The first carbonylation of perfluoroorganic compounds: The reactions of perfluorobenzocyclobutene and its perfluoroalkyl and pentafluorophenyl derivatives with CO in SbF5 medium

Article abstract of DOI:10.1016/j.jfluchem.2014.03.008

Carbonylation of polyfluorinated benzocyclobutenes with CO in an SbF ₅ medium readily proceeds at room temperature and atmospheric pressure and it is followed by four-membered ring transformations. Perfluorinated benzocyclobutene, 3- and 4-methyl-benzocyclobutenes add two CO molecules to form, after ring opening and following heterocyclization, isochromene derivatives. Hydrolysis of the reaction mixtures gives corresponding 4-carboxy-1H-isochromenes and 2-(carboxymethyl)benzoic acids. The carbonylation of perfluorinated 1-methyl-, 1-ethyl- and 1-isopropyl-benzocyclobutene gives salts of 2-(2-methylphenyl)alk-2-enoyl cations and the corresponding acids after hydrolysis of the latters. Perfluoro-1-phenylbenzocyclobutene in the reaction with CO-SbF₅ transforms into a salt of perfluro-4-phenylisochromenyl cation, its hydrolysis gives perfluro-4-phenylisochromen-1-one.

Full text of DOI:10.1016/j.jfluchem.2014.03.008

Journal of Fluorine Chemistry 162 (2014) 71–77
Contents lists available at ScienceDirect
Journal of Fluorine Chemistry
journal homepage: www.elsevier.com/locate/fluor
The first carbonylation of perfluoroorganic compounds: The reactions
of perfluorobenzocyclobutene and its perfluoroalkyl and
pentafluorophenyl derivatives with CO in SbF5 medium
a
Yaroslav V. Zonov a,b, Victor M. Karpov a, , Vyacheslav E. Platonov
*
a N.N. Vorozhtsov Novosibirsk Institute of Organic Chemistry, Novosibirsk 630090, Russia
b Novosibirsk State University, Novosibirsk 630090, Russia
A R T I C L E I N F O
A B S T R A C T
Article history:
Carbonylation of polyfluorinated benzocyclobutenes with CO in an SbF5 medium readily proceeds at
room temperature and atmospheric pressure and it is followed by four-membered ring transformations.
Perfluorinated benzocyclobutene, 3- and 4-methyl-benzocyclobutenes add two CO molecules to form,
after ring opening and following heterocyclization, isochromene derivatives. Hydrolysis of the reaction
mixtures gives corresponding 4-carboxy-1H-isochromenes and 2-(carboxymethyl)benzoic acids. The
carbonylation of perfluorinated 1-methyl-, 1-ethyl- and 1-isopropyl-benzocyclobutene gives salts of 2-
(2-methylphenyl)alk-2-enoyl cations and the corresponding acids after hydrolysis of the latters.
Perfluoro-1-phenylbenzocyclobutene in the reaction with CO–SbF5 transforms into a salt of perfluro-4-
phenylisochromenyl cation, its hydrolysis gives perfluro-4-phenylisochromen-1-one.
Received 14 February 2014
Received in revised form 20 March 2014
Accepted 22 March 2014
Available online 13 April 2014
Keywords:
Carbonylation
Carbocations
Perfluorinated benzocyclobutenes
Antimony pentafluoride
ß 2014 Elsevier B.V. All rights reserved.
1. Introduction
reported for cation reactions of polyfluorinated benzocyclobutenes
[10,11]. Irreversible ring opening would lead to complete
Fluorinated organic compounds are important to fundamental
organic chemistry [1] and its applications, particularly to materials
science, biomedicine and agriculture [1–3]. Of obvious interest are
comparative studies of fluorocarbon derivatives and their hydro-
carbon congeners.
conversion of starting benzocyclobutene even if the concentration
of B in the equilibrium is quite low. Therefore, a study into the
behavior of polyfluorinated benzocyclobutenes in the CO–SbF5
system seemed worthwhile.
In this work, we report on the carbonylation of perfluorinated
Many examples of carbonylation reactions of hydrocarbon
alcohols, alkyl halides, alkenes etc. proceeding by the addition of
CO to the cations generated from these compounds in acid
systems are known [4–6]. At the same time, even though there is
a wide variety of fluorinated cation reactions in the literature,
there are no examples of carbon monoxide addition to these
fluoro-cations. Decarbonylation of fluorinated acyl halides in the
presence of Lewis acids are the only known examples [7–9]; and
they may be influenced by thermodynamic factors that shift the
equilibrium of the carbonylation reactions to starting com-
pounds.
benzocyclobutene and a number of its perfluoroalkyl and
pentafluorophenyl derivatives under the action of carbon monox-
ide in the presence of SbF5.
2. Results and discussion
2.1. Reactions of polyfluorobenzocyclobutenes with CO–SbF5
We have found that carbonylation of polyfluorinated benzo-
cyclobutenes in an SbF5 medium readily proceeds at room
temperature and atmospheric pressure and it is followed by
four-membered ring transformations. Thus, perfluorobenzocyclo-
butene (1) reacts with CO in the presence of SbF5 to form a solution
of salts of perfluorinated (1H-isochromen-4-yl)(oxo)methyl (2)
and 4-fluorocarbonylisochromenyl (3) cations and perfluoro-4-
methylisochromen-1-one (4). Hydrolysis of the reaction mixture
gives perfluoro-4-carboxy-1H-isochromene (5) and 2-(carboxy-
methyl)-3,4,5,6-tetrafluorobenzoic acid (6) with admixture of
compound 4 (Scheme 2).
Polyfluorinated benzocyclobutenes appear to be a promising
study of carbonylation reactions with carbon monoxide in the
presence of SbF5. For example, carbonylation of cation A would
give B leading to four-membered ring opening (Scheme 1) as
*
Corresponding author. Tel.: +7 3833 30 6943; fax: +7 3832 34 4752.
E-mail address: karpov@nioch.nsc.ru (V.M. Karpov).
http://dx.doi.org/10.1016/j.jfluchem.2014.03.008
0022-1139/ß 2014 Elsevier B.V. All rights reserved.

72
Y.V. Zonov et al. / Journal of Fluorine Chemistry 162 (2014) 71–77
CFCF3
CF3
C2F5
CF2
+
C2F
F
C2F
F
5 CF3
5 CF2
SbF5
F
F
F
F
F
Ref. [10]
+
CF3
CF2
+
R
+
R
F
CO
R
R
SbF5
CO
CO
+
F
F
F
CR'R"
R"
R"
R'
R"
R'
R'
B
A
Scheme 1. Cationic ring opening of polyfluorobenzocyclobutenes.
CO
+
+
CO
COF
COF
F
CO
+
F
-
_
+
+
CO
F
CO
-
F
2
F
F
F
F
F
F
F
O
+
CF2
7
8
9
10
COF
+
SbF5, -F
CO
CF3
F
F
F
SbF5
70oC
CO, SbF5
20oC
4
F
F
F
F
F
F
O
O
O
+
F
O
1
3 (8%)
4 (15%)
2 (content 77%)
H2O
CF3
COOH
CH2COOH
F
F
+
+
F
F
O
O
COOH
O
6 (45%)
4 (6%)
5 (49%)
Scheme 2. Carbonylation reaction of perfluorobenzocyclobutene (1).
The transformations of compound 1 can be rationalized by
The reactions of perfluorinated 4-methyl- and 3-methyl-
benzocyclobutenes (11) and (12) with CO–SbF5 proceed regiose-
lectively and result in the formation of perfluorinated 6-methyl-
and 8-methyl-4-carboxy-1H-isochromenes (13) and (14) together
with 4-trifluoromethyl- and 6-trifluoromethyl-2-(carboxy-
methyl)-trifluorobenzoic acids (15) and (16), respectively (Scheme
3). Thus, the carbonylation occurs at the CF2 group in meta-
position to the CF3 group. This selectively arises from a higher
relative stability of the corresponding perfluoromethylbenzocy-
clobuten-1-yl cations without electron withdrawing CF3 group in
Scheme 2. At first, compound 1 with SbF5 generates cation 7 [12]
which adds a molecule of CO to form acyl cation 8. The latter
isomerizes to benzocyclobutenyl cation 9 which adds a second
molecule of CO to form cation 10. Opening of the four-
membered ring in cation 10 followed by heterocyclization gives
cation 2. Compounds 2–4 in the SbF5 solution appear to be in
equilibrium. This assumption is confirmed by formation of
similar mixture of the products in the reaction of individual
compound 4 with SbF5.
H2SO4, 70oC
98%
COOH
1) CO, SbF5,
CF3
CH2COOH
CF3
CF3
20oC
F
F
F
F
+
+
F
2) H2O
O
COOH
11
13
15
COOH
1) CO, SbF5,
20oC
2) H2O
CH2COOH
F
F
F
F
F
O
COOH
CF3
CF3
CF3
12
14
16
H2SO4, 70oC
93%
Scheme 3. Regioselectiv carbonylation of perfluoromethylbenzocyclobutenes 11 and 12.

Y.V. Zonov et al. / Journal of Fluorine Chemistry 162 (2014) 71–77
73
COOH
Despite the bulky perfluoroisopropyl group perfluoro-1-iso-
propylbenzocyclobutene (29) also readily reacts with CO–SbF5 to
form perfluoro-2-(2-methylphenyl)isopent-2-enoic acid (30), after
hydrolysis (Scheme 6).
1) CO, SbF5,
20oC
2) H2O
CF3
F
F
F
F
F
CF3
17
19
Perfluoro-1-phenylbenzocyclobutene (31) in the reaction with
CO–SbF5 forms a salt of perfluoro-4-phenylisochromenyl cation
(32) (Scheme 6), acyclic products of the four-membered ring
opening were not found. Hydrolysis of the reaction mixture gives
mainly perfluoro-4-phenylisochromen-1-one (33). Transforma-
tion of compound 31 into 32 apparently proceeds via addition of
CO molecule to perfluoro-1-phenylbenzocyclobutenyl cation (34)
[21] with subsequent formation of cation 35 after the ring opening.
Its further intramolecular cyclization gives isochromenyl cation
32. The reaction of compound 31 with CO–SbF5 readily proceeds at
room temperature but elevation of the reaction temperature up to
70 8C allows by-products formation to be decreased.
H2O
CO, SbF5
+
CO
CF3
F
SbF5
20oC
CF2
F
F
O
+
F
CF3
20
18
-
-
_
+
F
_
+
F
+
CF2
COF
CF2
2.2. Structure of compounds
F
F
F
+
The structures of the compounds were established by HRMS,
elemental analysis and spectral characteristics. Assignment of
signals in the 19F NMR spectra of compounds was made on the
basis of chemical shifts of the signals, their fine structure and
integral intensities. Compounds 4, 6, 20 [15], 24, 25 [16] were
identified by comparison of the 19F NMR data with data for
authentic samples.
The structures of E- and Z-isomers of compounds 23, 26 and 27
were defined on the base of through-space J(F,F) values between
closely located fluorine atoms. Thus, JCF3(E)–F(6), JCF3(E)–CF3(2) in the
Z-isomers are equal to 2 Hz whereas in the 19F NMR spectra of the
E-isomers JCF3(Z)–F(6), JCF3(Z)–CF3(2) are not observed. On the other
hand, JF(E)–CF3(2) = 3–3.5 Hz are measured in the 19F NMR spectra of
the E-isomers.
O
CF2
F
F
Scheme 4. Carbonylation reaction of perfluoro-1-methylbenzocyclobutene (17).
the resonance positions [13,14]. When heated with sulfuric acid,
isochromenes 13 and 14 are converted into homophthalic acids 15
and 16 in a high yield.
Perfluoro-1-methylbenzocyclobutene (17) in the reaction with
CO–SbF5 transforms into a salt of perfluoro-2-(2-methylphenyl)-
propenoyl cation (18) (Scheme 4). Its hydrolysis gives perfluoro-2-
(2-methylphenyl)propenoic acid (19). At room temperature cation
18 gradually undergoes heterocyclization to form perfluoro-4-
methylisochromenyl cation (20).
Perfluoro-1-ethylbenzocyclobutene (21) reacts with CO–SbF5
in a similar way to form a solution of a salt of perfluoro-2-(2-
methylphenyl)but-2-enoyl cation (22) (Scheme 5). The latter in
contrast to cation 18 does not undergo the cyclization at room
temperature. However, hydrolysis of the reaction mixture gives
not only perfluoro-2-(2-methylphenyl)but-2-enoic acid (23), but
also small amounts of perfluorinated 4-fluorocarbonyl-3-methy-
lisochromen-1-one (24) and 4-carboxy-3-methylisochromen-1-
one (25) as by-products of hydrolysis. Methanolysis of the solution
of the salt of cation 22 gives methyl perfluoro-2-(2-methylphe-
nyl)but-2-enoate (26) together with methyl 3,4,4,4-tetrafluoro-2-
(3,4,5,6-tetrafluoro-2-methoxycarbonylphenyl)but-2-enoate (27)
and 5,6,7,8-tetrafluoro-4-methoxycarbonyl-3-trifluoromethyliso-
chromen-1-one (28).
COX
RZ
RE
6
C
C
5
4
F
2R
3
Assignment of signals in the 19F NMR spectra of isochromenyl
cations 3 and 32 was made by analogy with that for cation 20 [15].
Chemical shifts and J(F,F) values in the 19F NMR spectra of acyl
cations 2, 18 and 22 are in agreement with those for perfluoro-2-
methylpropenoyl cation [17]; the structures of these cations are
additionally confirmed by isolation of the corresponding hydroly-
sis or metanolysis products.
+
COX
CO
COOH
CFCF3
CF3
CF2CF3
CF3
CO, SbF5
20oC
CFCF3
H2O
F
F
F
F
F
+
O
O
CF3
21
22
:
81
19
24 (X=F), 25 (X=OH)
23
COOCH3
CF3
COOCH3
CFCF3
CF3
COOCH3
CH3OH
CFCF3
F
F
F
+
+
O
COOCH3
O
:
:
61
13
27
26
26
28
Scheme 5. Reaction of perfluoro-1-ethylbenzocyclobutene (21) with CO.

74
Y.V. Zonov et al. / Journal of Fluorine Chemistry 162 (2014) 71–77
COOH
CF3
CF3
CF3
4.1. Reaction of perfluorobenzocyclobutene (1) with CO–SbF5
CF(CF3)2
1) CO, SbF5,
20oC
2) H2O
F
F
F
(1). A mixture of compound 1 (0.558 g) and SbF5 (3.323 g) (molar
ratio, 1:6.8) was intensively stirred under CO atmosphere at
room temperature for 2.5 h. The mixture was treated with cold
5% hydrochloric acid, extracted with mixture CH2Cl2–Et2O
(4:1) and dried over MgSO4. The extract contained compounds
4, 5 and 6 in the ratio 6:49:45. The solvents were distilled off,
the residue was dissolved in CH2Cl2 and extracted with
aqueous solution of NaHCO3. The aqueous extract was
acidified with HCl, extracted with CH2Cl2 and then with
Et2O. The extracts were dried over MgSO4. The solvents were
distilled off to give 0.231 g (yield 34%) of acid 5 (from CH2Cl2
extract) and 0.199 g (yield 35%) of acid 6 (from Et2O extract).
(2). A mixture of compound 1 (0.365 g) and SbF5 (2.553 g) (molar
ratio, 1:8) was intensively stirred under CO atmosphere at
room temperature for 3 h. 19F NMR spectrum of the resulting
solution contained signals of compound 4 and cations 2 and 3
in the ratio 15:77:8.
F
29
30
yield 53%
C6F5
C6F5
C6F5
F
F
CO, SbF5
70oC
H2O
F
F
F
O
O
O
+
F
31
SbF5 -F
33
yield 70%
32
-
_
+
-
F
C6F5
CO
C6F5
+
C6F5
CO
+
F
F
F
F
CO
F
+
CF2
35
34
4.1.1. Perfluoro-4-carboxy-1H-isochromene (5)
Scheme 6. Carbonylation reaction of perfluorinated 1-isopropyl- and 1-
phenylbenzocyclobutenes 29 and 31.
mp 125–126 8E (CCl4). IR (KBr)
(O55C–C55C); 1527, 1500 [fluorinated aromatic ring (FAR)]. 1H NMR
(CDCl3):
9.7 (bs, COOH). 19F NMR (CDCl3):
n
, cmÀ1: 1737, 1717, 1684
d
d
À51.8 (d, 2F, F-1),
À76.8 (ddddt, 1F, F-3), À135.8 (ddddt, 1F, F-5), À139.6 (dtddd, 1F,
F-8),À147.7 (ddddt, 1F, F-6), À153.4 (ddddt, 1F, F-7); J1,3 = 1,
J1,5 = 1.5, J1,6 = 1.5, J1,7 = 1, J1,8 = 21, J3,5 = 3.5, J3,6 = 1.5, J3,7 = 4.5,
J3,8 = 2.5, J5,6 = 20.5, J5,7 = 5, J5,8 = 12.5, J6,7 = 20, J6,8 = 8, J7,8 = 21.5.
HRMS m/z, 301.9809 (M+). Calcd. for C10 HF7O3 = 301.9808.
3. Conclusion
The behaviour of perfluorinated benzocyclobutene and
a
number of its alkyl and phenyl derivatives under the action of
carbon monoxide in the presence of SbF5 was investigated. It was
found that these compounds undergo carbonylation/four-mem-
bered ring opening tandem reactions and this is the first example
of carbonylation of perfluorinated compounds. Benzocyclobu-
tenes, which do not contain substituents in the aliphatic cycle, add
two CO molecules to form, after ring opening and following
heterocyclization, isochromene derivatives. Carbonylation of 3-
and 4-methyl-benzocyclobutenes occurs regioselectively at the
CF2 group in meta-position to the CF3 group. 1-Alkyl- and 1-
phenyl-benzocyclobutenes undergo monocarbonylation at the
substituted position of the four-membered ring to form finally
salts of 2-(2-methylphenyl)alk-2-enoyl and 4-phenylisochrome-
nyl cations, respectively.
4.2. Reaction of perfluoro-4-methylisochromen-1-one (4) with SbF5
A solution of compound 4 (0.364 g) in SbF5 (1.592 g) (molar
ratio, 1:6.1) was heated at 70 8C for 5 h, then SO2ClF (0.2 g) was
added at 0 8C and 19F NMR spectrum of the solution was measured
at 20 8C. The spectrum contained signals of compound 4 and
cations 2 and 3 in the ratio 11:80:9. The solution was poured into
5% hydrochloric acid and extracted with mixture CH2Cl2–Et2O
(4:1). The extract was dried over MgSO4. Distillation of the solvent
gave 0.280 g of a mixture of compounds 4, 5 and 6 in the ratio
8:36:56.
4.2.1. Perfluoro(1H-isochromen-4-yl)(oxo)methyl cation (2)
19F NMR (SbF5–SO2ClF):
d
À16.7 (s, 1F, F-3), À38.1 (d, 2F, F-1,
4. Experimental
J1,8 = 14), À131.1 (m, 1F, F-8 or F-5), À131.3 (m, 1F, F-5 or F-8),
À138.9 (m, 1F, F-6 or F-7), À143.1 (m, 1F, F-7 or F-6).
Analytical and spectral measurements were carried out in the
Collective Chemical Service Center of SB RAS. IR spectra were
taken on a Bruker Vector 22 IR spectrophotometer. 19F and 1H
NMR spectra were recorded on a Bruker AV 300 instrument
(282.4 MHz and 300 MHz, respectively). Chemical shifts are
4.2.2. 4-Fluorocarbonyl-perfluoroisochromenyl cation (3)
19F NMR (SbF5–SO2ClF):
d 56.0 (dd, 1F, COF, JCOF–F(5) = 33, JCOF–
F(3) = 18,), À20.0 (d, 1F, F-1, J1,8 = 80), À64.1 (m, 1F, F-3), À99.0 (m,
1F, F-6), À111.3 (dm, 1F, F-8, J1,8 = 80), À126.8 (m, 1F, F-5), À135.6
(m, 1F, F-7).
given in d
ppm from CCl3F (19F) and TMS (1H), J values in Hz; C6F6
and SO2ClF (À162.9 and 99.9 ppm from CCl3F), CHCl3 and
acetone-d5 (7.24 and 2.04 ppm from TMS) were used as internal
standards. The molecular masses of the compounds were
determined by high-resolution mass-spectrometry on a Thermo
Electron Corporation DFS instrument (EI 70 eV). Contents of
products in the reaction mixtures were established by 19F NMR
spectroscopic data.
Antimony pentafluoride was distilled at atmospheric pressure
(bp 142–143 8C); carbon monoxide was prepared by decomposi-
tion of formic acid in concentrated sulfuric acid and it was
additionally dried with the latter. The starting compounds were
obtained according to Refs.: 1 and 17 [18]; 21 [12]; 29 [19];
mixture of isomers 11 and 12 [18,20]. The reactions were carried
out in glassware at atmospheric pressure.
4.3. Reaction of perfluorinated 4-methyl- and 3- methyl-
benzocyclobutenes (11) and (12) with CO–SbF5
(1). Analogously to procedure 4.1. a mixture of compounds 11 and
12 (0.667 g) and SbF5 (3.403 g) (molar ratio, 0.92:0.08:6.9)
gave mixtures of compounds 13 and 14 in the ratio 92:8
(0.430 g, yield 55%) and compounds 15 and 16 in the ratio 93:7
(0.100 g, yield 15%). The former mixture was sublimed (125 8C,
3 Torr), subsequent recrystallization from CCl4 gave 0.185 g of
acid 13.
(2). Analogously to procedure 4.1. a mixture of compounds 11 and
12 (0.540 g) and SbF5 (3.088 g) (molar ratio, 0.23:0.77:7.8)
gave mixtures of compounds 13 and 14 in the ratio 26:74

Y.V. Zonov et al. / Journal of Fluorine Chemistry 162 (2014) 71–77
75
(0.250 g, yield 39%) and compounds 15 and 16 in the ratio
30:70 (0.166 g, yield 30%). The former mixture was sublimed
(115 8C, 3 Torr), subsequent recrystallization from CCl4 gave
0.056 g of acid 14.
(2). A mixture of compound 17 (0.430 g) and SbF5 (2.500 g) (molar
ratio, 1:8) was intensively stirred under CO atmosphere at
room temperature for 1 h and then kept at this temperature.
19F NMR spectrum of the resulting solution contained signals
of cations 18 and 20 in the ratio 68:32 (after 6 h) and 25:75
(after 24 h).
4.3.1. Perfluoro-4-carboxy-6-methyl-1H-isochromene (13)
mp 107.5–109.5 8E (CCl4). IR (KBr)
1649 (O55C–C55C); 1500, 1427 (FAR). 1H NMR (CDCl3):
COOH). 19F NMR (CDCl3):
À77.2 (m, 1F, F-3), À112.7 (qdm, 1F, F-5), À132.0 (qdd, 1F, F-7),
À140.3 (dtdd, 1F, F-8); JCF3(6)–F(5) = 23, JCF3(6)–F(7) = 23, J1,8 = 21,
J3,7 = 5, J3,8 = 2.5, J5,8 = 16.5, J7,8 = 21. HRMS m/z, 351.9775 (M+).
Calcd. for C11 HF9O = 351.9777. Anal. Calcd for C11HF9O3: C, 37.5;
H, 0.3; F, 48.6%. Found: C, 37.5; H, 0.4; F, 48.3%.
n
, cmÀ1: 1740, 1720, 1680,
d
9.3 (bs,
4.5.1. Perfluoro-2-(2-methylphenyl)propenoyl cation (18)
d
À53.6 (d, 2F, F-1), À58.0 (t, 3F, CF3-6),
19F NMR (SbF5):
d
5.7 (d, 1F, F-Z or F-E, JZ,E = 158), À5.1 (d, 1F, F-Z
or F-E, JZ,E = 158), À53.9 (d, 3F, CF3-2, J CF3(2)–F(3) = 24), À125.2 (m,
1F, F-6), À128.7 (m, 1F, F-3), À136.2 (m, 1F, F-4 or F-5), À140.5 (m,
1F, F-5 or F-4).
4.5.2. Perfluoro-2-(2-methylphenyl)propenoic acid (19)
mp 66.5–68 8E. IR (CCl4)
C55C); 1526, 1485 (FAR). 1H NMR (CDCl3):
NMR (CDCl3):
(ddq, 1F, F-E), À135.0 (dm, 1F, F-6), À137.3 (qddd, 1F, F-3),
À148.2 (ddd, 1F, F-5), À150.5 (ddd, 1F, F-4); JZ,E = 33.5, JE,6 = 3,
JCF3(2)–F(E) = 3, JCF3(2)–F(3) = 23, J3,4 = 21, J3,5 = 8, J3,6 = 11, J4,5 = 20,
J4,6 = 6, J5,6 = 22. HRMS m/z, 323.9831 (M+). Calcd. for
n
, cmÀ1: 1742, 1707, 1680 (O55C–
4.3.2. Perfluoro-4-carboxy-8-methyl-1H-isochromene (14)
d
11.6 (bs, COOH). 19F
mp 138–141 8E (CCl4). IR (KBr)
(O55C–C55C); 1509, 1479 (FAR). 1H NMR (CDCl3):
19F NMR [CO(CD3)2]:
À80.1 (m, 1F, F-3), À123.1 (ddm, 1F, F-5), À128.8 (qddd, 1F, F-7),
À147.7 (ddm, 1F, F-6); JCF3(8)–F(1) = 21, JCF3(8)–F(7) = 31, J3,7 = 3.5,
J5,6 = 20, J5,7 = 15, J6,7 = 20. HRMS m/z, 351.9779 (M+). Calcd. for C11
HF9O3 = 351.9777. Anal. Calcd for C11HF9O3: C, 37.5; H, 0.3; F,
48.6%. Found: C, 37.2; H, 0.3; F, 49.0%.
n
, cmÀ1: 1742, 1651, 1630
d
À57.7 (dd, 3F, CF3-2), À60.4 (d, 1F, F-Z), À60.6
d
8.5 (bs, COOH).
d
À52.7 (q, 2F, F-1), À53.9 (dt, 3F, CF3-8),
C
10HF9O2 = 323.9827. Anal. Calcd for C10HF9O2: C, 37.1; H, 0.3;
F, 52.8%. Found: C, 37.0; H, 0.5; F, 52.8%.
4.6. Reaction of perfluoro-1-ethylbenzocyclobutene (21) with CO–
SbF5
4.4. Reaction of perfluorinated 4-carboxy-6-methyl-1H-isochromene
(13) and 4-carboxy-8-methyl-1H-isochromene (14) with H2SO4
(1). A mixture of compound 21 (0.515 g) and SbF5 (2.543 g) (molar
ratio, 1:8) was intensively stirred under CO atmosphere at
room temperature for 3 h. 19F NMR spectrum of the resulting
solution contained signals of cation 22 (E:Z = 92:8). The
solution was kept at room temperature for 46 h but no
significant changes in the spectrum were observed. The
mixture was treated with cold 5% hydrochloric acid, extracted
with CH2Cl2 and dried over MgSO4. The solvent was distilled
off to give 0.483 g of a product containing ꢀ85% of compounds
23 (E:Z = 82:18), 24 and 25 in the ratio 81:7:12. The product
was dissolved in CH2Cl2 and extracted with aqueous solution
of NaHCO3. The aqueous extract was acidified with HCl,
extracted with CH2Cl2 and dried over MgSO4. The solvent was
distilled off, and recrystallization of a residue from hexane
gave 0.098 g of a mixture of compounds 23 (E:Z = 99:1), 25 and
5,6,7,8-tetrafluoro-3-hydroxy-3-trifluoromethyl-3,4-dihy-
droisochromen-1-one [16] in the ratio 84:2:14.
(1). A mixture of compound 13 (0.085 g) and 90% sulfuric acid
(3.4 g) was heated at 70 8C for 5 h. The mixture was poured
into water, extracted with Et2O and dried over MgSO4. The
solvent was distilled off and the residue was sublimed (150 8C,
3 Torr) to give 0.072 g (yield 98%) of acid 15.
(2). Analogously to previous procedure compound 14 (0.020 g)
gave acid 16 (0.016 g, yield 93%).
4.4.1. 2-(Carboxymethyl)-3,5,6-trifluoro-4-trifluoromethylbenzoic
acid (15)
mp 174–176.5 8E. IR (KBr)
1472, 1433 (FAR). 1H NMR [CO(CD3)2]:
2H, CH2). 19F NMR [CO(CD3)2]:
1F, F-3), À135.2 (qd, 1F, F-5), À139.8 (dd, 1F, F-6); JCH2–F(3) = 2,
JCF3(4)–F(3) = 22, JCF3(4)–F(5) = 23.5, J3,6 = 15, J5,6 = 20.5. Anal. Calcd for
C10 H4F6O4: F, 37.7%. Found: F, 37.8%.
n
, cmÀ1: 1726, 1695 (C55O); 1489,
9.3 (bs, 2H, COOH), 3.94 (d,
À55.8 (t, 3F, CF3-4), À117.0 (qdm,
d
d
(2). A mixture of compound 21 (0.548 g) and SbF5 (2.050 g) (molar
ratio, 1:6) was intensively stirred under CO atmosphere at
room temperature for 2 h. The mixture was poured into CH3OH
(20 ml) at 0 8C, then treated with cold 5% hydrochloric acid
(200 ml), extracted with CH2Cl2 and dried over MgSO4. The
solvent was distilled off to give 0.570 g of a product containing
ꢀ85% of compounds 26 (E:Z = 86:14), 27 (E:Z = 62:38) and 28
in the ratio 61:13:26. Silica gel column chromatography
(CHCl3 as eluent) gave 0.275 g (yield 45%) of compound 26
(E:Z = 88:12), 0.040 g (yield 7%) of compound 27 (E:Z = 81:19),
0.054 g (yield 10%) of compound 28.
4.4.2. 2-(Carboxymethyl)-3,4,5-trifluoro-6-trifluoromethylbenzoic
acid (16)
mp 169–171 8E. IR (KBr)
1516, 1475 (FAR). 1H NMR [CO(CD3)2]:
2H, CH2). 19F NMR [CO(CD3)2]:
1F, F-3), À133.7 (dqd, 1F, F-5), À156.4 (dd, 1F, F-4); JCH2–F(3) = 2,
JCF3(6)–F(5) = 19.5, J3,4 = 20, J3,5 = 13.5, J4,5 = 20. HRMS m/z, 302.0002
(M+). Calcd. for C10 H4F6O4 = 302.0008.
n
, cmÀ1: 1724, 1697, 1664 (C55O);
9.3 (bs, 2H, COOH), 3.83 (d,
À55.6 (d, 3F, CF3-6), À126.1 (ddm,
d
d
4.5. Reaction of perfluoro-1-methylbenzocyclobutene (17) with CO–
SbF5
4.6.1. Perfluoro-2-(2-methylphenyl)but-2-enoyl cation (22)
Mixture of two isomers, ratio E:Z = 92:8.
(1). A mixture of compound 17 (0.580 g) and SbF5 (2.356 g) (molar
ratio, 1:5.6) was intensively stirred under CO atmosphere at
room temperature for 1 h. The mixture was treated with cold
5% hydrochloric acid, extracted with CH2Cl2 and dried over
MgSO4. The solvent was distilled off to give 0.525 g of a
product containing ꢀ85% of acid 19. It was dissolved in hexane
(5 ml) and filtrated, subsequent sublimation (110 8C, 10 Torr)
and recrystallization from hexane gave 0.188 g (yield 30%) of
acid 19.
E-22: 19F NMR (SbF5):
d
À21.6 (s, 1F, F-E), À53.7 (d, 3F, CF3-2,
JCF3(2)–F(3) = 22,), À68.93 (d, 3F, CF3-Z, JCF3(Z)–F(E) = 7), À124.3 (m, 1F,
F-6), À127.1 (m, 1F, F-3), À134.3 (m, 1F, F-4 or F-5), À139.5 (m, 1F,
F-5 or F-4).
Z-22 19F NMR (SbF5):
d
À31.2 (s, 1F, F-Z), À52.8 (d, 3F, CF3-2,
JCF3(2)–F(3) = 24,), À66.9 (bs, 3F, CF3-E), À123.7 (m, 1F, F-6), À127.9
(m, 1F, F-3), À134.1 (m, 1F, F-4 or F-5), À139.6 (m, 1F, F-5 or F-4).

76
Y.V. Zonov et al. / Journal of Fluorine Chemistry 162 (2014) 71–77
4.6.2. Perfluoro-2-(2-methylphenyl)but-2-enoic acid (23)
The solvent was distilled off to give 0.751 g of a product containing
ꢀ70% of acid 30. The product was dissolved in CH2Cl2 extracted
with aqueous solution of NaHCO3. The aqueous extract was
acidified with HCl, extracted with CH2Cl2 and dried over MgSO4.
The solvent was distilled off and the residue was sublimed (90 8C,
3 Torr) to give 0.483 g (yield 53%) of acid 30.
Mixture of acid 23 (two isomers, E:Z = 99:1) with compounds
25 and 5,6,7,8-tetrafluoro-3-hydroxy-3-trifluoromethyl-3,4-dihy-
droisochromen-1-one in the ratio 84:2:14. 1H NMR (CDCl3):
(bs, COOH).
d 8.6
E-23: 19F NMR (CDCl3):
d
À57.5 (dd, 3F, CF3-2), À67.6 (d, 3F, CF3-
Z), À97.7 (qdq, 1F, F-E), À135.3 (dddd, 1F, F-6), À136.6 (qddd, 1F, F-
3), À147.2 (ddd, 1F, F-5), À149.7 (ddd, 1F, F-4); JCF3(Z)–F(E) = 5.5,
4.7.1. Perfluoro-2-(2-methylphenyl)isopent-2-enoic acid (30)
J
E,6 = 5, JCF3(2)–F(E) = 3.5, JCF3(2)–F(3) = 21.5, J3,4 = 20.5, J3,5 = 8.5,
mp 59–62 8E. IR (CCl4)
1H NMR (CDCl3): 11.0 (bs, COOH). 19F NMR (CDCl3):
n
, cmÀ1: 1732 (C55O); 1528, 1481 (FAR).
J3,6 = 10.5, J4,5 = 20.5, J4,6 = 6, J5,6 = 21.5.
Z-23: 19F NMR (CDCl3):
d
d
À56.8 (d, 3F,
d
À56.9 (d, 3F, CF3-2), À69.8 (dqd, 3F,
CF3-2), À61.1 (qm, 3F, CF3-E), À61.3 (q, 3F, CF3-Z), À134.2 (dm, 1F,
F-6), À135.4 (dqdd, 1F, F-3), À146.4 (ddd, 1F, F-5), À148.4 (ddd, 1F,
F-4); JCF3(E)–CF3(Z) = 8, JCF3(2)–F(3) = 17.5, J3,4 = 20.5, J3,5 = 9,
J3,6 = 11, J4,5 = 20.5, J4,6 = 6.5, J5,6 = 21.5. HRMS m/z, 423.9759
(M+). Calcd. for C12HF13O2 = 423.9763. Anal. Calcd for C12HF13O2:
C, 34.0; H, 0.2; F, 58.2%. Found: C, 33.8; H, 0.5; F, 58.2%.
CF3-E), À100.0 (q, 1F, F-Z) À135.0 (m, 1F, F-6), À136.9 (m, 1F, F-3),
À147.5 (m, 1F, F-5), À149.5 (m, 1F, F-4); JCF3(E)–F(Z) = 7.5, JCF3(E)–
F(6) = 2, JCF3(E)–CF3(2) = 2, JCF3(2)–F(3) = 21.
4.6.3. Methyl perfluoro-2-(2-methylphenyl)but-2-enoate (26)
Mixture of two isomers, ratio E:Z = 88:12, liquid. IR (CCl4)
cmÀ1: 2957 (CH); 1759, 1742 (C55O); 1526, 1485 (FAR).
n,
4.8. Reaction of perfluoro-1-phenylbenzocyclobutene (31) with CO–
SbF5
E-26: 1H NMR (CDCl3):
d
3.81 (s, CH3).19F NMR (CDCl3):
d
À57.4
(dd, 3F, CF3-2), À67.6 (d, 3F, CF3-Z), À103.2 (qdq, 1F, F-E), À135.3
(dddd, 1F, F-6), À136.9 (qddd, 1F, F-3), À147.6 (ddd, 1F, F-5),
À150.2 (ddd, 1F, F-4); JCF3(Z)–F(E) = 6, JE,6 = 5, JCF3(2)–F(E) = 3, JCF3(2)–
F(3) = 21.5, J3,4 = 20.5, J3,5 = 8.5, J3,6 = 11, J4,5 = 20.5, J4,6 = 6,
J5,6 = 21.5.
(1). A solution of compound 31 in SbF5 was prepared by the
reaction of benzocyclobutene 1 (0.518 g) with C6F5H (0.361 g)
in SbF5 (3.152 g) (molar ratio, 1:1:7) [21]. The solution was
intensively stirred under CO atmosphere at 70 8C for 4 h, then
SO2ClF (0.4 g) was added at 0 8C and 19F NMR spectrum of the
resulting solution was measured at 20 8C. The spectrum
contained signals of cation 32. The mixture was treated with
cold 5% hydrochloric acid, extracted with CH2Cl2 and dried
over MgSO4. The solvent was distilled off to give 0.852 g of a
residue containing ꢀ90% of compound 33. Sublimation
(110 8C, 3 Torr) and subsequent recrystallization of the residue
from hexane gave 0.588 g (yield 70%) of compound 33.
Z-26: 1H NMR (CDCl3):
d
3.83 (s, CH3). 19F NMR (CDCl3):
d
À56.9
(d, 3F, CF3-2), À69.7 (dqd, 3F, CF3-E), À104.3 (q, 1F, F-Z) À135.1 (m,
1F, F-6), À137.3 (qddd, 1F, F-3), À148.0 (ddd, 1F, F-5), À150.1 (ddd,
1F, F-4); JCF3(E)–F(Z) = 8, JCF3(E)–F(6) = 2, JCF3(E)–CF3(2) = 2, JCF3(2)–
F(3) = 21.5, J3,4 = 20.5, J3,5 = 8, J3,6 = 11, J4,5 = 20.5, J4,6 = 6,
J5,6 = 21.5. HRMS (mixture of two isomers) m/z, 387.9948 (M+).
Calcd. for C12H3F11O2 = 387.9952.
4.6.4. Methyl 3,4,4,4-tetrafluoro-2-(3,4,5,6-tetrafluoro-2-
(2). Analogously to previous procedure the reaction of compound
methoxycarbonylphenyl)but-2-enoate (27)
31, prepared from benzocyclobutene 1 (0.530 g), C6F5H
Mixture of two isomers, ratio E:Z = 81:19, liquid. IR (CCl4)
cmÀ1: 2957 (CH); 1751, 1738 (C55O); 1517, 1481 (FAR).
n
,
(0.370 g) and SbF5 (3.159 g) (molar ratio, 1:1:6.8), with CO
at room temperature (4 h) gave 0.890 g of a product, which
contained ꢀ60% of compound 33 together with unidentified
impurities.
E-27: 1H NMR (CDCl3):
NMR (CDCl3):
d F
3.89 (s, 3H, CH3), 3.79 (s, 3H, CH3).19
d
À67.6 (d, 3F, CF3-Z), À107.4 (dq, 1F, F-E), À135.3
(ddd, 1F, F-3), À136.3 (dddd, 1F, F-6), À148.8 (ddd, 1F, F-5), À151.2
(ddd, 1F, F-4); JCF3(Z)–F(E) = 6.5, JE,6 = 7, J3,4 = 21.5, J3,5 = 8, J3,6 = 11.5,
J4,5 = 21.5, J4,6 = 5.5, J5,6 = 21.5.
4.8.1. Perfluoro-4-phenylisochromenyl cation (32)
19F NMR (SbF5–SO2ClF):
d
À19.5 (dm, 1F, F-1, J1,8 = 78), À68.2
Z-27: 1H NMR (CDCl3):
NMR (CDCl3):
À69.2 (dd, 3F, CF3-E), À107.3 (qd, 1F, F-E), À135.0
(ddd, 1F, F-3), À136.5 (ddddq, 1F, F-6), À148.9 (ddd, 1F, F-5),
Z,6 = 3,
d
3.89 (s, 3H, CH3), 3.81 (s, 3H, CH3). 19
F
(m, 1F, F-3), À103.4 (m, 1F, F-6), À114.7 (dm, 1F, F-8, J1,8 = 78),
À134.2 (m, 1F, F-5), À136.4 (m, 2F, F-ortho), À137.6 (m, 1F, F-7),
À144.8 (tm, 1F, F-para, Jpara,meta = 19), À158.7 (m, 2F, F-meta).
d
À151.1 (ddd, 1F, F-4); JCF3(E)–F(Z) = 8, JCF3(E)–F(6) = 2,
J
J3,4 = 21.5, J3,5 = 8, J3,6 = 11.5, J4,5 = 20, J4,6 = 5.5, J5,6 = 22. HRMS
(mixture of two isomers) m/z, 378.0127 (M+). Calcd. for
4.8.2. Perfluoro-4-phenylisochromen-1-one (33)
mp 76–77 8C (hexane). IR (CCl4)
n
, cmÀ1: 1798 (C55O); 1681
À76.1 (ddtdd, 1F, F-3),
C
13H6F8O4 = 378.0133.
(C55C); 1508 (FAR). 19F NMR (CDCl3):
d
À131.4 (dddd, 1F, F-8), À139.5 (m, 2F, F-ortho), À141.2 (dddd, 1F,
F-6), À145.8 (ddddt, 1F, F-5), À151.6 (tt, 1F, F-para), À153.9 (dddd,
1F, F-7), À161.9 (m, 2F, F-meta); Jpara,meta = 21, Jpara,ortho = 2.5,
Jortho,3 = 4, Jortho,5 = 2, J3,5 = 4.5, J3,6 = 2, J3,7 = 5.5, J3,8 = 3, J5,6 = 20,
J5,7 = 2.5, J5,8 = 14, J6,7 = 20.5, J6,8 = 13.5, J7,8 = 20.5. HRMS m/z,
401.9734 (M+). Calcd. for C15F10O2 = 401.9733.
4.6.5. 5,6,7,8-tetrafluoro-4-methoxycarbonyl-3-
trifluoromethylisochromen-1-one (28)
mp 88–89 8C (hexane). IR (CCl4)
(C55O); 1636 (C55C); 1508 (FAR). 1H NMR (CDCl3):
19F NMR (CDCl3):
n
, cmÀ1: 2957 (CH), 1787, 1757
3.95 (s, CH3).
d
d
À68.5 (s, 3F, CF3), À129.8 (ddd, 1F, F-8), À138.4
(ddd, 1F, F-5), À140.7 (ddd, 1F, F-6), À147.7 (ddd, 1F, F-7); J5,6 = 20,
J5,7 = 5, J5,8 = 14, J6,7 = 20.5, J6,8 = 13, J7,8 = 20. HRMS m/z, 343.9905
(M+). Calcd. for C12H3F7O4 = 343.9914. Anal. Calcd for C12H3F7O4: C,
41.9; H, 0.9; F, 38.6%. Found: C, 41.5; H, 1.2; F, 38.3%.
Acknowledgments
We gratefully acknowledge the Russian Foundation for Basic
Researches (project no. 10-03-00120) for financial support.
4.7. Reaction of perfluoro-1-isopropylbenzocyclobutene (29) with
CO–SbF5
References
A mixture of compound 29 (0.848 g) and SbF5 (2.922 g) (molar
ratio, 1:6.3) was intensively stirred under CO atmosphere at room
temperature for 2 h. The mixture was treated with cold 5%
hydrochloric acid, extracted with CH2Cl2 and dried over MgSO4.
[1] (a) R.D. Chambers, Fluorine in Organic Chemistry, Blackwell Publishing, Oxford,
2004;
(b) P. Kirsch, Modern Fluoroorganic Chemistry. Synthesis, Reactivity,
Applications, Wiley-VCH, Weinheim, 2004;

Y.V. Zonov et al. / Journal of Fluorine Chemistry 162 (2014) 71–77
77
(c) R.E. Banks, B.E. Smart, J.C. Tatlow (Eds.), Organofluorine Chemistry: Principles
and Commercial Applications, Plenum, New York, NY, 1994.
[2] (a) M.L. Tang, Z. Bao, Chem. Mater. 23 (2011) 446–455;
(b) F. Babudri, G.M. Farinola, F. Naso, R. Ragni, Chem. Commun. (2007) 1003–
1022.
[11] V.M. Karpov, T.V. Mezhenkova, V.E. Platonov, V.R. Sinyakov, J. Fluorine Chem. 117
(2002) 73–81.
[12] V.M. Karpov, T.V. Mezhenkova, V.E. Platonov, G.G. Yakobson, J. Fluorine Chem. 28
(1985) 121–137.
[13] G.A. Olah, C.U. Pittman, J. Am. Chem. Soc. 88 (1966) 3310–3313.
[14] Yu.V. Pozdnyakovich, V.D. Shteingarts, J. Fluorine Chem. 4 (1974) 283–296.
[15] Ya.V. Zonov, T.V. Mezhenkova, V.M. Karpov, V.E. Platonov, Zh. Org. Khim. 44
(2008) 1675–1679;
Ya.V. Zonov, T.V. Mezhenkova, V.M. Karpov, V.E. Platonov, Russ. J. Org. Chem. 44
(2008) 1652–1656 (English translation).
[16] Ya.V. Zonov, V.M. Karpov, V.E. Platonov, J. Fluorine Chem. 128 (2007) 1065–
1073.
[17] V.F. Snegirev, M.V. Galakhov, V.A. Petrov, K.N. Makarov, V.I. Bakhmutov, Izv. Akad,
Nauk SSSR Ser. Khim. (1986) 1318–1325;
[3] (a) I. Ojima (Ed.), Fluorine in Medicinal Chemistry and Chemical Biology, Wiley-
Blackwell, Oxford, UK, 2009, pp. 291–311;
(b) J.-P. Begue, D. Bonnet-Delpon, Bioorganic and Medicinal Chemistry of
Fluorine, Wiley, Hoboken, 2008;
(c) S. Purser, P.R. Moore, S. Swallow, V. Gouverneur, Chem. Soc. Rev. 37 (2008)
320–330;
(d) F. Leroux, P. Jeschke, M. Schlosser, Chem. Rev. 105 (2005) 827–856;
(e) C.D. Hewitt, M.J. Silvester, Aldrichim. Acta 21 (1988) 3–10.
[4] Ya.T. Eidus, A.L. Lapidus, K.V. Puzitskii, B.K. Nefedov, Usp. Khim. 42 (1973) 442–
470;
Ya.T. Eidus, A.L. Lapidus, K.V. Puzitskii, B.K. Nefedov, Russ. Chem. Rev. 42 (1973)
199–213 (English translation).
[5] J.A. Falbe (Ed.), New Syntheses with Carbon Monoxide, Springer, New York, NY,
1980.
V.F. Snegirev, M.V. Galakhov, V.A. Petrov, K.N. Makarov, V.I. Bakhmutov, Bull.
Acad. Sci. USSR, Div. Chem. Sci. 35 (1986) 1194–1200 (English translation).
[18] V.E. Platonov, T.V. Senchenko, G.G. Yakobson, Zh. Org. Khim. 12 (1976) 816–821;
V.E. Platonov, T.V. Senchenko, G.G. Yakobson, J. Org. Chem. USSR 12 (1976) 818–
823 (English translation).
[6] O. Farooq, M. Marcelli, G.K.S. Prakash, G.A. Olah, J. Am. Chem. Soc. 110 (1988) 864–
867.
[19] V.M. Karpov, T.V. Mezhenkova, V.E. Platonov, G.G. Yakobson, Izv. Akad Nauk SSSR
Ser. Khim. (1985) 2315–2323;
[7] G.A. Olah, P.v.R. Schleyer (Eds.), Carbonium Ions, vol. 5, Wiley-Interscience, New
York, NY, 1976.
[8] C.G. Krespan, V.A. Petrov, Chem. Rev. 96 (1996) 3269–3301.
[9] V.V. Bardin, G.G. Furin, G.G. Yakobson, Zh. Org. Khim. 20 (1984) 567–573;
V.V. Bardin, G.G. Furin, G.G. Yakobson, Russ. J. Org. Chem. 20 (1984) 514–519
(English translation).
V.M. Karpov, T.V. Mezhenkova, V.E. Platonov, G.G. Yakobson, Bull. Acad. Sci. USSR,
Div. Chem. Sci. 34 (1985) 2143–2150 (English translation).
[20] V.R. Sinyakov, Ya.V. Zonov, V.M. Karpov, I.V. Beregovaya, V.E. Platonov, Zh. Org.
Khim. 49 (2013) 1026–1034;
V.R. Sinyakov, Ya.V. Zonov, V.M. Karpov, I.V. Beregovaya, V.E. Platonov, Russ. J.
Org. Chem. 49 (2013) 1010–1018 (English translation).
[10] V.M. Karpov, T.V. Mezhenkova, V.E. Platonov, G.G. Yakobson, Izv. Akad, Nauk SSSR
Ser. Khim. (1990) 1114–1120;
[21] V.M. Karpov, T.V. Mezhenkova, V.E. Platonov, V.R. Sinyakov, L.N. Shchegoleva, Zh.
Org. Khim. 38 (2002) 1210–1217;
V.M. Karpov, T.V. Mezhenkova, V.E. Platonov, G.G. Yakobson, Bull. Acad. Sci. USSR,
Div. Chem. Sci. 39 (1990) 1000–1004 (English translation).
V.M. Karpov, T.V. Mezhenkova, V.E. Platonov, V.R. Sinyakov, L.N. Shchegoleva,
Russ. J. Org. Chem. 38 (2002) 1158–1165 (English translation).