Tin-functionalized azobenzenes as nucleophiles in stille cross-coupling reactions

Article abstract of DOI:10.1021/jo402598u

The metalation of azobenzene by halogen-metal exchange typically leads to a reduction of the azo group to give hydrazine derivatives as major byproducts, instead of the desired metalated azobenzene species. In cross-coupling reactions, azobenzenes therefore usually serve as electrophiles, which greatly limits the scope of the reaction. To solve this problem, we have developed a mild and fast method to stannylate azobenzenes in high yields. This research shows that these stannylated azobenzenes can be used as nucleophilic components in Stille cross-coupling reactions with aryl bromides. The cross-coupling products were obtained in high yields ranging from 70 to 93%. With this reversal of the nucleophilic and electrophilic components, cross-coupling products are now accessible in which the aromatic rings coupled to the azobenzene bear functional groups that are sensitive to metalation.

Full text of DOI:10.1021/jo402598u

Article
pubs.acs.org/joc
Tin-Functionalized Azobenzenes as Nucleophiles in Stille
Cross-Coupling Reactions
Jan Strueben,† Paul J. Gates,‡ and Anne Staubitz*,†
†Otto-Diels-Institute for Organic Chemistry, University of Kiel, Otto-Hahn-Platz 4, 24098 Kiel, Germany
‡School of Chemistry, University of Bristol, Cantock’s Close, Bristol BS8 1TS, U.K.
S
* Supporting Information
ABSTRACT: The metalation of azobenzene by halogen−
metal exchange typically leads to a reduction of the azo group
to give hydrazine derivatives as major byproducts, instead of
the desired metalated azobenzene species. In cross-coupling
reactions, azobenzenes therefore usually serve as electrophiles,
which greatly limits the scope of the reaction. To solve this
problem, we have developed a mild and fast method to stan-
nylate azobenzenes in high yields. This research shows that
these stannylated azobenzenes can be used as nucleophilic
components in Stille cross-coupling reactions with aryl
bromides. The cross-coupling products were obtained in high
yields ranging from 70 to 93%. With this reversal of the nucleophilic and electrophilic components, cross-coupling products are
now accessible in which the aromatic rings coupled to the azobenzene bear functional groups that are sensitive to metalation.
reduction of ortho-, meta-, and para-halogenated azobenzenes
to the halogenated hydrazine analogues in 75−91% yield.17
However, no other metallic groups in the para position that can
be used for cross-coupling reactions have been described to
date.18
INTRODUCTION
■
Azobenzene and its derivatives are bistable systems (with their
trans and their cis form)1 that are comparatively resistant to
photobleaching2 and thermal decomposition.3 Therefore, they
are widely used as effective photoswitchable units in molecular
systems and polymers4 with potential applications ranging
from biochemical5 and medical uses,6 to photoswitchable bulk
materials,7 to smart surfaces8 and lithographic applications.9 A
further interest lies in their potential to couple both mechanical
and photochemical work in one molecule.1a,7,10 The effective
functionalization of azobenzenes, which enables their con-
nection to other functional molecules or tuning photophysical
properties, is therefore essential.
The most versatile approach to functionalize aromatic rings
to form CC bonds is by transition-metal-catalyzed cross-
coupling methods. While the use of halogen- or pseudohal-
ogen-functionalized azobenzenes as the electrophilic compo-
nent in this type of reaction is an established synthetic route,11
there are hardly any nucleophilic azobenzenes known. While a
metalation ortho to the azo group is facilitated by directed
ortho-metalation,12 yields are typically low. Moreover, this
feature cannot be used for the synthesis of para-substituted ana-
logues. For para-substituted azobenzenes, pinacol borane sub-
stituted azobenzene derivatives have been described that served
as a nucleophile in Suzuki cross-coupling reactions.13 Lithiated
species can be prepared in situ by halogen−metal exchange, but
the reported yields are very low (maximal 42% for para14 and
47% for ortho15). For Grignard compounds, a patent exists that
reports the synthesis of the symmetrical para-magnesiated
azobenzene.16 However, elemental magnesium in combination
with ammonium chloride has been successfully used for the
The main synthetic challenge of preparing para-metalated
azobenzenes is their low tolerance toward reductive reaction
conditions: Most protocols for introducing nucleophilic func-
tional groups are based on deprotonation followed by trans-
metalation. Alternatively, and as in the case of azobenzenes, a
halogen−metal exchange on compounds such as the para-
halogenated azobenzene 1 with an organolithium reagent to the
4-lithioazobenzene 2 can be used, followed by a transmetalation
with the desired metal chloride or semimetal chloride (Scheme 1).
However, a prominent side reaction that is impossible to avoid
using the highly nucleophilic alkylorganolithium reagents is the
reduction of the azo group to hydrazine derivatives (3 and 4).19
In this work, an easy-to-use methodology to obtain tin func-
tionalized azobenzene derivatives in very high yields18 and their
applicability for Stille cross-coupling reactions is presented.
RESULTS AND DISCUSSION
■
Although stannylation reactions of arenes are most often per-
formed by a transmetalation from the corresponding organo-
lithium or organomagnesium compounds, another method for
introducing trialkyl tin groups is by a cross-coupling reaction.20
In such a reaction, an aryl halide is treated with a distannane,
R3Sn−SnR3, using a transition-metal catalyst. For the
Received: December 4, 2013
Published: February 6, 2014
© 2014 American Chemical Society
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Scheme 1. Halogen−Lithium Exchange (Left); Concomitant
Reduction of the Azobenzene 1 to Hydrazine Derivatives 6
and 7 (Middle and Right)14,19
Table 1. Optimization of the Stannylation of 4-Iodo-4′-
methylazobenzene (10) with 2 mol % of [Pd(PPh3)4] as
Catalyst
a
entry
R
solvent
T (°C)
time
48 h
yield (%)
b
c
1
Me
Me
Me
Me
Me
Me
Me
Me
Me
Me
Me
toluene
DMF
reflux
46
b
2
70
9 h
91
84
b
3
DMSO
pyridine
toluene
dioxane
THF
70
13 h
b
4
70
11 h
54
b
5
70
14 h
64
b
6
70
12 h
39
b
7
70
16 h
43
d
8
DMF
150
10 min
10 min
10 min
10 min
21
d
9
DMSO
pyridine
toluene
150
27
d
10
11
150
82
d
150
>99
c
95
d
12
13
14
Me
dioxane
THF
150
10 min
10 min
35 min
94
d
Me
150
>99
d
c
stannylation of nitrophenyl iodide 8,21 n-hexabutyldistannane
as the electrophile and [Pd(PPh3)4] as the catalyst in toluene at
reflux conditions, a similar reaction had been reported to give
product 9 in a yield of 63% (Scheme 2).
n-Bu
toluene
150
74
a
GC yield unless noted otherwise; calibrated with the product 11
b
using triisoprobylbenzene as the internal calibration standard. Con-
ventional heating. Isolated yield. Microwave heating.
c
d
Scheme 2. Stannylation of p-Nitrophenyl Bromide (8) with
Hexa-n-butyldistannane to 9 in 63% Yield21
to adequate yields (54%, entry 5 and 64% entry 1, respectively),
starting material could still be observed. Dioxane and THF as
solvents led to the lowest yields under those reaction con-
ditions (39% and 43%, entries 6 and 7) and did not lead to a
full conversion of the starting material. However, in all reactions
no side products were observed, only the starting material 10
and product 11. As byproduct formation was not a significant
issue at 70 °C, and in order to reduce the reaction time and
increase the product yield, the reaction was performed at higher
temperatures in a sealed reaction vessel in a microwave at
150 °C (Table 1, entries 8−13). For each solvent, the reaction
was stopped after 10 min; the mixture was analyzed by GC to
determine the conversion of the starting material. Although no
decomposition or side products could be observed under any of
the reaction conditions, there were significant differences with
respect to conversion compared to the reactions performed at
70 °C. Under the microwave conditions, DMF and DMSO,
which were the most suitable solvents for conventional heating
at 70 °C, only led to low yields of 21% and 27% (Table 1,
entries 8 and 9). Toluene and THF, on the other hand, led to
an almost quantitative product yield of >99% each, respectively
(Table 1, entries 11 and 13). The same was true for dioxane
and pyridine (Table 1, entries 12 and 10). The optimized con-
ditions for the monostannylation of azobenzene in toluene
(Table 1, entry 11) were transferred to the coupling reaction of
n-hexabutyldistannane to azobenzene in toluene in the
microwave to give 12. Although THF also led to a quantitative
yield for the stannylation, toluene was selected because this
solvent does not carry the risk of forming explosive peroxides
when stored.22 The stannylation of 10 to give 12 was moni-
tored by thin layer chromatography because we could not
observe a signal in GC/MS. After 10 min, the reaction was
incomplete, presumably due to the lower reactivity of
To establish reaction conditions for a stannylation of azo-
benzene using such a cross-coupling reaction, initially those
same reaction conditions for azobenzene 10 as the electrophilic
component were used and hexamethyldistannane as elec-
trophile. Compound 11 could be isolated in a yield of 46%
(Table 1, entry 1): the trimethylstannylazobenzene 11 proved
stable to pH neutral silica gel, and therefore, it could be easily
purified by removal of the catalyst with a filtration column. This
product 11 was then used as a calibrant for the development of
a gas chromatographic (GC) method which allowed us to
monitor the reaction and optimize it efficiently. The reaction
was performed in different solvents at different temperatures
with [Pd(PPh3)4] as catalyst (Table 1). For the thermal reac-
tions at 70 °C, the reaction progress was monitored in regular
intervals, and the time given in Table 1 represents the time
when no further conversion was observed. The reaction turned
out to be highly temperature and solvent dependent. For all
solvents, a reaction temperature of 70 °C using conventional
heating led to significantly longer reaction times than heating to
150 °C in the microwave. (Compare Table 1, entries 2−7, with
entries 8 to 13, respectively.) At 70 °C, DMF and DMSO
turned out to be very suitable for the reaction to give a full con-
version and a GC yield of 91% and 84%, respectively (Table 1,
entries 2 and 3). Although pyridine and toluene as solvents led
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(n-Bu)3Sn−Sn(n-Bu3) as compared to the sterically more
accessible Me3Sn−SnMe3. The reaction time in the microwave
was therefore increased to 35 min. Although the product could
be isolated in an adequate yield of 74% (Table 1, entry 14), the
purification required a Kugelrohr distillation at very high tem-
peratures of 180 °C and a pressure of 9.4 × 10−2 mbar, which
probably led to a partial decomposition of the material.
Having established high-yielding reaction conditions, 4,4′-
bis(iodo)azobenzene (13) was stannylated with both hexam-
ethyldistannane and n-hexabutyldistannane to give products 14
and 15 (Scheme 3). As for the monostannylated species, the
Stille cross-coupling reactions with the tri-n-butylstannyl
azobenzene derivative were also performed with the electron-
deficient para-nitrophenyl bromide (16) and the electron-rich
3-methoxyphenyl bromide (28). As this starting material, tri-n-
butylstannyl azobenzene could not be observed by our GC/
MS, the reaction was monitored by thin-layer chromatography.
The yields with this nucleophile were significantly lower than
the product yield with the methylstannylated species: Product
17 could only be isolated in a yield of 71% (18% lower than
with nucleophile 11), and product 31 gave only a yield of 54%
(25% lower than with nucleophile 11) (Table 2, entries 1 (b)
and 7 (b)). We attribute this to the lower reactivity of the
nucleophile 12 to the increased sterical hindrance of the n-butyl
groups as compared to the methyl groups.
Several of the aryl bromides were used for the Stille coupling
reactions in this work are highly sensitive to typical metalation
reactions using organolithium or organomagnesium reagents.
For example, nitrophenyl bromide and cyanophenyl bromide
(16, 18) are unstable when treated with n-butyllithium, and the
aldehyde-, ketone-, and ester-functionalized aryl bromides (19,
24, and 2624) would be attacked by the butyl nucleophile and
be reduced. To circumvent this problem, protection groups can
be used, but as this increases the number of steps in a synthesis,
and causes more waste, it is advantageous if protection groups
can be avoided in a synthetic process. With our newly devel-
oped stannylated azobenzenes, aromatic bromides bearing
these sensitive functional groups (nitro, cyano, carbonyl) can
now be used as the much more easily accessible electrophilic
cross-coupling partner for a carbon−carbon bond formation
with an azobenzene. The carbonyl functionalized arylbromides
gave yields from 73 to 87% (Table 2 entries 3, 5, and 6).
Scheme 3. Distannylation of 4,4′-Bis(iodo)azobenzene (13)
with 4 mol % of [Pd(PPh3)4] as a Catalyst to Products 14
and 15
bis-trimethylstannylated azobenzene (14) was obtained in a
very good yield of 81%. For the bis(tri-n-butyl)stannylated
species 15, the isolated yield was somewhat lower, at 61%.
Whereas the methylstannylated azobenzenes could be purified
by filtration on silica and subsequent evaporation of the hexa-
methyldistannane without a noticeable loss of yield, 15 had to
be performed by Kugelrohr distillation. We assume that this
process caused the observed lower yield for this product.
With both the trimethylstannyl- and tri-n-butylstannylazo-
benzenes 11 and 12 in hand, we explored their usefulness in
Stille cross-coupling reactions with various aromatic bromides
as reaction partners (Table 2). All of these Stille coupling reac-
tions were performed at a moderate temperature of 70 °C. The
reaction progress was monitored by GC/MS, and the reaction
was terminated when no starting material could be detected any
more. Initially, all reactions were performed with the trimeth-
ylstannylazobenzene 11 as the nucleophile. As expected, the use
of electron-deficient benzene derivatives gave excellent yields:
para-bromonitrobenzene (16) was fully converted within 6 h,
giving the product 17 in a yield of 89% (Table 2, entry 1).
Similarly, the electron-deficient para-cyanobenzene bromide 18
and para-bromophenyl methyl ketone 19 required only a short
reaction time of 8 h to give the products 20 and 21 in good
yields of 82% and 87%, respectively. The reaction worked
equally well for electron-rich aromatic cycles. Although the
electrophile 24 bears an aldehyde group, the furan heterocycle
is very electron rich, which explains the longer reaction time of
16 h which was required for this compound, and a yield of 73%
of 25 could be isolated. Electron-rich electrophiles typically
required longer reaction times of more than 19 h (compare, for
example, 3-methoxy bromide 28, 2,4-dimethoxide 29, and
benzodioxolyl bromide 30 with bromobenzene 22; Table 2,
entries 7, 8, 9, and 4), but the furan ester 27 showed a full
conversion after only 14 h and could be isolated in a yield of
84% (Table 2, entry 6). In reactions with bromothiophenes, 2-
bromothiophene (34) gave 35 with a yield of 88% in only 11 h,
which is significantly faster compared to the less reactive23 3-
bromothiophene (36) with a reaction time of 14 h to give 37 in
an isolated yield of 83%. (Table 2, entries 10, and 11). The
para-bromoaniline 38 showed full conversion to product 39
after 19 h with a yield of 72%.
CONCLUSION
■
In conclusion, we have developed a highly efficient method-
ology to prepare mono- and distannylated azobenzene deri-
vatives with tri-n-butylstannyl and trimethylstannyl substituents.
This reaction involves a palladium-catalyzed cross-coupling
reaction with reagents of the type R3Sn−SnR3 and the easily
accessible iodinated azobenzene derivatives as starting materi-
als. It was also shown that such tin-functionalized azobenzenes
are effective nucleophiles in Stille cross-coupling reactions
with a range of functionalized aryl bromides. Electron-rich and
electron-deficient electrophiles were efficiently cross-coupled in
similarly high yields ranging from 70 to 93%. Of particular
interest are electrophiles FG−Ar−Br with functional groups
(FG) such as aldehydes, ketones, or nitro groups. Previously,
corresponding nucleophiles, FG−Ar−M, could be reacted with
the easily accessible azobenzene halides using established pro-
cedures. However, such compounds FG−Ar−M can be difficult
to prepare in this form if a metalation procedure would lead to
an attack of the nucleophile on these electrophilic functional
groups. Because azobenzene stannanes are now available, these
electrophiles can be used as the much more easily accessible
electrophilic component. This newly established method there-
fore complements the established procedures by introducing a
new synthon for nucleophilic azobenzenes.
EXPERIMENTAL SECTION
■
All reagents used were commercially available and used without further
purification. For their purities see the Supporting Information. All
solvents that were used in the stannylation and Stille reactions were
dried prior to use. For the exact drying procedures see Supporting
Information.
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Table 2. Stille Coupling of Various Aryl Bromides with 4-Methyl-4′-(trimethylstannyl)azobenzene (11) or 4-Methyl-4′-(tri-n-
butylstannyl)azobenzene (12)
a
b
Reaction with 4-methyl-4′(trimethylstannyl)azobenzene. Reaction with tri-n-butylstannylazobenzene.
All preparations for the stannylation reactions were performed in a Representative Procedure for the Optimization of the
nitrogen-filled glovebox and carried out in a sealed vial under nitrogen.
All Stille couplings were carried out using standard Schlenk techniques
under a nitrogen atmosphere.
Stannylation of 4-Iodo-4′-methylazobenzene. Thermal Synthe-
ses. A solution of 4-iodo-4′-methylazobenzene (177 mg, 550 μmol),
hexamethyldistannane (180 mg, 550 μmol), [Pd(PPh3)4] (12.7 mg,
2 mol %), and the internal reference 1,3,5-triisopropylbenzene
(112 mg, 550 μmol) in toluene (4 mL) was heated to 70 °C. In reg-
ular intervals, a sample (0.1 mL) was taken from the reaction vial and
filtered through a short column of silica (5 × 3 mm; eluent: DCM)
and a PTFE syringe filter (pore size = 45 μm) before the sample was
analyzed by GC.
All NMR spectra were recorded on a 500 MHz spectrometer (with
1
respect to the proton resonance). H NMR and 13C NMR spectra
were referenced against the solvent residual proton signals (1H) or the
solvent itself (13C). 119Sn NMR spectra were referenced externally
against CDCl3.
The exact assignment of the peaks was performed by two-
1
1
dimensional NMR spectroscopy such as H COSY, H/13C HSQC,
1
or H/13C HMBC when possible.
For all other reaction conditions, the parameters were varied as
All microwave syntheses were performed on a Biotage Initiator+ SP
Wave (Organic Synthesis Mode). The temperature was measured with
an external IR sensor during microwave heating.
specified in Table 1.
Microwave syntheses. A solution of 4-iodo-4′-methylazobenzene
(177 mg, 0.55 mmol), hexamethyldistannane (180 mg, 0.55 mmol),
[Pd(PPh3)4] (12.7 mg, 2 mol %), and the internal reference 1,3,5-
triisopropylbenzene (112 mg, 550 μmol) in toluene (4 mL) was
heated to 150 °C in a microwave apparatus. Every 10 min, a sample
(0.1 mL) was taken from the reaction vial and filtered through a short
column of silica (5 × 3 mm; eluent: DCM) and a PTFE syringe filter
(pore size = 45 μm), before the sample was analyzed by GC.
For all other reaction conditions, the parameters were varied as
specified in Table SI 1 (Supporting Information).
Due to the toxicity of organotin compounds,25 certain precautions
should be observed: All manipulations should be performed in a well-
vented fume cupboard, wearing standard eye protection, lab coats,
and nitrile gloves with the following specification: EN374-1:2003
“Protection against chemical splash.”
Care needs to be taken to observe proper disposal procedures for all
waste products, according to the health and safety procedures in place.
Under no circumstances should these compounds be disposed of in
the drains or allowed to leak in any other way into the environment.
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4-Iodo-4′-methylazobenzene (10).26
heated to 150 °C for 35 min in a microwave apparatus, and then the
solvent was evaporated. The crude product was dissolved in DCM and
filtered over a short plug of silica. The solvent was evaporated, and the
remaining oil was purified by Kugelrohr distillation (9.43 × 10−2 mbar,
150 °C, 20 min). The residue was dissolved in DCM and filtered over
a short plug of silica. After evaporation of the solvent, 998 mg (74%)
1
of a red oil was obtained. H NMR (500 MHz, CDCl3): 7.87 (d, 3J =
This compound has also been synthesized by J. Tour and co-workers
using a different method.26 para-Toluidine (26.0 g, 243 mmol) was
dissolved in DCM (100 mL) at 20 °C. A solution of K2SO5 (300 g,
488 mmol) in water (600 mL) was added, and the reaction was stirred
for 4 h at 20 °C. The dark green organic phase was separated and
washed with 1 M hydrochloric acid (2 × 150 mL), a saturated aque-
ous solution of hydrogen carbonate (1 × 150 mL), and water (1 ×
200 mL). The organic phase was dried over magnesium sulfate. After
evaporation of the solvent and without further purification, a solution
of 4-iodoaniline (53.2 g, 243 mmol) in acetic acid (250 mL) was added
to the crude product and stirred for 16 h at 20 °C. The precipitate was
separated by filtration and dissolved in DCM (50 mL). The solution
was washed 1 M hydrochloric acid (2 × 150 mL), a saturated solu-
tion of hydrogen carbonate in water (1 × 150 mL), and water (1 ×
200 mL). The combined organic phases were dried over magnesium
sulfate. After evaporation of the solvent, 42.3 g (54%, lit.26 93%) of an
8.1 Hz, 2 H, H-3, 3′), 7.86 (d, 3J = 8.2 Hz, 2 H, H-6, 6′), 7.65 (d, 3J =
8.2 Hz, 2 H, H-7,7′), 7.33 (d, 3J = 8.1 Hz, 2 H, H-2, 2′), 2.34 (s, 3 H,
H-9), 1.66−1.49 (m, 6 H, H-11), 1.41−1.32 (m, 6 H, H-12), 1.19−
1.05 (m, 6 H, H-10), 0.91 (t, 3J = 7.3 Hz, 9 H, H-13) ppm. 13C NMR
(126 MHz, CDCl3): 152.7 (C-5), 150.9 (C-4), 146.7 (C-8), 141.3
(C-1), 137.1 (C-7, 7′), 129.7 (C-2, 2′), 122.9 (C-6,6′) 122.8 (C-3,3′),
29.1 (C-11), 27.4 (C-12), 21.5 (C-9), 13.7 (C-13), 9.7 (C-10) ppm.
119Sn NMR (187 MHz, CDCl3): −39.9 ppm. IR (ATR): 3020 (w),
2955 (s), 2921 (s), 2870 (m), 2852 (m), 1600 (w), 1462 (m), 1376
(m), 1964 (m), 1013 (m), 960 (m), 873 (m), 829 (s), 658 (s) cm−1.
HRMS (ESI-FTMS) m/z: [M + H]+ calcd for [C25H38N2Sn + H]+
487.2130, found 487.2149.
4,4′-Bis(iodo-)azobenzene (13).28
orange solid was obtained. Mp: 160 °C (lit.27 mp 160 °C. H NMR
1
3
3
(500 MHz, CDCl3): 7.86 (d, J = 8.5 Hz, 2 H, H-3,3′), 7.83 (d, J =
3
3
8.5 Hz, 2 H, H-7,7′), 7.64 (d, J = 8.5 Hz, 2 H, H-6,6′), 7.32 (d, J =
8.5 Hz, 2 H, H-2,2′), 2.45 (s, 3H, H-9) ppm. 13C NMR (125 MHz,
CDCl3): 152.0 (C-5), 150.6 (C-4), 142.0 (C-1), 138.3 (C-7,7′), 129.8
(C-2,2′), 124.4 (C-6,6′), 123.0 (C-3,3′), 97.2 (C-8), 21.5 (C-9) ppm.
IR (ATR): 3021 (w), 2978 (w), 2912 (w), 1600 (m), 1381 (m), 1105
(m), 1065 (m), 830 (s), 763 (s), 711 (s), 512 (s) cm−1. HRMS (CI-
sector) m/z: [M + H]+ calcd for [C13H11N2I + H]+ 323.0045, found
323.0050.
This compound has also been synthesized by Roncali and co-
workers.28 For the preparation of the catalyst, copper chloride (5.00 g,
45.7 mmol) was dissolved in pyridine (50 mL), and the mixture was
stirred for 30 min at 20 °C. An insoluble residue which remained
was removed by filtration before addition of 4-iodoaniline (14.3 g,
65.3 mmol) in one portion to the solution. The reaction mixture was
stirred for 9 h at 20 °C while air was bubbled through the reaction
mixture with the help of a frit. Then, diethyl ether (150 mL) was
added, and the organic phase was washed with water (3 × 200 mL),
2 M aqueous hydrochloride acid (2 × 200 mL), and water (1 ×
100 mL). The organic phase was dried over sodium sulfate. After
evaporation of the solvent, the crude product was recrystallized from
ethanol to give 8.64 g (61%, lit.28 87%) of a red solid. Mp: 210 °C
4-Methyl-4′-(trimethylstannyl)azobenzene (11).
4-Iodo-4′-methylazobenzene (3.00 g, 9.31 mmol), hexamethyldistan-
nane (3.05 g, 9.31 mmol), and [Pd(PPh3)4] (215 mg, 186 μmol,
2 mol %) were dissolved in toluene (19 mL). The reaction vessel was
heated to 170 °C for 10 min in a microwave. The solvent was
evaporated, and the crude product was purified by filtration through
silica with cyclohexane/ethyl acetate (v/v, 2/3). The solvent and the
remaining hexamethyldistannane were evaporated at 6.4 × 10−2 mbar
and 70 °C over the course of 17 h. 3.12 g (95%) of red solid was
(lit.28 mp 210−211 °C). H NMR (500 MHz, CDCl3): δ= 8.07 (d,
1
3J = 8.7 Hz, 4 H, H-2, 2′), 7.79 (d, 3J = 8.7 Hz, 4 H, H-3,3′) ppm. 13
C
NMR (126 MHz, CDCl3): 151.8 (C-4), 138.4 (C-2, 2′), 124.5 (C-3,
3′), 98.1 (C-1) ppm. IR (ATR): 3078 (w), 1560 (m), 1573 (m), 1469
(m), 1393 (m), 1156 (m), 1051 (m), 1001 (m), 833 (s), 539 (s) cm−1.
HRMS (CI-sector) m/z: [M + H]+ calcd for [C12H8N2I2 + H]
434.8855, found 434.8848.
1
3
4,4′-Bis(trimethylstannyl)azobenzene (14).
obtained. Mp: 62 °C. H NMR (500 MHz, CDCl3): 7.87 (d, J =
8.2 Hz, 2 H, H-3, 3′), 7.84 (d, 3J = 8.3 Hz, 2 H, H-6, 6′), 7.66 (d, 3J =
8.3 Hz, 2 H, H-7,7′), 7.32 (d, 3J = 8.2 Hz, 2 H, H-2, 2′), 2.45 (s, 3 H,
H-10), 0.35 (s, 9 H, H-9) ppm. 13C NMR (126 MHz, CDCl3): 152.7
(C-5), 150.9 (C-4), 146.7 (C-8), 141.5 (C-1), 136.6 (C-7, 7′), 129.7
(C-2, 2′), 122.9 (C-6, 6′) 122.9 (C-3, 3′), 21.5 (C-10) −9.5 (C-9) ppm.
119Sn NMR (187 MHz, CDCl3): −25.25 ppm. IR (ATR): 3023 (w),
2980 (w), 2910 (w), 1600 (w), 1380 (w), 1156 (m), 1065 (m), 1010
(m), 830 (s), 783 (s), 711 (s), 526 (s) cm−1. HRMS (ESI-FTMS) m/
z: [M + H]+ calcd for [C16H20N2Sn + H]+ 361.0721, found 361.0735.
4-Methyl-4′-(tributylstannyl)azobenzene (12).
4,4′-Bis(iodo)azobenzene (1.00 g, 2.30 mmol), hexamethyldistannane
(1.15 g, 4.61 mmol), DMSO (2 mL), and [Pd(PPh3)4] (106 μg,
92.0 μmol, 4 mol %) were dissolved in toluene (19 mL). The reac-
tion vessel was heated to 150 °C for 10 min in a microwave apparatus.
Then the solvent was evaporated, and the crude product was purified
by filtration on silica with DCM. The solvent was evaporated, and the
remaining hexamethyldistannane was evaporated at 5.3 × 10−2 mbar
and 70 °C over 39 h. 946 mg (81%) of red solid was obtained. Mp:
1
3
54 °C. H NMR (500 MHz, CDCl3): 7.87 (d, J = 8.2 Hz, 4 H, H-3,
3′), 7.65 (d, 3J = 8.2 Hz, 4 H, H-2, 2′), 0.34 (s, 18 H, H-5) ppm. 13C
NMR (126 MHz, CDCl3): 152.8 (C-4), 147.1 (C-1), 136.6 (C-2, 2′),
122.0 (C-3, 3′), −9.4 (C-5) ppm. 119Sn NMR (187 MHz, CDCl3): δ =
−25.20 ppm. IR (ATR): 3067 (w), 3024 (w), 2987 (w), 2917 (w),
1925 (w), 1437 (m), 1383 (m), 1306 (m), 1067 (m), 1011 (m), 831
(s), 761 (s), 583 (s), 508 (s) cm−1. HRMS (ESI-FTMS) m/z: [M +
H]+ calcd for [C18H26N2Sn2 + H]+ 511.0213, found 511.0227.
4-Iodo-4′-methylazobenzene (1.00 g, 3.10 mmol), hexa-n-butyldistan-
nane 1.80 g (3.10 mmol), and [Pd(PPh3)4] (64.3 g, 55.0 μmol,
2 mol %) were dissolved in toluene (19 mL). The reaction vessel was
1723
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4,4′-Bis(tributylstannyl)azobenzene (15).
1067 (m), 832 (m), 1065 (m), 832 (s), 760 (s), 583 (s), 520 (s) cm−1.
HRMS (ESI-FTMS) m/z: [M + H]+ calcd for [C19H15N3O2 + H]+
318.1237, found 318.1245.
4-(4-Cyanophenyl-) 4′-methylazobenzene (20).
4-Methyl-4′-(trimethylstannyl)azobenzene (200 mg, 0.557 mmol), 4-
bromobenzonitrile (101 mg, 0.557 mmol), [Pd(PPh3)4] (12.9 mg,
11.2 μmol, 2 mol %), copper chloride (165 mg, 1.67 mmol, 3 equiv),
and lithium chloride (142 mg, 3.34 mmol, 6 equiv) were dissolved in
DMF (15 mL). The reaction mixture was heated to 70 °C for 8 h.
After the reaction mixture had cooled to 20 °C, chloroform (80 mL)
was added, and the mixture was extracted with 2 M hydrochloric acid
(2 × 150 mL), a saturated sodium carbonate solution (1 × 150 mL),
and water (1 × 150 mL). The combined organic phases were dried
over sodium sulfate. The product was purified by column
chromatography on silica with toluene as eluent. The solvent was
evaporated, and an orange solid (136 mg, 82%) was obtained. Mp:
207 °C. 1H NMR (500 MHz, CDCl3): 8.01 (d, 3J = 8.7 Hz, 2H, 6, 6′),
4,4′-Bis(iodo)azobenzene (1.00 g, 2.30 mmol), hexa-n-butyldistannane
(2.67 g, 4.60 mmol), DMSO (2 mL), and [Pd(PPh3)4] (106 mg,
92.0 μmol, 4 mol %) were dissolved in toluene (19 mL). The reac-
tion vessel was heated to 150 °C for 60 min in a microwave apparatus.
The solvent was evaporated. The crude product was dissolved in DCM
and filtered over a short plug of silica. The solvent was evaporated, and
the remaining oil was purified by removing the byproducts by
Kugelrohr distillation (7.93 × 10−2 mbar, 150 °C, 30 min). The
residue was dissolved in DCM and filtered over a short plug of silica.
1.07 g (61%) of a red oil was obtained. 1H NMR (500 MHz, CDCl3):
3
3
7.85 (d, J = 8.2 Hz, 4H, H-3, 3′), 7.64 (d, J = 8.2 Hz, 4H, H-2, 2′),
1.66−1.49 (m, 12H, H-7), 1.41−1.32 (m, 12 H, H-8), 1.19−1.05 (m,
3
12 H, H-6), 0.91 (t, J = 7.3 Hz, 18 H, H-9) ppm. 13C NMR (126
3
MHz, CDCl3): 152.7 (C-4), 147.0 (C-1), 137.1 (C-3, 3′), 121.8 (C-2,
2′), 29.1 (C-7), 27.4 (C-8), 13.7 (C-9), 9.7 (C-6) ppm. 119Sn NMR
(187 MHz, CDCl3): −40.0 ppm. IR (ATR): 3074 (w), 2955 (s), 2922
(s), 2850 (s), 1921 (w), 1462 (m), 1376 (m), 1064 (m), 1012 (m),
830 (s) cm−1. HRMS (ESI-FTMS) m/z: [M + H]+ calcd for
7.86 (d, J = 8.2 Hz, 2H, 3, 3′), 7.81−7.74 (m, 6 H, H-7, 7′, 10, 10′,
11, 11′), 7.36 (d, 3J = 8.2 Hz, 2H, 2,2′), 2.45 (s, 3 H, CH3) ppm. 13
C
NMR (125 MHz, CD2Cl2) (due to significant signal overlap in the 1H
NMR spectrum, it was impossible to assign all 13C NMR signals using
HMQC and HMBC): 152.7 (C-5), 150.9 (C-4), 144.65, 142.4 (C-1),
141.3, 132.8 (C-2, 2′), 123.0, 128.2, 127.9, 123.5 (C-6, 6′), 123.0 (C-3,
3′), 118.9, 111.6, 21.4 (C-14) ppm. IR (ATR): 3071 (w), 3045 (w),
2925 (w), 2225 (m), 1598 (m), 1417 (m), 1378 (m), 1233 (m), 1208
(m), 1158 (m), 1109 (m), 1003 (m), 830 (s), 718 (m), 639 (m), 546
(s) cm−1. HRMS (CI-sector) m/z: [M + H]+ calcd for [C20H15N3 +
H]+ 298.1344, found 298.1347.
+
[C36H62N2Sn2 + H] 763.3030, found 763.3023.
4-(4-Nitrophenyl)-4′-methylazobenzene (17).
4-(4-Acetylphenyl)-4′-methylazobenzene (21).
Method A. 4-Methyl-4′-(trimethylstannyl)azobenzene (200 mg,
0.557 mmol), para-nitrophenyl bromide (113 mg, 0.557 mmol),
[Pd(PPh3)4 (12.9 mg, 0.011 mmol, 2 mol %), copper chloride (165 mg,
1.67 mmol,] 3 equiv), and lithium chloride (142 mg, 3.34 mmol, 6 equiv)
were dissolved in DMF (15 mL). The reaction mixture was heated to
70 °C for 6 h. After the reaction mixture had cooled to 20 °C,
chloroform (80 mL) was added, and the mixture was extracted with 2 M
hydrochloric acid (2 × 150 mL), a saturated sodium carbonate solution
(1 × 150 mL), and water (1 × 150 mL). The combined organic phases
were dried over sodium sulfate. The product was recrystallized from
EtOH. Orange crystals (157 mg, 89%) were obtained.
4-Methyl-4′-(trimethylstannyl)azobenzene (200 mg, 0.557 mmol), 4-
bromoacetophenone (111 mg, 0.557 mmol), [Pd(PPh3)4] (12.9 mg,
11.2 μmol, 2 mol %), copper chloride (165 mg, 1.67 mmol, 3 equiv),
and lithium chloride (142 mg, 3.34 mmol, 6 equiv) were dissolved in
DMF (15 mL). The reaction mixture was heated to 70 °C for 8 h.
After the reaction mixture had cooled to 20 °C, chloroform (80 mL)
was added, and the mixture was extracted with 2 M hydrochloric acid
(2 × 150 mL), a saturated sodium carbonate solution (1 × 150 mL),
and water (1 × 150 mL). The combined organic phases were dried
over sodium sulfate. The solvent was evaporated. The crude product
was purified by column chromatography with toluene as eluent.
Method B. 4-Methyl-4′-(tri-n-butylstannyl)azobenzene (270 mg,
0.557 mmol), para-nitrophenyl bromide (113 mg, 0.559 mmol),
[Pd(PPh3)4] (12.9 mg, 11.2 μmol, 2 mol %), copper chloride (165 mg,
1.67 mmol, 3 equiv), and lithium chloride (142 mg, 3.34 mmol,
6 equiv) were dissolved in DMF (15 mL). The reaction mixture was
heated to 70 °C for 17 h. After the reaction mixture had cooled to
20 °C, chloroform (80 mL) was added, and the mixture was extracted
with 2 M hydrochloric acid (2 × 150 mL), a saturated sodium car-
bonate solution (1 × 150 mL), and water (1 × 150 mL). The
combined organic phases were dried over sodium sulfate. The solvent
was evaporated. The crude product was recrystallized from cyclo-
hexane. An orange solid (125 mg, 71%) was obtained. Mp: 104 °C. 1H
NMR (500 MHz, CDCl3): 8.24 (d, 3J = 8.9 Hz, 2 H, H-11, 11′), 7.93
1
An orange solid (152 mg, 87%) was obtained. Mp: 186 °C. H NMR
3
(500 MHz, CD2Cl2): 8.05 (d, J = 8.6 Hz, 2 H, H-11, 11′), 8.01 (d,
3J = 8.7 Hz, 2 H, H-6, 6′), 7.86 (d, 3J = 8.1 Hz, 2 H, H-3, 3′), 7.82 (d,
3J = 8.7 Hz, 2 H, H-7, 7′), 7.79 (d, 3J = 8.6 Hz, 2 H, H-10, 10′), 7.36
(d, 3J = 8.1 Hz, 2 H, H-2, 2′), 2.63 (s, 3 H, H-14), 2.45 (s, 3 H, H-15)
ppm. 13C NMR (125 MHz, CDCl3): 197.7 (C-13), 152.75 (C-5),
151.2 (C-4), 144.9 (C-9), 142.50 (C-8), 142.4 (C-1), 136.8 (C-12),
130.2 (C-2, 2′), 129.3 (C-11, 11′), 128.4 (C-7, 7′), 127.6 (C-10, 10′),
123.7 (C-6, 6′), 123.2 (C-3, 3′), 26.9 (C-14), 21.6 (C-15) ppm. IR
(ATR): 3348 (w), 3027 (w), 2917 (w), 2859 (w), 1925 (w), 1674 (s),
1599 (m), 1355 (m), 1231 (m), 957 (m), 824 (s), 720 (m) cm−1.
HRMS (ESI-FTMS) m/z: [M + Na]+ calcd for [C21H18N2O + Na]+
337.1311, found 337.1318.
3
3
(d, J = 8.7 Hz, 2H, H-6, 6′), 7.78 (d, J = 8.2 Hz, 2H, H-3,3′), 7.71
(d,3J = 8.9 Hz, 2H, H-10, 10′), 7.68 (d, 3J = 8.7 Hz, 2H, H-7, 7′), 7.26
3
(d, J = 8.2 Hz, 2 H, H-2, 2′), 2.37 (s, 3 H, H-13) ppm. 13C NMR
(125 MHz, CD2Cl2): 152.8 (C-5), 150.8 (C-4), 147.4 (C-12), 146.6
(C-9), 142.1 (C-8), 140.6 (C-1), 129.8 (C-2, 2′), 128.1 (C-7, 7′), 127.9
(C-10, 10′), 124.2 (C-11, 11′), 123.5 (C-6, 6′), 123.0 (C-3, 3′), 21.6
(C-13) ppm. IR (ATR): 3023 (w), 2979 (w), 2912 (w), 1382 (m),
1724
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4-Phenyl-4′-methylazobenzene (23).
4-(5-Methylcarboxylate-2-furanyl)-4′-methylazobenzene (27).
4-Methyl-4′-(trimethylstannyl)azobenzene (200 mg, 0.557 mmol),
bromobenzene (87,5 mg, 0.557 mmol), [Pd(PPh3)4] (12.9 mg, 11.2
μmol, 2 mol %), copper chloride (165 mg, 1.67 mmol, 3 equiv), and
lithium chloride (142 mg, 3.34 mmol, 6 equiv) were dissolved in DMF
(15 mL). The reaction mixture was heated to 70 °C for 10 h. After the
reaction mixture had cooled to 20 °C, chloroform (80 mL) was added,
and the mixture was extracted with 2 M hydrochloric acid (2 ×
150 mL), a saturated sodium carbonate solution (1 × 150 mL), and
water (1 × 150 mL). The combined organic phases were dried over
sodium sulfate. The solvent was evaporated. The crude product was
purified by column chromatography with toluene as eluent. The
solvent was evaporated, and an orange solid (141 mg, 93%) was
4-Methyl-4′-(trimethylstannyl)azobenzene (200 mg, 0.557 mmol), 5-
bromo-2-furancarboxylic acid methyl ester (114 mg, 0.557 mmol),
[Pd(PPh3)4] (12.9 mg, 11.2 μmol, 2 mol %), copper chloride (165 mg,
1.67 mmol, 3 equiv), and lithium chloride (142 mg, 3.34 mmol, 6
equiv) were dissolved in DMF (15 mL). The reaction mixture was
heated to 70 °C for 14 h. After the reaction mixture had cooled to 20
°C, chloroform (40 mL) was added, and the mixture was extracted
with water (3 × 150 mL). The combined organic phases were dried
over sodium sulfate. The product was purified by column
chromatography with toluene as eluent. The solvent was evaporated,
1
and an orange solid (185 mg, 84%) was obtained. Mp: 161 °C. H
NMR (500 MHz, CD2Cl2): 7.98 (d, 3J = 8.8 Hz, 2 H, H-7, 7′), 7.93 (d,
3J = 8.8 Hz, 2 H, H-6, 6′), 7.85 (d, 3J = 8.3 Hz, 2 H, H-3, 3′), 7.35 (d,
3J = 8.3 Hz, 2 H, H 2, 2′), 7.28 (d, 3J = 3.6 Hz, 1 H, H-11), 6.90 (d, 3J
= 3.6 Hz, 1 H, H-10), 3.91 (s, 3 H, H-14), 2.45 (s, 3 H, H-15) ppm.
13C NMR (125 MHz, CD2Cl2): 159.3 (C-13), 156.98 (C-12), 152.9
(C-8), 151.2 (C-4), 144.7 (C-9), 142.6 (C-1), 131.8 (C-5), 130.2 (C-2,
2′), 125.8 (C-7, 7′), 123.7 (C-6, 6′), 123.4 (C-3, 3′), 120.3 (C-11),
108.6 (C-10), 52.2 (C-14), 21.6 (C-15) ppm. IR (ATR): 3403 (w),
3243 (w), 3122 (w), 3034 (w), 2954 (m), 2848 (2), 1703 (s), 1527
(m), 1299 (s) 1133 (s), 1027 (m), 992 (m), 923 (m), 808 (s), 759 (s),
553 (s) cm−1. HRMS (ESI-FTMS) m/z: [M + Na]+ calcd for
[C19H16N2O3 + Na]+ 343.1053, found 343.1062.
1
3
obtained. Mp: 198 °C. H NMR (500 MHz, CD2Cl2): 7.99 (d, J =
8.7 Hz, 2 H, H-6, 6′), 7.86 (d, 3J = 8.1 Hz, 2 H, H-3, 3′), 7.78 (d, 3J =
8.7 Hz, 2 H, H-7, 7′), 7.70 (dd, J = 8.5, 1.2 Hz, 2 H, H-10, 10′), 7.49
3
(t, J = 8.5 Hz, 2 H, H-11, 11′), 7.42−7.38 (m, 1 H, H-12), 7.36 (d,
3J = 8.1 Hz, 2 H, H-2, 2′), 2,45 (s, 3 H, H-15) ppm. 13C NMR (125
MHz, CD2Cl2): 152.2 (C-5), 151.24 (C-4), 143.8 (C-8), 142.3 (C-1),
140.5 (C-9), 130.2 (C-2, 2′), 129.3 (C-11, 11′), 128.3 (C-12), 128.1
(C-7, 7′), 127.5 (C-10, 10′), 123.6 (C-6, 6′), 123.2 (C-3, 3′), 21.6
(C-13) ppm. IR (ATR): 3023.7 (w), 2914 (w), 1599 (m), 1483 (m),
1405 (m), 1157 (m) 847 (s), 763 (s), 687 (s) cm−1. HRMS (CI-
sector) m/z: [M + H]+ Calcd for [C19H16N2 + H]+ 273.1392; Found
273.1391.
4-(3-Methoxyphenyl-) 4′-methylazobenzene (31).
4-(2-Furanyl-5-aldehyde)-4′-methylazobenzene (25).
Method A. 4-Methyl-4′-(trimethylstannyl)azobenzene (200 mg,
0.557 mmol), 1-bromo-5-methoxybenzene (104 mg, 0.557 mmol),
[Pd(PPh3)4] (12.9 mg, 11.2 μmol, 2 mol %), copper chloride (165 mg,
1.67 mmol, 3 equiv), and lithium chloride (142 mg, 3.34 mmol,
6 equiv) were dissolved in DMF (15 mL). The reaction mixture was
heated to 70 °C for 19 h. After the reaction mixture had cooled to
20 °C, chloroform (80 mL) was added, and the mixture was extracted
with 2 M hydrochloric acid (2 × 150 mL), a saturated sodium
carbonate solution (1 × 150 mL), and water (1 × 150 mL). The
combined organic phases were dried over sodium sulfate. The solvent
was evaporated, and the crude product was purified by column chro-
matography with toluene as solvent. An orange solid (133 mg, 79%)
was obtained.
4-Methyl-4′-(trimethylstannyl)azobenzene (200 mg, 0.557 mmol), 2-
bromo-5-furaldehyde (97.5 mg, 0.557 mmol), [Pd(PPh3)4] (12.9 mg,
11.2 μmol, 2 mol %), copper chloride (165 mg, 1.67 mmol, 3 equiv),
and lithium chloride (142 mg, 3.34 mmol, 6 equiv) were dissolved in
DMF (15 mL). The reaction mixture was heated to 70 °C for 16 h.
After the reaction mixture had cooled to 20 °C, chloroform (80 mL)
was added, and the mixture was extracted with 2 M hydrochloric acid
(2 × 150 mL), a saturated sodium carbonate solution (1 × 150 mL),
and water (1 × 150 mL). The combined organic phases were dried
over sodium sulfate. The product was purified by column chro-
matography with DCM as eluent. The solvent was evaporated, and an
orange solid (155 mg, 73%) was obtained. Mp: 163 °C. 1H NMR (500
MHz, CD2Cl2): 9.67 (s, 1 H, H-13) 7.98 (m, 4 H, H-6,6′,7,7′), 7.85
(d, 3J = 8.2 Hz, 2 H,H- 3,3′), 7.38−7.34 (m, 3 H, 2, 2′, H-11), 6.99 (d,
3J = 3.7 Hz, 1 H, H-12), 2.45 (s, 3 H, H-14) ppm. 13C NMR (125
MHz, CD2Cl2): 177.3 (C-13), 158.4 (C-9), 153.0 (C-12), 152.7 (C-5),
150.9 (C-4), 142.5 (C-1), 131.0 (C-8), 129.0 (C-2, 2′), 127.9 (C-11),
126.0 (C-7, 7′), 123.5 (C-6, 6′), 123.1 (C-3, 3′), 109.1 (C-10), 21.4
(C-15) ppm. IR (ATR): 3137 (w), 3057 (w), 2922 (w), 2852 (w),
1683 (s), 1477 (s), 1257 (s), 1151 (s), 1104 (s), 1040 (s), 966 (s), 815
(s), 546 (m), 529 (m) cm−1. HRMS (ESI-FTMS) m/z: [M + Na]+
calcd for [C18H14N2O2 + Na]+ 313.0947, found 313.0956.
Method B. 4-Methyl-4′-(tri-n-butylstannyl)azobenzene (270 mg,
0.557 mmol), 1-bromo-5-methoxybenzene (104 mg, 0.557 mmol),
[Pd(PPh3)4] (12.9 mg, 11.2 μmol, 2 mol %), copper chloride (165 mg,
1.67 mmol, 3 equiv), and lithium chloride (142 mg, 3.34 mmol, 6
equiv) were dissolved in DMF (15 mL). The reaction mixture was
heated to 70 °C for 25 h. After the reaction mixture had cooled to
20 °C, chloroform (80 mL) was added, and the mixture was extracted
with 2 M hydrochloric acid (2 × 150 mL), a saturated sodium
carbonate solution (1 × 150 mL), and water (1 × 150 mL). The
combined organic phases were dried over sodium sulfate. The solvent
was evaporated. The crude product was purified by Kugelrohr dis-
tillation (150 °C, 9.74 × 10−2 mbar) for 30 min. The residue was puri-
fied by column chromatography. An orange solid (91 mg, 54%) was
1
3
obtained. Mp: 81 °C. H NMR (500 MHz, CD2Cl2): 7.99 (d, J =
8.7 Hz, 2 H, H-6, 6′), 7.87 (d, 3J = 8.1 Hz, 2 H, H - 3, 3′), 7.78 (d, 3J =
8.7 Hz, 1 H, H-7, 7′), 7.40 (t, 3J = 7.9 Hz, 1 H, H-13), 7.36 (d, 3J = 8.1 Hz,
1725
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2 H, H-2, 2′), 7.28 (ddd, J = 8.2, 2.6, 0.9 Hz, 1 H, H-14), 7.22 (dd, J =
2.6 Hz, 0.9 Hz, 1 H, H-10), 6.95 (ddd, J = 8.2, 2.6, 0.9 Hz, 1 H, H-12),
3.88 (s, 3 H, H-15), 2.46 (s, 3 H, H-16) ppm. 13C NMR (125 MHz,
CD2Cl2): 160.6 (C-11), 152.3 (C-5), 151.3 (C-4), 143.7 (C-8), 142.7
(C-1), 142.0 (C-9), 130.3 (C-10), 130.2 (C-13), 128.2 (C-7, 7′), 123.6
(C-6, 6′), 123.2 (C-3, 3′), 120.0 (C-2,2′), 113.7 (C-12), 113.2 (C-10),
55.7 (C-15), 21.6 (C-16) ppm. IR (ATR): 3080 (w), 3009 (w), 2947
(m), 2844 (m), 1925 (w), 1598 (s), 1583 (s), 1480 (s), 1295 (s), 1213
(s), 1152 (s), 1023 (s), 846 (s), 836 (s), 823 (s) 780 (s), 736 (m), 692
(s)545 (s) cm−1. HRMS (CI-sector) m/z: [M + H]+ calcd for
[C20H18N2O + H]+ 303.1497, found 303.1500.
108.7 (C-10), 107.5 (C-14), 101.7 (C-15), 21.4 (C-16) ppm. IR
(ATR): 3022 (w), 2912 (w), 2859 (w), 2720 (w), 1920 (w), 1696
(m), 1477 (m), 1242 (m), 1307 (m), 823 (s), 731 (m), 547 (m) cm−1.
HRMS (CI-sector) m/z: [M + H]+ calcd for [C20H16N2O2 + H]+
317.1290, found 317.1291.
4-(2-Thiophene-yl)-4′-methylazobenzene (35).
4-(2,5-Methoxyphenyl)-4′-methylazobenzene (32).
4-Methyl-4′-(trimethylstannyl)azobenzene (200 mg, 0.557 mmol),
2-bromothiophene (90.8, 0.557 mmol), [Pd(PPh3)4] (12.9 mg,
11.2 μmol, 2 mol %), copper chloride (165 mg, 1.67 mmol, 3 equiv),
and lithium chloride (142 mg, 3.34 mmol, 6 equiv) were dissolved in
DMF (15 mL). The reaction mixture was heated to 70 °C for 11 h.
After the reaction mixture had cooled to 20 °C, chloroform (80 mL)
was added, and the mixture was extracted with 2 M hydrochloric acid
(2 × 150 mL), a saturated sodium carbonate solution (1 × 150 mL),
and water (1 × 150 mL). The combined organic phases were dried
over sodium sulfate. The crude product was dissolved in DCM and
filtered through a short plug of silica. The solvent was evaporated and
dried at 5 × 10−2 mbar for 19 h. An orange solid (173 mg, 88%) was
4-Methyl-4′-(trimethylstannyl)azobenzene (200 mg, 0.557 mmol), 1-
bromo-2,5-dimethoxybenzene (120 mg, 0.557 mmol), [Pd(PPh3)4]
(12.9 mg, 11.2 μmol, 2 mol %), copper chloride (165 mg, 1.67 mmol,
3 equiv), and lithium chloride (142 mg, 3.34 mmol, 6 equiv) were
dissolved in DMF (15 mL). The reaction mixture was heated to 70 °C
for 26 h. After the reaction mixture had cooled to 20 °C, chloroform
(80 mL) was added, and the mixture was extracted with
2 M hydrochloric acid (2 × 150 mL), a saturated sodium carbonate
solution (1 × 150 mL), and water (1 × 150 mL). The combined
organic phases were dried over sodium sulfate. The crude product was
purified by column chromatography with toluene as solvent. The sol-
vent was evaporated, and an orange solid (137 mg, 74%) was obtained.
Mp: 74 °C. 1H NMR (500 MHz, CD2Cl2): 7.94 (d, 3J = 8.7 Hz, 2 H,
H-6, 6′), 7.86 (d, 3J = 8.3 Hz, 2 H, H-3, 3′), 7.70 (d, 3J = 8.7 Hz, 2 H,
H-7, 7′), 7.36 (d, 3J = 8.3 Hz, 2 H, H-2, 2′), 6.98 (d, 3J = 8.9 Hz, 1 H,
H-11), 6.97 (d, 4J = 2.9 Hz, 1 H, H-14), 6.91 (dd, J = 8.9, 3.2 Hz, 1 H,
H-12), 3.82 (s, 3 H, H-15), 3.79 (s, 3 H, H-16), 2.46 (s, 3 H, H-17)
ppm. 13C NMR (125 MHz, CD2Cl2): 154.7 (C-10), 152.3 (C-5), 151.3
(C-13), 151.2 (C-4), 142.6 (C-1), 142.0 (C-8), 131.1 (C-7, 7′), 130.6
(C-2, 2′), 130.2 (C-9), 123.6 (C-3, 3′), 123.1 (C-6, 6′), 117.3 (C-14),
114.5 (C-12), 113.6 (C-11), 57.0 (C-15), 56.6 (C-16), 22.0 (C-17)
ppm. IR (ATR): 3093 (w), 3001 (w), 2954 (m), 2830 (m), 1593 (m),
1488 (s), 1459 (s), 1296 (m), 1208 (s), 1178 (s), 1023 (s), 1010 (s),
852 (s), 811 (s), 727 (s). HRMS (CI-sector) m/z: [M + H]+ calcd for
[C21H20N2O2 + H]+ 333.1603, found 333.1604.
1
obtained. Mp: 153 °C. H NMR (500 MHz, CD2Cl2): 7.93 (d, J =
8.7 Hz, 2 H, H-6,6′), 7.84 (d, J = 8.2 Hz, 1H, H-3, 3′), 7.78 (d, J =
8.7 Hz, 2 H, H-7, 7′), 7.46 (dd, J = 3.6, 1.1 Hz, 1 H, H-10), 7.38 (dd,
J = 5.1, 1.1 Hz, 1 H, H-12), 7.35 (d, J = 8.2 Hz, 1H, H- 2,2′), 7.14 (dd,
J = 5.1, 3.6 Hz, 1 H, H-11), 2.45 (s, 3 H, H-13) ppm. 13C NMR (125
MHz, CD2Cl2): 152.1 (C-5), 151.2 (C-4), 143.8 (C-9), 142.3 (C-1),
137.0 (C-8), 130.2 (C-2), 128.8 (C-11), 126.7 (C-7), 126.3 (C-12),
124.5 (C-10), 123.8 (C-6), 123.2 (C-3), 21.6 (C-13) ppm. IR (ATR):
3104 (w), 3027 (w), 2914 (m), 2856 (m), 1928 (w), 1735 (m), 1598
(m), 1157 (m), 1110 (m), 1011 (m), 846 (s), 824 (s), 780 (s), 728
(s), 690 (m), 531 (s) cm−1. HRMS (CI-sector) m/z: [M + H]+ calcd
for [C17H14N2S + H]+ 279.0956, found 279.0953.
4-(3-Thiophene-yl)-4′-methylazobenzene (37).
4-(5-1,3-Benzodioxolyl)-4′-methylazobenzene (33).
4-Methyl-4′-(trimethylstannyl)azobenzene (200 mg, 0.557 mmol), 3-
bromothiophene (90.8 mg, 0.557 mmol), [Pd(PPh3)4] (12.9 mg,
11.2 μmol, 2 mol %), copper chloride (165 mg, 1.67 mmol, 3 equiv), and
lithium chloride (142 mg, 3.34 mmol, 6 equiv) were dissolved in DMF
(15 mL). The reaction mixture was heated to 70 °C for 14 h. After the
reaction mixture had cooled to 20 °C, chloroform (80 mL) was added,
and the mixture was extracted with 2 M hydrochloric acid (2 × 150
mL), a saturated sodium carbonate solution (1 × 150 mL), and water
(1 × 150 mL). The combined organic phases were dried over sodium
sulfate. The crude product was dissolved in DCM (5 mL) and filtered
through a short plug of silica. The solvent was evaporated, and the
product was dried at 5 × 10−2 mbar for 21 h. An orange solid (164 mg,
83%) was obtained. Mp: 178 °C. 1H NMR (500 MHz, CD2Cl2): 7.95
(d, J = 8.7 Hz, 2 H, H-6, 6′), 7.84 (d, J = 8.1 Hz, 2 H, H-3, 3′), 7.78 (d,
J = 8.7 Hz, 2 H, H-7, 7′), 7.62 (dd, J = 2.9, 1.4 Hz, 1 H, H-10), 7.50
(dd, J = 5.0, 1.4 Hz, 1 H, H-11), 7.46 (dd, J = 5.0, 2.9 Hz, 1 H, H-12),
7.35 (d, J = 8.1 Hz, 2 H, H-2, 2′), 2.45 (s, 3 H, H-13). 13C NMR (125
MHz, CD2Cl2): 152.0 (C-5), 151.2 (C-4), 142.2 (C-1), 141.8 (C-9),
138.4 (C-8), 130.2 (C-2), 127.3 (C-7), 127.0 (C-12), 126.6 (C-11),
123.7 (C-6), 123.1 (C-3), 121.8 (C-10), 21.6 (C-13) ppm. IR (ATR):
3104 (w), 3027 (w), 2914 (m), 2856 (m), 1928 (w), 1735 (m), 1598
(m), 1157 (m), 1110 (m), 1011 (m), 846 (s), 824 (s), 780 (s), 728
4-Methyl-4′-(trimethylstannyl)azobenzene (200 mg, 0.557 mmol),
5-bromo-1,3-benzodioxole (112 mg, 0.557 mmol), [Pd(PPh3)4]
(12.9 mg, 0.011 mmol, 2 mol %), copper chloride (165 mg, 1.67 mmol,
3 equiv), and lithium chloride (142 mg, 3.34 mmol, 6 equiv) were
dissolved in DMF (15 mL). The reaction mixture was heated to 70 °C
for 24 h. After the reaction mixture had cooled to 20 °C, chloroform
(80 mL) was added, and the mixture was extracted with water (3 ×
150 mL). The combined organic phases were dried over sodium
sulfate. The crude product was purified by column chromatography
with toluene as solvent. A red solid (123 mg, 70%) was obtained. Mp:
1
3
144 °C. H NMR (500 MHz, CD2Cl2): 7.95 (d, J = 8.5 Hz, 2 H,
6,6′), 7.84 (d, 3J = 8.2 Hz, 2 H, 3,3′), 7.69 (d, 3J = 8.5 Hz, 2 H, 7,7′),
7.35 (d, 3J = 8.2 Hz, 2 H, 2,2′), 7.19−7.16 (m, 2 H, H-11, 14), 6.84 (d,
3J = 7.8 Hz, 1 H, H-10), 6.03 (s, 2 H, H-15), 2.45 (s, 3 H, H-16) ppm.
13C NMR (125 MHz, CD2Cl2): 151.7 (C-5), 151.0 (C-4), 148.5 (C-
12), 147.9 (C-13), 143.3 (C-8), 141.9 (C-1), 134.5 (C-9), 129.9 (C-2,
2′), 127.5 (C-7, 7′), 123.3 (C-3, 3′), 122.9 (C-6, 6′), 121.0 (C-11),
(s), 690 (m), 531 (s) cm−1
. HRMS (CI-sector) m/z:
[M + H]+ calcd for [C17H14N2S + H]+ 279.0956, found 279.0951.
1726
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The Journal of Organic Chemistry
Article
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4-Methyl-4′-(trimethylstannyl)azobenzene (200 mg, 0.557 mmol), 4-
bromo-N,N-dimethylaniline (111 mg, 0.557 mmol), [Pd(PPh3)4]
(12.9 mg, 11.2 μmol, 2 mol %), copper chloride (165 mg, 1.67 mmol,
3 equiv), and lithium chloride (142 mg, 3.34 mmol, 6 equiv) were dis-
solved in DMF (15 mL). The reaction mixture was heated to 70 °C for
19 h. After the reaction mixture had cooled to 20 °C, chloroform
(80 mL) was added, and the mixture was extracted with 2 M hydrochloric
acid (2 × 150 mL), a saturated sodium carbonate solution (1 × 150 mL),
and water (1 × 150 mL). The combined organic phases were dried over
sodium sulfate. The solvent was evaporated, and the crude product was
purified by column chromatography with toluene as solvent. An orange
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solid (126 mg, 72%) was obtained. Mp: 124 °C. H NMR (500 MHz,
CD2Cl2): 7.93 (d, 3J = 8.6 Hz, 2 H, H-6, 6′), 7.83 (d, 3J = 8.3 Hz, 2 H,
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3
3
H-3, 3′), 7.73 (d, J = 8.6 Hz, 1 H, H-7, 7′), 7.60 (d, J = 8.9 Hz, 2 H,
H-10, 10′), 7.36 (d, 3J = 8.0 Hz, 2 H, H-2, 2′), 7.36 (d, 3J = 8.9 Hz, 2 H,
H-11, 11′), 3.01 (s, 6 H, H-13), 2.44 (s, 3 H, H-14) ppm. 13C NMR
(125 MHz, CD2Cl2):2 151.3 (C-5), 151.2 (C-4), 150.9 (C-12), 143.9
(C-8), 141.9 (C-1), 130.1 (C-2,2′), 129.7 (C-9), 128.0 (C-7,7′), 126.7
(C-10,10′), 123.6 (C-6,6′), 123.0 (C-7,7′), 112.9 (C-11,11′), 40.6
(C-13,13′), 21.6 (C-14) ppm. IR (ATR): 3031 (w), 2921 (w), 2852 (w),
2801 (w), 1591 (m), 1537 (m), 1492 (m), 1364 (m), 1286 (m), 1210
(m), 1153 (m), 950 (m), 826 (s), 812 (s) 716 (m) cm−1. HRMS (CI-
sector) m/z: [M + H]+ calcd for [C21H21N3 + H]+ 316.1814, found
316.1809.
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H.; Shiraga, T.; Kimura, T.; Kato, M.; Viswanathan, N. K.; Kumar, J.;
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ASSOCIATED CONTENT
■
́
(10) (a) Bleger, D.; Yu, Z.; Hecht, S. Chem. Commun. 2011, 47,
S
* Supporting Information
12260. (b) Thies, S.; Sell, H.; Schuett, C.; Bornholdt, C.; Naether, C.;
Tuczek, F.; Herges, R. J. Am. Chem. Soc. 2011, 133, 16243.
(c) Bushuyev, O. S.; Singleton, T. A.; Barrett, C. J. Adv. Mater.
2013, 25, 1796.
GC calibration, purities of the compounds used, drying pro-
1
cedures for the solvents, and H and 13C NMR spectra for all
compounds This material is available free of charge via the
Internet at http://pubs.acs.org.
(11) See, for example: (a) Nguyen, T.-T.-T.; Turp, D.; Wang, D.;
̈
Nolscher, B.; Laquai, F.; Mullen, K. J. Am. Chem. Soc. 2011, 133,
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̈
AUTHOR INFORMATION
Corresponding Author
*E-mail: astaubitz@oc.uni-kiel.de.
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■
Notes
The authors declare no competing financial interest.
ACKNOWLEDGMENTS
■
This project was supported by the Special Research Area 677
“Function by Switching” of the Deutsche Forschungsgemein-
schaft (DFG), Project C10.
(14) Kozlecki, T.; Syper, L.; Wilk, K. A. Synthesis 1997, 681.
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