Perfluorooctylation of aromatic compounds with perfluoroazooctane upon 185 nm irradiation in a two-phase system

Article abstract of DOI:10.1039/a707171j

An efficient method for the preparation of perfluorooctylated aromatic and heteroaromatic compounds has been developed using photolysis of perfluoroazooctane (E)-1 upon 185 nm irradiation. The use of a two-phase system made up of a perfluorohexane solution of (E)-1 and a suitable aromatic compound has proved efficient for this reaction. The photolytic mechanism of (E)-1 upon 185 nm irradiation has also been investigated on the basis of quantum yield, light intensity dependence and UV-VIS spectra. It is shown that (E)-1 is photoisomerized by one photon absorption to the (Z)-isomer, with subsequent extrusion of nitrogen by a second photon absorption.

Full text of DOI:10.1039/a707171j

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Perfluorooctylation of aromatic compounds with perfluoroazooctane
upon 185 nm irradiation in a two-phase system
Takako Nakamura* and Yoshinori Koga
National Institute of Materials and Chemical Research, Higashi 1-1, Tsukuba, Ibaraki 305,
Japan
An efficient method for the preparation of perfluorooctylated aromatic and heteroaromatic compounds
has been developed using photolysis of perfluoroazooctane (E)-1 upon 185 nm irradiation. The use of a
two-phase system made up of a perfluorohexane solution of (E)-1 and a suitable aromatic compound has
proved efficient for this reaction. The photolytic mechanism of (E)-1 upon 185 nm irradiation has also
been investigated on the basis of quantum yield, light intensity dependence and UV–VIS spectra. It is
shown that (E)-1 is photoisomerized by one photon absorption to the (Z)-isomer, with subsequent
extrusion of nitrogen by a second photon absorption.
Introduction
Perfluoroalkyl-containing organic compounds have attracted
much attention in the field of medicinal chemistry and material
science because of their unique properties, due to the presence
of fluorine atoms, namely biological activity and water/oil
1
repellent properties. Thermal- and photo-induced perfluoro-
alkyl radicals have been used in organofluorine chemistry to
2
introduce perfluoroalkyl groups into organic compounds. The
photochemical generation of perfluoroalkyl radicals from
perfluoroalkyl halides, perfluoroalkanesulfonyl chlorides, and
perfluoroalkylmetallic reagents have been reported by many
2
workers to date.
The photochemistry of azo compounds has been extensively
3
investigated in the field of organic chemistry. It is well known
that azoalkanes are photolyzed to give carbon-centered radicals
upon UV irradiation, accompanied by extrusion of nitrogen
4
which is inert to the reaction system. Few reports on per-
Fig. 1 UV–VIS spectrum of (E)-1 in hexane
fluoroalkylation of organic compounds by photolysis of per-
5
fluoroalkylazo compounds have been published, though they
are potentially useful for the photochemical formation of
perfluoroalkyl radicals.
8
fluorooctyl radicals [reaction (1)]. These results indicate that
In this work we report a novel perfluorooctylation of aro-
matic and heteroaromatic compounds by photolysis of per-
fluoroazooctane [(E)-1] and its photolytic mechanism upon 185
nm irradiation.
C8F17
hν
C16F34 + N2
(1)
N
N
2
C8F17
(
E)-1
1
85 nm irradiation (π–π* excitation) is much more effective for
Results and discussion
the denitrogenation of (E)-1 than >300 nm irradiation (n–π*
excitation). Adam et al. also reported that cyclic azoalkanes
decomposed to generate alkyl radicals upon 185 nm irradiation
7
Perfluoroazooctane [(E)-1] was prepared according to pro-
6
cedures in the literature. The UV–VIS spectrum of (E)-1 in
(
π–π* excitation), whereas they were slow to extrude nitrogen
hexane is shown in Fig. 1, indicating that the azo group of (E)-1
absorbs light at λmax 188 nm (π–π*) and λmax 380 nm (n–π*). As
the π–π* absorption of (E)-1 has a much more intense molar
9
upon > 300 nm irradiation (n–π* excitation).
An attempt was made to introduce a perfluorooctyl radical
into benzene by 185 nm photolysis of (E)-1. Benzene solutions
of (E)-1 (1 × 10 mol l ) were irradiated with 185 nm light
under a nitrogen atmosphere at room temperature [reaction
(2)]. The time-course results of this photoreaction, monitored
Ϫ1
Ϫ1
extinction coefficient (ε 23 100 l mol cm ) than the n–π*
absorption (ε 30), it was expected that vacuum ultraviolet
Ϫ3
Ϫ1
7
irradiation would lead an effective denitrogenation of (E)-1.
Ϫ3
Ϫ1
A hexane solution of (E)-1 (1 × 10 mol l ) was irradiated
at >300 nm light with a high pressure mercury lamp for 24 h. It
was shown by GC–MS and UV–VIS analyses of this reaction
mixture that the photolysis of (E)-1 by extrusion of nitrogen
was negligible, and that trans–cis isomerization of 1 was the
main reaction occurring during the irradiation. On the other
hand, decomposition of (E)-1 was effected completely with the
use of 185 nm light from a low pressure mercury lamp for 7 h to
give perfluorohexadecane (2) from the recombination of per-
C8F17
C8F17 C8F17
hν
(
E )-1
+ 2 +
+ (C8F17H) (2)
C8F17
N
NC8F17
3
4
by GC–MS with nonane and undecane as internal standards,
are shown in Fig. 2. After irradiation of the benzene solution
for 7 h, the desired perfluorooctylbenzene (3) was obtained in
10
J. Chem. Soc., Perkin Trans. 2, 1998
659

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Table 1 Preparation of perfluorooctyl aromatic and heteroaromatic
compounds with (E)-1 in two-phase system upon irradiation
a
Product
Yield (%)
3
5
6
7
8
9
70
b
66c
74
d
52
75
70
65
1
0
a
b
Yields of this photoreaction are not optimized. Isomer distribution:
c
o-, 33%; m-, 35%; p-, 32%. Isomer distribution: o-, 40%; m-, 23%; p-,
3
d
7%. Isomer distribution: o-, 42%; m-, 25%; p-, 33%.
Fig. 2 Photolysis of (E)-1 in benzene with a low pressure mercury
lamp. ᭿, 1; ᭺, 2; ᮀ, 3; ᭝, 4.
1
3% yield, together with 2 in 1% yield. It is assumed that the
low yield of 3 is due to the formation of tetrakis(perfluoro-
8
octyl)hydrazine (4) by the reaction of remaining (E)-1 with5
perfluorooctyl radicals, which are present in low concentration
due to the absorption of most of the photons by benzene in this
11
reaction.
In order to improve the efficiency of the generation of per-
fluorooctyl radicals, we needed to find solvents which have no
absorption of 185 nm light and are relatively unreactive
towards the resulting radicals. Recently the use of perfluoro-
hexane as an inert reaction medium has been reported for
12
Fig. 3 Light intensity dependence of the formation of 2 by photolysis
of (E)-1 upon 185 nm irradiation
organic reactions.
Additionally, perfluoroalkanes have
absorption bands in vacuum ultraviolet region (<160 nm) and
are easy to remove from the reaction mixture in vacuo after
1
3
irradiation. Therefore, perfluorohexane was selected as a suit-
able solvent for our purposes. Since hydrocarbon compounds
are generally insoluble in perfluorohexane, a two-phase sys-
tem, consisting of benzene, the substrate, as the upper phase
and a perfluorohexane solution of (E)-1 as the lower phase, was
adopted for this reaction. In general, two-phase systems, con-
sisting of two immiscible solvents, play an important role in
scope and limitation of this method, various benzene deriva-
tives and heteroaromatic compounds were examined as sub-
strates in a similar manner to that described above [reactions (3)
and (4)]. As shown in Table 1, perfluorooctylated aromatic
R
R
14
15
organic syntheses and extraction chemistry.
A two-phase system, consisting of a perfluorohexane solu-
hν
C8F17
+
(C8F17H)
(3)
(E )-1/C6F14
+
Ϫ3
Ϫ1
tion of (E)-1 (1 × 10 mol l ) and benzene was prepared. Only
the perfluorohexane phase was irradiated with a low pressure
mercury lamp at room temperature under a nitrogen atmos-
phere. After irradiation for 2 h, the two-phase system was con-
verted to a homogeneous solution by the addition of 1,1,2-
trichloro-1,2,2-trifluoroethane. It was shown that perfluoro-
octylbenzene (3) was obtained in 70% yield as a desired product
by GC–MS analysis of the solution with n-nonane as an
internal standard [reaction (3)]. This result indicates that the
use of two-phase system for perfluorooctylation of benzene has
improved the reaction efficiency. The reaction mechanism for
the two-phase system was estimated on the basis of GC–MS
and UV–VIS analyses before and after irradiation. Before
irradiation it was shown that the upper phase (benzene) did not
contain (E)-1 at all and that the lower phase contained a small
amount of benzene (ca. 10 mol l ). After irradiation the
product 3 was obtained in a ratio of 4:1 from the perfluorohex-
ane and benzene phases, respectively. From these results it
seems likely that photolysis of (E)-1 occurred in the perfluoro-
hexane phase generating the perfluorooctyl radical, which
reacted with the small amount of benzene dissolved in the per-
fluorohexane phase or at the interface of the two phases to give
3
5
6
R = H
R = Me
R = OMe
7 R = Cl
C8F17
hν
(
E )-1/C6F14
+
+
(C8F17H)
(4)
X
X
8
X = O
9 X = S
0 X = NH
1
compounds 3, 5–10 were obtained in moderate to good yields.
These results suggest that this two-phase method is generally
applicable to the preparation of perfluoroalkyl aromatic
compounds.
In order to investigate the photolytic mechanism of (E)-1
upon 185 nm irradiation, the quantum yield and the light inten-
sity dependence of the formation of 2 by photolysis of (E)-1
were examined in perfluorohexane (1 × 10 mol l ). The azo
compound (E)-1 was photodenitrogenated upon 185 nm irradi-
ation in a quantum yield (ΦϪazo = 0.15) based upon cis–trans
isomerization of cis-cyclooctene (Φ = 0.32) as an actino-
meter. As shown in Fig. 3, the formation of 2 was proportional
to the square of 185 nm light intensity. These results suggest
that (E)-1 was photolyzed upon 185 nm irradiation by two-
Ϫ4
Ϫ1
Ϫ3
Ϫ1
16
3
. This reaction system takes advantage of an efficient gener-
cis–trans
7
ation of perfluorooctyl radicals in the lower phase for a shorter
irradiation time, compared with photolysis of (E)-1 in benzene,
and of a continuous supply of benzene from the upper phase to
perfluorooctyl radicals in the lower phase. To investigate the
17
photon absorption in perfluorohexane. This consideration is
6
60
J. Chem. Soc., Perkin Trans. 2, 1998

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Fig. 5 Light intensity dependence of the formation of 2 by photolysis
of (Z)-rich 1 upon 185 nm irradiation
Fig. 4 UV–VIS spectra after 0, 5, 10, 15 and 20 min upon 185 nm
irradiation of (E)-1
irradiation without any filters unless otherwise noted. In the
case of only 185 nm irradiation a 185 nm bandpass filter (185
nm transmission 17.2%, Acton Research Corp. 185-HR-25) was
employed with the lamp described above. For >300 nm irradi-
ation a 500 W high pressure mercury lamp (Ushio USH-500D)
through a filter (Toshiba UV-31) and for 370 nm irradiation a
consistent with Kaupp and Sauerland’s report that high inten-
sity irradiation with a XeCl excimer laser (308 nm) gave a high
concentration of pentafluoroethyl radicals by two photon
11
absorption of perfluoroazoethane during the pulse.
In addition, UV–VIS spectral analysis of this photoreaction
at 5 min intervals indicated that the intensity of the absorption
band of azo group at 380 nm decreased with increase of irradi-
ation time and that a broad absorption band which appeared at
longer wavelength decreased gradually (Fig. 4). The azo group
5
3
00 W Xe lamp (Ushio UI-501C) through filters (Toshiba UV-
5 and UV-D33S filter) were used. UV–VIS spectra were meas-
19
ured on a Shimadzu UV-265. F NMR spectra were recorded
on a Varian Gemini 300BB and mass spectra on a Shimadzu
GCMS-QP1000EX.
of (E)-1 has an absorption band at λ 380 nm, whereas that of
max
11
(
Z)-1 absorbs at λmax 435 nm. These spectral changes indicate
Benzene and toluene, spectroscopy grade from Merck
KGaA, perfluorohexane from PCR INCORPORATED, and
cis-cyclooctene from Merck KGaA were used without purifica-
tion. Anisole, chlorobenzene, furan, thiophene and pyrrole
from Tokyo Chemical Industry Co., Ltd. were purified by distil-
lation before use.
that trans–cis isomerization of (E)-1 and the subsequent deni-
trogenation of resulting (Z)-1 took place during the irradiation.
In order to confirm the speculated mechanism of this two-
photon process it was necessary to investigate the light intensity
dependence of the photolysis of pure (Z)-1 upon 185 nm
irradiation. However, as it was difficult to obtain pure (Z)-1
from the direct preparation and isomerization of (E)-1, (Z)-
rich 1 was used (trans:cis = 1.00:1.46), which was obtained
Photolysis of (E)-1 in benzene
Ϫ3
Ϫ1
Ϫ3
A benzene solution of (E)-1 (1 × 10 mol l ; 3 ml) under a
nitrogen atmosphere was irradiated with a low pressure mer-
cury lamp for 7 h or with a high pressure mercury lamp for 24 h.
The reaction mixture was analyzed by GC–MS with internal
standards (n-nonane and n-undecane).
from photoisomerization of (E)-1 in cyclohexane (5 × 10 mol
Ϫ1
l ) upon 370 nm irradiation with Xe lamp through filters for 2
11
h at room temperature. As shown in Fig. 5, the slope of the
formation of 2 against various light intensities was 1.4, showing
that this photolysis of (Z)-rich 1 consisted of the mixture of
one- and two-photonic processes. These results suggest that
7
(
E)-1 is photoisomerized to (Z)-1 by one photon absorption,
Photolysis of (E)-1 in two-phase system
Ϫ3
Ϫ1
followed by photodecomposition of (Z)-1 to perfluorooctyl
radicals by another photon absorption (Scheme 1).
A perfluorohexane solution of (E)-1 (1 × 10 mol l ; 2 ml)
and aromatic compounds (1 ml) were prepared under a nitro-
gen atmosphere. After only the perfluorohexane phase had been
irradiated with a low pressure mercury lamp for 2 h, 1,1,2-
trichloro-1,2,2-trifluoroethane (1 ml) was added in the vessel,
and then the homogeneous solution was analyzed by GC–MS.
The products were characterized by co-injection of authentic
C8F17
C8F17
hν
hν
2 C8F17• + N2
(E)-1
N
N
(Z )-1
1
8
samples prepared according to literature procedures on a
GC–MS.
Scheme 1
In conclusion, an efficient method for the preparation of
perfluorooctylated aromatic compounds was developed using
photolyis of (E)-1 upon 185 nm irradiation. The use of a
two-phase system containing a perfluorohexane solution of
Determination of 185 nm photolysis quantum yield of (E)-1
Ϫ3
A degassed perfluorohexane solution of (E)-1 (1.01 × 10 mol
Ϫ1
l ; 3 ml) were irradiated with a low pressure mercury lamp
2
(
E)-1 and aromatic compounds was useful for the efficient gen-
(1 cm ) equipped with a 185 nm bandpass filter for 5 min. A
Ϫ4
eration of perfluorooctyl radicals and subsequent perfluoro-
octylation of aromatic compounds. It was found that photolysis
of (E)-1 took place via trans–cis isomerization and denitrogen-
ation of (Z)-1 via a two-photon process.
perfluorohexane solution of cis-cyclooctene (2.48 × 10 mol
Ϫ1
l ; 3 ml) were irradiated for an actinometer (Φcis–trans = 0.32) in
a similar manner as mentioned above. The quantum yield was
determined by comparison with isomerization of cis-
cyclooctene.
Experimental
All photolyses and isomerization were carried out in quartz
cuvettes at room temperature. The light sources were a 60 W
low pressure mercury lamp (Eikosha EL-S-SQ-60) for 185 nm
Light intensity dependence of the formation of 3
Perfluorohexane solutions of (E)-1 were irradiated with a low
pressure mercury lamp equipped with ND filters (60, 49, 30 and
17%) for 20 min.
J. Chem. Soc., Perkin Trans. 2, 1998
661

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Preparation of (Z)-rich 1
9 W. Adam and T. Openländer, Angew. Chem., Int. Ed. Engl., 1986,
Ϫ2
Ϫ1
25, 661.
A hexane solution of (E)-1 (1.01 × 10 mol l , 30 ml) was
irradiated at 370 nm light with a 500 W Xe lamp for 2 h. The
reaction mixture was dried in vacuo to remove solvent. The
ratio of trans- and cis-isomer was determined by F NMR
measurement (trans:cis = 1.00:1.46) in CDCl (internal stan-
1
1
0 K. J. L. Paciorek, R. H. Kratzer, J. H. Nakahara and H. M. Atkins,
J. Fluorine Chem., 1991, 53, 355.
1 It has been reported that tetrakis(perfluoroethyl)hydrazine was
formed during photolysis of perfluoroazoethane which acts as a
good radical scavenger. G. Kaupp and O. Sauerland, J. Photochem.
Photobiol. A: Chem., 1991, 56, 375.
19
3
dard:CFCl ). The (Z)-rich 1 prepared was used for photo-
3
1
1
2 S. M. Pereira, G. P. Savage and G. W. Simpson, Synth. Commun.,
reaction without purification.
1
995, 25, 1023.
3 M. B. Robin, in Higher Excited States of Polyatomic Molecules,
Academic Press, New York, 1974, vol. 1, p. 178–191; A.
Maciejewski, J. Photochem. Photobiol. A: Chem., 1990, 51, 87.
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Paper 7/07171J
Received 3rd October 1997
Accepted 27th November 1997
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6
62
J. Chem. Soc., Perkin Trans. 2, 1998