated, the residue was purified by silica gel column
chromatography (petroleum ether/ethyl acetate=20:1–5:1) to
afford the desired products.
Acknowledgements
We are grateful for the financial support provided by the
National Natural Science Foundation of China (21871100), the
Fundamental Research Funds for the Central Universities
(2017KFYXJJ166, 2019kfyRCPY096), Huazhong University of
Science and Technology (HUST), Hubei Technological Innova-
tion Project (2019ACA125), and the Opening Fund of Hubei
Key Laboratory of Bioinorganic Chemistry and Materia Medica
(No. BCMM201805). We also thank the Analytical and Testing
Center of HUST, Analytical and Testing Center of the School of
Chemistry and Chemical Engineering (HUST) for access to
their facilities.
Scheme 5. Control experiments.
(C6F5)3 ·H2O,[16] β-hydroxyketone is activated to form
intermediate A after elimination of one molecule of
H2O, the following Friedel-Crafts reaction of inter-
mediate A with indole gives intermediate B, which
affords the final product 3a after tautomerization
together with the regeneration of catalyst.
In summary, we have developed a novel, highly
efficient catalytic system that compatible with dehy-
dration of β-hydroxyl ketones, affording synthetically
valuable 4-(1H-indol-3-yl)butan-2-one derivatives
under mild conditions. Notable, boron catalyst showed
special catalytic activity to facilitate the dehydration of
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