Friedel-crafts arylation for the formation of C sp 2 iC sp 2 Bonds: A route to unsymmetrical and functionalized polycyclic aromatic hydrocarbons from aryl triazenes

Article abstract of DOI:10.1002/anie.201206578

Rings, rings, rings: A Friedel-Crafts intramolecular arylation for the synthesis of polycyclic aromatic hydrocarbons from aryl triazenes has been achieved. Polycyclic aromatic compounds consisting of five-, six-, and seven-membered rings can be prepared using this method. Copyright

Full text of DOI:10.1002/anie.201206578

Angewandte
Chemie
DOI: 10.1002/anie.201206578
Polycyclic Hydrocarbons
À
Friedel–Crafts Arylation for the Formation of Csp2 Csp2 Bonds: A Route
to Unsymmetrical and Functionalized Polycyclic Aromatic
Hydrocarbons from Aryl Triazenes**
Jun Zhou, Weijun Yang, Binjie Wang, and Hongjun Ren*
Because of their special optical and electronic properties, the
efficient synthesis of unsymmetrical and functionalized poly-
cyclic aromatic hydrocarbons (PAHs) is becoming much more
important.[1] The transitionmetalcatalyzed cross-coupling
reactions of organometallic reagents with organic halides
are popular methods for the formation of Csp2 Csp2 bonds.[2]
À
Thus, transition-metal-catalyzed annulation reactions of
functionalized precursors, such as intramolecular direct
arylations (Scheme 1, path a),[3] the annulation of arynes[4]
(path b), double-coupling reactions of organodimetallic
reagents with dihalides[5] (paths c and d), sequential C H
À
arylation with o-biphenylboroxin and FeCl3 oxidation
(path e),[6] and [2+2+2] or [4+2] cycloaddition reactions,[7]
provide effective synthetic transformations for the synthesis
of functionalized PAHs. Moreover, oxidative dehydrogen-
ation coupling reactions are also a feasible way to construct
PAHs, and FeCl3 is a popular oxidant.[8]
Compared to transition-metal-catalyzed crosscoupling
reactions, the Friedel-Crafts reaction is a classic carbon-
carbon bond (Csp2Csp3 bond) formation reaction that can be
carried out in the presence of a Lewis acid catalyst, such as
BF3, FeCl3, or AlCl3, to couple an aromatic ring with an alkyl
halide or acyl halide through a carbocation intermediate.[9]
À
With respect to Csp2 Csp2 bond formation, the Friedel-Crafts
reaction shows limited utility, owing to the instability of the
phenyl cation and a bottleneck in generation.[10] Recently, in
a new take on the reaction, Allemann and co-workers
reported an acid-catalyzed, silane-fueled Friedel–Crafts cou-
pling of fluoroarenes (path f).[11,12] This catalytic process
Scheme 1. Strategies for the synthesis of PAHs. pin=pinacol, Tf=tri-
fluoromethanesulfonyl, TMS=trimethylsilyl.
À
À
includes C F bond activation and C C bond formation.
Arenediazonium salts, an excellent precursor for gener-
ation of phenyl cation and radical intermediates, were
thoughly investigated by Zollinger et al.[13] and others.[14] As
a protected arenediazonium salt equivalent, aryltriazene has
(X = halides, CN, and N3) bonds, however, few examples were
reported for the construction of Csp2 Csp2 bonds.[15] In our
À
previous work, triazene unit acts as a leaving group in the
presence of BF3·OEt2, so as to synthesize a number of highly
substituted carbazoles, dibenzofurans, and dihydrobenzofur-
ans in a regioselective manner.[16] Thus, we had the idea that
aryltriazene could potentially act as an aryl cation precursor.
If this precursor was attacked by another aryl, a new aromatic
ring would form, which would provide an effective synthesis
of a series of PAHs.
À
been used as a critical synthon for the construction of C X
[*] J. Zhou, W. Yang, B. Wang, Prof. Dr. H. Ren
Department of Chemistry, Zhejiang University
38 Zheda Road, Hangzhou 310027 (P. R. China)
E-mail: renhj@zstu.edu.cn
Prof. Dr. H. Ren
Department of Chemistry, Zhejiang Sci-Tech University
Hangzhou 310018 (P. R. China)
Herein, we report a Friedel-Crafts intramolecular aryla-
tion for the synthesis of polycyclic aromatic hydrocarbons
from aryl triazenes. This approach involves treating readily
available aryl triazenes (Scheme 1) with BF3·OEt2, which
serves as a Lewis acid to promote the formation of a phenyl
cation. This highly activated phenyl cation intermediate is
then be attacked by a meta-substituted aryl group, followed by
intramolecular arylation. This method can be used to
[**] We are grateful for financial support from the National Nature
Science Foundation of China (21272003) as well as a Scholarship
Award for Excellent Doctoral Student granted by the Ministry of
Education (2011).
Supporting information for this article is available on the WWW
under http://dx.doi.org/10.1002/anie.201206578.
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Table 1: Synthesis of six-membered rings from aryltriazenes.[a]
Entry
Substrate
Product
Yield [%]
Entry
Substrate
Product
Yield [%]
1[b]
2[b]
3
1a: R=CO2Et
1b: R=CN
1c: R=CH3
2a: R=CO2Et
2b: R=CN
2c: R=CH3
77
65
64
10
11
1j: R=CO2Et
1k: R=CH3
2j: R=CO2Et
2k: R=CH3
84
91
4[b]
5[b]
6
1d: R=CO2Et
1e: R=CN
1 f: R=CH3
2d: R=CO2Et
2e: R=CN
2 f: R=CH3
91
79
88
12
1l
2l
53
7
8
1g: R=CO2Et
1h: R=CH3
2g: R=CO2Et
2h: R=CH3
88
66
13
1m
2m
73
9
1i
2i
55
14
15
1n: R=CO2Et
1o: R=CH3
2n: R=CO2Et
2o: R=CH3
82
79
[a] Reaction conditions: aryltriazene (0.5 mmol) and BF3·OEt2 (1.0 mmol) in CH2Cl2 (20 mL) for 30 min at room temperature. [b] Reaction run for 12 h
at 408C.
regioselectively generate a range of functionalized polycyclic
architectures.
(1a–c) and CN (1d–e), as substituents on triazene moiety
retarded the cyclization rate and reduced the yields. However,
prolonging the reaction time and elevating the reaction
temperature increased the yields (1a–b, 1d–e). The heter-
oatom-containing substrate 1i also reacts well to afford the
anticipated annulation compound 2i in 55% yield. A number
of heteroatom-containing substrates, including benzofurans
(1j–m) and indoles (1n–o), have also been employed in this
annulation process and all afforded polycyclic materials
successfully (2j–o) in 53–91% yields.
Owing to their utilization as synthetic precursors of
coannulenes and as model fluorescent compounds for their
high emission efficiency, the preparation of functionalized
fluoranthenes has been valuable.[17] Our arylation cyclization
reaction can be extended to prepare functionalized fluoran-
thenes. After treating aryl triazenes 3a and 3b with BF3·OEt2,
Initially, we treated biaryl triazene 1a (Table 1) with
BF3·OEt2 (2.0 equiv) in CH2Cl2 at room temperature over-
night, it provided the desired cyclization product 2a in 47%
yield. When we carried out the reaction in CH2Cl2 under
reflux, the yield increased to 77%. The scope and limitations
of this Lewis-acid-mediated Friedel-Crafts annulation process
were examined using various biaryl triazenes. This annulation
process can be extended to substrates bearing both electron-
withdrawing groups (EWG) and electron-donating groups
(EDG). The OCH3-substituted biaryl triazene substrates (1d–
h) are often produced in high yields in a short time, since the
electron-rich aromatic rings serve as a nucleophile to attack
the triazene aryl moiety; however, with CO2Et-substituted
(1a–c) aryl groups, the yield decreased. EWGs, such as CO2Et
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Scheme 2. Formation of fluoranthene derivatives 4a and 4b.
Scheme 4. Synthesis of pconjugated heterocyclic compound 8.
fluoranthenes 4a and 4b were generated in 62% and 67%
yield, respectively (Scheme 2).
Seven-membered ring compounds are often difficult to
access by classical routes. Recently, Greaney and Pintori[18]
À
reported an intramolecular oxidative C H coupling for
medium-ring synthesis using an indole system catalyzed by
[Pd(OAc)2], however, three equivalents of [Cu(OAc)2] was
used. Considering this problem, we tried to adjust our metal-
free intramolecular arylation method to generate seven-
membered rings. After treatment of aryl triazene 5 with
BF3·OEt2 in CH2Cl2 at room temperature for 30 min, the
annulated seven-membered ring heterocyclic compound 6
was obtained in 40% yield (Scheme 3).
Scheme 5. Investigation of the reaction mechanism.
scavengers,[20] were added to the reaction mixture, the desired
compound 2g was formed in 88–95% yield, but the by-
product 2gg was slightly decreased to 1–2% yield (Figure 1).
In this reaction, the initial interaction between the terminal
nitrogen atom of the triazene and the Lewis acid generates
a diazonium salt[21] which decomposes through aryl cations
and aryl radicals. This result implies that the desired product
2g could form through an aryl cation mechanism, whereas by-
product 2gg could form through a radical pathway.
In summary, we have developed a mild Friedel-Crafts
intramolecular arylation approach to the regioselective syn-
thesis of unsymmetrical and functionalized polycyclic aro-
Scheme 3. Synthesis of annulated seven-membered ring heterocyclic
compound 6.
As hetero-p-conjugated molecules show dramatically
different optical and electronic properties, we also tried to
expand our reaction to construct polycyclic frameworks with
pyrrole. Upon treatment of aryl triazene 7, which was easily
synthesized from 4-amino-3,5-dibromo-benzoic acid ethyl
ester by a Paal–Knorr condensation and a Suzuki cross-
coupling reaction, with BF3·OEt2 in CH2Cl2 at room temper-
ature for 30 min, the double cyclization compound 8 (with the
common name ullazine[19]) was obtained in 60% yield
(Scheme 4).
To explore the reaction mechanism for the synthesis of
PAHs, the following control experiments were performed
under the standard reaction conditions. When treated the
substrate 1g with BF3·OEt2 in CH2Cl2, the reaction proceeded
well and the desired compound 2g was obtained in 88% yield,
along with the by-product 2gg in 12% yield (Scheme 5).
When (2,2,6,6-Tetramethyl-piperidin-1-yl)oxyl (TEMPO),
1,1-diphenylethylene, or 2,6-di-tert-butyl-4-methylphenol
(BHT), all of which are known to be effective radical
Figure 1. Influence of different radical scavengers. Reaction conditions:
1g (0.5 mmol), BF3·OEt2 (1.0 mmol) and radical scavenger (0.5 mmol)
in CH2Cl2 (20 mL) for 30 min at room temperature. Yields shown are
of isolated products after chromatography. BHT=2,6-di-tert-butyl-4-
methylphenol, TEMPO=(2,2,6,6-Tetramethyl-piperidin-1-yl)oxyl.
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matic hydrocarbons (PAHs) in good to excellent yields. These
reactions are carried out under mild conditions with good
tolerance for a variety of functional groups. Mechanistic
investigations indicate that the reaction involves a highly
activated phenyl cation intermediate. Further investigations
into the reaction mechanism and synthetic applications are
underway.
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Experimental Section
Aryltriazene 1g (0.5 mmol, 1.0 equiv) was dissolved in CH2Cl2
(20 mL), the solution was stirred at room temperature, and then
BF3·OEt2 (127 mL, 2.0 equiv) was added dropwise. The mixture was
stirred for 30 min and monitored by thin layer chromatography. Upon
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Received: August 15, 2012
Published online: November 4, 2012
Keywords: annulation · aryl triazenes · fluoranthenes ·
.
Friedel-Crafts arylation · polycycles
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