Novel [4 + 2]-benzannulation to access substituted benzenes and polycyclic aromatic and benzene-fused heteroaromatic compounds

Article abstract of DOI:10.1021/ol501683v

A common [4 + 2]-benzannulation of Morita-Baylis-Hillman acetates of acetylenic aldehydes with boronic acids has been developed for the synthesis of aromatic and heteroaromatic compounds through tandem allylic substitution/hydroarylative cycloisomerization process. This method provides a facile and general route to substituted benzenes, naphthalenes, other polycyclic aromatics, and various benzene-fused heteroaromatic compounds such as benzofuran, benzothiophene, indole, and carbazoles.

Full text of DOI:10.1021/ol501683v

Letter
pubs.acs.org/OrgLett
Novel [4 + 2]-Benzannulation To Access Substituted Benzenes and
Polycyclic Aromatic and Benzene-Fused Heteroaromatic Compounds
Chada Raji Reddy,* Uredi Dilipkumar, and Motatipally Damoder Reddy
Division of Natural Products Chemistry, CSIR-Indian Institute of Chemical Technology, Hyderabad 500607, India
S
* Supporting Information
ABSTRACT: A common [4 + 2]-benzannulation of Morita−
Baylis−Hillman acetates of acetylenic aldehydes with boronic
acids has been developed for the synthesis of aromatic and
heteroaromatic compounds through tandem allylic substitu-
tion/hydroarylative cycloisomerization process. This method
provides a facile and general route to substituted benzenes,
naphthalenes, other polycyclic aromatics, and various benzene-
fused heteroaromatic compounds such as benzofuran,
benzothiophene, indole, and carbazoles.
annulation of conjugated enynes leading to benzene derivatives
under thermal or acid-catalysis conditions (eq 1).3 In 1996,
Yamamoto and co-workers first reported the palladium-catalyzed
intermolecular [4 + 2]-benzannulation of conjugated enynes
with enynophiles (eq 2).4 Later, this pioneering work was widely
explored by Yamamoto, Gevorgyan, and other research groups
for the preparation of diversely substituted benzenes using
preconstructed conjugated enynes as the four-carbon unit and
alkynes with an activating group (alkene or alkyne) as the C2
synthon.4,5 Herein, we report a novel [4 + 2]-benzannulation
using Morita−Baylis−Hillman (MBH) acetates of acetylenic
aldehydes as the C4 precursor10 and aryl or vinyl boronic acids as
the C2 synthon (eq 3). This approach offers a significant
advantage by giving an access to substituted benzenes as well as
to polycyclic aromatic hydrocarbons and benzene-fused
heterocyclic compounds from easily accessible substrates.
he development of new synthetic methods to substituted
Taromatic, polyaromatic, and heteroaromatic compounds
continues to command extensive interest due to their wide range
of applications.1 Benzannulation is one of the important
reactions for the construction of substituted benzene ring, and
over the years, various benzannulation reactions have been
developed for diversely substituted benzene and fused-benzene
compounds.2−9 Among these, [4 + 2]-benzannulation of enynes
with alkynes has received substantial attention to construct the
functionalized benzenes from acyclic starting materials (Figure
1).3−5 Danheiser et al. demonstrated the intramolecular [4 + 2]-
The synthesis of naphthalenes and polycyclic aromatics from
MBH adducts was reported by Kim and others involving
multistep reactions.11 We envisaged that MBH acetate of
acetylenic aldehyde will undergo allylic substitution with boronic
acid to give conjugated enyne, which would undergo hydro-
arylative cycloisomerization to lead the benzannulated product.
To test the hypothesis, MBH acetate 1a and phenyl boronic acid
(2a) were chosen as model substrates. From the optimization
studies (see the Supporting Information), we were pleased to
find that the [4 + 2]-benzannulation product, naphthalene 3aa,
was accomplished in 89% yield using 5 mol % of Pd(OAc)2,
Na2CO3 in acetonitrile/H2O, and DBU reaction conditions
(entry 1, Table 1). Encouraged by the above result, we examined
the substrate scope of this reaction by using a variety of MBH
acetates of acetylenic aldehydes (Table 1). A facile [4 + 2]-
benzannulation was observed for the reaction of 1-naphthyl
Received: June 11, 2014
Figure 1. [4 + 2]-Benzannulation reaction of enynes.
© XXXX American Chemical Society
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Letter
a
Table 1. [4 + 2]-Benzannulation of MBH Acetates of
Acetylenic Aldehydes with 2a
Scheme 1. Synthesis of Substituted Naphthalenes
a
a
Reaction conditions: MBH acetate 1 (1 equiv), Ph-B(OH)2 2a (1.1
a
Reaction schemes along with the structure of boronic acid are shown
in Supporting Information.
equiv), Pd(OAc)2, (5 mol %), Na2CO3 (1 equiv), DBU (1 equiv),
CH3CN (5 mL), H2O (0.1 mL), rt to 85 °C. Isolated yield.
b
Table 2. Synthesis of Substituted Benzenes Using Vinyl
Boronic Acids
a
MBH acetate 1b with 2a to give the substituted naphthalene 3ba
in 88% yield (entry 2, Table 1). Similarly, 2-thiophene-yl MBH
acetate 1c was also successful in reacting with 2a to provide the
naphthalene 3ca 86% yield (entry 3, Table 1). The reactions of
MBH acetates 1d (4-Me-Ph), 1e (4-Cl-Ph), and 1f (3-CF3-Ph)
with 2a were studied independently, and it was found that the
reactions gave the corresponding naphthalenes 3da−fa,
respectively, in good yield (entries 4−6, Table 1). Notably, the
reaction of MBH acetate 1g bearing a styryl group on alkyne with
2a proceeded very well and led to the expected naphthalene 3ga
in 60% yield (entry 7, Table 1). However, the reaction of MBH
acetate 1h having an n-propyl group on the alkyne functionality
with 2a provided the enyne intermediate 1HA instead of the
expected naphthalene (entry 8, Table 1).
We next evaluated the reactivity of diversely substituted
phenylboronic acids, and the results are summarized in Scheme
1. Remarkably, all the phenylboronic acids investigated efficiently
participated in [4 + 2]-benzannulation with 1a under the
optimized conditions to afford the corresponding naphthalenes
in good yields. The formation of naphthalenes 3ab−ag revealed
that various substitutions such as 4-MeO (2b), 4-CN (2c), 4-
morpholine (2d), 4-(Ph)2N (2e), 4-CF3 (2f), and 2-MeOCH2
(2g) on phenylboronic acid were found to be suitable for the
tandem reactions. 3,5-Dimethoxyphenylboronic acid (2h) and
2,4-dimethoxyphenylboronic acids (2i) readily underwent [4 +
2] benzannulation with 1a to produce naphthalenes 3ah (88%)
and 3ai (84%), respectively. Likewise, 2,3-dioxopyranylphenyl-
boronic acid (2j) also provided the respective naphthalene 3aj in
good yield (Scheme 1).
b
entry
boronic acid (4)
time (h)
benzene (5)
yield (%)
1
2
3
4
R = Ph, 4a
5
4
6
5
5a
5b
5c
5d
80
72
80
78
R = 4-F-C6H4−, 4b
R = n-C6H13, 4c
R = t-Bu−, 4d
a
Reaction conditions: MBH acetate 1a (0.38 mmol), (E)-vinylboronic
acid 4 (0.42 mmol), Pd(OAc)2, (5 mol %), Na2CO3 (0.38 mmol),
DBU (0.38 mmol), CH3CN (5 mL), H2O (0.1 mL), rt to 85 °C.
b
Isolated yield.
participated in the [4 + 2]-benzannulation to afford the
corresponding trisubstituted benzenes 5c and 5d, respectively,
in good yield (entries 3 and 4, Table 2).
The reaction scope was further extended to the synthesis of
polycyclic aromatic hydrocarbons (Scheme 2). Interestingly, the
treatment of 1a with naphthalene-1-boronic acid (6a) under the
described reaction conditions smoothly gave the substituted
phenanthrene 7a in 82% yield. In contrast to the reported
methods, wherein the middle ring of the phenanthrene was
generally constructed,6d,h,12 in the present method the terminal
ring was built. To make further higher numbered rings,
phenanthren-4-ylboronic acid (6b) and pyren-1-ylboronic acid
(6c) were independently subjected to the [4 + 2]-benzannula-
tion with 1a, and gratifyingly, both reactions ensued well to give
substituted benzo[c]phenanthrene 7b (83%) and benzo[pqr]-
tetraphene 7c (81%), respectively. Thianthren-1-ylboronic acid
(6d) was well suited for the benzannulation with 1a to give
benzo[a]thianthrene 7d in 71% yield. Additionally, dibenzo-
[b,d]furan-4-ylboronic acid (6e) and dibenzo[b,d]thiophene-4-
ylboronic acids (6f) were also treated with 1a under the
developed one-pot tandem reaction conditions to obtain the
Next, it occurred to us that the use of vinylic boronic acids in
the present annulation reaction is also feasible, which allows the
synthesis of substituted benzenes. To this end, the [4 + 2]-
benzannulation of 1a with (E)-styrylboronic acid (4a) was first
examined, and to our delight, the formation of 1,3,5-
trisubstituted benzene 5a (81%) was observed (entry 1, Table
2). (E)-(4-Fluorostyryl)boronic acid (4b) was found to be
suitable in providing 5b in 72% yield (entry 2, Table 2). Notably,
aliphatic vinylboronic acids 4c and 4d were also effectively
B
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a
Substituted boronic acid, (5-acetylthiophene-3-yl)boronic acid
(8f) was also efficient in producing 2, 5, 7-trisubstituted
benzo[b]thiophene 9f in 72% yield. Delightfully, the use of
(1H-indol-3-yl)boronic acid (8g) in the [4 + 2]-benzannulation
with 1a provided the corresponding 9H-carbazole 9g in 75%
yield. The benzannulation of (1-benzyl-1H- pyrrolo[2,3-b]-
pyridin-3-yl)boronic acid (8h) with 1a was also easily progressed
to offer the 9-benzyl-9H-pyrido[2,3-b]indole (9h) in 72% yield.
Further, the access to substituted dibenzo[b,d]furan 9i and
dibenzo[b,d]thiophene 9j was fruitful from the reaction of 1a
with respective boronic acids. These results clearly demonstrate
the use of MBH acetates of acetylenic aldehydes in [4 + 2]-
benzannulation with boronic acids toward the synthesis of a
diverse range of heterocyclic frameworks. Interestingly,
benzannulation of [1,1′-biphenyl]-4,4′-diyldiboronic acid (10)
with 1a gave the resulting 2,2′-binaphthalene 11 in 72% yield
(Scheme 4).
Scheme 2. Synthesis of Polycyclic Aromatic Compounds
Scheme 4. Synthesis of Bis-naphthlene Derivative
a
The structure of boronic acid for each product is shown in the
Supporting Information.
corresponding naphtho[1,2-b]benzofuran 7e and benzo[b]-
naphtho[2,1-d]thiophene 7f, respectively.
After achieving the above success, we next embarked on the
synthesis of bicyclic or tricyclic benzene-fused heterocycles
(Scheme 3). Thus, the reaction of N-Boc-pyrrolyl-2-boronic acid
a
Scheme 3. Synthesis of Benzene-Fused Heteroaromatics
A plausible mechanism for the present [4 + 2]-benzannulation
is illustrated in Scheme 5. First, the MBH acetate 1a undergoes a
Scheme 5. Plausible Reaction Pathway
palladium-catalyzed allylic substitution with 2a to form (Z)-
alkene13 3a via a π-allyl palladium intermediate. Addition of DBU
would facilitate the deprotonation to generate the intermediate
3b, followed by 6-exo-dig carbocyclization and subsequent
protonation provides 3c. Finally, a 1,7-hydrogen shift driven
aromatization would lead to the formation of benzannulated
product 3aa. The isolation of intermediate 3a and its treatment
with DBU/acetonitrile provided the product 3aa, which supports
that the palladium catalyst has no role in the cycloisomerization
reaction.
In summary, a general strategy for the synthesis of substituted
benzenes, polycyclic aromatic hydrocarbons, as well as benzene-
fused heterocycles has been developed, starting from the reaction
of MBH acetates of acetylenic aldehydes with aryl/heteroaryl or
vinyl boronic acid. By means of the studied examples, the
potential use of MBH acetates of acetylenic aldehydes as a C4-
synthon was demonstrated in the construction of benzene ring
through a [4 + 2]-annulation involving tandem allylic
substitution/hydroarylative cycloisomerization reactions. The
use of boronic acids as C2-synthon is an added advantage, due to
a
The structure of boronic acid for each product is shown in the
Supporting Information.
(8a) with MBH acetate 1a under tandem allylic substitution/
hydroarylative cycloisomerization conditions yielded the 4,6-
disubstituted N-Boc-indole 9a in 79% yield. Equally, furan-2-
ylboronic acid (8b) and thiophene-2-ylboronic acid (8c)
underwent smooth benzannulation upon treatment with 1a to
produce the corresponding 4,6-disubstituted benzofuran 9b and
benzo[b]thiophene 9c, respectively, in good yield. It is worth
mentioning that furan-3-ylboronic acid (8d) and thiophene-3-
ylboronic acid (8e) can also be employed in [4 + 2]-
benzannulation with 1a, giving an access to 5,7-disubstituted
benzofuran 9d and benzo[b]thiophene 9e in excellent yield.
C
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their easy accessibility. Therefore, the disclosed method may find
applications in the synthesis of not only aromatics and
polyaromatics but also various other benzannulated products
from the corresponding (hetero)aryl/vinylboronic acids. To the
best of our knowledge, there is no such method described in the
literature for the construction of benzene ring on a boronic acid
unit.
ASSOCIATED CONTENT
* Supporting Information
■
S
1
Experimental procedures, characterization details, and H and
13C NMR spectra of new compounds. This material is available
free of charge via the Internet at http://pubs.acs.org.
AUTHOR INFORMATION
Corresponding Author
■
*E-mail: rajireddy@iict.res.in.
Notes
The authors declare no competing financial interest.
ACKNOWLEDGMENTS
■
U.D. and M.D.R. thank the Council of Scientific and Industrial
Research (CSIR)−New Delhi for the award of fellowships.
C.R.R. is grateful to the Department of Science and Technology,
New Delhi, for funding the project (SR/S1/OC-66/2011).
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D
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