Pd-catalyzed decarboxylative Heck vinylation of 2-nitrobenzoates in the presence of CuF2

Article abstract of DOI:10.3762/bjoc.6.43

A new protocol for the decarboxylative Heck vinylation of benzoic acids is disclosed. In the presence of a catalyst system generated in situ from Pd(OAc)₂ (2 mol %), CuF₂ (2 equiv), and benzoquinone (0.5 equiv) in NMP, a wide range of olefins were coupled with various 2-nitrobenzoates at 130 °C with the release of carbon dioxide to afford the corresponding vinyl arenes in good yields.

Full text of DOI:10.3762/bjoc.6.43

Pd-catalyzed decarboxylative Heck vinylation of
2-nitrobenzoates in the presence of CuF2
Lukas J. Gooßen*, Bettina Zimmermann and Thomas Knauber
Full Research Paper
Open Access
Address:
Beilstein J. Org. Chem. 2010, 6, No. 43.
Department of Chemistry, Organic Chemistry, Technische Universität
Kaiserslautern, Erwin-Schrödinger-Strasse, Geb. 54, D-67663
Kaiserslautern, Germany
doi:10.3762/bjoc.6.43
Received: 22 February 2010
Accepted: 10 April 2010
Published: 03 May 2010
Email:
Lukas J. Gooßen* - goossen@chemie.uni-kl.de
Associate Editor: J. A. Porco Jr.
* Corresponding author
© 2010 Gooßen et al; licensee Beilstein-Institut.
License and terms: see end of document.
Keywords:
carboxylic acids; copper; decarboxylation; Heck vinylation; palladium
Abstract
A new protocol for the decarboxylative Heck vinylation of benzoic acids is disclosed. In the presence of a catalyst system gener-
ated in situ from Pd(OAc)2 (2 mol %), CuF2 (2 equiv), and benzoquinone (0.5 equiv) in NMP, a wide range of olefins were coupled
with various 2-nitrobenzoates at 130 °C with the release of carbon dioxide to afford the corresponding vinyl arenes in good yields.
Introduction
The palladium-catalyzed coupling of olefins with aryl or vinyl reduce the problematic salt load of these transformations. In this
substrates, developed by Mizoroki [1] and Heck [2,3] in the context, De Vries developed decarbonylative Heck reactions of
1970s, is one of the most important, reliable and generally benzoic homoanhydrides – a salt-free process for Heck vinyl-
applicable reactions for carbon–carbon bond formation in ations, which however, requires recycling of the arene-
organic synthesis. Over the last decade, several highly active carboxylate leaving group [17,18]. We developed a process in
catalyst systems have been developed for these reactions [4,5]. which benzoic acids are activated by esterification with elec-
Usually, aryl halides are employed as starting materials [6-9], tron-poor phenols with the loss of water [19]. The esters are
but other aryl sources such as aryl triflates [10-12], diazonium then subjected to a decarbonylative Heck reaction to yield vinyl
salts [13,14], arylsulfonyl halides [15] and aroyl chlorides [16] arenes, CO and the phenols, which can be used in further reac-
have also been used. Several protocols were recently described tion cycles. In a related approach, the benzoic acids activated as
that draw on widely available carboxylic acid derivatives as isopropenyl esters undergo a decarbonylative Heck reaction that
alternative aryl sources. This not only extended the substrate yields the vinyl arenes along with CO and acetone [20]. Since
basis of Heck reactions, but also opened up opportunities to no base is added, only volatile by-products are generated
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Beilstein J. Org. Chem. 2010, 6, No. 43.
instead of waste salts, and the amount of solvent can be minim- employed as oxidant [24]. However, their protocol is strictly
ized. Myers and co-workers introduced another version of the limited to highly activated benzoates. We herein disclose an
Heck reaction in 2002, in which they converted mostly electron- alternative protocol for the Pd-catalyzed decarboxylative Heck
rich aromatic carboxylic acids and alkenes to vinyl arenes with reaction, in which copper(II) fluoride is utilized as the oxidant.
the release of CO2 [21-23]. This decarboxylative Heck reaction This way, various nitrobenzoic acids are successfully coupled
allows the direct conversion of benzoic acids without prior with several olefins to give the corresponding vinyl arenes in
activation, but requires the addition of stoichiometric amounts moderate to good yields.
of both base and an oxidant. In the original protocol, an excess
of silver carbonate (3 equiv) was added to fulfil these functions. Results and Discussion
The work of Myers represents a milestone in the development In our search for a silver-free catalyst system for decarboxy-
of palladium-catalyzed decarboxylative coupling reactions. lative Heck reactions, we chose the conversion of 2-nitroben-
However, his original protocol suffers from several drawbacks, zoic acid (1a) with styrene (2a) to yield 2-nitro-stilbene (3aa) as
including the relatively narrow substrate scope, the high load- a model reaction, and investigated the catalytic activity of
ings of the Pd-catalyst, and the use of silver salts in over stoi- various combinations of palladium precursors, additives and
chiometric quantities. Very recently, Su published an improved oxidants (Table 1). The choice of an electron-deficient benzoic
procedure for this transformation, in which p-benzoquinone is acid was motivated by our intention to overcome substrate
Table 1: Reaction of 2-nitrobenzoic acid with styrene.
Entry Pd.-cat. (mol %)
Oxidant (mmol)
Additive
Solvent
3aa [%]a
4a [%]a
1
2
Pd(acac-F6)2 (5 mol %)
Ag2CO3 (1.5 mmol)
-
DMF/DMSO
52
45
14
57
0
29
31
24
22
56
0
"
Ag2CO3 (1.0 mmol)
-
"
3
"
Ag2CO3 (0.5 mmol)
-
"
4
Pd(OAc)2 (2 mol %)
Ag2CO3 (1.0 mmol)
-
"
5
-
Ag2CO3 (0.05 mmol)
-
"
6
Pd(OAc)2 (10 mol %)
-
-
"
0
7
Pd(CF3CO2)2 (5 mol %)
-
-
"
0
0
8
Pd(OAc)2 (2 mol %)
BQ (2.0 mmol)
-
"
0
0
9
"
"
"
"
"
"
"
"
"
"
"
"
Cu2O (1.0 mmol)
-
"
0
96
65
48
30
65
16
17
18
17
17
19
8
10
11
12b
13b
14b
15b
16e
17e
18e
19e
20e
Cu2CO3 (1.0 mmol)
-
"
0
CuF2 (2.0 mmol)
-
"
27
40
13c
44
46
60
59
67
65
89
"
-
"
CuF2 (0.5 mmol)
-
"
CuF2 (2.0 mmol)
3 Å MSd
"
"
"
NMP
"
" / phen.
"
"
"
"
"
"
" / bipy.
"
" / tan.
" / NQ (0.5 mmol)
" / BQ (0.5 mmol)
"
"
Conditions: 2-Nitrobenzoic acid (1.00 mmol), styrene (1.50 mmol), Pd-cat., oxidant, solvent (3 mL, 5% of DMSO), 20 h, 120 °C.
aYields were determined by GC analysis, with n-tetradecane as internal standard;
bpotassium 2-nitrobenzoate (1.00 mmol);
calong with 13% 2,2′-dinitrobiphenyl;
d3 Å MS (500 mg);
epotassium 2-nitrobenzoate (1.00 mmol), styrene (1.50 mmol), N-ligand (4 mol %). MS = molecular sieves, phen. = 1,10-phenanthroline, bipy. = 2,2′-
bipyridine, tan. = 1,4,5-triazanaphthalene, NQ = 1,4-naphthoquinone, BQ = p-benzoquinone.
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Beilstein J. Org. Chem. 2010, 6, No. 43.
limitations observed in Myers’ original protocol, which can be Nilsson and Cohen in protodecarboxylation reactions [34-37].
understood on the basis of the proposed reaction mechanism. In Thus, we investigated several copper bases in place of silver
his process, decarboxylation takes place at the palladium, and carbonate. With copper(I) oxide and copper(II) carbonate,
the role of the silver is restricted to acting as an oxidizing agent. nitrobenzene (4a) was formed predominantly, while the desired
Palladium has been found to be an effective mediator in the Heck product was observed only in trace quantities (entries 9
protodecarboxylation of particularly electron-rich, ortho-substi- and 10). When copper(II) fluoride was used, we obtained 27%
tuted benzoic acids, e.g. 2,6-dimethoxybenzoic acid, whereas of the product along with 48% of the undesired protode-
electron-poor derivatives are not easily decarboxylated. The carboxylation product (entry 11).
palladium-catalyzed protodecarboxylation reported by
Kozlowski [25], and the palladium-catalyzed decarboxylative Even after careful exclusion of moisture and the use of
cross-couplings described by Forgione, Bilodeau and Liu preformed potassium 2-nitrobenzoate, the yields remained
[26,27] are limited to particularly activated, mostly 2,6-disubsti- unsatisfactory (41%) and the protodecarboxylation could not
tuted or heterocyclic derivatives. We conclude from the fact that effectively be suppressed (entry 12). Another unwanted
Myers’ protocol – in contrast to the silver-free version by Su – byproduct, i.e. 2,2′-dinitrobiphenyl, appeared when reducing the
has a somewhat broader scope, that it allows for an additional quantity of the copper fluoride to less than 1 equivalent, which
catalytic cycle in which the decarboxylation occurs at the silver results from homocoupling of the arylcopper intermediate
cocatalyst. Our desired decarboxylative Heck reaction of non- (entry 13).
activated benzoates would have to proceed via this alternative
catalytic cycle.
The selectivity for the vinyl arene product was improved by
adding molecular sieves to the reaction medium (entry 14). We
We started our search for an active catalyst using the system were now able to switch from a DMF/DMSO mixture to the
employed by Myers, and observed the formation of the desired less toxic solvent N-methyl-2-pyrrolidone (NMP) without a
product, albeit in the expected low yields. After some optimiz- decrease in yields (entry 15). Next, we investigated the influ-
ation, we were able to convert the electron-poor 2-nitrostilbene ence of ligands on the performance of the catalyst system, and
(3aa) in high yields even when lowering the palladium catalyst discovered that phosphines impede the decarboxylation step. In
loading from 20 mol % to 5 mol %, and the amount of silver contrast, nitrogen-containing ligands had a beneficial effect
carbonate from 3 to 1.5 equivalents (entry 1). A further reduc- (entries 16–18). In the presence of 1,4,5-triazanaphthalene, the
tion to 1 or even 0.5 equivalents led to decreased yields (entries Heck product was formed in 67% yield (entry 18). We
2 and 3). We next investigated several palladium sources and subsequently tested several oxidants in an attempt to prevent the
found that palladium acetate gave yields comparable to the formation of palladium(0) species, which we assumed to be
substantially more expensive palladium hexafluoroacetyl- catalytically inactive in this process. This proved to be a good
acetonate (entry 4). In all reactions, nitrobenzene (4a) was strategy, as the addition of p-benzoquinone resulted in higher
formed as a byproduct. We assumed that it arises from a yields of the Heck product, while at the same time suppressing
competing silver-catalyzed protodecarboxylation as also the formation of the undesired nitrobenzene (4a) (entries 19 and
observed in decarboxylative cross-coupling reactions [28-33]. 20). Thus, under optimized conditions, potassium 2-nitro-
Control experiments confirmed that under these reaction condi- benzoate was decarboxylated and coupled with styrene in the
tions, 2-nitrobenzoic acid does not protodecarboxylate in the presence of 2 mol % palladium acetate, 4 mol % 1,4,5-
presence of palladium alone with or without an oxidant present, triazanaphthalene, 2 equivalents of copper fluoride, 0.5 equiva-
and that this side reaction is mediated by silver, irrespective of lents of p-benzoquinone, and excess molecular sieves in NMP
the presence of palladium (entries 5–8). In contrast, 2,6-di- at 120 °C, yielding the vinyl arene 3aa in 89%.
methoxybenzoic acid can be decarboxylated both by 5 mol %
palladium trifluoroacetate and by 5 mol % silver salts yielding Based on our mechanistic studies of this and other decarboxy-
2,6-dimethoxybenzene in 75% and 99% yield, respectively. lative couplings, we propose the mechanism detailed in
This finding further supports our theory that the catalyst cycle Scheme 1. In the first step, the benzoate decarboxylates with
outlined by Myers for electron-rich benzoic acids, in which formation of an aryl metal species. In contrast to Myers’ mech-
silver(I) acts as only an oxidant and is not involved in the anism, which has been confirmed for electron-rich substrates
decarboxylation step, may not hold true for electron-deficient [38], this step proceeds at a silver or copper mediator rather
benzoic acids. Instead, an additional decarboxylation catalyst is than at the palladium center. The resulting aryl residue is trans-
required to convert these substrates into the corresponding aryl ferred to a Pd(II) center with formation of an arylpalladium
metal species. Besides silver(I), we expected that copper salts species. Insertion of the olefin furnishes an alkylpalladium
should also mediate this step, as this metal has been used by species which, after internal rotation, liberates the vinyl arene
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Beilstein J. Org. Chem. 2010, 6, No. 43.
Scheme 1: Proposed mechanism for the decarboxylative Heck reaction.
product via β-hydride elimination. The resulting palladium cases, the reaction proceeded with high selectivities for the (E)-
hydride species is converted back to the initial Pd(II) catalyst by configured linear products. Other isomers were detected in trace
reaction with the copper.
amounts only.
We next investigated the scope of the new protocol with regard Next, we examined the substrate range with regard to the
to the olefin substrate. As shown in Table 2, styrenes, acrylate carboxylate substrate and found that the new protocol is particu-
esters, and even acrylamides were coupled in good yields. In all larly suitable for 2-nitro-substituted carboxylates (Table 3).
Table 2: Variation of the olefin.
Entry
Olefin
Product
Yield [%]a
1
90
2
87
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Beilstein J. Org. Chem. 2010, 6, No. 43.
Table 2: Variation of the olefin. (continued)
3
81
86
88
85
71
4
5
6
7
Conditions: Potassium 2-nitrobenzoate (1.50 mmol), olefin (1.00 mmol), copper(II) fluoride (2.00 mmol), palladium(II) acetate (0.02 mmol), 1,4,5-
triazanaphthalene (0.04 mmol), p-benzoquinone (0.50 mmol), 3 Å molecular sieves (350 mg), NMP, 20 h, 130 °C;
aisolated yields.
Various compounds of this type were converted to stilbenes in substrates with 2-methoxy or 2-fluoro substituents, which are
good yields. In contrast, arenecarboxylates with a nitro-group in highly reactive in other decarboxylative couplings mediated by
meta- or para-position gave unsatisfactory results. Interestingly, palladium alone, gave low yields in this process. Taking into
Table 3: Variation of the benzoic carboxylate derivative.
Entry
Carboxylate
Product
Yield [%]a
1
40
2
90
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Beilstein J. Org. Chem. 2010, 6, No. 43.
Table 3: Variation of the benzoic carboxylate derivative. (continued)
3
4
5
85
93
44
6
0
7
8
(7)
(2)
Conditions: Potassium carboxylate 1a–i (1.50 mmol), styrene (1.00 mmol) copper(II) fluoride (2.00 mmol), palladium(II) acetate (0.02 mmol), 1,4,5-
triazanaphthalene (4 mol %), p-benzoquinone (0.50 mmol), 3 Å molecular sieves (350 mg), NMP, 20 h, 130 °C.
aIsolated yields; yields in parentheses are GC-yields.
account that 2-nitrobenzoates are among the best substrates for the remainder of the catalytic cycle becomes rather delicate.
copper-mediated decarboxylation processes, this finding further Current work is directed toward the development of new cata-
supports our hypothesis that the decarboxylation takes place at lyst systems that would allow the coupling of a broader range of
the copper (entry 7 and 8).
aromatic carboxylic acids.
Experimental
Conclusion
In conclusion, we were able to show that decarboxylative Heck General remarks
reactions can also be performed with carboxylic acids that Reactions were performed in oven-dried glassware under a
cannot be decarboxylated at a palladium catalyst. The coupling nitrogen atmosphere containing a Teflon-coated stirrer bar and
of electron-poor 2-nitrobenzoates was achieved by combining a dry septum, unless otherwise specified. Solvents were purified
copper-mediated decarboxylation with a palladium-catalyzed by standard procedures prior to use. All reactions were
Heck coupling. The new mechanistic pathway is likely to open monitored by GC using n-tetradecane as an internal standard.
up new perspectives for decarboxylative Heck reactions by Response factors of the products with regard to n-tetradecane
overcoming some intrinsic substrate limitations. However, due were obtained experimentally by analyzing known quantities of
to the fact that the decarboxylation of the benzoate and coup- the substances. GC analyses were carried out using an
ling of the resulting aryl residue now take place at a different HP-5 capillary column (Phenyl Methyl Siloxane 30 m × 320 ×
metal center, the balance between the decarboxylation step and 0.25, 100/2.3-30-300/3) and a time program beginning with
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Beilstein J. Org. Chem. 2010, 6, No. 43.
2 min at 60 °C, followed by a 30 °C/min ramp to 300 °C, and 13C NMR (101 MHz, CDCl3): δ = 136.4 (s), 130.1 (s), 130.0
then a 3 min hold at this temperature. Column chromatography (s), 129.7 (s), 128.8 (s), 128.6 (s), 127.0 (s), 124.1 (s), 121.1 (s),
was performed using a Combi Flash Companion-Chromato- 17.3 (s) ppm; MS (EI), m/z (%): 222 (8), 207 (10), 193 (10),
graphy-System (Isco-Systems) and RediSep packed columns 152 (10), 133 (74), 104 (100); IR (NaCl): = 3027 (w), 1602
(12 g).
(w), 1524 (s), 1367 (s), 958 (m), 780 (m) cm−1; Anal. Calcd. for
C15H13NO2: C = 75.30, H = 5.48, N = 5.85. Found C = 75.40,
H = 5.43, N = 5.75.
General procedure for the synthesis of the
potassium carboxylates
A 250 mL, two-necked, round-bottomed flask was charged with 4-Methyl-2-nitrostilbene (6ca) [CAS: 1054567-62-2]
the carboxylic acid (20 mmol) and ethanol (20 mL). To this, a was synthesized from potassium 4-methyl-2-nitrobenzoate (1c)
solution of potassium tert-butoxide (2.24 g, 20 mmol) in (329 mg, 1.50 mmol) and styrene (2a) (104 mg, 1.00 mmol).
ethanol (20 mL) was added dropwise over 2 h. After complete Purification by column chromatography (SiO2, hexane / ethyl
addition, the reaction mixture was stirred for a further 1 h at acetate 4:1) gave 6ca as a yellow oil (220 mg, 90%). 1H NMR
room temperature. The gradual formation of a white precipitate (CDCl3, 400 MHz): δ = 7.91 (d, J = 8.5 Hz, 1H), 7.63 (d, J =
was observed. The resulting solid was collected by filtration 16.3 Hz, 1H), 7.51–7.56 (m, 3H), 7.35–7.40 (m, 2H), 7.31 (d, J
through a 7-cm Büchner funnel, washed sequentially = 7.2 Hz, 1H), 7.18 (d, J = 8.5 Hz, 1H), 7.05 (d, J = 16.0 Hz,
with ethanol (2 × 10 mL) and cold (0 °C) diethyl ether (10 mL), 1H), 2.46 (s, 3H) ppm; 13C NMR (101 MHz, CDCl3): δ = 145.9
transferred to a round-bottomed flask, and dried at 2 × (s), 136.7 (s), 133.5 (s), 133.3 (s), 128.8 (s), 128.7 (s), 128.5 (s),
10−3 mmHg to provide the corresponding potassium 127.1 (s), 125.0 (s), 124.1 (s), 21.5 (s) ppm; MS (EI), m/z (%):
carboxylates 1a–i in 70–98% yield.
222 (32), 207 (19), 194 (24), 165 (20), 133 (100), 77 (45); IR
(NaCl): = 3059 (w), 2923 (w), 1605 (m), 1581 (m), 1515 (s),
1343 (s) cm−1.
General procedure for the decarboxylative
Heck vinylation
An oven dried 20 mL crimp-top vial equipped with a septum 5-Methyl-2-nitrostilbene (6da) [CAS: 861631-64-3]
cap and a stirring bar was charged with potassium carboxylate was synthesized from potassium 5-methyl-2-nitrobenzoate (1d)
1a–i (1.50 mmol), copper(II) fluoride (203 mg, 2.00 mmol), (329 mg, 1.50 mmol) and styrene (2a) (104 mg, 1.00 mmol).
palladium(II) acetate (4.58 mg, 0.02 mmol), 1,4,5-triazanaph- Purification by column chromatography (SiO2, hexane / ethyl
thalene (5.25 mg, 0.04 mmol), p-benzoquinone (54.0 mg, acetate 4:1) gave 6da as a yellow oil (205 mg, 85%). 1H NMR
0.50 mmol) and 3 Å molecular sieves (350 mg, powdered, and (CDCl3, 400 MHz): δ = 7.92 (d, J = 8.3 Hz, 1H), 7.66 (d, J =
dried in the microwave). The reaction vessel was closed, evacu- 16.1 Hz, 1H), 7.53–7.62 (m, 3H), 7.39–7.47 (m, 2H), 7.31–7.38
ated and filled with nitrogen three times. A stock solution of the (m, 1H), 7.19 (d, J = 8.3 Hz, 1H), 7.08 (d, J = 16.1 Hz, 1H),
corresponding coupling partner 2a–g (1.00 mmol) and the 2.48 (s, 3H) ppm; 13C NMR (101 MHz, CDCl3): δ = 145.5 (s),
internal GC standard n-tetradecane (50 µL) in NMP (2.0 mL) 143.7 (s), 136.3 (s), 133.1 (s), 132.8 (s), 128.3 (s), 128.2 (s),
was added with a syringe, and the resulting mixture stirred at 128.0 (s), 126.6 (s), 124.5 (s), 123.6 (s), 21.0 (s) ppm; MS (EI),
130 °C for 24 h. The reaction solution was then cooled to room m/z (%): 240 (12) [M−], 223 (16), 195 (31), 179 (15), 134 (89),
temperature, diluted with ethyl acetate and filtered through 104 (100); IR (NaCl): = 3060 (w), 3025 (w), 1605 (m), 1581
Celite / SiO2. The solution was washed successively with (m), 1514 (s), 1340 (s) cm−1.
aqueous HCl (1N, 20 mL), sodium hydrogen carbonate solution
(20 mL), and brine (20 mL), dried over MgSO4, filtered, and 5-Methoxy-2-nitrostilbene (6ea) [CAS: 879124-26-2]
the solvents removed in vacuo. Purification of the residue by was synthesized from potassium 5-methoxy-2-nitrobenzoate
column chromatography (SiO2, hexane / ethyl acetate gradient) (1e) (353 mg, 1.50 mmol) and styrene (2a) (104 mg,
gave the corresponding product.
1.00 mmol). Purification by column chromatography (SiO2,
hexane / ethyl acetate 4:1) gave 6ea as a yellow solid (238 mg,
3-Methyl-2-nitrostilbene (6ba) was synthesized from 93%). 1H NMR (CDCl3, 400 MHz): δ = 8.07 (d, J = 9.2 Hz,
potassium 3-methyl-2-nitrobenzoate (1b) (329 mg, 1.50 mmol) 1H), 7.72 (d, J = 16.0 Hz, 1H), 7.54 (d, J = 7.5 Hz, 2H),
and styrene (2a) (104 mg, 1.00 mmol). Purification by column 7.35–7.43 (m, 2H), 7.32 (d, J = 7.5 Hz, 1H), 7.13 (d, J = 2.4 Hz,
chromatography (SiO2, hexane / ethyl acetate 4:1) gave 6ba as 1H), 7.01 (d, J = 16.0 Hz, 1H), 6.86 (dd, J = 8.9, 2.7 Hz, 1H),
a yellow oil (96 mg, 40%). 1H NMR (CDCl3, 400 MHz): δ = 3.92 (s, 3H) ppm; 13C NMR (101 MHz, CDCl3): δ = 163.2 (s),
7.59 (d, J = 8.1 Hz, 1H), 7.47 (d, J = 7.3 Hz, 2H), 7.33–7.39 (m, 141.1 (s), 136.5 (s), 136.3 (s), 133.7 (s), 128.8 (s), 128.6 (s),
3H), 7.30 (d, J = 7.1 Hz, 1H), 7.19 (d, J = 7.6 Hz, 1H), 127.7 (s), 127.2 (s), 126.0 (s), 124.9 (s), 114.0 (s), 113.2 (s),
7.10–7.17 (m, 1H), 6.92–6.99 (m, 1H), 2.33 (s, 3H) ppm; 113.0 (s), 56.0 (s) ppm; MS (EI), m/z (%): 238 (12), 165 (18),
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Beilstein J. Org. Chem. 2010, 6, No. 43.
7. Reetz, M. T.; Lohmer, G.; Schwickardi, R. Angew. Chem., Int. Ed.
1998, 37, 481–483.
149 (51), 121 (35), 106 (100), 77 (34); IR (KBr): = 1579 (m),
1506 (s), 1333 (s), 1269 (m), 1232 (m); 1076 (m) cm−1; mp
65–66 °C.
doi:10.1002/(SICI)1521-3773(19980302)37:4<481::AID-ANIE481>3.0.
CO;2-I
8. Selvakumar, K.; Zapf, A.; Beller, M. Org. Lett. 2002, 4, 3031–3033.
doi:10.1021/ol020103h
5-Fluoro-2-nitrostilbene (6fa) was synthesized from potassium
5-fluoro-2-nitrobenzoate (1f) (335 mg, 1.50 mmol) and styrene
(2a) (104 mg, 1.00 mmol). Purification by column chromato-
graphy (SiO2, hexane / ethyl acetate 9:1) gave 6fa as a yellow
solid (106 mg, 44%). 1H NMR (CDCl3, 400 MHz): δ = 8.04
(dd, J = 9.1, 5.0 Hz, 1H), 7.63 (d, J = 16.1 Hz, 1H), 7.54 (d, J =
7.3 Hz, 2H), 7.33–7.44 (m, 4H), 7.03–7.12 (m, 2H) ppm;
13C NMR (101 MHz, CDCl3): δ = 163.5 (s), 136.4 (s), 136.1
(s), 135.1 (s), 129.0 (s), 128.9 (s), 127.7 (s), 127.3 (s), 122.9 (s),
115.1 (s), 114.8 (s), 114.7 (s), 114.5 (s), 100.0 (s) ppm; MS
(EI), m/z (%): 226 (32), 197 (17), 184 (13), 171 (16), 137 (100),
91 (32); IR (KBr): = 1616 (m), 1578 (m), 1597 (s), 1347 (m),
1272 (m), 952 (m) cm−1; Anal. Calcd. for C14H10FNO2: C =
69.13, H = 4.14, N = 5.76. Found C = 68.96, H = 4.30, N =
5.77; mp 66–67 °C.
9. Pröckl, S. S.; Kleist, W.; Gruber, M. A.; Kohler, K.
Angew. Chem., Int. Ed. 2004, 43, 1881–1882.
doi:10.1002/anie.200353473
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Supporting Information File 1
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Experimental Section: The synthesis, purification and
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