Aromatic homologation by non-chelate-assisted RhIII-catalyzed c-H functionalization of arenes with alkynes

Article abstract of DOI:10.1002/anie.201310723

Larger condensed arenes are of interest owing to their electro- and photochemical properties. An efficient synthesis is the catalyzed aromatic annulation of a smaller arene with two alkyne molecules. Besides difunctionalized starting materials, directed C-H functionalization can be used for such aromatic homologation. However, thus far the requirement of either pre-functionalized substrates or suitable directing groups were limiting this approach. Herein, we describe a rhodium(III)-catalyzed method allowing the use of completely unbiased arenes and internal alkynes. The reaction works best with copper(II) 2-ethylhexanoate and decabromodiphenyl ether as the oxidant combination. This aromatic annulation tolerates a variety of functional groups and delivers homologated condensed arenes. Aside from simple benzenes, naphthalenes and higher condensed arenes provide access to highly substituted and highly soluble acenes structures having important electronic and photophysical properties. Multi-ring circus: The aromatic homologation of unfunctionalized, directing-group-free arenes with internal alkynes is promoted by a rhodium(III) catalyst in conjugation with suitable oxidants. The method tolerates a variety of functional groups and delivers highly substituted and highly soluble condensed arenes. These products provide access to acene structures with important electronic and photophysical properties.

Full text of DOI:10.1002/anie.201310723

.
Angewandte
Communications
DOI: 10.1002/anie.201310723
À
C H Activation
Aromatic Homologation by Non-Chelate-Assisted RhIII-Catalyzed
À
C H Functionalization of Arenes with Alkynes**
Manh V. Pham and Nicolai Cramer*
Abstract: Larger condensed arenes are of interest owing to
their electro- and photochemical properties. An efficient
synthesis is the catalyzed aromatic annulation of a smaller
arene with two alkyne molecules. Besides difunctionalized
À
starting materials, directed C H functionalization can be used
for such aromatic homologation. However, thus far the
requirement of either pre-functionalized substrates or suitable
directing groups were limiting this approach. Herein, we
describe a rhodium(III)-catalyzed method allowing the use of
completely unbiased arenes and internal alkynes. The reaction
works best with copper(II) 2-ethylhexanoate and decabromo-
diphenyl ether as the oxidant combination. This aromatic
annulation tolerates a variety of functional groups and delivers
homologated condensed arenes. Aside from simple benzenes,
naphthalenes and higher condensed arenes provide access to
highly substituted and highly soluble acenes structures having
important electronic and photophysical properties.
Scheme 1. Acene annulation by an undirected double oxidative RhIII-
À
catalyzed C H functionalization.
boronic acid substrates and methods to access them, the
requirement of a functionalized starting material represents
a restricting limitation. In the view of efficiency and step-
economy, the use of a totally unbiased aromatic substrate is
therefore highly desirable.
L
arger condensed arenes and heteroarenes have received
Recently, RhIII-directed C H functionalization[7] has
À
much interest due to their electro- and photochemical
properties, rendering them attractive materials for organic
electronics and luminescence applications.[1] The introduction
of multiple substituents/side chains generally makes these
compounds more soluble and stable, as well as allowing for
modulation and improvement of their fluorescence and
charge mobility properties.[2] A modular and attractive
method for their synthesis is an arene homologation that
provides the next higher acene by the condensation of an
aromatic ring with two alkyne units. A range of methods and
different starting materials have been devised. These com-
prise metal-catalyzed reactions of difunctionalized starting
materials[3] and, synthetically more efficiently, monofunction-
emerged as valuable tool to access polycondensed aromatic
ring systems.[5c,8] However, reactions lacking any directing
group on the substrate,[9] and thus allowing the use of
unbiased arenes, are scarce[10] and would be a highly desirable
tool (Scheme 1). Herein, we report a RhIII-catalyzed homo-
logation of directing group-free arenes and alkynes operating
À
by double C H functionalization under oxidative conditions.
The influence of the different reaction parameters was
initially evaluated with 5-decyne (1a) as the limiting reactant,
and naphthalene (2a) as aromatic acceptor (Table 1). Under
carefully optimized conditions, 60% of anthracene 3aa is
obtained using [(Cp*RhCl2)2] (Cp* = pentamethylcyclopen-
tadienyl) as catalyst and a combination of copper(II) 2-
ethylhexanoate and decabromo diphenyl ether as oxidants
(entry 1). Both the electron-poor complex reported by
Tanaka et al.[11] and simple RhCl3 were less efficient
(entries 3–4). The employed oxidant combination is essential
for the success of the annulation. The high solubility of
copper(II) 2-ethylhexanoate in apolar solvents makes it
largely superior to Cu(OAc)2 (entry 5). The perbromoben-
zene co-oxidant introduced by Glorius et al.[12] improved the
yield, however the separation of its degradation products
from 3aa is quite tedious (entry 6). The addition of sub-
stoichiometric amounts of decabromodiphenyl ether (deca-
BDE), an inexpensive and widely available flame retard-
ant,[13] mitigates this issue. Silver salts were detrimental for
the reaction outcome (entries 8–9). Moreover, both acidic
and basic additives decrease the yield significantly
(entries 10–11). The reaction proceeded equally well in
cyclohexane and less efficiently in dichloroethane, iPr2O,
À
alized educts, whereas the second C C bond formation is
initiated by a C H bond functionalization.[4,5] So far, the most
À
general method using monofunctionalized starting materials
is the RhIII-catalyzed annulation of arylboronic acids and
internal alkynes reported by Satoh and Miura (Scheme 1).[6]
However, despite the availability of a wide range of aryl
[*] M. V. Pham, Prof. Dr. N. Cramer
Laboratory of Asymmetric Catalysis and Synthesis, EPFL SB ISIC
LCSA
BCH 4305, 1015 Lausanne (Switzerland)
E-mail: Nicolai.cramer@epfl.ch
Homepage: http://isic.epfl.ch/lcsa
[**] This work was supported by the Swiss National Science Foundation
(137666) and the European Research Council under the European
Community’s Seventh Framework Program (FP7 2007–2013)/ERC
Grant agreement no. 257891.
Supporting information for this article is available on the WWW
under http://dx.doi.org/10.1002/anie.201310723.
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[a]
À
Table 1: Optimization studies of the C H annulation reaction.
Entry
Changes from standard conditions
Yield [%][b]
1
2
3
4
5
6
7
8
–
60[c]
0
42
23
5
53
20
15
3
without [(Cp*RhCl2)2]
2.5 mol% [{Me3(CO2Et)2CpRhCl2}2]
5 mol% RhCl3·3H2O
200 mol% Cu(OAc)2·H2O
with 50 mol% C6Br6
without (C6Br5)2O
with 10 mol% AgSbF6
with 200 mol% AgOPiv
with 200 mol% Cs2CO3
with 200 mol% PivOH
in cyclohexane
in DCE
in iPr2O
in tBuOH
with conventional heating
at 1408C
9
10
11
12
13
14
15
16
17
18
3
33
57
40
50
51
42
45
40
with 0.2 mmol 2a
[a] Standard conditions: 1a (0.20 mmol), 2a (0.50 mmol), [(Cp*RhCl2)2]
(2.50mmol), 0.20 mmol Cu(2-ethylhexanoate)2, 0.03 mmol (C6Br5O)2,
1.0m in heptane at 1608C microwave heating for 3 h; [b] Determined by
1H NMR spectroscopy with an internal standard; [c] Yield of isolated
3aa. DCE=1,2-dichloroethane, Piv=pivaloyl.
Scheme 2. Scope of the annulation process for monocyclic arenes.
Reaction conditions: 0.20 mmol 1, 2.00 mmol arene 4, 2.50 mmol
[(Cp*RhCl2)2], 0.20 mmol Cu(2-ethylhexanoate)2, 0.03 mmol (C6Br5)2O,
1608C microwave heating for 3 h; All yields are isolated pure 5.
[a] 2.3:1 meta/ortho. [b] 74% yield was obtained upon 40-fold scale up
(8.0 mmol 1a). [c] Reaction performed with 0.01 mmol (C6Br5)2O;
1.5:1 linear/angular isomer. [d] At 1208C for 12 h. [e] At 1408C for 6 h.
[f] Reaction performed with 5.00 mmol [(Cp*RhCl2)2] at 1808C.
and tBuOH (entries 12–15). Conventional heating, or low-
ering the reaction temperature or the equivalents of naphtha-
line reduces the yield as well (entries 16–18).
To explore the scope of the optimized process, we
evaluated different simple benzenes (4; Scheme 2). The
annulation of benzene and toluene gave the corresponding
naphthalenes 5aa and 5ab in 80% and 79% yield, respec-
tively. The substrate o-xylene delivers product 5ad in com-
parable yield, whereas m-xylene is not a competent substrate.
Halobenzenes[14] and other electron-poor derivatives display
an enhanced reactivity. Notably, esters and trifluoroacetyl
groups bearing both carbonyl directing groups, also yield the
distal activation product. Anisole, exemplifying an electron-
rich substrate, provides naphthaline 5an in somewhat reduced
yields. Aside from different internal alkyl alkynes,[15] w-
functionalized alkynes and diaryl alkynes can be used and
provide naphthalenes 5bi–gi. The reaction is also scalable
(8.0 mmol of alkyne 1a provides a 74% yield (1.25 g) of 5ai).
Besides the formation of substituted naphthalenes, the
method was further employed for the synthesis of a range of
anthracenes from their corresponding naphthalenes
(Scheme 3). The reaction is tolerant to a- and b-substitution
of the substrate. In these cases, the activation exclusively
takes place at the unsubstituted ring. Notably, a methyl ester
does not act as directing group (3af). Reaction with unsym-
metrically substituted alkyl aryl alkynes (1h and 1i) provide
the corresponding anthracenes 3ha and 3ia with a regioselec-
tivity clearly favoring an orientation of the aromatic sub-
stituents at the 2,3-position over the 2,4-position.[16] We next
tested larger polycyclic aromatic substrates for their viability
in this reaction. Such ring systems are at the center of interest
due to their physical properties, which are favorable for
applications in materials, such as organic electronics.[1] In this
respect, fluorene,[17] dibenzofurane,[18] dibenzothiophene,[19]
triphenylene,[20] thianthrene,[21] phenanthrene,[22] 9,10-dihy-
droanthracene,[23] 9H-xanthene, and 9,9-spirobi[fluorene][24]
participated well in the process to give the corresponding
annulated products 3ag–ao. Notably, 9,10-dichloroanthracene
gave tetracene 3ap when appropriate precautions are taken
to exclude light and oxygen during isolation.[25]
A common problem for material sciences is the dramat-
ically reduced solubility of larger polyarenes, which hampers
exploitation of the properties of the products, and makes the
fabrication of devices challenging.[2] To overcome this, the
installation of solubilizing side chains is required. In this
respect, the reported direct annulation method opens con-
venient opportunities for the direct introduction of such
solubilizing substituents or other functionalized tails. For
example, tetracene 3ap displays excellent solubility in pure
hydrocarbon solvents (minimum 290 mgmLÀ1 in hexane at
238C). We then measured its photophysical properties in n-
hexane as solvent and obtained values for 3ap in the expected
range for tetracenes (Figure 1).[25]
We propose the following scenario for the reaction
mechanism (Scheme 4): First, a non-chelation-assisted metal-
ation[26] of the napthalene 2a leads to aryl rhodium inter-
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Scheme 4. Mechanistic proposal for the annulation reaction (X=Cl,
Br, or 2-ethylhexanoate).
Incorporation of the second molecule of alkyne provides 9 or
9’, followed by reductive elimination, which expels annulated
product 3aa. Reoxidation by CuII then regenerates the
Cp*RhIII catalyst. Based on the report of Satoh and Miura
for the annulation of aryl boronic acids,[6] we assume the first
undirected intermolecular metalation is the challenging and
rate-limiting step of the overall transformation.
Scheme 3. Scope of the annulation process for polycyclic arenes.
Reaction conditions: 1 (0.20 mmol), arene 2 (0.50 mmol),
[(Cp*RhCl2)2] (2.50 mmol), Cu(2-ethylhexanoate)2 (0.20 mmol),
(C6Br5)2O (0.03 mmol), 1.00m in heptane, 1608C microwave heating
for 3 h; All yields shown are of isolated 3. [a] Aryl groups are 23:8:1
2,3-/2,4-/1,4-. [b] Aryl groups are 30:10:1 2,3-/2,4-/1,4-. [c] 2:1 linear/
angular isomer. [d] 3.7:1 linear/angular isomer. [e] 0.50m in heptane.
[f] 3:1 linear/angular isomer. [g] reaction performed with 5.00 mmol
[(Cp*RhCl2)2] in heptane 0.50m for 6 h.
In summary, we reported Cp*RhIII-catalyzed non-chelate-
À
assisted C H activation of a broad range of simple and
condensed arenes. The reaction with internal alkynes results
in annulative condensation to give the homologated acene.
The method is suited for larger aromatic substrates of interest
to material science and allows installation of solubilizing
chains and functional tails.
Experimental Section
Typical procedure for 3aa: [(Cp*RhCl2)2] (1.54 mg, 2.50mmol),
decabromodiphenyl ether (28.8 mg, 0.03 mmol), copper(II) 2-ethyl-
hexanoate (70.0 mg, 0.20 mmol), and naphthalene (64.1 mg,
0.50 mmol) were weighed into a 0.5 mL microwave vial equipped
with a magnetic stir bar and sealed. The vial was evacuated and
backfilled with nitrogen. 5-decyne (36.4 mL, 0.20 mmol) and 0.2 mL
heptane were then added. The mixture was heated to 1608C in
a microwave apparatus. After 3 h, the reaction mixture was cooled to
238C and the volatiles were evaporated in vacuo. The residue was
purified by chromatography on silica gel, eluting with pentane to yield
24.0 mg (60% yield) of 3aa.
Figure 1. Absorbance spectra (c) and fluorescence emission spectra
(a) of tetracene 3ap (5ꢀ10À5 m in n-hexane). Excitation wavelength:
450 nm.
Received: December 10, 2013
Published online: February 19, 2014
Keywords: acenes · alkynes · C-H activation · naphthalenes ·
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rhodium
mediate 6. Subsequent migratory insertion of the first
molecule of alkyne gives 7, which in turn triggers an intra-
À
molecular C H activation leading to rhodacycle 8; this
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