Mechanochemical oxidation of fluorinated anilines to symmetric azobenzenes

Article abstract of DOI:10.1016/j.jfluchem.2018.10.012

A solventless mechanochemical methodology for the oxidation of electron-poor fluorinated anilines to the corresponding symmetrical azo compounds has been developed. In this process phenyliodine(III)diacetate and inexpensive calcium hypochlorite demonstrat

Full text of DOI:10.1016/j.jfluchem.2018.10.012

Accepted Manuscript
Title: Mechanochemical oxidation of fluorinated anilines to
symmetric azobenzenes
Authors: Ivan Andreosso, Antonio Papagni, Luca Vaghi
PII:
S0022-1139(18)30265-3
DOI:
Reference:
https://doi.org/10.1016/j.jfluchem.2018.10.012
FLUOR 9237
To appear in:
FLUOR
Received date:
Revised date:
Accepted date:
28-8-2018
5-10-2018
19-10-2018
Please cite this article as: Andreosso I, Papagni A, Vaghi L, Mechanochemical oxidation
offluorinatedanilinestosymmetricazobenzenes, JournalofFluorineChemistry(2018),
https://doi.org/10.1016/j.jfluchem.2018.10.012
This is a PDF file of an unedited manuscript that has been accepted for publication.
As a service to our customers we are providing this early version of the manuscript.
The manuscript will undergo copyediting, typesetting, and review of the resulting proof
before it is published in its final form. Please note that during the production process
errors may be discovered which could affect the content, and all legal disclaimers that
apply to the journal pertain.

Mechanochemical oxidation of fluorinated anilines to symmetric azobenzenes
a
a
a*
Ivan Andreosso , Antonio Papagni , Luca Vaghi
a
Department of Materials Science, University of Milano-Bicocca, Via Cozzi 55, 20125 Milano,
Italy
*
Corresponding author. Tel.: +39-02-6448-5234; e-mail: luca.vaghi@unimib.it
Graphical abstract
Highlights



Highly fluorinated symmetric azobenzenes were obtained in good yields and
purity.
The mechanochemical procedure allows short reaction times and avoids
solvents.
Phenyliodine(III)diacetate and calcium hypochlorite as oxidants avoid the use of
environmental unfriendly metals.
ABSTRACT
A solventless mechanochemical methodology for the oxidation of electron-poor fluorinated
anilines to the corresponding symmetrical azo compounds has been developed. In this process
phenyliodine(III)diacetate and inexpensive calcium hypochlorite demonstrate to be good
oxidants, providing the desired fluorinated azobenzenes in satisfactory yields, within short
reaction times and avoiding the use of environmental unfriendly metal reagents.
Keywords
fluorinated
azobenzenes;
mechanochemistry;
oxidation;
fluorinated
anilines;
phenyliodine(III)diacetate; calcium hypochlorite.
1
. Introduction
Azobenzenes have found important applications as pigments and dyes [1-4], indicators [5], in
medicine [6] and as additives in food industry. Furthermore, the azo moiety exhibits a unique
1

photochemical response, light induced interconversion between trans and cis isomers makes
them ideal compounds for innovative applications in material science [7,8]. Within this contest
fluorinated azobenzenes display an important role since, because of their peculiar isomerization
properties [9,10], they have recently successfully used to cause a macroscopic crystal shaping
induced by light irradiation [11], and they were even applied in the preparation of
photoresponsive molecularly imprinted polymers [12].
Many synthetic methods to afford azo compounds are reported in the literature [13,14], and a
part of them are developed to prepare symmetric azobenzenes by oxidative coupling of aromatic
amines or by reductive homodimerization of nitroarenes. In the case of electron-poor anilines the
oxidative homocoupling is carried out by using stoichiometric amount of toxic and
environmentally unfriendly metals such as mercury, lead and manganese to achieve satisfactory
yields. In the past Pausacker has investigated a metal-free organic reagent namely
phenyliodine(III)diacetate (PIDA) [15], the oxidation of primary anilines afforded the
corresponding azo compounds in variable yields, depending on the substrate. In this context an
improved procedure exploiting PIDA and ethanol as solvent has been achieved recently [16].
With almost all oxidants cited, the azobenzene formation is proposed to proceed via
monoelectronic oxidation, producing a nitrogen centred radical cation, whose N-N coupling
forms the corresponding hydrazobenzene which, in turn, is further oxidized to the final azo
product.
Preparation of symmetric fluorinated azobenzenes was studied in the early 70s by Birchall [17],
he afforded good results using Pb(OAc) in refluxing benzene (in this case also a good amount of
4
the corresponding phenazine is formed) and calcium hypochlorite (bleaching powder) in
refluxing carbon tetrachloride. Previously has been reported the oxidation of pentafluoroaniline 2
(Scheme 1) with an aqueous solution of NaClO, however, due to limited solubility of aniline in
water, the corresponding azo derivative was recovered in a modest 20% yield [18]. A slight
+
-
improvement of yields (up to 28%) has been obtained using Bu
4
N HSO
4
as phase transfer
catalyst [19]. Recently, Minakata and co-workers have reported an efficient protocol for the
synthesis of symmetric and nonsymmetric aromatic azo compounds exploiting the in-situ
generation of tert-butyl hypoiodite from t-BuOCl and sodium iodide, in inert atmosphere [20].
This protocol has been applied to the oxidation of 2, the corresponding azo derivative was
obtained in lower yields with respect to electron-richer anilines. In 2010 Stolle reported the
oxidation of anilines under mechano-chemistry [21], using KMnO
4
as oxidant.
Mechanochemical methodologies have lately attracted the attention of synthetic organic chemists
as a valid alternative to conventional solution procedures [22-24]. The reagents are put directly
2

in contact to each other in a dedicated apparatus and the reaction is promoted by mechanical
energy provided by the impacts of the balls inside the reactor (jar).
One of our main research activities entails the synthesis of polyfluorinated aromatic and
heteroaromatic compounds as possible candidates for biological and material science
applications. Recently, we achieved the preparation of non-symmetric fluorinated acridines to be
tested against resistant bacteria via an innovative micellar-type methodology [25], and we
exploited the nucleophilic aromatic substitution typical of fluoroarenes to prepare Donor–
Acceptor–Donor (D–A–D) and Acceptor–Donor–Acceptor (A–D–A) fluorinated compounds to
be employed in the production of oligomers for organic photovoltaics and optoelectronics [26].
In this context a simple and efficient method to gain a set of symmetric polyfluorinated
azobenzenes by oxidation of the corresponding anilines was needed.
2
. Results and discussion
High level of fluorination is expected to reduce the nitrogen oxidability of anilines that, in turn,
is reflected on their transformation into azo derivatives in term of overall conversions and yields.
We tested the methodologies that are reported to be more effectiveness to gain highly fluorinated
azobenzenes, those reported by Birchall [17] and K
27]. In our experience only the Birchall’s procedures afforded satisfactory and reproducible
yields, however, the over stoichiometric use of Pb(OAc) in addition to the tricky workup due to
the presence of lead salts make this methodology not attractive. We tried Ca(ClO) in refluxing
3
Fe(CN)
6
/ KOH in refluxing water/ethanol
[
4
2
chloroform or dichloromethane and no reaction was observed, the desired products have been
observed only with refluxing carbon tetrachloride for 3-4 hours as reported [17]. Anyway,
calcium hypochlorite, as a solid and inexpensive reagent, looks suitable for the mechano-
chemical approach. PIDA has been also taken in consideration as another possible oxidant under
mechano-chemical condition, due to its crystalline form, moreover, its efficacy in the oxidation
of anilines to azo derivatives has already been proved in the past in solution processes [15,16].
Both oxidants, PIDA and Ca(ClO) , have been selected being free of transition metal and
2
relatively at low costs. Here we describe our results in the oxidation of a series of perfluorinated
anilines with these two oxidants in a ball mill.
Five
polyfluorinated
anilines
with
decreasing
fluorination
grade
namely
2
,3,5,6-tetrafluoro-4-(trifluoromethyl)-aniline 1, pentafluoroaniline 2, 2,3,5,6-tetrafluoroaniline
, 2,4,6-trifluoroaniline 4 and 2,6-difluoroaniline 5 have been chosen for this study, their
3
structures and the structures of expected symmetrical azo products are depicted in Scheme 1.
3

Scheme1. Reagents (1-5) and products (6-10) of the mechanochemical oxidation of
polyfluoroanilines.
1
, the electron-poorest aniline, has been selected as model substrate to optimise the reaction
conditions (Table 1). The best molar ratio respect to the substrate was found to be 1.5 equivalents
for PIDA (Table 1, entry 2) and 1.2 equivalents for Ca(ClO) (Table 1, entry 5), 1:1 ratio is not
2
sufficient to achieve complete conversion, as previously described by Pausacker in the case of
PIDA in solution [15]. Increasing the ratios of the two reagents did not resulted in better yields
(Table 1, entries 3 and 6), while in any case the transformation was completed after 30 minutes,
as indicated by TLC monitoring. For sake of completeness we decided to test some other non-
metal oxidant that has been reported to achieve this reaction, even if not with fluorinated anilines
[
28-30], obviously excluding unstable and high-reactive reagents non-compatible with the
milling conditions. Therefore, Oxone, Na and NaBO were experimented, but no trace of
2
S
2
O
8
3
the desired product 6 was detected (Table 1, entries 7, 8 and 9).
Table 1. Preliminary experiments for the mechanochemical oxidation of 1
4

a
Entry
Oxidant
PhI(OAc)
PhI(OAc)
PhI(OAc)
Equivalents
Yield %
1
2
3
4
5
6
7
8
2
2
2
1
1.5
2
38
55
56
44
56
54
-
Ca(ClO)
Ca(ClO)
Ca(ClO)
Oxone
2
1
2
1.2
1.5
1
2
Na
NaBO
PhI(OAc)
2
S
2
O
8
1
-
9
3
1
-
b
1
1
1
0
2
1.5
1.2
1
48
-
b
c
1
Ca(ClO)
2
2
PhI(OAc)
2
traces
a
b
2 2 2
Conditions: Aniline (1 g), ball milling, 15 Hz, 30 min. Isolated yields. CH Cl , 40 °C, 24 h, N
. cEtOH -20 to 60 °C 1h [16].
With the optimized experimental conditions, the reactor was then charged with 1 g of aniline, 1.5
equiv of PIDA or 1.2 equiv of Ca(ClO) respect to the aniline and shaken, the results are reported
in Table 2. Accordingly, 1, the electron-poorest aniline, requires 30 minutes of milling affording
in 55% and 56% isolated yields with PhI(OAc) (Table 2, entry 1) and Ca(ClO) (Table 2,
2
6
2
2
entry 5), respectively. In the other cases the oxidation reaction was faster and after 10 min no
trace of the starting materials was detected, thus aniline 2 provides decafluoroazobenzene 7 in
3
2% yield with PIDA (Table 2, entry 2) and 41% yield with Ca(ClO) (Table 2, entry 6). Good
2
results were achieved for the oxidation of 3 with both oxidants, yielding 63% and 51 % of pure
isolated product 8 (Table 2, entries 3 and 7). Conversely, the oxidation of more nucleophilic 4
and 5 was worse, giving the corresponding azo-derivatives 9 (Table 2, entries 4 and 9) and 10
(
Table 2, entry 5) in markedly lower yields. The oxidation of 5 with Ca(ClO) gave an intractable
2
mixture of unidentified products (Table 2, entry 8). Indeed, with this oxidant the chlorination of
the aromatic ring as well as the oxidation to quinoid species cannot be excluded, especially with
the more electron-rich anilines 4 and 5 [19].
Table 2. Yields in azobenzenes without grinding auxiliaries
a
Entry
Substrate Oxidant (equiv)
Time
Product
Yield %
55
1
2
3
4
5
6
7
8
1
2
3
4
5
1
2
3
PhI(OAc)
PhI(OAc)
PhI(OAc)
PhI(OAc)
PhI(OAc)
2
2
2
2
2
(1.5)
(1.5)
(1.5)
(1.5)
(1.5)
30 min
10 min
10 min
10 min
10 min
30 min
10 min
10 min
6
7
32
8
63
9
18
10
6
31
Ca(ClO)
Ca(ClO)
Ca(ClO)
2
2
2
(1.2)
(1.2)
(1.2)
56
7
38
8
51
5

9
4
5
Ca(ClO)
Ca(ClO)
2
2
(1.2)
(1.2)
10 min
9
8
b
1
0
10 min
10
-
a
b
Conditions: Aniline (1 g), ball milling at 15 Hz. Isolated yields. The reaction yielded an intractable mixture of products.
From these results, it appears evident that both PIDA and Ca(ClO)
corresponding azo derivatives with polyfluorinated and less electron-rich anilines. Anyway
Ca(ClO) provides unsatisfactory results with electron-richer anilines when compared with
2
give good yields in the
2
PIDA. In any case, specifically for electron-poor anilines, the mechanical chemical oxidation
provides comparable yields to solution protocols, but the reactions are faster, and the use of
reaction solvents is completely avoided. From the reaction work-up procedure point of view,
solvents need to be used to extract the reaction products from the crude at the end of ball-milling
process and further purified by dry column chromatography. The recovery of reaction products
from the ball-miller is sometime quite cumbersome, therefore, we decided to add an inert
inorganic material into the reactor, namely SiO (70-200 μm) to obtain a solid dispersion of
2
reaction products at the end ball-milling process suitable to be directly charged on the
chromatographic column for their purification. Of course, it is expected that the dispersion of
reagents onto an inert solid auxiliary (grinding auxiliary) will lower the oxidant efficacy,
resulting in higher reaction times and in the use of more oxidant to observed reasonable aniline
conversion. Indeed, in the case, the ratio of the oxidants needed to be raised to 2.0 equivalents
for PIDA and 1.5 equivalents in the case of Ca(ClO)
2
and the reaction times were increased up to
2
h for electron-poor amine 1 and 1 h for the others. The results are depicted in table 3.
Table 3. Yields in azobenzenes with SiO
2
as grinding auxiliary
a
Entry
Substrate Oxidant (equiv)
Time
2 h
1 h
1 h
1 h
1 h
2 h
1 h
1 h
1 h
1 h
Product
Yield %
37
1
2
3
4
5
6
7
8
9
1
2
3
4
5
1
2
3
4
5
PhI(OAc)
PhI(OAc)
PhI(OAc)
PhI(OAc)
PhI(OAc)
2
2
2
2
2
(2.0)
(2.0)
(2.0)
(2.0)
(2.0)
6
7
41
8
44
9
25
10
6
34
Ca(ClO)
Ca(ClO)
Ca(ClO)
Ca(ClO)
Ca(ClO)
2
2
2
2
2
(1.5)
(1.5)
(1.5)
(1.5)
(1.5)
47
7
33
8
34
9
13
1
0
10
18
a
Conditions: Aniline (1 g), SiO
2
(8 g), ball milling at 15 Hz. Isolated yields.
As reported in Table 3, the use of the grinding auxiliary doesn’t affect dramatically the overall
yield of each reactions. However it is noted that the presence of silica in the reactions with PIDA
6

increased the yields of oxidation of the anilines bearing a fluorine in para-position (from 32% to
1% for 2 and from 18% to 25% for 4), maybe slowing down the reaction kinetics would inhibit
the formation of by-products, and decreased the yields of less reactive compounds i.e., 1 and 3.
This trend is not observed with Ca(ClO) , in this case the use of a grinding auxiliary lowers the
4
2
yields in all the experiments except for the most electron-rich 5, where the desired product has
been isolated in 18% yield (table 3, entry 10), whereas, under neat conditions, no azobenzene 10
has been recovered. Although the slight drop of the final yield, the handling of the crude reaction
is much simpler when the milling auxiliary is employed, making this procedure suitable for an
easy and fast purification on a silica pad.
3
. Conclusions
In summary, we have described a simple and convenient procedure for the mechanochemical
oxidation of highly fluorinated anilines to the corresponding symmetric azobenzenes, in solvent
free conditions and without environmental unfriendly metal-containing reagents. The yields are
in line with previously reported metal- mediated methodologies. Generally, the use of silica gel
as grinding auxiliary implies a small decrease in isolated yields, but the easiness of purification
makes this the methodology of choice.
4
. Experimental
4
.1 General
2
,3,5,6-tetrafluoro-4-(trifluoromethyl)-aniline, 2,3,5,6-tetrafluoroaniline, 2,4,6-trifluoroaniline
and 2,6-difluoroaniline were purchased from Tokyo Chemical Industry Co.;
phenyliodine(III)diacetate and 2,3,4,5,6-pentafluoroaniline were purchased from Fluorochem
Co.; calcium hypochlorite, dichloromethane, n-hexane and Silica gel (60A, 63-200 μm) were
purchased from Aldrich Chemical Co. All reagents and solvents were used as received.
Reactions were performed in a SPEX® 8000M mixer/mill using 50 mL grinding reactor (ZrO
and 4 milling balls (1 cm diameter, ZrO ). Melting points were measured with a Stanford
Research Systems Optimelt apparatus. IR spectra were recorded with a Perkin Elmer Spectrum
2
)
2
1
13
19
1
00 FT-IR spectrometer equipped with universal ATR sampling accessory. H, C and F NMR
were recorded with Bruker AVANCE III HD 400MHz spectrometer. GC were executed with a
Perkin Elmer Clarus 500 gas chromatograph. Mass spectra were recorded with a Perkin Elmer
Clarus 560 S mass spectrometer.
7

4
.2 General procedure for the oxidation with PIDA without grinding auxiliaries.
Fluorinated aniline (1 g) and PIDA (1.5 equiv) were added to the zirconia reactor equipped with
four zirconia milling balls (d = 1 cm). The mixture was shaken at 15 Hz for the time stated in the
text. The residue was collected with dichloromethane, SiO was added to the solution and the
2
solvent was removed under reduced pressure. Purification over a short pad of silica gel eluting
with n-hexane afforded the desired azobenzene in a pure form.
4
.3 General procedure for the oxidation with Ca(ClO)
2
without grinding auxiliaries.
Fluorinated aniline (1 g) and Ca(ClO) (1.2 equiv) were added to the zirconia reactor equipped
2
with four zirconia milling balls (d = 1 cm). The mixture was shaken at 15 Hz for the time stated
in the text. The residue was collected with dichloromethane, the organic solution was washed
with water (3 x 100 mL), dried over Na
2
SO
4
, SiO was added to the resulting solution and the
2
solvent was removed under reduced pressure. Purification over a short pad of silica gel eluting
with n-hexane afforded the desired azobenzene in a pure form.
4
.4 General procedure for the oxidation with PIDA with SiO as grinding auxiliary.
2
Fluorinated aniline (1 g), PIDA (2.0 equiv) and silica gel (60Å, 63-200 μm, 8 g) were added to
the zirconia reactor equipped with four zirconia milling balls (d = 1 cm). The mixture was
shaken at 15 Hz for the time stated in the text. The powdery residue was directly put over a short
pad of silica gel, elution with n-hexane afforded the desired azobenzene in a pure form.
4
.5 General procedure for the oxidation with Ca(ClO)
2
with SiO
2
as grinding auxiliary.
Fluorinated aniline (1 g), Ca(ClO) (2.0 equiv) and silica gel (60Å, 63-200 μm, 8 g) were added
2
to the zirconia reactor equipped with four zirconia milling balls (d = 1 cm). The mixture was
shaken at 15 Hz for the time stated in the text. The powdery residue was directly put over a short
pad of silica gel and eluted with n-hexane. The collected organic solution was then washed with
water (3 x 100 mL), dried over Na
2
SO and the solvent was removed under reduced pressure to
4
afford the desired azobenzene in a pure form.
4
.6 Characterization of isolated azobenzenes.
Bis[2,3,5,6-tetrafluoro-4-(trifluoromethyl)-phenyl]-diazene (6). Orange solid. m.p. 116-117 °C.
IR (ATR): 1650, 1605, 1489, 1444, 1403, 1332, 1267, 1221, 1157, 1053, 994, 906, 869, 821,
99, 715, 594 cm . C NMR (101 MHz, CDCl
) δ 146.1 – 143.2 (m, 1JCF = 278.9), 141.8 –
-
1 13
7
1
3
38.9 (m, 1JCF = 265.6), 134.3 – 134.1 (m), 124.5 – 116.2 (m, 1JCF = 275.2), 112.0 – 111.4 (m).
8

19F NMR (376 MHz, CDCl
47.36 (m). MS (EI) m/z = 462 [M]+.
) δ -56.45 (tt, J = 22.1, 2.3 Hz), -138.73 – -139.06 (m), -147.20 – -
3
1
Bis(pentafluorophenyl)-diazene (7). Orange solid. m.p. 140-141 °C; IR (ATR): 1643, 1606,
-1
1
502, 1408, 1381, 1321, 1272, 1184, 1146, 1124, 1056, 1021, 999, 977, 799, 747, 611, 561 cm .
13C NMR (101 MHz, CDCl
) δ 144.3 – 141.3 (m, 1JCF = 261.8), 142.8 – 139.9 (m, 1JCF = 264.8),
3
39.5 – 136.6 (m, 1JCF = 253.7), 128.3 – 128.1 (m). F NMR (376 MHz, CDCl
19
) δ -148.18 – -
1
1
3
48.32 (m), -148.46 (tt, J = 20.9, 3.4 Hz), -161.13 – -161.30 (m). MS (EI) m/z = 362 [M]+.
Bis(2,3,5,6-tetrafluorophenyl)-diazene (8). Orange-red solid. m.p. 124-125 °C. IR (ATR): 3122,
3
9
098, 1711, 1643, 1609, 1523, 1501, 1446, 1409, 1379, 1279, 1262, 1178, 1129, 1069, 1037,
-1 1
52, 855, 803, 745, 717, 701, 662, 582 cm . H NMR (400 MHz, CDCl
3
) δ 7.26 (tt, J = 9.5, 7.1
13
Hz, 2H). C NMR (101 MHz, CDCl
38.8 (m), 132.8 (t, J = 7.9 Hz), 107.9 (t, J = 22.8 Hz). F NMR (376 MHz, CDCl
td, J = 15.2, 5.3 Hz), -149.65 (td, J = 15.2, 5.3 Hz). MS (EI) m/z = 326 [M]+.
3
) δ 146.3 (dddd, J = 251.4, 23.2, 10.1, 5.1 Hz), 141.7 –
1
9
1
3
) δ -138.03
(
Bis(2,4,6-trifluorophenyl)-diazene (9). Orange-yellow solid. m.p. 115-117 °C. IR (ATR): 3113,
2
7
928, 1704, 1639, 1611, 1590, 1517, 1451, 1362, 1340, 1302, 1223, 1177, 1126, 1049, 999, 848,
-1
1
13
84, 750, 658, 624, 588, 575 cm . H NMR (400 MHz, CDCl
3
) δ 6.90 – 6.83 (m, 4H). C NMR
(
101 MHz, CDCl
3
) δ 163.2 (dt, J = 255.4, 15.1 Hz), 156.5 (ddd, J = 263.4, 15.1, 6.5 Hz), 129.0 –
1
9
1
28.6 (m), 101.5 (ddd, J = 25.9, 24.7, 4.0 Hz). F NMR (376 MHz, CDCl ) δ -102.65 (t, J = 8.2
3
Hz), -116.41 (d, J = 8.2 Hz). MS (EI) m/z = 290 [M]+.
Bis(2,6-difluorophenyl)-diazene (10). Orange solid. m.p. 83-84 °C. IR (ATR): 3109, 3071, 1939,
1
9
7
1
860, 1769, 1614, 1587, 1471, 1357, 1330, 1297, 1285, 1246, 1221, 1153, 1130, 1058, 1018,
-1 1
75, 885, 829, 786, 736, 691, 669, 600 cm . H NMR (400 MHz, CDCl ) δ 7.44 – 7.36 (m, 2H),
3
1
3
.12 – 7.06 (m, 4H). C NMR (101 MHz, CDCl
3
) δ 155.6 (dd, J = 261.0, 4.0 Hz), 131.5 (t, J =
1
9
0.3 Hz), 112.7 (s), 112.6 (dd, J = 21.0, 2.8 Hz). F NMR (376 MHz, CDCl
3
) δ -121.24 (s). MS
(
EI) m/z = 254 [M]+.
Acknowledgements
We would like to thank Dr. Giorgio Patriarca for NMR analysis, Prof. Giancarlo Cravotto
Dipartimento di Scienza e Tecnologia del Farmaco, Università degli Studi di Torino) for
(
preliminary support and Prof. Giovanni Palmisano (Dipartimento di Scienza e Alta Tecnologia,
9

Università degli Studi dell’Insubria) for his fine suggestions. Ivan Andreosso would thank
CORIMAV (Consortium for the Research of Advanced Materials between Pirelli and Milano-
Bicocca University) for its support within the PCAM European Doctoral Program. This research
did not receive any specific grant from funding agencies in the public, commercial, or not-for-
profit sectors.
Appendix A. Supplementary data
1
19 13
H, F, C NMR, ATR FT-IR and GC-MS of isolated azobenzenes.
References
[
1] K. Hunger, Industrial Dyes: Chemistry, Properties, Applications, Wiley-VCH,
Weinheim, 2003.
[
2] H. Zollinger, Color Chemistry: Syntheses, Properties and Applications of Organic Dyes
and Pigments, Wiley-VCH: Weinheim, 1987.
[
[
[
[
[
[
[
3] P.F. Gordon, P. Gregory, Organic Chemistry in Colour, Springer, New York, 1983.
4] R.G. Anderson, G. Nickless, Analyst 92 (1967) 207-238.
5] P.N.D. Ashutosh, J.K. Mehrotra, Colourage 26 (1979) 25.
6] W.J. Sandborn, Am. J. Gastroenterol 97 (2002) 2939-2941.
7] H.M.D. Bandara, S.C. Burdette, Chem. Soc. Rev. 41 (2012) 1809-1825.
8] A. Emoto, E. Uchida, T. Fukuda, Polymers 4 (2012) 150-186.
9] D. Blꢀger, J. Schwarz, A.M. Brouwer, S. Hecht, J. Am. Chem. Soc. 134 (2012)
2
0597−20600.
[
[
10] C. Knie, M. Utecht, F. Zhao, H. Kulla, S. Kovalenko, A.M. Brouwer, P. Saalfrank, S.
Hecht, D. Bléger, Chem. Eur. J. 20 (2014) 16492–16501.
11] O.S. Bushuyev, A. Tomberg, T. Friꢁꢂiꢃ, C.J. Barrett, J. Am. Chem. Soc. 135 (2013)
1
2556−12559.
[
[
[
[
[
12] Q. Tang, C. Gong, M.H.W. Lam, X. Fu, Sensors and Actuators B 156 (2011) 100–107.
13] F. Hamon, F. Djedaini-Pilard, F. Barbot, C. Len, Tetrahedron 65 (2010) 10105-10123.
14] E. Merino, Chem. Soc. Rev. 40 (2011) 3835–3853.
15] K.H. Pausacker, J. Chem. Soc. (1953) 1989-1990.
16] K. Monir, M. Ghosh, S. Mishra, A. Majee, A. Hajra, Eur. J. Org. Chem. 5 (2014) 1096–
1
102.
[
[
[
17] J.M. Birchall, R.N. Haszeldine, J.E.G. Kemp, J. Chem. Soc. C (1970), 449-455.
18] J. Burdon, C.J. Morton, D.F. Thomas, J. Chem. Soc. (1965) 2621-2627.
19] J.J. Deadman, M. Jarman, R. McCague, R. McKenna, S. Neidle, J. Chem. Soc., Perkin
Trans. 2 (1989) 971-976.
[
[
20] Y. Takeda, S. Okumura, S. Minakata, Angew. Chem. Int. Ed. 51 (2012) 7804–7808.
21] R. Thorwirth, F. Bernhardt, A. Stolle, B. Ondruschka, J. Asghari, Chem. Eur. J. 16
(
2010) 13236–13242.
22] A. Stolle, T. Szuppa, S.E.S. Leonhardt, B. Ondruschka, Chem. Soc. Rev. 40 (2011)
317–2329.
[
2
[
[
[
23] R.B.N. Baig, R. S. Varma, Chem. Soc. Rev. 41 (2012) 1559–1584.
24] G. Wang, Chem. Soc. Rev. 42 (2013) 7668-7700.
25] L.Vaghi, A. Sanzone, M. Sassi, S. Pagani, A. Papagni, L. Beverina, Synthesis 50 (2018)
1
621-1628.
1
0

[
26] P. Coghi, A. Papagni, R. Po, A. Calabrese, A. Tacca, A. Savoini, M. Stuknyte, New J.
Chem. 39 (2015) 3615-3623.
[
[
[
[
27] E. Leyva, E. Monreal, C. Medina, S. Leyva, Tetrahedron Lett. 38 (1997) 7847–7848.
28] Y. Shima, J. Matsuo, Tetrahedron Lett. 57 (2016) 4066-4069.
29] P. Sims, J. Chem. Soc. (1958) 44-47.
30] S. M. Mehta, M. V. Vakilwala, J. Am. Chem. Soc. 74 (1952) 563-564.
1
1