Acid-Promoted Cross-Dehydrative Aromatization for the Synthesis of Tetraaryl-Substituted Pyrroles

Article abstract of DOI:10.1021/acs.orglett.5b03240

Tetraaryl-substituted pyrroles are one important class of luminophores. In this work, an acid-promoted cross-dehydrative aromatization between benzoin and deoxybenzion was developed. This transformation provides an efficient and straightforward path for the synthesis of various aryl group substituted tetraarylpyrroles. Good to excellent yields were obtained through the easy operation with acetic acid and ammonium acetate.

Full text of DOI:10.1021/acs.orglett.5b03240

Letter
pubs.acs.org/OrgLett
Acid-Promoted Cross-Dehydrative Aromatization for the Synthesis of
Tetraaryl-Substituted Pyrroles
Xudong Wu,† Ke Li,† Siyuan Wang,† Chao Liu,*,†,‡ and Aiwen Lei*,†
†College of Chemistry and Molecular Sciences, Institute for Advanced Studies (IAS), Wuhan University, Wuhan 430072, P. R. China
‡State Key Laboratory for Oxo Synthesis and Selective Oxidation, Suzhou Research Institute of LICP, Lanzhou Institute of Chemical
Physics (LICP), Chinese Academy of Sciences, Lanzhou 730000, P. R. China
S
* Supporting Information
ABSTRACT: Tetraaryl-substituted pyrroles are one impor-
tant class of luminophores. In this work, an acid-promoted
cross-dehydrative aromatization between benzoin and deoxy-
benzion was developed. This transformation provides an
efficient and straightforward path for the synthesis of various aryl group substituted tetraarylpyrroles. Good to excellent yields
were obtained through the easy operation with acetic acid and ammonium acetate.
Scheme 1. Approach To Access Tetraaryl-Substituted Pyrrole
romatic rings are important building blocks existing in
Anumerous pharmaceuticals, natural products, and func-
tional materials.1 Among various aromatic compounds, five- and
six-membered aromatic rings are the most common system in the
chemical world. Tremendous efforts have been devoted to
construct aromatic rings during the past decades.2 Normally, six-
membered aromatic cycle can be constructed through cyclo-
trimerization of alkyne or nitrile or the Dies−Alder reaction
followed by aromatization.3 A five-membered aromatic cycle can
usually be built via the classic Paal−Knorr synthesis or transition-
metal-catalyzed cycloisomerization-type or cycloaddition-type
reactions.4 The Paal−Knorr synthesis is a typical dehydrative
aromatization process with H2O as the side product.5 However, it
usually requires uneasily accessible 1,4-dione as the precursor.6
Pyrroles are one of the most frequently appearing five-
membered rings in many important molecules.7 Recently, with
the development of electroluminescent materials, some tetraar-
ylpyrroles were discovered to have remarkable fluorescence,
which shows potential application in organic electronic devices.8
However, until now, most of the reported synthetic methods
could only selectively afford symmetric tetraaryl-substituted
pyrroles, which restrict further application of those types of
molecules in materials chemistry.4,9 In 1938, Davidson
successfully prepared a symmetric tetraphenylpyrrole from
benzoin.10 It is known that benzoin is a proper precursor of
desoxybenzoin.11 Inspired by these results, we deduced that
whether the direct cross-condensation of benzoin and
desoxybenzoin could construct important tetraaryl pyrroles
with four different aryl groups. Herein, we demonstrated an acid-
promoted cross-dehydrative aromatization for the synthesis of
unsymmetrical tetraaryl-substituted pyrroles (Scheme 1).
the amount of 2c was increased to 1.33 equiv, the unsymmetrical
tetraaryl-substituted pyrrole 3ac was obtained in 98% yield
(Scheme 2).
Scheme 2. Initial Attempt To Access Unsymmetrical
Tetraaryl-Substituted Pyrrole
Furthermore, the reaction conditions were applied to the
synthesis of other unsymmetrical tetraaryl-substituted pyrroles.
First, different benzoins were tested as substrates to react with 1a
(Scheme 3). Both electron-donating and electron-withdrawing
substituents on the benzene ring of benzoin were well tolerated
under the current conditions (Scheme 3, 3ab, 3ac). Even the
cyano group was also well tolerated (Scheme 3, 3ad). Benzoin
substituted with halogens such as F, Cl, and Br afforded the
corresponding products in excellent yields (Scheme 3, 3ae−ag).
It is worthy of noting that functional groups such as cyano, fluoro
always show the potential of improving the photovoltaic
In our initial attempt, 1,2-diphenylethan-1-one (1a) and 4,4′-
bis(trifluoromethyl)benzoin (2c) were selected as the substrates
in the presence of ammonium acetate in AcOH at 120 °C to
prepare the unsymmetric tetraaryl-substituted pyrrole 3ac. To
our delight, the desired product 3ac was successfully obtained in
70% yield. When the temperature was decreased to 100 °C and
Received: November 10, 2015
© XXXX American Chemical Society
A
DOI: 10.1021/acs.orglett.5b03240
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Letter
Scheme 3. Acid-Promoted Cross-Dehydrative Aromatization
of 1a with Different Benzoins
Scheme 4. Acid-Promoted Cross-Dehydrative Aromatization
of Different 1,2-Diarylethan-1-ones with 2a
a
a
a
Reaction conditions: 1a (0.3 mmol), 2 (0.4 mmol), NH4OAc (4.5
mmol), AcOH (2.0 mL), 100 °C, 12 h. The products were obtained in
b
c
isolated yields. The reaction was conducted under 120 °C. 2 (0.5
mmol) was used.
properties of materials. Moreover, the halogens such as Br and
Cl, usually provide possibilities for further functionalization.
Some benzoin that the phenyl groups were replaced by other aryl
cycles were also investigated. Benzoins with naphthalene and
thiophene substituents were able to furnish the desired products
in good yields (Scheme 3, 3ah and 3ai), while 2,2′-furoin only
afforded the target product in 21% yield, which might due to its
instability under the standard conditions (3aj).
a
Reaction conditions: 1 (0.3 mmol), 2a (0.4 mmol), NH4OAc (4.5
mmol), AcOH (2.0 mL), 100 °C, 12 h. The products were obtained in
isolated yields. 1 (0.15 mmol), 2a (0.4 mmol), PhMe (1.0 mL) was
added.
b
Scheme 5. Synthesis of Tetraaryl-Substituted Pyrrole with
Four Different Substituents via Cross-Dehydrative
Aromatization
In the next step, a series of 1,2-diarylethan-1-ones were
investigated to extend the substrate scope (Scheme 4). The
reactions of desoxybenzoin with p- and o-methyl substitutents
showed good reaction efficiency in this acid-promoted cross-
dehydrative aromatization reaction (Scheme 4, 3ba and 3ca).
Electron-rich substituents on the benzene ring were also well
compatible (Scheme 4, 3da and 3ea). The desoxybenzoins with
halide substituents such as F, Cl, and Br afforded the
corresponding products in good yields (Scheme 4, 3fa−ia).
Ethanone substituted with α-naphthyl groups was also suitable in
this transformation (Scheme 4, 3ja). Carbazole and dibenzofuran
are also important structure motifs in material chemistry because
of their stability and special properties. Interestingly, unsym-
metrical tetraaryl-substituted pyrroles containing the block of
carbazole or dibenzofuran were synthesized successfully by
adding 1.0 mL of toluene as a cosolvent to increase the solubility
(3ka, 3la).
At last, the synthesis of tetraaryl-substituted pyrrole with four
different aromatic substituents was also explored through this
acid-promoted cross-dehydrative aromatization transformation.
4-Chlorobenzoin was used to react with desoxybenzoin bearing
two different aryl substituents. The desired pyrroles with four
different aryl substitutents were obtained with two regioisomers
due to the tautomerism of benzoin as expected (Scheme 5).
It is obvious that Ar1 and Ar2, respectively, bound to the C1
and C2 positions of the produced pyrrole. However, due to the
tautomerism of benzoin, it is not clear that whether the Ar group
Reaction conditions: 1 (0.3 mmol), 2 (0.4 mmol), NH4OAc (4.5
mmol), AcOH (2.0 mL), 100 °C, 12 h. Isolated yields.
bound to the carbonyl goes to the C4 position of the product
pyrrole. To confirm this, 2k and 2l were, respectively, subjected
to reaction with 1a (Scheme 6). Both reactions resulted in the
B
DOI: 10.1021/acs.orglett.5b03240
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Scheme 6. Comparison Experiments
Scheme 8. Applications of the Obtained Tetraaryl Pyrroles
same products 3ak1 and 3ak2, yet with different ratios. When 2k
was applied as the substrate (p-Cl phenyl was bound to the
carbonyl), 3ak2 was the major product, in which the p-Cl phenyl
was at the C4 position. When 2l was applied as the substrate (Ph
was bound to the carbonyl), 3ak2 was the major product, in
which the Ph group was at the C4 position. Those results
indicated that the carbonyl carbon of 2 became the C4 carbon of
the produced pyrrole and the tautomerism of benzoin derivatives
led to the minor product of the isomers.
In summary, we have developed a facile and highly efficient
approach for the construction of unsymmetrical tetraaryl-
substituted pyrroles via acid-promoted cross-dehydrative aroma-
tization. In this strategy, two monocarbonyl compounds, instead
of the difficult-to-synthesize 1,4-diketone, were used as
substrates to construct tetraaryl-substituted pyrroles with H2O
as the side product. Various functional groups and heterocycles
were tolerated.
On the basis of those results, a tentative reaction pathway was
proposed in Scheme 7. Benzoin 2 initially reacts with NH4OAc to
Scheme 7. Tentative Reaction Pathway
ASSOCIATED CONTENT
* Supporting Information
■
S
The Supporting Information is available free of charge on the
ACS Publications website at DOI: 10.1021/acs.orglett.5b03240.
Experimental details and spectral data for all compounds
(PDF)
AUTHOR INFORMATION
Corresponding Authors
■
*E-mail: chaoliu@licp.cas.cn.
*E-mail: aiwenlei@whu.edu.cn.
Notes
The authors declare no competing financial interest.
result in 4, which could be isomerized into α-amino ketone 5.
Further condensation of 5 with 1 results in imine 6 followed by a
tautomerism to enamine 7. An intramolecular cyclization via an
electrophilic addition to the carbonyl group forms 8. Then, an
acid promoted dehydrative aromatization of 8 releases the
desired pyrrole 3. The isomerized product might result from the
initial tautomerism of benzoin 2.
Author: Next, it is known that the N−H bond of the tetraaryl-
substituted pyrroles seem to be too active to be implemented on
photoelectric devices directly.12 Both N-alkylation and boryla-
tion are important methods for modifying the tetraaryl-
substituted pyrrole to the desired molecular materials. N-
Alkylation can stabilize the molecule and improve the photo-
voltaic properties of materials to some extent.13 The alkylation
reaction of 1aa with iodoethane and bromohexane all proceeded
smoothly (Scheme 8). Furthermore, boron is a strong electron-
withdrawing atom. Formation of an N−B bond can greatly
change the photovoltaic property of tetraaryl-substituted
pyrrole.14 N-Borylpyrroles showed larger stocker shift and better
fluorescence efficiency in PL spectra compared with N-
alkylpyrroles. The N-borylation reaction of 3aa was also
explored. A 26% yield of the desired product 5a was obtained
with B(Mes)2F as the boron source (Scheme 8).
ACKNOWLEDGMENTS
■
This work was supported by the 973 Program (2012CB725302),
the National Natural Science Foundation of China (21390400,
21520102003, 21272180, 21302148), the Hubei Province
Natural Science Foundation of China (2013CFA081), the
Research Fund for the Doctoral Program of Higher Education
of China (20120141130002), and the Ministry of Science and
Technology of China (2012YQ120060). The Program of
Introducing Talents of Discipline to Universities of China (111
Program) is also appreciated.
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